Can ethanol, which can be made from corn, prove to be a credible clean fuel alternative to gasoline for piston engines? Experience is positive, but doubts remain
As the aviation industry searches far and wide for replacements for petroleum, ethanol has emerged as a potential general aviation fuel that is reliable and clean. But there are significant hurdles still to be overcome before ethanol gains widespread acceptance as an alternative to aviation gasoline.
"While ethanol is a very good fuel for piston engine aircraft, it's not suitable for jets," says Dr Max Shauck, a pioneer for ethanol in aviation. But he adds: "In 26 years of testing we have found it is the best fuel there is." By the time ethanol fuelled Shauck's historic 1989 flight across the Atlantic Ocean in a single-engined Velocity, he had already been an advocate of the fuel for more than 10 years. He promoted the fuel for crop dusters in Brazil, where one third of the transport fuel is now ethanol, and where in October 2004 Embraer's Ipanema received the world's first certification for a production aircraft using 100% ethanol fuel.
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Embraer subsidiary Nieva began delivering ethanol-fuelled Ipanemas to Brazilian operators in 2005 |
As director of the Institute for Air Science at Baylor University in Waco, Texas, Shauck first earned supplemental type certification (STC) for 100% ethanol in a Cessna 152, then in 2000 he obtained an STC enabling the crop-dusting Piper Pawnee to fly on both avgas and ethanol. That was the first dual-fuel certification granted by the US Federal Aviation Administration.
For use as a fuel, ethanol - or ethyl alcohol, the same as that found in alcoholic drinks - is most often made from corn. It has its limitations and advantages for piston engines, but will not work in gas turbines because its energy density and specific energy are too low. Aircraft range and payload would be greatly limited, and ethanol's low flashpoint, at just 12ºC (53.6°F), would present safety dangers. Design considerations negate those drawbacks in piston aircraft, Shauck says. "What I do in my air shows is fly an aircraft that's been modified to take advantage of the higher octane. I fly at a 10.5:1 compression ratio and I get much better performance."
A bonus is the doubling of time between engine overhauls. "There is less vibration when you use ethanol as a fuel," says Shauck. "That's a function of the larger range of flammability. All of the fuel is consumed in the initial spark. With gasoline that's not the case, because you have a very narrow range of flammability. It goes back into the cylinder in liquid form and then it ignites later than it should." Aircraft range is less with ethanol than with avgas, but planning ahead overcomes that, he argues.
Skip Byrnes, fuels programme manager for the FAA, says decreased engine power is one of many issues with using ethanol that still need thorough testing. "It takes a lot more fuel flow to produce the same power," he says. "Frankly, we're not as far along as Dr Shauck is in proposing that ethanol be the complete replacement [for avgas]. We just have not tested enough here to be confident saying that yet."
In December, the FAA released the findings of its first extended test of aviation-grade ethanol (AGE) in a piston aeroengine, which confirmed the decrease in power that other researchers had reported. The 150h endurance test of a Lycoming IO-360C modified with adjusted ignition timing and increased fuel flow showed the engine produced 2.3% higher peak power on AGE-85 than on iso-octane reference fuel, but peak power required 56% more fuel mass flow (Flight International, 2-6 January).
The fuel used in the test, AGE-85, is "a blend of at least 85% ethanol denatured with 2% gasoline, less than 1% biodiesel, and the rest pentane isomerate", says the FAA, which has been sponsoring research on the fuel as an alternative to widely used 100LL avgas at South Dakota State University. "Using AGE-85 would increase fuel weight by 9% above that of 100LL for an equivalent volume of fuel and would reduce operating range by 35% from 100LL," the report concludes.
Decreased power may be acceptable considering the benefits of cleaner emissions and domestic production. General aviation is exempt from the US ban on leaded fuel, but efforts to develop a suitable unleaded avgas are still under way and ethanol could be an answer. The FAA's endurance test did not study emissions, but other studies have found lower levels of carbon monoxide and volatile organic compounds with ethanol than with avgas.
The 150h endurance test pushed the engine to maximum temperatures to test wear, performance, materials compatibility, range, efficiency, oil dilution and deposit formation. "The engine experienced normal wear except for the exhaust valve face and stems, which showed a hammered effect. There were minimal combustion and fuel system deposits and a moderate level of intake valve deposits," the report states. "We need a lot more endurance testing on ethanol," says Skip Byrnes.
Corrosion concerns
The biggest concern is ethanol's uncertain effect on fuel systems. Corrosion is one reason there was so much opposition to a Congressional proposal last year to require petroleum fuels in the USA to have a 10% mix of renewable fuel by 2010. The "Ten by Ten Act" was a proposed amendment to the Clean Air Act that ignited fierce opposition in 2006 across general aviation in the USA. Opponents hoped aviation could be exempted, but then Senator James Inhofe, an Oklahoma Republican and pilot, announced in July he was killing the bill in his committee.
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Vanguard Squadron's Van's RV-3A aerobatic kitplanes burn ethanol made from South Dakota corn |
"It looks like there are a lot of aircraft here," said Inhofe, owner of a Grumman Tiger and a Van's RV-7, at last year's AirVenture show in Oshkosh, "but I can take you to one parking lot in the Pentagon that has twice as many cars. So there aren't enough aircraft to make that a problem with air quality."
Inhofe's move came after Experimental Aircraft Association (EAA) president Tom Poberezny pointed out in a letter to Congress that the FAA prohibits STC holders from using ethanol blends. "Despite several attempts by EAA and others, ethanol-blended gasoline, of all grades, has not been able to meet the FAA flight safety fuel certification standards," he wrote. "EAA, Cessna and FAA studies have proven that ethanol-blended fuels are harmful to recreational and general aviation aircraft and their fuel system components (rubber lines, fuel pumps, rubber seals and fuel tanks). Vapour lock is also a critical flight safety issue caused by the use of ethanol-blended fuel in aircraft engines."
Republican Congressman Gil Gutknecht of Minnesota, author of the Ten by Ten Act, lost his seat in the November 2006 elections, but not before Congress heard the concerns, says his spokesman Jon Yarian. "This kind of change and this sort of legislation is going to ruffle feathers," he says. "There is going to be some disruption, but obviously we feel that the experts and scientists in the industry can mitigate that risk and make this workable for everybody, including the aviation industry."
Ian Walsh, vice-president and general manager of Textron's Lycoming piston-engine business, says general aviation companies need to adapt to alternative fuels. "Not in the short term, but more in the long term," he says. "Domestically, I think it's too early. Overseas I think it's going to be a real issue."
Micheal Wagner has tested 5% ethanol blends in Rotax engines as head of the Austrian manufacturer's aircraft centre, and plans to test higher ethanol-content mixes. "In the European community there is a three-year mandate to increase alcohol in fuels, and of course we have to follow that," he says, adding that ethanol is easy to work with and is good to the engine. "Combustion is stable. It burns nicely. It's clean. Performance is good. If you tune the fuel/air mixture right you will not easily kill an engine with it. For the engine, it's much better than avgas."
Reducing emissions
The presence of oxygen in the blended fuel is easily dealt with, Wagner says. "We have to adjust the air/fuel ratio on the carburettor setting, and the injector setting has to be checked. The detonator should be no problem." The only trouble with the fuel, he says, is its corrosive effect on some plastic and rubber components between the fuel tank and engine. "Everything that's in contact with the fuel - injectors, fuel pump, O-rings, sealants, gaskets - has to be resistant."
The debate baffles Steve Thompson who, as a pilot for the Vanguard Squadron, uses 100% ethanol in his aerobatic aircraft's Lycoming IO-320 engine. "We've been operating since 1993 on 100% ethanol," he says, adding: "Just 10% ethanol - virtually any aircraft engine will burn that. It's more hype than anything that's making all the noise." The downside, he says, "is you've got to convert your fuel injector and get it to flow more. Then there's the matter of getting the fuel itself." The squadron is sponsored by the South Dakota Corn Council, which supplies locally produced ethanol.
The council, Shauck and other supporters are quick to point to the reduced emissions of ethanol, which would help meet looming greenhouse gas deadlines around the world. Shauck concedes there are slight increases in emissions of nitrogen oxides. "But there's a decrease in carbon monoxide, a decrease in volatile organic compounds, and a very important thing is that the use of ethanol sequesters CO2. You get about the same amount of CO2 coming out but the CO2 that is sequestered by the growing of biomass to create the fuel offsets that."
There are other drawbacks to ethanol, points out John Heimlich, vice-president and chief economist of the Air Transport Association, the trade body for US airlines. "It freezes in pipelines. It's one thing to have fuel it's another to get it into the aircraft."
Source: Flight International
