[mdlug-discuss] OBD fuel monitor fine tuning

[Robert Meier]

Drew,

> 2007:09:16 to 2007:10:20
>      Actual use:   43.833 gallons
>      Tabulated:    40.301 gallons
>      tabulated short by:   8.1 percent

> 2007:10:20 to 2007:10:25
>      Actual use:   9.652 gallons
>      Tabulated:    8.895 gallons
>      tabulated short by:   7.8 percent

>      So the software seems to be pretty consistently short in its 
> estimate by about 8 percent, which is
> better than I had any right to expect.

Were both of the above measurements at the same fuel station?

Have you considered trying the same and different fuel stations to
discover/deny a correlation?

> Of course, I should probably get a measurement of the density of 
> gasoline (weigh a measured gallon) myself...

A gallon of avgas masses 6 pounds under specified conditions.

I understand that typical gasoline density varies from about 5.5 to 6
pounds per gallon depending on temperature (expands with rising
temperature), air pressure (expands with decreasing pressure),
humidity (expands with decreasing relative humidity).

> Is there an official, more "correct" way to apply the Fuel Trim numbers? 
> Where might it be found?

Still looking, I've been unable to find a better description than
copied by http://en.wikipedia.org/wiki/OBD-II_PIDs .

Please let me know if you find a better description.

My currently uncertain understanding is that the fuel trims (my
Saturn SL2 has only 1 bank) are PI states in the fuel metering
/oxygen sensor servo loop.
<fuel rate> = A <throttle position>
            + B <pre-catalytic-oxygen-sensor>
	    + C <post-catalytic-oxygen-sensor>
            - D <short-term-trim> - E <long-term-trim> - F
	    + G <servo-noise>
d<long-term-trim>/dt = H <short-term-trim>
                     + I <pre-catalytic-oxygen-sensor>
                     + J <post-catalytic-oxygen-sensor> - K
d<short-term-trim>/dt = L <pre-catalytic-oxygen-sensor>
                      + M <post-catalytic-oxygen-sensor> - N

So far, I've been unable to find documentary confirmation of the above,
and unable to calculate the above feedback coefficients.
So far, OBDGauge seems too slow to permit coefficient calculations.

Reporting,
-- 
Robert Meier

  "The idea that an arbitrary naive human should be able to properly use a
   given tool without training or understanding is even more wrong for
   computing than it is for other tools (e.g. automobiles, airplanes, guns,
   power saws)."
	-- Doug Gwyn