Why does air conditionning use up petrol in your car?

[ManiacPony]

It seriously affects your fuel consumption, but why? Isn't the alternator is always producing electricity? And surely most of this is usually wasted once the battery is recharged???

[Kev_UK]

Briefly, it makes the engine work harder. I have the belt taken off on my 5 litre chevy. I believe when off it freewheels but engages when the a/c is on. I know it uses valuable bhp. There is a website called "how it works" you can search for it on google.

[Ewood27]

This site will tell you about air conditioning. It's mainly about a/c in buildings, but the same principles apply.
As for the alternator, I'm no expert, but I understand that it does rotate all the time the engine is running. When there is a load being drained off the battery, though, and it needs building up again, the alternator has to work against increased resistance, so it takes more engine power to force it to rotate. That's enough to make you use more throttle to maintain your speed, so you use more petrol. I try to use my a/c only when I'm limited to 30 or 40 mph, and switch it out when I'm likely to want to accelerate.

[akaceefax]

ac works by compressing a fluid and then releasing it to cool the air comming into your car. in car ac the compressor will use large amounts of current which is generated by the momentum of the engine while the car is running this puts extra resistive force on the engine which will decrease the torque available for moving the car. It is like driving up hill all the time

[Headtime]

A friend gave me a lift in his car and was saying much the same thing. I was sceptical at first, but I can see the reasoning that by having the A/C on it requires extra power, and that must come from the engine. I suspect, though that the amount of extra petrol it uses is negligable.

[AllYourBase]

I lose about 3-4 MPG on my car when using the A/C. I can tell this from the fuel computer.

[norfolk boy]

Just a side note a friend of mine has a discovery,whilst being a passenger at a junction he wanted a quick getaway so he turned the a/c off.When asked why he replied "the bloody thing will stall".With him being a man who tolerates fools lightly i don't disbelieve it.

[Pottsy]

I have an Audi A4 and if you have the aircon on and accelerate hard, the ecu susses that you want as much power as possible and cuts the aircon until you ease off the accelerator. There is a distinct increase in performance when the aircon cuts out, indicative of the amount of power it saps from the engine.

[chrislangham]

It is the old principal of the conservation of energy. You cannot get energy out that you did not put in. The energy that the air con uses must come from somewhere, it this case the petrol. Energy in = energy out x efficiency