Engine Cooling

Dr Jehad Yamin

Why Cooling?• To prevent excessive overheating of any part of the cylinder

which may give rise to pre-ignition.• The strength of the material used for various parts of the engine

usually decreases as engine temperature increases.• To prevent the overheating of the lubricating oil which

deteriorates its properties and hence causes piston seizure.• If the piston head is kept at high temperature, there would be

loss in volumetric efficiency and consequently in power due to reduction in the density of the fresh charge.

• High local temperatures may result in high thermal stresses due to uneven expansion of various parts of the engine which may also result in cracking.

Disadvantages of Cooling

• At low engine temperature the starting of the engine becomes difficult.

• At low temperatures, the corrosion becomes significant which reduces the engine life.

• At low engine temperature, the sulphurous and sulphuric acids resulting from the combustion of the fuel attack the cylinder barrel and causes it to corrode.

• It is due to these and other reasons that the engine must always be kept hot enough to assure smooth and efficient operation.

Cooling Systems• Air cooling. • In this system, air alone is used to cool the engine. • This is used in small engines or whenever it is important to

have lesser weight engines e.g. aircraft, agricultural and industrial engines, motorcycles.

• In these engines, the cylinder and cylinder head is finned to increase its surface area hence promote more of heat transfer.

• The height of the fins controls the distance between the cylinders and hence the engine size.

• The fin height is usually kept between 15-25 mm, further, heavily thermally-stressed parts must be adequately finned.

• Water cooling.• In this system, water alone is used to cool the engine.• This is used in automobile engines. In this system the cylinder and its

head are enclosed in a water jacket where the water flows to produce the cooling effect.

• The local velocity of the water and its bulk rate of flow are important. Further, the flow paths as well as the local velocity should be carefully designed to cool the highly thermally-stressed parts.

• For cold starting anti-freezing agents are used such as kerosene, wood alcohol, sugar solution, calcium or magnesium chloride glycerin …etc.

• The various methods used to circulate the water around the cylinder are

• (1) Thermo-syphon cooling, • (2) Thermostat cooling, • (3) Pressurized water cooling, • (4) Evaporative cooling and • (5) Cooling with thermostatic regulator.

Advantages of air cooling Advantages of water cooling

1. Eliminate the use of water hence smaller engine size and weight.

1. Due to higher latent heat of water, more heat can be removed from locally heated spots.

2. Simpler engine design. 2. Higher volumetric efficiency.

3. Less sensitive to climatic changes. 3. Higher specific output engines becomes possible and poses no problems with water cooling.

4. Better warm-up performance. 4. Water-cooled engine can be placed anywhere in the vehicle.

5. Reduced carbon deposits due to higher cylinder temperature.

6. Easier control of cooling system.

7. Can tolerate some degree of damage.

Disadvantages of air cooling Disadvantages of water cooling

1. Combustion noise is high. 1. Increased weight and size of engine.

2. Lower volumetric efficiency due to higher cylinder head temperature.

2. Requires more maintenance.

Radiator• The main function of the radiator is to reject coolant

heat to the atmosphere. • The name “Radiator” is a misnomer because the heat

transfer from the coolant to the air is done by conduction and forced convection and not by radiation.

• The cooling effect is achieved by dispersing the heated coolant into fine streams through the radiator matrix so that relatively small quantities of coolant are brought in contact with large metal surface areas which in turn are cooled by the stream of air.

Fan• The purpose of the fan is to maintain an

adequate air flow across the radiator matrix especially at low car speeds and under engine idling conditions.

• The basic disadvantages of using the cooling fan are :

• (1) Rising the noise level, • (2) Increase in power consumption, and • (3) Tendency to overcool the engine.

Quantity of Cooling water

• The quantity of cooling water required is given by the following equation :

 Mf = QBP /(K * ∆T) 

Where : • QBP is heat equivalent of BP transferred to coolant, • ∆T is the permissible temperature rise in cooling water, and • K is a constant depending upon the compression ratio and

specific water consumption.

• Solved example.• Compare the quantity of cooling water required

for a 100 kW petrol and diesel in which the water is allowed to raise in its temperature by 30 oC while passing through the jackets.

• In petrol engine, the percentage of energy going to the coolant is 30% of the total energy input to the engine, and that for the diesel engine is 26%.

• The thermal efficiency for petrol engine is 26% and that for the diesel engine is 31%. Take K = 4.186.

• Given : Case (I) : Petrol engine : BP = 100 kW, Thermal eff. = 0.26, percentage Heat

loss to coolant = 30%. The total energy input to engine = BP/thermal eff. = 100 / 0.26 = 384.615 kJ/sThe amount of heat lost to coolant = 384.615 * 0.3 = 115.385 kJ/sHence, the amount of cooling water required is = 115.385/(30 * 4.1868)

= 0.918 kg/s Case (II) : Diesel engine : BP = 100 kW, Thermal eff. = 0.31, percentage Heat

loss to coolant = 26%. The total energy input to engine = BP/thermal eff. = 100 / 0.31 = 322.58 kJ/sThe amount of heat lost to coolant = 322.58 * 0.26 = 83.87 kJ/sHence, the amount of cooling water required is = 83.87 /(30 * 4.1868)

= 0.668 kg/s