One of the high points of summer for aviators is a trip to Wisconsin for EAA AirVenture Oshkosh.
The week-long show, traditionally held at the end of July, is a time to see what’s new and what has changed in the general aviation world.
I always go with a list of companies I want to talk to and questions I want to ask. At the 2024 show, my number one question was: “What happened to the University of North Dakota training fleet that led to exhaust valve recession when run on an unleaded Swift fuel?”
I have written often in the past about the possibility of exhaust valve recession in aircraft engines if all leaded fuels are banned, forcing the entire general aviation fleet to use unleaded fuels.
Many people involved in the GA business ignored the possibility, thinking that if the GA industry switches to hardened exhaust valve seats — like the automotive world — the problem would be solved. This, of course, ignores the fact that all automobile engines are liquid cooled and operate at a much lower load factor than spark ignition piston engines in aircraft.
Then came the news from the University of North Dakota that it was going to use a 94-octane unleaded fuel for much of its fleet. And guess what? They started seeing exhaust valve recession on several aircraft after just a few months.
While I was at AirVenture I asked numerous technical people — from engine manufacturers to engine overhaul shops to fuel suppliers — what they thought the cause of the problem was at UND.
What surprised me was that almost everyone I asked had a very definitive answer as to the cause, and almost every answer was different from the others.
The first answer was that the fuel contained aromatics and since they burn slower than alkylates, they would be hotter at the exhaust valve/seat interface. This would lead to high seat temperatures and corresponding seat wear.
There is some truth to this, but refineries have been adding aromatics to 100LL since the early 1970s when 100LL was introduced.
The Swift fuel could contain a higher level of aromatics, but that should not have been a major contributing factor.
The second answer was that the Swift fuel had inadequate octane performance and the engines at UND were knocking enough to elevate the exhaust valve seats, which led to the recession problem.
But on teardown, there was no sign on the pistons that there had been knocking on any of the engines.
The UND program is a very well-run operation and if there was significant knocking, I would think that they would take steps to alleviate the problem.
Again, this could be a contributing factor, but there’s no evidence that it was the major cause.
The third reason given was that UND was operating its engines at peak EGTs and overheating the valves and seats.
The planes were leaned fairly aggressively, but they had full instrumentation with multi-point EGTs, so I do not believe they were running them above the normal range.
The fourth reason given was that these engines were designed to run on leaded fuels, so the results were expected.
The facts are that the UND planes were flown exclusively on unleaded fuel. They were well maintained and flown by the book. They experienced exhaust valve recession.
Another uncomfortable fact is that if the GA world is switched over to unleaded fuels, we will see additional aircraft experience much more severe conditions in the real world.
Meanwhile the march towards an unleaded general aviation continues.
Officials at General Aviation Modifications Inc. (GAMI) continue to work on distributing the company’s unleaded avgas, G100UL, which was approved by the FAA in September 2022 through the Supplemental Type Certificate (STC) process.
The folks at Swift are working on an STC for the company’s unleaded candidate and are also preparing it for an ASTM ballot.
Swift’s 100R unleaded avgas candidate is based on aviation alkylate, plus a needed amount of ETBE (Ethyl Tertiary Butyl Ether), plus an exhaust valve seat recession preventative additive. The company’s representatives at Oshkosh would not disclose what the additive is or if it was used in the UND flight test.
1)
With respect to valve seat recession and the use of unleaded fuels – – there is very good data from the FAA William J. Hughes Technical Center.
See the following two reports:
DOT/FAA/CT-TN89/33
160 BHP Lycoming engines with 8.5:1 CR. One engine operated on autogas and the other on 100LL. See figure 5. At the end of 150 hours, the valve seat recession was very slightly increased for the avgas engine, compared to the autogas engine. That is, the unleaded fuel was the same, or very slightly better than the leaded fuel.
ar99-70 AFETF Aero Commander testing:
” . . . there has been no evidence of accelerated wear with the use of unleaded fuels.”
Aero Commander with Lycoming Engines, comparing left engine to right engine.
THAT is the research data. Believe the data!
*************
2)
Operating at peak EGT does not result in the highest temperature for the valve and valve seat areas. While that conclusion is highly “intuitive” – there is good data to the contrary! This subject was extensively researched in 1943-44 at Wright Patterson when the Army Air Force was evaluating lean of peak (they did not call it that) operation for military aircraft to increase range. They instrumented up valves with with thermocouples and ran the engine through a wide range of rich to lean fuel-air ratios and recorded the valve temperatures.
(Also, see the data from the Advanced Pilot Engine Management Seminar series, still available on line.)
The valve and valve seat area reach the maximum temperate around 25 to 50 degrees RICH of PEAK EGT. NOT at peak EGT, as is commonly assumed. The reason for that result is that the CHT reaches its maximum CHT at 25-50 degrees rich of peak. The valves have to “heat sink” into the cylinder head. The valves are “closed” (and heat sinking into the cylinder head) about 75% of the time during each 720 degrees of crankshaft rotation.
3)
There is available test data (submitted to and approved by the FAA) that demonstrates that aircraft engines with 8.5:1 compression ratios and timing more advanced than ~ 20 degrees BTDC, will detonate on ASTM 94UL avgas at cruise power settings. The detonation observed in that testing is moderate, but not typically severe. However, it is sufficient over time (due to repetitive, high frequency combustion pressure “spikes”) to be a competent cause for the observed VSR at UND.
Note, detonation at that level does not leave visible evidence on the face of the piston as has otherwise been suggested – – provided the cylinder head temperatures remain within in their normal operating range.
All of the comments above are based on actual data.
The data supports the following conclusion:
The problem at UND was not due to a lack of lead.
The problem at UND was due to a lack of “octane.”
George Braly
Mr. Braly,
Thanks for the info and references.
To find the reports, do a search in the technical library;
https://f10011.eos-intl.net/F10011/OPAC/Search/SimpleSearch.aspx
Regards,
JimH. Cessna 175B
EPA… A 3-Letter Dinosaur agency whose ‘EXTINCTION EVENT’ should have been from the get-go… and William Ruckleshaus should have been fired OUT OF THE GATE and prohibited from attending the UNESCO Rio Earth Summit in 1992, from which AGENDA 21 originated under the Commie eyes of the world-wide Communist capers at the United Nations and co-convenors Maurice Strong and Edmund de Rothschild! In addition, there’s no such thing as “FOSSIL” fuels. Oil creation is a natural occurring event inside the bowels of Mother Earth. Get over your deep indoctrination and cognitive dissonance.
Not really up on technical aspects of the problem. My experience of doing thousands of valve jobs on automotive engines was such that failure was from carbon build-up preventing seating. Cause was fuel management, not driver adjustable in a car, operating temp issues, and/or oil consumption.
Or, galling of valve stems due to valve materials not hard enough for unleaded fuel, which the automotive sector seemed to have transitioned into compatibility without much ado.
Well.!! The California Gov. will be signing into law SB 1193, banning 100LL on Dec. 31, 2030. …. so, the folks working on an unleaded 100 octane fuel have 5+ years, or we park most of our GA aircraft.
This bill is linked to the FAA reauthorization law in 49 U.S. Code § 47107 section [22].
So, times a-wasting..!!
Most low compression engines can use an 80 octane fuel, but cannot have any ethanol, as in the mogas stc.
Flying on MoGas since the 1980s, you can imagine how much I think of this whole thing. Auto gas at airports would certainly go a long way to lowering lead emissions. Enough planes in the fleet could use it to make a difference. Must be big money in this whole program.
All this hype is caused by the EPA. Their medaling for the sake of money.
We weren’t having as many problems before the EPA was invented! The convoluted thinking of the greeneeze has not one bit of evidence that general aviation operators contribute to excess material into the atmosphere. General aviation is not even a drop in the bucket of lead produced in America. Besides no one is going to pick it up and eat any of the imaginary stuff. I’ve been working around aircraft most of my life and I’m 82 years old in a couple of months. Name me the people who have died from aircraft exhaust lead??
There’s no commen sense left with the EPA!!
Valve recession is not new. Its impact is realized as a result of valve/seat material, localized operating temperatures, operational loads, and fuel composition. TEL has been a choice for fuel blends due to its octane boosting characteristics and relatively low ash production during combustion. What some called a “lubricant” others called “lead deposits.” Regardless, its combustion chamber characteristics are predictable and air-cooled engines perform well with its use.
One data point is not a trend and blends of aromatics can vary, but one observation by this author can attest to the wearing away of a valve face around the perimeter of the valve while the seat exhibited a rough, spotted texture as a result of micro-welding of valve material to the seat interface. This, from an IO-360 in a C-172 used for primary training operating on Swift UL94.
This does not imply that the fuel is to blame. Too many variables exist and it is still only one data point.
Racing engines using high-octane blends containing distillates of alcohol address the issue using exotic seat and valve materials to slow the recession and subsequent valve failure. Liquid cooled engines control the localized temperatures in combination with hardened valves and seats. Large bore air-cooled engines usually use an additive; If not TEL, then manganese or ETBE, or a similar blend of octane boosting materials. In the end, it’s all a compromise. One that should be vetted in the test cell, not in the field.
Ben —
There is a lot of conjecture here. Please let me try to clarify some important points from your article.
Aromatics were not the cause. UL94 uses < 20% aromatics – well within normal avgas ranges. This level causes no performance issues in these piston aircraft.
Detonation was not the problem. There was plenty of octane in UL94, and no evidence of detonation was found.
As a matter of policy, the flight school said they chose to operate cruise at Peak EGT for sustained periods to achieve "Best Economy"- as they routinely operate using new equipment all under warranty. Obviously, operating at peak EGT places the highest heat stress on the exhaust valves vs. operating "rich of peak" where cooler EGT's may better preserve engine life. Also be advised, our understating is that Lycoming does not authorize operating piston engines at "lean of peak" – please check their guidance.
Yes, 100R contains an anti-valve seat recession additive. It will set the standard for all unleaded avgas on a global scale. Yes, 100R successfully completed over 350-hours of FAA-conforming endurance and durability tests — and the majority of our engine parts were found to be within "NEW Wear Limits". The additive worked very well under the highest stress levels – including peak EGT's!! Our anti-VSR additive has now been extensively FAA-tested over the past 3+ years in our 100R formulation. 100R unleaded avgas will become FAA and ASTM International approved soon and will be rolled-out globally in the coming months to replace UL94.
Chris D'Acosta
CEO – Swift Fuels, LLC
“Obviously, operating at peak EGT places the highest heat stress on the exhaust valves”
Serious question. Trying to learn here.
How does heat stress cause recession?
If you were to remove a valve seat from an engine that showed signs of recession and put it on a bench next to a new seat, how would they differ from each other?
Would the recessed seat be thinner or not? What else would be different?
If thinner, would that support the “no lead causes wear” theory or the “no lead allows ‘micro welding’ and material is plucked away from the seat and vaporized” theory?
If dimensionally identical, would that support the “high ICPs and or detonation hammered the seat deeper into the head” theory? (I would imagine any extruded aluminum flash from this process would be vaporized away).
This reminds me of the discussion about asymmetrical patterns on valves. “Is the hot spot and crack on the valve where the gasses were escaping or was that where contact was still being made?”
I can tell you with pretty high certainty that’s where contact was still being made. I’ve set valves like that on my bench (vertically, stem up) and you can clearly see daylight under the hot area / area with the start of the small crack. That was the last part still making contact with the seat.
Ben,
I have 2 questions, if you would answer ?
1. is there any research on what the minimum amount of TEL is needed in avgas for our low compression engines…7.3:1 for my GO-300 D.
2. if I was to ‘wash out’ the ethanol from our CA 91 octane autogas, with 10 % ethanol,
would I get 89 octane gas ?
[ 109 x 0.1] + [ 89 x 0.9 ] = 91
Also, is there a reliable process to do this ?
Thanks
JimH. N8234T
It doesn’t matter what works for your specific engine as the final recipe, even if it includes a reduced amount of TEL, must meet the needs of the entire fleet.