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
(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.