How much horsepower does the a/c rob?

[oilyriser]

The power used by the a/c increases with engine speed. Also, the a/c only runs a ceratin %age of the time depending on the climate settings.

The effect of windows down depends on how far down they are. Some cars give good circulation by lowering both windows a few inches, on one side. You don't need to roll down all the windows all the way, like they usually do in those tests.

 

[eljefino]

quote:

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Originally posted by oilyriser:
The power used by the a/c increases with engine speed. Also, the a/c only runs a ceratin %age of the time depending on the climate settings.

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I was under the impression that the A/C compressor will run until it almost freezes up, then it cycles off. CCOT compressors have a variable output so they'd drag less for a bit.

At idle speed, the compressor wouldn't be compressing as much over a period of time, not freezing up, and working harder for every revolution. In other words, the compressor will do the same amount of work per minute at a certain ambient temperature no matter what it's revolving at, above a minimum when its duty cycle is 100%. More engine RPM means less drag per rev as well as a shorter gear for the speed you're going at... so delaying shifting will really make the car accellerate faster with the air on.

My saturns (with CCOT) bog like crazy starting off from a light, but revving above 3k rpm clears things up.

All theorized as I understand it.

 

[bulwnkl]

quote:

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Originally posted by XS650:
5/30 = .16666 A 17% increase in power, and a resulting 17% increase in fuel consumption with aresulting 17% loss in economy

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Well, the 17 percent increase in consumption does not yield a 17% loss in economy just as a 100% increase in consumption does not yield a 100% loss in economy. It yields a 50% loss in economy. That's why I posted what I did, although I can see my approach was not a great one. With the relatively small percentage we're talking about (17), the end mpg figure is still fairly similar for your example.

The anecdotes of people not noticing much mileage difference are primarily from folks with large engines in large, heavy vehicles, often with relatively poor aerodynamics and relatively poor mileage in the first place. Not all, but primarily. Certainly newer system designs will alter efficiencies, but the fact remains that A/C uses more than 0.2hp.

 

[XS650]

quote:

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Originally posted by bulwnkl:
Certainly newer system designs will alter efficiencies, but the fact remains that A/C uses more than 0.2hp.

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I think they average more like about 1 to 2 hp in a small-mid sized car on the highway once the car is cooled down.

 

[bulwnkl]

I'll agree with that. Which leaves most drivers (city-dwellers in traffic) with the considerably higher power figures I listed earlier since the car often fails to get completely cooled down, and there isn't the nice, high airflow through the radiator to raise the efficiency of the system.

 

[92saturnsl2]

IMO the A/C does have a significant effect on power output, but moreso at lower RPM's.

I usually have to shift at 3000rpm rather than the normal 2500rpm just for the car to feel the same in city traffic. The hot air further complicates the problem, because as we all know warmer air is less dense = less power. If I had to guess, I'd say about 20% of the power is robbed under 2500rpm.

I've got a 12valve 2.8L VW VR6 engine (very torque heavy), and it still sucks pulling through 1000-2500rpm driving with the A/C on in stop and go traffic.

When I'm cruising in high gear however (highway type driving) I don't even know it's there.

 

[kenw]

If (as discussed previously) a/c consumes on average 6hp, then it certainly makes sense that it is more noticeable at slower speeds/RPMs. That's where your engine is making the FEWEST HP, and 6 hp is a larger % so therefore more noticeable. At higher RPM, your engine is closer to it's max rated HP and 6hp is a smaller %.

You don't notice it at high speed because it is a lower % of available power. The HP loss is the same regardless of how fast you are moving. The % loss changes.

 

[Clement]

6hp sounds reasonable. My little Hyundai only puts out 105hp, and thats at 5500rpm! For economy purposes I try to shift at about 3000rpm, which is also the max torque point (106lb/ft). I calculated it once, forgot the formula, but for most of my driving, I think i'm only using 50-60 hp. 6hp is therefore a big hit!

The AC really kills the power when driving around town, not as noticeable on the highway.

Funny thing though, if I'm accelerating, you get a burst of power when the compressor cycles off!