Combination oil ring

A combination oil ring installed in an oil ring groove of a piston for an internal combustion engine and comprising:

• a pair of segments having outer peripheral surfaces sliding on an inner wall of a cylinder;
• and an expander spacer arranged between the pair of segments and pressing the outer peripheral surfaces of the pair of segments against the inner wall of the cylinder, wherein:
• a shape of the outer peripheral surface in an arbitrary longitudinal section of the segment has, in a radial direction from each outer peripheral end portion of faces forming a segment width toward the inner wall of the cylinder, curved shapes symmetric while forming a pair on an engine combustion chamber side and on a side distant from an engine combustion chamber in the segment width direction, and the shape of the outer peripheral surface on a side of an outer peripheral vertex in contact with the inner wall when inserted into the cylinder has an asymmetrical shape in the segment width direction across a portion of the outer peripheral vertex, which is a shape continuous to the curved shapes;
• the asymmetrical shape takes a first intermediate line passing through a center of the segment width;
• at an outer peripheral tip end part where a contour curve of the outer peripheral surface in the segment longitudinal section is traced, of two positions on the contour curve corresponding to a position at a distance of 3 μm from the outer peripheral vertex toward the inner peripheral side in the segment radial direction, a position on the engine combustion chamber side is a position a1 and a position on the side distant from the engine combustion chamber is a position b1;
• when a length of a line segment between the position a1 and the position b1 is L and an intermediate line of the line segment of the length L is a second intermediate line, the second intermediate line is located on the side more distant from the engine combustion chamber than the first intermediate line, and the outer periphery vertex of the segment is located on the second intermediate line or on the side more distant from the engine combustion chamber than the second intermediate line;
• the contour curve of the outer peripheral surface in the segment longitudinal section is traced from the outer peripheral vertex of the segment to at least 0.025 mm toward the inner peripheral side in the segment radial direction so that a symmetrical shape taking both end sides in an axial direction as a pair exists on inner peripheral side positions in the segment radial direction;
• when the contour curve of the asymmetrical shape at the segment outer peripheral tip end part is sectioned into a curved portion sandwiched between the outer peripheral vertex and a distance of 1.5 μm from the outer peripheral vertex toward the inner peripheral side in the segment radial direction and a contour portion sandwiched between the distance of 1.5 μm and a distance of 3.0 μm from the outer peripheral vertex toward the inner peripheral side in the segment radial direction, a first contour section, a second contour section, and a third contour section are set from the engine combustion chamber side in the segment width direction;
• the first contour section is provided to be a part of a linear shape or a part of a quadratic curve shape starting from a first end portion on the engine combustion chamber side of the second contour section;
• the second contour section has a flat part at a intermediate thereof, and is provided in a shape composed of a part of a linear shape or a part of a quadratic curve shape from an end portion on the engine combustion chamber side in the axial direction of the flat part and continuous to the first contour section, and composed of a part of a quadratic curve shape from an end portion on the side distant from the engine combustion chamber in the axial direction of the flat part and continuous to the third contour section;
• the third contour section is provided to be a part of a quadratic curve shape continuous to the second end portion;
• a surface roughness of the asymmetrical portion of the segment outer peripheral surface is 0.6 μmRp or less;
• a length on the position al side and a length on the position b1 side of the line segment L divided by a line in the radial direction orthogonal to a line segment between the position a1 and the position b1 of the outer peripheral tip end part in the contour curve of the segment outer peripheral surface and passing through the outer peripheral vertex, are L1 and L2 respectively;
• further, when at two positions on the contour curve at the distance of 1.5 μm toward the inner peripheral side in the segment radial direction, a position on the engine combustion chamber side is a position a2 and a position on the side distant from the engine combustion chamber is a position b2, a length of a line segment between the position a2 and the position b2 is L3, and a length in the axial direction of the flat part in the second contour section is L4, conditions of 0.05 mm ≦L≦0.15 mm, L1/L≧0.5, L3/L≦0.76, and 0<L4≦0.05 mm are satisfied;
• when an angle formed between a first straight line passing through the position a1 and through the position a2 and the axial direction of the cylinder is an angle θ1, a condition of 3 degrees ≦θ1≦6 degrees is satisfied;
• and
• when an angle formed between a second straight line passing through the position b1 and through the position b2 and the axial direction of the cylinder is an angle θ2, a condition of 9 degrees ≦θ2 is satisfied;
• and
• the pair of segments are installed in the oil ring groove so that the respective outer periphery vertexes are located on the side distant from the engine combustion chamber.