Gas exchange fluctuations may lead to operation instability of the free-piston engine (FPE), because the engine has a coupling effect between the gas exchange and the piston motion, which is different from a conventional engine. It is, therefore, important to study the gas exchange instability to ensure the stable operation of the engine. This study focuses the effect of variable piston position when injecting on the gas exchange stability of a looped scavenging FPE, and a systematic full-cycle gas exchange model is presented by developing a dynamic model, a combustion model, and a multidimensional scavenging model. Meanwhile, an iterative method between motion and scavenging is used to solve the model numerically for gas exchange simulation, and the effects of variable piston position when injecting on piston motion, thermodynamic in cylinder, and gas exchange process are studied. The results indicate that retarding injection induces instability of gas exchange duration which first increases and then decreases, and there is an injection position which is most conducive to the increase of capture efficiency of gas exchange, although the scavenging efficiency is lowest under this condition. This result also suggests that more complete exhaust may be achieved by a different injection position.

中文翻译：

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喷射位置变化对自由活塞发动机气体交换稳定性的耦合影响

气体交换波动可能会导致自由活塞发动机（FPE）的运行不稳定，因为该发动机与传统发动机不同，气体交换和活塞运动之间存在耦合效应。因此，研究气体交换不稳定性对于保证发动机的稳定运行具有重要意义。本研究重点关注喷射时可变活塞位置对循环扫气FPE气体交换稳定性的影响，通过建立动力学模型、燃烧模型和多维扫气模型，提出了系统的全循环气体交换模型。同时，采用运动与扫气迭代方法对模型进行数值求解，进行气体交换模拟，研究了喷油时活塞位置变化对活塞运动、缸内热力学和气体交换过程的影响。结果表明，延迟喷射会导致气体交换持续时间不稳定，先增加后减少，并且存在一个最有利于气体交换捕集效率提高的喷射位置，尽管该工况下扫气效率最低。该结果还表明，通过不同的喷射位置可以实现更完全的排气。