The race to develop more fuel-efficient and less polluting cars has picked up pace as gas prices, exacerbated by a series of devastating hurricanes in the United States, hit record levels this year and as worries heighten over the impact of global warming on climate patterns.

Most auto makers agree that fuel-cell cars powered by hydrogen produced with renewable energy sources are the end game since they would rely on no fossil fuels and emit only pure water. But most said the technology was at least a decade away.

"The only breakthrough technology is fuel cell because this is the one that guarantees you are out of oil dependency," Carlos Ghosn, who heads both France's Renault SA (RENA.PA: Quote, Profile, Research) and Japanese partner Nissan Motor Co. (7201.T: Quote, Profile, Research), told reporters this week.

"But this will take a very long time."

In the meantime, auto makers are caught in a war of words over what the best medium-term answer is, adding more possibilities such as dual-fuel engines, "plug-in" electric hybrids, diesel hybrids, and even the once-forsaken pure electric vehicles in a bid to gain an edge over their rivals.

Late to join the hybrid party, many European car makers touted the advantages of clean diesel systems, which they argued cost less and got better mileage than gasoline-electric hybrids depending on driving habits.

"Despite the big public debate right now, it will just be a niche technology," BMW AG (BMWG.DE: Quote, Profile, Research) Chief Executive Helmut Panke said, forecasting hybrids to account for no more than 5 percent of all cars in the long term.

German peer Volkswagen AG (VOWG.DE: Quote, Profile, Research) agreed that much of the hype around hybrids was unwarranted, suggesting instead that the developing synthetic fuels could prove more effective in cleaning tail-pipe emissions, especially when used to power diesel engines.

"We need alternative energy sources, not just improvement in powertrain," Research Director Wolfgang Steiger said.

He added the know-how in processing synthetic fuels, which are extracted or fabricated from solid earth materials rich in hydrocarbons, could come in handy for hydrogen production when fuel-cell vehicles arrive in earnest.

SETBACK FOR FUEL CELLS

On the other hand, Toyota Motor Corp. (7203.T: Quote, Profile, Research) is eager for hybrids to take off, adding the powertrain option on more models. Among them will be the Lexus LS flagship sedan, a hybrid concept of which was unveiled at the auto show on Thursday.

In addition to working on hybrid and fuel-cell technology, other Japanese auto makers such as Honda Motor Co. (7267.T: Quote, Profile, Research) and Nissan are scrambling to improve control systems on internal combustion engines to achieve better fuel economy.

"With all these possibilities on the horizon, it's tough to predict what the future is going to hold," Ford Motor Co. (F.N: Quote, Profile, Research) Director Mary Ann Wright said.

"But we believe there are going to be many solutions, and that's why we're taking a portfolio approach."

Larry Burns, vice president of research and development at General Motors Corp. (GM.N: Quote, Profile, Research), said this proliferation of powertrains was holding back progress in fuel-cell technology as auto makers stretch their limited resources thin and wide.

"I think we need to think hard about making the automobile dramatically simpler with hydrogen and fuel cells," Burns told Reuters, noting that a fuel-cell propulsion system had one-tenth as many moving parts as an internal combustion engine.

While skeptics of fuel-cell vehicles often bring up high infrastructure costs, GM, one of the most vocal proponents of the technology, said it would only take $10 billion to $15 billion to make hydrogen available to 70 percent of the U.S. population.

GM expects to develop a fuel-cell propulsion system that would be at least as functional and affordable as today's gasoline engines by 2010.

The world's biggest car maker won't say when it would begin mass-producing the vehicles, but Burns said that if 500,000 to 1 million were built, they would "cost no more than a gasoline car."

At the Tokyo Motor Show, GM is displaying its Sequel concept, the only fuel-cell car in the world that can run 300 miles (483 km) between refueling.

DaimlerChrysler AG (DCXGn.DE: Quote, Profile, Research), another major player in the field, debuted a prototype car that can run either on fuel cells or on a lithium ion battery and can crank out 115 horsepower. Fully charged, the family sized car has a range of 400 km.

"It's going to take collaboration to transform the industry," Burns said.

"The world is on a trajectory where it's becoming even more urgent to get this done, whether it's 9/11, the Iraq war, the explosive growth of China's economy or hurricanes that expose the vulnerability of energy infrastructures.

"We need to move faster rather than slower."