Research On Characteristics And Control Strategy Of Hydraulic Hybrid Vehicle With Hydraulic Common Pressure Rail

The hydraulic common pressure rail vehicle driving system has big power density and low cost. The driving system is suitable for heavy vehicles, such as urban bus and special vehicle. In the hydraulic common pressure rail vehicle driving system, the power source and the load can be separated and controlled individually. This characteristic of the structure gives the driving system bigger control range. For fully take use of the advantages of the driving system, it is essential for optimizing the driving system structure, study the system operating characteristics and making the relevant control strategy.Firstly, based on the operating principle of hydraulic common pressure rail driving system, structure optimization is conducted and the self-adaptive driving system based on the pressure cross-feedback is proposed. The operating process of the self-adaptive driving system is discussed. The engine model, variable hydraulic pump model, hydraulic accumulator model, hydraulic transformer model, hydraulic motor model, pilot operated check valve model and vehicle longitudinal dynamics model are built respectively. These models are validated by the test. The reliability for the followed research is guaranteed.For research the operating characteristics of the driving system deeply, the response characteristics, system stiffness and regenerative braking characteristics are conducted. For studying the response of the driving system when switch between different modes, the method of mathematical simulation is adopted. Both the dynamic process and response impact factors are analyzed. And the results are validated by the test. The robustness of the driving system is also studied. Based on the driving system dynamic equations, the analytical expression of the system stiffness is built. In terms of the stiffness expression, the impact factors of the stiffness are obtained. Through comparing with the pump-controlled motor system, the results indicate that the stiffness of hydraulic transformer controlled motor system is lower than the stiffness of pump-controlled motor system. Through the analysis of regenerative braking characteristics, the variations of system brake pressure and energy recovery percentage impacted by hydraulic transformer controlled angle are discussed. The comparison of regenerative braking characteristics between hydraulic transformer-controlled motor system and pump-controlled system is also investigated.On the basis of the driving system model and operating characteristics investigation, the fuzzy PI control method is presented. With this controlled method, hydraulic transformer-controlled motor system can realize better control performance in bigger range. Simultaneously, the proposed control method can resist the negative characteristic effectively and guarantee the stability of hydraulic transformer. The system rule-based control strategy is presented. Through the engine-pump control based on the pressure of hydraulic common pressure rail and the hydraulic transformer control based on the transformer ratio-flow ratio, the operating point of the engine is optimized and the fuel economy is improved.