Hydrogen-fuelled cars are not likely to be commercially viable until at least 2015, despite a two-year test here in which a fleet of test cars performed better than expected.

In the test, which began in 2004, six Mercedes-Benz A-class subcompact test cars each clocked an average of 22,500km and 750 hours.

One particular car clocked more than 1,000 hours at the end of the two years.

"That's better than our expectations of 600 hours," said Dr Christian Mohrdieck, DaimlerChrysler's director of fuel-cell drive systems development.

Dr Mohrdieck said DaimlerChrysler will extend its Singapore trial for another year, using four of the six cars.

"The second phase is to test the limits of the fuel-cell stack," he said, referring to the 'heart' of the car, which combines hydrogen in its tank with oxygen in the air to produce the electricity that powers the car's motor.

It is also the costliest part of the car. For the fuel-cell stack to last the lifetime of a car, Dr Mohrdieck said, it has to work for between 4,000 and 5,000 hours.

DaimlerChrysler is expected to launch its second-generation fuel-cell car - in the form of the bigger B-class - by 2009. That car will be more powerful and have a longer range than the A-class, which can travel for about 160km between refuellings. It will be equipped with a fuel-cell stack that lasts 2,000 hours.

Asked why there are only four cars remaining in the test, Dr Mohrdieck said the German government wants more test cars in Germany.

Partners in the second-phase test are the National Environment Agency, BP Singapore and German airline Lufthansa. The other two members of the first-stage trial - Conrad Centennial Hotel and French tyre maker Michelin - will not take part.

The cars, which emit only water from their tailpipes, currently cost at least 10 times as much as conventional vehicles of the same size. They are heavier than normal cars, but perform just as well in the city.

They are expected to incur similar fuel bills as a family sedan, but may be costlier to maintain.

Only one hydrogen refuelling station remains in Singapore, off Buona Vista Road. The other, in Upper East Coast Road, has closed.