Reply-To: <glcasey@gte.net>
Reply-To: lancair.list@olsusa.com
From: "Gary Casey" <glcasey@gte.net>
To: <lancair.list@olsusa.com>
Subject: BMEP again
Date: Wed, 2 Jan 2002 04:57:46 -0800
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>>And for the sake of clarity, I am sure you understand that the concept of
increasing net torque by reducing negative pre-TDC torque by delaying
ignition, resulting in lowered peak pressure, made possible by
highly-homogeneous charge characteristics and fast-burn chamber technology
is
neither new nor revolutionary. While closing the control loop on combustion
pressure by means of difficult-and-EXPENSIVE-to-certify computer magic is
certainly an interesting way to proceed, others have chosen to achieve those
goals by simpler (certifiable) means.<<

I feel the need to jump back in just for the fun of it and I apologize in
advance for prolonging the agony of this thread.  I include the above post
just to remind us of the subject.

Yes, just as Jack said, BMEP IS a useful number that allows us to compare
engines that might have different displacements - just as specific power,
rated rpm and a host of other "contrived numbers" are useful.  Actually none
of these numbers are contrived because they are all calculated from actual
measured parameters and, therefore are "actual," not "contrived."  How can
one engine produce the same power with a lower peak pressure?  FMEP
reduction, while desirable, is difficult to accomplish and has only a small
effect.  So the only way to make a big difference is to  change the shape of
the cylinder pressure curve.  The easiest way is to retard the ignition
timing, which reduces the pressure before TDC and puts the peak cylinder
pressure at a point where the piston is already on its way "down."
Unfortunately, the theoretical thermal efficiency of the Otto cycle is
inherently linked to the peak pressure since it assume constant-volume
combustion.  Therefore, just reducing the peak pressure will reduce the
thermal efficiency.  Then we have a real conundrum (I tried to use
"epistemological" since I went to the trouble to look it up, but couldn't
come up with the right circumstance) as we dearly want to both increase
efficiency (specific fuel consumption - oops, another "contrived" number)
AND reduce peak pressure.  But why reduce peak pressure?  We have to look at
what causes wear or breakage of engines.  High peak pressure causes high
contact pressure between the top ring and cylinder, wearing out both before
their time.  It also increases the stress on the cylinder itself and the
piston, increasing the risk of breakage.  It also increases thermal stress
by increasing the heat rejection rate to the piston and cylinder head.  The
only real way to get more efficiency with lower pressure is to increase the
rate of combustion, allowing the spark to be retarded while still keeping
the peak pressure in the same place.  I submit that changing the combustion
chamber shape and charge turbulence is difficult without changing the design
of the engine itself.

As I understand it (you have to kind of read between the line of the
hyperbole to find it) the purpose of the "Prism" system is to keep the peak
cylinder pressure at the same crank angle regardless of operating
condition - certainly a worthy goal.  Unfortunately, our aircraft engines
don't run over that wide a range of conditions, so I don't think the peak
combustion pressure angle moves around a lot anyway.  Unless you try to run
very lean or run a naturally-aspirated engine at very high altitude.  And
even then if you have no control over the mixture or mixture distribution.
Most fuel injected engines have relatively good control over mixture and an
electronically controlled engine has control over the mixture and ignition
timing.  And running "very" lean is not very useful because the power drops
off rapidly when leaner than LBT (Lean Best Torque - oops, another
"contrived" actual number) and we usually want more power, not less.

Gary Casey


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