Compressor Control

Compressor
When AC is requested, the clutch is energized and the pulley drives the shaft. The journal and the swash plate turn with the shaft, and the angled swash plate produces reciprocating movement of the pistons. Vapor from the inlet pressure chamber is drawn into the cylinder, compressed, and discharged into the outlet pressure chamber, producing a flow around the refrigerant circuit.

The flow rate through the compressor is determined by the length of the piston stroke, which is controlled by the tilt angle of the swash plate. The tilt angle of the swash plate is controlled by the servo pressure and compressor inlet pressure acting on the pistons during their induction stroke. A relative increase of inlet pressure over servo pressure moves the pistons along their cylinders to increase the tilt angle, the piston stroke and the flow rate. Similarly, a relative decrease of inlet pressure over servo pressure moves the pistons along their cylinders to reduce the tilt angle, the piston stroke and the flow rate.

The control valve regulates the servo pressure in the crankcase as a function of inlet pressure, so that the flow rate of the compressor matches the thermal load at the evaporator, i.e. the more cooling required in the passenger compartment, the higher the thermal load and flow rate. Servo pressure varies between inlet pressure and inlet pressure �0.07 bar (�1psi).

As the refrigerant flows through the evaporator and absorbs heat (i.e. as the thermal load increases) the pressure of the vapor entering the compressor increases. In the control valve, the increased inlet pressure causes the diaphragm and push rod to close the ball valve. The resulting reduction in crankcase pressure, together with the increase in inlet pressure, moves the swash plate to a higher tilt angle and increases the piston stroke and the flow through the compressor. When the thermal load of the evaporator decreases, the subsequent decrease in pressure of vapor entering the compressor causes the control valve to open. This increases swash plate crankcase pressure, which reduces the tilt angle of the swash plate and the flow through the compressor.