Aircraft technical Basics: TM 1-405, Aircraft Aircraft Engines, 1941: III - Description and Construction of Engine Units

SECTION III - DESCRIPTION AND CONSTRUCTION OF ENGINE UNITS

18. General. - In the construction of an internal-combustion engine, reliability of the working parts is of major importance. This generally requires the use of strong, and at times heavy materials which result in a bulky and heavy engine. The major problem in aircraft engine design is to construct the parts strong and light and still retain reliability. All moving parts must be carefully machined and balanced to reduce vibration and fatigue to a minimum, especially in engines which operate at a relatively high speed. In the construction of aircraft engines every detail must be worked out to obtain a unit which is of good design, reliable in operation, low in weight per horsepower and comparatively economical in fuel and oil consumption. The description and construction principles of the major units of an engine given in the paragraphs below apply particularly to aircraft engines.

19. Cylinders. - a. The cylinder is the most important unit of an engine and is that part in which the power is developed to accomplish work. Basically, the type of cylinder designed for aircraft engines has not changed to any great extent during the past few years and is still known as the I or overhead valve type. Structurally, there has been a vast number of detailed improvements, particularly in the air-cooled cylinder.

b. An air-cooled cylinder is made up of two major parts: a forged steel barrel and a cast aluminum alloy head. The alloy head is screwed and shrunk on the steel barrel which incorporates a base flange near its lower end for fastening it to the engine crankcase. By reference to figure 15. it may be noted that the cylinder head combustion chamber is more or less spherical in shape. This feature aids materially in suppressing detonation. Cooling fins are effectively distributed from a point near the bottom of the steel barrel to the top of the valve rocker boxes. Engine manufacturers are constantly improving fin design to increase total fin area because of high temperatures in high output engines. Provisions are made in the cylinder head for the installation of two spark plugs approximately diametrically opposite each other. A certain portion of the cylinder barrel extends down below the base flange into the crankcase when the cylinder is installed in position. This decreases the over-all diameter of the engine without sacrificing piston bearing surface and provides for a very rigid base flange attachment. In late type cylinders the inside surfaces of the barrels are processed or nitrided to obtain a glass hard wall to reduce wear. Bronze valve seat inserts are being replaced with stellite-faced steel inserts in many instances, to resist warping and pitting of the seat.

Figure: 15  Air-cooled cylinder.

c. In liquid-cooled engines several steel cylinder barrels are securely installed in a cast aluminum alloy assembly which incorporates passages and jacket space for the circulation of the coolant around the cylinders and valves. The complete unit is called a cylinder bank assembly. There is usually one bank for each row of cylinders. The cylinder base flange is part of the lower jacket assembly and although the upper jacket or head assembly appears to be detachable, once it is installed it actually becomes an integral part of the cylinder bank and is not to be removed, except when replacing worn cylinder barrels. The combustion chamber may be either spherical or roof-shaped and incorporates bronze or stellite-faced steel valve seat inserts.

20. Valves and valve springs. - The purpose of cylinder valves is to open and close the passageways to admit fresh gases into the cylinders and to expel burned gases. The poppet valve (fig. 16) is the conventional type used in aircraft cylinders.

Figure 16.-Poppet valves.

a. Generally, valves are constructed of tungsten or silchrome steel. which have the desirable quality of retaining strength at high temperatures. The head is that part of the valve which opens and closes the passageways. It has a ground circumferential face which rests on the valve seat in the cylinder when in a closed position. A thin layer of stellite fused to a stainless steel valve face greatly retards corrosion, pitting, and warping and is particularly desirable for exhaust valves in high-performance engines. By grinding the face of this type of valve at a slightly less angle than that of the valve seat (approximately 1/2 of 1°), a seat approaching a line contact is obtained which improves the life of the valve. The stem is that part of the valve which acts as a pilot for the valve head when operating. The guide for the valve stem is located in the cylinder head. Some exhaust-valve stems are hollow and filled with a salt solution, mercury, or metallic sodium to assist in reducing the operating temperature of the valve head. Exhaust valves, particularly in air-cooled engines, are constructed with heavier heads and larger diameter stems than intake valves to withstand the high temperatures at which they operate.

b. The valves are opened by cams and closed by coiled springs. In aircraft engines, two or more springs may be used on each valve. The use of multiple valve springs promotes operating safety, equalizes side pressure on the valve stem, permits a higher spring tension within a restricted area, and prevents valve-spring surging. The most important advantage of multiple valve springs is the elimination of valve spring surging. When one spring tends to vibrate at cam speeds of 1/2, 1/3, 1/4, etc.. of its natural period, the other spring or springs, having a different vibrational period, tend to "damp out" the vibration. The valve springs are compressed to position over the valve stems and securely held in place by a locking device.

21. Valve mechanism.    a. The valve mechanism of an engine consists of the parts which operate the valves, the number of parts depending upon the arrangement of the cylinders on the crankcase and the location of the valves. In aircraft engines the valves are located in the head of the cylinder to obtain maximum efficiency.

b. In single-row radial engines, the valve mechanism consists of a cam plate located in the nose section of the crankcase and driven by the crankshaft indirectly through a cam-drive gear, cam follower, push rod, and rocker arm. The cam plate, as shown in the nose section in figure 17, employs two rows, each containing four cam surfaces, one row operating the intake valves and the other row operating the exhaust valves.

(1) In double-row radial engines two cam plates are employed, one in the nose section which operates the valves of the front row of cylinders, and the other in the rear section for the rear row of cylinders. In some instances a single-row cam plate is used in place of a two-row cam plate. In such cases both valves remain open an equal number of degrees of crankshaft travel.

(2) Hardened steel cam follower rollers, running on the cam plate or ring, operate rocker arms through the medium of push rods. The rocker arms mounted in rocker boxes on the cylinder head operate the valves. Figure 18 shows the arrangement of the push-rod and rocker-arm assembly of a radial engine.

c. In V-type aircraft engines the valve mechanism consists of one or two camshaft assemblies mounted over the valves on the head of each cylinder bank.

Figure 17.- Radial engine valve mechanism.

(1) When two camshafts are used, one operates the intake valves and the other operates the exhaust valves. Both camshafts are usually driven by one common idler gear, which in turn is driven by the crankshaft indirectly through a gear train. Where each cam operates two valves, a T-shaped tappet is interposed between the cam and the two valve stems to provide a means for the cam to operate the two valves simultaneously.

FIGURE 18.-Radial engine push-rod and rocker-arm assembly.

(2) When only one camshaft is used on each cylinder bank, it is driven directly by the crankshaft through a vertical driveshaft and conventional bevel gears. Each intake and exhaust cam operates two intake and two exhaust valves through a unique rocker arm arrangement.

(3) Camshafts are generally machined from a silchrome steel forging. with the cams heat-treated to obtain a hard-wearing surface where they contact the valve tappets or rollers. Light. alloy bearings attached to the cylinder head support the camshafts and are lubricated through the hollow camshaft journals.

22. Piston assemblies. - a. The piston in an engine serves as a plunger in admitting a fuel charge into the cylinder, compressing the mixture, transmitting the work accomplished by combustion to the crankshaft and expelling the burned gases from the cylinder. Engine, pistons are constructed of a light aluminum alloy which reduces operating stresses to a minimum and permits a rapid conduction of heat away from the piston head to adjoining engine parts for radiation. This results in a comparatively low piston operating temperature. A low temperature of the piston results in a low heat transfer to the incoming charge during the intake stroke thereby increasing the volumetric efficiency of the engine and permitting a high compression ratio without detonation. Pistons are forged or die-cast under pressure, producing a strong, durable unit. Each piston is carefully machined to a uniform weight to reduce vibration to a minimum. A typical piston assembly is shown in figure 19.

(1) The piston head may be flat, convex, or concave, and in some instances recesses are machined in flathead pistons for use in high compression engines to permit the valves to open without interference. Insofar as efficiency of operation is concerned, it makes little difference which type is used. The inside of the head is usually ribbed for strength and to permit a high heat transfer to the. crankcase.

(2) There are usually four grooves in the piston head to accommodate three compression rings and one oil-control ring. The lowest of these four grooves is for the oil-control ring. It is drilled through at several points to allow surplus oil from the cylinder wall to be forced into the crankcase by the oil-control ring. A groove in the piston skirt, below the piston pin, accomodates an oil-wiper ring which assists the oil-control ring in preventing excessive oil consumption.

Figure 19. Typical piston assembly.

(a) The sidewalls, or the skirt of the piston, act as a guide or bearing surface for the head and incorporates the piston pin bosses, which are of heavy construction and usually ribbed to carry the piston pin load.

b. The purpose of piston rings is to prevent leakage of gas pressure from the combustion chamber and to reduce to a minimum the seepage of oil into the combustion chamber. The rings fit into the grooves of the piston which hold them square against the cylinder walls. The majority of piston rings are constructed of a good grade of grey cast iron. They must be capable of exerting sufficient spring pressure against the cylinder wall to perform their function with a minimum of friction. The gap in the piston ring, where it butts together when in position in the cylinder, may be diagonal step or butt cut. Only a small portion of gas seepage occurs at the piston ring gap, regardless of the shape of cut. Piston rings are concentric and are of uniform thickness (fig. 20).

Figure 20. Diagonal cut , concentric piston ring,

c. The oil-control ring is usually similar in construction to a compression ring. However, some manufacturers use two thin rings as shown in figure 21 (2). They are milled out at intervals on the lower side to permit seepage of the surplus oil from the cylinder wall to flow freely back into the crankcase through the drilled holes in the groove. An oil-wiper ring, beveled on its outside circumference is shown in figure 21(4). It is installed with the bearing edge toward the crankcase to allow he ring to wipe off surplus oil from the. cylinder wall as the piston moves on its outward stroke. Modern types of compression rings are similar to oil-wiper rings. but with only a slight bevel, as shown in figure 21(2). This design was found necessary with the advent of nitralloy cylinder barrels in order to hasten proper seating of the ring. This ring becomes more efficient as its life increases.

FIGURE 21 - Cross section of piston rings.

d. The piston pin connects the piston assembly to the connecting rod and is machined from a. nickel steel alloy forging, case-hardened, and ground. The pin is made hollow for lightness. The type used in aircraft pistons is free to move in bearings in the piston and in the small end of the connecting rod, and is known as the full floating type. It may be held in place by aluminum plugs or spring locks which prevent it from scoring the cylinder walls. Oscillating and stationary types of piston pins are used principally in automotive engines. The oscillating type is fastened securely to the small end of the connecting rod and oscillates in the piston boss. The stationary type is securely fastened in the piston boss and the small end of the connecting rod oscillates on the pin. The latter types mentioned do not have as much bearing surface as the full floating pin.

23. Connecting rods. - a. The connecting rod is that part which connects the piston assembly to the crankshaft, it transmits the power developed from combustion on the piston to the crankshaft. Connecting rods are classified according to their shape and arrangement on the crankshaft. They also vary in cross section. some being tubular and others I or H section. Three general types, classified by their bearing arrangement on the crankshaft, are the plain, the articulated and the forked and blade types. Connecting rods are usually constructed of chromium-nickel steel. The small end of the connecting rod is connected to the piston by the piston pin, and the large end is connected to the crankpin of the crankshaft.

b. The plain type of connecting rod is a single rod of conventional design and is seldom used in high output aircraft engines, because it necessitates the use of a long, heavy crankshaft. resulting in a heavy engine. Side by side mounting of plain type connecting rods is often used in small opposed type aircraft engines.

e. The articulated type of connecting rod is used extensively in radial- and V-type aircraft engines and is constructed of a master rod having one or more short rods attached to the large end with link pins. This arrangement permits the construction of a short, light-weight engine, without sacrificing reliability of operation.

d. The forked and blade type of connecting rod may also be employed on V-type engines and consists of two rods operating together on each crankpin. One rod is a blade type and the other a forked type. In some V-type engines the forked rod carries the crankpin bearing while the blade rod oscillates between the forks on the outside surface of the bearing. On other V-type engines the blade rod carries the crankpin bearing, the forked rod straddling the blade rod on the outside surface of the bearing.

Figure. 22.  -V-type engine connecting rods

24. Crankshaft assemblies. - a. The crankshaft is that part of the engine which receives the power developed on the piston and in aircraft engines, delivers it to the propeller in a rotary motion. The three most commonly used crankshafts are the 360°, 180°. and 120° types. All types are usually constructed of chrome-nickel steel. The crank journal is that part which rotates in the main bearings, and the crankarm or cheek is that part which connects the crankpin to the journal. Figure 24 illustrates the various types of crankshafts.

Figure 23. - Radial-type engine articulated connecting rod.

Figure 24.  - Various types of crankshafts..

(1) The 360°-type crankshaft is conventional for all single-row radial aircraft engines. This type of crankshaft (fig. 25) employs two counterweights, one on each crankcheek, to counteract torsional vibration. In high-powered engines, a dynamic damper or pendulum counterweight is mounted on the rear crankcheek in place of the conventional rigidly mounted counterweight. The pendulous mass is free to oscillate in a restricted arc and in the plane of rotation of the counterweight. In this action the dynamic damper has the potential ability to reduce torsional vibration to zero. Recent development inorporates two dynamic dampers of this type crankshaft to aid materially in reducing propeller stresses.

Figure 25.-A typical 360° crankshaft

(2) The 180° type of crankshaft may incorporate two or four crank throws, each throw or pair of throws arranged, around the crank journal, 180° apart. Crankshafts used in double-row radial engines employ two crank throws. The two center crank throws balance each other; therefore, only two counterweights or dynamic dampers are employed to counteract torsional vibration. Figure 26 shows a conventional 180` crankshaft for double-row radial engines.

Figure 26. - Typical 180° crankshaft

(3) The 120° type of crankshaft is conventional for 6-cylinder in-line engines and 12-cylinder V engines, in which case 6 crank throws are necessary. Late types of six-throws, 120° crankshafts employ counterweights on each crankcheek to reduce torsional vibration.

b. In geared aircraft engines, the crankshaft drives the propeller shaft through reduction gears at a predetermined ratio to improve propeller efficiency. Due to the numerous types of reduction gears employed by the various manufacturers, reference must be made to specific handbooks to obtain a complete description of the type incorporated in each engine. As a general rude, the following ratios are used : 2: 1, 3: 2, 7: 5 , 4: 3, 8:5, 16: 11. The first figure designates the engine speed and the second the propeller speed.

FIGURE 27. -- Various types of radial bearings.

25. Bearings. - c. Bearings are classified in three groups, plain, roller, and ball. The purpose of bearings is to reduce, insofar as practicable, metallic friction to a minimum. Figure 27 illustrates representative types of radial bearings.

b. Plain type bearings are generally employed in engines as main, connecting-rod, camshaft, and driveshaft bearings, because of their reliability and the ease with which they may be adjusted. Plain type bearings may be constructed in two parts, an upper and a lower half, and securely fastened together by bolts or screws; or in one piece, pressed or shrunk in position. These bearings are usually constructed of a nonferrous metal, such as copper-lead, babbitt, aluminum, brass, or bronze; or a combined nonferrous and ferrous metal, such as babbitt or copper-lead, and steel. For heavy-duty, high-speed work, such as encountered in connecting-rod and main bearings, a babbitt or copper-lead alloy is used as the bearing surface and is backed with bronze or steel for strength. For light-duty, slow-speed work, such as encountered in camshaft or driveshaft bearings, plain babbitt, bronze, brass, or aluminum is satisfactory.

FIGURE 28.--Types of thrust bearings

c. Roller bearings may be used as main bearings in the construction of radial aircraft engines, but in other type engines they are used only in accessories, such as starters and superchargers. Roller bearings can be made more adaptable for heavy loads than ball bearings due to their greater surface contact, but at a sacrifice of increased friction. However, roller bearings cause less friction than plain bearings.

d. Ball bearings are used extensively in aircraft engine construction, particularly in engine accessories, such as ignition units, generators, starters, and superchargers. Annular ball bearings are generally constructed of hardened steel balls operating between an inner and outer ball race and are assembled in such manner that the balls cannot fall out of position. Less friction occurs in ball bearings than either the plain or roller bearings. Ball bearings require less lubrication, although the highly polished ball surfaces are subjected to corrosion and pitting, especially when not in constant use.

e. In aircraft engines, annular ball bearings are used as thrust bearings. Figure 28(1) illustrates a type of ball thrust bearing (generally employed in pairs) which takes thrust in one direction only and is commonly used in supercharger drives, whereas the thrust bearing illustrated in figure 28(2) takes thrust in both directions and is commonly used on propeller driveshafts. These bearings are nonadjustable and in addition to taking thrust will support substantial radial loads.

26. Internal blowers or superchargers. - a. Internal blowers were originally incorporated in radial engines to distribute the fuel and air mixture uniformly to all cylinders, a function it still performs; however, with the advent of improved fuels, the speed ratio has been increased to such an extent that the blowers are called superchargers.

b. The blower or supercharger consists of a dynamically and statically balanced light alloy impeller operating within an aluminum alloy housing incorporating fuel charge passages from the carburetor to the intake pipes and forming part of the engine crankcase assembly. Either a vaneless or vaned diffuser type plate is used in conjunction with the impeller to convert the high velocity of the charge, caused by the high rotational speed of the impeller, to pressure before entering the intake pipes and cylinders. The impeller is mounted on a steel shaft and is driven through suitable gearing by the crankshaft. Figure 29 illustrates a vaned type diffuser plate and impeller mounted in position.

c. Due to the high speed at which an impeller is driven, it is necessary to incorporate some means of relieving the stresses in the impeller drive gears when the engine is suddenly accelerated or decelerated.

Figure 29.  -  Vaned diffuser type plate and impeller.

This is usually accomplished by a flexible coupling between the engine crankshaft and impeller gearing. This coupling, sometimes called a spring drive, relieves gear stresses because of its cushioning effect by the transmission of the rotative force to the driven gear through springs spaced concentrically around the driving shaft. In some engines a coupling using engine oil under pressure for cushioning effect is used. The oil is forced between vanes connected to the crank-shaft by means of suitable gearing which transmits the load to the drive gear of the supercharger by means of anchored weights attached to the gear.

d. The speed ratio at which the impeller is driven by the crankshaft varies according to the diameter of the impeller and the degree of supercharging desired. In general. small diameter impellers are driven at ratios as high as 14: 1 and large diameter impellers at ratios as high as 10:1. The first figure designates the impeller speed and the second, the engine speed. In addition, the large diameter impeller may have wide or narrow vanes (fig. 30).

e. Some modern high performance engines, used for high altitude flying, are equipped to operate on either a low or high blower ratio. The pilot may change from a low setting to a high setting when an altitude is reached that requires additional supercharging to maintain comparatively high manifold pressure.

Figure 30. - Impellers

27. Crankcase assemblies.   a. The crankcase of an engine forms the foundation upon which the entire engine is assembled, including the various accessories such as starters, generators, superchargers, etc. Due to the wide difference in design of the conventional radial- and V-type aircraft engines, each one is treated separately.

b. Radial engine crankcases are made up of a number of parts or sections bolted together into one compact unit. As a general rule, each section is constructed of light aluminum alloy, and in most cases, forged or die-cast for strength. There are usually five sections in a radial engine crankcase unit; the nose or front section, the main or power section, the mounting section, the supercharger section. and the accessory section. Figure 31 illustrates the various sections of a radial engine crankcase. Late type main or power sections are constructed of steel to further increase strength.

(1) The nose or front section usually incorporates the valve tappets and their guides. the crankshaft or propeller shaft thrust bearing, the propeller control valve, the drilled oil passages for the lubrication of the various operating parts, and in the geared engine, it encloses the reduction gears in addition to the cam mechanism.

(2) The main or power section to which the cylinders are attached, is usually made of two symmetrical forgings joined in the plane of the center line of the cylinders by long through bolts. The main crankshaft bearings and the cam drive gear assembly are located in this section.

(3) The mounting section is located immediately behind the main section, incorporating the mounting lugs which provide for the attachment of the engine to the mounting ring. This section usually forms the front wall of the supercharger diffuser and distributor chamber and carries the tangential ports of the intake pipes leading to the cylinders.

(4) The supercharger section usually carries the supercharger diffuser plate, carburetor, fuel pump, gun synchronizers, and vacuum pump. It also acts as a housing for the accessory drive gears.

(5) The accessory section usually forms the rear crankcase cover to which most of the engine accessories, such as magnetos, generator, and starter are attached.

c. V-type engine crankcases are made up of two parts, consisting of an upper and lower half. The upper half forms the foundation upon which the parts of the engine are assembled and is constructed of light aluminum alloy, forged or die-cast for rigidity and strength. This section incorporates the crankshaft main bearings and incloses the crankshaft, thrust bearings, connecting rods and, in geared engines, the reduction gears. The lower section, or oil pan, bolts to the upper half to form an oiltight compartment around the crankshaft and connecting-rod assembly.

d. The increase in the number of accessories attached to the engine has led to the development of a remote engine driven gear box. This gear box may be placed in a convenient position close to the engine and used to drive the many accessories which now appear to "clutter up" the rear of an engine installation.

28. Intake manifolds. - a. Intake manifolds and induction pipes are the parts of an engine which distribute the fuel and air charge to the various cylinders and provide a chamber for the fuel to vaporize and thoroughly mix with the air before being admitted into the cylinder. The design and number of intake manifolds or pipes naturally depends upon the type of the engine and the number of cylinders to which they are attached.

FIGURE 31.-Typical radial engine crankcase.

b. In radial engines, individual intake pipes connect the super-charger or blower section chamber to the intake ports of the cylinders and are constructed of a thin light alloy or sheet steel. By reference to the intake pipe (fig. 32(1)), it will be noted that the cylinder end is shaped in a symmetrical curve to provide a minimum of restriction to the flow of the incoming fuel charge. The lower end of the intake pipe is inserted into the tangential ports of the blower section and made gastight by the use of a rubber ring and a locking nut. This arrangement permits the intake pipe to move with the elongation and contraction of the cylinder, thus preventing distortion.

Figure 32. - Radial engine intake pipe and liquid-cooled engine manifold.

c. Manifolds are employed to conduct and distribute the mixture charge in V-type and in-line engines. These manifolds are made of cast aluminum alloy and conduct the fuel-air mixture from the carburetor or supercharger outlet to the cylinders, where it is divided at each intake port. Two or more manifolds are generally used, each cast to accommodate groups of three cylinders as illustrated in figure 32(2). Suitable gaskets and hose connections are employed to assure gastight connections. A jacket may be provided in some manifold installations to allow either the coolant or the lubricating oil to circulate around the intake passages. The heat given off in either case aids in the vaporization of the fuel charge.

29. Exhaust manifolds.  a. The primary function of the exhaust manifold is to conduct, with a minimum of back pressure, exhaust gases, hot carbon flakes. etc.. into the slipstream with minimum hazard to the airplane and pilot. Some of its additional functions may be the supplying of heat to the induction system and to the cockpit or cabin heater for use in cold weather.

b. The simplest exhaust piping is made up of a short steel tube extending slightly rearward from each exhaust port. This minimizes back pressure and exhaust valve temperatures. The disadvantages in the use of individual exhaust stacks are:

(1) More exhaust noise.

(2) Elimination of means for transmitting heat to the induction system and other auxiliary devices.

(3) Increased fire hazard.

(4) Danger of sudden cooling of exhaust valves during side slip maneuvers of the airplane.

(5) Failure to conduct injurious gases away from the airplane.

Figure 33.  - Front ring exhaust manifold installation.

FIGURE 34. - Rear-type exhaust manifold

c. In some radial engines, stainless steel exhaust manifolds of the tangential type are used (figs. 33 and 34). These types are generally known as collector rings, and collect and conduct exhaust gases wherever desired. These collector rings may be installed on the front side of the cylinders or on the rear side, as desired.

(1) The front type consists of a large collector ring installed over he nose section of the engine into which the exhaust gases of each cylinder are expelled at a tangent through individual connecting pipes. An expanding joint is provided in each pipe to allow for expansion differences without distortion. A large common outlet pipe, streamlined downward and backward, forms an integral part of the ring. The front type of manifolding lends itself readily to supplying heat to the induction system, cockpit, and is also adaptable for use with an external exhaust driven supercharger installation.

(2) Minor variations from the front-type ring exhaust manifold permit installation on the rear of an engine, usually with the outlet opening extending out through the side or bottom of the engine cowling. This location permits better cooling of the cylinders, especially on high-powered engines.

d. In V-type aircraft engines two stainless steel exhaust manifolds are used, one attached to each cylinder bank. Individual openings attached to each exhaust port lead the exhaust gases to a common outlet chamber which becomes larger in diameter as it tapers back and out toward its outlet opening (fig. 35). For the installation of an exhaust-driven supercharger the two outlet openings of the exhaust manifolds on a V-type engine are joined together and the exhaust gases directed through one common outlet into the supercharger nozzle box.

Figure 35. - V-type engine exhaust manifold

30. Coolant pumps. a. Coolant pumps for liquid-cooled engines are of the centrifugal type. This type is used in preference to the plunger, vane, and gear types, because it has high capacity at low pressure. The centrifugal-type pump consists of a light alloy flanged circular plate, an impeller, and a light alloy casting or housing. The impeller is installed with a minimum working clearance for obvious reasons. It is driven by the crankshaft through suitable gearing and forces the inflowing liquid outward through the tangential housing openings and attached manifolds into the cooling jackets. Figure 36 illustrates a typical centrifugal-type pump.

b. Packing glands are provided in the pump housing surrounding the driveshaft to prevent leakage of the coolant. This packing is manufactured of a suitable material and impregnated with graphite to minimize wear on the driveshaft The packing gland nut is adjustable to take up wear of the packing material whenever required.

Figure 36. - Centrifugal-type coolant pump.

c. The capacity of the coolant pump depends upon its size and the speed at winch the impeller is driven, usually 100 gallons per minute being the minimum rate.

31. Oil pumps and relief valves, - a. All aircraft engine lubricating systems require oil pumps to circulate oil under pressure to the various working parts and to scavenge the engine of surplus oil. For this purpose the gear type of pump (fig. 37) is generally used. In this type a multiple of steel gears are contained in one housing for a pressure pump and either one or two scavenging pumps. When only one scavenging pump is used it must incorporate larger sized gears than those in the pressure pump to insure positive scavenging of the oil from the crankcase. Each pump or set of gears is assembled in independent recesses of the unit but on the same shaft, in order to permit one shaft to drive both pumps simultaneously. The pumps are driven by the crankshaft through suitable gearing.

b. To regulate the pressure of the oil circulated to the various working parts of the engine, a relief valve is incorporated on the discharge side of the pressure pump. In some instances provisions are made to install the relief valve in the oil-pump assembly. As a general rule, oil pressure relief valves may be regulated by increasing or decreasing the tension of a spring acting on a valve. This is accomplished by turning an adjustable screw provided in the assembly. Figure 38 shows an oil pump in which provisions are made to install a relief valve. In instances where oil pressure is required to operate hydraulic-controlled propellers, the relief valve is usually located at the end of the main pressure line. This insures positive pressure to the propeller before the relief valve functions.

In some engines provisions are made to drive an additional oil pump for the operation of hydraulic systems. This pump is of the gear type similar in construction to a conventional oil-pressure pump.

32. Fuel pumps. a. Most conventional aircraft fuel systems incorporate a fuel pump driven directly or indirectly by the engine crankshaft. Early type engines used the two-gear type pump similar in construction to the oil-pressure shown in figure 37. However, due to the low viscosity of gasoline and the high column of fuel in long fuel lines which must be delivered at positive pressure and high capacity to the carburetor, the vaned-type pump (fig. 39) is considered more efficient than other types at the same operating speed. Fuel pumps are usually driven at crankshaft speed.

Figure 37. -  Typical gear pump.

b. The vaned-type pump consists of a cylindrical aluminum body in which a steel liner has been inserted to minimize wear. The sliding vanes, sleeve, and shaft assembly fit into the steel liner eccentrically so that when rotated they cause a suction at the intake side or port of the pump body. Provision is made at the drive end of the pump to prevent fuel leakage around the shaft by an adjustable lock screw and cork gland, or a special automatic spring take-up metal disk which does not require the use of packing material. In case of leakage, a drain is incorporated in the pump whereby the fuel may be directed overboard.

Figure 38 .- Typical oil pump.

c. Most modern fuel pump assemblies incorporate a relief and/or bypass valve. With this assembly, the use of a material number of plumbing units is eliminated. The fuel pump drive and mounting flange is usually installed in the rear, or accessory drive section, however, some V-type engines it is located in the vertical camshaft driveshaft housing. The pump may be mounted directly to the mounting flange or it may be conveniently located in the aircraft and operated by a remote flexible driveshaft assembly.

33. Vacuum pumps. - a. Due to the increased use of gyro instruments which require suction or subatmospheric pressure in their operation and to the unreliability and inadequate capacity of venturi tubes, an engine-driven vacuum pump has been designed to obtain uniform and adequate pressures. By the use of a suitable cock, either the venturies or the vacuum pump may be used independently as a safety factor in case trouble develops in either system.

FIGURE 39. - Vaned-type fuel pump.

b. The conventional vacuum pump (fig. 40) is of the vaned type. It consists of a cylindrical aluminum body in which a steel liner is inserted to minimize wear. The sliding vanes, sleeve, and shaft assembly fit into the steel liner eccentrically in such a manner that when rotated they cause a suction at the intake side, or port. and a pressure at the exhaust side, or port, of the pump body. It will be noted from the above description that a vaned-type vacuum pinup is almost identical with a vaned-type fuel pump, except in external appearance. An adjustable spring loaded relief valve is incorporated in the suction line between the pump and gyro instruments and may be regulated to furnish the proper amount of subatmospheric pressure to the various units. A restricted orifice in the pump body is connected to the pressure side of the engine oiling system to furnish proper lubrication.

Figure: 40. --- Vaned type vacuum pump and relief valve.

c. In order to provide a satisfactory means of exhausting the waste oil from the vacuum pump, either one of four methods may be employed. In some cases the waste oil is conducted into the slipstream through suitable piping with the air exhaust from the pump. Another method is to lead the waste oil and air to the carburetor air intake, and a third is to connect the pump exhaust line to the exhaust manifold of the engine where the exhaust pressure is less than 1 inch Hg. In the fourth method, the oil and air exhaust passes into an oil separator which permits the air to flow out into the atmosphere and the oil to flow back into the tank or engine.

d. The various types of engines used and the availability of a suitable drive for the vacuum pump necessitate several designs of mounting flanges and pump driveshaft ends. On V-type engines, the pump is usually mounted on the magneto driveshaft housing especially designed for this purpose. All late-type radial engines incorporate a special mount built on the rear crankcase section for installing and driving the pump. AlI types of vaned pumps are designed to operate in either direction, provided the intake and exhaust ports are properly connected.

34. Engine accessories. - The engine accessories, such as starters, generators, carburetors, magnetos, distributors, spark plugs, and ignition wiring, are of such importance as to require a separate manual for their description and operation.