Question

what percentage does fuel usage increase by using A/C

Please answer this specifically,If i take all variables out of this question.say i started my car ,let it idle for 1 hour and measured fuel used.Then repeat procedure ,this time with A/C knob in the on position.Now again ,what is the fuel used this time?thats my root question.all the searching im doing and all answers are focused on other circumstances like aero dynamics ,and speed,.
I knoe the A/C compressor is driven by the same belt that drives all other engine componants,i realize some extra load would be added to the turning of the pulley system when the A/C compressor is engaged.That,I believe is the true and real answer Im after,this extra load cause the engine to work harder ,and usen more fuel,and again ,how much fuel is that?if anyone has a good answer please help.Ive been in a debate for a long time with some freinds on this.I personally,dont believe it could increase fuel usage by more than 5%.
thank you again

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Comments
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Answer
It's more complicated than that.
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yes

thanx bob
i have an idea now,i guess its not a fixed answer ,its a reletively grey area,with lots of options ,and conditions.
thanks

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It's a lumpy curve. In ASCII it looks like this.

From prior readings I think it lumps closer to the 10% mark.

8%
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**********
****
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**
*
*
*
20%

Hope that helps.

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Answer
Fuel Use with Aircon

Most car aircon systems will have a thermostat to control the supply air temperature. If not you can adjust the supply air temperature to suit the required condition. A car aircon system works the same way as any other heating or cooling system. So the amount of energy, or fuel used will vary according the the demand on the system. If you have four people in the car and it is sitting in the car in the midday sun the demand will be much higher than it would if you are sitting in the car alone, perhaps shielded from the sun too.
You are right that there will be energy used by the mechanical components of a car aircon system but only when the compressor is engaged because the system is requiring cooling. When there is no demand the compressor spins without load so the fuel use is minimal.
I doubt that fuel useage be would be more than 5% especially with a large engine doing the work.

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