Future Fuels, Part 2

When 100LL phases out, the replacement will almost certainly be a 100-octane equivalent. But which one?


If you’re the owner of an airplane—certified or Experimental—that requires 100-octane avgas, you’ve certainly suffered pangs of worry about keeping the thing fed with fuel for the next decade. While the agita is understandable given escalating pressure to eliminate tetraethyl lead, the view from inside the fuels industry is quite different. Piston aviation fuel is still a big enough business—$1 billion-plus a year—to attract companies to serve the market. Bottom line: there will be an unleaded replacement for 100LL, it’s just that no one yet knows what it will be, how much it will cost or who will make it.

In part one of this series, in the May 2013 issue of KITPLANES®, we outlined the regulatory challenges for developing and fielding a new piston aviation fuel. In this follow-up article, we’ll examine some of the specific fuel candidates.

New Game, New Rules

With EPA moving forward with potential lead emissions restrictions, the FAA got busy last year in building a certification framework for an unleaded avgas replacement. Last year, the FAA’s Unleaded Avgas Transition rules committee finished its work and has stood up a new bureaucracy called AIR-20 to oversee fuel certification efforts, with an 11-year worst-case timeline, although most of the work is expected to be completed in half that time or less.

AIR-20 is gearing up to test and certify at least two high-profile candidate fuels—Swift Fuel and GAMI G100UL—plus another additive-based replacement fuel developed by the chemist who created a popular oil additive called CamGuard. Although nothing has come out of the ground publically yet, now that the FAA has provided a certification framework, it’s possible that the major oil companies—Shell, BP, ExxonMobil, Chevron and ConocoPhillips—will invest in their own fuel projects.

In Europe, French refiner Total is making some inroads with 91UL, a fuel that’s essentially made of the same aviation alkylates as 100LL, but without the lead. Three fuel choices—100LL, mogas and 91UL are becoming common on European airports, with 91UL priced between 100LL and mogas. Although 91UL meets an ASTM spec and could be refined and sold in the U.S., FBOs have expressed little interest in dual-fuel distribution that would require additional tanks and pumps. The FAA and members of the UAT-ARC believe a single-fuel 100LL replacement is most likely, even if it isn’t a direct drop in.

And don’t count out mogas, which was popular during the late 1980s when Petersen Aviation and EAA offered hundreds of autogas STCs on popular engines and airframes and not just Lycomings and Continentals. But by the 1990s, the boom was all but dead, a victim of both a smaller price delta between mogas and avgas, but mainly due to the flood of ethanol Congress mandated to be added as an oxygenate to motor fuels.

Because of ethanol’s affinity for water, all of the STCs require mogas with no ethanol added or E0. Thanks to enough E0 demand for boats and recreational uses, mogas for aviation is available, albeit not easily.

Which of these fuels will become the dominant replacement for 100LL is anyone’s guess at this juncture. In this article, we’ll run down the pros and cons of each, but the fuels market remains too murky to make long-term decisions on what might or might not be available. All we can say is that there’s high probability that some form of 100-octane unleaded fuel will be available.

Swift Fuel is a binary blend of mesitylene and isopentane. So far, it’s been made only in pilot-plant quantities.

Swift Fuel

Swift Fuel came out of the ground in mid-2008, the fruit of a research project by Swift Enterprises, a tech startup in Indiana with close ties to nearby Purdue University. Swift stirred the pot to a high boil with initial claims of a fuel that could be made from biomass, would perform better than 100LL and would cost about half as much to produce which, at the time, portended of $2 avgas. In the intervening years, economic reality has tempered the initial claims, but Swift is still in the game pitching and about to enter the FAA’s newly erected certification maze.

Without lead to foul plugs and valve guides, Swift Fuel leaves cylinders deposit-free.

Although Swift said its fuel could be made from biomass sources, it’s technically not a bio-fuel, but a binary chemical fuel composed of mesitylene (trimethylbenzine) and isopentane, a highly volatile hydrocarbon. In its original patents, Swift described a process that used biomass such as switchgrass or sorghum to produce acetone, which is the primary feedstock. The acetone is then catalytically reacted to produce the two binary chemicals blended into the finished fuel, with a butanol byproduct.

From the outset, Swift’s research has tilted toward the catalytic stage of production forward, not production of the acetone itself. Although Swift says it hasn’t entirely ruled out making fuel itself, it envisions scaling and perfecting the process and selling it to other companies for actual production. Such companies would, presumably, be on their own to acquire the acetone feedstock via bio methods, traditional petrochemical refining or from natural gas feedstock. With natural gas plentiful and prices soft, acetone from this source could bode well for Swift Fuel’s price.

Still, even though Swift says its fuel will be competitive with 100LL, its economics remain unproven. But its performance looks promising. The FAA’s initial testing showed that Swift Fuel has 13 percent higher heat content than run-of-the-refinery 100LL, with octane ranges between 102 and 103 MON, again typical of avgas. Swift doesn’t meet the ASTM D910 spec for avgas and so will be certified to an emerging ASTM standard. It still needs to find willing capital to manufacture the fuel.

Swift fuel continues to undergo test-cell and flight testing.

As of early 2013, Swift was contracting to build a proof-of-concept pilot plant capable of at least 10,000 gallons a month, according to Swift’s Jon Zuilkowski, who has been working on the project since 2008. Further, Swift has identified two potential candidates to run a fleet fuel test on a Cessna 172 STC’d to burn Swift Fuel. But unlike GAMI, Swift doesn’t intend to sell STCs directly to owners so they’ll have Swift Fuel as an option. The STC is merely to allow fleet tests for data collection purposes.

Swift’s Jon Ziulkowski: STCs for fleet testing of Cessna 172s were expected sometime in 2013.


General Aviation Modifications Inc. entered the fuel fray in early 2010, with a proposed avgas replacement called G100UL. This fuel is basically aviation alkylate dosed with a higher percentage of aromatic hydrocarbons, such as the mesitylene found in Swift Fuel or some form of xylene, a readily available solvent made from catalytic reforming of petroleum, although it can also be made from coal.

GAMI has experimented with many blends of these two chemicals and its reports indicate MON octane ratings similar to 100LL minimum specs under ASTM D911. (Typical FBO 100LL tests between 101 and as much as 107 MON, according to ASTM research.)

As with Swift, G100 is being made in pilot-plant volume. GAMI is seeking approvals for a wide number of models under the STC process.

GAMI’s George Braly says that the economics—as with Swift Fuel, largely unproven—may favor xylene as the primary blendstock because mesitylene isn’t widely available.

High aromatic content fuels potentially pose at least two challenges. This class of chemicals have high solvent properties and can react with seals and o-rings, softening them. Second, at cold temperatures, they can be difficult to vaporize, resulting in starting problems. Thus far, GAMI’s soak tests of various materials, including composite fuel tanks, hasn’t revealed significant material softening issues. The FAA is accepting materials compatibility testing already done on G100UL by Cirrus.

Initial cold starting has been similarly successful, but GAMI has further testing planned. Braly says the FAA has also approved some of its flight testing, although more is planned. “We’ve got a couple of in-house tests and a 150-hour block test on the Cirrus engine [to go]. That’s it.” Braly says.

GAMI’s George Braly testing G100UL. “We’ve got a couple of in-house tests and a 150-hour block test on the Cirrus engine [to go]. That’s it.”

The FAA has historically required fuel suppliers to seek ASTM approvals before fuels can be officially certified for aircraft use. However, rather than follow that route exclusively, GAMI sought to certify its fuel via an STC process. It has spent more than three years and hundreds of thousands of development dollars seeking the STC and as of early 2013, the FAA had approved the last of GAMI’s STC test plans. Braly says the initial STC will cover Cirrus aircraft and discussions are in progress to eventually extend that approval to all models and all engines requiring 100-octane or less, contingent on some additional testing.

G100UL has been tested extensively in Cirrus composite tanks, with no evidence of materials softening.

Bottom line: Once the STC is approved and expanded, it’s conceivable that the entire fleet would be covered to burn G100UL.

Then what? “We don’t want to be in the refinery business,” says Braly, “the goal is to issue reasonable and non-discriminatory license agreements to anyone who wants to manufacture the fuel.” That could be so-called tea-kettle small refiners or even majors who may want to produce G100UL if they can make money on it.

Price to the end user is more difficult to predict, but Braly told us he has had small quantities of G100UL delivered to his Ada, Oklahoma research facility for $5.50 a gallon. Larger quantities would almost certainly push the price lower so Braly believes the finished fuel will retail for about 50 cents or $1 higher than current avgas. (As of mid-January, the U.S. national average price of 100LL was $5.90, according to www.airnav.com.)

ASL’s Additive

Chemist Ed Kollin developed the well-regarded oil additive, CamGuard, and has now turned his efforts to an octane additive for aviation fuel. Unlike Swift and G100UL, Kollin’s material would be a direct drop-in for tetraethyl lead, not a clean-sheet blended 100-octane fuel.

Kollin told us early in 2013 that the proprietary compound he has developed delivers the required octane, but he’s working to overcome two significant challenges: cost and aging.

“I’ve got some molecules that do the things I want them to do, it’s just a matter of getting them made at a price that’s reasonable. Right now, it’s five times the cost of where I think it needs to be,” Kollin says. His comment goes to the core problem with TEL, other than its toxicity: It’s cheap and it provides a potent octane boost.

Long-term stability has also proven challenging, Kollin said. “I’ve been surprised at some of those results. This type of chemistry has been stable in other applications, but in fuel, you add a little water and a little copper metal and it just falls apart,” he says. While these problems may be addressed with additional research, Kollin says he can’t put a timeline on when his fuel additive would be ready for testing and certification.


For some owners—and that includes certified aircraft, Experimentals and LSAs—automotive fuel is proving to be the ideal choice, and it’s not just price driving the decision. Although approved for 100LL, Rotax engines have slightly lower maintenance incidence when running unleaded fuel and the same applies to some traditional aircraft engines in which lead fouling is a nuisance, Lycoming’s O-235 being one example. Only about 110 airports sell mogas, but the price delta against avgas is hardly trivial. The national average airport price of mogas, according to www.airnav.com, is about $4.55, nearly a buck-and-a-half cheaper than avgas. It’s even possible to find it a dollar cheaper than that at a few airports.

Mogas remains an option for many owners, especially those of Experimentals. However, it’s available only at about 110 airports in the U.S.

According to Todd Petersen, who pioneered mogas STCs during the 1980s, interest in automotive fuel for aviation is increasing slightly. He says during the last year, he’s sold more STCs for mogas than the year before, many going to aircraft in Europe where mogas is common on airports. In response to worries about continued avgas availability, and to serve budget-minded LSA and Experimental owners, a few airports have added mogas pumps, but some have also removed it because overall mogas availability on airports doesn’t seem to be increasing.

According to www.flyunleaded.com, the best mogas coverage is in the Midwest, especially Wisconsin and Minnesota, which between them, have about 25 airports dispensing mogas. On the downside, other states and regions have so little airport mogas availability to make the choice impractical, unless you’re based at an airport that has it.

Hauling your own is an option for the ambitious, but to do it safely requires the right equipment, preferably an approved tank and pump or at least groundable containers.

This is an easy option for Rotax owners, who can burn widely available unleaded fuel blended with 10 percent ethanol, E10. Ethanol-free premium—E0—may be more widely available than is apparent from the sparse number of airports dispensing it, because there’s a modest demand for it in the recreational marine industry. Generally, where there are marinas, there’s E0 and some specialty suppliers serving the classic car market are also selling it. In California, a start-up called Clear Gas (www.cleargas.com) is trying to push into both the marine and aviation markets with E0.

But make no mistake, this is an uphill battle. According the U.S. Energy Information Administration, refinery output of unleaded, non-ethanol gasoline—known as finished conventional gasoline—has been in sharp decline since 2007, when the Energy Independence Security Act required refiners to blend more ethanol at the same time the government was offering $6 billion in blender tax credits.

“You can still get E0,” says Todd Petersen, “but it’s just hard. Once in a while, I get a call from someone like on Long Island, in New York. I can’t do much for them because it all has ethanol there. I sometimes worry how much longer it will be available.”

The E0 market appears to be somewhat regional in that refiners who have a market for it and whose refineries are configured to produce it, will. But the economics strongly tilt toward blendstock for oxygenate blending (BOB) which is what gasoline is before the ethanol is blended at the terminal, not finished gasoline ready for market. Many refiners have backed off their reformers to favor lower-octane BOB production over finished conventional gasoline without ethanol. The addition of ethanol boosts the octane to required specs.

Other Options

In Europe, French refiner Total is having some success marketing a product called 91UL, which is essentially 100LL without the lead. Total told us it’s distributing throughout Europe and in the United Kingdom, with retail prices set between mogas and 100LL. (In Europe, that’s still north of $8 a gallon.)

91UL is an ASTM-spec gasoline, so it could be refined by anyone interested in the market. Whether the U.S. market could support a two-fuel system is questionable. Lycoming general manager Michael Kraft told us it’s conceivable that 91UL could find a niche or regional market, but it’s unlikely to find wide distribution in the U.S.

Also in Europe, boutique refiner Hjelmco Oil has for years distributed a fuel called 91/96UL which it claims is suitable for 90 percent of the piston aircraft engines in the world fleet. It has a minimum MON of 90.8, compared to min-spec 100LL at 99.6 MON. The Hjelmco product is actually competitive with Total’s 91UL, but neither is likely to gain a market foothold in the U.S.

Three years ago, Continental Motors was pushing another ASTM-spec fuel called 94UL. While this fuel would be suitable for the vast majority of engines in the fleet, including some of Continental’s high-output powerplants, it has gained no support in the U.S. market thus far. In 2010, when Continental was pushing for 94UL as a solution, a grassroots group called Clean 100 Coalition formed specifically to lobby against 94UL and for a 100-octane solution.

94UL lacks the octane for legacy high-compression engines that still require 100-octane and, according to the FAA, the fleet comprising these engines burns nearly 70 percent of all the avgas consumed in the U.S.

Although the unleaded replacement for 100LL won’t be a drop-in, it’s expected to use the same FBO pumping equipment.

Where It’s Going?

There’s little point in painting an unrealistically rosy picture here. Aviation gasoline production is in decline, diminishing from 337 million gallons in 1990 to 230 million gallons in 2009, according to the Energy Information Administration. Demand has picked up slightly since 2010 and appears to be leveling out. While that represents less than a single day’s worth of automotive gasoline production, it’s still a large enough volume to amount to a $1.3 billion-a-year business with a high profit margin for refineries. General aviation’s contribution to the overall economy, according to the FAA, is at least $150 billion.

Everyone we speak to in the industry seems to agree that despite 100LL’s tiny volume, there’s too much profit there to walk away from.

But which fuel will rise to the fore? As of early 2013, that’s not obvious yet, although if GAMI is successful with its wide STC approach, it may have a marketable fuel first. The major oil companies typically don’t share their research plans with the public, but we do know that Chevron was recently awarded a patent for a xylene-based high-aromatic fuel similar to GAMI’s G100. Other majors may have background research underway that they’ll reveal when the timing is right.

For the time being, builders making engine choices can reasonably assume that for those who need it, there will be an unleaded 100-octane choice within five years and possibly a niche or regional market for a lower-octane fuel such as mogas or 91UL. We just can’t predict who the providers will be.


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