Air con and fuel consumption

Specific discussion about the E89 2009 Z4 (sDrive35is, sDrive35i, sDrive30i, sDrive23i)
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mcbutler
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Air con and fuel consumption

Post by mcbutler » Tue Mar 13, 2018 4:53 pm

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...
Last edited by mcbutler on Tue Mar 13, 2018 6:33 pm, edited 1 time in total.

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Crazy Harry
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Air con and fuel consumption

Post by Crazy Harry » Tue Mar 13, 2018 5:31 pm

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'

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ph001
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Air con and fuel consumption

Post by ph001 » Tue Mar 13, 2018 5:32 pm

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%,
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Busterboo
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Air con and fuel consumption

Post by Busterboo » Tue Mar 13, 2018 6:09 pm

Looked at another way, when it's hot, do you sit there and sweat, thinking, 'Mmm, this is good. Really saving fuel'?
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mcbutler
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Air con and fuel consumption

Post by mcbutler » Tue Mar 13, 2018 6:36 pm

ph001 wrote:
Tue Mar 13, 2018 5:32 pm
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%,
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..

mcbutler
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Air con and fuel consumption

Post by mcbutler » Tue Mar 13, 2018 6:41 pm

ph001 wrote:
Tue Mar 13, 2018 5:32 pm
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%,
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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mr wilks
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Air con and fuel consumption

Post by mr wilks » Tue Mar 13, 2018 7:15 pm

Idle curiousity prompts me to ask what the MPG figures were :?
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