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Re: The gauge works (archive)

[ Follow Ups ] [ 5-series (E34) Message Board ] [ Msg. Board FAQ ]

Posted by paul on November 05, 1999 at 15:15:55:

In Reply to: Re: The guage works like this - posted by Andre on November 04, 1999 at 09:43:51:

Andre is correct.
Clearly both fuel flow and distance traveled are a component of "miles per gallon" and need to be present to operate the gauge. Offering a copy of the diagnostic manual does not change this mathematical relationship nor prove any unspoken technical expertise.


: I have to disagree. If you calculate the estimated fuel flow at ~2000 rpm under full throttle from a standing start, you would find that it is equal to the fuel flow at cruise at ~3000 rpm, which is about 80 mph. I don't have the reference texts to do the calcs now, but any complete treatment of engine dynamics and control will illustrate this. Now look at the guage at these conditions - is it the same? No.

: The guage's reaction to a change in throttle positions supports both a short term averaging AND that it does not simply follow fuel flow. When you stomp on the accellerator, the fuel flow almost instantly (milliseconds) goes to max fuel flow from some lower cruise value (the injector duty cycle increases within 4 ms on my car, based on oscilliscope readings). The guage does not react all at once, but goes down _much_ slower. 4 secs may be too high, but nothing in the guage's behaviour or electronics (I have measured the inputs / outputs of the DME, trans computer, and instrument panel so I can change a few things) supports a simple fuel flow meter.

: Like it or think it's cheesy, the guage is not something BMW got out of the JC Whitney catalog.

: : I don't think this is right. The MPH have nothing to do with this MPG indication. It's solely based on the quantity of fuel that's been injected. The indication you see on the MPG meter is directly related to this signal. It is actually a MPG scale put on a fuel flow meter.
: : And there's definetely no 4 sec averaging/response time involved. Any change in trottle opening/load on the engine gives an immediate change in readout.
: : Luc.

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