JP7529635B2 - Upper segment and oil ring - Google Patents

Description

The present invention relates to an upper segment for use in an oil ring, and to an oil ring.

Internal combustion engines installed in general automobiles adopt a configuration in which piston rings including a compression ring (pressure ring) and an oil ring are attached to 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 to prevent piston seizure caused by 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 making the R of the upper arc of the inner circumferential surface larger than the R of the lower arc when the outermost diameter part of the sliding part (outer circumferential surface) of the segment is positioned below the center of the axial width of the segment, thereby preventing the segment from tilting (for example, Patent Document 1).

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

In a three-piece combination oil ring, the posture of the segments changes as the piston reciprocates. In conventional oil rings, a gap may form between the top surface of the upper segment of the pair of segments that is located on the combustion chamber side and the top wall of the ring groove of the piston. Oil may pass through this gap and flow out of the ring groove to the combustion chamber side (oil rising), which may 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 relates to an oil ring that is fitted in a ring groove formed in a piston of an internal combustion engine, and of a pair of segments that are arranged side by side in the axial direction of the oil ring and biased radially outward by a spacer expander, the upper segment is arranged on the combustion chamber side of the internal combustion engine, the inner circumferential surface of the upper segment, in a cross section perpendicular to the circumferential direction of the upper segment, is located radially inside the upper segment relative to other portions of the inner circumferential surface and includes an inner circumferential apex that has a smallest diameter on the inner circumferential surface, and the inner circumferential apex is located closer to the combustion chamber than a central 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 upper segment.

According to the present invention, by making the inner peripheral surface of the upper segment eccentric in a cross section perpendicular to the circumferential direction of the upper segment, the contact pressure between the upper surface of the upper segment and the upper wall of the ring groove can be increased and oil leakage can be suppressed. As a result, according to the present invention, the sealing performance of the oil ring can be improved and oil consumption can be reduced.

In addition, in the present invention, the inner peripheral surface may have a curved shape in a cross section perpendicular to the circumferential direction of the upper segment, such that it moves away from the central axis of the upper segment as it moves from the inner peripheral apex toward the crank chamber of the internal combustion engine.

In the present invention, the inner peripheral surface may have a curved shape that is convex radially inward of the upper segment, including the inner peripheral apex, in a cross section perpendicular to the circumferential direction of the upper segment.

In addition, in the present invention, the inner peripheral surface may have a shape that is inclined in a cross section perpendicular to the circumferential direction of the upper segment so as to move away from the central axis of the upper segment from the inner peripheral apex toward the crank chamber of the internal combustion engine.

In addition, in the present invention, the distance between the inner circumferential apex and the center position of the inner circumferential surface in the axial direction of the upper segment may be more than 2.5% and less than 50% of the axial width of the upper segment.

In addition, in the present invention, the distance between the inner circumferential apex and the center position of the inner circumferential surface in the axial direction of the upper segment may be 5% or more and less than 50% of the axial width of the upper segment.

The present invention may also be an oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine, comprising a pair of segments arranged side by side in the axial direction of the oil ring, and a spacer expander that biases the pair of segments radially outward, and the inner circumferential surface of the upper segment of the pair of segments that is arranged on the combustion chamber side of the internal combustion engine includes an inner circumferential apex that is located radially inside the upper segment relative to other parts 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 upper segment, and the inner circumferential apex is located closer to the combustion chamber than the center position of the inner circumferential surface in the axial direction of the upper segment.

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. 4 is a cross-sectional view for explaining the shape of an inner circumferential surface of an upper segment according to the embodiment. FIG. 2 is a perspective view of a portion of a spacer expander according to an embodiment. FIG. 2 is a partial cross-sectional view of the oil ring according to the embodiment. 11 is a cross-sectional view for explaining a contact state of an upper segment in the embodiment. FIG. FIG. 4 is a cross-sectional view for explaining an example of the shape of the inner circumferential surface of the lower segment. 11 is a cross-sectional view perpendicular to the circumferential direction of an upper segment according to Modification 1. FIG. 11 is a cross-sectional view perpendicular to the circumferential direction of an upper segment according to Modification 2. FIG. 11 is a cross-sectional view perpendicular to the circumferential direction of an upper segment according to a comparative example. FIG.

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 illustrates a cross section along the axial direction of the oil ring 40. FIG. 1 is a schematic diagram illustrating the oil ring 40 in a simplified form. As shown in FIG. 1, in the internal combustion engine 100, a piston gap PC1 is formed by ensuring a predetermined separation distance between an inner wall surface 10a of a cylinder 10 and an outer peripheral surface 20a of a piston 20 mounted in the cylinder 10. In addition, a ring groove 30 is formed in the outer peripheral 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 peripheral 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.

Hereinafter, as shown in FIG. 1, the direction along the central axis of the oil ring 40 (axial direction) is defined as the "up-down direction". In addition, in the axial direction of the oil ring 40, the combustion chamber side (upper side in FIG. 1) in the internal combustion engine 100 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 addition, in the following description of the oil ring 40, unless otherwise specified, the "circumferential direction" refers to the circumferential direction of the oil ring 40, the "radial direction" refers to the radial direction of the oil ring 40, and the "axial direction" refers to the axial direction of the oil ring 40. The circumferential direction, radial direction, and axial direction of the oil ring 40 respectively coincide with the circumferential direction, radial direction, and axial direction of the pair of segments 1 and 2. In addition, 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 "use state". In addition, 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 symmetric in the axial direction, and an "eccentric barrel shape" in which the apex is away from the center of the axial width of the ring and is asymmetric in the axial direction. In addition, in this specification, the term "tapered shape" refers to a surface shape that is inclined away from the central axis of the ring (i.e., expanding in diameter) toward the crank chamber side, or a surface shape that is inclined toward the central axis of the ring (i.e., contracting in diameter) toward the crank chamber side.

[segment]
The pair of segments 1 and 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.
Of these, 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 for the pair of segments 1, 2 include stainless steel and spring steel. Note that a chemical conversion coating, a nitriding coating, etc. may be formed on the inner circumferential surfaces of the upper segment 1 and the lower segment 2.

As shown in FIG. 1, the upper segment 1 includes a first outer peripheral surface 11 provided on the outer peripheral side, a first inner peripheral surface 12 (corresponding to the inner peripheral surface of the upper segment according to the present invention) provided on the inner peripheral side, a first upper surface 13 facing the combustion chamber side, and a first lower surface 14 facing the crank chamber side. The first upper surface 13 and the first 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 first upper surface 13 and the first lower surface 14. However, the first upper surface 13 and the first lower surface 14 do not have to be parallel. As shown in FIG. 1, the upper segment 1 is provided so that the first 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 first outer peripheral surface 11 as a sliding surface.

Figure 2 is a cross-sectional view for explaining the shape of the first inner circumferential surface 12 of the upper segment 1 according to the embodiment. In Figure 2, a cross section perpendicular to the circumferential direction of the upper segment 1 is shown. In Figure 2, the symbol A1 indicates the central axis of the upper segment 1. Furthermore, the symbol CL1 indicates a line passing through the center of the axial width of the upper segment 1. Furthermore, the symbol MP1 indicates the central position in the axial direction of the first inner circumferential surface 12 (the vertical central position of the axial width).

2, the first inner circumferential surface 12 includes an inner circumferential apex P1. In a cross section perpendicular to the circumferential direction of the oil ring 40, the inner circumferential apex P1 is located radially inward from other parts of the first inner circumferential surface 12, and is the part with the smallest diameter in the first inner circumferential surface 12. The first inner circumferential surface 12 has a barrel shape. That is, in a cross section perpendicular to the circumferential direction, the first inner circumferential surface 12 is curved so as to be convex radially inward, including the inner circumferential apex P1. Furthermore, the inner circumferential apex P1 is located above (combustion chamber side) the vertical center position MP1 of the axial width of the first inner circumferential surface 12, so that the first inner circumferential surface 12 is asymmetric in the vertical direction (axial direction). In other words, the first inner circumferential surface 12 has an eccentric barrel shape. Here, the region above the inner circumferential apex P1 is the upper inner circumferential region 121, and the region below the upper inner circumferential region 121 is the lower inner circumferential region 122. The lower inner circumferential region 122 includes the inner circumferential apex P1. The upper inner circumferential region 121 is curved so that the diameter increases from the inner circumferential apex P1 toward the upper side, and the lower inner circumferential region 122 is curved so that the diameter increases from the inner circumferential apex P1 toward the lower side. In other words, the first inner circumferential surface 12 is formed so that the diameter increases in the axial direction as it moves away from the inner circumferential apex P1.

As shown in FIG. 1, the lower segment 2 includes a second outer peripheral surface 21 provided on the outer peripheral side, a second inner peripheral surface 22 provided on the inner peripheral side, a second upper surface 23 facing the combustion chamber side, and a second lower surface 24 facing the crank chamber side. The second upper surface 23 and the second lower surface 24, which are the axial end surfaces of the lower segment 2, are parallel to each other, and the axial width of the lower segment 2 is determined by the second upper surface 23 and the second lower surface 24. The lower segment 2 is provided so that the second 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 second outer peripheral surface 21 as a sliding surface. The second 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.

[Spacer expander]
The spacer expander 3 is provided between the pair of segments 1, 2 as shown in FIG. 1, and biases the pair of segments 1, 2 radially outward (i.e., toward the inner wall surface 10a of the cylinder 10) when in use.

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 oil ring 40 (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 the 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 provided perpendicular to the axial direction and 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 adjacent to each other in the axial direction. As a result, the spacer expander 3 is formed in a wavy shape in the axial direction.

Figure 4 is a partial cross-sectional view of the oil ring 40 according to the embodiment. In Figure 4, a cross section perpendicular to the circumferential direction of the oil ring 40 is shown. As shown in Figure 4, 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 in the radial direction perpendicular to the axial direction. The upper ear portion 311 is formed to stand on the inner side of the upper base portion 310 in the radial direction and protrudes upward relative to the upper base portion 310. In addition, the upper ear portion 311 is arranged to be located radially inward from the upper segment 1 in the used state, and presses the upper segment 1 radially outward by abutting against the first inner circumferential surface 12 of the upper segment 1. The pressing surface S1 of the upper ear 311 that abuts against the first inner peripheral surface 12 is inclined in a cross section perpendicular to the circumferential direction of the oil ring 40 so as to move away from the central axis of the oil ring 40 (i.e., so as 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 from the upper base portion 310. In addition, the upper support portion 312 supports the upper segment 1 by abutting against the first lower surface 14 of the upper segment 1 in the usage state. The support surface S2 of the upper support portion 312 that abuts against the first inner peripheral surface 12 extends in the radial direction. In addition, a through hole 34 through which oil can pass is formed at the base of the upper ear 311. Note that the through hole 34 may not be formed. In addition, the upper piece according to the present invention may not have the upper support portion 312.

As shown in FIG. 4, 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 second inner peripheral surface 22 of the lower segment 2. The pressing surface S3 of the lower ear portion 321 that abuts against the second inner peripheral surface 22 is inclined in a cross section perpendicular to the circumferential direction of the oil ring 40 so as to approach the central axis of the oil ring 40 (i.e., to reduce in diameter) as it moves downward. In addition, the lower support portion 322 supports the lower segment 2 by abutting against the second upper surface 23 of the lower segment 2 in the usage state. The support surface S4 of the lower support portion 322 that abuts against the second inner peripheral surface 22 extends in the radial direction. A through hole 34 is also formed at the base of the lower ear portion 321. Note that the through hole 34 does not have to be formed. Furthermore, the lower piece according to the present invention does not have to have the lower support portion 322.

The shape of the spacer expander according to the present invention is not limited to the above.

[About oil seals]
5 is a cross-sectional view for explaining the contact state of the upper segment 1 in the embodiment. In FIG. 5, the state of the upper segment 1 during the upward stroke of the piston 20 is illustrated. The reference character CP1 in FIG. 5 indicates a contact portion (hereinafter, referred to as the inner peripheral contact portion CP1) of the first inner peripheral surface 12 with the pressing surface S1 of the upper ear portion 311 of the spacer expander 3. As shown in FIG. 5, the first inner peripheral surface 12 contacts the pressing surface S1 of the spacer expander 3 in the lower inner peripheral region 122. At this time, since the inner peripheral apex P1 is located above the vertical center position MP1 of the axial width of the first inner peripheral surface 12, the first inner peripheral surface 12 contacts the pressing surface S1 of the spacer expander 3 at a position higher than when the inner peripheral apex P1 is located at the vertical center of the axial width of the first inner peripheral surface 12. In other words, the position of the inner peripheral contact portion CP1 is shifted upward compared to when the first inner peripheral surface 12 has a symmetric barrel shape. This increases the contact pressure between the first upper surface 13 of the upper segment 1 and the upper wall 301 of the ring groove 30, improving the sealing performance at the contact area, thereby suppressing oil leakage from the ring groove 30 through the gap between the first upper surface 13 of the upper segment 1 and the upper wall 301 of the ring groove 30 into the combustion chamber.

[Action and Effects]
As described above, the oil ring 40 according to the embodiment is configured such that, in a cross section perpendicular to the circumferential direction, the inner circumferential apex P1 of the first inner circumferential surface 12 of the upper segment 1 is located above the vertical center position MP1 of the axial width of the first inner circumferential surface 12. In other words, the first inner circumferential surface 12 has an eccentric shape. This increases the contact surface pressure between the first upper surface 13 of the upper segment 1 and the upper wall 301 of the ring groove 30, thereby suppressing oil leakage. As a result, according to the embodiment, the sealing performance of the oil ring 40 can be improved and oil consumption can be reduced.

Furthermore, the first inner peripheral surface 12 of the upper segment 1 has a barrel shape that is curved to be convex radially inward, including the inner peripheral apex P1, in a cross section perpendicular to the circumferential direction. Therefore, the first inner peripheral surface 12 comes into contact with the pressing surface S1 of the spacer expander 3 via a barrel curved surface. However, the shape of the inner peripheral surface of the upper segment in the present invention is not limited to a barrel shape. In addition, the inner peripheral apex may be flat.

As shown in FIG. 2, the axial width of the upper segment 1 (the width of the upper segment 1 in the axial direction) is h1. When the first upper surface 13 and the first lower surface 14 are parallel, h1 is equal to the axial width of the first inner peripheral surface 12. The axial distance between the inner peripheral apex P1 and the vertical center position MP1 (inner peripheral apex deviation) is d1. The ratio of d1 to h1 is α. Since the first inner peripheral surface 12 has an inner peripheral eccentric shape, 0<d1<h1/2, 0<α<50%. At this time, α is preferably more than 2.5% and less than 50%, and more preferably 5% or more and less than 50%. By setting α in the above range, the contact surface pressure between the first upper surface 13 of the upper segment 1 and the upper wall 301 of the ring groove 30 can be further increased, and the sealing performance of the oil ring 40 can be further improved. However, the relationship between h1 and d1 in the present invention is not limited to the above. In addition, due to manufacturing errors in the vertical position of the inner apex of the segments, α varies by about 2%.

The shape of the second inner circumferential surface 22 of the lower segment 2 is not particularly limited, but the second inner circumferential surface 22 may also be eccentric. FIG. 6 is a cross-sectional view for explaining an example of the shape of the second inner circumferential surface 22 of the lower segment 2. FIG. 6 illustrates a cross section perpendicular to the circumferential direction of the lower segment 2. In FIG. 6, the symbol A2 indicates the central axis of the lower segment 2. The symbol CL2 indicates a line passing through the center of the axial width of the lower segment 2. The symbol MP2 indicates the central position in the axial direction of the second inner circumferential surface 22 (the vertical central position of the axial width). As shown in FIG. 6, the second inner circumferential surface 22 includes an inner circumferential apex P2 that is located radially inward of the lower segment relative to other portions of the second inner circumferential surface 22 and has the smallest diameter on the second inner circumferential surface 22 in a cross section perpendicular to the circumferential direction of the lower segment 2, and the inner circumferential apex P2 may be located below the central position MP2 (on the crank chamber side). This makes it possible to further increase the contact surface pressure between the second lower surface 24 of the lower segment 2 and the lower wall 302 of the ring groove 30, thereby further improving the sealing performance of the oil ring 40.

[Modification]
Modified examples of the upper segment according to the embodiment will be described below. In the following description, differences from the above-described upper segment 1 will be mainly described, and similar configurations will be denoted by the same reference numerals and detailed description will be omitted. The upper segments 1A and 1B described below can be configured with an oil ring 40 in place of the upper segment 1.

[Modification 1]
7 is a cross-sectional view perpendicular to the circumferential direction of the upper segment 1A according to the first modification. As shown in FIG. 7, the upper segment 1A differs from the upper segment 1 in that the upper inner circumferential region 121 and the lower inner circumferential region 122 of the first inner circumferential surface 12 have a tapered shape. Specifically, in a cross-section perpendicular to the circumferential direction, the upper inner circumferential region 121 is inclined away from the central axis A1 of the upper segment 1A as it moves upward from the inner circumferential apex P1, and the lower inner circumferential region 122 is inclined away from the central axis A1 as it moves downward from the inner circumferential apex P1. Note that although the inner circumferential apex P1 of the first modification is rounded, the inner circumferential apex may be flat.

[Modification 2]
Fig. 8 is a cross-sectional view perpendicular to the circumferential direction of the upper segment 1B according to the second modification. As shown in Fig. 8, the upper segment 1B differs from the upper segment 1 in that the first inner circumferential surface 12 has a tapered shape as a whole. In a cross-section perpendicular to the circumferential direction, the first inner circumferential surface 12 is inclined away from the central axis A1 of the upper segment 1B as it moves downward from the inner circumferential apex P1. In the upper segment 1B, d1 is less than h1/2 and α is less than 50%. Although the inner circumferential apex P1 of the second modification is also rounded, the inner circumferential apex may be flat.

[Other Modifications]
The inner peripheral shapes of the upper segments 1, 1A, and 1B described above can be appropriately combined. For example, in the first inner peripheral surface 12 of the upper segment 1 shown in FIG. 2, the upper inner peripheral region 121 may be an inclined surface. Specifically, the upper inner peripheral region 121 may be an inclined surface (tapered surface) inclined so as to move away from the central axis A1 of the upper segment 1 as it moves upward from the inner peripheral apex P1, and the lower inner peripheral region 122 may be a curved surface (R surface) curved so as to move away from the central axis A1 as it moves downward from the inner peripheral apex P1. Also, in the first inner peripheral surface 12 shown in FIG. 2, the lower inner peripheral region 122 may be an inclined surface inclined so as to move away from the central axis A1 as it moves downward from the inner peripheral apex P1.

[Sealing performance evaluation]
The sealing performance of the upper segment according to the embodiment was evaluated by analysis. In the analysis, a state in which an oil ring is assembled in a ring groove of a piston of an internal combustion engine was assumed, and the contact surface pressure between the upper surface of the upper segment and the upper wall of the ring groove was derived. The analysis was performed for each of the oil rings using the upper segments according to the following Examples 1 to 5 and Comparative Example.

[Example]
The oil rings according to Examples 1 to 5 used a segment similar to the upper segment 1 shown in FIG. 2. In Example 1, d1 = 0.01 mm, h1 = 0.4 mm, and α = 2.5%. In Example 2, d1 = 0.02 mm, h1 = 0.4 mm, and α = 5.0%. In Example 3, d1 = 0.08 mm, h1 = 0.4 mm, and α = 20%. In Example 4, d1 = 0.08 mm, h1 = 0.4 mm, and α = 20%.
In Example 5, d1 = 0.08 mm, h1 = 0.3 mm, and α = 26.7%.

[Comparative Example]
Fig. 9 is a cross-sectional view perpendicular to the circumferential direction of an upper segment 1X according to a comparative example. As shown in Fig. 9, the first inner circumferential surface 12 of the upper segment 1X according to the comparative example has a symmetrical barrel shape with an inner circumferential apex P1 located in the vertical center of the axial width of the first inner circumferential surface 12. In the comparative example, d1 = 0 mm, h1 = 0.4 mm, and α = 0%.

[Analysis result]
The analysis results are shown in Table 1. The contact pressure was calculated by calculating the average contact pressure of the contact portion between the top surface of the upper segment and the top wall of the oil groove, and expressed as a ratio to the comparative example.

As shown in Table 1, it can be seen that the contact surface pressure is higher in Examples 1 to 5 than in the comparative example. This confirmed that the sealing performance is improved by positioning the inner circumferential apex above the vertical center of the axial width of the inner circumferential surface. It was also confirmed that the contact surface pressure is significantly increased by setting α to 5% or more.

＜Other＞
Although the preferred embodiments of the present invention have been described above, the above-mentioned various aspects can be combined to the extent possible.

100: Internal combustion engine 10: Cylinder 20: Piston 30: Ring groove 40: Oil ring 1: Upper segment 11: First outer peripheral surface 12: First inner peripheral surface (an example of an inner peripheral surface according to the present invention)
13: First upper surface 14: First lower surface 2: Lower segment 3: Spacer expander 311: Upper ear portion P1: Inner circumferential apex portion S1: Pressing surface

Claims (5)

An oil ring is fitted in a ring groove formed in a piston of an internal combustion engine, and among a pair of segments arranged side by side in the axial direction of the oil ring and biased radially outward by a spacer expander, an upper segment is provided on a combustion chamber side of the internal combustion engine,
The upper and lower surfaces, which are axial end surfaces of the upper segment, are parallel to each other,
an inner circumferential surface of the upper segment 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 upper segment and that has a minimum diameter on the inner circumferential surface,
the inner peripheral apex is located closer to the combustion chamber than a center position of the inner peripheral surface in the axial direction of the upper segment in a cross section perpendicular to a circumferential direction of the upper segment,
the inner circumferential surface has a lower inner circumferential region that is curved in a cross section perpendicular to a circumferential direction of the upper segment so as to move away from a central axis of the upper segment from the inner circumferential apex toward a crank chamber of the internal combustion engine, and is configured to come into contact with the spacer expander at the lower inner circumferential region.
Upper segment. the inner circumferential surface has a curved shape that is convex toward a radially inward direction of the upper segment, the curved shape including the inner circumferential apex, in a cross section perpendicular to a circumferential direction of the upper segment.
The upper segment of claim 1 . a distance in an axial direction of the upper segment between the inner circumferential apex and a center position of the inner circumferential surface is more than 2.5% and less than 50% of an axial width of the upper segment;
An upper segment according to claim 1 or 2. a distance between the inner circumferential apex and a center position of the inner circumferential surface of the upper segment in the axial direction is equal to or greater than 5% and less than 50% of an axial width of the upper segment;
An upper segment according to any one of claims 1 to 3. An oil ring that is fitted to a ring groove formed in a piston of an internal combustion engine,
a pair of segments arranged side by side in an axial direction of the oil ring; and a spacer expander that biases the pair of segments radially outward,
an upper segment of the pair of segments that is provided on a combustion chamber side of the internal combustion engine has an upper surface and a lower surface that are axial end surfaces parallel to each other, and an inner circumferential surface of the upper segment includes an inner circumferential apex portion that is located radially inside the upper segment relative to other portions of the inner circumferential surface and has a minimum diameter on the inner circumferential surface in a cross section perpendicular to a circumferential direction of the upper segment,
the inner peripheral apex is located closer to the combustion chamber than a center position of the inner peripheral surface in the axial direction of the upper segment in a cross section perpendicular to a circumferential direction of the upper segment,
the inner circumferential surface has a lower inner circumferential region that is curved in a cross section perpendicular to a circumferential direction of the upper segment so as to move away from a central axis of the upper segment from the inner circumferential apex toward a crank chamber of the internal combustion engine, and is configured to come into contact with the spacer expander at the lower inner circumferential region.
Oil ring.

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