Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Tue, 07 May 2002 23:49:14 -0400
Message-ID: <redirect-1232288@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: FADEC vs. PRISM
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<<From
my initial understanding of PRISM vs. FADEC, instead of looking at EGT's
(fadec, an indication of the trend of the last few engine cycles), the
prism system looks directly at the combustion event by means of
combustion pressure and looks where the peak occurs to adjust timing, as
well as looking for 5kHz "knock" ripples after the peak.  This seems
like a dream come true system if it truly works.>>

The PRISM and FADEC systems have different objectives, so in my opinion,
can't be directly compared.  The FADEC is what it says - a complete
electronic engine management system.  As such it combines the control of
ignition timing and fuel delivery to presumably optimized both over the
normal operating range.  Because of a automotive-type fuel system it will
handle cold and hot starts, idle control, etc.  The PRISM system is an
ignition-only systems that uses pressure signals to adapt the ignition
timing to whatever fuel control parameters the operator chooses.  The
decisions of ROP, LOP, etc. are taken away from the FADEC operator, but are
encouraged by the PRISM system.  The PRISM system doesn't do anything about
the shortcomings of the existing mechanical fuel systems regarding hot
starts, cold enrichment, etc.  Then one step up from the standard mags is
the LASAR system that has an open-loop control of ignition timing.  It
doesn't attempt to compensate for different mixtures, but does add ignition
timing at low manifold pressures, improving high altitude (naturally
aspirated) performance.  It is the only one that is not "all-electric" so
doesn't require any kind of back-up electrical power source as it backs up
to the standard magnetos.  It looks to me like if you are looking to
optimize cruise fuel economy and power the PRISM system gives you a better
chance.  If you want car-like operation (kick the tires light the fire and
go) then the FADEC system will probably do it.  If you are looking for just
an incremental improvement over the standard system the LASAR is the logical
choice.  Just my opinion.

Gary Casey
ES project, home brew FADEC