When fuel is supplied to an engine for combustion then the heat generated increases the engine temperature. It has been observed that the properties of metals change if the temperature is raised beyond certain limits and that increased temperature damages the engine. Hence it is essential to control the rise in temperature by providing certain means for cooling the engine in automobiles. This article discusses the various methods of cooling employed in engines.
Air Cooling
In case of the air cooling system or the direct cooling system, air is brought in contact with the engine parts. However, the specific heat of air is low (0.24), and therefore a large amount of air is needed to cool the engine parts.
Also air is not a good conductor of heat and it carries heat by the convection method only. It is for this reason that the air contact area is increased by providing fins. Fins are provided over the cylinder. In an automobile, such air-cooled engines are exposed to the atmosphere for cooling by the draughts obtained due the motion of the vehicle.
In case of scooters in which engines are covered, air blowers are used to force the cool air over the fins. The fins are made of aluminium which is a good conductor of heat and the shape increases the surface area for cooling. Due to inertia a certain time elapses before the heat begins to flow from the cylinder to the cooling fins. Since the heat is not quickly dissipated due to the characteristics of air, the cylinder temperature is higher in air-cooled engines than in water-cooled engines.
Advantages of Air Cooing.
1. The air cooling system is lighter in weight, i.e. less weight-power ratio.
2. It does not require a radiator and a water pump.
3. No antifreeze’s agents are required as in water-cooled engines.
4. No salt and mud deposits are formed in water-cooled engines.
5. Air-cooled engines are cheaper.
6. The same applies to cooling systems applied in server farms like EC fans for rack-mount servers.
Disadvantages.
I. The engine is not cooled efficiently and so runs hot. 2. Due to non-uniform cooling, temperature stresses are developed in the engine parts. 3. Engines arc noisier. 4. It needs an impeller to blow air over the fins. This impeller is noisy and it absorbs more power in case of scooters. in which engines are covered.
2. Water Cooling The water cooling system is an indirect cooling system. Water is cheap and easily available. Its specific heat being greater than that of air, water can absorb more heat than air. Water absorbs heat and dissipates it into the atmospheric air. Therefore, the same water can be circulated by a pump and used again. Figure 9.2 shows a water cooling system. Water is forced by a water pump in the water jacket of the engine. Hot water comes out from the jacket of the engine and is passed into the radiator. The vertical radiator tubes are brought in contact with the atmospheric air. Water passing through these tubes gets cooled and is collected at the bottom of the radiator. The water pump recirculates water through the water jacket of the engine. There is also a fan which is driven by the engine or electrically as in a modem car. The fan draws air and blows it over the engine parts. Thus the partial vacuum created behind the radiator. increases the pressure difference across the radiator and more air flows through the narrow gaps between the radiator tubes. Thus the quantity of air passing through the radiator is increased. This air drawn by the fan is blown over the engine body and the engine temperature is reduced.
The forced circulation of water does not allow its temperature to rise to the boiling point. Scale formation takes place in boiling water. Scales are the deposits of salts and mud and are a bad conductor of heat. Therefore scale deposition leaves hot spots in the combustion chamber which may lead to detonation and pre-ignition in a petrol engine. Hence forced circulation of water is essential in the engines. Advantages of Forced Cooling I. Forced cooling reduces the size of the radiator. Therefore a smaller radiator is needed. 2. Water under pressure is forced into every corner of the water jacket. Therefore no hot spots are left in the combustion chamber. 3. This system gives a rapid cooling effect. 4. Scale formation does not take place. 5. Pressurised cooling has an advantage that its boiling temperature is increased and its heat absorption capacity is also increased.
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