Long Downhill Braking and Energy Recovery of Pure Electric Commercial Vehicles

Long Downhill Braking and Energy Recovery of Pure Electric Commercial Vehicles

Nowadays, the development trend of new energy vehicles is rapid, and pure electric commercial vehicles have attracted much attention in terms of energy saving, environmental protection and economy. With the continuous development of electric motor technology, the speed and torque that can be provided by automotive electric motors are also increasing, which enables the electric motor to provide continuous braking for a long period of time when the commercial vehicle is driving downhill for a long period of time so that the friction braking force can be reduced, the brake thermal degradation can be reduced and the braking energy can be recovered, which improves the energy utilization rate [1].
The braking energy recovery rate is mainly determined by the driving conditions of the vehicle and the energy recovery control strategy. Control strategies can be formulated based on different driving conditions of the vehicle to distribute the frictional braking force of the front and rear axles and the regenerative braking force of the motor in order to improve the braking energy recovery rate. Chenghu Ni et al. [2] proposed to use the drive motor back-dragging torque as the auxiliary braking torque when the vehicle is traveling long downhill and verified the feasibility through a dynamic model. Haichao Lan et al. [3] proposed a dynamic planning-based joint braking control strategy for long downhill driving of electric commercial vehicles using a hierarchical control method for the braking force to effectively reduce the braking load to be borne by the brake. Jiujian Chang et al. [4] proposed an EMB-based braking energy recovery control strategy for pure electric vehicles to maximize the recovery of braking energy and effectively improve the braking energy recovery rate. Peilong Shi et al. [5] proposed a method for constructing and recognizing the long downhill braking conditions of heavy-duty trucks, which provides a basis for the continuous braking system to intervene or withdraw from the active control, and the results show that it is able to effectively identify the braking status of the vehicle. Takuya Yabe et al. [6] established a simulation model of the whole vehicle based on Matlab/Simulink to clarify the influence of motor capacity and battery current on the regeneration energy. By connecting the speed difference between the vehicle and the motor, it is assumed that in the ideal condition, the lateral motion of the vehicle is not considered, and the variable transmission ratio is selected to optimize the braking energy recovery rate of the vehicle. In the study of Wei Zhang et al. [7], based on Matlab/Simulink and ADVISOR, the vehicle model is established, and the regenerative braking priority control strategy is adopted to distribute the axle load and braking force on the uphill and downhill road slope to maximize the recovery of braking energy. In the study of Zhe Li et al. [8], based on the driver’s braking intention, the braking mode is determined by fuzzy theory and logical threshold method, and the braking energy is recovered by fuzzy control rules with road slope, braking intensity and speed as input parameters and braking force proportional coefficient as output parameters. Longlong Wei et al. [9] proposed a brake power distribution control strategy for front and rear wheels based on braking intent recognition, which takes the effect of retarder on brake power distribution into account and maximizes the braking energy recovery.

In the above studies, there is a lack of research for rear-axle drive, continuous braking by electric motors and between friction braking. Most of the traditional commercial vehicles need to be equipped with retarders to provide auxiliary braking and absorb part of the kinetic energy of the vehicle when traveling downhill. In contrast, pure electric commercial vehicles are themselves driven by electric motors, which directly provide auxiliary braking to avoid the need for retarders that can reduce the vehicle’s own mass and recover braking energy. While the electric motor acts as an auxiliary braking device, the friction braking force of the front and rear axles of the vehicle is distributed in a variable ratio to make it close to the ideal braking force distribution curve, which can improve the braking efficiency compared with the traditional fixed-ratio distribution method. In this paper, a brake force distribution control strategy based on the rear-axle-drive vehicle is proposed, which divides the long downhill braking into two processes,: firstly, the vehicle decelerates to the long downhill constant driving speed in the shortest possible time, and then the electric motor provides the main braking force, and the friction brake provides the residual braking force, which then controls the vehicle to go downhill at a constant speed.

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