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Variable Displacement Compressor: How It Works

Victor is a businessman, engineer, and teacher. He is interested in car repair and maintenance.

Unlike the old fixed displacement compressor (FDC), the variable displacement compressor (VDC) automatically varies its pumping capacity to meet air-conditioning demands. When the car’s cabin temperature is high, it increases its refrigeration capacity until the desired temperature is reached. Once the desired temperature is reached, it automatically reduces its capacity to maintain the desired temperature.

With a VDC, there is no jerking of the engine brought about by the switching on and off of the compressor clutch (as in FDC). In fact, some VDCs have no clutch at all. This results in a very smooth operation and improvement in fuel consumption.


Two Types of Variable Displacement Compressor (Fig. 1a and Fig. 1b)

There are two commonly used types of VDCs: the internally controlled variable displacement compressor (ICVDC) and the externally controlled VDC (ECVDC). Fig. 1a shows an ICVDC and Fig. 1b shows an ECVDC. They have basically the same internal structure. They differ only on the manner the displacement control valve is actuated. In an ICVDC the control valve is actuated by the refrigerant pressure at the suction chamber of the compressor by means of a bellows or diaphragm. In an ECVDC, actuation of the control valve is done by the engine’s ECU or by an external electronic module by means of a solenoid actuator. Note the wiring harness of the solenoid actuator (Fig. 1b). The solenoid is inside the valve.

Externally controlled variable displacement compressors (ECVDC) have far better control of the piston displacement, and hence the temperature, compared to ICVDC. This makes the clutch entirely unnecessary in ECVDC, as shown in Fig. 1b.

The Internal Structure of a Variable Displacement Compressor (VDC) (Fig. 2)


Internally Controlled Variable Displacement Compressor (ICVDC): Uncharged and Not Running (Fig. 3)

When the compressor is not charged with refrigerant, the swash plate is held at the minimum-angle position by the spring on the shaft (Fig. 3). The bellows of the displacement control valve (DCV) is at expanded condition, closing the low side port while opening the high side port.


ICVDC Charged but not Running (Fig. 4)

When the system is charged and the compressor is not running the pressure in all chambers of the compressor is the same. This pressure causes the bellows of the DCV to contract, opening the low side port, while closing the high pressure port. See Fig. 4.


ICVDC Charged and Running—Reaching Its Maximum Displacement (Fig. 5)

The swash plate’s slightly slanted position creates a small displacement at the compression chamber of the compressor (Fig. 4). When the compressor runs , the swash plate which rotate with the shaft wobbles a little. This wobbling action causes the pistons to move back and forth at a short stroke.

On each suction stroke of the pistons, a small volume of refrigerant is sucked into the compression chamber through the suction reed valve, which is then pumped out into the discharge chamber through the discharge reed valve on each compression stroke of the pistons. This increases the pressure at the discharge chamber of the compressor, while reducing the pressure at the suction chamber. Since the low-side port of the displacement control valve (DCV) is open at this stage, the same reduced pressure is present at the control chamber.


The refrigerant pressure at the control chamber and the spring around the shaft exert a combined force (F2) at the back of each piston. As pumping continues, there comes a time (and this won’t be long) when the force at the back of each piston (F2) is lesser than the force exerted on the head of each piston (F1) by the refrigerant in the compression chamber.

By the time F1 is greater that F2, the pistons opposite the pivot are pushed to the left by the resultant force (F1 minus F2). This Increases the swash plate angle and of course, the piston displacement. In effect, more refrigerant is sucked from the suction chamber and pumped out into the discharge chamber. This increases further the pressure at the discharge chamber and decreases the pressure at the suction chamber. And since the low-side port of the control valve is still open at this stage, the pressure at the control chamber is also reduced. This increases the resultant force on the piston head (F1 minus F2), which in effect increases the swash plate angle and the piston displacement.

This increase in capacity (displacement) continues until the swash plate is at the maximum angle position. In this case, the compressor reaches its maximum capacity.

ICVDC Adjusts to Its Minimum Displacement (Fig. 6)

As the compressor continues to run at maximum capacity (displacement), the cabin temperature will continue to decrease until the desired temperature is reached. At this stage the low-side pressure (suction chamber pressure) is low enough to cause the billows of the DCV to expand, closing the low-side port and opening the high-side port of the DCV (Fig. 6). This channels the high pressure refrigerant to the control chamber. At this stage, the force at the back of the pistons (F2) is greater than the force at the head of the pistons (F1). This decreases the piston displacement.


Fuel Efficiency

In actual operation, when the air-conditioning system has stabilized and the desired temperature is reached, the piston displacement is neither maximum nor minimum. It is at a dispacement just enough to maintain a stable cabin temperature. The displacement will increase only when there is a demand for it, like when the door is opened, but will return to its stable condition once the temperature is stabilized.

The larger the displacement, the harder it is to turn the compressor shaft and the more engine power is needed. Correspondingly, the smaller the displacement, the lesser engine power is needed. Lesser engine power requirement means lesser fuel consumption. And since the compressor's displacement is not maximum at normal condition, lesser engine power is needed. Hence, lower fuel consumption.

A Common Problem With Internally Controlled Variable Displacement Compressors (ICVDC)

A problem we encountered with ICVDCs is the loss of cooling as the engine is revved up. At idle cooling is just right. Using a manifold gage, you will notice that as the engine is revved up the pressure at the low side starts to rise and that of the high side starts to decrease. It is as if the clutch has disengage, but it has not.

It is very likely that the compressor has still enough pumping capacity. It may just be that the compressor prematurely decreases its displacement (capacity) resulting in the loss of cooling as the engine is revved up. There may be a number of reasons why this is so, but one thing is sure . . . the displacement control valve prematurely closes the low side port, at the same time opening the high side port as the engine is revved up. We’ve successfully implemented a solution to this problem which is addressed in “Adjusting an Internally Controlled Variable Displacement Compressor or Converting It to Function as a Fixed Displacement Compressor.”

See you there.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: What is the feedback mechanism by which the cabin temperature is communicated to the Internally Controlled VD Compressor?

Answer: The engine's ECU receives the signal from the temperature sensor which senses the temperature of the air coming from the evaporator, and it also receives the signal from the temperature control switch which you use to control the cabin temperature. Using these two signals, the ECU sends a Pulse-Width-Modulated (PWM) signal to the Control Valve of the compressor. If more cooling is needed, the ECU increases the duty cycle of the PWM signal. If cabin cooling exceeds what has been set at the temperature control switch, the ECU decreases the duty cycle of the PWM signal. The duty cycle of the PM signal determines the pumping capacity of the compressor. Increasing the duty cycle increases the pumping capacity.

Question: Is it ok to drive a variable compressor before it’s charged? Came with oil in it. Have to drive it about 50 miles to get it charged after replacing the entire ac system.

Answer: If your new variable displacement compressor has a clutch (internally-controlled type), then go ahead. This type of compressor won't run even if the engine is running if the system is not charged, hence, no damage can occur. This is because the pressure switch shuts off the power supply to the compressor clutch preventing it from engaging. But just to sure, turn off the ac switch.

If your new compressor has no clutch (externally-controlled type), let me ask you these questions first:

1) Is the newly installed compressor a new one (fresh from the box)?

2) Is the oil in your compressor factory filled-in?

3) Did you flush your AC system?

If the answer to each of these questions is YES, then am against your driving your car for 50 miles to get it charged. The amount of oil that comes with the new compressor is so little and, with system flushed, could easily be dispersed out of the compressor, thus starving your compressor with the needed lubrication. Please note that a clutchless compressor is continuously running while the engine is running.