ppl_06_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT

PRIMARY TASK OF THE LUBRICATION SYSTEM.

R e d u c i n g F r i c t i o n .

Engine components which move against each other are subject to friction. Friction of this kind causes engine power to be wasted, and leads to wear of the engine components.

Friction can be reduced by introducing a lubricant between the moving surfaces. In the case of aviation engines the lubricant is oil. An engine that relies on ‘Pressure Lubrication’ requires the oil to be forced between the moving parts at relatively high pressure. Alternatively the components can be ‘Splash Lubricated’: in a system whereby the moving parts are splashed by oil which has been thrown around the crankcase of the engine by the moving parts themselves.

The primary task of the oil in an engine lubrication system is to reduce friction and wear. But the oil also has a number of secondary functions which are extremely important.

SECONDARY TASKS OF THE LUBRICATION SYSTEM.

Co o l i n g .

Perhaps the most important of the secondary tasks is that of cooling. Oil flowing through the engine absorbs heat which is then dissipated through an oil cooler, one of which is shown in Figure 5.1. Without the oil cooler, the temperature of the oil would rise uncontrollably until the oil broke down and lost its lubricating ability.

Cl e a n i n g .

As the	oil flows through the engine
it carries away the by-products of
combustion and thus cleans the engine
too. A filter like the one shown in Figure
5.2 will remove contaminants from		Figure 5.1 An Oil Cooler.
the oil.	In this way the oil acts as a

detergent. Most of the components of the engine are ferrous in nature and will, if left alone in a damp oxygenated atmosphere, corrode, or oxidise. The oil prevents this corrosion by excluding the oxygen from the surface of the metal.

Figure 5.2 An Oil Filter.

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Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT IO N

T h e O i l s a s a H y d r a u l i c Me d i u m .

The oil can act as a hydraulic medium, for instance, when used as the hydraulic fluid in a variable pitch propeller mechanism, or within hydraulic tappets.

T h e O i l a s a n In d i c a t i n g Me d i u m .

Oil can also act as an indicating medium, giving information to the pilot about engine condition and power output, by, for instance, showing the oil pressure and temperature on gauges in the cockpit.

TYPE OF LUBRICATION SYSTEMS.

There are two types of lubrication system in common use. They are the wet-sump system and the dry-sump system.

Most light ‘non-aerobatic’ aircraft, like the one shown in Figure 5.3, use the wet-sump system, where the oil is stored in the bottom of the engine, in an engine component which is usually called the sump. It is from the word “sump” that we get the names of the

two lubrication systems. Figure 5.3 A Wet-Sump Engine.

THE WET-SUMP SYSTEM.

The use of a wet-sump simplifies the construction of the engine, but leads to a number of disadvantages which we will discuss later. In the wet-sump engine the oil is circulated by the pressure pump. The oil then passes through a high pressure filter into internal drillings in the crankcase. These internal drillings are called the ‘oil gallery’.

Ducts in the oil gallery transfer the pressure oil to provide lubrication for the various drives in the accessory casing and also the main crankshaft bearings.

Figure 5.4 Oil grooves in the crank case main bearings and crankshaft.

Annular grooves (see Figure 5.4) are cut in the main bearings and these are aligned with transverse drillings in the crankshaft itself. The annular grooves fill with pressure oil, which is forced through the transverse drillings in the crankshaft, via the crank pins, into the big end bearings (see Figure 5.5).

Oil droplets which are squeezed from between the bearing surfaces will collide with the crankshaft as it rotates, causing the inside of the crankcase to be filled with a fine oil mist. This oil mist will provide splash lubrication for the cylinder walls, the pistons, the small end bearings, gear teeth and valve mechanisms.

Lubrication for the valve gear rocker arms is fed through external oil pipelines.

74

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT

Di s a d v a n t a g e s o f T h e W e t - Su m p Sy s t e m .

The first disadvantage of the wet-sump system is that there are lubrication difficulties during some aerobatic manoeuvres, such as stall turns and inverted flight. During extreme manoeuvres, oil is thrown around inside the engine sump. This causes parts of the engine to be under-lubricated, which can damage components. Likewise, parts of the engine can be over-lubricated, which can be equally hazardous, potentially causing a damaging build up of oil pressure.

	The second disadvantage of the wet-
	sump system is that the temperature
	of the oil is more difficult to control,	Figure 5.5 Annular grooves in the Big End	Inverted
	because it is stored within the engine
		bearings.
			flight can be
	in the sump, which itself is hot.
			particularly
	The third disadvantage of the wet-sump system is that the oil becomes oxidised and		dangerous to a wet-sump
			engine.

blackened by the continual stirring action of the crankshaft and connecting rods. This means that the oil and the oil filter have to be changed much more often than would be the case in a dry-sump engine.

Finally, the amount of oil available is limited by the sump capacity. The sump of a wet-sump engine is the container for the oil supply, and its size is determined by the original engine design, which in practical terms cannot be changed.

THE DRY-SUMP SYSTEM.

The dry-sump system overcomes all of the problems of a wet-sump system by storing the oil in a remotely mounted tank.

An oil

temperature sensor is

located

downstream of the oil tank.

Figure 5.6 A Dry Sump Lubrication System.

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Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT IO N

Sy s t e m Co m p o n e n t s .

The oil tank is made of sheet metal, suitably baffled internally in order to stop the oil surging around while the aircraft is manoeuvring.

T h e T a n k .

An oil pressure

pump must be able to

supply a minimum specified oil pressure when the engine is running slowly and the oil temperature is very high.

Figure 5.7 An Oil Tank.

Whenever possible the tank is placed above the level of the engine to ensure a gravity feed into the engine. The tank forms a reservoir of oil large enough for the engine’s requirements, plus an air space.

The air space is very important, allowing for several potentially hazardous lubrication situations. For instance, the airspace permits the expansion of the oil when it gets hot, as it inevitably will. Also frothing of the oil due to aeration as it flows around the engine will occur, requiring a greater volume of space in the tank.

Another situation which requires the air space arises every time the engine is shut down. After engine shutdown, the walls of the crankcase are saturated with oil that drains into the sump. The oil will remain there until the engine is restarted, when the scavenge pump, which is fitted at the bottom of the sump, will return the oil to the oil tank. There has to be sufficient room in the tank to accommodate the returned oil.

Finally, if the engine has any oil operated devices fitted to it, such as a variable pitch propeller, the displaced oil caused by these mechanisms has to go somewhere, and the tank serves as the receptacle for it.

T h e P r e s s u r e P u m p .

The pressure pump (see Figure 5.8) consists of two deep-toothed spur gears rotating in a close fitting pump casing. It is fitted on the accessory casing and is driven from the crankshaft by gears.

Oil pressure is sensed at the outlet of the pressure pump (see Figure 5.6).

76

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT

Unless it is controlled, the oil pressure will vary depending on the speed of the pump, the temperature of the oil and the resistance offered by the engine components.

The pump must be able to supply a minimum oil pressure under the most adverse running conditions; that is, when the engine is running slowly and the oil inlet temperature is very high. Consequently, there is an inherent danger that under normal running conditions, oil pressure may be too high and damage the engine or its components.

Figure 5.8 A Pressure Pump.

Therefore, to maintain the oil pressure constant during normal engine running conditions, and also to prevent excessive oil pressure in the system, a pressure relief valve is fitted across the inlet and the outlet pipes of the pump.

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. A coarse wire mesh suction filter is fitted between the tank and the pressure pump. It is designed to remove large solid particles from the oil before it enters the pressure pump, and thus prevent damage to the pump.

The pressure filter is fitted downstream of the pressure pump, before the oil enters the engine. The pressure filter is designed to remove very small solid particles before the oil passes to the bearing surfaces, again preventing damage to the engine.

The scavenge pump returns the oil which has passed through the engine into the sump back to the oil tank. Essentially the scavenge pump is the same design as the pressure pump, but larger. This is because in order to maintain a dry-sump, the scavenge pump needs to be of a larger capacity than the pressure pump.

T h e O i l Co o l e r .

Oil is used as a cooling medium in the engine. If the oil gets too hot then it will fail as a lubricant and, as a consequence, the engine will also fail. To prevent the oil temperature becoming too high, an oil cooler is introduced into the system (see Figure 5.1). The oil cooler consists of a matrix block which forces the oil into a thin film as it passes through it. The cooler matrix is exposed to the flow of cold slipstream air which is directed through the cowlings.

Engines with a high power output may have shutters fitted to the oil cooler. The shutters are used to control the flow of air through the cooler. They can be manual or automatic in operation.

Prior to engine start, the engine oil will normally be cold and viscous. The cooler matrix is fairly delicately constructed and can easily be damaged by high internal pressures. To prevent damage to the cooler matrix, an anti-surge valve, which is sensitive to oil pressure, is placed in parallel with the oil cooler (see Figure 5.6). The anti-surge valve allows cold thick oil to by-pass the cooler and return directly to the tank.

In order to

maintain the oil pressure

constant

during normal engine running conditions, and to prevent excessive oil pressure in the system, an oil pressure relief valve is fitted.

The scavenge

pump has the same design

as the pressure pump but has a larger capacity.

In order

to prevent damage to

the oil cooler

matrix, an anti-surge valve, which is sensitive to oil pressure, is placed in parallel with the oil cooler.

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Figure 5.10 Oil 80 and Oil W80.

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT IO N

QUALITIES AND GRADES OF OIL.

Viscosity is defined as the measure of the internal friction of a fluid.

Different engines operating in different ambient conditions will need different grades of oil. The grade of an oil is mainly dictated by its viscosity.

Viscosity is defined as the measure of the internal friction of a fluid. A liquid that flows freely has a low viscosity, and a fluid that flows sluggishly has a high viscosity.

There are various standards employed to determine the viscosity of oils in order that different oils may be compared.

Figure 5.9 A Viscosity Table.

The two standards which are generally

employed in aviation are the ‘Society of Automobile Engineers’ or SAE; and the

‘Saybolt Universal’. Both systems use numbers to denote the viscosity, the lower the number, the thinner the oil.

Some oils have two viscosity values. For example, SAE 15/50. Oils with two viscosity values are called ‘Multigrade Oils’. Multigrade oils have the characteristics of low viscosity at low temperatures, and high viscosity at high temperatures.

The type of oil normally used in aircraft piston engines is mineral based. However, some engine manufacturers have trialed and approved the use of

‘Semi-Synthetic’ oils.

If an oil contains no additives it is called a ‘Straight’ oil. A straight oil can be recognised by the fact that it only has an identification number. For instance,

the bottle with the blue label marked ‘Oil 80’ in Figure 5.10 contains a straight oil. Note that the oil has a viscosity rating of SAE 40. Generally speaking, a straight oil is used only when running in new engines. However, there are certain requirements of some engines that straight oils cannot meet.

To satisfy such requirements, additives must be mixed with the oil. These additives can take the form of anti-oxidants, detergents and oiliness agents.

Oils with additives are called ‘Compound’ oils. A compound oil of the same viscosity as the straight oil shown previously, SAE 40, would be kept in a container marked W80, like the one with the red label in Figure 5.10. This particular oil is used where specific cleaning qualities are required.

78

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT

OIL PRESSURE AND TEMPERATURE.

Oil pressure and temperature readings, when interpreted correctly, are indicative of the mechanical integrity of engine operation. A scan of the aircraft instruments must always include these vital items. The information they give may enable the pilot to take action which will prevent a bad situation getting much worse.

The oil contents must be verified

during the pre-flight check to ensure there is sufficient oil for the flight.

Figure 5.11 Engine Instruments.

As part of the pre-flight check, the pilot must verify that the oil contents are sufficient for the flight that is to be undertaken. The oil filler cap must be secure and a thorough check made to ensure that there are no oil leaks.

After engine start the oil pressure guage must indicate the correct oil pressure within a specified time. If the engine is started from cold, the oil pressure may initially seem excessively high. However, as long as the oil pressure drops to normal as the engine warms up, the initial high pressure reading can

be considered acceptable.

Figure 5.12 Oil Contents Check.

If the aircraft has a dry-sump engine, its oil contents must be checked immediately after the engine has stopped: realistically within a few minutes of shut down. This ensures that the tank contents are recorded accurately before the oil migrates into the sump under the influence of gravity. If, on the other hand, the engine has a wetsump, a period of 15 to 20 minutes must elapse before the contents are checked, to allow all oil to return to the sump.

After a wet

sump engine is shut down,

15 to 20

minutes must elapse before the oil contents are checked.

HYDRAULICING.

Radial and inverted engines can suffer from a condition known as ‘hydraulicing’.

Hydraulicing happens if oil accumulates between the piston and the cylinder head in the cylinders at the bottom of the engine.

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Figure 5.13 High Oil Pressure.

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT IO N

To prevent

‘hydraulicing’ on some radial and

inverted engines, the engine must be rotated by pulling through on the propeller, after confirming that the magnetos are ‘OFF’ before the propeller is touched.

Because oil is incompressible, if it is trapped between the piston and the cylinder head on the compression stroke, a hydraulic lock will be formed. If this happens, severe damage may occur when the starter motor is being used to start the engine. The piston might break, the connecting rod might bend, the cylinder may be torn from the crankcase, or the crankshaft may break.

To prevent hydraulicing, the engine must be rotated by turning the propeller, after confirming that the magnetos are OFF before the propeller is touched.

OIL SYSTEM MALFUNCTIONS.

There are four main indications associated with a failure of the lubrication system:-

•Oil pressure too high

•Oil pressure too low

•Fluctuating oil pressure

•Oil pressure falling to zero

O i l P r e s s u r e T o o H i g h .

Too high an oil pressure, with normal oil temperature, may be caused by the pressure relief valve being set incorrectly. But this is unlikely unless the component has been worked on just prior to the flight. If, on the other hand, the oil temperature is low, the high oil pressure may be caused by the relatively high viscosity of the oil at low temperatures making it difficult for the oil to pass through the engine. The oil pressure relief valve may be unable to control the situation completely until the oil temperature rises. But, as long as

the oil pressure drops to normal as the engine warms up, the initial high pressure can be considered acceptable. However, if the pressure indication is very high, and remains high even after the oil temperature gauge has indicated a significant temperature rise the engine must be shut down to prevent damage.

O i l P r e s s u r e T o o

Too low an oil pressure, with normal or even low oil temperature, could once again mean that the oil pressure relief valve has been set incorrectly. But again this is unlikely unless the component has been worked on just prior to the flight. The problem here is more likely to be excessive clearance in the bearings due to wear. Potentially even worse is an oil leak from the output side of the

pressure pump.

Figure 5.14 Low Oil Pressure.

80

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT

If the minimum oil pressure cannot be maintained, the engine must be shut down in order to prevent damage.

If the oil pressure is too low, with an associated high oil temperature, then this is probably due to the temperature causing the viscosity of the oil to drop below the viscosity value at which the oil pressure relief valve can cope. Remember, the task of the relief valve is to maintain constant engine oil pressure, regardless of engine R.P.M. or oil temperature.

If the engine is fitted with oil cooler shutters, they can be opened in an attempt to lower the oil temperature. If oil cooler shutters are not fitted, the pilot should consider either reducing engine power, or lowering the nose to increase airspeed. The mixture may also be enriched. Any or all of these actions can be taken to cool the engine and, thus, cool the oil, bringing its viscosity back up to the value where the lubrication system can function normally. Once again though, if the minimum oil pressure cannot be maintained, then the engine must be shut down to prevent damage.

F l u c t u a t i n g O i l P

Small fluctuations of the oil pressure gauge needle, either side of the correct pressure, indication may be symptomatic of a sticking pressure relief valve.

A sticking	pressure relief valve
would initially	cause the pressure to

build up beyond the optimum level. Subsequently, when the pressure reached a value high enough to overcome the “stiction” of the valve, the valve would open, causing the pressure to fall momentarily below the correct level. Then the cycle would start again.

Large fluctuations of the oil pressure gauge needle, will probably be caused by insufficient oil in the system. If the engine has been leaking oil, the oil level will eventually drop to a point where the pressure pump cannot sustain its output. In such a situation pressure will drop momentarily until the pump draws in more oil. At this point the pressure will rise again, if only for a moment. Again, if the minimum oil pressure cannot be maintained, the engine must be shut down to prevent damage.

r e s s u r e .

Figure 5.15 Fluctuating Oil Pressure.

Figure 5.16 Zero Oil Pressure.

O i l P r e s s u r e F a l l i n g t o Z e r o .

If the oil pressure falls suddenly to zero, the pilot needs to quickly work out what has happened. If the minimum oil pressure cannot be maintained, the engine must be shut down immediately to prevent damage. The probable cause of a zero pressure indication is either failure of the pressure pump or a catastrophic loss of oil from the system.

81

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 5 : ENG INE LU BR ICAT IO N Q U EST IO NS

R e p r e s e n t a t i v e P P L - s t y l e q u e s t i o n k n o w l e d g e o f t h e Lu b r i c a t i o n Sy s t e m .

1.By what activation method does the valve which allows oil to either flow through or by-pass a serviceable engine oil cooler work?

a.Temperature activated

b.Pressure activated

c.Manually activated

d.Electrically activated

2.Excessive oil pressure can be resolved by using:

a.An oil pressure relief valve

b.A thermal cut-out

c.A filter by-pass valve

d.An oil tank overflow

3.The prevention of excessive oil pressure in an aircraft engine is assured by:

a.Ensuring that the engine does not exceed the red-line RPM value.

b.The engine’s high capacity pressure pump.

c.The engine’s oil pressure relief valve.

d.The engine’s filter by-pass valve.

4.Where is the sensor of the engine oil temperature gauge located?

a.Within the exhaust sections of the engine

b.Before the oil cooler

c.After passing through the oil cooler, but before reaching the hot sections of the engine

d.In the same position as the pressure sensor for the oil pressure gauge

5.The scavenge pump:

a.Is of a similar design as the pressure pump but has a smaller capacity

b.Is located at the exit of the oil cooler

c.Has a fine mesh filter at its outlet to protect the oil cooler matrix

d.Is of the same design as the pressure pump but has a larger capacity

Question	1	2	3	4	5
Answer

T h e a n s w e r s t o t h e s e q u e s t i o n s c a n b e f o

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