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Old 10-09-2008, 08:19 PM   #10 (permalink)
cfg83
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Join Date: Dec 2007
Location: Southern California
Posts: 5,864

1999 Saturn SW2 - '99 Saturn SW2 Wagon
Team Saturn
90 day: 40.49 mpg (US)
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Hello -

(apologies in advance for some of my "custom" acronyms. I am "programming" the variable names in the post)

I am trying to determine an approximate Instant MPG calculation for my scantool software. All of the following is based on this OBD 2007 documentation blurb :

OBD 2007 - Installation Guide
http://www.glmsoftware.com/Documenta...de20080822.pdf - Page 15 of PDF
Quote:
fuel consumption can be calculated, if a vehicle supports Pid 0x0D Vehicle Speed and Pid 0x10 Mass Air Flow. All vehicles support vehicle speed and nearly all vehicles support MAF.

MAF, the mass of air moving into the engine is measured in grams per second. A petrol/gasoline engine burns the air/fuel mixture at the stoichiometric air fuel ratio of 14.64:1. The oxygen sensors in a modern vehicle are designed in conjunction with both the short and long term fuel trims to maintain this ratio. Under ideal conditions for every 14.64 grams of air an engine will consume 1 gram of petrol. We can therefore calculate FuelFlowGramsPersSec = MAF/Air Fuel ratio. We can convert the previous figure to FuelFlowLitresPerSecond by dividing FuelFlowGramsPerSec by the density of petrol/gasoline.

We can then readily convert that figure to FuelFlowLitresPerHour -- i.e. litres/hour. Fuel consumption is then simply SpeedKmsPerHour/FuelFlowLitresPerHours -- i.e. litres/kms.
Here are my inputs :

- Intake Manifold Pressure (MAP) units: inHg
- Vehicle Speed units: MPH

Here are my constants/conversions :

- AFR = Air/Fuel Ratio = 14.64
Source: http://www.glmsoftware.com/Documenta...de20080822.pdf

- "DOG" = Density of Gas (g/cm3) = (0.71 +0.77) / 2 = 0.74
Source: Gasoline - Wikipedia, the free encyclopedia

- "M2K" = Convert Miles to Kilometers = 1.609344
Source: Convert miles to km - Conversion of Measurement Units

- "KML2MPG" = Convert KM/L to MPG = 2.35214583
Source: WikiAnswers - How many km to 1 litre of gas

- "SPH" = Seconds Per Hour = 3600

- "MPL" = Milliliters Per Liter = 1000

Steps to convert with example inputs from real OBD II data logging :

- Intake Manifold Pressure (inHg) = 17.542
- Vehicle Speed (MPH) = 52.533

1 - Convert MPH to KMH
=> MPH * M2K = 52.533 * 1.609344 = 84.54366835 KMH

2 - "FFG" = Calculate FuelFlowGrams/Sec
=> MAP / AFR = 17.542 / 14.64 = 1.198224044 FFG

3 - "FFL" = Calculate FuelFlowLitres/Second
=> (FFG / DOG) / MPL = (1.198224044 / 0.74) / 1000 = 0.001619222 FFL

4 - LPH = Calculate Liters Per Hour
=> FFL * SPH = 0.001619222 * 3600 = 5.829198051 LPH

5 - KML = Calculate KiloMeters / Liter
=> KMH / LPH = 84.54366835 / 5.829198051 = 14.50348189 KML

6 - Calculate MPG
=> KML * KML2MPG = 14.50348189 * 2.35214583 = 34.11430445 MPG

The MPG number looks "ballpark" correct to me, subject to programming and testing. The most iffy part to me is step 3, where I divide by 1000 to get the "FFL". I am doing this because in step 2, it is not clear that the FFG is in Liters.

Also, the OBD 2007 algorithm calls for a MAF sensor. In my S-Series Saturn, I *think* I have a MAP sensor. My question here is, what's the difference? Do I need to convert from MAP to MAF?

Based on the steps above, here is the output I get. This is based on the data logging from thread #1 :

Code:
,,INPUT,INPUT,STEP 1,STEP 2,STEP 3,STEP 4,STEP 5,STEP 6,
YYYMMDD,HHMMSS,MAP (inHg):,Speed (MPH):,Speed (KMH),FuelFlowGrams/Sec ,FuelFlowLitres/Second,Liters per hour (LPH),KMH/LPH,MPG
20080407,201137,17.542,52.533,84.54366835,1.198224044,0.001619222,5.829198051,14.50348189,34.11430445
20080407,201242,11.647,54.867,88.29987725,0.795560109,0.001075081,3.870292424,22.81478183,53.66369394,
20080407,201348,19.197,55.35,89.0771904,1.311270492,0.001771987,6.379153744,13.96379425,32.84488043,
20080407,201455,15.507,53.233,85.67020915,1.059221311,0.00143138,5.152968542,16.62540892,39.10538626,
20080407,201601,21.542,53.817,86.61006605,1.471448087,0.001988443,7.158396101,12.09908824,28.45881994,
20080407,201709,17.98,55.133,88.72796275,1.228142077,0.001659651,5.974745237,14.85050144,34.93054504,
20080407,201814,12.02,53.733,86.47488115,0.821038251,0.001109511,3.994240142,21.64989537,50.92371111,
20080407,201919,12.173,50,80.4672,0.831489071,0.001123634,4.045081967,19.89260061,46.79029757,
20080407,202023,14.567,52.8,84.9733632,0.995013661,0.001344613,4.840607,17.55427846,41.29022288,
20080407,202128,14.065,52.383,84.30226675,0.960724044,0.001298276,4.673792645,18.03722868,42.42619223
20080407,202234,15.575,49,78.857856,1.06386612,0.001437657,5.175564909,15.2365698,35.83863412
20080407,202339,14.21,50.017,80.49455885,0.970628415,0.00131166,4.721976074,17.04679515,40.09654813
20080407,202446,16.237,44.483,71.58844915,1.109084699,0.001498763,5.395547187,13.26806099,31.20841433
20080407,202549,16.167,50.583,81.40544755,1.104303279,0.001492302,5.372286221,15.15285005,35.64171307
20080407,202653,13.59,42.583,68.53069555,0.928278689,0.001254431,4.515950377,15.17525434,35.69441122
20080407,202758,15.017,43.383,69.81817075,1.025751366,0.00138615,4.990141781,13.99121985,32.90938943
20080407,202901,15.563,45.067,72.52830605,1.063046448,0.001436549,5.171577315,14.02440718,32.98745088
20080407,203003,15.853,46.233,74.40480115,1.082855191,0.001463318,5.267944174,14.12406789,33.22186739
20080407,203104,15.34,44.533,71.66891635,1.047814208,0.001415965,5.097474524,14.05969093,33.0704434
20080407,203207,16.358,48.1,77.4094464,1.117349727,0.001509932,5.435755428,14.24078906,33.49641259
20080407,203307,17.822,48.517,78.08054285,1.217349727,0.001645067,5.922241914,13.18428797,31.01136798
20080407,203409,17.785,48.25,77.650848,1.214822404,0.001641652,5.909946832,13.13900957,30.90486658
20080407,203515,23.225,52.183,83.98039795,1.586407104,0.002143793,7.717656181,10.88159358,25.59509496
20080407,203622,21.535,54.367,87.49520525,1.470969945,0.001987797,7.156070004,12.22671176,28.75900907
20080407,203724,15.088,41.4,66.6268416,1.030601093,0.001392704,5.013735047,13.28886369,31.25734531
20080407,203829,19.203,46.683,75.12900595,1.311680328,0.001772541,6.381147541,11.7735886,27.69319733
20080407,203934,16.125,48.533,78.10629235,1.101434426,0.001488425,5.358329641,14.57661204,34.28631722
20080407,204037,12.637,44.033,70.86424435,0.86318306,0.001166464,4.199268941,16.87537649,39.69334643
20080407,204148,11.128,48.117,77.43680525,0.76010929,0.001027175,3.697828977,20.94115378,49.25664754
20080407,204253,11.532,43.35,69.7650624,0.787704918,0.001064466,3.83207798,18.20554351,42.82209326
20080407,201137,17.542,52.533,84.54366835,1.198224044,0.001619222,5.829198051,14.50348189,34.11430445
20080407,201242,11.647,54.867,88.29987725,0.795560109,0.001075081,3.870292424,22.81478183,53.66369394
20080407,201348,19.197,55.35,89.0771904,1.311270492,0.001771987,6.379153744,13.96379425,32.84488043
20080407,201455,15.507,53.233,85.67020915,1.059221311,0.00143138,5.152968542,16.62540892,39.10538626
20080407,201601,21.542,53.817,86.61006605,1.471448087,0.001988443,7.158396101,12.09908824,28.45881994
20080407,201709,17.98,55.133,88.72796275,1.228142077,0.001659651,5.974745237,14.85050144,34.93054504
20080407,201814,12.02,53.733,86.47488115,0.821038251,0.001109511,3.994240142,21.64989537,50.92371111
20080407,201919,12.173,50,80.4672,0.831489071,0.001123634,4.045081967,19.89260061,46.79029757
20080407,202023,14.567,52.8,84.9733632,0.995013661,0.001344613,4.840607,17.55427846,41.29022288
20080407,202128,14.065,52.383,84.30226675,0.960724044,0.001298276,4.673792645,18.03722868,42.42619223
20080407,202234,15.575,49,78.857856,1.06386612,0.001437657,5.175564909,15.2365698,35.83863412
20080407,202339,14.21,50.017,80.49455885,0.970628415,0.00131166,4.721976074,17.04679515,40.09654813
20080407,202446,16.237,44.483,71.58844915,1.109084699,0.001498763,5.395547187,13.26806099,31.20841433
20080407,202549,16.167,50.583,81.40544755,1.104303279,0.001492302,5.372286221,15.15285005,35.64171307
20080407,202653,13.59,42.583,68.53069555,0.928278689,0.001254431,4.515950377,15.17525434,35.69441122
20080407,202758,15.017,43.383,69.81817075,1.025751366,0.00138615,4.990141781,13.99121985,32.90938943
20080407,202901,15.563,45.067,72.52830605,1.063046448,0.001436549,5.171577315,14.02440718,32.98745088
20080407,203003,15.853,46.233,74.40480115,1.082855191,0.001463318,5.267944174,14.12406789,33.22186739
20080407,203104,15.34,44.533,71.66891635,1.047814208,0.001415965,5.097474524,14.05969093,33.0704434
20080407,203207,16.358,48.1,77.4094464,1.117349727,0.001509932,5.435755428,14.24078906,33.49641259
20080407,203307,17.822,48.517,78.08054285,1.217349727,0.001645067,5.922241914,13.18428797,31.01136798
20080407,203409,17.785,48.25,77.650848,1.214822404,0.001641652,5.909946832,13.13900957,30.90486658
20080407,203515,23.225,52.183,83.98039795,1.586407104,0.002143793,7.717656181,10.88159358,25.59509496
20080407,203622,21.535,54.367,87.49520525,1.470969945,0.001987797,7.156070004,12.22671176,28.75900907
20080407,203724,15.088,41.4,66.6268416,1.030601093,0.001392704,5.013735047,13.28886369,31.25734531
20080407,203829,19.203,46.683,75.12900595,1.311680328,0.001772541,6.381147541,11.7735886,27.69319733
20080407,203934,16.125,48.533,78.10629235,1.101434426,0.001488425,5.358329641,14.57661204,34.28631722
20080407,204037,12.637,44.033,70.86424435,0.86318306,0.001166464,4.199268941,16.87537649,39.69334643
20080407,204148,11.128,48.117,77.43680525,0.76010929,0.001027175,3.697828977,20.94115378,49.25664754
20080407,204253,11.532,43.35,69.7650624,0.787704918,0.001064466,3.83207798,18.20554351,42.82209326
Pending programming and testing, do you think I am on the right track?

CarloSW2
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Last edited by cfg83; 10-16-2008 at 06:40 PM..
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