Cold-Start and Mode-Transition Strategy for Passively Centered Free-Piston Generators

In free-piston generators utilizing passive gas-dynamic centering, piston rotation plays a critical role in steady-state operation by enabling a load-bearing gas film and contactless centering. However, during cold start, combustion-induced gas pressure is absent, rendering the passive self-stabilization mechanism ineffective. This paper proposes a holistic cold-start and mode-transition strategy in which initial piston rotation is actively initiated via a controlled rotating electromagnetic field generated by a specially designed linear generator stator. Axial motion of the shuttle is deliberately suppressed during the initial phase until a stable gas-dynamic wedge forms in the piston–cylinder clearance. Only after this hydrodynamic "lift-off" condition is met does the system transition to axial excitation, combustion initiation, and standard resonant operation. The proposed approach seamlessly integrates electromagnetic actuation, passive gas-dynamic centering, and resonant free-piston dynamics into a unified startup architecture without introducing auxiliary mechanical subsystems.