Basic Piston Engine
A piston (or reciprocating) engine is a device for converting the heat energy of a fuel into mechanical energy, by internal combustion. The principles which govern the relationship between pressure, temperature. and volume. in a gas are stated in the laws of boyles' and Charles', and these principles are applicable to the operation of a piston engine.
In a piston engine, a fuel/air mixture is drawn or forced into a cylinder, compressed and ignited, thus increasing temperature and pressure; acts on a piston and forces it down in the cylinder. The linear movement of the piston is converted into rotary movement by the engine mechanism. Piston engines are designed to operate on a 2- stroke or 4- stroke cycle, but since the vast majority of aircraft engines operate according to the 4 stroke or "otto" cycle which is named after in inventor.
Principle of operation
The movement of the piston from its highest to its lowest position in a cylinder is known as “stroke” and corresponds to one half of a revolution of its crankshaft. Two upward and two downward strokes make up the complete cycle, and purpose of each stroke is as follows:-
When the piston is at the top of its stroke, an inlet valve in the cylinder head is opened, and as the piston travels down to the bottom of its stroke, the combustible mixture of fuel and air is drawn into the cylinder. The valve closes when a piston reaches the bottom of the stroke.
As the piston travels up to the top of its stroke both the inlet valves and the exhaust valve are closed and the combustible gas is compressed in the cylinder.
As the piston commences its second downward stroke the combustible mixture is electrically ignited (by means of a magneto and sparking plug) and gas expands thus building up pressure and forcing the piston down.
The exhaust valve in the cylinder head now opens, and as the piston continue its second upward stroke the burnt gases are forced out through the exhaust port to atmosphere. At the completion of this stroke exhaust valve is closed.
WORKING OF A PISTION ENGINE
The theoretical 4- stroke cycle is very inefficient, for several reasons and must be modified, to produce acceptable power. The main factors which necessitate these modifications are inertia of the gases, burning rate of fuel/air mixture and the ineffective crank angle, the last being defined as angular position of the crank shaft when, for large angular movement of the crankshaft at both ends of the stroke; the linear movement of the piston is small. Ideally best power will be produced by varying the valve timing (i.e. the times at which the valves open and close in relation to the crankshaft position) according to the rotational speed of the engine, but the mechanism necessary would result in such increased weight and complication that the valves of an a/c engine are usually timed to provide the greatest efficiency at cruising speed. The actual timing of the valves on a particular engine is often illustrated in the form of a dig known as 'valve timing diagram'.
VALVE TIMING DIAGRAM
The terms Top Dead Centre (T.D.C.) and Bottom Dead Centre (B.D.C.) are used to define the positions of the crankshaft when piston is exactly at the top or bottom of its stroke respectively. For the induction stroke, the opening of the inlet valve is initiated before T.D.C. to ensure that it is partially open when the piston commences its downward stroke, so reducing the lag between the piston and the gases.
The inlet valve closes after B.D.C. to take advantage of the inertia of the incoming, gases and fill the cylinder as completely as possible. Movement of the piston for a short period after B.D.C. is in sufficient to oppose the incoming gases before the valve closes. Although the fuel/air mixture burns quickly, combustion is not instantaneous. The ignition is therefore arranged to occur before T.D.C. at the end of the compression stroke, so that maximum pressure is achieved shortly after T.D.C. on the power stroke.
The exhaust valve opens before B.D.C. on the power stroke when most of the expansion due to combustion has taken place, and further useful work is limited by the ineffective crank angle residual gas pressure initiates scavenging of the burnt gases through the exhaust port.
The exhaust valve closes after T.D.C. to make use of the inertia of the outgoing gases to completely scavenge the cylinder and to assist in overcoming the inertia of the incoming gases. The number of degrees of crankshaft movement by which valve opening precedes B.D.C. or T.D.C. is known as “valve lead”, and the number of degrees of crankshaft movement by which valve opening follows BDC or TDC is known as “Valve lag”. The period when both inlet and outlet valve are open together is known as Valve OverLap.
There are different arrangement of cylinders according to the requirements. Air cooled in line horizontally opposed and radial engines, are all widely used on civil a/c because of their general reliability and economy. Liquid cooled, Veeengines were widely used on military a/c because of their high power output and low frontal area but are rarely found on civil a/c.
A radial engine has an odd no of cylinders usually not more than nine arranged radially around the crank case. If greater power is required, two banks of cylinders are used, each cylinder in the rear row being located midway between two front row cylinders to ensure adequate cooling. The crankshaft of the radial engine has only one throw for each bank of cylinders, and all the connecting rods are attached to the single crankpin via a master rod. This fact also dictates the firing order of the engine. On a seven cylinder engine a firing stroke is required every of crankshaft movement, and since the angle between cylinders is the firing order can only be alternate cylinders in the direction of rotation i.e. 1,3,5,7,2,4,6. To balance the heavy mass of the master rod assembly, counter weight and fitted to the crankshaft, and it is also usual to fit vibration dampers to minimize the effects of any residual vibration. On engine with two banks of cylinders, the crankshaft throws are arranged at 1800 to each other.
(a) Except for sleeve valve engines, the valves are operated by a cam drum which is concentric with and driven by the crankshaft. The cam drum has two rows of cams, one for the inlet valves and one for the exhaust valves. On seven cylinder and nine cylinder engines , there are four equally spaced cams in each row, and the drum rotates at 1/8 engine speed ; on three cylinders and five cylinder engines, two equally spaced cams on each row, with the drum rotating at 1/4 engine speed would be suitable.
(b) Taking a seven cylinder radial engine as example, when the inlet valve on No.1 cylinder is open, the next inlet valve to open is on No 3 cylinder ( Since this is the next cylinder in the firing order ).The cams are 900 apart and the drum must therefore rotate through an angle of (12 x 6/7)0 [the angle between No1 & No.3 cylinder is (102 6/7)0 ] in the direction of rotation to open the required value on No.3 cylinder speed of rotation of the cam drum must be 812 ¸ = engine speed ( operation of cam drum on seven cylinder engine is shown in fig.4)
On nine cylinder engine the spacing of the cylinders is at 400 and successive valves open at every 800 of crankshaft movement. Since the cams are 900 apart, the cam drum must rotate in the opposite direction of rotation to the crankshaft, but still at 1/ 8 engine speed [(90-80]/80] =1/8
In line engines usually have four or six cylinders arranged in an upright or inverted row along the crankcase it is not usual to have more than six cylinders, because of the difficulty of cooling the rear cylinders and the length of the crankshaft which would be required. In a four cylinder engine, four power strokes occur every two revolutions of the crankshaft, and must be evenly spaced to provide smooth running with the firing order of 1,3,4,2 or 1,2,4,3. The camshaft which is a shaft having a cam for each valve in the engine, would be driven from the crankshaft at half engine speed and would operate the valves by means of push rods, and rockers. Each of the eight cams (two to each cylinders) would be located on the cam shaft to open and close an inlet or exhaust valve in relation to the particular firing order and valve timing prescribed for that engine. If the engine had six cylinders, there would be six power strokes every two revolutions of the crankshaft and a cylinder would have to fire every 1200 of the crankshaft movement. This would necessitate a crankshaft with throws (i.e. the offset portions of the crankshaft containing the crankpins) arranged accordingly. Suitably arranged cams would be provided. On the camshaft, which would still be driven at half the engine speed. The firing orders of six cylinder engine is generally 1,4,2,6,3,5 but a different order could be used and the crankshaft would be arranged differently.
Horizontally Opposed Engines
The cylinders of a horizontally opposed engine (usually four or six) are arranged in horizontal banks on opposite sides of crankcase. Most engines have individual connecting rods operating on separate crankpins, thus the cylinders are staggered as shown in fig.3(c). A single camshaft is located either above or below the crankshaft, and is driven at half engine speed to operate the valves in both banks of cylinders. On some engines the inlet valve cams are shared by opposite cylinders, so that the camshaft of six cylinder engine may have a total of 9 cams, six separate exhaust cams and three shared inlet cams. To minimize the length of the engine, a four cylinder engine may have three main (crankshaft) bearing and a six cylinder engine may have four. Because six firing strokes occur every two revolutions of crankshaft of a six cylinder engine, the throws of the crankshaft must be arranged at 1200 to each other. In the six cylinder engine shown in fig.3(d) the firing order would normally be 1,4,5,2,3,6, but different firing orders would be possible on engines with different crankshaft and cam arrangements.
The V-type engine has the cylinders arranged on the crankcase in two rows or banks forming the letter V with an angle between the banks of 90, 60 or 45 degrees. There is always an even number of cylinders in each row.
Since the two banks of cylinders are opposite each other, two sets of connecting rods can operate on same crankpin, thereby reducing the weight as compared with the in-line engine. The frontal area is only slightly greater that of the in-line type, here the engine cowling can be streamlined to reduce drag. If the cylinders are above the crankshaft the engine is known as upright V-type, but if the cylinders are below the crankshaft, it is known as inverted V-type. Better pilot visibility and a short 1/a are possible if the engine is inverted.
ENGINE CYLINDER ARRANGEMENT
Otto cycle is the standard cycle for internal combustion engine (spark ignition (SI)). The sequence of process in the elementary operation of the S.I. engines is given below. With reference to fig. below where, the sketches of the engine and indicator diagram are given :-
Process 1-2 : Intake
The inlet valve is open, the piston moves to right admitting fuel-air mixture into the cylinder at constant pressure.
Process 2-3 : Compression
Both the valves are closed, the piston compresses the combustion to the minimum volume.
Process 3-4 : Combustion
The mixture is then ignited by means of a spark, combustion takes place and there is an increase in temperature and pressure.
Process 4-5 : Expansion
The products of combustion do work on the piston which moves to the right and the pressure and temperature of gases decrease.
Process 5-6 : Blow down
The exhaust valve opens and the pressure drops to the initial pressure.
Process 6-1 : Exhaust
With the exhaust valve open, and the piston moves inwards to expel the combustion product from the cylinder at constant pressure.
The series of processes as described above constitute a mechanical cycle, and not the thermodynamic cycle. The cycle is completed in four strokes. It consists of two reversible adiabatic and two reversible isochores.
Air is compressed in process 1-2 reversibly and adiabatically. Heat is then added to air irreversibly at constant volume in process 2-3. Work is done by air in expanding reversibly and adiabatically in process 3-4. Heat is then rejected by air reversibly at constant volume in process 4-1, and the system (air) comes back to its initial state. Heat transfer processes have been substituted for the combustion and blow down processes of the engine. The intake and exhaust processes of the engine cancel each other.
The mechanical efficiency of an engine is measured by the ratio of the shaft output or brake horse power to the indicated horse power or power developed in cylinders. e.g. if the ratio of the bhp to ihp is 9:10, then the mechanical efficiency of the engines is 90%. In determining the mechanical efficiency only the losses suffered by the energy that has been delivered to the pistons is considered.
Thermal efficiency is a measure of the heat losses suffered in converting the heat energy of the fuel into the mechanical working. In fig below, the heat dispelled by the cooling system represents 25% the heat carried away by the exhaust gases represents 40%, the mechanical work on the piston to overcome friction and pumping losses represents 5% and the useful work at the propeller shaft represents 30% of the heat energy of the fuel. The thermal efficiency of air engine is the ratio of heat developed into the useful work to the heat energy of the fuel. It may be based on either bhp or ihp and is represented by a formula in this manner.
Indicated thermal efficiency = ihp x 33000 .
weight of fuel X Heat value burned per min. (Btu) 788
The formula for brake horse power is also same with the word “brake inserted in place of indicated on both sides of the formula.
Volumetric efficiency is the ratio of the volume of fuel air charge, burned by the engine at atmospheric pressure and temperature to the piston displacement. If the cylinder of an engine draws in a charge of fuel and air having a volume at standard atmospheric pressure and temperature which is exactly equal to the piston displacement of the cylinder, the cylinder has a volumetric efficiency of 100%.
Volumetric efficiency may be expressed as a formula then;
Volumetric efficiency = Volumetric charge at atmospheric pressure
Factors which tend to decrease volumetric efficiency are improper timing of the valves fuel air manifolds having too small a diameter and many bends, the use of air which has been raised to a high temperature from any cause, two high a temperature in the c.c. incomplete scavenging of the burned gases during the exhaust stroke, and excessive speed. One or more of these factors may exist at various times.
Basically any condition which tends to slow or reduces the flow of air into the engine will reduce volumetric efficiency. Improper timing of the valves affects volumetric efficiency because the intake valve must be as wide open as possible when the piston starts the intake stroke. The exhaust valve must be closed precisely at the contact when the exhaust gases stop flowing out of the c.c.
At high engine rpm the volumetric efficiency of the engine decreases because of the friction produced by the intake, carburettor intake manifold, and valve ports. The effect of the friction increases as the air velocity increases. When the throttle is partially closed, volumetric efficiency decreases in accordance with the degree of closing.
A leak in the intake manifold would tend to increase volumetric efficiency because more air would be entering the engine, however it will also cause a lean mixture.
Maximum volumetric efficiency is obtained when the throttle is wide open and the engine is operating under a full load. A naturally aspirated engine always has a volumetric efficiency of less than 100% on the other hand, the supercharged engine often is operated at a volumetric efficiency of more than 100% because the super charger compresses the air before it enters the cylinder. The volumetric efficiency of naturally aspirated is less than 100% for two principal reasons.
1) The bends, obstruction and surface roughness inside the intake system cause substantial resistance to the airflow, thus reducing air pressure below atmospheric in the intake manifold;
2) The throttle and the carburettor venturi provide restriction across which a pressure drop occurs.
Aircraft engines may be cooled either by air or by liquid; however, there are few liquid-cooled engines still in operation in the United States. We shall therefore devote most of our discussion to the air cooled types.
Excessive heat is undesirable in any internal combustion engine for three principal reasons
1) It adversely affects the behaviour of the combustion of the fuel-air charge.
2) It weakens and shortens the life of the engine parts, and
3) It impairs lubrication.
If the temperature inside the engine cylinder is too great the fuel mixture will be preheated and combustion will occur before the proper time. Premature combustion causes detonation, “knocking” and other undesirable conditions.
It will also aggravate the over heated condition and is likely to result in failure of pistons and valves. The strength of many of the engine parts depends on their heat treatment. Excessive heat weakens such parts and shortens their life. Also, the parts may become elongated, warped or expanded to the extent that they freeze or lock together and stop the operation of the engine. Excessive heat “cracks” the lubricating oil, lowers its viscosity, and destroys its lubricating properties.
In an air-cooled engine, thin metal fins project from the outer surface of the walls and heads of the engine cylinders. When air flows over the fins, it absorbs the excess heat from the cylinders and carries it into the atmosphere. Deflector baffles fastened around the cylinders direct the flow of air to obtain the maximum cooling effect. The baffles are usually made of aluminium sheet. They are called pressure baffles because they direct airflow caused by ram air pressure.
The operating temperature of the engine can be controlled by movable cowl flaps located on the engine cowling. On some airplanes, these cowl are manually operated by means of a switch which controls an electric actuating motor.
On other airplanes they can be operated either manually or by means of a thermostatically controlled actuator.
In the assembly of the engine baffling system, great care must be taken to see that the pressure baffles around the cylinders are properly located and secured. An improperly installed or loose baffle can cause a hot spot to develop with the result that the engine may fail. The proper installation of baffles around the cylinders of a twin-row radial engine. It will be observed that the baffling maintains a high-velocity airstream close to the cylinder and through the cooling fins. The baffles are attached by means of screws, bolts, spring hooks, or special fasteners.
Cylinder cooling is accomplished by carrying the heat from the inside of the cylinders to the air outside the cylinders. Heat passes by conduction through the metal walls and fins of the cylinder assembly to the cooling airstream which is forced into contact with the fins by the baffles and cowling. The fins on the cylinder head are made of the same material and are forged or cast as part of the head. Fins on the steel cylinder barrel are of the same metal as the barrel in most instances are machined from the same forging as the barrels. In some cases the inner part of the cylinder is a steel sleeve and the cooling fins are made as a part of a muff or sleeve, shrunk fitted on the outside of the inner sleeve. A large amount of the heat developed in an engine cylinder is carried to the atmosphere with the exhaust. This amount varies from 40 to 45 percent, depending upon the design of the engine.
The proper adjustment of valve timing is the most critical factor in heat rejection through the exhaust. In the operation of a helicopter, the ram air pressure is usually not sufficient to cool the engine, particularly when the craft is hovering. For this reason, a large engine driven fan is installed in a position to maintain a strong flow of air across and around the cylinders and other parts of the engine. Helicopters powered by turbine engines do not require the external cooling fan.
The principal advantages of air cooling are that
1) The weight of the air-cooled engine is usually less than that of a liquid-cooled engine of the same horsepower because the air-cooled engine does not need a radiator, connecting hoses and lines, and the coolant liquid;
2) The air-cooled engine is less affected by cold-weather operations:
3) The air-cooled engine in military airplanes is less vulnerable to gunfire. If an enemy bullet or bomb fragment strikes the radiator, hose, or lines of a liquid cooled engine, it is obvious that its cooling system will leak and soon cause a badly overheated engine.
Liquid-cooled engines are rarely found in the United States aircraft today; however, the power plant technician should have some understanding of the principal elements of such systems. A liquid cooling system consists of the liquid passages around the cylinders and other hot spots of the engine, a radiator by which the liquid is cooled, a thermostatic element to govern the amount of cooling applied to the liquid, a coolant pump for circulating the liquid, and the necessary connecting pipes and hoses. If the system is sealed, a relief valve is required to prevent excessive pressure and a sniffier valve is necessary to allow the entrance of air to prevent negative pressure when the engine is stopped and cooled off.
Water was the original coolant for liquid-cooled engines. Its comparatively high freezing point (320F) (00C) and its relatively low boiling point (2120 F) (1000 C), made it unsatisfactory for the more powerful engines used in military applications. The liquid most commonly used for liquid-cooled engines during world war II was ethylene glycol or a mixture of ethylene glycol and water. Pure ethylene glycol has a boiling point of about 3500F (1760C) and a slushforming freezing point of about 00F (-17.780 C) at sea level. This combination of high boiling point and low freezing point made it a satisfactory coolant for aircraft engines.
Fuel used in Piston Engine
There are two main types of fuel used in aircraft, aviation gasoline, which is used in piston engines, and aviation kerosene, which is used in turbo-jet and turbo-propeller engines. It is most important that the correct type and grade of fuel, as indicated in the appropriate maintenance manual, should be used.
Aviation gasoline (AVGAS) is the lighter of the two fuels, having a relative density of approximately 0.72. The only grade of AVGAS generally available is grade 100L, which has an octane rating of 100 and a low lead content. Where different grades of fuel were previously specified for use in a particular engine, the use of AVGAS 100L may necessitate additional checks and maintenance to be carried out. Automobile fuel must not be used instead of non-lead aviation fuel.
Gasoline has powerful solvent properties, and it is essential that it does not come into contact with certain components such as transparent panels and tyres. Personal contact may also result in skin infections, and it should be noted that some of the additives used in gasoline are poisonous
Plain, ball or roller bearings may be used at various positions in an engine, depending on the magnitude and direction of the load which they are required to accept. Plain bearings have a greater load-bearing capacity than either ball or roller bearings, and are generally used in a place where the radial load is high. A plain bearing usually consists of a pair of semi-circular steel shells which are lined with a non-ferrous alloy; this in turn may be faced with a white metal. In most cases each half of the bearing is pegged or otherwise located to prevent rotation in its support, and receives its oil supply through drilling in the supporting member. Some plain bearings, such as those fitted to the crankpins on radial engines, are completely circular and fully-floating, and oil is supplied to both sides of the bearing, thus providing two bearings faces. Although plain bearings are generally fitted in positions where the load is mainly radial, plain bearings can be made capable of accepting axial loads, and are sometimes used to transmit the propeller thrust. In these cases the bearing has a flange on each side, which forms a bearing face normal to the shaft axis, and limits axial movement. Plain bearings must be pressure lubricated in order to maintain an oil film between the mating parts, and prevent damage to their surfaces.
Ball bearings are used in many places where radial loads are light, and where axial positioning is important. Heavy roller bearings are used as main crankshaft bearings on radial engines, and ball bearings are frequently used as thrust bearings on propeller shafts and on the crankshafts of direct-drive engines Ball and roller bearings do not, generally, require pressure lubrication, and are frequently lubricated by splash; however, oil jets may be used in locations where lubrication is particularly critical.
The crankcase is, usually, the largest single component of an engine. It provides the mounting faces for the cylinders, reduction gear, sump, and accessories, supports the crankshaft, provides oil ways for the lubricating oil, and carries the mounting for attachment of the engine to the airframe. A crankcase is, therefore, of complicated shape, and it is usually cast from aluminium or magnesium alloys, which provide the strength and rigidity required, without unnecessary weight. Some crankcases are in two or more parts, which are bolted and dowelled together. A typical crankcase for a horizontally-opposed engine is illustrated in figure 8, which shows that the two halves are joined at a vertical plane passing through the crankshaft centre line. With radial engines the joint is on the plane normal to the crankshaft centre line and passing through the centres of all cylinders in one bank, each portion of the crankcase supporting one of the main bearings.
Studs are fitted to the crankcases for the attachment of all components, except that in the case of horizontally opposed engines with staggered cylinders, the positions of some cylinder holding-down points coincide with the main bearing supports, and through-bolts are used at these locations.
The sump may be considered as part of the crankcase, and may be a casting in light alloy, or may be fabricated from sheet steel. The sump contains a drain plug, and may also house the scavenge filter. A dip-stick, housed in the crankcase, provides a means of checking the sump oil level.
All crankcase face joints are sealed to prevent oil leakage, but a vent at the top of the crankcase is ducted overboard to relieve internal pressure. In general, cylinder mounting flanges are sealed with an “O” ring, sump joint faces are sealed with a cork or composition gasket, and other joint faces are sealed with paper gaskets or jointing compound.
The crankshaft is the heaviest single component of the engine and is usually forged from an alloy steel in order to resist the high stresses imposed during operation. The crankshafts of in-line and horizontally-opposed engines are machined from a single forging, with hollow crankpins and journals, and drilled webs to provide passageways for the lubricating oil. The crankshaft of a single-row radial engine is generally made from two forging, the separate parts being joined at the crankpins; this is because of the difficulty of providing a split bearing capable of accommodating all the connecting rods on a single crank pin. Typical crankshafts are illustrated in figure 9.
Lubricating oil is ducted from the oil pressure pump, through crankcase oil passages, to each of the crankshaft main bearing supports. This oil passes though holes in the bearings to lubricate the journals, and through radial holes in the journals into the hollow shaft. From there it flows through drilling in the webs to the connecting rod big-end bearings and then escapes from these bearings to be thrown by centrifugal force into the pistons and cylinders. On some engines, oil jets at the crankshaft main bearings spray oil into the cylinders. At its front end, a crankshaft may have a flange or splined portion to which the propeller is attached, or it may be internally splined in order to drive the propeller reduction gear through a quill shaft; the quill shaft is designed to twist under torsional loads so as to smooth out the power impulses. A gear or a quill shaft is generally attached to the rear end of the crankshaft, for the purpose of driving the camshaft and accessories, but some of these may be driven from the propeller reduction gear.
A cylinder must, generally, provide the hard bearing surface on which the piston slides, must be strong enough to resist the pressures produced by the combustion of the mixture, and must dissipate the heat produced in the combustion chamber. Aluminium alloy has god strength and have-dissipation properties and is generally used for cylinder heads, but its surface is not hard enough to resist abrasive wear, and therefore the cylinder barrels are generally made from a steel alloy. An exception is the sleeve valve engine, in which the piston operates inside a steel alloy sleeve, and aluminium alloy is used for the cylinder barrel. Poppet valve guides and rocker bearings fig 10., are made from bronze or similar material, and the valve seats are made from steel in order to resist the hammering of the valves. Sparking plugs may be fitted into bronze inserts, which are screwed and pegged into the cylinder head, but in some engines thread inserts are used, and are installed directly into the head.
On air-cooled engines, which invariably have individual cylinders, the cylinder head and barrel are finned to present a large cooling surface to the airflow, the spacing and size of the fins depending on the amount of heat which must be dissipated. The cylinder head is usually screwed and shrunk onto the barrel to make a permanent assembly, but on some engines the head may be removable and bolted to the cylinder barrel or secured by studs extending from the crankcase. A copper gasket between the head and barrel prevents gas leakage.
On water-cooled in-line engines, the one-piece aluminium-alloy cylinder block has a detachable head, and steel liners in which the pistons operate. The whole assembly is attached to the crankcase by studs or bolts which pass right through the head and block, a copper gasket preventing gas leakage between the liners and head, and a flexible seal round each liner preventing coolant leakage from the block. Coolant flowing round the liners, and through passageways in the head, absorbs and removes excess heat.
Lubrication of the cylinder bores is generally by oil mist and spray from the connecting rod bearings, but oil jets at the crankshaft bearings may be used. Cylinder bores are often honed in such a manner as to result in a pattern of microscopic grooves which permit the retention of small quantity of oil on the walls. The rocker bearings (and, in the case of some in-line engines, the overhead camshaft bearings) are usually pressure lubricated by oil ducted from the crankcase, and the splash oil released from these bearings is used to lubricate the valve stems, guides and springs. In some small engines the rocker arms oscillate in roller bearings which are hand lubricated at specified intervals, whilst on inverted engines the rocker cover may be partially filled with oil, to splash lubricate all the cylinder head components. A typical air-cooled cylinder is shown in figure.10.
Connecting rods convert the reciprocating motion of the pistons to the rotary motion of the crankshaft. They require considerable strength and rigidity, and are generally aluminium alloy or steel forging of “H” section. On horizontally opposed and in-line engines, the bearing at the crankpin end (big end) is usually a split plain bearing similar to those used at the crankshaft main bearings ( figure 11.). The connecting rod small end is usually fitted with a bronze bush and attached to the piston with a hollow steel gudgeon pin. On radial engines only one connecting rod (the master rod) in each bank of cylinders is mounted directly on to the crankpin fig.12., and usually has a fully floating bearing.
The connecting rods on the other cylinders (known as articulated rods) are connected to flanges on the master rod big-end by hollow steel wrist pins which are similar to the gudgeon pins at the connecting rod small end. Big end bearings are pressure lubricated through drilling in the hollow crankpins from the main oil pressure supply, and smallend (and wrist pin) bearings are usually lubricated by splash oil through holes in the connecting rods.
Pistons are subjected to high pressures and temperatures, and to rapid acceleration and deceleration. They must, therefore, be strong yet light, and capable of conducting away some of the heat generated in the combustion chamber; they are generally machined from forging of high strength aluminium alloy.
Pistons are attached to their connecting rods by means of a gudgeon pin, which is often free to rotate in both the piston and connecting rod, and may be supported in bronze bushes fitted internally on each side of the piston; axial movement of the gudgeon pin is usually prevented by a circlip fitted at each end, or by an end pad of soft metal, which bears against the wall of the cylinder.
Since a piston, being made from aluminium alloy, expands more than the cylinder barrel (which is normally steel alloy), a working clearance between these components is essential, and a number of piston rings are fitted into grooves in the piston. These rings are generally made from cast iron or alloy steel, are split to permit assembly, and have a gap between their ends to allow for expansion; a side clearance between the groove and ring is also essential.
Rings are often free in their grooves, and are assembled with the gaps of alternate rings spaced 180o apart, but in some cases rotation is prevented by a peg in case the gas leakage from the combustion chamber, and are generally fitted above the gudgeon pin, whilst scraper rings (also known as oil control rings) are designed to remove oil from the cylinder walls, and are generally fitted below the gudgeon pin.
Piston heads may be flat or slightly domed for strength, or may be concave in order to provide a combustion chamber which is as nearly spherical as possible. In some cases it may also be necessary to have recesses in the head of the piston, to provide clearance for the open valves when the piston is at the top of the exhaust stroke.
Lubrication of the gudgeon pin bearings is provided by splash oil, through holes drilled in the gudgeon pin bosses, and drainage of the oil removed from the cylinder walls by the piston scraper rings, is provided by radial holes drilled through the piston from the base of the piston ring groves.
On a few engines, the inlet and exhaust ports in the cylinder are opened and closed by means of a cylindrical sleeve fitted between the cylinder barrel and the piston. The sleeve, of hardened steel, is driven by a crank, which is geared to the crankshaft, and ports in the sleeve uncover the cylinder inlet and exhaust ports at the appropriate times. Timing of the opening and closing of the ports in the sleeve is set by manufacturer, and no adjustments are possible at field level. The main advantage of this method is reputed to be the increased volumetric efficiency resulting from the lack of obstruction to the incoming and out-going gases.
These valves are in fig.10. fitted to the majority of aircraft piston engines; they operate under arduous conditions and may be made from a variety of steel alloys. Exhaust valves, which are subjected to the highest temperatures, often have a larger diameter stem than inlet valves, the stem being hollow and partially filled with sodium to transfer heat away from the valve head. Valve heads are ground to form a face which mates with the valve seat and forms a gas tight seal. The ends of the valve stems are grooved to secure a split collect, which holds the spring retaining collar in position, and are hardened at the tip to provide a bearing surface for the rocker arm. Each valve is closed by two or more coil springs, which are concentrically mounted and coiled in opposite directions; the fitting of two or more springs having different vibration frequencies prevents the valve from bouncing on its seat when it closes.
Valve stems slide in valve guides fitted in the cylinder head, which are generally lubricated by splash from the rocker gear. The inner ends of the valve guides are often fitted with a seal, to prevent the leakage of oil into the inlet and exhaust ports of the cylinder.
Valve Operating Mechanism
Poppet valves are opened by a mechanical linkage from the cam shaft or cam drum, and closed by the valve springs. As the appropriate cam is rotated, its lobe pushes a tappet, which in turn activates a push rod, which transmits movement to the rocker arm; the rocker arm pivots on its bearing and pushes on the end of the valve stem to open the valve. When the cam has passed its points of maximum lift, the valve springs return the mechanism to its original position and close the valve. This type of mechanism is generally used on horizontally-opposed and radial engines, and also on some in-line engines. On other in-line engines the camshaft is mounted on the cylinder heads, and operates directly on the rocker arms to open the valves; these are known as “overhead camshaft” engines.
Camshafts are fitted to all horizontally-opposed and inline engines, and are driven through spur or bevel gearing from the crankshaft, at half engine speed. They are made from alloy steel and are supported in plain bearings which are pressure lubricated from the engine oil system. The cams are shaped and positioned so as to open and close their associated valves at the correct time, and their faces are hardened to provide a good bearing surface.
Cam drums are used in most radial engines, and have two rows of cams (one for the inlet valves and one for the exhaust valves). They are made from steel, and are mounted on a bearing around the front of the crankshaft and driven, by a gear train from the crankshaft, at the required speed and in the required direction of rotation. The cam drum bearing is generally pressure lubricated by the engine oil system.
The purpose of a tappet is to transfer the motion of a cam to its associated push rod. Tappets may be fitted either directly into the crankcase or in bronze guides in the crankcase. They are often purely mechanical devices comprising a rod with a hardened pad or roller at the cam end, and a hardened socket at the push rod end. To ensure that the valves close properly when the engine is running, in spite of expansion of the cylinder, a means of adjustment is provided to enable a predetermined clearance to be maintained in the valve operating mechanism when the valve is closed; this is known as tappet clearance. Most modern light aircraft engines are fitted with hydraulic tappets. (fig.13). This type of tappet consists basically of a body and a plunger, with an internal spring and non return valve, and a push rod socket.
During operation, pressure oil supplied to the tappet is picked up by a groove round the body when the tappet is near the outer end of its stroke. This oil lubricates the tappet bearing surface and enters the plunger reservoir through a port in the plunger wall; it then passes through the push rod socket and hollow push rod to lubricate the rocker mechanism. If clearance is present in the valve opening mechanism when the tappet is resting on the cam dwell, the spring in the tappet body pushes the plunger outwards to eliminate this clearance, the non-return valve opening to allow oil to pass into the body reservoir. As the cam lobe commences to push on the tappet, the non-return valve closes and a hydraulic lock is formed, transmitting motion to the push rod. In this way clearance is eliminated from the mechanism; valve closure is unaffected, since the force applied by the tappet spring is much less than that of the valve springs.
Push rods are usually steel tubes, with hardened steel fittings at each end to mate with the tappet and rocker arm. These fittings are usually drilled to allow lubricating oil to pass to the rocker arm. The push rods are surrounded by push rod covers, which may be steel or aluminium alloy tubes, and which are fitted with seals at the cylinder head and crankcase; the crankcase seal usually being spring-loaded to permit assembly.
A rocker arm pivots on a steel shaft, which may be held in mounting in the cylinder head or in pedestals which are bolted to the cylinder head. Rocker arms are generally made from alloy steel, with a hardened face at the valve end, and an adjusting screw with hardened socket at the push rod end. On some engines, mounting and adjustment are by means of a ball or roller pivot bearing, which is mounted in an eccentric bush. Oil from the hollow push rod is often fed through the drilling in the rocker arm to lubricate the rocker arm bearing. On other engines the rockers may be splash lubricated.
Propeller Reduction Gear
The purpose of a reduction gear is to reduce engine speed to a speed suitable for efficient operation of the propeller. The various types of reduction gears are illustrated in figure 14. Epicyclic (sometimes known as “planetary) reduction gears are always used on radial engines, and spur gear reduction gears are generally used on in-line engines, but either type may be fitted to horizontally-opposed engines.
A propeller shaft is normally supported in roller bearings, and propeller thrust is transferred to the engine by means of a ball thrust bearing. On some small engines, however, the propeller may be supported in plain bearings, the thrust being taken by a thrust washer placed between a flange on the propeller shaft, and one of the bearing supports.
Lubrication of plain bearings is by pressure feed from the normal engine lubrication system, and lubrication of ball and roller bearings, and of gears, is by oil spray nozzles and splash.
A number of components such as magnetos, oil pumps, fuel pumps, starter and engine speed indicator drive, are required for normal operation of the engine, and , in addition, accessories such as hydraulic pumps, pneumatic pump and electrical generators may be required to power the aircraft systems. All these components are driven at a suitable speed by gearing from the engine crankshaft, and are generally attached to the engine crankcase. In some cases, however, the aircraft system components are fitted to a remotely mounted gearbox, which is driven by an extension shaft from the rear of the engine crankshaft. Accessories are often coupled to their driving gears by means of a quill shaft, which is designed to shear in the event of failure of the accessory, thus preventing damage to the engine.
Lubrication of accessory drive plain bearings is generally by lubrication system pressure, through ductings in the crankcase, and of ball and roller bearings, by crankcase splash; remotely mounted gearboxes are generally self contained, the casing being partially filled with oil, and lubrication effected by splash.
Mechanically driven pumps may be used for a number of purposes on an engine; centrifugal pumps are used to circulate coolant, gear -type pumps are used to provide oil at high pressure for engine lubrication, and diaphragm pumps are sometimes used to supply fuel to the carburettor. Other types of pumps are used to power various aircraft systems.
A centrifugal pump consists of an impeller, which is rotated inside a housing. The working fluid rotates with the impeller, and centrifugal forces acting on this fluid cause it to flow to the outside of the housing, and more fluid is drawn into the eye of the impeller. This provides a low pressure circulation through the system, and, since it is not a positive displacement pump, neither a pressure relief valve nor a by-pass is required.
These pumps consist of two meshing gears, which rotate in a close-fitting housing (fig.15). One gear is driven from the engine, and as it rotates it carries the other gear round with it, and fluid is carried round the casing between the gear teeth. These pumps are known as positive displacement pumps, a definite volume of fluid being delivered for each revolution of the gears. Any restriction in the delivery line (such as will normally be provided by the bearings) will result in a build-up of pressure, and a relief valve is required. Relief valves are adjusted to maintain a predetermined pressure on the delivery side of the pump, and any excess fluid is bypassed to the inlet side of the pump or to the sump; in some engines a second relief valve is fitted after the main pressure relief valve, to provide a low-pressure lubrication system for certain components. Engine oil pressure and scavenge pumps are generally driven by a common shaft and mounted in adjoining housings, the gears of the scavenge pump being longer than those of the pressure pump to ensure complete scavenging of the sump.
In a diaphragm pump, a rotating cam in the engine acts indirectly on a diaphragm (usually of rubberized fabric), and causes it to reciprocate. This motion , in conjunction with lightly spring-loaded inlet and outlet valves, can be used, for example, to pump fuel to the carburettor. In the pump illustrated in figure 16, a spring keeps the plunger cap in contact with the cam, and a second spring, inside the plunger, limits the delivery pressure by restricting the movement of the diaphragm. Construction of these pumps varies in detail, and in some pumps a filter bowl is suspended below the pump, the inlet valve being located inside the filter; in other pumps the plunger may be operated indirectly by a cranked lever.
PARTS OF PISTON ENGINE
Power Calculation and Measurement
Power. IMEP, therefore, has a definite relationship to Indicated Power, and, in a similar way, is composed of components representing Friction Power and Bake Power.
These components are known as Friction MEP (FMEP) and Brake MEP (BMEP), and can be used for calculating Friction Power and Brake Power respectively. Similarly, if Indicated Power and rev/min are known, IMEP can be calculated
IMEP = k x Indicated Power rev/min
and if Brake Power and rev/min are known, then BMEP can be calculated
BMEP = k x Brake Power rev/min
Engine operation must be confined within cylinder pressure and crankshaft speed limitations, which are determined by the manufacturer. Various combinations of these parameters could be used to produce any particular power output, and the most economical would be the use of low rev/min to minimize friction and high cylinder pressure to produce the power required. On most engines, since cylinder pressure is related to manifold pressure, adequate control is provided by operating within prescribed manifold pressure and rev/min limitations, but on large engines where economy is particularly vital, closer control of cylinder pressure becomes necessary. IMEP is related directly to peak cylinder pressure and to Indicated Power, so that control of IMEP would ensure operating at safe cylinder pressures; however, Indicated Power is difficult to measure and other means must be used.
FMEP varies according to peak cylinder pressure and internal power requirements (different supercharger ratios, etc.), and can be measured throughout the engine speed range. The relationship between BMEP and IMEP can, therefore, be determined for any operating conditions, and since Brake Power can easily be measured by fitting a torque meter to the engine, operation at safe cylinder pressures can be achieved by imposing BMEP limitations for the various operating conditions. Manufactures conduct tests to ascertain the BMEP which is equivalent to the maximum safe cylinder pressure for any set of operating conditions, and also provide sets of tables showing the range of BMEP and rev/min setting which will give particular power outputs. The pilot may then select the power settings for the power output he requires, ensuring that the BMEP is within the limit prescribed for the particular operating conditions.
Alternatively, using the formula
BMEP = k x Brake Power rev/min
the pilot may calculate the rev/min necessary to achieve the power he requires at maximum permissible IMEP.
Any rapid reduction in rev/min when operating at maximum BMEP, would result in the cylinder pressure limit being exceeded. When adjusting power, therefore, manifold pressure should be reduced before decreasing rev/min, and rev/min should be increased before raising manifold pressure.
The power developed in an engine cylinder can be calculated from the cylinder dimensions and the average pressure on the piston during the power stroke. The force exerted on the piston will be the average pressure multiplied by the area of the piston, and the work done (force x distance) will be this force multiplied by the length of the stroke. The power developed in the cylinder can then be calculated by multiplying the work done by the number of power strokes (N) per unit time. In the case of single cylinder engine, “N” will be the crank shaft rotational speed divided by 2, and in the case of a multi-cylinder engine “N” will be the crankshaft rotational speed X no.of cylinders. When using Imperial units, power is usually quoted in horsepower (hp) (1 hp = 33,000 ft lbf/min) and when using SI units, power is usually quoted in kilowatts (kw) (1 hp = 0.746 kw). Thus the Indicated Power of an engine can be calculated from the formula :-
PLAN hp or PLAN kW
where P = pressure on piston (lbf/in or N/m)
L = length of stroke (ft or m)
A = area of piston (in or m )
N = number of power strokes /min.
For any particular engine the cylinder capacity is fixed, so that a constant (k) could be used to replace all the
invariable quantities in the formula for Indicated Power, which could then be simplified to :-
P X rev/min
where k is 33,000 - or 60,000
L X A X 1/2 no.of cylinders L X A X 1/2 no.of cylinder as appropriate.
It can then be seen that Indicated Power for a particular engine varies directly as the cylinder pressure and engine speed, an increase in either giving an increase in Indicated Power.
The Brake Power, or shaft power, of an engine is the power actually delivered to the propeller, and represents the Indicated Power reduced in quantity by the power required to overcome friction and to drive the engine accessories.
Power used internally is known as Friction Power, and the relationship between Brake Power and Indicated Power, expressed as a percentage, is known as the Mechanical Efficiency of the engine.
The output of an engine is obtained by measuring the torque of the propeller shaft. When calculating the work done on the piston, work was taken as force x distance (in a straight line); when measuring the work done by the propeller shaft, the torque can be thought of as a force “F” acting at a distance “r” from the axis of the shaft. If the system rotates once, the force can be regarded as having travelled one circumference of a circle of radius r,
i.e. work done per revolution = F2 pr
or, as torque = Fr, then work = torque X 2p
Brake Power can then be calculated if the speed of rotation is known. Using Imperial units the Brake Power becomes :-
torque (lbf ft) X 2p X rev/minhp
and using SI units it becomes :-
torque (N-m) X 2p X rev/min kW
Again, using a constant (C) for the invariable quantities, Brake Power become
torque X rev/min, and it can be seen that it varies directly with torque and engine speed.
Mean Effective Pressure
The average pressure exerted on the piston during the power stroke is known as the Mean Effective Pressure (MEP). The actual pressures can be measured, and are generally reproduced on an Indicator Diagram. The shaded areas represent work done on the piston during the induction and power strokes, and unshaded areas below the curve represent work done on the piston during the compression and exhaust strokes. The sum of the shaded areas, less the sum of the unshaded areas, represent useful work, and when this area is confined to the power stroke, the pressure coordinate becomes the Indicated MEP (IMEP), and may be used for calculating Indicated.
Propeller shaft torque is generally measured at the reduction gear. As the crankshaft gear rotates, it drives the propeller pinions, and these exert a thrust on the fixed gear teeth, tending to rotate the fixed gear in the opposite direction to the crankshaft gear; this thrust is directly proportional to power output. To measure the thrust applied to the fixed gear, the gear is allowed to float, and is attached to the structure through pistons and oil-filled cylinders, as shown in figure. Engine oil pressure to these cylinders is boosted by a torque meter pump, and each cylinder is fitted with a bleed back to the engine oil system.
At low engine power, thrust on the “fixed” gear is at a minimum and the bleed port is fully open, resulting in a low oil pressure in the system and a low reading on the torque meter pressure gauge. As power is increased the thrust on the “fixed” gear increases, and the pistons are forced further into their cylinders. The bleed ports are reduced in size by movements of the pistons, and the oil pressure in the system increases to balance the thrust on the fixed gear. The torque meter gauge may be calibrated directly in BMEP, or in units of oil pressure.