SGII MPG Calculation
 

Does anyone know how the SGII calculates instant MPG? No injector pulse information runs through the OBDII so I know they dont use that. My best guess is using the fuel/air ratio and the engine rpm but I cant figure out an equation that would work.

Here are a couple approximate ways to calculate fuel flow. http://en.wikipedia.org/wiki/Fuel_in...rom_pulsewidth

I should consider (Fuel flow rate) ≈ (throttle position) × (rpm) × (cylinders) for my bike.

Nobody I know has (or should) reverse engineer the SG, so it is anyones guess exactly how they do it. But there are plenty of threads here and on the net that discuss it and go into varying degrees of accuracy and complexity.

i was thinking that it may use gal. per hour, # of cylinders, and speed.

I don't know how it calculates MPG, but I am using trip values Xgauges for MAF and AFR and so far it looks more precise than the SGII MPC calculation. I'll know for sure in july when I take a long highway trip to Boston.

It uses airflow and calculates fuel flow by stoic ratio. Not the best, especially with varying air/fuel ratios like Tas's TDI, but good enough for standard gas engines. After initial calibration, mine's within 1-2% every time.

This information is from Scangauge, by way of Wayne at Cleanmpg.

I don't know the math but I do know that it always is a little low when compared to actual use when calc'd at the fillup. Heaven forbid I should forget to enter the new tank when filling up. It will take 2 tanks to get it straightened out. I really just use it as a yardstick to hunt for the highest numbers, mpg, while driving which in the end give me the best mpg when calc'd per tank. It has worked so far for someone who is tech challenged.

homeworkhome53

Once you get it calibrated, write down the offset on the fillup screen. You can re-enter that later and you're all set. Just do the fillup process, and then do it again, and the 2nd time you can adjust the % offset.

I do this every time I move the unit between cars, which is fairly frequent.

It's a heavily guarded secret, kept in the same safe as the Colonel's recipe for extra crispy.

Thanks

homeworkhome53

It depends on whether your car has a MAF sensor or not.

MAF Sensor:

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
= 710.7 * VSS / MAF

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

No MAF (Uses MAP and Absolute Temp to approximate MAF):

IMAP = RPM * MAP / IAT
MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

MAP - Manifold Absolute Pressure in kPa
IAT - Intake Air Temperature in Kelvin
R - Specific Gas Constant (8.314 JM/K)
MM - Average molecular mass of air (28.97 g/M)
VE - volumetric efficiency measured in percent
ED - Engine Displacement in liters

This method requires tweaking of the VE for accuracy.

I have no idea if these are the methods the SGII uses. The accuracy will be off due to approximations (AFR ~14.7, etc). All the information is from the brilliant mind of Bruce Lightner. :thumbup:

- LostCause

i don't know if the scangauge can read the injector pw info, but it is available from the obd2 port

Are you saying that OBD2 carries injector pulse-width info? Really? Are you sure? Because every OBD2 reference I've come across seems to omit that information (at least as far as the general OBD2 standard is concerned). Can you point me to a reference which indicates otherwise? It would be very helpful.

It seems that certain makers provide the info, but in a propietary manner that's not easy to get at. I think a helpful summary of the situation is here: http://www.mp3car.com/vbulletin/engi...ctor-duty.html

well i'm a ford tech so i can see it on ford scan tools, like the NGS, WDS, and IDS. I don't have much experience with aftermarket scan tools besides a scangauge. So i know its there, maybe aftermarket companies have not figured out the software or ford will not let that out.

I'm speaking only of fords because thats what i work on, other manufactures maybe different i'm not sure

Thanks for the quick response.

OK, that fits what I thought (as expressed in that other thread I cited), that certain makers provide it, but they do it in a proprietary manner that's not easy to read unless you get them to tell you how to do it.

I am a professional automotive tech and I have experience with several aftermarket obd2 scantools. obd2 does provide fuel pw on most models. Im not sure if the device in question uses it, but it should in order to properly calculate fuel usage. It might however just use rpm and load and mph readings to calculate a rough estimated instant fuel usage figure. The complications of using the pw to calculate the fuel usage are injector latency injector size and fuel pressure. The latency is the ammount of time it takes the injector to open and close when the signal goes to it from the ecu. The injector size is used to calculate the flow rate based on the pressure differential over the injector. The fuel pressure, while on most cars should be between 40 and 45 psi, is variable and most cars, almost all in fact do not have a fuel pressure sensor that could be logged to use it in propper calculations for fuel usage. Some cars run at higher fuel pressure, some lower, making it hard for any aftermarket device to properly work on all vehicles.

Some of the obd2 scan tool type devices i have used are...

Snap-On Solis - Awesome setup, obd0, obd1,obd2,abs,airbags,everything

AutoEnginuity Cable- Again awesome setup, primarily obd2

Tactrix Cable- Unbelievable logging capability, samples roughly 12 times per second when used on obd2 mitsubishis with the evoscan program and uses the factory communication protocols to get data that isnt even supported by obd2

CarChip- great tool for logging and testing intermittant issues

I think your comments are very helpful; thanks for chiming in.

But my understanding is that this is done in a proprietary manner. Which means the data is hard or impossible to get at unless you pay a license fee to the manufacturer. Which I think is what scantool-makers generally do. But not SG.

I'm pretty sure SG does not read injector data.

I thought that most (all?) EFI cars have a fuel pressure regulator, to make sure that pressure across the injector is constant.

But I think that for any particular vehicle, fuel pressure is a constant. Which means that it should only be necessary for the user to enter a parameter which represents the dynamic flow rate for his injectors. (The idea of dynamic flow rate, as compared with static, is that the former takes latency into account.)

That parameter could be found in various ways, including trial and error. But once it's found, it should not be necessary to worry about fuel pressure. It already takes fuel pressure into account. Assuming there is a fuel pressure regulator that holds fuel pressure constant.

I have a feeling that when these tools show you injector data, the data is delivered via "factory communication protocols." In other words, I think the injector data is not "supported by obd2."