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Date: Sun, 02 Jan 2005 19:46:30 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: Re: airspeed vs. power
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From Walter
<<OK, in injected engines, there is an enrichment feature in the linkage
between the throttle and mixture.  As the throttle is moved the mixture
changes.  After WOT throttle is achieved (before the throttle is
completely forward) any remaining throttle addition does not add any
more MP, but does continue to add fuel....

As you continue to reduce throttle, the mixture and throttle plate
begin to move together.  From this point on, the mixture will remain
fairly close to the set F:A ratio with changes in MP, but this only
occurs after the initial FF reduction has taken place.  Some
installations do a better job of this than others.  In some (Lycomings)
the addition of throttle results in not quite as much fuel addition so
that the mixture leans a tad as throttle is added and richens a tad as
throttle is reduced.  It's a quirk of their system and actually works
BACKWARDS to how Lycoming says it works!  We've tested it and have
advised them of the results of the tests.>>

There might be some confusion among the listers on carburetor vs. fuel
injection and Lycoming (Bendix/Precision Airmotive) systems and Continental.
On the Lycoming fuel injection system, with which I'm quite familiar, there
is no mechanical linkage that affects the air/fuel ratio.  What looks like a
mechanical link is there to control the idle fuel flow and it stops
affecting the fuel flow by the time the throttle is perhaps 1/3 open.  The
high load fuel flow is independent of throttle position and is normally
calibrated to be a fixed ratio of air flow (the mixture doesn't change with
throttle opening or with air flow changes).  There is a phenomena that
changes the air/fuel ratio, primarily on 4-cylinder engines, in that the air
flow pulsations can get quite large and can possibly go negative and these
pulsations are more pronounced at full throttle.  The result is that full
throttle mixtures are sometimes richer than part throttle mixtures.  This
effect is very small with 6-cylinder engines.  On Continental systems the
fuel flow is primarily controlled by throttle position, so the previous
remarks don't apply.

Gary Casey