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 consumption. 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 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
(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 reduces 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.
assembly, the cylinder head is expanded by heating and then screwed down on the cylinder barrel which Cast aluminum head Intake valve Exhaust valve Combustion chamber Piston Connecting rod Forged steel barrel FIGURE 1-14. Cutaway view of the cylinder assembly.
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 constructed 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 +h Cylinder Heads Stainless ing 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 18
known as the right bank and the left bank, as viewed from the accessory end.
The numbering of engine cylinders is shown in figure 1-15. Note that the cylinder numbering of the opposed engine shown begins with the right rear as No. 1, and the left rear as No. 2. The one forward of No. 1 is No. 3; the one forward of No. 2, is No. 4, and so on. The numbering of opposed engine cylinders is by no means standard.
Some manufacturers number their cylinders from the rear and others from the front of the engine.
Always refer to the appropriate engine manual to determine the correct numbering system used by the manufacturer.
18 16 6 15 5 5 4 3 2 1 10 Single-row radial Double-row radial Opposed FICURE 1-15. 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. I 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 engines can usually be listed in pairs of cylinders, as each pair fires across the center main bearing.
The firing order of six-cylinder opposed engines is 1-4-5-2-3-6. The firing order of one model four-cylinder opposed engine is 1-4-2-3, but on another model it is 1-3-2-4.
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 1-3-5-2-4, 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 1-3-5-7-9-2-4-6-8.
Double-Row Radial Engines On a double-row radial engine, the firing order is somewhat complicated. The firing order is arranged with the firing impulse 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 1 to 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, subtract 5 from 8 to get 3, add 9 to 3 to get 12, subtract 5 from 12 to get 7, 19
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, begin with any number from 1 to 18 and add 11 or subtract 7. For example, beginning with 1, add 11 to get 12; 11 cannot 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 11 to 2 to get 13, and continue this process for 18 cylinders.
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
- Face Mushroom type Head Tulip type Semi-tulip type Tulip type FIGURE 1-16. 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-nickel steel. Exhaust valves are usually made of nichrome, silchrome, or cobalt-chromium 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 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 20
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.
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 VALVE-OPERATING MECHANISM stem key, close each valve and push the valve mecha- For a reciprocating engine to operate properly, nism in the opposite direction when the cam roller each valve must open at the proper time, stay open or follower rolls along a low section of the cam ring.
for the required length of time, and close at the Adjusting screw proper time. Intake valves are opened just before Rocker arm the piston reaches top dead center, and exhaust valves remain open after top dead center. At a particular Lock Roller screw instant, therefore, both valves are open at the same Valve time (end of the exhaust stroke and beginning of / spring the intake stroke). This valve-overlap permits better volumetric efficiency and lowers the cylinder - guide Valve operating temperature. This timing of the valves is - Port controlled by the valve-operating mechanism.
- Seat The valve lift (distance that the valve is lifted off its seat) and the valve duration (length of time the Push rod valve is held open) are both determined by the shape of the cam lobes.
Typical cam lobes are illustrated in figure 1-17. Return oil 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 • Tappet cam lobe to permit the rocker arm to be eased into Tappet contact with the valve tip and thus reduce the shock guide load which would otherwise occur.
Case The valve-operating mechanism consists of a cam Pressure oil ring or camshaft equipped with lobes, which work Cam roller / Cam lobe Cam ring Cam track Camshaft Cam ramps FIGURE 1-18. Valve-operating mechanism FIGURE 1-17. Typical cam lobes. (radial engine).
Tappet Camshaft Push rod Valve spring FIGURE 1-19. Valve-operating mechanism (opposed engine).
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 1-3-5-7-9-2-4-
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 impulses. Therefore, to obtain proper relation of valve operations and firing order, it is necessary to drive the cam opposite the crankshaft rotation.
Cam gear twice size 5 cylinders 7 cylinders 9 cylinders of crankshaft gear. Camshaft lobe Operates 1/2 speed.
1/8 1/10 With crankshaft.
1/4| 1/6| 1/8 Opposite crankshaft.
FIGURE 1-20. Radial engines, cam ring table.
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 ÷ by 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 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 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 crank-
(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 Camshaft Timing gear Crankshaft raan Crankshaft gear FIGURE 1-21. Cam drive mechanism 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 23
Oil pressure chamber Plunger spring Push rod socket Oil hole Oil supply chamber Push rod shroud tube Tappet body Push rod Cylinder Cam Ball check valve Plunger FIGURE 1-22. Hydraulic valve tappets.
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.
One 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 21
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 radial loads, 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 bearings 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 Roller Ball FICURE 1-23. Bearings.
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 25
propeller decreases rapidly. The general practice has been to provide reduction gearing for propellers of engines whose speeds are above 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.
(3) Spur and pinion.
The planetary reduction gear systems are used gines. 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 planetary 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 In some engines, the bell gear is mounted on the propeller shatt, 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 bevel 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 gears is mounted in a cage attached to the end of the propeller shaft.
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 design. The drive and the fixed gears are generally supported by heavy-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. Splined and flanged shafts are identified by SAE numbers.
The propeller shaft of most low-power output engines is forged as part of the crankshaft.
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 gases 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- 26
Bell gear
- Sun gear Pinion Spur-planetary Bevel planetary
• Driven gear Crankshart
• Drive gear Spur and pinion FIGURE 1-24. Reduction gears.
Intake valve Spark plug Exhaust valve to allow the burned gases to escape, and the momen- A valve in the top or head of the cylinder opens tum 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.
T.D.C. The valve allowing for the escape of the burning Combustion exhaust gases is called the exhaust valve, and the chamber valve which lets in the fresh charge of the fuel/air mixture is called the intake valve. These valves Piston are opened and closed mechanically at the proper times by the valve-operating mechanism.
Stroke The bore of a cylinder is its inside diameter. The Connecting stroke is the distance the piston moves from one end rod 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).
Cylinder B.D.C.
tlange 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 Top center 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 Crankshaft engine completes the operating cycle in one revolution of the crankshaft.
Bottom center FIGURE 1-25. 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 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.
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.