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Thread: Could this be a method to calculate diesel mpg?

  1. #11
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    Ok, I thought you were saying that the fuel consumption signal was OBD accessible.

    So it sounds to me that if you want to compute fuel consumption based on OBD-II data alone, you're gonna have to measure and reverse-engineer something. Either come up with a plot of MAF vs. TPS vs. actual fuel, or measure the flow caracteristics of the injectors. Neither will be simple.

    Do you have a chart showing all of the various data that are available from a diesel's OBD-II port? There must be some way of reading injector duration from it.

  2. #12
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    Data is described in SAE J1979, which is not available on the internet.
    Most of the data (pids) are described here: http://en.wikipedia.org/wiki/OBD-II_PIDs
    This applies to vehicles produced until 2006. In 2007 more than 50 new pids were defined which may have some information needed for this problem, but that information is not publicly available, and only used in 2007 and newer vehicles.

    With the available pids there will be problems. Pid 10 (Maf airflow) is insufficient. Maximum is 655 grams/second and big diesels may use more air than that.
    Injection duration is not present within the 2002 specification.
    Fuel temperature is also not present within this spec.
    You tell me how it should be done. I don't think it's possible for a diesel, which is what I have been saying all along.

  3. #13
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    If you want a one-size-fits-all solution, running entirely off OBD-II, with no external data inputs and no external references, then yeah, it's probably impossible.

    If you're willing to do a little bit of app-specific configuration, then I don't see it being too hard.

    Looks like the fuel pressure problem is solved for us: PID 23, Fuel Rail Pressure, Diesel (0 - 655,350 kPa) ought to cover that.

    Either PID 04, Calculated engine load value or PID 43, Absolute load value ought to be useful. I'm not sure whether they are considering "load" to be a static value (independent of RPM, IOW, reflective of instantaneous torque and thus correlating to injector duration) or a dynamic value (varying with RPM, IOW, reflective of HP and thus correlating to injector duty cycle %) but either way, you can do some empirical observation and then solve for actual injector duration based on one of 'em without needing to know airflow, TPS, or anything else.

    Is fuel temp really that big of a variable? Honest question. If it is, then I'm guessing you can solve it based on CLT and be "close enough" for our purposes.

    So let's see. Fuel pressure? Check. Injector duration? Check. Fuel temp? Check(ish).

    At that point, all you need are the lag characteristics and static mass flowrate curve of the injectors themselves, and you've got MPG.


    Still haven't found any references to an injector firing more than once per cycle. I can't imagine how that would be either physically possible or practically useful.

  4. #14
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    Have you read the article I gave the link for?
    I'll quote:
    Unlike a conventional port fuel injected petrol engine, where the amount of fuel injected can be considered to be directly proportional to the injector opening time, a diesel injector will vary in mass flow depending on the difference between the injection and combustion chamber pressures, the density of the fuel (which is temperature dependent), and the dynamic compressibility of the fuel. The specified injector duration must therefore take these factors into account.

    Diesel fuel injectors do not add the fuel for a combustion cycle in one event, instead they operate in up to four different modes. The first is pre-injection, a short duration pulse which reduces combustion noise and Oxides of Nitrogen (NOx) emissions. The bulk of the fuel is then added in the main injection phase, before the injector is turned off momentarily before then adding a post-injection amount of fuel. This post-injection reduces soot emissions. Finally, at up to 180 crankshaft degrees later, a retarded post-injection can occur. The latter acts as a reducing agent for an NOx accumulator-type catalytic converter and/or raises the exhaust gas temperature for the regeneration of a particulate filter.

  5. #15
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    Quote Originally Posted by p2psmurf View Post
    Have you read the article I gave the link for?
    Yes, though I glossed over the multi-event part. That's quite interesting, and is definitely a feature unique to common-rail systems. Learn something new every day. Doesn't matter for our purposes though if we forget about measuring pulsewidth directly and instead take load data from the ECU.


    Unlike a conventional port fuel injected petrol engine, where the amount of fuel injected can be considered to be directly proportional to the injector opening time, a diesel injector will vary in mass flow depending on the difference between the injection and combustion chamber pressures, the density of the fuel (which is temperature dependent), and the dynamic compressibility of the fuel. The specified injector duration must therefore take these factors into account.
    Yep, and if we use the load computation from the OBD-II datastream, then all this will have already been accounted for. Now that I think about it, it's even simpler than I stated above.

    We don't need to concern ourselves at all with fuel pressure and temperature, chamber pressure, yadda yadda. I started overcomplicating this after your first response, and I failed to step back and appreciate the actual simplicity of the problem. Load, be it in a petrol engine, a diesel, a jet turbine or a reaction rocket, is all about fuel mass per unit time. It really is exactly as the OP described it.

  6. #16
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    MAF on diesel engine are totally useless. Every diesel engine can run without the MAF, in fact, the only reason why there is a MAF on diesel engine is to regulate the EGR valve. A good chiptuner will do a MAF delete and a EGR delete at the same time.

    Diesel engine (with injection pump or common rail) principle is totally different than gas engine. There is no Air/fuel ratio with diesel. As soon as you inject more fuel, throttle will automatically raise. It is possible to get, with some software, some indication of the fuel consumption, but these are calculations based on some parameters that the ECU is using to run the engine efficiently. If I look at the PID standard, I would expect the ECU to determine his fuel consumption using the Throttle position and timing advance especially.

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