The function of a heat engine is to transform energy which is supplied by heat transfer into mechanical energy and make it available for doing useful work...Any system which enables a substance or working agent to undergo a cyclic process by receiving and rejecting energy by heat transfer with some of this energy converted into work is called a heat engine.

A heat engine is, strictly, one which involves the transfer of energy in the form of heat and work but with no transfer of matter - it is a closed circuit system.  A closed circuit steam power plant and a closed circuit gas turbine are heat engines but an, open circuit, internal combustion engine is not a heat engine.

For the majority of heat engines the end product is a rotating shaft.  A shaft is effectively a conduit through which work is transmitted.

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.

Q_i + W_i = Q_o + W_o

Also the net work done by the engine is equal to (the work out - the work expended ).

W = W_o - W_i.....= Q_i - Q_o

The efficiency of the heat engine is the ratio of the useful work output and the energy input i.e.

All heat engines have the following essential components

All other parts of the heat engine are provided for the successful operation and functioning of the three essential components identified above.