Free-piston engine linear generators (FPELGs) are an innovative linear power device that exhibits the distinctive dynamics of pistons and performance of free-piston engines. Furthermore, the single-cylinder/single-piston FPELG structure type has more advantages than other FPELG structure types, including a straightforward structure and ease of control. However, when coupled with various rebound devices, the operational characteristics and piston and engine performance of single-cylinder/single-piston FPELGs are quite different. Therefore, this paper aims to quantitatively compare the dynamics of the piston and engine performance of single-cylinder/single-piston FPELGs coupled with various types of rebound device. The results indicate that when the equivalent stiffness of the gas spring is greater than that of the mechanical spring, the operating frequency of the piston of the FPELG coupled with a gas spring will be higher than that when coupled with a mechanical spring. During the compression stroke, the piston velocity of a FPELG coupled with a mechanical spring changes linearly, while the piston velocity of a FPELG coupled with a gas spring changes nonlinearly. FPELGs coupled with gas springs have shorter compression and expansion durations compared to those coupled with mechanical springs. In addition, the indicated powers of FPELGs coupled with ideal gas springs and mechanical springs are 1.5 kW and 1.3 kW, respectively. However, due to leakage, the thermal efficiency of a FPELG coupled with an actual gas spring is reduced by approximately 2.5% compared with the FPELG coupled with the ideal gas spring. Furthermore, the operation frequency of the piston is positively correlated with the stiffness of the mechanical spring. In addition, as the stiffness of the mechanical spring increases, the combustion process of the engine becomes close to an isovolumetric process. The changes in piston dynamics and engine performance when increasing the initial gas pressure of the gas spring are similar to those when increasing the stiffness of the mechanical spring.

Due to their unique operating characteristics and high thermal efficiencies, free-piston engine linear generator (FPELG) has attracted research interest in recent years, thanks to the crankshaft absence. The FPELG has three typical structure configurations: single-cylinder single-piston, dual-cylinder dual-piston, and opposed-piston. These structure configurations have different benefits owing to their distinctive piston operating characteristics and engine performances. This paper analysed the piston operating characteristics and engine performances of the three typical FPELG structure configurations using a quantitative comparison method. The results revealed that the output power of the dual-cylinder dual-piston FPELG was the highest, at 3.7 kW. The opposed-piston FPELG exhibited the highest indicated thermal efficiency of 34%. In addition, the indicated power of FPELG was negatively correlated with the heat transfer area. The friction loss of each FPELG was < 5% of the overall energy consumption. Our study comprehensively analysed the various FPELG structure configurations and highlighted their advantages.