Multi-objective optimization of design and control parameters for hybrid electric-hydraulic propulsion systems considering the effects of battery degradation, mass increase, and energy loss

To address the increasingly severe environmental and energy crises, electric vehicles are occupying a growing proportion of the automotive market. In pure electric vehicles, frequent starting and braking conditions lead to a reduction in battery lifespan. The adoption of an "electric-hydraulic" hybrid power system can effectively mitigate this issue. However, the increase in vehicle mass due to the integration of hydraulic systems may affect the overall efficiency of the vehicle. In this study, we integrated a hydraulic energy storage system into a midsized pure electric sport utility vehicle (SUV), forming an "electric-hydraulic hybrid" power system. By comprehensively consider factors such as the extension of battery life, mass increase and energy efficiency, a multi-objective problem for the hydraulic energy storage system's parameters was formulated. The optimization objectives include minimizing battery degradation, reducing the total weight of the energy storage system, and decreasing overall vehicle losses under typical operating conditions. The key parameters of the hydraulic energy storage system and the torque distribution coefficients over typical operating time profiles were considered as the optimization variables. A genetic algorithm was employed to optimize the system parameters, resulting in a Pareto front of the solutions for the hydraulic energy storage system and torque distribution parameters. The system parameters corresponding to the minimum values of the different objective functions were comparatively analyzed. The system parameters that yielded minimal battery degradation were selected to simulate the charging and discharging currents under typical operating conditions. Throughout the operating cycle, the maximum and average discharging currents decreased by 12.7% and 22.9%, respectively, whereas the maximum and average charging currents decreased by 27.4% and 45%, respectively. According to the simulation results, the integration of a hydraulic energy storage system significantly suppressed charging and discharging currents, particularly during frequent acceleration and deceleration conditions under typical operating cycles. A test bench was established and braking experiments were conducted. The effectiveness of the hydraulic energy storage system in reducing battery current was preliminarily validated.