At this point, hybrid buses really aren't anything new. Anyone living in a city with a bus system of any size has probably seen or ridden on a hybrid bus at some point. Most of the thousands of hybrid transit buses in service today are of the parallel variety similar in architecture to the Toyota Prius. Dutch firm e-Traction has developed a series hybrid bus using wheel motors. The system uses an engine driving a generator to produce electricity for a battery that powers the motors.
Compared to parallel hybrids like the General Motors Two Mode system, the series hybrid eliminates the weight and complexity issues of a transmission. For an application like a bus, the added unsprung mass of the wheel motors doesn't really impact the ride and handling the way it would in a car. Parallel hybrid buses boost mileage by about 25-30 percent while a series hybrid can push that to 50 percent.

Efficiency is boosted further by having a GPS system input that shuts down the generator and lowers the power when operating in congested areas where less speed is needed.

[Source: Technology Review]

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