The search for an alternative to leaded avgas is gathering pace. Europe is backing diesels, but the USA is still seeking a viable unleaded avgas.

Within a decade, the aviation gasoline burned by most of the world's general aviation aircraft could be a thing of the past. While the search for a true replacement continues, the supply of lead additive for avgas is in danger of drying up as global demand for leaded fuel evaporates.

In the USA, where aviation is the last user of leaded fuel, industry expects 100-octane low-lead (100LL) avgas to be unavailable or extremely expensive within 10 years. While lower-octane unleaded alternatives are already available, a high-octane unleaded avgas able to replace 100LL totally has yet to emerge.

In Europe, where scarcity is already driving up avgas prices, attention is focusing on new diesel-cycle aero-engines capable of burning the same Jet-A kerosene as turbines. But the US goal remains a single unleaded avgas that can replace 100LL without requiring modification to the aircraft or its engine.

"The existing GA fleet must have a single fuel satisfying the octane requirements of the entire fleet," says the US Aircraft Owners and Pilots Association (AOPA). "Airport operators and fixed-base operators cannot afford to have multiple storage and distribution systems for multiple types of fuel because the quantities are much too small to be economical."

Similarly, fuel companies are not inclined to produce multiple grades of avgas. Alow-octane unleaded fuel, 82UL, was approved in 1998 to replace hard-to-find 80/87 avgas in low-compression engines. Around 70% of the single-engine fleet could run on 82UL. All but about 10% of the total GA fleet could run on a higher-octane unleaded avgas which meets the 91/98-grade aviation fuel specification that was reactivated in December. But the goal of a single direct replacement for 100LL remains to be achieved.

Market forces

"There is no deadline for 100LL to disappear, but economics will end it anyway," says Earl Lawrence, Experimental Aircraft Association (EAA) vice president of government and industry relations. Fuel companies are increasingly unwilling to produce and distribute the relatively small quantities of leaded fuel required by aviation. The only plant still producing the tetra-ethyl lead (TEL) additive, located in the UK, is expected to shut down in 2005, leaving only stockpiles. Air BP and Exxon Mobile are among the fuel companies working on a direct unleaded replacement for 100LL. "They both have fuels almost there. Whether they are economically viable is the question," says Lawrence. Reactivation of the 91/98 specification "provides a safety net in case lead is not available and the new fuels do not work", he says. "In the worst case, we can develop engines and engine control systems that can operate on 91."

While work towards a direct replacement for 100LL continues, the 91/98 specification will provide a certification fuel for developers working to make existing engines run on lower-octane fuel. An unleaded avgas meeting the specification, 91/96UL, is already refined and distributed in limited quantities in northern Europe by Sweden's Hjelmco Oil. The fuel is approved for use in all Textron Lycoming engines under 150kW (200hp), without loss of performance. "But 91 is not a complete solution," says Lawrence.

The problem with finding a single-fuel solution lies with the high-output piston engines which power about 30% of the GA fleet, but consume about 70% of the avgas produced, most of which require the performance that 100LL provides. TEL is added to avgas to increase its octane rating and, according to AOPA, "realistically, no other additives have been found that come close to the performance of 100LL".

Europe, faced with near-term economic and environmental concerns about the continued availability and acceptability of leaded avgas, appears to be placing its hopes for replacing 100LL on diesel engines burning jet fuel. At least two new diesel aero-engines have been certificated in Europe in the past year and programmes to develop diesel-powered light aircraft are under way on both sides of the Atlantic.

Advantages claimed for modern digitally controlled, turbocharged diesels over conventional engines include 30% to 40% lower specific fuel consumption, turbine-like single-lever control and constant-rpm operation, simplified maintenance and longer overhaul intervals. In Europe, the wider availability and lower price of Jet A fuel compared with avgas are also factors which favour diesels.

In November, Austria's Diamond Aircraft began flight-testing the DA40-TDI version of its Diamond Star four-seater, powered by a Thielert TAE 135 direct-injection turbo-diesel. The German-made 100kW engine is scheduled for European certification this month, with aircraft certification planned for October. Initially, the aircraft and engine will only be marketed in Europe. According to Diamond, replacing the basic DA40's 135kW LycomingIO-360 with a diesel results in fuel cost savings of 80%, when the lower fuel consumption and fuel price are combined.

Similar savings are claimed for SMA Engines' SR305, also a flat-four, direct-injection, turbo-diesel but rated at 170kW. The Jet A-burning engine received Euro-pean certification in April last year, and US approval is imminent. Ajoint venture between EADS, Renault and Snecma, SMA is working with EADS Socata to certificate the SR305 in the TB20 four-seat light aircraft. Engine production is to begin in France "no later than September", says Al Beech, sales director of SMA's US arm.


In the USA, SMA is working with Embry-Riddle Aeronautical University to re-engine the Cessna 182 with the SR305, and award of a supplemental type certificate (STC) is expected in the third quarter. In addition, the company is working with Cirrus Design and Maule Air on new diesel-powered versions of their light aircraft. SMA says the Cirrus SR21tdi and Maule MX-9-230 are scheduled for certification in the fourth quarter, but Cirrus says it will wait until production engines are available before beginning its programme.

Single fuel or diesels

"The biggest market in the USA is the aftermarket," says Beech. "The original equipment manufacturers are hesitant to go into new technology. Cessna, [New] Piper and Raytheon will wait and see, but they will not wait very long. They are already talking to us."

In the USA, where fuel cost and availability are less factors than in Europe, he believes diesels will sell initially to customers interested in new technology. "The key is to get a couple of STCs under our belt and then start flying people," he says.

Criticisms levelled against diesels include high cost and lack of support. Beech admits the SR305's base price is $52,000, against $25,000-$30,000 for a comparable Teledyne Continental avgas engine, but adds that the target time between overhauls (TBO) is 3,000h, compared to 2,000h for a piston engine.

Tackling the cost issue was the principal goal of NASA's General Aviation Propulsion (GAP) programme, under which Continental began development of a 150kW diesel intended as a direct replacement for its IO-360 piston engine. After much delay, the technology demonstrator is expected to fly in a Cessna 337 testbed in February, replacing the forward engine. GAP's goals for the diesel engine include a 50% lower cost, 75% longer TBO and 25% lower fuel consumption compared with the piston engine.

The GAP programme, under which NASA was also sponsoring the development of a small turbofan by Williams International, ended in 2000, but work on both engines has continued under co-operative research and development agreements which are scheduled to end soon. Continental is then expected to take its new diesel to certification, with the engine likely to become commercially available within the next two to three years.

Even if the cost and support issues can be overcome, the desire for a single fuel could still stand in the way of the USA following the European lead towards diesel engines. "I don't know where diesels will go," says the EAA's Lawrence. "The US goal is one fuel. If we have to have separate aviation diesel [and unleaded avgas] supplies we are back to the old problem."

Source: Flight International