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Cylinder Barrel or Block Made of alloy steel, the Cylinder resists the pressure of combustion and provides a working surface for the piston. The cylinders are usually secured to the crankcase by studs and nuts. One end of the cylinder is sealed by the Cylinder Head, the movable piston sealing the other end. Figure 2.14 Figure 2.13 Valve Springs Valve Guides Valves Camshafs Spark Plug Cylinder Head Cylinder Barrel Figure 2.14 About 30% of the heat generated during combustion is transferred to the cylinders.

To cool the cylinder there are two cooling methods used. Liquid Cooling has jacket around the cylinders to allow for the flow of a liquid around them and carry the heat away. Air-cooled engines, have fins machined onto the cylinder to increase the surface area in contact with air, which is used to dissipate the heat. Piston Engines - General 2 28 2 Piston Engines - General The Cylinder Head The cylinder head is generally made of aluminium alloy to improve heat dissipation.

It seals one end of the cylinder to provide a combustion chamber for the mixture. The cylinder head accommodates the Valves, Valve Guides and Sparking Plugs, and supports the valve Rocker Arms. Valve Seats are cut into the cylinder head, which form gas tight seals with the valves. The cylinder head may be detachable but more commonly it is screwed and shrunk onto the cylinder.

Valve Guide - guides the valve in a straight path and keeps the valve concentric to its seat. Usually the valve guide is pressed into the cylinder head. Valve Seat - ground to form a gas tight seal with the face of the valve, cut at various angles (30° or 45°). Valves - inlet and exhaust valves open and close the passages for the induction and scavenging of the gases.

The face of the valve is accurately machined to the same angle as the valve seat. The valve and seat are then lapped until a full contact is obtained. Exhaust valve stems are sometimes hollow and partly filled with sodium to assist in cooling. They may be flat, trumpet or mushroom shape.

Valve Springs - made of special spring steel, to ensure that the valves remain closed except when operated by the cams. The springs are of the helical coil type, the usual practice being for two springs to be fitted to each valve, one inside the other. This provides a Safety Factor and helps to eliminate Valve Bounce. The springs are held compressed between the cylinder head and the valve spring cap, the latter being located on the valve stem by split collets.

Valve Operating Gear The valve operating gear consists of a Camshaft, Figure 2.15, (or camshafts) driven from the crankshaft at Half Crankshaft Speed regardless of how many cylinders there are, or how they are arranged. The camshaft is designed so as to have one Cam Lobe to control the opening of each valve. The camshaft is driven at half crankshaft speed because each valve is only required to open and close once per working cycle, that is to say, once every two revolutions of the crankshaft. The angular position of the lobes on the camshaft of an aircraft engine is fixed, causing the amount of valve lead, valve lag and valve overlap to remain constant, irrespective of changing engine speed.

The fact that the camshaft is driven by the crankshaft means that valve opening and closing angles are referred to with respect to crankshaft rotation, not camshaft rotation. (See valve timing diagrams.) Valve Clearance To ensure that the valves close fully, it is necessary for there to be a Valve (or Tappet) Clearance. This is a small gap measured between the Rocker Pad and the Valve Tip.

The valves are continuously heated by combustion and expand at a greater rate than the rest of the operating mechanism. As the engine heats up, the small gap, or valve clearance, shown in Figure 2.15, allows the valve to expand at its own rate. 2 29 Piston Engines - General 2 Piston Engines - General Figure 2.15 Valve clearance The valve clearance becomes smaller but the valve still remains shut. The valve clearance is measured between the rocker pad and the valve tip by feeler gauges and there is provision made on the rocker arm for the clearance to be adjusted.

Excessive valve clearance will cause the valve to open late and close early. Too little clearance will have the opposite effect of causing the valves to open early and close late and may even prevent the valves closing at all, thereby producing an event called Popping back into the Carburettor. The same effect can be caused by an inlet valve which is sticking in its guide. Some designs of engine use Hydraulic Tappets.

These are self-adjusting and operate with no clearance and thus there is no tappet noise. A hydraulic tappet is made in two main parts, one sliding within the other. Oil, which is supplied under pressure, causes the tappet to lengthen and take up any clearance when the engine is running. The Sump The sump is a casing attached to the base of the crankcase, it collects the lubricating oil after it has passed through the engine.

With some lubricating systems the sump also acts as the oil reservoir and all the oil is contained within it. A filter is housed in the sump to trap any debris in the oil, so preventing damage to the oil pumps. Piston Engines - General 2 30 2 Piston Engines - General The Carburettor The Carburettor meters the air entering the engine and adds the required amount of fuel as a fine spray under all conditions of engine running. For an aircraft engine the correct mixture must be supplied regardless of altitude or attitude of the aircraft.

An Injector can be fitted instead of a carburettor on some engines. They are attached to the base of the crankcase, metal pipes connect the outlet from the carburettor or injector to the cylinders. This is called the Induction Manifold. The waste gases after combustion are carried away from the cylinders by the Exhaust System.

The exhaust consists of steel pipes connected to each of the cylinders. The pipes from each cylinder usually connect up and go into one or two pipes which then carry the hot gases outside the aircraft to atmosphere. The Accessory Housing or Wheelcase For the engine to operate supporting systems are needed, and they may need power to drive them. Oil Pumps, Fuel Pumps, Superchargers and Magneto Ignition systems are fitted to the Accessory Housing and driven via gears by the crankshaft.

The housing casing is bolted to the rear of the crankcase which encloses the gear train and provides mounting pads for the ancillary equipment, Figure 2.16. A Starter Motor can be connected to the housing to initially rotate the crankshaft and start the cycle of operation. Figure 2.16 Accessory housing The accessory housing can also provide the drive to power aircraft systems such as Electrical Generation, Hydraulics and Pneumatic systems. 2 31 Piston Engines - General 2 Piston Engines - General Some engines may also have a Gearbox fitted between the crankshaft and the propeller.

This is a Reduction Gearbox to reduce the speed of propeller rotation. For the propeller to operate efficiently a comparatively low speed is required. For the engine to develop its full power, it must turn at high speed. So that the engine and the propeller can both operate efficiently the reduction gearbox may be required.

Two typical types of reduction gearing are Spur Gear and Planetary Gears. The lower powered engines have the propeller connected directly onto the crankshaft. These are called Direct Drive engines. Aero-engines are classified by Cylinder Arrangement, Type of Drive, Direction of Rotation, Cylinder Capacity, Cooling Method, Fuel System Type and whether they are supercharge or normally aspirated.

An example of light aircraft engine is depicted below. Figure 2.17 Figure 2.17 shows the Textron Lycoming model AEIO 540 L1B5. The model number is used to define the engine. AE Aerobatic Engine.

I Injected Fuel System O Horizontally opposed Cylinder Arrangement. 540 Cylinder displacement = 540 cubic inches. L Left hand Rotation. 1B5 Modifications from basic model.

This type of model numbering system is used by most manufacturers. If the letters G and S were included it would imply the engine was geared and supercharged. Questions 2 32 2 Questions Questions 1. The temperature of the gases within the cylinder of a four-stroke engine during the power stroke will: a. be constant b. decrease c. increase d. follow Charles’s Law 2.

The number of revolutions of the crankshaft required to complete a full cycle in a four-stroke engine is: a. 6 b. 4 c. 2 d. 8 3. The inlet valve opens before TDC in the exhaust stroke to: a. increase the pressure in the cylinder on completion of the induction stroke.

b. reduce engine vibration c. allow the incoming mixture to mix with a certain proportion of the exhaust gases d. induce a greater amount of mixture into the cylinder 4. The correct working cycle of a four-stroke engine is: a. exhaust, power, induction, compression b. compression, power, exhaust, induction c. induction, power, compression, exhaust d. power, exhaust, compression, induction 5. Valve overlap is incorporated in the valve timing of a piston engine to: a. improve volumetric efficiency b. reduce wear on the big end bearings c. increase the engines compression ratio d. prevent a weak cut when the engine is accelerated rapidly 6. With an increase in the rotational speed of a four-stroke engine, the valve overlap: a. increases b. decreases c. remains constant d. increases up to ground idle and thereafter decreases 7.

In a normally aspirated engine, exhaust back pressure: a. decreases as an aircraft climbs and thereby reduces the rate of decline of the engine power output b. increases as an aircraft climbs and thereby reduces the engine power output c. is affected by the power lever position d. decreases as an aircraft descends and thereby improves the engine power output 2 33 Questions 2 Questions 8. When the spark ignites the mixture: a. the explosion pushes the piston down b. the mixture changes from rich to weak forward of the flame front c. complete combustion occurs within 8 to 10 microseconds d. temperature and pressure increase within the cylinder 9. If the volume of a quantity of gas is halved during compression: a. its pressure is approximately doubled b. its temperature remains constant c. its mass is approximately doubled d. its pressure is approximately halved 10. The term Indicated Mean Effective Pressure refers to: a. the maximum working pressure in the engine cylinder b. the average pressure within the cylinder during the four cycles c. the pressure achieved during compression d. the minimum working pressure applied to the piston during the cycle 11.

The degrees of rotation to complete a full cycle on a nine cylinder engine will be: a. 180 b. 360 c. 720 d. 80 12. The firing interval of a six cylinder horizontally opposed engine will be: a.

180 b. 120 c. 60 d. 360 13. Which of the following statements would be correct for a double banked radial engine? a.

There will always be an odd number of cylinders b. Radial engines are generally liquid-cooled c. The linear distance from TDC to BDC will accommodate two throws d. Radial engines cannot suffer from hydraulicing 14. On a four cylinder engine with a total volume of 9600 cc, bore area of 100 cm² and a crank throw of 10 cm, what would the Compression Ratio be? a.

7:1 b. 8:1 c. 24:1 d. 6:1 Questions 2 34 2 Questions 15. With an increase in outside air temperature, specific fuel consumption will: a. increase b. decrease c. stay the same d. stay the same for all temperatures up to and including 15°C and thereafter increase 16. Combustion, in a four-stroke engine, theoretically occurs at: a. a constant pressure b. a constant temperature c. a constant volume d. a constant velocity 17.

In a convergent duct: a. the pressure and velocity increase, the temperature decreases b. the pressure and temperature decrease, the velocity increases c. the temperature and velocity increase, the pressure decreases d. the pressure and velocity remain constant, the temperature decreases 18. During the compression stroke: a. the temperature of the gases remains constant b. the volume of the gases increases c. the mass of the mixture decreases d. the mass of the mixture remains constant 19. From Top Dead Centre (TDC) to Bottom Dead Centre (BDC) on the practical power stroke: a. the temperature of the gases rises for a short time then decreases b. the pressure of the gases remains constant c. the temperature of the gases decreases from TDC to BDC d. the density of the gas remains constant 20. In a divergent duct: a. the velocity and temperature increase, the pressure decreases b. the temperature and pressure increase, the velocity decreases c. the temperature and pressure decrease, the velocity increases d. the velocity and temperature decrease, the pressure increases 21.

During the induction stroke: a. the mixture becomes weaker b. the volume of the gases becomes smaller c. the temperature of the gases reduces d. the pressure of the gases increases 2 35 Questions 2 Questions 22. Ideally, maximum pressure is attained within the cylinder: a. when combustion is complete b. at the end of the compression stroke c. during the period of valve overlap d. when combustion temperature is at a minimum 23. The power output of an internal combustion engine: a. is proportional to the volume of mixture induced into the cylinder b. increases with increased humidity c. falls as the charge temperature falls d. is proportional to the weight of the mixture induced into the cylinder 24. During the period of valve overlap: a. the action of the exhaust gases flowing past the exhaust valve increases the pressure within the cylinder b. the temperature of the exhaust gases increases the mass of incoming mixture c. the action of the exhaust gases flowing out past the exhaust valve tends to reduce the pressure in the cylinder d. the crankshaft is moving past Bottom Dead Centre 25.

The power output of an internal combustion engine can be increased by: a. increasing the area of the cylinder b. increasing the length of the stroke c. increasing the engine rpm d. all of the above Engine Components 26. Valve Overlap is: a. the number of degrees of camshaft rotation during which the inlet and exhaust valves are open at the same time b. the number of degrees of crankshaft movement during which the inlet and exhaust valves are open at the same time c. the distance the piston travels while the inlet valve remains open after BDC d. the number of degrees of crankshaft rotation during which the inlet and exhaust valves are open at the same time around BDC 27. The purpose of a valve spring is to: a. close the valve b. cause a snap opening of the valve c. allow the valve timing to vary with changing rpm d. maintain the valve clearance within tolerance Questions 2 36 2 Questions 28. Excessive blue smoke from the exhaust of an engine that has been warmed up to normal operating temperature may indicate that: a. the mixture is too rich b. the oil pressure relief valve has stuck in the open position c. the piston rings are worn or stuck in their grooves d. the oil pressure is too low 29.

The camshaft of a horizontally opposed four-stroke engine rotates at: a. twice engine speed b. engine speed c. twice magneto speed d. half engine speed 30. A reduction gear is fitted: a. between the camshaft and the propeller b. between the pushrods and the valves c. between the crankshaft and propeller d. between the connecting rod and the crankshaft 31. Prolonged use of low rpm could cause contamination of the: a. oil filter b. spark plug c. carburretor d. oil pump 32. If the Starter Engaged Light remains on after engine start, you should: a. shut the engine down immediately b. ignore it if it remains on for longer than 30 seconds c. shut the engine down if the light remains on for more than 30 seconds d. shut the engine down if the light remains on for more than 60 seconds 33.

The crankshaft of typical in-line four cylinder aircraft engine: a. rotates at half the speed of the camshaft b. will have the crank throws spaced 90 degrees apart c. allows a firing order of 1-3-4-2 d. will not flex or twist 34. Two valve springs are fitted to each valve: a. to minimize camshaft wear b. to allow a greater cam rise c. to prevent valve rotation d. to reduce valve bounce 35. Excessive valve clearance: a. will prevent the valve closing completely b. is eliminated when the engine reaches working temperature c. will cause the valve to open early and close late d. will cause the valve to open late and close early 2 37 Questions 2 Questions 36. Valve lead occurs when: a. the inlet valve opens before bottom dead centre b. the exhaust valve opens before the inlet valve c. the exhaust valve opens before top dead centre d. the inlet valve opens before top dead centre and the exhaust valve opens before bottom dead centre 37.

Insufficient tappet clearance at the inlet valve would cause: a. the valve to open early and close late b. the valve to open late and close early c. the mixture in that cylinder to be weak d. misfiring 38. The length of the stroke is: a. equal to the length of the cylinder b. determined by the size of the piston c. equivalent to twice the crank throw d. inversely proportional to the engine power output 39. Tappet clearance is measured between the: a. push rod and the valve tip b. valve tip and the rocker pad c. valve spring and the rocker pad d. valve tip and the rocker cover 40. The number of revolutions required to complete the induction and compression stroke in a six cylinder four-stroke engine is: a.

1 b. 2 c. 6 d. 4 41. The purpose of a crankcase breather is to: a. maintain the oil tank pressure at atmospheric b. prevent distortion of the crankcase c. allow the oil to breathe d. prevent pressure building up inside the crankcase 42. Tappet clearance is provided in a piston engine to: a. adjust the valve timing b. allow for expansion of the valve gear as the engine warms up c. allow for manufacturing tolerances d. prevent valve bounce Questions 2 38 2 Questions 43.

Piston rings are manufactured from cast iron: a. because it has a negative coefficient of expansion b. to take advantage of its extreme malleability c. because of its self-lubricating qualities d. to take advantage of its brittleness 44. The exhaust valves: a. are opened directly by the action of push rods which are in turn operated by cams on the crankshaft b. are less affected by the heat of combustion than the inlet valves c. are opened by the valve springs and closed by the rocker gear d. sometimes have their stems partly filled with sodium to assist cooling 45. Hydraulic valve tappets are used on some engines to: a. eliminate valve bounce b. eliminate constant valve adjustment and checks c. give a more positive closing action d. give a more positive opening action Terms and Definitions 46. The swept volume of a cylinder is: a. the area of the bore × the stroke b. the area of the cylinder cross section × the cylinder length c. half of the clearance volume d. the total volume + the piston volume 47.

The thermal efficiency of a piston engine can be increased by: a. increasing the rpm b. increasing the combustion chamber volume c. advancing the ignition point into the direction of rotation d. increasing the compression ratio 48. A normally aspirated engine is one which: a. has four cylinders b. is not supercharged c. is never air-cooled d. is all of the above 49. The Compression Ratio of an engine may be defined as the: a. swept volume + clearance volume ÷ swept volume b. swept volume + clearance volume ÷ clearance volume c. total volume - clearance volume ÷ clearance volume d. swept volume ÷ (swept volume + clearance volume) 2 39 Questions 2 Questions 50. An engine has a total volume of 2100 cm3 and a swept volume of 1800 cm3.

Its compression ratio is: a. 7:6 b. 6:1 c. 7:1 d. 6:7 51. Volumetric efficiency may be defined as: a. the ratio of the volume of the mixture drawn into the cylinder during normal engine working, to the volume of the mixture which would be required to fill the cylinder under normal temperatures and pressures b. the ratio of the volume of air and the volume of fuel drawn into the cylinder c. the ratio of the volume of one of the cylinders to the volume of all of the cylinders in the engine d. the efficiency with which the air and fuel mix together in the cylinder 52.

The ratio of the power produced by an engine to the power available in the fuel is known as the: a. specific fuel consumption b. indicated horsepower c. volumetric efficiency d. thermal efficiency 53. Specific Fuel Consumption (SFC) a. is the inability of the internal combustion engine to use any fuel other than that specified by the manufacturer b. becomes greater as the efficiency of the engine improves c. is the weight of fuel used by an engine per unit horsepower per unit time d. increases in proportion to the thermal efficiency 54. Brake Horsepower is: a. theoretical power in the cylinder b. useful power at the propeller c. power lost in the engine d. power required to slow the aircraft down 55. A method of improving Volumetric Efficiency is: a. valve overlap b. the use of carburettor heat c. weakening the mixture d. to make the mixture richer Answers 2 40 2 Answers Answers 1 2 3 4 5 6 7 8 9 10 11 12 b c d b a c a d a b c b 13 14 15 16 17 18 19 20 21 22 23 24 c d a c b d a b c a d c 25 26 27 28 29 30 31 32 33 34 35 36 d b a c d c b a c d d d 37 38 39 40 41 42 43 44 45 46 47 48 a c b a d b c d b a d b 49 50 51 52 53 54 55 b c a d c b a 41 3 Chapter Piston Engines - Lubrication Function of the Lubrication System .

.43 The Wet and Dry Sump Lubricating Systems . .43 The Oil Tank . .45 The Suction Filter . .45 The Pressure Pump .

.46 The Check Valve. .46 The Pressure Filter. .46 The Scavenge Pump . .47 Oil Cooler .

.47 Lubrication Monitoring Instruments. .47 Viscosity . .48 Viscosity Grade Numbering . .48 Types of Oil .

.49 Operational Considerations . .50 Questions . .51 Answers . .54 Piston Engines - Lubrication 3 42 3 Piston Engines - Lubrication 3 43 Piston Engines - Lubrication 3 Piston Engines - Lubrication Function of the Lubrication System The components that make up a piston engine are subjected to high loads, high temperatures, and high speeds.

The component parts are generally made of metals, and as the moving parts of the engine slide against each other, there is a resistance to their movement. This is called Friction. The friction will increase as the load, temperature and speed increases, the movement of the components also produces Wear which is the loss or destruction of the metal components. Both friction and wear can be reduced by preventing the moving surfaces coming into contact by separating them with a material/substance which has lower frictional properties than the component parts.

This is referred to as a Lubricant. A lubricant can come in many forms. Greases, powders and some solid materials. However it is in the form of Oils with which this chapter will concentrate on.

The oil can be forced between the moving parts, called Pressure Lubrication or the components can be Splash Lubricated. The Primary task of the lubrication system of the engine is to Reduce Friction and component Wear, it also has a number of secondary functions. Of these perhaps the most important is the task of Cooling. The flow of oil through the engine helps to dissipate the heat away from the internal components of the engine.

As the oil flows through the engine it also carries away the by-products of the combustion process and cleans the engine. The internal metal components are protected against Corrosion by the oil, which also acts a Hydraulic Medium reducing the shock loads between crankshaft and bearing and so reducing vibration. The oil can provide the power source for the operation of a hydraulic variable pitch propeller. The oil system can be used to give an indication of the power being developed by the engine, and its condition.

The oil system’s use as an Indicating Medium is of great importance to the pilot as it can give an early warning of mechanical failure or loss of power. It should be remembered that an increase in friction will cause an increase in Friction Horsepower, and therefore a reduction in the Brake Horse Power developed by the engine. The Reduction in Friction and Wear by the lubricant is of prime importance, but the secondary functions of Cooling, Cleaning, Protection, Hydraulic and Indicating Mediums should not be ignored. The Wet and Dry Sump Lubricating Systems There are two lubrication systems in common use, these are the Wet Sump and Dry Sump systems.

The system used is normally dependant on the power output of the engine, and role of the aircraft. The principle of lubrication of the engine is the same whichever system is used, the principle difference between the two systems being the method used to store the supply of oil. Most light, non-aerobatic aircraft engines use the Wet Sump system. In this system the oil is stored in the bottom or sump of the engine.

This simplifies construction but has a number of disadvantages: a) Lubrication difficulties arise during manoeuvres. The oil enters the crankcase, is flung around by the revolving shafts with possible over-oiling of the engine, inverted flight being particularly hazardous. Piston Engines - Lubrication 3 44 3 Piston Engines - Lubrication b) The temperature of the oil is more difficult to control as it is stored within the hot engine casing. c) The oil becomes contaminated and oxidizes more easily because of the continual contact of the oil with hot engine.

d) The oil supply is limited by the sump capacity. The Dry Sump system overcomes the above problems by storing the oil in a remotely mounted Tank. As previously stated the principle of oil supply is the same for both systems. A Pressure Pump circulates the oil through the engine, and so lubricates the moving parts.

In a dry sump system, Scavenge Pumps then return the oil to the tank to prevent the engine sumps flooding. The arrangement of the oil systems in different aircraft engines varies widely, however the functions of all such systems are the same. A study of one system will clarify the general operation and maintenance requirements of other systems. The principal units in a typical reciprocating engine oil system includes an Oil Tank (dry sump), Oil Filters, Pressure and Scavenge Pump, Oil Cooler (radiator), an Oil Pressure and Oil Temperature Gauge, plus the necessary interconnecting oil lines, which are all shown in the Figure 3.1 This shows a dry sump system, for a wet sump system the oil tank is not used, and there is a single pump, the pressure pump.

The following paragraphs state the function of the main components of the system. Figure 3.1 Dry sump lubrication 3 45 Piston Engines - Lubrication 3 Piston Engines - Lubrication The Oil Tank Oil tanks are made of sheet metal, suitably baffled and strengthened internally to prevent damage due to the oil surging during manoeuvres. The tank is placed wherever possible at a higher level than the engine to give a gravity feed to the pressure pump, and forms a reservoir of oil large enough for the engine’s requirements, plus an air space. The air space allows for: a) The increased oil return when starting the engine.

When the engine is stopped after a previous run, the walls of the crankcase are saturated with oil which will drain into the sump. The oil will remain there until the engine is started, when the scavenge pump will return it to the tank. b) The expansion of the oil, and therefore its greater volume as the oil absorbs heat from the bearings c) ‘Frothing’ due to aeration of the oil. d) The displacement of oil from the variable pitch propeller and other automatic controlling devices.

The hot pot (hot well) forms a separate compartment within the tank. Its purpose is to reduce the time taken to raise the temperature of the oil when starting the engine from cold by restricting the quantity of oil in circulation when the oil is cold and viscous. The hot pot consists of a cylinder of metal fitted above the oil outlet to the engine, thus the oil must be inside the hot pot to be able to reach the pressure pump. When starting, the level of oil in the hot pot drops, uncovering a ring of small diameter ports.

These ports offer a great resistance to the flow of cold thick oil so that very little passes to the inside of the hot pot. The oil is returned from the engine to the inside of the hot pot and is recirculated. As the hot oil is returned to the tank some of its heat raises the temperature of the walls of the hot pot. The oil in the immediate vicinity is heated and thins so that the ports offer less resistance to the flow of the thinner oil, and progressively more and more oil is brought into circulation.

The oil is filtered by the suction filter before passing to the pressure pump. When feathering propellers are fitted, the lower ring of feed ports to the hot pot are placed above the bottom of the tank, this provides a feathering reserve of oil even if the main tank has been emptied through the normal outlet, as would occur if the main feed pipeline was to develop a leak or completely fail. The scavenge oil returning to the tank is passed by an internal pipeline over a de-aerator plate to the inside of the hot pot. The plate separates the air from the oil to reduce frothing.

The tank is vented through the crankcase breather to prevent oil losses during excessive frothing conditions. The Suction Filter A coarse wire mesh filter is fitted between the tank and pressure pump. It is designed to remove large solid particles from the oil before it enters the pressure pump and so prevent damage. Piston Engines - Lubrication 3 46 3 Piston Engines - Lubrication The Pressure Pump The pump consists of two deep toothed spur gears rotating in a close fitting pump casing driven via the accessory housing.

Oil is carried either side of the casing in the space between the gear teeth, and is made to flow. The outlet side of the pump is enclosed and restriction to flow is given from the engine components to be lubricated. This gives a rise in system pressure. The actual oil pressure obtained will depend on the Speed of the Pump, the Temperature of the Oil and the Resistance offered by the Components.

The capacity of the pump must be such that it will supply a minimum oil pressure under its most adverse running conditions of low turning speed and high inlet oil temperature. As a consequence of this, under normal running conditions the increased flow would tend to cause a dangerously high oil pressure. Very high pressures are prevented by a Pressure Relief Valve (PRV) across the inlet and outlet connections which limits maximum pressure in the system. When the pressure reaches a predetermined figure, the valve opens and sufficient oil is returned to the inlet side of the pump to limit the maximum oil pressure.

In operation the engine will have range of operating pressure related to engine speed from idle to maximum rpm. The Check Valve (Non-return Valves, or One-way Valves) The oil tank may be at a higher level than the pressure pump to provide a gravity feed. When the engine is stopped and the oil is hot and thin, there is sufficient pressure from the gravity feed to force the oil through the clearances in the pressure pump so that the oil tank would drain into the crankcase and the engine would be flooded with oil. This feature of dry-sump operation is sometimes referred to as over-oiling.

To prevent this a check valve is fitted. This consists of either an lightly sprung loaded valve, or electrically-operated shut off valve (SOV) which will hold back the oil until the engine is started. The Pressure Filter The pressure filter is fitted downstream of the pressure pump before the oil enters the engine and is designed to remove very small solid particles before the oil passes to the bearing surfaces. A spring loaded relief valve is fitted to bypass the filter element when the oil is cold, or if the element becomes blocked.

It will also protect the engine if the pressure pump breaks up. Figure 3.2 Spur gear pump 3 47 Piston Engines - Lubrication 3 Piston Engines - Lubrication The Scavenge Pump The Scavenge Pump returns the oil by pumping it from the sump back to the tank. When the engine is stopped the oil in the crankcase will drain into the sump. As the engine is started there will be a quantity of oil, which, if the pumps were the same size, would not be removed.

Therefore, to maintain a dry sump it is necessary for the scavenge pump to be of a larger capacity than the pressure pump. In practice the scavenge pump capacity is 25% - 50% larger than that of the pressure pump. Oil Cooler The use of oil for cooling the internal components of the engine has already been emphasized. If the oil itself gets too hot, it could fail as a lubricant.

To prevent its temperature rising too high a cooler is introduced in to the system. The oil cooler consists of a matrix or tube block, which spreads the oil in a thin film and subjects it to cooling air. The matrix is built up of round tubes, the ends of which are expanded and shaped to form hexagons to form a surface for soldering the tubes together. The matrix itself is bonded into the oil cooler jacket by soldering the flats of the tubes to the inner shell of the cooler jacket.

When starting the engine from cold, the cooler matrix will be full of cold thick oil, and to force the oil through the small oilways of the cooler would require a very high pressure. To prevent damage to the cooler an Anti-surge Valve is fitted to by-pass the matrix when the oil is cold. The temperature of the oil is affected by three factors: a) The amount of heat generated in the engine (power). b) The temperature of the cooling air.

c) The rate at which air flows through the cooler. In some light aircraft the flow of air through the cooler is simply dependant on the forward speed of the aircraft in flight, and the airflow from the propeller whilst the aircraft is on the ground. In certain conditions of flight, where high power is used with low forward speed e.g. a climb, care must be taken to prevent overheating the oil.

The flight manual will recommend climb speeds that should ensure adequate cooling. Higher powered aircraft will be fitted with shutters behind the cooler to control the flow of air through the cooler. This would be closed at start up to allow the engine oil temperature to rise quickly (cold oil increases internal friction), and then be opened to maintain the temperature. In flight the shutters will close off again as the temperature of the air reduces at altitude.

Control of the shutters can be manual or automatic. Diesel engine lubrication systems are typically ‘Wet-Sump’ and would definitely include an oil cooler because of the need to dissipate the additional heat generated by the diesel engine. Lubrication Monitoring Instruments The importance of maintaining the correct Oil Temperature has been explained in the paragraphs above. The other parameters of the oil system monitored are Pressure and Quantity.

Piston Engines - Lubrication 3 48 3 Piston Engines - Lubrication The temperature of the oil in a piston engine is measured at the inlet to the engine pressure pump. Most aircraft use an electrical sensor to indicate the temperature to a flight deck gauge. Temperatures in the region of 85°C would be considered normal. Oil pressure is sensed at the outlet side of the engine driven pressure pump.

The pressure will depend on the size and loading of the engine, 50-100 psi being a typical value. The sensor can be electrical or a direct reading mechanical system. Both temperature and pressure sensing systems are covered in the Engine Instruments, Book 5. It is mandatory that oil temperature and pressure are indicated on the flight deck.

Oil quantity may be displayed. If not displayed there will be a facility for checking the quantity prior to flight, either by the use of a dip stick or sight glass. Correct oil temperature and pressure during engine operation are perhaps the most important indicators the pilot has of engine condition. Indications outside of operating limits could be indicative of impending engine failure.

Viscosity The varying load, power and outside air temperatures that aircraft engines operate at require oils with differing properties. Thickness of the oil is a very important factor, and is known as the oil’s Viscosity or Grade. Viscosity is defined as the measure of a fluid’s internal friction, or its resistance to flow. A liquid that flows freely has a low viscosity (thin oil) and one which is sluggish has a high viscosity (thick oil).

The viscosity of an oil will change with changes in Temperature. An increase in temperature will Reduce viscosity and vice versa. The engine’s operating temperature will vary considerably from the time when it is started from cold, to running at high power for long periods of time. The oil’s viscosity must stay within required limits to do its job, this range of temperature is termed its Viscosity Index.

Viscosity Grade Numbering There are various standards employed to determine the viscosity or thickness of oils. They all provide a datum by which differing oils can be compared. These methods measure the time taken for a fixed quantity of oil at a given temperature to flow through an orifice or jet of a given size. There are two standards that are generally employed in aviation to indicate the viscosity of oils.

These are the Society of Automotive Engineers, (SAE) and the Saybolt Universal systems. Both systems use numbers to indicate the viscosity. The lower the viscosity number, the thinner the oil. COMMERCIAL SAE NO.

SAYBOLT UNIVERSAL 30 60 40 80 50 100 60 120 It can be seen that the SAE number is half that of the Saybolt Universal system. Lighter loaded engines use a Low Viscosity or thin oil, whereas higher powered engines with higher loading 3 49 Piston Engines - Lubrication 3 Piston Engines - Lubrication would require a High Viscosity or thick oil. As previously stated the climate in which the engine operates also has an influence on the viscosity. For example a light aircraft operating within the UK during winter may use an 80 grade oil, and in summer it would use a 100 grade.

The choice being dependent on the average ambient temperature. The use of too high a viscosity oil at too low a temperature can cause problems during starting. There are in use oils that have two viscosity values - SAE 15W/50. These oils are called Multi -grade Oils.

They would give the characteristics of low viscosity at low temperatures, and high viscosity at higher temperatures. Types of Oil The type of oil used in aircraft piston engines is normally mineral based. If the oil contains no additives it is called a Straight oil. To meet certain requirement of engine operation, additives can be added to the oil.

These take the form of anti-oxidants, detergents and oiliness agents. These oils are called Compound oils. The two oils are identified by the viscosity numbering system, and if a compound oil the addition of letters or lettering. A bottle or can containing a straight oil with a viscosity of 80, would have only the number 80 marked on it.

A compound oil of the same viscosity may be marked AD 80 or W 80. The actual lettering varies with manufacturer. The letters AD stand for Ashless Dispersant, and is oil with specific qualities for cleaning. Generally straight oil is only used when running in new engines, or for specific engine installations.

As previously stated piston engines normally use mineral based oils, however some engine manufacturers have trialed and approved the use of Semi-synthetic Oils (Figure 3.3). Figure 3.3 Types of oil compound, multigrade and straight oil Piston Engines - Lubrication 3 50 3 Piston Engines - Lubrication Operational Considerations Indications of oil pressure and temperature give the pilot a good idea of the mechanical integrity of the engine. Of course the pilot must then interpret these indications correctly. On radial and inverted engines the pilot’s knowledge of the lubrication system is required even before starting the engines.

These engine can suffer from a problem called Hydraulicing, where oil accumulates in the lower cylinders between piston and cylinder head. As oil is incompressible damage to the engine could occur as the piston moves on the compression stroke. Prior to starting, these engines should be pulled through the cycle by use of the propeller, to ensure no hydraulic lock has occurred, (Confirm magnetos are OFF before turning engine). On starting positive engine oil pressure should be indicated within a specified time.

(Piper Warrior 30 seconds). If the engine is started from cold the oil pressure could be excessively high. This would be Normal as long as it drops to within its normal range as the engine warms up. Correct engine operating pressure and temperatures are dependent on each other.

High oil temperature could give low pressure. The oil pressure should be within its operating range at the correct operating temperature. Fluctuations in pressure could be the result of low oil levels, or system faults. Low pressure at normal temperature would indicate imminent engine failure, and a landing should be made as soon as possible.

A problem that can occur during starting in very cold weather is Coring. It is caused by the fact the cold viscous oil does not flow correctly through the engine. It should be remembered that an important task of the oil is to cool. The reduction in flow rate will not dissipate the heat being generated in the engine.

The result is that the Oil temperature rapidly rises, but this is only locally at the point of sensing. The problem is that the majority of the oil is Cold. To overcome coring oil cooler flaps should be Closed, this will initially increase temperature but should improve flow particularly through the cooler, and then bring temperatures down. It should be appreciated that as the oil is used to lubricate the moving parts of the engine, the oil will come in contact with the combustion gases.

Sealing of the valves and pistons is not 100% and as a result some oil will be burnt and the engine will therefore have an oil consumption rate. Ignoring external leakage, oil consumption varies between engines. A light aircraft would use around 1 pint per hour. A consumption rate greater than this would indicate wear in the engine.

The oil contents should always be checked prior to flight. If the engine has a Dry Sump system, the contents should be checked immediately after the engine has stopped, (realistically within a few minutes of shutdown). This ensures that the tank contents are recorded accurately before the oil migrates under gravity down into the engine sump. Large piston engines have oil tanks fitted with a check valve which is underneath the oil tank and closes under spring pressure or by an electrically operated actuator on engine shutdown.

The closing of the check valve prevents oil migration into the sump. The Wet Sump system is the opposite. A period of a least 15-20 minutes should have elapsed before the contents are checked in a similar fashion to motor cars. In any event, the oil level is checked after a period of time.

3 51 Questions 3 Questions Questions 1. From the following list select the correct combination of statements. The primary tasks of lubrication are to: 1. reduce friction 2. cool the engine 3. clean the engine 4. reduce component wear 5. act as a hydraulic medium a. 1 and 3. b.

2 and 5. c. 1 and 4. d. 1 and 5. 2. In a piston engine dry sump oil system, the oil temperature and pressure are sensed: a. when the oil is leaving the sump. b. for the temperature when the oil is leaving the tank, and for the pressure when the oil is leaving the pressure pump.

c. for the oil temperature when the oil is entering the tank and for the pressure when it is entering the pressure pump. d. at the same point. 3. Oil returning to the oil tank is filtered by: a. the oil pressure filter.

b. the oil tank filter. c. a micron size multi-bore filters assembly. d. the scavenge filter. 4.

Engine oil pressure is: a. low at idle rpm and high at high rpm. b. controlled by the oil cooler. c. substantially decreased when the oil pressure relief valve opens. d. relatively unaffected by engine speed.

5. The purpose of the crankcase breather is to: a. maintain the pressure in the oil tank at atmospheric pressure. b. ease the task of the oil scraper ring. c. prevent pressure building up inside the crankcase.

d. prevent distortion of the crankcase. 6. The most probably cause of small fluctuations in the oil pressure would be: a. lack of oil. b. the pressure relief valve sticking.

c. air in the oil tank. d. the scavenge pump working at a greater capacity than the pressure pump. Questions 3 52 3 Questions 7. The extra space in the oil tank is to cater for: a. frothing and aeration of the oil as it passes through the engine.

b. fire protection. c. the accommodation of extra oil contents on long duration flights. d. anti-surge action. 8.

The scavenge pump system in a lubrication system has: a. a bypass in case of blockage. b. a smaller capacity than the pressure pump. c. a bifurcated tertiary drive system. d. a larger capacity than the pressure pump.

9. In a “wet sump” oil system, the oil is contained in the: a. engine and tank. b. tank and oil cooler. c. sump and tank.