Too Lean?

[gsxrpilot]

14 minutes ago, EricJ said:

I think that's a bit deceptive, since it's a matter of perspective or reference.   If you're on the rich side of the red box or red fin and start leaning, you can certainly hurt things by going leaner, which would be too lean compared to where it was.   That's pretty much what carusoam said, if you stop in the box while leaning you're too lean compare to the richer side of the box.   Can you fix it by going outside of the box on the lean side?   Sure, and that point was made in carusoam's #2.    So you're right, but #1 was correct as stated.

Nope, the question was, can you be too lean. And the answer is No. Your example is of being either not lean enough or not rich enough. It's not possible to damage the engine by running either too rich or too lean. 

And the reason for this is that the only "reference/perspective" or way to measure Rich or Lean is from Peak EGT. From peak EGT you cannot be either too rich or too lean, only not enough in either direct.

[EricJ]

3 minutes ago, gsxrpilot said:

Nope, the question was, can you be too lean. And the answer is No. Your example is of being either not lean enough or not rich enough. It's not possible to damage the engine by running either too rich or too lean. 

And the reason for this is that the only "reference/perspective" or way to measure Rich or Lean is from Peak EGT. From peak EGT you cannot be either too rich or too lean, only not enough in either direct.

It's semantics, which was my point.    You can damage the engine by running in the red box or red fin.     It that region too rich or too lean?   Neither answer describes the problem.    It is TOO LEAN compared to the region rich of it and TOO RICH compared to the region lean of it.   carusoam captured it properly in his statements, so I was just pointing out that "correcting" his characterization was a little misleading.

[gsxrpilot]

What's misleading is saying that you can measure "too" lean from whatever position you make up. If you're gonna speak of something, then be correct about it. And it's not semantics, it's physics, math, and chemistry. 

Lean is measured from peak EGT and that is the only position/constant that matters.

It's lean of peak, not lean of wherever you feel like starting. Sorry.

[Shadrach]

1 hour ago, EricJ said:

It's semantics, which was my point.    You can damage the engine by running in the red box or red fin.     It that region too rich or too lean?   Neither answer describes the problem.    It is TOO LEAN compared to the region rich of it and TOO RICH compared to the region lean of it.   carusoam captured it properly in his statements, so I was just pointing out that "correcting" his characterization was a little misleading.

Risk of engine damage anywhere on the lean side of peak EGT is highly improbable.  The setting might be on the hot side but detonation unlikely.  A way to test this in your yourself (I have) is to run a contant MP and lean to 100ROP let temps stabilize then lean to peak.  The result in my case has always been a CHT drop after leaning from 100ROP to Peak EGT.   

[EricJ]

13 minutes ago, gsxrpilot said:

What's misleading is saying that you can measure "too" lean from whatever position you make up. If you're gonna speak of something, then be correct about it. And it's not semantics, it's physics, math, and chemistry. 

Lean is measured from peak EGT and that is the only position/constant that matters.

It's lean of peak, not lean of wherever you feel like starting. Sorry.

I don't think there's a strict physical or chemical definition of the boundary between "lean" or "rich".   There's "peak EGT" as one reference point, there's also "stoichiometric" (NOT necessarily the same as peak EGT), there are AFR references if you're using an O2 sensor (which is an attempt to find stoichiometric operation), there's asymptotic CHT, fuel flow, specific fuel consumption, propeller load, etc., etc.   The CHT, EGT, and power output curves all peak at difference places.   Which should one use to define "too rich" or "too lean"?   It'll depend entirely on who you're talking to and what you're trying to optimize.  Most engine manufacturer power charts just show a continuous region and what's happening where and sometimes point out the direction of the trend of richer or leaner.   Most don't define a point and say "this side is lean" and "this side is rich", partly because it's not that simple and partly because those terms are relative depending on what you're trying to achieve.   Stoichiometric operation might be "too lean" for some optimizations and "too rich" for others.   Likewise peak EGT.  Lycoming doesn't seem to universally define "lean" anywhere that I can find, and the only lean limitations that I can find, i.e., definitions of "too lean", are lean limits that are specified by fuel flow, not EGT.   So I don't think you're correct in asserting that EGT is the supreme reference point in dividing between rich and lean operation.   Personally I don't think there is one because it's always relative to what you're trying to optimize.

This is also why using EGT strictly as a primary operating parameter is often discouraged, because it doesn't really tell the whole tale.  I suspect this is also why Lycoming has avoided the whole ROP/LOP discussions, because it's a rabbit hole in many ways and not worth getting tangled up in.   I appreciate that perspective.   It does wind up being important for aviators that operate reciprocating engines because it is one of the finer calibrated engine instruments that we have available to us that can be useful in tuning operation settings.

So, as an engineer I know the value of geeking out over details, and I also know the value of really understanding the limitations of the terms and the usefulness to the broader audience.   Engine tuning is a pretty deep science that goes way past what we can do with an EGT gauge, but since it is the most reactive tuning instrument that many of us have available in the cockpit it often gets the most attention. 

I'll stand by my previous statements, and I'm not at all surprised that this is yet again subject of discussion.   I think this particular point is pretty much semantic hair-splitting.

[EricJ]

4 hours ago, Shadrach said:

Risk of engine damage anywhere on the lean side of peak EGT is highly improbable.  The setting might be on the hot side but detonation unlikely.  A way to test this in your yourself (I have) is to run a contant MP and lean to 100ROP let temps stabilize then lean to peak.  The result in my case has always been a CHT drop after leaning from 100ROP to Peak EGT.   

Yes, generally as the mixture is leaned from full rich power peaks first, then CHT, then EGT.    So once you're leaning past peak EGT everything is getting cooler, and the curves generally steepen up so they're getting cooler faster with smaller adjustments towards increasingly lean.    I think at lower altitude (higher MAPs) where the power output is higher the risks from detonation and increased head pressures are the biggest issues.    If we had head pressure gauges we'd probably be tuning with those, but, alas, we have EGTs and much slower-responding CHT indications, so that's what we try to make do with.   If we ever do get to switch to unleaded fuel we might get to use O2 sensors, and then we can argue about how to use those effectively and what AFR should be for what flight phase/altitude/etc., etc... 

[carusoam]

Fortunately for Austin...

1) Continental doesn’t mind writing ops pages for both ROP and LOP... at least for their IO550...

2) Rocket Engineering writes some pretty complete POH performance inserts...at least for the standing O... both ROP and LOP...

3) Expect the same level of documentation is available for the Rocket... 

4) APS is a great resource for hands on engine operation classes... they can even measure ICPs in their instrumented engine... (lab)

5) Goof the ignition timing up in a turbo bird... a piston gets melted somewhere by the time you reach TPA...

6) The JPI is pretty good at supplying data for things not working so well...

7) See if you can get your hands on actual STC documents for the Rocket.

When does the Rocket arrive?

Best regards,

-a-

[Austintatious]

4 hours ago, carusoam said:

Fortunately for Austin...

1) Continental doesn’t mind writing ops pages for both ROP and LOP... at least for their IO550...

2) Rocket Engineering writes some pretty complete POH performance inserts...at least for the standing O... both ROP and LOP...

3) Expect the same level of documentation is available for the Rocket... 

4) APS is a great resource for hands on engine operation classes... they can even measure ICPs in their instrumented engine... (lab)

5) Goof the ignition timing up in a turbo bird... a piston gets melted somewhere by the time you reach TPA...

6) The JPI is pretty good at supplying data for things not working so well...

7) See if you can get your hands on actual STC documents for the Rocket.

When does the Rocket arrive?

Best regards,

-a-

Thanks you. 

I have the STC info and performance chart on the rocket currently.  Which is where I was getting my info from.

It is undergoing an extensive annual ad repairs at Don Maxwells.  We expect that to take about a month and then we will close on it.  Might be sticking it strait into the paint shop after that or might just fly it ugly for a while.

 

[Shadrach]

6 hours ago, EricJ said:

Yes, generally as the mixture is leaned from full rich power peaks first, then CHT, then EGT.    So once you're leaning past peak EGT everything is getting cooler, and the curves generally steepen up so they're getting cooler faster with smaller adjustments towards increasingly lean.    I think at lower altitude (higher MAPs) where the power output is higher the risks from detonation and increased head pressures are the biggest issues.    If we had head pressure gauges we'd probably be tuning with those, but, alas, we have EGTs and much slower-responding CHT indications, so that's what we try to make do with.   If we ever do get to switch to unleaded fuel we might get to use O2 sensors, and then we can argue about how to use those effectively and what AFR should be for what flight phase/altitude/etc., etc... 

Agree with all of the above with an emphasis on the fact that my CHTs have fallen significantly by peak EGT. I don’t really think detonation is a risk in any normally aspirated certified aeroengine during level flight. Maybe in climb if you’re totally hamfisted or are trying to induce  detonation. It’s pretty cool that even with our WWII era tech that we get better BSFC than most auto engines save for a handful of specialty designs and modern DFI turbo diesels.  A little tech developement would go a long way.

[Shadrach]

4 hours ago, Austintatious said:

Thanks you. 

I have the STC info and performance chart on the rocket currently.  Which is where I was getting my info from.

It is undergoing an extensive annual ad repairs at Don Maxwells.  We expect that to take about a month and then we will close on it.  Might be sticking it strait into the paint shop after that or might just fly it ugly for a while.

 

Fly it ugly for a little while…It’s character building and it will give you a familiarity with the plane pre-paint so that you have a sense that the plane has been properly assembled and rigged post-paint.

Edited April 2, 2019 by Shadrach
spelling

[PT20J]

I can think of a couple of cases where it is possible to be too lean:

1. The BSFC curve has a minimum that is pretty constant over a range of LOP EGTs. But, if you get leaner than that, the fuel flow continues to decrease, and your MPG actually decreases. Doesn't hurt anything, but it's not as efficient -- you'll burn more gas to get to the destination later. 

2.  I experimented with LOP in a Beaver on floats with a R-985. The mixture distribution on this engine is about the best you could get -- single row of cylinders with a central supercharger and equal length intake tubes to each cylinder. You can pull it back and just feel the power decrease until it gets so lean that there is still fire when the intake valve opens and it backfires. That's almost idle cutoff! I had an interesting discussion with Walter Atkinson on leaning this engine (he used to own a Beech 18 with R-985s). Just shy of very slight roughness is about 50 LOP (the planes I was flying didn't have EGTs). But you can't always operate the float Beaver there on a hot day. The airspeed is so low (around 90 Kts) that the engine won't stay cool LOP. Apparently as the airspeed falls off, the cooling decreases faster than the combustion chamber heat decreases. The Beaver doesn't have cowl flaps.

Skip

[Shadrach]

16 hours ago, PT20J said:

I can think of a couple of cases where it is possible to be too lean:

1. The BSFC curve has a minimum that is pretty constant over a range of LOP EGTs. But, if you get leaner than that, the fuel flow continues to decrease, and your MPG actually decreases. Doesn't hurt anything, but it's not as efficient -- you'll burn more gas to get to the destination later. 

2.  I experimented with LOP in a Beaver on floats with a R-985. The mixture distribution on this engine is about the best you could get -- single row of cylinders with a central supercharger and equal length intake tubes to each cylinder. You can pull it back and just feel the power decrease until it gets so lean that there is still fire when the intake valve opens and it backfires. That's almost idle cutoff! I had an interesting discussion with Walter Atkinson on leaning this engine (he used to own a Beech 18 with R-985s). Just shy of very slight roughness is about 50 LOP (the planes I was flying didn't have EGTs). But you can't always operate the float Beaver there on a hot day. The airspeed is so low (around 90 Kts) that the engine won't stay cool LOP. Apparently as the airspeed falls off, the cooling decreases faster than the combustion chamber heat decreases. The Beaver doesn't have cowl flaps.

Skip

Skip, Do you know why the onset of roughness occurs so early (~50LOP) in that engine? Is it inherent to that design? Perhaps perhaps due to lower compression ratio?

[PT20J]

3 hours ago, Shadrach said:

Skip, Do you know why the onset of roughness occurs so early (~50LOP) in that engine? Is it inherent to that design? Perhaps perhaps due to lower compression ratio?

Since the plane didn't have an EGT, I used Walter's suggestion. Here's the quote from his email,  "If you lean till roughness and richen to smoothness on an R-985 you will end up about 50dF LOP. "

Frankly, I never found real "roughness". The power just fell off with reduced mixture strength until it either backfired or stopped. The way I ended up doing it was moving the mixture control toward lean fairly rapidly by a fixed amount (the throttle quadrant has the word  M I X T U R E written on it vertically next to the lever, so I used the letters as an index) and noting if there was a noticeable power drop. If not, I'd go to full rich, wait a bit, and do it again moving it further lean. Once I got to a perceptible power drop I'd leave it there or sometimes enrich slightly and watch the CHT. On some planes this would work out fine and the the CHT would stay below 200 deg C (392 F). On others, it would get too hot. I think the variation was in the cooling airflow. The air exits an annular area between the cowling and the firewall dishpan. These planes were all built in the '50s and '60s and the cowlings have been banged around over the years. I noticed that the gap is tighter in places around the circumference on some planes than on others and this likely restricts the cooling airflow a bit. But, that's just a guess.