MAF and Calculated VE

[Dave Plummer]

What happens when the calculated VE is signicantly different than the MAF
reading?  Who wins, and/or what does the PCM do?  I'm asking for a '94 LT1
in particular, but in general as well...

- Dave

> -----Original Message-----
> From: Scot Sealander [mailto:Sealand at clarityconnect.com]
> Sent: Saturday, February 05, 2000 4:31 AM
> To: gmecm at diy-efi.org
> Subject: Re: MAF and Calculated VE
> 
> 
> 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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