Need for Engine Cooling:
From the viewpoint of converting heat into mechanical energy, it follows that if each piston accomplished its power stroke starting at the temperature of combustion, which for spark ignition is in the region of 2500°C, such an engine would in theory be highly efficient. To exploit this in practice, however, would impose unacceptably high temperatures on the materials of the working cylinder. In descending order, the melting points of cast iron cylinders, steel valves and aluminium pistons are about 1800°C, 1350°C and 900°C respectively, hence all these temperatures would be exceeded in the absence of any cooling.
The behavior of metals at elevated temperatures also differs from that at normal temperatures. Apart from losing some of their strength they can exhibit a condition known as creep, in which the metal deforms slowly and continuously at a constant stress. Excessively high temperatures would in any event cause breakdown of the cylinder lubricating oil films, resulting in the overheated working parts losing their running clearances and seizing up.
For these above mentioned reasons the engine must be provided with a system of cooling, so that it can be maintained at its most efficient practicable operating temperature. This generally means that the temperature of the cylinder walls should not exceed about 250°C, whereas the actual temperature of the cylinder gases during combustion may, as previously noted, reach many times this figure.
Conversely, there is no merit in operating the engine too cool since this would reduce thermal efficiency and therefore increase fuel consumption. It would also increase oil dilution and hasten corrosion wear of the engine.
It is perhaps worth recalling an observation made many years ago by D.R. Pye, a one-time leading engine specialist, that ‘One may go so far as to say that if an engine is not within a narrow margin of overheating at some point or other it is not working at its maximum capacity; or, expressed otherwise, that a change of condition which reduces the cylinder temperatures at critical points under given conditions of running, will raise the limit of power output of which the cylinder is capable.’
Methods of heat transfer
It is a common experience that heat always flows from a hotter to a cooler substance, and a physical principle that the greater the temperature difference between them the more rapidly is the heat transferred, until both substances attain the same temperature. That is, the loss of heat from one substance is equal to the gain in heat by the other. The process of heat transfer can take place in three ways, all of which are encountered in the engine cooling system, and they may be summarized as follows:
Conduction occurs most readily when heat energy is supplied to a solid substance, through which it is transmitted by direct contact or collision of molecules in the substance itself. The resulting molecular vibration increases as the temperature of the substance is raised and decreases when the temperature is lowered. An insulator is a substance that is a relatively poor conductor of heat and therefore acts as a thermal barrier.
Convection occurs when heat energy is supplied to a liquid or a gas and produces density changes in it. These promote natural convection currents, because the lower-density warmer matter rises to replace the higher-density cooler matter which descends. The process becomes one of forced convection with enhanced heat transfers when the circulation of the liquid is assisted by a pump, or that of the air by a fan.
Radiation occurs when heat energy is transferred by wave motion, there being neither contact of the substances nor molecular vibration within them. Radiant heat energy is emitted by all substances and may be either absorbed or reflected by others. Their ability to emit, absorb or reflect heat depends upon the color and nature of the surfaces concerned, black rough ones being best for absorption and light polished ones best for reflection.
In practice, motor vehicle engines are designed either for indirect cooling by air through the medium of water or, less commonly, for direct cooling by air. Expressed in everyday language, these two systems are known simply as water cooling and air cooling respectively. Each system possesses certain advantages over the other.
Advantages of water cooling:
 Cooling is more uniform because heat is transferred with greater rapidity from the engine metal surfaces to water than it is to air. This can be explained by the specific heat of water being higher than that of air, so that if equal masses of water and air each receive the same quantity of heat, the water will experience the least rise in temperature. Consequently, there will be a more rapid transfer of heat as a result of the higher mean temperature differences between the metal surfaces and the water in contact with them.
Water Cooled Engine
 Cooling is more constant because the time taken for the water to rise through a given temperature range is longer than that for the same mass of air. This property of the water possessing a greater thermal inertia than the air can be advantageous in maintaining a more nearly constant operating temperature – that is, when the rate of heat emission equals the rate at which heat is generated in the engine.
 Interior heating for the vehicle is improved because outside air may be directed through a heat exchanger matrix, which can be conveniently heated by circulating a portion of the engine coolant through it.
Advantages of air cooling:
 Warm-up of the engine is more rapid because of the lower specific heat of the air cooling medium; heat is less readily transferred from the engine metal to the air being circulated around it, as a result of their lower mean temperature difference.
Air Cooled Engine
 The system is inherently more reliable because air cooling is immune to either freezing or boiling of the coolant around the cylinder heads and cylinders, and to the loss of coolant. It is also free from any build-up of corrosive products that can restrict coolant passages.
 Less maintenance is required in service because there is neither the requirement to check the cooling medium for level and condition, nor the need to inspect rubber connecting hoses for signs of leakage.