Mythbusters on auto Air-conditioning

[Just thinking]

I recently watched the episode where a comparrison was made to see which was more fuel efficient -- driving with the windows down (producing drag) vs. driving with them up and using the air-conditioner.

The testing involved driving an SUV at speeds 55 and 45 mph on a test track. What I found astonishing (although not by too much) was not that driving with the windows down was more fuel efficient, but that under even ideal conditions at these steady speeds the vehicles got no better than 12 mpg. And this point was not even mentioned in the episode as being important. And then we (as a nation) complain that oil is too expensive???

 

[mummymonkey]

I can remember driving up Mt Olympus in Cyprus in an old Mazda with a couple of mates. We had a fair load of grub and beer and we had to switch off the air conditioning several times just to get up the hill.

 

[LTC8K6]

I thought the tests were pretty unrealistic.

I still think you will do better with the A/C on.

I thought they went too slow, and they just left the A/C wide open rather than setting it at a comfortable level.

 

[kevin]

I thought the tests were pretty unrealistic.

I still think you will do better with the A/C on.

I thought they went too slow, and they just left the A/C wide open rather than setting it at a comfortable level.

yeah those were my thoughts on that episode too. Especially the speed. Supossedly the windows open causes more drag and the faster you go the more drag you would see. I think speeds of 60-70 mph are much more typical for highway driving at least in the midwest.

Also I thought the worry about the tires blowing out was weird. I've done 12 hour drives in cars at 70-80 mph without problems, with only stops for the restroom and lunch.

 

[kevin]

they did another gas mpg test with some trucks testing tailgate up or tailgate down for best mpg that I thought was much better done. And tailgate up is better.

 

[Hindmost]

This topic was later revisited. The benefits are related to speed...at low speeds when the drag is less, it is better to have the windows open and the a/c off...at higher speeds with greater drag, using the a/c is more fuel efficient.

glenn

 

[casebro]

Their samples are waaay tooo small. Two guys, two trucks. They didn't even switch trucks and redo the tests. Bad science.

 

[pgwenthold]

yeah those were my thoughts on that episode too. Especially the speed. Supossedly the windows open causes more drag and the faster you go the more drag you would see. I think speeds of 60-70 mph are much more typical for highway driving at least in the midwest.

Also I thought the worry about the tires blowing out was weird. I've done 12 hour drives in cars at 70-80 mph without problems, with only stops for the restroom and lunch.

This sort of illustrates the folly of the whole question. If you are worried about your gas milage, the question isn't whether to run the AC or open the windows. It's what speed to drive.

I have about an hour commute to work each day (and back again), of which 45 minutes or so is on open highway. When I started doing this, I used to fit more into the 60 - 70 mph that you speculate for highway driving in the midwest. That is a reasonable characterization of the stretch I drive. I used to go about 65 and was not the fastest nor the slowest.

Lately, I have slowed down. Now I run the cruise at about 58 mph. It costs me about 4 minutes extra on the drive, but since I have started slowing it down I discover that I am getting about 2 mpg better gas milage than I did before.

The small difference between windows and AC isn't the problem when you are going fast, where the efficiency is lower. And it doesn't cost you much in time, just a couple of minutes usually, and, moreover, I don't care if I see a cop, because he isn't going to bother with me at 58 mph. In terms of benefit, given two tanks of gas each week, and gas prices of 2 - $2.50, I figure I can save about $4 - $5 a week in gas, or $200 - $250 a year. Just by slowing down.

 

[Art Vandelay]

The other question is which is more effective? If the outside temperature is the same as the inside, then what's the point of opening the windows? In fact, any air from the outside is going to be moving relative to you, so it's going to heat the car up.

 

[Simon Bridge]

This topic was later revisited. The benefits are related to speed...at low speeds when the drag is less, it is better to have the windows open and the a/c off...at higher speeds with greater drag, using the a/c is more fuel efficient.

glenn

Strictly speaking, this would also depend on the temperature of the air.

When you wind the windows down, you are letting hot air from outside into the car. It feels cooler than the air in the car because, as it is moving, it can cool you by convection (and evaporating sweat - so humidity plays a role too).

I suspect the air inside the air conditioned car was cooler.

The test needed to check the cooling vs the additional fuel use as well. The windows-down car would set the internal temp that the conditioner is to try to maintain - and the steady state should have arrived before measurements taken.

In short - poorly controlled.

This is generally the case for these guys. They have only so much time to make one of their eps and to air it, so there are some things they just cannot do. There will also be budget restraints (not per-test but per-season) and the tests have to look good on TV as well as be understandable to station execs/producers. (At least - these guys must believe that their idea of the TV audience will want to watch these things - and that advertisers are prepared to pay for the timeslots).

TV is just not a good medium for reporting scientific investigations.

 

[Simon Bridge]

Cooling: there is a neet experiment for you to do at home.

Heat up something (with a hole in it for a thermometer) by boiling in water, then suspend it in still air for a (actually quite a long) time. Keep track of the temperature at regular intervals as the thing cools down and plot this.

Do it again, only this time introduce a small desk fan.

One may suggest that the cooling is slower (it will be an exponential, so the cooling factor is taken from the slope of a log graph) for the fan-forced test because the air is moving, thus delivering kinetic energy via collisions to the object, thus adding heat ...

The temperature of the room will be a factor. The room temp will be a bit higher after the first test. This should also decrease cooling for the second test.

However - cooling rate is increased with the fan. A result not surprising. In fact, the fan-forced cooling is optimal (Newtonian) cooling - where the cooling rate depends only on the difference between room temp and the object temp.

This is because, as the object cools, it's heat goes to the local body of air. This will gradually move away (convection). Since this is gradual, the air immediately around the object is warmer than the mean room temp. However, a fan will blow this hot air away quickly, replacing it with lower temperature air.

The experiment can be repeated with ice, and a rate of heating considered.
In this case, the ice heats faster under the fan! And for exactly the same reasons.

 

[EatatJoes]

"What I found astonishing (although not by too much) was not that driving with the windows down was more fuel efficient, but that under even ideal conditions at these steady speeds the vehicles got no better than 12 mpg."

Well, I've read in Consumer Report magazines that the stated MPG for most vehicles is way off. The industry calculates the gas milage based on average driving conditions from, like, twenty years ago. We don't drive the same now.

Many automobiles today spend 62 percent of their annual miles in city stop-and-go traffic, where fuel economy is the lowest. The EPA formula still uses a 55/45 percent city/highway ratio to calculate combined fuel economy. - from www.consumerreports.com

When Consumer Reports did their testing they found that the actual MPG ranged from 21% better than claimed MGP to 28% worse. City MPG is usually much lower than what is claimed. For example, the hybrid Honda Civic has a claimed City MPG of 48, it's actually more like 26 MPG. Not that I would complain if my car got that kind of gas milage in the city, but if I was expecting much more, I'd be ticked off.

 

[kevin]

Well, I've read in Consumer Report magazines that the stated MPG for most vehicles is way off. The industry calculates the gas milage based on average driving conditions from, like, twenty years ago. We don't drive the same now.

They are, but this was a comparitive A/C to no A/C on the same vehicle model. The estimated MPG from the manufacturer wasn't involved.

For one part they did use a computer model from the manufacturer to get a running estimate that disagreed with their results (but I think it was run under quite significant different conditions).

The computer in my Prius gives a pretty accurate MPG number based on my refueling amounts, but isn't close to the sticker estimated MPG (I get 40 MPG roughly depending on how maniac I drive.) I would actually get better milage in my Prius if I moved further away from work.

 

[Hindmost]

Strictly speaking, this would also depend on the temperature of the air.

When you wind the windows down, you are letting hot air from outside into the car. It feels cooler than the air in the car because, as it is moving, it can cool you by convection (and evaporating sweat - so humidity plays a role too).

I suspect the air inside the air conditioned car was cooler.

The test needed to check the cooling vs the additional fuel use as well. The windows-down car would set the internal temp that the conditioner is to try to maintain - and the steady state should have arrived before measurements taken.

agreed...it would really take reams of data to test this completely. There would be variations in cars, climate, clouds, sun, humidity, driving conditions..etc. I am going to use my a/c anyhow since I consider it one of the best inventions of all time.

glenn

 

[coalesce]

In my Toyota Highlander, driving with the back windows down in the summer as opposed to using the AC is not an option. The amount of road noise and especially the air reverberating in back of me is unbearable, worse than any sedan I've ever ridden in or driven. Does any other SUV driver have this problem as well?

Michael

 

[davefoc]

On pgwenthold's slowing down comment:
I also have slowed my driving down of late. I am semi-retired and don't have the pressure of having to work at specific times for fairly long hours anymore so driving slower, eliminating the possibility of speeding tickets, listening to some talk radio and saving a bit on gas seems like a reasonable thing to do.

But I still drive between 65 and 70 most of the time. When I tried to slow down much below that there were all sorts of minor annoyances. There are a lot of people that think that even in the slow lane drivers aren't supposed to drive below 65 mph and they let you know that by driving up your butt and tailgating you for awhile.

And then there is the general annoyance of driving in the slow lane. Most people who drive in California are fairly adept at getting on the freeways but a fair number aren't. Some people get on the freeway by getting to the end of the on ramp and just blindly plowing onto the freeway. And quite a few people use the on ramp to accelerate to about 35 mph and then finish their acceleration while they are on the freeway. I am pretty sure that braking and accelerating for these people reduces fuel economy and increases car maintenace expenses more than any small savings recovered from using the slow lane to drive slower.

On the EPA mileage estimate inaccuracy:
I have beaten the EPA highway mileage estimates for every vehicle I have owned. Currently I own a Honda Prelude that was rated at 27 mpg and I get between 29 and 33 mpg for a 75% highway mile mix. The only things I do are to keep my tires inflated and to drive with as little braking as possible meaning that I try to be aware of slowing traffic and allow the car to slow itself down before I need to use the brakes. I also own a Honda accord that gets about 30 mpg when I drive it on the highway. My wife gets about 25 mpg for mostly around town driving. Both of those numbers are better than the EPA estimates.

My buddy owns a Toyota Prius and while he doesn't quite get the rated 56 mpg he still gets well over 50 mpg on the highway.

 

[Bigt]

Their samples are waaay tooo small. Two guys, two trucks. They didn't even switch trucks and redo the tests. Bad science.

It's entertainment, not science.

 

[shecky]

Interesting. My first car back in the 80s was a Chevrolet Sprint, a absolutely reliable car that got a real 50 MPG on the highway. And it was about the cheapest car on the market. Unfortunately, no A/C, but amazingly economical car be even today's hybrid standards.

 

[Just thinking]

Interesting. My first car back in the 80s was a Chevrolet Sprint, a absolutely reliable car that got a real 50 MPG on the highway. And it was about the cheapest car on the market. Unfortunately, no A/C, but amazingly economical car be [sic] even today's hybrid standards.

Yes -- I had a 1988 Honda CRX Si that got over 38 mpg on a 75% highway 25% city jaunt of over 200 miles. Again, no AC. But my friend's father just bought a Toyota hybrid that's giving him well over 40 mpg. This car is nice -- and peppy; nothing like a Sprint or CRX without AC. Yes, some cars of the past got what some hybrids are getting today, but these are far better and more realistic everyday cars.

 

[Rocky]

In my Toyota Highlander, driving with the back windows down in the summer as opposed to using the AC is not an option. The amount of road noise and especially the air reverberating in back of me is unbearable, worse than any sedan I've ever ridden in or driven. Does any other SUV driver have this problem as well?

Michael

Yes, in my '03 Toyota 4Runner. I like to demonstrate the oscillations to unsuspecting people. It is just subsonic but it can make it hard to breath. You cant miss the feeling on your body. It works best (worst?) with one rear window down.

-R