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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 to a very smooth operation and improvement in fuel consumption.

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variable-displacement-compressors

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)

Fig. 2 shows the internal components of a VDC.

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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.

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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.

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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.

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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.

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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 signal from the temperature sensor which senses the temperature of the air coming from the evaporator, and it also receives signal from the temperature control switch which you use to control the cabin temperature. Using these two signals, the ECU sends a Pulse-Wave-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.

Comments

Bwire Johnson on April 13, 2020:

Very educative thank you very much for such information.

Honey on March 19, 2020:

First of all, thanks a lot for the explanation. I have one query, why we encounter more noise and vibrations in VDC as compared to FDC ?

Bless on February 17, 2020:

Hi Victor,

Thank you for your article on Variable displacement compressor. It would be great help if you could assist me with diagnosing a problem with my car AC. it is a 2007 Mercedes ML350, the compressor in it failed (Pulley broke off) few months back and I bought an aftermarket compressor from China through Aliexpress, after installation it was working alright fro the last two months and then failed making rattling noise. It is not cooling anymore, I removed the serpentine belt and found the pulley is rotating and sort of getting stiff..however it seems it is not broken and still spinning, is it something broken inside? is this fixable or need to replace? Does the reed valve failure or something else can cause the rattling noise? Look forward to hear from you.

Kind Regards,

Blesson Thomas

Francisco on March 12, 2019:

Just came up with a solution for adapting the electronic control of ECVDC for a BMW E60 pre LCI 2004, i'm not responsible for anything you could break or catch fire, this is just a concept and i'm already testing it in my own car, you need to know basic electronic knowledge to build the circuit so better get someone to help you if you don't, the circuit it's pretty basic and works good so far after 3 weeks of testing, it's based on the idea of a voltage comparator, first i use a RC filter to transform the PWM signal to a linear voltage value representing the continuous range of the duty cycle, i searched what the service manual says about the PWM signal from the A/C computer and it's a 400Hz signal with a duty cycle range of 2-100%, i put a 1K resistor to trick the computer that the much lower resistance solenoid valve was still connected and put as little current posible, next put this output to a LM393 comparator with a ref voltage of 3.3V that it's ~25% duty cycle so if the PWM signal it's greater or equal to 3.3V ref or 25% dc will put the output of the chip low and activate a relay connected to a power PNP transistor, you can simulate and learn more at this url http://tinyurl.com/y69mvzgb hope my information helps you :)

Abbakar lamido on January 21, 2019:

hello sir am very glad about ur information thank you sir ! but I want to joined ur company on net thank you sir !

Francisco on September 12, 2018:

thanks for your time and sharing this useful information it would be really cool if can make a tutorial of how to replace a externally controlled variable displacement compressor with a fixed one, how do you convert the pwm signal to on/off clutch signal, another cool thing would be force the ecvd compressor to run on max when duty cycle is greater than 40% or 30% and turn it off when the pwm signal duty cycle drops from 40% or 30% i live in a tropical weather country and we require the ac to blow ice cold all the time no need for "temperature control" just the coldest air the damn thing can blow, I know that the pwm signal drops to 0V duty cycle is less than 10% when the ac is off or the ECU request to turn it off so it might work well using all the sensors the system already has to prevent the system freeze i think you can convert the signal using a low pass filter resistor capacitor to smooth the pwm signal and then make it on/off signal, regards!

sanouni on July 24, 2018:

hi fred

would you please help me.

i have problem with noise of AC compressor in my company .

our compressor is variable without clutch (swash plate).

i need emergency general test plan in this regards

thanks in advance if you send any document in following address

hamidreza.sanouni@ikap.co.ir

Victor Mangubat (author) from Philippines on July 21, 2018:

Hi Fred,

7SEU17c compressor is an Externally-Controlled Variable Displacement Compressor and, most probably, has no clutch. Its compression capacity is controlled by a PWM (pulse-wave modulated) signal from the ECU. PWM signal continuously varies between zero and 12 volts.

10PA17c has a clutch, which requires 12 volts (or thereabout) to turn it on, and zero volt to turn it off. Connecting the wires with PWM signal to the clutch terminal(s) may cause the clutch to slip, and in turn cause no cooling at all. And even if miraculously the clutch won't slip, there's no way the compressor can be turned off automatically when the engine is heavily loaded.

So to answer your question, it is not plug and play, but it can be done. I've replaced the Externally-Controlled Variable Displacement Compressors with Fixed Displacement Compressors (with clutch) in all of my taxi units and they're working fine. The original compressor costs $280, while the replacement (with clutch) costs $65.

Fred Ez on June 19, 2018:

I want to replace 7SEU17c on my Mercedes W211 with 10PA17c series.

Does this require any electrical rewiring or just plug and play ?

Zeyar on January 09, 2017:

Hi, I have a car that service manual says it uses variable displacement compressor SD7V16, however on the used car i bought the compressor is SE7H13 which i learnt is not a variable displacement type. The compressor does not have intermittently on-off cycle too, which i guess the previous owner changed it. Car's service manual says it uses ECU's electronic pulse to control displacement. So, Is it possible that previous owner could have just plugged in the current compressor without modifications? If i change it back to SD7V16, will it work just fine?

Joe on November 02, 2016:

Hi. 2006 BMW650i Clutchless AC compressor won't pressurize. Disconnected Electronic Control Valve harness and tested it to read 18ohms. DC supply reads 5volts then goes up to 14volts shortly after engine start while connected. Minor rattle seems to be coming from the compressor when voltage shifts from 5V to 14V as if it's trying to engage but nothing happens. I'm suspecting either the compressor or its Electronic Control Valve to have failed. What do you think? Any other possibilities causing the AC not to work?

BTW I already replaced the pressure sensor and vacuumed it and recharged it with r134a before I started suspecting the compressor itself.

Ron on September 07, 2016:

is there a replacement icdvc valve for a mercedes benz as the compressor itself seems fine

Dr Haroon on August 24, 2016:

Hello. My car ac keeps on tripping. Its a toyota corolla 2013 and has a ICVD (densto tse14c) i believe. It just does it by itself .... trips for a couple of minutes or maybe more but when its working it cools like frozen ice.

Somebody said the compressor has carbon particles and needs to be replaced. I dont think thats the issue .... is i the control valve and ECU issue ? im really confused as this is the 2nd compressor which I just installed (new).

Victor Mangubat (author) from Philippines on May 08, 2015:

Hi Dave,

If your Avensis has an Internally-Controlled Variable Displacement (ICVD) compressor, it is very likely that it has a clutch. Without the clutch, it would be difficult to enable and disable the compressor when the aircon system is turned on or off. Moreover, most Engine Management Systems disables the compressor when the engine is overloaded and without the clutch this is impossible to implement.

Clutchless compressors are those that are Externally Controlled, where total control of the compressor is done by the ECU via the Control Valve.

With regards to the 'tick over', there are a number of possible causes. One thing I can suggest is to check if your car has a window type of refrigerant pressure sensor, the one that triggers the turning off of the compressor when the pressure is low and when the pressure is high. It could be possible that the sensor is sensing a high pressure due to overcharging or insufficient cooling.

Dave Webster on May 01, 2015:

I have a Toyota Avensis with a Variable Displacement compressor, the system works OK and is topped up, but at tick over the compressor constantly loads and unloads together with the condenser cooling van every 10 seconds approx. If there is no clutch why should it do this?