Effect of air conditioner on mileage

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.

seems to confirm...

JRE

Hello,

As our cars get older, scoring of the compressor surfaces will cause loss of cooling ability and create additional drag. This is because the esther oil has slowly escaped or is breaking down. Sure you could add some more esther but esther may not be enough... enter Molybdenum Disulphide.

If you were to go down to the local parts house today you will find products that promise 10 degree cooler air; a performance booster. Its simply more R134 and 2oz of moly. Moly acts like little bearings and reduce friction quite well. So give it a try. Add one of these "boosters" and see if things improve for you. It should but dont expect your guage to change.

As for your guage - not it is not vacuum sensing, its driven by the ecu. It is quite possible that the engineers programmed it for the worse case senario, thats when the belt driven fan is locked up. That could be another 5 -15 hp loss on top of what the compressor uses. Oh! And lets not forget the additional power the alternator needs to generate to run all of the fans too; that's another power loss!

;-) Blake

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. ;-)

I reckon that the aircon reduced my mpg from around 50 to 47 on a regular 60 mile run I do. Oh and that is on a 1 yr and 11 day old

320D.

-- John Perry

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,

I seriously doubt that the AC alone sucked up 3 mpg. I suspect that if you made the same run several times, some with and some without the AC blowing, the average consumption rate would be almost identical - with the variation being closer to .25 mpg. If you get 50 most of the time, then the AC should turn in a number of about 49.75. I think there have to be other factors in the changes you report, long time standing in traffic, mashing the pedals harder, starting the car then going back inside to pee and look for your cell phone, those sorts of things.

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.

"J Strickland" wrote

It's a bit steep, but not impossible, I think. Losing 3 mpg at 50 is not the same as losing 3 mpg at 20, for example. Percentage-wise, going from 50 to 47 mpg, you only worsened your mileage by 6% (still a lot though). Going from 20 to 17 mpg, your mileage worsened by 15% (terrible).

For me it's easier to see this using metric units: Going from 50 to 47 mpg, means he's using only 0.4 liters more per each 100 km. Going from 20 to 17 mpg, means he'd be using 2.5 liters more per each 100 km.

Also, the weaker the engine, the more visible the effect of A/C on mileage will be. Finally, there are so many different factors (wind speed and direction, for example), that I agree, he should have done more runs and averaged out the results to minimize the error.

Regards,

Pete

I found a relatively authoritative web page that indicates that an auto's a/c compressor uses about 5hp:

's more than my 2nd estimate, but certainly less than my 1stguess of 15hp (seems that may have been accurate 25 yearsago however).

Floyd

Well, I really appreciate all of the informed commentary that this little posting of mine has generated.

For the record, my analog MPG needle drops at least 10 indicated units when I turn on the AC, and gets them back immediately when it is turned off.

Many have commented that this needle readout is calculated from actual consumption data, but I contend that it is really manifold pressure. At idle, in Park, no motion (such as just being started first thing in the morning) the readout is 'infinite' MPG, i.e. pegged on the high end of the MPG scale. This is exactly the way manifold pressure gauges on airplanes work, it is the only thing in my experience that works this way.

(Car again is 1996 750iL with about 110K miles).

Thanks to all.

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

Your contention is very nice but WRONG.

The needle uses actual injection pulse width and car speed to calculate the MPG. The system knows the AMOUNT of fuel being injected into the engine (pulse width x injector volume x fuel pressure) and knows the speed the vehicle is going at so the calculation is "elementary"..

At

Well, now you have a NEW experience. You might try finding actual data or facts, they help a lot in these sort of discussions. I have no idea why the system is programmed to "infinite" - but you might consider that to actually be "no MPG.." instead.

If you want to see instantaneous data - your on-board-computer can give you MPG (probably two different displays) - and pressing the reset button will effectively give you the instantaneous data. This is calculated from the same formula as drives the needle in your speedo.

How it works isn't effected at all by the miles on it. BMW has used the same meter and formula driving it since they introduced it back in.. probably early '80's.

Infinite guages were required by Canadian law; US is the opposite - 0 mpg whilst standing still.

You contend incorrectly. Here's why. If you were rolling to a stop, the needle will point to full scale Max. MPG, but when you are actually stopped, the needle points to full scale Min. MPG, but the manifold pressure hasn't changed.

You say it is pointed full scale MAX while not moving, but the fact is that it is pointed full scale MIN.

On my car, a '94 3 Series, the graduation on the scale is not nearly adequate to see the change you are talking about - the movement to show 10 mpg can be anywhere from 7 to 13 mpg.

The real way to check is to drive without the air on for a significant distance, then fill up and see what the mileage is, then drive again with the air off and fill up and see what the mileage is. My guess is that the two fill ups will be within 1 mpg of each other.

But it goes to zero when the car comes to a stop but the engine still running? A vacuum gauge would read near maximum.