working fluid

In one very popular form ammonia gas is the "working fluid."

The constant pressure cycle is so called because heat is added to the working fluid at constant pressure.

The constant volume cycle is so called because the heat required is added to the working fluid at constant volume.

At the higher limit of pressure, heat is added while the working fluid expands at a constant pressure.

A gas engine is a heat engine in which the working fluid is atmospheric air and the fuel an inflammable gas.