# AC 65-12A: Airframe & Powerplant Mechanics Powerplant Handbook - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Historical - Approved: January 1, 1976 - Last reviewed: October 22, 2024 - This change: Canceled by Memo signed by John Barbagallo and Richard Mathews; dated 05/21/2015. - Family: AC 65-12 - 14 CFR parts: 1 - Pages: 503 - PDF: https://openairdocs.org/files/sha256/45/90/45903ceaafc6d4122cababc37cb6b995c57a31b49e74277bce38f45b772e4049 - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/614BD958F4D610DF862569EE0077E5F0.0001 - DRS record last changed: November 22, 2025 - Page: https://openairdocs.org/ac/65-12 ## Versions - [AC 65-12A](https://openairdocs.org/ac/65-12) (Historical), approved Jan 1, 1976, 503 pages. Canceled by Memo signed by John Barbagallo and Richard Mathews; dated 05/21/2015. ## Text Each page's text from the PDF; open the PDF at a page with #page=N. ### Page 1 Airframe df Powerplant **MECHANICS** **POWERPLANT HANDBOOK US. DEPARTMENT OF TRANSPORTATION** **FEDERAL AVIATION ADMINISTRATION 4** ### Page 2 **AIRFRAME AND POWERPLANT** **MECHANICS** **POWERPLANT HANDBOOK** **DEPARTMENT OF TRANSPORTATION** FEDERAL AVIATION ADMINISTRATION Flight Standards Service First Edition 1971 First Revision 1976 For sale by the Superintendent of Documents. U.S.Government Printing O5ce Washington. D.C. 20402 ### Page 3 - - PREFACE This advisory circular is printed for persons preparing for a mechanics certificate with a powerplant rating. It is intended to provide basic information on principles, fundamentals, and technical procedures in the areas relating to the powerplant rating. This is designed to serve as a guide for students enrolled in a formal course of instruction, as well as the individual who is studying on his own. This volume is devoted to an explanation of the units which make up each of the systems that bring fuel, air, and ignition together in an aircraft engine for combustion. It also contains information on engine construction features, lubrication systems, exhaust systems, cooling systems, cylinder removal and replacement, compression checks, and valve adjustments. Because there are so many different types of aircraft engines in use today, it is reasonable to expect minor differences to exist in like system components. To avoid undue repetition, the practice of using representative systems and units is carried out throughout the advisory circular. Subject-matter treatment throughout the text is from a generalized point of view and its use can be supplemented by reference to manufacturer's manuals and other textbooks if more detail is desired. This advisory circular is not intended to replace, substitute for, or supersede official regulations or manufacturer's instructions which should be consulted for final authority. Grateful acknowledgement is extendei to the manufacturers of engine, propellers, and powerplant accessories for their cooperation in making material available for inclusion in this handbook. This handbook contains material for which a copyright has been issued. Copyright material is used by special permission of United Aircraft Corporation, Pratt and Whitney Aircraft Division, and may not be extracted or reproduced without permission 'of the copyright owner. The advancements in aerdnautical technology dictate that this advisory circular be brought up-to-date periodically. It has been updated, errors have been corrected, new material has been added, and some material has been rearranged to improve the usefulness of the handbook. We would appreciate, however, having errors brought to our attention, as well as suggestions for improving its usefulness. Your comments and suggestions will be retained in our files until such time as the next revision is completed. A,ddress all correspondence relating to this handbook to: Department of Transportation Federal Aviation Administration Flight Standards National Field Office P.O. Box 25082 Oklahoma City, Oklahoma 73125 The companion advisory circulars to AC 65-12A are the Airframc and Fowerplant Mechanics General Handbook, AC 65-9A, and the Airframc and Powerplant llechanics Airframe Handbook, AC 65-15A. ### Page 4 **CONTENTS PREFACE ........................................................** **CONTENTS .......................................................** CHAPTER 1.THEORY AND CONSTRUCTION OF AIRCRAFT ENGINES General ..................................................... Comparison of Aircraft Powerplants ............................. Types of Reciprocating Engines ................................. Reciprocating Engine Design and Construction .................... Crankshafts .................................................. Connecting Rods ............................................. Pistons ..................................................... Piston Rings ................................................ Cylinders .................................................... Cylinder Numbering .......................................... Firing Order ................................................ Valves ...................................................... Valve-Operating Mechanism .................................... Bearings .................................................... Propeller Reduction Gearing ................................... Propeller Shafts .............................................. Reciprocating Engine Operating Principles ....................... Operating Cycles ............................................. Four-Stroke Cycle ............................................ Reciprocating Engine Power and Efficiencies ...................... Efficiencies .................................................. Turbine Engine Construction ................................... Air Entrance ................................................ Accessory Section ............................................ Compressor Section ........................................... Combustion Section ........................................... Major Subassemblies .......................................... Turboprop En, mines ........................................... ' Turboshaft En, u~nes ........................................... ' Turbofan En, mines ............................................ ' Turbine Engine Operating Principles ............................ Thrust ...................................................... Gas Turbine Engine Performance ............................... CHAPTER 2.-INDUCTION AND EXHAUST SYSTEMS Reciprocating Engine Induction Systems .......................... Induction System Icing ........................................ Internally Driven Superchargers ................................. Externally Driven Superchargers ................................ Turbosupercharger System for Large Reciprocating Engines ......... Turbocharger ................................................ ### Page 5 CHAPTER 2.-(Cont.) Sea Level Boosted Turbocharger System … 86 Turbocompound Systems for Reciprocating Engines … 89 Turbojet Engine Inlet Duct Systems … 90 Turboprop Compressor Inlets … 94 Reciprocating Engine Exhaust Systems … 95 Reciprocating Engine Exhaust System Maintenance Practices … 98 Turbine Engine Exhaust Ducts … 101 Turboprop Exhaust System … 102 Thrust Reversers … 103 Engine Air-Inlet Vortex Destroyer … 106 **CHAPTER 3.-ENGINE FUEL AND FUEL METERING SYSTEMS** Fuel System Requirements … 109 Turbine Engine Fuel System-General Requirements ............... 1.19 Fuel Quantity Indicating Units .................................. 174' Basic Fuel System … 110 Fuel Metering Devices for Reciprocating Engines … 110 Carburetion Principles … 113 Carburetor Systems … 115 Carburetor Types … 116 Carburetor Icing … 116 Float-Type Carburetors … 117 Pressure Injection Carburetors … 122 Stromberg PS Series Carburetor … 134 Direct Fuel-Injection Systems … 136 Carburetor Maintenance … 142 Fuel System Inspection and Maintenance … 146 Water Injection System for Reciprocating Engines … 147 Jet Fuel Controls … 149 Coordinator … 153 Fuel Control … 153 Speed-Sensitive Control … 154 Fuel Manifold Drain Valve … 15-1 System Operation … 154 Hydromechanical Fuel Control … 156 Fuel Control Description … 156 Fuel Scheduling System … 163 Water Injection Re-Set System … 167 Jet Fuel Control Maintenance … 167 Engine Fuel System Components … 169 Water or Coolant Injection … 174 **CHAPTER 4.-ENGINE IGNITION AND ELECTRICAL SYSTEMS** Reciprocating Engine Ignition Systems … 177 Battery Ignition System … 177 Magneto Ignition System Operating Principles … 177 Auxiliary Ignition Units … 190 Spark Plugs … 195 Reciprocating Engine Ignition System Maintenance and Inspection … 196 Magneto Ignition Timing Devices … 197 Checking the Internal Timing of a Magneto … 201 ### Page 6 ......................................s~al~adoq paads lu~lsuo3 EEE ............................... yelx!y 3qBq uo pasn sla11adold TEE ..................................... sia11adoq JO uoye3y !ssel3 TEE ........................................... sla11ado.x~30 sadd~ 6ZE .......................................... uo!le~ado lallado~d LZE ...................................... sald!3u!ld la~~adold ?seaSZS ..................................................... leraua3SZE S1I3'1?3dO'tId-'t UBLdVH3 ....................................... 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Bu!punol3 putl Bu!puoa SSZ ..................... ~43018leu!una~ 01 sBn? ~tlu!urma~, Buy3auuo3 VSZ ................................... sqedax Bup~ds iC3uaoQ.raur3 ESZ ....................................... aIqq pue anfi 2u!dd!.u~ OSZ ........................................ qqe3 pue aqA Buyn3 6PZ ................................. sa~pung aqfi Bu!d~ PUB Bu!3eq wz .................................. suralsd~ 1tl3!"3a13 lueldla~od SEZ sluauoduro3 ruals.4~ uoy!u21 JO uo!1elplsu1 pue 'a3ueualu!e~ 'pourax SEZ ...................... a3ueualu!ely putl uoy3adsu1 uralsdg uo!@~ PEZ ................................ suralsd~ uo!l!u81 au!Bug au!qlnL OEZ .......................................... ~azd~euy au!oQq aqL 822 ........................... a~wualu!e~ PUB uo!a3adsu1Bnld qmdsEIZ .................. sragu!~ lolnqys!a uraasdg uo!sua~,-q%~ Bu!ur!~ 802 (3~03)-'0 IEIUVH3 ### Page 7 CHAPTER 7.-(Cont. ) Feathering … 335 Hamilton Standard Hydromatic Propellers … 338 Hydraulic Governors … 344 Propeller Synchronization … 347 Propeller Ice Control Systems … 347 Propeller Inspection and Maintenance … 349 Propeller Vibration … 350 Blade Tracking … 350 Checking and Adjusting Propeller Blade Angles … 350 Propeller Balancing … 352 Servicing Propellers … 353 Turboprop Propeller … 354 Blade Cuffs … 358 **CHAPTER 8.-ENGINE REMOVAL AND REPLACEMENT** General … 359 Reasons for Removal of Reciprocating Engines … 359 Preparation of Reciprocating Engines for Installation … 361 QECA Buildup of Radial Engines … 361 Inspection and Replacement of Powerplant External Units and Systems 364 Preparing the Engine for Removal … 365 Removing the Engine … 368 Hoisting and Mounting the Engine for Installation … 369 Preparation of Engine for Ground and Flight Testing … 372 Propeller Check … 373 Checks and Adjustments after Engine Run-up and Operation ........ 37% Removal and Installation of an Opposed-Type Engine … 374 Engine Installation … 375 Turbojet Powerplant Removal and Installation … 375 Installation of Turbojet Engines … 377 Rigging. Inspections. and Adjustments … 378 Turboprop Powerplant Removal and Installation … 380 Helicopter Engine Removal and Installation … 381 Rigging and Adjustment of Helicopter QECA … 382 Engine Mounts … 382 Preservation and Storage of Engines … 386 Corrosion Preventive Treatment … 387 Engine Shipping Containers … 388 Inspection of Stored Engines … 390 Preservation and De-preservation of Gas Turbine Engines … 390 **CHAPTER 9.-ENGINE FIRE PROTECTION SYSTEMS** General … 391 Reciprocating Engine Fire Protection Systems … 391 Fire Zones … 394 Fire Extinguishing Agents ..................................... 391. Turbine Engine Fire Protection Systems … 397 Typical Multi-Engine Fire Protection System … 399 Fire Detection System Maintenance Practices … 403 Fire Extinguisher System Maintenance Practices … 406 Turbojet Aircraft Fire Protection System (Saberliner) … 407 ... Vlll ### Page 8 CHAPTER 10.-ENGINE MAINTENANCE AND OPERATION Reciprocating Engine Overhaul .................................. General Overhaul Procedures ................................... Cylinder Assembly Reconditioning ............................... Crankshaft Inspection ......................................... Connecting Rods ............................................. Block Testing of Reciprocating Engines .......................... Reciprocating Engine Operation ................................ Basic Engint. Operatins Principals .............................. Factors Affecting Engine Operation .............................. Engine Troubleshooting ....................................... Cylinder Maintenance ......................................... Cylinder Compression Tests .................................... Cylinder Removal ............................................ Cylinder Installation ......................................... Valve and Valve Mechanism .................................... Cold Cylinder Check ......................................... Turbine Engine Maintenance ................................... Combustion Section .......................................... Inspection and Repair of Turbine Disc .......................... Commercial Ratings .......................................... Engine Instrumentation ....................................... Turbojet Engine Operation .................................... Ground Operation ............................................ Engine Shutdown ............................................. Troubleshooting Turbojet Engines ............................... Turboprop Operation ......................................... Troubleshooting Procedures for Turboprop Engines ................ Jet Calibration Test Unit ...................................... Troubleshooting Aircraft Tachometer System ...................... Spectrometric Oil Analysis Program ............................. ### Page 9 **CHAPTER 1** **THEORY AND CONSTRUCTION OF AIRCRAFT ENGINES** GENERAL For an aircraft to remain in level unaccelerated flight, a thrust must be provided that is equal to and opposite in direction to the aircraft drag. This thrust, or propulsive force, is provided by. a suitable type of heat engine. All heat engines have in common the ability to convert heat energy into mechanical energy, by the flow of some fluid mass through the engine. In all cases, the heat energy is released at a point in Engine Type Major Means of Compression Turbojet. Turbine-driven compressor. Turboprop. Turbine-driven compressor. Ramjet. Ram compression due to high flight speed. Pulse-jet. Compression due to combustion. Reciprocating. Reciprocating action of pistons. Rocket. Compression due to combustion. the cycle where the pressure is high, relative to atmospheric. These engines are customarily divided into groups or types depending upon: (1) The working fluid used in the engine cycle, (2) the means by which the mechanical energy is transmitted into a propulsive force, and (3) the method of compressing the engine working fluid. The types of engines are illustrated in figure 1-1. Engine Working Propulsive Fluid Working Fluid Fuel/air mixture. Same as engine working fluid. Fuel/air mixture. Ambient air. Fuel/air mixture. Same as engine working fluid. Fuel/air mixture. Same as engine working fluid. Fuel/air mixture. Ambient air. Oxidizer/fuel Same as engine mixture. working fluid. FIGURE1-1. Types of engines. The propulsive force is obtained by the displacement of a working fluid (not necessarily the same fluid used within the engine) in a direction opposite to that in which the airplane is propelled. This is an application of Newton's third law of motion. Air is the principal fluid used for propulsion in every type of powerplant except the rocket, in which only the byproducts of combustion are accelerated and displaced. The propellers of aircraft powered by reciprocating or turboprop engines accelerate a large mass of air through a small velocity change. The fluid (air) used for the propulsive force is a different quantity than that used within the engine to produce the mechanical energy. Turbojets, ramjets, and pulse-jets accelerate a smaller quantity of air through a large velocity change. They use the same working fluid for propulsive force that is used within the engine. A rocket carries its own oxidizer rather than using ambient air for combustion. It discharges the gaseous byproducts of combustion through the exhaust nozzle at an extremely high velocity. Engines are further characterized by the means of compressing the working fluid before the addition of heat. The basic methods of compression are : (1)The turbine-driven compressor (turbine engine). (2) The positive displacement, piston-type com- ### Page 10 pressor (reciprocating engine). (3) Ram compression due to forward flight speed (ramjet). (4) Pressure rise due to combustion (pulse-jet and rocket). A more specific description of the major engine types used in commercial aviation is given later in this chapter. COMPARISON OF AIRCRAFT POWERPLANTS In addition to the differences in the methods employed by the various types of powerplants for producing thrust, there are differences in their suitability for different types of aircraft. The following discussion points out some of the important characteristics which determine their suitability. General Requirements All engines must meet certain general requirements of effkiency, economy, and reliability. Besides being economical in fuel consumption, an aircraft engine must be economical (the cost of original procurement and the cost of maintenance) and it must meet exacting requirements of efficiency and low weight per horsepower ratio. It must be capable of sustained high-power output with no sacrifice in reliability; it must also have the durability to operate for long periods of time between overhauls. It needs to be as compact as possible, yet have easy accessibility for maintenance. It is required to be as vibration free as possible and be able to cover a wide range of power output at various speeds and altitudes. These requirements dictate the use of ignition systems that will deliver the firing impulse to the spark plugs or igniter plugs at the proper .time in all kinds of weather and under other adverse conditions. Fuel-metering devices are needed that will deliver fuel in the correct proportion to the air ingested by the engine regardless of the attitude, altitude, or type of weather in which the engine is operated. The engine needs a type of oil system that delivers oil under the proper pressure to all of the operating parts of the engine when it is running. Also, it must have a system of damping units to damp out the vibrations of the engine when it is operating. Power and Weight The useful output of all aircraft powerplants is thrust, the force which propels the aircraft. Since the reciprocating engine is rated in b.hp. (brake horsepower) and the gas turbine engine is rated in pounds of thrust, no direct comparison can be made. However, since the reciprocating engine/propeller combination receives its thrust from the propeller, a comparison can be made by converting the horsepower developed by the reciprocating engine to thrust. If desired, the thrust of a gas turbine engine can be converted into t.hp. (thrust horsepower). But it is necessary to consider the speed of the aircraft. This conversion can be accomplished by using the formula : thrust x aircraft speed (m.p.h.) t.hp. = 375 mile-pounds per hour The value 375 mile-pounds per hour is derived from the basic horsepower formula as follows: 1hp. =33,000 ft.-lb. per minute. 33,000 x 60 =1,980,000 ft.-lb. per hour. 1,980,000 =375 mile-pounds per hour. 5,280 One horsepower equals 33,000 ft.-lb. per minute or 375 mile-pounds per hour. Under static conditions, thrust is figured as equivalent to approximately 2.6 pounds per hour. If a gas turbine is producing 4,000 pounds of thrust and the aircraft in which the engine is installed is traveling at 500 m.p.h., the t.hp. will be: It is necessary to calculate the horsepower for each speed of an aircraft, since the horsepower varies with speed. Therefore, it is not practical to try to rate or compare the output of a turbine engine on a horsepower basis. The aircraft engine operates at a relatively high percentage of its maximum power output throughout its service life. The aircraft engine is at full power output whenever a takeoff is made. It may hold this power for a period of time up to the limits set by the manufacturer. The engine is seldom held at a maximum power for more than 2 minutes, and usually not that long. Within a few seconds after lift-off, the power is reduced to a power that is used for climbing and that can be maintained for longer periods of time. After the aircraft has climbed to cruising altitude, the power of the engine (s) is further reduced to a cruise power which can be maintained for the duration of the flight. If the weight of an engine per brake horsepower (called the specific weight of the engine) is decreased, the useful load that an aircraft can carry and the performance of the aircraft obviously are increased. Every excess pound of weight carried by ### Page 11 an aircraft engine reduces its performance. Tremendous gains in reducing the weight of the aircraft engine through improvement in design and metallurgy have resulted in reciprocating engines now producing approximately 1 hp. for each pound of weight. Fuel Economy The basic parameter for describing the fuel economy of aircraft engines is usually specific fuel consumption. Specific fuel consumption for turbojets and ramjets is the fuel ffow (lbs./hr.) divided by thrust (lbs.), and for reciprocating engines the fuel flow (IbsJhr.) divided by brake horsepower. These are called "thrust specific fuel consumption" and "brake specific fuel consumption," respectively. Equivalent specific fuel consumption is used for the turboprop engine and is the fuel ffow in pounds per hour divided by a turboprop's equivalent shaft horsepower. Comparisons can be made between the various engines on a specific fuel consumption basis. At low speed, the reciprocating and turbopropeller engines have better economy than the turbojet engines. However, at high speed, because of losses in propeller efficiency, the reciprocatihg or turbopropeller engine's efficiency becomes less than that of the turbojet. Figure 1-2 shows a comparison of average thrust specific fuel consumption of three types of engines at rated power at sea level. Flight Mach number FIGURE1-2. Camparison of fuel consumption for three types of engines at rated power at sea level. Durability and Reliability Durability and reliability are usually considered identical factors since it is difficult to mention one without including the other. An aircraft engine is reliable when it can perform at the specified ratings in widely varying flight attitudes and in extreme weather conditions. Standards of powerplant reliability are agreed upon by the FAA, the engine manufacturer, and the airframe manufacturer. The engine manufacturer ensures the reliability of his product by design, research, and testing. Close control of manufacturing and assembly procedures is maintained, and each engine is tested before it leaves the factory. Durability is the amount of engine life obtained while maintaining the desired reliability. The fact that an engine has successfully completed its type or proof test indicates that it can be operated in a normal manner over a long period before requiring overhaul. However, no definite time interval between overhauls is specified or implied in the engine rating. The TBO (time between overhauls) varies with the operating conditions such as engine temperatures, amount of time the engine is operated at high-power settings, and the maintenance received. Reliability and durability are thus built into the ### Page 12 engine by the manufacturer, but the continued reliability of the engine is determined by the maintenance, overhaul, and operating personnel. Careful maintenance and overhaul methods, thorough periodical and preflight inspections, and strict observance of the operating limits established by the engine manufacturer will make engine failure a rare occurrence. Operating Flexibility Operating flexibility is the ability of an engine to run smoothly and give desired performance at all speeds from idling to full-power output. The aircraft engine must also function efficiently through all the variations in atmospheric conditions encountered in widespread operations. Compactness To effect proper streamlining and balancing of an aircraft, the shape and size of the engine must be as compact as possible. In single-engine aircraft, the shape and size of the engine also affect the view of the pilot, making a smaller engine better from this standpoint, in addition to reducing the drag created by a large frontal area. Weight limitations, naturally, are closely related to the compactness requirement. The more elongated and spreadout an engine is, the more difficult it becomes to keep the specific weight within the allowable limits. Powerplant Selection Engine specific weight and specific fuel consumption were discussed in the previous paragraphs, but for certain design requirements, the final powerplant selection may be based on factors other than those which can be discussed from an analytical point of view. For that reason, a general discussion of powerplant selection is included here. For aircraft whose cruising speeds will not exceed 250 m.p.h. the reciprocating engine is the usual choice. When economy is required in the low-speed range, the conventional reciprocating engine is chosen because of its excellent efficiency. When high-altitude performance is required, the turbosupercharged reciprocating engine may be chosen because it is capable of maintaining rated power to a high altitude (above 30,000 feet). In the range of cruising speeds from 180 to 350 m.p.h. the turbopropeller engine performs better than other types of engines. It develops more power per pound of weight than does the reciprocating engine, thus allowing a greater fuel load or payload for engines of a given power. The maximum overall efficiency of a turboprop powerplant is less than that of a reciprocating engine at low speed. Turboprop engines operate most economically at high altitudes, but they have a slightly lower service ceiling than do turbosupercharged reciprocating engines. Economy of operation of turboprop engines, in terms of cargo-ton-miles per pound of fuel, will usually be poorer than that of reciprocating engines because cargo-type .aircraft are usually designed for lowspeed operation. On the other hand, coit of operation of the turboprop may approach that of the reciprocating engine because it burns cheaper fuel. Aircraft intended to cruise from high subsonic speeds up to Mach 2.0 are powered by turbojet engines. Like the turboprop, the turbojet operates mo'st e~ciently at high altitudes. High-speed, turbo- *i jet-propelled aircraft fuel economy, in terms of miles per pound of fuel, is poorer than that attained at low speeds with reciprocating engines. However, reciprocating engines are more complex in operation than other engines. Correct operation of reciprocating engines requires about twice the instrumentation required by turbojets or turboprops, and it requires several more controls. A change in power setting on some reciprocating engine installations may require the adjustment of five controls, but a change in power on a turbojet requires only a change in throttle setting. Furthermore, there are a greater number of critical temperatures and - pressures to be watched on reciprocating engine installations than on turbojet or turboprop installations. TYPES OF REC~PROCAT~~ ENGINES Many types of reciprocating engines have been designed. However, manufacturers have developed some designs that are used more commonly than others and are therefore recognized as conventional. Reciprocating engines may be classified according to cylinder arrangement with respect to the crankshaft (in-line, V-type, radial, and opposed) or according to the dethod of cooling (liquid cooled or air cooled). Actually, all engines are cooled by transferring excess heat to the surrounding air. In air-cooled engines, this heat transfer is direct from the cylinders to the air. In liquid-cooled engines, the heat is transferred from the cylinders to the coolant, which is then sent through tubing and cooled within-a radiator placed in the airstream. The radiator must be large enough to cool the liquid efficiently. Heat is transferred to air more slowly than it is to a liquid. Therefore, it is necessary to ### Page 13 provide thin metal fins on the cylinders of an aircooled engine in order to have increased surface for sufficient heat transfer. Most aircraft engines are air cooled. In-line Engines An in-line engine generally has an even number of cylinders, although some three-cylinder engines have been constructed. This engine may be either liquid cooled or air cooled and has only one crankshaft, which is located either above or below the cylinders. If the engine is designed to operate with the cylinders below the crankshaft, it is called an inverted engine. The in-line engine has a small frontal area and is better adapted to streamlining. When mounted with the cylinders in an inverted position, it offers the added advantages of a shorter landing gear and greater pilot visibility. The in-line engine has a higher weight-to-horsepower ratio than most other engines. With increase in engine size, the air cooled, in-line type offers additional handicaps to proper cooling; therefore, this type of engine is, to a large degree, confined to low- and mediumhorsepower engines used in light aircraft. Opposed or 0-type Engines The opposed-type engine, shown in figure 1-3, has two banks of cylinders directly opposite each other with a crankshaft in the center. The pistons of both cylinder banks are connected to the single crankshaft. Although the engine can be either liquid cooled or air cooled, the air-cooled version is used predominantly in aviation. It can be mounted with the cylinders in either a vertical or horizontal position. The opposed-type engine has a low weight-tohorsepower ratio, and its narrow silhouette makes it ideal for horizontal installation on the aircraft wings. Another advantage is its comparative freedom from vibration. V-type Engines In the V-type engines, the cylinders are arranged in two in-line banks generally set 60' apart. Most of the engines have 12 cylinders, which are either liquid cooled or air cooled. The engines are designated by a V, followed by a dash and the piston displacement in cubic inches, for example, V-1710. Radial Engines The radial engine consists of a row, or rows, of cylinders arranged radially about a central crankcase (see figure 1-4). This type of engine has proven to be very rugged and dependable. The number of cylinders composing a row may be either three, five, seven, or nine. Some radial engines have two rows of seven or nine cylinders arranged radially about the crankcase. One type has four rows of cylinders with seven cylinders in each row. FIGURE Opposed engine. 1-3. FIGURE Radial en,' uine.1-4. The power output from the different sizes of radial engines varies from 100 to 3,800 horsepower. **RECIPROCATING ENGINE DESIGN AND CON-** STRUCTION The basic parts of a reciprocating engine are the crankcase, cylinders, pistons, connecting rods, valves, valve-operating mechanism, and crankshaft. In the head of each cylinder are the valves and spark ### Page 14 plugs. One of the valves is in a passage leading from the induction system; the other is in a passage leading to the exhaust system. Inside each cylinder Every internal combustion engine must have certain basic parts in order to change heat into mechanical energy. An intake valve is needed to let the into the cylinder. 8 The piston, moving within the cylinder, forms one of the walls of the combustion The piston has nngs which seal the gases in the cylinder, preventing any loss of powel around the sides of the piston. **FIGURE1-5.** Crankcase Sections I Ichamber. The foundation of an engine is the crankcase. It contains the bearings in which the crankshaft revolves. Besides supporting itself, the crankcase must provide a tight enclosure for the lubricating oil and must support various external and internal mechanisms of the engine. It also provides support for attachment of the cylinder assemblies, and the is a movable piston connected to a crankshaft by a connecting rod. Figure 1-5 illustrates the basic parts of a reciprocating engine. The cylinder forms a part of the chamber in which the /fuel is compressed and burned. / An exhaust valve is needed to let the exhaust gases out. The connecting rod forms a link between the piston and the crankshaft. The crankshaft and connecting rod change the straight line motion of the piston to a rotary turning motion. The crankshaft in an aircraft en ine also absorbs the power or work from all the cylin5ers and transfers it to the propeller. Basic parts of a reciprocating engine. powerplant to the aircraft. It must be sufficiently rigid and strong to prevent misalignment of the crankshaft and its bearings. Cast or forged aluminum alloy is generally used for crankcase construction because it is light and strong. Forged steel crankcases are used on some of the high-power output engines. The crankcase is subjected to many variations of ### Page 15 vibrational and other forces. Since the cylinders are fastened to the crankcase, the tremendous expansion forces tend to pull the cylinder off the crankcase. The unbalanced centrifugal and inertia forces of the crankshaft acting through the main bearing subject the crankcase to bending moments which change continuously in direction and magnitude. The crankcase must have sufficient stiffness to withstand these bending moments without objectional deflections. If the engine is equipped with a propeller reduction gear, the front or drive end will be subjected to additional forces. In addition to the thrust forces developed by the propeller under high-power output, there are severe centrifugal and gyroscopic forces applied to the crankcase due to sudden changes in the direction of flight, such as those occurring during maneuvers of the airplane. Gyroscopic forces are, of course, particularly severe when a heavy propeller is installed. Radial Engines The engine shown in figure 1-6 is a single-row, nine-cylinder radial engine of relatively simple construction, having a one-piece nose and a twosecticn main crankcase. Crankcase front main **INOSE POWER** **SECTION I SECTION FIGURE1-6.** The larger twin-row engines are of slightly more complex construction than the single-row engines. For example, the crankcase of the Wright R-3350 engine is composed of the crankcase front section, four crankcase main sections (the front main, the front center, the rear center, and the rear main sections), the rear cam and tappet housing, the supercharger front housing, the supercharger rear housing, and the supercharger rear housing cover. Pratt and Whitney engines of comparable size incorporate the same basic sections, although Supercharger rear housing cover **I SUPERCHARGER I ACCESSORY** I SECTION I SECTION Engine sections. the mnstruction and the nomenclature differ considerably. Nose Section The shape of nose sections varies considerably. In general, it is either tapered or round in order to place the metal under tension or compression instead of shear stresses. A tapered nose section is used quite frequently on direct-drive, low-powered engines, because extra space is not required to house the propeller reduction gear. It is usually cast of ### Page 16 either aluminum alloy or magnesium since the low power developed and the use of a lightweight propeller do not require a more expensive forged nose section. The nose section on engines which develop from 1,000 to 2,500 hp. is usually rounded and sometimes ribbed to get as much strength as possible. Aluminum alloy is the most widely used material because of its adaptability to forging processes and its vibration-absorbing characteristics. - The design and construction of the nose section is an important factor since it is subjected to a wide variation of forces and vibration. For instance, if the valve mechanism is located in front of the cylinders, the vibration and forces occurring at the tappet and guide assembly are applied near the flanged portion of the case. The forces created by the propeller reduction gear are applied to the case as a whole. Careful vibration surveys are conducted during the experimental testing of newly designed engines to see that these conditions will not become detrimental throughout the operating range of the engine. The mounting of the propeller governor varies. On some engines it is located on the rear section, although this complicates the installation, especially if the propeller is operated or controlled by oil pressure, because of the distance between the governor and propeller. Where hydraulically operated propellers are used, it is good practice to mount the governor on the nose section as close to the propeller as possible to reduce the length of the oil passages. The governor is then driven either from gear teeth on the periphery of the bell gear or by some other suitable means. Since the nose section transmits many varied forces to the main or power section, it must be properly secured to transmit the loads efficiently. It also must have intimate contact to give rapid and uniform heat conduction, and be oiltight to prevent leakage. This is usually accomplished by an offset or ground joint, secured by studs or capscrews. On some of the larger engines, a small chamber is located on the bottom of the nose section to collect the oil. This is called the nose section oil sump. Power Section On engines equipped with a two-piece master rod and a solid-type crankshaft, the main or power crankcase section may be solid, usually of aluminum alloy. The front end of this section is open when the diaphragm plate in which the front main bearing is mounted is removed. The knuckle pins may be removed through this opening by a suitable knucklepin puller. The master rod is then removed by disassembling the split end and pulling the shank out through the master rod cylinder hole. There is also an engine equipped with this crankshaft and master rod arrangement which uses a split case crankcase held together by through bolts. The split main section (aluminum or magnesium alloy) may be slightly more expensive, but permits better control over the quality of the casting or forging. The split main section is generally necessary when a solid master rod and a split-type crankshaft are used. This portion of the engine is often called the power section, because it is here that the reciprocating motion of the piston is converted to rotary motion of the crankshaft. Because of the tremendous loads and forces from the crankshaft assembly and the tendency of the cylinders to pull the crankcase apart, especially in extreme conditions when a high-powered engine is detonated, the main crankcase section must be very well designed and constructed. It is good practice to forge this section from aluminum alloy to obtain uniformity in the, density of the metal and maximum strength. One large engine uses a forged alloy steel main section, which is slightly heavier but has very great strength. The design of the forged sections is usually such that both halves can be made in the same die in order to reduce manufacturing costs. Any variations can be taken care of during the machining operation. The two halves are joined on the center line of the cylinders and held together by suitable high-strength bolts. The machined surfaces on which the cylinders are mounted are called cylinder pads. They are provided with a suitable means of retaining or fastening the cylinders to the crankcase. The general practice in securing the cylinder flange to the pad is to mount studs in threaded holes in the crankcase. On engines equipped with the steel main section, capscrews are being utilized because the threads may be tapped in the stronger material and are not as likely to be stripped or stretched by installing and removing threaded members. The inner portion of the cylinder pads are sometimes chamfered or tapered to permit the installation of a large rubber O-ring around the cylinder skirt, which effectively seals the joint between the cylinder and the crankcase pads against oil leakage. Because oil is thrown about the crankcase, ### Page 17 especially on inverted in-line and radial-type engines, the cylinder skirts extend a considerable distance into the crankcase sections to reduce the flow of oil into the inverted cylinders. The piston and ring assemblies, of course, have to be arranged so that they will throw out the oil splashed directly into them. As mentioned previously, the nose section is secured to one side of the main section unit, and the diffuser or blower section is attached to the other side. Diffuser Section The diffuser or supercharger section generally is cast of aluminum alloy, although, in a few cases, the lighter magnesium alloy is used. Mounting lugs are spaced about the periphery of this section to attach the engine assembly to the engine mount or framework provided for attaching the powerplant to the fuselage of single-engine aircraft or to the wing nacelle structure of multiengine aircraft. The mounting lugs may be either integral with the diffuser section or detachable, as in the case of flexible or dynamic engine mounts. The mounting arrangement supports the entire powerplant including the propeller, and therefore is designed to provide ample strength for rapid maneuvers or other loadings. Because of the elongation and contraction of the cylinders, the intake pipes which carry the mixture from the diffuser chamber through the intake valve ports are arranged to provide a slip joint which must be leakproof. The atmospheric pressure on the outside of the case of an unsupercharged engine will be higher than on the inside, especially when the engine is operating at idling speed. If the engine is equipped with a supercharger and operated at full throttle, the pressure will be considerably higher on the inside than on the outside of the case. If the slip joint connection has a slight leakage, the engine may idle fast due to a slight leaning of the mixture. If the leak is quite large, it may not idle at all. At open throttle, a small leak probably would not be noticeable in operation of the engine, but the slight leaning of the fuel/air mixture might cause detonation or damage to the valves and valve seats. On some radial engines, the intake pipe has considerable length and on some in-line engines, the intake pipe is at right angles to the cylinders. In these cases, flexibility of the intake pipe or its arrangement eliminates the need for a slip joint. In any case, the engine induction system must be arranged so that it will not leak air and change the desired fuel/air ratio. Accessory Section The accessory (rear) section usually is of cast construction, and the material may be either aluminum alloy, which is used most widely, or magnesium, which has been used to some extent. On some engines, it is cast in one piece and provided with means for mounting the accessories, such as magnetos, carburetors, and fuel, oil, and vacuum pumps, and starter, generator, etc., in the various locations required to facilitate accessibility. Other adaptations consist of an aluminum alloy casting and a separate cast magnesium cover plate on whidh the accessory mounts are arranged. Recent design practice has been toward standardizing the mounting arrangement for the various accessories so that they will be interchangeable on different makes of engines. For example, the increased demands for electric current on large aircraft and the requirements of higher starting torque on powerful engines have resulted in an increase in the size of starters and generators. This means that a greater number of mounting bolts must be provided and, in some cases, the entire rear section strengthened. Accessory drive shafts are mounted in suitable bronze bushings located in the diffuser and rear sections. These shafts extend into the rear section and are fitted with suitable gears from which power takeoffs or drive arrangements are carried out to the accessory mounting pads. In this manner the various gear ratios can be arranged to give the proper drive speed to magneto, pump, and other accessories to obtain correct timing or functioning. In some cases there is a duplication of drives, such as the tachometer drive, to connect instruments located at separate stations. The accessory section provides a mounting place for the carburetor, or master control, fuel injection pumps, engine-driven fuel pump, tachometer generator, synchronizing generator for the engine analyzer, oil filter, and oil pressure relief valve. Accessory Gear Trains Gear trains, containing both spur- and bevel-type gears, are used in the different types of engines for driving engine components and accessories. Spurtype gears are generally used to drive the heavier loaded accessories or those requiring the least play or backlash in the gear train. Bevel gears permit angular location of short stub shafts leading to the various accessory mounting pads. ### Page 18 Practically all high-powered engines are equipped with a supercharger. From 75 to 125 hp. may be required to drive the supercharger. The acceleration and deceleration forces imposed on the supercharger gear train when opening and closing the throttle make some kind of antishock device necessary to relieve excessive loads. The current practice on large radial engines is to use a main accessory drive gear which is fitted with several springs between the rim of the gear and drive shaft. This device, called the spring-loaded accessory drive gear, permits absorption of forces of high magnitude, preventing damage to the accessory gear trains. When an engine is equipped with a two-speed supercharger, the oil-pressure operated clutches act as shock absorbers to protect the supercharger gear train. On the low-powered, opposed, and in-line engines, the accessory gear trains are usually simple arrangements. Many of these engines use synthetic rubber or spring couplings to protect the magneto and generator gear trains from excessive loads. Opposed and In-line Types The crankcases used on engines having opposed or in-line cylinder arrangements vary in form for the different types of engines, but in general they are approximately cylindrical. One or more sides are surfaced to serve as a base to which the cylinders are attached by means of capscrews, bolts, or studs. These accurately machined surfaces are frequently referred to as cylinder pads. The crankshaft is carried in a position parallel to the longitudinal axis of the crankcase and is generally supported by a main bearing between each throw. The crankshaft main bearings must be supported rigidly in the crankcase. This usually is accomplished by means of transverse webs in the crankcase, one for each main bearing. The webs form an integral part of the structure and, in addition to supporting the main bearings, add to the strength of the entire case. The crankcase is divided into two sections in a FIGURE1-7. Typical opposed engine exploded into component assemblies. ### Page 19 longitudinal plane. This division may be in the plane of the crankshaft so that one-half of the main bearing (and sometimes camshaft bearings) are carried in one section of the case and the other half in the opposite section. (See figure 1-7.) Another method is to divide the case in such a manner that the main bearings are secured to only one section of the case on which the cylinders are attached, thereby providing means of removing a section of the crankcase for inspection without disturbing the bearing adjustment. CRANKSHAFTS The crankshaft is the backbone of the reciprocating engine. It is subjected to most of the forces developed by the engine. Its main purpose is to Crankpin transform the reciprocating motion of the piston and connecting rod into rotary motion for rotation of the propeller. The crankshaft, as the name implies, is a shaft composed of one or more cranks located at specified points along its length. The cranks, or throws, are formed by forging offsets into a shaft before it is machined. Since crankshafts must be very strong, they generally are forged from a very strong alloy, such as chromium-nickelmolybdenum steel. A crankshaft may be of single-piece or multipiece construction. Figure 1-8 shows two representative types of solid crankshafts used in aircraft engines. The four-throw construction may be used either on four-cylinder horizontal opposed or four-cylinder in-line engines. FIGURE Solid types of crankshafts. 1-8. The six-throw shaft is used on six-cylinder in-line engines, 12-cylinder V-type engines, and six-cylinder opposed engines. Crankshafts of radial engines may be the singlethrow, two-throw, or four-throw type, depending on whether the engine is the single-row, twin-row, or four-row type. A single-throw radial engine crankshaft is shown in figure 1-9. KO matter how many throws it may have, each crankshaft has three main parts-a journal, crankpin, and crank cheek. Counterweights and dampers, although not a true part of a crankshaft, are usually attached to it to reduce engine vibration. The journal is supported by, and rotates in, a main bearing. It serves as the center of rotation of the crankshaft. It is surface-hardened to reduce wear. The crankpin is the section to which the connecting rod is attached. It is off-center from the main journals and is often called the throw. Two crank cheeks and a crankpin make a throw. When a force is applied to the crankpin in any direction other than parallel or perpendicular to and through the center line of the crankshaft, it will cause the crankshaft to rotate. The outer surface is hardened by nitriding to increase its resistance to wear and to provide the required bearing surface. The crankpin is usually hollow. This reduces the total weight of the crankshaft and provides a passage for the transfer of lubricating oil. The hollow crankpin also serves as a chamber for collecting sludge, carbon deposits, and other foreign material. Centrifugal force throws these substances to the outside of the chamber and thus keeps them from reaching the connecting-rod bearing surface. On some engines a passage is drilled in the crank cheek to allow oil ### Page 20 / weightsCounterweight FIGURE1-9. A single-throw radial engine crankshaft. from the hollow crankshaft to be sprayed on the cylinder walls. The crank cheek connects the crankpin to the main journal. In some designs, the cheek extends beyond the journal and carries a counterweight to balance the crankshaft. The crank cheek must be of sturdy construction to obtain the required rigidity between the crankpin and the journal. In all cases, the type of crankshaft and the number of crankpins must correspond with the cylinder arrangement of the engine. The position of the cranks on the crankshaft in relation to the other cranks of the same shaft is expressed in degrees. The simplest crankshaft is the single-throw or 360" type. This type is used in a single-row radial engine. It can be constructed in one or two pieces. Two main bearings (one on each end) are provided when this type of crankshaft is used. The double-throw or 180' crankshaft is used on double-row radial engines. In the radial-type engine, one throw is provided for each row of cylinders. Crankshaft Balance Excessive vibration in an engine not only results in fatigue failure of the metal structures, but also causes the moving parts to wear rapidly. In some instances, excessive vibration is caused by a crankshaft which is not balanced. Crankshafts are balanced for static balance and dynamic balance. A crankshaft is statically balanced when the weight of the entire assembly of crankpins, crank cheeks, and counterweights is balanced around the axis of rotation. 'When testing the crankshaft for static balance, it is placed on two knife edges. If the shaft tends to turn toward any one position during the test, it is out of static balance. A crankshaft is dynamically balanced when all the forces created by crankshaft rotation and power impulses are balanced within themselves so that little or no vibration is produced when the engine is operating. To reduce vibration to a minimum during engine operation, dynamic dampers are incorporated on the crankshaft. A dynamic damper is merely a pendulum which is so fastened to the crankshaft that it is free to move in a small arc. It is incorporated in the counterweight assembly. Some crankshafts incorporate two or more of these assemblies, each being attached to a different crank cheek. The distance th~ pendulum moves and its vibrating frequency correspond to the frequency of the power irnpulses pf the engine. When the vibration frequency of the crankshaft occurs, the pendulum oscillates out of time with the crankshaft vibration, thus reducing vibration to a minimum. Dynamic Dampers The construction of the dynamic damper used in one engine consists of a movable slotted-steel counterweight attached to the crank cheek. Two spoolshaped steel pins extend into the slot and pass through oversized holes in the counterweight and crank cheek. he' difference in the diameter between the pins and the holes provides a pendulum effect. An analogy of the functioning of a dynamic damper is shown in figure 1-10. CONNECTING RODS The connecting rod is the link which transmits forces between the piston and the crankshaft. Connecting rods must be strong enough to remain rigid under load and yet be light enough to reduce the inertia forces which are produced when the rod and piston stop, change direction, and start again at the end of each stroke. There are three types of connecting-rod assemblies: (1) The plain-type connecting rod, (2) the fork-and-blade cbnnecting rod, and (3) the masterand-articulated-rod assembly. (See figure 1-11.) Master-and-Articulated Rod Assembly The master-and-articulated rod assembly is commonly used in radial engines. In a radial engine the piston in one cylinder in each row is connected to the crankshaft by a master rod. All other pistons in the row are connected to the master rod by an ### Page 21 If a simple pendulum is given a series of regular impulses at a speed corresponding to its natural frequency (using a bellows to simulate a power impulse in an engine) it will commence swinging, or vibrating, back and forth from 'the impulses. Another pendulum, suspended from the first, would absorb the impulses and swing itself, leaving the first stationary. The dynamic damper is a short pendulum hung on the crankshaft and tuned to the frequency of the power impulses to absorb vibration in the same manner. 1-10.FIGURE Principles of a dynamic damper. articulated rod. In an 18-cylinder engine which has two rows of cylinders, there are two master rods and 16 articulated rods. The articulated rods are constructed of forged steel alloy in either the I- or H-shape, denoting the cross-sectional shape. Bronze bushings are pressed into the bores in each end of the articulated rod to provide knuckle-pin and piston-pin bearings. The master rod serves as the connecting link between the piston pin and the crankpin. The crankpin end, or the "big end," contains the crankpin or master rod bearing. Flanges around the big end provide for the attachment of the articulated rods. The articulated rods are attached to the master rod by knuckle pins, which are pressed into holes in the master rod flanges during assembly. A plain bearing, usually called a piston-pin bushing, is installed in the piston end of the master rod to receive the piston pin. When a crankshaft of the split-spline or splitclamp type is employed, a one-piece master rod is used. The master and articulated rods are assembled and then installed on the crankpin; the crankshaft sections are then joined together. In engines that use the one-piece type of crankshaft, the big end of the master rod is split, as is the master rod bearing. The main part of the master rod is installed on the crankpin; then the bearing cap is set in place and bolted to the master rod. The centers of the knuckle pins do not coincide with the center of the crankpin. Thus, while the crankpin center describes a true circle for each revolution of the crankshaft, the centers of the knuckle pins describe an elliptical path (see figure The elliptical paths are symmetrical about a center line through the master rod cylinder. It can be seen that the major diameters of the ellipses are not the same. Thus, the link rods will have varying degrees of angularity relative to the center of the crank throw. Because of the varying angularity of the link rods and the elliptical motion of the knuckle pins, all pistons do not move an equal amount in each cylinder for a given number of degrees of crank throw movement. This variation in piston position between cylinders can have considerable effect on engine operation. To minimize the effect of these factors on valve and ignition timing, the knucklepin holes in the master rod flange are not equidistant from the center of the crankpin, thereby offsetting to an extent the effect of the link rod angularity. Another method of minimizing the adverse effects on engine operation is to use a compensated magneto. In this magneto the breaker cam has a number of lobes equal to the number of cylinders on the engine. To compensate for the variation in piston position due to link rod angularity, the breaker cam lobes are ground with uneven spacing. This allows the breaker contacts to open when the piston is in the correct firing position. This will be further outlined during the discussion on ignition timing in Chapter 4. Knuckle Pins The knuckle pins are of solid construction except for the oil passages drilled in the pins, which lubri- ### Page 23 FIGURE1-12. Elliptical travel path of knuckle pins in an articulated rod assembly. cate the knuckle-pin bushings. These pins may be installed by pressing into holes in the master rod . . flanges so that they are prevented from turning in the master rod. Knuckle pins may also be installed with a loose fit so that they can turn in the master rod flange holes, and also turn in the articulating rod bushings. These are called "full-floating" knuckle pins. In either type of installation a lock .. plate on each side retains the knuckle pin and prevents a lateral movement of it. Plain-Type Connecting Rods Plain-type connecting rods are used in in-line and opposed engines. The end of the rod attached to the crankpin is fitted with a cap and a two-piece bearing. The bearing cap is held on the end of the rod by bolts or studs. To maintain proper fit and balance, connecting rods should always be replaced in the same cylinder and in the same relative position. Fork-and-Blade Rod Assembly The fork-and-blade rod assembly is used primarily in ~-;~~e The forked rod is split at the engines. crankpin end to allow space for the blade rod to fit between the prongs. A single two-piece bearing is used on the crankshaft end of the rod. PISTONS The piston of a reciprocating engine is a cylindrical member which moves back and forth within a steel cylinder. The piston acts as a moving wall within the combustion chamber. As the piston moves down in the cylinder, it draws in the fuel/air ' mixture. As it moves upward, it compresses the charge, ignition occurs, and the expanding gases force the piston downward. ~his'force is transmitted to the crankshaft through the connecting rod. On the return upward stroke, the piston forces the exhaust gases from the cylinder. Piston Construction The majority of aircraft engine pistons are machined from aluminum alloy forgings. Grooves are machined in the outside surface of the piston to receive the piston rings, and cooling fins are provided on the inside of the piston for greater heat transfer to the engine oil. Pistons may be either the trunk type or the slipper type; both are shown in figure 1-13. Slippertype pistons are not used in modern, high-powered engines because they do not provide adequate strength or wear resistance. The top face of the piston, or head, may be either flat, convex, or concave. Recesses may be machined in the piston head to prevent interference with the valves. As many as six grooves may be machined around the piston to accommodate the compression rings and oil rings. (See figure 1-13.) The compression rings are installed in the three uppermost grooves; the oil control rings are installed immediately above the piston pin. The piston is usually drilled at the oil control ring grooves to allow surplus oil scraped from the cylinder walls by the oil control rings to pass back into the crankcase. An oil scraper ring is installed at the base of the piston wall or skirt to prevent excessive oil consumption. The portions of the piston walls that lie between each pair of ring grooves are called the ring lands. In addition to acting as a guide for the piston head, the piston skirt incorporates the piston-pin bosses. The piston-pin bosses are of heavy construction to enable the heavy load on the piston head to be transferred to the piston pin. Piston Pin The piston pin joins the piston to the connecting rod. It is machined in the form of a tube from a nickel steel alloy forging, casehardened and ground. The piston pin is sometimes called a wristpin because of the similarity between the relative motions of the piston and the articulated rod and that of the human arm. The piston pin used in modern aircraft engines is the full-floating type, so called because the pin is ### Page 24 Aluminum plug Piston Slipper type I /TrUn: type \ Flat head Recessed head Concave head Dome head FIGURE Piston assembly and types of pistons. 1-13. free to rotate in both the piston and in the connecting rod piston-pin bearing. The piston pin must be held in place to prevent the pin ends from scoring the cylinder walls. In earlier engines, spring coils were installed in grooves in the piston-pin bores at either end of the pin. The current practice is to install a plug of relatively soft aluminum in the pin ends to provide a good bearing surface against the cylinder wall. PISTON RINGS The piston rings prevent leakage of gas pressure from the combustion chamber and reduce to a minimum the seepage of oil into the combustion chamber. The rings fit into the piston grooves but spring out to press against the cylinder walls; when properly lubricated, the rings form an effective gas seal. Piston Ring Construction Most piston rings are made of high-grade cast iron. After the rings are made, they are ground to the cross section desired. They are then split so that they can be slipped over the outside of the piston and into the ring grooves which are machined in the piston wall. Since their purpose is to seal the clearance between the piston and the cylinder wall, they must fit the cylinder wall snugly enough to provide a gastight fit; they must exert equal pressure at all points on the cylinder wall; and they must make a gastight fit against the sides of the ring grooves. Gray cast iron is most often used in making piston rinp. However, many other materials have been tried. In some engines, chrome-plated mild steel piston rings are used in the top compression ring groove because these rings can better withstand the high temperatures present at this point. Compression Ring The purpose of the compression rings is to prevent the escape of gas past the piston during engine operation. They are placed in the ring grooves immediately below the piston head. The number of compression rings used on each piston is determined ### Page 25 by the type of engine and its design, although most aircraft engines use two compression rings plus one or more oil control rings. The cross section of the ring is either rectangular or wedge shaped with a tapered face. The tapered face presents a narrow bearing edge to the cylinder wall which helps to reduce friction and provide better sealing. Oil Control Rings Oil control rings are placed in the grooves immediately below the compression rings and above the piston pin bores. There may be one or more oil control rings per piston; two rings may be installed in the same groove, or they may be installed in separate grooves. Oil control rings regulate the thickness of the oil film on the cylinder wall. If too much oil enters the combustion chamber, it will burn and leave a thick coating of carbon on the combustion chamber walls, the piston head, the spark plugs, and the valve heads. This carbon can cause the valves and piston rings to stick if it enters the ring grooves or valve guides. In addition, the carbon can cause spark plug misfiring as well as detonation, preignition, or excessive oil comumption. To allow the surplus oil to return to the crankcase, holes are drilled in the piston ring grooves or in the lands next to these grooves. Oil Scraper Ring The oil scraper ring usually has a beveled face and is installed in the groove at the bottom of the piston skirt. The ring is installed with the scraping edge away from the piston head or in the reverse position, depending upon cylinder position and the engine series In the reverse position, the scraper ring retains the surplus oil above the ring on the upward piston stroke, and this oil is returned to the crankcase by the oil control rings on the downward stroke. CYLINDERS The portion of the engine in which the power is developed is called the cylinder. The cylinder provides a combustion chamber where the burning and expansion of gases take place, and it houses the piston and the connecting rod. There are four major factors that need to be considered in the design and construction of the cylinder assembly. These are: (1) It must be strong enough to withstand the internal pressures developed during engine operation. (2) It must be constructed of a lightweight metal to keep down engine weight. (3) It must have good heat-conducting properties for efficient cooling. (4) It must be comparatively easy and inexpensive to manufacture, inspect, and maintain. The head is either produced singly for each cylinder in air-cooled engines, or is cast "in-block" (all cylinder heads in one block) for liquid-cooled engines. The cylinder head of an air-cooled engine is generally made of aluminum alloy, because aluminum alloy is a good conductor of heat and its light weight reduce! the overall engine weight. Cylinder heads are forged or die-cast for greater strength. The inner shape of a cylinder head may be flat, semispherical, or peaked, in the form of a house roof. The semispherical type has proved most satisfactory because it is stronger and aids in a more rapid and thorough scavenging of the exhaust gases. The cylinder used in the air-cooled engine is the overhead valve type shown in figure 1-14. Each cylinder is an assembly of two major parts: (1) The cylinder head, and (2) the cylinder barrel. At assembly, the cylinder head is expanded by heating and then screwed down on the cylinder barrel which Cast aluminum headIntake valve Exhaust valve Y rod barrel L-J FIGURE1-14. Cutaway view of the cylinder assembly. ### Page 26 has been chilled; thus, when the head cools and contracts, and the barrel warms up and expands, a gastight joint results. While the majority of the cylinders used are conslructed in this manner, some are one-piece aluminum alloy sand castings. The piston bore of a sand cast cylinder is fitted with a steel liner which extends the full length of the cylinder barrel section and projects below the cylinder flange of the casting. This liner is easily removed, and a new one can be installed in the field. Cylinder Heads The purpose of the cylinder head is to provide a place for combustion of the fuel/air mixture and to give the cylinder more heat conductivity for adequate cooling. The fuel/air mixture is ignited by the spark in the combustion chamber and commences burning as the piston travels toward top dead center on the compression stroke. The ignited charge is rapidly expanding at this time, and pressure is increasing so that as the piston travels through the top dead center position, it is driven downward on the power stroke. The intake and exhaust valve ports are located in the cylinder head along with the spark plugs and the intake and exhaust valve actuating mechanisms. After casting, the spark plug bushings, valve guides, rocker arm bushings, and valve seats are installed in the cylinder head. Spark plug openings may be fitted with bronze or steel bushings that are shrunk and screwed into the openings. Stainless steel Heli-Coil spark plug inserts are used in many engines currently manufactured. Bronze or steel valve guides are usually shrunk or screwed into drilled openings in the cylinder head to provide guides for the valve stems. These are generally located at an angle to the center line of the cylinder. The valve seats are circular rings of hardened metal which protect the relatively soft metal of the cylinder head from the hammering action of the valves and from the exhaust gases. The cylinder heads of air-cooled engines are subjected to extreme temperatures; it is therefore necessary to provide adequate fin area, and to use metals which conduct heat rapidly. Cylinder heads of air-cooled engines are usually cast or forged singly. Aluminum alloy is used in the construction for a number of reasons. It is well adapted for casting or for the machining of deep, closely spaced fins, and it is more resistant than most metals to the corrosive attack of tetraethyl lead in gasoline. The greatest improvement in air cooling has resulted from reducing the thickness of the fins and increasing their depth. In this way the fin area has been increased from approximately 1,200 sq. in. to more than 7,500 sq. in. per cylinder in modern engines. Cooling fins taper from 0.090'in. at the base to 0.060 in. at the tip end. Because of the difference in temperature in the various sections of the cylinder head, it is necessary to provide more cooling-fin area on some sections than on others. The exhaust valve region is the hottest part of the internal surface; therefore, more fin area is provided around the outside of the cylinder in this section. Cylinder Barrels In general, the cylinder barrel in which the piston operates must be made of a high-strength material, usually steel. It must be as light as possible, yet have the proper characteristics for operating under high temperatures. It must be made of a good bearing material and have high tensile strength. The cylinder barrel is made of a steel alloy forging with the inner surface hardened to resist wear of the piston and the piston rings which bear against it. This hardening is usually done by exposing the steel to ammonia or cyanide gas while the steel is very hot. The steel soaks up nitrogen from the gas which forms iron nitrides on the exposed surface. As a result of this process, the metal is said to be nitrided. In some instances the barrel will-have threads on the outside surface at one end so that it can be screwed into the cylinder head. Some air-cooled cylinder barrels have replaceable aluminum cooling fins attached to them, while others have the cooling fins machined as an integral part of the barrel. CYLINDER NUMBERING Occasionally it is necessary to refer to the left or right side of the engine or to a particular cylinder. Therefore, it is necessary to know the engine directions and how cylinders of an engine are numbered. The propeller shaft end of the engine is always the front end, and the accessory end is the rear end, regardless of how the engine is mounted in an aircraft. When referring to the right side or left side of an engine, always assume you are viewing it from the rear or accessory end. As seen from this position, crankshaft rotation is referred to as either clockwise or counterclockwise. Radial engine cylinders are numbered clockwise as viewed from the accessory end. In-line and V-type engine cylinders are usually numbered from the rear. In V-engines, the cylinder banks are ### Page 27 known as the right bank and the left bank, as No. 2, is No. 4, and so on. The numbering of viewed from the accessory end. opposed engine cylinders is by no means standard. The numbering of engine cylinders is shown in Some manufacturers number their cylinders from figure 1-15. Note that the cylinder numbering of the rear and others from the front of the engine. the opposed engine shown begins with the right Always refer to the appropriate engine manual to rear as No. 1, and the left rear as No. 2. The one determine the correct numbering system used by forward of NO. 1 is No. 3; the one forward of the manufacturer. Single-row radial Double-row radial Opposed FIGURE1-15. Numbering of engine cylinders. Single-row radial engine cylinders are numbered clockwise when viewed from the rear end. Cylinder No. 1 is the top cylinder. In double-row engines, the same system is used, in that the No. 1 cylinder is the top one in the rear row. No. 2 cylinder is the first one clockwise from No. 1, but No. 2 is in the front row. No. 3 cylinder is the next one clockwise to No. 2, but is in the rear row. Thus, all odd-numbered cylinders are in the rear row, and all even-numbered cylinders are in the front row. FIRING ORDER The firing order of an engine is the sequence in which the power event occurs in the different cylinders. The firing order is designed to provide for balance and to eliminate vibration to the greatest extent possible. In radial engines the firing order must follow a special pattern, since the firing impulses must follow the motion of the crankthrow during its rotation. In in-line engines the firing orders may vary somewhat, yet most orders are arranged so that the firing of cylinders is evenly distributed along the crankshaft. Six-cylinder inline engines generally have a firing order of 1-5-3-6-2-4. Cylinder firing order in opposed ewgines can usi~ally be listed in pairs of cylinders, as each pair fires across the center main bearing. The firing order of six-cylinder opposed engines is 14-5-2-3-6. The firing order of one model four-cylinder opposed engine is 1-4-2-3, but on another model it is 1-3-24. Single-Row Radial Engines On a single-row radial engine, first, all the oddnumbered cylinders fire in numerical succession; then the even-numbered cylinders fire in numerical succession. On a five-cylinder radial engine, for example, the firing order is 1-3-5-24, and on a seven-cylinder radial engine it is 1-3-5-7-2-4-6. The firing order of a nine-cylinder radial engine is 1-3-5-7-9-246-8. Double-Row Radial Engines On a double-row radial engine, the firing order is somewhat complicated. The firing order is arranged with the firing impulsr occurring in a cylinder in one row and then in a cylinder in the other row; therefore, two cylinders in the same row never fire in succession. An easy method for computing the firing order of a 14-cylinder, double-row radial engine is to start with any number from 1to 14, and add 9 or subtract 5 (these are called the firing order numbers), whichever will give an answer between 1 and 14, inclusive. For example, starting with 8, 9 cannot be added since the answer would then be more than 14; therefore, suhract 5 from 8 to get 3, add 9 to 3 to get 12, subtract 5 from 12 to get 7, ### Page 28 subtract 5 from 7 to get 2, and so on. The firing order numbers of an 18-cylinder, double-row radial engine are 11 and 7; that is, Legin with any number from 1 to 18 and add 11 or subtract 7. For example, beginning with 1, add 11to get 12; 11cannot be added to 12 because the total would he more than 18, so subtract 7 to get 5, add 11 to 5 to get 16, subtract 7 from 16 to get 9, subtract 7 from 9 to get 2, add 11to 2 to get 13, and continue this process for 18 cylinders. VALVES The fuel/air mixture enters the cylinders through the intake valve ports, and burned gases are expelled through the exhaust valve ports. The head of each valve opens and closes these cylinder ports. The valves used in aircraft engines are the conventional poppet type. The valves are also typed by their shape and are called either mushroom or tulip because of their resemblance to the shape of these plants. Figure 1-16 illustrates various shapes and types of these valves. Large Hardened tip IIollo\~.-lwntl Jlushroom JIu\hroorn ty~x tLIW Tulip type Scmi-tulip type Tulip type FIGURE1-16 Valve types. Valve Construction The valves in the cylinders of an aircraft engine are subjected to high temperatures, corrosion, and operating stresses; thus, the metal alloy in the valves must be able to resist all these factors. Because intake valves operate at lower temperatures than exhaust valves, they can be made of chrome-nickcl steel. Exhaust valves are usually made of nichrome, silchrome, or cobalt-chromium steel. The valve head has a ground face which forms a seal against the ground valve seat in the cylinder head when the valve is closed. The face of the valve is usually ground to an angle of either 30' or 45'. In some engines, the intake-valve face is ground to an angle of 30°, and the exhaust valve face is ground to a 45' angle. Valve faces are often made more durable by the application of a material called stellite. About 1/16 inch of this alloy is welded to the valve face and ground to the correct angle. Stellite is resistant to high-temperature corrosion and also withstands the shock and wear associated with valve operation. Some engine manufacturers use a nichrome facing on the valves. This serves the same purpose as the stellite material. The valve stem acts as a pilot for the valve head and rides in the valve guide installed in the cylinder head for this purpose. The valve stem is surfacehardened to resist wear. The neck is the part that forms the junction between the head and the stem. The tip of the valve is hardened to withstand the hammering of the valve rocker arm as it opens the valve. A machined groove on the stem near the tip receives the split-ring stem keys. These stem keys form a lock ring to hold the valve spring retaining washer in place. Some intake and exhaust valve stems are hollow and partially filled with metallic sodium. This material is used because it is an excellent heat conductor. The sodium will melt at approximately 208' F., and the reciprocating motion of the valve circulates the liquid sodium and enables it to carry away heat from the valve head to the valve stem, where it is dissipated through the valve guide to the cylinder head and the cooling fins. Thus, the operating temperature of the valve may be reduced as much as 300' to 400' F. Under no circumstances should a sodium-filled valve be cut open or subjected to treatment which may cause it to rupture. Exposure of the sodium in these valves to the outside air will result in fire or explosion with possible personal injury. The most commonly used intake valves have solid stems, and the head is either flat or tulip shaped. Intake valves for low-power engines are usually flat headed. In some engines, the intake valve may be the tulip type and have a smaller stem than the exhaust valve, or it may be similar to the exhaust valve but ### Page 29 - - - have a solid stem and head. Although these valves - are similar, they are not interchangeable since the faces of the valves are constructed of different material. The intake valve will usually have a flat milled on the tip to identify it. VALVE-OPERATING MECHANISM For a reciprocating engine to operate properly, each valve must open at the proper time, stay open for the required length of time, and close at the proper time. Intake valves are opened just before the piston reaches top dead center, and exhaust valves remain open after top dead center. At a particular instant, therefore, both valves are open at the same time (end of the exhaust stroke and beginning of the intake stroke). This valve-overlap permits better volumetric efficiency and lowers the cylinder operating temperature. This timing of the valves is controlled by the valve-operating mechanism. The valve lift (distance that the valve is lifted off its seat) and the valve duration (length of time the valve is held open) are both determined by the shape of the cam lobes. Typical cam lobes are illustrated in figure 1-17. The portion of the lobe that gently starts the valveoperating mechanism moving is called a ramp, or step. The ramp is machined on each side of the cam lobe to permit the rocker arm to be eased into contact with the valve tip and thus reduce the shock load which would other^''~seoccur. The valve-operating mechanism consists of a cam ring or camshaft equipped with lobes, which work Camshaft against a cam roller or a cam follower. (See figs. 1-18 and 1-19.) The cam follower, in turn, pushes a push rod and ball socket, which, in turn, actuates a rocker arm which opens the valve. Springs, which slip over the stem of the valves and which are held in place by the valve-spring retaining washer and stem key, close each valve and push the valve mechanism in the opposite direction when the cam roller or follower rolls along a low section of the cam ring. Valve . guide - Seat FIGURE1-18. Valve-operating mechanism FIGURE1-17. Typical cam lobes. (radial engine). ### Page 30 FIGURE Valve-operating mechanism (opposed engine). 1-19. Cam Ring The valve mechanism of a radial engine is operated by one or two cam rings, depending upon the number of rows of cylinders. In a single-row radial engine one ring with a double cam track is used. One track operates the intake valves; the other, the exhaust valves. The cam ring is a circular piece of steel with a series of cams or lobes on the outer surface. The surface of these lobes and the space between them (on which the cam rollers ride) is known as the cam track. As the cam ring revolves, the lobes cause the cam roller to raise the tappet in the tappet guide, thereby transmitting the force through the push rod and rocker arm to open the valve. In a single-row radial engine, the cam ring is usually located between the propeller reduction gearing and the front end of the power section. In a twin-row radial engine, a second cam for the operation of the valves in the rear row is installed between the rear end of the power section and the supercharger section. The cam ring is mounted concentrically with the crankshaft and is driven by the crankshaft at a reduced rate of speed through the cam intermediate drive gear assembly. The cam ring has two parallel sets of lobes spaced around the outer periphery, one set (cam track) for the intake valves and the other for the exhaust valves. The cam rings used may have four or five lobes on both the intake and the exhaust tracks. The timing of the valve events is determined by the spacing of these lobes and the speed and direction at which the cam rings are driven in relation to the speed and direction of the crankshaft. The method of driving the cam varies on different makes of engines. The cam ring can be designed with teeth on either the inside or outside periphery. If the reduction gear meshes with the teeth on the outside of the ring. the cam will turn in the direction of rotation of the crankshaft. If the ring is driven from the inside, the cam will turn in the opposite direction from the crankshaft. This method is illustrated in figure 1-18. A study of figure 1-20 will show that a four-lobe cam may be used on either a seven-cylinder or ninecylinder engine. On the seven-cylinder it will rotate in the same direction as the crankshaft, and on the nine-cylinder, opposite the crankshaft rotation. On the nine-cylinder engine the spacing between cylinders is 40°, and the firing order is 1-3-5-7-9-24 6-8. This means that there is a space of 80' between firing impulses. The spacing on the four lobes of the cam ring is 90°, which is greater than the spacing between in~pulses. Therefore, to obtain proper relation of valve operations and firing order, it is necessary to drive the cam opposite the crankshaft rotation. ### Page 31 - - ------ 5 cylinders 7 cylinders 9 cylinders * U1 L "7 n"l" z l"l" z$ E 2 & 2 & Direction of rotation 2% zci: V, 2% 3 1/6 4 1/8 5 1/10 Withcrankshaft. 2 1/4 3 1/6 4 1/8 Opposite crankshaft. FIGURE Radial engines, cam ring table. 1-20. Using the four-lobe cam on the seven-cylinder engine, the spacing between the firing of the cylinders will be greater than the spacing of the cam lobes. Therefore, it will be necessary for the cam to rotate in the same direction as the crankshaft. A formula that sometimes is used in figuring cam speed is: Cam ring speed = 1/2 tby the number of lobes on either cam track. One-half is the speed at which the cam would operate if it were equipped with a single lobe for each valve. It is divided by the number of lobes, which will determine how much the speed will have to be reduced. In a twin-row, 14-cylinder radial engine which has seven cylinders in each row or bank, the valve mechanism may consist of two separate assemblies, one for each row. It could be considered as two seven-cylinder engines in tandem having the firing impulses properly spaced or lapped. For instance, in a twin-row engine, two four-lobe cam rings may be used. The cams are driven by gears attached to the crankshaft through gear teeth on the periphery of each cam. Camshaft The valve mechanism of an opposed engine is operated by a camshaft. The camshaft is driven by a gear that mates with another gear attached to the crankshaft (see figure 1-21). The camshaft always rotates at one-half the crankshaft speed. As the camshaft revolves, the lobes cause the tappet assembly to rise in the tappet guide, transmitting the force through the push rod and rocker arm to open the valve. Tappet Assembly The tappet assembly consists of: (1) A cylindrical tappet, which slides in and out in a tappet guide installed in one of the crankcase sections around the cam ring. (2) A cam follower or tappet roller, which follows the contour of the cam ring and lobes. (3) A tappet ball socket or push rod socket. (4) A tappet spring. The function of the tappet assembly is to convert Cam gear twice size -of crankshaft gear. Camshaft lobe Crankshaft gear FIGURE Cam drive mechanism 1-21. opposed-type aircraft engine. the rotational movement of the cam lobe into reciprocating motion and to transmit this motion to the push rod, rocker arm, and then to the valve tip, opening the valve at the proper time. The purpose of the tappet spring is to take up the clearance between the rocker arm and the valve tip to reduce the shock load when the valve is opened. A hole is drilled through the tappet to allow engine oil to flow to the hollow push rods to lubricate the rocker assemblies. Hydraulic Valve Tappets Some aircraft engines incorporate hydraulic tappets which automatically keep the valve clearance at zero, eliminating the necessity for any valve clearance adjustment mechanism. A typical hydraulic tappet (zero-lash valve lifter) is shown in figure 1-22. When the engine valve is closed, the face of the tappet body (cam follower) is on the base circle or back of the cam, as shown in figure 1-22. The light plunger spring lifts the hydraulic plunger so that its outer end contacts the push rod socket, exerting a light pressure against it, thus eliminating any clearance in the valve linkage. As the plunger moves outward, the ball check valve moves off its seat. Oil from the supply chamber, which is directly connected with the engine lubrication system, flows in and fills the pressure chamber. As the camshaft rotates, the cam pushes the tappet body and the hydraulic lifter cylinder outward. This ### Page 32 Oil pressure chamber FIGURE Hydraulic valve tappets. 1-22. action forces the ball check valve onto its seat; thus, the body of oil trapped in the pressure chamber acts as a cushion. During the interval when the engine valve is off its seat, a predetermined leakage occurs between plunger and cylinder bore which compensates for any expansion or contraction in the valve train. Immediately after the engine valve closes, the amount of oil required to fill the pressure chamber flows in from the supply chamber, preparing for another cycle of operation. Push Rod The push rod. tubular in form, transmits the lifting force from the valve tappet to the rocker arm. A hardened-steel ball is pressed over or into each end of the tube. One ball end fits into the socket of the rocker arm. In some instances the balls are on the tappet and rocker arm. and the sockets are on the push rod. The tubular form is employed because of its lightness and strength. It permits the engine lubricating oil under pressure to pass through the hollow rod and the drilled ball ends to lubricate the ball ends, rocker-arm bearing, and valve-stem guide. The push rod is enclosed in a tubular housing that extends from the crankcase to the cylinder head. Rocker Arms The rocker arms transmit the lifting force from the cams to the valves. Rocker arm assemblies are supported by a plain, roller. or ball bearing. or a combination of these, which serves as a pivot. Generally one end of the arm bears against the push rod and the other bears on the valve stem. Orie end of the rocker arm is sometimes slotted to accommodate a steel roller. The opposite end is constructed with either a threaded split clamp and locking bolt or a tapped hole. The arm may have an adjusting screw for adjusting the clearance between the rocker arm and the valve stem tip. The screw is adjusted to the specified clearance to make certain that the valve closes fully. Valve Springs Each valve is closed by two or three helical-coiled springs. If a single spring were used, it would vibrate or surge at certain speeds. To eliminate this difficulty, two or more springs (one inside the other) are installed on each valve. Each spring will therefore vibrate at a different engine speed, and rapid damping out of all spring-surge vibrations during engine operation will result. Two or more springs also reduce danger of weakness and possible failure by breakage due to heat and metal fatigue. The springs are held in place by split locks installed in the recess of the valve spring upper retainer or washer, and engage a groove machined into the valve stem. The functions of the valve springs are to close the valve and to hold the valve securely on the valve seat. Hydraulic Valve Lifters Hydraulic valve lifters are normally adjusted at the time of overhaul. They are assembled dry (no lubrication), clearances checked, and adjustments are usually made by use of pushrods having different lengths. A minimum and maximum valve clearance is established. Any measurement between these extremes is acceptable but approximately half way between the extremes is desired. Hydraulic valve lifters require less maintenance, are better lubricated, and operate more quietly than the screw adjustment type. BEARINGS A bearing is any surface which supports, or is supported by, another surface. A good bearing must be composed of material that is strong enough to withstand the pressure imposed on it and should 24 ### Page 33 permit the other surface to move with a minimum of friction and wear. The parts must be held in position within very close tolerances to provide efficient and quiet operation, and yet allow freedom of motion. To accomplish this, and at the same time reduce friction of moving parts so that power loss is not excessive, lubricated bearings of many types are used. Bearings are required to take radialloads, thrust loads, or a combination of the two. There are two ways in which bearing surfaces move in relation to each other. One is by the sliding movement of one metal against the other, and the second is for one surface to roll over the other. The three different types of bearings in general use are plain, roller, and ball (see figure 1-23). Plain Bearings Plain bearings are generally used for the crankshaft, cam ring, camshaft, connecting rods, and the accessory drive shaft bearings. Such bearings are usually subjected to radial loads only, although some have been designed to take thrust loads. Plain bearings are usually made of nonferrous (having no iron) metals, such as silver. bronze, aluminum, and various alloys of copper, tin, or lead. Master rod or crankpin bearings in some engines are thin shells of steel, plated with silver on both the inside and the outside surfaces and with lead-tin plated over the silver on the inside surface only. Smaller bearings, such as those used to support various shafts in the accessory section, are called bushings. Porous Oilite bushings are widely used in this instance. They are impregnated with oil so that the heat of friction brings the oil to the bearing surface during engine operation. Ball Bearings A ball bearing assembly consists of grooved inner and outer races, one or more sets of balls, and, in bearings designed for disassembly, a bearing retainer. They are used for supercharger impeller shaft bearings and rocker arm bearings in some engines. Special deep-groove ball bearings are used in aircraft engines to transmit propeller thrust to the engine nose section. Roller Bearings Roller bearings are made in many types and shapes, but the two types generally used in the aircraft engine are the straight roller and the tapered roller bearings. Straight roller bearings are used where the bearing is subjected to radial loads only. In tapered roller bearings, the inner- and outer-race bearing surfaces are cone shaped. Such bearing will withstand both radial and thrust loads. Straight roller bearings are used in high-power aircraft engines for the crankshaft main bearings. They are Plain Ball FIGURE Bearings.1-23. also used in other applications where radial loads are high. PROPELLER REDUCTION GEARING The increased brake horsepower delivered by a high-horsepower engine results partly from increased crankshaft r.p.m. It is therefore necessary to provide reduction gears to limit the propeller rotation speed to a value at which efficient operation is obtained. Whenever the speed of the blade tips approaches the speed of sound, the efficiency of the ### Page 34 propeller decreases rapidly. The general practice has been to provide reduction gearing for propellers of engines whose speeds are abobe 2,000 r.p.m., because propeller efficiency decreases rapidly above this speed. Since reduction gearing must withstand extremely high stresses, the gears are machined from steel forgings. Many types of reduction gearing systems are in use. The three types (fig. 1-24) most commonly used are: (1) Spur planetary. (2) Bevel planetary. 13) Spur and pinion. The planetary reduction gear systems are used with radial and opposed engines. and the spur and pinion system is used with in-line and V-type engines. Two of these types, the spur planetary and the bevel planetary. are discussed here. The spur planetary reduction gearing consists of a large driving gear or sun gear splined (and sometimes shrunk) to the crankshaft. a large stationary gear, called a bell gear, and a set of small spur planetary pinion gears mounted on a carrier ring. The ring is fastened to the propeller shaft, and the ~lanetary gears mesh with both the sun gear and the stationary bell'or ring gear. The stationary gear is bolted or splined to the front-section housing. When the engine is operating, the sun gear rotates. Because the planetary gears are meshed with this ring, they also must rotate. Since they also mesh with the stationary gear. they will walk or roll around it as they rotate, and the ring in which they are mounted will rotate the propeller shaft in the same direction as the crankshaft but at a reduced speed. In some engines, the bell gear is mounted on the propeller shaft, and the planetary pinion gear cage is held stationary. The sun gear is splined to the crankshaft and thus acts as a driving gear. In such an arrangement, the propeller travels at a reduced speed, but in opposite direction to the crankshaft. In the belel planetary reduction gearing system. the driving gear is machined with beveled external teeth and is attached to the crankshaft. A set of mating bevel pinion gcars is mounted in a cage attached to the end of the shaft.~ro~eller The pinion gears are driven by the drive gear and walk around the stationary gear, which is bolted or splined to the front-section housing. The thrust of the bevel pinion gears is absorbed by a thrust ball bearing of special desip. The dribe and the fixed gears are generally supported by healy-duty ball bearings. This type of planetary reduction assembly is more compact than the other one described and can therefore be used where a smaller propeller gear step-down is desired. PROPELLER SHAFTS Propeller shafts may be three major types; tapered, splined, or flanged. Tapered shafts are identified by taper numbers. Splin.ed and flanged shafts are identified by SAE numbers. The propeller shaft of most low-power output engines is forged as part of the crankshaft. It is tapered and a-milled slot is provided so that the propeller hub can be keyed to the shaft. The keyway and key index of the propeller are in relation to the #1 cylinder top dead center. The end of the shaft is threaded to receive the propeller retaining nut. Tapered propeller shafts are common on older and in-line engines. The propeller shaft of a high-output engine generally is splined. It is threaded on one end for a propeller hub nut. The thrust bearing, which absorbs propeller thrust, is located around the shaft and transmits the thrust to the nose-section housing. The shaft is threaded for attaching the thrustbearing retaining nut. On the portion protruding from the housing (between the two sets of threads), splines are located to receive the splined propeller hub. The shaft is generally machined from a steelalloy forging throughout its length. The propeller shaft may be connected by reduction gearing to the engine crankshaft, but in smaller engines the propeller shaft is simply an extension of the engine crankshaft. To turn the propeller shaft, the engine crankshaft must revolve. Flanged propeller shafts are used on medium or low powered reciprocating and turbojet engines. One end of the shaft is flanged with drilled holes to accept the propeller mounting bolts. The installation may be a short shaft with internal threading to accept the distributor valve to be used with a controllable propeller. The flanged propeller shaft is a normal installation on most approved reciprocating engines. **RECIPROCATING ENGINE** OPERATING PRINCIPLES A study of this section will help in understanding the basic operating principles of reciprocating engines. The principles which govern the relationship between the pressure, volume, and temperature of pases are the basic principles of engine operation. An internal-combustion engine is a device for converting heat energy into mechanical energy. Gasoline is vaporized and mixed with air, forced or drawn into a cylinder, compressed by a piston, and then ignited by an electric spark. The conversion of the resultant heat energy into mechancial energy and then into work is accomplished in the cylinder. Figure 1-25 illustrates the various engine compo- ### Page 35 uo!u!d pue lnds ### Page 36 Spark plug Intake valve \ a Exhaust valve \ \ Crankshaft FIGURE1-25. Components and terminology of engine operation. nents necessary to accomplish this conversion and also presents the principal terms used to indicate engine operation. The operating cycle of an internal combustion reciprocating engine includes the series of events required to induct, compress, ignite, burn, and expand the fuel/air charge in the cylinder, and to scavenge or exhaust the byproducts of the combustion process. When the compressed mixture is ignited, the resultant gases of combustion expand very rapidly and force the piston to move away from the cylinder head. This downward motion of the piston, acting on the crankshaft through the connecting rod, is converted to a circular or rotary motion by the crankshaft. A valve in the top or head of the cylinder opens to allow the burned gases to escape, and the momentum of the crankshaft and the propeller forces the piston back up in the cylinder- where it is ready for the next event in the cycle. Another valve in the cylinder head then opens to let in a fresh charge of the fuel/air mixture. The valve allowing for the escape of the burning exhaust gases is called the exhaust valve, and the valve which lets in the fresh charge of the fuel/air mixture is called the intake valve. These valves are opened and closed mechanically at the proper times by the valve-operating mechanism. The bore of a cylinder is its inside diameter. The stroke is the distance the piston moves from one end of the cylinder to the other, specifically, from T.D.C. (top dead center) to B.D.C. (bottom dead center), or vice versa (see figure 1-25). OPERATING CYCLES There are two operating cycles in general use: (1) The two-stroke cycle, and (2) the four-stroke cycle. At the present time, the two-stroke-cycle engine is fast disappearing from the aviation scene and will not be discussed. As the name implies, two-stroke-cycle engines require only one upstroke and one downstroke of the piston to complete the required series of events in the cylinder. Thus the engine completes the operating cycle in one revolution of the crankshaft. Most aircraft reciprocating engines operate on the four-stroke cycle, sometimes called the Otto cycle after its originator, a German physicist. The fourstroke-cycle engine has many advantages for use in aircraft. One advantage is that it lends itself readily to high performance through supercharging. In this type of engine, four strokes are required to complete the required series of events or operating cycle of each cylinder, as shown in figure 1-26. Two complete revolutions of the crankshaft (720°) are required for the four strokes; thus, each cylinder in an engine of this type fires once in every two revolutions of the crankshaft. FOUR-STROKE CYCLE In the following discussion of the four-strokecycle engine operation, it should be realized that the timing of the ignition and the valve events will vary considerably in different engines. Many factors influence the timing of a specific engine, and it is most important that the engine manufacturer's recommendations in this respect be followed in maintenance and overhaul. The timing of the valve and ignition events is always specified in degrees of crankshaft travel. ### Page 37 FIGURE1-26. Four-stroke cycle. In the following paragraphs, the timing of each event is specified in terms of degrees of crankshaft travel on the stroke during which the event occurs. It should be remembered that a certain amount of crankshaft travel is required to open a valve fully; therefore, the specified timing represents the start of opening rather than the full-open position of the valve. Intake Stroke During the intake stroke, the piston is pulled downward in the cylinder by the rotation of the crankshaft. This reduces the pressure in the cylinder and causes air under atmospheric pressure to flow through the carburetor, which meters the correct amount of fuel. The fuel/air mixture passes through the intake pipes and intake valves into the cylinders. The quantity or weight of the fuel/air charge depends upon the degree of throttle opening. The intake valve is opened considerably before the piston reaches top dead center on the exhaust stroke, in order to induce a greater quantity of the fuel/air charge into the cylinder and thus increasc the horsepower. The distance the ~alve may be opened before top dead center, however. is limited by several factors, such as the possibility that hot gases remaining in the cylinder from the prelious cycle may flash back into the intake pipe and the induction system. In all high-power aircraft engines, both the intake and the exhaust valves are off the Palve seats at top dead center at the start of the intake stroke. As mentioned above. the intake valve opens before top dead center on the exhaust stroke (valve lead). and the closing of the exhaust valve -is delayed considerably after the piston has passed top dead center and has started the intake stroke (valve lag). This timing is called valve overlap and is designed to aid in cooling the cylinder internally by circulating the cool incoming fuel/air mixture, to increase the amount of the fueliair mixture induced into the cylinder, and to aid in scavenging tlie byproducts of combustion. The intake valve is timed to close about 50' to 75O past bottom dead center on the compression stroke depending upon the specific engine. to allow the momentum of the incoming gases to charge the cylinder more conlpletely. Because of tlie comparatively large volume of the cylinder above the piston when the piston is near bottom dead center. the slight upward travel of the piston during this time does not hale a great effect on the incoming flow of gases. This late timing can be carried too far because the gases may be forced back through the intake valve and defeat the purpose of the late closing. Compression Stroke After the intake valve is closed. the continued upward travel of the piston compresses tlie lucliair mixture to obtain the desired burning and expansion characteristics. The charge is fired by means of an electric spark as the piston approaches top dead center. The timc of ignition will vary from 20° to 3S0 before top dead center, depending upon the requirt,mcwts of the specific engine. to ensure complctc combustion of the charge by the tirnc the piston is slightly past the top dead center position. Many factors affect ignition timing. and thc engine manufacturer has expended considcrablc tirnc in research and testing to determine the best setting. All engines incorporate deviccs lor adjusting the ignition timing, and it is most important that the ignition system be timed according to tlir rnginc. manufacturer's recommendations. Power Stroke As the piston mohes through the top dead center position at the end of the compression strokc and starts down on the power stroke, it is pushed downward by the rapid expansion of the burning gasrs within the cylinder head with a force that can be greater than 15 tons (30,000 p.s.i.) at maximum power output of the engine. The temperature of these burnning gases map he betwem 3.000° and 4,000°F. ### Page 38 As the piston is forced downward during the power stroke by the pressure of the burning gases exerted upon it, the downward movement of the connecting rod is changed to rotary movement by the crankshaft. Then the rotary movement is transmitted to the propeller shaft to drive the propeller. As the burning gases are expanded, the temperature drops to within safe limits before the exhaust gases flow out through the exhaust port. The timing of the exhaust valve opening is determined by, among other considerations, the desirability of using as much of the expansive force as possible and of scavenging the cylinder as completely and rapidly as possible. The valve is opened considerably before bottom dead center on the power stroke (on some engines at 50° and 75O before B.D.C.) while there is still some pressure in the cylinder. This timing is used so that the pressure can force the gases out of the exhaust port as soon as possible. This process frees the cylinder of waste heat after the desired expansion has been obtained and avoids overheating the cylinder and the piston. Thorough scavenging is very important, because any exhaust products remaining in the cylinder will dilute the incoming fuel/air charge at the start of the next cycle. Exhaust Stroke As the piston travels through bottom dead center at the completion of the power stroke and starts upward on the exhaust stroke, it will begin to push the burned exhaust gases out the exhaust port. The speed of the exhaust gases leaving the cylinder creates a low pressure in the cylinder. This low or reduced pressure speeds the flow of the fresh fuel/air charge into the cylinder as the intake valve is beginning to open. The intake valve opening is timed to occur at 8O to 55O before top dead center on the exhaust stroke on various engines. RECIPROCATING ENGINE POWER AND EFFICIEN- CIES All aircraft engines are rated according to their ability to do work and produce power. This section presents an explanation of work and power and how they are calculated. Also discussed are the various efficiencies that govern the power output of a reciprocating engine. Work The physicist defines work as "Work is force times distance. Work done by a force acting on a body is equal to the magnitude of the force multiplied by the distance through which the force acts.'' Work (W) =Force (F) x Distance (D) Work is measured by several standards, the most common unit is called foot-pound. If a 1-pound mass is raised 1 foot. 1 ft.-lb. (foot-pound) of work has been performed. The greater the mass and the greater the distance, the greater the work. Horsepower The common unit of mechanical power is the hp. (horsepower). Late in the 18th century, James Watt, the inventor of the steam engine, found that an English workhorse could work at the rate of 550 ft.-lb. per second, or 33,000 ft.-lb. per minute, for a reasonable length of time. From his observations came the hp., which is the standard unit of power in the English system of measurement. To calculate the hp. rating of an engine. divide the power developed in ft.-lb. per minute by 33.000, or the power in ft.-lb. per second by 550. ft.-lb. per min. hp. = . or 33,000 ft.-lb. per sec. -- 550 As stated above, work is the product of force and ditsance, and power is work per unit of time. Consequently, if a 33.000-lb. weight is lifted through a vertical distance of 1ft. in Imin., the power expended is 33,000 ft.-lb. per min., or exactly 1hp. Work is performed not only when a force is applied for lifting; force may be applied in any direction. If a 100-lb. weight is dragged along the ground, a force is still being applied to perform work, although the direction of the resulting motion is approximately horizontal. The amount of this force would depend upon the roughness of the ground. If the weight were attached to a spring scale graduated in pounds. then dragged by pulling on the scale handle. the amount of force required could be measured. Assume that the force required is 90 lbs., and the 100-lb. weight is dragged 660 ft. in 2 min. The amount of work performed in the 2 min. will be 59,400 ft.-lb.. or 29.700 ft.-lb. per min. Since 1hp. is 33,000 ft.-lb. per min., the hp. expended in this case will be 29.700 divided by 33,000, or 0.9 hp. Piston Displacement When other factors remain equal. the greater the piston displacement the greater the maximum horsepower an engine will be capable of developing. When a piston moves from bottom dead center to top dead center. it displaces a specific volume. The volume displaced by the piston is known as piston displacement and is expressed in cubic inches for most American-made engines and cubic centimeters for others. ### Page 39 The piston displacement of one cylinder may be obtained by multiplying the area of the cross section of the cylinder by the total distance the piston moves in the cylinder in one stroke. For multicylinder engines this product is multiplied by the number of cylinders to get the total piston displacement of the engine. Since the volume (V) of a geometric cylinder equals the area (A) of the base multiplied by the altitude (H), it is expressed mathematically as: V=A x H. For our purposes, the area of the base is the area of the cross section of the cylinder or of the piston top. Area Of A Circle To find the area of a circle it is necessary to use a number called pi. This number represents the ratio of the circumference to the diameter of any circle. Pi cannot be found exactly because it is a never-ending decimal, but expressed to four decimal places it is 3.1416, which is accurate enough for most computations. The area of a circle, as in a rectangle or triangle, must be expressed in square units. The distance that is one-half the diameter of a circle is known as the radius. The area of any circle is found by squaring the radius and multiplying by pi. (7). The formula is expressed thus: A=7;K2. Where A is the area of a circle; pi is the givrn constant; and r is the radius of the circle, which is equal to I/i2 the ,diameter or R= -D 2 Example Compute the piston displacement of the PR7A 14 cylinder engine having a cylinder with a 5.5 inch diameter and a 5.5 inch stroke. Formulas required are : A =T;R2 V=A x H Total V=V x N (number of cylinders) Substitute values into these formulas and complete the calculation. n R=- R=5.5/2=2.752 A=xR2 Ar3.1416 (2.75 s 2.75) A=3.1416 x 7.5625 =23.7584 sq. inches V=A x H V=23.7584 s 5.5 V= 130.6712 Total V=V x IY Total V=130.6'712 s 14 Total V= 1829.3968 Rounded off to the next whole number total piston displacement equals 1830 cu. in. Another method of calculating the piston displacement uses the diameter of the piston instead of the radius in the formula for the area of the base. A=l/r (7) (D)2 Substituting A=Y$ x 3.1416 x 5.5 x 5.5 A=.7854 s 30.25 A=23.758 square inches. From this point on the calculations are identical to the preceding example. Compression Ratio All internal-combustion engines must compress the fuel/air mixture to receive a reasonable amount of work from each power stroke. The fuel/air charge in the cylinder can be compartd to a coil spring, in that the more it is compressed the more work it is potentially capable of doing. The compression ratio of an engine (srr figure 1-27) is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke. This comparison is expressed as a ratio. hence the term "compresion ratio." Compression ratio is a controlling factor in the maximum horsepower developed by an engine. but it is limited by present-day fuel grades and the high engine speeds and manifold pressures required for takeoff. For example, if there arc 140 cu. in. of spacr in the cylinder when thr piston is at the bottom and there are 20 cu. in. of space wlwn the piston is at the top of the stroke, the compression ratio would be 140 to 20. If this ratio is elpressed in fraction form, it would be 140/20, or 7 to 1, usually represented as 7 :l. To grasp mow thoroughly the limitation placed on compression ratios, manifold pressure and its effect on compression pressurrs should be understood. Manifold pressure is the alerage absolute pressure of the air or fuel/air charge in the intake manifold and is measured in units of inches of mercury (Hg) . Manifold pressure is drprndent on engine speed (throttle setting) and supc~cllarging. The engine-driven internal supercharger (bIower) and the externaI exhaust-driven suprrcharger (turbo) are actually centrifugal-type air compressors. Thr operation of these superchargers increases the weight of the charge entering the cylinder. When either one or both are used with the aircraft cngine. the manifold pressure may be considerably higher than the pressure of the outsidc atmosphere. The advantage of this condition is that a greater amount of charge, is forced into a gihen cylindel \olumr. and a greater power results. ### Page 40 Clearance Lu FIGURE1-27. Compression ratio and manifold pressure determine the pressure in the cylinder in that portion of the operating cycle when both valves are closed. The pressure of the charge before compression is determined by manifold pressure, while the pressure at the height of compression (just prior to ignition) is determined by manifold pressure times the compression ratio. For example, if an engine were operating at a manifold pressure of 30" Hg with a compression ratio of 7:1, the pressure at the instant before ignition would be approximately 210" Hg. However, at a manifold pressure of 60" Hg the pressure would be 420" Hg. Without going into great detail, it has been shown that the compression event magnifies the effect of varying the manifold pressure, and the magnitude of both affects the pressure of the fuel charge just before the instant of ipition. If the pressure at this time becomes too high, premature ignition or knock will occur and produce overheating. One of the reasons for using engines with high compression ratios is to obtain long-range fuel Compression ratio. economy, that is, to convert more heat energy into useful work than is done in engines of low compression ratio. Since more heat of the charge is converted into useful work, less heat is absorbed by the cylinder walls. This factor promotes cooler engine operation, which in turn increases the thermal efficiency. Here, again, a compromise is needed between the demand for fuel economy and the demand for maximum horsepower without knocking. Some manufacturers of high-compression engines suppress knock at high manifold pressures by injecting an antiknock fluid into the fuel/air mixture. The fluid acts primarily as a coolant so that more power can be delivered by the engine for short periods, such as at takeoff and during emergencies, when power is critical. This high power should be used for short periods only. indicated Horsepower The indicated horsepower produced by an engine is the horsepower calculated from the indicated mean effective pressure and the other factors which ### Page 41 affect the power output of an engine. Indicated L = Length of the stroke in ft. or in fractions of horsepower is the power developed in the combus- a foot. tion chambers without reference to friction losses A = Area of the piston head or cross-sectional within the engine. area of the cylinder, in sq. in. This horsepower is calculated as a function of the r.p.m.N = Number of power strokes per minute; -.actual cylinder pressure recorded during engine 2 operation. To facilitate the indicated horsepower K = Number of cylinders. In the formula above, the area of the piston timescalculations, a mechanical indicating device, attached to the engine cylinder, scribes the actual the indicated mean effective pressure gives the force acting on the piston in pounds. This force multi-pressure existing in the cylinder during the complete operating cycle. This pressure variation can be plied by the length of the stroke in feet gives the represented by the kind of graph shown in figure work performed in one power stroke, which, multi- 1-28. Notice that the cylinder pressure rises on plied by the number of power strokes per minute, the compression stroke, reaches a peak after top gives the number of ft.-lb. per minute of work center, then decreases as the piston moves down on produced by one cylinder. Multiplying this result the power stroke. Since the cylinder pressure varies by the number of cylinders in the engine gives the during the operating cycle, an average pressure, amount of work performed, in ft.-lb., by the engine. line AB, is computed. This average pressure, if Since hp. is defined as work done at the rate of applied steadily during the time of the power stroke, 33,000 ft.-lb. per min., the total number of ft.-lb. would do the same amount of work as the varying of work performed by the engine is divided by pressure during the same period. This average 33,000 to find the indicated horsepower. pressure is known as indicated mean effective pres- Example sure and is included in the indicated horsepower Given :calculation with other engine specifications. If the Indicated meancharacteristics and the indicated mean effective preseffective pressure (P) = 165 lbs./sq. in.sure of an engine are known, it is possible to calcu- Stroke (L) = 6 in. or .5 ft.late the indicated horsepower rating. Bore = 5.5 in.The indicated horsepower for a four-stroke-cycle r.p.m. = 3,000engine can be calculated from the following formula, No. of cylinders (K) = 12in which the letter symbols in the numerator are arranged to spell the word "plank" to assist in PLANKIndicated hp. = memorizing the formula: 33,000 ft.-lbs./min. Find indicated hp. :PLANKIndicated horsepower = A is found by using the equation33.003 Where : A = 1/&D2 P = Indicated mean effective pressure in p.s.i. A = 1/4 x 3.1416 x 5.5 x 5.5 + CompressionCompression +I-- PowerPower ExhaustExhaust +- IntakeIntake ++ - r + - U C.- Indicated mean effective pressure .-C (IMEP) e 2 2 t4 E II Bottom center Top center BC Piston travel FIGURE Cylinder pressure during power cycle.1-28. ### Page 42 =23.76 sq. in. N is found by multiplying the r.p.m. by 1/2: N =1/2 x 3,000 =1,500 r.p.m. Now, substituting in the formula: 165 x .5 x 23.76 x 1,500 x 12 Indicated hp. = 33,000 ft.-lbs./min. Brake Horsepower The indicated horsepower calculation discussed in the preceding paragraph is the theoretical power of a frictionless engine. The total horsepower lost - in overcoming friction must be subtracted from the indicated horsepower to arrive at the actual horsepower delivered to the propeller. The power delivered to the propeller for useful work is known as b.hp. (brake horsepower). The difference between indicated and brake horsepower is known as friction horsepower, which is the horsepower required to overcome mechanical losses such as the pumping action of the pistons and the friction of the pistons and the friction of all moving parts. In practice, the measurement of an engine's b.hp. involves the measurement of a quantity known as 'torque, or twisting moment. Torque is the product of a force and the distance of the force from the axis about which it acts, or Collar Torque =Force x Distance (at right angles to the force). Torque is a measure of load and is properly expressed in pound-inches (1b.-in.) or pound-feet (1b.-ft.) and should not be confused with work, which is expressed in inch-pounds (in.-lbs.) or footpounds (ft.-lbs.) . There are a number of devices for measuring torque, of which the., Prony brake, dynamometer, and torquemeter are .examples. Typical of these devices is the Prony brake (figure 1-29), which measures the usable power output of an engine on a test stand. It consists essentially of a hinged collar, or brake, which can be clamped to a drum splined to the propeller shaft. The collar and drum form a friction brake which can be adjusted by a wheel. An arm o,f a known length is rigidly attached to or is a part of the hinged collar and terminates at a point which bears on a set of scales. As the propeller shaft rotates, it tends to carry the hinged collar of the brake with it and is prevented from doing so only by the arm that bears on the scale. The scale reads the force necessary to arrest the motion of the arm. If the resulting force registered on the scale is multiplied by the length of the arm, the resulting product is the torque exerted by the rotating shaft. Example: If the scale registers 200 lbs. and the length of the arm is Length of arm 3.18 feet FIGURE1-29. Typical Prony brake. ### Page 43 3.18 ft., the torque exerted by the shaft is: 200 Ib. x 3.18 ft. =636 1b.-ft. Once the torque is known, the work done per revolution of the propeller shaft can be computed without difficulty by the equation: Work per revolution =2x x torque. If work per revolution is multiplied by the r.p.m., the result is work per minute, or power. If the work is expressed in ft.-lbs. per min., this quantity is divided by 33,000; the result is the brake horse- Dower of the shaft. In other words: Power = Work per revolution x r.p.m. Work per revolution x r.p.m. and b.hp. = 33,000 9 2~ x force on the scales (Ibs.) x length of arm (ft.) x r.p.m. or b.hp. = 33,000 Example Given : Force on scales =200 lbs. Length of arm =3.18 ft. r.p.m. =3,000 7; =3.1416. Find b.hp. : Substituting in equation- As long as the friction between the brake collar and propeller shaft drum is great enough to impose an appreciable load on the engine, but is not great enough to stop the engine, it is not necessary to know the amount of friction between the collar and drum to compute the b.hp. If there were no load imposed, there would be no torque to measure, and the engine would "run away." If the imposed load is so great that the engine stalls, there may be considerable torque to measure, but there will be no r.p.m. In either case it is impossible to measure the b.hp. of the engine. However, if a reasonable amount of friction exists between the brake drum and the collar and the load is then increased, the tendency of the propeller shaft to carry the collar and arm about with it becomes greater, thus imposing a greater force upon the scales. As long as the torque increase is proportional to the r.p.m. decrease, the horsepower delivered at the shaft remains unchanged. This can be seen from the equation in which 27; and 33,000 are constants and torque and r.p.m. are variables. If the change in r.p.m. is inversely proportional to the change in torque, their product will remain unchanged. Therefore, b.hp. remains unchanged. This is important. It shows that horsepower is the function of both torque and r.p.m., and can be changed by changing either torque or r.p.m., or both. Friction Horsepower Friction horsepower is the indicated horsepower minus brake horsepower. It is the horsepower used by an engine in overcoming the friction of moving parts, drawing in fuel, expelling exhaust, driving oil and fuel pumps, and the like. On modern aircraft engines, this power loss through friction may be as high as 10 to 15% of the indicated horsepower. Friction and Brake Mean Effective Pressures The IMEP (indicated mean effective pressure), discussed previously, is the averase pressure produced in the combustion chamber during the operating cycle and is an expression of the theoretical, frictionless power known as indicated horsepower. In addition to completely disregarding power lost to friction, indicated horsepower gives no indication as to how much actual power is, delivered to the propeller shaft for doing useful work. However, it is related to actual pressures which occur in the cylinder and can be used as a measure of these pressures. To compute the friction loss and net power output, the indicated horsepower of a cylinder may be thought of as two separate powers, each producing a different effect. The first power overcomes internal friction, and the horsepower thus consumed is known as friction horsepower. The second power, known as brake horsepower, produces useful work at the propeller. Logically, therefore, that portion of IMEP that produces brake horsepower is called BMEP (brake mean effective pressure). The remaining pressure used to overcome internal friction is called FMEP (friction mean effective ~ressure) . This is illustrated in figure 1-30. IMEP is a useful expression of total cylinder power output, but is not a real physical quantity; likewise, FMEP and BMEP are theoretical but useful expressions of friction losses and net power output. Although BMEP and FMEP have no real existence in the cylinder, they provide a convenient means of representing pressure limits, or rating engine performance throughout its entire operating range. This is true since there is a relationship between IMEP, BMEP, and FMEP. ### Page 44 One of the basic limitations placed on engine operation is the pressure developed in the cylinder during combustion. In the discussion of compression ratios and indicated mean effective pressure, it was found that, within limits, the increased pressure resulted in increased power. It was also noted that if th cylinder pressure was not controlled Power stroke + FIGURE Powers and pressures. 1-30. within close limits, it would impose dangerous internal loads that might result in engine failure. It is therefore important to have a means of determining these cylinder pressures as a protective measure and for efficient application of power. If the b.hp. is known, the BMEP can be computed by means of the following equation: b.hp. x 33,000BMEP = LANK Example Given : b.hp. =1,000 Stroke =6 in. Bore =5.5 in. r.p.m. =3,000 No. of cyls. =12. Find BMEP: Find length of stroke (in ft.) : L =0.5. Find area of cylinder bore: A = 1/4.?rD2 =0.7854 x 5.5 x 5.5 =23.76 sq. in. Find No. of power strokes per min.: N =1/2 x r.p.m. =1/2 x 3,000 =1,500. Then substituting in the equation: i,ooo 33,000BMEP = .5 x 23.76 x 1,500 x 12 =154.32 Ibs. per sq. in. Thrust Horsepower Thrust horsepower can be considered as the result of the engine and the propeller working together. If a propeller could be designed to be 100% efficient, the thrust- and the brake-horsepower would be the same. However, the efficiency of the propeller varies with the engine speed, attitude, altitude, temperature, and airspeed, thus the ratio of the thrust horsepower and the brake horsepower delivered to the propeller shaft will never be equal. For example, if an engine develops 1,000 b.hp., and it is used with a propeller having 85 percent efficiency, the thrust horsepower of that engine-propeller combination is 85 percent of 1,000 or 850 thrust hp. Of the four types of horsepower discussed, it is the thrust horsepower that determines the performance of the engine-propeller combination. EFFICIENCIES Thermal Efficiency Any study of engines and power involves consideration of heat as the source of power. The heat produced by the burning of gasoline in the cylinders causes a rapid expansion of the gases in the cylinder, and this, in turn, moves the pistons and creates mechanical energy. It has long been known that mechanical work can be converted into heat and that a given amount of heat contains the energy equivalent of a certain amount of mechanical work. Heat and work are ### Page 45 - - theoretically interchangeable and bear a fixed relation to each other. Heat can therefore be measured in work units (for example, ft.-lbs.) as well as in heat units. The B.t.u. (British thermal unit) of heat is the quantity of heat required to raise the temperature of 1lb. of water loF. It is equivalent to 778 ft.-lbs. of mechanical work. A pound of petroleum fuel, when burned with enough air to consume it completely, gives up about 20,000 B.t.u., the equivalent of 15,560,000 ft.-lbs. of mechanical work. These quantities express the heat energy of the fuel in heat and work units, respectively. The ratio of useful work done by an engine to the heat energy of the fuel it uses, expressed in work or heat units, is called the thermal efficiency of the engine. If two similar engines use equal amounts of fuel, obviously the engine which converts into work the greater part of the energy in the fuel (higher thermal efficiency) will deliver the greater amount of power. Furthermore, the engine which has the higher thermal efficiency will have less waste heat to dispose of to the valves, cylinders, pistons, and cooling system of the engine. A high thermal efficiency also means a low specific fuel consumption and. therefore. less fuel for a flight of a given distance at a given power. Thus. the practical importance of a high thermal eficiency is threefold, and it constitutes one of the most desirable features in the performance of an aircraft en,' rime. Of the total heat produced, 25 to 30% is utilized for power output; 15 to 20% is lost in cooling (heat radiated from cylinder head fins) ; 5 to 10% is lost in overcoming friction of moving parts; and 40 to 45% is lost through the exhaust. Anjthing which increases the heat content that goes into mechanical work on the piston, which reduces the friction and pumping losses, or which reduces the quantity of unburned fuel or the heat lost to the engine parts, increases the thermal efficiency. The portion of the total heat of combustion which is turned into mechanical work depends to a great extent upon the compression ratio. Compression ratio is the ratio of the piston displacement plus combustion chamber space to the combustion chamber space. Other things being equal, the higher the compression ratio, the larger is the proportion of the heat energy of combustion turned into useful work at the crankshaft. On the other hand. increasing the compression ratio increases the cylinder head temperature. This is a limiting factor, for the extremely high temperature created by high compression ratios causes the material in the cylinder to deterioriate rapidly and the fuel to detonate. The thermal efficiency of an engine may be based on either b.hp. or i.hp. and-is represented by the formula: Indicated thermal efficiency = i.hp. x 33,000 weight of fuel burned/min. x heat value x 778' The formula for brake thermal efficiency is the same as shown above, except the value for b.hp. is inserted instead of the value for i.hp. Example An engine delivers 85 b.hp. for a period of 1hr. and during that time consumes 50 Ibs. of fuel. Assuming the fuel has a heat content of 18,800 B.t.u. per Ib., find the thermal efficiency of the en,' mine : Brake thermal efficiency = 0.23 or 23%. Reciprocating engines are only about 34% thermally efficient; that is. they transform only about 34% of the total heat produced by the burning fuel into mechanical energy. The remainder of the heat is lost through the exhaust gases, the cooling system, and the friction within the engine. Thermal distribution in a reciprocating engine is illustrated in figure 1-31. Mechanical EfFiciency Mechanical efficiency is the ratio that shows how much of the power developed by the expanding gases in the cylinder is actually delivered to the Mechanical efficiency = -. output shaft. It is a comparison between the b.hp. and the i.hp. It can be expressed by the formula: b.hp. i.hp. Brake horsepower is the useful power delivered to the propeller shaft. Indicated horsepower is the total hp. develo@d in the cylinders. The difference between the two is f.hp. (friction horsepower), the power lost in overcoming friction. The factor that has the greatest effect on mechanical efficiency is the friction within the engine itself. The friction between moving parts in an engine remains practically constant throughout an engine's speed range. Therefore, the mechanical efficiency of an engine will be highest when the engine is running at the r.p.m. at which maximum b.hp. is developed. Mechanical efficiency of the average aircraft reciprocating engine approaches 90%. ### Page 46 Heat released by combustion. 40-458 is camed 15-2016 is removed out with exhaust. into useful power. FIGURE1-31. Thermal distribution in an engine. Volumetric Efficiency Volumetric efficiency, another engine efficiency, is a ratio expressed in terms of percentages. It is a comparison of the volume of fuel/air charge (corrected for temperature and pressure) inducted into the cylinders to the total piston displacement of the engine. Various factors cause departure from a 100% volumetric efficiency. The pistons of an unsupercharged engine displace the same volume each time they sweep the cylinders from top center to bottom center. The amount of charge that fills this volume on the intake stroke depends on the existing pressure and temperature of the surrounding atmosphere. Therefore, to find the volumetric efficiency of an engine, standards - for atmospheric pressure and temperature had to be established. The U.S. standard atmosphere was established in 1958 and provides the necessary pressure and temperature values to calculate volumetric efficiency. The standard sea-level temperature is 59' F. or 15' C. At this temperature the pressure of one atmosphere is 14.69 lbs./sq. in., and this pressure will support a column of mercury 29.92 in. high. These standard sea-level conditions determine a standard density, and if the engine- draws in a volume of charge of this density exactly equal to its piston displacement, it is said to be operating at 100% volunietric efficiency. An engine drawing in less volume than this has a volumetric efficiency lower than 100%. An engine equipped with a high-speed internal or external blower may have a volumetric efficiency greater than 100%. The equation for volumetric efficiency is: Volume of charge (corrected for Volumetric - temperature and pressure) efficiency - Piston displacement Many factors decrease volumetric efficiency; some the- qe are: (1) Part-throttle operation. (2) Long intake pipes of small diameter. (3) Sharp bends in the induction system. (4) Carburetor air temperature too high. (5) Cylinder-head temperature too high. (6) Incomplete scavenging. (7) Improper valve timing. Propulsive EfFiciency A propeller is used with an engine to provide thrust. The engine supplies b.hp. through a rotating shaft, and the propeller absorbs the b.hp. and converts it into thrust hp. In this conversion, some power is wasted. Since the efficiency of any machine is the ratio of useful power output to the power input, propulsive efficiency (in this case, propeller efficiency) is the ratio of thrust hp. to b.hp. On the average, thrust hp. constitutes approximately 80% of the b.hp. The other 20% is lost in friction and slippage. Controlling the blade angle of the propeller is the best method of obtaining maximum propulsive efficiency for all conditions encountered in flight. During takeoff, when the aircraft is moving at low speeds and when maximum power and thrust are required, a low propeller blade angle will give maximum thrust. For high-speed flying or diving, the blade angle is increased to obtain maximum thrust and efficiency. The constant-speed propeller is used to give required thrust at maximum efficiency for all flight conditions. TURBINE ENGINE CONSTRUCTION In a reciprocating engine the functions of intake, compression, combustion, and exhaust all take place in the same combustion chamber; consequently, each must have exclusive occupancy of the chamber during its respective part of the combustion cycle. A significant feature of the gas turbine engine, however, is that a separate section is devoted to each ### Page 47 functicn, and all functions are performed simultaneously without interruption. A typical gas turbine engine consists of: An air inlet. Compressor section. Combustion section. Turbine section. Exhaust section. Accessory section. The systems necessary for starting, lubrication, fuel supply, and auxiliary purposes, such as anti-icing, cooling, and pressurization. The major components of all turbine engines are basically the same; however, the nomenclature of the component parts of various engines currently in use will vary slightly due to the difference in each manufacturer's terminology. These differences are reflected in the applicable maintenance manuals. The greatest single factor influencing the construction features of any gas turbine engine is the type compressor (axial flow or centrifugal flow) for which the engine is designed. Later in the chapter a detailed description of compressors is given, but for the time being examine figures 1-32 and 1-33. Notice the physical effect the two types of compressors have on engine construction features. It is obvious that there is a difference in their length and diameter. Note that in the axial-flow engine the air inlet -Engine mount fitting St;linlcss stet.1 fire seal Accessory case Air inlet FIGURE1-32. Axial-flow engine. Accessory Combustion case Exhaust duct FIGURE1-33. Centrifugal-flow engine. ### Page 48 duct is one of the major engine components; on the other hand, in the centrifugal-flow engine, air enters the air inlet and is directed to the compressor inducer vanes through circumferential inlets located in front and back of the impeller. The inlets are screened to prevent entry of foreign objects that could cause serious damage to the metal components if allowed to enter the compressor. The accessories of the two types of en,' mines are located at different points on the engines. This is necessary because of engine construction. The front of the axial-flow engine is utilized for air entrance; consequently, the accessories must be located elsewhere. Other than the features previously mentioned, there is little visual dissimilarity between the remaining major components of the two engines. AIR ENTRANCE The air entrance is designed to conduct incoming air to the compressor with a minimum energy loss resulting from drag or ram pressure loss; that is, the flow of air into the compressor should be free of turbulence to achieve maximum operating efficiency. Proper design contributes materially to aircraft performance by increasing the ratio of compressor discharge pressure to duct inlet pressure. The amount of air passing through the engine is dependent upon three factors: (1) The compressor speed (r.p.m.) . (2) The forward speed of the aircraft. (3) The density of the ambient (surrounding) air. Inlets may be classified as: Nose inlets, located in the nose of the fuselage, or powerplant pod or nacelle. Wing inlets, located along the leading edge of the wing, usually at the root for singleengine installations. Annular inlets, encircling, in whole or in part, the fuselage or powerplant pod or nacelle. Scoop inlets, which project beyond the immediate surface of the fuselage or nacelle. Flush inlets, which are recessed in the side of the fuselage, powerplant pod, or nacelle. There are two basic types of air entrances in use: the single entrance and the divided entrance. Generally, it is advantageous to use a single entrance with an axial-flow engine to obtain maximum ram pressure through straight flow. It is used almost exclusively on wing or external installations where the unobstructed entrance lends itself readily to a single, short, straight duct. A divided entrance offers greater opportunity to diffuse the incoming air and enter the plenum chamber with the low velocity required to utilize efficiently a double-entry compressor. (The plenum chamber is a storage place for ram air, usually asso- FIGURE1-34. Accessory location on a centrifugal-flow engine. ### Page 49 ciated with fuselage installations.) It is also advantageous when the equipment installation or pilot location makes the use of a single or straight duct impractical. In most cases the divided entrance permits the use of very short ducts with a resultant small pressure drop through skin friction. ACCESSORY SECTION The accessory section of the turbojet engine has various functions. The primary function is to provide space for the mounting of accessories necessary for operation and control of the engine. Generally, it also includes accessories concerned with the aircraft, such as electric generators and fluid powerpumps. Secondary functions include acting as an oil reservoir and/or oil sump, and housing the accessory drive gears and reduction gears. The arrangement and driving of accessories have always been major problems on gas turbine engines. Driven accessories are usually mounted on common pads either ahead of or adjacent to the compressor section, depending on whether the engine is centrifugal flow or axial flow. Figures 1-34 and 1-35 illustrate the accessory arrangement of a centrifugalflow engine and an axial-flow engine, respectively. The components of the accessory section of all centrifugal- and axial-flow engines have essentially the same purpose even though they often differ quite extensively in construction details and nomenclature. The basic elements of the centrifugal-flow engine accessory section are (1) the accessory case, which has machined mounting pads for the engine-driven accessories. and (2) the gear train, which is housed within the accessory case. The accessory case may be designed to act as an oil reservoir. If an oil tank is utilized, a sump is usually provided below the front bearing support for the drainage and scavenging of oil used to lubricate bearings and drive gears. The accessory case is also provided with adequate tubing or cored passages for spraying lubricating oil on the gear train and supporting bearings. The gear train is driven by the engine rotor through an accessory drive shaft gear coupling, which splines with a shaft gear and the rotor assembly compressor hub. The reduction gearing within the case provides suitable drive speeds for each engine accessory or component. Because the rotor ACCE \FLOW DIVIDER AND DRAIN VALVE FIGURE1-35. Accessory arrangement on an axial-flow engine. ### Page 50 operating r.p.m. is so high, the accessory reduction gear ratios are relatively high. The accessory drives are supported by ball bearings assembled in the mounting pad bores of the accessory case. The components of an axial-flow engine accessory section are an accessory gearbox and a power takeoff assembly, housing the necessary drive shafts and reduction gears. Figure 1-36 shows the location of the accessory gearbox. Turbine and afterburner Compressor section 1 diffuser section -IAfterburner section - Accessory gearbox ( B ) FIGURE1-36. (A) Accessory gearbox mounted beneath the compressor; (B) Accessory gearbox mounted beneath the front bearing support. Although the close relationship of the accessory gearbox and the power takeoff necessitates their being located near each other, two factors affect the location of gearboxes. They are engine diameter and engine installation. Designers are forever striving to reduce engine diameter to make the engine more streamlined, thereby increasing aircraft performance by reducing drag. Also, engine installation in a particular aircraft may dictate the location or re-arrangement of the accessory gearboxes. The accessory gearbox has basically the same functions as the accessory case of the centrifugalflow engine. It has the usual machined mounting pads for the engine accessories, and it houses and supports the accessory drive gear trains. Also included are adequate tubing and cored passages for lubricating the gear trains and their supporting bearings. The accessories usually provided on en,' wines are the fuel control with its governing device; the highpressure fuel pump (s) ; oil pressure pump and scavenge pump (s) ; auxiliary fuel pump and sometimes a starting fuel pump; and several engine accessories including starter, generator, and tachometer. Although these accessories are for the most part essential, the particular combination of engine-driven accessories depends upon the use for which the engine is designed. The accessories mentioned above (except starters) are the engine-driven type. Also associated with the engine systems are the nondriven accessories, such as ignition exciters, fuel or oil filters, barometric units, drip valves, compressor bleed valves, and relief valves. COMPRESSOR SECTION The compressor section of the turbojet engine has many functions. Its primary function is to supply air in sufficient quantity to satisfy the requirements of the combustion burners. Specifically, to fulfill its purpose, the compressor must increase the pressure of the mass of air received from the air inlet duct and then discharge it to the burners in the quantity and at the pressures required. A secondary function of the compressor is to supply bleed-air for various purposes in the engine and aircraft. The bleed-air is taken from any of the various pressure stages of the compressor. The exact location of the bleed ports is, of course, dependent on the pressure or temperature required for a particular job. The ports are small openings in the compressor case adjacent to the particular stage from which the air is to be bled; thus, varying degrees of pressure or heat are available simply by tapping into the appropriate stage. Air is often bled from the final or highest pressure stage, since at this point, pressure and air temperature are at a maximum. At times it may be necessary to cool this high-pressure air. If it is used for cabin pressurization or other purposes where excess heat would be uncomfortable or detrimental, the air is sent through a refrigeration unit. Bleed air is utilized in a wide variety of ways, including driving the previously mentioned remotedriven accessories. Some of the current applications of bleed air are: (1) Cabin pressurization, heating, and cooling. (2) Deicing and anti-icing equipment. (3) Pneumatic starting of engines. (4) Auxiliary drive units (ADU) . ### Page 51 (5) Control-booster servosystems. (6) Power for running instruments. The compressor section's location depends on the type of compressor. Figures 1-32 and 1-33 have already illustrated how the arrangement of engine components varies with compressor type. In the centrifugal-flow engine, the compressor is located between the accessory section and the combustion section; in the axial-flow engine the compressor is located between the air inlet duct and the combustion section. Compressor Types The two principal types of compressors currently being used in turbojet aircraft engines are centrifugal flow and axial flow. The compressor type is a means of engine classification. Much use has been made of the terms "centrifugal flow" and "axial flow" to describe the engine and compressor. However, the terms are applicable to the flow of air through the compressor. In the centrifugal-flow engine, the compressor achieves its purpose by picking up the entering air and accelerating it outwardly by centrifugal action. In the axial-flow engine, the air is compressed while continuing in its original direction of flow, thus avoiding the energy loss caused by turns. From inlet to exit the air flows along an axial path and is compressed at a ratio of approximately 1.25:l per stage. The components of each of these two types of compressors have their individual functions in the compression of air for the combustion section. Centrifugal-flow Compressors The centrifugal-flow compressor consists basically of an impeller (rotor), a diffuser (stator), and a compressor manifold, illustrated in figure 1-37. The two main functional elements are the impeller and the diffuser. Although the diffuser is a separate unit and is placed inside and bolted to the manifold; the entire assembly (diffuser and manifold) is often referred to as the diffuser. For clarification during compressor familiarization, the units are treated individually. The impeller is usually made from forged aluminum alloy, heat-treated, machined, and smoothed for minimum flow restriction and turbulence. In some types the impeller is fabricated from a single forging. This type impeller is shown in figure 1-37 (A). In other types the curved inducer vanes are separate pieces as illustrated in figure 1-38. The impeller, whose function is to pick up and accelerate the air outwardly to the diffuser, may be either of two types-single entry or double entry. Both are similar in construction to the reciprocating engine supercharger impeller, the double-entry type being similar to two impellers back to back. However, because of the much greater combustion air requirements in turbojet engines, the impellers are larger than supercharger impellers. The principal differences between the two types of impellers are the size and the ducting arrangement. The double-entry type has a smaller diameter, but is usually operated at a higher rotational speed to assure sufficient airflow. The single-entry impeller permits convenient ducting directly to the impeller eye (inducer vanes) as opposed to the more complicated ducting necessary to reach the rear side of the double-entry type. Although slightly more efficient in receiving air, the single-entry impeqer must be large in diameter to deliver the same quantity of air as the double-entry type. This, of course, increases the overall diameter of the engine. Included in the ducting for double-entry compressor engines is the plenum chamber. This chamber is necessary for a double-entry compressor because the air must enter the engine at almost right angles to the engine axis. Therefore, the air must, in order to give a positive flow, surround the engine compressor at a positive pressure before entering the compressor. Included in some installations, as a necessary part of the plenum chamber, are the auxiliary airintake doors (blow-in doors). These blow-in doors admit air to the engine compartment during ground operation, when air requirements for the engine are in excess of the airflow through the inlet ducts. The doors are held closed by spring action when the engine is not operating. During operation, however, the doors open automatically whenever engine compartment pressure drops below atmospheric pressure. During takeoff and flight, ram air pressure in the engine compartment aids the springs in holding the doors closed. The diffuser is an annular chamber provided with a number of vanes forming a series of divergent passages into the manifold. The diffuser vanes direct the flow of air from the impeller to the manifold at an angle designed to retain the maximum amount of energy imparted by the impeller. They also deliver the air to the manifold at a velocity and pressure satisfactory for use in the combustion ### Page 52 Impeller Compressor manifold FIGURE1-37. (A) Components of a centrifugal compressor; (B) Air outlet-elbow with turning vanes for reducing air pressure losses. chambers. Refer to figure 1-37 (A) and notice the bustion chambers. The manifold will have one arrow indicating the path of airflow through the outlet port for each chamber so that the air is evenly diffuser, then through the manifold. divided. A compressor outlet elbow is bolted to each The compressor manifold shown in figure 1-37 of the outlet ports. These air outlets are constructed (A) diverts the flow of air from the diffuser, which in the form of ducts and are known by a variety of is an integral part of the manifold, into the com- names, such as air outlet ducts, outlet elbows, or ### Page 53 Compressor rear inducer rn FIGURE1-38. Double entry impeller with inducer vanes as separate pieces. combustion chamber inlet ducts. Regardless of the terminology used, these outlet ducts perform a very important part of the diffusion process; that is, they change the radial direction of the airflow to an axial direction, where the diffusion process is completed after the turn. To help the elbows perform this function in an efficient manner, turning vanes (cascade vanes) are sometimes fitted inside the elbows. These vanes reduce air pressure losses by presenting a smooth, turning surface. (See figure 1-37 (B) .) Axial-flow Compressor The axial-flow compressor has two main elements, a rotor and a stator. The rotor has blades fixed on a spindle. These blades impel air rearward in the same manner as a propeller because of their angle and airfoil contour. The rotor, turning at high speed, takes in air at the compressor inlet and impels it through a series of stages. The action of the rotor increases the compression of the air at each stage and accelerates it rearward through several stages. With this increased velocity, energy is transferred from the.compressor to the air in the form of velocity energy. The stator blades act as diffusers at each stage, partially converting high velocity to pressure. Each consecutive pair of rotor and stator blades constitutes a pressure stage. The number of rows of blades (stages) is determined by the amount of air and total pressure rise required. The greater the number of stages, the higher the compression ratio. Most present-day engines utilize from 10 to 16 stages. The stator has rows of blades, or vanes, dovetailed into split rings, which are in turn attached inside an enclosing case. The stator vanes project radially toward the rotor axis and fit closely on either side of each stage of the rotor. ### Page 54 The compressor case, into which the stator vanes are fitted, is horizontally divided into halves. Either the upper or lower half may be removed for inspection or maintenance of rotor and stator blades. The function of the vanes is twofold. They axe designed to receive air from the air inlet duct or from each preceding stage of the compressor and deliver it to the next stage or to the burners at a workable velocity and pressure. They also control the direction of air to each rotor stage to obtain the maximum possible compressor blade efficiency. Shown in figure 1-39 are the rotor and stator elements of a typical axial-flow compressor. The rotor blades are usually preceded by an inlet guide vane assembly. The guide vanes direct the airflow into the first stage rotor blades at the Front compressor Rear compressoi stator casing stator casing Compresso~Compressor rear frmnc~ rotor I I Rear Front compressor compre FIGURE149. Airflow through a can-annular chamber. The function of the turbine nozzles is twofold. First, after the combustion chamber has introduced the heat energy into the mass airflow and delivered - it evenly to the turbine nozzles, it becomes the job of the nozzles to prepare the mass air flow for driving the turbine rotor. The stationary blades or vanes of the turbine nozzles are contoured and set at such an angle that they form a number of small nozzles discharging the gas at extremely high speed; thus, the nozzle converts a varying portion of the heat and pressure energy to velocity energy which FIGURE Stator element of the FIGURE Rotor element of the 1-50. 1-51. turbine assembly. turbine assembly. ### Page 63 can then be converted to mechanical energy through the rotor blades. The second purpose of the turbine nozzle is to deflect the gases to a specific angle in the direction of turbine wheel yotation. Since the gas flow from the nozzle must enter the turbine blade passageway while it is still rotating, it is essential to aim the gas in the general direction of turbine rotation. The turbine nozzle assembly consists of an inner shroud and an outer shroud between which are fixed the nozzle vanes. The number of vanes employed vary with different types and sizes of engines. Figure 1-52 illustrates typical turbine nozzles featuring loose and welded vanes. (A) Tuhine nozzle vane aswmbly with loow Fitting vanes; (B) Turbine nozzle vane assembly with welded vanes. FIGURE1-52. Typical turbine nozzles. The blades or vanes of the turbine nozzle may be assembled between the outer and inner shrouds or rings in a variety of ways. Although the actual elements may vary slightly in their configuration and construction features, there is one characteristic peculiar to all turbine nozzles; that is, the nozzle vanes must be constructed to allow for thermal expansion. Otherwise, there would be severe distortion or warping of the metal components because of rapid temperature changes. The thermal expansion of turbine nozzles is accomplished by one of several methods. One method necessitates the vanes being assembled loosely in the supporting inner and outer shrouds. (See figure 1-52 (A) .) Each vane fits into a contoured slot in the shrouds, which conforms with the airfoil shape of the vane. These slots are slightly larger than the vanes to give a loose fit. For further support the inner and outer shrouds are encased by an inner and an outer support ring, which give increased strength and rigidity. These support rings also facilitate removal of the nozzle vanes as a unit; otherwise, the vanes could fall out as the shrouds were removed. Another method of thermal expansion construction is to fit the vanes into inner and outer shrouds; however, in this method the vanes are welded or riveted into position. (See figure 1-52(B) .) Some means must be provided to allow for thermal expansion; therefore, either the inner or the outer shroud ring is cut into segments. These saw cuts dividing the segments will allow sufficient expansion to prevent stress and warping of the vanes. The rotor element of the turbine section consists essentially of a shaft and a wheel. (See figure 1-51.) The turbine wheel is a dynamically balanced unit consisting of blades attached to a rotating disk. The disk, in turn, is attached to the main powertransmitting. shaft of the engine. The jet gases leaving the turbine nozzle vanes act on the blades of the turbine wheel, causing the assembly to rotate at a very high rate of speed. The high rotational speed imposes severe centrifugal loads on the turbine wheel, and at the same time the elevated temperatures result in a lowering of the strength of the material. Consequently, the engine speed and temperature must be controlled to keep turbine operation within safe limits. The turbine disk is referred to as such when in an unbladed form. When the turbine blades are installed, the disk then becomes the turbine wheel. The disk acts as an anchoring component for the turbine blades. Since the disk is bolted or welded to the shaft, the blades can transmit to the rotor shaft the energy they extract from the exhaust gases. The disk rim is exposed to the hot gases passing through the blades and absorbs considerable heat from these gases. In addition, the rim also absorbs heat from the turbine buckets (blades) by conduction. Hence, disk rim temperatures normally are high and well above the temperatures of the more remote inner portion of the disk. As a result of ### Page 64 these temperature gradients, thermal stresses are added to the rotational stresses. There are various means to relieve, at least partially, the aforementioned stresses. One such means is to bleed cooling air back onto the face of the disk. Another method of relieving the thermal stresses of the disk is incidental to blade installation. A series of grooves or notches, conforming to the blade root design, are broached in the rim of the disk. These grooves attach the turbine blades to the disk, and at the same time space is provided by the notches for thermal expansion of the disk. Sufficient clearance exists between the blade root and the notch to permit movement of the turbine blade when the disk is cold. During engine operation, expansion of the disk decreases the clearance. This causes the bucket root to fit tightly in the disk rim. The turbine shaft, illustrated in figure 1-51, is usually fabricated from alloy steel. It must be capable of absorbing the high torque loads that are exerted when a heavy axial-flow compressor is started. The methods of connecting the shaft to the turbine disk vary. In one method, the shaft is welded to the disk, which has a butt or protrusion provided for the joint. Another method is by bolting. This method requires that the shaft have a hub which matches a machined surface on the disk face. The bolts then are inserted through holes in the shaft hub and anchored in tapped holes in the disk. Of the two methods, the latter is more common. The turbine shaft must have some means for attachment to the compressor rotor hub. This is usually accomplished by a spline cut on the forward end of the shaft. The spline fits into a coupling device between the compressor and turbine shafts. If a coupling is not used, the splined end of the turbine shaft may fit into a splined recess in the compressor rotor hub. This splined coupling arrangement is used almost exclusively with centrifugal compressor engines, while the axial compressor engine may use either of these described methods. There are various ways of attaching turbine blades or buckets, some similar to compressor blade attachment. The most satisfactory method used is the fir-tree design shown in figure 1-53. The blades are retained in their respective grooves by a variety of methods; some of the more common ones are peening, welding, locktabs, and riveting. Figure 1-% shows a typical turbine wheel using rivets for blade retention. FIGURE Turbine blade with fir-tree design 1-53. and lock-tab method of blade retention. FIGURE Riveting method of turbine 1-54 blade retention. The peening method of blade retention is used frequently in various ways. One of the most common applications of peening requires a small notch to be ground in the edge of the blade fir-tree root prior to the blade installation. After the blade is inserted into the disk, the notch is filled by the disk metal, which is "flowed" into it by a small punchmark made in the disk adjacent to the notch. The tool used for this job is similar to a center punch. Another method of blade retention is to construct the root of the blade so that it will contain all the elements necessary for its retention. This method, illustrated in figure 1-55, shows that the blade root has a stop made on one end of the root so that the blade can be inserted and removed in one direction only, while on the opposite end is a tang. This tang is bent to secure the blade in the disk. Turbine blades may be either forged or cast, depending on the composition of the alloys. Most ### Page 65 Tang is peened over to secure --\ FIGURE1-55. Turbine bucket, featuring tang method of blade retention. blades are precision-cast and finish-gound to the desired shape. Most turbines are open at the outer perimeter of the blades; however, a second type called the shrocded turbine is sometimes used. The shrouded turbine blades, in effect, form a band around the outer perimeter of the turbine wheel. This improves efficiency and vibration characteristics, and permits lighter stage weights; on the other hand, it limits turbine speed and requires more blades (see figure 1-56). In turbine rotor construction, it occasionally becomes necessary to utilize turbines of more than one stage. A single turbine wheel often cannot absorb enough power from the exhaust gases to drive the components dependent on the turbine for rotative power, and thus, it is necessary to add additional turbine stages. A turbine stage consists of a row of stationary vanes or nozzles, followed by a row of rotating blades. In some models of turboprop engine, as many as five turbine stages have been utilized successfully. It should be remembered that, regardless of the number of wheels necessary for driving engine components, there is always a turbine nozzle preceding each wheel. As was brought out in the preceding discussion of turbine stages, the occasional use of more than one turbine wheel is warranted in cases of heavy rotational loads. It should also be pointed out that the same loads that necessitate multiple-stage turbines often make it advantageous to incorporate multiple compressor rotors. In the single-stage rotor turbine (figure 1-57), the power is developed by one rotor, and all enginedriven parts are driven by this single wheel. This arrangement is used on engines where the need for low weight and compactness predominates. FIGURE Single-stage rotor turbine. 1-57. In the multiple-rotor turbine the power is developed by two or more rotors. It is possible for each turbine rotor to drive a separate part of the engine. For example, a triple-rotor turbine can be so arranged that the first turbine drives the rear half of the compressor and the accessories, the second turbine drives the front half of the compressor, and the third turbine furnishes power to a propeller. (See figure 1-58.) FIGURE1-56. Shrouded turbine blades. FIGURE1-58. Multiple-rotor turbine. ### Page 66 FIGURE1-59. Dual-rotor turbine for split-spool compressor. The turbine rotor arrangement for a dual-rotor turbine, such as required for a split-spool compressor, is similar to the arrangement in figure 1-58. The difference is that where the third turbine is used for a propeller in figure 1-58, it would be joined with the second turbine to make a two-stage turbine for driving the front compressor. This arrangement is shown in figure 1-59. The remaining element to be discussed concerning turbine familiarization is the turbine casing or housing. The turbine casing encloses the turbine wheel and the nozzle vane assembly, and at the same time gives either direct or indirect support to the stator elements of the turbine section. It always has flanges provided front and rear for bolting the assembly to the combustion chamber housing and the exhaust cone assembly, respectively. A turbine casing is illustrated in figure 1-60. FIGURE Turbine casing assembly. 1-60. Exhaust Section The exhaust section of the turbojet engine is made up of several components, each of which has its individual functions. Although the components have individual purposes, they also have one common function: They must direct the flow of hot gases rearward in such a manner as to prevent turbulence and at the same time impart a high final or exit velocity to the gases. In performing the various functions, each of the components affects the flow of gases in different ways as described in the following paragraphs. The exhaust section is located directly behind the turbine section and ends when the gases are ejected at the rear in the form of a high-velocity jet. The components of the exhaust section include the exhaust cone, tailpipe (if required), and the exhaust or jet nozzle. Each component is discussed individually. The exhaust cone collects the exhaust gases discharged from the turbine buckets and gradually converts them into a solid jet. In performing this, the velocity of the gases is decreased slightly and the pressure increased. This is due to the diverging passage between the outer duct and the inner cone; that is, the annular area between the two units increases rearward. The exhaust cone assembly consists of an outer shell or duct, an inner cone, three or four radial hollow struts or fins, and the necessary number of tie rods to aid the struts in supporting the inner cone from the outer duct. The outer shell or duct is usually made of stainless steel and is attached to the rear flange of the turbine - case. This element collects the exhaust gases and delivers them either directly or via a tailpipe to the jet nozzle, depending, of course, on whether or not a tailpipe is required. In some engine installations a tailpipe is hot needed. For instance, when the engine is installed in nacelles or pods, a short tailpipe is all that is required, in which case the exhaust duct and exhaust nozzle will suffice. The duct must be constructed to include such features as a predetermined number of thermocouple bosses for installing tailpipe temperature thermocouples, and there must also be the insertion holes for the supporting tie rods. In some cases, tie rods are not used for supporting the inner cone. If such is the case, the hollow struts provide the sole support of the inner cone, the struts being spot-welded in position to the inside surface of the duct and to the inner cone, respectively (see figure 1-61). The radial struts actually have a twofold function. They not onIy support the inner cone in the exhaust ### Page 67 - - - - ~nner c0ne.J struts A FIGURE1-61. Exhaust collector with welded support struts. duct, but they also perform the important function of straightening the swirling exhaust gases that would otherwise leave the turbine at an angle of approximately 45'. The centrally located inner cohe fits rather closely against the rear face of the turbine disk, preventing turbulence of the gases as they leave the turbine wheel. The cone is supported by the radial struts. In some configurations a small hole is located in the exit tip of the cone. This hole allows cooling air to be circulated .from the aft end of the cone, where the pressure of the gases is relatively high, into the interior of the cone and consequently against the face of the turbine wheel. The flow of air is positive, since the air pressure at the turbine wheel is relatively low due to rotation of the wheel; thus air circulation is assured. The gases used for cooling the turbine wheel will return to the main path of flow by passing through the clearance between the turbine disk and the inner cone. The exhaust cone assembly is the terminating component of the basic engine. The remaining components (the tailpipe and jet nozzle) are usually considered airframe components. The tailpipe is used primarily to pipe the exhaust gases out of the airframe. The use of a tailpipe imposes a penalty on the operating efficiency of the engine in the form of heat and duct (friction) losses. These losses materially affect the final velocity of the exhaust gases and, hence, the thrust. The tailpipe terminates in a jet nozzle located just forward of the end of the fuselage. Most installations use a single direct exhaust as opposed - to a dual exit exhaust to obtain the advantages of low-weight, simplicity, and minimum duct losses (see figure 1-62). (A) Single exit tailpipe e FIGURE (A) Single exit tailpipe, 1-62. (B) Divided exit tailpipes. The tailpipe is usually constructed so that it is semiflexible. Again the necessity for this feature is dependent on its length. On extremely long tailpipes, a bellows arrangement is incorporated in its construction, allowing movement both in installation and maintenance, and in thermal expansion. This eliminates stress and warping which would otherwise be present. The heat radiation from the exhaust cone and tailpipe could damage the airframe components surrounding these units. For this reason, some means of insulation had to be devised. There are several suitable methods of protecting the fuselage structure; two of the most common are insulation blankets and shrouds. The insulation blanket, illustrated in figures 1-63 and 1-64, consists of several layers of aluminum foil, each separated by a layer of fiber glass or some other suitable material. Although these blankets protect the fuselage from heat radiation, they are primarily used to reduce heat losses from the exhaust system. The reduction of heat losses improves engine performance. A typical insulation blanket ### Page 68 r Insulation blanket Ram air - Induction air ---L FIGURE1-63. Exhaust system insulation blanket. and the temperatures in the exhaust section are shown in figure 1-64. This blanket contains fiber glass as the low conductance material and aluminum foil as the radiation shield. The blanket should be suitably covered to prevent its becoming soaked with oil. The heat shroud consists of a stainless steel envelope enclosing the exhaust system (see figure 1-65). r--1 The exhaust or jet nozzle imparts to the exh,aust gases the all-important final boost in velocity. The jet nozzle, like the tailpipe, is not included as part of the basic powerplant, but is supplied as a component of the airframe. The nozzle is attached to the rear of the tailpipe, if a tailpipe is required, or to the rear flange of the exhaust duct if a tailpipe is not necessary. There are two types of jet nozzle design. They Cooling air I 120°F. t -Stainless steel shroud -350°F. I 8-Fiberglass I 1 -Aluminum foil Fiberglass--I I Silver foil I I Jet tailpipe 900°F. L,,,,J Exhaust gas 1000°F. FIGURE14. Insulation blanket with the temperatures which would be obtained at the various locations shown. ### Page 69 Flush louvers cooling air inlet Exhaust system shroud - I7 Cooling air elector FIGURE1-65. Exhaust system shroud. are the converging design, for subsonic gas velocities, and the converging-diverging design for supersonic gas velocities. These are discussed in greater detail in Chapter 2; "Induction and Exhaust Systems." The jet nozzle opening may be either fixed-area or variable-area. The fixed-area is the simpler of the two jet nozzles. Since there are no moving parts, any adjustment in nozzle area must be made mechanically. Adjustments in nozzle area are sometimes necessary because the size of the exit orifice will directly affect the operating temperature of the engine. When necessary, a fixed-area nozzle can be adjusted in one of several ways. One method of changing nozzle area is to use inserts, which fit inside the nozzle and are held in place by screws. The inserts are of varying curvatures and sizes. The different size inserts allow a change in nozzle area to be made in varying increments. Thus, through experience a mechanic can run the engine at maximum speed with one combination of inserts, check the temperature, and substitute another combination to make up a temperature deficiency or remedy an excess temperature situation. MAJOR SUBASSEMBLIES The assemblies included in the discussion that follows are integral parts of, or a combination of, the components which comprise the major sections of a turbojet engine. Diffuser The diffuser is the divergent section of the.engine. It has the all-important function of changing highvelocity compressor discharge air to static pressure. This prepares the air for entry into the burner cans at low velocity so that it will burn with a flame that will not blow out. Air Adapters The air adapters of the centrifugal compressor are illustrated in figure 1-37 along with the diffuser. The purpose of the air outlet ducts is to deliver air from the diffuser to the individual can-type combustion chambers. In some instances, fuel nozzles or igniter plugs are also mounted in the air outlet duct. Engine Rotor The engine rotor is a combination of the compressor and turbine rotors on a common shaft. The common shaft is provided by joining the turbine and compressor shafts by a suitable method. The engine rotor is supported by bearings, which are seated in suitable bearing housings. Main Bearings The main bearings have the critical function of supporting the main engine rotor. The number of bearings necessary for proper engine support will, for the most part, be decided by the length and weight of the engine rotor. The length and weight are directly affected by the type of compressor used in the engine. Naturally, a split-spool axial compressor will require more support than a centrifugal compressor. Probably the minimum number of bearings required would be three, while some of the later models of split-spool axial compressor engines require six or more. The gas turbine rotors are usually supported by either ball or roller bearings. Hydrodynamic or slipper-type bearings are receiving some attention for use on turbine powerplants where operating rotor speeds approach 45,000 r.p.m. and where excessive bearing loads during flight are anticipated. (See figure 1-66.) In general, the ball or roller anti- ### Page 70 Ball bearing Slipper I~rnring FIGURE1-66. Type of main bearings used for gas turbine rotor support. friction bearings are preferred largely on the basis that they: Offer little rotational resistance. Facilitate precision alignment of rotating elements. Are relatively inexpensive. Are easily replaced. Withstand high momentary overloads. Are simple to cool, lubricate, and maintain. Accommodate both radial and axial lods. Are relatively resistant to elevated temperatures. The main disadvantages are their vulnerability to foreign matter and tendency to fail without appreciable warning. Usually the ball bearings are positioned on the compressor or turbine shaft so that they can absorb any axial (thrust) loads or radial loads. Because the roller bearings present a larger working surface, they are better equipped to support radial loads than thrust loads. Therefore, they are used primarily for this purpose. A typical ball or roller bearing assembly includes a bearing support housing, which must be strongly constructed and supported in order to carry the radial and axial loads of the rapidly rotating rotor. The bearing housing usually contains oil seals to prevent the oil leaking from its normal path of flow. It also delivers the oil to the bearing for its lubrication, usually through spray nozzles. The oil seals may be the labyrinth or thread (helical) type. These seals also may be pressurized to minimize oil leaking along the compressor shaft. The labyrinth seal is usually pressurized, but the helical seal depends solely on reverse threading to stop oil leakage. These two types of seals are very similar, differing only in thread size and the fact that the labyrinth seal is pressurized. Another type of oil seal used on some of the later engines is the carbon seal. These seals are usually spring loaded and are similar in material and application to the carbon brushes used in electrical motors. Carbon seals rest against a surface provided to create a sealed bearing cavity or void; thus, the oil is prevented from leaking out along the shaft into the compressor airflow or the turbine section. ### Page 71 Figure 1-67 illustrates an oil seal of the springloaded carbon type. The ball or roller bearing is fitted into the bearing housing and may have a self-aligning feature. If a bearing is self-aligning, it is usually seated in a spherical ring, thus allowing the shaft a certain amount of radial movement without transmitting stress to the bearing inner race. Bcaring inner raw FIGURE1-67. Carbon oil seal. The bearing surface is usually provided by a machined journal on the appropriate shaft. The bearing is usually locked in position by a steel snapring, or other suitable locking device. The rotor shaft also provides the matching surface for the oil seals in the bearing housing. These machined surfaces are called lands and fit in rather close to the oil seal, although not actually touching. If rubbing occurs, eventual wear and leakage will result. TURBOPROP ENGINES The turboprop (turbo-propeller) engine is a combination of a gas turbine and a propeller. Turboprops are basically similar to turbojet en,' ~lnes in that both have a compressor, combustion chamber(s), turbine, and a jet nozzle, all of which operate in the same manner on both en,' mines. However, the difference is that the turbine in the turboprop engine usually has more stages than that in the turbojet engine. In addition to operating the compressor and accessories, the turboprop turbine transmits increased power forward, through a shaft and a gear train, to drive the propeller. The increased power is generated by the exhaust gases passing through additional stages of the turbine. Refer to figure 1-58, which shows a multiple-rotor turbine with coaxial shafts for independent driving of the compressor and propeller. Although there are three turbines utilized in this illustration, as many as five turbine stages have been used for driving the two rotor elements, propeller, and accessories. The exhaust gases also contribute to engine power output through jet reaction, although the amount of energy available for jet thrust is considerably reduced. Since the basic components of the turbojet and the turboprop engines differ only slightly in design features, it should be fairly simple to apply acquired knowledge of the turbojet to the turboprop. The typical turboprop engine can be broken down into assemblies as follows: (1) The power section assembly, which contains the usual major components of gas turbine engines (compressor, combustion chamber, turbine, and exhaust sections). (2) The reduction gear or gearbox assembly which contains those sections peculiar to turboprop configurations. (3) The torquemeter assembly, which transmits the torque from the engine to the gearbox of the reduction section. (4) The accessory drive housing assembly. These assemblies are illustrated in figure 1-68. The turboprop engine can be used in many different configurations. It is often used in transport aircraft, but can be adapted for use in single-engine aircraft. TURBOSHAFT ENGINES A gas turbine engine that delivers power through a shaft to operate something other than a propeller is referred to as a turboshaft engine. Turboshaft engines are similar to turboprop engines. The power takeoff may be coupled directly to the engine turbine, or the shaft may be driven by a turbine of its own (free turbine) located in the exhaust stream. The free turbine rotates independently. This principle is used extensively in current production turboshaft engines. The turboshaft engine is currently being used to power helicopters. ### Page 72 1- Yower section assembly - Torquemeter e b 7 Turbine assembly - Reduction gear assembly 4 - - kccessory clrivei housing assembly FIGURE Turboprop engine major components. 1-68. TURBOFAN ENGINES The turbofan gas turbine engine (figure 1-69) is, in principle, the same as a turboprop, except that the propeller is replaced by a duct-enclosed axialflow fan. The fan can be a part of the first-stage compressor blades or can be mounted as a separate set of fan blades. The blades can be mounted forward of the compressor, or aft of the turbine wheel. The general principle of the fan engine is to convert more of the fuel energy into pressure. With more of the energy converted to pressure, a greater product of pressure times area can be achieved. One of the big. advantages is that the turbofan produces this additional thrust without increasing fuel flow. The end result is savings in fuel with the consequent increase in range. Because more of the fuel energy is turned into pressure in the turbofan engine, another stage must be added in the turbine to provide the power to drive the fan, and thus increase the expansion through the turbine. This means that there will be less energy left over and less pressure in back of the turbine. Also, the jet nozzle has to be larger in area. The end result is that the main engine does not develop as much jet nozzle thrust as a straight turbojet en,' -me. The fan more than makes up for the dropoff in thrust of the main engine. Depending on the fan design, it will produce somewhere around 50% of the turbofan engine's total thrust. In an 18,000 Ib. thrust engine about 9,000 lbs. will be developed by the fan and the remaining 9,000 Ibs. by the main engine. The same basic turbojet engine without a fan will develop about 12,000 Ibs. of thrust. FIGURE Forward turbofan engine. 1-69. ### Page 73 FIGURE Forward-fan turbofan engine installation. 1-70. Two different duct designs are used with forwardfan engines. The air leaving the fan can be ducted overboard (figure 1-70), or it can be ducted along the outer case of the basic engine to be discharged through the jet nozzle. The fan air is either mixed with the exhaust gases before it is discharged or it passes directly to the atmosphere without prior mixing. Turbofans, sometimes called fanjets, are becoming the most widely used gas turbine engine. The turbofan is a compromise between the good operating efficiency and high-thrust capability of a turboprop and the high-speed, high-altitude capability of a turbojet. TURBINE ENGINE OPERATING PRINCIPLES The principle used by a turbojet engine as it provides force to move an airplane is based on Newton's law of momentum. his law shows that a force is required to accelerate a mass; therefore, if the engine accelerates a mass of air, it will apply a force on the aircraft. The propeller and turbojet engines are very closely related. The propeller generates thrust by giving a relatively small acceleration to a large quantity of air. The turbojet engine achieves thrust by imparting greater acceleration to a smaller quantity of air. The mass of air is accelerated within the engine by the use of a continuous-flow cycle. Ambient air enters the inlet diffuser where it is subjected to changes in temperature, pressure, and velocity due to ram effect. The compressor then increases pressure and temperature of the air mechanically. The air continues at constant pressure to the burner section where its temperature is increased by combustion of fuel. The energy is taken from the hot gas by expanding through a turbine which drives the compressor, and by expanding through a tailpipe designed to discharge the exhaust gas at high velocity to produce thrust. The high-velocity jet from a turbojet engine may be considered a continuous recoil, imparting force against the aircraft in which it is installed, thereby producing thrust. The formula for thrust can be derived from Newton's second law, which states that force is proportional to the product of mass and acceleration. This law is expressed in the formula: F=MxA where ; F =Force in pounds. M =Mass in slugs. A =Acceleration in ft. per sec. per sec. In the above formula "mass" is similar to "weight," but it is actually a different quantity. Mass refers to the quantity of matter, while weight refers to the pull of gravity on that quantity of matter. At sea level under standard conditions, 1 lb. of mass will have a weight of 1lb. To calculate the acceleration of a given mass, the gravitational constant is used as a unit of comparison. The force of gravity is 32.2 ft./sec.2 (or feet per second squared). This means that a free-falling 1-lb. object will accelerate at the rate of 32.2 feet per second each second that gravity acts on it. Since the object mass weighs 1 lb., which is also the actual force imparted to it by gravity, we can assume that a force of 1lb. will accelerate a 1-lb. object at the rate of 32.2 ft./sec.2. Also, a force of 10 Ibs. will accelerate a mass of 10 Ibs. at the rate of 32.2 ft./sec.2. This is assuming ### Page 74 --- there is no friction or other resistance to overcome. It is now apparent that the ratio of the force (in pounds) is to the mass (in pounds) as the acceleration in ft./sec2 is to 32.2. Using M to represent the mass in pounds, the formula may be expressed thus : F A M-g or, F=- MA g where ; F =Force. M =Mass. A =Acceleration. g =gravity. In any formula involving work, the time factor must be considered. It is convenient to have all time factors in equivalent units; i.e., seconds, minutes, or hours. In calculating jet thrust, the term "pounds of air per second" is convenient, since the time factor is the same as the time in the force of gravity, namely, seconds. THRUST Using the foregoing formula, compute the force necessary to accelerate a mass of 50 lbs., 100 ft./ sec.2, as follows: F = 155 lb. This illustrates that if the velocity of 50 lbs. of mass per sec. is increased by 100 ft./sec.2, the resulting thrust is 155 lbs. Since the turbojet engine accelerates a mass of air, the following formula can be used to determine jet thrust: where; F =Force in lbs. Ms =Mass flow in lbs./sec. =Inlet velocity. v1 v2 =Jet velocity (exhaust). v2 V1 =Change in velocity; difference between inlet velocity and jet velocity. =Acceleration of gravity, or g 32.2 ft./sec2. As an example, to use the formula for changing the velocity of 100 lbs. of mass airflow per sec. from 600 ft./sec. to 800 ft./sec., the formula can be applied as follows: F =621 pounds. As shown by the formula, if the mass airflow per second and the difference in the velocity of the air from the intake to the exhaust is known, it is easy to compute the force necessary to produce the change in the velocity. Therefore, the jet thrust of the engine must be equal to the force required to accelerate the airmass through the engine. Then, by using the symbol "T" for thrust pounds, the formula becomes : It is easy to see from this formula that the thrust of a gas turbine engine can be increased by two methods: first, by increasing the mass flow of air through the engine, and second, by increasing the jet velocity. If the velocity of the turbojet engine remains constant with respect to the aircraft, the jet thrust will decrease if the speed of the aircraft is increased. This is because V1 will increase in value. This does not present a serious problem, however, because as the aircraft speed increases, more air enters the engine, and jet velocity increases. The resultant net thrust is almost constant with increased airspeed. The Brayton cycle is the name given to the thermodynamic cycle of a gas turbine engine to produce thrust. This is a varying volume constantpressure cycle of events and is commonly called the constant-pressure cycle. A more recent term is continuous combustion cycle. The four continuous and constant events are the intake, compression, expansion (includes power), and exhaust. These cycles will be discussed as they apply to a gas turbine engine. In the intake cycle, air enters at ambient pressure and a constant volume. It leaves the intake at an increased pressure and a decrease in volume. At the compressor section, air is received from the intake at an increased pressure, slightly above ambient, and a slight decrease in volume. Air enters ### Page 75 the compressor where it is compressed. It leaves the compressor with a large increase in pressure and decrease in volume. This is caused by the mechanical action of the compressor. The next step, the expansion, takes place in the combustion chamber by burning fuel which expands the air by heat. The pressure remains relatively constant, .but a marked increase in volume takes place. The expanding gases move rearward through the turbine assembly and are converted from velocity energy to mechanical energy by the turbine. The exhaust section, which is a convergent duct, converts the expanding volume and decreasing pressure of the gases to a final high velocity. The force created inside the jet engine to keep this cycle continuous has an equal and opposite reaction (thrust) to move the aircraft forward. Bernoulli's principle (whenever a stream of any fluid has its velocity increased at a given point, the pressure of the stream at that point is less than the rest of the stream) is applied to the jet engine through the design of the air ducts. The two types of ducts are the convergent and the divergent. The convergent duct increases velocity and decreases pressure. The divergent duct decreases velocity and increases pressure. The convergent principle is usually used for the tailpipe and exhaust nozzle. The divergent principle is.used in the compressor where the air is slowing and pressurizing. GAS TURBINE ENGINE PERFORMANCE Thermal efficiency is a prime factor in gas turbine performance. It is the ratio of net work produced by the engine to the chemical. energy supplied in the form of fuel. The three most important factors affecting the thermal efficiency are turbine inlet temperature, compression ratio, and the component efficiencies of the compressor and turbine. Other factors that affect thermal efficiency are compressor inlet temperature and burner efficiency. Figure 1-71 shows the effect that changing compression ratio has on thermal efficiency when compressor inlet temperature and the component efficiencies of the compressor and turbine remain constant. The effect that compressor and turbine component efficiencies have on thermal efficiency when turbine and compressor inlet temperatures remain constant is shown in figure 1-72. In actual operation, the turbine engine tailpipe temperature varies directly with turbine inlet temperature at a constant "1 T - temperature Compression ratio FIGURE The effect of compression ratio on 1-71. thermal efficiency. compression ratio. R.P.M. is a direct measure of compression ratio; therefore, at constant r.p.m. maximum thermal efficiency can be obtained by maintaining the highest possible tailpipe temperature. Since engine life is greatly reduced at a high turbine inlet temperature, the operator should not exceed the tailpipe temperatures specified for continuous operation. Figure 1-73 illustrates the effect of turbine inlet temperature on turbine bucket life. In the previous discussion, it has been assumed that the state of the air at the inlet to the com- Compression ratio FIGURE Turbine and compressor efficiency 1-72. vs. thermal efficiency. ### Page 76 0 100 PO0 300 400 500 Turbine bucket life in hours FIGURE Effect of turbine inlet temperature on 1-73. turbine bucket life. pressor remained constant. Since the turbojet engine is a practical application of a turbine engine, it becomes necessary to analyze the effect of varying inlet conditions on the power ~roduced. The three principal variables that affect inlet conditions are the speed of the aircraft, the altitude of the aircraft, and the ambient temperature. To make the analysis simpler, the combination of these three variables can be represented by a single variable, called "stagnation density." The power produced by a turbine engine is proportional to the stagnation density at the inlet. The next three illustrations show how changing the density by varying altitudes, airspeed, and outside air temperature affects the power level of the engine. Figure 1-74 shows that the thrust output improves rapidly with a reduction in OAT (outside air temperature) at constant altitude, r.p.m., and airspeed. This increase occurs partly because the energy required per pound of airflow to drive the compressor varies directly with the temperature, thus leaving more energy to develop thrust. In addition, the thrust output will increase since the air at reduced temperature has an increased density. The increase in density causes the mass flow through the engine to increase. The altitude effect on thrust, as shown in figure 1-75, can also be discussed as a density and temperature effect. In this case, an increase in altitude causes a decrease in pressure and temperature. Since the temperature lapse rate is less than the Constant altitude, r.p.m., & airspeed DEC STD INC Outside air temperature FIGURE1-74. Effect of OAT on thrust output. pressure lapse rate as altitude is increased, the density is decreased. Although the decreased temperature increases thrust, the effect of decreased density more than offsets the effect of the colder temperature. The net result of increased altitude is a reduction in the thrust output. The effect of airspeed on the thrust of a turbojet engine is shown in figure 1-76. To explain the airspeed effect, it is first necessary to understand the effect of airspeed on the factors which combine to produce net thrust. These factors are specific thrust and engine airflow. Specific thrust is the pounds of net thrust developed per pound of airflow 50,000 Ft Altitude FIGURE1-75. Effect of altitude on thrust output. ### Page 77 FIGURE1-76. Effect of airspeed on net thrust. per second. It is the remainder of specific gross thrust minus specific ram drag. As airspeed is increased, the ram drag increases rapidly. The exhaust jet velocity remains relatively constant; thus the effect of the increase in airspeed results in decreased specific thrust as shown in figure 1-76. In the low-speed range, the specific thrust decreases faster than the airflow increases and causes a decrease in net thrust. As the airspeed increases into the higher range, the airflow increases faster than the specific thrust decreases and causes the net thrust to increase until sonic velocity is reached. The effect of the combination on net thrust is illustrated in figure 1-77. Ram Recovery A rise in pressure above existing outside atmospheric pressure at the engine inlet, as a result of the TAS Lncrease - FIGURE1-77. Effect of airspeed on specific thrust and total engine airflow. forward velocity of an aircraft, is referred to as ram. Since any ram effect will cause an increase in compressor entrance pressure over atmospheric, the resulting pressure rise will cause an increase in the mass airflow and jet velocity, both of which tend to increase thrust. Although ram effect increases the engine thrust, the thrust being produced by the engine decreases for a given throttle setting as an aircraft gains airspeed. Therefore, two opposing trends occur when an aircraft's speed is increased. What actually takes place is the net result of these two different effects. An engine's thrust output temporarily decreases as aircraft speed increases from static, but soon ceases to decrease; towards the high speeds, thrust output begins to increase again. ### Page 78 RECIPROCATING ENGINE INDUCTION SYSTEMS The induction system of an aircraft reciprocating engine consists of a carburetor, an airscoop or ducting that conducts air to the carburetor, and an intake manifold. These units form a long curved channel which conducts air and the fuel/air mixture to the cylinders. These three units of a typical induction system are usually supplemented by a temperature-indicating system and temperature-controlling unit in some form of alternate air valve and a carburetor heat source. Additionally, a system for compressing the fuel/air mixture may be included. Since many engines installed in light aircraft do not use any type of compressor or supercharging device, induction systems for reciprocating engines can be broadly classified as supercharged or naturally aspirated (nonsupercharged) . Nonsupercharged lnduction Systems The nonsupercharged engine is commonly used in light aircraft. The induction systems of these engines may be equipped with either a carburetor or a fuel-injection system. If a carburetor is used, it may be a float-type or a pressure-type carburetor. If fuel injection is used, it will normally be either a constant flow or a pulsed system. Figure 2-1 is a diagram of an induction system used in a nonsupercharged engine equipped with a carburetor. In this induction system, carburetor cold air is admitted at the leading edge of the nose cowling below the propeller spinner, and is passed through an air filter into air ducts leading to the carburetor. An air valve is located at the carburetor for selecting an alternate warm air source to prevent carburetor icing. The cold-air valve admits air from the outside air scoop for normal operation and is controlled by a control knob in the cc17;pit. The warm-air valve admits warm air from - engine compartment for operatio- durir.g icing mditions and is spring loaded t he "closed" pt sition. When the cold air door is closed, engine suction opens the spring- CHAPTER 2 INDUCTION AND EXHAUST SYSTEMS loaded warm-air valve. If the engine should backfire with the warm-air valve open, spring tension automatically closes the warm-air valve to keep flames out of the engine compartment. The carburetor air filter is installed in the air scoop in front of the carburetor air duct. Its purpose is to stop dust and other foreign matter from entering the engine through the carburetor. The screen consists of an aluminum alloy frame and a deeply crimped screen, arranged to present maximum screen area to the airstream. The carburetor air ducts consist of a fixed duct riveted to the nose cowling and a flexible duct between the fixed duct and the carburetor air valve. The carburetor air ducts provide a passage for cold, outside air to the carburetor. Air enters the system through the ram-air intake. The intake opening is located in the slipstream so the air is forced into the induction system, giving a ram effect. The air passes through the ducts to the carburetor. The carburetor meters the fuel in proportion to the air and mixes the air with the correct amount of fuel. The carburetor can be controlled from the cockpit to regulate the flow of air and, in this way, power output of the engine can be controlled. The carburetor air temperature indicating system shows the temperature of the air at the carburetor inlet. If the bulb is located at the engine side of the carburetor, the system measures the temperature of the fuel/air mixture. Additional Units of the Induction System The units of a typical induction system previously discussed satisfy the needs of the engine insofar as its ability to produce power is concerned. There are two additional units that add nothing to help the engine do its work but are vital to efficient engine operation. One unit is the preheater; the other is the fluid deicing unit. Induction system ice can be prevented or eliminated -37 raisin; '1~.t2mperature of the air that passes tiirougk the qstem, using a preheater lo- ### Page 79 rr Carburetor\ Air filter - bulb I Air intake duct / Cold air actuator lever Warm air = Cold air FIGURE Nonsupercharged induction system using a carburetor. 2-1. cated upstream near the induction system inlet and just as dangerous as the most advanced stage of well ahead of the dangerous icing zones. Heat is induction system ice. Increasing the temperature usually obtained through a control valve that opens of the air causes it to expand and decrease in the induction system to the warm air circulating in density. This action reduces the weight of the the engine compartment. When there is danger of charge delivered to the cylinder and causes a induction system icing, move the cockpit control noticeable loss in power because of decreased volutoward the "hot" position until a carburetor air metric efficiency. In addition, high intake air temtemperature is obtained that will provide the neces- perature may cause detonation and engine failure, sary protection. especially during takeoff and high-power operation. Throttle ice or any ice that restricts airflow or Therefore, during all phases of engine operation, reduces manifold pressure can best be removed by the carburetor temperature must afford the greatest using full carburetor heat. If the heat from the protection against icing and detonation. When engine compartment is sufficient and the application there is no danger of icing, the heat control is has not been delayed, it is only a matter of a few normally kept in the "cold" position. It is best to minutes until the ice is cleared. If the air tempera- leave the control in this position if there are ture in the engine compartment is not high enough particles of dry snow or ice in the air. The use of to be effective against icing, the preheat capacity heat may melt the ice or snow, and the resulting can be increased by closing the cowl flaps and moisture may collect and freeze on the walls of increasing engine power. However, this may prove the induction system. ineffective if the ice formation has progressed so To prevent damage to the heater valves in the far that the loss of power makes it impossible to case of backfire, carburetor heaters should not be generate sufficient heat to clear the ice. used while starting the engine. Also, during ground Improper or careless use of carburetor heat can be operation only enough carburetor heat should be used ### Page 80 to give smooth engine operation. The carburetor air inlet temperature gage must be monitored to be sure the temperature does not exceed the maximum value specified by the engine manufacturer. On some aircraft the basic deicing system is supplemented by a fluid deicing system. This auxiliary system consists of a tank, a pump, suitable spray nozzles in the induction system, and a cockpit control unit. This system is intended to clear ice whenever the heat from the engine compartment is not high enough to prevent or remove ice. The use of alcohol as a deicing agent tends to enrich the fuel mixture, but at a high-power output such slight enrichment is desired. At low throttle settings, however, the use of alcohol may over-enrich the mixture; therefore, alcohol should be applied with great care. INDUCTION SYSTEM ICING A short discussion concerning the formation and the place of formation of induction system ice (fig. 2-2) is helpful to the mechanic, even though he is not normally concerned with operations that occur only when the aircraft is in flight. But the mechanic should know something about induction system 9Fuel evaporation ice FIGURE2-2. Types of induction system ice. icing because of its effect on engine performance. Even when inspection shows that everything is in proper working order, induction system ice can cause an engine to act erratically and lose power in the air, yet the engine will perform perfectly on the ground. Many engine troubles commonly attributed to other sources are actually caused by induction system icing. Induction system icing is an operating hazard because it can cut off the flow of the fuel/air charge or vary the fuel/air ratio. Ice can form in the induction system while an aircraft is flying in clouds, fog, rain, sleet, snow, or even clear air that has a high moisture content (high humidity). Induction system icing is generally classified in three types: (1) Impact ice, (2) fuel evaporation ice, and (3) throttle ice. Chapter 3 discusses types of icing in more detail. To understand why part-throttle operation can lead to icing, the throttle area during this operation must be examined. When the throttle is placed in a partly closed position, it, in effect, limits the amount of air available to the engine. The glide which windmills a fixed-pitch propeller causes the engine to consume more air than it normally would at this same throttle setting, thus aggravating the lack of air behind the throttle. The partly closed throttle, under these circumstances, establishes a much higher than normal air velocity past the throttle, and an extremely low preszure area is produced. The low-pressure area lowers the temperature of the air surrounding !he throttle valves by the same physical law that raises the temperature of air as it is compressed. If the temperature in this air falls below freezing and moisure is present, ice will form on the throttles and nearby units in much the same manner that impact ice forms on units exposed to below freezing temperatures. Throttle ice may be minimized on engines equipped with controllable-pitch propellers by the use of a higher than normal BMEP (brake mean effective pressure) at this low power. The high BMEP decreases the icing tendency because a large throttle opening at low engine r.p.m. partially removes the temperature-reducing obstruction that part-throttle operation offers. Induction System Filtering While dust is merely an annoyance to most individuals, it is a serious source of trouble to an aircraft engine. Dust consists of small particles of hard, abrasive material that can be carried into the ### Page 81 engine cylinders by the very air the engine breathes. It can also collect on the fuel-metering elements of the carburetor, upsetting the proper relation between airflow and fuel flow at all powers. It acts on the cylinder walls by grinding down these surfaces and the piston rings. It then contaminates the oil and is carried through the engine, causing further wear on the bearings and gears. In extreme cases an accumulation may clog an oil passage and cause oil starvation. Although dust conditions are most critical at ground level, dust of sufficient quantity to obscure a pilot's vision has been reported in flight. In some parts of the world, dust can be carried to extremely high altitudes. Continued operation under such conditions without engine protection will result in extreme engine wear and produce excessive oil consumption. When operation in dusty atmosphere is necessary, the engine can be protected by an alternate induction system air inlet which incorporates a dust filter. This type of air filter system normally consists of a filter element, a door, and an electrically operated actuator. When the filter system is operating, air is drawn through a louvered access panel that does not face directly into the airstream. With this entrance location, considerable dust is removed as the air is forced to turn and enter the duct. Since the dust particles are solid, they tend to continue in a straight line, and most of them are separated at this point. Those that are drawn into the louvers are easily removed by the filter. In flight, with air filters operating, consideration must be given to possible icing conditions which may occur from actual surface icing or from freezing of the filter element after it becomes rainsoaked. Some installations have a spring-loaded filter door which automatically opens when the filter is excessively restricted. This prevents the airflow from being cut off when the filter is clogged with ice or dirt. Other systems use an ice guard in the filtered-air entrance. The ice guard consists of a coarse-mesh screen located a short distance from the filtered-air entrance. In this location the screen is directly in the path of incoming air so that the air must pass through or around the screen. When ice forms - on the screen, the air, which has lost its heavy moisture particles, will pass around the iced screen and into the filter element. The efficiency of any filter system depends upon proper maintenance and servicing. Periodic removal and cleaning of the filter element is essential to satisfactory engine protection. Induction System Inspection and Maintenance The induction system should be checked for cracks and leaks during all regularly scheduled engine inspections. The units of the system should be checked for security of mounting. The system should be kept clean at all times, since pieces of rags or paper can restrict the airflow if allowed to enter the air intakes or ducts, and loose bolts and nuts can cause serious damage if they pass into the engine. On systems equipped with a carburetor air filter, the filter should be checked regularly. If it is dirty or does not have the proper oil film, the filter element should be removed and cleaned. After it has dried, it is usually immersed in a mixture of oil and rust-preventive compound. The excess fluid should be allowed to drain off before the filter element is reinstalled. Induction System Troubleshooting The following chart provides a general guide to the most common induction system troubles. **PROBABLE CAUSE** 1. Engine fails to start- (a) Induction system obstructed. (b) Air leaks. 2. Engine runs rough- (a) Loose air ducts. (b) Leaking intake pipes. **ISOLATION PROCEDURE** (a) Inspect airscoop and air ducts. (b) Inspect carburetor mounting and intake pipes. (a) Inspect air ducts. (b) Inspect intake pipe packing nuts. **CORRECTION** (a) Remove obstructions. (b) Tighten carburetor and repair or replace intake pipe. (a) Tighten air ducts. (b) Tighten nuts. ### Page 82 **PROBABLE CAUSE ISOLATION PROCEDURE CORRECTION** (c) Engine valves sticking. (d) Bent or worn valve push rods. 3. Low power- (a) Restricted intake duct. (b) Broken door in carburetor air valve. (c) Dirty air filter. 4. Engine idles improperly- (a) Shrunken intake packing. (b) Hole in intake pipe. (c) Loose carburetor mounting. (c) Remove rocker arm cover and check valve action. (dl Inspect push rods. (a) Examine intake duct. (b) Inspect air valve. (c) Inspect air filter. (a) Inspect packing for proper fit. (b) Inspect intake pipes. (c) Inspect mount bolts. (c) Lubricate and free sticking valves. -. (dj Replace worn or damaged push rods. (a) Remove restrictions. (b) Replace air valve. (c) Clean air filter. (a) Replace packing. (b) Replace defective intake pipes. (c) Tighten mount bolts. Supercharged Induction Systems Supercharging systems used in reciprocating engine induction systems are normally classified as either internally driven or externally driven (turbosupercharged) . Internally driven superchargers compress the fuel/air\mixture after it leaves the carburetor, while externally driven superchargers (turbochargers) compress the air before it is mixed with the metered fuel from the carburetor. Each increase in the pressure of the air or fuel/air mixture in an induction system is called a stage. Superchargers can be classified as singleastage, two-stage, or multi-stage, depending on the number of times compression occurs. Superchargers may also operate at different speeds. Thus, they can be referred to as singlespeed, two-speed, or variable-speed superchargers. Combining the methods of classification provides the nomenclature normally used to describe supercharger systems. Thus, from a simple single-stage system that operates at one fixed speed ratio, it is possible to progress to a single-stage, two-speed, mechanically clutched system or a single-stage, hydraulically clutched supercharger. Even though two-speed or multi-speed systems permit varying the output pressure, the system is still classified as a single-stage of compression if only a single impeller is used, since only one increase (or decrease) in compression can be obtained at a time. INTERNALLY DRIVEN SUPERCHARGERS Internally driven superchargers are used almost exclusively in high-horsepower reciprocating engines. Except for the construction and arrangement of the various types of superchargers, all induction systems with internally driven superchargers are almost identical. The reason for this similarity is that all modern aircraft engines require the same air temperature control to produce good combustion in the engine cylinders. For example, the temperature of the charge must be warm enough to ensure complete fuel vaporization and, thus, even distribution; but at the same time it must not be so hot that it reduces volumetric efficiency or causes detonation. With these requirements, all induction systems that use internal-driven superchargers must include pressure .and temperature-sensing devices and the necessary units required to warm or cool the air. Single-Stage, Single-Speed Supercharger Systems The simple induction system shown in figure 2-3 is used to explain the location of units and the path of the air and fuel/air mixture. Air enters the system through the ram air intake. The intake opening is located so that the air is forced into the induction system, giving a ram effect. The air passes through ducts to the carburetor. The carburetor meters the fuel in proportion to the ### Page 83 Intake air duct Carburetor heat valve Carburetor air temperature bulb FIGURE2-3. Simple induction system. air and mixes the air with the correct amount of fuel. The carburetor can be controlled from the cockpit to regulate the flow of air. In this way, the power output of the engine can be controlled. The manifold pressure gage measures the pressure of the fuel/air mixture before it enters the cylinders. It is an indication of the performance that can be expected of the engine. The carburetor air temperature indicator measures either the temperature of the inlet air or of the fuel/air mixture. Either the air inlet or the mixture temperature indicator serves as a guide so that the temperature of the incoming charge may be kept within safe limits. If the temperature of the incoming air at the entrance to the carburetor scoop is 100' F., there will be approximately a SO0 F. drop in temperature because of the partial vaporization of the fuel at the carburetor discharge nozzle. Partial vaporization takes place and the air temperature falls due to absorption of the heat by vaporization. The final vaporization takes place as the mixture enters the cylinders where higher temperatures exist. The fuel, as atomized into the airstream which flows in the induction system, is in a globular form. The problem, then. becomes one of uniformly breaking up and distributing the fuel remaining in, globular form to the various cylinders. On engines equipped with a large number of cylinders, the uniform distribution of the mixture becomes a greater problem, especially at high engine speeds when full advantage is taken of large air capacity. One method of improving fuel distribution is shown in figure 2-4. This device is known as a distribution impeller. The impeller is attached directly to the end of the rear shank of the crankshaft by bolts or studs. Since the impeller is attached to Primer nozzle Large diameter Diffuser chamber Cranks (hunterweight Manifold ring Slip Slip joint, Rubber gasketjoint Gland nut FIGURE2-4. Distribution impeller arrangement used on a radial engine. ### Page 84 the end of the crankshaft and operates at the same speed, it does not materially boost or increase the pressure on the mixture flowing into the cylinders. But the fuel remaining in the globular form will be broken up into finer particles as it strikes the impeller, thereby coming in contact with more air. This will create a more homogeneous mixture with a consequent improvement in distribution to the various cylinders, especially on acceleration of the engine or when low temperatures prevail. When greater pressure is desired on the fuel/air niixture in the induction system to charge the cylinders more fully, the diffuser or blower section contains a high-speed impeller. Unlike the distribution impeller, which is connected directly to the crankshaft, the supercharger, or blower impeller, is driven through a gear train from the crankshaft. The impeller is located centrally within the diffuser chamber. The diffuser chamber surface may be any one of three general designs: (1) Venturi type (figure 24), (2) vaned type (A of figure 2-5), or (3) airfoil type (B of figure 2-5). The venturi-type diffuser is equipped with plain surfaces, sometimes more or less restricted sectionally to form the general shape of a venturi between the impeller tips and the manifold ring. This type has been most widely used on medium-powered, supercharged engines or those in which lower volumes of mixtures are to be handled and where turbulence of the mixture between the impeller tips and the manifold chamber is not critical. On large-volume engines ranging from 450 hp. upwards, in which the volume of mixture is to be handled at higher velocities and turbulence is a more important factor, either a vane or airfoil type diffuser is widely used. The vanes or airfoil section straightens the airflow within the diffuser chamber to obtain an efficient flow of gases. The intake pipes on early model engines extended in a direct path from the manifold ring to the intake port on the cylinder. In the more recent designs, however, the intake pipes extend from the manifold -. ring on a tangent and the pipe is curved as it extends toward the intake port, which has also been streamlined or shaped to promote efficient flow of gases into ithe cylinder. This reduces turbulence to a mmimum. This has been one of the important methods of increasing the breathing capacity or volume of air which a given design of engine might handle. Increases in supercharger efficiency have been one of the major factors in increasing the power output of modern engines. The gear ratio of the impeller gear train varies from approximately 6:l to 12:l. Impeller speed on an engine equipped with a 10:l impeller gear ratio operating at 2,600 r.p.m. would be 26,000 r.p.m. This requires that the impeller unit be a high-grade forging, usually of aluminum alloy, carefully designed and constructed. Because of the high ratio of all supercharger gear trains, considerable acceleration and deceleration forces are created when the engine speed is increased or decreased rapidly. This B. Vaned type Airfoil section type A. FIGURE2-5. Supercharger diffuser design (vaned and airfoil section types). ### Page 85 necessitates that the impeller be splined on the shaft. In addition, some sort of spring-loaded or antishock device must be incorporated in the gear train between the crankshaft and impeller. An oil seal is usually provided around the impeller shaft just forward of the impeller unit. The functions of the seal on this unit are to minimize the passage of lubricating oil and vapors from the crankcase into the diffuser chamber when the engine is idling and to minimize the leakage 'of the fuel/air mixture into the crankcase when the pressure on the mixture is high at open throttle. The clearance between the diffuser section and the impeller is obtained by varying the length of the oil seal or the thickness of spacers commonly called shims. Close clearance is necessary to give the greatest possible compression of the mixture and to eliminate, insofar as possible, leakage around the fore and aft surfaces of the impeller. The impeller shaft and intermediate drive shaft assemblies may be mounted on antifriction ball or roller bearings or on friction-type bushings. The impeller shaft and gear are usually forged integrally of very high grade steel. The impeller end of the shaft is splined to give as much driving surface as possible. The intermediate shaft and large and small gears also are one piece. Both of these units are held within very close running balance or dynamic limits due to the high speeds and stresses involved. Single-Stage, Two-SpeedSupercharger Systems Some aircraft engines are equipped with internally driven superchargers which are single-stage, twospeed systems. The impeller in such systems can be driven at two different speeds by means of clutches. A schematic of such a supercharger is shown in figure 2-6. This unit is equipped with a means of driving the impeller directly from the crankshaft at a ratio of 10:1, which is accomplished by moving the control in the cockpit, thereby, applying oil pressure throughthe high-speed clutch and thus locking the entire intermediate gear assembly. This is called "high blower" and is used above a specified altitude ranging from 7,000 to 12,000ft. Below these levels, the control is positioned to release the pressure on the high-speed clutch and apply it to the low-speed clutch. This locks the sun pinion of the small planetary gear. The impeller then is driven through the spider and shaft assembly on which the planetary pinions are cause? to rotate by the large bell gear. In this case, the i;:~peller is drivr at a r~tio of approximately 7:l relative to crankshatt speed. (See figure 2-7.) This condition is called "low blower" and is used during takeoff and for all altitudes below those at which best efficiency can be obtained in "high blower." In effect, this gives two engines in one. It improves the power output characteristics over a range of operating conditions varying from sea level to approximately 20,000 ft. Naturally, a device of this kind complicates and increases considerably the initial and maintenance costs of the engine. A higher grade fuel is also required to withstand the additional pressures and, in some cases, higher temperatures created within the combustion chamber due to more complete fuel charging of the cylinder. The addition of this unit also complicates the operation of the powerplant because it requires more attention and adds to the variables which must be controlled. Another example of a two-stage, two-speed supercharger system is shown in figure 2-8, where the blower and intermediate rear sections are opened to show their internal construction. In this example, the blower case supports the engine in the aircraft. It has eight pads on the outer circumference for the engine mounting brackets. A liner in the center of the case accommodates the oil seal rings in the impeller shaft front ring carrier. The blower case houses the impeller, which is driven by clutches at either 7.15 or 8.47 times crankshaft speed. An annulus around the case delivers the fuel and air mixture' from the impeller to 14 ports in the case. Attached to each of these ports is an intake pipe through which the fuel and air mixture proceeds to the inlet valve of its cylinder. Intermediate Rear Case The intermediate rear case houses the impeller drive gear train and supports a vaned diffuser (see figure 2-9). The impeller ratio selector valve is mounted on a pad on the top left of the case. From the carburetor mounting flange on the top of the case, a large duct leads down to carry the intake air to the impeller. The fuel transfer pipe from the carburetor connects with a passage in the case behind the carburetor mounting flange. This passage leads to the fuel feed valve. The fuel feed valve delivers fuel to the fuel'slinger, which mixes the fuel with the intake air. The cover and diaphragm assembly of the fuel feed valve are mounted on the forward side of the carburetor flange. An accelerating pump is fastened to a pad on the right side of the case just below the carburetor flange. At the lowest point in ### Page 86 FIGURE Schematic diagram of two-speed supercharger in high ratio. 2-6. the carburetor air duct, drilled passages lead down to the automatic fuel drain valve in the bottom of the case, which discharges any fuel that may accumulate while the engine is being started. The intermediate rear case also houses the dualratio clutches and the impeller ratio selector valve. Both a high(8.47:l)-and a low(7.15:l)-ratio clutch are mounted on each of two shafts, one on each side of the impeller shaft. These shafts are supported at the front end by bushings in the rear case. The shafts are driven by the accessory drive gear through pinions splined to the shafts. The clutch cones are splined to the clutch shafts and, when engaged, drive the clutch gears, which, in turn, drive the spur gears on the impeller shaft. The selector valve directs pressure oil to oil chambers between the cones and the gears of either the two low or the two high clutches. The oil pressure causes the cones to engage the segments that, in turn, engage the gears of whichever pair of clutches are selected to drive the impeller. Drain oil from the disengaged pair of clutches is forced back through the selector valve and discharged into the intermediate rear case. To assist in cleaning sludge out of the clutches, ### Page 87 each clutch gear is equipped with a creeper gear having one more tooth than the clutch gear. A bleed hole in the creeper gear itself alines momentarily with each of the bleed holes in the corresponding clutch gear. The pressure oil within the engaged clutch spurts out, carrying the sludge with it. EXTERNALLY DRIVEN SUPERCHARGERS Externally driven superchargers are designed to deliver compressed air to the inlet of the carburetor or fuel/air control unit of an engine. Externally driven superchargers derive their power from the energy of engine exhaust gases directed against some form of turbine. For this reason, they are commonly called turbosuperchargers or turbochargers. **TURBOSUPERCHARGER SYSTEM FOR LARGE** RECIPROCATING ENGINES In some high-altitude aircraft the internal super- " charger is supplemented by an external turbosupercharger driven by a portion of the exhaust gas from the aircraft engine. This type of supercharger is mounted ahead of the carburetor as shown in figure 2-10 to pressurize the air at the carburetor Spring loaded Accessory / drive gear -driven by 2-7.FIGURE Schematic diagram of two-speed supercharger in low ratio. ### Page 88 Intake pipe ports Diffuser vanes Impeller ratio FIGURE Blower and intermediate rear sections. 2-8. inlet. If the air pressure entering the carburetor is maintained at approximately sea level density throughout the aircraft's climb to altitude, there will be none of the power loss experienced in aircraft not equipped with turbos. However, this type of supercharger imposes an induction system requirement not needed in other supercharger installations. As air moves through the turbo, its temperature is raised because of compression. If the hot air charge is not properly cooled before it reaches the internal supercharger, the second stage of supercharging will produce a final charge temperature that is too great. The air in turbo-equipped induction systems is cooled by an intercooler (figure 2-10), so called because it cools the charge between compression stages rather than after the last stage. The hot air flows through tubes in the intercooler in much the same manner that water flows in the radiator of an automobile. Fresh outside air, separate from the charge, is collected and piped to the intercooler so that it flows over and cools the tubes. As the induction air charge flows through the tubes, heat is removed and the charge is cooled to a degree that the engine can tolerate without detonation occurring. Control for the cooling air is provided by intercooler shutters which regulate the amount of air that passes over and around the tubes of the intercooler. The typical turbosupercharger is composed of three main parts: (1) The compressor assembly. (2) The exhaust gas turbine assembly. (3) The pump and bearing casing. These major sections are shown in figure 2-11. In addition to the major assemblies, there is a baffle between the compressor casing and the exhaust-gas turbine that directs cooling air to the pump and bearing casing, and also shields the compressor from the heat radiated by the turbine. In installations where cooling air is limited, the baffle is replaced by a regular cooling shroud that receives its air directly from the induction system. The compressor assembly (A of figure 2-11) is made up of an impeller, a diffuser, and a casing. The air for the induction system enters through a circular opening in the center of the compressor casing, where it is picked up by the blades of the impeller, which gives it high velocity as it travels outward toward the diffuser. The diffuser vanes ### Page 89 FIGURE2-9. Intermediate rear and rear case sections. Internally driven supercharger. FIGURE2-10. Induction system with turbosupercharger. ### Page 90 direct the airflow as it leaves the impeller and also converts the high velocity of the air to high pressure. Motive power for the impeller is furnished through the impeller's attachment to the turbine wheel shaft of the exhaust-gas turbine. This complete assembly is referred to as the rotor. The rotor revolves on the A. Compressor assembly ball bearings at the rear end of the pump and bearing casing and the roller bearing at the turbine end. The roller bearing carries the radial (centrifugal) load of the rotor, and the ball bearing supports the rotor at the impeller end and bears the entire thrust (axial) load and part of the radial load. The exhaust gas turbine assembly (B of figure 2-11) consists of the turbine wheel (bucket wheel), nozzle box, butterfly valve (waste gate), an8 cooling cap. The turbine wheel, driven by exhaust gases, drives the impeller. The nozzle box collects and directs the exhaust gases onto the turbine wheel, and the waste gate regulates the amount of exhaust gases directed to the turbine by the nozzle box. The cooling cap controls a flow of air for turbine cooling. The waste gate (figure 2-12) controls the volume of the exhaust gas that is directed onto the turbine and thereby regulates the speed of the rotor (turbine and impeller). B. Exhaust gas turbine assembly C. Pump and bearing cash:: FIGURE2-11. Main sections of a typical turbosupercharger. FIGURE.^-12. Waste gate assembly. If the waste gate is completely closed, all the exhaust gases are "backed up" and forced through the nozzle box and turbine wheel. If the waste gate is partially closed, a corresponding amount of exhaust gas is directed to the turbine. The nozzles of the nozzle box allow the gases to expand and reach high velocity before they contact the turbine wheel. The exhaust gases, thus directed, strike the cuplike buckets, arranged radially around the outer edge of the turbine, and cause the rotor (turbine and impeller) to rotate. The gases are then exhausted overboard through the spaces between the buckets. When the waste gate is fully open, nearly all of the exhaust gases pass overboard through the tailpipe. ### Page 91 TURBOCHARGER An increasing number of engines used in light aircraft are equipped with externally driven supercharger systems. These superchargers are powered by the energy of exhaust gases and are usually referred to as "turbocharger" systems rather than turbos supercharger^." On many small aircraft engines, the turbocharger system is designed to be operated only above a certain altitude; for example, 5,000 ft., since maximum power without supercharging is available below that altitude. The location of the air induction and exhaust systems of a typical turbocharger system for a small aircraft is shown in figure 2-13. Induction Air System The induction air system shown in figure 2-14 consists of a filtered ram-air intake located on the side of the nacelle. An alternate air door within the nacelle permits compressor suction to automatically admit alternate air (heated engine compartment air) if the induction filter becomes clogged. The alternate air door can be operated manually in the event of filter clogging. A separately mounted exhaustdriven turbocharger is included in each air induction system. The turbocharger is automatically controlled by a pressure controller, to maintain manifold pressure at approximately 34.5 in. Hg from sea level to the critical altitude (typically 16,000 ft.) regardless of temperature. The turbocharger is completely automatic, requiring no pilot action up to the critical altitude. Controllers and Waste-Gate Actuator The waste-gate actuator and controllers use engine oil for power supply. (Refer to turbocharger system schematic in figure 2-15.) The turbocharger is controlled by the waste gate and waste-gate actuator, an absolute pressure and a rate-of-change controller. A pressure ratio controller controls the waste-gate actuator above critical altitude (16,000 ft.). The waste-gate bypasses the engine exhaust gases around the turbocharger turbine inlet. The waste-gate actuator, which is physically connected to the waste gate by mechanical linkage, controls the position of the waste-gate butterfly valve. The absolute pressure controller and the rate-of-change FIGURE2-13. Turbocharger induction and exhaust systems. ### Page 92 Induction system balance tube Induction manifold left hand bank i Induction manifold right hand Ram filtered air > Waste Waste Alternate air-door Waste gate [Turbocharger ( normally closed ) Induction system balance tube Induction manifold left hand bank Waste gate actuator lternate air door (open) Code ---Mechanical linkage Ram air Compressed air Oil pressure ---- Oil return Ram filtered air Heated air FIGURE2-14. Induction air system schematic. ### Page 93 controller have a two-fold function: (1) The absolute pressure controller controls the maximum turbocharger compressor discharge pressure (34 k.5 in. Hg to critical altitude, approximately 16,000 ft.) ; and (2) the rate-of-change controller controls the rate at which the turbocharger compressor discharge pressure will increase. SEA-LEVEL BOOSTED TURBOCHARGER SYSTEM Some turbocharger systems are designed to operate from sea level up to their critical altitude. These engines, sometimes referred to as sea-level boosted engines, can develop more power at sea level than an engine without turbocharging. Figure 2-16 is a schematic of a sea level booster turbocharger system. This system is automatically regulated by three components shown in the schematic: (1) The exhaust bypass valve assembly, (2) the density controller, and (3) the differential pressure controller. It should be noted that some turbocharger systems are not equipped with automatic control devices. They are similar in design and operation to the system shown in figure 2-16, except that the turbocharger output is manually controlled. By regulating the waste gate position and the "fully open" and "closed" positions (figure 2-16), a constant power output can be maintained. When the waste gate is fully open, all the exhaust gases are directed overboard to the atmosphere, and no air is compressed and delivered to the engine air inlet. Conversely, when the waste gate is fully closed, a maximum volume of exhaust gases flows into the turbocharger turbine, and maximum supercharging is accomplished. Between these two extremes of waste gate position, constant power output can be achieved below the maximum altitude at which the system is designed to operate. A critical altitude exists for every possible power setting below the maximum operating ceiling, and if the aircraft is flown above this altitude without a corresponding change in the power setting, the waste gate will be automatically driven to the "fully closed" position in an effort to maintain a constant power output. Thus, the waste gate will be almost fully open at sea level and will continue to move toward the "closed" position as the aircraft climbs, in order to maintain the preselected manifold pressure setting. When the waste gate is fully closed (leaving only a small clearance to prevent sticking) the manifold pressure will begin to drop if the aircraft continues to climb. If a higher power setting cannot be selected, the turbocharger's critical altitude has been reached. Beyond this altitude, the power output will continue to decrease. The position of the waste gate valve, which determines power output, is controlled by oil pressure. Engine oil pressure acts on a piston in the waste gate assembly which is connected by linkage to the waste gate valve. When oil pressure is increased on the piston, the waste gate valve moves toward the "closed" position, and engine output power increases. Conversely, when the oil pressure is decreased, the waste gate valve moves toward the "open" position, and output power is decreased. The position of the piston attached to the waste gate valve is dependent on bleed oil which controls the engine oil pressure applied to the top of the piston. Oil is returned to the engine crankcase through two control devices, the density controller and the differential pressure controller. These two controllers, acting independently, determine how much oil is bled back to the crankcase, and thus establishes the oil pressure on the piston. The density controller is designed to limit the manifold pressure below the turbocharger's critical altitude, and regulates bleed oil only at the "full throttle" position. The pressure- and temperaturesensing bellows of the density controller react to pressure and temperature changes between the fuel injector inlet and the turbocharger compressor. The bellows, filled with dry nitrogen, maintains a constant density by allowing the pressure to increase as the temperature increases. Movement of the bellows re-positions the bleed valve, causing a change in the quantity of bleed oil, which changes the oil pressure on top of the waste gate piston. (See figure 2-16.) The differential pressure controller functions during all positions of the waste gate valve other than the "fully open" position, which is controlled by the density controller. One side of the diaphragm in the differential pressure controller senses air pressure upstream from the throttle; the other side samples pressure on the cylinder side of the throttle valve (figure 2-16). At the "wide open" throttle position when the density controller controls the waste gate, the pressure across the differential pressure controller diaphragm is at a minimum and the controller spring holds the bleed valve closed. At "part throttle" position, the air differential is increased, opening the bleed valve to bleed oil to the engine crankcase and re-position the waste gate piston. ### Page 96 Thus, the two controllers operate independently to control turbocharger operation at all positions of the throttle. Without the overriding function of the differential pressure controller during part-throttle operation, the density controller would position the waste gate valve for maximum power. The differential pressure controller reduces injector entrance pressure and continually re-positions the valve over the whole operating range of the engine. The differential pressure controller reduces the unstable condition known as "bootstrapping" during part-throttle operation. Bootstrapping is an indication of unregulated power change that results in the continual drift of manifold pressure. This condition can be illustrated by considering the operation of a system when the waste gate is fully c!osed. During this time, the differential pressure controller is not modulating the waste gate valve position. Any slight change in power caused by a change in temperature or r.p.m. fluctuation will be magnified and will result in manifold pressure change since the slight change will cause a change in the amount of exhaust gas flowing to the turbine. Any change in exhaust gas flow to the turbine will cause a change in power output and will be reflected in manifold pressure indications. Bootstrapping, then, is an undesirable cycle of turbocharging events causing the manifold pressure to drift in an attempt to reach a state of equilibrium. Bootstrapping is sometimes confused with the condition known as "overboost," but bootstrapping is not a condition which is detrimental to engine life. An overboost condition is one in which manifold pressure exceeeds the limits prescribed for a particular engine and can cause serious damage. Thus, the differential pressure controller is essential to smooth functioning of the automatically controlled turbocharger, since it reduces bootstrapping by reducing the time required to bring a system into equilibrium. There is still a great deal more throttle sensitivity with a turbocharged engine than with a naturally aspirated engine. Rapid movement of the throttle can cause a certain amount of manifold pressure drift in a turbocharged engine. This condition, less severe than bootstrapping, is called "overshoot." While overshoot is not a dangerous condition, it can be a source of concern to the pilot or operator who selects a particular manifold pressure setting only to find it has changed in a few seconds and must be reset. Since the automatic controls cannot respond rapidly enough to abrupt changes in throttle settings to eliminate the inertia of turbocharger speed changes, overshoot must be controlled by the operator. This can best be accomplished by slowly making changes in throttle setting, accompanied by a few seconds' wait for the system to reach a new equilibrium. Such a procedure is effective with turbocharged engines, regardless of the degree of throttle sensitivity. Turbocharger System Troubleshooting Table 1includes some of the most common turbocharger system malfunctions, together with their cause and repair. These troubleshooting procedures are presented as a guide only and .should not be substituted for applicable manufacturer's instructions or troubleshooting procedures. **TURBOCOMPOUND SYSTEMS FOR RECIPROCAT-** ING ENGINES The turbocompound engine consists of a conventional, reciprocating engine in which exhaust-driven turbines are coupled to the engine crankshaft. This system of obtaining additional power is sometimes called a PRT (power recovery turbine) system. It is not a supercharging system, and it is not connected in any manner to the air induction system of the aircraft. The PRT system enables the engine to recover power from the exhaust gases that would be otherwise directed overboard. Depending on the type of engine, the amount of horsepower recovered varies with the amount of input power. An average of 130 horsepower from each of three turbines in a system is typical for large reciprocating en,' vines. A power recovery turbine's geared connection to the engine crankshaft is shown in figure 2-17. Typically there are three power-recovery turbines on each engine, located 120' apart. They are numbered, viewed from the rear of the engine, in a clockwise direction. Number 1turbine is located in the 3-o'clock position, and number 3 turbine in the 11-o'clock position. Turbine position in relationship to the exhaust system of the various cylinders on an 18-cylinder engine is shown in the schematic of figure 2-18. The exhaust collector nozzle for each segment of cylinders (figure 2-18) directs the exhaust gases onto the turbine wheel. The turbine wheel shaft transmits the power to the engine crankshaft through gears and a fluid coupling. The fluid coupling prevents torsional vibration from being transmitted to the crankshaft. Power recovery turbine systems, because of weight and cost considerations, are used exclusively on very large reciprocating engines. ### Page 97 TABLE 1. Troubleshooting turbocharger system. Trouble Probable Cause Remedy Aircraft fails to reach Damaged compressor or Replace turbocharger. critical altitude. turbine wheel. Exhaust system leaks. Repair leaks. Faulty turbocharger bearings. Replace turbocharger. Waste gate will not close fully. Refer to "waste gate" in the trouble column. Malfunctioning controller. Refer to "differential controller" in the trbuble column. Engine surges. Bootstrapping. Ensure engine is operated in proper range. Waste gate malfunction. Refer to "waste gate" in the trouble column. Controller malfunction. Refer to "differential controller" in the trouble column. Waste gate will not Waste gate bypass valve Replace bypass valve. close fully. bearings tight. Oil inlet orifice blocked. Clean orifice. Controller malfunction. Refer to "controller" in the trouble column. Broken waste gate linkage. Replace linkage and adjust waste gate for proper opening and closing. Waste gate will Oil outlet obstructed. Clean and reconnect oil return not open. line. Broken waste gate linkage. Replace linkage and adjust waste gate opening and closing. Controller malfunction. Refer to "controller" in the trouble column. Differential controller Seals leaking. Replace controller. malfunctions. Diaphragm broken. Replace controller. Controller valve stuck. Replace controller. Density controller Seals leaking. Replace controller. malfunctions. Bellows damaged. Replace controller. Valve stuck. Replace controller. TURBOJET ENGINE INLET DUCT SYSTEMS flow is necessary to avoid compressor stall and exces- Although no direct parallel can be drawn, the sive internal engine temperatures at the turbine. turbine engine air-inlet duct is somewhat analogous The high energy enables the engine to produce an to +he air induction system of reciprocating engines. optimum amount of thrust. Normally, the air-inlet The engine inlet and the inlet ducting of a turbine duct is considered an airframe part, and not a part engine furnish a relatively distortion-free, high- of the engine. However, the duct is so important energy supply of air, in the required quantity, to the to engine performance that it must be considered in face of the compressor. A uniform and steady air- any discussion of the complete engine. ### Page 98 Exhaust gases- I 4 r~ FIGURE2-17. Transmission of turbine power to crankshaft. Bottom of EngineTop of Engine FIGURE2-18. Schematic diagram of a PRT system. A gas turbine engine consumes six to 10 times as engine and aircraft performance, especially at high much air per hour as a reciprocating engine of airspeeds. Inefficiencies of the duct result in sucequivalent size. The air-entrance passage is corre- cessively magnified losses through other components spondingly larger. Furthermore, it is more critical of the en,' cine. than a reciprocating engine airscoop in determining The inlet duct has two engine functions and one ### Page 99 - - aircraft function. First, it must be able to recover as much of the total pressure of the free airstream as possible and deliver this pressure to the front of the engine with a minimum loss of pressure or differential. This is known as "ram recovery" or, sometimes, as "total pressure recovery." Secondly, the duct must uniformly deliver air to the compressor inlet with as little turbulence and pressure variation as possible. As far as the aircraft is concerned, the duct must hold to a minimum the drag effect, which it creates. Pressure drop or differential is caused by the friction of the air along both sides of the duct and by the bends in the duct system. Smooth flow depends upon keeping the amount of turbulence to a minimum as the air enters the duct. The duct must have a sufficiently straight section to ensure smooth, even airflow within. The choice of configuration of the entrance to the duct is dictated by the location of the engine within the aircraft and the airspeed, altitude, and attitude at which the aircraft is designed to operate. There are two basic types of inlet ducts, the single-entrance duct and the divided-entrance duct. With ducts of any type, careful construction is very essential. ~ood workmanship is also needed when an inlet duct is repaired. Surprisingly small amounts of airflow distortion can result in appreciable loss in engine efficiency or can be responsible for otherwise unexplainable compressor stalls. Protruding rivet heads or poor sheet metal work can pIay havoc with an otherwise acceptable duct installation. Single-Entrance Duct The single-entrance type of duct is the simplest and most effective because the duct inlet is located directly ahead of the engine and aircraft in such a position that it scoops undisturbed air. Figure 2-19 illustrates the single-entrance duct on a singleengine, turbojet aircraft. Also, the duct can be built either in a straight configuration or with only relatively gentle curvatures. In a single-engine aircraft installation where the engine is mounted in the fuselage, the duct is necessarily long. While some pressure drop is occasioned by the long duct, the condition is offset by smooth airflow characteristics. In multi-engine installations, a short, straight duct, or one that is nearly straight, is a necessity. Although this short, straight duct results in minimum pressure drop, the engine is apt to suffer from inlet turbulence, especially at slow airspeeds or high angles of attack. FIGURE Aircraft with single-entrance duct. 2-19. Divided-Entrance Duct The requirements of high-speed, single-engine aircraft, in which the pilot sits low in the fuselage and close to the nose, render it difficult to employ the single-entrance duct. Some form of a divided duct which takes air from either side of the fuselage may be required. This divided duct can be either a wingroot inlet or a scoop at each side of the fuselage, as shown in figure 2-20. Either type of duct presents more problems to the aircraft designer than a singleentrance duct because of the difficulty of obtaining sufficient airscoop area without imposing prohibitive amounts of drag. Internally, the problem is the same as that encountered with the single-entrance duct; that is, to construct a duct of reasonable length, yet with as few bends as possible. The wing-root inlet on aircraft on which the wing is located fairly far aft presents a design problem because, although short, the duct must have considerable curvature to deliver air properly to the compressor inlet. Scoops at the sides of the fuselage are often used. These side scoops are placed as far forward as possible to permit a .gradual bend toward the compressor inlet, making the airflow characteristics approach those of a single-entrance duct. A series of small rods is sometimes placed in the sidescoop inlet to assist in straightening the incoming airflow and to prevent turbulence. Variable-Geometry Dud The main function of an inlet duct is to furnish the proper amount of air to the engine inlet. In a typical turbojet engine, the maximum airflow requirements are such that the Mach number of the airflow directly ahead of the face of the engine is ### Page 100 u A Side Inlet \ wing Inlet FIGURE Two types of divided-entrance ducts. 2-20. about 0.5, or a little less. Therefore, under practically all flight conditions except takeoff or landing, the velocity of the airflow as it enters the air-inlet duct must be reduced through the duct before the same air is ready to enter the compressor. To accomplish this, inlet ducts are designed to function as diffusers, and thus decrease the velocity and increase the static pressure of the air passing through them. For subsonic multi-engine aircraft, a normal inlet duct, therefore, increases in size, front to rear, along the length of the duct, as illustrated in figure 2-21. A supersonic diffuser progressively decreases in area in the downstream direction. Therefore, a supersonic inlet duct will follow this general config- FIGURE Divergent subsonic inlet duct. 2-21. Divergent subsonic section FIGURE Supersonic inlet duct. 2-22. uration until the velocity of the incoming air is reduced to Mach 1.0. The aft section of the duct will then commence to increase in area, since this part must act as a subsonic diffuser. (See figure 2-22.) In practice, inlet ducts for supersonic aircraft will only follow this general design insofar as practical, depending upon the design features of the aircraft. For very high speed aircraft, the inside area of configuration of the duct will be changed by a mechanical device as the speed of the aircraft increases or decreases. A duct of this type is usually known as a variable-geometry inlet duct. Two methods are used to diffuse the inlet air and slow the inlet airflow at supersonic flight speeds. One is to vary the area, or geometry, of the inlet duct either by using a movable restriction, such as a ramp or wedge, inside the duct. Still another system is some sort of a variable airflow bypass arrangement which extracts part of the inlet airflow from the duct ahead of the engine. In some cases, a combination of both systems is used. The other method is the use of a shock wave in the airstream. A shock wave is a thin region of discontinuity in a flow of air or gas, during which the speed, pressure, density, and temperature of the ### Page 101 air or gas undergo a sudden change. Stronger shock waves produce larger changes in the properties of the air or gas. A shock wave is willfully set up in the supersonic flow of the air entering the duct, by means of some restriction or small obstruction which automatically protrudes into the duct at high flight Mach numbers. The shock wave results in diffusion of the airflow, which, in turn, slows down the velocity of the airflow. In at least one aircraft installation, both the shock method and the variable-geometry method of causing diffusion are used in combination. The same device which changes the area of the duct also sets up a shock wave that further reduces the speed of the incoming air within the duct. The amount of change in duct area and the magnitude of the shock are varied automatically with the airspeed of the aircraft. With a variable-geometry inlet, the so-called inlet "buzz" which sometimes occurs during flight at high Mach numbers can often be prevented by changing the amount of inlet-area variation that takes place when the variable-geometry inlet system is in operation. The "buzz" is an airflow instability which occurs when a shock wave is alternately swallowed and regurgitated by the inlet. At its worst, the condition can cause violent fluctuations in pressure through the inlet, which may result in damage to the inlet structure or, possibly, to the engine itself. A suitable variable-geometry duct will eliminate the "buzz" by increasing the stability of the airflow within the inlet duct. Bellmouth Compressor Inlets Although not a duct in the true sense of the word, a bellmouth inlet is usually installed on an engine being calibrated in a ground test stand, to lead the outside static air to the inlet guide vanes of the compressor. This type of inlet is easily attached and removed. It is designed with the single objective of obtaining very high aerodynamic efficiency. Essentially, the inlet is a bell-shaped funnel having carefully rounded shoulders which offer practically no air resistance (see figure 2-23]. Duct loss is so slight that it is considered zero. The engine can, therefore, be operated without the complications resulting from losses common to an installed aircraft duct. Engine performance data, such as rated thrust and thrust specific fuel consumption, are obtained while using a bellmouth compressor inlet. Usually, the inlets are fitted with protective screening. In this case, the efficiency FIGURE Bellmouth compressor inlet. 2-23. which is lost as the air passes through the screen must be taken into account when very accurate engine data are necessary. TURBOPROP COMPRESSOR INLETS The air inlet on a turboprop is more of a problem than that on a turbojet because the propeller drive shaft, the hub, and the spinner must be considered in addition to the usual other inlet design factors. The ducted spinner arrangement (figure 2-24A) is generally considered the best inlet design of the turboprop engine as far as airflow and aerodynamic characteristics are concerned. However, the ducted spinner is heavier. and is more difficult to maintain and to anti-ice than the conventional streamline spinner arrangement which is frequently used. A conical spinner, which is a modified version of the streamline spinner, is sometimes employed. In either event, the arrangement of the spinner and the inlet duct is similar to that shown in figure 2-24B. When the nose section of the turboprop engine is offset from the main axis of the engine. an arrangement similar to that in figure 2-242 may be employed. compressor-Inlet Screens The appetite of a gas turbine for nuts, small bolts, rags, small hand tools, and the like, is well known. To prevent the engine from readily ingesting such items, a compressor-inlet screen is sometimes placed across the engine air inlet at some location along the inlet duct. ### Page 102 FIGURE2-24. Turboprop compressor inlets. The advantages and disadvantages of a screen of this type vary. If the engine is readily subject to internal damage, as would be the case for instance, of an engine having an axial compressor fitted with aluminum compressor blades, an inlet screen is almost a necessity. Screens, however, add appreciably to inlet-duct pressure loss, and are very susceptible to icing. Failures .due to fatigue are also a problem. A failed screen can sometimes cause more damage than no screen at all. In some instances, inlet screens are made retractable and may be withdrawn from the airstream after takeoff or whenever icing conditions prevail. Such screens are subject to mechanical failure, and add both weight and bulk to the installation. In large engines having steel or titanium compressor blades which do not damage easily, the disadvantages of compressor screens outweigh the advantages, so they are not generally used. Turbofan Engine Fan Sections Although some turbofan engines have their fan section, or blades, integral with the turbine, aft of the combustion chamber, other versions are usually constructed with the fan at the forward end of the compresser. In dual-compressor engines, the fan is integral with the relatively slow-turning, lowpressure compressor, which allows the fan blades to rotate at low tip speed for best fan efficiency. The forward fan permits the use of a conventional air-inlet duct, resulting in low inlet-duct loss. The forward fan reduces engine damage from ingested foreign material because much of any material that may be ingested will be thrown radially outward, and will pass through the fan discharge rather than through the main part of the engine. The fan consists of one or more stages of rotating blades and stationary vanes that are somewhat larger than the forward stages of the compressor to which they are attached. The air accelerated by the fan tips forms a secondary airstream which is ducted overboard without passing through the main engine. The air which passes through the center of the fan becomes the primary airstream through the engine itself (see figure 2-25). The air from the fan exhaust, which is ducted overboard, may be discharged in either of two ways: (1) To the outside air through short ducts, directly hehind the fan, as shown in figure 2-26 and in the sketch of a bifurcated duct configuration, figure 2-27, or (2) ducted all the way to the rear of the engine, where it is exhausted to the outside air in the vicinity of the engine tailpipe. RECIPROCATING ENGINE EXHAUST SYSTEMS The reciprocating engine exhaust system is fundamentally a scavenging system that collects and disposes of the high-temperature, noxious ### Page 103 FIGURE Airflow through a forward-fan engine. 2-25. FIGURE Typical forward-fan turbofan 2-26. engine installation. gases as they are discharged by the engine. Its basic requirement is to dispose of the gases with complete safety to the airframe and the occupants of the aircraft. The exhaust system can perform many useful functions, but its first duty is to provide protection against the potentially destructive action of the exhaust gases. Modern exhaust systems, though comparatively light, adequately resist high temperatures, corrosion, and vibration to provide long, trouble-free operation with a minimum of maintenance. There are two general types of exhaust systems in use on reciprocating aircraft engines: the short stack (open) system and the collector system. The short stack system is generally used on nonsupercharged engines and low-powered engines where noise level is not too objectionable. The collector system is used on most large nonsupercharged engines and on all turbosupercharged engines and installations where it would improve nacelle streamlining or provide easier maintenance in the nacelle area. On turbosupercharged engines the exhaust gases must be collected to drive the turbine FIGURE Bifurcated duct configuration. 2-27. compressor of the supercharger. Such systems have individual exhaust headers which empty into a common collector ring with only one outlet. From this outlet, the hot exhaust gas is routed via a tailpipe to the nozzle box of the turbosupercharger to drive the turbine. Although the collector system raises the back pressure of the exhaust system, the gain in horsepower from turbosupercharging more than offsets the loss in horsepower that results from increased back pressure. The short stack system is relatively simple, and its removal and installation consists essentially of removing and installing the holddown nuts and clamps. In figure 2-28, the location of the exhaust system components of a horizontally opposed engine is shown in a side view. The exhaust system in this installation consists of a down-stack from each cylinder, an exhaust collector tube on each side of the engine, and an exhaust ejector assembly protruding aft and down from each side of the firewall. The down-stacks are connected to the cylinders with high-temperature locknuts and ### Page 104 FIGURE Exhaust system of a horizontally 2-28. opposed engine. secured to the exhaust collector tube by ring clamps. A cabin heater exhaust shroud is installed around each collector tube. (See figure 2-29.) The collector tubes terminate at the exhaust ejector openings at the firewall and are tapered to deliver the exhaust gases at the proper velocity to induce an airflow through the exhaust ejectors. The exhaust ejectors consist of a throat and duct assembly which utilizes the pumping action of the exhaust gases to induce a flow of cooling air through all parts of the engine compartment. Radial Engine Exhaust Collector Ring System Figure 2-30 shows the exhaust collector ring installed on a lkylinder radial engine. The collector ring is a welded corrosion-resistant steel assembly manufactured in seven sections, with each section collecting the exhaust from two cylinders. The sections are graduated in size (figure 2-31). The small sections are on the inboard side, and the largest sections are on the outboard side at the point where the tailpipe connects to the collector ring. Each section of the collector ring is bolted to a bracket on the blower section of the engine, and is partly supported by a sleeve connection between the collector ring ports and the short stack on the engine exhaust ports. The exhaust tailpipe is joined to the collector ring by a telescoping expansion joint, which allows enough slack for the removal of segments of the collector ring without - removing the tailpipe. A. Upper sheet jacket. B. Heat exchanger collector tube. C. Lower sheet jacket. FIGURE2-29. Exploded view of heater exhaust shroud assembly. A. Clamp assembly 1).Engine diaphragm B. Telescoping flange E. Clamp assembly C. Main exhaust segment F. Clevis pin & washer FIGURE2-30. Installed exhaust collector ring. ### Page 105 FIGURE2-31. Exhaust collector ring. The exhaust tailpipe is a welded, corrosionresistant steel assembly consisting of the exhaust tailpipe and, on some aircraft, a muff-type heat exchanger. Manifold and Augmentor Exhaust Assembly Some radial engines are equipped with a combination exhaust manifold and augmentor assembly. On a typical 18-cylinder engine, two exhaust assemblies and two augmentor assemblies are used. Each manifold assembly collects exhaust gases from nine cylinders and discharges the gases into the forward end of an augmentor assembly. Four stacks of each manifold assembly are siamese exhaust stacks, each receiving the exhaust from two cylinders (see figure 2-32). The firing order of the two cylinders exhausting into each exhaust stack is as widely separated as possible. Front row cylinders are connected to stacks by port extensions. This type of exhaust manifold is manufactured from corrosion-resistant steel, and has either a plain sandblast or a ceramic-coated finish. The exhaust gases are directed into the augmentor bellmouths. The augmentors are designed to produce a venturi effect to draw an increased airflow over the engine to augment engine cooling. An augmentor vane is located in each tailpipe. When the vane is fully closed, the cross-sectional area of the tailpipe is reduced by approximately 4570. The augmentor vanes are operated by an electrical actuator, and indicators adjacent to the augmentor vane switches in the cockpit show vane positions. The vanes may be moved toward the "closed" position to decrease the velocity of flow through the augmentor to raise the engine temperature. **RECIPROCATING ENGINE EXHAUST SYSTEM** MAINTENANCE PRACTICES Any exhaust system failure should be regarded as a severe hazard. Depending on the location and type of failure, an exhaust system failure can result in carbon monoxide poisoning of crew and passengers, partial or complete loss of engine power, or an aircraft fire. Exhaust system failures generally reach a maximum rate of occurrence at 100 to 200 hours of aircraft operating time. More than 50% of all exhaust system failures occur within 400 hours. Exhaust System Inspection While the type and location of exhaust system components vary somewhat with the type of aircraft, the inspection rkquirements for most reciprocating engine exhaust systems are very similar. The following paragraphs include a discussion of the most common exhaust system inspection items and procedures for all reciprocating engines. Figure 2-33 shows the primary inspection areas of three types of exhaust systems. Before the removal and installation of representative exhaust systems are discussed, a precaution to be observed when performing maintenance on any exhaust system should be mentioned. Galvanized or zinc-plated tools should never be used on the exhaust system, and exhaust system parts should never be marked with a lead pencil. The lead, zinc, or galvanized mark is absorbed by the metal of the exhaust system when heated, creating a distinct change in its molecular structure. This change softens the metal in the area of the mark, causing cracks and eventual failure. After a complete exhaust system has been installed, the air induction scoop or duct, the fuel drain lines, the cowl flaps, and all pieces of engine cowl are installed and secured. When these items have been inspected for security, the engine is operated to allow the exhaust system to heat up to ### Page 106 FIGURE2-32. Exhaust manifold installation. normal operating temperatures. The engine is then by a flat gray or a sooty black streak on the pipes shut down and the cowling removed to expose the in the area of the leak. An exhaust leak is usually exhaust system. the result of poor alignment of two mating exhaust Each clamped connection and each exhaust port system members. When a leaking exhaust conconnection should be inspected for evidence of nection is discovered, the clamps should be loosened exhaust gas leakage. An exhaust leak is indicated and the leaking units repositioned to ensure a gas- ### Page 107 A FIGURE Primary inspection (A. Sep-2-33. areas arate system; B. Crossover-type system; C. Exhaust/augmentor system.) tight fit. After repositioning, the system nuts should be re-tightened enough to eliminate any looseness without exceeding the specified torque. If tightening to the specified torque does not eliminate looseness, the bolts and nuts should be replaced, since they have probably stretched. After tightening to the specified torque, all nuts should be safetied. With the cowling removed, all necessary cleaning operations can be performed. Some exhaust units are manufactured with a plain sandblast finish. Others may have a ceramic-coated finish. Ceramic-coated stacks should be cleaned by degreasing only. They should never be cleaned with sandblast or alkali cleaners. During the inspection of an exhaust system, close attention should be given to ali external surfaces of the exhaust system for cracks, dents, or missing parts. This also applies to welds, clamps, supports, and support attachment lugs, bracing, slip joints, stack flanges, gaskets, and flexible couplings. Each bend should be examined, as well as areas adjacent to welds; and any dented areas or low spots in the system should.be inspected for thinning and pitting due to internal erosion by combustion products or accumulated moisture. An ice pick or similar pointed instrument is useful in probing suspected areas. The system should be disassembled as necessary to inspect internal baffles or diffusers. If a component of the exhaust system is inaccessible for a thorough visual inspection or is hidden by nonremovable parts, it should be removed and checked for possible leaks. This can often best be accomplished by plugging the openings of the component, applying a suitable internal pressure (approximately 2 p.s.i.), and submerging it in water. Any leaks will cause bubbles that can be readily detected. The procedures required for an installation inspection are also performed during most regular inspections. Daily inspection of the exhaust system usually consists of checking the exposed exhaust system for cracks, scaling, excessive leakage, and loose clamps. Muffler and Heat Exchanger Failures Approximately half of all muffler and heat exchanger failures can be traced to cracks or ruptures in the heat exchanger surfaces used for cabin and carburetor heat sources. Failures in the heat exchanger surface (usually in the outer wall) allow exhaust gases to escape directly into the cabin heat system. These failures, in most cases, are caused by thermal and vibration fatigue cracking in areas of stress concentration. Failure of the spot-welds which attach the heat transfer pins can result in exhaust gas leakage. In addition to a carbon monoxide hazard, failure of heat exchanger surfaces can permit exhaust gases ### Page 108 to be drawn into the engine induction system, causing engine overheating and power loss. Exhaust Manifold and Stack Failures Exhaust manifold and stack failures are usually fatigue failures at welded or clamped points; for example, stack-to-flange, stack-to-manifold, and crossover pipe or muffler connections. Although these failures are primarily fire hazards, they also present carbon monoxide problems. Exhaust gases can enter the cabin via defective or inadequate seals at firewall openings, wing strut fittings, doors, and wing root openings. Internal Muffler Failures Internal failures (baffles, diffusers, etc. ) can cause partial or complete engine power loss by restricting the flow of the exhaust gases. As opposed to other failures, erosion and carburization caused by the extreme thermal conditions are the primary causes of internal failures. Engine backfiring and combustion of unburned fuel within the exhaust system are probable contributing factors. In addition, local hot-spot areas caused by uneven exhaust gas flow can result in burning, bulging, or rupture of the outer muffler wall. Exhaust Systems with Turbocharger When a turbocharger or a turbosupercharger system is included, the engine exhaust system operates under greatly increased pressure and temperature conditions. Extra precautions should be taken in exhaust system care and maintenance. During high-pressure altitude operation, the exhaust system pressure is maintained at or near sea level values. Due to the pressure differential, any leaks in the system will allow the exhaust gases to escape with torch-like intensity that can severely damage adjacent structures. A common cause of malfunction is coke deposits (carbon buildup) in the waste gate unit causing erratic system operation. Excessive deposit buildups may cause the waste gate valve to stick in the "closed" position, causing an overboost condition. Coke deposit buildup in the turbo itself will cause a gradual loss of power in flight and low manifold pressure reading prior to takeoff. Experience has shown that periodic de-coking, or removal of carbon deposits, is necessary to maintain peak efficiency. Clean, repair, overhaul, and adjust the system components and controls in accordance with the applicable manufacturer's instructions. Augmentor Exhaust System On exhaust systems equipped with augmentor tubes, the augmentor tubes should be inspected at regular intervals for proper alignment, security of attachment, and general overall condition. Even where augmentor tubes do not contain heat exchanger surfaces, they should be inspected for cracks along with the remainder of the exhaust system. Cracks in augmentor tubes can present a fire or carbon monoxide hazard by allowing exhaust gases to enter the nacelle, wing, or cabin areas. Exhaust System Repairs It is generally recommended that exhaust stacks, mufflers, tailpipes, etc., be replaced with new or reconditioned components rather than repaired. Welded repairs to exhaust systems are complicated by the difficulty of accurately identifying the base metal so thet the proper repair materials can be selected. Changes in composition and grain structure of the original base metal further complicate the repair. However, when welded repairs are necessary, the original contours should be retained; the exhaust system alignment must not be warped or otherwise affected. Repairs or sloppy weld beads which protrude internally are not acceptable as they cause local hot-spots and may restrict exhaust gas flow. When repairing or replacing exhaust system components, the proper hardware and clamps should always be used. Steel or low-temperature, selflocking nuts should not be substituted for brass or special high-temperature locknuts used by the manufacturer. Old gaskets should never be re-used. When disassembly is necessary, gaskets should be replaced with new ones of the same type provided by the manufacturer. TURBINE ENGINE EXHAUST DUCTS The term "exhaust duct" is applied to the engine exhaust pipe, or tailpipe, which connects the turbine outlet to the jet nozzle of a non-afterburning engine. Although an afterburner might also be considered a type of exhaust duct, afterburning is a subject in itself which is discussed later in this chapter. If the engine exhaust gases could be discharged directly to the outside air in an exact axial direction at the turbine exit, an exhaust duct might not be necessary. This, however, is not practical. A larger total thrust can be obtained from the engine if the gases are discharged from the aircraft at a ### Page 109 higher velocity than is permissible at the turbine outlet. An exhaust duct is therefore added, both to collect and straighten the gas flow as it comes from the turbine and to increase the velocity of the gases before they are discharged from the exhaust nozzle at the rear of the duct. Increasing the velocity of the gases increases their momentum and increases the thrust produced. An engine exhaust duct is often referred to as the engine tailpipe; although the duct, itself, is essentially a simple, stainless steel, conical or cylindrical pipe. The assembly also includes an engine tailcone and the struts inside the duct. The tailcone and the struts add strength to the duct, impart an axial direction to the gas flow, and smooth the gas flow. Immediately aft of the turbine outlet, and usually just forward of the flange to which the exhaust duct is attached, the engine is instrumented for turbine discharge pressure. One or more pressure probes are inserted into the exhaust duct to provide adequate sampling of the exhaust gases. In large engines, it is not practical to measure the internal temperature at the turbine inlet, so the engine is often also instrumented for exhaust gas temperature at the turbine outlet. Conventional Convergent Exhaust Nozzle The rear opening of a turbine engine exhaust duct is called the exhaust nozzle (figure 2-34). The nozzle acts as an orifice, the size of which determines the density and velocity of the gases as they emerge from the engine. In most non-afterburning engines the exhaust nozzle area is quite critical. Adjusting the area of the exhaust nozzle will change both the engine performance and the exhaust gas temperature. Some engines are trimmed to their correct exhaust gas temperature by altering the exhaust nozzle area. .Exhaust nozzle When this is the case, small tabs, which may be bent as required, are provided on the exhaust duct at the nozzle opening; or small, adjustable pieces called "mice" are fastened, as needed, around the perimeter of the nozzle to change the area. Convergent-Divergent Exhaust Nozzle Whenever the engine pressure ratio is high enough to produce exhaust gas velocities which might exceed Mach 1 at the engine exhaust nozzle, more thrust can be gained by using a convergentdivergent type of nozzle (figure 2-5). The advantage of a convergent-divergent nozzle is greatest at high Mach numbers because of the resulting higher pressure ratio across the engine exhaust nozzle. To ensure that a constant weight or volume of a gas will flow past any given point after sonic velocity is reached, the rear part of a supersonic exhaust duct is enlarged to accommodate the additional weight or volume of a gas that will flow at supersonic rates. If this is not done, the nozzle will not operate efficiently. This section of the exhaust duct is known as divergent. When a divergent duct is used in combination with a conventional exhaust duct, it is called a convergentdivergent exhaust duct. In the convergent-divergent, or C-D nozzle, the convergent section is designed to handle the gases while they remain subsonic, and to deliver the gases to the throat of the nozzle just as they attain sonic velocity. The divergent section handles the gases, further increasing their velocity, after they emerge from the throat and become supersonic. TURBOPROP EXHAUST SYSTEM In a typical turboprop exhaust system, the exhaust gases are directed from the turbine section L Subsonic 1 Supersonic LGas attains sonic. vclocitv FIGURE2-34. Conventional convergent exhaust duct. FIGURE2-35. Convergent-divergent exhaust duct (nozzle). ### Page 110 of the engine to the atmosphere through a tailpipe assembly. In a typical installation the tailpipe assembly is mounted in the nacelle and is attached at its forward end to the firewall. The forward section of the tailpipe is funnel shaped and surrounds but does not contact the turbine exhaust section. This arrangement forms an annular gap which serves as an air ejector for the air surrounding the engine hot section. As the high-velocity exhaust gases enter the tailpipe, a low pressure effect is produced which causes the air around the engine hot section to flow through the annular gap into the tailpipe. An exhaust tailpipe of this type is usually manufactured in two sections (see figure 2-36). Both the forward funnel-shaped section and the rear section are made of corrosion-resistant steel, and a corrosion-resistant, high-temperature clamp secures the two sections together in a gastight joint. The mounting flange welded to the forward edge of the forward tailpipe section mates to the engine side of the firewall and is secured to it with screws. An integral bellows section permits expansion between the firewall and two fixed bearing fittings, which can be adjusted to move the tailpipe in a vertical plane. The rear section of the tailpipe is secured to the airframe by two support arms, one on each side of the tailpipe. The support arms are attached to the upper surface of the wing in such a way that free movement fore and aft is permitted to compensate for expansion. The tailpipe assembly is wrapped in an insulating blanket to shield the surrounding area from the high heat produced by the exhaust gases. Such blankets may be made of stainless steel laminated sheet on the outside and fiber glass on the inside. THRUST REVERSERS The difficult problem of stopping an aircraft after landing greatly increases with the higher airspeeds and greater gross weights, common to most of the larger, modern aircraft, which result in higher wing loadings and increased landing speeds. In many instances, wheel brakes can no longer be entirely relied upon to slow the aircraft within a reasonable distance, immediately after touchdown. The reversible pitch propeller has solved the problem for reciprocating-engine- and turboprop-powered airplanes. Commercial turbojet aircraft, however, must rely upon reversing the thrust produced by their engines. An engine thrust reverser (see figure 2-37) not only provides a ground-speed braking force, but, if suitable, is desirable for in-flight use prior to landing. Some means of slowing the airspeed and increasing the rate of sink during descent, such as a dive brake, some form of wing spoiler, or a thrust reverser that can be used while airborne, is almost a necessity for turbojet aircraft. Many forms of thrust reversers have been proposed, and quite a number have been tested with a considerable degree of success. The most successful thrust reversers can be divided into two categories, the mechanical-blockage type and the aerodynamic-blockage type. Mechanical blockage is accomplished by placing a removable obstruction in the exhaust gas stream, usually somewhat to the rear of the nozzle. The engine exhaust gases are mechanically blocked and diverted at a suitable angle in the reverse direction by an inverted cone, half-sphere, or other means of obstruction, which is placed in position to reverse the flow of exhaust gases. In the aerodynamic-blockage type of thrust reverser, thin airfoils or obstructions are placed in the gas stream, either along the length of the exhaust duct or immediately aft of the exhaust nozzle. In one adaptation of the aerodynamic reverser, vanes inside the duct create swirling of the gases in a manner which centrifuges them into a cascade of turning vanes. At least one current-model, commercial turbojet aircraft uses a combination of the mechanical-blockage and FIGURE2-35. Two-se~tion turboprop exhaust tailpipe. FIGURE2-37. Operation of the thrust reverser. ### Page 111 aerodynamic-blockage type reversers. A thrust reverser must not affect engine operation either when the reverser is operating or when it is not. It must be able to withstand high temperatures, and must be mechanically strong, relatively light in weight, reliable, and "fail-safe." When not in use, it should not add appreciably to the engine frontal area, and must be streamlined into the configuration of the engine nacelle. To satisfy the minimum braking requirements after landing, a thrust reverser should be able to produce in reverse at least 50% of the full forward thrust of which the engine is capable. The clamshell-type of mechanical-blockage reverser (figure 2-38) adequately satisfies most of these requirements and, in one form or another, has been adopted for use on non-afterburning engines. At throttle positions below idle, the reverser operates to form a turning barrier in the path of escaping exhaust gases, which, in turn, nullifies and reverses the forward thrust of the engine. Throttle positions below idle cause the engine to accelerate in controllable amounts up to full r.p.m. so that either partial or full reverse thrust may be used at will. When the reverser is not in use, the clamshell doors retract and nest neatly around the engine exhaust duct, usually forming the rear section of the engine nacelle. Most thrust reversers in use at this time are combined with an engine exhaust silencer. Engine Noise Suppressors Aircraft powered by large turbojet engines require some sort of silencing device or noise suppressor for the engine exhaust gases when operating from airports located in or near thickly populated areas. Two types of noise suppressors are used, one being a portable device, separate from the aircraft, for use on the ground by maintenance activities; it is positioned at the rear of an engine FIGURE2-38. Mechanical blockage thrust reverser. whenever prolonged engine operation is anticipated. The other type of noise suppressor is an integral, airborne part of the aircraft engine installation or engine tailpipe. Only this latter form of suppressor, which primarily suppresses engine noise during takeoff, climb, approach, and landing, will be discussed here. It is generally accepted that the amount of sound attenuation required for turbojet aircraft will be the amount necessary to moderate the engine noise to a level which will be no more objectionable than the noise produced by a reciprocating engine and propeller combination, operating under similar conditions. Although the amount of attenuation necessary is usually about 12 decibels, the manner in which the noise of a turbojet aircraft can be reduced to a level that is as acceptable as that of a reciprocating-engine aircraft is not simple to determine. The propeller, which is a major source of noise in reciprocating-engine aircraft, has a noise pattern which rises sharply to a maximum level as the plane of the propeller passes an individual on the ground, and then drops off almost as sharply after the propeller has gone by. The turbojet aircraft produces a sharp rise in noise, which reaches a peak after the aircraft has passed an individual on the ground, and is at an angle of approximately 45' to him. The noise then persists at a high level for a considerable period of time, as compared with that of a reciprocating engine with a propeller (see figure 2-39). There are three sources of noise involved in the operation of a gas turbine engine. The engine air intake and vibration from engine housing are sources of some noise, but the noise thus gener- - ated does not compare in magnitude with that produced by the engine exhaust as illustrated in figure 2-40. The noise produced by the engine exhaust is caused by the high degree of turbulence of a high-velocity jet stream moving through a relatively quiet atmosphere. For a distance of a few nozzle diameters downstream behind the engine, the velocity of the jet stream is high, and there is little mixing of the atmosphere with the jet stream. In this region, the turbulence within the high-speed jet stream is a very fine grain turbulence, and produces relatively high-frequency noise. Farther downstream, as the velocity of the jet stream slows down, the jet stream mixes with the atmosphere, and turbulence of a coarser type begins. Compared with noise from other portions of ### Page 112 -- 4 engine turbojet, 160,000 to 2W,000Ibs. transport-type aircraft. 3 eg 80 /I I I I I I -60 -20 0 20 60 100 140 Time -seconds FIGURE2-39. Noise levels. the jet stream, noise from this portion has a much lower frequency. As the energy of the jet stream finally is dissipated in large turbulent swirls, a greater portion of the energy is converted into noise. The noise generated as the exhaust gases dissipate is at a frequency near the low end of the audible range. The lower the frequency of the noise, the greater the distance that it will travel. This means that the low-frequency noises will reach an individual on the ground in greater volume than the high-frequency noises, and hence will be more objectionable. High-frequency noise is weakened more rapidly than low-frequency noise, both by distance and the interference of buildings, terrain, and atmospheric disturbances. A deepvoiced, low-frequency foghorn, for example, may be heard much farther than a shrill, high-frequency whistle, even though both may have the same overall volume (decibels) at their source. Noise levels vary with engine thrust and are proportional to the amount of "work done by the engine on the air which passes through it. An engine having relatively low airff ow but high thrust due to high turbine discharge (exhaust gas) temperature, pressure, and/or afterburning will produce a gas stream of high velocity and therefore high noise levels. A larger engine, handling more air, will be quieter at the same thrust. Thus, the noise level can be considerably reduced by operating the engine at lower power settings, and large engines operating at partial thrust will be less noisy than smaller engines operating at full thrust. Compared with a turbojet, a turbofan version of the same engine will be quieter during takeoff. The noise level produced by a fan-type engine is less, principally because the exhaust gas velocities ejected at the engine tailpipe are slower than those for a turbojet of comparative size. Fan engines require a larger turbine to provide additional power to drive the fan. The large turbine, which usually has an additional turbine stage, reduces the velocity of the gas and therefore reduces the noise produced, because exhaust gas noise is proportional to exhaust gas velocity. The exhaust from the fan, itself, is at a relatively low velocity, and therefore does not create a noise problem. Because of the characteristic of low-frequency noise to linger at a relatively high volume, effective noise reduction for a turbojet aircraft must be w Most of the noise radiates from this low frrquency turbulence region D = Nozzle diameter FIGURE240. Turbojet exhaust noise pattern. ### Page 113 achieved by revising the noise pattern or by changing the frequency of the noise emitted by the jet nozzle. The noise suppressors in current use are either of the corrugated-perimeter type, shown in figure 2-41, or the multi-tube type, shown in figure 242. Both types of suppressors break up the single, main jet exhaust stream into a number of smaller jet streams. This increases the total perimeter of the nozzle area and reduces the size of the eddies created as the gases are discharged into the open air. Although the total noise-energy remains unchanged, the frequency is raised considerably. The size of the eddies scales down, at a linear rate, with the size of FIGURE Rear view of corrugated-perimeter 2-41. noise suppressor. the exhaust stream. This has two effects. First, the change in frequency may put some of the noise above the audibility range of the human ear, and, secondly, high frequencies within the audible range, while perhaps more annoying, are more highly attenuated by atmospheric absorption than are low frequencies. Thus, the falloff in intensity is greater and the noise level is less at any given distance from the aircraft. ENGINE AIR-INLET VORTEX DESTROYER When turbojet engines are operating on the ground, an engine air-inlet vortex can sometimes FIGURE Rear view of multi-tube 2-42. noise suppressor. FIGURE Inlet vortex destroyer jet stream. 2-43. ### Page 114 form between the engine air inlet and the ground. This vortex can cause a strong suction force capable of lifting small foreign objects from the ground into the engine inlet. The ingestion of such debris can cause eng,ine damage or even failure. To minimize the ingestion of runway debris, some turbojet engines are equipped with an engine air-inlet vortex destroyer. This destroyer is a small jet stream directed downward from the lower leading edge of the noise cowl to the ground to destroy the swirling vortex base. Figure 2-43 illustrates the general direction and size of the vortex-destroying jet blast. Figure 2-44 is a diagram showing the location of the jet stream nozzle and the control valve. Bleed air from the engine is used as the vortexdestroying air stream. It is controlled by a valve located in the nose cowl. The control valve is usually a two-position valve that is opened by a landing gear safety switch. The valve closes when the aircraft leaves the runway and the weight of the aircraft is removed from the landing gear. /Jet stream nozzle FIGURE24. Location of vortex destroyer components. ### Page 115 **CHAPTER 3** **ENGINE FUEL AND FUEL METERING SYSTEMS** FUEL SYSTEM REQUIREMENTS Improvements in aircraft and engines have increased the demands on the fuel system, making it more complicated and increasing the installation, adjustment, and maintenance problems. The fuel system must supply fuel to the carburetor or other metering device under all conditions of ground and air operation. It must function properly at constantly changing altitudes and in any climate. The system should be free of tendency to vapor lock, which can result from changes in ground and in-flight climatic conditions. On small aircraft a simple gravity-feed fuel system consisting of a tank to supply fuel to the engine is often installed. On multi-engine aircraft, complex systems are necessary so that fuel can be pumped from any combination of tanks to any combination of engines. Provisions for transferring fuel from one tank to another may also be included on large aircraft. Vapor Lock Normally the fuel remains in a liquid state until it is discharged into the air stream and then instantly changes to a vapor. Under certain conditions, however, the fuel may vaporize in the lines, pumps, or other units. The vapor pockets formed by this premature vaporization restrict the fuel flow through units which are designed to handle liquids rather than gases. The resulting partial or complete interruption of the fuel flow is called vapor lock. The three general causes of vapor lock are the lowering of the pressure on the fuel, high fuel temperatures, and excessive fuel turbulence. At high altitudes, the pressure on the fuel in the tank is low. This lowers the boiling point of the fuel and causes vapor bubbles to form. This vapor trapped in the fuel may cause vapor lock in the fuel system. Transfer of heat from the engine tends to cause boiling of the fuel in the lines and the pump. This tendency is increased if the fuel in the tank is warm. High fuel temperatures often combine with low pressure to increase vapor formation. This is most apt to occur during a rapid climb on a hot day. As the aircraft climbs, the outside temperature drops, but the fuel does not lose temperature rapidly. If the fuel is warm enough at takeoff, it retains enough heat to boil easily at high altitude. The chief causes of fuel turbulence are sloshing of the fuel in the tanks, the mechanical action of the engine-driven pump, and sharp bends or rises in the fuel lines. Sloshing in the tank tends to mix air with the fuel. As this mixture passes through the lines, the trapped air separates from the fuel and forms vapor pockets at any point where there are abrupt changes in direction or steep rises. Turbulence in the fuel pump often combines with the low pressure at the pump inlet to form a vapor lock at this point. Vapor lock can become serious enough to block the fuel flow completely and stop the engine. Even small amounts of vapor in the inlet line restrict'the flow to the engine-driven pump and reduce its output pressure. To reduce the possibility of vapor lock, fuel lines are kept away from sources of heat; also, sharp bends and steep rises are avoided. In addition, the volatility of the fuel is controlled in manufacture so that it does not vaporize too readily. The major improvement in reducing vapor lock, however, is the incorporation of booster pumps in the fuel system. These pumps keep the fuel in the lines to the engine-driven pamp under pressure. The slight pressure on the fuel reduces vapor formation and aids in moving a vapor pocket along. The booster pump also releases vapor from the fuel as it passes through the pump. The vapor moves upward through the fuel in the tank and out the tank vents. To prevent the small amount of vapor which remains in the fuel from upsetting its metering action, vapor eliminators are installed in some fuel systems ahead of the metering device or are built into this unit. ### Page 116 BASIC FUEL SYSTEM The basic parts of a fuel system include tanks, booster pumps, lines, selector valves, strainers, engine-driven pumps, and pressure gages. A review of Chapter 4 in the General Handbook will provide some information concerning these components. Additional information is presented later in this chapter. Generally, there are several tanks, even in a simple system, to store the required amount of fuel. The location of these tanks depends on-both the fuel system design and the structural design of the aircraft. From each tank, a line leads to the selector valve. This valve is set from the cockpit to select the tank from which fuel is to be delivered to the engine. The booster pump forces fuel through the selector valve to the main line strainer. This filtering unit, located in the lowest part of the system, removes water and dirt from the fuel. During starting, the booster pump forces fuel through a bypass in the engine-driven pump to the metering device. Once the engine-driven pump is rotating at sufficient speed, it takes over and delivers fuel to the metering device at the specified pressure. The airframe fuel system begins with the fuel tank and ends at the engine fuel system. The engine fuel system usually includes the enginedriven pumps and the fuel metering systems. In aircraft powered with a reciprocating engine, the fuel metering system consists of the air- and fuelcontrol devices from the point where the fuel enters the first control unit until the fuel is injected into the supercharger section, intake pipe, or cylinder. For example, the fuel metering system of the Continental 10-470L engine consists of the fuel/air control unit, the injector pump, the fuel manifold valve, and the fuel discharge nozzles. On the Pratt and Whitney R-1830-94 engine, the fuel metering system consists of the carburetor, the fuel feed valve, and the carburetor accelerating pump. In the latter case, the fuel feed valve and the accelerating pump are mounted on the engine and are engine manufacturer's parts. However, they still constitute a part of the basic fuel metering system. The fuel metering system on current reciprocating engines meters the fuel at a predetermined ratio to airflow. The airflow to the engine is controlled by the carburetor or fuel/air control unit. The fuel metering system of the gas turbine engine consists of a jet fuel control and n17y extend to and include the fuel nozzles. On some .,rboprop engines a temperature datum valve 'e a .:t of the engine fuel system. The rate of fuel delivery is a function of air mass flow, compressor inlet temperature, compressor discharge pressure, r.p.m., and combustion chamber pressure. The fuel metering system must operate satisfactorily to ensure efficient engine operation as measured by power output, operating temperatures, and range of the aircraft. Because of variations in design of different fuel metering systems, the expected performance of any one piece of equipment, as well as the difficulties it can cause, will vary. **FUEL METERING DEVICES FOR RECIPROCATING** ENGINES This section explains the systems which deliver the correct mixture of fuel and air to the engine combustion chambers. In the discussion of each system, the general purpose and operating principles are stressed, with particular emphasis on the basic principles of operation. No attempt is made to give detailed operating and maintenance instructions for specific types and makes of equipment. For the specific information needed to inspect or maintain a particular installation or unit, consult the manufacturer's instructions. The basic requirement of a fuel metering system Typical F/A mixture curve -float-type carburetor Rich F/A Low Airflow in lbs/hr High /~~~ical F/A mixture curve-pressure injection carburetor Rich Lean Low Aimow in Ihs/hr Hieh FIGURE Fuel/air mixture curves. 3-1. ### Page 117 is the same, regardless of the type of system used or the model engine on which the equipment is installed. It must meter fuel proportionately to air to establish the proper fuel/air mixture ratio for the engine at all speeds and altitudes at which the engine may be operated. In the fuel/air mixture curves shown in figure 3-1, note that the basic best power and best economy fuel/air mixture requirements for all reciprocating engines are approximately the same. A second requirement of the fuel metering system is to atomize and distribute the fuel from the carburetor into the mass airflow in such a manner that the air charges going to all cylinders will hold similar amounts of fuel so that the fuel/air mixture reaching each cylinder is of the same ratio. Carburetors tend to run richer at altitude than at ground level, because of the decreased density of the airflow through the carburetor throat for a given volume of air per hour to the engine. Thus, it is necessary that a mixture control be provided to lean the mixture and compensate for this natural enrichment. Some aircraft use carburetors in which the mixture control is operated manually. Other aircraft employ carburetors which automatically lean the carburetor mixture at altitude to maintain the proper fuel/air mixture. The rich mixture requirements for an aircraft engine are established by running a power curve to determine the fuel/air mixture for obtaining maximum usable power. This curve (figure 3-2) is plotted at 100-r.p.m. intervals from idle speed to takeoff speed. Since it is necessary in the power range to add fuel to the basic fuel/air mixture requirements to keep cylinder-head temperatures in a safe range, High Brake HP Low FIGURE Power versus fuel/air mixture curve. 3-2. the fuel mixture must become gradually richer as powers above cruise are used. (See figure 3-1.) In the power range, the engine will run on a much leaner mixture, as indicated in the curves. However, on the leaner mixture, cylinder-head temperature would exceed the maximum permissible temperatures and detonation would occur. The best economy setting is established by running a series of curves through the cruise range, as shown in the graph in figure 3-3, the lo? point (auto-lean) in the curve being the fuel/air mixture where the minimum fuel per horsepower is used. In this range the engine will operate normally on slightly leaner mixtures and will obviously operate on richer mixtures than the low-point mixture. If a mixture leaner than that specified for the engine is used, the leanest cylinder of the engine is apt to backfire, because the slower burning rate of the lean mixture results in a continued burning in the cylinder when the next intake stroke starts. Fuel/Air Mixtures Gasoline and other liquid fuels will not burn at all unless they are mixed with air. If the mixture is to burn properly within the engine cylinder, the ratio of air to fuel must be kept within a certain range. It would be more accurate to state that the fuel is burned with the oxygen in the air. Seventy-eight percent of air by volume is nitrogen, which is inert and does not participate in the combustion process, and 21% is oxygen. Heat is generated by burning the mixture of gasoline and oxygen. Nitrogen and gaseous byproducts of combustion absorb this heat energy and turn it into power by expansion. The mixture proportion of fuel and air by weight is of extreme importance to' engine performance. The High Specific fuel consumption Low Lean Fuel/air mixture Rich FIGURE Specific fuel consumption curve. 3-3. ### Page 118 characteristics of a given mixture can be measured in terms of flame speed and combustion temperature. The composition of the fuel/air mixture is described by the mixture ratio. For example, a mixture with a ratio of 12 to 1 (12:l) is made up of 12 lbs. of air and 1 lb. of fuel. The ratio is expressed in weight because the volume of air varies greatly with temperature and pressure. The mixture ratio can also be expressed as a decimal. Thus, a fuel/air ratio of 12:l and a fuel/air ratio of 0.083 describe the same mixture ratio. Air and gasoline mixtures as rich as 8:l and as lean as 16:l will burn in an engine cylinder. The engine develops maximum pbwer with a mixture of approximately 12 parts of air and 1part of gasoline. From the chemist's point of view the perfect mixture for combustion of fuel and air would be 0.067 lb. of fuel to 1 lb. of air (mixture ratio of l5:l). The scientist calls this chemically correct combination a stoichiometric mixture (pronounced stay-key-o-metric) . With this mixture (given sufficient time and turbulence), all the fuel and all the oxygen in the air will be completely used in the combustion process. The stoichiometric mixture produces the highest combustion temperatures because the proportion of heat released to a mass of charge (fuel and air) is the greatest. However, the mixture is seldom used because it does not result in either the greatest economy or the greatest power for the airflow or manifold pressure. If more fuel is added to the same quantity of air charge than the amount giving a chemically perfect mixture, changes of power and temperature will occur. The combustion gas temperature will be lowered as the mixture is enriched, and the power will increase until the fuel/air ratio is approximately 0.0725. From 0.0725 fuel/air ratio to 0.080 fuel/air ratio the power will remain essentially constant even though the combustion temperature continues downward. Mixtures from 0.0725 fuel/ air ratio to 0.080 fuel/air ratio are called best power mixtures, since their use results in the greatest power for a given airflow or manifold pressure. In this fuel/air ratio range, there is no increase in the total heat released, but the weight of nitrogen and combustion products is augmented by the vapor formed with the excess fuel; thus, the working mass of the charge is increased. In addition, the extra fuel in the charge (over the stoichiometric mixture) speeds up the combustion process, which provides a favorable time factor in converting fuel energy into power. Enriching a fuel/air ratio above 0.080 results in the loss of power besides reduction of temperature, as the cooling effects of excess fuel overtake the favorable factor of increased mass. The reduced temperature and slower rate of burning lead to an increasing loss of combustion efficiency. If, with constant airflow, the mixture is leaned below 0.067 fuel/air ratio, power and temperature will decrease together. This time, the loss of power is not a liability but an asset. The purpose in leaning is to save fuel. Air is free and available in limitless quantities. The object is to obtain the required power with the least fuel flow and to let the air consumption take care of itself. A measure of the economical use of fuel is called SFC (specific fuel consumption), which is the lbs. of fuel per hr. per hp. Thus, SFC =lbs. fuel/hr./ hp. By using this ratio, the engine's use of fuel at various power settings can be compared. When leaning below 0.067 fuel/air ratio with constant airflow, even though the power diminishes, the cost in fuel to support each horsepower hour (SFC) also is lowered for a while. While the mixture charge is becoming weaker, this loss of strength occurs at a rate slower than that of the reduction of fuel flow. This favorable tendency continues until a mixture strength known as best economy is reached. With this fuel/air ratio, the required hp. is developed with the least fuel flow, or, to put it another .way, a given fuel flow produces the most power. The best economy fuel/air ratio varies somewhat with r.p.m. and other conditions, but, for cruise powers on most reciprocating engines, it is sufficiently accurate to define this range of operation as being from 0.060 to 0.065 fuel/air ratios with retard spark, and from 0.055 to 0.061 fuel/air ratios with advance spark. These are the most commonly used fuel/air ratios on aircraft where manual leaning is practiced. Below the best economical mixture strength, power and temperature continue to fall with constant airflow while the SFC increases. As the fuel/air ratio is reduced further, combustion becomes so cool and slow that power for a given manifold pressure becomes so low as to be uneconomical. The cooling effect of rich or lean mixtures results from the excess fuel or air over that needed for combustion. Internal cylinder cooling is obtained from unused fuel when fuel/air ratios above 0.067 are used. The same function is per- ### Page 119 formed by excess air when fuel/air ratios below 0.067 are used. Varying the mixture strength of the charge produces changes in the engine operating condiition affecting power, temperature, and sparktiming requirements. The best power fuel/air ratio is desirable when the greatest power from a given airflow is required. The best economy mixture results from obtaining the given power output with the least fuel flow. The fuel/air ratio which gives most efficient operation varies with engine speed and power output. In the graph showing this variation in fuel/air ratio (figure 3-l), note that the mixture is rich at both idling and high-speed operation and is lean through the cruising range. At idling speed, some air or exhaust gas is drawn into the cylinder through the exhaust port during valve overlap. The mixture which enters the cylinder through the intake port must be rich enough to compensate for this gas or additional air. At cruising power, lean mixtures save fuel and increase the range of the airplane. An engine running near full power requires a rich mixture to prevent overheating and detonation. Since the engine is operated at full power for only short periods, the high fuel consumption is not a serious matter. If an engine is operating on too lean a mixture and adjustments are made to increase the amount of fuel, the power output of the engine increases rapidly at first, then gradually until maximum power is reached. With a further increase in the amount of fuel, the power output drops gradually at first, then more rapidly as the mixture is further enriched. There are specific instructions concerning mixture ratios for each type of engine under various operating conditions. Failure to follow these instructions will result in poor performance and often in damage to the engine. Excessively rich mixtures result in loss of power and waste of fuel. With the engine operating near its maximum output, very lean mixtures will cause a loss of power and under certain conditions, serious overheating. When the engine is operated on a lean mixture, the cylinder head temperature gage should be watched closely. If the mixture is excessively lean, the engine may backfire through the induction system or stop completely. Backfire results from slow burning of the lean mixture. If the charge is still burning when the intake valve opens, it ignites the fresh mixture and the flame travels back through the combustible mixture in the induction system. CARBURETION PRINCIPLES Venturi Principles The carburetor must measure the airflow through the induction system and use this measurement to regulate the amount of fuel discharged into theairstream. The air measuring unit is the venturi, which makes use of a basic law of physics: as the velocity of a gas or liquid increases, the pressure decreases. As shown in the diagram of the simple venturi (figure 34), it is a passageway or tube in which there is a narrow portion called the throat. As the air speeds up to get through the narrow portion, its pressure drops. Note that the pressure in the throat is lower than that in any other part of the venturi. This pressure drop is proportional to the velocity and is, therefore, a measure of the airflow. The basic operating principle of most carburetors depends on the differential pressure between the inlet and the venturi throat. Application of Venturi Principle to Carburetor The carburetor is mounted on the engine so that air to the cylinders passes through the barrel, the part of the carburetor which contains the venturi. The size and shape of the venturi depends on the requirements of the engine for which the carburetor is designed. A carburetor for a high-powered engine may have one large venturi or several small ones. The air may flow either up or down the venturi, depending on the design of the engine and the carburetor. Those in which the air passes downward are known as downdraft carburetors, and those in which the air passes upward are called updraft carburetors. As air passes through the throat of the venturi, there.is an increase in velocity and a drop in pressure Venturi throat FIGURE34. Simple venturi. ### Page 120 Air can be drawn through a rubber tube by placing one end in the mouth and exerting a sucking action. Actually, the pressure inside the tube is lowered and atmospheric pressure pushes air into the open end. Air flows through the induction system in the same manner. When a piston moves toward the crankshaft on the intake stroke, the pressure in the cylinder is lowered. Air rushes through the carburetor and intake manifold to the cylinder due to the higher pressure at the carburetor intake. Even in a supercharged engine operating at high manifold pressure, there is still a low pressure at the engine side of the carburetor. Atmospheric pressure at the air intake pushes air through the carburetor to the supercharger inlet. The throttle valve is located between the venturi and the engine. Mechanical linkage connects this valve with the throttle lever in the cockpit. By means of the throttle, airflow to the cylinders is regulated and controls the power output of the engine. It is the throttle valve in your automobile carburetor which opens when you "step on the gas." Actually, more air is admitted to the engine, and the carburetor automatically supplies enough additional gasoline to maintain the correct fuel/air ratio. The throttle valve obstructs the passage of air very little when it is parallel with the flow. This is the wide-open position. Throttle action is illustrated in figure 3-5. Note how it restricts the airflow more and more as it rotates toward the closed position. Metering and Discharge of Fuel In the illustration showing the discharge of fuel into the airstream (figure 34), locate the inlet through which fuel enters the carburetor from the engine-driven pump. The float-operated needle valve regulates the flow through the inlet and, in this way, maintains the correct level in the fuel chamber. This level must be slightly below the FIGURE Throttle action. 3-5. From fuel FIGURE Fuel discharge. 3-6. ### Page 121 outlet of the discharge nozzle to prevent overflow when the engine is not running. The discharge nozzle is located in the throat of the venturi at the point where the lowest drop in pressure occurs as air passes through the carburetor to the engine cylinders. Thus, there are two different pressures acting on the fuel in the carburetora low pressure at the discharge nozzle and a higher (atmospheric) pressure in the float chamber. The higher pressure in the float chamber forces the fuel through the discharge nozzle into the airstream. If the throttle is opened wider to increase the airflow to the engine, there is a greater drop in pressure at the venturi throat. Because of the higher differential pressure, the fuel discharge increases in proportion to the increase in airflow. If the throttle is moved toward the "closed" position, the airflow and fuel flow decrease. The fuel must pass through the metering jet (figure 3-6) to reach the discharge nozzle. The size of this jet determines the rate of fuel discharge at each differential pressure. If the jet is replaced with a larger one, the fuel flow will increase, resulting in a richer mixture. If a smaller jet is installed, there will be a decrease in fuel flow and a leaner mixture. CARBURETOR SYSTEMS To provide for engine operation under various loads and at different engine speeds, each carburetor has six systems: (1) Main metering. (2) Idling. (3) Accelerating. 14) Mixture control. (5) Idle cutoff. (6) Power enrichment or economizer. Each of these systems has a definite function. It may act alone or with one or more of the others. The main metering system supplies fuel to the engine at all speeds above idling. The fuel discharged by this system is determined by the drop in pressure in the venturi throat. A separate system is necessary for idling because the main metering system is unreliable at very low engine speeds. At low speeds the throttle is nearly closed. As a result, the velocity of the air through the venturi is low and there is little drop in pressure. Consequently, the differential pressure is not sufficient to operate the main metering system, and no fuel is discharged from this system. Therefore, most carburetors have an idling system to supply fuel to the engine at low engine speeds. The accelerating system supplies extra fuel during increases in engine power. When the throttle is opened to obtain more power from the engine, the airflow through the carburetor increases. The main metering system then increases the fuel discharge. During sudden acceleration, however, the increase in airflow is so rapid that there is a slight time lag before the increase in fuel discharge is sufficient to provide the correct mixture ratio with the new airflow. By supplying extra fuel during this period, the accelerating system prevents a temporary leaning out of the mixture and gives smooth acceleration. The mixture control system determines the ratio of fuel to air in the mixture. By means of a cockpit control, the mechanic, pilot, or engineer can select the mixture ratio to suit operating conditions. In addition to these manual controls, many carburetors have automatic mixture controls so that the fuel/air ratio, once it is selected, does not change with variations in air density. This is necessary because, as the airplane climbs and the atmospheric pressure decreases, there is a corresponding decrease in the weight of air passing through the induction system. The volume, however, remains constant, and, since it is the volume of airflow which determines the pressure drop at the throat of the venturi, the carburetor tends to meter the same amount of fuel to this thin air as to the dense air at sea level. Thus, the natural tendency is for the mixture to become richer as the airplane gains altitude. The automatic mixture control prevents this by decreasing the rate of fuel discharge to compensate for the decrease in air density. The carburetor has an idle cutoff system so that the fuel can be shut off to stop the engine. This system, incorporated in the manual mixture control, stops the fuel discharge from the carburetor completely when the mixture control lever is set to the "idle cutoff" position. In any discussion of the idle cutoff system, this question usually comes up: Why is an aircraft engine stopped by shutting off the fuel rather than by turning off the ignition? To answer this question, it is necessary to examine the results of both methods. If the ignition is turned off with the carburetor still supplying fuel, fresh fuel/air mixture continues to pass through the induction system to the cylinders while the engine ij coasting to a stop. If the engine is excessively hot, this combustible mixture may be ignited by local hot spots within the combustion chambers, and the engine may keep on running or kick backward. Again, the mixture may pass out of the cylinders ### Page 122 - - unburned but be ignited in the hot exhaust manifold. More often, however, the engine will come to an apparently normal stop but have a combustible mixture in the induction passages, the cylinders, and the exhaust system. This is an unsafe condition since the engine may kick over after it has been stopped and seriously injure anyone near the propeller. On the other hand, when the engine is shut down by means of the idle cutoff system, the spark plugs continue to ignite the fuel/air mixture until the fuel discharge from the carburetor ceases. This alone should prevent the engine from coming to a stop with a combustible mixture in the cylinders. Some engine manufacturers suggest that just before the propeller stops turning, the throttle be opened wide so that the pistons can pump fresh air through the induction system, the cylinders, and the exhaust system as an added precaution against accidental kick-over. After the engine has come to a complete stop, the ignition switch is turned to the "OF'position. The power enrichment system automatically increases the richness of the mixture during highpower operation. In this way, it makes possible the variation in fuel/air ratio necessary to fit different operating conditions. Remember that at cruising speeds a lean mixture is desirable for economy reasons, while at high-power output the mixture must be rich to obtain maximum power and to aid in cooling the engine. The power enrichment system automatically brings about the necessary change in the fuel/air ratio. Essentially, it is a valve which is closed at cruising speeds and opens to supply extra fuel to the mixture during highpower operation. Although it increases the fuel flow at high power, the power enrichment system is actually a fuel saving device. Without this system, it would be necessary to operate the engine on a rich mixture over the complete power range. The mixture would then be richer than necessary at cruising speed to ensure safe operation at maximum power. The power enrichment system is sometimes called an "economizer" or a "power compensator." Although the various systems have been discussed separately, the carburetor functions as a unit. The fact that one system is in operation does not necessarily prevent another from functioning. At the same time that the main metering system is discharging fuel in proportion to the airflow, the mixture control system determines whether the resultant mixture will be rich or lean. If the throttle is suddenly opened wide, the accelerating and power enrichment systems act to add fuel to that already being discharged by the main metering system. CARBURETOR TYPES In the discussion of the basic carburetor principles, the fuel was shown stored in a float chamber and discharged from a nozzle located in the venturi throat. With a few added features to make it workable, this becomes the main metering system of the float-type carburetor.- This type of carburetor, complete with idling, accelerating, mixture control, idle cutoff, and power enrichment systems, is probably the most common of all carburetor types. However, the float-type carburetor has several distinct disadvantages. In the first place, imagine the effect that abrupt maneuvers have on the float action. In the second place, the fact that its fuel must be discharged at low pressure leads to incomplete vaporization and difficulty in discharging fuel into some types of supercharged systems. The chief disadvantage of the float carburetor, however, is its icing tendency. Since the float carburetor must discharge fuel at a point of low pressure, the discharge nozzle must be located at the venturi throat, and the throttle valve must be on the engine side of the discharge nozzle. This means that the drop in temperature due to fuel vaporization takes place within the venturi. As a result, ice readily forms in the venturi and on the throttle valve. A pressure-type carburetor discharges fuel into the airstream at a pressure well above atmospheric. This results in better vaporization and permits the discharge of fuel into the airstream on the engine side of the throttle valve. With the discharge nozzle located at this point, the drop in temperature due to fuel vaporization takes place after the air has passed the throttle valve and at a point where engine heat tends to offset it. Thus, the danger of fuel vaporization icing is practically eliminated. The effects of rapid maneuvers and rough air on the pressure-type carburetors are negligible since its fuel chambers remain filled under all operating conditions. CARBURETOR ICING There are three general classifications of carburetor icing that are common for all aircraft: (1) Fuel evaporation ice. (2) Throttle ice. (3) Impact ice. Fuel evaporation ice or refrigeration ice is formed because of the decrease in air temperature resulting from the evaporation of fuel after it is introduced ### Page 123 into the airstream. It frequently occurs in those systems where the fuel is injected into the air upstream from the carburetor throttle, as in the case of float-type carburetors. It occurs less frequently in the systems in which the fuel is injected into the air downstream from the carburetor. Engines employing spinner or impeller injection of fuel are free of this type of ice, except those that have turning vanes (to change the direction of flow) at the entrance to the impeller. In this type, ice can be deposited on the turning vanes. Refrigeration ice can be formed at carburetor air temperatures as high as 100" F. over a wide range of atmospheric humidity conditions, even at relative humidities well below 100%. Generally, fuel evaporation ice will tend to accumulate on the fuel distribution nozzle, the turning vanes, and any protuberances in the carburetor. This type of ice can lower manifold pressure, interfere with fuel flow, and affect mixture distribution. Throttle ice is formed on the rear side of the throttle, usually when the throttle is in a partially "closed" position. The rush of air across and around the throttle valve causes a low pressure on the rear side; this sets up a pressure differential across the throttle, which has a cooling effect on the fuel/air charge. Moisture freezes in this low-- pressure area and collects as ice on the low-pressure side. Throttle ice tends to accumulate in a restricted passage. The occurrence of a small amount of ice may cause a relatively large reduction in airflow and manifold pressure. A large accumulation of ice may jam the throttles and cause them to become inoperable. Throttle ice seldom occurs at temperatures above 38' F. Impact ice is formed either from water present in the atmosphere as snow, sleet, or subcooled liquid water, or from liquid water which impinges on surfaces that are at temperatures below 30' F. Because of inertia effects, impact ice collects on or near a surface that changes the direction of the airflow. This type of ice may build up on the carburetor elbow, as well as the carburetor screen and metering elements. The most dangerous impact ice is that which collects on the carburetor screen and causes a very rapid throttling of airflow and power. In general, danger from impact ice exists only when ice forms on the leading edges of the aircraft structure. Under some conditions ice may enter the carburetor in a comparatively dry state and will not adhere to the screen or walls; therefore, it will not affect engine airflow or manifold pressure. This ice may enter the carburetor and gradually build up internally in the carburetor air metering passages and affect carburetor metering characteristics. FLOAT-TYPE CARBURETORS A float-type carburetor consists essentially of a main air passage through which the engine draws its supply of air, a mechanism to control the quantity of fuel discharged in relation to the flow of air, and a means of regulating the quantity of fuel/air mixture delivered to the engine cylinders. The essential parts of a float-type carburetor are illustrated in figure 3-7. These parts are: (1) The float mechanism and its chamber. (2) The main metering system. (3) The idling system. (4) The mixture control system. (5) The accelerating system. (6) The economizer system. Float Mechanism A float chamber is provided between the fuel supply and the metering system of the carburetor. The float chamber provides a nearly constant level of fuel to the main discharge nozzle. This level is usually about 1/8 in. below the holes in the main discharge nozzle. The fuel level must be maintained slightly below the discharge nozzle outlet holes to provide the correct amount of fuel flow and to prevent fuel leakage from the nozzle when the engine is not operating. The level of fuel in the float chamber is kept nearly constant by means of a float-operated needle valve and a seat. The needle seat is usually made of bronze. The needle valve is constructed of hardened steel, or it may have a synthetic rubber section which fits the seat. With no fuel in the float chamber, the float drops toward the bottom of the chamber and allows the needle valve to open wide. As fuel is admitted from the supply line, the float rises and closes Cle valve when the fuel reaches a predetermined level. When the engine is running and fuel is being drawn out of the float chamber, the valve assumes an intermediate position so that the valve opening is just sufficient to supply the required amount of fuel and keep the level constant. With the fuel at the correct level, the discharge rate is controlled accurately by the air velocity through the carburetor and the atmospheric pressure on top of the fuel in the float chamber. A vent or small opening in the top of the float chamber allows air to enter or leave the chamber as the level ### Page 124 Fuel inlet Float chamber FIGURE3-7. A float-type carburetor. of fuel rises or falls. This vent passage is open into the engine air intake; thus, the air pressure in the chamber is always the same as that existing in the air intake. Main Metering System The main metering system supplies fuel to the engine at all speeds above idling and consists of: (1) A venturi. (2) A main metering jet. (3) A main discharge nozzle. (4) A passage leading to the idling system. (5) The throttle valve. Since the throttle valve controls the mass airflow through the carburetor venturi it must be considered a major unit in the main metering system as well as in other carburetor systems. A typical main metering system is illustrated in figure 3-8. The venturi performs three functions: (1) Proportions the fuel/air mixture, (2) decreases the pressure at the discharge nozzle, and (3) limits the airflow at full throttle. The fuel discharge nozzle is located in the carburetor barrel so that its open end is in the throat or narrowest part of the venturi. A main metering orifice, or jet, is placed in the .iirflow FIGURE3-8. Main metering system. ### Page 125 fuel passage between the float chamber and the discharge nozzle to limit the fuel flow when the throttle valve is wide open. When the engine crankshaft is revolved with the carburetor throttle open, the low pressure created in the intake manifold acts on the air passing through the carburetor barrel. Due to the difference in pressure between the atmosphere and the intake manifold, air will flow from the air intake through the carburetor barrel into the intake manifold. The volume of airflow depends upon the degree of throttle opening. As the air flows through the venturi, its velocity increases. This velocity increase creates a low pressure area in the venturi throat. The fuel discharge nozzle is exposed to this low pressure. Since the float chamber is vented to atmospheric pressure, a pressure drop across the discharge nozzle is created. It is this pressure difference, or metering force, that causes fuel to flow from the discharge nozzle. The fuel comes out of the nozzle in a fine spray, and the tiny particles of fuel in the spray quickly vaporize in the air. The metering force in most carburetors increases as the throttle opening is increased. A pressure drop of at least 0.5 in. Hg is required to raise the fuel in the discharge nozzle to a level where it will discharge into the airstream. At low engine speeds where the metering force is considerably reduced, the fuel delivery from the discharge nozzle would decrease if an air bleed (air metering jet) were not incorporated in the carburetor. The decrease in fuel flow in relation to airflow is due to two factors: (1) The fuel tends to adhere to the walls of the discharge nozzle and break off intermittently in large drops instead of forming a fine spray, and (2) a part of the metering force is required to raise the fuel level from the float chamber level to the discharge nozzle outlet. The basic principle of the air bleed can be explained by simple diagrams as shown in figure 3-9. In each case, the same degree of suction is applied to a vertical tube placed in the container of liquid. As shown in A, the suction applied on the upper end of the tube is sufficient to lift the liquid a distance of about 1-in. above the surface. If a small hole is made in the side of the tube above the surface of the liquid, as in B, and suction is applied, bubbles of air will enter the tube and the liquid will be drawn up in a continuous series of small slugs or drops. Thus, air "bleeds" into the tube and partially reduces the forces tending to retard the flow of - liquid through the tube. However, the large opening at the bottom of the tube effectively prevents any great amount of suction from being exerted on the air bleed hole or vent. Similarly, an air bleed hole which is too large in proportion to the size of the tube would reduce the suction available to lift the liquid. If the system is modified by placing a metering orifice in the bottom of the tube and air is taken in below the fuel level by means of an air bleed tube, a finely divided emulsion of air and liquid is formed in the tube, as shown in C. In a carburetor, a small air bleed is led into the fuel nozzle slightly below the fuel level. The open end of the air bleed is in the space behind the venturi wall, where the air is relatively motionless and approximately at atmospheric pressure. The FIGURE Air bleed principle. 3-9. ### Page 126 low pressure at the tip of the nozzle not only draws fuel from the float chamber but also draws air from behind the venturi. Air bled into the main metering fuel system decreases the fuel density and destroys surface tension. This results in better vaporization and control of fuel discharge, especially at lower engine speeds. The throttle, or butterfly valve, is located in the carburetor barrel near one end of the venturi. It provides a means of controlling engine speed or power output by regulating the airflow to the engine. This valve is a disk which can rotate on an axis, so that it can be turned to open or close the carburetor air passage. Where more than one throttle valve is necessary, they may be attached to the same throttle shaft or to separate shafts. In the latter case, it is necessary to check the uniformity of opening or synchronization. Idling System With the throttle valve closed at idling speeds, air velocity through the venturi is so low that it cannot draw enough fuel from the main discharge nozzle; in fact, the spray of fuel may stop altogether. However, low pressure (piston suction) exists on the engine side of the throttle valve. In order to allow the engine to idle, a fuel passageway is incorporated to discharge fuel from an opening in the low pressure area near the edge of the throttle valve. This opening is called the idling jet. With the throttle open enough so that the main discharge nozzle is operating, fuel does not flow out of the idling jet. As soon as the throttle is closed far enough to stop the spray from the main discharge nozzle, fuel flows out the idling jet. A separate air bleed, known as the idle air bleed, is included as part of the idling system. It functions the same as the main air bleed. An idle mixture adjusting device is also incorporated. A typical idling system is illustrated in figure 3-10. Mixture Control System As altitude increases, the air becomes less dense. At an altitude of 18,000 ft. the air is only half as dense as it is at sea level. This means that a cubic foot of space contains only half as much air at 18,000 ft. as at sea level. An engine cylinder full of air at 18,000 ft. contains only half as much oxygen compared to a cylinder full of air at sea level. The low pressure area created by the venturi is dependent upon air velocity rather than air density. The action of the venturi draws the same volume of fuel through the discharge nozzle at a high altitude as it does at a low altitude. Therefore, the fuel mixture becomes richer as altitude increases. This can be overcome either by a manual or an automatic mixture control. On float-type carburetors two types of purely manual or cockpit controllable devices are in general use for controlling fuel/air mixtures, the needle type and the back-suction type. The two types are illustrated in figures 3-11and 3-12. FIGURE Idling system. 3-10. Full rich FIGURE Needle-type mixture control system. 3--11. ### Page 127 FIGURE3-12. Back-suction type mixture control system. With the needle-type system, manual control is provided by a needle valve in the base of the float chamber (figure 3-11). This can be raised or lowered by adjusting a control in the cockpit. Moving the control to "rich" opens the needle valve wide, which permits the fuel to flow unrestricted to the nozzle. Moving the control to "lean" closes the valve part way and restricts the flow of fuel to the nozzle. The back-suction type mixture control system is the most widely used. In this system (figure 3-12) a certain amount of venturi low pressure acts upon the fuel in the float chamber so that it opposes the low pressure existing at the main discharge nozzle. An atmospheric line, incorporating an adjustable valve, opens into the float chamber. When the valve is completely closed, pressures on the fuel in the float chamber and at the discharge nozzle are almost equal and fuel flow is reduced to maximum lean. With the valve wide open, pressure on the fuel in the float chamber is greatest and fuel mixture is richest. Adjusting the valve to positions between these two extremes controls the mixture. The quadrant in the cockpit is usually marked "lean" near the back end and "rich" at the forward end. The extreme back position is marked "idle cutoff" and is used when stopping the engine. On float carburetors equipped with needle-type mixture control, placing the mixture control in idle cutoff seats the needle valve, thus shutting off fuel flow completely. On carburetors equipped with back-suction mixture controls, a separate idle cutoff line, leading to the extreme low pressure on the engine side of the throttle valve, is incorporated. (See the dotted line in figure 3-12.) The mixture control is so linked that when it is placed in the "idle cutoff" position, it opens another passage which leads to piston suction; when placed in other positions, the valve opens a passage leading to the atmosphere. To stop the engine with such a system, close the throttle and place the mixture in the "idle cutoff" position. Leave the throttle? closed until the engine has stopped turning over and then open the throttle completely. Accelerating System When the throttle valve is opened quickly, a large volume of air rushes through the air passage of the carburetor. However, the amount of fuel that is mixed with the air is less than normal. This is because of the slow response rate of the main metering system. As a result, after a quick opening of the throttle, the fuel/air mixture leans out momentarily. To overcome this tendency, the carburetor is equipped with a small fuel pump called an accelerating pump. A common type of accelerating system used in float carburetors is illustrated in figure 3-13. It consists of a simple piston pump operated through linkage, by the throttle control, and a line opening into the main metering system or the carburetor barrel near the venturi. When the throttle is closed, the piston moves back and fuel fills the cylinder. If the piston is pushed forward slowly, the fuel seeps past it back into the float chamber, but if pushed rapidly, it will emit a charge of fuel and enrich the mixture in the venturi. Economizer System For an engine to develop maximum power at full throttle, the fuel mixture must be richer than for cruise. The additional fuel is used for cooling the engine to prevent detonation. An economizer is essentially a valve which is closed at throttle settings below approximately 60 to 70% of rated power. This system, like the accelerating system, is operated by the throttle control. A typical economizer system, as shown in figure 3-14, consists of a needle valve which begins to open when the throttle valve reaches a predetermined point near the wide-open position. As the throttle continues to open, the needle valve is opened further and additional fuel flows through it. This additional fuel supplements the flow from the main metering jet direct to the main discharge nozzle. ### Page 128 Economizer opening automatically controlled by supercharger pressure FIGURE A pressure-operated economizer 3-15. I system. I cruising is provided by reducing the effective pres- FIGURE Accelerating system. 3-13. sure acting on the fuel level in the float compartment. With the throttle valve in cruising position, suction is applied to the float chamber through an economizer hole and back-suction economizer channel and jet. The suction thus applied to the float chamber opposes the nozzle suction applied by the venturi. Fuel flow is reduced, thus leaning the mixture for cruising economy. Another type mixture control system uses a metering valve which is free to rotate in a stationary metering sleeve. Fuel enters the main and idling systems through a slot cut in the mixture sleeve. Fuel metering is accomplished by the relative position between one edge of the slot in the hollow metering valve and one edge of the slot in the metering sleeve. Moving the mixture control to reduce the size of the slot provides a leaner mixture for metering jet / I altitude compensation. PRESSURE INJECTION CARBURETORS FIGURE A needle-valve type economizer 3-14. Pressure injection carburetors are distinctly dif- system. ferent from float-type carburetors, as they do not incorporate a vented float chamber or suction pickup A pressure-operated economizer system is shown from a discharge nozzle located in the venturi tube. in figure 3-15. This type has a sealed bellows Instead, they provide a pressurized fuel system that located in an enclosed compartment. The compartis closed from the engine fuel pump to the discharge ment is vented to engine manifold pressure. When nozzle. The venturi serves only to create pressure the manifold pressure reaches a certain value, the differentials for controlling the quantity of fuel to bellows is compressed and opens a valve in a carbuthe metering jet in proportion to airflow to theretor fuel passage, supplementing the normal quanengine.tity of fuel being discharged through the main nozzle. Typical Injection Carburetor Another type of economizer is the back-suction The injection carburetor is a hydromechanical system shown in figure 3-16. Fuel economy in device employing a closed feed system from the fuel ### Page 129 Main air bleed Back suction Main metering 1 1j; Air FIGURE Back-suction economizer system. 3-16. pump to the discharge nozzle. It meters fuel through fixed jets according to the mass airflow through the throttle body and discharges it under a positive pressure. The illustration in figure 3-17 represents a pressure-type carburetor simplified to the extent that only the basic parts are shown. Note the two small passages, one leading from the carburetor air inlet to the left side of the flexible diaphragm and the other from the venturi throat to the right side of the diaphragm. When air passes through the carburetor to the engine, the pressure on the right of the diaphragm is lowered because of the drop in pressure at the venturi throat. As a result, the diaphragm moves to the right, opening the fuel valve. Pressure from the engine-driven pump then forces fuel through the open valve to the discharge nozzle, where it sprays into the airstream. The distance the fuel valve opens is determined by the difference between the two pressures acting on the diaphragm. This difference in pressure is proportional to the airflow through the carburetor. Thus, the volume of airflow determines the rate of fuel discharge. The pressure injection carburetor is an assembly of the following units: (1) Throttle body. (2) Automatic mixture control. (3) Regulator unit. (4) Fuel control unit; some are equipped with an adapter. Throttle Body The throttle body contains the throttle valves, main venturi, boost venturi, and the impact tubes. All air entering the cylinders must flow through the ### Page 130 fuel can be added to meet the engine dkmands under all conditions. As air flows through the venturis, its velocity is increased and its pressure is decreased (Bernoulli's principle). This low pressure is vented to the lowpressure side of the air diaphragm (chamber B, figure 3-18), in the regulator assembly. The impact tubes sense carburetor inlet air pressure and direct it to the automatic mixture control, which measures the air density. From the automatic mixture control the air is directed to the high-pressure side of the air diaphragm (chamber A). The pressure differential of the two chambers acting upon the air diaphragm is known as the air metering force which opens the fuel poppet valve. The throttle body controls the airflow with the throttle valves. The throttle valves may be either rectangular or disk shaped, depending on the design of the carburetor. The valves are mounted on a shaft which is connected by linkage to the idle valve and to the throttle control in the cockpit. A throttle stop limits the travel of the throttle valve and has an FIGURE Pressure-type carburetor. adjustment which sets engine idle speed. 3-17. Regulator Unitthrottle body; therefore, it is the air control and measuring device. The airflow is measured by vol- The regulator (figure 3-18) is a diaphragm-conume and by weight so that the proper amount of trolled unit which is divided into five chambers and Sealing diaphragm Balance diaphragm valve )hragm Air diaphragm . strainer Mixture control Fuel diaphragm \ \ Fuel inlet bleeds Fuel pressure gage connection FIGURE Regulator unit. 3-18. ### Page 131 contains two regulating diaphragms and a poppet valve assembly. Chamber A is regulated air-inlet pressure from the air intake. Chamber B is boost venturi pressure. Chamber C contains metered fuel pressure controlled by the discharge nozzle or fuel feed valve. Chamber D contains unmetered fuel pressure controlled by the opening of the poppet valve. Chamber E is fuel pump pressure, controlled by the fuel pump pressure relief valve. The poppet valve assembly is connected by a stem to the two main control diaphragms. The carburetor fuel strainer, located in the inlet to chamber E, is a fine mesh screen through which all the fuel must pass as it enters chamber D. The strainer must be removed and cleaned at scheduled intervals. The purpose of the regulator unit is to regulate the fuel pressure to the inlet side of the metering jets in the fuel control unit. This pressure is automatically regulated according to the mass airflow to the engine. Referring to figure 3-18, assume that for a given airflow in lbs./hr. through the throttle body andventuri, a negative pressure of one-fourth p.s.i. is established in chamber B. This tends to move the diaphragm assembly and the poppet valve in a direction to open the poppet valve, permitting more fuel to enter chamber D. The pressure in chamber C is held constant at 5 p.s.i. (10 p.s.i. on some installations) by the discharge nozzle or impeller fuel feed valve. Therefore, the diaphragm assembly and poppet valve will move in the open direction until the pressure in chamber D is 5-1/4 p.s.i. Under these pressures, there is a balanced condition of the diaphragm assembly with a pressure drop of onefourth psi. across the jets in the fuel control unit (auto-rich or auto-lean) . Fuel Control Unit The fuel control unit (figure 3-19) is attached to the regulator assembly and contains all metering jets and valves. The idle and power enrichment valves, together with the mixture control plates, select the jet combinations for the various settings, i.e., auto-rich, auto-lean, and idle cutoff. If nozzle pressure (chamber C pressure) rises to 5-1/2 p.s.i., the diaphragm assembly balance will be upset, and the diaphragm assembly will move to open the poppet valve to establish the necessary 5-3/4 p.s.i. pressure in chamber D. Thus, the onefourth p.s.i. differential between chamber C and chamber D is re-established, and the pressure drop Manual Fuel control unit wer enrichment jet FIGURE3-19. Fuel control unit. across the metering jets will remain the same. If the fuel inlet pressure is increased or decreased, the fuel flow into chamber D will tend to increase or decrease with the pressure change, causing the chamber D pressure to do likewise. This will upset the balanced condition previously established, and the poppet valve and diaphragm assembly will respond by moving to increase or decrease the flow to re-establish the pressure at the one-fourth p.s.i. differential. When the mixture control plates are moved from auto-lean to auto-rich or vice versa, thereby selecting a different set of jets or cutting one or two in or out of the system, the fuel flow changes. However, when the mixture position is altered, the diaphragm and poppet valve assembly will reposition to maintain the established pressure differential of one-fourth p.s.i. between chambers C and D, maintaining the established differential across the jets. Under low-power settings (low airflows), the difference in pressure created by the boost venturi is not sufficient to accomplish consistent regulation of the fuel. Therefore, an idle spring, shown in figure 3-18, is incorporated in the regulator. As the poppet valve moves toward the closed position, it contacts the idle spring. The spring holds the poppet valve off its seat far enough to provide more fuel than is needed for idling. This potentially over-rich mixture is regulated by the idle valve. At idling . speed the idle valve restricts the fuel flow to the ' proper amount. At higher speeds it is withdrawn from the fuel passage and has no metering effect. Vapor vent systems are provided in these carbu- ### Page 132 retors to eliminate fuel vapor created by the fuel pump, heat in the engine compartment, and the pressure drop across the poppet valve. The vapor vent is located in the fuel inlet (chamber E) or, on some models of carburetors, in both chambers D and E. The vapor vent system operates in the following way. When air enters the chamber in which the vapor vent is installed, the air rises to the top of the chamber, displacing the fuel and lowering its level. When the fuel level has reached a predetermined position, the float (which floats in the fuel) pulls the vapor vent valve off its seat, permitting the vapor in the chamber to escape through the vapor vent seat, its connecting line, and back to the fuel tank. If the vapor vent valve sticks in a closed position or the vent line from the vapor vent to the fuel tank becomes clogged, the vapor-eliminating action will be stopped. This will cause the vapor to build up within the carburetor to the extent that vapor will pass through the metering jets with the fuel. With a given-size carburetor metering jet, the metering of vapor will reduce the quantity of fuel metered. This will cause the fuel/air mixture to lean out, usually intermittently. If the vapor vent valve sticks open or the vapor vent float becomes filled with fuel and sinks, a continuous flow of fuel and vapor occurs through the vent line. It is important to detect this condition, as the fuel flow from the carburetor to the fuel supply tank may cause an overflowing tank with resultant increased fuel consumption. To check the vent system, disconnect the vapor vent line where it attaches to the carburetor, and turn the fuel booster pump on while observing the vapor vent connection at the carburetor. Move the carburetor mixture control to auto-rich; then return it to idle cutoff. When the fuel booster pump is turned on, there should be an initial ejection of fuel and air followed by a cutoff with not more than a steady drip from the vent connection. On installations where a fixed bleed from the D chamber is connected to the vapor vent in the fuel inlet by a short external line, there should be an initial ejection of fuel and air followed by a continuing small stream of fuel. If there is no flow, the valve is sticking closed; if there is a steady flow, it is sticking open. The purpose of the fuel control unit is to meter and control the fuel flow to the discharge nozzle. The basic unit consists of three jets and four valves arranged in series, parallel, and series-parallel hookups. (See figure 3-19.) These jets and valves receive fuel under pressure from the regulator unit and then meter the fuel as it goes to the discharge nozzle. The manual mixture control valve controls the fuel flow. By using prope? size jets and regulating the pressure differential across the jets, the right amount of fuel is delivered to the discharge nozzle, giving the desired fuel/air ratio in the various power settings. It should be remembered that the inlet pressure to the jets is regulated by the regulator unit and the outlet pressure is controlled by the discharge nozzle. The jets in the basic fuel control unit are the autolean jet, the auto-rich jet, and power enrichment jet. The basic fuel flow is the fuel required to run the engine with a lean mixture and is metered by the auto-lean jet. The auto-rich jet adds enough fuel to the basic flow to give a slightly richer mixture than best power mixture when the manual mixture control is in the "auto-rich" position. The four valves in the basic fuel control unit are: (1) The idle needle valve. (2) The power enrichment valve. (3) The regulator fill valve. (4) The manual mixture control. The functions of these valves are as follows: The idle needle valve meters the fuel in the idle range only. It is a round, contoured needle valve or a cylinder valve placed in series with all other metering devices of the basic fuel control unit. The idle needle valve is connected by linkage to the throttle shaft so that it will restrict the fuel flowing at low-power settings (idle range). The manual mixture control is a rotary disk valve consisting of a round stationary disk with ports leading from the auto-lean jet, the auto-rich jet, and two smaller ventholes. Another rotating part, resembling a cloverleaf, is held against the stationary disk by spring tension and rotated over the ports in that disk by the manual mixture control lever. All ports and vents are closed in the "idle cutoff" position. In the "auto-lean" position, the ports from the auto-lean jet and the two ventholes are open. The port from the auto-rich jet remains closed in this position. In the "auto-rich" position, all ports are open. The valve plate positions are illustrated in figure 3-20. The three positions of the manual mixture control lever make it possible to select a lean mixture or a rich mixture, or to stop fuel flow ### Page 133 Full rich Channel from auto rich jet Channel from auto lean jet Auto rich @ Vent for space above valve - Idle cutoff FIGURE Manual mixture control valve 3-20. plate positions. entirely. The "idle cutoff" position is used for starting or stopping the engine. During starting, fuel is supplied by the primer. The regulator fill salve is a small poppet-type valve located in a fuel passage which supplies chamber C of the regulator unit with metered fuel pressure. In idle cutoff, the flat portion of the cam lines up with the valve stem, and a spring closes the valve. This provides a means of shutting off the fuel flow to chamber C and thus provides for a positive idle cutoff. The power enrichment valve is another poppet-type valve. It is in parallel with the auto-lean and auto-rich jets, but it is in series with the power enrichment jet. This valve starts to open at the beginning of the power range. It is opened by the unmetered fuel pressure overcoming metered fuel pressure and spring tension. The power enrichment valve continues to open wider and wider during the power range until the combined flow through the valve and the autorich jet exceeds that of the power enrichment jet. At this point the power enrichment jet takes over the metering and meters fuel throughout the power range. (5) Carburetors equipped for water injection are modified by the addition of a derichment valve and a derichment jet. The derichment valve and derichment jet are in series with each other and parallel with the power enrichment jet. The carburetor controls fuel flow by varying two basic factors. The fuel control unit, acting as a pressure-reducing valve, determines the metering pressure in response to the metering forces. The regulator unit, in effect, varies the size of the orifice - through which the metering pressure forces the fuel. It is a basic law of hydraulics that the amount of fluid that passes through an orifice varies with the size of the orifice and the pressure drop across it. The internal automatic devices and mixture control act together to determine the effective size of the metering passage through which the fuel passes. The internal devices, fixed jets, and variable power enrichment valve are not subject to direct external control. Automatic Mixture Control (AMC) The automatic mixture control unit (figure 3-21) Intake air fl' Throttle valve FIGURE3-21. Automatic mixture control and throttle body. ### Page 134 consists of a bellows assembly, calibrated needle, and seat. The purpose of the automatic mixture control is to compensate for changes in air density due to temperature and altitude changes. The automatic mixture control contains a metallic bellows, sealed at 28 in. Hg, absolute pressure, which responds to changes in pressure and temperature. In the illustration, note that the automatic mixture control is located at the carburetor air inlet. As the density of the air changes, the expansion and contraction of the bellows moves the tapered needle in the atmospheric line. At sea level, the bellows is contracted and the needle is not in the atmospheric passage. As the aircraft climbs and the atmospheric pressure decreases, the bellows expands, inserting the tapered needle farther and farther into the atmospheric passage and restricting the flow of air to chamber A of the regulator unit (figure 3-18). At the same time, air leaks slowly from chamber A to chamber B through the small bleed (often referred to as the back-suction bleed, or mixture control bleed). The rate at which air leaks through this bleed is about the same at high altitude as it is at sea level. Thus, as the tapered needle restricts the flow of air into chamber A, the pressure on the left side of the air diaphragm decreases; as a result, the poppet valve moves toward its seat, reducing the fuel flow to compensate for the decrease in air density. The automatic mixture control can be removed and cleaned, provided the lead seal at the point of adjustment is not disturbed. Adapter Unit The purpose of the adapter is to adapt the carburetor to the engine. This unit may also contain the discharge nozzle and the accelerating pump (see figure 3-22). On engines using fuel feed valves, however, the discharge nozzle is eliminated, since the fuel feed valve serves the same purpose and is built into the engine. Where a spinner injection discharge valve is used in place of'the discharge nozzle, the accelerating pump is usually housed on the side of the throttle body, and the adapter is then nothing more than a spacer and has no working parts. The discharge nozzle is a spring-loaded valve which maintains metered fuel pressure. Before fuel can pass through the discharge nozzle, enough pressure must be built up against the diaphragm to overcome the tension of the spring which is on the air side of the diaphragm. The diaphra,m then rises, lifting the attached valie, and the fuel is sprayed out the nozzle. Secured to the nozzle is a diffuser which FIGURE3-22. Adapter. is designed to improve distribution and atomization of the fuel into the airstream. There are three types of diffusers used in adapter-mounted discharge nozzles-the rake, the bar, and the bow tie. Two other diffusers built into some engines are the slinger ring used with the fuel feed valve and the spinner ring used with the spinner injection discharge valve. The acceleration pump is used to compensate for the inherent lag in fuel flow during rapid acceleration of the engine. Pressure Injection Carburetor Systems The pressure-type carburetor, like the float-type carburetor, contains a main metering system, an idling system, an accelerating system, a mixture control system (both manual and automatic), an idle cutoff system, and a power enrichment system. A schematic representation of a pressure injection carburetor, PD series, is illustrated in figure 3-23. Main Metering System Perhaps the most noticeable feature of the main metering system is the double venturi. In figure 3-23, note that the lower opening of the boost venturi is near the throat of the main one. Thus, the drop in pressure within the main venturi causes an acceleration in airflow through the boost venturi and, consequently, a still greater pressure drop at its throat. As a result, a greater differential in pressure and a greater air metering force are obtained. To make use of this metering force, the throat of the boost venturi and the inlet to the main venturi are connected to the air chambers of the carburetor. There are two of these air chambers, A and B. ### Page 135 FIGURE3-23. Schematic representation of a pressure injection carburetor, PD series. Chambers C and D are fuel chambers. The passages from chamber B to the throat of the boost venturi can be easily traced. The other passage is less direct. It leads from chamber A to the space behind the main venturi. The impact tubes vent this space to the pressure at the carburetor air inlet. In some induction systems, the pressure at the carburetor air inlet is slightly above atmospheric because of ram effect. If there is a turbo or other supercharger ahead of the carburetor, the inlet pressure is considerably higher than atmospheric under certain operating conditions. In all systems, the drop in pressure at the throat of the boost venturi is proportional to the airflow. . . This causes a lowering of the pressure in chamber B. Thus the pressures in the two air chambers differ, impact pressure in chamber A and boost venturi suction in chamber B. This pressure difference is a measure of airflow. The air-chamber diaphragm moves in the direction of the lower pressure (to the right), opening the poppet valve. This allows fuel, delivered to the carburetor inlet under pressure from the engine-driven pump, to enter chamber D. This fuel passes through the control unit to the discharge nozzle, where it forces the valve off its seat and sprays into the airstream. The pressure in the fuel discharge line is determined by setting of the nozzle. The pressure at which the engine-driven pump delivers fuel to the carburetor varies with different models. However, it is always well above the discharge nozzle setting. Chamber C is filled with fuel at the same pressure as that in the discharge line. The function of chamber C is to allow fuel to be discharged under pressure and to compensate for variations of pressures in the discharge line. The discharge nozzle acts as a relief valve to hold this pressure relatively constant regardless of the volume of fuel being discharged. This metered fuel pressure acts on the left side of the fuel diaphragm. The fuel admitted to chamber D through the poppet valve exerts pressure against the opposite side of the diaphragm. The unrnetered fuel pressure in chamber D varies with the position of the poppet valve and the rate of fuel discharge. During engine operation, it is higher than that in chamber C. Thus, there are two forces acting on the poppet valve: (1) A force on the air diaphragm (difference in pressure between chambers A and B) tending to open the valve, and (2) a force on the fuel diaphragm (difference in pressures between chambers D and C) tending to close the valve. The poppet valve moves toward the open position until the fuel pressure in chamber D is high enough to make these forces balance. This balance is reached when the fuel discharge and the airflow are in the correct proportion. ### Page 136 To prevent air in the fuel from upsetting the metering of the carburetor, there is a vapor separator at the fuel inlet. A small float and needle valve are positioned in the vapor separator chamber. When there are no vapors in the chamber, the float is raised and holds the needle valve closed. As . vapors gather, the fuel level in the chamber drops, lowering the float until the needle valve opens. The vapors then escape through the vent line to one of the fuel tanks. This action is not an opening and closing process, but one in which the needle takes an intermediate position which allows vapors to escape as fast as they gather. Idling System In the pressure-type carburetor, the fuel follows the same path at idling as it does when the main metering system is in operation. Because of the low velocity of the air through the venturi, however, the differential pressure on the air diaphragm is not sufficient to regulate the fuel flow. Instead, the idle spring in chamber D holds the poppet valve off its seat to admit fuel from the carburetor inlet during operation at idling speeds. The inset in figure 3-23 shows how the idle valve meters this fuel from chamber D to the discharge nozzle. This valve is connected to the throttle linkage in such a manner that it regulates the fuel flow only during the first few degrees of throttle opening. At higher speeds, it is withdrawn from the fuel passage and has no metering effect. Accelerating System The accelerating pump is entirely automatic in operation. In the illustration in figure 3-24, note that the vacuum passage connects the pump vacuum chamber with the air passage at the engine side of the throttle valve. The air pressure at the engine side of the throttle valve varies with the throttle position. When the throttle is nearly closed and is restricting the airflow to the engine, the pressure is low because of piston suction. In the left-hand drawing, the throttle is nearly closed, and, as a result, the low pressure in the vacuum chamber has caused the diaphragm to move to the right. This compresses the spring and stores fuel on the left side of the diaphragm. When the throttle is opened, the pressure on the engine side of the throttle valve increases. As a result, the low pressure in the vacuum chamber is lost, and the spring moves the diaphragm to the left, discharging the fuel stored during the lower throttle setting. This fuel, added to that from the main metering system, compensates for the sudden increase in airflow and gives smooth acceleration. Mixture Control System The mixture control system contains both automatic and manually operated units. The automatic mixture control varies the air pressure in chamber A to compensate for changes in air density. The manual mixture control provides for selecting the fuel/air ratio to fit engine operating conditions. \valve spring Valve diaphragmAirflow duect~on Discharge nozzlevalve Loaded Discharged FIGURE3-24. A single-diaphragm accelerating pump. ### Page 137 The automatic mixture control contains a sealed, metallic bellows, which responds to changes in pressure and temperature. As the density of the air changes, the expansion and contraction of the bellows moves the tapered needle in the atmospheric line. At sea level, the bellows is contracted and the needle is withdrawn from the atmospheric passage. Refer to the illustration showing the sea level condition in figure 3-25. As the aircraft climbs and the atmospheric pressure decreases, the bellows expands, inserting the tapered needle farther and farther into the atmospheric passage and restricting the flow of air to chamber A. At the same time, air leaks slowly from chamber A to chamber B through the small bleed. Thus, as the tapered needle restricts the flow of air into chamber A, the pressure on the left side of the air diaphragm decreases; as a result, the poppet valve moves toward its seat, reducing the fuel flow to compensate for the decrease in air density. In the illustration of the manual mixture control, figure 3-23, note that fuel enters the control unit from chamber D, passes through it, and out to the discharge nozzle. The path of the fuel through the control unit is determined by the cloverleaf valve. The movable disk of this valve is rotated from the cockpit by means of a mixture control lever, which is linked to the lever on the carburetor. The position that the movable disk takes when the cockpit control is set to auto-lean is shown in figure 3-26. Note that the large opening in the fixed disk is partially uncovered. This allows the fuel delivered to the control unit from chamber D to pass through the auto-lean metering jet, the auto-lean Automatic mixture contrrl / Impact tubes Contour needle /' Back suction line At sea level b Air diaphragm Contour needle inserted into Zeduction of pressure --in chamber A reducesa /' opening At altitude FIGURE3-25. Automatic mixture control system. ### Page 138 channel, and the opening in the fixed disk into the able disk determines which jets will meter the fuel. discharge line. The rate of flow depends on the size of the jet and the fuel pressure from chamber D. The cloverleaf valve has no metering effect since the opening in the fixed disk is larger than the metering jet. In figure 3-27 note that, in the "auto-rich" setting, the movable disk uncovers two additional openings in the fixed disk. Fuel then flows into the discharge line through both the automatic lean and the automatic rich metering jets. Thus, for each pressure, the fuel flow increases and a richer mixture is supplied to the engine. In all mixture settings, the position of the mov- IAuto rich channel - The fuel discharge then depends on the size and number of jets through which fuel can pass and on the fuel pressure as determined by the pressure drop in the carburetor venturi. In the "auto-lean" and "auto-rich" settings of the manual mixture control, the automatic mixture control varies the fuel pressure with changes in air density. Thus, during (6 auto-lean" and "auto-rich" operation, the rate of fuel discharge depends on three factors: (1) The volume of airflow through the venturi, (2) the air density, and (3) the setting of the manual mixture control. FIGURE Manual mixture control, auto-lean setting. 3-26. FIGURE Manual mixture control, auto-rich setting. 3-27. ### Page 139 Idle Cutoff System phra,~ with the automatic lean channel. Thus, To stop the engine, the manual mixture control lever in the cockpit is set to idle cutoff. This rotates the cloverleaf valve to the corresponding position. As shown in figure 3-28, the movable disk then covers all the openings and completely stops the fuel discharge. Power Enrichment System The power enrichment system is illustrated in figure 3-29. Note that the power enrichment valve is diaphragm operated. The chamber to the left of the diaphragm is connected to chamber D. A passage connects the chamber to the right of the diametered fuel pressure (the pressure at the discharge nozzle and in chamber C), acts on the right side of the diaphragm. At low engine speeds, the metered fuel pressure plus the force of the spring holds the valve on its seat. As the poppet valve opens farther at higher engine speeds, the pressure in chamber D increases. When this pressure, acting on the left side of the diaphragm, is high enough to overcome the combined forces of the spring and the metered fuel pressure, the power enrichment valve opens. Note how this increases the fuei flow to the discharge nozzle to provide a richer mixture for operation at high power output. FIGURE3-28. Manual mixture control in idle cutoff. FIGURE3-29. Power enrichment system. The text continues at https://openairdocs.org/ac/65-12.md?from=140 --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.