Power generation analysis of opposed-piston free piston engine generator with coupled simulation

Opposed-piston free-piston engine generators (FPEGs) are promising compact range-extender power units for hybrid vehicle applications, but their inherently asymmetric piston motion leads to strong coupling among piston dynamics, in-cylinder pressure, and the electromagnetic response of the linear generator. To address this issue, this study develops a multi-field coupled simulation model that uses measured cylinder pressure as the input for piston dynamics and integrates a finite-element model of the linear motor through the piston motion state as the coupling interface. The model is validated using a motor reverse-traction power generation test, in which the simulated three-phase coil voltages show errors below 10% for most measured data points. A real-time velocity feedback control strategy is then implemented to regulate piston motion under asymmetric operating conditions, achieving maximum displacement and speed errors of 0.25 mm and 0.05 m/s, respectively. Under FPEG operation, the simulation reveals an asymmetric electromagnetic force response, with a 123.45 N difference between the magnitudes of the positive and negative force peaks. A comparative analysis of FPEG, conventional crank, and sinusoidal piston trajectories shows that the FPEG trajectory yields the lowest motor losses, with an ohmic loss of 188.89 W, a hysteresis loss of 3.67 W, and an eddy-current loss of 1.19 W. The FPEG trajectory achieves an electromechanical conversion efficiency of 88.67% and an instantaneous peak output power of 5.40 kW. This work provides a validated coupled simulation framework for evaluating the electromagnetic characteristics, loss distribution, and power generation performance of opposed-piston FPEGs for compact hybrid powertrain applications.