"We do anticipate that at least one of the buses will run on a central London route so that we can see how they work in the middle of the city and so Londoners can actually get a look at these buses," a TFL spokesman said.

The project has the backing of the European Commission, which in December last year awarded a grant of 18.5 million euros ($18.3 million) to the nine cities involved in the pilot: London, Madrid, Barcelona, Amsterdam, Hamburg and Stuttgart in Germany, Porto in Portugal, Stockholm and Luxembourg.

The three 70-seater buses to be supplied by EvoBus UK, part of German auto firm Daimler Chrysler , will arrive in early 2003 for everyday use in the smoggy capital, decked out in the familiar bright red livery of London's famous double-decker buses but with green technology under the hood.

"Basically there are a lot of short-term measures to reduce both toxic emissions and the carbon dioxide emissions from vehicles. In the longer run, people are looking for zero-emission vehicles and although you can use batteries...the range associated with those vehicles is not very high, so the great hope of the motor industry is the hydrogen fuel cell," Roger Higman, senior transport campaigner of UK green lobby Friends of the Earth.

The fuel cells in the buses combine hydrogen with oxygen releasing electrical energy, heat and water, and the only emission is harmless steam, unlike combustion engines that run on fossil fuels like gasoline and pump out the so-called greenhouse gases thought to contribute to the depletion of the earth-shielding ozone layer.

GREEN MACHINES

David Jollie, editor of FuelCellToday.com, a web-based industry resource centre for fuel cell technology, said: "Fuel cells have the potential given the right fuel to be more efficient, no matter what, than diesel or gasoline engines."

Fuel cells produce energy like a battery but require no recharging and never run down.

The cell itself is like a sandwich in which the top slice of bread is smeared with hydrogen and the bottom slice with oxygen.

The filling, a thin plastic membrane, is coated with the precious metal platinum to speed up the electrochemical reaction.

Inside the cell, hydrogen atoms at the positively charged electrode shed their electrons and pass through the platinum membrane to react with the oxygen at the negatively charged electrode, creating an electrical current to power the vehicle.

Although still in their infancy, the cells use fuel more efficiently than combustion engines and this greater efficiency means getting better mileage per unit of fuel, which should cut fuel bills, experts say.

"The other main benefit they (fuel cells) have, that is particularly relevant for London, where we're looking at local air quality being a big issue and also carbon dioxide, is local fuel emission...they produce less carbon dioxide and hopefully, therefore running costs can be lower," FuelCellToday.com's Jollie said.

NO POLLUTION

Because fuel cells rely on chemistry rather than combustion, emissions from this type of system would still be much smaller than exhaust from the cleanest fuel combustion processes.

"All of our existing bus fleet use traditional fossil fuels. But our new buses are all fitted to meet Euro III standards, the older buses are now matching Euro II standards, taxis also are being made a lot cleaner," the TFL spokesman said, referring to the European Union's tough regulations to restrict vehicle fuel emissions.

Traditional combustion engines have high-temperature processes to burn fuel and run the vehicle which all adds to the amount and toxicity of the emissions.