In remaining (70%) must be removed from

In Engine when fuel is burned heat
is produced. Additional heat is also generated by friction between the moving
parts. All the heat produced by the combustion of fuel in the engine cylinders
is not converted into useful power at the crankshaft. Only approximately 30% of
the energy released is converted into useful work. The remaining (70%) must be
removed from the engine to prevent the parts from melting. For this purpose
Engine have cooling mechanism in engine to remove this heat from the engine
some heavy vehicles uses water-cooling system and almost all two wheeler uses
Air cooled engines, because Air-cooled engines are only option due to some advantages
like lighter weight and lesser space requirement. In an internal combustion
engine, the expansion of the high-temperature and high-pressure gases produced
by combustion applies direct force to some component of the engine, such as
pistons, turbine blades, or a nozzle. This force moves the component over a
distance, generating useful mechanical energy. We know that in case of Internal
Combustion engines, combustion of air and fuel takes place inside the engine
cylinder and hot gases are generated. The temperature of gases will be around
2300-2500°C. This is a very high Temperature and may result into burning of oil
film between the moving parts and may result its seizing or welding of same.
So, this temperature must be reduced to about 150-200°C at which the engine
will work most efficiently.

 However, cooling beyond optimum limits is not
desirable, because it decreases the overall efficiency due to the following

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Thermal efficiency is decreased due to more loss
of heat to the cylinder walls.

The vaporization of fuel is less; this results
in fall of combustion efficiency.

Low temperatures increase the viscosity of
lubrication and hence more piston friction is encountered, thus decreasing the
mechanical efficiency.