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It takes the only 2 inputs needed; current injector duty cycle and current speed, and does a simple calculation to determine immediate MPG. It might even just use a simple lookup table. X duty cycle at Y speed = MPG. Now all you do is average that out over time to get your average MPG.douglee25 said:On my BMW M3, it not only has the avg. MPG but it also calculates the realtime MPG with a sweeping gauge. Anyone know the how that works?
Doug
Mileage is not in the OBDII specification. If there is a reader that calculates MPG, it will calculate it this same way, just by reading the data from the ECU, rather than getting it directly from the sensors.Oznog said:The pulse width is the most practical way to do it if you do not have OBD2. I believe there are off the shelf products to read mpg from OBD2 if you have it.
RPM has no bearing on MPG (except for your note below), only duty cycle and speed. Speed refers to the distance the car travels, not the engine speed. So you still need two inputs to get MPG.You do not need 2 inputs for the pulse width thing. The injector signal's frequency shows both RPM and duty cycle.
This is a good point. Though, it seems like a pretty simple matter to remove the rampup time from each on pulse in the period.It is imperfect in that you have no information on how the injector fires, no calibration. In the real world the solenoid takes a finite time to turn on and off the fuel stream. This will mean the flow at say 25% duty will be different at different RPM since the turn-on and turn-off times become a larger percentage of the pulse when you have narrower pulses.