Lycoming's views on best economy settings

Mike,

there was an article in
Flying magazine (p. 74-75, 7/02, inset article, J.Mac) , where there
was some sort of lead crystalline deposit (lead oxybromide) forming in
_turbo_ engines only in LOP operations.

Complete, total poppycock, as many sources available on the web show.


Running lean, by a moment of carelessness (pilots have lots of
things to tend to) invites catastrophic trouble in any engine.

Yes? Says who?

--
Thomas Borchert (EDDH)

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.

"Tom Sixkiller" wrote
They don't even out a power imbalance, they just make all the cylinders
reach peak EGT at the same mixture setting.

Which likely produces a balancing of power, no?

No.

What happens is this - the engines have crappy, obsolete induction
systems, by design. By crappy induction systems, I mean they deliver
a different amount of air to each identical cylinder. Modern engines
(meaning auto engines) don't have this problem because their induction
systems are designed by people who understand fluid mechanics and
model the air flow in the induction system, making changes until all
cylinders have equal (to a reasonable tolerance) air flow at all
normal operating conditions. This process doesn't occur in aero
engines because their induction systems were designed decades ago,
before CFD tools were generally available. I suppose they could be
tweaked now, but there are two things preventing this - it would
require changes in the design of the induction system, which would
mean getting the FAA to recertify the engines, and neither Lycoming
nor Continental have design engineers on staff anymore.

The GAMI 'fix' for the problem is really a bandaid solution - instead
of actually fixing the real problem, you measure the extent of it
(with your all-cylinder EGT) and then change the bores of the
injectors until the fuel distribution is off in exactly the same way
as the air distribution. In fact, since both Lycoming and Continental
make injectors with a variety of bores, you could do this yourself
(though probably not legally).

The problem here is that once you've installed the GAMI's, you STILL
don't have equal power output on each cylinder. Each cylinder is
getting a different amount of air, and the injectors make sure it gets
just the right amount of fuel to go with that amount of air, so each
cylinder produces a different amount of power at every stroke.

The real question is - is that any worse than what happens in normal
operation? A little. Remember, best power mixture is a little bit
rich of peak. Let's say we leaned to peak on the leanest cylinder
(meaning the one that gets the most air). Since it gets the most air,
it should produce the most power. However, the other cylinders are a
little rich of peak, and that compensates a little.

How important is all this? Probably not very. After all, it's not
like the cyclinders are firing simultaneously and we're depending on
the forces of the power strokes to cancel out.

Michael

Michael,

The GAMI 'fix' for the problem is really a bandaid solution - instead
of actually fixing the real problem, you measure the extent of it
(with your all-cylinder EGT) and then change the bores of the
injectors until the fuel distribution is off in exactly the same way
as the air distributio


That's not quite what GAMI says, IIRC. They claim the fuel nozzle specs
from TCM and Lyc are so vague that the fuel delivered will vary widely
between cylinders - and that's what's also evened out.

--
Thomas Borchert (EDDH)

"Mike Rhodes" wrote in message
news
As for Lycoming recommending against LOP, there was an article in
Flying magazine (p. 74-75, 7/02, inset article, J.Mac) , where there
was some sort of lead crystalline deposit (lead oxybromide) forming in
_turbo_ engines only in LOP operations. That deposit would cause a
"light" detonation, and eventually destroy the engine. The deposit
apparently does not form in normally aspirated engines, regardless of
mixture. Lead oxybromide was also found to harm the rod and
crankshaft bearings.

Junk Science more than likely.

http://www.avweb.com/news/columns/182152-1.html
http://www.avweb.com/news/columns/182153-1.html

If true, I would think this would be common knowledge, and pilots
would not have to run to Lycoming for it. And other authorities would
not suggest lean in turbo engines. (Do they?)
Running lean, by a moment of carelessness (pilots have lots of
things to tend to) invites catastrophic trouble in any engine. And
for the pilot to get in that habit in normally-aspirated engines can,
after the pilot upgrades, apparently inflict harm on turbo engines.
Anyone else familiar with this?

--Mike

On 29 Jun 2004 22:13:49 -0700, (John Clear) wrote:

In article ,
Mike Rhodes wrote:

As for Lycoming recommending against LOP, there was an article in
Flying magazine (p. 74-75, 7/02, inset article, J.Mac) , where there
was some sort of lead crystalline deposit (lead oxybromide) forming in
_turbo_ engines only in LOP operations.

I've snipped the rest since it is full of old wives tales. The
theory of lead oxybromide came from a poorly investigated accident
in Austrailia.

John Deakin analyzes the accident, and Flying's coverage of it.

Accident: http://www.avweb.com/news/columns/182152-1.html

Flying's coverage: http://www.avweb.com/news/columns/182153-1.html

Deakin also covers LOP in alot of his articles, specifically the
ones titled 'Where should I run my engine?' He goes into the
science of how an engine actually works, and examines how the
'your engine will burn up if you do that' OWTs relate to reality.

All of Deakin's articles: http://www.avweb.com/news/columns/182146-1.html

John

So I can ignore all the hysterics and lean to roughness, then enrichen
it to smoothness. And we all should do so in any piston engine, as
long as the power is markedly below 75%.

Mike

On Wed, 30 Jun 2004 13:16:14 -0500, Mike Rhodes
wrote:

So I can ignore all the hysterics and lean to roughness, then enrichen
it to smoothness. And we all should do so in any piston engine, as
long as the power is markedly below 75%.

Mike

What Deakin suggests is that you get yourself a multi cylinder EGT
guage that can tell you what the temperatures are for the EGT and CHT
for ALL the cylinders.

Without that instrument and even with a single point EGT guage, you
have no idea where the CHT's are when you lean by that method. He
frequently characterized the typical Lycoming/Continental engine as a
group of cylinders flying along loosely in formation because the
temperature readings from one cylinder to the other can vary so much
you'd think they were from some other engine.

Maybe you've leaned to a safe settng but maybe not. Deakin advocates
knowing for sure. Seems like good, albeit expensive advice.

Corky Scott

In article ,
Mike Rhodes wrote:
Deakin also covers LOP in alot of his articles, specifically the
ones titled 'Where should I run my engine?' He goes into the
science of how an engine actually works, and examines how the
'your engine will burn up if you do that' OWTs relate to reality.

All of Deakin's articles: http://www.avweb.com/news/columns/182146-1.html

So I can ignore all the hysterics and lean to roughness, then enrichen
it to smoothness. And we all should do so in any piston engine, as
long as the power is markedly below 75%.

Without an engine analyzer, you have know way of knowing how bad
the fuel/air mixture is in each cylinder. Running at the standard
50F rich of peak EGT puts you right in the 'Red Zone'. Running
100-150F ROP is a better place to run the engine if you can't run
LOP smoothly. Most non-fuel injected engines have such large
differences in fuel/air mixture between cylinders that they can't
be run LOP smoothly.

At lower power settings, it doesn't matter much where you run your
engine since lower power means lower heat and pressure.

Deakin does a much better job of explaining all this.

John
--
John Clear - http://www.panix.com/~jac

Thomas Borchert wrote
The GAMI 'fix' for the problem is really a bandaid solution - instead
of actually fixing the real problem, you measure the extent of it
(with your all-cylinder EGT) and then change the bores of the
injectors until the fuel distribution is off in exactly the same way
as the air distributio

That's not quite what GAMI says, IIRC. They claim the fuel nozzle specs
from TCM and Lyc are so vague that the fuel delivered will vary widely
between cylinders - and that's what's also evened out.

That's disingenuous (on their part, not yours).

It is true that the specs on the fuel nozzles are vague. It is also
true that some of the engines out there are not set up properly with
respect to fuel flow. It is NOT true that this is the major problem.

When setting up an injected engine, there is a classic "coke bottle"
test. Basically, you unscrew the nozzles from the jugs, stick them
into coke bottles, then run the boost pump. After a few minutes, you
have a very good idea of how the fuel distribution is working.
Everyone knows this part of it.

The part that used to be common knowledge, but isn't anymore, is that
(at least with Lycomings) there are different bores of fuel nozzle.
All of them meet spec. By playing with the diameters of the nozzles
(putting smaller ones on the jugs that flow too much and larger ones
on the ones that flow too little) you can very easily make the fuel
flows right - without GAMI's. If you're not too terribly worried
about legalities, you can open up the bores yourself. Like you said -
the specs are so vague, nobody will ever know even if your nozzles are
inspected. In fact, the old style Lyc nozzles were two-part affairs
where the actual nozzle portion was a piece inserted into the bleeder.

If the big issue was fuel rather than air flow, there would not be any
need for test flights and expensive instrumentation. The coke bottle
test would be all you would need. However, that is simply not the
case.

This procedure actually works very well for the baby injected Lycs -
IO-320/360. The air distribution on those is so good that there's no
need to match the fuel flows to the individual jugs, only to each
other. This is why it can be so difficult to lean Lyc O-320/360
engines. We've all met the engine that can be leaned so much that
there is obvious power loss - and it's still running smooth. There is
a tendency to blame the pilot ("you just can't feel the roughness")
but the reality is that some of them have such even air flow (and with
carbureted engines that means the fuel flow is even too, since it's
vapor) that you simply can't feel the onset of roughness.

Such is not the case with the big Continentals. In fact, if you do a
coke bottle test with one of those engines that has GAMI's, you will
see differences in the fuel flows.

Michael

Michael opined

Thomas Borchert wrote
The GAMI 'fix' for the problem is really a bandaid solution - instead
of actually fixing the real problem, you measure the extent of it
(with your all-cylinder EGT) and then change the bores of the
injectors until the fuel distribution is off in exactly the same way
as the air distributio

That's not quite what GAMI says, IIRC. They claim the fuel nozzle specs
from TCM and Lyc are so vague that the fuel delivered will vary widely
between cylinders - and that's what's also evened out.

That's disingenuous (on their part, not yours).

It is true that the specs on the fuel nozzles are vague. It is also
true that some of the engines out there are not set up properly with
respect to fuel flow. It is NOT true that this is the major problem.

When setting up an injected engine, there is a classic "coke bottle"
test. Basically, you unscrew the nozzles from the jugs, stick them
into coke bottles, then run the boost pump. After a few minutes, you
have a very good idea of how the fuel distribution is working.
Everyone knows this part of it.

The part that used to be common knowledge, but isn't anymore, is that
(at least with Lycomings) there are different bores of fuel nozzle.
All of them meet spec. By playing with the diameters of the nozzles
(putting smaller ones on the jugs that flow too much and larger ones
on the ones that flow too little) you can very easily make the fuel
flows right - without GAMI's. If you're not too terribly worried
about legalities, you can open up the bores yourself. Like you said -
the specs are so vague, nobody will ever know even if your nozzles are
inspected. In fact, the old style Lyc nozzles were two-part affairs
where the actual nozzle portion was a piece inserted into the bleeder.

If the big issue was fuel rather than air flow, there would not be any
need for test flights and expensive instrumentation. The coke bottle
test would be all you would need. However, that is simply not the
case.

This procedure actually works very well for the baby injected Lycs -
IO-320/360. The air distribution on those is so good that there's no
need to match the fuel flows to the individual jugs, only to each
other. This is why it can be so difficult to lean Lyc O-320/360
engines. We've all met the engine that can be leaned so much that
there is obvious power loss - and it's still running smooth. There is
a tendency to blame the pilot ("you just can't feel the roughness")
but the reality is that some of them have such even air flow (and with
carbureted engines that means the fuel flow is even too, since it's
vapor) that you simply can't feel the onset of roughness.

Such is not the case with the big Continentals. In fact, if you do a
coke bottle test with one of those engines that has GAMI's, you will
see differences in the fuel flows.

GAMI claims something else is going on. Fuel from the upstream injectors is
leaking into the downstream intake ports, making the downstream cylinders
richer than the upstream cylinders.

AS to intake air distribution, I would assume that it is like the rest of
aviation, reliable junk.


-ash
Cthulhu for President!
Why vote for a lesser evil?