In an internal combustion engine, two compression rings are fitted in piston ring grooves formed in an outer peripheral surface of a piston at a side of a combustion chamber, and one oil ring is fitted in a piston ring groove at a side opposite to the combustion chamber. An example of the oil ring is a so-called three-piece oil ring that includes a spacer expander and a pair of side rails. The spacer expander exerts a biasing force in the radially outward direction of the ring. The side rails are set at the axial ends of the spacer expander, respectively, and are biased by the space expander radially outwardly, or toward an inner peripheral surface of a cylinder.
In recent years, a demand has increased that fuel consumption should be reduced in the internal combustion engines for automobiles. The reduction in friction to piston rings has therefore been greatly required. To meet this demand, a thin axial width and low tension in the oil ring has been designed. Three-piece oil ring including a pair of side rails and a spacer expander is increasingly used in gasoline engines, because they help to reduce the oil consumption. The three-piece oil ring excels in sealability with respect to a piston ring groove, because the spacer expander pushes both side rails onto the inner peripheral surface of the cylinder. However, the reduction in friction decreases the tension (to 0.20 N/mm to 0.25 N/mm, i.e., a value obtained by dividing total tension of two compression rings and one oil ring by a bore diameter). Consequently, the sealing property of the three-piece oil ring becomes insufficient, inevitably increasing the oil consumption.
The spacer expander of the three-piece oil ring has seating tabs on the inner peripheral surface thereof. The seating tabs extend almost in the axial direction of the ring and push the inner peripheral surface of each side rail (see, for example, Patent Document 1: Japanese Patent Publication No. Sho-35-16503. Since the seating tabs extend parallel to the axis of the ring, the side rails cannot exert a sufficient side pressure on a radially extending wall surface of the piston ring groove. Inevitably, the side rails become floppy in the piston ring groove and may move in the axial direction of the piston. Further, oil may easily flow into the gap between the side rail and top surface of the piston ring groove. This causes an increase in oil consumption.
To solve this problem, seating tabs having so-called a predetermined seating tab angle has been proposed in which the seating tabs protrude in a direction inclined with respect to the axial direction of the ring. Thus inclined, the seating tabs urge the side rails radially outwardly and in a direction intersecting the radial direction. The seating tabs therefore urges the side rails to be in pressure contact with the radially extending wall of the piston ring groove, providing a side pressure on the radially extending wall of the piston ring groove and thus restraining mobility of the side rails. This can suppress the oil consumption. The spacer expander has side-rail supporting portions on the outer peripheral surface. The side-rail supporting portions protrude in the axial direction and support axial end face of the first and second side rails. The side-rail supporting portions hold the side rails at a specific fulcrum position when the side rails are assembled in the piston ring groove and so inclined as described above. (See, for example, Non-Patent Document 1: “Piston Rings for Use in Automobile, p. 33, FIG. 2.30.” published by Sankaido Co. Ltd., Editors' Committee of Automobile Piston Rings, dated October 1997.
A piston is formed with a drain hole that opens to a bottom of the piston ring groove. Through the drain hole, oil that has flowed into the piston ring groove is circulated into a crankcase. A width of the piston ring groove in the axial direction becomes smaller in accordance with decrease in thickness of the three-piece oil ring. Hence, a volume of the space between the ring groove and either side rail becomes extremely small. In addition, because of the enhancement in the sealing property of the side rails, which has been achieved by the above-mentioned seating tab angle, sticking the side rails or the expander to a combustion residue accumulated in the piston ring groove may occur, causing phenomenon known as “sticking”. This may degrade oil recirculation. Thus, improvement on sealing property of the side rails and the reduction in lubricant consumption are contradicting requirement to each other, particularly in the low-tension oil ring which has a small width in the axial direction.