Heat engine is a device which converts heat energy into mechanical work by exploiting the temperature difference between a hot source and a cold sink. Heat is transferred from the source to the sink, through the working substance of the engine. During this transfer, some of the heat energy is converted into work. The working substance may be a gas or a liquid.
Heat Engine Principle
For the typical heat engine, as shown, the working agent is assumed to complete a cyclic process. There is no net gain or loss of energy within the boundary (Δ E = 0). This is in accordance with the First Law of Thermodynamics or the law of conservation of energy.
Qi + Wi = Qo + Wo
Also the net work done by the engine is equal to (the work out - the work expended ).
W = Wo - Wi.....= Qi - Qo
The efficiency of the heat engine is the ratio of the useful work output and the energy input i.e.