Carmakers are going back to the drawing board in the hunt for fuel-saving technologies as hopes that electric vehicles will be the silver bullet for CO2 emissions look increasingly forlorn.
There is a growing awareness that conventional hybrids and slow-selling battery cars simply won't be enough to meet rigid EU emissions limits.
Among those showing off new ideas at the Geneva car show this week, Volkswagen presented its diesel-electric XL1 - a low-slung two-seater that burns less than a liter of fuel per 100 kilometers - while PSA Peugeot Citroen rolled out a compressed-air hybrid.
Automakers are broadly on track to meet the interim goal of trimming vehicles' average CO2 output to 130 grams per kilometer by 2015. But drastic steps are needed to meet the 95 gram target set for 2020 and the potential for tougher standards after that.
"We can't get the necessary gains we need with traditional technology any more. We're seeing a real break with the past," Peugeot innovation chief Jean-Marc Finot said in an interview.
Arthur Wheaton, automotive expert at Cornell University, offers a succinct summing up of the problem. "Battery technology has not been able to resolve the century-old problem of too much weight and limited range capability," he said.
Despite the billions spent by the likes of Renault-Nissan to develop electric cars, optimism about their future has "dampened considerably", KPMG said in a survey in January.
World leader Toyota, which launched the Prius hybrid in 1997, dropped plans for broader sale of the battery-powered eQ last September, saying it had misread demand.
GM's Opel scrapped plans for a fully electric Adam subcompact, citing high costs, while VW's luxury Audi brand shelved the electric R8 coupe and Nissan slashed the price of its Leaf after disappointing sales.
"Demand for electric cars isn't where we thought it would be," said Francois Bancon, Nissan's upstream development chief. "We're in a very uncertain phase, and everyone's a bit lost."
For automakers battered by Europe's prolonged market slump, the investment costs are a big concern. Several have joined forces to develop new technologies, most offering some degree of "hybridization" of combustion engine and electric power.
"By now we would have seen a standardization based on the pure electric car if it had turned out to be the solution," said Guillaume Faury, Peugeot's executive vice president for research and development. "That's why we're seeing so many micro-hybrids, mild hybrids, full hybrids, rechargeable hybrids, range extenders and battery cars."
Another response has been to shrink engines, removing cylinders and adding turbochargers to maintain horsepower.
VW's XL1, which draws heavily on aerodynamics, is powered by a 0.8 liter twin-cylinder engine. That substantially undercuts the fuel consumption of the 1 liter three-cylinder Up! mini, VW's smallest and cheapest production car to date.
Peugeot's Hybrid Air system, developed with German supplier Robert Bosch, will use a separate hydraulic motor driven by nitrogen compressed by energy recovered from braking.
Longer-term relief may come from cars driven by hydrogen fuel cells, which can cover much longer distances on a single top-up and refuel more quickly than battery cars.
Fuel-cell vehicles, in common with rechargeable models such as Nissan's Leaf, are propelled by electric motors. Instead of a battery, however, a "stack" of cells combines hydrogen with oxygen to generate the electricity.
Daimler, Ford and Nissan have announced joint plans to launch affordable fuel-cell cars within five years, while Toyota and BMW aim to do so by 2020.
But even if those goals are met, initial sales volumes are unlikely to make a significant contribution to the next round of EU-mandated CO2 cuts, experts say.
To make up the difference, carmakers have little choice but to squeeze more gains from existing engines as the costs and risks of developing breakthrough technologies are too high for most, said Klaus Stricker, a consultant with Bain & Company.
"I don't expect anything new to come into play in the next five to ten years," Stricker said.
Output of the XL1 - VW is planning to build 250 this year - will be too low to make a dent in the German group's fleet emissions any time soon. But the vehicle, touted by its maker as the world's most fuel-efficient production car, could be used by VW to push for "supercredits" with the European Commission.
Supercredits allow manufacturers to produce a quota of cars that exceed the CO2 target if they also make vehicles with very low emissions. German carmakers have most to gain from this because they could reduce the changes to their luxury cars.
Their poorer mass-market cousins, however, face a more fundamental challenge.
"There's more and more regulation, but customers want to pay less and less," Nissan's Bancon said. "So we have to cut prices and increase technology content - that's the headache we're faced with."
(Additional reporting by Barbara Lewis in Brussels; Editing by David Goodman)