GA and the Environment: Plant Power

What's the answer to avgas?

July 1, 2009

The original claims describing a bio-fuel replacement for avgas sounded too good to be true.

Swift Enterprises, a small start-up firm in Indiana, announced last year that it had developed an unleaded, domestically produced, higher-octane aviation fuel that could be manufactured and produced at a far lower cost than avgas. It promised its bio-fuel would reduce emissions 20 percent and increase aircraft range up to 15 percent.

Laboratory tests by the FAA confirm that Swift’s avgas replacement will work in current piston aircraft engines, even the high-compression, turbocharged varieties that are particularly susceptible to detonation. (Teledyne Continental Motors has begun flight tests using Swift fuel.) The bio-fuel burns cleanly, has acceptable vapor pressures, and even offers some range and performance advantages over traditional avgas. So far Swift has only produced its product in tiny batches. And the unit costs for making it that way are extraordinarily high.

U.S. refiners currently sell about 250 million gallons of leaded avgas a year. That may sound like a lot, but it’s far less than a day’s production of unleaded auto fuel. The world’s aviation emissions—including airlines and military jets—account for about 3 percent of all carbon-dioxide emissions, and general aviation is a tiny fraction of that amount. But avgas is one of the few fuels that still contain lead, and a combination of new regulations, environmental considerations, and economics may soon make leaded fuels untenable.

Swift plans to use switchgrass as its fuel source. An abundant, fast-growing plant that used to cover much of the American heartland before corn, soy, wheat, and other crops took over, switchgrass has a higher energy output than food crops, and using switchgrass in bulk won’t drive up food prices. Swift is building a manufacturing facility near Purdue University Airport in Indiana where company officials hope to prove the merits of high-volume fuel production.

Since Swift fuel is unleaded, it can travel in the same delivery pipelines refiners use to deliver auto gas. Avgas must be segregated from other petroleum products throughout the production and delivery process and stored in separate containers. Trucks, rail cars, and vehicles that carry leaded avgas can’t be used for unleaded fuels. Swift officials say they expect some cost savings based on its ability to use the existing transportation infrastructure for other unleaded fuels. FAA certification of Swift fuel, or any other bio-fuel, is likely to take several years at least. While Swift pursues FAA approval, it’s also selling its specialized fuel to car, motorcycle, and even air racers willing to pay a premium for the added performance they say it brings.

Diesel aircraft engines got a black eye last year when German manufacturer Thielert sought bankruptcy protection after the founder was accused of financial misdeeds. The company’s troubles dealt a serious blow to Diamond Aircraft and other aircraft manufacturers that depended on Thielert for a steady supply of engines and parts, as well as owners of aircraft with Thielert engines. Cessna had already announced plans to manufacture Thielert-powered 172 Skyhawks for the world market when Thielert imploded.

But despite the turmoil Thielert caused the GA industry, the company proved that modern diesel engines are technically feasible, and there’s a demand for the relatively simple, highly efficient powerplants that can run on kerosene-based jet fuel.

Diamond has partnered with Austro Engines to replace Thielerts on its ultra-efficient DA42 twins. Also, French manufacturer SMA is continuing its certification efforts for its own purpose-built diesel engine that has logged many hundreds of test hours in a Cessna 182 and Maule aircraft. U.S. engine manufacturers Lycoming and Continental have studied diesel engines intensively, although neither has brought one to market.

The simplest solution to moving away from lead in aviation fuel would be to continue manufacturing avgas—but without the lead. Such a fuel is currently produced in Europe as 91/96UL and about 70 percent of the piston-powered U.S. aircraft fleet could use it today without any engine or airframe modifications. Piston aircraft engines don’t require lead for lubrication. It’s there to raise octane levels and prevent detonation in high-compression and turbocharged engines that operate at elevated temperatures and internal pressures.

Such high-compression piston engines comprise about 30 percent of the current U.S. piston fleet. But because they are typically found on business aircraft that fly more often and consume more fuel on an hourly basis, those engines account for roughly 70 percent of avgas sales. AOPA has long been committed to finding a single fuel that can serve as a safe, reliable avgas replacement for the entire piston fleet. Petroleum producers and aircraft engine manufacturers have been experimenting with unleaded avgas and say the results are encouraging. But they haven’t announced plans to approve it for everyday aircraft use.

U.S. engine manufacturers and aftermarket firms have long been studying electronic engine controls that would allow piston aircraft to operate on today’s avgas, auto fuel, or any combination of the two. None of those efforts have produced a product ready for FAA certification yet—but such tools may be getting close.

George Braly, founder of General Aviation Modifications Inc. (GAMI), has said he plans to fly an aircraft with a turbocharged, high-compression engine using unleaded fuel and electronic controls to a major aviation event this year. GAMI has long been developing an electronic system known as PRISM that would allow high-compression piston aircraft to use unleaded fuel and improve engine performance at the same time.

The PRISM system is designed to continuously monitor and control cylinder pressure and combustion timing for optimal efficiency. Company officials say the PRISM system would replace both magnetos on a standard aircraft engine with an equally reliable, redundant system. PRISM is designed to give pilots a steady stream of information about the amount of power their engine is producing as well as internal temperatures and pressures. It senses changing temperatures and pressures and automatically adjusts spark plug timing for peak performance.

Other aviation firms have explored variable timing and electronic engine controls for aircraft, but GAMI appears closest to gaining FAA certification of a product designed to allow the use of unleaded fuels in turbocharged, high-compression aircraft engines.

The U.S. Air Force and commercial airlines including Air New Zealand, Continental, Japan Air Lines, and Virgin Atlantic have performed high-profile test flights using bio-fuels made from algae, coconut oil, and other products—but for different reasons. The U.S. Air Force uses far more jet fuel than any other military service branch and wants to become less dependent on foreign sources of oil. USAF officials have said they plan to approve a 50/50 mix of petroleum and synthetic fuel for its jet aircraft fleet. The Defense Advanced Research Projects Agency is working on similar programs to find a bio-fuel alternative to JP-8 jet fuel.

Airlines are looking to bio-fuels to reduce costs, increase supply, and reduce carbon emissions. Virgin Atlantic flew a partially bio-fuel powered Boeing 747-400 on a test flight from London to Amsterdam this year. The fuel, produced from coconut oil and other sources, was designed to dramatically lower carbon dioxide emissions.

Turbine engines are less finicky about fuels than piston engines, but turbines—and the fuel they use—must operate at extreme temperatures. And there is promise that current research may yield new fuels and new tools for current GA engines, which will power GA aircraft into the future.

E-mail the author at dave.hirschman@aopa.org.