A friction force measurement system was developed to clarify the tribological mechanism of the piston assembly. In order to adopt merits of the floating liner(movable bore) method and the rig for connecting rod-crank mechanism, a conventional spark ignition engine was modified. Measurement system was designed to restrict the secondary motion of the piston and to control temperatures of the cylinder wall and lubricant. Reliability of the friction data could be improved by considering the vibration of the engine block and the dynamic characteristics of the system. Friction forces of the piston assembly components or ring pack were measured with the experimental parameters such as oil viscosity, engine speed, model of ring and roughness of bore. The twist stiffness of piston rings could be evaluated through the test of dry friction.
The friction forces which were measured independenatly under flood lubricant supply, were classified into five frictional modes with regard to the combination of predominant lubrication regimes(boundary, mixed and hydrodynamic lubrication)and stroke region(mid-stroke and dead centers). The modes could be identified on the Stribeck diagrams where the friction coefficients are evaluated both at the mid-stroke and at the dead centers. For the cases of the compression rings, most of the steady operating conditions fails into the mode where the hydrodynamic lubrication is dominant throughout the entire stroke except for the dead centers. Frictional power loss is also minimized in this condition. The rings showed an optimized behavior in the viewpoint of wear and frictional loss. However, mixed lubrication regime is predominant on the oil control ring for the basic function of scraping in the condition.
Comparing the friction force of the ring pack and the sum of friction forces which were measured with only one ring among the top, second and oil control rings, the ringpack's is greater than the other under hydrodynamic lubrication modes. The phenomenon seems from that the lower rings restrict the lubricant to upper rings in ring pack. Therefore, friction force of each ring in ring pack was evaluated from the friction forces of ring combinations. Oil starvation was observed on the compression rings when the hydrodynamic lubrication is dominant in the mid-stroke. In particular, the phenomenon is severe on the top ring.
The current test results of coefficient of friction, frictional modes and frictional force could be directly used in developing and verifying lubrication analysis models of piston ring.