Analytical evaluation of fitted piston compression ring: modal behaviour and frictional assessment

Abstract

Piston compression rings are thin, incomplete circular structures which are subject to complex motions during a typical 4-stroke internal combustion engine cycle. Ring dynamics comprises its inertial motion relative to the piston, within the confine of its seating groove. There are also elastodynamic modes, such as the ring in-plane motions. A number of modes can be excited, dependent on the net applied force. The latter includes the ring tension and cylinder pressure loading, both of which act outwards on the ring and conform it to the cylinder bore. There is also the radial inward force as the result of ring-bore conjunctional pressure (i.e. contact force). Under transient conditions, the inward and outward forces do not equilibrate, resulting in the small inertial radial motion of the ring. The conjunctional friction, comprising viscous shear of the lubricant and any boundary friction as the result of direct interaction of surfaces also act on the ring, as well as the inertial force in the axial direction of the cylinder. Therefore, ring motions are quite complex. However, with properly fitted rings, the radial modal behaviour of the ring is the most important. This provides an opportunity to determine the in-situ ring shape analytically by assuming a series of quasi-static steps in which the balance between ring tension and pressure induced forces with the instantaneous contact force is assumed. The resulting ring shape yields the ring-bore gap, allowing the determination of frictional losses for a given bore out-of-roundness and surface topography. A subsequent analysis based upon one dimensional lubricated conjunction for certain ring configurations enables evaluation of lubricant flow and any chance of oil loss and blow-by. This fully analytical as opposed to computationally intensive numerical analysis is verified with FEA. Copyright © 2011 SAE International.