Compressor torque computing method, air-conditioning system and engine control apparatus

An air conditioning apparatus includes a refrigerant circulation circuit that has a compressor. A driving status of the compressor is detected by first detector. A first calculator (81) calculates a theoretical torque and a driving efficiency of the compressor based on information from the first detector. A second calculator (82) calculates a necessary torque required for driving the compressor. The second calculator calculates the necessary torque based on the theoretical torque.