Lycoming's views on best economy settings

I was searching via Google for informational schematics, or even just
a written description, on Lycoming's fuel injection system because
it's different from Continental's.

I came across Lycoming's website in which a number of technical and
informational articles reside.

One of them
http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/operation/leaningEngines.html

discusses various mixture settings for their fuel injected engines.

Here is item no. 5 from that article, it's a description of how to
achieve "best power" and "best economy" when an EGT guage is
available:

5. The exhaust gas temperature (EGT) offers little improvement in
leaning the float-type carburetor over the procedures outlined above
because of imperfect mixture distribution. However, if the EGT probe
is installed, lean the mixture to 100oF on the rich side of peak EGT
for best power operation. For best economy cruise, operate at peak
EGT. If roughness is encountered, enrich the mixture slightly for
smooth engine operation. ***editorial note*** If you do this (richen
slightly from peak), the only place for the cylinderhead temperatures
to go is up. Slightly to the rich side of peak on the graph described
below is where the cylinderhead temps peak. So it would pay to be
careful how much you richen from the peak setting, if the engine is
running rough at that point. The Cessna 172 I rent has the EGT guage,
but no CHT guage. So you can richen from peak and drone happily along
not knowing that you are cooking the cylinderheads.

This particular section of the website includes a nice graph, which,
if you are familiar with John Deakin's "Mixture Magic" article, will
look very familiar. It should because all fixed spark engines will
show the exact same performance curves for things like EGT, CHT,
percent power and BSFC.

What was interesting to me is if you look at where peak EGT is
occuring and then go down to the BSFC curve is, you will see that you
have not yet reached the lowest BSFC. That doesn't occur until you've
leaned a bit further. But then you would be **LOP**. Notice it does
not take much additional leaning to get the lowest BSFC the engine is
capable of producing.

Here's the interesting part: At peak EGT, the cylinderhead temps are
already starting down from their peak, and the downward curve is
pretty steep once you get to peak EGT.

Just a tiny bit more leaning and the cylinderhead temps dive down
another full ten degrees, while the EGT has hardly gone down at all.

While this is going on, the percentage of power is dropping off too,
which is why economy cruise is slower than best power, of course.

Lycoming finishes the graph with the following statement: "Textron
Lycoming does not recommend operating on the lean side of peak EGT."

Yet as their own graph shows, best economy is ONLY achieved lean of
peak. What extremely interesting to me is that the difference between
peak and lean of peak where best economy occurs is only a matter of a
very few degrees EGT.

Remember, when you are cruising at 60% power, you cannot hurt the
engine no matter where you set the mixture control. You can't burn
valves or cook the cylinderheads or cause detonation, it just isn't
producing enough power to do that. Lycoming themselves recommends
that for maximum engine life, cruise power should be limited to 65%
and CHT's kept below 400 F. But since the instrument panel doesn't
include a CHT guage, the only way to avoid high temps is to be way
rich, or at peak EGT or below.

Why Lycoming recommends against LOP operation is a mystery. MUCH
cooler CHT's and less fuel being burned... what am I missing here?
Does the engine run roughly at this setting? Only those who try LOP
will know.

Corky Scott

wrote in message
...
Why Lycoming recommends against LOP operation is a mystery. MUCH
cooler CHT's and less fuel being burned... what am I missing here?
Does the engine run roughly at this setting? Only those who try LOP
will know.

http://www.lycoming.textron.com/supp...ps/SSP700A.pdf

sets out their case, at least for larger turbos. (I'm not suggesting I
endorse it.)

Another thing to bear in mind is that mixture distribution is imperfect in
most factory engines. If you operate at a steep part of the power vs
mixture curve, small differences in mixture mean large imbalances of power
between the cylinders. That can't be good for the engine. The point of
Braly's Gamijectors is to even out the mixture distribution so that the
imbalance disappears.

Julian Scarfe

"Julian Scarfe" wrote in message
...
The point of
Braly's Gamijectors is to even out the mixture distribution so that the
imbalance disappears.


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

"Stan Prevost" wrote in message
...

"Julian Scarfe" wrote in message
...
The point of
Braly's Gamijectors is to even out the mixture distribution so that the
imbalance disappears.


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?

Also, IIRC, it leads to the piston achieving ignition at the optimal point
in the stroke (at LOP??).

"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)

"Stan Prevost" wrote in message
...
The point of
Braly's Gamijectors is to even out the mixture distribution so that the
imbalance disappears.

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

Which they do by ensuring the same actual fuel/air mixture at a given
mixture setting. Further, since the difference in fuel/air mixture at a
given mixture setting plays a big part in the power differences between each
cylinder, why wouldn't the Gamijectors help improve the power imbalance?

Pete

wrote in message
...
Lycoming finishes the graph with the following statement: "Textron
Lycoming does not recommend operating on the lean side of peak EGT."

Yet as their own graph shows, best economy is ONLY achieved lean of
peak. What extremely interesting to me is that the difference between
peak and lean of peak where best economy occurs is only a matter of a
very few degrees EGT.

Corky,

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

Considering the wear on the engines moving parts that provides, maybe George
Patterson is right, that they'd like us to buy new engines? :~) I also
wonder how they are trying to cover their legal asses if they were spreading
wrong (negligent) information for all these years, or merely just PP QC on
their engines??

Tom
--
"Flying an airplane is just like riding
a bike -- it's just a lot harder to put
baseball cards in the spokes" -- Capt. Rex Cramer

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.

On Tue, 29 Jun 2004 12:55:39 -0400,
wrote:

[trim]

Lycoming finishes the graph with the following statement: "Textron
Lycoming does not recommend operating on the lean side of peak EGT."

Yet as their own graph shows, best economy is ONLY achieved lean of
peak. What extremely interesting to me is that the difference between
peak and lean of peak where best economy occurs is only a matter of a
very few degrees EGT.

Remember, when you are cruising at 60% power, you cannot hurt the
engine no matter where you set the mixture control. You can't burn

That since an aircraft engine will not usually detonate at the lower
power. But turbo engines, I've read, may have difficulty...

valves or cook the cylinderheads or cause detonation, it just isn't
producing enough power to do that. Lycoming themselves recommends
that for maximum engine life, cruise power should be limited to 65%
and CHT's kept below 400 F. But since the instrument panel doesn't
include a CHT guage, the only way to avoid high temps is to be way
rich, or at peak EGT or below.

Why Lycoming recommends against LOP operation is a mystery. MUCH
cooler CHT's and less fuel being burned... what am I missing here?
Does the engine run roughly at this setting? Only those who try LOP
will know.

Corky Scott

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.
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