Air Con. & MPG

[melinx]

Three weeks ago during a 180 mile run in the unseasonably hot weather I finally turned on the air con as the day warmed up.

I had done about 80 miles, the MPG had settled and was varying between 34.8 and 35.2. held at a steady 70 MPH by the cruise control.

After switching on the air con. I had a nice cool interior and the MPG continued to indicate 34.8 to 35.2 for the rest of the motorway journey

So is the reputedly high fuel consumption of air conditioning another urban myth

[Dave the Builder]

Trip computers tel fibs 
Leave the air con off and make the wife pay for 99 flakes all round 

[melinx]

Trip computers tel fibs 

Mine doesn't I've verified it from fill up to fill up

On motorway runs particularly the error is less than 2%; predominantly urban use, it's about 5%.

I've never managed to work out why there should be an increased error for urban driving

[Whippit]

the pumps running whether its switched on or not so can't see how it would make that much difference.

[melinx]

It isn't ! it's freewheeling until the electric clutch on the pump is switched on and the pump actually starts pumping

[Dave the Builder]

Trip computers tel fibs 

Mine doesn't I've verified it from fill up to fill up

On motorway runs particularly the error is less than 2%; predominantly urban use, it's about 5%.

I've never managed to work out why there should be an increased error for urban driving

Ok , I stand corrected
Trip computers tel fibs are  economical with the truth ,between 2 and 5 % 
you'd have to test with the heater blower on, then re test with heater blower and air con , and factor in any variants (lights on, colder or warmer outside,same weight of fuel,same route taken on test,driver must do the same speeds at same locations.) and mamy many other factors to make it comparable.
IIRC top gear did a test once.

[melinx]

you'd have to test with the heater blower on, then re test with heater blower and air con , and factor in any variants (lights on, colder or warmer outside,same weight of fuel,same route taken on test,driver must do the same speeds at same locations.) and mamy many other factors to make it comparable.
IIRC top gear did a test once.

Thanks for the suggestions Dave

I'm going to stick with the tests I've done

A fairly simple test is to run the engine with the air con on until the engine is fully warm and the air con. is settled (it works quite hard until it has been on a few minutes) then switch to instantaneous consumption, switch off the air con. and see if the instantaneous reading drops (mine doesn't)

That test isn't definitive because it's not telling you what it's doing at 'full chat' but that's what happens when the air con is working quite effectively sitting in a traffic jam (the pump compensates at higher speeds because it has variable displacement pistons)

[The Duke]

So is the reputedly high fuel consumption of air conditioning another urban myth

Its certainly no worse than driving around with the window open 

[bstardchild]

So is the reputedly high fuel consumption of air conditioning another urban myth

Its certainly no worse than driving around with the window open 

Spot on

AC v Windows down in mpg terms AC better on fuel every time

[nightmare]

I was wondering if anyone would point that out.

Driving with windows open at 50+ = 5%-8% increase in fuel consumption on average.
Its even worse with the sunroof as well.

Sunroof tilted doesn't seem to make any difference with windows closed.

Driving with aircon on = 0%-2% increase in fuel consumption. At least thats what I've found.

[diplomat2.6]

Don't forget also that the Harrison V5 is a variable displacement compressor as are most these days ie the hotter it is, the more work it does.

I had an xj8 for 3 years and the ac was always on but didn't work too hard unless it was hot. In winter the mpg was always worse (18mpg vs 22mpg) as it took a lot longer to heat up.  Personally I usually savour the journey when the ac is on and therefore use less fuel by driving more slowly

[nightmare]

It also keeps the interior drier, so less condensation/misting in winter.

[melinx]

Three weeks ago during a 180 mile run in the unseasonably hot weather I finally turned on the air con as the day warmed up.

I had done about 80 miles, the MPG had settled and was varying between 34.8 and 35.2. held at a steady 70 MPH by the cruise control.

After switching on the air con. I had a nice cool interior and the MPG continued to indicate 34.8 to 35.2 for the rest of the motorway journey

So is the reputedly high fuel consumption of air conditioning another urban myth

Confirmation of my findings: Read the section on "Does the AC use more petrol".

http://www.airconditioningforcars.co.uk/ACpage01.htm

The rest of the link also makes interesting reading

[bootie3367]

That bodes well for me then.... I still have a complete system waiting for me to fit it!

[PeterC]

Yeah but, bootie3367, think of the extra weight you will be lugging around. Joking - I dont have to worry as I don't have A/C. Sadly I don't often need it.

Melinx's points make sense. Same with central heating. Once the desired temperature is reached then the only remaining use for energy is to overcome thermal leakage. In the case of A/C thats thermal gain, body heat but also humidity (so dont breathe !).

I can believe that opening the windows would change the (hopefully) good aerodynamics.

[Einarrson]

Hmm... Almost makes me regret deleting mine... Still I reckon all the AC gear must weigh about 10KGs.