Gas saving myth: Turn off your AC?
Fueleconomy.gov says "operating the air conditioner on 'Max' can reduce MPG by roughly 5-25 percent compared to not using it." Consumer Reports and Edmunds.com says you will lose 1 MPG. That's the low end of the government estimate of 5 percent if you have a car that does 20 MPG. Is it fair to call it a myth, as CNN claims?
Putting the windows down did not change mileage enough for Consumer Reports and Edmunds to measure any change in MPG. So, that's a myth. There is SOME gas savings and putting down the window does not burn a lot of gas. I guess the real question is how did the government get 25 percent reduction in mileage by turning on the AC? The AC on the car must have looked like this :D
I like AC and I hate the noise when I open the window. There are a lot more gadgets in cars today like GPS, DVD etc. It does take energy to recharge the battery. I am a big fan of recharging your battery with solar devices. I say have two batteries one for the engine and another for everything else. People could charge the batteries that powers the AC and radio etc by plugging it in at night.
Actually, now that I think about it, scratch that. Lots of people might not plug-in PHEVs. I doubt they would go through the hassle of independently charging the battery for the AC, heat, radio, etc. Okay, maybe hypermillers would do it. It might be cool if the car companies came up with hyper miler, bare bones edition of cars with little features like that. I would buy one. Even if it's a lower MPG car vs a hybrid.
[Source: CNN]
Reader Comments (Page 1 of 1)
Me 8:36AM (4/22/2008)
Some of the information is incorrect.
First, your alternator in fact uses the same amount of the engine's energy whether your battery is low or full. As long as it's working properly, your alternator is fully capable of charging your car's battery without the help of a solar charger.
Second, the MythBusters episode only tested the relative gas consumption between driving with your windows down vs. windows up w/AC on. It did not test gas consumption with the AC on vs. off.
Third, using AC *DOES* reduce engine horsepower, because the torque has to overcome resistance from the compressor. Your engine then has to work harder, which requires more fuel. With the AC off, the AC clutch is disengaged and the engine power is focused more on moving the vehicle.
Finally, electrical devices do NOT cause increased gas use, because there's no additional drain on the engine. While the engine is running, the alternator is always turning, and always producing power to the battery. The engine doesn't have to work any harder to turn the alternator when the battery needs to be charged.
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Whopper 10:58AM (4/22/2008)
Me, you are not correct. The alternator may be spinning continually, but it is not drawing the same power from the engine all of the time. The alternator is controlled by an internal regulator which provides voltage to the stator. When the battery is low and more charging is required the regulator increased the voltage available to the stator. This increases the magnetic field which permits the rotor to generate more power output. When the battery is fully charged the regulator reduces the voltage to the stator and the alternator's output is reduced. Because, when the stator is at full voltage, the magnetic field is strongest it takes more torque to drive the alternator. When the stator voltage is low the only drag created by alternator is mechanical friction of the bearings and drive belt and windage by the alternator fan.
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rgseidl 5:06PM (8/19/2007)
A/C can actually increase fuel consumption by 25% if:
a) it's very basic A/C unit without any feedback control,
b) the vehicle been baking in the sun on a hot summer's day,
c) you put the A/C on maximum setting, and
d) you then putter around town or get stuck in a traffic jam.
Another context is very high relative humidity on a hot day, which makes you sweat profusely.
These are fairly extreme conditions but those are exactly when you really need A/C for safety rather than just comfort. Heat and humidity stress reduces reaction times and tends to make drivers more aggressive.
VW - perhaps others, too - now offers an in-cabin CO2 sensor. Standard A/Cs are set up to provide a sufficient volume of cool, fresh air to support a full complement of passengers. With the sensor, the volume flow is adapted to the number of people actually in the car, which reduces drafts and saves quite a bit of fuel.
The next step is a combo of special glass formulations and coatings that is transparent in the visible spectrum but opaque in the infrared. That way, the inside never heats up to unbearable temperatures to begin with. Saint-Gobain claims it has the technology, but it is still quite expensive and longevity remains to be proven.
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small-wee-wee 5:17PM (8/19/2007)
http://en.wikipedia.org/wiki/MythBusters_(season_3)#AC_vs._Windows_Down
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derek.hofmann 5:23PM (8/19/2007)
There is a discussion at http://ask.metafilter.com/42167/Hot-window-breeze-or-cool-car-breeze about this (including a reference to that Mythbusters episode). Looks like it depends on the car and how fast you're going (the faster you drive, the more drag caused by an open window). Bottom line, get yourself a ScanGauge and measure it for yourself.
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MikeW 5:35PM (8/19/2007)
A small 400 mile trip. Window all the way down 28mpg@70, windows all the was up 31mpg@70.
Using recirculation reduces the duty cycle, because it is cooling air that cooler than outside.
Doesn't Honda have an automatic system for partial air recirculation in the MDX, to boost mileage.
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Lascelles 7:31PM (8/19/2007)
Repeats in a week. I will check it out. Write a follow up if I find something new.
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jg3 8:46PM (8/19/2007)
Test your own car, don't rely on sources that haven't driven your car in your climate on local fuel, no matter how reliable the source...too many factors. I've had cars that had very noticeable decreases in mpg, and others with no measurable difference whatsoever. One thing that IS often lost is that MAX doesn't mean max energy used (as I once erroneously believed). Max really means "recirculate" as many foreign models correctly label the controls. Setting your car to MAX instead of NORM will actually save both energy and wear and tear on the system, as the compressor will have to work less (not more as the misnomer suggests).
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gnoble 11:37AM (8/20/2007)
Using A/C is a huge drain on your engine, taking on average about 6hp of engine power. I remember reading another site w/ this same debate and a guy who claimed to be a NASCAR mechanic said that the first thing they always take out of cars when they're converting them to racers is the A/C. If you're ever driving at high speeds and suddenly turn your A/C on full blast you'll notice a decrease in power - especially in lower HP cars.
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detroit9000 11:39AM (8/20/2007)
Are you serious?
This post was written by someone who lacks a remedial understanding of automobiles. That's not a problem, unless it's an automotive periodical. There are a few sentence fragments in there too, so I hope they can play guitar or cook.
A/C uses a compressor which compresses gas. The system later expands that gas in an endothermic process to chill a fluid which chills fins of a radiator in the cabin. The HVAC blows air over those fins.
The compressor requires energy to compress the gas. This energy is supplied in the form of rotational motion from a pulley. That pulley gets its rotational energy from a belt, which is connected to a pulley on the engine.
The A/C uses a helluva lot more energy than yer nav system or radio does. It's like comparing the energy needed to run a hand pump to pump water to the energy needed to move a drum-stick to beat a drum. You get a lot of "sound" from the drum. It might sound loud to you. It might seems like a sensory experience equal in intensity to that of pushing the pump to pump water. But the energies are vastly different.
Currently, all the known implementations of A/C use some variation of this process. On the horizon are nanotechnologies would could reduce energy consumption, but they're pretty far off. They probably won't be implemented until their patents expire.
Seriously. Are we getting this science retarded? Because I don't want to learn to speak an Indian language to talk about cars at a higher level than I did in the 3rd grade.
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Donald Bush 11:55AM (8/20/2007)
SoCool is a new company dedicated to solar powered systems to help save the environment, save money for users, and provide "COOL" Products to the people.
We have created the worlds first solar powered air conditioner system (Helios) that fits into a standard sun roof footprint.
Helios Pays for itself! - Save 5-6 Miles/Gallon by not using the engine driven A/C system. Save the environment by preventing the emissions from that 5-6 Mpg per vehicle becoming pollution.
Helios helps prevent harm to children and pets - Helios will keep the internal heat of an automobile at safe levels for children and pets. So accidental cases where children or pets are left in vehicles need not end in tragedy.
Helios is convenient - Imagine leaving your vehicle in the mall car park for several hours on a hot day, and returning to a cool passenger cabin without needing to switch on maximum AC and swelter until it kicks in.
http://www.travelsocool.com/
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Lascelles 12:21PM (8/20/2007)
Detroit9000, The gov website says electrical accessories. I know the AC is different than a radio but I thought the group of features was worth mentioning. I will check out the Mythbusters and look into this in much more detail but I really find it hard to believe that if I put down my window and max the AC, my car's MPG will be cut in half. I am not saying these thing take up no energy at all but that REALLY sounds like a lot to me.
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Culex 10:00AM (8/21/2007)
I'm driving a Civic Hybrid (2003 model) and there is definitely a MPG hit when the AC is on. Windows open I can routinely be in the low to mid 50s on MPGs. AC on and windows closed and I'm in the upper 40's. This is on a 25 mile commute at around 50 mph most of the way and also includes a number of stop lights. So, in my car, for me, AC means more gas used.
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Lascelles 10:01AM (8/21/2007)
There is a hit to MPG if you go above 50 MPH. I wonder if Mythbusters tested a car, going that fast without AC or just varied the speed, assuming no impact from speed?
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youknowit 10:48AM (8/21/2007)
I am not a mechanic or engineer, but when A/C is on in my current and previois cars I have to push the gas pedal a lot harder to get up to speed (like on on-ramps to a highway). My basic understanding, and some common sense, would lead me to believe that if I am driving the car harder and pushing the engine harder, I am probably using more gas. Right?
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pamela lorraine 2:24AM (8/22/2007)
being a city courier i really notice the extra fuel usage with the ac on. at speed it used slightly more fuel with the widows down i corrected this by fitting weather shields to both front doors which produced fuel economy the same as windows up.but i would really be interested in a solar powered ac not only for my comfort but in australia at christmas time whe deliver a lot of perishables like chocolates and other food stuffs a full time ac would stop these from being damaged as its summer down here
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steven 1:26PM (8/22/2007)
I think the dead giveaway here is "at highway speeds". It is all the other speeds we drive on a day-to-day basis the uses more of the the gas!
OK, how about this one? What about if you drive you pickup with the tailgate down? Do you save any gas and would it somehow compensate for the tailgate crash damage you'd get during a rear-ended when the car behind you hits the tailgate and not your bumper!!!! If a Mini hits you what does that do to thier windscreen?. Sounds like a MythBuster's special if they have not already done it.
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naggs 12:42PM (8/24/2007)
they did it, trucks are more aerodynamic with the tailgate up
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