A gas engine is a heat engine in which the working fluid is atmospheric air and the fuel an inflammable gas.
The constant pressure cycle is so called because heat is added to the working fluid at constant pressure.
In this particular apparatus there are two channels O O for the outlet of the working fluid and one, I, for the inlet.
At the higher limit of pressure, heat is added while the working fluid expands at a constant pressure.
In this cycle adiabatic compression is assumed to raise the temperature of the working fluid from the lowest to the highest point.
In one very popular form ammonia gas is the "working fluid."
The constant volume cycle is so called because the heat required is added to the working fluid at constant volume.