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Discussion Starter · #1 ·
Just curious how this onboard device work.

The distance that a car run can be accurately counted
by knowing the diameter of tire and # of time revolutions.
But how is the amount of burnt gas measured? The onboard
fuel level display is very rough according to my experience.
Or the consumed gas is metered through the fuel injector
pulse width?
 

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These days fuel flow is indirectly measured by the PCM knowing the amount of air entering the engine via the MAF. Since the engine is running closed loop fuel control 99.5% of the time, the air/fuel ratio sits at 14.7:1 nearly all the time, so it is easy for the PCM to calculate amount of fuel based on anount of air entering the engine.

Back in the 80s, fuel flow for what Ford called the "Tripminder" was calculated based on injector pulse width and the known injector flow rate. Personally, I think the older system gave more accurate numbers than the later air flow based systems.
 

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Discussion Starter · #5 ·
I agree with you. The old type fuel consumption measurement is
more accurate by knowing the real-time fuel pressure and pulse width.

The indirect air mass based measurement purely depends on the
accuracy of MAF sensor. It seems a lot of MAF sensor tried to
over-estimate the amount of air passing it, resulting in a larger
measurement value of air mass. Therefore the calculated MPG
is always lower than it actually is.

In most interstate driving, when cruise control is set and the car
is run constantly at a certain speed, the fuel control is close-looped.
However, in most city driving when the gas pedal has to be adjusted
frequently according to road and traffic conditions, the fuel control
is probably in open loop. Just my 2 cents.

These days fuel flow is indirectly measured by the PCM knowing the amount of air entering the engine via the MAF. Since the engine is running closed loop fuel control 99.5% of the time, the air/fuel ratio sits at 14.7:1 nearly all the time, so it is easy for the PCM to calculate amount of fuel based on anount of air entering the engine.

Back in the 80s, fuel flow for what Ford called the "Tripminder" was calculated based on injector pulse width and the known injector flow rate. Personally, I think the older system gave more accurate numbers than the later air flow based systems.
 

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Modern OBDII cars are in closed loop most of the time. Only WOT or very near causes open loop. Newer cars, with "air fuel ratio" sensors, AKA wide band O2 sensors are starting to be in closed loop virtually 100% of the time.
 
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