Bentley is pinning its green future on more efficient petrol engines powered by biofuel together with lighter-weight body construction rather than a radical switch to diesel or hybrid powertrains.

The company announced at Geneva a plan to move all its line of sports cars and saloons to a corporate fleet average of 120g/km on a 'well-to-wheel' basis by 2012. Currently its range averages 450g/km on a well-to-wheel basis.

"This is a major step in the history of Bentley," said chief executive Franz-Josef Paefgen, "reflecting the increasing expectation from our customers around the world for performance motoring with fuel-efficient engines."

New engine management systems and improved transmissions will cut around 15% from tailpipe emissions, which currently average 400g/km across the range.

Bentley said: "A new powertrain will be introduced by 2012, delivering a further 40% reduction in fuel economy."

Details of this engine weren't revealed, but its is widely interpreted to mean a mild-hybrid using stop-start technology and de-clutching alternator, also running on second generation biofuel.

Bentley expects second generation biofuels to be widely available around the world by then. Gen 2 biofuels are created from waste material and don't compete with production of food crops.

Bentley's chief engineer Uli Eichhorn rules out the imminent arrival of diesel and full-hybrid power.

"We don't think our customers, particularly in North America and Europe, are ready for a diesel Bentley," he said.

Although chairman Franz-Josef Paefgen refuses to rule out a diesel, Eichhorn said: "Not ruling it out, does not mean it is our top priority."

Hybrid powertrains don't fit the typical useage cycle of a Bentley, according to Eichhorn. "They are very good in cities and stop-start driving, but that's not where our cars are used."

Julian Rendell