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Old 07-27-2010, 12:43 AM   #1 (permalink)
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Air Density

I've used the MPGuino for over a year. I've done nothing to the vehicle to add farings/etc but have realized almost 10% fuel efficiency improvement by running the tires at 35+ PSI rather than 25-28.

My vehicle has a characteristic 'best' fuel economy (6.2 L/100km) which is eventually reached given a long enough trip - on golden trips it has been 5.8 L/100km (!), but often it won't go below 6.5 L/100km - and these on the same trip and the same roads, just different days - why?

Watching the Instant readings fluctuate (and being a pilot) I've often thought about the wind resistance of the vehicle - which (besides the vehicle profile) is a function of air density (temperature, pressure, humidity) and headwind component. It would be interesting to be able to obtain a "current air resistance" reading from the MPGuino. Perhaps airspeed is sufficient, comparing that to ground speed from the VSS. Air density is easily calculated given the proper sensors, headwind component (airspeed) with more difficulty with some sort of aircraft-type pitot tube. I've was experimenting with pressure sensors a few months ago.

Does anyone have any comments about this idea?

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Old 07-27-2010, 06:56 AM   #2 (permalink)
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How would you use this readout to get better mileage?
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Old 07-27-2010, 09:37 AM   #3 (permalink)
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You won't adjust your driving style based on air density, but I do adjust based on airspeed. Rather than running constant road speed, I run roughly constant road load, speeding up slightly when airspeed drops, and slowing down for a headwind.

I do this without a gauge. But if you want to throw a simple pitot tube in your car, why not? It would be cool if the readout were in the same units as your speedometer, but even if you left it in KPa, you'd be able to see winds.
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Old 07-27-2010, 10:39 AM   #4 (permalink)
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...replace your "hood ornament" with a "pitot tube" and see what numbers you get!

..."ground" speed (wheels) vs. "air" speed (pitot tube data).
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Old 07-27-2010, 11:34 AM   #5 (permalink)
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Yeah, sounds like a fun project. I don't have a hood ornament, but a magnetic antenna mount would help me make one

I read that there are ways to calculate air resistance of a vehicle, but it appears to require a lot of knowledge of fluid dynamics. By doing some math with groundspeed vs airspeed (and perhaps an accelerometer or GPS to take road grade into account) the vehicle's wind resistance would come out as a number? ... good information on which to base further experimentation on drag-reducing mods to the vehicle.

But if you really want to improve vehicle mileage, I suggest only travelling in the direction of the wind
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Old 07-27-2010, 12:17 PM   #6 (permalink)
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If you want to measure your CdA, coastdown testing is probably the best way to do it. Here's an instructable: Measure the drag coefficient of your car

Also, here's an academic project that aspired to analyze fuel consumption, vehicle speed, and GPS data to spit out aero drag, rolling resistance, and engine efficiency. Sadly, nothing ever came of it, and we haven't heard from the author since spring.
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Old 07-27-2010, 02:39 PM   #7 (permalink)
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mluckham,

As others have stated...you can't do much about airspeed even when you know what it is. Also easily calculated with ground speed plus or minus relative wind (known heading and speed from aviation weather). I guess you could use it to help justify your bad run or tank in all headwinds driving. ;-)

Knowing pressure could be helpful though. Using a magnehelic <sp?> could get you pressure at various points on your vehicle. See how low it can go when drafting vs how high is it during normal open air driving. What is the low pressure in your wake? Reducing your pressure wake (size of your wake) makes your more efficient. You could maybe measure the difference of aerodynamic modifications like a boat tail or kammback. Measure the pressure difference just inside your grill or engine compartment to see the effect of a grill block.

Just some thoughts or ideas....
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Old 07-27-2010, 03:37 PM   #8 (permalink)
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...from the days of propellor aircraft: to determine vehicle "yaw" vs. wind direction, simply tape a limp piece of white string onto your hood...when it's pointing 'straight' back at you, you're flying/driving directly "into" the wind.
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Old 07-27-2010, 05:11 PM   #9 (permalink)
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has anyone ever used a MAF or MAP sensor instead of a pitot tube? not to get air speed, but just to measure air flow, generally: e.g., you could compare the values by ducking in/out of a truck's slipstream.
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Old 07-27-2010, 05:37 PM   #10 (permalink)
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Quote:
Originally Posted by nickdigger View Post
has anyone ever used a MAF or MAP sensor instead of a pitot tube? not to get air speed, but just to measure air flow, generally: e.g., you could compare the values by ducking in/out of a truck's slipstream.
A pitot tube requires ram air AND a static pressure source (dual source) and displays the difference. Very sensitive device measuring small amount of pressure differences.

Aren't MAF and MAP single source sensors? I don't think they have enough resolution for anything beyond their intended purpose. I'm just guessing on this point though.

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