more 747 questions

[Bruce Plecan]

Subject: Re: more 747 questions


>> I'm still confused.  This table lists desired A/F ratios, right?  But at
>> WOT you're not measuring A/F because you're ignoring the O2 sensor. 
>> Having a table of A/F values seems to imply closed loop operation in
>> that you measure A/F, compare to what's in the table, and adjust if
>> necessary.
>
>Steve, and others,
>  If you haven't ran across it yet, the following URL offers
>some super insight. It describes the GN system but I
>expect it will apply to all delco systems. The ECM
>learns differences required to achieve appropriate A/F
>mixtures at part throttle conditions (and sets the Block
>Learn Multipliers) and applies the same multiplier
>even when you go WOT.  In other words "open loop" is
>not entirely based on a pre-programmed map. The BLMs
>provide a learned fudge-factor.
>See
>http://www.thrasher-ep.com/cal_hints.htm
>For some almost as good O2 info, see
>http://ni.umd.edu/gnttype/www/ecmpage.html
>Now for my probably wrong interpretion of the whole
>shebang:
>What I'm beginning to conclude is that the O2 is 
>only of a feedback mechanism to prove that what the 
>ECM thinks its doing is really what's happening. For 
>example, the ECM enleans and then checks O2 to see 
>that it really worked. It richens and checks. If that 
>works it can figure (guess?) that it is close to 
>stoich and that XX  amount of extra fuel would be required to go 
>to 12:1. Open loop just eliminates the check and
>gives it the authority to do even more precision 
>guesswork.
>Gurus..am I just slow, or slow and stupid too? Regardless,
>the Thrasher page is worth a read for 101-ers.
>-greg


Couple other little points.  The ecm uses the O2 sensor readings
as a switch.  ie comparator. Just for instance lets say 14.7:1 
occurs at .440v, as measured by the O2 sensor.  To maintain a
14.7:1 ratio, the ecm determines a pulse width that averages 
50% of the O2 response time to be above the .44, and 50%
below that.  
  OK now with that said, your driving around at sea level +10'.
Ya drive up to the maountains, and find your self at 10,000' 
above sea level.  In trying to maintain this 14.7 the ecm has had
to use a fudge factor.  All of a sudden, you have the need for
speed, and go to WOT.  Which do you suppose would be most
accurate, using a leaner correction value, or just a map value?.
The WOT AFR is a target value for the ecm to try and acheive. 
Cheers
Bruce