There's also the the power required to run the electric cooling fans, which run whenever the AC compressor is on... I believe the ones in my Contour take 40 amps, about 480 watts, or a bit over 1/2HP.
How much horsepower does the a/c rob?
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I have had some cars and trucks noticibly effected by the AC being on notably a 96 dodge dakota and a 1989 4 banger cavalier. Nothing I currenty drive has any tangible effect from running that AC.. since it's always in the 90's during the day I aint going to bother trying a tank with and a tank without type experiments.. I love my AC. Keeps the dust out and keeps me from smelling.
I had a 1994 3300 V6 and currently have a 1998 3800 V6, both in a mini van. In both cases when I run the A/C I lose 3 or 4 MPG (from 28 to 24 or 25 MPG). On the flip side, I had a 1962 chevy wagon with a 230 or 235 CI straight 6, I added an A/C unit that would freeze you out of the car and there was no difference in MPG with or withiout the A/C. Go figure!
A 2 ton househould unit that takes 4800 Watts would have an EER of 2.5. I doubt you could find a new AC nearly that inefficient. An EER under 10 isn't even legal for a central air system many places. someplaces. Are you confusing starting current x voltage (Sears motor HP
Even an automotive AC would have an EER of 5 or better.
The relatively small differences in gas mileage with and without AC on should raise a red flag for you that automotive AC system power consumption is lower than folk lore says it is.
On my 3,600 miles vacation I kept tract of my millage carefully and measured the following. When the air conditioner was on 100 %, cruse set at 74 mph, 90 degrees ambient temp the gas millage wa 27.6 mpg. Same conditions air off, 85 degrees ambient, windows open the gas millage was 27.3. It seems that at higher speeds that the air conditioner load is about the same as windows open. Only explanations is car is more streamlined with the window closed?? Car is a Subaru Forester or box if you will. ed
My 59HP Jetta diesel is useless with the A/C on. The A/C works just so so, and it feels like it robs about 40 HP. I turn it off except when I'm on flat terrain at cruising speed with no shifting necessary.
In my 193HP XJ I have recorded my highest ever MPG with the A/C on. Same with the 165HP Subaru. I don't think the effect on those vehicles is significant. The effect of winter fuel is much greater based on my measurements.
In my 193HP XJ I have recorded my highest ever MPG with the A/C on. Same with the 165HP Subaru. I don't think the effect on those vehicles is significant. The effect of winter fuel is much greater based on my measurements.
There have been claims made that running at highway speeds with the air conditioner on and the windows closed in more efficient than driving with the windows open.
If you hold your arm out the window at 74 mph, there is a lot of drag on it. Maybe 10 lbs.
74 mph = 109 ft/sec.
10 lb x 109 ft/sec = 1.98 hp
AC at highway speeds = open windows is in the realm of the highly probably. Unless you are one of those that believes an AC takes 7 to 40 hp
EER is BTU/hr divided by watts.quote:
Originally posted by XS650:
A 2 ton househould unit that takes 4800 Watts would have an EER of 2.5. I doubt you could find a new AC nearly that inefficient. An EER under 10 isn't even legal for a central air system many places. someplaces. Are you confusing starting current x voltage (Sears motor HP) with running power?
Even an automotive AC would have an EER of 5 or better.
So the example would be 24000/4800=5 EER (hmm)
A 10 EER unit would consume only 2400 watts, so I was mistaken (20 amps may have been the startup amperage or the minimum circuit ampacity; the label isn't in the easiest place to get to).
One thing to keep in mind is that an automotive AC unit usually cycles on and off, and so won't cosume the full amount of power all the time.
I've not noticed small differences in fuel economy with the AC on. Not 1-2 mpg, more like 3-4mpg but that was during our recent 97 degree, high-humidity heat wave, which likely resulted in a higher duty cycle for the AC unit (higher outdoor temps also reduce the EER don't they?)
Both my Corolla's feel like I threw out an aircraft carrier anchor when the A/C turns on.
The TV show Mythbuster's tested the "myth" that using the A/C on the highway will result in less mpg than having the windows down. I don't remember the details but they got much better mileage with the A/C on.
The TV show Mythbuster's tested the "myth" that using the A/C on the highway will result in less mpg than having the windows down. I don't remember the details but they got much better mileage with the A/C on.
Well, let's see what other red flags we can raise. The above assumption regarding hp usage vs. fuel economy is very, very obviously improper. Your premise above says that increasing the hp required to move your Accord-sized vehicle down the road to 60hp (pulling a small trailer up a slight incline, for example) would have a 30/30 = 100% hit in fuel economy. 0 mpg when pulling a small trailer up a slight incline? Even at WOT at the hp peak you'll get some number of miles out of each gallon consumed. Surely that simple exercise is sufficient to discard this ridiculous model of fuel economy vs. hp.
A/C does use ~5hp (commonly 5-10 depending upon a wide variety of factors), and driving with the windows open does not use as much fuel as the A/C.
Well that statement depends on many factors, mainly the speed, aerodynamics of the car in question, drag coeffecient created with the airflow being compromised by the window being down or even partially down. In other words, there are a lot of variables. Go back and read the experiences of folks seeing no difference in mileage with A/C on or off.
The head pressure sensor to cycle or bleed off pressure of the compressor mitigates the higher RMP = more power rob theory posted earlier.
I watched this segment with eager anticipation, hoping that it would finally give an answer to this question. To my horror, they tested two Ford Explorers at 45(!) MPH and went farther in the vehicle with all the windows open. (The test track deemed it unsafe to run an Explorer around the test track banking at 55MPH. Yikes!) So all they accomplished was to prove that one Explorer tooling around at 45 MPH gets better mileage with the windows open than another one with the AC blasting full on. They completely (and conveniently) ignored the question of what happens at freeway speeds, and so only added fire to the myth. Even if they had answered the question about what happens at the much higher drag at 55MPH, it wouldn't be useful. I would get rear ended if I slowed down to 70MPH on a posted 55MPH...
Hmmmmmmmmm, I remembered that show to be the other way. Maybe I'm thinkin of some article I read in the newspaper that said using the A/C at highway speeds results in better mpg. Oh well. I think the mytbusters did mention that highway speeds would be different.
This is just as bad is the tail gate up-vs- down argument. I am a tailgate up AC on at 70 mph type of guy. of course rolling agressive 33" tires and racking a kayak in the wind makes the ac seem trivial to me.
5/30 = .16666 A 17% increase in power, and a resulting 17% increase in fuel consumption with aresulting 17% loss in economy
30/30 = 1 = a 100% increase in power and twice the fuel consumption, or 15 mpg.
I think you could pick a speed that supported your argument, no matter which side you were on
I'm a certified flipflopper; what can I say!quote:
Originally posted by XS650:
I think you could pick a speed that supported your argument, no matter which side you were on
Actually they did the test in a flawed manner, and their results were that windows down a/c off had better mpg.
The windows down/ A/C on debate has a lot of variables; power to weight ratio compared to drag co efficients of different body styles.
The windows down/ A/C on debate has a lot of variables; power to weight ratio compared to drag co efficients of different body styles.
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