'Sticky chains' promise crash-safe fuel
Chemical engineers have produced an additive for jet fuel that reduces the risk of violent explosions, but - at least in laboratory tests - does not compromise engine function.
They say it could cut the likelihood of fireballs erupting when aircraft and other vehicles crash, or are attacked.
It contains very long polymer molecules which stop the fuel forming tiny, explosive droplets, but break when it flows through pipes - and then rejoin.
It is described in the journal Science.
"Initial engine tests showed no adverse effect on performance and our hope is that these polymers will save lives in fatal crashes in aviation and ground transportation," said lead author Julia Kornfield, of the California Institute of Technology.
Efforts to produce explosion-prevention additives, including a major initiative following a deadly runway collision in Tenerife in 1977, have been hampered by twin problems relating to the chemistry of hydrocarbon fuels.
The obvious choice for such an additive is a long, chain-like molecule, which causes the liquid to hang together in bigger, less flammable droplets. But a fuel also has to be pumped and filtered and these forces readily tear up those long molecules into smaller, ineffective fragments - and, crucially, if the polymer remains intact and resists those forces, then it will probably hamper fuel handling or engine performance.
Prof Kornfield and her colleagues have solved this dilemma by designing very long "megasupramolecules" that break up when the fuel flows through pipelines, but then reassemble.
This rebuilding occurs thanks to "sticky ends" which the team designed in a theoretical model, before manufacturing and testing the additive. This design was no easy task, because the snapped ends must join back together to make long, straight chains, without forming circular molecules.
Once they had a polymer that looked promising, the team added it to both diesel and jet fuel and tested how well it prevented explosions - both before and after putting it through a fuel pump 50 times.
The results were promising.
"We shot a projectile at 140mph (225km/h) at a small fuel tank and observed the resulting mist using high-speed imaging," said Prof Kornfield, who worked with collaborators at the Jet Propulsion Laboratory in Pasadena to develop the tests.
"We had three continuously burning propane torches deliberately in the path of the mist to make sure that it would ignite."
In fuel treated with a small quantity of megasupramolecules, the droplets in the mist were bigger and any tiny ignitions quickly extinguished themselves - whereas the untreated fuel, predictably, turned almost instantly into a fireball.
Tests in generator motors and a diesel engine showed that the fuel was just as efficient with the additive and even produced less soot than untreated diesel.
The team is still investigating how the molecules might affect regular petrol - which, because of its very low flashpoint, is particularly explosive. But Prof Kornfield said they were "cautiously optimistic".
If future development goes well, she added, the additive could be available for commercial use in two years, for diesel fuel, and five to seven years for aviation fuel.
Andrew Cooper, a chemistry professor at the University of Liverpool, said that engineering a new additive based on the principles of polymer chemistry was "an interesting take" on the problem - and that carrying the concept from design through to testing was impressive.
"It's a nice example of basic polymer physics and polymer chemistry translating into something that is, in principle, very useful," he told the BBC.
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