Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Wed, 21 Aug 2002 10:30:14 -0400
Message-ID: <redirect-1701653@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: PRISM vs FADEC
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I can't help it George, I have to respond to:

<<Here is the issue.  Compared to the traditional continuous flow port
injection systems - - NOBODY - - has been able to point to any performance
enhancement for any internal combustion engine operated with pulsed
sequential port injection in an application where the engine operates for
extended periods of time at constant power with the RPM at or above 2000
RPM.
...
So... why do it?   Just for the engineering challenge?  Just to add 10 lbs
of wiring and a couple of 50 pin connectors?  Just to add a half dozen new
failure modes and  thousands of lines of software code to be certified?  And
re-certified every time you make a change in the "map" of the F/A ratios ?>>

You are precisely right on the first part.  I spend a lot of my career
evaluating and developing different fuel injection systems for automotive
use and we looked in great depth at the various ways to squirt the fuel into
the engine.  We even had a "Bendix" RSA system on a car and could almost get
it to pass emissions - remember that for a few years the RSA system was the
thing to have at Indy.  Anyway, the only real advantage of a pulsed system
is that it provides a highly accurate way to control the fuel flow with
electronics.  It really has little to do with how the fuel gets into the
engine.  The only advantage of sequential injection is that it allows less
acceleration enrichment to be used to get the engine to be smooth coming off
idle, thereby reducing hydrocarbon (HC) emissions.  It also allows a slower
stable idle speed.  George is right, above maybe 1500 rpm or so there is no
measurable difference.  Air bleed injectors can offer a real advantage under
light load conditions, but those can be incorporated regardless of the type
of injection.

Why do it?  The question is probably why do electronics at all, not why do
pulsed injectors.  And I think the only reason is to remove the engine
management workload from the pilot - this includes cold start, hot start,
hot fuel handling and other peripheral advantages.  Actually, the same
reason electronics has been used in cars, but the incentive is less because
the aircraft engine runs essentially constant speed and is devoid of
emission concerns - for the moment.  Think of the way you operate a car
engine compared to the way you operate an aircraft engine - that's the
difference.  How important is that difference to you?  One of the incentives
to use the PRISM system is that it adapts the spark advance to whatever the
air/fuel ratio actually is, taking away some of the requirement for
precision in the control of fuel flow, which remains manual.  However, it
can't fix most of the other fuel system weaknesses mentioned above.

Gary Casey
C177RG, no electronics
ES project, with electronics