From William Neuman's article:
The city’s other hybrid buses run like hybrid cars. They run off battery power some of the time and diesel or (in the case of many cars) gas engines at other times. And the braking action helps charge the batteries.
The test bus is different in that it runs on battery power all the time. It has a diesel engine, but that is used only to charge the battery, although the bus also uses the brakes for that purpose. The diesel engine is different too. It is a turbine engine.
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