Series Hybrids
Some European municipal busses have been designed as hybrids. One full-sized model uses a small 2litre diesel producing around 100 horsepower. (A conventional bus has 200-300bhp.) This engine is not connected to the drive wheels. It simply powers a 75 kilowatt generator and charges quite large batteries under the vehicle floor. The bus is designed to cover its downtown route on battery power alone. Once in the suburbs, the diesel engine starts, recharging the batteries. This greatly reduces urban noise and pollution. Moreover, because the engine is not connected to the drive wheels, it can be located anywhere in the bus, freeing up vehicle design. This design is called a series hybrid. It must use the electric motor to move, but its energy can be supplied either from the battery or from the generator. The diesel-electric rail locomotive is similar to this, but has no battery. The Japanese commuter train mentioned above is also a series hybrid, as is the yard switching locomotive manufactured by the Canadian company “Railpower”.
The big advantage of series hybrids is that they have the same amount of regenerative braking power available as they have for acceleration. Also, because the internal combustion engine is not directly connected to the wheels, they need no transmission, clutch or fluid drive. In addition to saving on the cost and weight of components which are not there, the designer does not have to place the internal combustion engine in a specific position lined up with the drive wheels. Since its only connection to the electric motor is electrical, it can be anywhere.