Back in June 2023, the University of North Dakota (UND) Aviation program transitioned its training fleet from 100LL to UL94, a 94-octane unleaded fuel.
In October, after logging 46,000 hours using UL94, UND maintenance personnel identified abnormal valve seat recession in its Lycoming engines. Following a risk assessment, UND announced its decision to revert back to 100LL.
That led Lycoming Engines to conduct a “root cause analysis” of the issue, including further testing of UL94.
“This analysis has determined that the engine components and airframe components conform to specification and are not believed to contribute to valve seat recession,” Lycoming officials said in an April 12, 2024, press release. “However, testing has also demonstrated that, under certain conditions, use of UL94 may impact valve seat recession.”
Analysis indicates that aromatic concentration tolerances for UL94 may contribute to valve seat recession in certain flight profiles, Lycoming officials pointed out.
“Aromatic limits are not directly specified by ASTM D7547 and elevated aromatic concentration may result in slower flame speed, radiant heat from particulates, and particulate abrasiveness to valve seats that may contribute to valve seat recession,” they continued.
Lycoming officials noted they are working with the FAA to evaluate the sensitivity of piston engines to aromatics in fuels.
If the further analysis indicates it is needed, Lycoming officials said they will work with the industry to “further define” ASTM D7547 to “provide appropriate guidance to operators regarding how leaning techniques can impact valve seat recession with high aromatic fuels.”
Officials said that the company “remains committed” to the FAA’s Eliminate Aviation Gasoline Lead Emissions (EAGLE), an effort to find an unleaded fuel to replace 100LL by 2030.
Most fuel sold in the United States, including 100LL and both unleaded fuels now available to general aviation — from Swift Fuels and General Aviation Modifications Inc. (GAMI) — contain aromatics.
Interestingly, the AOPA Baron was at Sun N Fun 2024. There was significant (enough to rub off on your fingers) soot in the exhaust pipe from the engine running G100UL. There was no soot in the exhaust pipe running leaded Avgas. Related?
Another aircraft fuel crook !!! How long will all this continue ?? Haven’t gotten the price of unleaded fuel up enough to use it as is ? It appears to me someone is still unhappy with the present price of ul fuel for aircraft, still wanting to continue raising the price !!! That is exactly what this is still doing in a more orthodox manner, and I’m afraid this will continue until it reaches untouchable heights. It will reach extinct levels, and no one will pay for that price of a gallon of fuel. Only in America !!!
UL94 is supposed to be the same as 100LL without the lead. It wasn’t G100UL, for example.
So… what aromatics are in UL94 that aren’t in 100LL?
“After logging 46,000 hours…..”.
This is aggregate hours of an entire test fleet. But how many hours of operation of one particular engine were required to realize valve seat recession? That number would be more realistic and meaningful!
Benzene, Toluene, Ethylbenzene, and Xylene (BETX) occur naturally in crude oil. Each have qualities that enhance the usefulness of the fuel in turbine and reciprocating engines. Increasing the percentages of one or more of these naturally occurring elements as a replacement for tetraethyl lead (TEL) can slow the flame front across the combustion chamber and decrease risk of detonation. These aromatics can also leave a sooty byproduct easily observed in the exhaust of both turbine and internal combustion engines. BETX does have a purpose in fuel. So does TEL. It’s impossible to form a viable analysis around UND’s findings, or that of Textron Lycoming, until more is revealed about the testing and the proportional involvement of the blend. Nevertheless, it sure speaks to the need for cautioned use of unleaded fuels in engines that previously performed as expected on 100LL.
I read a report that the GAMI, G100UL uses some amount of Xylene to boost the octane rating. It wold be interesting to know the testing that Gami did at ‘cruise’ power and leaned for max power and lean of peak.
Did Gami examine the exhausts valve seats ?