Development of Conformability Model of Piston Rings with Consideration of Their Three-Dimensional Torsional Distortions and Fourier Series Representation of Cylinder Bore Geometry

Conformability of a piston ring is a measure of the ring ability to conform to the ever-present cylinder bore deviations from a perfect circle by elastic deformation of the ring. Conformability is usually defined as the limit of bore distortion at which the clearance between the piston ring and bore can be kept at zero by elastic deformation of the piston ring. Retention of good conformability is important for ring/bore sealing and reduction of oil passing and blow-by.

The most known technique for conformability analysis was developed almost sixty years ago and considers only two-dimensional deformation of the ring in its plane by the force of piston ring elasticity. It is known, however, that the piston rings with asymmetrical cross-section, a prevalent type of the modern compression piston rings, undergo a complicated three-dimensional installation displacements (twist) which affect many functions of the rings. Piston rings with a symmetrical cross-section will also produce displacement normal to the ring plane if for example they have taper face and thus are loaded in the plane off the centroidal axis of the ring. Hence a need exists for improvement of the conformability analysis to account for torsional distortions of the rings.

This paper, which is based on the work of the authors demonstrated in the preceding SAE conferences and papers, illustrates development of a model of conformability of the piston rings with arbitrary cross-section that, unlike the conventional model, takes into account three-dimensional torsional distortion of the rings. As a measure of verification of the analysis torsional distortions of a piston ring calculated with the formulae developed are compared favorably with the values of ring twist measured experimentally.