When the vapor injection is turned “on”, the system works as a two-stage cycle. From Figure 12b it can be seen that the injected vapor from state “7” is mixed with the low-stage discharged vapor from state “9”, reaching state “8”, and then starts the high-stage compression process. States “ 9' ” and “ 2' ” stand for the low-stage and high-stage discharge states, respectively, if the compression process is isentropic. Low-stage and high-stage isentropic and volumetric efficiencies are calculated by Equation (7) through Equation (10):