A heat engine is a device that transforms the chemical energy of fuel into thermal energy
and utilizes this energy to produce mechanical work.
Heat engines can be classified in two categories:
(1) Internal combustion engine (I.C. Engine).
(2) External combustion engine.
In an external combustion engine, the combustion of fuel takes place outside the engine
cylinder whereas in case of an internal combustion engine, the combustion of fuel takes place
in the engine cylinder.
In internal combustion engine the working substance (fluid) is the products of combustion
of air-fuel mixture. Internal combustion engines find wide applications in automobile sector,
power boats, ships, locomotives and small power plants etc.
The common examples of external combustion engines are: steam engine and steam
turbine where the working fluid is steam. In case of an external combustion engine, the
combustion of fuel takes place in a boiler where steam is produced by using the heat energy
of combustion. Steam thus produced is utilized to produce power when steam acts on the
piston of a steam engine cylinder or on the blades of a steam turbine. Another example of an
external combustion engine is a closed cycle gas turbine in which heat of combustion from an
outside furnace is transferred to air (or gas), which is used in a gas turbine. In external
combustion engine the working fluid is entirely different from air-fuel mixture.
The advantages of external combustion engines are:
Use of low cost fuel (solid, liquid or gas)
High starting torque
Used for electric power generation.
In case of an internal combustion engine, the thermal energy resulting from the combustion
of fuel is converted directly to produce mechanical power. Since combustion takes place inside
the engine cylinder, very high temp. (2000°C) is produced inside the cylinder. It is therefore
necessary to remove some portion of the heat from the cylinder to prevent damage to the
metal of the cylinder either by circulating the water through jacket around the cylinder or by
cooling it through atmospheric air. The thermal efficiency of an internal combustion engine is
high as compared to a steam engine plant as internal combustion engines do not have losses
that arise due to use of boilers, condensers etc., in steam engine plant.