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CHAPTER 4

ENGINE COOLING

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CH AP T ER 4 : ENG INE CO O LING

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

Customer: Oleg Ostapenko E-mail: [e-mail] CH AP T ER 4 : ENG INE CO O

	AIR COOLING.
	The piston engine is a heat engine.
	Its purpose is to convert the energy
	released from burning fuel into
	mechanical energy, and, by so doing,
	generate usable power. The thermal
	efficiency of a light aircraft piston petrol
	engine may reach 30 percent. This,
	however, means that 70 percent of
	the heat energy released by the fuel is
	wasted. Around 40 percent will leave
	with the exhaust gas, but 30 percent
	stays behind to raise the temperature	Figure 4.1 A Light Aircraft Engine.
	of the engine components and the oil.

O v e r h e a t i n g .

If nothing were done to alleviate the rising temperature of the engine, several problems could occur:-

• Engine components could start to fail structurally.

• The oil could break down and lose its lubricating properties. (You will learn more about this in the chapter on lubrication.)

• The fuel could ignite as soon as it enters the cylinder, before the spark plug fires; this is called pre-ignition. Pre-ignition causes loss of power and may exacerbate the overheating problem.

	The incoming mixture may get too hot, which	Figure 4.2 Engine
	will cause unstable combustion, usually called
		Overheating.
	detonation, or knocking. (You will learn more

about this in the chapter on carburation.) Detonation will also cause loss of power and further overheating, and possible damage to the engine.

O v e r c o o l i n g .

The problems associated with overheating are perhaps obvious, but what problems can be caused by overcooling? We have first to remember that maximum thermal efficiency is only achieved at high engine temperatures, so the engine needs to be kept as hot as possible without causing the problems mentioned in the paragraph on

‘Overheating’ .

A low engine temperature also increases the viscosity (the internal friction) of the oil which will reduce the Brake Horse Power. A pilot must, therefore, keep the engine temperature, and thus the oil temperature, fairly high, but not so high that the oil breaks down.

Figure 4.3 Engine

Overcooling.

Detonation

will cause loss of power,

overheating, and possible damage to the engine.

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CH AP T ER 4 : ENG INE CO O LING

In order to burn, the fuel must vaporise. Very low engine temperatures would prevent complete fuel vaporisation. Some fuel would not be burnt, reducing the power output of the engine.

COOLING SYSTEMS.

In order to work efficiently, the engine needs to be at the highest temperature consistent with safe operation.

To maintain the engine temperature within a range which will satisfy its working requirements a cooling system is required. Most light aircraft engines are air-cooled. The simplicity of an air-cooled cooling system makes it virtually maintenance free and it is much lighter than the alternative, the liquid cooling system. However, the liquid cooling system is more efficient, gives more precise control of engine temperature and generates less drag. Many high performance piston engines, therefore, especially those which powered military aircraft during and after World War 2, employed liquid cooling systems.

FACTORS AFFECTING COOLING.

Am b i e n t Ai r T e m p e r a t u r e .

One of the major factors governing the efficiency of an air-cooled system is the ambient air temperature. Although the ambient air temperature can vary widely with changes in climate and altitude, it must be borne in mind that dissipation of the heat will be more rapid as air temperature decreases.

Ai r s p e e d .

The speed of the airflow is the other major factor affecting the efficiency of the air cooling system. Uncontrolled, the speed of the airflow over the cylinders will vary with the speed of the aircraft and cause the temperature of the cylinders to fluctuate accordingly. Obviously, air-cooling is least effective at high power settings and low airspeed. Consequently, engine temperatures have to be monitored closely in the climb, especially if the climb is prolonged.

En g i n e Ma t e r i a l s .

Severalmethodsofimprovingtheefficiency of cooling systems can be employed. Firstly, the engine components are made of materials with a high conductivity, for instance cylinder heads are sometimes made from aluminium alloys, aluminium being a good conductor of heat.

F i n s .

The walls of the cylinders are finned to increase the cooling area (see Figure 4.4). The fins are thin in section and may be extended at local hotspots, such as the exhaust ports, to increase their area, in order to achieve an even temperature.

Figure 4.4 The cylinder is finned to increase the cooling area.

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O i l .

The oil used to lubricate the engine is also a cooling medium, so the amount of oil in the engine must be within limits.

T h e En g i n e Co w l i n g .

The engine is streamlined by surrounding it with a cowling. This encloses the engine within a controllable environment and also reduces drag.

Ba f f l e s .

Baffles (see Figure 4.5) are directional air guides which direct the airflow in an efficient manner around the cylinders. Baffles must be close-fitting and are sealed against the cowling to ensure that all of the airflow is over the cylinders and that each part of the cylinder is supplied with equal amounts of cooling air.

4 : ENG INE CO O

As well as

lubricating the engine,

oil plays an

important part in cooling the engine.

Co w l F l a p s O r G i l l s .

Figure 4.5 Cowl Flaps.

Cowl flaps and Gills (see Figure 4.5) can be fitted to the cowlings to ensure control of the temperature. They are usually manually controlled on light aircraft, and, when open, the airflow over them causes a pressure drop which accelerates the airflow within the cowlings.

PROCEDURES TO ENSURE ADEQUATE COOLING.

Prior to flight, the air inlet and outlet to the cowlings must be checked to ensure that they are clear of obstruction.

The condition of baffles and cowling must be checked.

The operation of the cowl flap or gill, if fitted, must be checked.

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CH AP T ER 4 : ENG INE CO O LING

Check the engine oil level. Remember that as well as lubricating the engine, oil plays an important part in engine cooling.

At high power settings and low airspeeds, such as those occurring at take off, the cowl flaps (gills) should be selected open in order to increase the flow rate over the cylinders, and so increase cooling.

In the climb and cruise the cowl flaps or gills can be adjusted to maintain the engine temperature at the optimum.

In descent, engine power is reduced and less heat is generated. A very rapid descent will overcool the engine, and this can lead to what is known as Thermal Shock. Thermal Shock is the name given to cracking as a result of rapid temperature change. Overcooling in the descent can cause engine components to contract at different rates, and, thus, can result in their failure. Therefore the cowl flaps (gills) should be closed during descent to reduce the cooling effect. The cowl flaps (gills) should be opened on final approach in case the pilot is required to go around. If no cowl flaps (gills) are fitted, the pilot should increase engine RPM for a brief period, at regular intervals during the descent, in order to avoid overcooling of the engine.

During ground running, engine oil temperatures and cylinder head temperatures should be closely monitored to ensure the limitations are not exceeded. Although initially after engine start the cowl flaps (gills) should be closed to encourage quick engine warm up, they should subsequently be opened to prevent overheating.

During flight, especially during a long climb at full power, monitor the oil temperature and cylinder head temperature closely. If the cylinder head temperature becomes excessive, there are several methods of cooling the engine. Obviously, as has just been stated, the cowl flaps (gills) can be opened. Perhaps less obviously, the mixture can be made richer. With a rich mixture, the excess fuel will evaporate and cool the charge. A reduction in engine power will tend to lower engine temperatures. An increase in forward speed will have the same effect.

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

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

CH AP T ER 4 : ENG INE CO O LING Q U

R e p r e s e n t a t i v e P P L - t y p e q u e s t i o n s t o t e k n o w l e d g e o f En g i n e Co o l i n g .

1.Which of the following actions would help to improve engine cooling during the climb in an aircraft fitted with an air cooled engine?

a.Leaning the air/fuel mixture

b.Closing the cowl flaps

c.Increasing power and reducing speed

d.Decreasing power and increasing speed

2.Which of the following design features would not increase the effectiveness of the air-cooling of an aircraft piston engine?

a.The fitting of baffles and directional air ducts

b.“Finning” the cylinders

c.Manufacturing major engine components using low conductivity materials

d.Fitting cowl flaps and gills

Question 1 2

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 u n d a t

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CHAPTER 5

ENGINE LUBRICATION

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

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