| Confronted with the conflicts between increasing depletion of oil resources and people's increasing demand for energy, energy-saving electric vehicles become a hot research. At present, the major domestic and foreign automotive companies are actively developing electric vehicles. The limited driving range is the main factor that restricts the industrialization of electric vehicles. In order to improve the driving rang of electric vehicles, on the one hand, we should research and develop new batteries with high specific energy; on the one hand, based on the present battery resources, we should improve vehicle energy utilization. Regenerative braking is an effective method to improve the energy utilization of electric vehicles.Regenerative braking is also known as energy feedback braking. The basic principle are as follows: In regenerative braking, the vehicle inertial energy transmit from the wheels pass through transmission system to the motor, the electric motor acts as a generator, charge the power battery. In this way, some braking energy can be stored in the power battery and recycled to drive the vehicle. At the same time, the motor braking torque exert to the driving wheel through transmission system, resulting in braking force.In driving cycles of city, the average speed of buses is not high, vehicle load changes significantly, the need for starting and braking is quite frequently, if the part of energy which is lost in the process of braking can be recycled, the range of electric vehicle will be greatly enhanced. The United States test results about the practical application of electric vehicle shows that regenerative braking can prolong the range of electric vehicle by 10% to 30%.The use of regenerative braking for energy recovery is a great energy-saving advantage of electric vehicles compared to traditional vehicles. Under assurance of braking safety, maximizing the recovery of braking energy is focused on regenerative braking research. To increase the proportion of electric motor braking force is an effective method to improve the energy recovery efficiency. The distribution of braking force is the key question on the study of regenerative braking control strategy, including front and rear axle braking force distribution, and motor braking force and mechanical braking force distribution. The front and rear axle braking force distribution influences the utilization of road adhesion coefficient and braking stability. Mechanical braking and regenerative braking force distribution determines how much energy can be recovered.When the motor regenerative braking imposed, the front and rear axle braking force distribution coefficient changes; the braking force distribution is no longer a fixed proportion of the traditional mechanical brake system. In order to ensure the braking performance, braking stability and braking security, the braking force distribution coefficient should have a reasonable range. Under conditions of meeting braking regulations and assurance of braking safety, how to maximize the recovery of braking energy is the main problem studied in this paper. That is the regenerative braking control strategy for electric vehicles. For pneumatic braking system without ABS (Anti-Lock Braking Systems), this paper proposed a method to determine motor regenerative braking force, developed regenerative braking control strategies, and analyzed energy recovery effect of different regenerative braking control strategy through simulation. Main contents and conclusions are as follows:(1) Analyze the impact factors and constraints of regenerative braking. By analyzing the process of regenerative braking energy loss, summed up the impact factors of regenerative braking are driving environment, driving form of vehicle, electric motor, battery, control strategy, and so on. This paper mainly researches regenerative braking control strategy, three constraints is given in detail. To develop regenerative braking control strategy, both the ECE regulations, motor peak torque and battery power limitations are taken into account. According to the speed, brake strength and battery SOC to determine the value of the dynamic motor regenerative braking force, rather than just according to the speed or brake strength to distribute motor braking force.(2) Analyze the mechanical braking characteristics of electric bus. The purpose is to determine a reasonable extent of front and rear axle braking force under assurance of meeting ECE braking regulations. According to the automotive theory, this paper analyzed the relationship between front and rear axle braking force distribution, determined synchronous adhesion coefficient both at full load and no-load. The utilization adhesion coefficient and braking efficiency also have been analyzed. Both front and rear axle utilization adhesion coefficient meet the ECE regulations. In urban road commonly used, the braking efficiency are high both at full load and no-load. In order to ensure the braking performance, braking stability and braking security, the braking force distribution coefficient should have a reasonable range. Under assurance of meeting ECE braking regulations, this paper determined the braking force distribution coefficient range, which provided a theoretical basis for developing regenerative braking control strategy.(3) Develop regenerative braking control strategy. To study regenerative braking control strategy, the key question is to determine motor regenerative braking force and braking force distribution. A method to determine motor regenerative braking force is proposed in detail in this paper, mainly taking into account three constraints: ECE braking regulations, the motor peak torque and battery power. The weight factor of vehicle velocity and battery SOC also have been taken into account to determine the motor regenerative braking force, in order to ensure the motor braking force change gently rather than suddenly to withdraw and impose. The brake feel of driver and crew comfort also can be improved. In this paper, parallel regenerative braking control strategy and series regenerative braking control strategy are developed separately based on the structure of regenerative braking system.(4) Construct simulation model of regenerative braking system. In order to verify the energy recovery effect of regenerative braking control strategy, a simulation model for regenerative braking system is constructed in Matlab/Simulink, mainly including the vehicle dynamics model, electric motor model, power battery model and control strategy model. Driving conditions have been taken into account, so it is facilitate to conduct the simulation analysis both in conventional braking condition and in urban cycle condition. The simulation model provides the necessary platform to verify the regenerative braking control strategy.(5) Simulation verification of regenerative braking control strategy. The simulation analysis for regenerative braking control strategy is conducted based on the simulation model. The simulation results show that, applying parallel control strategy for regenerative braking, the energy of battery recycling is very limited, only about 5% to a maximum. For the regenerative braking system of electric bus, the energy recovery is higher in the middle-speed, medium brake strength and medium battery SOC. Series regenerative braking system is superior to parallel regenerative braking system in the energy recovery. In order to improve the energy recovery, while minimizing the cost, this paper presented two improved control strategy: series regenerative braking control strategy with rear axle and improved parallel regenerative braking control strategy. Simulation shows that the improved control strategy effectively improves the energy recovery.
|
PDF Full Text Request