Air con and fuel consumption

M

mcbutler

Veteran
 Plymouth
Hi All,

My car is a 23i and I do regular trips of over 300 miles to visit poorly parent.

Obviously its boring on the motorway so I decided to do some 'experiments' with the instantaneous fuel consumption indicator and air con. The reason being I had been told use of air con increases fuel consumption!

Well, they are damn right, after picking a flat stretch of motorway I set the car on cruise at 70mph and noted the instantaneous readout. I then engaged the air conditioning to see what the difference was.

Can you believe 10 mpg less with air con engaged!!!, thats a 25% reduction in economy......

Worth bearing in mind, open the windows I reckin in future...
 
Don't you mean increases fuel consumption /reduces mpg?

Why not try the experiment again with the air con off and windows down - I bet that will shock you. Most wagons now have air con to save fuel - the disruption to air flow from an open window causes too much drag and fuel burn. What's that old saying 'you don't get ouwt for nouwt'
 
Something not right there (was there a big headwind on one run and not the other for example). Car a/c takes around 4kW (5hp) max so that's about 2.5% of your total engine output. No way it should affect economy by 25%,
 
ph001 said:
Something not right there (was there a big headwind on one run and not the other for example). Car a/c takes around 4kW (5hp) max so that's about 2.5% of your total engine output. No way it should affect economy by 25%,
Click to expand...
I did it twice, on a long flat run on the M62 and also on the M5.
Both times i did no air con followed by air con on, then air con off again. Both runs hundreds of miles apart showed pretty much the same reading..
 
ph001 said:
Something not right there (was there a big headwind on one run and not the other for example). Car a/c takes around 4kW (5hp) max so that's about 2.5% of your total engine output. No way it should affect economy by 25%,
Click to expand...
You are probably right (theoretically) but give it a go and see what you get..
Does the power drawn change with engine RPM? probably yes as the compressor speed/belt drag/internal resistance is directly linked to engine speed.
 
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