Currently, most of the commercially available hybrid transit city buses have a series hybrid powertrain topology. The series topology is probably the easiest solution to hybridize a city bus (Ehsani et al., 2010). This is because all the traction power is produced only by an electric motor or by several electric motors and the engine-generator (gen-set), with energy storage, provides the power for the traction. As there is no mechanical link between the engine and the wheels, placement of the components is quite flexible and the engine control is not dependent on the vehicle’s speed. In addition, the optimization of the energy management can be practically done on the basis of the gen-set use for the driving power and recharge power needs of the energy storage. Fig. 1 presents the simplified layouts for the configurations of the conventional, hybrid and electric
powertrains.