Performance trade-offs of free-piston engine generator through trajectory optimization and segmented trajectory-tracking strategy: Stability, indicated thermal efficiency and net generation efficiency

Free-piston engine generators eliminate the crankshaft connecting rod mechanism of conventional internal combustion engines, offering the advantage of a variable motion trajectory, but suffering from poor operational stability and weak disturbance rejection. Therefore, this paper proposes a segmented trajectory optimization method based on the Gauss pseudospectral method and an interior-point nonlinear programming solver, and obtains multiple optimal solutions with objectives of maximizing indicated thermal efficiency, improving net generation efficiency, and enhancing stability. To evaluate the performance under simulated operating conditions, the optimized trajectories are used as reference trajectories for trajectory-tracking control systems. The results show that under stable conditions, the segmented trajectory-tracking control strategy increases the indicated thermal efficiency by 2.1% and reduces the net generation efficiency by 1.3% compared with the same situation without tracking. The system exhibits better combustion and power generation stability under combustion disturbances. The indicated thermal efficiency and net generation efficiency are simultaneously improved by 4.2% and 1.2% respectively, and the tracking error of the dead-center position is reduced from 0.0065 m to 0.0005 m compared with the same situation without tracking.