Combination Piston and Piston Ring for Reducing Crevice Volume

Abstract

Embodiments of a combination piston and piston ring reduce the volume of a crevice defined between the circumferential wall of a cylinder and a portion of the outer wall of a piston within the cylinder, the portion extending down from the top of the piston. The combination piston and piston ring is configured such that pressurized fluid within the cylinder reliably urges a radial face of the piston ring to sealingly engage the circumferential wall of the cylinder, even as the piston reciprocates within the cylinder.

What is claimed is: 
     
         1 . A combination piston and piston ring for sealing a gap between a circumferential surface of a piston of a reciprocating machine and a wall of a cylinder of the machine within which the piston reciprocates along an axis between upper and lower limits, wherein the upper limit is reached by a top surface of the piston with each reciprocation, the combination piston and piston ring comprising:
 a piston having a corner cut-out including a circumferentially disposed ridge having a top surface, and being configured to produce a radially disposed gap between the piston and the cylinder, the gap extending downward from the top surface of the piston;   a piston ring having a ring-shaped region and a retaining feature, wherein (i) the piston ring is configured to reside in the peripheral corner cut-out of the piston, wherein the radially disposed gap between the piston and the cylinder extends downward from the top surface of the piston by a distance that is at least as large as an axial height of the piston ring; and (ii) the retaining feature is configured to project into a corresponding groove of the piston;   the piston ring having:   a top surface exposed to fluid pressure, a radial contact surface facing the cylinder wall, a radially inward surface, opposite the radial contact surface and exposed to fluid pressure, and an axial surface, directly opposite the top surface of the piston ring, in contact with the top surface of the piston's ridge during the upward motion of the piston; and   wherein the top surface of the piston ring, the radial contact surface, and the radially inward surface are shaped so that the fluid pressure causes a net force by which the radial contact surface is urged radially against the cylinder wall and the axial surface is urged axially against the ridge.   
     
     
         2 . The combination piston and piston ring according to  claim 1 , wherein the piston ring, in a vertical section thereof, has a U-shaped profile, including an upper member and a lower member and a bridge member connecting the upper and lower members, the U being horizontally disposed so that the bridge member is proximate to the wall of the cylinder, the top surface of the upper member corresponding to the top surface of the piston ring, the lower member corresponding to the retaining feature, the upper and lower members defining a notch therebetween configured to receive the ridge of the piston. 
     
     
         3 . The combination piston and piston ring according to  claim 1 , wherein the piston contains a recess that lies below the top surface of the piston, and wherein a secondary seal is partially disposed within the recess between the piston and the piston ring, the secondary seal being configured to be loaded against a surface selected from the group consisting of a surface of the piston, a surface of the piston ring, and combinations thereof, by pressure of any fluid between the piston ring and the piston. 
     
     
         4 . The combination piston and piston ring according to  claim 1 , wherein the radial contact surface has a parabolic profile. 
     
     
         5 . The combination piston and piston ring according to  claim 2 , the ring having a set of circumferentially disposed gaps, wherein each gap extends axially from the bottom of the ring through the lower member. 
     
     
         6 . The combination piston and piston ring according to  claim 5 , wherein each gap extends axially from the bottom of the ring through the bridge member. 
     
     
         7 . The combination piston and piston ring according to  claim 5 , wherein the axial length of the radial contact surface of the piston ring is shorter than the axial length of the piston ring.