While green car enthusiasts wait for the next generation of hybrids that use different fuel sources or rely more on electricity, big cities are now placing orders for buses that combine hybrid controls, diesel engines, and electric drives. Yesterday, Daimler AG received an order for 850 of its Orion VII hybrid diesel-electric buses from MTA, the agency that oversees public transit in New York. The new order will double the MTA’s diesel hybrid bus fleet to 1,700, making it the largest in the world.

The Orion VII buses are powered by BAE Systems’ HybriDrive technology, which are about 30 percent more efficient than conventional diesel models. Unlike today’s consumer hybrid vehicles, the buses are propelled solely by an electric motor that is powered by a diesel-driven generator. The diesel engine’s only function is to power a generator that produces electricity for the motor.

Daimler claims the buses burn cleaner than conventional diesels, producing about 90 percent less soot. That’s because the diesel engine runs at a nearly constant speed to power the generator, rather than the varying speeds of a conventional bus as it starts and stops. Additionally, the buses offer regenerative braking, which means the drive motor is used to slow the bus. This effectively turns the motor into a generator that produces more power to be stored in batteries, and later used to move the wheels.

The new order provides 745 new buses for New York City Transit, and 105 for its sister agency, MTA Bus. It will be fulfilled by 2010. In a related story, the city of Ottawa’s public transit agency ordered 202 of the same hybrid-electric diesel buses.