FW: Re: fuel map discussion

Ciciora Steve sciciora at aztec.al.bldrdoc.gov
Wed Jul 13 20:32:00 GMT 1994

John Lusky writes:

>I believe if you play with the math a little more, you will find >that you can
eliminate a bit of it.

I agree.  I'm not shure where to draw the line between fast and less readable
and slower and easer for someone else to understand.  

> The basic equation I'd use would be 
> OT = BPC * 1/AF * VE * ATC, where OT = injector on time, AF = 
> desired air/fuel ratio, VE = volumetric efficiency, ATC = air temp
> correction (note to John: I made that acronym up off the top of my
> head), BPC = a constant that makes all of this work.  VE is a
> function of MAP,RPM and comes from a lookup table.  BPC is a
> constant or could come from a lookup table.  AF would probably
> come from one of several lookup tables.

I believe the above approach above will work, but it is not how I think I
understand it.  I'm not sure why VE would be in it's own 3D look up table.  The
VE of an engine is not something I have a good feel for, know how to calculate
or know how to measure.  Please correct me if I'm wrong, but besides some
sensors (thermistors) VE and the fuel injectors are the only non-linear things
in the equation.  I envision a 3-D 'fuel map' (for steady state conditions)
that would have RPM on one side, a combination of MAP and air temp on the other
side, and would output the base pulse width.  Other factors (cold start, power,
etc) would modify this base pulse width.  If it would help clear things up,
mabe the fuel map would output the amount of fuel (in grams or something) which
would go to another table that accounted for the non-linear fuel injectors (and
output the pulse duration in ms).  I doubt that this would help since the
injector response time changes with age.  Na, forget what I just said about a
second table to linearize the injectors.  Right now I don't think its a good
idea anymore (tomorrow I'll change my mind... probably).
  It it were not for VE, the fuel map would be a flat slab with no nice looking
shape.  I think what I described is what you described with the VE map
  I think for 'tuning' an engine, having one 3D fuel map would be easer for
someone to play with and see the results.  "Seems a bit lean between 4k and
4.5k rpm, I'll just push up these data points a little..."

  Correct me if I'm wrong, but wouldn't you want 14.7 A/F most of the time (all
rpms and pressures) except under cold start, power, etc?  Would my base 'fuel
map' be to get you at 14.7 A/F and other modifiers pull you away from 14.7?

  These are just my opinions, and I make no claims as to their accuracy.  I
just made them up.
- Steven Ciciora

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