Re: MAF and Calculated VE

[Scot Sealander <Sealand@clarityconnect.com>]

Walter Sherwin wrote:
 
> Hmmm......what sort of help with the SD equation.  Would be interested
> either on or off list, whichever you prefer.

The aim of SD code is to *calculate* the amount of air mass in the 
cylinder.  The code then injects the amount of fuel at the desired AFR. 
If the air mass is not calculated accurately, it makes the job of 
injecting the proper amount of fuel much harder.  You may see strange 
things in open loop AFR tables trying to correct for that.

There were a number of things that needed improving....  So where to 
start.  :-)

The Turbo Grand Prix (TGP) uses an air temp sensor mounted in the
intake manifold.  It was my experience that the only time the sensor 
really told you actual air temp was during periods of high air flow.
At idle, and other low flow periods, the sensor gave a temp that seemed 
too warm.  I was having a tough time trying to keep the fueling errors 
(as reported by the BLM) to a minimum.  GM seems to agree with this,
as the air temp compensation table does not do much real compensation.
There has been a previous post that stated that the air temp 
compensation in the Syclone/Typhoon does not do much.  Notice
that the Sy/Ty code also uses a manifold-mounted air temp sensor. The
code in both cases justs seems to rely on the O2 feedback to get fueling 
correct during closed loop.

So the air temp sensor was moved to the intake air stream after the
intercooler.  This also presents a challenge, as the air temp does rise
as it flows through the engine, with that rise based on how fast the
air is passing through the engine.  What the ECM needs to know, is the 
actual air temp in the cylinder, and that is pretty hard to *measure*.

To try and get around this, I set up a table that contained the
theoretical air density change with temp, and then set up a variable
called "CylAirTemp" to address that table.  CylAirTemp is based
on engine temp, incoming air temp, engine air flow, how long the engine
has been idling, etc.  The recent very large delta T's (-15 deg C air
temp and usually 93-95 deg C engine temp) allowed for a practical test 
of the model.  The fueling error is now *much* lower, but there is 
still work to do.

I presently don't have EGR enabled, so fueling is not compensating
for that.  As soon as this non-EGR portion is done, the EGR fueling will
be addressed.  My goal is to have the BLM running in the 126-128 range 
for all operating conditions.  With this goal, the fueling will not need
the O2 sensor to be "tight".

The VE tables were off by quite a bit.  Also, resolution at/near
idle was a problem, so I expanded the VE table over the whole operating
range.  VE also changes rapidly around/near the idle area, so I added
a VE filter to the look up tables raw VE value when the engine was at 
idle.  Seems to idle better, but there may be some more work to be done 
in the O2 feedback controls at idle. 

The TGP code also has a fuel algorithm error in it.  The effects of
it may or may not appear.... 

Anybody feel like just getting a MAF sensor?  ;-)

There is more, but this is getting long....

So where does this leave those that use a 747 for their ECM?  The truck
ECM has no air temp sensor, so it relies on engine temp to determine
what the cylinder air temp is.  This is normally not a problem, as 
the stock truck uses exhaust-manifold-heated air and a thermostat
in the air cleaner to try and send a near fixed air temp to the engine.
If you now use that truck ECM with a "cold air" intake, your calculated
air mass in the cylinder will have errors that vary with outside air
temp....  That is, if you tune it for an 80 deg F day, it will be
lean on a zero degree F day.  As always, this depends on just how the
calibration is set up.

Scot Sealander

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