 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Car Forum / BMW Cars / August 2005Effect of air conditioner on mileage
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 "Sun God" wrote > 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? 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" <escape2music@hotmail.com> wrote > "Sun God" wrote >> [quoted text clipped - 8 lines] > larger/stronger the engine (such as yours), the less it's affected by > the additional resistance introduced by the A/C compressor. 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 >> "Sun God" wrote >>> [quoted text clipped - 16 lines] > > 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. > >> "Sun God" wrote > >>> [quoted text clipped - 21 lines] > I know its different for every car, given its weight and aerodynamics, but > just a general one. Well, since aerodynamic drag is so important at such speeds, the power required varies as the square of the speed. Jim >Well, since aerodynamic drag is so important at such speeds, the power >required varies as the square of the speed. >Jim > 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 > Well, since aerodynamic drag is so important at such speeds, the power > required varies as the square of the speed. > Jim 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 > Two things: first, turn off HTML - these newsgroups are text. Not sure why you are seeing it as HTML. I sent it as text. Here's the proof from the message properties: > Content-Type: text/plain; > 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. Have you ever seen a car where running A/C decreased the mpg by this much? Cheers, Pete > "fbloogyudsr" wrote >> [quoted text clipped - 10 lines] > > Have you ever seen a car where running A/C decreased the mpg by this much? 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 Floyd, is it I that am in HTML? How do I turn it off in OE6? Thanks on all points. >> "Sun God" wrote >>> [quoted text clipped - 16 lines] > > Floyd "Sun God" <twc9529261982@earthlink.net> wrote > Floyd, is it I that am in HTML? How do I turn it off in OE6? I'm not sure; might have been Pete. Tools/Options, on the Send tab, select text. Floyd > 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. 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.  Signature*A fool and his money are soon partying * Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. "Dave Plowman (News)" <dave@davenoise.co.uk> wrote > As they say in the UK, bollocks. 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 > > As they say in the UK, bollocks. > 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. ;-> 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. ;-) Signature*Never kick a cow pat on a hot day * Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. >> "Sun God" wrote >> [quoted text clipped - 16 lines] > > Floyd 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, SignatureRob Munach, PE Excel Engineering PO Box 1264 Carrboro, NC 27510 > 10-15 HP? I bet the compressor on my home's A/C takes less than 2 HP - > based on the amps it pulls. 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. > Why would a car's compressor use so much more energy? 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. Signature*What boots up must come down * Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. > Rob Munach <xlengr@mindspring.com> wrote: >> 10-15 HP? I bet the compressor on my home's A/C takes less than 2 HP - [quoted text clipped - 13 lines] > heat of the engine. 5kW is about 6.5 bhp. An AC system is far less > efficient. 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 > My beloved 1996 750il has only an analog "mpg" needle which I believe is a > manifold pressure guage based on how it reacts. [quoted text clipped - 8 lines] > > Tom in MN, USA 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. >> My beloved 1996 750il has only an analog "mpg" needle which I believe is >> a manifold pressure guage based on how it reacts. [quoted text clipped - 16 lines] > The 750i is of course a V12, but I would have thought the marginal effect > of the AC on mpg was even less. 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. "Trey" <treydog90spam@hotmail.com> wrote in message news:2S3Ke.4921 > "R. Mark Clayton" <nospamclayton@btinternet.com> wrote in message >> "Sun God" <twc9529261982@earthlink.net> wrote in message [quoted text clipped - 23 lines] > AC on, and calculate the miles the old fission way (at the pump) then > drive the same route with the AC off. 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. > 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. You do get more cooling when the car is running as opposed to idling. >> 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 [quoted text clipped - 10 lines] > 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. 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. > My beloved 1996 750il has only an analog "mpg" needle which I believe is a > manifold pressure guage based on how it reacts. [quoted text clipped - 8 lines] > > Tom in MN, USA > 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. If you were consuming gas at this rate, you'd go through a 20 gallon tank in just 80 miles. :) Cheers, Pete 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. >> 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. [quoted text clipped - 5 lines] > > Pete > 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. 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 >>> 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. [quoted text clipped - 5 lines] >> >> Pete >> 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 [quoted text clipped - 4 lines] > > OK, so it's a quarter of 1 mpg, not quarter of gallon per mile. :) Yes, a quarter of one mile per gallon, which is much different than a quarter of a gallon per mile. >>> 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 [quoted text clipped - 7 lines] >Yes, a quarter of one mile per gallon, which is much different than a >quarter of a gallon per mile. 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 http://www.redoak.co.uk http://www.eze-buy.co.uk >>>> 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 [quoted text clipped - 11 lines] > regular 60 mile run I do. Oh and that is on a 1 yr and 11 day old > 320D. 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. >> 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. > > I seriously doubt that the AC alone sucked up 3 mpg. 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 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. Sun God wrote: > Many have commented that this needle readout is calculated from actual > consumption data, but I contend that it is really manifold pressure. 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 > 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. 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. > (Car again is 1996 750iL with about 110K miles). 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. > Well, I really appreciate all of the informed commentary that this little > posting of mine has generated. [quoted text clipped - 9 lines] > 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. 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. > Many have commented that this needle readout is calculated from actual > consumption data, but I contend that it is really manifold pressure. But it goes to zero when the car comes to a stop but the engine still running? A vacuum gauge would read near maximum. Signature*Growing old is inevitable, growing up is optional * Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. > 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. 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. :) Signature-bob- ______________________ OSX, BeOS, Linux, & OS/2 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 |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.