The dream of smog-free cities served by whispering fleets of zero-emission cars could still be decades away from becoming reality.
Internal combustion engines have established such a stranglehold on the automotive industry that they will probably outlast everyone alive today, automotive specialists suggest.
Only limited numbers of non-polluting cars that run on hydrogen-powered fuel cells or that burn hydrogen rather than petrol or diesel are on the road, and even their most vociferous supporters say they will stay in the slow lane to the future.
"We have 900 million passenger cars and light trucks on the road and 51 million units of annual production capacity for those vehicles. ..TEXT "It would thus take decades to replace all vehicles with combustion engines by those with fuel cells, even if the fuel cell production would be sufficient," said Thomas Weber, head of research at DaimlerChrysler.
Mr Weber believes the realistic approach will be to keep screwing down emissions from standard engines in steps so that they get as close to zero emissions as possible.
Real zero-emissions driving is possible only with fuel cells or electric cars. These vehicles have largely flopped and their batteries remain a problem.
"Battery technology is the Achilles' heel of electric vehicles today. If battery manufacturers could make a breakthrough then the electric car could see a renaissance," Mr Weber said.
How often you can drain and then recharge batteries before they wear out remains a major problem area.
Fuel cells use the interaction between hydrogen and oxygen to produce electricity that powers the car while emitting only water vapour.
Fuel cell technology is making rapid progress, but still costs far too much and lacks the network of filling stations that motorists will need to tank up.
"Infrastructure is a big question mark," said Takis Athanasopoulos, former executive vice-president of Toyota in Europe and still a senior company adviser.
He said that much of the hydrogen used today is produced with the help of traditional fossil fuels.
"This technology is not yet clean technology," he noted.
Toyota is the world market leader in hybrid vehicles which reduce fuel consumption and thus emissions by yoking an electric motor and battery to standard engines.
The cars can run on battery power alone at low speeds and recharge by capturing energy released in braking.
As an abundant, clean energy source, hydrogen is a natural choice for a world facing a limited supply of increasingly expensive fossil fuels. Estimates differ widely on when fuel cells will catch on after initial estimates were far too bold.
DaimlerChrysler thinks commercial sales may start around 2012 but that fuel cell cars will have a single-digit share of the new car market by 2020 at the earliest.
Other industry experts think even that is too ambitious unless governments force the issue by levying London-style congestion fees on polluting cars.
Charles Stone, research head at Canadian fuel-cell company Ballard Power Systems, acknowledged that cost and durability were the biggest challenges.
"If it had a market share of 50 per cent by 2050, I would die a very happy man," he said.
The recent rise in the price of oil has increased pressure on carmakers to illustrate their green credentials.
BMW has just announced it will launch a hydrogen-burning 7 Series executive car early next year, but it costs so much that selected customers can only lease it.
Carmakers and suppliers admit they are unsure what form their industry's future will take and are worried about putting all their eggs in one technology basket.
"Actually nobody knows at the moment where this is going to end. I believe you have to have a very broad investment in technology because probably all of these (technologies) are going to be required as we go forward," Ford of Europe president John Fleming said last week.
Karl-Thomas Neumann, head of German tyre maker and car parts group Continental's automotive division, cautioned: "This is not something where someone will just turn a switch,
"Emissions will be driven down and fuel consumption will be driven down with the existing engines. At the same time we will see the whole spectrum of hybrids, from microhybrids to full hybrids."
Microhybrids save fuel by turning engines off at stop lights and restarting them when drivers step on the gas.
Mr Weber thought fuel cells would first gain use in urban areas and in fleets - for instance, buses that return to a depot every night.
"This solves the challenge of infrastructure and fuelling stations. The technology will be used in rural populated areas.
"But for now it makes little sense to implement it coast to coast in the US."