proportional fueling
02-13-2007, 10:34 AM
#2
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The fuel control loop depends on mixture feedback.
Most systems use switching mode, time above /
time below, because this plays well with the stock
style O2 sensors and is fairly robust to noise, sensor
slowness etc.
Depending on switching events to be found, in enough
quantity that they are real-time useful, wants the gas
to move above/below mixture threshold enough to make
the sensor output switch often (see insufficient
switching codes). To make this happen consistently
the PCM adds, subtracts, adds some proportionate
amount of fuel to the straight calculated shot. I call
this "dithering". By deliberately jacking it around
constantly you get a busy chattering waveform and
life is good. Sort of.
Too much "proportion" though, and you spend all
your time -too- rich or -too- lean, and little or no
time balanced on the true ideal mixture. You get
surging and stumbling, you get constant STFT
movement etc.
Too little and you may get larger trim excursions
as transitions are not found, maybe get switch
codes (but these usually come from sensors being
too cool or not hot soon enough).
If your NBO2 waveforms are pointy sawtooth shape
then you are pretty much OK. If they are squared off
and bottoming / pegging then you have too much
proportion, for the airflow you are at. See airflow
mode.
Most systems use switching mode, time above /
time below, because this plays well with the stock
style O2 sensors and is fairly robust to noise, sensor
slowness etc.
Depending on switching events to be found, in enough
quantity that they are real-time useful, wants the gas
to move above/below mixture threshold enough to make
the sensor output switch often (see insufficient
switching codes). To make this happen consistently
the PCM adds, subtracts, adds some proportionate
amount of fuel to the straight calculated shot. I call
this "dithering". By deliberately jacking it around
constantly you get a busy chattering waveform and
life is good. Sort of.
Too much "proportion" though, and you spend all
your time -too- rich or -too- lean, and little or no
time balanced on the true ideal mixture. You get
surging and stumbling, you get constant STFT
movement etc.
Too little and you may get larger trim excursions
as transitions are not found, maybe get switch
codes (but these usually come from sensors being
too cool or not hot soon enough).
If your NBO2 waveforms are pointy sawtooth shape
then you are pretty much OK. If they are squared off
and bottoming / pegging then you have too much
proportion, for the airflow you are at. See airflow
mode.
