Engine Cycles

For a constant mass of gas, the operation of a heat engine is a repeating cycle and its PV diagram will be a closed figure. The idea of an engine cycle is illustrated below for one of the simplest kinds of cycles. If the cycle is operated clockwise on the diagram, the engine uses heat to do net work. If operated counterclockwise, it uses work to transport heat and is therefore acting as a refrigerator or a heat pump.