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 O-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 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 1-4. Radial engine.
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.
FIGURE 1-3. Opposed engine. In the head of each cylinder are the valves and spark 5
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 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 Every internal combustion engine must have certain basic the chamber in which the parts in order to change heat into mechanical energy. /fuel is compressed and burned.
into the cylinder. An intake valve is needed to let the fuel/air to let the exhaust gases out. An exhaust valve is needed The piston, moving within the cylinder, forms one of the walls of the combustion chamber. The connecting rod forms a link The piston has rings which seal the gases between the piston and the in the cylinder, preventing any loss ot crankshaft.
power around the sides of the piston.
The crankshaft and connecting rod change the straight line motion of the piston to a rotary turning motion. The crankshaft in an aircraft engine also absorbs the power or work from all the cylinders and transfers it to the propeller.
FIGURE 1-5. Basic parts of a reciprocating engine.
Crankcase Sections 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 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 6
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 twosection main crankcase.
• Cylinder Supercharger Piston housing Crankcase front main Crankcase section ' rear main section Front Rear ВОЗОВ® Propeller reduction gears | Cam drum Connecting assembly rod Crankshaft Impeller Diffuser plate Supercharger rear housing cover
NOSE POWER SUPERCHARGER ACCESSORY
SECTION SECTION SECTION SECTION FIGURE 1-6. Engine sections.
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 the construction 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
nose section.
se crankshaft are used.
le engine. < ce c- 02 ce rs re or 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.
nes equip pe with two-piece master rod The inner portion of the cylinder pads are somelid-type crankshaft, the main or ower times chamfered or tapered to permit the installasection may be solid, usually of aluminum tion of a large rubber O-ring around the cylinder front end of this section is open when skirt, which effectively seals the joint between the the diaphragm plate in which the front main bearing cylinder and the crankcase pads against oil leakage.
is mounted is removed. The knuckle pins may be Because oil is thrown about the crankcase, 8
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 which 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 aircratt 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.
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 Starter Crankcase (left half) Magneto 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 Prop shaft Connecting rod Crankshaft Cylinder Camshaft Tachometer Generator Accessory case assembly Crankcase (right half) Induction system Oil pump Oil sump FIGURE 1-7. Typical opposed engine exploded into component assemblies.
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 Journal 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.
180° 120° 12(° 2,5 1,6 20 FIGURE 1-8. Solid types of crankshafts.
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.
No 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 11
Crankpin Journal Crankcheck Damping Counterweight weights FIGURE 1-9. 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 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.
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 the pendulum moves and its vibrating frequency correspond to the frequency of the power impulses of 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. The 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 connecting 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 12
Pendulum
- length 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.
FIGURE 1-10. 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 1-12).
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- 13
Blade rod Fork rod
SOLID TYPE FORK AND BLADE ROD
MASTER ROD Articulating rod Knuckle pin lock plate Knuckle pin Piston pin end Bronze bushing Shank - Bearing shells lined with bearing material Crimp or pinch Connecting rod bolts Cap
SPLIT TYPE MASTER ROD
PLAIN ROD FIGURE 1-11. Connecting rod assemblies.
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. This force is transmitted to the crankshaft through the connecting rod.
On the return upward stroke, the piston forces the exhaust gases from the cylinder.
FIGURE 1-12. 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 V-type engines. The forked rod is split at the 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 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.
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 15
Compression rings Oil control rings Piston pin piston pin boss Aluminum plug Oil scraper ring Piston Slipper type Trunk type nAsr Flat head FICURE 1-13. Piston assembly and types of pistons.
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 rings. 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 16