Regenerative Braking System for Series Hybrid Electric City Bus

*State Key Lab. of Automotive Energy and Safety Tsinghua University, Beijing, China 100084 Phone: 010-62771839 Regenerative Braking Systems (RBS) provide an efficient method to assist hybrid electric buses achieve better fuel economy while lowering exhaust emissions. This paper describes the design and testing of three regenerative braking systems, one of which is a series regenerative braking system and two of which are parallel regenerative braking systems. The existing friction based Adjustable Braking System (ABS) on the bus is integrated with each of the new braking systems in order to ensure bus safety and stability. The design of the RBS is facilitated by Simulink [1] which is used to build an interactive, multi-domain simulator that is allows parametric variation of vehicle speed, State of Charge (SOC) for the batteries, and the maximum current to be allowed to the batteries from the RBS. The required braking forces as a function of wheel speed are modeled using dSpace[2]. A Hardwire-in-the-Loop (HIL) experimental setup is used for component testing, followed by road testing using the Chinese Urban Bus Driving Cycle. Results indicate that all three braking systems provide some energy recovery, with the serial RBS providing the best combination of energy recovery with acceptable drivability, safety and stability. Overall results confirm that regenerative braking systems can recover significant braking energy while operating in a safe and predictable manner.