Displacement control apparatus and method for variable displacement compressor

A control valve (46) is located in a variable displacement compressor used in a refrigerant circuit. The control valve (46) operates such that the pressure difference (PdH-PdL) between two pressure monitoring points (P1, P2) in the refrigerant circuit seeks a predetermined target value. A controller (81) determines the target value of the pressure difference (Pdh-PdL) in accordance with external information that represents a required cooling performance of the refrigerant circuit. The target value of the pressure difference (PdH-PdL) is represented by the duty ratio of voltage applied to the control valve (46). When the depression degree of the acceleration pedal is greater than a predetermined level, the controller (81) sets a limit value of the duty ratio. The controller (81) changes the duty ratio to the limit value to limit the compressor displacement only when the limit value is set and the duty ratio is greater than the limit value. Therefore, the compressor torque does not hinder quick acceleration of the vehicle. Also, the occasions in which the cooling performance is lowered below a demanded level are minimized while preventing the compressor torque from hindering quick accelerations of the vehicle.