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#3371373 - 05/14/14 11:10 AM Re: AC Compressor Load on Engine [Re: A_Harman]
440Magnum Offline


Registered: 02/01/09
Posts: 6256
Loc: Texas
Originally Posted By: A_Harman
Setting a higher temperature on the A/C control would make the compressor clutch cycle off more often, which would save fuel.


Actually, almost NO automotive systems work that way. The HVAC maintains the temperature of the evaporator coil at about 35 degrees F (or as close as it can get). If you set the temperature to warmer, it may slow the fan down on fully automatic climate controlled systems and that does help. But on most systems, it just blends in some warm air from the heater core and does nothing to reduce the load on the compressor.

Fan speed always has more effect- slower fan speed means that the compressor doesn't have to run as much (or if its a variable-displacement compressor, it can run at a lower displacement) to keep the evaporator coil at the target temperature.

Even older systems (for example, Chrysler Airtemp with the old V2 and RV2 compressors) ran the compressor all the time, but used an evaporator-pressure regulator valve and variable thermal expansion valve to "throttle" the system. The EPR valve would begin to block the suction side of the compressor when the returning refrigerant temperature got down to about freezing, which reduced the compressor head pressure, and in turn reduced the load on the crank. But mainly, it made the system stupid-slow to respond to a high load. All my EPR systems got converted to cycling-clutch systems when they got converted to R-134a.
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#3371476 - 05/14/14 12:47 PM Re: AC Compressor Load on Engine [Re: Chris142]
zzyzzx Offline


Registered: 05/18/12
Posts: 1640
Loc: Baltimore, Maryland, USA
Originally Posted By: Chris142
now lets throw a variable displacement compressor into the mix


I suspect that not enough people have one of those to comment. I do, and when driving that car I just leave it on. On my other car witha normal compressor that turns on and off, I only have the A/C on when not accelerating, since turning the A/C on is roughly equivalent to a "drop anchor" button.

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#3371523 - 05/14/14 02:11 PM Re: AC Compressor Load on Engine [Re: actionstan]
accent2012 Offline


Registered: 02/08/12
Posts: 441
Loc: Orlando, FL
On my older car, turning on the AC noticeably bogged the engine making it run at higher RPMs in idle. Acceleration time would also grow by a second or two. IF I'm on the highway and wanted to pass someone in that car I had to turn it off so as not to take too long lol. It felt like the car lost some weight lol.

Would cycling the AC clutch all the time cause it to wear out faster? If that's the case, wouldn't it just be better to run the AC at max so the clutch doesn't engage?

I was told this bogging down the engine was much worse in the older days. I knew someone who drove a first-generation Ford Fiesta from the 80s with AC. The car would lose half of its power with it engaged!


Edited by accent2012 (05/14/14 02:13 PM)
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#3371531 - 05/14/14 02:24 PM Re: AC Compressor Load on Engine [Re: accent2012]
zzyzzx Offline


Registered: 05/18/12
Posts: 1640
Loc: Baltimore, Maryland, USA
Originally Posted By: accent2012
I was told this bogging down the engine was much worse in the older days. I knew someone who drove a first-generation Ford Fiesta from the 80s with AC. The car would lose half of its power with it engaged!


That could be said for all or most 4 cylinder cars of the era yes.

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#3371539 - 05/14/14 02:34 PM Re: AC Compressor Load on Engine [Re: zzyzzx]
dishdude Offline


Registered: 11/14/08
Posts: 3283
Loc: Phoenix
Originally Posted By: zzyzzx
Originally Posted By: accent2012
I was told this bogging down the engine was much worse in the older days. I knew someone who drove a first-generation Ford Fiesta from the 80s with AC. The car would lose half of its power with it engaged!


That could be said for all or most 4 cylinder cars of the era yes.


I had a '95 Neon that when new, I couldn't tell when the compressor cycled. By around 150-160k, I'd lose a ton of power each time the compressor cycled.

I think smaller displacement engines that don't have a lot of low end torque are the ones that really get bogged down.

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#3371546 - 05/14/14 02:44 PM Re: AC Compressor Load on Engine [Re: actionstan]
exranger06 Offline


Registered: 01/25/08
Posts: 2567
Loc: Guilford, CT
Most cars have a wide open throttle relay that will automatically turn the AC compressor off when you're accelerating hard.
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#3372471 - 05/15/14 12:36 PM Re: AC Compressor Load on Engine [Re: zzyzzx]
artificialist Offline


Registered: 09/23/07
Posts: 6770
Loc: Florida
Originally Posted By: zzyzzx
Originally Posted By: Chris142
now lets throw a variable displacement compressor into the mix


I suspect that not enough people have one of those to comment. I do, and when driving that car I just leave it on. On my other car witha normal compressor that turns on and off, I only have the A/C on when not accelerating, since turning the A/C on is roughly equivalent to a "drop anchor" button.

Really? I'm used to seeing them on the GM 2.2 OHV, 3100, 3400, and all Ecotec engines. They were so common, that it was the first time I saw a clone of a compressor made brand new in China.

Anyway, the burden the A/C put on the car varied by compressor design. My father installed retrofit kits in the late 1960s and early 1970s. The kits were not recommended for I6 and smaller engines, and that was because they had a huge 1-cylinder compressor.

Dad did have a car where the AC was an enormous burden. It was a 1987 Hyundai Excel. The A/C compressor disengaged at full throttle, and because of how weak that engine was, there were plenty of times when he had no A/C at all.
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#3372878 - 05/15/14 10:02 PM Re: AC Compressor Load on Engine [Re: actionstan]
1 FMF Offline


Registered: 08/12/02
Posts: 1505
Loc: CT
Originally Posted By: actionstan
Hello All,

I was wondering.. when you turn on the AC in a vehicle and the compressor starts putting load on the engine.. does it put the same load on the engine all of the time? Or does it depend on how fast the blower motor is running.

For example if you use the AC but keep the blower on low, will you save fuel/load on the engine, V.S. running the blower on high with the AC on?

Just wondering because it may make more sense to run it high.. then turn it off when I am comfortable enough, then run it high again. I have a 99 SL1 and the compressor basically completely kills its power, and in the 2013 it idles rougher with the AC on so I like to minimize use as much as possible.


Thanks,


there are two types of compressors, clutch cycling and variable displacement.

for either one, the load that the compressor puts on the engine is directly proportional to the heat the refrigerant is moving from the evaporator to the condenser and how well the condenser is shedding that heat. so on a hot day with your blower on high- the most hot air is flowing over the evaporator coil to be cooled, putting the most heat in the system. now if your parked and you notice your radiator fans kick on high speed, that's because of all the heat in the refrigerant and it's unable to be shed at the condenser easily- high side pressure of the AC system is very high and that pressure difference from what the compressor is pumping is the load placed on your engine.

on a variable displacement compressor which is what most cars have now, i don't know what model years the clutch cycling got mostly phased out, the compressor varies its output (pressure) based on internal valving to keep low side pressure above 30 psi for r134a refrigerant. so when heat load is low the compressor swash plate is more flat and it's pistons pump less, when heat load is high the swash plate is angled most and the pistons pump at max capacity, as heat builds in the system and high side pressure rises, that pressure between low side and high side of what the compressor is causing is the load placed on the engine. you will never hear a normally operating variable displacement compressor clutch cycle off unless it's turned off or you're low on refrigerant and the clutch is tripped by the low pressure sensor.

in a clutch cycling orifice tube system, when it does cycle off under normal conditions that's because the heat load on the system is very low causing that so just prior to it cycling off the compressor is placing the least amount of load on the engine it could be.

and in either case a blower setting on low vs high puts less mass heat flow over the evaporator coil = less heat put into refrigerant = lower system pressure = less load on engine regardless of outside temperature.

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#3373790 - 05/17/14 12:07 AM Re: AC Compressor Load on Engine [Re: actionstan]
Barkleymut Offline


Registered: 01/27/04
Posts: 2431
Loc: Richmond, VA
I learned to drive in a '82 Corolla station wagon with AT. With the AC on I often wondered if I could get up certain hills. At stoplights the AC quit running until you gave it some gas. That little car was tough, but very unpleasant to drive.
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#3373849 - 05/17/14 05:17 AM Re: AC Compressor Load on Engine [Re: actionstan]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 14767
Loc: Sunny Florida
Sounds like my Dad's old Corona. The AC clunked on and half your HP vanished!

At least it was cold. But the constant vibe and cutting in and out was annoying as the dickens...
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#3373864 - 05/17/14 06:03 AM Re: AC Compressor Load on Engine [Re: actionstan]
Colt45ws Offline


Registered: 08/15/06
Posts: 7408
Loc: Central Washington
My old '98 Cavalier had a 2.2L OHV w/ 3-speed AT and the A/C button was a turbo button. I remember reaching over and hitting that button all the time when I was driving to school or work in Las Vegas. Also made it vibrate a lot at idle. I think it was literally a 20-21 second 0-60 vs. 14 or 15 seconds. It was horrible. I wished it had a WOT cutout.


Edited by Colt45ws (05/17/14 06:08 AM)
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