Engine cooling is the process of maintaining temperature in an engine by using either air or liquid.
As engines generate power they also generate heat because they are not perfectly efficient. The engine must therefore be cooled to prevent some components getting so hot that materials or lubricants fail.
Although some waste heat goes out with exhaust gases in most conventional internal combustion engines, further cooling is needed.
Basic principles
Most internal combustion engines are "air-cooled" or "liquid-cooled". Each principle has advantages and disadvantages, and particular applications may favour one over the other. For example, most cars and trucks use liquid-cooled engines, while most small airplane engines are air-cooled.
Most liquid-cooled engines use a mixture of water and other chemicals such as antifreeze and rust inhibitors. Some use no water at all, instead using a liquid, such as propylene glycol or the combination of propylene glycol and ethylene glycol. Although the term "liquid-cooled" is used, most air-cooled engines also use some liquid oil cooling, and most liquid-cooled engines subsequently cool the hot liquid using air.
Conductive heat transfer is proportional to the temperature difference between materials. If an engine metal is at 300°C and the air is at 0°C, then there is a 300°C temperature difference for cooling. An air-cooled engine uses all of this difference.
In contrast, a liquid-cooled engine might dump heat from the engine to a liquid, heating the liquid to 150°C which is then cooled with 0°C air. Thus, in each step, the liquid-cooled engine has half the temperature difference and so may need as much as twice the cooling area.
For all these reasons, it is difficult to make generalizations about air-cooled or liquid-cooled engines. Air-cooled Volkswagen kombis are known for sometimes "eating engines", with both rapid wear in normal use and sometimes sudden failure when driven in hot weather. On the other hand, air-cooled Deutz diesel engines are known for reliability even in extreme heat, and are often used in situations where the engine runs unattended for months at a time.
It is usually more difficult to get either low emissions or low noise from an air-cooled engine, two reasons why most road vehicles use liquid-cooled engines. It is also often difficult to build large air-cooled engines, so nearly all air-cooled engines are under 500 kW, whereas large liquid-cooled engines exceed 80 MW (Wärtsilä-Sulzer RTA96-C 14-cylinder diesel).
It can be more difficult to build a light liquid-cooled engine for a given power output, and weight is one reason air-cooling is common in aircraft engines, although reliability through simplicity and easy access to cool air are two other reasons.