MICHAEL PHELAN / LONDON
Aviation fuel has hardly altered in half a century, but many believe it is about time it did, with removal of avgas lead a high priority
While engine manufacturers have constantly striven for improved performance and lower emissions, one component of propulsion systems has shown a remarkable resistance to change since jets first flew. Aviation fuels have remained essentially the same for half a century, but producers are now looking to incorporate performance boosters and replace leaded avgas. But first there are enormous regulatory hurdles to overcome.
The most common types of jet fuel standards, Jet A and A-1, were drawn up 50 years ago, and are similar except for the lower freeze point of -47°C (-53°F) of the internationally available A-1 versus -40°C for the US standard Jet A. Freeze point is defined as the highest temperature at which solid particles can exist in the fuel, and is critical for cold weather and high-altitude performance. In the same period, lower lead content varieties have been introduced to piston engine avgas, but developments are not as advanced as in automotive fuels.
Shell Aviation, one of the world's largest aviation fuel providers, speaks for the industry when it says the main barrier to introducing improvements to fuels is the tortuous regulatory procedures. Mike Farmery, global technical and quality manager at Shell Aviation, says composition changes such as fuel additives can take between seven and 10 years to be approved.
Farmery says performance-enhancing additives hold promise for the future of jet fuel. Shell is testing its Additive 101 on three KLM Boeing 747-400s, and hopes it will reduce emissions and component wear. Developed by US-based BetzDearborn, the additive is in service with the US Air Force.
Paul Bogers, Shell Aviation project manager for differentiated fuels, says that although originally developed as a thermal stability enhancer for military JP-8 fuel, the benefits should also be seen in commercial operations. "There's a perception that these big engines have no coking problems, but at higher than optimum temperatures and thrust levels you still get fuel injector clogging," he says. Coking is exacerbated by impurities in the fuel, so Additive 101 includes a dispersant to keep particulates in solution. It also provides 56°C additional temperature capacity, says Bogers, who describes Additive 101 as the first performance-based additive for jet fuel.
Engine wear reduction
BP Air is also enthusiastic about the ability of higher thermal capacity fuels to reduce engine wear. "In addition to increasing the heat sink characteristics of the fuel, thermal stability additives have been seen to reduce engine deposits, decreasing maintenance requirements," it says. But the challenges are tough. "The prize is to produce a suitable fuel at a reasonable price which does not adversely affect engine and airframe or fuel-handling characteristics," BP says.
Farmery sees the additive market as the most logistically feasible way to develop current fuels and sees companies using the additives as product differentiators. "For Additive 101, we only need very small quantities - about 256 parts per million [ppm], or 1/4000 of total fuel volume," he says. "It could be offered on a customer-by-customer basis for a premium," he says.
Shell will have to show the benefits of the product to convince airlines that it is worthwhile. "We saw significant benefits in engine wear by adding it to JP-8, but we're not sure what to expect on commercial engines where the maintenance interval is 20,000 hours instead of 200 hours," says Bogers. The KLM engines have run about 17,000-18,000h with the additive since December 2000, and by the end of this year Shell hopes to have its first good look at them. "We expect the benefits may be more flight-cycle dependent than flight- hour dependent, so maybe the short-haul market will be interested," he says.
Even if Shell can show the benefits of its Additive 101, gaining regulatory approval can be difficult. Farmery says manufacturers must ensure their engines run on "the lowest common denominator of fuel", so often the additives can present problems.
He cites an example of a failed attempt to get approval for an additive which allowed dramatic improvements in fuel pumpability, cutting losses and improving capacity in the long pipelines into airports. "By adding just 2ppm, we could see a 40% improvement in pumping efficiency due to drag reduction in the pipes," he says. After a lengthy approval attempt, however, engine maker General Electric found that the additive affected the cold-start performance of some engines, so the idea was abandoned.
Additives may help engine performance, but regulatory groups such as the US Environmental Protection Agency are often more interested in removing substances already in fuel. Sulphur content is set to become an issue, although there are questions about its effect. Sulphur contributes to the particulate emissions from aircraft, which can play a role in the development of contrails behind a jet. These have been linked to effects on local weather patterns and climate change. BP says the issue for fuel producers is removing the sulphur economically while maintaining the fuel's performance characteristics. "The original equipment manufacturers will be concerned about maintaining reliability as sulphur reduction could alter a fuel's lubricity," the company says. Farmery says that although there is no convincing environmental case, general perception is that sulphur content needs to be reduced.
Although jet fuel constitutes most of the aviation fuel used, there is no less effort to bring avgas standards into the 21st century. Avgas, a close relative of automotive gasoline, has the dubious distinction of being the only leaded fuel still in widespread use. It has a high tetraethyl lead content - even the 1980s Grade 100LL (100 octane low-lead) specification has a 0.56g/litre (0.075oz/USgal) lead content, compared to about 0.1g/litre for leaded automotive fuel.
Lead benefits
Lead is a good octane booster, and older engines cannot maintain their performance with unleaded fuels. "There is an 82UL [82-octane unleaded] that works with about 70% of piston-engined aircraft, but not with high performance turbocharged or supercharged types," says Farmery.
Unfortunately for the UL avgas, Farmery says there is a 30:70 split within the piston- powered fleet, with larger high-performance engines accounting for 30% but consuming 70% of avgas. For twin-engined pistons, the allowable performance margin for single-engine diversions means the performance loss associated with UL fuel can be prohibitive, he adds.
While the replacement of lead without performance losses is the "holy grail" of avgas production, Farmery sees two potential options. "There's the effort to increase use of aerodiesel, and there's the potential to use electronic engine management to maximise the performance of the lower-octane UL fuels," he says. He sees the first as impractical as it requires the costly replacement of engines, but says the second may be feasible. "Many operators could live with a small power reduction. For example, in Germany there's a trend for using automotive UL gas in light aircraft, although that's not particularly advisable," says Farmery. There are also doubts as to the relevance of octane ratings for avgas. "It's an old measurement system based on the anti-knocking qualities of fuel, and the ratings don't mean very much on newer engines. We need to look more carefully at the behaviour of fuels during combustion," he says.
Despite technical difficulties, however, it is expected that public pressure will lead to the elimination of leaded avgas in Europe and the USA within 10 years.
Source: Flight International
