General Automotive DiscussionGeneral automotive discussion and chat. Honda, Toyota, Chevrolet, Ford. It doesn't matter, just talk about it here.
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Ok, maybe this is a stupid question but I can't figure it out.
Me and my roommate were talking and he asked me why does the car lose power and use more gas when the A/C is on. I'd never thought about it. I was going to say that the compressor is running but, then I remembered, the compressor is always running since the belt always spins the compressor. I know it uses power from the battery (alternator) but that shoudn't matter since my amps don't bother my gas mileage any. Can someone explain it?
Also, would it matter, gas mileage-wise, if it's on low or hi?
Sorry for the dumb question but I always took it as a given that it lowers your range, just never thought of why.
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when the A/C is off the compressor is just Idoling. (Not compressing anything). and Yes, pulling more power from the alternator will also suck more power from the engine (Pulling more power causes more resistance in the alternator making it harder for the engine to turn it). and no, it dosn't matter if the Fan is on High or low, the compressor is still running at the same speed.
Basically, running the compressor takes around 8 horsepower or so. If you put it hi, even more and the car will lag since its used to more hp being available.
think about it, the compressor is run off of the engine, meaning that its spinning as fast as the engine is (No matter what), the controller in the cab only contorlls an electric fan that blows the cold air on to you. So the settings high and low mean notheing to the engine, just to you. Now if our cars had an A/C and a MAX A/C like some cars do then there is a difference.
when the A/C is off the compressor is just Idoling. (Not compressing anything).
This is correct. When the compressor turns on (via the a/c button in the car), the compressor provides resistance. When off, it is just spinning--like in neutral.
I am not sure about the fan speed. I THINK it is separate from the compressor. The fan would, though, take power from the alternator, I think. Check out www.howstuffworks.com and go to the auto section.
Somehow I doubt we would lose as much as 10hp with the A/C on. I hope not, because it's on 24/7 when I drive where I'm at!!! I know that studies have shown you'll actually lose more mileage driving with the windows down at freeway speeds than with the A/C on, due to drag.
Yeah. .what everyone already said. Plus. . I guess that it is possible that the windows down would create different drag and might affect your mileage at highway speeds. Interesting idea..
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Originally posted by Zzyzx think about it, the compressor is run off of the engine, meaning that its spinning as fast as the engine is (No matter what), the controller in the cab only contorlls an electric fan that blows the cold air on to you. So the settings high and low mean notheing to the engine, just to you. Now if our cars had an A/C and a MAX A/C like some cars do then there is a difference.
As usual, Zzyzx, you're very right. . . .
However, I believe (on other vehicles) that MAX AC setting merely closes-off the vent to fresh air. This is something we do by pushing a button, but it accomplishes the same thing. It merely reciculates the air within the cabin, but continues to run it through the cooling coils to further cool it. This IS a much more efficient way to cool the cabin, but after a bit the air will start to get funky. . . (Many owner's manuals will state that this setting is best used to initially cool the cabin down to a comfortable level, then it's suggested that fresh air be used the rest of the time.)
Either way, the compressor (as you said) is still spinning with the engine. . . regardless. That MAX AC setting merely rechannels air to further cool it within the cabin.
So anyone with a Ford or Chevy can use the MAX AC setting as much as they want. It won't matter to the engine. As for us, you can hit the recirculate button as much as you want. . . some difference. Just after about 5-10 minutes you'll probably want to go back to fresh air.
Originally posted by AZD17A2 Somehow I doubt we would lose as much as 10hp with the A/C on. I hope not, because it's on 24/7 when I drive where I'm at!!! I know that studies have shown you'll actually lose more mileage driving with the windows down at freeway speeds than with the A/C on, due to drag.
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howstuffworks.com says with the windows down you lose around 8-12% on the tank of gas
id still rather have the windows down on a nice day than the A/C. and drag depends on how far down your windows are. so if youre cruising on the highway you dont really need them all the way down...like not even a quarter down is perfect.
just saying. and i was gonna say about the resistance of the compressor point also, but got beat to it.
i can't stand having the windows down on the highway, all that noise, its annoying, but going 40mph or less, really nice day outside, i dont mind my windows opened a bit.
i dont know the technical things about the a/c compressor and what not, but the last time i went on a long trip, i had the a/c running full a lot of the way, and i got great gas mileage, 450miles on the tank i recall. in any case, i'd rather be comfortable than save a few bucks at the pump.
Riddle me this, Batman! 
(Many owner's manuals will state that this setting is best used to initially cool the cabin down to a comfortable level, then it's suggested that fresh air be used the rest of the time.)
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