The overall isothermal efficiency of the compressor is defined as the ratio of

• work required to compress the air isothermally to the actual work required to compress the air for the same pressure ratio
• isothermal power to the shaft power or B.P. of the motor or engine required to drive the compressor
• volume of free air delivery per stroke to the swept volume of the piston
• isentropic power to the power required to drive the compressor