LONDON, — Mayor Ken Livingstone has announced that Transport for London is adding six of the hybrids to the city's bus fleet.

The buses, dubbed the "Prius bus" by the mayor, will run on Route 360 through central London and use a 1.9L clean diesel engine that would be more at home in a family car to keep the electric motor which powers the bus fully charged.

Initial tests show the buses improve fuel efficiency by 40%, reduce CO2 emissions by 38%, nitrous oxide by 89% and carbon monoxide by 83%.

They are also considerably quieter than their traditional diesel equivalents due to reduced noise from the much smaller engine.

"These cleaner, greener hybrid buses are at the forefront of environmentally friendly technology," said the mayor.

"They will help London cut pollutant emissions and so contribute to tackling climate change and improve the city's air, as well as offering passengers a smoother, quieter journey.

"Air pollution affects the quality of life of a large number of Londoners causing around 1,000 premature deaths and a similar number of hospital admissions every year.

"I am determined to improve air quality through reducing transport emissions and if this hybrid bus trial is successful we will expand their use across London."

The buses each cost in the region of £200,000, compared with around £120,000 for a comparable single-decker vehicle.

William Wright, chairman of the Wright Group which built the buses, said that over their predicted lifespan of ten years the saving made on fuel bills were likely to pay for the initial premium.

"As order numbers go up the premium will come down and fuel prices are likely to stay high for some time to come," he said.

"As far as the technology is concerned it's a robust technology, we've been running them for three years in tests.

"It's pushing the frontiers and we've done everything we possibly can to make them reliable."

As well as the six hybrids, six conventional diesel buses will continue to ply Route 360, making direct comparisons in performance fairly straightforward.

But in a city of over seven million, the impact made by six buses will be little more than a drop in the ocean.

"You've got to start somewhere," Mike Weston, operations director for London Buses, said.

"The point is to prove the technology before telling everyone they've got to replace the existing fleet with hybrids.

"The benefit for the environment is considerable and we are hoping that if we can show they work we might encourage others to give them a go too.

"The bus network in London leads the way in the U.K. and beyond in innovation -- this is another good example of technology we hope might bring benefits to the urban environment in the near to medium term future."