NEW YORK – The largest transit agency in North America has ordered 850 diesel-electric buses and will have the world’s largest fleet of hybrids when the vehicles hit the streets of New York in 2010.

The Metropolitan Transit Authority’s latest, and biggest, order for Daimler’s Orion VII buses will double the number of hybrids in its fleet. Diesel-electric vehicles will comprise almost half of the rolling stock used by the agency’s New York City Transit and MTA Bus systems once the vehicles are delivered.

Daimler said the buses use about 30 percent less fuel than conventional models. The Orion VII is a series hybrid, meaning it is powered entirely by an electric motor. The small diesel engine runs a generator that charges the vehicle’s lithium-ion battery. The buses also use regenerative braking in which braking forces produce current to charge the batteries. Daimler claims the Orion VII emits 90 percent less soot than conventional buses because the diesel engine runs at a nearly constant speed.

Daimler said OC Transpo of Ottawa has ordered 202 Orion VII buses, bringing to 2,600 the number of Orion buses ordered since it launched the hybrid model four years ago.

Texas,Chicago, Philadelphia, and San Francisco are among the major American cities experimenting with hybrid buses. London wants to replace its entire fleet of 8,000 buses with hybrid models beginning in 2012, a move officials said would cut carbon dioxide emissions by 200,000 tons annually.