heat engine

In thermodynamics, heat engines convert thermal energy into mechanical work repetitively.

The conversion process starts at a high operating temperature $T_H$ and ends up at a lower operating temperature $T_L$. In the process, the input heat $Q_H$ which is used to raise the temperature to $T_H$ is converted partially to work. Heat not converted to work is released as exhaust — $Q_L$. The engine’s temperature is then raised once more, starting the cycle all over again.

The change is thermal energy is 0 as the temperature always returns to the starting point, $T_H$, thus according to first law of thermodynamics, $Q_H=W+Q_L$.

In a heat engine, the net work $W_{net}=\Delta P\Delta V$ (area enclosed by the cycle on PV diagram).