On Tue, 29 Jun 2004 14:33:56 -0700, "Tom Sixkiller"
wrote:

Isn't it also true that LOP optimizes the point in the stroke that the
cylinder achieves the peak ignition point?

Yup. I think what you are refering to is the Peak Power Pulse (PPP),
which needs to occur at or about 16 degrees After Top Dead Center
(ATDC). This is vitally important during takeoff as high temps and
pressures inside the engine at that point can cause destruction of the
engine. But during cruise, high temperatures can cause the engine to
have a shortened TBO.

I should make it clear I'm referring only to NON turbocharged engines
in the information below.

The situation is a bit complex and is also the result of aircraft
engines having fixed timing. Since the timing is fixed, the only way
to assure that the PPP occurs at the proper 16 degrees ATDC is by
varying the mixture. The Fuel Air Mixture (FAM) burns more slowly on
either side of the stochiometrically correct ratio of approximately
14.7 to 1 in mass.

Takeoff power is where bad things happening cause BIG problems, so the
engineers optimized the engine for this particular regime. Since the
timing is fixed at 26 or so degrees Before Top Dead Center (BTDC), and
the takeoff RPM is known, the speed of the piston is also known. The
engineers calculated that in order to cause the PPP at 16 degrees
ATDC, the mixture must be overly rich. The over rich mixture delays
the PPP just long enough for it to occur at about 16 degrees ATDC.
Why is it necessary for the PPP to occur there? Because at that
point, the connecting rod has swiveled past TDC and the PPP is working
to force the crankshaft throw downward. If the PPP occurs closer to
TDC, the PPP cannot apply the downward force to the crankshaft, it has
nowhere to go and heat and pressure skyrocket. If the PPP occurs
BEFORE TDC, this is a worst case scenario called pre-ignition and
assures the quick destruction of the engine.

THIS is why running the engine rich on takeoff is necessary. The
extra rich mixture doesn't keep the engine running cool by hosing down
the cylinders, it keeps it cool by making sure the PPP occurs at 16
degrees ATDC.

As I mentioned in a previous post, if there was a way to vary the
timing on the engine, there would be no need to use an overly rich
mixture for takeoff or any other time because the PPP could be kept at
16 degrees ATDC regardless the rpm or power setting or mixture. Well
that's not really true, the mixture would still affect burn rate, but
the timing could vary to make the PPP occur where we want it anyway.

During cruise, the engine is slowed down. If you have a constant
speed prop, you can slow the prop down, but it does not necessarily
mean that you reduce the throttle. Deakin is an advocate of leaving
the throttle full forward because as he quoted a friend of his: "I
didn't buy a fast airplane to fly slow."

So even though the engine is producing less power because it's been
slowed down some, it's still making heat and now the pistons are
moving more slowly. Because they are moving more slowly, the PPP is
now occuring closer to TDC. If you lean out the mixture to something
close to the ideal of 14.7 to 1, the mixture will burn as fast as is
physically possible and the PPP will be very close to TDC. Physics is
physics, even though the air is now pushing through the engine MUCH
faster than was occuring during takeoff and climb, you can produce
high cylinderhead temperatures by using a not quite lean enough
mixture setting. This is what Deakin called the "RED ZONE" and he
warns pilots to avoid mixture settings between LOP and Best Power.

So he advocates either running the engine significantly rich of peak
(in the best power zone), to slow down the burning, or leaning it past
peak again due to the slower burning lean mixture. As we said, either
side of ideal and the rate of burn slows down.

Deakin is also leery of allowing cylinderhead temps to get anywhere
near 400 degrees because aluminum begins to soften at that point.
Lycoming does not worry about temps being that high. They recommend
"400 degrees or below." Deakin strongly suggests not exceeding 380
degrees.

So what Deakin is advocating is setting the mixture where you can fly
the fastest for the best fuel burn and lowest engine temperatures
possible. Note: LOP won't produce the fastest cruise speed, nor will
it produce the best economy, but he feels it's the most reasonable
compromise in that it for sure won't hurt the engine because the
engine will be running cooler.

Corky Scott

PS, I am not an engine expert. I used to be an auto mechanic and am a
lifetime motorhead but all the information above is from John Deakin's
columns, and he got the information from Pratt and Whitney, Lycoming,
Continental and GAMI and their test stand work. The information
stands the test of critical review.