## 1 Introduction  
The braking energy can be recovered and recycled by the regenerative braking system in the hybrid vehicle, which was propitious to save energy, reduce the emission of polluting gases, and protect the environment [1; 2]. The hydraulic hybrid vehicle is an important type of the hybrid vehicle, which takes the internal-combustion engine as a power source, the hydraulic fluid as energy transfer medium, and the controllable valve as regulator [3; 4]. It has the advantages of a large driving force, excellent fuel economy, long service life, fast response, and flexible controllability, which make it the promising candidate vehicle type for most of the construction machineries and some special vehicles [5; 6]. Therefore, analysis, simulation, and optimization of the influencing factors for energy recovery rate of regenerative braking system not only be meaningful to improve economic and environmental effect of the hydraulic hybrid vehicle [7; 8], but also can provide significant and valuable guidance for the other kinds of hybrid vehicles. The researches on the influencing factors of other kinds of hybrid vehicles supply the meaningful references for this study [9; 10; 11; 12; 13]. A novel cyber-physical system approach was reported by [PERSON] et al. [9] to determine how to optimally adapt the automatic control of an intelligent electric vehicle to driving styles, and the experimental results validated feasibility and effectiveness of the proposed codesign optimization approach. Meanwhile, a novel continuous observation method was proposed based on the artificial neural networks to quantitatively analyze and recognize the brake intensity [10]. In addition, the experimental results demonstrated feasibility and accuracy of the proposed hybrid learning methods. Furthermore, a novel probabilistic estimation method of the brake pressure had been developed for the electrified vehicles based on the multilayer artificial neural networks (ANNs) with the Levenberg-Marquardt backpropagation (LMBP) training algorithm [11], feasibility and accuracy of which were validated by the conducted experiments. Moreover, the regenerative braking contribution to energy efficiency improvement of the electrified vehicles was investigated by the theoretical analysis, analog simulation, and vehicle tests [12]. [PERSON] and [PERSON] had proposed a new cooperative control of regenerative braking and friction braking called the \"combined braking\" for the rear-wheel-driven series hybrid electric vehicle [13], and the simulation results under the urban driving and across the Modified Indian Driving Cycle and vehicle road testing results showed that the proposed combined braking could regenerate more than twice the braking energy of normal parallel braking. Although research targets of these studies were not the hydraulic hybrid vehicle, the used methods in theory, in simulation, and in experiment provided helpful references for investigation on influencing factors for the energy recovery rate of regenerative braking system in the hydraulic hybrid vehicle. The mainly present researches on the influencing factors for energy recovery rate, focus on the selection of auxiliary power source, which includes the battery [14], supercapacitor [15], mechanical flywheel [16], hydraulic accumulator [17], and so on. Among the auxiliary power sources, hydraulic accumulator has the advantages of a high efficiency, simple structure, high reliability, and a low cost [18]. Moreover, the hydraulic accumulator can restrain the impact and remain the steady when the forklift truck, excavator, loader, bulldozer, or other vehicles carry some heavy goods [19]. Therefore, the hydraulic accumulator is a promising auxiliary power source for the hydraulic hybrid vehicle. Relative to system structure of the parallel connection or the mixed connection, the series connection can achieve a higher energy recovery rate [20].