Compressor HVAC: Description and Operation

The compressor in an automotive A/C system serves two important functions:
- It creates a low-pressure zone at the compressor inlet, to draw refrigerant vapor from the evaporator.
- It compresses the low-pressure refrigerant vapor into a high-pressure vapor and sends it toward the condenser.

BMW A/C systems use various types of compressors. These include:
- Seiko-Seiki rotary vane compressor
- Nippondenso swash-plate design







The Seiko-Seiki type is a five-vane rotary compressor. It consists of a shaft with vanes, that maintain contact with the inner wall of a cavity. The cavity is shaped like an ellipse. As the shaft rotates, oil pressure and centrifugal force push out on the vanes, so that their outer edges stay in contact with the cavity. This creates spaces where the volume is expanded and contracted, to draw refrigerant vapor in, compress it, and force it out.







When the space between the shaft and the cavity is large, the pressure is low. Refrigerant vapor is drawn into the space. When the vane passes the inlet port, the space is sealed off; no more refrigerant vapor can be drawn in. As the vane sweeps through 180°, the space shrinks, compressing the refrigerant. The refrigerant is then forced out through reed valves, to the discharge ports.







The Nippondenso compressor is a five-cylinder swash-plate compressor. The swash plate is set on an angle, and it rotates with the shaft. As it goes through one complete revolution, it drives pistons from one end of their travel to the other, and back again. As it drives a piston forward in its cylinder; the piston compresses the refrigerant in the cylinder The compressed refrigerant is then discharged. As the swash plate pulls a piston back, the piston draws refrigerant into the cylinder.







COMPRESSOR CLUTCH
The compressor pulley is driven by a belt from the crankshaft; a compressor clutch is used to engage/disengage the pulley and driveshaft. The clutch is electromagnetic. When power is provided to the clutch, the clutch engages and rotates the compressor drive shaft. When the power is cut off, the clutch disengages and the compressor pulley freewheels On BMW A/C systems, the compressor is cycled on and off, according to evaporator temperature; it is also cycled off at full-throttle, standing start acceleration conditions.







A diode is used to prevent induced current and voltage from damaging the control module when the clutch solenoid is disengaged. The diode allows the voltage spike created when the clutch solenoid is disengaged, to flow in a loop back through the solenoid coil, until the energy is dissipated. Different vehicles use different control modules to control the compressor clutch.







The compressor clutch is unit replaceable.