Cooling system is designed to keep the engine at its optimal operating temperature. It cools down the engine when the optimal temperature is exceeded. Automobile engine is a heat engine and apparently it is a waste of energy to remove heat from an engine but it is due to the reliability requirements and metallurgical properties of the metals used in the engine construction that excessive heat has to be removed.
Engine Performance
Engine performance is directly linked to the cooling system. Engine uses a rich mixture when the engine (or coolant) is relatively cool and then moves to lean the mixture as engine heats up. If an engine is overheating, the ECU or PCM would reduce the ignition advance and/or fuel injection pulse duration, both leading to a loss of power. Engine temperature is measured indirectly through the coolant temperature.
Choice of Coolant
A coolant is also known as anti-freeze in everyday parlance but it gives an incomplete picture of its functions. A coolant should not freeze in winter when temperatures are below zero degree Celsius but it should also have a higher boiling point (at least higher than water, which is 100°C at sea level). Additionally, it should have a low vapor pressure, high specific heat capacity, high enthalpy of vaporization and very low electrical conductivity. Pure water has a high specific heat capacity but has a high vapor pressure (unsuitable), a lower boiling point (unsuitable), high freezing point (unsuitable) and unsuitable electrical conductivity. Therefore, water is mixed with other chemical coolants like glycols to produce an acceptable mixture. Water can be used temporarily though, in case the engine develops a coolant leak on the road and you need to top up the coolant. In Pakistan (1990s), it was common to see truck drivers (very rugged Bedford trucks) pouring fresh water into hot radiators on the road side. Not only water is not a good candidate as a coolant (except in some races) but cold water should not be poured into a hot engine.
