Effect of air conditioner on mileage

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My beloved 1996 750il has only an analog "mpg" needle which I believe is a manifold pressure guage based on how it reacts.
On a long cruise this past weekend I played with the air conditioner. If
you belive the MPG needle, the A/C takes fully as much power as 10 MPG on the meter, at 60 mph.
Does this make sense to anyone, or do I have a problem?
Thanks to all
Tom in MN, USA
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"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
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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
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wrote

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.
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wrote

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

IIRC, the torque requirement varies as a function of the square of the speed, so power would vary as a modified function of the *cube* of the speed. http://www.nsxprime.com/FAQ/Miscellaneous/TorqueHPSpeed.htm seems to confirm...
JRE
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Jim wrote:

Yes, that is true. Which brings about something germane to the original question that I have heard a few times recently. In the past it was always assumed that you could improve your gas mileage simply by not turning on the AC. More recent advice is that it is more efficient at highway speeds to run the AC than it is to drive with the windows down due to the increased drag. Of course there are a number of variables such as the speed, efficiency of the particular AC unit, and aerodynamic drag coefficient of the car in question...
-Fred W
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"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
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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
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Floyd, is it I that am in HTML? How do I turn it off in OE6?
Thanks on all points.
wrote

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I'm not sure; might have been Pete.
Tools/Options, on the Send tab, select text.
Floyd
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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.
--
*A fool and his money are soon partying *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Nice of you to lambaste me before reading my subsequent post backing off that one. If you'd use a better newsreader like OEX you might not make that mistake. ;->
Floyd
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With my present work schedule I comment on posts as I read them. ;-)
BTW, you'll have no chance to try Pluto, but it still knocks all others senseless. ;-)
--
*Never kick a cow pat on a hot day *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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fbloogyudsr wrote:

10-15 HP? I bet the compressor on my home's A/C takes less than 2 HP - based on the amps it pulls. Why would a car's compressor use so much more energy?
Regards,
--
Rob Munach, PE
Excel Engineering
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It's electrically driven? Not on any petrol engined car I've seen.
One figure which sticks in the mind was some spec from the first Silver Shadow Rolls Royce of the '60s - the first R-R with standard fit AC. It was said to use about the same power as the then Mini - about 28 bhp. Modern systems are probably more efficient although not so powerful, but I doubt it's down to 2 bhp.

Well, a heating system in most cars is about 5kW. Derived from the waste heat of the engine. 5kW is about 6.5 bhp. An AC system is far less efficient.
--
*What boots up must come down *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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I found a relatively authoritative web page that indicates that an auto's a/c compressor uses about 5hp: http://www.ornl.gov/info/ornlreview/rev28_2/text/trabar1.htm That's more than my 2nd estimate, but certainly less than my 1st guess of 15hp (seems that may have been accurate 25 years ago however).
Floyd
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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.
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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.
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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.
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