Variable Displacement A/C Compressor


(Delphi Automotive illustration)

by Jacques Gordon

When a belt-driven piston-type A/C compressor is engaged, it produces a noticeable ‘thump’ and a reduction in engine power. This can be eliminated by keeping the compressor  engaged all the time and controlling refrigerant flow by varying the displacement of the compressor.

The variable displacement compressor is an axial piston design, with the pistons driven by a wobble plate or a swash plate. Since the angle of that plate determines the length of the piston stroke, changing that angle changes the length of the stroke, therefore changing the amount of refrigerant pumped (displaced) on each stroke. The plate angle is controlled with linkage and springs, and it’s adjusted by changing refrigerant pressure in the compressor housing. So the key to controlling displacement is controlling housing pressure.

When housing pressure is increased, pressure on the back side of the pistons keeps them “higher” in their bores, closer to the cylinder head. This reduces the angle of the swash plate and shortens the stroke, reducing displacement. When housing pressure decreases, a spring pushes the adjusting linkage away from the cylinder head, increasing plate angle and lengthening the piston stroke to increase displacement. Housing pressure is controlled by a valve with ports and passages that connect to the suction (low-side) and  discharge (high-side) chambers of the compressor head.

Two different types of control valve are used; mechanical and electronic. The mechanical valve has a precision diaphragm that senses low-side pressure. When the cabin is warm, evaporator temperature increases, which increases low-side pressure and collapses the diaphragm. A port opens to vent housing pressure to the suction side of the compressor head. This decreases housing pressure and increases piston stroke, increasing refrigerant flow through the system.

As evaporator temperature decreases, so does low-side pressure. The diaphragm expands to close the low-side vent port and at the same time open a port that admits high-side pressure into the housing. Higher pressure reduces piston stroke and refrigerant flow volume. Remember, changing flow volume doesn’t change pressure, so a diaphragm-type control valve remains stable.


Delphi’s mechanical displacement control valve is actually two valves in the same body. When low-side pressure is high, the bellows collapses (towards the left). The cone valve opens and pressure vents from the wobble plate case (compressor housing) through the by-pass hole and to the suction side of the compressor head, reducing housing pressure. When low-side pressure is low, the bellows expands to close the cone valve and open the ball valve. Pressure vents from the discharge cavity (high-side) to the wobble plate case, increasing housing pressure.

Around 2001, DENSO introduced a pulse-width-modulated solenoid valve to control housing pressure, based on information from temperature and pressure sensors in the refrigerant system. With a computer controlling the valve’s duty cycle, compressor displacement can be used to control evaporator temperature, rather than the other way around.

The mechanical control valve is still used in some systems because it’s inexpensive and reliable, but it’s range of control is limited. Electronic control valves are becoming more common, and on many applications there is no clutch, so the compressor runs continuously. Displacement can be reduced to about 1 percent when cooling is not needed, keeping seals lubricated, minimizing oil pooling and preventing other kinds of damage that result from long periods of inactivity. Ultimately there is less load on the engine when the A/C is in use, reducing the system’s contribution to tailpipe emissions. As mileage and emissions regulations continue to tighten, we can expect to see electronically-controlled variable displacement compressors on more new models.



The Mobile Air Conditioning Society’s blog has been honored as the best business to business blog in the Automotive Aftermarket by the Automotive Communications Awards and the Car Care Council Women’s Board!

When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.

If you’re a service professional and not a MACS member yet, you should be, for more information.

You can E-mail us at . To locate a Mobile Air Conditioning Society member repair shop in your area. Click here  to find out more about your car’s mobile A/C and engine cooling system.

The 34th annual Mobile Air Conditioning Society (MACS) Worldwide Training Conference and Trade Show, Power Up  will take place January 16-18 2014 at the Sheraton New Orleans.


About macsworldwide

Mobile Air Conditioning Society (MACS) Worldwide Founded in 1981, MACS is the leading non-profit trade association for the mobile air conditioning, heating and engine cooling system segment of the automotive aftermarket. Since 1991, MACS has assisted more than 600,000 technicians to comply with the 1990 U.S. EPA Clean Air Act requirements for certification in refrigerant recovery and recycling to protect the environment. The Mobile Air Conditioning Society (MACS) Worldwide’s mission is clear and focused--as the recognized global authority on mobile air conditioning and heat transfer industry issues.
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10 Responses to Variable Displacement A/C Compressor

  1. Geetha says:

    Good illustration! This blog is helpful for understanding about refrigerated air compressor’s processes and solutions.

  2. Pat Hill says:

    How do i know if it’s a variable compressor on the car ? They look pretty similar.

  3. Alwalid Abdelkarim yousif says:

    now I understand how varaible compressor V-5 & V-7 works.
    thank you very much

  4. Pingback: Variable Displacement Compressor | Automotive Airflow

  5. Imtiaz Farid says:

    I have a Toyota Allion 2012 model car that has Variable Displacement A/C compressor. Sometime my car A/C blows hot air. Is it due to malfunction of the compressor? Or it is a common phenomena? How it can be repaired? Please advice.

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