In reply to a message from XJRengineer sent Mon 16 Mar 2009:
Very interesting, my knowledge of the GM ODBII and its ECM would
have me believe the following. It depends on how the engine is
tuned. Clearly if tuned to run open loop in speed density mode you
would have total control of ignition timing. This is usually done
by making changes to the timing maps in the ECM.
In closed loop, the GM ECM looks at the knock sensor after each
plug is fired. If no knock is detected the timing is advanced (1/2
degree) for that cylinder the next time that plug is fired. If the
ECM hears a knock then the timing for that cylinder is retarded (1
degree) the next time that plug is fired. This process is
continuous and done individually for each cylinder. If a knock is
detected for the same cylinder 5 times in succession then that
event is recorded as a timing fault. AND � if you have the
capability to write into the ECU�s NVRAM you can change the size of
the steps that are used to advance and retard the timing.
So, in the case of the GM ECU. In closed loop, there would not be
a change in the timing if the crank position sensor were moved. On
the other hand there would clearly be a change in timing if a
higher octane fuel is used.
From your description and intrinsic knowledge of the AJ16’s ECU,
that is apparently not the case. So, to help me understand what is
really going on, what is the logic associated with the knock
sensors and their effect on timing?
One other note.
Several years ago around 1995 I built a device that could measure
the exact position of the crank as well as when (in relation to the
number of degrees after top dead center) peak combustion chamber
The goal was to dynamically adjust the timing to insure that the
plugs fired at the right time (regardless of RPM, fuel octane, or
speed of the flame front, etc.) to achieve Peak Combustion Chamber
Pressure at the best position of the crankshaft. I had a formula
to calculate the best position of the crank. It included the
stroke, length of connecting rod, and other stuff I’ve forgotten.
I do remember it was 42.5 degrees after top dead center for each
cylinder for the engine I was using as a test mule .
The real trick was measuring the peak pressure. The way I did this
was to apply a relatively low DC voltage across the sparkplug after
the plug was fired. As I recall it was 400volts or thereabouts.
While this potential was applied (starting shortly after TDC and
ending at 60 degrees ATDC I would measure the amount of current
that was conducted by the plasma (fire) in the cylinder. When the
plasma was at it’s most dense it would conduct the most current.
Taking note of the crank position relative to TDC for that cylinder
when the conductance was at it’s highest, I would adjust the
ignition timing in one degree increments until I got the Peak
Combustion Chamber Pressure at 42.5 degrees.
Continued in following post
SpeedyPAL 1995 XJR
Milford / OH, United States
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