It is a device which converts continuously heat energy into mechanical energy in a cylic process.
As shown in the fig, a heat energy has the following essential parts :
(i) Source: It is heat reservoir at higher temperature T1 It is supposed to have infinite thermal capacity so that any amount of heat can be drawn from it without changing the temperature.
(ii) Sink : It is heat reservoir at a lower temperature T2. It has also infinite thermal capacity so that any amount of heat can be added to it without changing its temperature.
(iii) Working substance: Working substance is any material (solid, liquid or gas) which performs mechanical work when heat is supplied to it. For example, a mixture of fuel vapour and air is used in a gasoline or a diesel engine or steam in a steam engine.
Working: In every cycle of operation, the working substance absorbs a definite amount of heat Q1 from the source of higher temperature T1, converts a part of this heat energy into mechanical work W and rejects the remaining heat Q2 to sink at lower temperature T2. The work done W in a cycle is transferred to the environment by some arrangement e.g, the working substance maybe in a cylinder with a moving piston that transfers mechanical energy to the wheels of a vehicle via a shaft.
Efficiency of heat engine: The efficiency of a heat engine is defined as the ratio of the net work done by the engine in one cycle to the amount of heat absorbed by the working substance from the source.
As the working substance returns to its initial state after completing one cycle, there is no change in its internal energy. Hence by first law of thermodynamics, net heat absorbed in a cycle = work done
i.e Q1 - Q2 = W
The efficiency of heat engine is given by
Hence η = 1 - Q2/Q1
Efficiency of a heat engine is always less than unity. Clearly, when Q2 = 0, η= 1 or 100%.
