Effect of air conditioner on mileage

"Sun God" wrote

Not a chance in hell that an A/C reduces mpg by 10, especially at steady hwy cruising, when the effect of A/C on mileage is very little, and the larger/stronger the engine (such as yours), the less it's affected by the additional resistance introduced by the A/C compressor.

Also, reduction of mpg from 40 to 30 is not the same as reduction of mpg from 20 to 10, so you need to be more specific - what did the gauge exactly show? In any case, your gauge is not working properly or there is something wrong with your A/C unit if it puts so much strain on such a big engine.

Cheers,

Pete

"Pete" wrote

Two things: first, turn off HTML - these newsgroups are text.

An A/C compressor takes 10-15 HP. Since it takes around 15-20hp to maintain cruise at 70mph or so, it's entirely possible that the compressor cycling on will drop mpg from 30 to 20.

Floyd

Do you happen to have a HP requirement curve relative to speed? I would be interested to see what that curve looks like, going form 50mph to 100 mph. I know its different for every car, given its weight and aerodynamics, but just a general one.

I actually measured this on a long trip in a former 735iSE using the computer. The effect was to reduce the mpg by just one [imperial] mile to the gallon. The other noticeable effect is that the idle speed is raised a couple of hundred rpm.

The 750i is of course a V12, but I would have thought the marginal effect of the AC on mpg was even less.

"fbloogyudsr" wrote

Not sure why you are seeing it as HTML. I sent it as text. Here's the proof from the message properties:

Have you ever seen a car where running A/C decreased the mpg by this much?

Cheers,

Pete

Floyd, is it I that am in HTML? How do I turn it off in OE6?

Thanks on all points.

"Pete" wrote

In reviewing my post, I think I'm going to back off some. Since a home a/c unit runs around 3/4 to probably 3 or so for a whole-house unit, it's unlikely a car a/c compressor uses more. So if we're talking 20 hp to maintain cruise and 2 hp for the compressor, that's 10% or a drop from 30mpg to

27mpg. Of course it's not linear...

Floyd

Well, since aerodynamic drag is so important at such speeds, the power required varies as the square of the speed. Jim

"Sun God" wrote

I'm not sure; might have been Pete.

Tools/Options, on the Send tab, select text.

Floyd

How accurate is that analog fuel meter? I would think the most accurate way to know is if you were to burn a full take on a given route with the AC on, and calculate the miles the old fission way (at the pump) then drive the same route with the AC off. If you could stand it. try driving for a week, in your normal routine without the AC, then a week with, and see what numbers you get.

How does the compressor handle the RPM range of the engine? I'm sure its designed to keep the air cold at idle, it just above idle, so what about then you are, say, 1000 rpm below redline? is there a bypass valve so it doesn't over pressure? I'm sure it puts a higher load on the engine as RPMs increase.

What I did was drive up the A1 in the UK when I was going on holiday and compare the mpg shown by the trip computer with the AC on and the AC off over stretches of tens of miles. The trip computer is fairly accurate, and the effect was repeatable and predictable.

You do get more cooling when the car is running as opposed to idling.

Seems I recall reading a few years back that this type of test might be problematical because of the increased air resistance when driving with the windows open. The theory was that mileage would actually be worse without the A/C. Of course, if you can stand the heat with no open windows or A/C...

Tom

The effect of AC on fuel consumption is minimal, I'm thinking it's going to be about 1/4 gallon per mile, give or take a bit.

That "manifold pressure guage" is really a guage that measures the injector timing to calculate the actual fuel demand. I wouldn't use this guage for what you are trying to figure out, I'd use the On Board Computer's CONSUM feature. I used to think the manifold pressure guage was precisely that, but when you think about how it goes to Max Scale when you take you foot off of the gas and put it on the brake, then drops to Min Scale as the car rolls to a stop, then you have to see that it isn't a vacuum guage at all. What is happening is that the computer calculates how much fuel is in demand at that precise moment, then extrapolates out how far you could go on a gallon if nothing else changed. It doesn't know that you are rolling to a stop and therfore there is no way you can go 99 miles without touching the gas pedal, but it if you actually managed to keep going without using the gas pedal, then you could go as far as the needle says on a gallon.

The On Board Computer looks at fuel demand over time and distance, and gives an average. If you could travel a stretch of road at 80 without the AC on, then come back and do the same stretch at the same speed with the AC on, then you could compare the two averages, and there would not be an appreciable difference. There would certainly NOT be a 10mpg difference.

"J Strickland" wrote

If you were consuming gas at this rate, you'd go through a 20 gallon tank in just 80 miles. :)

Cheers,

Pete

As they say in the UK, bollocks.

It will make a vast difference to MPG if used in heavy stop start traffic since it uses the same power at idle as at speed. (near enough) But no BMW does 30 mpg in heavy stop start traffic.

If you go to a more realistic 15 mpg in those conditions, it will make something like 2 mpg or less difference.

Where you might expect 30mpg as an average - which will be open road driving - the difference as a percentage will be less.

If one gets 25 mpg with the AC off, and 24.75 mpg with it on, then the difference in range on a tank of gas would be 3.75 miles, assuming 15 gallon fill up.

I don't know what numbers you were running, but you were not running a quarter gallon per mile.

"J Strickland" wrote

OK, so it's a quarter of 1 mpg, not quarter of gallon per mile. :)

Otherwise, if you use up 1/4 of a gallon per mile, then that means after

4 miles you used up 1 gallon, and after 40 miles you used up 10 gallons, and so on...

Pete

Yes, a quarter of one mile per gallon, which is much different than a quarter of a gallon per mile.

Pete just pulled your leg. Your last sentence has the "gallon" & "mile" reversed. I'm sure you meant to say the difference would be 1/4 mile per gallon difference, not 1/4 gallon per mile. :)