Message-ID: <001701bf2d1a$3d21fb20$e3818ed1@pavilion>
From: "J. N. Cameron" <CIC@centurytel.net>
To: "Lancair List" <lancair.list@olsusa.com>
Subject: Exhaust popping;  converting capacitative to resistive fuel level
Date: Fri, 12 Nov 1999 08:27:03 -0600
Mime-Version: 1.0

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    Interesting, this exhaust popping.  The stock exhaust system for the ES
as of a couple of years ago was pretty small-bore, maybe 1-3/4", and the
factory prototype popped like crazy at low power / rich mix settings, like
on final.  I had Dawley aviation build me an exhaust system that was
properly graduated at each cylinder, ending up at 2-1/4" diameter (which
increases the cross-sectional area by 65%).  I also had about two feet of
extra pipe under the belly, as I had planned to bring the pipes together in
the midline and run them into a muffler.  With the extra couple of feet,
mine never popped once.  Later, however, when I abandoned the muffler
project and cut the pipes back to the usual position, several inches below
and behind the cowl outlets, I started to get the popping at low power /
rich mix.  My guess is that it's simply a matter of how much the exhaust
temperature drops from the cylinder to the end of the exhaust pipe.  With a
shorter, smaller pipe, the temp is still high when it mixes with the air, so
there is probably spontaneous combustion of the over-rich exhaust gas.  With
a longer, larger pipe, the temp probably drops enough to avoid that.  [I was
recently assured that the ES exhaust system being sold by Lancair has been
re-designed -- I'll have a look at it when I pick up Kit #2 in Feb.]

    How many of you noticed the extra 18" or so of exhaust pipe that sticks
straight back, parallel to the belly, on the Cirrus SR20? [Recent article in
one of the flying mags.]   Seems to me it wouldn't cause much extra drag, it
would probably eliminate the popping, and would also make the cabin quieter.
[When I cut the Y off my ES pipes, the cabin noise increased significantly.]

    To Pat Weston, trying to convert gauges designed for resistive sensors
to work with capacitative sensors will be a major electronic design project.
The resistive sensors are stone simple, and just produce a variable voltage
out to the meter.  To use a capacitance element for level sensing, you make
the capacitor (the probes, in this case) part of an oscillator circuit.
When the material between the plates of the capacitor, which in the fuel
probes are the outer tube and an inner rod, changes, e.g. from air (empty)
to fuel (full), since the dielectric constants of air and avgas are very
different, the value of the capacitor changes.   This changes the frequency
of the oscillator.  Next step is to convert the frequency to a voltage (not
a big deal, really, as there are low-cost chips on the market that do this).
In the E.I. unit, there is obviously a microprocessor that stores a table of
voltage vs. level data (which you enter when you first calibrate, or
program, the unit).  That takes care of non-linearities resulting both from
the inherent non-linearity of the sensing system and the irregular geometry
of the tank and probe installation.

    I share your dislike for digital readouts on a lot of things, but in my
experience the E.I. fuel level gauges worked very nicely. .... Except --
Nearly everyone I've talked to has found that when you key the mic to
transmit on your COM's, the E.I. level gauge will go nuts.  You can add
extra shielding and grounding 'til you're blue in the face, and they still
go nuts.  My best guess is that the probes act like antennas, and that their
oscillator circuit picks up the interference.  E.I. obviously know about the
problem, as they get sorta vague on the telephone and mumble about how
they're going to have another look at it real soon now.  Don't know if they
have.  It's only a minor nuisance, really, as they settle back down in a
short time.  You won't want the low level alarm hooked to anything audible,
however, because every time you announce your turn to short final, the E.I.
level gauge will blast you at a time when you don't need spurious alarms
distracting your attention.

Jim Cameron  (once & future ES)


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