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Thread: how to calculate diesel fuel consumption?

  1. #1
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    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?

  2. #2
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    Our downloads page has a reference sheet for all of the CALC PIDs, this should explain how everything is calculated.
    http://www.palmerperformance.com/downloads.php

  3. #3
    Creator - OBDuino Magister's Avatar
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    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?

  4. #4
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    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.

  5. #5
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    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
    Thanks in advance.

  6. #6
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    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).

  7. #7
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    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?

  8. #8
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    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.

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