Octane Is The ‘Prima Donna’ Of Avgas, Engine OEM Says

Octane is the “prima donna” of the effort to develop a high-octane unleaded fuel for the wide variety of piston-engine aircraft in the U.S. but it is just one of many fuel characteristics to consider, says Jennifer Miller, Lycoming Engines senior director of engineering.

“Octane is merely the ability of a fuel to withstand compression in an engine without detonating,” Miller said July 27, during a presentation at EAA AirVenture Oshkosh. “When you burn fuel, you want a nice, smooth increase in pressure and when you have detonation you get sudden explosions in your combustion chamber and that could be a very bad day. With runaway detonation you can burn holes in your pistons.”

Miller briefed pilots on fuel constituents and characteristics as the general aviation (GA) industry and the FAA pursue development of a high-octane, unleaded avgas to replace 100 Low Lead (100LL), which is needed by roughly 30% of the U.S. piston-engine aircraft fleet to safely operate. Two partnerships are developing candidate fuels under the Piston Aviation Fuels Initiative (PAFI), a long-running industry-government testing program, and two through the FAA supplemental type certification (STC) process, which is proprietary between the agency and the fuel developers.

“I’m not necessarily referring to any of the candidate fuels,” Miller said, emphasizing that caveat. “I would not share anybody’s IP, I won’t comment on anybody’s formula. I won’t give my opinion. I’m here merely to talk about what your engine cares about.”

Echoing a message that Lycoming Engines Senior Vice President Shannon Massey delivered during a media briefing in June, Miller said the density, volatility, freezing point, stability, compatibility, engine durability and producibility of a new fuel are important considerations in addition to octane rating.

“Octane is important,” she allowed. “I think it’s the prima donna of this discussion. It’s really [the reason] why replacing 100 Low Lead is so difficult. But if we’re chasing octane with these candidate fuels, it’s really important that you don’t lose sight of other important characteristics. Octane is not a measure of any other quality of a fuel.”

Aircraft with high-horsepower, high-compression piston engines require high-octane avgas; many of them are mission-oriented “workhorses” used to ferry supplies to remote populations, patrol borders and support military operations, Lycoming says. The engine manufacturer, based in Williamsport, Pennsylvania, holds type certificates for 650 engine models. There are more than 100,000 Lycoming type certificate engines in the GA aircraft fleet, of which more than half require a higher-octane fuel, Massey told reporters.

Most of the Lycoming-powered training aircraft fleet is powered by lower compression engines that should be able to run on lower-octane fuel, Miller said during AirVenture.

The higher the octane rating of a fuel, the higher its resistance to detonation. The fuel additive tetra-ethyl-lead (TEL) is an octane enhancer that is used in blue-dyed 100LL, the most common type of avgas.

In the U.S., leaded gasoline was gradually phased out and finally banned in 1996 for use in vehicles other than aircraft, race cars, farm equipment and marine engines. Piston aircraft that operate on leaded fuel are the largest remaining source of lead emissions into the air, contributing 70% of the lead being emitted annually, according to the U.S. Environmental Protection Agency (EPA). 

Types Of Octane Enhancers

Miller described four categories of octane enhancers: aromatics, oxygenates, aromatic amines and metals, including lead and the compound Methylcyclopentadienyl Manganese Tricarbonyl (MMT), each with advantages and disadvantages for engines, materials, or the environment. MMT has been approved for use in small quantity in automobile fuel but is known to leave manganese-containing deposits on engine and emissions system components. “I am sure there will be discussions about the manganese in some of the [avgas] candidate fuels and its tendency to cause deposits,” Miller said.

The industry standard specification for avgas is ASTM D910; another specification, D5059, describes standard test methods for determining the concentration of lead and manganese in gasoline. A different standards organization—SAE—governs piston-oil specifications.

FAA type certificate data sheets for Lycoming engines reference Service Instruction 1070, “Specified Fuels For Spark-Ignited Gasoline Aircraft Engine Models,” which identifies approved fuels that can be used when refueling a Lycoming-powered aircraft. Making any change to the 1070 document would require a full recertification effort, Miller said.

In her presentation, Miller recounted the historical irony of Thomas Midgley, Jr., a General Motors mechanical and chemical engineer who advanced the use of TEL in gasoline and of chlorofluorocarbons—or Freon—as refrigerants in the 1920s, introducing two of the world’s most disastrous inventions for the environment.

To avoid similar unforeseen consequences, the level of detail and due diligence underpinning industry specifications should be applied in evaluating unleaded avgas candidates, Miller said. “In particular we have a lot of new novel additives that we’re evaluating for the replacement of avgas,” she said. “An important question to ask the fuel producers that are presenting their candidate fuels is: are you going to create another problem? Are you [using] a more toxic additive that nobody is looking at right now but in 10 years the EPA will be on our necks again?”

Fuel Developers Report Progress

On July 24 at AirVenture, the executive committee of the Eliminate Aviation Gasoline Lead Emissions (EAGLE) initiative invited representatives of the four developers of candidate high-octane unleaded fuels to report on their progress. The EAGLE committee consists of top executives of major GA industry associations and the FAA’s Aircraft Certification Service. Each of the fuel developers was allowed 10 minutes for their presentations.

The PAFI developers are LyondellBasell-VP Racing and Afton Chemical-Phillips 66. The STC developers are Swift Fuels and General Aviation Modifications Inc. (GAMI). 

GAMI received broad FAA authorization of its G100UL fuel across the U.S. piston-engine aircraft fleet in September 2022; the small engineering company known for developing precision fuel injectors and aftermarket turbochargers is trying to gain acceptance of G100UL by engine and aircraft manufacturers and fuel suppliers. It is the only candidate fuel developer that has not sought a product specification through the ASTM process.

“The industry has been working on a high-octane unleaded avgas for over 20 years,” George Braly, GAMI’s co-founder, told the AirVenture audience. “We need to stop loving this problem and just fix it. General Aviation Modifications Inc. has fixed the problem.” 

He alleged that there has been “a lot of false emphasis—deceptive if not outright false—publicity and comments that are being made by people and institutions that do not want to see this happen.”

Braly had previously informed BCA that he has arranged with Vitol, a blending company in Houston, to produce the high-octane unleaded fuel and approached avgas suppliers Avfuel, Epic, Titan and World Fuel Services to send rail cars there to load it. He says Cirrus Aircraft and Robinson Helicopter have tested the GAMI fuel and engine manufacturers have been offered supplies if they sign non-disclosure agreements.

“We have engaged an extremely large, existing producer of aviation jet fuel [that] formerly produced 100 Low Lead,” Braly said at AirVenture. “They’re in the process of making ready a four-million-gallon production tank in which they intend to begin producing G100UL as soon as they get the new filters and equipment. And we have made a written offer to all the existing distributors of jet fuel and 100 Low Lead to make arrangements with Vitol to move that fuel to the California market because that’s where the environmental activism is the strongest and the risk to the pilots that require high-octane fuel is the greatest.”

Representing the Afton Chemical-Phillips 66 collaboration, Enrico Lodrigueza, Phillips 66 regional fuel quality director, said the team’s candidate fuel in 2021 received ASTM product specification D8434, “Standard Specification for Unleaded Aviation Gasoline Test Fuel Containing Organo-metallic Additive.” The candidate fuel is “substantially similar” to ASTM D910-specification fuel but instead of lead uses a manganese-based octane enhancer, Lodrigueza said. The fuel has completed detonation testing on a Lycoming engine at the FAA Technical Center and emissions testing with the National Research Council Canada, he reported.

Dual Routes To Fleet Approval
During the separate Lycoming presentation on July 27, questioners asked Miller to explain the differences between developers seeking approval of their fuels through the STC route and those undergoing the PAFI process. The STC developers “started with a company spec so they didn’t have to share their IP with industry when they were developing these candidate fuels and that’s appropriate—it’s allowed, but you still have to demonstrate at the same level of due diligence [of] ASTM D910,” she said.

“If you’re a fuel producer and you think you have a really special secret sauce and you don’t want anybody to know what it is, you go the STC route so that you don’t have to socialize your formula,” Miller explained. “ASTM is a really broad, cross-functional group. In addition to engine and airframe OEMs, you’ve got Exxon, Shell, Phillips, Chevron—all of those folks in the same room, a lot of operations folks, chemists. They’re going to eventually have to ask questions about what’s in your fuel as you develop a spec.”

When an unleaded fuel successfully completes the PAFI testing regime, the FAA will issue a fleetwide authorization, allowing its use in aircraft.
 
“The nice thing about PAFI is at the end of the day, the FAA administrator says PAFI has done significant testing, that the fuel they validated under PAFI is effectively a drop-in replacement,” for 100LL, Miller said. “That was the industry’s initial goal for PAFI. What we really wanted was that golden ticket, so that Lycoming didn’t have to change all our STCs, that we didn’t have to do all of the cert testing. 

“Unfortunately, what we’ve learned is that there is no such thing as a drop-in replacement,” she added. “Every candidate fuel is different in one way or another. What we need to do is understand what the differences are and mitigate those differences and the risks to the fleet.”