Rotational Degree-of-Freedom Localization: A Universal Passive Centering Architecture for Free-Piston Machines

This paper presents a universal passive stabilization mechanism applicable to a wide range of free-piston engines and linear generators: the localization of the rotational degree of freedom (DOF) exclusively within the piston assembly. Free-piston topologies generally suffer from the lack of kinematic guidance, leading to piston slap and friction. Conventional solutions (magnetic or external gas bearings) add significant complexity. The proposed architecture mechanically constrains the translator shaft (carrying the linear load) against rotation while enabling free piston spin via low-friction thrust bearings. This decoupling allows the system to utilize gas- induced torque—generated during both combustion and motoring phases—to achieve hydrodynamic centering and gyroscopic stiffness. CFD validation confirms hydrodynamic film thickness hfilm > 5-10 μm at 30 Hz, sufficient for zero-contact operation with gyroscopic precession rates stabilizing tilt angles