Is the ratio of the polytropic compression energy transferred to the gas to the actual energy transferred to the gas.
The polytropic or small stage efficiency of a compressor is defined as the ratio of the actual differential work done (enthalpy change) on the fluid to the isentropic differential work done on the flowing through the compressor.
Why Polytropic efficiency: Adiabatic efficiency makes thermodynamic sense for cycle analysis. Changing Adiabatic efficiency with varying number of identical compression stages, does not describe fluid mechanics.