Although the principle of fuel cells is older than the internal-combustion engine (it goes back to 1839), until now they have been so expensive that they were used only in space rockets and some military applications. But they are inherently twice as efficient as internal-combustion engines, capable of converting 40% of the energy in their fuel into movement.

A fuel cell produces electricity from hydrogen via a complex electro-chemical process in which hydrogen gives off electricity as it turns into water. Since hydrogen is such a dangerous gas, a widespread hydrogen distribution network seems out of the question. Car makers solve this problem by using a feedstock such as methanol or gasoline. The feedstock will then be converted on board the vehicle into hydrogen, which in turn will pass through the cell to produce the electricity to power the vehicle. Because of this complication, the feul cell car is not quite emission-free: it still produces a little carbon dioxide as the methanol or gasoline is converted into hydrogen.

Some environmentalists accuse the car industry of going slow on electric cars to maintain its profits. Others think the industry isa prisoner of its past, and should not be pursuing battery-driven electric cars at all. Amory Lovins of the Rocky MountainInstitute in Colorado, whose "negawatts" idea (that investment in energy conservation will often yield greater returns thaninvestment in generating plant) got him a reputation as the "inventor" of energy conservation, calls for a radical type oflightweight "hypercar". He wants cars to be so light that they have to be anchored in high winds.

In the early days of mass production, only steel was strong enough for the chassis and later monocoque shells of their bodies. That made them so heavythat they needed powerful (and heavy) engines to provide enough acceleration. Yet most of the time the power is not required. "Only about 16% of the average engine's power is typically needed for highway driving, 4% for city driving," he wrote in a much-noted pamphlet a few years ago. He thinks cars "for sustainable transport" could have super-strong bodies moulded easily in one stage from plastic composites, like aircraft tails. "We need to design cars less like tanks and more like airplanes."