Identification and analysis on the variation sources of a dual-cylinder free piston engine generator and their influence on system operating characteristics

Stable combustion is a huge challenge for dual-cylinder free-piston engine generator (FPEG). Combustion variations are easy to occur, because the piston movement is only affected by the forces acting on the moving parts, and there is no mechanical structure limit. This paper has investigated the sources of cyclic and cylinder-to-cylinder variations for dual-cylinder FPEGs, and further analysed their effect on the piston movement characteristics in operation and engine's performance by using an established working model. This study reveals that FPEG has specific anti-interference properties but is easily affected by external conditions and operating parameters. The operation of the FPEG system should be limited to a specific range rather than fixed at a fixed compression ratio like a traditional internal combustion engine. The simulation results showed that the FPEG system returns to a stable operating state within just a few cycles for one-time combustion variations if there are no misfires, regardless of whether they occur in one cylinder or two cylinders. The operating centre deviates from the geometric centre when continuous variations occur in just one cylinder. The compression ratios and operating frequency decrease when combustion variations occur, but the compression ratio decreases significantly in normal cylinders. When continuous variations occur in two cylinders, the compression ratio, operating frequency, and free-piston engine performance are significantly reduced.