AC Compressor Load on Engine

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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,
 
Yes the blower motor makes a slight difference.

More air blowing over the evaporator makes it more efficient.

Your saturn has a CCOT compressor which can cause a variable load, just enough to not ice itself over. Give it more BTUs to shed, more work to do, it will drag the engine more.

Also, pumping away heat is an absolute thing that has to happen "X units" per minute. When the engine/ compressor is turning less RPM, more power from each rev is taken for the AC. Also, the cam and other engine design factors make more torque at ~2300 RPM than it does at idle.

In a nutshell, this makes the car bog wicked in summer weather.

How to test this will be iffy though, we don't know if or how often a compressor is maxxed out.
 
Setting a higher temperature on the A/C control would make the compressor clutch cycle off more often, which would save fuel.
 
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.


I agree. You're better off running the fan at a higher speed and setting the A/C Temperature Control Switch to a slightly warmer setting.
 
Keep the temperature all the way cold (no need to add heated air) and adjust the fan speed for comfort. If you want to play with it for better fuel economy, kick the fan speed up all the way up during coast-down.
 
Running the fan on low speed & using recirculating air setting will cause the compressor to kick off more often-which will help fuel economy-somewhat. I usually try to use vent & lower a rear window a 1/2 inch or so, until it gets too hot to take it!
 
Originally Posted By: Merkava_4
When you turn your fan speed up, your compressor will cycle on more frequently.


No it wont. All the way low will cause it to cycle more depending on ambient air.
 
Some family members of mine blast the A/C for a few minutes until it cools down, then turn everything off (blower motor and A/C pump) until it warms up again. To me, all this does is make it muggy in the car and very annoying. If its really hot, I'll leave it on the coldest setting and use the blower motor speed to regulate the temperature. If it is just muggy, I'll adjust the temp control a little warmer and leave the fan speed on MED and forget it. I think GM A/C pumps have a variable output depending on need, so get it cool fast and then just maintain it on low. My old Camry takes a lot of power away with the air on too, but it gets it cool so whatever the car does, I just live with it.
 
The system is designed the way it is designed. Removing heat from the cabin takes tremendous amount of energy and that is why most low power econoboxes take quite a big hit in fuel economy and power. The only way to minimize it is to not use the AC, otherwise there is nothing one can do.

Besides, if you want your AC system to run trouble free for years to come, you HAVE to use it periodically to lubricate all the components and seals, even during winter.
 
Originally Posted By: 1kickbuttranger
Originally Posted By: Merkava_4
When you turn your fan speed up, your compressor will cycle on more frequently.


No it wont. All the way low will cause it to cycle more depending on ambient air.

Interesting, I noticed my AC was cycling quite a bit yesterday with a low fan speed, and an ambiant of 78F. I'll have to try it on a higher fan speed tonight and see what it does. It does cause quite a hit on instantaneous mileage on the scangauge, like 40mpg to 34mpg, but I think the ECU knows when the AC kicks on as it compensates without me changing throttle position, and the car doesn't slow down.
 
Don't know what compressor my 2010 Hyundai has but when I run the AC, the mileage hit is so little.
It gets cold, just doesn't get cold fast (have to drive a bit) like my other two vehicles which they have a more noticeable mpg hit.
It must be small and efficient, just takes a little running time to cool the air coming out the vents.

Anyway I mostly (if vehicle been sitting hot in the sun) start out at max AC which kicks in recirculate with fan on high to start, or if really hot inside, turn it on and open windows for a little to flush out the hot air (while moving) non recirculate, then close windows and run max until inside feels better, then run mix (under dash and on dash vent flow) and work fan speed as needed.
For driver, the left/right dash vents, some air flow gets blocked by the steering wheel - they could have done better. And wish when running air down below would cool my feet (off work with boots...hot feet) as well as my other vehicles.
 
Oddly, my Mazda (2012 Mazda3 Skyactiv) does not seem to suffer a fuel economy hit when using A/C. This makes it different from other cars I have owned. However, it does bog the engine noticeably, especially from launch, in hot weather. Since I like it cool and this is Texas I just turn it on and leave it on.
 
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.
 
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.
 
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!
 
Last edited:
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.
 
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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