During the combustion of the air-fuel mixture, an enormous amount of heat is produced inside the engine cylinder and the temperature as high as 2500C may be reached by the burning gases. The temperature is so high that it will break the lubricating film between the moving parts, weld the moving parts or may cause any mechanical breakage of the engine parts. Hence, this temperature must be reduced by some means to such a value, about 200°C-250°C, at which the engine may work efficiently. Too much cooling would, however, the lower thermal efficiency of the engine. Thus, the purpose of the cooling system is to keep the engine at its most efficient operating temperature at all engine speeds and all driving conditions. About 15% of the total heat produced is utilized for useful work at the crankshaft. The remaining amount of heat is absorbed in, removed by exhaust gases and taken by the cooling system. The cooling system is designed to remove about 30 to 35% of the heat produced in the engine cylinder. When the combustion takes place, the cylinder walls, cylinder head, piston, and valves are heated. Their temperature should not reach excessive values.
|Engine Cooling System|
They must be cooled by some to a desirable temperature. It is also to be noted that the engine is quite inefficient when cold. The cooling system is so designed that it prevents cooling’ until the engine reaches its normal operating temperature. When the engine warms up, the cooling system begins
to function. It cools rapidly when the engine is too hot and it cools slowly or not at all when the engine is cooled or is warming up. Most engines are designed to operate in a definite temperature range which will ensure correct clearance between parts, promote vaporization of the fuel, keep the oil at its best viscosity and prevent the condensation of harmful vapor. Thus, the duty of the cooling system is to keep the engine from getting too hot- not to keep it cool.
Cooling beyond permissible limits is not desirable, because it decreases the overall efficiency due to the following reasons:
- Thermal efficiency is decreased due to more loss of heat to the cylinder walls.
- The vaporization of fuel is less, it decreases combustion efficiency.
- The viscosity of lubricant increases at low temperature, it increases friction.
Although more cooling improves volumetric efficiency, the above factors result in a decrease in overall efficiency.
Although various metals are used for the construction of cylinder and piston, but for better engine cooling, aluminum alloy is the best for various reasons:
- Aluminum is a much better heat conductor than cast iron or steel. For the same temperature difference, it will conduct about three times the quantity of heat, while copper is better still and will conduct six times the quantity of heat.
- Aluminum alloys enable the use of higher compression ratio, facilitate better cooling and lighter, the engine weight because of aluminum as only one-third of the density of cast iron.
- Concerning strength properties, a suitable aluminum alloy containing 9 to 12% copper, has tensile strength equivalent to that of cast iron.
PROPERTIES OF AN EFFICIENT COOLING SYSTEM
- The efficient cooling system removes 30 to 35% of the heat generated in the combustion chamber. Too much removal of the heat decreases the thermal efficiency of the engine.
- It removes heat at a fast rate when the engine is hot, and at a slow rate when the engine is started until the engine reaches its normal operating temperature.
METHODS OF COOLING
There are the following four methods of engine cooling:.
- Air cooling.
- Water cooling.
- Liquid cooling.
- Steam cooling.
Most automatic engines use air cooling and water cooling methods. Liquid cooling and steam cooling methods are rarely used in actual practice.