Does airconditioning effect my fuel consumption?

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    cannedhams on 13/01/2014 - 19:57

    That works out to about 8L per 100k. Presumably that is city driving in an auto?

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  Lavec on 13/01/2014 - 14:32

  My car tells me I use 0.6L per hour at idle without air con and about 1.1L per hour with air con on, that equates to be around 75c per hour to run the air con. Once I'm moving it changes to L/100km and it's harder to work out the affect but I assume it's the same as for idle.

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  Jasmine2323 on 13/01/2014 - 23:24

  Great information everyone. Thanks for these details and sharing this. Awesome.

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  Kkuba on 14/01/2014 - 10:10

  Another thing to keep in mind, and you might notice this yourself, (particularly on older systems).

  If it is a hot day, the speed of the vehicle will increase the effectiveness of the AC system. Increase in airflow through the condenser will decrease the temperature of the refrigerant going to the TX valve, which in turn decreases the temperature of the refrigerant going through the evaporator, therefore colder air coming out of the vents. You will most notice this when comparing say, idle to suburban speeds. Then if you compare suburban speeds to highway speeds, the increase in benefit reduces in magnitude.

  There is a fan that helps with this when you are stationary, simulating airflow through the as if you were driving, but the fan will not even compare in airflow to when the car is moving.

  Whether it increases the efficiency, I would have to look back at some text books. Because I suspect that with decreased airflow (and therefore higher temperature of refrigerant entering the expansion valve) the effectiveness of the AC system might be lower, but it almost might be doing less work because the refrigerant will be warmer, and therefore less dense, which could result in (close to) a zero net difference in efficiency. But I would have to confirm this. Would someone like to elaborate?

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    _Bruce_ on 14/01/2014 - 12:38

    In a very rough approximation the decreased airflow means that the air that does come out is cooler. The engery used or cabin temperature doesn't change much, but the lower fan speed means that you don't cool down as quickly.

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      Kkuba on 14/01/2014 - 15:49

      When I talk about air flow, "Increase in airflow through the condenser", this means the air flow that goes through the front grill of the car when you are travelling, it is used to cool the radiator, and also the part of the air conditioning system that gets hot (a by-product of the operation of air conditioning). A condenser is the part that (as the name suggests) condenses the refrigerant, before it goes into the cabin to expand and therefore cool the vehicle.

      The less you cool the refrigerant, the less cold it will be when it goes the the TX valve, and therefore the less it cools the air that is coming out of the vents in the cabin.

      Don't want to get too much into the thermodynamics of AC systems, but in conclusion, less airflow to the condenser, will result in warmer air coming out of the vents into the cabin.

      I think you might have been confused with the air going over the evaporator.

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        _Bruce_ on 14/01/2014 - 15:51

        Sorry, fair point I read your post too quickly.

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  lead_org on 15/01/2014 - 22:40

  A Toyota Engineer at the Toyota plant in Melbourne told me that for new cars if you are driving over 80 km/h, it is better to have aircon on than to have the windows fully open. Some of newer cars with better aerodynamics, can achieve this at 70 km/h.

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    onlyletters on 16/01/2014 - 07:50

    having the windows open creates a drag which transfers to the transmission and then to the engine.

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