# Control Strategies for Piston Trajectory in Ionic Compressors for Hydrogen Storage
## 1 Introduction
Controlling the flow of the hydraulic oil inlet and the outlet of the hydraulic cylinder can achieve the purpose of controlling the movement speed of the free piston, which not only allows the piston to move according to a specific trajectory but also greatly reduces the probability of the piston colliding with the cylinder and the subsequent hazards. Several studies have shown that the piston's trajectory is a crucial factor in determining the performance of the designed machine. [PERSON] et al. [23] analyzed and simulated an HCCI combustion process of ammonia. The results showed that the most effective piston trajectory for the HCCI combustion of ammonia was the symmetric trajectory, the thermal efficiency of which was about 58.1%. [PERSON] et al. [24] obtained the optimal motion curve of a piston via simulations based on the proposed model, considering variations in the energy and mass conservations of the reciprocating compressor. [PERSON] et al. [25] designed a new type of two-piston linear compressor for which findings indicate that the highest performance was observed when the system operated following a triangular trajectory. Numerous studies, as mentioned above, demonstrate that the free piston's trajectory has a significant influence on the system's performance. The hydraulic drive system provides the possibility of flexibly regulating the movement of the solid piston of an ionic compressor [26]. Hydraulic drive systems are widely used in various industries and have been studied by many researchers for achieving high levels of control accuracy and low levels of energy consumption. [PERSON] et al. [27] improved the response performance of a free piston in a proposed counter-rotating engine using a combination of the Gray mode-modified energy equation feedforward and PID control. [PERSON] et al. [28] experimentally verified that high tracking accuracy was achieved using the proposed nonlinear motion control method in the case of the large dynamic nonlinearity of the hydraulic system. [PERSON] et al. [29] proposed an adaptive controller for hydraulic drive units, combining the state observation of the polynomial nonlinear extension and a linearization control method based on the adaptive feedback signal, which improved the performance of the impedance control in the hydraulic system. [PERSON] et al. [30] proposed a control method to increase the energy efficiency of a hydraulic system by addressing the mismatch issue between the desired and designed power supplies based on a waterjet cutting machine. [PERSON] et al. [31] proposed a control strategy for the cylinder's position based on valves through the adaptive backsliding solution, which enhanced the accuracy and anti-interference ability of the hydraulic loading system. However, a study on the piston trajectory control of ionic compressors is not reported in the open literature. Therefore, the novelty of this study is (I) to propose three control strategies for the piston's trajectory in an ionic liquid compressor, (II) to investigate the compressor's performance using transient numerical modeling, and (III) to obtain an optimal control strategy by comparing the performances of different control strategies. The research method and the results of this paper provide a reference for the design and operation of ionic compressors.