A car AC compressor is used to convert low pressure refrigerant gas into high pressure gas. The AC compressor is driven by a drive belt connected to the crankshaft in non-hybrid vehicle and by an electric motor in many hybrid cars.
In a non-hybrid engine, the drive belt rotates the car AC compressor pulley any time the engine is running. But the pulley only operates the AC compressor when the driver requests AC. At that point, the HVAC control system activates a magnetic clutch assembly to connect the AC pulley to the AC compressor drive shaft. The drive shaft then moves the pistons inside the compressor to draw in and compress the refrigerant.