Study on the influence of operating stroke on combustion and emission characteristics of opposed single-cylinder free piston generator under direct injection

In recent years, opposite single-cylinder free piston generator (OSFPG) has become the focus of research on improving energy utilization efficiency and addressing energy shortage owing to its large power per unit volume, adjustable compression ratio, and high motion stability, etc. Therefore, in this paper, a physical prototype platform for OSFPG is built, and orthogonal test is performed for comprehensive study on the influence of different operating stroke on the working characteristics of OSFPG. The results show with increase of the operating stroke, the stagnant combustion period can be shortened by at least 35 %, but the effect on the rapid combustion period is insignificant. Especially, The SOOT emissions of OSFPG are basically zero and do not change with the operating stroke. However, the NO_x emissions continue to increase by up to 23 % with the increase of the stroke in the study. Importantly, the increase of operating stroke can significantly improve the thermal efficiency of OSFPG by up to 17 %, but the increase rate decreases as the ignition time is continuously advanced. In the study, the indicated thermal efficiency of OSFPG was as high as 41.1 %.