Heat engine

In thermodynamics, heat engine refers to a system capable of converting the heat input to the system into mechanical work.
eg.: Steam engine.
In thermodynamic analysis, while solving problems a typical heat engine is represented as follow.

Representation of heat engine.

Here the heat engine acts between source and sink.Source will be at high temperature
compare to sink hence flow of heat takes place from source to sink with engine acting in between.
Here Q_S refers to heat supplied
         Q_R refers to heat rejected with respect to heat engine.
The most important parameter of heat engines is their efficiency.Efficiency of an heat engine is given by
the ratio of output to input.Here output is W and input is Q_S.

Efficiency of an heat engine.