Sliding mechanism

Abstract

A sliding mechanism is applied between a piston of an engine capable of being automatically and temporarily stopped and an inner circumferential surface of a cylinder. The lubricating oil for an engine contains an organic molybdenum-based friction modifier. The sliding mechanism is provided with an annular oil ring having a pair of outer peripheral surfaces, each of the pair of outer peripheral surfaces of the oil ring includes a curved surface having a cross-sectional shape curved convexly toward the outside in the radial direction, and the apex, which is the outermost point in the radial direction of the curved surface of the oil ring, is disposed in the center of the outer peripheral surface of the oil ring in the axial direction of the oil ring. The curved surface of the oil ring forms an arcuate surface that passes through a pair of points that are separated by 0.075 mm from the apex of the oil ring toward both sides in the axial direction of the oil ring, respectively, and are positioned on the inside in the radial direction at a fall of 10 [mu] m or more.

Description

Technical Field

[0001] The present disclosure relates to a sliding mechanism, and more particularly to a sliding mechanism between a piston and an inner peripheral surface of a cylinder applied to an engine capable of being temporarily stopped automatically. Background Art

[0002] In sliding parts used in automobiles and other applications, ion-plated films of chromium nitride (CrN, Cr2N), titanium nitride (TiN), chromium carbide (CrC), and other materials, such as DLC films, are formed on the sliding portions using film formation methods such as PVD and CVD, sometimes to improve wear resistance. Furthermore, it is known that the addition of friction modifiers such as MoDTC and MoDTP to the lubricating oil used in sliding parts can reduce friction and improve wear resistance (see, for example, Patent Documents 1 and 2).

[0003] Prior art literature Patent Literature Patent Document 1: Japanese Patent Application No. 2014-532841 Patent Document 2: Japanese Patent Application Laid-Open No. 2018-150434 Summary of the Invention

[0004] Problems to be solved by the invention

[0005] In recent years, the adoption of engines with an idle stop function and engines installed in hybrid vehicles that can automatically stop temporarily has been increasing in automobiles and other applications. Furthermore, to improve fuel economy, there is a trend toward using low-viscosity lubricants, which tends to thin the oil film on the inner surface of the cylinder where the outer surface of the piston ring slides in contact. In such engines, as the number of engine stops and starts increases, the frequency of the piston sliding speed becoming low increases, and therefore the frequency of the lubrication state between the outer surface of the piston ring and the inner surface of the cylinder becoming boundary lubrication (solid lubrication) increases. However, reducing friction and wear when the piston sliding speed is low has received relatively little attention compared to previous situations where the piston sliding speed is not low, and there is room for improvement.

[0006] An object of the present disclosure is to provide a sliding mechanism capable of achieving reduced friction and reduced wear under a condition where the piston sliding speed is low.

[0007] Means for solving problems

[0008] To address the above-mentioned issues, the present inventors conducted extensive research and, as a result, discovered a piston ring outer peripheral surface shape that is effective in reducing friction and wear even at low piston sliding speeds when using an engine lubricating oil containing an organic molybdenum-based friction modifier in a sliding mechanism between a piston and the inner peripheral surface of a cylinder used in an engine capable of automatic temporary stopping.

[0009] A sliding mechanism according to one embodiment of the present disclosure is a sliding mechanism applied to a piston and an inner peripheral surface of a cylinder of an engine capable of being automatically temporarily stopped, wherein the lubricating oil of the engine contains an organic molybdenum-based friction modifier, and the sliding mechanism comprises a plurality of piston rings, each of which is assembled in a plurality of ring grooves of the piston, the plurality of piston rings comprising an annular pressure ring and an annular oil ring, a pair of outer peripheral surfaces of the oil ring each comprising a curved surface having a cross-sectional shape that is convexly curved toward the radial outside, the vertex of the curved surface of the oil ring being the radially outermost point being arranged at the central portion of the outer peripheral surface of the oil ring in the axial direction of the oil ring, the curved surface of the oil ring forming an arc surface, the arc surface being formed by a pair of radially inner surfaces separated by 0.075 mm from the vertex of the oil ring to both sides of the oil ring in the axial direction and having a height difference of 10 μm or more. The outer peripheral surface of the pressure ring is constructed in the following manner (1) or (2): (1) the outer peripheral surface of the pressure ring protrudes toward the radial outside, and the vertex of the radially outermost point of the outer peripheral surface of the pressure ring is arranged at a position closer to the bottom dead center of the piston than the central part of the outer peripheral surface of the pressure ring in the axial direction of the pressure ring; (2) the outer peripheral surface of the pressure ring includes a curved surface with a cross-sectional shape that is convexly curved toward the radial outside, and the vertex of the radially outermost point of the curved surface of the pressure ring is arranged at the central part of the outer peripheral surface of the pressure ring or at a position closer to the bottom dead center of the piston than the central part of the pressure ring in the axial direction of the pressure ring, and the curved surface of the pressure ring forms an arc surface, which is separated by 0.3 mm from the vertex of the pressure ring to both sides of the axial direction of the pressure ring and is located radially inside with a drop of more than 2 μm.

[0010] In a sliding mechanism according to one embodiment of the present disclosure, the outer circumferential surfaces of the oil ring and the pressure ring are configured to have shapes that easily scrape lubricating oil from the inner circumferential surface of the cylinder, causing the oil film to thin when the piston slides at high speed and the proportion of the fluid-lubricated area is high during engine operation. These outer circumferential shapes of the oil ring and the pressure ring facilitate boundary lubrication at the sliding contact between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder, increasing the number of micro-contacts between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder. As a result, frictional heat increases at the sliding contact, facilitating the formation of a film of molybdenum disulfide, etc., from an organic molybdenum-based friction modifier. Therefore, in a sliding mechanism between a piston and the inner circumferential surface of a cylinder in an engine capable of automatic temporary stopping, using an engine lubricating oil containing an organic molybdenum-based friction modifier, friction and wear can be reduced even at low piston sliding speeds.

[0011] In one embodiment, a film containing chromium may be formed on the outer circumferential surfaces of the pressure ring and the oil ring. In this case, the presence of chromium facilitates the formation of a film of molybdenum disulfide or the like by the organic molybdenum-based friction modifier in the sliding contact area.

[0012] In one embodiment, the total surface pressure of the multiple piston rings against the inner circumferential surface of the cylinder may be greater than 0.85 MPa. In this case, the oil film on the inner circumferential surface of the cylinder becomes thinner than when the total surface pressure is less than 0.85 MPa, and thus, the phenomenon of the organic molybdenum-based friction modifier, such as molybdenum disulfide, forming a film in the sliding contact area, is more likely to occur.

[0013] In one embodiment, the combined roughness of the outer circumferential surfaces of the pressure ring and the oil ring, and the inner circumferential surface of the cylinder, may be Ra 0.1 μm or greater. In this case, compared to a case where the combined roughness is less than Ra 0.1 μm, minute contact between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder is likely to increase, thereby increasing the generation of frictional heat and making it more likely that a film of molybdenum disulfide, etc., formed by the organic molybdenum-based friction modifier, will form in the sliding contact area.

[0014] Another embodiment of the present invention relates to a sliding mechanism between a piston and an inner peripheral surface of a cylinder of an engine capable of being automatically temporarily stopped, wherein the lubricating oil of the engine contains an organic molybdenum-based friction modifier, and the sliding mechanism comprises a plurality of piston rings, which are respectively assembled in a plurality of ring grooves of the piston, and the plurality of piston rings have an annular pressure ring whose outer peripheral surface is configured in the following manner (1) or (2); (1) the outer peripheral surface of the pressure ring protrudes toward the radial outside, and the vertex as the radially outermost point of the outer peripheral surface of the pressure ring is arranged in the axial direction of the pressure ring. The outer peripheral surface of the pressure ring includes a curved surface with a cross-sectional shape that is convexly curved toward the radial outside, and the vertex of the radially outermost point of the curved surface of the pressure ring is arranged in the central part of the outer peripheral surface of the pressure ring or in a position closer to the bottom dead center of the piston than the central part of the pressure ring in the axial direction of the pressure ring. The curved surface of the pressure ring forms an arc surface, and the arc surface passes through a pair of points that are separated by 0.3 mm from the vertex of the pressure ring to both sides of the axial direction of the pressure ring and are located radially inside with a drop of more than 2 μm.

[0015] In another embodiment of the sliding mechanism disclosed herein, the outer circumferential surface and apex of the pressure ring are configured to have a shape that easily scrapes lubricating oil from the inner circumferential surface of the cylinder, causing the oil film to thin when the piston slides at a high speed and the proportion of the fluid-lubricated area is high during engine operation. This outer circumferential shape of the pressure ring facilitates boundary lubrication at the sliding contact between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder, increasing the number of micro-contacts between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder. As a result, frictional heat increases at the sliding contact, facilitating the formation of a film of molybdenum disulfide, etc., from an organic molybdenum-based friction modifier. Therefore, in a sliding mechanism between a piston and the inner circumferential surface of a cylinder in an engine capable of automatic temporary stopping, using an engine lubricating oil containing an organic molybdenum-based friction modifier, friction and wear can be reduced even at low piston sliding speeds.

[0016] Another embodiment of the sliding mechanism disclosed herein is a sliding mechanism applied to a piston and an inner peripheral surface of a cylinder of an engine capable of being automatically temporarily stopped, wherein the lubricating oil of the engine contains an organic molybdenum-based friction modifier, and the sliding mechanism includes a plurality of piston rings, each of which is assembled in a plurality of ring grooves of the piston, and the plurality of piston rings includes an annular oil ring having a pair of outer peripheral surfaces, the pair of outer peripheral surfaces of the oil ring each including a curved surface having a cross-sectional shape that is convexly curved toward the radial outside, the vertex of the radially outermost point of the curved surface of the oil ring being arranged at the central portion of the outer peripheral surface of the oil ring in the axial direction of the oil ring, the curved surface of the oil ring forming an arc surface, the arc surface passing through a pair of points located radially inwardly at a height difference of 10 μm or more and separated by 0.075 mm from the vertex of the oil ring to both sides of the axial direction of the oil ring.

[0017] In another embodiment of the sliding mechanism disclosed herein, the curved surface and apex of the oil ring are configured to be shaped to easily scrape lubricating oil from the inner circumferential surface of the cylinder, thinning the oil film, when the piston sliding speed is high and the proportion of the fluid lubrication area is high during engine operation. This outer circumferential shape of the oil ring facilitates boundary lubrication at the sliding contact between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder, increasing the number of micro-contacts between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder. As a result, frictional heat increases at the sliding contact, facilitating the formation of a film of molybdenum disulfide, etc., from an organic molybdenum-based friction modifier. Therefore, in a sliding mechanism between a piston and the inner circumferential surface of a cylinder in an engine capable of automatic temporary stopping, using an engine lubricating oil containing an organic molybdenum-based friction modifier, friction and wear can be reduced even at low piston sliding speeds.

[0018] In one embodiment of the sliding mechanism according to the above-described further aspect, a film containing chromium may be formed on the outer circumferential surface of the oil ring. In this case, the presence of chromium on the outer circumferential surface of the oil ring, which tends to have a higher surface pressure relative to the inner circumferential surface of the cylinder among the plurality of piston rings, facilitates and effectively detects the formation of a film of molybdenum disulfide, etc., by the organic molybdenum-based friction modifier, at the sliding contact portion of the oil ring.

[0019] Effects of the Invention

[0020] According to the sliding mechanism of various aspects of the present disclosure, it is possible to achieve reduced friction and reduced wear under conditions where the piston sliding speed is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a cross-sectional view schematically showing a sliding mechanism according to one embodiment. Figure 2 yes Figure 1 Cross-sectional view of the top ring. Figure 3 yes Figure 1 sectional view of the second ring. Figure 4 (a) Yes Figure 1 (b) is a cross-sectional view of the oil ring of (a). Figure 5 It is an icon Figure 2 A graph showing the relationship between the amount of top ring collapse and the average oil film thickness. Figure 6 It is an icon Figure 3 A graph showing the relationship between the amount of lower end collapse of the second ring and the average oil film thickness. Figure 7 It is an icon Figure 4A graph showing the relationship between the amount of oil ring collapse and the average oil film thickness. Figure 8 This is a Stribeck diagram showing schematic characteristics of the friction coefficient of the sliding mechanism according to one embodiment. Figure 9 It is a cross-sectional view of a top ring according to a modified example. Figure 10 It is a cross-sectional view of the second ring of the modification. Figure 11 is a cross-sectional view of the second ring of another modified example. Figure 12 (a) is an enlarged cross-sectional view of the outer peripheral surface of an oil ring according to a modified example. (b) is an enlarged cross-sectional view of the outer peripheral surface of an oil ring according to another modified example. Figure 13 It is an icon Figure 11 A graph showing the average oil film thickness of the oil ring. Figure 14 It is an icon Figure 1 Graph showing the relationship between the total surface pressure and the average oil film thickness of a ring set of multiple piston rings. Figure 15 It is an icon Figure 1 A graph showing the relationship between the combined roughness of the outer circumferential surfaces of multiple piston rings and the inner circumferential surface of the cylinder and the friction mean effective pressure of the ring group. DETAILED DESCRIPTION

[0022] The following describes embodiments of the present disclosure with reference to the accompanying drawings. In the following description, identical or corresponding elements are denoted by the same reference numerals, and duplicate descriptions are omitted. In the following description, "upper side" corresponds to the piston's top dead center side (the engine's combustion chamber side), and "lower side" corresponds to the piston's bottom dead center side (the engine's crankcase side).

[0023] The sliding mechanism disclosed herein is a sliding mechanism between the piston of an engine and the inner circumference of a cylinder, and is applied, for example, to an engine mounted on an automobile. The engine here is configured to be able to automatically stop temporarily during use of the automobile. Examples of the engine include an engine with an idle stop function and an engine mounted on a hybrid vehicle. The engine rotational speed is proportional to the piston sliding speed. Compared to the piston sliding speed when the engine is running at a specified rotational speed or above (for example, above 1500 rpm), the piston sliding speed during temporary stopping and starting of the engine is slow. The piston sliding speed refers to the speed of the reciprocating motion of the piston relative to the inner circumference of the cylinder.

[0024] Engine lubricating oils contain organic molybdenum-based friction modifiers. Examples of organic molybdenum-based friction modifiers include MoDTC (molybdenum dialkyldithiocarbamate) and MoDTP (molybdenum dialkyldithiophosphate). In this engine lubricating oil, for example, the MoDTC content is 2000 ppm or less. Engine lubricating oils with MoDTC contents exceeding 2000 ppm may also be used. Organic molybdenum-based friction modifiers are not limited to MoDTC and MoDTP; any organic molybdenum-based friction modifier can be used regardless of the type of base oil or the presence or absence of other additives, as long as it contains an organic molybdenum compound.

[0025] Figure 1 It is a cross-sectional view schematically showing a sliding mechanism according to one embodiment. Figure 1 The cross-sectional view is a cross-sectional view of a plurality of piston rings 1 along the axial direction. Figure 1 1 is a schematic cross-section of a portion of a piston 3 disposed in a cylinder in a state where a piston ring 1 is fitted into a ring groove 2. The axial direction of the piston ring 1 is the same direction as the reciprocating direction of the piston 3.

[0026] like Figure 1 As shown, a plurality of ring grooves 2 are formed on the outer peripheral surface 3a of the piston 3. The plurality of ring grooves 2 here are, from the upper side, a top ring groove 2a, a second ring groove 2b, and an oil ring groove 2c. A plurality of piston rings 1 are assembled in each of the plurality of ring grooves 2.

[0027] The multiple piston rings 1 include an annular pressure ring and an annular oil ring. The multiple piston rings 1 here are a top ring (pressure ring) 10 fitted into the top ring groove 2a, a second ring (pressure ring) 20 fitted into the second ring groove 2b, and an oil ring 30 fitted into the oil ring groove 2c. In other words, the sliding mechanism 100 includes multiple piston rings 1 assembled in the multiple ring grooves 2 of the piston 3. Each piston ring 1 slides against the cylinder inner circumferential surface 4, to which lubricating oil adheres, thereby performing functions such as sealing gas between the combustion chamber and the crankcase, scraping off lubricating oil, and forming an oil film. The multiple piston rings 1 assembled in the multiple ring grooves 2 slide against the cylinder inner circumferential surface 4 at a speed equal to the piston's sliding speed, via the lubricating oil film, as the piston reciprocates. The cylinder inner circumferential surface 4 is the inner wall of the cylinder bore.

[0028] [Top ring]

[0029] Figure 2 yes Figure 1 A cross-sectional view of the top ring. Figure 1 and Figure 2As shown, the top ring 10 includes an annular main body 11 and an opening (not shown) formed in a portion of the main body 11. The main body 11 includes a pair of side surfaces 12 and 13, an inner peripheral surface 14, and an outer peripheral surface 15. Side surfaces 12 and 13 are, for example, substantially perpendicular to the inner peripheral surface 14. In the following description, the direction connecting the side surfaces 12 and 13 is referred to as the width direction of the piston ring 1, and the direction connecting the inner peripheral surface 14 and the outer peripheral surface 15 is referred to as the thickness direction of the piston ring 1. The width direction of the piston ring 1 corresponds to the "up-down direction" and the "axial direction."

[0030] The main body 11 has a substantially rectangular cross-section with its long sides in the thickness direction and its short sides in the width direction. The main body 11 is formed using, for example, cast iron or steel containing multiple metal elements to have sufficient strength, heat resistance, and elasticity.

[0031] The surface of the main body 11 may also be modified to form a hard film. For example, the hard film is a physical vapor deposition film (PVD film) formed using the physical vapor deposition method (PVD). This allows for a hard film with sufficient hardness. The hard film is an ion-plated film or a diamond-like carbon (DLC) film containing at least one of titanium (Ti) and chromium (Cr) and at least one of carbon (C), nitrogen (N), and oxygen. Specific examples of the hard film include a titanium nitride film, a chromium nitride film, a titanium carbonitride film, a chromium carbonitride film, a chromium oxynitride film, a chromium film, or a titanium film. Among these, a chromium nitride film is preferably used when wear resistance and scratch resistance are important. It should be noted that the hard film may be a laminate, for example, including a chromium nitride film and a diamond-like carbon film. A hard film (film) containing chromium nitride (CrN) may also be formed on the outer peripheral surface 15 of the top ring 10.

[0032] The opening is a portion of the main body 11 that has been partially cut off, and is formed by a pair of opposing open end portions. Each of the pair of open end portions serves as a free end of the main body 11. The gap in the opening (opening gap) narrows when, for example, the top ring 10 is heated and thermally expands. When the top ring 10 is in use, the opening serves as a relief for the thermal expansion of the main body 11 caused by the temperature difference between the top ring 10 and the cylinder inner circumferential surface 4.

[0033] The outer peripheral surface 15 of the main body 11 of the top ring 10 will be described in detail. In the following description, the shape of the line related to the outer peripheral surface of each piston ring 1 refers to the shape of the line in the cross-section of the piston ring 1 unless otherwise specified.

[0034] For example, the outer circumferential surface 15 of the top ring 10 is provided with a curved surface 16 having a cross-sectional shape that curves radially outward in a convex manner. Curved surface 16 is, for example, an arcuate surface whose ends are the radially outer ends of side surface 12 and side surface 13. The center of curved surface 16 may be located midway between side surfaces 12 and 13. In this case, the outermost portion of the arcuate surface is located at the center of the width connecting side surfaces 12 and 13 on the outer circumferential surface 15 in the radial direction of the top ring 10. Specifically, vertex 17, the radially outermost point of curved surface 16 of the top ring 10, is located at center portion 15M of the outer circumferential surface 15 of the top ring 10 in the width direction (axial direction) of the top ring 10. Vertex 17 is the radially outward-most protruding portion of the outer circumferential surface 15 and forms a sliding contact portion with the cylinder inner circumferential surface 4. Vertex 17 extends along the entire circumference of the main body 11, forming an annular shape. The outer peripheral surface 15 of the top ring 10 has a symmetrical barrel shape that is symmetrical in the width direction with the apex 17 as a boundary.

[0035] It should be noted that the barrel shape refers to a curved surface that curves convexly toward the radially outer side of the piston ring 1 and includes the radially outermost portion of the piston ring 1. Barrel shapes include symmetrical barrel shapes and eccentric barrel shapes. A symmetrical barrel shape is a barrel shape in which the radially outermost portion (apex) of the piston ring 1 is located at the center of the width of the outer circumference of the piston ring 1. An eccentric barrel shape is a barrel shape in which the radially outermost portion (apex) of the piston ring 1 is located below the center of the width of the outer circumference of the piston ring 1 (closer to the crankcase).

[0036] The size of the arc surface of the curved surface 16 can be determined by the amount of edge collapse (the size of the step) in the radial direction of the top ring 10 between a point separated by a certain distance from the apex 17 in the width direction (for example, a point separated by 0.3 mm above and below) and the position of the apex 17. The edge collapse of the curved surface 16 can be, for example, not less than 2 μm and not more than 10 μm. The edge collapse of the curved surface 16 can also be, for example, not less than 3 μm and not more than 7 μm. The edge collapse of the curved surface 16 can also be, for example, not less than 4 μm and not more than 5 μm. That is, the curved surface 16 of the top ring 10 forms an arc surface, which is formed by separating 0.3 mm ( 0.3 mm) from the apex 17 of the top ring 10 to both sides in the width direction (axial direction) of the top ring 10. Figure 2 Lh1 is 0.6mm) and with a drop of 2μm or more ( Figure 2 D1) is a pair of points located radially inward.

[0037] [Second Ring]

[0038] Figure 3 yes Figure 1 A cross-sectional view of the second ring. Figure 1 and Figure 3As shown, the second ring 20 includes an annular main body 21 and an opening (not shown) formed in a portion of the main body 21. The main body 21 includes a pair of side surfaces 22 and 23, an inner peripheral surface 24, and an outer peripheral surface 25. The side surfaces 22 and 23 are, for example, substantially orthogonal to the inner peripheral surface 24. The opening is configured similarly to the opening of the main body 11 of the top ring 10 described above.

[0039] The main body 21 has a substantially rectangular cross-section with its long sides in the thickness direction and its short sides in the width direction. The main body 21 is formed using, for example, cast iron or steel containing multiple metal elements to have sufficient strength, heat resistance, and elasticity.

[0040] Similar to the body 11 of the top ring 10 , the body 21 may be modified to form a hard coating. A hard coating (film) containing chromium nitride (CrN) may also be formed on the outer peripheral surface 25 of the second ring 20 .

[0041] The outer circumferential surface 25 of the main body 21 of the second ring 20 will be described in detail. For example, the outer circumferential surface 25 of the second ring 20 includes a tapered surface 26. The tapered surface 26 is an inclined surface with a cross-sectional shape that protrudes radially outward as it approaches the lower side. For example, the tapered surface 26 extends so as to linearly connect the radially outer end of the side surface 22 with the radially outer end of the side surface 23. A chamfered portion 26a is formed at the upper end of the tapered surface 26, connecting to the radially outer end of the side surface 22. A lower end sag portion 27, or circular arc surface, is formed at the lower end of the tapered surface 26, connecting to the radially outer end of the side surface 23. The boundary between the linear portion of the tapered surface 26 and the lower end sag portion 27 is located at the outermost vertex 28 of the outer circumferential surface 25 in the radial direction of the second ring 20, and is located closer to the bottom dead center than the center between the side surfaces 22 and 23 in the width direction. Specifically, the outer circumferential surface 25 of the second ring 20 protrudes radially outward. Apex 28, the radially outermost point of the outer circumferential surface 25 of the second ring 20, is located closer to the bottom dead center of the piston 3 than the center portion 25M of the outer circumferential surface 25 of the second ring 20 in the width direction (axial direction) of the second ring 20. Apex 28 of the second ring 20 is located at a distance less than 50% of the width dimension of the second ring 20 (the widthwise distance between the side surfaces 22 and 23) relative to the bottom dead center of the second ring 20 (relative to the side surface 23). Apex 28 is the radially outward-most protruding portion of the outer circumferential surface 25 and forms a sliding contact area with the cylinder inner circumferential surface 4. Apex 28 extends entirely around the circumference of the main body 21, forming an annular shape.

[0042] The size of the arcuate surface of the lower end collapsing portion 27 can be determined by the amount of lower end collapsing (the size of the drop) from the position of the vertex 28 in the radial direction of the second ring 20 to the radially inner end of the arcuate surface. The amount of lower end collapsing can be, for example, 2 μm or less. The amount of lower end collapsing can also be, for example, greater than 0 μm and less than 1 μm. That is, the arcuate surface of the lower end collapsing portion 27 of the second ring 20 is formed by a drop (the size of the drop) of less than 2 μm from the position of the vertex 28 in the radial direction of the second ring 20. Figure 3 The axial dimension from the side surface 23 to the apex 28 may be, for example, 2 μm or less.

[0043] [Oil ring]

[0044] Figure 4 (a) Yes Figure 1 Cross-sectional view of the oil ring. Figure 4 (b) Yes Figure 4 (a) is an enlarged cross-sectional view of the outer peripheral surface of the oil ring. Figure 1 、 Figure 4 (a) and Figure 4 As shown in (b), the oil ring 30 is a three-piece oil control ring comprising a pair of side rails 31 and 32 and a spacer expander 30X disposed between the side rails 31 and 32. The outer circumferential surfaces 35 and 36 of the side rails 31 and 32 contact the cylinder inner circumferential surface 4. The inner circumferential surfaces 31a and 32a of the side rails 31 and 32 contact the ears of the spacer expander 30X. A spacer expander of a known structure can be used as the spacer expander 30X.

[0045] The axial width of the side rails 31 and 32 is, for example, approximately 0.35 mm. The widths of the side rails 31 and 32 may be the same or different from each other.

[0046] The material of the side rails 31 and 32 is, for example, stainless steel or carbon steel. The side rails 31 and 32 may also include a hard film (not shown) provided so as to cover at least the outer peripheral surfaces 35 and 36. The hard film of the side rails 31 and 32 may be the same hard film as that of the main body 11 of the top ring 10 described above. The outer peripheral surfaces 35 and 36 of the oil ring 30 may also be formed with a hard film (film) containing chromium nitride (CrN). Figure 4 As shown in (a), the side rails 31 and 32 have the same shape, for example. Hereinafter, the shape of the side rail 31 will be described, and the description of the shape of the side rail 32 will be omitted.

[0047] For example, the outer circumferential surface 35 of the side rail 31 includes a curved surface 37 with a cross-sectional shape that curves radially outward convexly. Curved surface 37 is, for example, an arcuate surface with its ends at the radially outer ends of side surface 33 and side surface 34. The center of curved surface 37 may be located midway between side surfaces 33 and 34. In this case, the outermost portion of the arcuate surface is located at the center of the width connecting side surfaces 33 and 34 on the outer circumferential surface 35 in the radial direction of the oil ring 30. In other words, the outer circumferential surfaces 35 and 36 of the oil ring 30 each include a curved surface 37 with a cross-sectional shape that curves radially outward convexly. The apex 38, the radially outermost point of the curved surface 37 of the oil ring 30, is located at the center portion 35M of the outer circumferential surfaces 35 and 36 of the oil ring 30 in the width direction (axial direction) of the oil ring 30. Apex 38 is the radially outward-most protruding portion of outer peripheral surface 35 and forms a sliding contact point with cylinder inner peripheral surface 4. Apex 38 extends entirely around the circumference of side rail 31, forming an annular shape. Outer peripheral surface 35 of oil ring 30 has a symmetrical barrel shape, symmetrical in the width direction with apex 38 as the boundary.

[0048] The size of the arcuate surface of the curved surface 37 can be determined by the radial collapse (the dimension of the drop) of the side rail 31 between a point separated by a certain distance from the apex 38 in the width direction (for example, a point separated by 0.075 mm above and below) and the position of the apex 38. The collapse of the curved surface 37 can be, for example, from 10 μm to 30 μm. The collapse of the curved surface 37 can be, for example, from 15 μm to 25 μm. The collapse of the curved surface 37 can be, for example, from 20 μm to 25 μm. In other words, the curved surface 37 of the oil ring 30 is formed as an arcuate surface defined by a pair of points located radially inward, separated by 0.075 mm from the apex 38 of the oil ring 30 in the width direction (axial direction) of the oil ring 30, with a drop of at least 10 μm.

[0049] Regarding the sliding mechanism 100 including the top ring 10, the second ring 20, and the oil ring 30 configured as described above, refer to Figures 5 to 7 The simulation results under the condition of low piston sliding speed are described. Figures 5 to 7The simulation results are as follows: in the sliding mechanism 100, the width dimension of the top ring 10 is set to 1.2 mm, the width dimension of the second ring 20 is set to 1.2 mm, and the width dimension of the oil ring 30 is set to 0.35 mm. A hard film (film) containing chromium nitride (CrN) is formed on the outer peripheral surface 15 of the top ring 10, the outer peripheral surface 25 of the second ring 20, and the outer peripheral surfaces 35 and 36 of the oil ring 30. The cylinder bore diameter is set to 86 mm, the reciprocating stroke of the piston 3 is set to 86 mm, the cylinder inner peripheral surface 4 is made of cast iron (material: FC250), low-viscosity engine oil (SAE 0W-8) is used as the engine lubricant, the engine lubricant oil temperature is set to 80°C, and the engine speed is set to 1200 rpm. The simulation was conducted under the operating conditions of electric motor drive.

[0050] Figure 5 It is an icon Figure 2 A graph showing the relationship between the amount of top ring collapse and the average oil film thickness. Figure 5 The horizontal axis is the amount of edge collapse of the top ring 10, and the vertical axis is the average oil film thickness between the outer peripheral surface 15 of the top ring 10 and the inner peripheral surface 4 of the cylinder. Figure 5 As shown in the figure, it can be seen that the average oil film thickness between the outer peripheral surface 15 of the top ring 10 and the cylinder inner peripheral surface 4 tends to decrease as the amount of edge collapse of the top ring 10 increases. This tendency is due to the fact that as the amount of edge collapse of the top ring 10 increases, the radius of curvature of the curved surface 16 of the outer peripheral surface 15 of the top ring 10 decreases, and the outer peripheral surface 15 becomes closer to a sharp point within the curved surface 16. This makes it easier for the lubricating oil on the cylinder inner peripheral surface 4 to be scraped off, and the oil film tends to become thinner.

[0051] Figure 6 It is an icon Figure 3 A graph showing the relationship between the amount of lower end collapse of the second ring and the average oil film thickness. Figure 6 The horizontal axis is the amount of collapse at the lower end of the second ring 20, and the vertical axis is the average oil film thickness between the outer peripheral surface 25 of the second ring 20 and the inner peripheral surface 4 of the cylinder. Figure 6 As shown, as the amount of undercut at the lower end of the second ring 20 decreases within a range of 2 μm or less, the average oil film thickness between the outer circumferential surface 25 of the second ring 20 and the cylinder inner circumferential surface 4 tends to decrease. This tendency corresponds to the fact that as the amount of undercut at the lower end of the second ring 20 decreases within a range of 2 μm or less, the outer circumferential surface 25 of the second ring 20 becomes acute-angled (approaching a sharp edge) near its apex 28. This makes it more likely that the lubricating oil on the cylinder inner circumferential surface 4 is scraped off, and the oil film tends to become thinner.

[0052] Figure 7 It is an icon Figure 4 A graph showing the relationship between the amount of oil ring collapse and the average oil film thickness. Figure 7The horizontal axis is the amount of edge collapse of the side rails 31 and 32 of the oil ring 30, and the vertical axis is the average oil film thickness between the outer peripheral surfaces 35 and 36 of the side rails 31 and 32 and the inner peripheral surface 4 of the cylinder. Figure 7 As shown, it can be seen that the average oil film thickness between the outer circumferential surfaces 35 and 36 of the side rails 31 and 32 and the cylinder inner circumferential surface 4 tends to decrease as the amount of edge collapse of the side rails 31 and 32 increases. This tendency is due to the fact that the radius of curvature of the curved surface 37 of the outer circumferential surfaces 35 and 36 of the side rails 31 and 32 decreases as the amount of edge collapse of the side rails 31 and 32 increases, and the outer circumferential surfaces 35 and 36 approach a sharp shape at the curved surface 37. This makes it easier for the lubricating oil on the cylinder inner circumferential surface 4 to be scraped off, and the oil film tends to become thinner.

[0053] Here, Figure 8 This is a Stribeck diagram showing schematic characteristics of the friction coefficient of the sliding mechanism according to one embodiment. Figure 8 The horizontal axis is the value obtained by dividing the product of the viscosity of the engine lubricating oil and the piston sliding speed by the vertical load (load) relative to the cylinder inner peripheral surface 4, and the vertical axis is the friction coefficient of the lubricating oil through the oil film between the multiple piston rings 1 and the cylinder inner peripheral surface 4.

[0054] exist Figure 8 In the figure, assuming the viscosity of the engine's lubricating oil and the vertical load are constant, the horizontal axis is proportional to the piston sliding speed. In this case, the piston sliding speed increases as the engine speed increases, corresponding to points on the right side of the horizontal axis. The piston sliding speed decreases as the engine speed decreases, corresponding to points on the left side of the horizontal axis. Figure 8 The vertical dashed lines Lu1 and Lu2 delineate the fluid lubrication region, the boundary lubrication region, and the mixed lubrication region between the fluid lubrication region and the boundary lubrication region. The left side of the horizontal axis of dashed line Lu1 corresponds to the boundary lubrication region. The right side of dashed line Lu2 corresponds to the fluid lubrication region. The portion of the horizontal axis between dashed lines Lu1 and Lu2 corresponds to the mixed lubrication region.

[0055] exist Figure 8 In the figure, solid line L1 corresponds to the hypothetical characteristics of the sliding mechanism 100 having the configuration of the above-described embodiment, using a low-viscosity lubricating oil containing an organic molybdenum-based friction modifier. Short-dashed line L2 corresponds to the known characteristics of a comparative example in which the outer peripheral surface shape of the plurality of piston rings 1 in the configuration of the above-described embodiment is restored to a known shape, relative to solid line L1. Long-dashed line L3 corresponds to the known characteristics of a comparative example in which a low-viscosity lubricating oil not containing an organic molybdenum-based friction modifier is used, relative to short-dashed line L2. Single-dotted dashed line L4 corresponds to the known characteristics of a comparative example in which the viscosity of the lubricating oil is high, relative to long-dashed line L3.

[0056] exist Figure 8 In the prior art, comparing the dashed line L4 with the long-dashed line L3, the long-dashed line L3 (low-viscosity lubricant) exhibits a lower friction coefficient in the fluid lubrication region to the right of the dashed line Lu2, and a higher friction coefficient in the mixed lubrication and boundary lubrication regions to the left of the dashed line Lu2. This comparison suggests that low-viscosity lubricants tend to increase the frequency of boundary lubrication (solid lubrication) between the multiple piston rings 1 and the cylinder inner circumferential surface 4. In engines not designed for automatic temporary stopping, there are many opportunities for operation at piston sliding speeds that occur in the fluid lubrication region, arguably embracing a design philosophy that prioritizes friction reduction in the fluid lubrication region. Furthermore, by incorporating an organic molybdenum-based friction modifier into the low-viscosity lubricant, the short-dashed line L2 achieves lower friction in the mixed lubrication and boundary lubrication regions to the left of the dashed line Lu2, compared to the long-dashed line L3. However, in an engine configured to be able to temporarily stop automatically, the chances of the engine being temporarily stopped and started are likely to increase, and the chances of the piston sliding speed being low are likely to increase. Therefore, the lubrication state between the outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder is more likely to become boundary lubrication (solid lubrication) than in an engine not configured to be able to temporarily stop automatically. Figure 8 In the example of , even with the short dashed line L2, there is room for achieving friction reduction in the boundary lubrication region.

[0057] Therefore, the present inventors have adopted a design concept that is different from the conventional design concept of prioritizing low friction in the fluid lubrication region and instead prioritizes low friction and low wear in the boundary lubrication region. They have intentionally adopted a configuration in which the outer peripheral surfaces of the plurality of piston rings 1 are configured so that the oil film of the lubricating oil on the cylinder inner peripheral surface 4 becomes thinner. With this configuration, even when the piston sliding speed is low, it is easy to observe the phenomenon of the organic molybdenum-based friction modifier forming a film of molybdenum disulfide in the sliding contact area. As a result, as Figure 8 As shown by the solid line L1 corresponding to the sliding mechanism 100 , it can be expected that friction can be reduced in the boundary lubrication region compared to the short-dashed line L2 .

[0058] As described above, in sliding mechanism 100, outer peripheral surfaces 15, 25, 35, and 36 are configured to be easily scraped off lubricating oil from cylinder inner peripheral surface 4, causing the oil film to become thinner when the piston slides at high speed and the proportion of the fluid lubrication area is high during engine operation. Consequently, the lubrication state at the sliding contact areas between outer peripheral surfaces 15, 25, 35, and 36 and cylinder inner peripheral surface 4 tends to become boundary lubrication, and minute contacts between outer peripheral surfaces 15, 25, 35, and 36 and cylinder inner peripheral surface 4 tend to increase. As a result, frictional heat increases at the sliding contact areas, facilitating the formation of a film of molybdenum disulfide, etc., from the organic molybdenum-based friction modifier. Therefore, in sliding mechanism 100 used between piston 3 and cylinder inner peripheral surface 4 in an engine capable of automatic temporary stopping, when using engine lubricating oil containing an organic molybdenum-based friction modifier, friction and wear can be reduced even at low piston sliding speeds.

[0059] In the sliding mechanism 100, a film containing chromium is formed on the outer peripheral surfaces 15, 25, 35, and 36. This facilitates the formation of a film of molybdenum disulfide or the like at the sliding contact portion by the organic molybdenum-based friction modifier due to the presence of the chromium component.

[0060] [Modification]

[0061] While the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. The present disclosure can be implemented in various forms represented by the above-described embodiments, with various modifications and improvements made based on the knowledge of those skilled in the art.

[0062] In the above embodiment, the top ring 10 is formed into a symmetrical barrel shape with the apex 17 as the boundary in the width direction, but it may also be an eccentric barrel shape. For example, Figure 9 is a cross-sectional view of a modified top ring. Figure 9The outer circumferential surface 15A of the top ring 10A is provided with a curved surface 16A having a cross-sectional shape that curves radially outward in a convex manner. The curved surface 16A is, for example, a portion of an arcuate surface whose center in the widthwise direction is located closer to the bottom dead center than the center between the side surfaces 12 and 13. Specifically, the outer circumferential surface 15A of the top ring 10A protrudes radially outward, and the vertex 17A, which is the radially outermost point of the outer circumferential surface 15A of the top ring 10A, is located closer to the bottom dead center of the piston than the center portion 15M of the outer circumferential surface 15A in the widthwise (axial) direction of the top ring 10A. The outer circumferential surface 15A of the top ring 10A has a barrel shape, with the radially outermost portion (the vertex 17A) located lower (closer to the crankcase) than the center portion 15M of the outer circumferential surface 15A in the widthwise direction of the top ring 10A (an eccentric barrel shape). The amount of edge collapse of the curved surface 16A of the top ring 10A is, for example, specified in the same manner as the amount of edge collapse of the curved surface 16 of the top ring 10. In this case, a connecting surface 18 having a different radius of curvature than that of the curved surface 16A may be interposed between the upper end of the curved surface 16A and the radially outer end of the side surface 12. A connecting surface 19 having a different radius of curvature than that of the curved surface 16A may be interposed between the lower end of the curved surface 16A and the radially outer end of the side surface 13. Furthermore, the amount of edge collapse of the curved surface 16A of the eccentric barrel-shaped top ring 10A may have different values ​​above (on the piston's top dead center side) and below (on the piston's bottom dead center side) the apex 17A.

[0063] In the above embodiment, the lower end collapsed portion 27 of the tapered surface 26 of the second ring 20 is directly connected to the radially outer end of the side surface 23, but a cutout portion may be provided between the lower end collapsed portion 27 and the radially outer end of the side surface 23. For example, Figure 10 is a cross-sectional view of the second ring of the modified example. Figure 10 As shown, a cutout 29A is formed in the main body 21A of the second ring 20A. The cutout 29A is a cutout portion formed at the corner formed by the side surface 23A and the outer peripheral surface 25A, and extends along the entire circumference of the main body 21A. The cutout 29A is formed, for example, by removing a portion of the main body 21A on the side surface 23A and the outer peripheral surface 25A side using a cutting, grinding, or lapping jig. Alternatively, the cutout 29A can be formed by plastically working the portion of the main body 21A using rolling or drawing. The cutout 29A has a first surface 29x facing the outer peripheral surface 25A and a second surface 29y facing the side surface 23A. The first surface 29x extends approximately parallel to the inner peripheral surface 24, while the second surface 29y extends approximately parallel to the side surfaces 22 and 23A. Therefore, the angle formed between the first surface 29x and the second surface 29y is approximately right. The lower end droop portion 27A formed by the second surface 29y and the outer peripheral surface 25A is configured similarly to the lower end droop portion 27 of the second ring 20. The second ring 20A has a so-called scraper-cut shape.

[0064] As another example, Figure 11 FIG is a cross-sectional view of the second ring of another modified example. Figure 11 As shown, on the outer peripheral surface 25B of the second ring 20B, the second surface 29z of the cutout portion 29B is non-parallel to the side surface 23A of the main body 21B. The second surface 29z is inclined so as to approach the side surface 23A as it moves radially outward from the second ring 20B. The lower end collapsible portion 27B formed by the second surface 29z and the outer peripheral surface 25B is configured similarly to the lower end collapsible portion 27 of the second ring 20. The second ring 20B has a so-called chamfered shape. Furthermore, the second ring 20B may also include an inclined surface 26b, which is more inclined relative to the width direction than the tapered surface 26, in place of the chamfered portion 26a.

[0065] In the above embodiment, the outer peripheral surfaces 35 and 36 of the oil ring 30 are formed into a symmetrical barrel shape with the vertex 38 as the boundary in the width direction, but the present invention is not limited to this example. Figure 12 (a) is an enlarged cross-sectional view of the outer peripheral surface of an oil ring according to a modified example. The outer peripheral surface 35A of the side rail 31A includes an abutment surface 37A substantially parallel to the width direction and an inclined surface 37B extending from the upper end of the abutment surface 37A toward the combustion chamber. The inclined surface 37B decreases in diameter toward the combustion chamber relative to the abutment surface 37A. The outer peripheral surface 35A of the side rail 31A has a so-called tapered rail shape.

[0066] As another example, Figure 12 (b) is an enlarged cross-sectional view of the outer peripheral surface of an oil ring according to another modified example. The side rail 31B includes an outer peripheral surface 35B, side surfaces 33 and 34, an inclined surface 35x between the outer peripheral surface 35B and side surface 33, and an inclined surface 35y between the outer peripheral surface 35B and side surface 34. For example, the outer peripheral surface 35B, inclined surface 35x, and inclined surface 35y have vertically symmetrical shapes in an axial cross-section. The outer peripheral surface 35B of the side rail 31B has a so-called barrel shape. For example, the front end of the outer peripheral surface 35B is formed by an arcuate surface with a predetermined radius of curvature. The inclined surface 35x is an inclined surface that linearly connects the end of the arcuate surface forming the front end of the outer peripheral surface 35B on the side surface 33 side to the radially outermost portion of the side surface 33 in an axial cross-section. The inclined surface 35y is an inclined surface that linearly connects the end of the arcuate surface forming the front end of the outer peripheral surface 35B on the side surface 34 side to the radially outermost portion of the side surface 34 in an axial cross-section. The angle formed by the surface perpendicular to the axial direction and the inclined surfaces 35x and 35y may be, for example, 30 to 50 degrees.

[0067] Figure 13 It is an icon Figure 12 Graph of the average oil film thickness of the oil ring. Figure 13 The horizontal axis and Figure 12 (a) and Figure 12(b) corresponds to the outer peripheral surface shape of the side rails 31A, 31B of the oil rings 30A, 30B, and the vertical axis is the average oil film thickness between the outer peripheral surfaces 35A, 35B of the side rails 31, 32 and the cylinder inner peripheral surface 4. Figure 13 As shown, Figure 12 (a) The tapered rail shape of the side rail 31A and Figure 12 The average oil film thickness of the barrel-shaped side rail 31B of (b) is less than 0.1 μm. Figure 4 Compared with the case where the edge collapse amount of the oil ring 30 in (b) is 10 μm, the average oil film thickness is smaller. Figure 12 (a) and Figure 12 The oil rings 30A and 30B in (b) are likely to scrape off the lubricating oil on the cylinder inner peripheral surface 4, and the oil film is likely to become thinner.

[0068] Alternatively, by increasing the total surface pressure of the multiple piston rings 1 relative to the cylinder inner peripheral surface 4, the lubricating oil on the cylinder inner peripheral surface 4 can be easily scraped off, frictional heat can be increased at the sliding contact portion, and the organic molybdenum-based friction modifier, such as molybdenum disulfide, can be easily formed into a film. The total surface pressure P here is the value obtained by adding the surface pressures of each piston ring 1 calculated using the following equation (1) for all the multiple piston rings 1. For the oil ring 30, the sum of the side rails 31 and 32 is used. In the following equation (1), P is the surface pressure on the outer periphery of the piston ring 1, Ft is the tension of the piston ring 1, D is the diameter of the cylinder bore, and h1 is the width dimension of the piston ring 1.

[0069] [Formula 1]

[0070]

[0071] For example, the total surface pressure P of the piston rings 1 against the cylinder inner peripheral surface 4 may be 0.85 MPa to 5.5 MPa. The total surface pressure P of the piston rings 1 against the cylinder inner peripheral surface 4 of the ring group may be 1.5 MPa to 5.5 MPa. Figure 14 It is an icon Figure 1 Graph showing the relationship between the total surface pressure P and the average oil film thickness of a ring group of multiple piston rings. Figure 14 The horizontal axis is the total surface pressure P of the ring group of multiple piston rings 1, and the vertical axis is the average oil film thickness between the ring group and the cylinder inner peripheral surface 4. The average oil film thickness of the ring group is the average of the four average oil film thicknesses of the top ring 10, the second ring 20, and the oil ring 30 (a pair of side rails 31, 32). Figure 14As shown, the average oil film thickness between the ring group and the cylinder inner circumferential surface 4 tends to decrease as the total surface pressure P of the ring group increases. This tendency corresponds to the fact that as the total surface pressure P of the ring group increases, the outer circumferential surfaces of the multiple piston rings 1 are pressed against the cylinder inner circumferential surface 4, which tends to scrape off the lubricating oil on the cylinder inner circumferential surface 4, and the oil film tends to become thinner. For example, when the total surface pressure P of the top ring 10, second ring 20, and oil ring 30 relative to the cylinder inner circumferential surface 4 is 0.85 MPa or greater, the oil film on the cylinder inner circumferential surface 4 becomes thinner than when the total surface pressure P is less than 0.85 MPa. Consequently, the phenomenon of the organic molybdenum-based friction modifier, such as molybdenum disulfide, forming a film in the sliding contact area, is more likely to be observed.

[0072] Alternatively, by increasing the composite roughness of the outer circumferential surface 15 of the top ring 10, the outer circumferential surface 25 of the second ring 20, and the outer circumferential surfaces 35 and 36 of the oil ring 30, and the cylinder inner circumferential surface 4, the lubricating oil on the cylinder inner circumferential surface 4 can be easily scraped off, the frictional heat in the sliding contact area can be increased, and the organic molybdenum-based friction modifier, such as molybdenum disulfide, can be easily formed into a film. The composite roughness Ra here is the left-hand side σ calculated by the following equation (2), and can be calculated for each piston ring 1. In the following equation (2), σr is the outer circumferential roughness Ra of the piston ring 1, and σl is the inner circumferential roughness Ra of the cylinder inner circumferential surface 4. The composite roughness Ra is calculated as the value of the piston ring 1 in its unused state (immediately after processing or in its factory-fresh state).

[0073] [Formula 2]

[0074]

[0075] For example, the combined roughness Ra may be greater than or equal to Ra0.1 μm and less than or equal to Ra0.5 μm. Figure 15 It is an icon Figure 1 A graph showing the relationship between the combined roughness of the outer circumferential surfaces of multiple piston rings and the inner circumferential surface of the cylinder and the average oil film thickness. Figure 15 The horizontal axis is the composite roughness Ra, and the vertical axis is the FMEP (Friction Mean Effective Pressure) of the ring group of multiple piston rings. Figure 15 As shown, when the composite roughness Ra is between Ra0.1 μm and Ra0.5 μm, the roughness of the outer peripheral surface of the piston ring 1 easily causes minute contact (protrusion contact) with the cylinder inner peripheral surface 4, resulting in a tendency for the FMEP (frictional force) of the outer peripheral surface of the piston ring 1 to increase. This increases the generation of frictional heat and the formation of a film of molybdenum disulfide, etc., in the sliding contact area caused by the organic molybdenum-based friction modifier.

[0076] In the above embodiment, the piston ring outer surface shape that is effective for reducing friction and wear even at low piston sliding speeds is employed for all of the top ring 10, the second ring 20, and the oil ring 30. However, this is not limiting. Such an outer surface shape may be employed for at least one of the piston rings 1, 10, 20, and 30. For example, the outer surface shapes described in the above embodiment and variations may be employed for the top ring and / or the pressure ring of the second ring, while a known outer surface shape may be employed for the oil ring. Alternatively, the outer surface shapes described in the above embodiment and variations may be employed for the oil ring, while a known outer surface shape may be employed for at least one of the top ring and the pressure ring of the second ring.

[0077] In the above embodiment, a film containing chromium is formed on the outer circumferential surface of the piston ring 1 for all of the top ring 10, the second ring 20, and the oil ring 30. However, this is not limiting. A film other than chromium (e.g., a DLC film) may be formed on at least one of the top ring 10, the second ring 20, and the oil ring 30. Furthermore, when the outer circumferential surface of the oil ring 30 is configured as in the above embodiment and its variations, a film containing chromium may also be formed on the outer circumferential surface of the oil ring 30. In this case, the presence of chromium on the outer circumferential surface of the oil ring 30, which tends to have a higher surface pressure relative to the cylinder inner circumferential surface 4 among the multiple piston rings 1, facilitates and effectively detects the formation of a film of molybdenum disulfide, etc., formed by the organic molybdenum-based friction modifier, at the sliding contact portion of the oil ring 30.

[0078] In the above embodiment, for example, in the case of the top ring 10, the curved surface 16 is formed as an arcuate surface with the radially outer end of the side surface 12 and the radially outer end of the side surface 13 as its two ends. However, this is not limiting to this example, and the shape of the outer peripheral surface away from the sliding contact portion may be modified. For example, a connecting surface comprising an arcuate surface different from the curved surface 16, a linear tapered surface, or the like may be interposed between the upper end of the curved surface 16 and the radially outer end of the side surface 12. A connecting surface comprising an arcuate surface different from the curved surface 16, a linear tapered surface, or the like may also be interposed between the lower end of the curved surface 16 and the radially outer end of the side surface 13. The same applies to the other piston rings 1.

[0079] In the above embodiment, the oil ring 30 is a three-piece oil control ring, but it may also be a two-piece oil control ring.

[0080] In the above embodiment, the side surfaces 22 and 23 of the second ring 20 are substantially perpendicular to the inner circumferential surface 24, but this is not limiting. For example, the second ring may have an inner cut surface (so-called inner bevel) formed by cutting off the corner where the side surfaces 22 and 23 intersect the inner circumferential surface 24.

[0081] The side faces 12 and 13 of the top ring 10 are substantially perpendicular to the inner circumferential surface 14, but this is not limiting. For example, the top ring may have an inner cut surface (so-called inner bevel shape) formed by cutting off the corner where the side faces 12 and 13 intersect the inner circumferential surface 14. Alternatively, the top ring may have a balance cut surface (so-called balance cut shape) formed by cutting off the corner where the side faces 12 and 13 intersect the outer circumferential surfaces 15 and 15A.

[0082] Description of labels

[0083] 1…Piston ring; 2…Ring groove; 3…Piston; 4…Cylinder inner circumference; 10, 10A…Top ring (pressure ring); 15, 15A, 25, 25A, 25B, 35, 35A, 35B, 36…Outer circumference; 15M, 25M, 35M…Center; 16, 16A, 37…Curved surface; 17, 17A, 28, 38…Vertex; 20, 20A, 20B…Second ring (pressure ring); 30, 30A, 30B…Oil ring; 100…Sliding mechanism.

Claims

1. A sliding mechanism, applied between a piston and an inner peripheral surface of a cylinder of an engine capable of being automatically temporarily stopped, wherein: The engine lubricating oil contains an organic molybdenum friction modifier, The sliding mechanism includes a plurality of piston rings, each of which is assembled in a plurality of ring grooves of the piston. The plurality of piston rings include an annular pressure ring and an annular oil ring. The pair of outer peripheral surfaces of the oil ring respectively include curved surfaces with a cross-sectional shape that is convexly curved toward the radial outside. The vertex, which is the radially outermost point of the curved surface of the oil ring, is disposed at the center portion of the outer peripheral surface of the oil ring in the axial direction of the oil ring. The curved surface of the oil ring forms an arc surface, which passes through a pair of points located radially inwards at a distance of 0.075 mm from the apex of the oil ring to both sides of the oil ring in the axial direction and with a height difference of 10 μm or more. The outer peripheral surface of the pressure ring is constructed in the following manner (1) or (2): (1) The outer peripheral surface of the pressure ring protrudes radially outward, The vertex, which is the radially outermost point of the outer peripheral surface of the pressure ring, is arranged at a position closer to the bottom dead center of the piston than the center of the outer peripheral surface of the pressure ring in the axial direction of the pressure ring; (2) The outer peripheral surface of the pressure ring includes a curved surface having a cross-sectional shape that is convexly curved toward the radial outside, The vertex as the radially outermost point of the curved surface of the pressure ring is arranged at the center of the outer peripheral surface of the pressure ring or at a position closer to the bottom dead center of the piston than the center of the pressure ring in the axial direction of the pressure ring. The curved surface of the pressure ring forms an arc surface passing through a pair of points separated by 0.3 mm from the apex of the pressure ring to both sides in the axial direction of the pressure ring and located radially inward with a height difference of 2 μm or more.

2. The sliding mechanism according to claim 1, wherein: A film containing chromium is formed on the outer peripheral surface of the pressure ring and the outer peripheral surface of the oil ring.

3. The sliding mechanism according to claim 1 or 2, wherein: A total surface pressure of the plurality of piston rings relative to the inner peripheral surface of the cylinder is greater than or equal to 0.85 MPa and less than or equal to 5.5 MPa.

4. The sliding mechanism according to claim 1 or 2, wherein: The combined roughness of the outer peripheral surface of the pressure ring, the outer peripheral surface of the oil ring, and the inner peripheral surface of the cylinder is greater than or equal to Ra 0.1 μm and less than or equal to Ra 0.5 μm.

5. A sliding mechanism between a piston and an inner peripheral surface of a cylinder of an engine capable of being automatically temporarily stopped, wherein: The engine lubricating oil contains an organic molybdenum friction modifier, The sliding mechanism includes a plurality of piston rings, each of which is assembled in a plurality of ring grooves of the piston. The plurality of piston rings include annular pressure rings whose outer peripheral surfaces are configured in the following manner (1) or (2): (1) The outer peripheral surface of the pressure ring protrudes radially outward, The vertex, which is the radially outermost point of the outer peripheral surface of the pressure ring, is arranged at a position closer to the bottom dead center of the piston than the center of the outer peripheral surface of the pressure ring in the axial direction of the pressure ring; (2) The outer peripheral surface of the pressure ring includes a curved surface having a cross-sectional shape that is convexly curved toward the radial outside, The vertex as the radially outermost point of the curved surface of the pressure ring is arranged at the center of the outer peripheral surface of the pressure ring or at a position closer to the bottom dead center of the piston than the center of the pressure ring in the axial direction of the pressure ring. The curved surface of the pressure ring forms an arc surface passing through a pair of points separated by 0.3 mm from the apex of the pressure ring to both sides in the axial direction of the pressure ring and located radially inward with a height difference of 2 μm or more.

6. A sliding mechanism between a piston and an inner peripheral surface of a cylinder of an engine capable of being automatically temporarily stopped, wherein: The engine lubricating oil contains an organic molybdenum friction modifier, The sliding mechanism includes a plurality of piston rings, each of which is assembled in a plurality of ring grooves of the piston. The plurality of piston rings include an annular oil ring having a pair of outer peripheral surfaces, The pair of outer peripheral surfaces of the oil ring respectively include curved surfaces with a cross-sectional shape that is convexly curved toward the radial outside. The vertex, which is the radially outermost point of the curved surface of the oil ring, is disposed at the center portion of the outer peripheral surface of the oil ring in the axial direction of the oil ring. The curved surface of the oil ring forms an arc surface passing through a pair of points separated by 0.075 mm from the apex of the oil ring to both sides in the axial direction of the oil ring and located radially inward with a height difference of 10 μm or more.

7. The sliding mechanism according to claim 6, wherein: A film containing chromium is formed on the outer peripheral surface of the oil ring.

Images