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.
By August, UND mechanics started seeing valve seat recession in a portion of the fleet’s Lycoming engines.
“Valve seat recession was seen in about 28% of the fleet after 400 hours of using UL94,” reported Lycoming’s Jennifer Miller.
In October, after logging 46,000 hours using UL94, UND decided to switch back to 100LL.
That led Lycoming Engines to conduct a “root cause analysis” of the issue, including further testing of UL94.
Lycoming officials were “put back on their heels” with this development, since the engines do not have a history of valve seat recessions, Miller said. Additionally, many Lycoming engines have been run successfully on unleaded mogas through an STC with Petersen Aviation or the Experimental Aircraft Association, she noted.
The company’s analysis continues, but has revealed “no smoking gun,” according to Miller.
In an April 12, 2024, press release, company officials noted that it’s not the engines that are contributing to the valve seat recession.
“However, testing has also demonstrated that, under certain conditions, use of UL94 may impact valve seat recession,” the release stated.
Analysis indicates that aromatic concentration tolerances for UL94 may contribute to valve seat recession in certain flight profiles, Lycoming officials said, noting that the problem — right now — seems to be isolated to the UND fleet.
“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,” Lycoming officials said.
Lycoming’s Miller said the company is working with the FAA and Swift Fuels, which produces UL94, to isolate the cause of the valve seat recession.
Part of that is evaluating 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.”
For more information: SwiftFuels.com, Lycoming.com, Aero.UND.edu
This is an update to a story posted April 16, 2024: Aromatics may be cause of valve seat recession after using unleaded fuel.
FAA “relying” on the manufacturers to come up with better engine designs and walk away from lead , but there are no incentives for Lycoming and others engine manufacturers to do so ! Here we are with engine design that has not changed in over 60 years while car engines have improved by leaps and bounds. Go figure!!
Great question, Mr. T Boyle, another great question from the G.A. audience !!!
Here is some info frm Google. ” Aromatic compounds or cyclic compounds have higher auto ignition temperature which would prevent the fuel mixture from catching fire during compression. This in turn increases the octane number.May 30, 2019.” Looks to me that it a necessary ingredient
It’s not that simple cars are liquid cooled and aircraft are air cooled total different effects on metal
Auto manufacturers figured out how to make hardened valve seats more than 50 years ago. One would think the metallurgy knowledge is readily available for aviation manufacturers to do the same. They have had no incentive to do so because the government (FAA) has continued to push the use of leaded fuel. Neither seems to have any inclination to change.
UL94 is supposedly 100LL without the lead.
How, then, does it have a different aromatic profile?