Originally Posted by fsae150

Sorry, wasn't paying attention when I posted earlier and I thought you were talking about an individual trying to calculate fuel flow based off IDCs and fuel rail pressure recorded from a obd II dongle, not the PCM calculations. That is more difficult because you don't necessarily know all of the injector characterization, such as flow curves, latency, etc., and even though the PCM calculates all of this very rapidly internally, it is computationally expensive to send it out to the rest of the CMs, so the values reported to the obd II network are nut updated as frequently as the internal calculations. However, yes, the fuel flow rate is calculated by the PCM fairly accurately, but in the reverse order of what you said. The PCM measures or estimates air flow first, then calculates the required fuel flow to achieve a target AFR and then calculates injector duty cycles required to achieve this. There are a lot of sources of error here, such as system dynamics and injector flow variation, so there are additional fuel trims applied to make sure the target AFR is actually achieved.

Fuel rail pulsations can still make an impact on fuel delivery because even though you can measure FRP at high speed, it is only measured at one spot. You get waves travelling through the fuel rail as a result of not only the fuel pump and various control valves, but also injector opening and closing, which can cause fuel pressure to fluctuate at each injector. Additionally, if you are running WOT, you can run into the issue of injectors flowing so much fuel that some injectors (especially the injectors furthest from the rail inlet) can run out of fuel. All of these issues make it very difficult for the lamen to calculate fuel flow rate based on IDCs and FRP, and you would be much better off using AFR and air flow to back out fuel flow. When real fuel flow rate measurements are required, like doing engine development work on a dyno, a fuel flow meter is installed.

I'm not sure exactly what Ford does to calculate MPG in our trucks.

Haha sorry to the OP for getting this thread so sideways!

Originally Posted by BlackBoost

There's too many variables that can't be accounted for to make it 100% accurate.


I think they should just have the green bar showing 0-30 MPG's but with no numbers. Just something that gives you an idea of what kind of mileage you're getting so that people can't complain. Either that or remove the decimal numbers.


I mean this screen:



Just remove all the MPG numbers and have a graphic instead, along with the DTE. Problem solved.