Petrol-electric hybrid buses are being road tested in parts of California as a direct alternative to traditional diesel engines. While oil burners are far more fuel efficient than petrol vehicles, they have a bad reputation on the emissions front, especially with NOx emissions - a real concern in many large, Californian cities. The petrol-electric hybrid powertrain returns only slightly better fuel efficiency than a diesel engine - 5 mpg versus 3 to 5 mpg - but harmful emissions are hugely reduced. This has led ISE, the bus hybrid electro-mechanical subsystem contractor, towards utilising a series hybrid model where the engine drives a generator that produces electricity to run the electric motors driving the wheels. For Grover City, the flat terrain suits the petrol-electric hybrid, whereas in hilly Oakland, the hybrid buses will use fuel-cells instead of an electric motor.
Analysis: Using a series hybrid configuration is a good idea to allow the engine to be swapped out for different models or fuel-cells or the like. I think they should be giving a modern clean diesel configuration a go, though, to get the best of both worlds. Especially if they could run it on biodiesel.