Car Air-conditioning uses up Fuel?

I have wondered this ever since my Mom told me. My Father also told me that you should not start a car with the AC on. What is up with this? Does a Car AC use up Gas? Can you leave it ‘on’ and turn the car on?

Yes, an air conditioner is a pump, it pumps heat from a cooler place to a hotter place, so naturally it requires energy and that energy comes for gasoline. That’s just like pumping water up hill. But as for starting the car with the a/c on, it’s not good for the machinery. Doubt if it affects the mileage that much just from starting but it’s not good for the clutch on the front of your a/c compressor.

As far as gas use increase from running the a/c while driving, at low speeds and in town stop-and-go type driving the a/c gobbles gasoline. But, at about 50 miles per hour there is break even point. Turbulence from having the windows rolled down increases gas consumption, and it increases with the square of the speed. The increased fuel use from using the a/c is balanced by the decreased turbulence of having the windows up at about 50 mph for most vehicles. It’s probably even less than that for 4 cylinder vehicles.

Well of course it uses up some energy, it can’t come from nowhere. The only energy source in the car is the fuel, except the nett gain got from the battery slowly dying.

But the energy lost to AC while driving is less than that lost by extra air resistance if you drive with the windows down instead.

I’ve not heard the one about not starting up with the AC on. I can only guess it’s to do with extra strain on the starter and battery, cranking the engine and the AC as well. I certainly wouldn’t do it if you are having trouble starting, but otherwise I can’t see a problem.

I think the average for A/C use is about 0.4 litres of fuel an hour but I imagine this varies a lot. I’d forgotten about turning off the A/C when you stop the car but I now recall an A/C mechanic told me that starting the car with the A/C on runs the risk of blowing the compressor. I obviously have ignored his advice but may start now.

I had it figured that starting the car with the compressor running meant using up battery juice - eventually, the battery will give up on you. I don’t think it especially uses up more fuel (while starting). As a habit, I turn off the AC before turning off the engine.

I don’t understand why you run the risk of blowing the compressor. Cranking the engine doesn’t send any excess power to the compressor, AFAIK.

That clutch on the front of your compressor free-wheels when the a/c is not use. When you turn on the a/c with the motor running the clutch brings the compressor online in a controlled manner. Starting with the a/c on puts a lot of strain on the clutch and ‘jerks’ the compressor into action. Repeatedly doing this will shorten the compressors life and increase the risk of a blown seal or leaky hose connection.

Accessories such as A/C, wipers, and turn signals are bypassed during starting (while cranking). The moment the key is released to the “run” position, these accessories are then enabled, and if their switches are “on” they will engage. Not much, if any, difference between leaving the A/C on while starting and turning it on 2 seconds after starting. I don’t see where it’s a significant concern.


…an A/C mechanic told me that starting the car with the A/C on runs the risk of blowing the compressor.

Hmmm. I’ve been an auto repair professional for 30 years, some of that as an A/C specialist, and I’ve never heard that before. I suspect he was misinformed.

Any of the accessories in your car will use up fuel (headlights, stereo, power windows, etc.), because fuel is the ultimate energy source in your car. The only thing you get for free is heat; but even then, the fan that brings heat from the engine into the cabin will use a tiny bit of fuel.

The A/C does a lot more work than your headlights or stereo, though, and it’s powered directly by the engine (like the alternator or water pump).

I agree with Gary T that it should be OK to start the car with the A/C on, because the clutch doesn’t engage until the engine is done starting.

Gary T is right about accessories being switched off upon startup. This is true for ever car I’ve owned, but I wouldn’t be surprised if some (epspecially older) cars did not have this load-reduction feature.

The purpose is to eliminate as much electrical and mechanical resistance as possible during startup, so that the starter has an easier time turing the engine over. The A/C compressor is a noticeable drag, so it gets disengaged when the key is in the “start” position. In my car, a Load Reduction Relay is tripped when the starter is engaged, and cuts power to headlights, blower fan, wipers, rear defrost, and A/C, among other things. If I start the car with the A/C on, I can usually hear the “clack” of the clutch being engaged when I return the key to the “on” position.

[anecdote]
I had a problem once that caused a terrible noise every time I pushed my clutch in. I spent a few days convinced that I had clutch problems, and was about to tear apart the drivetrain. It wasn’t until a few days later when I tried to drive it at night that I realized something else was wrong. I noticed that I had no headlights. Some more investigating made me realize I didn’t have a fan or windshield wipers, either.

It took a little more pondering, then the solution was clear. Turns out, my ignition switch was getting stuck in the start position after I released it. Since my car has a clutch interlock for the starter, the starter wasn’t constantly trying to turn the engine. But, every time I pushed the clutch to the floor, the circuit was competed. It engaged the starter motor against the already spinning flywheel, and made the noise from hell.

A good example of a tiny issue causing symptoms much larger than itself.
[/anecdote]

That’s all good information, and here’s a little more. Modern cars, built after the year (mumble,) automatically shut off the A/C compressor clutch when the engine is under a lot of load. That means on the interstate on-ramp, passing, going up a hard hill, or squealing your tires like a teenage boy in rut. So, if you’ve just stomped the go-pedal to avoid decorating the grill of a gravel truck, and you feel muggy air on your face, don’t panic. That’s the way it’s supposed to work.

With my first car (way older than any car any of you is driving now, except to a classic car rally) starting the engine with the A/C on was a sure-fire way to blow at least one fuse.

Of course I imagine electrical systems have also improved in the last 40 years.

The only thing I would add to what Gary T posted is that many if not most A/C systems also have a delay relay built in. The A/C compressor won’t engage until 5-10 seconds after engine start to allow the idle to stabilize, and prevent stalling right after engine start.

The Master has a partial answer.