What is a variable displacement air-conditioning compressor?

A brand new car air conditioning compressor

The word "displacement" refers to the amount of refrigerant gas the compressor moves or displaces in one revolution. This volume is measured in cubic centimetres, and it varies between air conditioning systems.

Until recently, all air conditioning systems used “fixed displacement” compressors that always pumped or displaced the same amount of refrigerant in one revolution. This meant that the only way to maintain the set temperature was to cycle the compressor “ON” and “OFF” to interrupt and restart the cooling process.

This not only wore out the compressor clutch but also wasted fuel every time the compressor restarted.

Variable displacement compressors work a little differently. In practice, their internal components can be adjusted to vary the amount of coolant they displace in one revolution. For example, if maximum cooling is required, the compressor can be set to pump the maximum amount of refrigerant.

If less cooling is required, the computer adjusts the compressor’s internal parts to pump less refrigerant per revolution.

As a practical matter, the amount of regfigerant the compressor displaces is directly related to the cooling effect the air conditioning system produces. Therefore, if the temperature in the car is stable, the compressor will only pump enough refrigerant to maintain that temperature.

If a lower temperature is required, the control system will change the compressor’s setting to pump more refrigerant, thus increasing the cooling effect. This system has some major advantages, these being that-

  • compressors and drive belts last longer
  • the engine uses less fuel to drive the compressor
  • set air conditioning temperatures are more stable
  • using less fuel creates fewer harmful exhaust emissions

Overall, air conditioning systems that use variable displacement compressors have fewer parts, meaning that such systems are more reliable and easier to control.

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