AC vs windows open mileage test
 

Ever since I saw the 'mythbusters' guys do an AC vs windows test I've wanted to do one with my hybrid. They ran 2 expeditions around a track at 45mph: one with all the windows wide open, and one with AC and closed windows. the windows open car got better mileage. But, they weren't travelling at real highway speeds, so I've been wondering how much open windows really hurt.

So this past saturday I had the oportunity to do a highway run where it was fairly flat (unlike where I live). I had to pick my kids up in Corning, NY. The highway there- Rt 17 runs for 50 miles between Binghamton & Elmira pretty flat. there are some slight grade ups & downs, but not much.
I ran 3 rounds of each venting option- driving at mainly 60mph- but varying slightly between 55-64mph with the grade. I figure 3 rounds each way would average out grade & wind variations. roads were dry, temp was 85. rt 17 is a divided 4 lane in this area (2 each way). speed limit was 65; I was passed, but traffic was light, and mostly going the limit, not above. going west, I had a suburban about 10 car lengths in front of me as a windbreak. he was travelling at 60mph.
3 venting options: AC w/ windows closed; no AC- windows closed; no AC diagonal windows- 1 each side of car- open 6"

Driving West - 2 adult passengers:
AC no windows: 56 52.6 56.7 avg: 55.1

no AC/ no windows: (hot!) 64.7 62.8 59.4 avg: 62.3

no AC/ 2 windows venting 65.1 55.6 55.6 avg: 58.7

Driving East - 2 adults, 2 kids:
AC/ no windows: 54.1 54.1 52 avg: 53.4

no AC/ no windows: 63.2 55.6 57.9 avg: 58.9

no AC/ 2 windows venting: 57.1 60.0 58.6 avg: 58.5

overall mpg averages:

AC/ windows closed: 54.25mpg
no AC/ windows closed: 60.6mpg
no AC/ 2 windows venting 6": 58.6mpg

thus confirming the mythbusting theory to me: AC takes alot of energy & is less efficient- at least with these small engined hybrids. I did these test on flat terrain, but I know with my usual driving over hilly roads, the car really works to go up a hill and run AC. I don't use AC much myself anyway- but I'm in the northeast & it doesn't get over 90 much here.
perhaps 40 years ago, when test were first run for fuel efficiency with AC, the bigger powerplants were more efficient with the windows shut and AC, as well as larger window opening volumes and a generally less aerodynamic form overall.

on a side note- this is my first full tank since the recall- I had my oil changed to mobil 1 as well & I got my best tank ever: 55mpg. I love this car- it just keeps getting better & better. I DREAD the return of really cold weather this winter!

Kate

Re: AC vs windows open mileage test
 

Kate,

thanks for the research. Looks like you covered most of the variables. I have always suspected that driving on the highway with the windows open would be as bad or worse as using the a/c, especially on recirc (vent closed) mode which allows the compressor to work less.

I have also wondered what the effect of drag caused by the rear window is. The old station wagons had rear windows that retracted completely, taking that out of the equation so they probably did a little better. I wish the rear windows of the Escapes could open and eliminate this drag, which at highway speeds has to be huge (driving with the side windows open that is). If you were able to vent at the rear of the car to reduce the pressure inside the car I think the mileage difference would increase with the windows open mileage improving.

I recently removed my roofracks and definitely noticed an increase in my highway mileage. The jury is still out on the city driving - I don't think it would make much difference at lower speed. Although I am looking at my best tank yet on the '06.
Good info. Thanks.

Re: AC vs windows open mileage test
 

Kate, nice job on the testing....:D

I had suspected as much, because the real-time MPG meter just goes WAY DOWN when I turn on the A/C, and correspondingly, I have not really EVER noticed it drop significantly when I merely rolled my windows down.

Living in Phoenix, this summer has been WAY HUMID ( for us, anyway ) and the A/C in my car has been on far more than last year, and my lowered MPG is a testament to that, dropping about 3-5 MPG per tank average this summer versus last because or using the A/C more. I have used the windows when I can, but the opps have been fewer this summer......

Re: AC vs windows open mileage test
 

Hi KLCarch:

___Nice post with excellent detail. Now I know why you are a hypermiler :D

___The larger the ICE, the less parasitic losses an A/C unit will have on it. The HCH’s smallish ICE takes a pretty big hit with A/C. As long as you know the actual hit, you can decide when A/C is mandatory and or when it is optional. I bet you will be using it more as an option from now on ;)

___Good Luck

___Wayne R. Gerdes
___Waynegerdes@earthlink.net

Re: AC vs windows open mileage test
 

Mythbusters ----- BUSTED... NOT!

Great project and report Kate, thanks!

If I have time I may try to replicate your test on my 05 Prius, since it is supposed to have a somewhat more efficient compressor and AC system. However, I would guess that no matter what, AC will use a lot of power and thus decrease mileage.

Once again thanks for a great post!
M

Re: AC vs windows open mileage test
 

Just as a quick note. I borrowed a HCH from my company motor pool and drove to Southern Pines (near Pinehurst) NC from Chattanooga, TN. I went through Knoxville, Asheville, Winston-Salem on the way. For starters, this is a most Hybrid unfriendly route, next I had my boss and he is used to a Jeep that gets considerably less gas mileage.

On the way over my boss drove 1/3 to 1/2 the way. When he took over the display read 46.1, when we filled up it was around 43. On the way back I drove the entire time and when we filled up the display and calculation was 46.1. I realize this is not good mileage, but considering it was traveling 60-80 (at least on my part) with the air conditioning on the entire time, it was not too bad.

If felt great to get home and jump into a HCH that displayed 55 though :D.

Billy

Re: AC vs windows open mileage test
 

People got similar results with insights over at insightcentral.net
" no AC/ no windows: (hot!)"
Karen, if you had the same weather we had here (MA), this is dedication above and beyond the call of duty! I wimp out and use AC when the temp get above 80 (mid 70s if its humid):embarass:
Thanks for the experiment.
--Walter

Re: AC vs windows open mileage test
 

This is great info.

I wonder what the effect is with windows wide open? How much more of a hit that is, than the 6" that Kate tested?

Speaking of windows open -- has anybody noticed how unbearable it is to drive the HCH at highway speeds with just the back windows open (all the way)? It sounds like the car is going to burst. Spooky. The only way to offset it is to open the front windows a bit also.

Re: AC vs windows open mileage test
 

Hey Kate...

Great post. I live in Syracuse, NY and know that trek well. I used to drive from Long Island to Ithaca, NY and sometimes took Rt 17 between Binghamton and Owego, but have also gone into into the Elmira-Corning area too.

We appreciate your information and post.

Thanks

Re: AC vs windows open mileage test
 

Nice testing, thanks.

I usually drive windows up no AC. I hate having the windows down for any reason. My hair blows in my face and you can't hear anything so the middle mileage option is out for me. I'll take window up vent only or windows up AC running every time.

Re: AC vs windows open mileage test
 

The thought that using the AC is more expensive in an economy (or hybrid) car versus a larger car (such as the Expeditions used on Mythbusters) is misleading via the way we compute fuel consumption -- miles per gallon, rather than gallons per mile. The AC will consume power (and thus gasoline) based on difference in temperature, number of passengers and sun intensity, and between different cars is effected by interior volume, window size and position, color of car, and window tinting, but it's effect is best measured in gallons per hour. Speed and distance have a minor effect.

Lets say that the hybrid AC consumes .25 gallons/hour, that it gets 40 mpg at 60 mph. Thats 1.5 gallons/hour. Adding the AC gives 1.75 gallons/hour and converting back we get 34.3 mpg. That's a loss of 5.7 mpg, or 16%.

Lets say the big Expedition, which has twice the interior volume, has an AC that consumes .5 gallons/hour. It gets 12 mpg at 60 mph. Thats 5 gallons/hour. Adding the AC gives 5.5 gallons/hour and converting back we get 10.9 mpg. That's a loss of 1.1 mpg or 9% which sure sounds much better.

But if we look at gallons per hour (or gallons per mile) we find that, of course, the AC is more costly in the Expedition, in fact twice as costly.

You can also see that the faster you go, the less the AC costs per mile. If you open windows, the drag will cost more at higher speeds. So there is probably a certain speed that represents the crossover point. I'd guess that in town the windows should be open and on an open highway they should be closed.

Re: AC vs windows open mileage test
 

This is mostly accurate given the recent data I've been seeing from my Graham miniscanner. In the case of the older 2001-03 Prius, at speeds below 42 mph, the engine has to run to operate the compressor. So typical idle, engine air flow runs about 2 g/sec but with the compressor, 2.45 g/sec. The problem is we don't see as much 0 g/sec with the engine off. But that changes once the car is going over 42 mph.

Over 42 mph, the engine runs regardless of load. This means the 0.45 extra g/sec is pretty much just part of the ordinary operation. However, it also means that as the engine load increases with higher speeds, the AC can push the engine into inefficient modes. What this means is cruising at 65 mph w/o AC is closer to cruising at 60 mph with AC.

It is important to use air recirculate to prevent cooling hotter external air than internal. Also, set the internal temperature control to as warm as is comfortable . . . avoid seeing condensation on the exterior glass.

As for air drag effects from open windows, if they are partially open so as to get a breeze versus a full blast, the effects appear to be minimal although I don't have numbers, yet.

GOOD LUCK!
Bob Wilson

Re: AC vs windows open mileage test
 

Might be interesting for someone to repeat this little exercise with one of the side window fairings like those from Lund (Link = http://www.autoventshade.com/home_avs.aspx. They claim an aerodynamic improvement, and come in chrome or tinted poly. I have thought about it, but it just doesn't get hot enough, long enough in the Puget Sound area.

Re: AC vs windows open mileage test
 

Even with an electric compressor, and the ICE off, the power has to come from somewhere, so there will be additional consumption later when the ICE is on again and recharging the traction battery. In fact, there will be a loss in efficiency (powering via the battery) so the average gallons/hour of AC use will increase where there is low speed EV operation mixed in. As you have observed with a 2001-2003 Prius, low speed AC operation is particulary onerous as you have about 5x the airflow (=gas consumption) you would need to run the AC alone! Even more reason to open the windows around town.

Re: AC vs windows open mileage test
 

Recently we've had some +100F days and it was unsafe to drive around without AC. So instead of taking the access road at 38 mph, I jumped on the divided, limited access expressway and tooled along at 50 mph. I only saw low 50s MPG but I was fresh, alert and it worked. Had I stayed on the access road at 38 mph, I'd have only gotten middle 50s with AC and heat stroke with without AC.

Bob Wilson

Re: AC vs windows open mileage test
 

Ahh.... I love the extreme heat. I was in Vegas two years ago when it was 119! I loved it! At midnight I went out to walk around and get a drink.... it was 112! Man I loved it.

It was 100+ here last week and I rolled the windows down and cruised on home. I wish it were like that more often.

Re: AC vs windows open mileage test
 

Pardon my ignorance Bob; but I've got two questions:

1) What are the units for g/sec? Grams/second?
2) How do you correlate air flow to fuel consumption? I think I know, to maintain a stochiometric ratio a higher air flow means also a higher fuel flow. Am I correct?

Re: AC vs windows open mileage test
 

Hi,
Sorry, I'm often so buried in the technology I often forget to 'speak human.'

Air flow is in grams per second based upon a sensor located just in front of the throttle plate. It is a critical part of injector timing to maintain a stochimoetric, 14.7 to 1, air to fuel ratio.

Take the air flow in grams per second and divide by 14.7 and you'll get the fuel flow rate in grams/second. However, this typically becomes a pretty small number:

2.00 g/sec -> 0.136 grams of fuel / second
2.45 g/sec -> 0.167 grams of fuel / second

When I posted the air flow I realized that it is technically more correct to post the fuel rate but it is often harder to visualize the difference between 0.136 and 0.167. So I used the source data, the air flow rate, which by inspection shows a 20% increase: 0.20 -> 10% of 2.00, thus 0.45 ~> 20% increase over 2.00. But it was somewhat sloppy engineering not to show the exact units and explain the relationship between air and gas flow.

Bob Wilson

Re: AC vs windows open mileage test
 

We can even enjoy some old fashioned engineering fun and re-calculate these units into something else.

Since gasoline's specific weight is about 0.7 grams/milliliter and there are 3785 milliliters per gallon, this translates into 2649 grams per gallon of gasoline.
Therefore 0.136 grams per second translates into 0.00005134 gallons per second, and 0.167 translates into 0.00006304 gallons per second.
Or put another way 19,477 seconds to consume one gallon of gas in the former case, and 15,862 seconds in the latter.

You may want to check my calculations, they were done in a hurry.

Re: AC vs windows open mileage test
 

Thanks for doing the hard work. In more useful units that would be .184 gallons/hour and .227 gallons/hour. The AC uses 0.042 gallons/hour, so if you are paying $3/gallon, that means less than 13 cents/hour for air conditioning. I would guess we could all afford the luxury!

Re: AC vs windows open mileage test
 

Hi,

My wife and I just drove from Huntsville to Nashville and back at 65-70 mph with AC. The trip mileage came in just about 49-50 MPG with the AC (I wasn't working on accuracy, just trying to get her to an appointment.) At ordinary cruising speeds of 65 mph, I would expect about 53-54 MPG. But consider this, the EPA highway rating (old style,) was only 45 MPG and they used a slower average speed, 48 mph, without AC.

AC isn't free but it sure is affordable . . . and makes the wife much happier.

Bob Wilson

Re: AC vs windows open mileage test
 

Doing a T-Test on your data shows that "AC no windows" average MPG is statistically different from the average MPG of the other two methods. However the averages for "no AC windows closed" and for "no AC 2 windows venting" are not statistically different.

Granted though, you do not have a lot of data points here.

Re: AC vs windows open mileage test
 

Very interesting data and testing.
Any one know if opening windows parallel would create much more drag? I'm asking, as a driver of a coupe, and would only be able to vent the front 2 windows.