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.
The only other option for here-and-now technology is hydrogen, with the first hydrogen cars now appearing in limited numbers. Hydrogen's big attraction is that an engine burning it emits only harmless water vapour.
BMW's Hydrogen 7  demonstration vehicle debuted last November, and the company is leasing out a few hundred for consumer trials in a sort of motoring beta test. The Hydrogen Seven is essentially no more than an existing BMW 760 limo with a barrel-sized insulated cryogenic tank in the boot. The engine is a normal big-car job, which runs happily enough on hydrogen rather than petrol although – in the current state of the technology – it loses a little power when doing so. It can also use the car's ordinary petrol tank, which is just as well as there are only seven hydrogen filling stations in Europe.
There are some snags in view for aspiring Hydrogen Seven drivers. For one, the cryogenic fuel will all boil off after nine days whether it is used or not. This also means that the car cannot be left parked in an enclosed space – a garage, for example – without risking a Hindenburg-style explosion. For another, the car will only go a bit over a hundred miles on a tank of hydrogen.
Most big firms don't see this kind of vehicle as the way ahead, preferring to use hydrogen to generate electricity in a fuel cell and then drive the car's wheels with hybrid-style motor-generator units.
Honda's FCX , due to beta-test from 2008, will probably be the first example in normal drivers' hands. It's already used by a few organisations as a Honda marketing tool. The FCX, however, involves very serious performance compromises.
Unlike the Hydrogen Seven, which handles pretty much like a normal Beemer in hydrogen mode, the FCX tops out at 80 horsepower. This has enabled it to act as the official pace car for the Los Angeles Marathon, but not for any motorsport events.
At the moment, hydrogen is at least as big an environmental nightmare as fossil fuel, as its manufacture requires not only power but natural gas. The Hydrogen Seven is estimated to have as much environmental impact under current production methods as a heavy diesel truck. However, the motor firms are looking forward to some future day when hydrogen would be cracked from water using cleanly-generated electricity.
When pressed, car execs usually speculate that this electricity might be from solar, wind or wave plants. Even radical greens, however, don't really think that these methods could yield enough power for current domestic/industrial use, let alone colossal amounts more to produce clean fuel for road transport (sample quote from Greenpeace (pdf) : "Renewable energy, combined with the smart use of energy, can deliver half of the world's energy needs by 2050"). This problem probably applies to ethanol production as well.
Few are saying so openly, but it appears that any truly zero-carbon car of the future would be ultimately dependent on nuclear power. ®