Renewable Fuels for Piston Aircraft

Up until recently small aircraft pilots were nervous that an affordable high-performance replacement fuel would not be available once leaded avgas was outlawed. That is, until they leaned about SwiftFuel — a drop-in replacement that so far has impressed manufacturers and the Federal Aviation Administration (FAA) as it marches toward approval as a commercial product. Swift Enterprises, the company that created SwiftFuel has roots in the labs of Purdue University and negotiations are underway to build a 250,000 gallon per year facility nearby.

So far SwiftFuel has passed every test thrown at it, but earning approval as an aircraft fuel is no easy feat. Consequences of a frozen fuel line or corroded gasket in an airplane are deadly. For the FAA to recognize any new fuel it must meet industry guidelines of ASTM International, which could grant a new specification to SwiftFuel by June.

Swift principal investigator Jon Ziulkowski explained that in the past 13 years, ASTM has “never seen anyone meet even half of the specifications. We meet 42 of the 44 specifications, so that's why we've captured [the] interest of the FAA," he said.

Ziulkowski said that the fuel is “chemically identical,” to its petroleum-based counterparts. Sugar beets and sorghum number among the potential green feedstocks, but “we're using your traditional corn ethanol plant at this point because it's there, it's available [and] it's cheap.”

SwiftFuel should not be confused with ethanol, notes Research Director John Rusek. "You can't use oxygenates as fuels. It just doesn't work. The energy is not there," he said.

The ethanol infrastructure, however, can be adapted to produce SwiftFuel, Ziulkowski expained. “We're actually converting your biomass output, your 50/50 wet ethanol midway through an ethanol plant. We're taking that and running it through our catalyst bed in the reactor. It actually rips apart the molecules and converts them into hydrocarbons.” The catalyst is key. “We're using a proprietary catalyst that doesn't require more than your standard atmospheric pressure and 100 degrees centigrade,” explained Ziulkowski.

Zuilkowski and Rusek stress that SwiftFuel is chemically identical to avgas (also called 100LL for “low lead”), but the new fuel emits 20 percent fewer pollutants than the old. Fuel characteristics feature zero sulphur emissions and a much lower freezing point, while offering a 15% range boost. Testing thus far shows no deterioration of plastic or rubber engine components.

FAA testing at the William J. Hughes Technical Center measured the Motor Octane Number (MON) of SwiftFuel at 104.1, higher than avgas and experimental non-leaded petroleum substitutes. In the final FAA report, it states: "On a volume basis, the Swift 702 fuel contained 13 percent more energy than the 100LL. Operation on the Swift 702 fuel resulted in an average decrease in volumetric fuel consumption of approximately 8 percent." 

The MON of 104.1 compares to avgas’s 99.5, referring to how precisely the fuel ignites.  The higher number also prevents the "knocking" that harms engines. Without it’s tetra-ethyl lead additive, avgas has a MON of 94. Most piston aircraft can operate at MON 91 or even 80, which is what most automobiles need.

At least 70 percent of the piston fleet doesn’t need the lead additive and can use automotive gasoline, but the 30 percent that needs high performance fuel are flown much more often — consuming 70 percent of the avgas sold. In 2008 that was 186 million gallons, or .14 percent of the gasoline used by American automobiles.

But the aircraft that can use automotive gasoline are losing that option, too, as ethanol blends become universal and percentage increases are likely. There's disagreement in the industry about how costly system modifications are to counter corrosive effects, but most pilots want nothing to do with ethanol. The Experimental Aircraft Association (EEA), a light aircraft trade group, is ready to give up its fight against blending ethanol into all auto gasoline. “That’s a losing battle,” said Doug MacNair, EAA vice president of government relations. “Frankly, it runs at cross purposes with the petroleum industry itself, who wants a product with universal distribution.”

As fuel supplies for these niche aircraft are squeezed from all sides, urgency is growing among pilots and manufacturers to find an avgas replacement and to find it quickly. New aircraft still need avgas, but in 2008 engine and aircraft manufacturers decided to leap toward alternatives together. The manufacturers are deciding now whether to go with diesel engines or to accept unleaded fuels, but none of that will help the existing fleet. Whether it's a variety of new fuels and new engines, or one drop-in alternative fuel acting as savior, experts agree fuel costs are about to go way up.

Many industry analysts are skeptical that SwiftFuel can succeed where other new fuels have failed, but each successful test renews optimism that a drop-in replacement could be possible — especially if the price is right. Avgas costs around $5 per gallon and Swift Enterprises claims it can attain manufacturing costs of $2 per gallon and retail prices of $4 per gallon, although those costs hinge on industrial-scale production since manufacturing costs in the lab hover around $60 per gallon today.

Ziulkowski pointed out how beneficial it is that almost no other researchers are trying to solve the avgas problem. “When you have really no competitors in the industry you can charge the same price and get away with it. We know we can deliver the fuel to the pump a lot cheaper than they do with 100LL currently,” he said.

Avgas is only used in small piston engines aircraft and never in jet engines. Jet aircraft fuel uses no lead and there are dozens of research labs attempting to create cost-effective alternative jet fuels.

Image: Avgas and jet fuel tanks behind fencing.  Credit: Jeffrey Decker

Jeff Decker is a freelance journalist covering energy and aviation and living in Oshkosh, Wisconsin.