Message-ID: <003001c1959e$f31cb200$4c6c8a90@direcpc.com>
From: "Fred Moreno" <FredMoreno@bigpond.com>
To: "Lancair list" <Lancair.list@olsusa.com>
Subject: Engine inquiries for George Braley
Date: Sat, 5 Jan 2002 10:44:52 +0800
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Hi George.  I'd like to pick you brain on an engine topic that I think
all readers can appreciate: engine smoothness.  (I am referring to the
actual vibration and smoothness levels one feels in cruise as opposed to
the "automatic rough" phenomena that I frequently experience during
night, IFR, over the mountains flight conditions.)

A couple of years ago I had a talk with the RAM folks (who do the work
with TSIO-520's in twin Cessnas) at OSH, and the fellow made the passing
comment that measurements of combustion pressures in these engines
showed that even in "steady state" operation the pressures recorded in
an individual cylinder varied substantially from cycle to cycle.  He
asserted that one could balance an engine and prop as much as you like,
but these pressure variations would give rise to a vibration harshness
that could never be controlled by balancing alone.

Seems reasonable to me.

I kept his comments in mind when I started reading some of the new
engine text books and learning a bit about electronic fuel injection and
the ability to more carefully control the mixture into each cylinder.  I
also note with interest the careful work done on  intake manifolds in
modern automotive engines.  This contrasts dramatically from the
induction system of my previous plane, a TR-182 with a carbureted,
turbocharged Lycoming O-540 engine.  This beast used the normal Lycoming
intake manifold (buried in part in the oil sump) with six intake tubes
radiating up to the  individual cylinders.  I had it fully instrumented,
and fought boredom on long flights fiddling with throttle butterfly
position, manifold pressure, RPM, mixture and anything else I could
think of while recording pages and pages of data which I would later
plot.  The EGT spread was simply awful under all conditions, and there
was not much you could do about it.  If you leaned it to "peak" (which
peak?) as recommended in the book, it burned some exhaust valves after
about 800 hours.  Burning more fuel saved exhaust valves, but resulted
in the same money consumption, but over a longer period of time.  And
this engine was a slouch, putting out only 235 horsepower from 540 cubic
inches.

The Continental 470 and 520 engines use a log manifold design under the
cylinders as opposed to the Malibu 520-BE and the 550 engines which have
a top intake manifold that has been more carefully designed to equalize
air flow and mixture.  The later engines are reputedly capable of MUCH
smoother operation suggesting the truth of assertion that mixture
uniformity contributes to more equal combustion events cycle to cycle,
and thus more smoothness.

So here is my current hypothesis which I would like you to comment upon.
In a primitive induction system (such as the Lycoming I used to fly or
the 470/520 bottom induction Continentals) designed just to get air to
the cylinders at minimum cost and complexity, the result is dynamic
behavior with pressure waves ricocheting from end to end in a fairly
chaotic fashion thus causing different amounts of air to arrive at a
particular intake valve, varying from cycle to cycle.  Thus, even with
carefully balanced fuel injection that squirts precisely the same amount
of fuel each cycle, there will be a difference in mixture strength cycle
to cycle because the air delivered to the cylinder varies in a
semi-random fashion.  Consequently, combustion speed, peak pressures,
total power delivered,  etc. all vary from cycle to cycle, and from
cylinder to cylinder in a semi-chaotic way varying over time as well as
cylinder to cylinder.

In my mind, this differs substantially from the nice, regular, standing
wave behavior one would see from a modern auto engine with electronic
fuel injectors and carefully designed intake manifolds with equal length
runners, carefully designed inlets, and other careful attention to
detail.  I gaze upon these new plastic intake manifolds with their
aerodynamic sophistication with some admiration.

So, my question (at last.....) - What kind of cycle to cycle variations
do you see in your engine testing?  I presume that your optical
instrumentation permits you to get fairly high quality traces of
cylinder pressure versus time (or rotation) and that with enough data
storage in your data acquisition system, you can see how an individual
cylinder behaves through a series of cycles.  (Then again, this IS
asking a lot of a measurement system.)  If so, what do you see in terms
of variation cycle to cycle?  How does this relate to intake manifold
design?  What other factors affect these variations?  And do these
variations show up in the form of engine smoothness (or lack of it) as
perceived by the pilot?

Related: would you care to comment on the famous "Lycoming knock"
reported by many?  For those not "in the know," this refers to the
tendency of Lycoming engines to cruise nicely, and then make about 10-20
knocking noises in a row which then disappears immediately after your
ears prick up in concern.  In my plane, the frequency seemed to
correspond to one cylinder making the offending noise a few times in a
row, and then returning to normalcy.  It happens often enough that it is
not just "automatic rough" (happens also day VFR), and it remains a
mystery as far as I know.

Your comments and contributions are most appreciated.  Keep up the good
work.

Fred Moreno

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