Energy-Management Strategy for Four-Wheel Drive Electrohydraulic Hybrid System with Optimal Comprehensive Efficiency

The four-wheel drive electric sport utility vehicle (SUV) requires high dynamic performance, and the front and rear axles are matched with a high-power motor. High-power motors operate under low-speed and low-torque conditions, with low efficiency and large power loss. To reduce the power loss under low-speed and low-load conditions, a hybrid system of front and rear dual motors and dual hydraulic pumps/motors is designed. A simulation model of a four-wheel drive SUV electrohydraulic hybrid system is constructed. Aiming at the optimal energy consumption, a dynamic programming algorithm is adopted to establish the driving control rules of the vehicle. Constrained by the Economic Commission for Europe Regulation No.13 (ECE R13), a braking-force distribution strategy for the front and rear axles is formulated. On the premise of satisfying the braking safety, regenerative braking is preferred, and the braking energy is recovered to the greatest extent possible. The optimal efficiency curve of the motor is identified, and an energy-management strategy based on the optimal efficiency curve of the motor is established. The comprehensive efficiency of the dual motor for driving and braking is defined, and the energy-management strategy with the optimal comprehensive efficiency of the dual motor is established. Under the New European Driving Cycle (NEDC) condition, the equivalent energy consumption per 100 km for the two energy-management strategies is 13.2208 kWh/100 km and 13.1507 kWh/100 km. The latter has a higher overall efficiency and less power loss. Fuzzy-logic control with the accumulator pressure and its variation as the input and the threshold speed as the output is proposed, which improves the energy-management strategy with the optimal comprehensive efficiency of the dual motor. The results show that the equivalent energy consumption per 100 km for the improved strategy is 13.1481 kWh/100 km, and the vehicle energy consumption is reduced. The system design and control strategy are validated.