Ethanol (ethyl alcohol) is currently the only car fuel made in large quantities from biomass, under fast-expanding initiatives to wean industrialised economies from dirty, costly oil.

Biomass sources such as corn, sugarcane and other plants are rich in potential energy, in the form of large chains of carbohydrates.

In their plant form, these long sugary molecules comprise six carbon atoms and six oxygen atoms.

But car engines like a leaner form of carbohydrate molecule -- one with between five and 15 carbon atoms and with very little oxygen.

Most ethanol facilities harness biology, using enzymes to break down starch and cellulose to glucose, which is then fermented by a common yeast, Saccharomyces cerevisiae, to produce ethanol and carbon dioxide.

But the process takes days and the fuel still has relatively high levels of oxygen, which reduces its energy density, makes it evoraporate readily and leaves it liable to water contamination by absorbing atmospheric humidity.

Distillation is needed to separate the fuel from water, and this process is energy-intensive.

Biofuel engineers at the University of Wisconsin at Madison believe they have found the answer in a process that results in 2,5-demethylfuran, or DMF, which volume for volume delivers 40 percent more energy than ethanol.

In addition, it is not soluble in water and is stable in storage.

Under the process, reported on Thursday in the British journal Nature, enzymes rearrange the plant carbohydrates into a highly oxygenated sugar, fructose.

The next step is to turn the fructose into an intermediate chemical, hydroxymethylfurfural, or HMF, by using an acid catalyst and a solvent with a low boiling point. This expels three oxygen atoms.

Under the final phase, the HMF is converted to DMF by exposing it to a copper-ruthenium catalyst that kicks out two more oxygen atoms and converts the gas to a liquid at a lower temperature, thus making it easier to use as a general fuel for transport.

More research is needed before the technology can be commercialised, according to lead researcher James Dumesic, a professor of chemical and biological engineering, who has previously won plaudits for work on HMF.

"There are some challenges that we need to address, but this work shows that we can produce a liquid transportation fuel from biomass that has energy density comparable to petrol," he says.

Biofuels are touted as a green alternative to traffic fuels derived from oil and gas.

Both biofuels and fossil fuels emit carbon dioxide (CO2), the principal greenhouse gas blamed for climate change.

The carbon emitted by fossil fuels is pumped out of the earth, where it has been stored for millions of years, and so it adds to atmospheric pollution.

But in biofuels, plants suck CO2 out of the atmosphere in order to grow, and this carbon is returned when the fuel is burned.

The process is renewable and environmentally-friendlier than fossils but not completely clean. Energy has to be used to harvest and process the biomass, and this makes biofuels carbon-positive, not carbon-neutral.

Another growing concern against biomass is the environmental impact of converting land, especially in Amazonia, to growing fuel crops. In addition, the rush to biofuels is having an impact on some food prices, as cornfields are given over to ethanol production.

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