09032010, 04:53 AM

#1 (permalink)

EcoModding Apprentice
Join Date: Aug 2008
Location: UK
Posts: 104
Thanks: 3
Thanked 7 Times in 5 Posts

outside air temperature and mpg
I have been logging the fuel economy and outside air temperature for a month or so; and thought I'd try to make sense of it. There's a nice trend, a linear fit gives mpg = (0.4*temperature) +70. Nice mix of imperial mpg and degrees centigrade  that's just what we use in the UK!
I found some figures for the "dynamic viscosity" of air and so compared figures for 10 degrees C and 20 degrees C.
Air  Absolute and Kinematic Viscosity
The effect on economy is 6% (78.5 and 74mpg (imp) at 20C and 10C respectively) ... and the viscosity changes by 5.3% (3.44 and 3.5).
Convincing  provided I've chosen the right type of viscosity...



Today



Other popular topics in this forum...



09032010, 09:17 AM

#2 (permalink)

Left Lane Ecodriver
Join Date: Aug 2008
Location: Buffalo, NY, USA
Posts: 2,258
Thanks: 79
Thanked 280 Times in 193 Posts

The benefits are more than just aero. There's faster warmup times, warmer tires, hotter air charge, and less frequent rain.
It is not necessary to concern yourself with viscosity. Aerodynamic drag varies linearly with air density. Without a datalogger, it's difficult to know how much energy you spend overcoming aerodynamic drag, versus inertia and rolling resistance, but you can use the Tools > Calculator link at the top of the page to concoct an estimate for steadystate driving (zero inertial loads).



09032010, 10:05 AM

#3 (permalink)

Hypermiler
Join Date: Dec 2007
Location: Texas
Posts: 2,290
Thanks: 553
Thanked 406 Times in 266 Posts

I concur that there is a linear relationship between temperature and mpg. Here are my results. This is 3 years of daily commuting, so it's as consistent as the real world can be. mpg is averaged at each point.
__________________
11mile commute: 100 mpg    Tank: 90.2 mpg / 1191 miles



09052010, 08:36 PM

#4 (permalink)

...beats walking...
Join Date: Jul 2009
Location: Tucson, AZ
Posts: 6,142
Thanks: 175
Thanked 1,477 Times in 1,089 Posts

...FWIW, I have 8months worth of AM (to work) and PM (to home) mpgvstemperature data showing similar results.
...there's a definite trendline difference:
• AM is cooler with slightly less traffic (at 0600 AM).
• PM is hotter with more traffic (at 17001800 PM).
...overall altitude difference is 220feet over 26.2 miles, all freeway at 65 mph.



09062010, 03:12 AM

#5 (permalink)

Pokémoderator
Join Date: Dec 2007
Location: Southern California
Posts: 5,864
Thanks: 439
Thanked 524 Times in 351 Posts

ecoxantia 
I agree that IAT is an important component of MPG. Here is what I found :
ElmScan + Customized Scantool Software = data logging!
http://ecomodder.com/forum/187883post20.html
Quote:
Ok, I now have a workable formula, thanks to post #9 :
Calculating MPG from VSS and MAF from OBD2  MP3Car.com
Quote:
... For the record, the first "oneline" MPG formula above, taken from my Circuit Cellar article, is off by 100! The "4.54" should in fact be "454". The correct formula is:
MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
MPG = 710.7 * VSS / MAF
Note that OBDII VSS reading is in kilometers/hour and MAF reading is grams/sec times 100.
This formula works very well in a modern automobile because the engine computer spends almost 100% of its time managing the fuelairratio to 14.7, which it can do very well because of the "closed loop" feedback from the O2 sensor(s).
In fact, the accuracy of this method has been proven in literally tens of thousands of gasolinepowered vehicles. Accuracy within a few percent is typical, often limited by the accuracy of the vehicle speed reading (i.e., VSS).
As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fillup" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine, MAF can be calculated from RPM, MAP and IAT. With VE, one can use the following formulas to calculate a synthetic "mass airflow" (MAF) in grams per second, all without a MAF sensor, using the "Ideal Gas Law", as follows:
IMAP = RPM * MAP / IAT
MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)
where manifold absolute pressure (MAP) is in kPa, intake air temp (IAT) is in degrees Kelvin, R is 8.314 J/°K/mole and the average molecular mass of air (MM) is 28.97 g/mole. Note that, in the above formula, the volumetric efficiency of the (4cycle!) engine is measured in percent and the engine displacement (ED) is in liters.
The VE of my 1999 7.4L Chevy Suburban is about 65%. Smaller, higher performance engines can have VE's of 85% or higher.
...


CarloSW2



11102012, 11:07 AM

#6 (permalink)

EcoModding Lurker
Join Date: Nov 2012
Location: Chicago
Posts: 2
Thanks: 0
Thanked 0 Times in 0 Posts

Quote:
Originally Posted by PaleMelanesian
I concur that there is a linear relationship between temperature and mpg. Here are my results. This is 3 years of daily commuting, so it's as consistent as the real world can be. mpg is averaged at each point.

I know this was from a while ago, but would you be able to post the data for your graph, in a spreadsheet or equivalent?



