Cooling
System
 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.
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