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Air conditioning unit use fuel ?

ang12el Jun 14, 2007

  1. ang12el

    ang12el Member

    Having had the bog standard car before and never having had an air con I was wondering say for example its bloody freezing inside the car and you want some heat in me old car and like many before you moved the dial from cold to warm and the engine heat was directed into the cabin .. now I have to stick the unit on and increase or decrease the temp < whats with this surely you are using fuel or NOT ? can someone give me the low down.. if you are using fuel it seems a good gimick or does the heat come from the same place engine ...? Pardon me if its a novice question but aint too well up on this aircon thingy still trying to get to grips with it

  2. Caesium

    Caesium My BM is fixed! VCDS Map User

    The heat into the cabin comes from a heat exchanger called a matrix, this is just like a small car radiator, air is blown through it and the air is heated by the coolant running through the matrix and therefore heats the air.
    The only fuel used is the fuel the engine is consuming while running.
  3. icenutter

    icenutter Member

    And cool air is provided through a compressor (like in a fridge) - which does use fuel. Although probably less than having a window open.

    Incidentally your car will (probably) have climate control which is a posh version of aircon. The idea is that the control unit sets the temperature output and speed based on the temperature you select, rather than you having to fiddle with the temp all of the time. So in the summer it goes crazy for a few minutes to get the temperature down to the required temperature, then blows in a steady stream of air in.
  4. simonwjones

    simonwjones Member

    I think what people mean is that when the aircon is set to low, the cold air compressor is engaged and because of the friction, drag etc used by this off the auxiliary belt, it can have a typical 5-10% decrase in mpg of the car.

    If you press the ECON button this turns OFF the air compressor and as such doesnt effect economy. It does mean thought that the temp of the air blown out will not be cold compared to when the ECON button is on and creating cold air as would happen with a standard fridge in your house

    Hope this explains it better


  5. enda1

    enda1 Member

    A car with aircon uses fuel for the following reasons.
    The compressor has to consume energy as work to compress the refridgerant gas and pump it round the air con circuit. This high pressure gas then is expanded and cooled down via a throttle valve or turbine. It expands and cools down. This cold air runs through a matrix/heat exchanger. The air entering the cabin runs over this matrix and cools down so you get cold air in the cabin.
    The amount of energy consumed is dependant on the conditions. if its 5C outside then the compressor will need little work to keep the cabin at 21C, then the electric clutch that drives the compressor via the drive belt from the engine will be disengaged most of the time.
    If its 35C then the compressor will need to work harder to keep the cabin at 21C, then the electric clutch that drives the compressor via the drive belt from the engine will be engaged most of the time.
    So the energy the compressor needs to do work comes via the drive belt and clutch and ultimately from the engine. So the engine load, demand for energy, is greater and the engine must consume more fuel to creat this extra energy.
    As to how much fuel it uses just look at the size of a normal well insulated fridge and check out the energy rating on the label. Now look at the cabin of you car which could have approx 20-100 time the volume with very poor insulation and you'll get an idea of the max amount of energy needed to keep the car cool.
    As to heating the car up that just uses the same system as normal cars in that the hot engine coolant is driven through another heat exchanger and the air passing over it heats up

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