MPG Calculation Question

JustinGray · 03-01-2005, 11:39 AM

Ok, so i was wondering how exactly the ecu calculates MPG. I assume it is a function of Pulsewidth, and speed . What i am wondering though, is it also a function of fuel pressure or does it just assume proper fuel pressure?

I ask because i am going to install the new turbo kit... now available for insights.... it uses a rising rate fueling system to increase fuel flow while on boost. I am hoping/wondering that the ecu takes fuel pressure into account when calculating MPG's cause then even when on boost my MPG's will be accurate!

I know that the car measures fuel pressure already, cause you can get it off an obdII scanner (which i am now going to pick up!). Heres to hoping the ecu uses fuel pressure in its calculation...

Rick · 03-02-2005, 06:19 PM

To my understanding it measures exhaust gas flow, which is porportional to fuel use. I do know that Willie's EFI (extra fuel injector) system does not take the 4th injector in to account.

Insightful Trekker · 03-02-2005, 07:43 PM

No fuel pressure is used in the calculation. Nor do I see on our Insights or know of any Honda that has a sensor for fuel pressure.

Fuel tank "pressure" is used under OBDII for EVAP leak testing. And is different from fuel pressure at the injectors.

My suspicion is that MPG is calculated by adding injector on time, which the ECM has to calculate anyway for correct fuel delivery. And since delivery pressure is regulated relative to MAP and the nozzle size is known, on time directly translates to a precise amount of fuel delivered. You simply have to add it up!

HTH!

Mike Dabrowski 2000 · 03-02-2005, 08:19 PM

I could be wrong, but to my understanding the fuel pump on any fuel injected motor puts out a constant pressure and is self regulating. If this is true, the fuel injector pulse width, rpm (pulse frequency), and possibly fuel temperature would be all that needs to be considered to compute MPG.
Mike

Insightful Trekker · 03-02-2005, 09:30 PM

The "pressure" must be regulated relative to the pressure in the manifold. Else a variable amount of fuel will be delivered given the same on time. E.g. a closed throttle plate will cause a lower manifold pressure and a larger amount of fuel would be delivered given constant fuel pressure and injector on time.

Of course it depends on where the injectors are located relative to the throttle plate. Most MPI (multi port injection) systems have one injector per cylinder and are located "in" the manifold. TBI (throttle body injection) uses a single or dual injector above the throttle plate.
Pressure also varies in a TBI system due to the venturi effect and must be accounted for.

Fuel temperature is a negligible effect and well within the dynamic range of the feedback system (O2 sensor range).

HTH!

JustinGray · 03-03-2005, 08:31 AM

Sad to hear that.... wish the car could measure the increased fuel pressure from the FMU and adjust the MPG accordingly. I knew it was a long shot.

So i guess that when you go into lean burn the ECU just chops your pulse widths down. I know that when injector pulse width gets too small you get inacurate fuel delivery. I was hoping that honda got around this problem by letting the fuel pressure drop for lean burn... guess not!

Oh well