This is what happens to a algorithm that uses pulse width, along with other engine measurements like rpm, tps, map/maf, O2. It is making inferences, and not measuring actual flow of fuel- ccm/m. If it knew the size of the injector (fuel flow capability) and the pulse width, it would be more likely to be accurate with lean burn. Just my thoughts.
Yeah, that's the problem (at least as I understand it): the SGII isn't actually measuring fuel consumption directly, it's inferring it from other measurements and some assumptions, like the A/F ratio. If your engine doesn't follow the assumptions it uses, the values will be wrong.
... For the record, the first "one-line" MPG formula above, taken from my Circuit Cellar article, is off by 100! The "4.54" should in fact be "454". The correct formula is:
Note that OBD-II VSS reading is in kilometers/hour and MAF reading is grams/sec times 100.
This formula works very well in a modern automobile because the engine computer spends almost 100% of its time managing the fuel-air-ratio to 14.7, which it can do very well because of the "closed loop" feedback from the O2 sensor(s).
In fact, the accuracy of this method has been proven in literally tens of thousands of gasoline-powered vehicles. Accuracy within a few percent is typical, often limited by the accuracy of the vehicle speed reading (i.e., VSS).
As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fill-up" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine, MAF can be calculated from RPM, MAP and IAT. With VE, one can use the following formulas to calculate a synthetic "mass air-flow" (MAF) in grams per second, all without a MAF sensor, using the "Ideal Gas Law", as follows:
where manifold absolute pressure (MAP) is in kPa, intake air temp (IAT) is in degrees Kelvin, R is 8.314 J/°K/mole and the average molecular mass of air (MM) is 28.97 g/mole. Note that, in the above formula, the volumetric efficiency of the (4-cycle!) engine is measured in percent and the engine displacement (ED) is in liters.
The VE of my 1999 7.4L Chevy Suburban is about 65%. Smaller, higher performance engines can have VE's of 85% or higher.
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