# how to calculate diesel fuel consumption?

• 08-17-2010, 11:52 AM
rs_zhou
how to calculate diesel fuel consumption?
Hi guys!

I am trying to evaluate real-time fuel consumption of diesel engine, and I have learned a post " Fuel economy / consumption for Diesel vehicles in DashCommand ", in this post,the author mentioned that

DashCommand should do fuel economy calculations for all gasoline and Diesel vehicles that comply to the OBD-II spec. ALL OBD-II complient vehicles must support the 5 PIDs that DashCommand needs for fuel economy, SAE.MAP, SAE.IAT, SAE.RPM, SAE.VSS, and SAE.LOAD_PCT. BTW, the last one is only needed for Diesel vehicles

What a useful hint for me to do my work!, however, I do not understand the true method(formulations) to use the 5 PIDs to calcuate the diesel fuel consumption.

Can anyone do me a favor to explain the formulations of the calculating?
• 08-17-2010, 01:10 PM
WestonAtPPE
Our downloads page has a reference sheet for all of the CALC PIDs, this should explain how everything is calculated.
• 12-01-2011, 04:52 PM
Magister
Yeah sorry old thread, I checked the PDF but it's not really explained what you do with LOAD and diesel :-/

Does this mean you simply multiply MAF by LOAD_PCT for diesel? MAF being ((RPM * MAP / IAT)/120)*(VE/100)*(ED)*(MM)/(R)

LOAD_PCT for a diesel engine is the "current fuel flow" / "peak fuel flow at WOT as a function of RPM" so basically LOAD_PCT for a diesel engine is the percentage of flow per a given rpm, right?
• 12-01-2011, 05:44 PM
WestonAtPPE
That is my understanding, LOAC_PCT is the percentage of maximum flow for the current RPM. I didn't personally write the fuel economy calculations for diesel, but it does just a simple multiplication with LOAD_PCT if I remember.

I don't know a ton about diesels, but just off the top of my head I would try to calculate it like this:

FuelFlow = (MassAirflow / IdealAfr) * (LoadPct / 100)

Not sure what the ideal AFR is for a diesel, but I'd try something like that first. Never owned a diesel, never tried to calculate MPG of one. Just my best guess.
• 02-15-2012, 06:21 PM
dgarrett
What is the ideal A/F ratio? Is that the A/F ratio at peak torque? My guess is something around 14.5 to 15: 1
• 02-15-2012, 06:46 PM
WestonAtPPE
For gasoline its 14.7 (:1), I don't know what the stoichiometric AFR for diesels is, you'd have to try and look it up (Google is a friend).
• 02-15-2012, 07:10 PM
dgarrett
Thanks,
I hear it is around 15:1 and diesel operate slightly rich at peak torque, less than peak torque it can be from 15:1 to 40:1. was the ideal A/R in you formula for peak torque?
• 02-15-2012, 07:28 PM
WestonAtPPE
I'm not sure diesel engines even have a stoichiometric AFR. Diesel engines handling fueling in an entirely different way than gasoline engines. The accelerator pedal dumps in more fuel, instead of moving a throttle valve. I don't think that airflow is restricted in a diesel.

I didn't write the fuel consumption formulas for our software, and I'm not entirely sure what all is involved in the calculation for diesels.