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The water may be used directly to cool the engine, but often has sediment, which can clog coolant passages, or chemicals, such as salt which can chemically damage the engine. Thus, engine coolant may be run through a heat exchanger that is cooled by the body of water.
In the case of an over-filled radiator (or header tank) pressure is vented by allowing a little liquid to escape. This may simply drain onto the ground or be collected in a vented container which remains at atmospheric pressure. When the engine is switched off, the cooling system cools and liquid level drops.
Water is inexpensive, non-toxic, and available over most of the earth's surface.Liquid cooling offers higher thermal conductivity than air cooling. Water has unusually high specific heat capacity among commonly available liquids at room temperature and atmospheric pressure allowing efficient heat transfer over distance with low rates of mass transfer.
Water-cooled engines remove this heat from around the cylinder head by surrounding it with a water jacket. In most familiar engines today, this water is circulated from the hot parts of the engine to a radiator, where it gives up its heat to the air. In early and low powered engines with hopper cooling there is little circulation.
As the boiling point of water increases with increasing pressure, these pressurised systems could run at a higher temperature without boiling. This increased both the working temperature of the engine, thus its efficiency, and also the heat capacity of the coolant by volume, allowing smaller cooling systems that required less pump power. [ 6 ]
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Adding water increases the mass being accelerated out of the engine, increasing thrust and it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect lets the engine run at higher RPM with more fuel injected and more thrust created without overheating.
Most modern internal combustion engines are cooled by a closed circuit carrying liquid coolant through channels in the engine block and cylinder head. A fluid in these channels absorbs heat and then flows to a heat exchanger or radiator where the coolant releases heat into the air (or raw water, in the case of marine engines).