JP7529709B2 - Oil Ring - Google Patents

Description

The present invention relates to an oil ring.

Internal combustion engines installed in general automobiles are configured with piston rings, including a compression ring (pressure ring) and an oil ring, fitted in the ring groove of the piston. In the axial direction of the piston, the compression ring is provided on the combustion chamber side, and the oil ring is provided on the crank chamber side, and they perform their functions by sliding on the inner wall surface of the cylinder. The oil ring has an oil seal function that suppresses the outflow of oil to the combustion chamber side (oil rising) by scraping off excess engine oil (lubricating oil) attached to the inner wall surface of the cylinder to the crank chamber side, and a function that prevents the piston and piston ring from burning due to the operation of the internal combustion engine by adjusting the amount of oil so that a lubricating oil film is appropriately maintained on the inner wall surface of the cylinder. The compression ring has a gas seal function that suppresses the outflow of combustion gas from the combustion chamber side to the crank chamber side (blow-by) by maintaining airtightness, and an oil seal function that suppresses oil rising by scraping off excess oil that the oil ring could not scrape off.

Here, a three-piece type combined oil ring is widely used as the oil ring, which combines a pair of segments (also called side rails) that slide on the cylinder inner wall surface with a spacer expander that urges the pair of segments against the cylinder inner wall surface. In relation to such combined oil rings, a technology has been disclosed that aims to improve oil seal performance by preventing the inclination of the segments by making the R of the upper arc of the inner circumferential surface larger than the R of the lower arc with respect to the width centerline when the outermost diameter part of the sliding part (outer circumferential surface) of the segments is positioned below the center of the axial width of the segments (for example, Patent Document 1).

International Publication No. 2020/158949 Utility Model Publication No. 61-157149 Jitszen No. 59-099154

Here, with conventional oil rings, oil passes through the gap between the top surface of the upper segment and the upper wall of the piston's ring groove and flows out of the ring groove into the combustion chamber (oil rising), which can increase oil consumption.

The present invention was made in consideration of the above-mentioned problems, and its purpose is to provide a technology that can improve the oil seal performance in a combined oil ring that includes a pair of segments and a spacer expander.

In order to solve the above problems, the present invention employs the following configuration: That is, the present invention provides an oil ring to be fitted in a ring groove formed in a piston of an internal combustion engine, the oil ring comprising an upper segment arranged on an upper side of the oil ring on the combustion chamber side in the axial direction, a lower segment arranged on a lower side of the oil ring on the crank chamber side independently of the upper segment, and a spacer expander arranged between the upper segment and the lower segment and generating a tension that causes the oil ring to protrude outward in the radial direction, the upper segment including an outer circumferential surface arranged on an outer circumferential side, an inner circumferential surface arranged on an inner circumferential side, an upper surface facing the upper side in the internal combustion engine, and a lower surface facing the lower side in the internal combustion engine, the spacer expander including a pressing surface arranged inward of the upper segment in the radial direction of the oil ring and pressing the inner circumferential surface by the tension, the inner circumferential surface being a surface that presses the inner circumferential surface of the oil ring against the inner circumferential surface of the oil ring. the oil ring includes an inner circumferential apex which is located radially inside the oil ring relative to other portions of the inner circumferential surface and has a smallest diameter on the inner circumferential surface, in a cross section perpendicular to the circumferential direction of the oil ring, the outer circumferential surface includes an outer circumferential apex which is located radially outside the oil ring relative to other portions of the outer circumferential surface and has a largest diameter on the outer circumferential surface, the inner circumferential apex is located closer to the combustion chamber than a center position of the inner circumferential surface in the axial direction of the upper segment, in a cross section perpendicular to the circumferential direction of the oil ring, the pressing surface is inclined so as to approach a central axis of the oil ring as it approaches the combustion chamber side, and when the inclination angle of the pressing surface with respect to the axial direction of the oil ring (i.e., ear angle) is θ, 1°≦θ≦20°.

In other words, the present invention makes the inner peripheral surface of the upper segment eccentric, and the inclination angle of the pressing surface that presses the upper segment of the spacer expander is 1°≦θ≦20°. This improves the oil seal performance of the upper segment and reduces oil consumption.

In the present invention, the inner peripheral surface may include an upper curved surface that curves from the inner peripheral apex toward the upper surface side, and a lower curved surface that curves from the inner peripheral apex toward the lower surface side, in a cross section perpendicular to the circumferential direction of the oil ring.

In the present invention, the radius of curvature of the arc adjacent to the inner apex on the upper curved surface may be equal to or less than the radius of curvature of the arc with the largest radius of curvature on the lower curved surface.

In addition, in the present invention, in a cross section perpendicular to the circumferential direction of the oil ring, the inner circumferential surface is formed by an upper arc and a lower arc that is provided below the upper arc and connected to the upper arc, and when the upper segment is installed on the spacer expander so as to be parallel to the radial direction of the oil ring, the intersection of the upper arc and the lower arc may be located above the intersection of the pressing surface and a lower virtual circle, which is a virtual circle formed by extending the lower arc.

In addition, in the present invention, in a cross section perpendicular to the circumferential direction of the oil ring, the inner circumferential surface is formed by an upper arc and a lower arc that is provided below the upper arc and connected to the upper arc, and when the upper segment is installed on the spacer expander so as to be parallel to the radial direction of the oil ring, the intersection of the upper arc and the lower arc may be located below the intersection of the pressing surface and a lower virtual circle, which is a virtual circle formed by extending the lower arc.

In the present invention, the inner peripheral surface may include an upper tapered surface that slopes from the inner peripheral apex toward the upper surface side, and a lower tapered surface that slopes from the inner peripheral apex toward the lower surface side, in a cross section perpendicular to the circumferential direction of the oil ring.

In addition, in the present invention, the inner peripheral surface includes an upper region above the inner peripheral apex and a lower region below the inner peripheral apex in a cross section perpendicular to the circumferential direction of the oil ring, and one of the upper region and the lower region may be formed as a curved surface and the other as a tapered surface.

In addition, in the present invention, the upper region may be formed as a tapered surface, and the lower region may be formed as a curved surface.

In addition, in the present invention, when the upper segment is installed on the spacer expander so as to be parallel to the radial direction of the oil ring, in a cross section perpendicular to the circumferential direction of the oil ring, in an XY coordinate system in which the radial outside of the oil ring is the positive X-axis and the axial upper side of the oil ring is the positive Y-axis, the position coordinates of the intersection of the inner peripheral surface and the pressing surface are (X1, Y1) and the position coordinates of the outer peripheral apex are (X2, Y2), cosθ(Y1-Y2)/(sinθ(X2-X1))>-0.14 may be satisfied. Note that cosθ(Y1-Y2)/(sinθ(X2-X1)) is more preferably -0.07 or more, and even more preferably 0 or more. Also, the upper limit of cosθ(Y1-Y2)/(sinθ(X2-X1)) is not particularly set, but from the viewpoint of manufacturing, it is preferable that it is 5.0 or less.

In addition, in the present invention, when the upper segment is installed on the spacer expander so as to be parallel to the radial direction of the oil ring, in a cross section perpendicular to the circumferential direction of the oil ring, the direction in which the outer peripheral surface descends is taken as positive with respect to the direction in which a moment acts about the intersection point between the inner peripheral surface and the pressing surface, the moment acting on the upper segment due to the axial component of the force acting on the inner peripheral surface from the pressing surface is taken as M1, and the moment acting on the upper segment due to the radial component of the force acting on the inner peripheral surface from the pressing surface is taken as M2, and M2/M1>-0.14 may be satisfied.

In the present invention, in a cross section perpendicular to the circumferential direction of the oil ring, the intersection between the inner peripheral surface and the pressing surface may be located above the outer peripheral apex.

The present invention can improve the oil sealing performance of the oil ring.

1 is a partial cross-sectional view of an internal combustion engine equipped with an oil ring according to an embodiment. FIG. 2 is a cross-sectional view of the oil ring according to the embodiment. FIG. 2 is a perspective view of a portion of a spacer expander according to an embodiment. 1 is a cross-sectional view for explaining a shape pattern 1 of an inner circumferential surface of an upper segment according to an embodiment. FIG. 1 is a diagram showing an example of a contact state between an inner peripheral surface of an inner peripheral shape pattern 1 and an upper ear portion. FIG. 4 is a cross-sectional view for explaining a shape pattern 2 of the inner circumferential surface of the upper segment according to the embodiment. FIG. 13 is a diagram showing an example of a contact state between an inner peripheral surface of the inner peripheral shape pattern 2 and an upper ear portion. FIG. 11 is a cross-sectional view for explaining a shape pattern 3 on the inner circumferential surface of the upper segment according to the embodiment. FIG. 13 is a diagram showing an example of a contact state between an inner peripheral surface of an inner peripheral shape pattern 3 and an upper ear portion. FIG. 11 is a cross-sectional view for explaining a shape pattern 4 on the inner circumferential surface of the upper segment according to the embodiment. FIG. FIG. 2 is a cross-sectional view for explaining a derived shape 1 of an inner peripheral surface. FIG. 2 is a cross-sectional view for explaining a derived shape 2 of the inner peripheral surface. 11 is a cross-sectional view for explaining a derived shape 3 of the inner peripheral surface. FIG. 11 is a cross-sectional view for explaining a derived shape 4 of the inner peripheral surface. FIG. A cross-sectional view for explaining a shape pattern 1 on the outer peripheral surface of an upper segment according to an embodiment. 11 is a cross-sectional view for explaining a shape pattern 2 on the outer peripheral surface of an upper segment according to an embodiment of the present invention. FIG. 11A and 11B are diagrams for explaining forces acting on an upper segment of an oil ring according to a conventional example. FIG. 4 is a diagram (1) for explaining the forces acting on the upper segment of the oil ring according to the embodiment. FIG. 4 is a diagram (2) for explaining the forces acting on the upper segment of the oil ring according to the embodiment. FIG. 4 is a diagram (3) for explaining the forces acting on the upper segment of the oil ring according to the embodiment. FIG. 4 is a diagram for explaining the forces acting on the upper segment of the oil ring according to the embodiment. Graph (1) showing the analysis results. Graph (2) showing the analysis results. Graph (3) showing the analysis results.

Below, preferred embodiments of the present invention will be described with reference to the drawings. Note that the configurations described in the following embodiments are not intended to limit the technical scope of the invention unless otherwise specified.

[overall structure]
FIG. 1 is a partial cross-sectional view of an internal combustion engine 100 including an oil ring 40 according to an embodiment. FIG. 1 is a schematic view illustrating the oil ring 40 in a simplified manner. FIG. 2 is a cross-sectional view of the oil ring 40 according to an embodiment. In FIG. 1 and FIG. 2, a cross section perpendicular to the circumferential direction of the oil ring 40 is illustrated. As shown in FIG. 1, in the internal combustion engine 100, a piston gap PC1 is formed by ensuring a predetermined separation distance between the inner wall surface 10a of the cylinder 10 and the outer circumferential surface 20a of the piston 20 mounted in the cylinder 10. In addition, a ring groove 30 is formed in the outer circumferential surface 20a of the piston 20. The ring groove 30 has an upper wall 301 formed on the combustion chamber side, a lower wall 302 formed on the crank chamber side and facing the upper wall 301, and a connecting wall 303 connecting the inner circumferential edges of the upper wall 301 and the lower wall 302. The oil ring 40 according to this embodiment is mounted in the ring groove 30.

The oil ring 40 is a sliding member that slides on the inner wall surface 10a of the cylinder 10 in association with the reciprocating motion of the piston 20. The oil ring 40 is a so-called three-piece type combination oil ring, and as shown in FIG. 1, it comprises a pair of segments 1, 2 and a spacer expander 3. The oil ring 40 is assembled to the piston 20 by being fitted into the ring groove 30.

In Fig. 2, the symbol A1 indicates the central axis of the oil ring 40. Fig. 2 illustrates a state before the oil ring 40 is attached to the piston 20, in which the upper and lower segments 1 and 2 are installed on the spacer expander 3 so as to be parallel to the radial direction of the oil ring 40. In the state of Fig. 2 (hereinafter also referred to as a stationary state), no external force acts on the upper and lower segments 1 and 2, and the upper and lower segments 1 and 2 are installed on the upper and lower sides of the spacer expander 3 without being affected by the ear angle θ described later. Hereinafter, as shown in Fig. 2, the direction along the central axis A1 of the oil ring 40 (axial direction) is defined as the "vertical direction". In addition, in the axial direction of the oil ring 40, the combustion chamber side of the internal combustion engine 100 (upper side in Fig. 1) is defined as the "upper side", and the opposite side, i.e., the crank chamber side (lower side in Fig. 1) is defined as the "lower side". In the following description of the oil ring 40, unless otherwise specified, the term "circumferential direction" refers to the circumferential direction of the oil ring 40 and the upper segment 1, the term "radial direction" refers to the radial direction of the oil ring 40 and the upper segment 1, and the term "axial direction" refers to the axial direction of the oil ring 40 and the upper segment 1. The circumferential direction, radial direction, and axial direction of the oil ring 40 and the upper segment 1 are the same as the circumferential direction, radial direction, and axial direction of the pair of segments 1 and 2, respectively. In this specification, the state in which the piston to which the piston ring is assembled is inserted into the cylinder as shown in FIG. 1 is referred to as the "used state". In this specification, the term "barrel shape" refers to a surface shape that is curved to be convex including the apex, and includes a "symmetric barrel shape" in which the apex is located at the center of the axial width of the ring and is axially symmetric with respect to a center line passing through the center of the axial width of the ring, and an "eccentric barrel shape" in which the apex is away from the center of the axial width of the ring and is axially asymmetric with respect to the center line. In addition, in this specification, a "tapered shape" refers to a surface shape that is inclined so as to move away from the central axis of the oil ring 40 (i.e., so as to expand in diameter) as it approaches the crank chamber side, or a surface shape that is inclined so as to move closer to the central axis of the oil ring 40 (i.e., so as to contract in diameter) as it approaches the crank chamber side.

[segment]
The pair of segments 1, 2 are formed in an annular shape around the axis of the oil ring 40, and are arranged side by side in the axial direction independently of each other. As shown in FIG. 1, of the pair of segments 1, 2, the upper segment 1 is located on the upper side (combustion chamber side), and the lower segment 2 is located on the lower side (crank chamber side). Examples of the material of the pair of segments 1, 2 include stainless steel and spring steel. Note that the inner peripheral surfaces of the upper segment 1 and the lower segment 2 may be formed with a chemical conversion coating, a nitriding coating, a resin coating, a PVD (Physical Vapor Deposition) coating, a DLC (Diamond-Like Carbon) coating, a chrome plating coating, or the like.

As shown in FIG. 1, the upper segment 1 includes an outer peripheral surface 11 provided on the outer peripheral side, an inner peripheral surface 12 provided on the inner peripheral side, an upper surface 13 facing the combustion chamber side, and a lower surface 14 facing the crank chamber side. The upper surface 13 and the lower surface 14, which are the axial end surfaces of the upper segment 1, are parallel to each other, and the axial width of the upper segment 1 is determined by the upper surface 13 and the lower surface 14. However, the upper surface 13 and the lower surface 14 do not have to be parallel. The upper surface 13 and the lower surface 14 may be partially parallel. As shown in FIG. 1, the upper segment 1 is provided so that the upper surface 13 faces the upper wall 301 of the ring groove 30, and slides on the inner wall surface 10a of the cylinder 10 with the outer peripheral surface 11 as a sliding surface.

As shown in FIG. 1, the lower segment 2 includes an outer peripheral surface 21 provided on the outer peripheral side, an inner peripheral surface 22 provided on the inner peripheral side, an upper surface 23 facing the combustion chamber side, and a lower surface 24 facing the crank chamber side. The upper surface 23 and the lower surface 24, which are the axial end surfaces of the lower segment 2, are parallel to each other, and the width of the lower segment 2 in the axial direction is determined by the upper surface 23 and the lower surface 24. However, the upper surface 23 and the lower surface 24 do not have to be parallel. The upper surface 23 and the lower surface 24 may be partially parallel. In addition, the lower segment 2 is provided so that the lower surface 24 faces the lower wall 302 of the ring groove 30, and slides on the inner wall surface 10a of the cylinder 10 with the outer peripheral surface 21 as a sliding surface. The inner peripheral surface 22 of the lower segment 2 has a symmetrical barrel shape. The shape of the lower segment 2 is not particularly limited. The shape of the inner peripheral surface 22 of the lower segment 2 may be a symmetrical barrel shape with the apex located in the center of the axial width of the ring, or it may be an eccentric barrel shape with the apex eccentric from the center of the axial width of the ring toward the combustion chamber side or the crankcase side.

[Spacer expander]
1, the spacer expander 3 is provided between a pair of segments 1 and 2. The spacer expander 3 generates tension so as to protrude radially outward, and urges the pair of segments 1 and 2 radially outward (i.e., toward the inner wall surface 10a of the cylinder 10) in a used state.

3 is a perspective view of a part of the spacer expander 3 according to the embodiment. As shown in FIG. 3, the spacer expander 3 is configured with a plurality of wavy periodic elements displaced in the axial direction, which are connected in the circumferential direction, and is formed in a wavy shape that continues in the circumferential direction when viewed in the radial direction of the spacer expander 3 (when viewed from the outside or inside in the radial direction). Specifically, the spacer expander 3 includes a plurality of upper pieces 31 and lower pieces 32 that are spaced apart in the axial and circumferential directions and alternately arranged in the circumferential direction, and a connecting piece 33 that connects the upper pieces 31 and lower pieces 32 adjacent to each other in the circumferential direction. The plurality of upper pieces 31 are arranged at intervals along the circumferential direction. The plurality of lower pieces 32 are provided so as to be located below the plurality of upper pieces 31 when the oil ring 40 is assembled to the piston 20 (i.e., in a used state). The plurality of lower pieces 32 are also arranged so that each of the lower pieces 32 is alternately arranged with each of the upper pieces 31 in the circumferential direction. The upper piece 31 and the lower piece 32 are spaced apart from each other in the axial direction. The connecting piece 33 connects the circumferential ends of the upper piece 31 and the lower piece 32 that are adjacent in the circumferential direction. As a result, the spacer expander 3 is formed in a wavy shape in the axial direction.

As shown in FIG. 2, the upper piece 31 includes an upper base portion 310, an upper ear portion 311, and an upper support portion 312. The upper base portion 310 extends radially perpendicular to the axial direction. The upper ear portion 311 is formed to stand on the inside of the upper base portion 310 in the radial direction and protrudes upward relative to the upper base portion 310. The upper ear portion 311 is provided so as to be located radially inward from the upper segment 1 in the usage state, and presses the upper segment 1 radially outward by abutting against the inner peripheral surface 12 of the upper segment 1. The pressing surface 3a of the upper ear portion 311 that abuts against the inner peripheral surface 12 is inclined in a cross section perpendicular to the circumferential direction so as to move away from the central axis A1 of the oil ring 40 (i.e., to expand in diameter) as it moves downward. The upper support portion 312 is formed to stand outward from the upper base portion 310 in the radial direction and protrudes upward relative to the upper base portion 310. The upper support portion 312 supports the upper segment 1 by contacting the lower surface 14 of the upper segment 1 in the use state. The support surface 3b of the upper support portion 312 that contacts the lower surface 14 extends in the radial direction. A through hole 34 through which oil can pass is formed at the base of the upper ear portion 311. The through hole 34 does not have to be formed. The upper piece 31 according to the present invention does not have to have the upper support portion 312. The upper base portion 310 and the upper support portion 312 may be inclined relative to the radial direction.

As shown in FIG. 2, the lower piece 32 includes a lower base portion 320, a lower ear portion 321, and a lower support portion 322. The lower base portion 320 extends in the radial direction perpendicular to the axial direction. The lower ear portion 321 is formed to stand on the inside of the lower base portion 320 in the radial direction and protrudes downward relative to the lower base portion 320. The lower ear portion 321 is provided so as to be located radially inward from the lower segment 2 in the usage state, and presses the lower segment 2 radially outward by abutting against the inner peripheral surface 22 of the lower segment 2. The pressing surface 3c of the lower ear portion 321 that abuts against the inner peripheral surface 22 is inclined in a cross section perpendicular to the circumferential direction so as to approach the central axis of the oil ring 40 (i.e., to reduce in diameter) as it moves downward. The lower support portion 322 supports the lower segment 2 by abutting against the upper surface 23 of the lower segment 2 in the usage state. The support surface 3d of the lower support portion 322 that abuts against the upper surface 23 extends in the radial direction. A through hole 34 is also formed at the base of the lower ear portion 321. The through hole 34 does not have to be formed. The lower piece 32 according to the present invention does not have to have the lower support portion 322. The lower base portion 320 and the lower support portion 322 may be inclined with respect to the radial direction.

As shown in FIG. 2, the inclination angle (hereinafter referred to as the ear angle) of the pressing surface 3a of the upper piece 31 of the spacer expander 3 with respect to the axial direction (i.e., the vertical direction) of the oil ring 40 is θ. In this embodiment, 1°≦θ≦20°. Note that the shape of the spacer expander according to the present invention is not limited to the above-mentioned one.

[Upper segment surface shape]
Next, the inner peripheral surface shape and the outer peripheral surface shape that can be adopted in the upper segment according to the present invention will be described using the upper segment 1 according to the embodiment as an example. Note that the surface shapes described below are merely examples and do not limit the present invention.

Figures 4 to 14 are cross-sectional views for explaining the shape pattern of the inner peripheral surface 12 of the upper segment 1 according to the embodiment. Also, Figures 15 and 16 are cross-sectional views for explaining the shape pattern of the outer peripheral surface 11 of the upper segment 1 according to the embodiment. Figures 4 to 16 show the state of the upper segment 1 when the oil ring 40 is installed on the spacer expander 3 so as to be parallel to the radial direction of the oil ring 40 before it is attached to the piston 20. Also, Figures 4 to 16 show a cross section perpendicular to the circumferential direction, and for convenience, the cross section hatching is omitted. Also, the symbol CL1 in Figures 4, 6, 8, 10 to 16 indicates a line (center line) passing through the center of the axial width of the upper segment 1. Also, the symbol MP2 in Figures 4, 6, 8, 10 to 16 indicates the central position in the axial direction of the inner peripheral surface 12 (the vertical central position of the axial width), and the symbol MP1 in Figures 15 and 16 indicates the central position in the axial direction of the outer peripheral surface 11 (the vertical central position of the axial width). In some of the shape patterns described below, similar configurations may be given the same reference symbols to avoid redundant explanations.

The symbol TP2 in Figures 4 to 14 indicates the inner circumferential apex. In a cross section perpendicular to the circumferential direction, the inner circumferential apex TP2 is located radially inward from other parts of the inner circumferential surface 12 and is the part of the inner circumferential surface 12 with the smallest diameter (i.e., closest to the oil ring central axis A1). In each shape shown in Figures 4 to 14, the inner circumferential apex TP2 is located above (on the combustion chamber side) the vertical center position MP2 of the axial width of the inner circumferential surface 12. In other words, the inner circumferential surface 12 according to the embodiment has an eccentric shape in which the inner circumferential apex TP2 is eccentric.

[Inner Circumference Shape Pattern 1]
4 is a cross-sectional view for explaining a shape pattern 1 (hereinafter, inner peripheral shape pattern 1) of the inner peripheral surface 12 of the upper segment 1 according to the embodiment. The inner peripheral surface 12 according to the inner peripheral shape pattern 1 has a barrel shape. That is, the inner peripheral surface 12 is curved so as to be convex radially inward, including the inner peripheral apex TP2, in a cross section perpendicular to the circumferential direction. Furthermore, since the inner peripheral apex TP2 is located above the vertical center position MP2, the inner peripheral surface 12 is asymmetric in the vertical direction (axial direction) with respect to the center line CL1. That is, the inner peripheral surface 12 has an eccentric barrel shape.

As shown in FIG. 4, the inner peripheral surface 12 according to the inner peripheral shape pattern 1 includes an inner peripheral end surface S1 including an inner peripheral apex TP2, a first connection surface S2 connecting the inner peripheral end surface S1 and the upper surface 13, and a second connection surface S3 connecting the inner peripheral end surface S1 and the lower surface 14. Here, the region of the inner peripheral end surface S1 above the inner peripheral apex TP2 is the upper curved surface S11, and the region below the inner peripheral apex TP2 is the lower curved surface S12. The upper curved surface S11 is curved so as to move away from the central axis A1 of the oil ring 40 (i.e., to expand in diameter) as it moves upward from the inner peripheral apex TP2, and the lower curved surface S12 is curved so as to expand in diameter as it moves downward from the inner peripheral apex TP2. In other words, the inner peripheral end surface S1 is formed so as to expand in diameter as it moves away from the inner peripheral apex TP2 in the axial direction.

Here, the symbol TA in FIG. 4 indicates the apex vicinity region. The apex vicinity region TA is a region from the inner apex TP2 to a predetermined distance radially outward. The range of the apex vicinity region TA is not particularly limited, but can be, for example, 30 μm to 50 μm radially outward from the inner apex TP2. The inner periphery end surface S1 shown in FIG. 4 is asymmetrical in the apex vicinity region TA. More specifically, the inner periphery surface 12 is asymmetrical in the vertical direction with respect to the center line CL1, and the apex vicinity region TA is asymmetrical in the vertical direction with respect to the virtual line L1 (described later). In addition, the inner periphery end surface S1 is formed by two arcs with different radii of curvature in the apex vicinity region TA. Note that the inner periphery shape pattern 1 may be formed by two or more arcs with different radii of curvature in the apex vicinity region TA, for example, it may be formed by three arcs. In this example, the inner peripheral end surface S1 is formed by an upper circular arc AR1 and a lower circular arc AR2 that is disposed below the upper circular arc AR1 and connected to the upper circular arc AR1 in a cross section perpendicular to the circumferential direction. In the example shown in FIG. 4, the upper circular arc AR1 is disposed above the inner peripheral apex TP2, and the lower circular arc AR2 is disposed below the inner peripheral apex TP2.

The symbol VC1 in FIG. 4 indicates an upper virtual circle, which is a virtual circle formed by extending the upper arc AR1. The upper arc AR1 is included in the upper virtual circle VC1. Also, VC2 indicates a lower virtual circle, which is a virtual circle formed by extending the lower arc AR2. The lower arc AR2 is included in the lower virtual circle VC2. Also, the symbol C1 in FIG. 4 indicates the center of the upper arc AR1 and the upper virtual circle VC1, C2 indicates the center of the lower arc AR2 and the lower virtual circle VC2, and L1 indicates a virtual line extending radially through the inner apex TP2. Also, the symbol CP1 indicates the intersection (connection point) of the upper arc AR1 and the lower arc AR2. Additionally, the radius of the upper arc AR1 and the upper virtual circle VC1 is R1, and the radius of the lower arc AR2 and the lower virtual circle VC2 is R2.

In the inner circumference shape pattern 1 shown in FIG. 4, the center C1 of the upper arc AR1 and the center C2 of the lower arc AR2 are both located on the virtual line L1, and the intersection point CP1 coincides with the inner circumference apex TP2. Therefore, the upper curved surface S11 is formed by the upper arc AR1, and the lower curved surface S12 is formed by the lower arc AR2. In addition, R1<R2, and the upper virtual circle VC1 is inscribed in the lower virtual circle VC2. Note that R1=R2 may also be satisfied. In the example shown in FIG. 4, R1=0.1 mm, R2=0.35 mm. However, the inner circumference shape pattern 1 is not limited to this. Note that R1 and R2 can be appropriately set in the range of 0.03 mm to 2.0 mm, but are not limited to this range. The upper virtual circle VC1 and the lower virtual circle VC2 intersect at only one point, the intersection point CP1, and have a common tangent at the intersection point CP1. Therefore, the upper arc AR1 and the lower arc AR2 are tangent to each other.

In the present invention, the upper curved surface and the lower curved surface may be formed by multiple arcs with different radii of curvature, as in derived form 2 shown in FIG. 12 described below. The upper curved surface and the lower curved surface may also have a shape in which the radius of curvature gradually changes toward the upper or lower side. When the radius of curvature gradually changes, a small portion of the curve near any point can be approximated by an arc whose radius is the radius of curvature at that point.

Here, from the viewpoint of improving the oil seal performance, it is preferable that the radius of curvature of the arc adjacent to the inner apex TP2 on the upper curved surface S11 is equal to or smaller than the radius of curvature of the arc with the largest radius of curvature on the lower curved surface S12. By satisfying such a condition, the amount of eccentricity of the inner apex TP2 with respect to the vertical center position MP2 can be increased. This allows the surface pressure on the inner periphery of the upper surface 13 of the upper segment 1 to be increased, as described later. However, the present invention is not limited to this condition. The inner periphery shape pattern 1 shown in FIG. 4 satisfies the above-mentioned condition because the radius of curvature R1 of the upper arc AR1, which is the arc adjacent to the inner apex TP2 on the upper curved surface S11, is smaller than the radius of curvature R2 of the lower arc AR2, which is the arc with the largest radius of curvature on the lower curved surface S12. The above-mentioned condition can also be applied to cases where the upper curved surface or the lower curved surface is formed of multiple arcs with different radii of curvature, or where the radii of curvature of the upper curved surface or the lower curved surface gradually change upward or downward. As an example, in the derived form 2 shown in Fig. 12 described later, the radius of curvature R1 of the upper arc AR1, which is an arc adjacent to the inner apex TP2 in the upper curved surface S11, is smaller than the radius of curvature R3 of the third arc AR3, which is an arc with the largest radius of curvature in the lower curved surface S12, and therefore this condition is satisfied.

The first connection surface S2 and the second connection surface S3 are so-called R surfaces (corner R) formed by chamfering, and are formed by circular arcs in a cross section perpendicular to the circumferential direction. Note that the first connection surface S2 and the second connection surface S3 are not essential components of the present invention.

5 is a diagram showing an example of the contact state between the inner peripheral surface 12 and the upper ear portion 311 according to the inner peripheral shape pattern 1. The reference symbol P1 in FIG. 5 indicates the intersection (contact point) between the inner peripheral surface 12 and the pressing surface 3a of the upper ear portion 311. Also, the reference symbol Par2 indicates the intersection point between the lower virtual circle VC2 and the pressing surface 3a. As shown in FIG. 5, the inner peripheral surface 12 contacts the pressing surface 3a of the upper ear portion 311 at the lower curved surface S12. Here, in the inner peripheral shape pattern 1, the intersection point CP1 between the upper arc AR1 and the lower arc AR2 is located above the intersection point Par2 between the lower virtual circle VC2 and the pressing surface 3a. Therefore, as shown by the intersection point P1 in FIG. 5, the inner peripheral surface 12 contacts the pressing surface 3a of the upper ear portion 311 at the lower arc AR2, and the intersection point P1 coincides with the intersection point Par2.

As described above, in a cross section perpendicular to the circumferential direction, the inner circumference shape pattern 1 has a shape in which the apex region TA is formed by two or more arcs with different radii of curvature, the center C1 of the upper arc AR1 and the center C2 of the lower arc AR2 are both located on the imaginary line L1, the upper arc AR1 and the lower arc AR2 are tangent continuous, and the intersection CP1 of the upper arc AR1 and the lower arc AR2 forms the inner circumference apex TP2.

[Inner Circumference Shape Pattern 2]
6 is a cross-sectional view for explaining a shape pattern 2 of the inner circumferential surface 12 of the upper segment 1 according to the embodiment (hereinafter, referred to as the inner circumferential shape pattern 2). The inner circumferential surface 12 according to the inner circumferential shape pattern 2 has an eccentric barrel shape similar to the inner circumferential shape pattern 1, and is vertically asymmetric in the apex vicinity region TA. More specifically, the inner circumferential surface 12 is vertically asymmetric with respect to the center line CL1, and the apex vicinity region TA is vertically asymmetric with respect to the virtual line L1. In addition, the inner circumferential shape pattern 2 differs from the inner circumferential shape pattern 1 shown in FIGS. 4 and 5 in that the center C2 of the lower circular arc AR2 is located above the center C1 of the upper circular arc AR1.

6, in the cross section perpendicular to the circumferential direction, the inner peripheral shape pattern 2 has a shape in which the apex region TA is formed by two or more arcs with different radii of curvature, the center C1 of the upper arc AR1 is located on the virtual line L1, the center C2 of the lower arc AR2 is located above the center C1, the upper arc AR1 and the lower arc AR2 are tangent continuous, and the position of the upper arc AR1 closest to the central axis A1 of the oil ring is the inner peripheral apex TP2. In the inner peripheral shape pattern 2, the intersection CP1 of the upper arc AR1 and the lower arc AR2 is located below the inner peripheral apex TP2. In the inner peripheral shape pattern 2, the upper part of the upper arc AR1 forms the upper curved surface S11, and the lower part of the upper arc AR1 and the lower arc AR2 form the lower curved surface S12. Furthermore, R1<R2, and the upper virtual circle VC1 is inscribed in the lower virtual circle VC2.

FIG. 7 is a diagram showing an example of a contact state between the inner circumferential surface 12 and the upper ear 311 according to the inner circumferential shape pattern 2. As shown in FIG. 7, the inner circumferential surface 12 contacts the pressing surface 3a of the upper ear 311 at the lower curved surface S12. Here, in the inner circumferential shape pattern 2, the intersection CP1 between the upper arc AR1 and the lower arc AR2 is located below the intersection Par2 between the lower virtual circle VC2 and the pressing surface 3a. Therefore, as shown by the intersection P1 in FIG. 7, the inner circumferential surface 12 can contact the pressing surface 3a of the upper ear 311 at the upper arc AR1. In the example shown in FIG. 7, the inner circumferential surface 12 contacts the pressing surface 3a at the upper arc AR1, but by making the ear angle θ of the upper ear 311 larger, the inner circumferential surface 12 can contact the pressing surface 3a at the intersection CP1 or the lower arc AR2.

[Inner peripheral shape pattern 3 of inner peripheral surface]
8 is a cross-sectional view for explaining a shape pattern 3 of the inner circumferential surface 12 of the upper segment 1 according to the embodiment (hereinafter, the inner circumferential shape pattern 3). The inner circumferential surface 12 according to the inner circumferential shape pattern 3 has an eccentric barrel shape, similar to the inner circumferential shape pattern 1. The inner circumferential shape pattern 3 is different from the inner circumferential shape patterns 1 and 2 shown in FIGS. 4 and 5 and the inner circumferential shape patterns 2 shown in FIGS. 6 and 7 in that, in a cross section perpendicular to the circumferential direction, the inner circumferential end surface S1 is formed by a single arc Ar1, and the apex vicinity region TA is vertically symmetrical. More specifically, the inner circumferential surface 12 is vertically asymmetrical with respect to the center line CL1, and the apex vicinity region TA is vertically symmetrical with respect to the virtual line L1.

As shown in FIG. 8, the inner circumference shape pattern 3 is shaped such that, in a cross section perpendicular to the circumferential direction, the apex region TA is formed by a single arc Ar1 having a radius of curvature R, the center c1 of the arc Ar1 is located on the imaginary line L1, and the position of the arc Ar1 closest to the central axis A1 of the oil ring is the inner circumference apex TP2. In addition, in the inner circumference shape pattern 3, the upper part of the arc Ar1 forms an upper curved surface S11, and the lower part of the arc Ar1 forms a lower curved surface S12. The symbol Vc1 in FIG. 8 indicates a virtual circle formed by extending the arc Ar1.

Figure 9 is a diagram showing an example of the contact state between the inner circumferential surface 12 and the upper ear 311 in the inner circumferential shape pattern 3. As shown by the intersection point P1 in Figure 9, in the inner circumferential shape pattern 3, the inner circumferential surface 12 comes into contact with the pressing surface 3a of the upper ear 311 in the lower part of the arc Ar1.

[Inner Circumference Shape Pattern 4]
FIG. 10 is a cross-sectional view for explaining a shape pattern 4 of the inner circumferential surface 12 of the upper segment 1 according to the embodiment (hereinafter, the inner circumferential shape pattern 4). The inner circumferential surface 12 according to the inner circumferential shape pattern 4 has an eccentric shape in which the inner circumferential apex TP2 is located above the vertical center position MP2, similar to the inner circumferential shape patterns 1 to 3. The inner circumferential shape pattern 4 is different from the inner circumferential shape patterns 1 to 3 described above in that the inner circumferential end surface S1 is formed by an upper tapered surface S13 and a lower tapered surface S14 having a tapered shape. Specifically, in a cross section perpendicular to the circumferential direction, the upper tapered surface S13 is inclined so as to move away from the central axis A1 of the oil ring 40 as it moves upward from the inner circumferential apex TP2, and the lower tapered surface S14 is inclined so as to move away from the central axis A1 of the oil ring 40 as it moves downward from the inner circumferential apex TP2. In the example shown in FIG. 10, the inner circumferential apex TP2 is R-shaped, but the inner circumferential apex TP2 may be flat. In the inner peripheral shape pattern 4, the inner peripheral surface 12 contacts the pressing surface 3a of the upper ear 311 at the lower tapered surface S14. Note that, when the ear angle θ is relatively small, the pressing surface 3a of the upper ear 311 may contact the inner peripheral apex TP2 of the upper segment 1. Here, assuming that the angle of the lower tapered surface S14 with respect to the axial direction (i.e., the vertical direction) of the oil ring 40 is θ1, it is preferable that θ≦θ1. Also, when θ=θ1, the lower tapered surface S14 of the inner peripheral surface 12 contacts the pressing surface 3a of the upper ear 311, suppressing fluttering (posture change) of the upper segment 1, but there is a possibility that the deposit resistance will decrease. Therefore, it is more preferable that θ<θ1. Note that the upper tapered surface S13 and the lower tapered surface S14 are formed in a straight line, but may be rounded. In this case, the lower tapered surface S14 can be regarded as a lower circular arc AR2, and the R surface forming the inner circumferential top portion TP2 can be regarded as an upper circular arc AR1.

[Derived forms]
The following describes variations of the above-mentioned inner peripheral shape patterns 1 to 4. The variations described below can be combined with any of the above-mentioned inner peripheral shape patterns 1 to 4 as far as possible.

[Variation 1]
Fig. 11 is a cross-sectional view for explaining a derived shape 1 of the inner circumferential surface 12. The derived shape 1 shown in Fig. 11 is a derived shape of the inner circumferential shape pattern 1, and the ratio of the radius of curvature R1 of the upper circular arc AR1 to the radius of curvature R2 of the lower circular arc AR2 is different from that of the inner circumferential shape pattern 1 shown in Figs. 4 and 5. In the derived shape 1, R1 = 0.04 mm, and R2 = 0.4 mm. However, the derived shape 1 of the inner circumferential surface 12 is not limited to this. Note that R1 and R2 can be appropriately set in the range of 0.03 mm to 2.0 mm, but are not limited to this.

[Variation 2]
FIG. 12 is a cross-sectional view for explaining a derived shape 2 of the inner peripheral surface 12. The derived shape 2 shown in FIG. 12 is a derivative of the inner peripheral shape pattern 1, and differs from the inner peripheral shape pattern 1 shown in FIG. 4 and FIG. 5 in that the apex vicinity area TA is formed by three arcs with different radii of curvature. In other words, another arc is interposed between the upper and lower arcs and the upper and lower surfaces. The inner peripheral end surface S1 of the derived shape 2 is formed in a cross section perpendicular to the circumferential direction by an upper arc AR1, a lower arc AR2 that is provided below the upper arc AR1 and connected to the upper arc AR1, and a third arc AR3 that is provided below the lower arc AR2 and connected to the lower arc AR2, in a cross section perpendicular to the circumferential direction. The reference symbol C3 in FIG. 12 indicates the center of the third arc AR3, and the reference symbol CP2 indicates the intersection (connection point) between the lower arc AR2 and the third arc AR3. The radius of curvature of the third arc AR3 is R3.

As shown in FIG. 12, in derived shape 2, R1<R2<R3, and the center C1 of the upper arc AR1, the center C2 of the lower arc AR2, and the center C3 of the third arc AR3 are all located on the imaginary line L1. In addition, the upper arc AR1 and the lower arc AR2 are tangent to each other, and the lower arc AR2 and the third arc AR3 are tangent to each other, and the intersection CP1 of the upper arc AR1 and the lower arc AR2 is the inner apex TP2. In derived shape 2, the upper curved surface S11 is formed by the upper arc AR1, and the lower curved surface S12 is formed by the lower arc AR2 and the third arc AR3.

In the example shown in FIG. 12, the radius of curvature of the inner peripheral end face S1 changes in stages, but the radius of curvature of the inner peripheral end face S1 may be shaped so that it changes gradually toward the upper or lower side. In the present invention, R1<R2<R3 does not have to be true, the centers C2 and C3 do not have to be located on the virtual line L1, and the connections between the arcs do not have to be tangent continuous.

[Variation 3]
Fig. 13 is a cross-sectional view for explaining a derived shape 3 of the inner circumferential surface 12. The derived shape 3 shown in Fig. 13 is a derived shape of the inner circumferential shape pattern 1, and differs from the shapes shown in Figs. 4 and 5 in that the second connection surface S3 is recessed. By making only the second connection surface S3 of the first connection surface S2 and the second connection surface S3 a recessed shape, it is possible to improve the distinguishability between the top and bottom of the upper segment 1. The second connection surface S3 may be formed in a curved shape or a tapered shape.

[Variation 4]
Fig. 14 is a cross-sectional view for explaining a derived shape 4 of the shape pattern 1 of the inner circumferential surface 12. The derived shape 4 differs from the shape pattern 1 shown in Fig. 4 and Fig. 5 in that the upper circular arc AR1 and the lower circular arc AR2 are not tangent continuous. That is, at the intersection CP1, the tangent to the upper circular arc AR1 and the tangent to the lower circular arc AR2 are different.

[Outer periphery shape pattern 1]
15 is a cross-sectional view for explaining a shape pattern 1 (hereinafter, outer circumferential shape pattern 1) of the outer circumferential surface 11 of the upper segment 1 according to the embodiment. The symbol TP1 indicates an outer circumferential apex. The outer circumferential apex TP1 is located radially outward from other portions of the outer circumferential surface 11 in a cross section perpendicular to the circumferential direction, and is a portion of the outer circumferential surface 11 with the maximum diameter. The outer circumferential surface 11 according to the outer circumferential shape pattern 1 has a barrel shape. That is, the outer circumferential surface 11 is curved so as to be convex radially outward, including the outer circumferential apex TP1, in a cross section perpendicular to the circumferential direction.

In the outer peripheral shape pattern 1 shown in FIG. 15, the outer peripheral apex TP1 is located at the vertical center position MP1, and the outer peripheral surface 11 is symmetrical in the vertical direction (axial direction) with respect to the center line CL1. In other words, the outer peripheral surface 11 of the outer peripheral shape pattern 1 has a symmetrical barrel shape. The outer peripheral surface 11 may have a constant radius of curvature over its entire area, or may have a shape in which the radius of curvature changes gradually.

[Outer periphery shape pattern 2]
Fig. 16 is a cross-sectional view for explaining a shape pattern 2 of the outer circumferential surface 11 of the upper segment 1 according to the embodiment (hereinafter, outer circumferential shape pattern 2). The outer circumferential surface 11 according to the outer circumferential shape pattern 2 differs from the outer circumferential surface 11 according to the outer circumferential shape pattern 1 shown in Fig. 15 in that it has an eccentric barrel shape. That is, in the outer circumferential shape pattern 2 shown in Fig. 16, the outer circumferential apex TP1 is located below the vertical center position MP1, and the outer circumferential surface 11 is asymmetric in the vertical direction (axial direction) with respect to the center line CL1.

[Force acting on the upper segment]
Next, the forces acting on the upper segment in the internal combustion engine 100 will be described. Fig. 17 is a diagram for explaining the forces acting on the upper segment 1A of the oil ring according to the conventional example. Figs. 18 to 21 are diagrams for explaining the forces acting on the upper segment 1 of the oil ring 40 according to the embodiment. Figs. 17 to 21 show cross sections perpendicular to the circumferential direction.

As shown in Figure 17, the upper segment 1A of the conventional oil ring has an outer peripheral surface 11 and an inner peripheral surface 12 that have a symmetrical barrel shape. In other words, the outer peripheral apex TP1 is located at the vertical center position MP1 of the outer peripheral surface 11, and the inner peripheral apex TP2 is located at the vertical center position MP2 of the inner peripheral surface 12.

In the upper segment 1 according to the embodiment shown in FIG. 18, the outer peripheral surface 11 has a symmetrical barrel shape with an outer peripheral shape pattern 1 as shown in FIG. 15, as in the conventional example, and the inner peripheral surface 12 has an eccentric barrel shape with an inner peripheral shape pattern 1 as shown in FIG. 4. In FIG. 18, the ear angle θ of the upper ear portion 311 is the same as θ in the conventional example in FIG. 17.

The upper segment 1 according to the embodiment shown in FIG. 19 has an outer peripheral surface 11 that has a symmetrical barrel shape with an outer peripheral shape pattern 1, similar to FIG. 18, and an inner peripheral surface 12 that has an eccentric barrel shape with an inner peripheral shape pattern 1, the same as FIG. 18. In FIG. 19, the ear angle θ of the upper ear portion 311 is smaller than the θ in FIG. 17 and FIG. 18.

The upper segment 1 according to the embodiment shown in FIG. 20 has an outer peripheral surface 11 that is a symmetrical barrel shape of the outer peripheral shape pattern 1, and an inner peripheral surface 12 that is an eccentric barrel shape of the derivative shape 1 of the inner peripheral shape pattern 1 shown in FIG. 11. In FIG. 20, the ear angle θ of the upper ear portion 311 is the same as θ in FIG. 17 and FIG. 18.

The upper segment 1 according to the embodiment shown in FIG. 21 has an outer peripheral surface 11 that has an eccentric barrel shape with an outer peripheral shape pattern 2 as shown in FIG. 16, and an inner peripheral surface 12 that has an eccentric barrel shape with an inner peripheral shape pattern 1 as shown in FIG. 18. In FIG. 21, the ear angle θ of the upper ear portion 311 is the same as the θ in FIGS. 17 and 18.

In addition, in Figures 17 to 21, an XY coordinate system is set in which the radial outside of the oil ring is the positive X-axis and the axial upper side of the oil ring is the positive Y-axis in a cross section perpendicular to the circumferential direction. In this example, the origin of the XY coordinate system is set on the center line CL1.

Here, in a cross section perpendicular to the circumferential direction, the direction in which the moment acts around the intersection point P1 between the inner circumferential surface 12 and the pressing surface 3a of the upper ear 311 is positive, and is represented by D1, in which the outer circumferential surface 11 descends. The opposite direction to D1, that is, the direction in which the outer circumferential surface 11 ascends, is negative, and is represented by D2. The moment acting on the upper segment 1 in the D1 direction acts to make the upper segment 1 tilt downward radially outward as shown in FIG. 1, and becomes a rotational force that presses the inner circumferential side of the upper surface 13 of the upper segment 1 against the upper wall 301 of the ring groove 30. In other words, the moment in the D1 direction is a rotational force that causes the upper segment 1 to maintain a downward tilt radially outward, and generates a side seal force on the inner circumferential side. On the other hand, the moment acting on the upper segment 1 in the D2 direction is a rotational force that acts to push up the outer circumferential side of the upper surface 13 of the upper segment 1, and may act to reduce the side seal force on the inner circumferential side.

The symbol F1 in Figs. 17 to 21 indicates the force that the upper segment receives from the ear of the spacer expander 3. F1 acts in the normal direction of the pressing surface 3a. Fy indicates the axial (Y-direction) component of F1, and Fx indicates the radial (X-direction) component of F1. Fy acts axially upward and mainly contributes to the side sealing force on the inner periphery side. Fx acts radially outward and mainly contributes to the oil scraping performance on the outer periphery surface. Here, if the tension generated by the spacer expander 3 is 2F, a force F that tries to push outward acts on each of the upper segment and the lower segment (not shown). 2F is the force (tension) of the spacer expander 3 toward the outer periphery in one cross section, so F is half of that. F is a positive value and acts in the radial direction (direction perpendicular to the axial direction). If the tangential tension of the oil ring is Ft and the bore diameter (nominal diameter of the ring) is D, then F can be calculated by the formula F = Ft/D. Furthermore, F acting on the upper segment is divided into a force F1 in the normal direction of the pressing surface 3a and a downward force parallel to the pressing surface 3a (not shown). In this case, F1, Fy, and Fx can be expressed by the following formulas.
F1=Fcosθ...Formula (1)
Fy=Fcosθsinθ...Formula (2)
Fx=Fcos ^2θ Equation (3)
Since 1°≦θ≦20°, F1, Fy, and Fx are positive values.

17 to 21, it is assumed that the outer peripheral surface 11 contacts the inner wall surface 10a of the cylinder 10 at the outer peripheral apex TP1. Here, when the upper segment 1 is pressed against the inner wall surface 10a of the cylinder 10 by Fx, which is a component force of F1, a reaction force Fr from the inner wall surface 10a acts radially inward on the outer peripheral apex TP1 of the outer peripheral surface 11. Fr can be expressed by the following equation and is a negative value.
Fr=-Fx...Formula (4)

In addition, in the stationary state, the position coordinates of the intersection point P1 between the inner peripheral surface 12 and the pressing surface 3a of the upper ear 311 in the XY coordinate system are (X1, Y1), the position coordinates of the outer peripheral apex TP1 are (X2, Y2), the coordinate difference between the intersection point P1 and the outer peripheral apex TP1 in the Y direction is ΔY, and the coordinate difference between the intersection point P1 and the outer peripheral apex TP1 in the X direction is ΔX. In addition, the moment acting on the upper segment 1 by Fy is M1. In addition, the moment acting on the upper segment 1 by Fr is M2. At this time, ΔY, ΔX, M1, and M2 can be expressed by the following formula. In addition, in an actual product, the shape of the upper segment 1 is obtained from the cross-sectional projection of the upper segment 1, the upper segment 1 is arranged horizontally, and the position coordinates of the intersection point P1 in the stationary state can be specified from the intersection point between the straight line having the separately measured ear angle and the inner peripheral surface 12 of the upper segment 1. Similarly, the position coordinates of the inner periphery apex TP2 in a stationary state can be determined from the innermost diameter portion of the upper segment 1 in the cross-sectional projection, and the position coordinates of the outer periphery apex TP1 in a stationary state can be determined from the outermost diameter portion.
ΔY=Y1-Y2...Formula (5)
ΔX=X2-X1...Formula (6)
M1=FyΔX=Fcosθsinθ(X2-X1)...Equation (7)
M2=FxΔY=Fcos ² θ(Y1-Y2)...Formula (8)

Here, in the radial direction, intersection point P1 is located inside outer circumferential apex TP1, so X2-X1>0. Therefore, according to equation (7), M1 is a positive value, and is a rotational force acting in the positive direction D1. In other words, moment M1 due to Fy acts to maintain the upper segment 1 in a position inclined downward radially outward, generating a side sealing force on the inner circumferential side.

On the other hand, in the axial direction, the intersection point P1 may be located above or below the outer circumferential apex TP1. When the intersection point P1 is located above the outer circumferential apex TP1, Y1-Y2>0 holds, so according to equation (8), M2 is a positive value and is a rotational force acting in the positive direction D1. In other words, it is a rotational force that increases the side sealing force on the inner circumferential side. When the intersection point P1 is located below the outer circumferential apex TP1, Y1-Y2<0 holds, so according to equation (8), M2 is a negative value and is a rotational force acting in the negative direction D2. In other words, it is a rotational force that decreases the side sealing force on the inner circumferential side.

In addition, Fx can be converted to an axial force Fz based on M2 using the following formula:
Fz=Fcos ² θ(Y1-Y2)/(X2-X1)...Formula (9)

In the case of the conventional example shown in FIG. 17, the intersection P1 between the inner circumferential surface 12 and the pressing surface 3a of the upper ear 311 is located below the outer circumferential apex TP1, so M2 is a negative value according to equation (8). In other words, the moment due to Fx acts in the negative direction D2. Therefore, the moment M2 due to Fx acts to reduce the side sealing force on the inner circumferential side of the upper segment 1A.

In the embodiment shown in FIG. 18, the intersection P1 is located below the outer periphery apex TP1, but since the inner periphery apex TP2 is eccentric upward, the vertical position of the intersection P1 is located higher than in the conventional example. In other words, by making the inner periphery surface 12 eccentric, the vertical position of the intersection P1 is brought closer to the outer periphery apex TP1, and the absolute value of the coordinate difference ΔY between the intersection P1 and the outer periphery apex TP1 in the Y direction is reduced. Therefore, according to formula (8), although M2<0, the absolute value of M2 is smaller than in the conventional example. As a result, the sum of M1 and M2 is larger than in the conventional example. As a result, the side sealing force can be increased in the example shown in FIG. 18 compared to the conventional example.

In the embodiment shown in FIG. 19, the ear angle θ of the upper ear 311 is made smaller than that in FIG. 18, so that the intersection point P1 is located above the outer periphery apex TP1. Therefore, Y1-Y2>0, and therefore M2>0 according to equation (8), and the moment M2 due to Fx acts in the positive direction D1. In other words, the moment M2 due to Fx acts to increase the side sealing force on the inner periphery side. This makes the sum of M1 and M2 larger than that in FIG. 18. As a result, in the example shown in FIG. 19, the side sealing force can be further increased compared to the conventional example.

In the example embodiment shown in FIG. 20, by making the shape of the inner circumferential surface 12 the derivative shape 1 described above, the intersection point P1 is located above the outer circumferential apex TP1. Therefore, according to equation (8), M2>0, and the moment M2 due to Fx acts to increase the side sealing force on the inner circumferential side. As a result, the example shown in FIG. 20 can further increase the side sealing force compared to the conventional example.

In the example embodiment shown in FIG. 21, the shape of the inner peripheral surface 12 and the shape of the outer peripheral surface 11 are both eccentric barrels, so that the intersection point P1 is located above the outer peripheral apex TP1. Therefore, according to equation (8), M2>0, and the moment M2 due to Fx acts to increase the side sealing force on the inner peripheral side. As a result, the side sealing force can be further increased in the example shown in FIG. 21 compared to the conventional example.

[Action and Effects]
As described above, in the oil ring 40 according to the embodiment, the inner circumferential apex TP2 of the inner circumferential surface 12 of the upper segment 1 is configured to be located above the vertical center position MP2 of the inner circumferential surface 12 in a cross section perpendicular to the circumferential direction. In other words, the inner circumferential surface 12 has an eccentric shape. Furthermore, the ear angle θ of the upper ear 311 of the spacer expander 3 is 1°≦θ≦20°. According to the oil ring 40 according to the embodiment configured in this manner, the vertical position of the intersection point P1 between the inner circumferential surface 12 of the upper segment 1 and the pressing surface 3a of the upper ear 311 can be shifted upward, compared to a conventional oil ring. As a result, according to the oil ring 40 according to the embodiment, the sealing performance of the upper segment 1 can be improved and oil consumption can be reduced. Specifically, the larger the eccentricity of the inner circumferential apex TP2, the larger the sum of the eccentricity of the inner circumferential apex TP2 and the outer circumferential apex TP1 becomes, so that the upper segment 1 is inclined downward significantly on the outer circumferential side, and the surface pressure on the inner circumferential side of the upper surface 13 tends to increase. This improves the sealing performance of the inner circumferential side of the upper surface 13. When the position of the inner circumferential apex TP2 is located above the upper ear 311, the vertical position of the intersection point P1 between the inner circumferential surface 12 of the upper segment 1 and the pressing surface 3a of the upper ear 311 does not change even if the eccentricity of the inner circumferential apex TP2 is increased. In addition, the smaller the ear angle θ, the smaller the downward inclination of the outer circumferential side of the upper segment 1, but the shorter the movement time until the upper surface 13 contacts the upper wall 301 of the ring groove 30 during the upward stroke (more specifically, the range in which the inertial force acts upward during the upward stroke). In other words, the upper surface 13 contacts the upper wall 301 of the ring groove 30 earlier, improving the sealing performance of the upper surface 13. Note that the narrower the gap (side clearance) between the upper surface 13 of the upper segment 1 and the upper wall 301 of the ring groove 30, the better the oil sealing performance, but the greater the risk of carbon sticks. Therefore, in this technology, it is preferable to set the side clearance within a range in which carbon sticking does not occur. Specifically, it is preferable that the difference between the axial width of the ring groove 30 and the axial width of the oil ring 40 is 30 μm to 60 μm. If it is smaller than this, carbon sticking is likely to occur, and if it is larger than this, the upper and lower segments 1 and 2 may ride up onto the spacer expander 3, which may hinder the behavior of the upper and lower segments 1 and 2. In addition, the moving distance and moving time required for the upper and lower segments 1 and 2 to seal the side clearance may increase. However, the present invention is not limited to this.

Here, the larger the moment M1 caused by Fy, the better the side sealing performance on the inner circumference side of the upper segment 1, but if this is too large, the friction between the upper segment 1 and the upper wall 301 of the ring groove 30 becomes excessive, and the oil scraping performance on the outer circumference side may decrease. There is also a possibility that the aggressiveness against the upper wall 301 of the ring groove 30 may increase. Therefore, it is preferable to keep Fy at a certain level. Taking this into consideration, the lower limit of θ is preferably θ≧2°, and more preferably θ≧5°. Also, θ
The upper limit of θ is preferably θ≦18°, and more preferably θ≦15°. If θ<1°, manufacturing is difficult, and if θ>20°, Fy becomes too large, increasing friction with the upper wall 301 of the ring groove 30, reducing the ability of the upper segment 1 to follow the cylinder 10, and as a result, reducing the oil scraping performance.

Furthermore, it is preferable that the ratio of the moment M1 due to Fy to the moment M2 due to Fx satisfies M2/M1>-0.14. The condition M2/M1>-0.14 can be expressed by the following formula from the above formulas (7) and (8).
cosθ(Y1-Y2)/(sinθ(X2-X1))>-0.14...Formula (10)

The ratio of M1 to M2 can also be replaced with the ratio of the axial force Fy derived from the above formula (2) to the axial force Fz derived based on M2 from the above formula (9). That is, the condition M2/M1>-0.14 can also be expressed by the following formula.
Fz/Fy>-0.14...Formula (11)
By satisfying formula (10) or formula (11), it is possible to suitably increase the side sealing force on the inner circumferential side of the upper segment 1. Note that cos θ(Y1-Y2)/(sin θ(X2-X1)) is more preferably set to be equal to or greater than -0.07, and even more preferably set to be equal to or greater than 0. There is no particular upper limit set for cos θ(Y1-Y2)/(sin θ(X2-X1)), but from the viewpoint of manufacturing, it is preferably set to be equal to or less than 5.0.

Furthermore, it is preferable that the moment M2 due to Fx is a positive value. By making M2>0, the side sealing force can be further increased. The condition of M2>0 can be expressed by the following formula, based on the above formula (8).
Fcos ² θ(Y1-Y2)>0...Formula (12)
In other words, when the tension generated by the spacer expander 3 is 2F and the force acting on the upper segment 1 is F, it is preferable to satisfy Fcos ² θ(Y1−Y2)>0.

As shown in Figures 19 to 21, by positioning the intersection P1 between the inner circumferential surface 12 and the pressing surface 3a above the outer circumferential apex TP1 in a cross section perpendicular to the circumferential direction, Y1-Y2>0 is satisfied. This makes it possible to make M2>0, further increasing the side sealing force.

[Sealing performance evaluation]
The sealing performance of the upper segment 1 of the oil ring 40 according to the embodiment was evaluated by analysis. In the analysis, a state in which the oil ring 40 is assembled in the ring groove 30 of the piston 20 of the internal combustion engine 100 was assumed, and the surface pressure at which the upper segment 1 comes into contact with the upper wall 301 of the ring groove 30 during the upward or downward stroke of the piston 20 and the movement time until the upper segment 1 comes into contact with the upper wall 301 of the ring groove 30 were derived. The analysis was performed for Comparative Example 1, Comparative Example 2, and Examples 1 to 11 shown in Table 1, and for each of the oil rings 40 using the upper segment 1 according to Example 2 and the Comparative Example.

In Table 1, the value in the "ear angle [°]" column is the ear angle θ of the upper ear 311 of the spacer expander 3. θ is measured directly from the shape obtained by using a shape measuring device at 50 times the vertical and horizontal magnifications. In Table 1, the value in the "total inner and outer circumference eccentricity [mm]" column is the axial distance between the inner circumference apex TP2 and the outer circumference apex TP1 in the stationary state. In Table 1, the value in the "Fz/Fy" column is the ratio of the axial force Fy derived from the above formula (2) to the axial force Fz derived from the above formula (9), and is derived by calculating Fz/Fy. In Table 1, the value in the "evaluation of the inner circumference surface pressure" column is the evaluation of the inner circumference surface pressure of the upper surface 13 of the upper segment 1. The average value of the surface pressure (the surface pressure at which the inner circumferential side of the upper surface 13 of the upper segment 1 presses against the upper wall 301 of the ring groove 30) during the upward and downward strokes of the piston 20 was evaluated. The evaluation was performed as follows: if the increase in surface pressure compared to Comparative Example 1 was less than 10%, "C", if it was 10% or more but less than 50%, "B", and if it was 50% or more, "A". In Table 1, the entry in the "Transfer Time Evaluation" column indicates the evaluation of the time until the upper surface 13 of the upper segment 1 contacts the upper wall 301 of the ring groove 30 during the downward stroke of the piston 20. The evaluation was performed as follows: if the reduction rate of the transfer time compared to Comparative Example 2 was less than 10%, "C", if it was 10% or more but less than 50%, "B", and if it was 50% or more, "A".

In Comparative Example 1, the lug angle was 10°, the total eccentricity between the inner and outer periphery was 0 mm, and Fz/Fy was −0.2.
In Comparative Example 2, the lug angle was 10°, the total eccentricity between the inner and outer periphery was 0.01 mm, and Fz/Fy was −0.17.
In Example 1, the lug angle was 10°, the total eccentricity between the inner and outer periphery was 0.02 mm, and Fz/Fy was −0.14.
In Example 2, the lug angle was 10°, the total eccentricity between the inner and outer periphery was 0.04 mm, and Fz/Fy was −0.07.
In Example 3, the lug angle was set to 10°, the total eccentricity between the inner and outer periphery was set to 0.06 mm, and Fz/Fy was set to 0.
In Example 4, the lug angle was 10°, the total eccentricity between the inner and outer periphery was 0.16 mm, and Fz/Fy was 0.33.
In Example 5, the lug angle was 1°, the total eccentricity between the inner and outer periphery was 0.01 mm, and Fz/Fy was 0.13.
In Example 6, the lug angle was 5°, the total inner and outer periphery eccentricity was 0.01 mm, and Fz/Fy was −0.14.
In Example 7, the lug angle was 15°, the total eccentricity between the inner and outer periphery was 0.04 mm, and Fz/Fy was −0.11.
In Example 8, the lug angle was 20°, the total eccentricity between the inner and outer periphery was 0.04 mm, and Fz/Fy was −0.13.
In Example 9, the ear angle is 2°, the total eccentricity of the inner and outer circumferences is 0.03 mm, and Fz/Fy is 0.2
The score was 8.
In Example 10, the lug angle was 7°, the total inner and outer periphery eccentricity was 0.04 mm, and Fz/Fy was −0.01.
In Example 11, the lug angle was 10°, the total eccentricity between the inner and outer periphery was 0.05 mm, and Fz/Fy was −0.04.

Figures 22 to 24 are graphs showing the analysis results. Figure 22 shows the relationship between the total inner and outer periphery eccentricity of the upper segment 1 and the surface pressure on the inner periphery of the upper surface 13 of the upper segment 1. In Figure 22, a comparison is made when the ear angle is 10°, and the relative values of the surface pressure of Comparative Example 2, Examples 1 to 4, and Example 11 to the surface pressure of Comparative Example 1 are shown. Figure 23 shows the relationship between the ear angle and the surface pressure on the inner periphery of the upper surface 13 of the upper segment 1 when the ear angle is 7° or more and 20° or less. In Figure 23, a comparison is made when the inner and outer periphery eccentricity of the upper segment 1 is 0.04 mm, and the relative values of the surface pressure of Examples 2, 7, 8, and 10 to the surface pressure of the inner periphery of the upper surface 13 of the upper segment 1 of Comparative Example 1 are shown. Figure 24 shows the relationship between the ear angle and the movement time until the upper segment 1 contacts the upper wall 301 of the ring groove 30 when the ear angle is less than 7°. In Figure 24, a comparison is made when the inner and outer periphery eccentricity of the upper segment 1 is 0.01 mm, and the relative values of the movement time of Examples 5, 6, and 9 are shown with respect to the movement time of Comparative Example 2.

As shown in FIG. 22, it was confirmed that the greater the total eccentricity of the inner and outer peripheries, the greater the surface pressure on the inner periphery of the upper surface 13 of the upper segment 1. Also, as shown in FIG. 23, it was confirmed that the greater the ear angle, the greater the surface pressure on the inner periphery of the upper surface 13 of the upper segment 1. Also, as shown in FIG. 24, it was confirmed that when the ear angle is relatively small, less than 7°, the smaller the ear angle, the shorter the travel time until the upper surface 13 of the upper segment 1 contacts the upper wall 301 of the ring groove 30 tends to be.

＜Other＞
Although the preferred embodiments of the present invention have been described above, the above-mentioned various forms can be combined as much as possible. For example, the shape of the inner peripheral surface of the upper segment according to the present invention may be a combination of a curved shape as described in FIG. 4 and the like and a tapered shape as described in FIG. 10. In other words, in the inner peripheral surface of the upper segment, one of the upper region above the inner peripheral apex and the lower region below the inner peripheral apex may be formed as a curved surface and the other may be formed as a tapered surface. Furthermore, in this case, the upper region of the inner peripheral surface may be formed as a tapered surface, and the lower region may be formed as a curved surface.

100: Internal combustion engine 10: Cylinder 20: Piston 30: Ring groove 40: Oil ring 1: Upper segment 11: Outer surface 12: Inner surface 13: Upper surface 14: Lower surface 2: Lower segment 3: Spacer expander 311: Upper ear portion 3a: Pressing surface TP2: Inner circumferential apex

Claims (6)

An oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine,
an upper segment disposed on an upper side of the oil ring on the combustion chamber side in the axial direction, a lower segment disposed on a lower side of the oil ring on the crank chamber side independently of the upper segment, and a spacer expander disposed between the upper segment and the lower segment and generating a tension that causes the oil ring to bulge outward in the radial direction,
the upper segment includes an outer circumferential surface provided on an outer circumferential side, an inner circumferential surface provided on an inner circumferential side, an upper surface facing the upper side in the internal combustion engine, and a lower surface facing the lower side in the internal combustion engine,
the spacer expander includes a pressing surface that is provided inward of the upper segment in a radial direction of the oil ring and presses the inner circumferential surface by the tension,
the inner circumferential surface includes an inner circumferential apex portion that is located radially inward of the upper segment relative to other portions of the inner circumferential surface in a cross section perpendicular to a circumferential direction of the oil ring and that has a minimum diameter on the inner circumferential surface,
the outer peripheral surface includes an outer peripheral apex portion that is located radially outward of the oil ring relative to other portions of the outer peripheral surface in a cross section perpendicular to a circumferential direction of the oil ring and that has a maximum diameter on the outer peripheral surface,
the inner circumferential apex is located closer to the combustion chamber than a center position of the inner circumferential surface of the upper segment in the axial direction in a cross section perpendicular to the circumferential direction of the oil ring,
the pressing surface is inclined so as to approach a central axis of the oil ring toward the combustion chamber in a cross section perpendicular to a circumferential direction of the oil ring,
When the inclination angle of the pressing surface with respect to the axial direction of the oil ring is θ,
1°≦θ≦20°,
the inner circumferential surface includes, in a cross section perpendicular to the circumferential direction of the oil ring, an upper curved surface that curves from the inner circumferential apex toward the upper surface side, and a lower curved surface that curves from the inner circumferential apex toward the lower surface side,
a radius of curvature of a circular arc adjacent to the inner circumferential apex on the upper curved surface is equal to or smaller than a radius of curvature of a circular arc having a largest radius of curvature on the lower curved surface;
Oil ring. An oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine,
an upper segment arranged on an upper side of the oil ring on the combustion chamber side in the axial direction, a lower segment arranged on a lower side of the oil ring on the crank chamber side independently of the upper segment, and a spacer expander arranged between the upper segment and the lower segment and generating a tension that causes the oil ring to bulge outward in the radial direction,
the upper segment includes an outer circumferential surface provided on an outer circumferential side, an inner circumferential surface provided on an inner circumferential side, an upper surface facing the upper side in the internal combustion engine, and a lower surface facing the lower side in the internal combustion engine,
the spacer expander includes a pressing surface that is provided on the inside of the upper segment in a radial direction of the oil ring and presses the inner circumferential surface by the tension,
the inner circumferential surface includes an inner circumferential apex portion that is located radially inward of the upper segment relative to other portions of the inner circumferential surface in a cross section perpendicular to a circumferential direction of the oil ring and that has a minimum diameter on the inner circumferential surface,
the outer peripheral surface includes an outer peripheral apex portion that is located radially outward of the oil ring relative to other portions of the outer peripheral surface in a cross section perpendicular to the circumferential direction of the oil ring and that has a maximum diameter on the outer peripheral surface,
the inner circumferential apex is located closer to the combustion chamber than a center position of the inner circumferential surface of the upper segment in the axial direction in a cross section perpendicular to the circumferential direction of the oil ring,
the pressing surface is inclined so as to approach a central axis of the oil ring toward the combustion chamber in a cross section perpendicular to a circumferential direction of the oil ring,
When the inclination angle of the pressing surface with respect to the axial direction of the oil ring is θ,
1°≦θ≦20°,
In a cross section perpendicular to a circumferential direction of the oil ring, the inner circumferential surface is formed by an upper circular arc and a lower circular arc that is provided below the upper circular arc and connected to the upper circular arc,
When the upper segment is installed in the spacer expander so as to be parallel to a radial direction of the oil ring, an intersection point between the upper arc and the lower arc is located above an intersection point between the pressing surface and a lower virtual circle, which is a virtual circle formed by extending the lower arc.
Oil ring. An oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine,
an upper segment arranged on an upper side of the oil ring on the combustion chamber side in the axial direction, a lower segment arranged on a lower side of the oil ring on the crank chamber side independently of the upper segment, and a spacer expander arranged between the upper segment and the lower segment and generating a tension that causes the oil ring to bulge outward in the radial direction,
the upper segment includes an outer circumferential surface provided on an outer circumferential side, an inner circumferential surface provided on an inner circumferential side, an upper surface facing the upper side in the internal combustion engine, and a lower surface facing the lower side in the internal combustion engine,
the spacer expander includes a pressing surface that is provided on the inside of the upper segment in a radial direction of the oil ring and presses the inner circumferential surface by the tension,
the inner circumferential surface includes an inner circumferential apex portion that is located radially inward of the upper segment relative to other portions of the inner circumferential surface in a cross section perpendicular to a circumferential direction of the oil ring and that has a minimum diameter on the inner circumferential surface,
the outer peripheral surface includes an outer peripheral apex portion that is located radially outward of the oil ring relative to other portions of the outer peripheral surface in a cross section perpendicular to the circumferential direction of the oil ring and that has a maximum diameter on the outer peripheral surface,
the inner circumferential apex is located closer to the combustion chamber than a center position of the inner circumferential surface of the upper segment in the axial direction in a cross section perpendicular to the circumferential direction of the oil ring,
the pressing surface is inclined so as to approach a central axis of the oil ring toward the combustion chamber in a cross section perpendicular to a circumferential direction of the oil ring,
When the inclination angle of the pressing surface with respect to the axial direction of the oil ring is θ,
1°≦θ≦20°,
In a cross section perpendicular to a circumferential direction of the oil ring, the inner circumferential surface is formed by an upper circular arc and a lower circular arc that is provided below the upper circular arc and connected to the upper circular arc,
When the upper segment is installed in the spacer expander so as to be parallel to a radial direction of the oil ring, an intersection point between the upper arc and the lower arc is located below an intersection point between the pressing surface and a lower virtual circle, which is a virtual circle formed by extending the lower arc.
Oil ring. An oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine,
an upper segment arranged on an upper side of the oil ring on the combustion chamber side in the axial direction, a lower segment arranged on a lower side of the oil ring on the crank chamber side independently of the upper segment, and a spacer expander arranged between the upper segment and the lower segment and generating a tension that causes the oil ring to bulge outward in the radial direction,
the upper segment includes an outer circumferential surface provided on an outer circumferential side, an inner circumferential surface provided on an inner circumferential side, an upper surface facing the upper side in the internal combustion engine, and a lower surface facing the lower side in the internal combustion engine,
the spacer expander includes a pressing surface that is provided on the inside of the upper segment in a radial direction of the oil ring and presses the inner circumferential surface by the tension,
the inner circumferential surface includes an inner circumferential apex portion that is located radially inward of the upper segment relative to other portions of the inner circumferential surface in a cross section perpendicular to a circumferential direction of the oil ring and that has a minimum diameter on the inner circumferential surface,
the outer peripheral surface includes an outer peripheral apex portion that is located radially outward of the oil ring relative to other portions of the outer peripheral surface in a cross section perpendicular to the circumferential direction of the oil ring and that has a maximum diameter on the outer peripheral surface,
the inner circumferential apex is located closer to the combustion chamber than a center position of the inner circumferential surface of the upper segment in the axial direction in a cross section perpendicular to the circumferential direction of the oil ring,
the pressing surface is inclined so as to approach a central axis of the oil ring toward the combustion chamber in a cross section perpendicular to a circumferential direction of the oil ring,
When the inclination angle of the pressing surface with respect to the axial direction of the oil ring is θ,
1°≦θ≦20°,
the inner circumferential surface includes, in a cross section perpendicular to a circumferential direction of the oil ring, an upper region above the inner circumferential apex and a lower region below the inner circumferential apex,
The upper region is formed as a tapered surface, and the lower region is formed as a curved surface.
Oil ring. An oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine,
an upper segment arranged on an upper side of the oil ring on the combustion chamber side in the axial direction, a lower segment arranged on a lower side of the oil ring on the crank chamber side independently of the upper segment, and a spacer expander arranged between the upper segment and the lower segment and generating a tension that causes the oil ring to bulge outward in the radial direction,
the upper segment includes an outer circumferential surface provided on an outer circumferential side, an inner circumferential surface provided on an inner circumferential side, an upper surface facing the upper side in the internal combustion engine, and a lower surface facing the lower side in the internal combustion engine,
the spacer expander includes a pressing surface that is provided on the inside of the upper segment in a radial direction of the oil ring and presses the inner circumferential surface by the tension,
the inner circumferential surface includes an inner circumferential apex portion that is located radially inward of the upper segment relative to other portions of the inner circumferential surface in a cross section perpendicular to a circumferential direction of the oil ring and that has a minimum diameter on the inner circumferential surface,
the outer peripheral surface includes an outer peripheral apex portion that is located radially outward of the oil ring relative to other portions of the outer peripheral surface in a cross section perpendicular to the circumferential direction of the oil ring and that has a maximum diameter on the outer peripheral surface,
the inner circumferential apex is located closer to the combustion chamber than a center position of the inner circumferential surface of the upper segment in the axial direction in a cross section perpendicular to the circumferential direction of the oil ring,
the pressing surface is inclined so as to approach a central axis of the oil ring toward the combustion chamber in a cross section perpendicular to a circumferential direction of the oil ring,
When the inclination angle of the pressing surface with respect to the axial direction of the oil ring is θ,
1°≦θ≦20°,
When the upper segment is installed in the spacer expander so as to be parallel to the radial direction of the oil ring, the direction in which the outer circumferential surface descends is defined as positive with respect to the direction in which a moment acts about the intersection point between the inner circumferential surface and the pressing surface in a cross section perpendicular to the circumferential direction of the oil ring,
A moment acting on the upper segment due to an axial component of a force acting from the pressing surface to the inner circumferential surface is defined as M1.
When the moment acting on the upper segment due to the radial component of the force acting from the pressing surface to the inner circumferential surface is M2,
M2/M1>-0.14 is satisfied.
Oil ring. In a cross section perpendicular to the circumferential direction of the oil ring, an intersection between the inner circumferential surface and the pressing surface is located above the outer circumferential apex.
An oil ring according to any one of claims 1 to 5 .

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