MP3Car.com - View Single Post - Calculating MPG from VSS and MAF from OBD2

lightner · 09-21-2007, 02:34 PM

Quote: Originally Posted by maeliosa

Hi, I'd like to add instantaneous fuel efficiency (MPG - miles per gallon) to OBD Gauge for Palm. I've only been able to find a single mention of how to calculate this from an article at Circuit Cellar http://www.circuitcellar.com/adverti...ghtner-183.pdf .

There, Bruce Lightner gives the formula:

MPG = (14.7 * 6.17 * 4.54 * VSS * 0.621371) / (3600 * MAF / 100)
= 710.7 * VSS / MAF

MPG - miles per gallon
14.7 grams of air to 1 gram of gasoline - ideal air/fuel ratio
6.17 pounds per gallon - density of gasoline
4.54 grams per pound - conversion
VSS - vehicle speed in kilometers per hour
0.621371 miles per hour/kilometers per hour - conversion
3600 seconds per hour - conversion
MAF - mass air flow rate in 100 grams per second
100 - to correct MAF to give grams per second

Is this what other software uses to calculate MPG? Are there other methods?

I just got my OBD2 All-In-One yesterday, and have just started playing around with it and OBD Gauge. I'm going to try and add the above formula to OBD Gauge and see what it gives. I'd really appreciate any info. Thanks

For the record, the first "one-line" 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 OBD-II 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 fuel-air-ratio 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 gasoline-powered 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 "fill-up" 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 air-flow" (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 (4-cycle!) 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.

Best regards,

Bruce D. Lightner
lightner@lightner.net
http://www.lightner.net/lightner/bruce/

09-21-2007, 02:34 PM	#9
lightner Newbie Join Date: Sep 2007 Posts: 2	Let's Get the MPG Formula Right! Quote: Originally Posted by maeliosa Hi, I'd like to add instantaneous fuel efficiency (MPG - miles per gallon) to OBD Gauge for Palm. I've only been able to find a single mention of how to calculate this from an article at Circuit Cellar http://www.circuitcellar.com/adverti...ghtner-183.pdf . There, Bruce Lightner gives the formula: MPG = (14.7 * 6.17 * 4.54 * VSS * 0.621371) / (3600 * MAF / 100) = 710.7 * VSS / MAF MPG - miles per gallon 14.7 grams of air to 1 gram of gasoline - ideal air/fuel ratio 6.17 pounds per gallon - density of gasoline 4.54 grams per pound - conversion VSS - vehicle speed in kilometers per hour 0.621371 miles per hour/kilometers per hour - conversion 3600 seconds per hour - conversion MAF - mass air flow rate in 100 grams per second 100 - to correct MAF to give grams per second Is this what other software uses to calculate MPG? Are there other methods? I just got my OBD2 All-In-One yesterday, and have just started playing around with it and OBD Gauge. I'm going to try and add the above formula to OBD Gauge and see what it gives. I'd really appreciate any info. Thanks For the record, the first "one-line" 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 OBD-II 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 fuel-air-ratio 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 gasoline-powered 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 "fill-up" 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 air-flow" (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 (4-cycle!) 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. Best regards, Bruce D. Lightner lightner@lightner.net http://www.lightner.net/lightner/bruce/