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When driving your car why does air conditioning use more petrol?

I know that when the engine is running the alternator generates electricity which is used to power the electrics, but if you are on the motorway doing 70mph, I would have thought no extra power was required? Also if you switch on all the car electrics when stationery, you don’t hear the engine reving more to produce more electricity. Can anyone explain?

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13 Responses to “When driving your car why does air conditioning use more petrol?”

  1. Me again said:

    The A/C uses a compressor that is connected to the engine. When you turn on the air, the engine runs the compressor. There’s no free lunch. Cold air costs.

  2. billy said:

    wel when a/c is used it takes more power from the eninge to drive the pump so it needs just a little more fuel to keep going. teh altonator is always spinning and is always producing electricity. inside it theres a voltage control which regualtes how much power it will produce. the optimum speed for producing the most power uaually at 2,000 engine rpm.

  3. junglejungle said:

    yep it does..

    if it’s warm. use the sunroof / windows

    your car IDLES when not being driven.. if u have a rev counter you will notice.. otherwise it will stall..

    if u have like amps etc u need to run the car, so the alternater is charging, if u play it without the ignition on/engine running.. well u’ll just drain the battery..

    it uses the battery/ fuel is the short answer.

    it won’t rev more, simply because it’s taking the charge from the battery.

    your car idles at about 800-900 rpm’s or so..

  4. PMack said:

    Your car contains an air conditioning compressor, and there is an engine driven belt that drives it, with an electrically actuated clutch to enable/disable it as needed. When you turn on the AC, the clutch engages, and now that compressor is putting a small (but noticeable) load on the engine, which is why your mileage goes down somewhat. Your revs don’t change at idle because todays cars automatically increase the throttle to maintain the optimum idle speed when the extra load occurs. The only electricity used is for the fan and to engage the clutch itself, the compressor is what is actually causing the cooling effect.

    Hope this helps.

    Just to clarify here – there are quite a few answers that state or imply the the AC is powered solely by the electricity generated by the alternator. This is simply NOT the case. The primary load on any AC system, whether it be in your home or in your car, is the compressor. In a home system, the compressor is driven by electricity (which is why they use quite a bit of electricity). In a car, electricity powers the fan and actuates the compressor clutch, but the compressor itself is driven by a belt from the engine, which is why is causes an increase in fuel consumption.

  5. Yoshi said:

    Modern cars are aerodynamically designed for least wind resistance, with the windows closed. When you open windows at 70 mph you increase the drag. Air conditioners are pretty efficient these days and depending on the car, it is probably more efficient to turn on the A/C and keep the windows closed. Even better, slow down to 50 or 60, but still keep the windows closed.

    An idling car produces enough electricity to run most features, but you may notice a change when you operate power windows because they are a big energy draw.

  6. tatrafan said:

    This is based on fact but with modern air con it is not even worth considering.
    Air con uses a compressor driven via a belt and therefore absorbs power from the engine.
    Modern air con pumps are very efficient and my 1600cc car will still do 45mpg on a run with the air con on.
    Another point to consider is that the amount of power absorbed by the air con is about the same as driving your car with the windows and sun roof open but much cooler.

  7. Johneye said:

    Don’t worry about the electrical draw, the power goes to drive the AC compressor. This isn’t all bad because with the windows up it makes the car more aerodynamically efficient and will actually save you gas.

  8. Data said:

    It is to do with kilojoules. A kilojoule is the amount of energy/power used in any type of force, the engine uses more kilojoules or kilowatts if it has more work to do. The harder the engine works, obviously the more fuel it will need.

    As for the electric dilemma, you would run out of fuel quicker with all the electrics on than you would than if they were all off, albeit fractionally, with the same amount of fuel.

  9. victor j said:

    because more stress is put on the engine. when air condtioning is on. so your driving 2 pistons in compressor and it is pumping the freon, thats the stress.

  10. Ron said:

    You have the answer in your question, “the alternator generates
    electricity”.

    The more electriciy that is needed, the harder the alternator has to work.

    You don’t get anything for nothing, every single thing in your car that uses electricity from your courtesy light,car lights ,radio, cd player, windows, cigarette lighter, central locking and especially air conditioning cost you more money in petrol, it’s as easy as that.

  11. mister ss said:

    you have to turn the AC compressor and that takes power from the engine thus using more gas, if your sitting in your drive turn the air on and you will hear the compressor kick in and your engine speed will drop for a second.

  12. jason c said:

    OK this is not a straight forward answers. The text book answer is that when the air con is running the magnetic coupling on the refrigeration compressor is engaged placing a load on to the engine resulting in higher fuel usage.
    However if you compare driving with the air con on and driving with the windows open, the air con option is still more cost effective.
    Ja.

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