Effect of variable stroke on combustion characteristics and stability of a free-piston engine generator

The free-piston engine generator (FPEG) features a variable stroke structure, offering significant advantages in multi-fuel adaptability and high-efficiency combustion technologies. Stroke control and design directly affect combustion regulation and system performance, and thus represent a key challenge in FPEG development. Existing studies on stroke variation mostly rely on numerical simulations, often neglecting fuel short-circuit losses and lacking experimental validation of actual combustion performance and stability. In this study, an experimental approach was employed to investigate the effects of stroke length variation (47–55 mm) on the combustion characteristics, system performance, and operational stability of an FPEG. The results show that shortening the stroke reduced the indicated work by 4.3 % from 55 mm to 47 mm, but increased the operating frequency by 14.7 %, resulting in a 10 % improvement in indicated power. Moreover, shorter stroke lengths enhanced both indicated thermal efficiency and fuel economy, with the indicated thermal efficiency increasing by 19.6 % and the minimum ISFC reaching 336 g/(kW·h). In terms of stability, the short-stroke configuration markedly reduced cycle-to-cycle variations in both combustion and system operation: COV_Pmax and COV_IMEP decreased by 34.5 % and 50.6 %, respectively, while COV_f dropped to 0.59 %.This study is the first to experimentally reveal the influence of stroke variation on FPEG performance, providing valuable data support for structural optimization and performance control of FPEG systems.