marc wiese wrote:

Subject: Engine design factors
From: Walter Atkinson


*All* of the fixed-timing engines are set up so that the timing places the
peak power pulse at approximately 15-16 degrees after top dead center where
the con-rod is at it's most efficient position. This occurs ONLY when the
engine is LOP. The slower fuel burn of LOP places the peak power pulse in
the *right place* only when LOP. When the engine with fixed timing is run
ROP at high power settings, the peak power pulse is earlier (9-10 degrees)
than the most efficient position, which also places more stress on the
engine's components.


You would have to believe that the engine was purposely set up to run
inefficiently to believe that the engineers planned it to run ROP. Or, it
is purely coincidental that LOP operation places the least stress and most
efficient operation on the engine.


Personally, I cannot believe it is an accident.


The data is in *huge* piles at the Mousewerks in Ada. LOP is clearly the
way the engines were designed to run--OR there is one hell of a
coincidence.


It is not an indefensible statement. The reason the high-power setting are
to be done *very* ROP is that all that extra fuel is used to *delay* the
fuel burn, which in turn, moves the peak power pulse aft of the 9-11
degrees after top dead center that high power, ROP operation causes. This
peak power pulse very close to TDC is risky as it leaves the margin for
detonation very small and the drive-train under a LOT of unnecessary
stress. The answer to moving this peak power pulse aft is to:


A) have variable timing (not possible up to now but as a result of the
Mousewerks, soon be possible)


B) Use LOP mixtures at high power settings (not possible up to now but as a
result of the Mousewerks, soon be possible) George, John and I have done
this and it works, but it is not for the uneducated, under-instrumented, or
faint of heart.


C) Run the mixture so rich that it delays the power pulse to an acceptable
location ATDC. (The solution used by ALL fixed timing aircraft engines --
ALL of them)


The use of C by ALL engines is a *compromise* which allowed the
fixed-timing engines to run at rated power without detonating and using
this later peak power pulse to allow the engine to run a little cooler. It
is not the coolness of the fuel which cools a very rich mixture, it is the
delaying of the fuel burn and of the peak power pule which puts less stress
on the cylinders and therefore runs a tiny bit cooler. As soon as variable
timing is possible, LOP operation at HIGH power settings is possible, and
detonation becomes a worry of the old technology, and almost any fuel can
be used, the we will have full time lean run engines, which is the way they
were designed to be run!


My comments were not trying to paint with a broad brush, but to bring to
the surface, the reason why we have been operating under these over-rich,
ROP compromises. Until now, full time LOP operation was difficult because
of the fixed timing and the lack of instrumentation. That is all changing,
but it does not change the fact that all these engines would have been run
LOP IF they could have been. That is the *design*... always has been. ROP
at Take-off was the compromise to keep the engine together under the
compromise of fixed timing!


Best regards,


Walter

LOP lean of peak? ROP rich of peak?

Paul Lamar
 
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