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Old 08-09-2014, 08:23 AM   #1
AVANTI R5
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Default Don’t Use A/C – Roll Up Windows & Wear an Ice Vest to Save Gas

Quote:
t’s always nice when you come across an answer that addresses a question that you’ve wondered about? When I saw that Vox, a relatively new site that says it has “the smartest thinkers, the toughest questions” to “explain” our confusing world to us, was running a post on which uses less fuel, running the A/C or opening the windows, I figured I could put the question to bed. While I did find out about the windows down vs air conditioning thing, I also found out that the smart thinkers over at Vox may not be as smart as they think they are.



The article, by Joseph Stromberg, answered my question in the headline, “Why rolling down your cars’ windows is more fuel efficient than using AC”, perhaps in line with that explaining thing. Stromberg’s primary source is a study published by the Society of Automotive Engineers in 2004, “Affect of Windows Down on Vehicle Fuel Economy as compared to AC load“. My first inkling that Stromberg wasn’t completely in command of his sources was when I checked out the study’s presentation. Vox’s writer said “In 2004,the Society of Automobile Engineers tested a full-size V8 sedan and SUV on a desert track and in a wind tunnel, with outside air temperatures around 86°F.” While the SAE indeed published the study, which bears an SAE logo on the title page, on that same page there is a GM logo. Most of the research that the SAE publishes is done by members for their employers, the SAE functions as a clearinghouse to disseminate the information to other engineers. If that logo didn’t make the study’s origin at GM obvious enough, it says that wind tunnel tests were done at GM’s aerodynamic wind tunnel facility according to GM’s internal program and that the road tests were done at GM’s desert proving grounds. A quick search on the lead author, William Hill, also shows that he’s an HVAC engineer at GM.





Okay, so maybe the guy doesn’t cover the automotive beat much and isn’t familiar with the SAE. Well, then there’s the question as to whether the Vox post accurately conveys what it says in the study. Stromberg writes, concerning the fact that running the A/C consumed more fuel than driving with the windows down (at least when the ambient temperature exceeded the A/C settings by at least 15-20 deg F), “The difference was very small for the sedan, and the gap did close even further at high speeds, but it didn’t disappear. For the SUV, the difference was much bigger, and actually increased at high speeds.” While the graphs for the two vehicle types indeed showed those differences between the sedan and SUV, the authors of the study concluded that the differences weren’t significant, not “much higher” as Vox has it: “Penalty of AC ON at higher ambient as compared to windows down is not significantly different for SUV or Sedan [5-10%]“.





Finally, there’s Stromberg’s suggestion, and I can’t tell if he’s being serious or not, that if you really want to save fuel, you should emulate what some hypermilers do. Since it’s true that cars and SUVs are more aerodynamic with the windows closed, some hypermilers will keep the windows up at all times, even in broiling heat, and stay cool by wearing vests with pockets for ice packets. He even links to places where you can buy them. Now using ice to stay cool is not unheard of in the automotive world. I’ve seen ice cubes poured into the racing suits of NASCAR drivers and many race cars today have cooling systems for the drivers, so maybe it’s not such a silly idea. Something the author says about how that ice is made, though, makes me believe that he doesn’t quite understand how energy works.
Some of the best hypermilers wear ice vests to stay cool without windows or AC. They’re not cheap, but you can get a good ice vest for a few hundred dollars, and it’ll pay for itself over the course of a few years of saving gas.
Instead of draining your fuel efficiency, its cooling power is drawn from the freezer you already have running in your house. So if minimizing gas use is your thing, forget about this silly debate, and slip on a vest filled with little ice packs that normal people use to keep food cool at a picnic.



Stromberg couches his suggestion in the context of concern over climate change. He seems to understand that running a refrigeration system in your car takes energy and that the bigger the cooling load, i.e. the greater the difference between the ambient temperature outside and the temperature that the car’s A/C is set to, the more fuel it takes to run the A/C compressor. However, at the same time he doesn’t seem to understand that the freezer in your home works on the same principles. While it’s not as silly as suggesting running the A/C off of a hood mounted windmill with a generator or other perpetual motion-like concepts there’s still no such thing as a free lunch and when you put those unfrozen ice packs, at room temperature, into your freezer that actually increases the load on the freezer’s compressor system. It’s pretty basic science. When I asked a scientist friend who does thermodynamic problems for fun if putting a jug of water in a freezer increases the electrical consumption of the compressor, he said, “Of course, that heat has to go somewhere, why would you ask?”, as though it was too simple to ask about. You might be saving fuel in your car, but you’re using more electricity at home. Overall, you might not really be saving any energy, or money (that would depend on how much you pay for fuel and electricity) at all and still end up wearing a cold soggy vest while you’re at it, with possibly no impact on climate change.




Either way, if it’s more than about 78 or 80 degrees outside and I’m driving, I’m going to have the A/C on, often with the temperature at its lowest setting, the fan at its highest setting, recirculating the air (or, as some car companies labeled the switch back in the early 1970s, Desert Air), just as the good Lord intended Schreiber men to do so, unless I’m running low on fuel.

Charts courtesy of the SAE.
Ronnie Schreiber edits Cars In Depth, a realistic perspective on cars & car culture and the original 3D car site. If you found this post worthwhile, you can get a parallax view at Cars In Depth. If the 3D thing freaks you out, don’t worry, all the photo and video players in use at the site have mono options. Thanks for reading – RJS
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Old 08-09-2014, 10:46 AM   #2
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Is this from the tightwad gazette ? Like Mr O says, put a couple lbs pressure extra in the tires to compensate for additional drag of AC compressor. The again we can all go back to those evaporation coolers I have in my VW bus.

Last edited by Masterauto; 08-09-2014 at 10:51 AM.
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Old 08-09-2014, 02:27 PM   #3
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I would note that not one of those graphs has any numbers on the y-axis, so the difference between one line and another could be as little as 0.0001mpg.

Besides which, if you really cared about fuel consumption, you wouldn't be driving a V8 SUV.
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Old 08-10-2014, 01:52 AM   #4
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The again we can all go back to those evaporation coolers I have in my VW bus.
How much for bus?
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Old 08-11-2014, 10:10 AM   #5
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Yeah, the original article being discussed is pretty sad.
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Old 08-11-2014, 10:14 AM   #6
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I don't have doors or a roof. Your graphs and words are useless to me.
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Old 08-11-2014, 12:26 PM   #7
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My DD's aerodynamics suffer when I turn the headlights on.
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Old 08-11-2014, 06:32 PM   #8
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Originally Posted by Masterauto View Post
Is this from the tightwad gazette ? Like Mr O says, put a couple lbs pressure extra in the tires to compensate for additional drag of AC compressor. The again we can all go back to those evaporation coolers I have in my VW bus.
Good ol' swamp cooler.
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Old 08-12-2014, 03:38 PM   #9
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Originally Posted by White out View Post
My DD's aerodynamics suffer when I turn the headlights on.
As far as I could tell, my X1/9 lost 1-2mpg when I turned the lights on at highway speeds...
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Old 08-12-2014, 04:18 PM   #10
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My top fuel dragster dropped from 10 gallons per 1/4 mile to 12 gallons per 1/4 mile when I opened the chutes.
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Old 08-12-2014, 05:49 PM   #11
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Wow, challenges the numbers of the study without any of their own...brilliant.

"You might be saving fuel in your car, but you’re using more electricity at home." Is he suggesting that putting a few ice packs in your freezer takes as much energy as cooling your car for whatever time you are wearing them? I would love to see the math on that....oh, wait, there isn't any provided.

" the authors of the study concluded that the differences weren’t significant, not “much higher” as Vox has it: “Penalty of AC ON at higher ambient as compared to windows down is not significantly different for SUV or Sedan [5-10%]“." I didn't read the original study, but there is clearly something missing here. There are specific measures for establishing significance. Unless there is a lot of variation, 5-10% can be pretty significant...up to 4mpg isn't significant?!

Waste of an article.
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Old 08-12-2014, 09:02 PM   #12
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Quote:
Originally Posted by Chin View Post
Wow, challenges the numbers of the study without any of their own...brilliant.
you might want to read the article a little more closely


Quote:
Originally Posted by Chin View Post
"You might be saving fuel in your car, but you’re using more electricity at home." Is he suggesting that putting a few ice packs in your freezer takes as much energy as cooling your car for whatever time you are wearing them? I would love to see the math on that....oh, wait, there isn't any provided.
I think his point was the freezer isn't free. With our numbers we have no idea how much fuel savings was gained by not running the A/C though.


Quote:
Originally Posted by Chin View Post
" the authors of the study concluded that the differences weren’t significant, not “much higher” as Vox has it: “Penalty of AC ON at higher ambient as compared to windows down is not significantly different for SUV or Sedan [5-10%]“." I didn't read the original study, but there is clearly something missing here. There are specific measures for establishing significance. Unless there is a lot of variation, 5-10% can be pretty significant...up to 4mpg isn't significant?!

Waste of an article.
As stated the vehicles tested were a V8 sedan and a SUV, so I doubt they were 40 MPG vehicles. More like 20 MPG ones. so the measured MPG would be 1-2 MPG. The author didn't make the significant statement, the authors of the study did.

SO out of 300 miles of driving you would save about $2. That would be about 5 hours of driving. My cool suit lasts for 30 minutes, so it would take 10 charges of those ice packs, at say 2 hours a piece. 200 KWH's of refreeze time at $.09 a KWh. I have no idea how much extra those packs would add, maybe $.20 total, but the result is a savings of maybe $1.80 for every 300 miles, and if the house is powered by a coal plant, even worse emissions that the car would go up for the extra load of the a/c.

I think the only point the author was trying to make is a $2 a month worth the hassle of dealing with an ice vest, and the fact it's probably making no impact at all on less emissions.
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Old 08-12-2014, 09:51 PM   #13
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Quote:
Originally Posted by mhoward1 View Post
My top fuel dragster dropped from 10 gallons per 1/4 mile to 12 gallons per 1/4 mile when I opened the chutes.
Pics of said dragster or I call foul.
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Old 08-12-2014, 10:39 PM   #14
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Old 08-13-2014, 01:04 AM   #15
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Old 08-13-2014, 07:54 AM   #16
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HHAHAA

Man, I had no idea I was about to see a Z!!!!

Well played sir!!!
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