Experimental investigation of dynamic characteristics and stability of a mechanically started free-piston engine

This study presents an experimental investigation of the dynamic characteristics and operational stability of a two-stroke, two-cylinder free-piston engine (FPE) employing a purely mechanical starting mechanism, eliminating the need for the conventionally used linear electric motor. The engine was successfully initiated on the first ignition attempt through a forced oscillation procedure, producing a peak in-cylinder pressure of approximately 8 bar during starting phase. Under free-running, no-load conditions, pronounced cycle-to-cycle variations were observed, with peak in-cylinder pressures ranging from 6 to 15 bar and a non-stationary top dead center (TDC) position. Operational instability was primarily attributed to two coupled mechanisms: piston impacts with the stroke-limiting device and ignition timing deviations associated with piston velocity. Quantitative analysis revealed a coefficient of variation (COV) of peak pressure of 24.8%, substantially exceeding those of the peak pressure location and TDC position (both approximately 9.6%). These results provide one of the first experimental dataset describing combustion variability in mechanically started FPEs and highlight critical parameters required for stable operation.