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Carmakers tout green motors in Geneva

But where's the juice ultimately coming from?

Car manufacturers have been at pains to tout a new green image in the run up to the 77th International Motor Show, which opens tomorrow in Geneva.

A lot of the environmentally-friendly concepts being exhibited are little more than vapourware for now, but some manufacturers have models actually in production.

Most famous of these, perhaps, is Toyota's Prius hybrid. The Prius shipped in Japan in 1997 and is nowadays available worldwide. It generates power using a petrol engine just like a normal car, but the power can be stored in a large battery and then used to drive the wheels electrically. This means the Prius' petrol engine can be run much more efficiently, and that it doesn't need to be as large as the one in a conventional car. Also, kinetic energy which would normally be wasted during braking can now be dumped into the battery for later use. The Prius shows to great advantage in stop-start city driving, where normal vehicles produce huge amounts of poisonous emissions.

Prius-type hybrids are now available from Audi, Honda, and Lexus and there is some interest from American manufacturers. Chevy, GMC, Dodge, and Ford hybrids are planned for the next two years. There is already a so-called "mild hybrid" option available for the Chevrolet Silverado pickup truck, but in this case the battery cannot be used to drive the car. Mild hybrids save small amounts of fuel by allowing drivers to shut their engines down more often, perhaps when coasting or sitting stopped with the air conditioning on.

Hybrid vehicles can achieve very impressive pollution reductions and good fuel economy, but in the end they generate their power by burning fossil fuel. Their carbon burden isn't radically different from that of a conventional car, and different technologies will be necessary to achieve truly green motoring.

One approach is the "plug-in" hybrid, where the car's battery can be charged from the mains overnight. The idea here is to achieve still less use of the internal-combustion engine by drawing grid electricity which may – in future, anyway – be generated by carbon-neutral means.

Unfortunately, the limits of current battery technology mean fully-electric vehicles with petrol-car performance aren't on the cards for now. And a plug-in hybrid is distressingly expensive, as it requires a big, capable battery pack to be of any use.

An alternative is the use of biofuels such as ethanol, derived from plant crops. Here the idea is that the growing plant will have absorbed carbon dioxide before being made into ethanol, so one can merrily burn ethanol and release carbon into the atmosphere with a clear conscience. And in this case, there is no need to accept any performance limitations whatsoever.

Swedish company Koenigsegg has adapted its CCX supercar to take ethanol rather than petrol, and achieved a huge increase in power owing to the biofuel's naturally higher octane rating. The new CCXR ethanol job is rated at a blistering 1018 horsepower.

But green sceptics point out that ethanol production is heavily dependent on US corn-farming subsidies, and that the process of turning corn into fuel uses a lot of energy, which at the moment is typically generated by burning fossil fuels – some say more fossil fuel than would have been required to drive the car in the first place. When transport costs, fertilisers and so forth are factored in, many would question ethanol's status as a green fuel though it would be hard to argue with in petrolhead performance terms.

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