Tribological characteristics of piston ring in a free-piston engine for linear generator

This research aims to obtain the piston ring tribological characteristics of a free-piston engine. Mathematical models of piston ring seal and lubrication were established based on thermodynamic equation and hydrodynamic lubrication equation. The lubrication performances of the piston ring were simulated; the effects of the piston motion to piston ring lubrication process were analyzed compared with a corresponding conventional engine. Results show that seal working time of free-piston ring is longer than conventional engine's during a cycle time, but the leakage loss is lower. Besides, the oil film thickness of piston ring in free-piston engine is thinner than that of crankshaft engine just around TDC, but other position is thicker. Moreover friction forces and friction work of piston ring in two engines are almost equivalent, but the peak friction power of free-piston engine is larger than crankshaft engine.