BAE Systems’ HybriDrive diesel-electric bus system with two options (HDS100 and HDS200 modular traction systems) has three components: a diesel engine coupled to a generator, an electric traction motor and the fixed ratio reduction gear. The diesel engine powers the generator, which starts the engine and gives power to the electric motor and energy storage system. The electric motor turns the wheels of the bus. When the bus slows down, the hybrid diesel electric motor functions as a generator, recapturing energy for later use. Lithium-ion batteries provide power for acceleration and store the power generated when the brakes are applied.