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Author Topic: Air conditioning and mileage: dueling news stories
snopes
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http://abcnews.go.com/2020/Autos/story?id=1274541

quote:
As for using the air conditioner, go ahead. It has little impact on gas mileage.
http://www.alternet.org/envirohealth/37882/?comments=view&cID=142322&pID=142025

quote:
Government tests have shown that running an air-conditioner can decrease a car's fuel efficiency by 4 miles per gallon. About 5.5 percent of the gasoline burned annually by America's cars and light trucks — 7 billion gallons — goes to run air-conditioners.

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Viliphied
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Through my own emperical observations in my own vehicles, you get better gas mileage with the a/c on than you do with the windows down.

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"I used to think I was a little unstable, then I met every girl I've ever dated." -- Mike Birbiglia

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zman977
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It's so hot in Illinois I'm useing the AC no matter what our milage is. Honestly though, we don't notice any difference.

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jamira
I'm Dreaming of a White Sale


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I can tell you it doesn't make a lick o' difference in a semi truck. A/C being more comfortable is the way to go.
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dlloyd
I Saw Three Shipments


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It makes about a 5% difference in a deisel Renault Grand Scenic. We have display that shows the mpg, flick on the AC and watch the numbers drop. That's with the windows up in both cases, by the way.
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Troberg
Angels Wii Have Heard on High


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quote:
It makes about a 5% difference in a deisel Renault Grand Scenic. We have display that shows the mpg, flick on the AC and watch the numbers drop. That's with the windows up in both cases, by the way.
Don't trust those gauges too much, they usually just measure intake manifold pressure, which is far from accurate. They can give a general hint, but not more.

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/Troberg

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dlloyd
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This one measures average mpg from mileage divided by fuel used. If the tank is fairly full you can really see the effect of the AC.
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Troberg
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quote:
This one measures average mpg from mileage divided by fuel used.
And fuel used is usually measured by intake manifold pressure.

Even if they were to measure the amount of fuel in the tank, that would also be fairly inaccurate, as the fuel sloshes around, the tank is far from having a simple form (the sensor just measures the height of the surface) and the measurement device in itself is not that accurate.

You can try to measure the fuel flow, but that's basically what they try to do when intake manifold pressure is measured, in a roundabout way, as it is cheaper, simpler and in most cases accurate enough for the average motorist.

I would almost (but only almost) be prepared to place a bet that it measures intake manifold pressure. If you look at the intake manifold, you can probably see the wire going there, leading you to the sensor. Or, if you are lazy, ask your mechanic the next time you visit (not that mechanics can be trusted, at least if you believe the song).

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/Troberg

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Lonely Mountain
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WARNING: LONG AND BORING POST AHEAD

WARNING: LONG AND BORING POST AHEAD

WARNING: LONG AND BORING POST AHEAD

I warned you

From my completely non-scientific, un-tested, "got an A- in high schools physics 10 years ago" point of view it's going to depend on the car and how fast you are going. Luckily I still have my high school physics book so hopefully I can explain this without sounding like an idiot.

The loss from AC is for the most part, constant. It takes a certain amount of horsepower to power the fan, compressor, etc. The loss from wind resistence or drag however increases by factor of 2. So everytime you double your speed, the drag quadruples.

You can determine the drag by the equation
F = 1/2 * (density of air, D) * (velocity squared, v2) * (area of effect, A) * (drag coeffcient, C)

We can ignore D because the density of air will be constant and so will C, the drag coeffcient which is determined by largely by the car's aerodynamics.

That leaves us with v2 * A. Rolling the windows down increases A, the area of effect. So, in theory any vehicle will have worse gas mileage with the windows down versus the AC if you go fast enough. In the real world, that speed may not be practical or even obtainable. A semi with a trailer would probably have to go Mach 1 for the windows to make a noticiable difference.

Well that's just the force created by the wind. To overcome force, you need power. Power is equal to Force * velocity, or using our simplified equation:

P = F * v = v3 * A

So now, everytime your speed doubles, the amount of power needed octuples. So lets use some hypothetical numbers. Let's just say rolling down your windows increases the area by .5 (assuming the interior is about 1/2 of the car) So everytime your speed doubles, the power extra power lost by the windows increases by 4 (8 / .5 = 4). So if at 20 mph the windows take up 1 hp, they will consume 4 hp at 40 mph and 24 hp at 80 mph.

So hypothetically, if you AC uses 10 hp to run, it is less effcieient to use it at 40 mph but more effcient at 80 mph. N

ow how much efficency this transates to depends on the power output of the vehicle. A semi truck can engines that put out 600 hp so a 14 hp difference probably won't even be noticiable. This number would be even lower considering that rolling down the windows on a large truck would add very little to the total area (A) of the truck so a difference of maybe 5-10hp would be closer. So jamira is right, a semi is just too big and too powerful to be affected either way so do whichever is more comforatable.

Now a compact or smaller mid-size car on the other hand can have an engine with as little as 80 hp so saving 14 hp (assuming you could even get the little bugger to 80 mph [Smile] ) would transalate into a huge difference in efficiency.

So I guess what I'm saying is that it depends. If you drive around a city all day, the AC will cost you. If you are driving 70-80 mph on interstates all day, then the AC is your friend.
[dunce]

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WildaBeast
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quote:
Originally posted by dlloyd:
It makes about a 5% difference in a deisel Renault Grand Scenic. We have display that shows the mpg, flick on the AC and watch the numbers drop. That's with the windows up in both cases, by the way.

Well, yeah. I don't think there's much debate over the fact that if all other conditions are the same, your mileage will drop when you turn on the AC. The question is whether turning on the AC (with the windows up) reduces your mileage more than rolling down the windows (with the AC off).

I thought we already pretty much determined that it's what Lonely Mountian said -- it depends on the car and how fast you're going.

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Grand Illusion
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The difference betweenm a/c and windows would also depend on how far you rolled your windows down. Why was Lonely Mountain the first person I've seen that somewhat covers that?

The other question I've had is how much does the fan speed have to do with fuel efficiency? Am I using more gas with the a/c with the fan on 4 than with the fan on 1? My logic says that I would, slightly, because the cool air needs to be replenished faster in the a/c unit if it's being fanned out faster.

Most of the time, when I've heard people say that having the a/c on is more efficient than having the windows down, it was a passenger of mine implying, "Don't be a cheapskate; turn on the air. I'm dying and I don't want to get wind blasted."

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Troberg
Angels Wii Have Heard on High


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Lonely Mountain: Perfect post! I think you got it all.

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/Troberg

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dlloyd
I Saw Three Shipments


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quote:
Originally posted by WildaBeast:
quote:
Originally posted by dlloyd:
It makes about a 5% difference in a deisel Renault Grand Scenic. We have display that shows the mpg, flick on the AC and watch the numbers drop. That's with the windows up in both cases, by the way.

Well, yeah. I don't think there's much debate over the fact that if all other conditions are the same, your mileage will drop when you turn on the AC. The question is whether turning on the AC (with the windows up) reduces your mileage more than rolling down the windows (with the AC off).
The original post was querying this statement:

quote:
As for using the air conditioner, go ahead. It has little impact on gas mileage.
No mention of windows.

quote:
I thought we already pretty much determined that it's what Lonely Mountian said -- it depends on the car and how fast you're going.
Are you including another thread here? There wasn't a whole lot of discussion in this thread prior to my post.
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Ophiuchus
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It could be fair to say that both the windows rolled down and the AC on drop your gas milage by quite a bit, but it is important to cool down when you are driving. Which one is really worse could be endlessly debated without a clear answer. Obviously if every window in your car is rolled all the way down that is worse than the AC, if 1 is halfway down, the AC is probably worse...

So, ultimately, just do what makes you most comfortable. Realize doing both at once is the worst but doing one or the other is about equally bad.

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Troberg
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quote:
So, ultimately, just do what makes you most comfortable.
Actually, it's all about speed. The AC has a constant power drain, propulsion increases with speed, speed increases drag from open windows.

At low speeds, windows are more efficient, at high speeds, AC is better and probably more comfortable. Exactly where the lines cross depends on the aerodynamics of the car, the age and type of AC and the size of the engine.

My guess would be that the drag from open windows is neglible until we reach speeds where aerodynamics and drags actually starts to matter, which probably is around highway speeds.

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/Troberg

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geminilee
The First USA Noel


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Is the A/C running off the battery? If so, how is it affecting milage at all. The belt to the alternator is running constantly, recharging the battery, whether the battery is being used or not. The drain on the engine comes from the belt, the battery does not have anything to do with horsepower, since you are not using the battery to propel the vehicle.

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Bassist
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In any car I've driven, the A/C runs off of the serpentine belt that is powered by the engine (the alternator may or may not be powered by the same belt, but the A/C is not totally battery driven). In my current car, for instance (four cylinder, manual transmission, 1.8 L displacement), there is a noticeable drop in performance when I engage the air conditioner. I often have to turn the A/C off when I'm merging onto highways or trying to climb a relatively steep hill.

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Troberg
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quote:
Is the A/C running off the battery?
As Bassist said, it's driven off the engine through a belt.

In any case, it doesn't matter. Even if it was run off the battery, it would still take as much power, as the alternator must produce more power to keep the battery charged, which makes the alternator require more power to operate.

quote:
The drain on the engine comes from the belt, the battery does not have anything to do with horsepower, since you are not using the battery to propel the vehicle.
You ARE using engine power to charge the battery.

When it comes to energy, there is no such thing as a free lunch. The energy has to come from somewhere and it has to be produced. If you take 1 kW from the battery, you need to put 1 kW in, which has to be produced in the alternator, which means that the engine has to produce one 1 kW to drive the alternator. This off course assuming that all parts work at 100% effieciency, which they are not, so more power needs to be generated for each link in that chain.

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/Troberg

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geminilee
The First USA Noel


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Yes, I know the belt is powered by the engine, but I thought the drain produced on the engine did not change in response to need. Like if you had one of those friction-powered lights on your bycicle, but turned off the bulb by a switch, the fact that it is clamped is producing resistance. It does not matter whether the bulb is using the electricity generated or not, the effort required is the same either way.

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Troberg
Angels Wii Have Heard on High


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quote:
Yes, I know the belt is powered by the engine, but I thought the drain produced on the engine did not change in response to need. Like if you had one of those friction-powered lights on your bycicle, but turned off the bulb by a switch, the fact that it is clamped is producing resistance. It does not matter whether the bulb is using the electricity generated or not, the effort required is the same either way.
Any generator will require more power to turn if you put more load on it. When you charge a battery, it's not as noticable, as the battery acts as a buffer, smoothing out the load fluctuations over time.

The same applies for the bike, but is harder to notice, since that tiny light bulb uses almost no power and much more power is used up in mechanical loss (mainly friction, as it needs to push firmly against the rim to avoid slipping). An AC uses a lot of power. Depending on who you ask and which model it is, it's usually something between 5-30 hp, or, if you prefer, 3700-22000 W, compared to a tiny light bulb that has maybe 5 W. 5 W is rougly how much power is needed to lift 7 grams 1 meter in 1 second, about as much energy as is needed to flip a coin. The power an AC use (the lowest estimate) is roughly equivalent to lifting 375 kg 1 meter in 1 second, or, if you prefer, the power needed to make a world class pole vault. The high estimate for the AC is enough to launch an average car 1.5 meters up into the air in 1 second. I think that it's obvious that we are looking at different scales here...

I've even seen applications where generators have been used as a dynamic brake by using a dummy device to change the load depending on how much you want to brake.

Edit: My calculations are of course assuming that the power can be harnessed efficiently, which is unlikely to happen in real life.

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/Troberg

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dlloyd
I Saw Three Shipments


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quote:
Originally posted by Troberg:
quote:
It makes about a 5% difference in a deisel Renault Grand Scenic. We have display that shows the mpg, flick on the AC and watch the numbers drop. That's with the windows up in both cases, by the way.
Don't trust those gauges too much, they usually just measure intake manifold pressure, which is far from accurate. They can give a general hint, but not more.
Thinking about this post again, while the average mpg displayed is slightly erratic in the first ten minutes or so following refuelling (and zeroing of the gauge), the display certainly appears to be quite accurate when compared with actual fuel economy... the figure it displays matches the figure I get when I do an off-the-cuff calculation at the petrol station.

Would you expect intake manifold pressure to be affected by the AC?

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Troberg
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quote:
Would you expect intake manifold pressure to be affected by the AC?
Yes. Anything that increase fuel consumption would require more fuel to be pressed into the cylinders, so intake manifold pressure will increase. It's not completely linear and somewhat hard to calibrate, which is why I said that it should not be looked at too closely.

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/Troberg

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dlloyd
I Saw Three Shipments


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Okay, I think I understand you now.

While you're not questioning the fact that using the AC increases fuel consumption, you're not convinced by the 5% figure.

I'm not sure I could come up with a fair test of how much AC influences fuel consumption. I can get anywhere between 45 and 60 mpg average, depending on how I drive.

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Troberg
Angels Wii Have Heard on High


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quote:
While you're not questioning the fact that using the AC increases fuel consumption, you're not convinced by the 5% figure.
I think it's difficult to measure it accurately without special equipment. Also, a fixed percentage is not enough, as it will be higher at low speeds and lower at high speeds (the AC consumption is constant, while propulsion consumption increase with speed).

I originally made the comment about how it's measured just as a side note, more as a warning about the method in general than in a specific case. For instance, don't trust that MPG you get when you try to figure out if you can skip the next gas station because the fuel will be enough to reach the followin station.

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/Troberg

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