Aircon Compressor HP

Honest John Back Room » Motoring discussion
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Wed 6 Aug 2008 21:54
Aircon Compressor HP - hypographamist
Hi
Maybe this question has been answered before, not seen it though..
I have a car with air conditioning, it's warm so I select the aircon, cold air
floods the car and I presume my engine is developing an extra 10hp or so
to power the aircon pump or compressor. So far very clear.

The car cools down and I find it a little too cool so the only option I can take
is to adjust the temperature dial for more heat. This action is manual but I'm sure
cars with climate control will be using servos to add warm water to the heater matrix
just as I adjust my dial. Now my question is this - is the aircon pump still using
10hp and some of the cold air is wasted or does the aircon pump adjust itself
to need less HP because the cold air requirement is reduced and if so how.
I control my car's engine with the throttle to produce the required power, is there
something in the aircon system to reduce the aircon pump's energy usage ?
Or maybe the compressor is switched on and off from the engine drive and inside
the car you don't notice the loading/unloading on the engine

My gut feeling is that most cars have a basic compressor and that selecting cold
air whether you use some or all of it means you pay full price of energy usage
in fuel economy.
If so it's not very green and efficient.
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Thu 7 Aug 2008 08:31
Aircon Compressor HP - L'escargot
.......... I presume my engine is developing an extra 10hp or so
to power the aircon pump or compressor. .........


Domestic air conditioning units are in the region of 1kW to 2 kW (2.7 hp) so I wouldn't have thought that a car aircon would take anything like 10 hp.
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Thu 7 Aug 2008 08:46
Aircon Compressor HP - ijws15
The compressor has a clutch on the end of it, should only run when needed.
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Thu 7 Aug 2008 10:12
Aircon Compressor HP - moonshine {P}

I could be wrong here and I'm prepared to stand corrected, but...

Yes, there is a clutch on the compressor, but when the air con is on, it's on - i.e the compressor runs full time. The air temp (when you have climate rather than air con) is adjusted by using the heater matrix to re-heat the air to the correct temp before it enters the cabin.

Yes, it sounds crazy and inefficient, but to my knowledge thats how most systems work.

As already said earlier, I doubt that air con would take as much as 10hp.
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Thu 7 Aug 2008 10:23
Aircon Compressor HP - Number_Cruncher
See;

www.honestjohn.co.uk/forum/post/index.htm?v=e&t=41...9


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Thu 7 Aug 2008 13:32
Aircon Compressor HP - craig-pd130
I don't know if all cars do it but on my old Volvo T4, the owner's manual clearly stated that under full throttle the aircon compressor disengages to make full power available. I tested it and it did so (well, the little "aircon on" light went off).

Perhaps power drain is getting towards double figures ....?

To balance this, I've repeatedly seen the statement that aircon uses less fuel than the drag caused by open windows / sunroof at 50+mph
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Fri 8 Aug 2008 00:03
Aircon Compressor HP - defender
I read somewhere that the average air con needs between 3 and 5 hp if its all working ok ,10 hp seems too high a figure to be a viable %age of a cars power
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Fri 8 Aug 2008 09:39
Aircon Compressor HP - hypographamist
I think I said 'extra 10hp or so'. Perhaps manufacturers fit different size compressors
to larger or smaller cars and so the HP requirement will vary.

What I do know is that every car I've driven with aircon the power drop off when it's
switched on is immediately apparent and stays off whilst ever the switch is actuated.
I had a SAAB turbo many years ago and in town it was virtually un-driveable due to
turbo lag and the low engine speed aircon requirement, very bad design.

I just hate the idea of wasting energy powering a compressor and then negating to some
extend, by adding more heat to the mix, the effects of the cold air produced by the
aircon.
It's akin to controlling your fridge temperature by shoving a fan heater through
the door...

I think I'll do some re-search into aircon design and fitment to cars.

The truth must be out there...
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Fri 8 Aug 2008 09:59
Aircon Compressor HP - L'escargot
What I do know is that every car I've driven with aircon the power drop
off when it's
switched on ......


I've not noticed that effect with my Focus.
I just hate the idea of wasting energy powering a compressor ....


You're not wasting energy. You're merely using energy in order to get the benefits of aircon.

....... and then negating to
some
extend by adding more heat to the mix the effects of the cold air produced
by the
aircon.


How are you adding heat?
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Fri 8 Aug 2008 11:31
Aircon Compressor HP - moonshine {P}
How are you adding heat?


My understanding is that cars with climate control adjust the temperature at the vents by passing the air over the heater matrix effectively re-heating the air.
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Fri 8 Aug 2008 11:36
Aircon Compressor HP - Mapmaker
How interesting. In my office I have a somewhat elderly portable aircon unit which makes a lot of noise but is not that effective. It is 15"x15"x30" and is rated at 1kW. It takes an age to do anything worthwhile - although I fully accept that my office is serveral times the size of a car.

Given the rapidity with which a car aircon unit chills, I am happy to accept that it is far more powerful than the one in my office.
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Fri 8 Aug 2008 12:45
Aircon Compressor HP - ifekas
Most modern car a/c compressors have variable displacement, so when there is less demand for cooling, there is less load on the engine. Some cars even have compressors which are clutchless and always running.

Having said that, cars sold in countries with tropical climates tend to have thermostats which just cycle the compressor on and off; this must be more efficient, but when one is at a standstill you do notice the car revs changing slightly when the compressor switches on/off.

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