Heat engines generate mechanical power by extracting energy from heat
flows, much as a water wheel extracts mechanical power from a flow of
mass falling through a distance. Engines are not perfectly efficient, so
more heat energy enters the engine than comes out as mechanical power;
the difference is waste heat and must be removed. Internal combustion
engines remove waste heat through cool intake air, hot exhaust gases,
and explicit engine cooling.
Engines with higher efficiency have more energy leave as mechanical
motion and less as waste heat. So, all heat engines need cooling to
Cooling is also needed because high temperatures damage engine materials
and lubricants. Internal-combustion engines burn fuel hotter than the
melting temperature of engine materials, and hot enough to set fire to
lubricants. Engine cooling removes energy fast enough to keep
temperatures low so the engine can survive.
Some high-efficiency engines and its
car parts run without
explicit cooling and with only accidental heat loss, a design called
as adiabatic. For example like this10,000 mile-per-gallon "cars" for the Shell
economy challenge are insulated, both to transfer as much energy as
possible from hot gases to mechanical motion, and to reduce reheat
losses when restarting. Such engines can achieve high efficiency but
compromise power output, duty cycle, engine weight, durability, and
Most internal combustion engines are fluid cooled using either air (a
gaseous fluid) or a liquid coolant run through a heat exchanger
(radiator) cooled by air.. The water may be used directly to cool the
engine, but often has sediment, which can clog coolant passages, or
chemicals, such as salt, that can chemically damage the engine. Marine
engines and some stationary engines have ready access to a large volume
of water at a suitable temperature. So, engine coolant may be run
through a heat exchanger that is cooled by the body of water.
There are many demands on a cooling system. One key is an engine fails
if just one part overheats. Therefore, it is vital that the cooling
system keep all parts at suitably low temperatures. Other demands
include cost, weight, reliability, and durability of the cooling system
Most liquid-cooled engines use a mixture of water and chemicals such as
antifreeze and rust inhibitors. Some use no water at all, instead using
a liquid with different properties, such as propylene glycol or a
combination of propylene glycol and ethylene glycol. Most "air-cooled"
engines use some liquid oil cooling, to maintain acceptable temperatures
for both critical engine parts and the oil itself, Most "liquid-cooled"
engines use some air cooling, with the intake stroke of air cooling the
combustion chamber. An exception is Winkle engines, where some parts of
the combustion chamber are never cooled by intake, requiring extra
effort for successful operation.
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. In each
step, the liquid-cooled engine has half the temperature difference and
so at first appears to need twice the cooling area.
After all, properties of the coolant (water, oil, or air) also affect
cooling. As for example like this, comparing water and oil as coolants,
one gram of oil can absorb about 55% of the heat for the same rise in
temperature. Oil has about 90% the density of water, so a given volume
of oil can absorb only about 51% of the energy of the same volume of
water. The thermal conductivity of water is about four times that of
oil, which can aid heat transfer. The viscosity of oil can be ten times greater than
water, increasing the energy required to pump oil for cooling, and
reducing the net power output of the engine.