Novel Free-Piston Engine Generator Configuration with Air Springs for Variable Frequency Control

The mover operating frequency of a free-piston engine generator (FPEG) relies on its natural frequency, which is determined by the mass of the mover and engine specifications. Increasing this frequency is crucial for improving power density. Further, variable frequency operation can effectively enhance efficiency under partial load conditions. To meet these demands, this paper proposes a novel FPEG configuration with opposing air springs to realize a controllable operating frequency. A control method that adjusts the generating load coefficient using the top-dead-center (TDC) position feedback is introduced to maintain operational stability during frequency transitions. The feasibility of the proposed configuration was initially evaluated through numerical simulations, which included a comparison with a conventional configuration. Subsequently, the proposed control method was experimentally validated using a test device that simulates the dynamics of the FPEG. Results confirmed that the operating frequency could be actively varied by adjusting the air spring pressure, and the proposed control method successfully maintained the TDC position with high accuracy during these transitions.