 The mass of air is followed as its volume, pressure and temperature change during the various thermodynamic steps.As the piston is capable of moving along the cylinder, the volume of the air changes with its position in the cylinder.The compression and expansion processes induced on the gas by the movement of the piston are idealised as reversible, i.e., no useful work is lost through turbulence or friction and no heat is transferred to or from the gas during those two processes.