Research on Braking Safety of Parallel Hybrid Electric Buses on Long Downhill Based on Gradient and Speed Change

When driving in mountainous areas, vehicles often encounter long downhill sections. Due to the large mass of bus and the drum brake with poor heat dissipation effect, it is easy for bus to produce braking thermal decay in long downhill section, which makes the vehicle out of control and causes safety accidents. The braking methods of parallel hybrid electric bus include drum braking, engine braking and regenerative braking, whose torque models are established in this paper. The coasting test in Trucksim is used to verify the correctness of the engine braking torque model. Based on coupling braking torque curve with vehicle speed in different gradient, the stable speed is determined and the shift strategy is proposed. The temperature rise model of brake drum is established to analyze the temperature change of brake drum during long downhill. Then, according to the ramp data of G22 freeway, the above models are simulated. The results show that shift strategy can make full use of engine braking torque and reduce the temperature rise by 136°C, which is equivalent to 38% of the temperature under drum braking only, effectively reducing the braking thermal decay and ensuring the driving safety. It is also found that regenerative braking also has a great meaning on reducing the temperature rise of brake drum, and the reasonable design of battery management system will make the battery SOC in the appropriate range, which can increase the regenerative braking time. So the viewpoint might provide a new idea to design the battery management system of electric commercial vehicle.