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?

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.

window cracked open
 

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.

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.

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

I haven't noticed a hit in MPG unless its over 65MPH in my car

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?:confused:

If it does add some parasitic drag at all, it's not too much.

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

With my dodge intrepid I didn't have a/c until the last year I owned it. It was standard MO in the summer to crack all 4 windows 1" or less with the fan on. The cabin air will get sucked out which will assist the fan in providing more air into the cabin.

I don't even run the fan when I have my windows cracked; the suction pulls plenty of air through the vents.

Check this:

Fuel Efficiency: AC On Vs. Windows Down - Business Insider

https://amp.businessinsider.com/imag...56-750-562.jpg

https://amp.businessinsider.com/imag...10-750-560.jpg

I'm going to have to measure mine. It can't be anywhere near 200W though since the entire HVAC system, including the rear window defogger, is on a single 15A fuse, so at 14.5V, that would be 217W total (fan and defogger both running) if the system was designed to run right up to the rating of the fuse (unlikely).

I'd be surprised if it was much more than 50W in my car.

Considering that it's fed by the battery with 12V DC, the entire system shouldn't operate with more than 180W.

Yes, when the car's not running, it's 12V. When it is running, the alternator is keeping the voltage up around 14.5V, hence my 217W figure.

I have been pondering about this. I try to use the windows as much as possible. But I really enjoy the AC. (I am half Finnish living in Turkey). I only use a jacket a couple of times a year (besides a motorbike jacket). Nowadays urban areas are so dense that i have a tshirt on even when it snowing. Time spent outside is always less than ten minutes and my thermal inertia can handle that (i do have a little bit of (20lbs) excess).

AC on full blast is rarely enough. It can be humid over here and i get soggy really fast.

There must be ways to increase AC efficiency.

I shall install a water misting system to the AC condenser when i get the chance.

I would like to experiment with AC systems. Increase the amount of refigerant in a reservoir, so regenerative braking can be used to refill it. That way the cooling can be virtually free. Unfortunately auto work here costs an arm amd a leg. A transmission swap or engine swap is unheard of. Some one doing it diy does not exist! There mayyy be that one guy in the whole country who has done it DIY.

Ps: on my car there is an ac disable button that cuts off the power to the compressor clutch.

I use it a lot, enable the compressor only when decelerating. Seemed to work ok untill the ac quit because of a burned clutch.

Paint the roof white.

I will install reflective foil once my transportation situation stabilises.

water misting
 

If you have ground level ozone,there is a risk that misting will create nitric acid which could chemically attack the condenser coil.

real world vs. tested
 

Your moderate speeds (= 55 mph limit for our American friends) lessens the impact too.


FYI, completely anecdotal, but in the same hot weather you're probably talking about, I did a trip at 80 km/h = 50 mph with both windows wide open, and then the same trip with both nearly closed. Same MPG on both trips. :D



Doesn't prove anything, of course.



I also tested it directly (repeated coastdown testing), and could detect the extra drag of one window down in more controlled conditions.

I do not think that our ground level ozone levels are high. Not many dc motors inside the hood to create arcs either.

How does one check ground level ozone levels?

It's often pointed out that drag effects become more noticeable at speeds above 80km/h, but I'm sure even a little below that speed they're not negligible at all.

Indeed, like any drag effect, it's probably proportional to the square of your speed. So at say 75km/h, I'd expect to see about 88% of the window drag I'd get at 80km/h.

Tried this out today; at 40-65 mph, the only open window configuration that didn't add drag (reduce mileage) was having only one window open about half an inch.

I finally got around to measuring what my HVAC blower uses. All tests were done with the car off, so voltage was only around 12V:

The difference between TotalPower and MotorPower is what is wasted in the speed control resistors. The power actually reaching the motor is approximately doubled with each increase in setting.

I was wrong about everything going through a 15A fuse. This is true for the low and medium settings, but when the blower is on high, a relay connects it almost directly to the battery through a 30A fuse.

EDIT: At 114W, running the fan adds about 0.06L/100km to my fuel consumption (based on 114W = 0.15hp = 0.04L/h = 0.06L/100km at 67km/h, which is my overall average speed). In US units, where the calculation is more complicated because we're working with reciprocals of fuel consumption, this costs me about 0.6mpg if I was averaging 47mpg.

Thank you for those measurements.

how does
 

Looking over at GOOGLE,one site is mentioning using a 405 nanometer photometer to detect ozone.
They also mention chemiluminescence detector technology,however they say that measurements can be highly variable.:o