fuel map discussion

Jonathan R. Lusky lusky at knuth.mtsu.edu
Thu Jul 7 22:38:19 GMT 1994

Ciciora Steve writes:
> Below is my first draft of a discussion of how I think fuel injection systems
> work.  I faithfully submit it for peer review...
> Subject: fuel map
[ math/chem stuff deleted ]

>   So by measuring the intake manifold pressure and the intake air temperature
> (hopefully at the same spot that the pressure was measured) we can calculate
> the amount of fuel needed for a given a/f ratio.  But wait!  This assumes a
> constant 100% volumetric effency.  We all know that most of the time it is
> less than 100%, and some times greater.  A good estimate is to assume
> volumetric effency changes with engine RPM and to correct the mass of air
> calculation with an RPM factor.  This is usually done with a 3D Fuel Map.  On
> the X axis we would have the mass of air, the Y axis we would have the engine
> RPM, and on the Z axis we output the amount of fuel needed.  This can be in
> grams per cycle or if we include the fuel injector correction factor in the
> fuel map, it can output pulse width.
I believe if you play with the math a little more, you will find that
you can eliminate a bit of it.  I don't think you actually need to know
or calculate the exact airflow, since all of its factors will be present
in the lookup tables.  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.
ATC I guess would be the temp in K?

> (ranges of values for map/temp)

Depends on the application...  forced induction will require a much
wider range on both of those.

> (corrections for cold start, acc, power mode, emmissions mode, fuel econ, etc)
> (what do you think?)

Power Enrichment is accomplished by changing the value for AF.
Cold start is accomplished by changing the value for AF (f[Temp]).
Emissions "mode" is closed loop around stoich.
Economy mode is probably also closed loop around stoich, with maybe
a higher threshold for power enrichment.

Jonathan R. Lusky  --  lusky at knuth.mtsu.edu
 "Turbos are nice but I'd rather be blown!"
    68 Camaro Convertible - 350 / TH350
       80 Toyota Celica - 20R / 5spd

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