Effect of Disturbances on the Operation Process of a Methane-Fueled Free-Piston Engine Generator

In this paper, the effect of disturbances on the operation process of a methane-fueled free-piston engine generator (FPEG) was experimentally investigated. Four disturbance sources, namely step change of external load, mixture flow rate fluctuation, random misfire of a cylinder, and elastic collision, were identified and applied to the FPEG. The results showed that the FPEG successfully achieved a steady-state operation with load. The maximum instantaneous electric power of 127 W and the average effective electric power of 38.9 W were obtained. When an external load was instantaneously disconnected, the engine frequency increased from 26.7 Hz to 31.3 Hz. The fluctuation amplitudes of induced voltage, pressure and compression ratio were 18.9%, 24.7% and 52.2% respectively in the disturbance. By contrast, when the external load was instantaneously connected, the corresponding values were 42.2%, 31.3% and 64.3% respectively, indicating that the instantaneous external load connection had a greater disturbance impact on the FPEG operation stability. Despite encountering the step change of external load, the FPEG can still restore stable operation and show good anti-disturbance ability. Compared with increasing mixture flow rate, reducing the mixture flow rate has a greater disturbance impact on the engine operation stability. Although random misfire of a cylinder will cause remarkable fluctuations in piston displacement and cylinder pressure, the FPEG will not stop running, but continues to work as a single-piston engine. Minor collision event may adversely affect the stability of engine operation, but will not lead to the FPEG shutdown. However, serious collision event may lead to ignition failure and shutdown accident.