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Old 07-14-2018, 02:22 PM   #1 (permalink)
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Slightly open window?

Hi everyone. First post in a long time ...

The common wisdom is that for city driving, and open window is more fuel efficient than A/C, and for highway, the opposite is true. I generally try to avoid using the A/C altogether, so usually just set the vent fan on high with the windows closed when on the highway.

Anyway, on one extremely hot day last week (in the midst of a run of very hot days), I felt the fan wasn't doing a very good job, so I cracked the window just slightly so the air already in the car had somewhere to go. This seemed to dramatically improve the ability of the fan to keep me cool.

I tried this for the next few days, and it doesn't seem to have had any measurable effect on fuel economy (my driving is mostly ~90km/h on country highways) as reported by Torque. It occurred to me that with just a very tiny gap of an opening, probably no air is coming IN to the car through the window. The positive pressure inside caused by the fan is blowing air OUT instead. What I feel when I hold my fingers near the gap seems to agree with this theory.

I haven't done any serious A-B-A testing of course, but my limited experience (a few days of driving compared to a almost a year of eco-driving before that), and my theoretical in-my-head analysis, suggests that this won't add any drag to the vehicle.

Anyone have any thoughts on the matter?

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Old 07-14-2018, 03:19 PM   #2 (permalink)
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No measured data to share, but my sense is you are correct in your assumptions.

Most vehicles have fairly high electrical draw just running the fans, perhaps around 200 W on high (measured the TSX at 220W). Even lower settings often have a high power consumption due to the resistors that many fans use to control speed. Factoring in the losses involved in generating that electricity, we might be looking at half a horsepower or more in required engine power.

A typical AC might take 3 HP when it runs, but since it runs intermittently (cycles on and off), the average power draw on the engine might be closer to 1.5 HP. The general rule is a 10% drop in MPG running the AC, though I suspect that also includes the power required to run the fans.

My best guess is that your fans drop MPG by about 4%, and if you were to run AC, it would drop it an additional 6%. If you have a gauge that measures fuel consumption at idle, you might be able to get a rough estimate of the extra amount of fuel needed to run the fans, as well as the extra amount to run the AC. You'd need to keep track of the % of time the AC is running vs % of time it is off to get an accurate average of the AC consumption. The other factor in AC power consumption is that it has to run a higher % of the time when cooling a heat-soaked car, and runs less frequently to maintain temperature in an already cooled cabin.

Cracking the windows is probably negligible depending on how open they are and the speed driven.
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Old 07-14-2018, 03:27 PM   #3 (permalink)
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window cracked open

Quote:
Originally Posted by stefanv View Post
Hi everyone. First post in a long time ...

The common wisdom is that for city driving, and open window is more fuel efficient than A/C, and for highway, the opposite is true. I generally try to avoid using the A/C altogether, so usually just set the vent fan on high with the windows closed when on the highway.

Anyway, on one extremely hot day last week (in the midst of a run of very hot days), I felt the fan wasn't doing a very good job, so I cracked the window just slightly so the air already in the car had somewhere to go. This seemed to dramatically improve the ability of the fan to keep me cool.

I tried this for the next few days, and it doesn't seem to have had any measurable effect on fuel economy (my driving is mostly ~90km/h on country highways) as reported by Torque. It occurred to me that with just a very tiny gap of an opening, probably no air is coming IN to the car through the window. The positive pressure inside caused by the fan is blowing air OUT instead. What I feel when I hold my fingers near the gap seems to agree with this theory.

I haven't done any serious A-B-A testing of course, but my limited experience (a few days of driving compared to a almost a year of eco-driving before that), and my theoretical in-my-head analysis, suggests that this won't add any drag to the vehicle.

Anyone have any thoughts on the matter?
As far as I know,tests have only been conducted with either fully-open,or fully-closed conditions.And Hucho reported that in wind tunnel testing,that the cabin ventilation ducting is closed as a matter of practice.So,so far,we have no laboratory,or CFD analysis.
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Old 07-14-2018, 03:40 PM   #4 (permalink)
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I got this tip off of Wayne Gerdes, but he opens the driver window 1 inch (ish) and the rear passenger window 1 inch (ish) and it creates a nice cross draft without any noticeable effect on aerodynamics. I use this technique and find it works quite well at moderate or high speeds. If you're stuck in bumper to bumper traffic and very low speeds you mine as well roll your windows down all of the way because aero will be fairly irrelevant. I never use the A/C or fans because of redpoint5's points.
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Old 07-14-2018, 04:21 PM   #5 (permalink)
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I go a little further and open the driver window more than the right rear... if my wife rides along she’ll open her window same as mine is, usually 3” in front and 2” rear... no huge drop in mileage from the windows cracked
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Old 07-14-2018, 08:59 PM   #6 (permalink)
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I haven't noticed a hit in MPG unless its over 65MPH in my car
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Old 07-15-2018, 02:17 AM   #7 (permalink)
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Not totally sure if adding weathershields would effectively decrease the drag at highway speeds if you crank the windows down one inch or two.
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Old 07-15-2018, 12:12 PM   #8 (permalink)
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Quote:
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Not totally sure if adding weathershields would effectively decrease the drag at highway speeds if you crank the windows down one inch or two.
But how much drag would the weathershields add?
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Old 07-15-2018, 02:51 PM   #9 (permalink)
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Quote:
Originally Posted by chumly View Post
But how much drag would the weathershields add?
If it does add some parasitic drag at all, it's not too much.
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Old 07-15-2018, 02:55 PM   #10 (permalink)
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Quote:
I haven't done any serious A-B-A testing of course, but my limited experience (a few days of driving compared to a almost a year of eco-driving before that), and my theoretical in-my-head analysis, suggests that this won't add any drag to the vehicle.
If you compare the amount of air entrained by the vehicles passage, with the air exchanged through the openings, there will be an orders of magnitude difference.

Cf: https://en.wikipedia.org/wiki/Meredith_effect

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