Here is the thing: I'm trying to estimate the fuel consumption of cars using OBDII data. It's quite simple if we can treat each fuel and its stoichiometric air fuel ratio separately, but when it turns to flex fuel cars I can't get a conclusion.

Is there any sensor that tells how much of each fuel is running?


How the instant fuel consumption is calculated for flex cars?

Please help.

  • I assume you're talking about cars that have 2 fuel systems (e.g. petrol and LPG)?
    – Hobbes
    Commented Sep 9, 2016 at 8:43
  • @Hobbes No, I'm talking about cars that runs with gasoline and alcohol at the same time, ie you can mix it. Commented Sep 11, 2016 at 4:05

1 Answer 1


"Flex Fuel" isn't an issue (I assume you mean E85 and similar) if you use injector duty cycle as the basis of your calculations. Total injector "on" time gives you fuel consumption, regardless of the AFR, or even if it was a stoichiometric burn.

If you are trying to find live efficiency data, this becomes more complex.

But simple fuel consumption is only a matter of injector(s) rate (cc/min, lb/hr, etc) and how long the injector stays open. Fuel injected can be assumed "consumed" even if not fully burned.

I do agree, that if you need to know exact number of moles of oxygen for your "flex" hydrocarbon juice, this will be nearly impossible. E85, for instance, is a variable blend of corn liquor moonshine (errrr, I meant ETOH or ethyl alcohol) and various x-tanes of "gasoline" hydrocarbons, along with some hygroscopically absorbed water. Problem is, goverment controls on E85 are not all that tight, nor is there much enforcement. I'm big into extreme turbocharging, and E85 is a great way to combat intake heat. But some E85 may be as low as 70% moonshi... ETOH. Bottom line, you need to know the number of carbon atoms in every style of hydrocarbon to determine ultimate stociometric burn, but even this is hotly debated as "stochio" burn does NOT mean the highest power or fuel efficeiency. But I'm rambling, and I don't think this is what you asked...

Also note that E85 ETOH blend (and "top-fuel" methanol) has a lower BTU content than pure "gasoline", for identical quantities. The value is in the higher compression and BMEP that can be achieved. Also also note that true E85 has an ideal A/F ratio of about 9.8. Clearly more gasoline drives this number higher, and more ETOH drives it lower. Virtually impossible to know exactly the true ratio of the blended "fuel", and the appropriate A/F ratio.

Still, consumption is # injectors X injector rate X IDC ("on time")

Fuel economy (efficiency) is mileage over the measured time period / consumption

(Which yields something along the line of "MPG" ... it's the reciprocal if you live where liters/100 kilometers is the normal unit...)

  • You still need to include the amount of fires per time period in your equation, don't you? Commented Sep 9, 2016 at 11:16
  • No, you don't. There's a common misconception that injectors only inject when the intake valve is open. Injector duty cycle tells all. IDC at 100% means the injector is wide open (which is very bad, because you can't get any richer and you might go lean). If you know the amount of time each injector is injecting, and the injector rate, you know "fuel" consumption. The only issue that the OP alluded to was that different hydrocarbons require different amounts of oxygen to oxidize. I mentioned this, but @dlu edited out that part as it wasn't clear. I need better skills!
    – SteveRacer
    Commented Sep 10, 2016 at 3:52
  • 1
    Dang did I get it wrong. A word that good deserves to be spelled correctly. Especially in such an esteemed forum.
    – dlu
    Commented Sep 10, 2016 at 4:00
  • 2
    @dlu yeah. I resmeble the remark. Reminds me of a sex scene from Bull Durham which can't be quoted here. Distilled: Do you want me to fix your car, or do you need me to spell "stochiometric" correctly??
    – SteveRacer
    Commented Sep 10, 2016 at 4:07
  • 1
    @dlu - Never, ever, trust autocorrect ... it just ... well ... isn't. It's got the "auto" in the bag ... but correct? Nope. Commented Sep 10, 2016 at 23:08

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