Liquid cooling systems are employed by most engines today. A typical automotive cooling system comprises
(1) a series of channels cast into the engine block and cylinder head, surrounding the combustion chambers with circulating water or other coolant to carry away excessive heat,
(2) a radiator, consisting of many small tubes equipped with a honeycomb of fins to radiate heat rapidly, that receives and cools hot liquid from the engine,
(3) a centrifugal-type water pump with which to circulate coolant,
(4) a thermostat, which maintains constant temperature by automatically varying the amount of coolant passing into the radiator, and
(5) a fan, which draws fresh air through the radiator.
For operation at temperatures below 32º F (0º C), it is necessary to prevent the coolant from freezing. This is usually done by adding some compound to depress the freezing point of the coolant. Alcohol formerly was commonly used, but it has a relatively low boiling point and evaporates quite easily, making it less desirable than organic compounds with a high boiling point, such as ethylene glycol. By varying the amount of additive, it is possible to protect against freezing of the coolant down to any minimum temperature normally encountered. Coolants contain corrosion inhibitors designed to make it necessary to drain and refill the cooling system only once a year.
Air-cooled cylinders operate at higher, more efficient temperatures, and air cooling offers the important advantage of eliminating not only freezing and boiling of the coolant at temperature extremes but also corrosion damage to the cooling system. Control of engine temperature is more difficult, however, and high-temperature-resistant ceramic parts are required when design operating temperatures are significantly increased.
Pressurized cooling systems with operating pressures up to 14 pounds per square inch (100 kilopascals) have been used to increase effective operating temperatures. Partially sealed systems using coolant reservoirs for coolant expansion if the engine overheats were introduced in 1970.
