Citing Valve Damage, UND Drops Unleaded Fuel And Returns To 100LL

FPK1

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"After an extensive trial, the University of North Dakota’s flight school has dropped Swift UL94 fuel and resumed use of 100LL. The school said ongoing maintenance monitoring of aircraft using UL94—almost exclusively Lycoming-powered Piper Archers and Seminoles—resulted in measurable exhaust valve recession. The school made the switch back to 100LL on Oct. 27."

 
This is not the first time I've heard concern about valve recession with long term use of unleaded fuel in aircraft engines. Most of the literature I've read forbids engine break in with unleaded for this reason, and suggests periodic doses of leaded fuel throughout the life of the engine.

My personal opinion is that aircraft owners and mechanics will have a bunch of new problems from running unleaded fuel that haven't been realized yet. The problems with leaded fuels are just well known at this point.
 
Yup….just like when all those auto gas STCs were sold back in the 80’s.
 
I thought Berto retired?

Probably running leaner. Only effecting exhaust valves.

" I don’t want to speculate on what it could be"...but we're going to speculate and change back to lead anyway.
 
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"Lycoming says it’s working with the FAA to further study the effects of aromatics on engines and notes that aromatics concentrations are not specified by the ASTM D7547 that applies to aviation gasoline"

Beginning to see why GAMI didn't go the ASTM route
 
why aren't aromatics a problem in automotive valve seats? Not being rhetorical, I'm legitimately trying to understand what Lyco is insinuating. Or is UL94 that wildly a different formulation than 94+ octane auto mogas. It is my understanding 100LL without the LL (nothing low about it, in the context of engines originally designed to run on 80/87 avgas) is UL 94 but I don't follow this kerfuffle that closely.
 
Per Mike Busch, they had a clear problem with valve recession in one cylinder on one engine of a twin - the other 7? 11? cylinders on the same aircraft were fine. They "measured" valve recession in some other engines - but apparently there was no "before and after" comparison to indicate that the valve issues started with the use of the unleaded fuel. Also, M. Busch suggests that their measurement technique may have issues.


(Of course, if one were to watch the video, one would do it at 2x speed...)

I would also mention that auto fuel STCs have been around since 1982. One would think that if unleaded fuel resulted in fiery death raining from the sky, it would have been somewhat apparent by now.
 
why aren't aromatics a problem in automotive valve seats?
Auto manufacturers tend to use hardened valve seat inserts. M. Busch discussed Lycoming / Contintental valve seat history to some extent in the previously linked video. Unfortunately, the fastest viewing speed that YouTube offers is 2x.
 
why aren't aromatics a problem in automotive valve seats? Not being rhetorical, I'm legitimately trying to understand what Lyco is insinuating. Or is UL94 that wildly a different formulation than 94+ octane auto mogas. It is my understanding 100LL without the LL (nothing low about it, in the context of engines originally designed to run on 80/87 avgas) is UL 94 but I don't follow this kerfuffle that closely.

Valve and seat material choice may be an issue. Of course, you’d think Lycoming would have considered that already, as they have published documentation stating that their engines will run on unleaded fuels. I also have a hard time believing that UND’s engines did not ingest ANY leaded fuel.

This may or may not be similar to the problems encountered in the automotive industry in the 1970s when unleaded auto fuel was introduced. If you do some reading on research they did back then it was discovered that the recession problems encountered were minimal, even with old style materials used, if the engine was broken in on leaded fuel.

So, what is Lycoming and the other cylinder manufacturers using for materials? Will they (we) keep shooting themselves in the foot by using materials that aren’t up to the task? It wouldn’t surprise me if that’s what happens.
 
Auto manufacturers tend to use hardened valve seat inserts. [snip] Unfortunately, the fastest viewing speed that YouTube offers is 2x.
I rather "Ctrl+F" it. If you could Professor, provide us mech/aero engineering graduates who begrudgingly strugglebussed through chem class, with the context as to why Lycoming doesn't, or conversely, why auto OEM use hardened seats?

Post 12 already jumped ahead of my implication, in that it wouldn't surprise me if this comes down to cost-cutting. In the case of crypt-keepers like Lycoming, shelling out the same old certified ish while charging 2024 markups because they can. They're certainly not likely to change their materials capex in an environment that's nowhere near at risk of overnight lead banning.
 
This may or may not be similar to the problems encountered in the automotive industry in the 1970s when unleaded auto fuel was introduced. If you do some reading on research they did back then it was discovered that the recession problems encountered were minimal, even with old style materials used, if the engine was broken in on leaded fuel.
I was in the auto repair industry during and after lead removal. It was serious problem. Have seen cases where valves receded so far into the heads the lifters ran out of travel, valves would no longer seat with a resulting misfire.

There aren't many of us around today that experienced it first-hand. With time people tend to forget.

Lower octane and detonation likely contributed to the valve issues. How much could be attributed to lack of lead and/or excessive heat could be debated. But one thing is for sure the engines did not like it. For about 5-7 years we pulled our hair out trying to get decent drivability out of fuel the engines were not designed to run on.
 
I rather "Ctrl+F" it.
:)

Historically, low turning, lower performance engines just had the valve seats machined into the cast iron head. Higher speeds, more performance means the valves are hitting harder. Higher exhaust temperatures don't help either. Busch talked about micro adhesion causing abrasive points that would wear the seats as the valves turned. There was something about the particular engines that UND was running and ignition timing that M.B. thought presented something of a worst case scenario for higher exhaust temperatures, which don't help the valves / seats either.

Automotive went with harder seats around the time of the switch to unleaded - also as aluminum heads became more common. Things like PCV, improved materials, etc. extended the life of engines enough that new wear things became more of an issue - used to be at 100K you were more than overdue for an overhaul. My POS Ford has put 200K in the mirror and still runs strong, doesn't use much oil.

Lycoming switched to harder seats some time ago (Busch said when, but I ain't a gonna go back and watch the whole thing just to find out (20 years?)) and has a more favorable attitude to unleaded. Continental made the switch significantly more recently and seems to be more unleaded hostile. At least that's the impression I get of his opinion.
 
Lower octane and detonation likely contributed to the valve issues. How much could be attributed to lack of lead and/or excessive heat could be debated. But one thing is for sure the engines did not like it. For about 5-7 years we pulled our hair out trying to get decent drivability out of fuel the engines were not designed to run on.
Retarding the spark timing was a good way to get catalysts to light faster or to get HC to react to air from the smog pump better by increasing exhaust temperatures. Cars didn't run rich all the time. Enough EGR to choke a horse. There was a lot of stuff going on.
 
Wouldn’t the UND engines have had the harder seats, then?
If my memory is serving me correctly on the timing, that would be affirmative.

But given that it was apparently one cylinder out of their whole fleet, and who knows when the problem started...
 
So we have lower tbo on our top ends. It’s worth it to save the world.

/sarc
 
Can we stop letting perfect be the enemy of good?
How about "100 Super Low Lead" fuel with 1/2 or 1/4 the lead content for the 80% of the fleet that can handle it.

Maybe a blanket approval from the FAA that allows you to mix alcohol free super unleaded pump gas in with 100LL to create 100SLL and run it in all airframes without having to have an STC?
Because we know that will never happen.
 
Per Mike Busch, they had a clear problem with valve recession in one cylinder on one engine of a twin - the other 7? 11? cylinders on the same aircraft were fine. They "measured" valve recession in some other engines - but apparently there was no "before and after" comparison to indicate that the valve issues started with the use of the unleaded fuel.
Sounds like there's no way to determine whether this is a new problem, or an old problem that was developing without being noticed until new scrutiny was applied.
 
...Aromatics like benzene, toluene and xylene are used to boost octane in fuel and in “elevated” concentrations may result in “particulate abrasiveness to valve seats that may contribute to valve seat recession.”
What sort of abrasive particulates does combusting aromatics create beyond that which aliphatics produce?
 
I’ll suggest that the UND test was n=1, statistically speaking. It’s not statistically valid, unless the engine makes, models, age of cylinder head manufacture (materials, suppliers, assembly) are varied. UND likely doesn’t have cylinder heads on their engines that are older or of different eras of Mfg, because the engines need to be replaced at a set TBO and they are all relatively newer because of the hours they fly. I would blame neither Swift nor Lycoming at this stage. We need more data.
 
I’ll suggest that the UND test was n=1, statistically speaking. It’s not statistically valid, unless the engine makes, models, age of cylinder head manufacture (materials, suppliers, assembly) are varied. UND likely doesn’t have cylinder heads on their engines that are older or of different eras of Mfg, because the engines need to be replaced at a set TBO and they are all relatively newer because of the hours they fly. I would blame neither Swift nor Lycoming at this stage. We need more data.

What we need is a constrained test (multiple tests would help too). From the sounds of things, the UND “test” was not.

We do this kind of stuff in the engine test lab in which I work all the time. One would think Lycoming and Continental would have done some unleaded fuel testing by now but they probably aren’t going to publish that information, which they paid for, for everyone to use freely, and pick it apart.
 
What we need is a constrained test (multiple tests would help too). From the sounds of things, the UND “test” was not.

We do this kind of stuff in the engine test lab in which I work all the time. One would think Lycoming and Continental would have done some unleaded fuel testing by now but they probably aren’t going to publish that information, which they paid for, for everyone to use freely, and pick it apart.
They do....and likely knew the results before any of this happened. :cool:
 
Isn't UL94 just 100LL with out the lead?

So the question is why aren't the valves seats hard enough. If there not then that must be down to Lycoming.
 
It is not that they are not hard enough. It seems that there are two carbs used. And the new one seems to run leaner at full power. So with the lower octane, it is running in the light detonation range.

So not the UL, but the 94 may be the issue.
 
It is not that they are not hard enough. It seems that there are two carbs used. And the new one seems to run leaner at full power. So with the lower octane, it is running in the light detonation range.

So not the UL, but the 94 may be the issue.

Weren't these engines originally designed for lower octane? What you're describing sounds more like improper leaning.
 
Weren't these engines originally designed for lower octane? What you're describing sounds more like improper leaning.
From post #6:

Lycoming Says Aromatics In Fuel, Leaning Techniques May Have Caused Valve Problems​

 
It is not that they are not hard enough. It seems that there are two carbs used. And the new one seems to run leaner at full power. So with the lower octane, it is running in the light detonation range.

So not the UL, but the 94 may be the issue.

Are you saying light detonation can possibly cause valve seat sinking? Without curling a top half rod bearing or breaking rings, or other havoc first? I never hear or read about that as an effect of detonation, but sounds like an interesting theory.

Upon quick search, there are about 2 dozen academic journal articles from 1971 to 1999 discussing valve seat issues (erosion) related to unleaded. Most of them are SAE Technical Papers.

One abstract (from 1971), which gives an idea of how this actually happens:

E000D021-E56E-4A40-815D-9B3368ED5E70.jpeg
 
Are you saying light detonation can possibly cause valve seat sinking?
I believe the theory is that an increase in exhaust temperature increases the valve recession.

The big questions are (assuming that M.Busch got the straight dope) is whether or not UND aircraft had an issue with valve recession or not. And, they did, was that valve recession was related to unleaded fuel or not.
 
Paul Bertorelli from AVweb clears it up for us "...valve seat recession doesn't appear to be a thing with unleaded fuel..."

 
Weren't these engines originally designed for lower octane? What you're describing sounds more like improper leaning.
Did you read what I wrote? One of the carbs used runs leaner. With 100 octane, things are OK. With 94, things are not OK.
 
These O-360 engines have STCs for 91 octane mogas in some configurations. For example, the O-360-A4M in the Cessna 172. If that caused valve recession one would expect to know this already since it's been out there for years.
Is there something different about 94 UL versus 91 auto gas?
 
AGAIN, it seems to be an issue with the octane and a DIFFERENT CARB THAT RUNS LEANER.
 
The fundamental problem with the UND trial of 94UL is that they did not conduct a proper control (say, dedicating a significant portion of their fleet to 100 LL for comparative analysis). This will make it more difficult to evaluate the cause of what they seem to be observing.
 
AGAIN, it seems to be an issue with the octane and a DIFFERENT CARB THAT RUNS LEANER.
Isn't the fuel:air ratio set by adjusting the mixture knob? What does it mean that the carb runs "leaner" if the operator can adjust the fuel-air mixture to the desired setting for best economy or power by ear (onset of lean misfire or to max rpm) or EGT?
 
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