Shell Announces Unleaded 100-Octane Fuel (Updated)

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

Shell Oil announced on Tuesday that it has developed an unleaded 100-octane piston-engine fuel to replace 100LL and hopes to achieve certification of the product within two to three years. Although the company has no hard numbers on pump price, it predicts that its new product will be comparable in price to 100LL.

Tim Shea, Shell’s VP for aviation fuels development, told AVweb on Tuesday that the fuel is the culmination of a 10-year internal research project to find an unleaded 100-octane fuel, a problem that has dogged the industry for more than three decades. Although Shell currently doesn’t directly refine piston avgas in North America, Shea said it intends to make the new fuel widely available, but he declined to describe any specific licensing terms. “Our plan is to make this fuel, once approved, widely available on a global basis. Whether that’s through Shell refineries or licensing, the plan is to make it available,” Shea said.

Traditional avgas is composed of what refiners call aviation alkylate, a blend of branched-chain hydrocarbons such as isomers of isooctane which, of themselves, have high octane and good anti-knock characteristics. Refiners add a small dose of tetraethyl lead to boost octane to a bit over 100 to meet the requirements of ASTM fuel spec D-910. Tetraethyl was banned from automotive fuels during the 1980s and there’s pressure to remove it from aviation fuels to achieve new, more stringent air quality standards. Shea explained that Shell is using an aviation alkylate base with a blend of aromatic compounds to deliver a fuel with performance characteristics almost identical to 100LL. 

“In our formulation direction, we started with what aviation gasoline looks like and then removed the lead. From there, the question was how do we maintain D-910’s physical properties while achieving the MON requirement for high-octane fuel? It’s fair to say it’s alkylate-based in its approach,” Shea said. If this approach sounds familiar, it should; General Aviation Modifications, Inc., one of two other companies proposing a 100LL replacement, is using similar formulation.

“A lot of the chemistry has been around, but what we’ve figured out is how to make the chemistry work in an aviation fuel. A lot of the existing molecules that people are well aware of struggle in aviation applications because when you use them, you really struggle with the low temperature properties of the fuel,” Shea explained.

To prove those properties, Shell will embark upon an intensive program that will include submission to ASTM International for a new piston-engine fuel spec. Shea said the new spec will be almost identical to D-910 in performance, but will vary slightly.

“In a physical property sense, we are extremely close. We meet every performance criteria and the two that we’re off, we’re off very slightly,” Shea said.

Following ASTM approval, Shell will submit its fuel to the FAA’s Piston Aviation Fuels Initiative program that emerged for the FAA Aviation Rulemaking Committee for unleaded fuel last year. Last summer, the FAA asked for fuel candidate submissions from the industry and Shell’s new product represents the first major oil company to announce a candidate fuel. GAMI and Swift Fuels are already well along in testing, but haven’t formally submitted to the FAA yet.

We were surprised to learn that Shell has been working the problem for a decade, but it now thinks the timing is right to move the fuel quickly through the certification process. “The FAA timelines are a little bit longer than ours. We think we can commercialize this fuel quicker than what the FAA has currently laid out,” Shea said. Shell will concentrate its testing on materials compatibility, which is always a concern with high-aromatic fuels because of potential seal and o-ring swelling caused by high concentrations of compounds such as toluene and xylene.                        

For its initial proving, Shell enlisted two OEMs, Lycoming Engines and Piper, both of whom have done basic testing on the new fuel. “From a performance perspective, it appears to be the equivalent of D-910 100LL fuel. From a materials compatibility perspective, we haven’t seen anything on the engine. But it’s beyond Lycoming’s expertise to judge that,“ said Michael Kraft, general manager of Lycoming. Kraft said Lycoming is contracted with Shell to examine and test the fuel. The company has tested the fuel in its most octane-demanding engine, the TIO-540-J2BD. Piper has flown the fuel in a Piper Saratoga.

Kraft said for Lycoming to certify fuel usage on its engines, it will need an ASTM spec for a basis. “We’re watching to see what Shell’s next step is with regard to ASTM. That will give us something to work with,” Kraft said. With an ASTM spec in hand, engine approvals are relatively straightforward for Lycoming, but the issue isn’t as simple as that. Hundreds of airframe models will also have to be approved and everyone in the industry is hoping for some kind of blanket approval.

Shea said configuring refineries to make the new fuel is essentially an overnight process. “Then the question becomes does FAA grant blanket, fleet-wide certification that would allow everyone to essentially switch overnight?,” he adds. To get as close to that as possible, Shea said Shell wants to make its replacement fuel look at much like a D-910 avgas as possible.

And what of price? “It’s a bit early stage, but our early estimates are that it will be comparable to the current leaded product,” Shea told us. “Historically, if you look where unleaded fuels have come to displace leaded fuel, the cost generally goes up a bit, but it should be within a very reasonable figure,” Shea added.