FW: basic fuel metering equations

Ciciora Steve sciciora at aztec.al.bldrdoc.gov
Fri Jul 1 23:19:48 GMT 1994

John Writes:

   Before the discussion on lookup tables really gets going, I would like to
make sure that I fully understand the basic concepts and equations needed to
control the injector portion of this project...
...This is getting lengthy...
Will expanding this help anyone?

Absolutly!!  I'm suprised that this type of discussion has not come up before. 
Although I'm not as good of a writer as you, I have tried to point the
discussion in this direction.  Two of my attempts got completly scrambled by
sending them from New Zealand.  What I'm hoping is that we can lay out the
project on the table (like your outline) and goups can work on sorting out
small portions.  Like calculating the density of air from MAP and intake temp. 
Or how to do a look up table.  This way the project can be broken up into
manageable chunks.  Below is my first pass at explaining my understanding of
fuel injection systems.  Not as complete as yours, and mabe not as clear, but
from a slightly different view point.  Damn!  I can't figure out how to 'cut
and paste' text in Microsoft Mail, so I'll have to wait until tuesday to post
my explanation of calculating air density.  
  This is geting more into implementation rather than theory, but how are we
going to scale the input sensors?  We could scale them in software or hardware.
 The hardware method would be more universal but less elagant.  What we could
do is scale the MAP and temp sensors to output 0-5V in the range of interest
(simple inst. amp circuit for gain and offset, can be done in one quad op-amp).
 The software black box will take MAP and Temp numbers from the A/D converter
(of the range of 0->255) and convert them to a air density number of the range
of 0->255 (arbatrary units).  The reson for not using calibrated units is to
keep maximum resolution in the minimum # of bits.  It would be wastefull to
measure temperature over the range of -50 -> 300 deg. F just because we can. 
Similar with MAP.  The range of values I see for MAP/Temp are different for my
engine/altitude than it is for your engine/sea level.
  The other way I see of doing things is the software scaling method.  What we
can do is use a 12 bit A/D converter and measure the entire useful range of the
sensors (-50 -> 300 deg F and all).  This 12 bit number can be scaled down to a
more usefull range for the software.  Or, heck, we could just do it all in
floating point and not wory about range/resoultion! (joke) :-)
  Am I making myself clear enough or just rambling on?
- Steven Ciciora

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