How does automobile A/C work?

[Wade Lippman]

"Car Talk" in today's newspaper says that when you turn A/C on, the compressor stays on. If you turn the temperature up, the system mixes outside air in to bring the temperature up, but the compressor stays on.
In a house, the thermostat turns the compressor on and off; I always assumed it was the same in a car.

Is "Car Talk" right? If so, why do they do it that way?

[Doug Dawson]

"Car Talk" in today's newspaper says that when you turn A/C on, the compressor stays on. If you turn the temperature up, the system mixes outside air in to bring the temperature up, but the compressor stays on.
In a house, the thermostat turns the compressor on and off; I always assumed it was the same in a car.

Is "Car Talk" right? If so, why do they do it that way?

The compressor is run off a belt connected to the crankshaft pulley (or some variation of that.) It's engaged or disengaged via an electrically operated clutch, so that it's only "compressing" when needed. The belt was historically the best way of transferring the needed power to the compressor.

Even in a house, the compressor would only cycle on or off as needed, regardless of the thermostat setting.

It's possible that some newer cars (Tesla etc.?) may use an electric compressor, but it still wouldn't actually be "compressing" all the time, because it would be a waste of power.

[Mike Henderson]

My experience with automobile air conditioners goes back a few years but Doug described how it worked back then. There was an electrically operated clutch that engaged and disengaged the compressor, as needed to cool the car.

It's possible that later cars would mix outside air with the air that flowed over the evaporator so that the air coming out of the ducts was not so cold, especially after the interior was cooled down. The early cars definitely did not do that - when it blew cold, it was cold. To moderate the cooling, the fan was turned down to a lower speed.

Tesla, and all other electric cars, uses an electric motor to drive the compressor, same as the way your compressor works in your home.

Mike

[Greg R Bradley]

On later cars the compressor is likely to run all the time. The older and lower end the car, the more likely it is to just run when the AC is "on". ANYTHING with any kind of climate control instead of just manual AC is certainly going to run it all the time. Most have a button where you can turn the compressor off but that is something that you might not want to do often as you would even lose effective defrost as the AC helps to get less humid air to clear the windshield faster.

Higher end cars started running the compressor all the time in the 1970s. Probably hit most cars by the 1990s particularly in cars that basically aren't available without AC.

[Tom M King]

Something must be left out here.

Without typing myself, this is copied, and pasted:
If the A/C switch in the passenger compartment is turned off, any air delivered by the in-dash blower should simply be the same temperature as the passenger compartment, that is not chilled. If you have the A/C switch off and yet cold air is being delivered, that means there is an electrical problem that is causing the system to remain on when it is supposed to be off.

If you are looking at the pulley side of the physical compressor on the vehicle, the external pulley will ALWAYS be turning when the engine is running and so you might mistakenly think the A/C is on when in fact all that is turning is the pulley. When you turn the A/C system "on", a clutch behind the pulley mates the pulley to the compressor itself and it is ONLY at that point that the compressor turns and begins the vapor-compression cycle that creates cooled air in the passenger compartment. If you look at the center of the pulley, when the a/c is off, the central portion of the pulley should be stationary. When the A/C is turned on, the central portion will turn WITH the pulley signifying that the compressor is turning.

[Grant Wilkinson]

On my 2013 Acura MDX with climate control, the AC compressor is definitely not always on. When it is not needed to cool air, as in winter, it cycles on and off to keep the heated air dry to keep the windshield clear. But, it is cycling on and off. When I have set the climate to control to cool the air in the Summer, the compressor cycles on and off as needed to keep the cabin temperature where I've set it. I have the option of turning off the compressor manually when I'm heating the air, but not only does it mean that the humidity in the cabin goes up and the windows fog up, but the manufacturer says that I run the risk of having the compressor eventually seize up from lack of lubrication. It is the refrigerant that lubricates the compressor and if it is not running, there is no refrigerant running through it.
None of this may be true for other makes and models.

[Doug Dawson]

On later cars the compressor is likely to run all the time. The older and lower end the car, the more likely it is to just run when the AC is "on". ANYTHING with any kind of climate control instead of just manual AC is certainly going to run it all the time. Most have a button where you can turn the compressor off but that is something that you might not want to do often as you would even lose effective defrost as the AC helps to get less humid air to clear the windshield faster.

Higher end cars started running the compressor all the time in the 1970s. Probably hit most cars by the 1990s particularly in cars that basically aren't available without AC.

It's all about pressure, which the compressor provides on the "high side" to expand the refrigerant via the expansion valve etc., to provide cooling by the heat cycle that you learned about in high school physics class. Just because the compressor cycles off doesn't mean there isn't still pressure on the high side to bleed out the expansion valve and cause cooling. So there's that.

But also: The compressor is sized to deal with the most extreme conditions the car is likely to see, which means grossly oversized for ordinary conditions. Particularly in "luxury" cars. Thus, it doesn't _have_ to be on all the time to cool the car. This continues particularly up to the present day, with the government-mandated pressing need for lower emissions and higher mpg (from less parasitic power loss.) There's just no need for it.

[Tom M King]

I'm still trying to figure out these AC compressors that don't have clutches. If it has a wire coming out from behind where the pulley is, as in this picture, that's to operate the clutch, which is inside the pulley. I don't know the total number of different types of compressors, but there is not nearly as many as there are different types of cars.
https://www.buyautoparts.com/autoparts/Acura/MDX/A-C_Compressor.html

I understand the part about one maybe staying on as long as the AC is turned on, but not about one running all the time.

[Doug Dawson]

I'm still trying to figure out these AC compressors that don't have clutches. If it has a wire coming out from behind where the pulley is, as in this picture, that's to operate the clutch, which is inside the pulley. I don't know the total number of different types of compressors, but there is not nearly as many as there are different types of cars.
https://www.buyautoparts.com/autoparts/Acura/MDX/A-C_Compressor.html

True.

That compressor is sold with the clutch. You can buy compressors without them sometimes (if your clutch is still good.) They can even be rebuilt if you can find the parts and the compressor wasn't burnt. But that's for the hardcore.

[Tom Stenzel]

"Car Talk" in today's newspaper says that when you turn A/C on, the compressor stays on. If you turn the temperature up, the system mixes outside air in to bring the temperature up, but the compressor stays on.
In a house, the thermostat turns the compressor on and off; I always assumed it was the same in a car.

Is "Car Talk" right? If so, why do they do it that way?

They're *almost* right. All the air passes through the evaporator cooling it and dehumidifying it. If the air is too cold some of the air is ducted through the heater core to warm it. If outside air was brought it you would have a humid mess.

The compressor will cycle on and off but it's based on pressure switches protecting the compressor.

That's how it's been on all the cars I've owned with air, even the 1984 Cavalier. The only auto air conditioner that I saw that cycled the compressor to control the temp was the aftermarket unit my brother installed in his '75 Tradesman.

The Town and Country I have now with climate control will first lower the fan speed but only so far. It will then warm the air with the heater.

If you take it out of auto mode there's a button to turn the air conditioner on and off. But the manual says the the air conditioner will run anyway if:

The dampers are set to defrost
The dampers are set to heat
The dampers are set to mix

Or all of the time. The most useless button I've ever seen.

On the Chevy pickup we had at work the air would come on if the recirc button was pushed. My foreman thought that was how the air was turned on. He had no idea what the big button with the snowflake on it was for.

-Tom

[Frank Pratt]

@Tom Stenzel has it right. I had a couple of of vehicles back in the day with aftermarket AC & they cycled the compressor to control the discharge temperature. Some systems do have a low limit temperature sensor as well as pressure sensors to cycle the compressor.

[Kev Williams]

first- to be sure, I'm no AC expert. That said, I have never in my life ever heard of an AC compressor that just runs indefinitely. For one thing, AC compressors requires a LOT of energy to run, even part time. Having one run constantly and adjusting air temp only by mixing air doesn't make sense, especially considering there's no downside to just powering down the compressor. If a compressor was allowed to run indefinitely it would likely soon overheat and fail -like the one in our old refrigerator- or they'll freeze up the evaporator coils -like the defective forward AC unit does in our motorhome.

Just for fun, I googled 2019 Mustang AC compressor, here's a pic,
404321
note that it has a fan clutch and an electric connector that operates it, just like car AC units have always had... I have to presume most cars still use similar compressors...

To the best of my knowledge, these compressor will run as long as the AC switch is on *AND* the output air isn't too cold (to prevent coil freezing) or too warm (indicating not enough refrigerant). The only purpose of mixing air is to adjust the temp inside the vehicle.

There may be variable speed or output (?) compressors in use, but they would likely be the exception, not the rule...

[Chris Parks]

This refers to climate control systems where the AC is left on year round and never turned off. In modern cars there is no heater tap and with the AC on as in climate controlled auto temperature systems the air is mixed as needed in the HVAC box with the air flaps controlling the temp. This is how multiple zones can be controlled in cars, just isolate those zones at the mixer (for want of a better term). Clever things are modern cars. It goes back a fair way as my 2009 Skoda had this system. In older cars when I wanted to keep the screen clear I would turn on the AC and manually mix the air from the heater to get to the temp I wanted and it worked surprisingly well.

[Tom Stenzel]

first- to be sure, I'm no AC expert. That said, I have never in my life ever heard of an AC compressor that just runs indefinitely. For one thing, AC compressors requires a LOT of energy to run, even part time. Having one run constantly and adjusting air temp only by mixing air doesn't make sense, especially considering there's no downside to just powering down the compressor. If a compressor was allowed to run indefinitely it would likely soon overheat and fail -like the one in our old refrigerator- or they'll freeze up the evaporator coils -like the defective forward AC unit does in our motorhome.

Just for fun, I googled 2019 Mustang AC compressor, here's a pic,
404321
note that it has a fan clutch and an electric connector that operates it, just like car AC units have always had... I have to presume most cars still use similar compressors...

To the best of my knowledge, these compressor will run as long as the AC switch is on *AND* the output air isn't too cold (to prevent coil freezing) or too warm (indicating not enough refrigerant). The only purpose of mixing air is to adjust the temp inside the vehicle.

There may be variable speed or output (?) compressors in use, but they would likely be the exception, not the rule...

Denso, a major OEM manufacturer, has made variable displacement (output) compressors for 20 years or more. GM with their AC Delco division started with variable displacement in the '90's. I read they switched back to fixed for awhile but now may use either depending on the application. Dunno what other companies are up to.

They still can have an electric clutch to turn them off if the pressures are too high/low or if the ambient air temp is low enough so the programmers decided it didn't need to run.

When I owned a SL1 Saturn I belonged to a Saturn forum when there was a dustup in the Vue area. Some tried to see what gave better mileage, windows up air on or windows down air off. Those that tried found they couldn't tell any difference at all. Then others chimed in that their compressor was engaged when they had turned it off. One had lifted his snow covered hood and found the compressor engaged and spinning. With computerized controls who can tell if that was a malfunction? Or did the programmers screw up? There's no telling.

-Tom

[Jason Roehl]

I can almost guarantee every vehicle A/C compressor has an electric clutch (save for the aforementioned Tesla). A/C takes a HUGE amount of power in a vehicle (as much as 5-10HP--a tour bus can have around 70 HP worth of cooling), so when you put the pedal to the metal, every vehicle in the last 50 years with A/C cuts out the clutch to give the engine all the available ponies.

On another note, your "NORMAL" A/C setting is bringing in outside air. The "MAX" setting closes the outside air intake and recirculates the cabin air. Many cars will run the A/C compressor in the "DEFROST" setting, regardless of whether the A/C button is on or off (if it has one).

[Tom M King]

Jason, I don't claim to know anything about AC systems, but do work on our own. What you just described is exactly how I always thought they worked. Even with the Tesla, I don't see any reason why the compressor would always be engaged. They don't use a belt clutch because it's just controlled by an electric switch to turn the built in electric motor on, or off.

Out of curiosity, I did a bit of Googling, but didn't find an exact answer. I did find pictures of the compressor, and this:
The Tesla AC compressor is fully electric, integrating both an efficient scroll compressor and and an electric motor. Other AC compressors require to be belt driven from the motor or engine. This compressor takes electric power from the battery pack. The unit does not require a clutch, as the electric motor can be switched on and off as desired.

[Tom Stenzel]

I can almost guarantee every vehicle A/C compressor has an electric clutch (save for the aforementioned Tesla).

Hi Jason,

See:

https://axleaddict.com/auto-repair/Variable-Displacement-Compressors

Both Cadillac and Chrysler use clutchless compressors and there's bound to be more.

So you're driving down the road in winter. Some ice hits the grill and puts a hole in the air conditioning condenser. The system loses its charge. Now the compressor has no lubrication and seizes. If the compressor has a clutch you won't notice a problem until spring. A clutchless system? You can end up stranded on the side of the road in zero degree F weather. Because the air conditioner broke! To me a clutchless compressor is a product of some truly bad minds.

-Tom

[Bill Dufour]

A Ford Escape hybred. wehn you slow down the engine shuts off so the ac shuts down. You have to flip a switch at startup so the engine does not shutdown all the time. Or maybe it is set ac to max. Not fun to have no ac when it is 115 during summer. I think latter models added a small electric compressor but it is very weak, can not keep up with even double digit temps.
In theory a swash plate compressor could run all the time in neutral with very little load on the engine. The plate would tilt off neutral as pressure required.
Most cars if the defroster is running the ac compressor is running too. this helps freeze water out of the air so it defrosts with dry warm air.
Bill D.

[Mike Henderson]

It's very clear that the cars that I worked on the AC (older cars) had clutches on the compressor.

1. You could see the outer part of the pulley turning and the inner part not turning when the compressor was not running.
2. There was an electric wire running to the compressor, just behind the pulley. If you disconnected it, the compressor did not run at all. The outer pulley was still turning.
3. You could hear the clutch kick in when the compressor started turning. It was a very distinctive sound and easily heard, especially if you had the hood up.
4. On an idling engine, you would detect a change in the idling RPMs when the clutch engaged. The engine would slow down and then mostly recover.
5. There was a difference in the temperature of the air coming out of the interior vents when the compressor was running and when it was not.

I can't say anything about the air conditioning in modern cars but I know for sure that older (70's and 80's) cars had compressors with clutches and the compressor was cycled depending on the cooling needs of the car interior.

Mike

[Kev Williams]

Hi Jason,

See:

https://axleaddict.com/auto-repair/Variable-Displacement-Compressors

Both Cadillac and Chrysler use clutchless compressors and there's bound to be more.

So you're driving down the road in winter. Some ice hits the grill and puts a hole in the air conditioning condenser. The system loses its charge. Now the compressor has no lubrication and seizes. If the compressor has a clutch you won't notice a problem until spring. A clutchless system? You can end up stranded on the side of the road in zero degree F weather. Because the air conditioner broke! To me a clutchless compressor is a product of some truly bad minds.

-Tom

Interesting-- so I just googled these variable compressors, and IMO they're pretty ingenious- when they're not pumping, they shouldn't draw any more HP off the engine than an idler pulley; and as to what happens if the system loses its charge, the main rotating assembly is always rotating and separately lubed (good bearings I'm assuming) from the pumping mechanism, and the pump only requires lube from within the refrigerant if it's actually pumping. The variable-angle 'swash plate' system requires system pressure to make it work, so if the system loses refrigerant, there's no pressure to move the swash plate, so no pumping going on. There will be no AC but the compressor unit shouldn't seize up and fail...

[Wade Lippman]

This has gotten rather too technical for me.

My take away is that the newspaper article is right; the A/C runs all the time it is turned on and regulates temperature through either lowering the fan or heating some air. Is that about it?

Does it take less hp to run the fan on low rather than high? (not for the fan electricity, but for load on the compressor)

[Doug Dawson]

This has gotten rather too technical for me.

My take away is that the newspaper article is right; the A/C runs all the time it is turned on and regulates temperature through either lowering the fan or heating some air. Is that about it?


No. (Short non-technical answer.) It would be informative to see the newspaper article you're referring to. It sounds like it may have been misinterpreted. OTOH, the tap-it brothers' minds have been fading for a long time, by their own admission. :^)

[Mike Henderson]

No. (Short non-technical answer.) OTOH, the tappet brothers' minds have been fading for a long time, by their own admission. :^)

I think one of them died a while back. Don't remember which one.

[Edit] Tom died in 2014. Ray is still alive. Both Tom and Ray were graduates of MIT. (https://en.wikipedia.org/wiki/Tom_and_Ray_Magliozzi)

Mike

[Ron Citerone]

From the cars I have owned with AC, I would say the compressor is not always on. I would never say 100% about anything as I have been wrong a few times when I said that, but would give it a 97% that the compressor was not always on on my cars with AC. You can hear and feel when they go on and off. If you stand there with the hood up you can tell when it goes on and off.

M 2 cents

[Doug Dawson]

Hi Jason,

See:

https://axleaddict.com/auto-repair/Variable-Displacement-Compressors

Both Cadillac and Chrysler use clutchless compressors and there's bound to be more.

Don't underestimate the immense technological inertia of the big car companies. If they don't have to change something, they will not change it. Change costs money. Sales, government imperatives, and financial considerations can explain everything they do, in the bigger picture.


So you're driving down the road in winter. Some ice hits the grill and puts a hole in the air conditioning condenser. The system loses its charge. Now the compressor has no lubrication and seizes. If the compressor has a clutch you won't notice a problem until spring. A clutchless system? You can end up stranded on the side of the road in zero degree F weather. Because the air conditioner broke! To me a clutchless compressor is a product of some truly bad minds.


It's interesting that it's coming from Chrysler, who IMO have a history of utter contempt for what happens to their vehicles after the warranty expires. Google the "chrysler concord sludge engine failure" fiasco from the early 2000's to witness that. My own MIL was dinged by that, cost her US$13k for a new engine after it failed (for that problem) at 30k miles. In the case of Cadillac, they may be gambling that the customer passes away before it becomes an issue.

[JERRY BRINKMAN]

"Car Talk" in today's newspaper says that when you turn A/C on, the compressor stays on. If you turn the temperature up, the system mixes outside air in to bring the temperature up, but the compressor stays on.
In a house, the thermostat turns the compressor on and off; I always assumed it was the same in a car.

Is "Car Talk" right? If so, why do they do it that way?

A large majority of vehicles today have cycling clutch orifice tube operating system. There is a low and a high pressure switch. The low pressure switch is closed above 30 psi to allow electromagnetic clutch engagement. The high pressure switch will open at about 300 psi to shut off clutch. With the system off normal internal pressure (the higher the outside temperature the higher the pressure). A 65 degrees it will be about 70 psi. thus both switches are closed in the circuit to allow clutch engagement. With the compressor running now that they are closed it will circulate Freon and the low side pressure will begin decrease in about 30 seconds it will be below 30psi and the low side switch will open and shut off clutch. The low side pressure slowly increase and close the switch allowing clutch engagement again. Just about all cars will turn the compressor on in defrost mode to dehumidify the air even if your car is covered in snow. Automatic control systems when on will run the compressor all the time but it will cycle off and on depending on Freon pressures. They temperature blend doors to control temperature, mixing cold and warm air to get the desired temperature. If you turn the system off, the compressor will not run. All systems have oil in them that is circulated by the Freon movement when on. If you loose enough Freon the low side pressure switch will not allow the compressor to come on to protect it from lack of oil circulation. The high side switch protects from excessive pressures that can cause damage. Generally 300 psi.

[Ken Combs]

I guess by now it's crystal clear that there are three answers: Yes No and Maybe

all depends on the make, model and year. Older add on units where always a cycling clutch type. All vehicles with zoned systems use blends of hot and cold air to 'fit' the settings of each zone. So they do run the compressor virtually all the time. But, on my Chrysler, there is a pushbutton that allows me to override that. Clutchless run all the time, but not necessarily pump all the time.

[Doug Dawson]

I guess by now it's crystal clear that there are three answers: Yes No and Maybe

all depends on the make, model and year. Older add on units where always a cycling clutch type. All vehicles with zoned systems use blends of hot and cold air to 'fit' the settings of each zone. So they do run the compressor virtually all the time.

No, they still don't. They cool to the coldest-set zone and adjust the blend doors to accommodate to the other zones, as best they can. (There's always warm air available from the heater core box in water-cooled vehicles.) But, as Jerry and I previously noted, it's all governed by whatever pressure is in the system, which controls how the compressor clutch cycles.

If you set the a/c to maximum full cooling, the compressor will still cycle. This (i.e. measuring the resulting cabin vent temperature) is part of how we would performance-test in the shop. A compressor that stayed on the whole time would be cause for alarm and concern, and protective equipment. 8^) (Or shutting down the car before the pressures get to that point...)

Then there are the oddball systems like Chrysler is claimed to still be using (keep those things far away from me...) and that's the point of confusion.

[Mel Fulks]

In the late 70's GM cars ran the compressor with AC,and with heat unless compressor was turned off by separate switch,

and with defog ( you could have compressor off....but the fog would stay! People who had always bought GM would ,as a passenger in a friends car ,comment "I wish my defogger worked like that". Don't know what they had after that. You can't make people
read directions.