 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Car Forum / Honda Cars / September 2006Influence of window opening vs. A/C use on fuel economy
It has become popular lately to claim that turning on A/C uses less fuel than opening windows. On a recent round trip to Kansas I used my handy Scan Gauge to check milage under different operating conditions. (What else is there to do driving across Missouri and Kansas?) I checked the calculated fuel economy over approximate ten mile segments on each tank of fuel. I used the cruise control and the calibrated Scan Gauge mph measurements. Usually I could go the whole ten miles without touching the gas or brake. I tried to be as consistent as possible. For open windows, I started out rolling them all the way down but after a couple segments of that, I decided that no one could stand the tornado effect for long distances so I tried various partial open positions which improved ventilation without being punishing. Generally this was the rears open 4 inches and the fronts either closed or open 3 inches. The vehicle, unfortunately, was not typical for most drivers: 1998 Odyssey 4 cylinder with a Thule car-top cargo box. Newer A/C systems and less drag-challenged vehicles may yield different results, but here goes... Test 1 8/31/06 76 mph I35 Southbound Hilly 22.8 mpg overall (measured) A/C Windows # Segments Ave. MPG Relative MPG Off Closed 4 19.83 100 Off Full Open 2 19.45 98 Off Part Open 2 18.90 95 On Closed 4 18.24 92 Test 2 9/4/06 75 mph I35 Northbound Hilly 19.7 mpg overall (measured) A/C Windows # Segments Ave. MPG Relative MPG Off Closed 5 21.62 100 Off Part Open 3 21.27 98 On Closed 1 20.90 97 Test 3 9/5/06 75 mph I35 Northbound Hilly 23.4 mpg overall (measured) A/C Windows # Segments Ave. MPG Relative MPG Off Closed 3 20.43 100 Off Part Open 2 20.65 101 On Closed 3 19.27 94 Test 4 9/5/06 73 mph I55 Northbound Flat 21.6 mpg overall (measured) A/C Windows # Segments Ave. MPG Relative MPG Off Closed 6 22.22 100 Off Part Open 5 21.68 98 On Closed 5 20.82 94 Overall, weighted by number of segments relative to windows up, A/C off, fuel efficiency was: Windows open 98% A/C on 94% In this test, turning on the A/C cost three times as much fuel as opening the windows. There was no indication that it made any difference whether the windows were wide open, rears down 4 inches or front and rear both down 3 - 4 inches, but there was limited testing of this, and the effect was small in any event. As noted before the vehicle may not be typical, but these were the results. One further point of interest; shortly after I started one segment I came upon a speed reduced work zone. I didn't use the segment in the above calculations but I noted that average speed was 63 mph and average fuel consumption was 26.4 mpg. It appears that the difference between going 60 and going 75 was about 4 mpg. > Overall, weighted by number of segments relative to windows up, A/C > off, fuel efficiency was: [quoted text clipped - 14 lines] > 63 mph and average fuel consumption was 26.4 mpg. It appears that the > difference between going 60 and going 75 was about 4 mpg. The Mythbusters did a controlled experiment, with the result being windows down = a lot better mileage. http://cartalk.com/board/showthreaded.php?Cat=0&Number=341160&page=6&vc=1 The link above leads to CarTalk's forums. The post notes that their computer models said the mileage should be practically the same. The real experiment showed otherwise. -- R Flowers Windows down messes up my wife's hair. A few miles per gallon is cheap for my peace of mind. Just my .02 Spdloader > Windows down messes up my wife's hair. Big haired wife? >> Windows down messes up my wife's hair. > > Big haired wife? Let me guess - you're not married? -- R Flowers > >> Windows down messes up my wife's hair. > > > > Big haired wife? > > Let me guess - you're not married? I am. I am fortunate to be married to a woman who doesn't worry about her hair. >> Let me guess - you're not married? > > I am. I am fortunate to be married to a woman who doesn't worry about > her hair. That's nice. Does she have very short hair? My wife doesn't really 'do' her hair, but still complains about the wind whipping it around her face. -- R Flowers > > I am. I am fortunate to be married to a woman who doesn't worry about > > her hair. > > That's nice. Does she have very short hair? Yes. She also doesn't hesitate to wear a scarf to keep it from whipping around. Nope. Just particular. Spdloader >> Windows down messes up my wife's hair. > > Big haired wife? > Windows down messes up my wife's hair. > > A few miles per gallon is cheap for my peace of mind. > > Just my .02 Yeah, you have to factor in the cost of a trip to the hairdresser too! >It has become popular lately to claim that turning on A/C uses less >fuel than opening windows. On a recent round trip to Kansas I used my >handy Scan Gauge to check milage under different operating conditions. >(What else is there to do driving across Missouri and Kansas?) (snip) Interesting information, but when it's 105 when I leave work, I'll use my AC and just pay the difference. Better than being covered in sweat and having all the dirt stuck to me. Average Hi temp for August in my area=100 degrees. Thanks, but I'll just keep the windows up!!! >It has become popular lately to claim that turning on A/C uses less >fuel than opening windows. On a recent round trip to Kansas I used my >handy Scan Gauge to check milage under different operating conditions. >(What else is there to do driving across Missouri and Kansas?) ... You sir are a gentleman and a scholar. However, the numbers are small enough, say 2%, that on a $50 tank of gas we're talking $1.00. And you sure can't hear the radio with the windows open more than a crack at speed. I do guess the window numbers would be more significant for an Accord without the antlers and big slab sides, might get it up to, oh, who knows, 5%?!? But presumably the A/C numbers would be the same, which would validate the claim which your experiment did not, but would still be just about $1.00/tank in the other direction. OTOH, windows would impose less overhead at slow speeds, say in stop and go, ... oh, where will it end?! J. > However, the numbers are small enough, say 2%, that on a $50 tank of > gas we're talking $1.00. And you sure can't hear the radio with the [quoted text clipped - 8 lines] > OTOH, windows would impose less overhead at slow speeds, say in stop > and go, ... oh, where will it end?! I'm with you on this one - yeah, there's a MEASURABLE difference, but is it really that NOTICEABLE to the average driver? And is the difference worth the comfort? It's kinda like the amusement I get from seeing people lining up or going out of their way to go to a gas station that has prices a few cents less per liter - with my 50 liter tank, a two-cent difference means a savings of a whole $1.00, IF I have to fill up from empty. It's just not worth $1.00 of my time to sit and wait in line or to go out of my way. >> However, the numbers are small enough, say 2%, that on a $50 tank of >> gas we're talking $1.00. And you sure can't hear the radio with the [quoted text clipped - 12 lines] >really that NOTICEABLE to the average driver? And is the difference worth the >comfort? The difference would be about $2 per tank for this vehicle or about fifty cents an hour. Well worth it when needed - which it really wasn't on this trip. If this test were done in an Accord or Civic or even the Ody without the roof box, the cost per hour might be similar but larger as a percentage of total fuel cost. >It's kinda like the amusement I get from seeing people lining up or going out of >their way to go to a gas station that has prices a few cents less per liter - >with my 50 liter tank, a two-cent difference means a savings of a whole $1.00, >IF I have to fill up from empty. It's just not worth $1.00 of my time to sit >and wait in line or to go out of my way. >>>However, the numbers are small enough, say 2%, that on a $50 tank of >>>gas we're talking $1.00. And you sure can't hear the radio with the [quoted text clipped - 20 lines] > the roof box, the cost per hour might be similar but larger as a > percentage of total fuel cost. Fair'nuff... my Accord gives me usually 500-550km on a tank (80% city driving), which works out to around 28mpg (alright for an '87 with almost 420,000km), so it really isn't worth the difference for me. >> It has become popular lately to claim that turning on A/C uses less >> fuel than opening windows. On a recent round trip to Kansas I used [quoted text clipped - 19 lines] > > J. It is good information, but the true differences are difficult to assess, as many segments are averaged together, so one cannot calculate the standard deviations for each group. It does seem that on 9/7 there was less head wind..... >>> It has become popular lately to claim that turning on A/C uses less >>> fuel than opening windows. On a recent round trip to Kansas I used [quoted text clipped - 24 lines] >deviations for each group. It does seem that on 9/7 there was less head >wind..... Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5. The behavior of the Scan Gauge is a little flaky in regards to measuring fuel consumption. I recalibrate it at every fuel stop and it can vary by 10% or more. Since the calibration changed between test sessions, you can't compare the absolute milage figures between sessions. Also, gradually changing terrain, elevation, wind and different test speeds complicate the situation. That is why I looked at relative fuel economy between the sessions. Below is the individual session data in case you or someone else wants to do more sophisticated analysis. Just keep the above in mind. Test 1 8/31 76 mph A/C Window m/g miles on closed 18.2 10.0 off closed 19.5 10.0 off wide opn 20.2 10.0 off closed 21.2 10.0 on closed 17.4 10.0 off closed 18.9 10.0 off wide opn 18.7 10.0 on closed 18.8 10.5 off closed 19.7 10.0 off F3 R3* 19.3 10.0 on closed 19.1 10.0 off R6 18.5 10.0 * F3 R3 = front window down 3", Rear window down 3" Test 2 9/4 75 mph A/C Window m/g miles off closed 21.0 10.0 off R4 20.0 10.0 off closed 22.5 8.7 on closed 20.9 10.0 off closed 22.4 11.4 off F2 R4 22.4 10.0 off F2 R4 21.4 11.4 off closed 21.7 10.0 off closed 21.0 10.4 Test 3 9/5 75 mph A/C Window m/g miles off closed 19.6 10.0 off R4 20.5 10.0 on closed 18.5 7.8 off closed 20.0 10.0 on closed 19.2 10.2 off R4 20.8 10.1 off closed 21.7 10.0 on closed 20.1 10.0 Test 4 9/5 73 mph flatland A/C Window m/g miles off closed 21.6 10.0 off F3 R4 21.1 10.2 on closed 21.2 10.0 off closed 22.5 10.0 off F2 R4 22.4 10.9 on closed 20.6 11.6 off closed 21.4 10.0 off F3 R4 21.7 10.0 on closed 20.6 10.0 off closed 22.8 13.7 off F3 R4 21.4 10.1 on closed 20.3 10.5 off closed 21.6 12.0 off F3 R4 21.8 11.0 on closed 21.4 10.0 off closed 23.4 13.0 >>>> It has become popular lately to claim that turning on A/C uses less >>>> fuel than opening windows. On a recent round trip to Kansas I used [quoted text clipped - 99 lines] > on closed 21.4 10.0 > off closed 23.4 13.0 My bad on the dates. 7.2% loss between air on and air off with windows closed, with almost identical standard deviations with decent sample sizes. It looks like with windows partially open is just about the same as them closed (.4% difference probably insignificant due to standard test error) until you get to the 6" opening in the rear (though there is only 1 sample), which is just about the same as with them wide open (again, small sample size). 6" to wide open the same as with the air on and closed. Probably could use more "open window" numbers just to verify, but it makes sense if you look at some simple HVAC calculations for orifice flow.... A 14" diameter that has about 154 sq/in of area (semi close guess to a car window, I haven't actually measured one) with a round edge discharge co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and almost 100K CFM at 20 psi (sharp edge drops numbers by 40%). I'm not sure about the effects of the angle of the flow on the opening and I haven't measured the air pressure entering a car through an open window moving at 70 mph, but I'm sure it would be quite high (and then multiple the CFM x 4!) That is a lot of drag...... I'll take the 7.2% loss without the wind howling in my ears! Thanks for the clarifications. Good real world application. > Good real world application. Ditto. What Gordon found appears to be consistent with some study (or a summary of a study) I read not too long ago. Tom and Ray of "Car Talk" may have cited it at their web site, if memory serves. Namely, even when moving at highway speeds, not using the A/C and leaving windows open for some cooling is best. Plenty of studies on the net on this. E.g. --- Several sources claim that closing windows and using a car's air conditioner will provide better fuel economy at freeway speeds than leaving the windows open without the air conditioner. However, FSEC tests showed this in not the case. In repeated evaluation at 65 miles per hour, our test car experienced 11% better fuel efficiency with no A/C and the windows open than using the air conditioner. We also found that closing windows at freeway speeds improved fuel efficiency by 2-3%. http://www.fsec.ucf.edu/Pubs/energynotes/en-19.htm --- The above site has a nice chart for freeway speed driving. The test vehicle was a 1986 VW GTI. OTOH, the damned people at FSEC.UCF appear to misquote a study done by edmunds.com, indicating that there's no difference at highway speeds. I suspect this all depends on the model and its wind resistance characteristics more than we'd like to admit. Perhaps the best approach is to perform one's own experiments with one's own car. Great job. Please test next without the luggage on top and replace 4 cylinder with 6. > It has become popular lately to claim that turning on A/C uses less > fuel than opening windows. On a recent round trip to Kansas I used my [quoted text clipped - 70 lines] > 63 mph and average fuel consumption was 26.4 mpg. It appears that the > difference between going 60 and going 75 was about 4 mpg. Great Post, it's something I always wanted to do but didn't. Question, during A/C winows closed, was the Vent on Re-Circulate of Fresh Air Position Thanks >Great Post, it's something I always wanted to do but didn't. >Question, during A/C winows closed, was the Vent on Re-Circulate of >Fresh Air Position >Thanks Fresh Air. > >Great Post, it's something I always wanted to do but didn't. > >Question, during A/C winows closed, was the Vent on Re-Circulate of > >Fresh Air Position > >Thanks > > Fresh Air. I thought the compressor worked less when the Vent is not always bringing in Warm air from outside, I wonder what difference that would have made. Personnally though, I worry about having it in Re-Circulate mode on a long trip and not getting enought Oxygen and staying alert. Bg >> >Great Post, it's something I always wanted to do but didn't. >> >Question, during A/C winows closed, was the Vent on Re-Circulate of [quoted text clipped - 9 lines] >long trip and not getting enought Oxygen and staying alert. >Bg I think it brings in some fresh air even in recirc mode. Regarding your point on the compressor; This car does not have a thermostat but I do sometimes sense that the compressor is shutting off sometimes. Not so much that I can say for sure. Does anyone know if this car cycles the compressor? > Regarding your point on the compressor; This car does not have a > thermostat but I do sometimes sense that the compressor is shutting > off sometimes. Not so much that I can say for sure. Does anyone know > if this car cycles the compressor? System pressure cycles the compressor, by nature of its design. Spdloader A/C is essentially a "minutes per gallon" question while driving represents more of a "miles per gallon" cost. I think it's certain that at low speeds the efficiency is better with the windows open and A/C off while at high speeds the opposite is true. Where that changeover point is undoubtedly varies widely from model to model, and the "high speed" regime may start above the speed limit for many cars. When the Mythbusters ran their test they used SUVs which drank a lot of gasoline anyway and probably didn't suffer much when the windows were open. In addition, the speed was so low A/C would be a clear loser. Mike > A/C is essentially a "minutes per gallon" question while driving > represents more of a "miles per gallon" cost. I think it's certain [quoted text clipped - 8 lines] > loser. > Mike I don't believe the original gas mileage matters, only the differences. Maybe one can assume that windows open would be less of an effect on a vehicle with more mass and available torque (unless the vehicle allowed a higher volume of air)? > I don't believe the original gas mileage matters, only the differences. > > Maybe one can assume that windows open would be less of an effect on a > vehicle with more mass and available torque (unless the vehicle allowed a > higher volume of air)? Vehicles with lower fuel economy are less sensitive to differences, because the losses are already pretty high. In the Toyota Prius fora people are shocked to find that running the heater can decrease the in-town fuel economy 10 mpg... the finer the edge the more quickly it dulls. Mike >> I don't believe the original gas mileage matters, only the differences. >> [quoted text clipped - 8 lines] > >Mike Running the heater? Isn't this just waste heat anyway? >>> I don't believe the original gas mileage matters, only the >>> differences. [quoted text clipped - 12 lines] > > Running the heater? Isn't this just waste heat anyway? I guess it has to do with the amount of juice needed to run the fan.......???? >>> Vehicles with lower fuel economy are less sensitive to differences, >>> because the losses are already pretty high. In the Toyota Prius fora [quoted text clipped - 6 lines] > I guess it has to do with the amount of juice needed to run the > fan.......???? The effect on the amount of time the engine runs can be pretty radical. The car has a display for the mpg over 5 minute intervals, so the first winter we had it I decided to see just how much the heater would drag it down. I opened the windows and turned the heater on full while driving in town. The previous five minute bars had been something like 40 or 45, while with the heater on full it dropped to 25! Even at 75 mph with the A/C on full and five adults in the car it never went below 30 in real life. Mike >>>> Vehicles with lower fuel economy are less sensitive to differences, >>>> because the losses are already pretty high. In the Toyota Prius [quoted text clipped - 16 lines] > never went below 30 in real life. > Mike Amazing. That's not something they advertise....... >> The effect on the amount of time the engine runs [in the Toyota Prius] >> can be pretty [quoted text clipped - 8 lines] > > Amazing. That's not something they advertise....... Nope - and I wouldn't if I were them, either. I think it will be increasingly common as we see more efficient cars, and especially if electric cars make inroads into everyday use. We all know it takes a lot of energy to make heat but in cars we assume there's always an excess of heat. Mike > >> The effect on the amount of time the engine runs [in the Toyota Prius] > >> can be pretty [quoted text clipped - 14 lines] > a lot of energy to make heat but in cars we assume there's always > an excess of heat. The old air-cooled Bugs didn't have an excess of heat either, so one option was a gasoline fueled cab heater. That doesn't sound real economical, but I'll bet it's more economical than having to fire up the engine to get heat. It doesn't sound real safe either, but if you maintained it it wasn't too bad. Earle >>> I don't believe the original gas mileage matters, only the differences. >>> [quoted text clipped - 12 lines] > > Running the heater? Isn't this just waste heat anyway? The Prius doesn't normally run the engine when the car is moving slowly or stopped, so the waste heat often falls short. The engine has to run more just to make heat. Mike >> I don't believe the original gas mileage matters, only the >> differences. Maybe one can assume that windows open would be less of an [quoted text clipped - 8 lines] > dulls. > Mike The raw number would be lower, but the percentages should be similar. >>> I don't believe the original gas mileage matters, only the >>> differences. Maybe one can assume that windows open would be less of an [quoted text clipped - 10 lines] > > The raw number would be lower, but the percentages should be similar. I think the discrepancy comes from working with the reciprocal of what we really want to measure: fuel per increment. For example, suppose it requires 5 gallons of gas to move the car 100 miles with the windows rolled up and A/C off. That is 20 mpg. If the window drag at some speed consumes 0.2 gallons in 100 miles at that speed the economy drops to 19.2 mpg for a loss of nearly 5%. If the drag is the same in a vehicle that requires 2 gallons to move the car 100 miles (50 mpg) the window drag drops it to 45.5 mpg, a nearly 10% hit. I'm not a big fan of the "liters per 100km" measurement but it works a lot better than mpg here. So, here's an example of how it goes - theoretically - with the window and A/C. Assume (for illustration) that at 50 mph the window drag consumes 0.1 gallon per hundred miles. Similarly, assume the A/C consumes 0.1 gallon per hour. For our hundred mile trip that would mean the windows would use 0.1 gallon and the A/C would use 0.2 gallons at 50 mph. If we increase the speed to 100 mph the window drag, increasing with the square of the speed, becomes 0.4 gallons for the one hour the trip takes while the A/C loss drops to 0.1 gallon for the hour instead of 0.2 gallons for two hours. For different vehicles the numbers would change; the A/C losses are higher for a larger vehicle with more glass and the window drag will certainly vary with body style. In the end, all that will change is the speed at which the A/C is more economical than windows down. >>>> I don't believe the original gas mileage matters, only the >>>> differences. Maybe one can assume that windows open would be less [quoted text clipped - 35 lines] > certainly vary with body style. In the end, all that will change is > the speed at which the A/C is more economical than windows down. The net effect will be determined by environmental conditions and choice. My preferences are usually determined by speed of travel and temperature/humidity conditions. <90° and lower humidity while driving on back roads at <50 mph would be open windows for me for the most part (general rule of thumb). >90° would most likely be AC all the time. Highway speeds would either be AC or vent, depending on the preferred comfort level at that specific time. I am not very concerned about the differences in fuel economy, only in comfort level (although it is always nice to know.....). > The net effect will be determined by environmental conditions and choice. > My preferences are usually determined by speed of travel and [quoted text clipped - 6 lines] > differences in fuel economy, only in comfort level (although it is always > nice to know.....). That's undoubtedly the bottom line. I used to live in Phoenix, and the windows don't open wide enough to make 117 F comfortable! Mike >> The net effect will be determined by environmental conditions and >> choice. My preferences are usually determined by speed of travel and [quoted text clipped - 11 lines] > > Mike Amen. |
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.