The Clausius Inequality is applicable to the operation of
all real engines. The ideal heat cycle does not result in a net change in entropy as the
increase in entropy during part of the cycle is compensated by an equal decrease during
the rest of the cycle.
In case of a real engine on the
other hand, there is a negative change in entropy over the entire heat cycle. This is
due to the fact that the temperature of the engine when heat is being added to it from
the source is less than the temperature of the source. As a result of this, the addition
of heat is an irreversible process and there is entropy created which here has to be
passed on to the cold reservoir.
Unlike an ideal engine, a
real engine cannot convert all the heat from the source into work and its efficiency is
less than one.
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