But automakers say such cars are unlikely to move out of the research lab any time soon, even as fuel efficiency becomes a must for more and more customers appalled by prices at the pump.

The main problem is that diesel hybrid cars cost too much to produce - thousands of dollars more than petrol-electric hybrids like Toyota Motor Corp's Prius, which is a sell-out U.S. hit.

"Diesel hybrid is one possible propulsion system that we are researching and testing, but no one can say whether this is the path to the future," said Edith Meissner, a spokeswoman for German-American carmaker DaimlerChrysler.

"It is certainly the most efficient method among the conventional powertrains ... but also the most expensive."

Standard diesel engines burn less fuel than petrol engines. Hybrid technology makes them even more frugal by letting cars run on stored electricity captured during braking and coasting.

But this lands a double whammy on costs. A diesel engine typically costs around 10 percent more than its petrol-driven cousin of similar power, even without the cost of adding an electric motor, batteries and the electronics to run them.

A Toyota diesel hybrid truck that went on sale in November 2003, for instance, costs around $10,000 more than its diesel-only version, a third more.

Whether consumers are ready to pay that kind of premium for super-efficient cars remains open. Toyota doesn't produce a passenger-car version of the diesel hybrid.

BETTER THAN FUEL CELLS?

A Bernstein Research study estimated that hybrid cars able to go 20 miles on electric power alone would still cost 20 percent more than conventional cars, even if manufacturing volumes rose to 100,000 a year by 2010. Fuel savings would recoup the price premium only after 300,000 miles - more than twice most cars' life expectancy.

Toyota, the Japanese carmaker that made petrol-electric hybrids a real hit, questions whether it is viable, practical or sensible to sell diesel hybrid passenger vehicles in the foreseeable future.

"Before adding a hybrid system to a diesel engine, we still think there's room for improvement in the diesel-only and gasoline-only engines in terms of fuel efficiency," a Toyota spokesman said. Honda Motor Co. Ltd., another force in hybrids, also played up the costs and played down the prospects of launching serial production of a diesel hybrid car.

General Motors, Ford and DaimlerChrysler all came up with fuel-sipping diesel hybrids in the 1990s under the U.S. government-backed Partnership for a New Generation of Vehicles, but never marketed them.

Still, some researchers think diesel hybrids could hold their own even against highly touted hydrogen fuel cells - widely seen as a candidate for powering a big portion of future cars, but which would need a huge energy infrastructure overhaul.

A study by the Massachusetts Institute of Technology's Laboratory for Energy and the Environment found that even with aggressive research, fuel-cell cars won't beat diesel hybrids on total energy use or greenhouse gas emissions by 2020.

It concluded that intense research on a diesel-engine hybrid could produce by 2020 a vehicle that is twice as efficient and half as polluting as fuel-cell cars including the emissions and energy used to deliver the fuel and make the vehicle.

Many carmakers are pressing on.

"It is more difficult to hybridize diesel, but we are going to show that it is possible," Jean-Martin Folz, chief executive of Europe's number-two carmaker PSA Peugeot Citroen, told an industry conference in Frankfurt last month.

A spokesman said PSA was working with two British firms to develop a diesel hybrid version of the Citroen Berlingo car, but had not yet decided whether to make it commercially.

Volkswagen will enter an experimental diesel hybrid car at the Michelin Challenge Bibendum in Shanghai next month, a contest for alternative powertrain vehicles.

"The technology is there and has been well known for years. The problem with hybrid is the battery technology," a VW spokesman said, noting no one yet has come up with a hybrid battery that lasts as long as the car it powers.