Transient dynamic cycle evolution and thermodynamic performance analysis of a free-piston engine generator

The free-piston engine generator (FPEG) is a promising hybrid power system. Eliminating the crankshaft mechanism, it features a variable piston dynamic cycle distinct from traditional engines. This paper investigates transient dynamic evolutions, identifies the stable operation zone, and further analyzes thermodynamic performance under various key design parameters. First, a coupled dynamic-thermodynamic model is developed for FPEG. Second, transient dynamic evolutions from start-up to combustion-generation stage are analyzed. The boundaries of operating parameters are identified to ensure the presence of limit cycles for stable operation. Subsequently, the thermodynamic performance is comprehensively evaluated. The indicated power and thermal efficiency improve as load resistance and excess air ratio decrease. The maximum and minimum performance points consistently occur on the overshooting and damping lines, respectively. A lower piston assembly mass and higher design compression ratio are recommended to achieve higher indicated power and thermal efficiency. A lower stroke-to-bore ratio leads to higher indicated power with a slight decrease in thermal efficiency. Additionally, the load coefficient should be adjusted to ensure stable operation according to the design parameters. This paper enhances the understanding of transient dynamic evolutions of the FPEG and provides guidance for prototype design aimed at achieving stable operation and improved output performance.