A new ASTM specification will be written for an unleaded fuel formulation. Both of the two companies working on formulations for lead-free avgas have submitted applications for ASTM standards for their fuel.
Even though Glenn Passavant, EPA's project manager for this issue, told the industry at AOPA Summit 2010 that "over the next several years we will start getting more monitoring data," suggesting a leisurely approach to the transition, key industry players don't think we have that kind of time. Engine manufacturers Lycoming and TCM are urging adoption of the new avgas standard within the next year.
Octane Boosters
There are a multitude of challenges in creating an unleaded alternative to 100LL, but chief among them is finding a replacement for the lead.
D-910 restricts the maximum amount of TEL in 100LL fuel to 2.0 milliliters per gallon. Real-world 100LL airport pump supplies almost always contain less lead, so our engines can clearly get by without that maximum amount.
The good thing is that TEL is not the only octane enhancer we know of. The bad news? Some are more hazardous than TEL. Beginning in 1992, methyl butyl tertiary ether (MBTE) was added to automobile gasoline to raise the octane number but was phased out beginning in 1999 due to its groundwater-polluting characteristics.
Mesitylene — also known as 1,3,5-trimethylbenzene — is an aromatic hydrocarbon that's been proved to provide adequate detonation protection for today's high-performance aircraft engines. Most of the mesitylene now available is manufactured in China. One estimate puts the quantity in the United States at less than six railroad tank cars. It's expensive on the bulk market. Yet it's a critical component of new avgas formulations being developed by both of the two companies vying to produce tomorrow's lead-free avgas.
New Fuels on the Horizon
One of the companies is forging ahead with a traditional petroleum-based approach. Its formulation uses mesitylene or a related aromatic, such as one of the forms of dimethylbenzene, in place of TEL to jack up the octane of high-grade aviation alkylate base stock. The other company will grow its supply of mesitylene by converting biomass via a proprietary catalytic process to produce sufficient quantities of this aromatic hydrocarbon.
General Aviation Modifications Inc. (GAMI) of Ada, Oklahoma, has developed a petroleum-centered plan for producing the new avgas: Add aromatic compounds such as mesitylene and/or one of the xylene offshoots to the high-quality alkylate base stock (94 octane) that's already in production as the base stock for today's TEL-laced 100LL. Xylene is a plastic precursor and is plentiful. Ta-da! An avgas that can be put into production using existing production facilities and matching up very well with almost all D-910 performance mandates.
