Alpha-N vs. Speed Density?

[steve ravet]

Alpha-N uses two inputs:  Throttle position (alpha) and RPM (n).  These
two numbers are used to estimate airflow, then fuel is injected based on
that airflow.

speed-density uses MAP (manifold absolute pressure/density) and RPM
(speed) as its main inputs, but usually incorporates other factors like
air temp.  From RPM you know the volume of air going thru the engine,
and from MAP you know the pressue, you can measure or guess at
temperature, and from all that you can calculate the mass of air going
into the engine.  Then inject fuel based on that.  In practice the
calculations are done ahead of time and put into a table that has RPM on
one axis, MAP on the other, and VE is the value returned.

alpha-n is the most basic type of EFI, AFAIK only used on engines where
you can't really measure MAP (throttle/cylinder, lots of overlap, etc.).

Bob Cunningham wrote:
> 
> This may be a rudimentary question, but could someone explain the
> difference between Speed-Density and Alpha-N as a strategy of controlling
> the injection system?  Please keep it simple, I don't really even
> understand what each of the terms mean.
> 
> Thanks,
> -Bob Cunningham
> 
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-- 
Steve Ravet
steve.ravet at arm.com
ARM,Inc.
www.arm.com
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