Slide mechanism

Abstract

To provide a slide mechanism which can reduce and target at reducing friction in a state where a piston slide speed is lowered.SOLUTION: A slide mechanism between a piston and a cylinder inner surface is applied to an engine that can automatically temporarily stopped. A lubricant oil of engine contains an organic molybdenum-type friction modifier. The slide mechanism includes a pair of annular oil rings having an outer surface, and each of the outer surface of one pair of the oil rings includes a curved surface having a curved cross-sectional surface projecting radially outward. An apex which is the radially outermost point of the curved surface of the oil ring is positioned at a center of the outer surface of the oil ring in an axial direction of the oil ring. The curved surface of the oil ring forms an arc surface that passes through a pair of points positioned at 0.075 mm away from the apex of the oil ring on both sides of an axial direction of the oil ring and at height difference of 10 μm or more on a radially inner side.SELECTED DRAWING: Figure 1

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 that is applied to an automatically pauseable engine. [Background technology]

[0002] In sliding parts used in automobiles, etc., ion plating films such as chromium nitride (CrN, Cr2N), titanium nitride (TiN), chromium carbide (CrC), and DLC films are formed on the sliding parts by film formation methods such as PVD and CVD, and wear resistance is improved. It is also known that friction modifiers such as MoDTC and MoDTP are added to the lubricating oil used in the sliding parts to reduce friction and improve wear resistance (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2014-532841 [Patent Document 2] JP 2018-150434 A Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, the use of engines with an idling stop function and engines that can be automatically stopped, such as engines installed in hybrid vehicles, has been increasing in automobiles and the like. In addition, there is a tendency to use low-viscosity lubricating oils to reduce fuel consumption, and the oil film is likely to become thin on the inner peripheral surface of the cylinder where the outer peripheral surface of the piston ring slides. In such engines, as the number of engine stoppages and starts increases, the frequency with which the piston sliding speed becomes low increases, and the frequency with which the lubrication state between the outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder becomes boundary lubrication (solid lubrication) increases. However, the reduction in friction and wear in a situation where the piston sliding speed is low has not received as much attention as compared to the conventional situation where the piston sliding speed is not low, and there is room for improvement.

[0005] An object of the present disclosure is to provide a sliding mechanism that can achieve low friction and low wear in a situation where the piston sliding speed is low. [Means for solving the problem]

[0006] In order to solve the above problems, the present inventors have conducted extensive research and have found that, in a sliding mechanism between a piston and an inner peripheral surface of a cylinder applied to an engine that can be automatically temporarily stopped, when an engine lubricating oil containing an organic molybdenum-based friction modifier is used, the shape of the outer peripheral surface of a piston ring is effective in reducing friction and wear even in a situation where the piston sliding speed is low.

[0007] A sliding mechanism according to one aspect of the present disclosure is a sliding mechanism between a piston and an inner peripheral surface of a cylinder that is applied to an engine that can be automatically temporarily stopped, the engine lubricating oil containing an organic molybdenum-based friction modifier, and the engine is provided with a plurality of piston rings that are respectively assembled into a plurality of ring grooves of the piston, the plurality of piston rings having an annular pressure ring and an annular oil ring, each of a pair of outer peripheral surfaces of the oil ring includes a curved surface having a cross-sectional shape that is convexly curved radially outward, and an apex that is the outermost point in the radial direction of the curved surface of the oil ring is curved in an axial direction of the oil ring. The pressure ring is located at the center of the outer peripheral surface of the oil ring, and the curved surface of the oil ring forms an arc surface passing through a pair of points located 0.075 mm away from the apex of the oil ring on both sides of the axial direction of the oil ring and with a drop of 10 μm or more radially inward, and the outer peripheral surface of the compression ring is configured as follows (1) or (2): (1) The outer peripheral surface of the compression ring protrudes radially outward, and the apex, which is the radially outermost point of the outer peripheral surface of the compression ring, is located on the bottom dead center side of the piston in the axial direction of the compression ring. (2) The outer peripheral surface of the compression ring includes a curved surface having a cross-sectional shape that is convexly curved radially outward, and the apex, which is the radially outermost point of the curved surface of the compression ring, is located in the axial direction of the compression ring, either at the center of the outer peripheral surface of the compression ring or on the side of the piston's bottom dead center relative to the center of the compression ring, and the curved surface of the compression ring forms an arc surface that passes through a pair of points that are 0.3 mm away from the apex of the compression ring on either side of the axial direction of the compression ring and with a drop of 2 μm or more radially inward.

[0008] In the sliding mechanism according to one embodiment of the present disclosure, the outer circumferential surface of the oil ring and the outer circumferential surface of the pressure ring are configured to have a shape that easily scrapes off the lubricating oil on the inner circumferential surface of the cylinder and easily makes the oil film thin when the piston sliding speed is fast and the ratio of the fluid lubrication region is high during engine operation. Due to such an outer circumferential surface shape of the oil ring and the outer circumferential surface shape of the pressure ring, the lubrication state at the sliding contact portion between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder is easily boundary lubrication, and the micro-contact between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder is easily increased. As a result, frictional heat increases at the sliding contact portion, and film-like molybdenum disulfide and the like are easily formed from the organic molybdenum friction modifier. Therefore, in a sliding mechanism between the piston and the inner circumferential surface of the cylinder that is applied to an engine that can be automatically temporarily stopped, when an engine lubricating oil containing an organic molybdenum friction modifier is used, it is possible to achieve low friction and low wear even in a situation where the piston sliding speed is low.

[0009] In one embodiment, a coating containing chromium may be formed on the outer circumferential surface of the compression ring and the outer circumferential surface of the oil ring, In this case, the presence of chromium makes it easier for the phenomenon of film-like molybdenum disulfide or the like to be formed from the organic molybdenum friction modifier at the sliding contact portion to occur.

[0010] In one embodiment, the total surface pressure of the piston rings against the cylinder inner peripheral surface may be 0.85 MPa or more. In this case, the oil film on the cylinder inner peripheral surface becomes thinner than when the total surface pressure is less than 0.85 MPa, so that the phenomenon in which a film-like molybdenum disulfide or the like is formed from the organic molybdenum friction modifier at the sliding contact portion is more likely to occur.

[0011] In one embodiment, the composite roughness of the outer circumferential surface of the compression ring and the outer circumferential surface of the oil ring and the inner circumferential surface of the cylinder may be Ra 0.1 μm or more. In this case, compared with the case where the composite roughness is less than Ra 0.1 μm, micro-contact between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder is more likely to increase, so that frictional heat is more likely to be generated and the phenomenon in which a film-like molybdenum disulfide or the like is formed from the organic molybdenum-based friction modifier at the sliding contact portion is more likely to occur.

[0012] A sliding mechanism according to another aspect of the present disclosure is a sliding mechanism between a piston and an inner surface of a cylinder that is applied to an engine that can be automatically temporarily stopped, wherein the engine's lubricating oil contains an organic molybdenum-based friction modifier, and the engine is provided with a plurality of piston rings that are respectively assembled into a plurality of ring grooves of the piston, and the plurality of piston rings have annular compression rings whose outer peripheral surfaces are configured as follows (1) or (2): (1) The outer peripheral surface of the compression ring protrudes radially outward, and the apex, which is the radially outermost point of the outer peripheral surface of the compression ring, is positioned toward the bottom dead center of the piston in the axial direction of the compression ring rather than the center of the outer peripheral surface of the compression ring. (2) The outer peripheral surface of the compression ring includes a curved surface having a cross-sectional shape that is convexly curved radially outward, and the apex, which is the radially outermost point of the curved surface of the compression ring, is located in the axial direction of the compression ring, either at the center of the outer peripheral surface of the compression ring or on the side of the piston's bottom dead center relative to the center of the compression ring, and the curved surface of the compression ring forms an arc surface that passes through a pair of points that are 0.3 mm away from the apex of the compression ring on either side of the axial direction of the compression ring and with a drop of 2 μm or more radially inward.

[0013] In the sliding mechanism according to another aspect of the present disclosure, the outer peripheral surface and the apex of the pressure ring are configured to have a shape that easily scrapes off the lubricating oil on the inner peripheral surface of the cylinder and easily makes the oil film thin when the piston sliding speed is high and the ratio of the fluid lubrication region is high during engine operation. Due to such an outer peripheral surface shape of the pressure ring, the lubrication state at the sliding contact portion between the outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder is easily boundary lubrication, and the micro-contact between the outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder is easily increased. As a result, frictional heat increases at the sliding contact portion, and film-like molybdenum disulfide and the like are easily formed from the organic molybdenum friction modifier. Therefore, in the sliding mechanism between the piston and the inner peripheral surface of the cylinder applied to an engine that can be automatically temporarily stopped, when an engine lubricating oil containing an organic molybdenum friction modifier is used, it is possible to achieve low friction and low wear even in a situation where the piston sliding speed is low.

[0014] A sliding mechanism according to yet another aspect of the present disclosure is a sliding mechanism between a piston and an inner circumferential surface of a cylinder that is applied to an engine that can be automatically temporarily stopped, wherein the lubricating oil for the engine contains an organic molybdenum-based friction modifier, and the engine is provided with a plurality of piston rings that are respectively assembled into a plurality of ring grooves of the piston, the plurality of piston rings having an annular oil ring including a pair of outer circumferential surfaces, each of the pair of outer circumferential surfaces of the oil ring includes a curved surface having a cross-sectional shape that is convexly curved radially outward, an apex that is the radially outermost point of the curved surface of the oil ring is located at the center of the outer circumferential surface of the oil ring in the axial direction of the oil ring, and the curved surface of the oil ring forms an arc surface that passes through a pair of points that are located 0.075 mm away from the apex of the oil ring on both sides of the axial direction of the oil ring and with a drop of 10 μm or more radially inward.

[0015] In the sliding mechanism according to yet another aspect of the present disclosure, the curved surface and apex of the oil ring are configured to be shaped so that the lubricating oil on the inner peripheral surface of the cylinder is easily scraped off and the oil film is easily thinned when the piston sliding speed is high and the ratio of the fluid lubrication region is high during engine operation. Due to such an outer peripheral surface shape of the oil ring, the lubrication state at the sliding contact portion between the outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder is easily boundary lubrication, and the micro-contact between the outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder is easily increased. As a result, frictional heat increases at the sliding contact portion, and film-like molybdenum disulfide and the like are easily formed from the organic molybdenum friction modifier. Therefore, in the sliding mechanism between the piston and the inner peripheral surface of the cylinder applied to an engine that can be automatically temporarily stopped, when an engine lubricating oil containing an organic molybdenum friction modifier is used, it is possible to achieve low friction and low wear even in a situation where the piston sliding speed is low.

[0016] In one embodiment of the sliding mechanism according to the above-mentioned further aspect, a coating containing chromium may be formed on the outer peripheral surface of the oil ring. In this case, the presence of chromium on the outer peripheral surface of the oil ring, which among the multiple piston rings tends to have a higher surface pressure against the inner peripheral surface of the cylinder, makes it easier to effectively realize the phenomenon in which a film-like molybdenum disulfide or the like is formed from the organic molybdenum-based friction modifier at the sliding contact portion of the oil ring. Effect of the Invention

[0017] According to the sliding mechanisms according to various aspects of the present disclosure, it is possible to achieve low friction and low wear in situations where the piston sliding speed is low. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a cross-sectional view illustrating a sliding mechanism according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of the top ring of FIG. 1. [Diagram 3] FIG. 2 is a cross-sectional view of the second ring of FIG. [Figure 4]2(a) is a cross-sectional view of the oil ring of Fig. 1. (b) is an enlarged cross-sectional view of the outer circumferential surface of the oil ring of (a). [Diagram 5] 3 is a graph illustrating the relationship between the amount of sagging of the top ring in FIG. 2 and the average oil film thickness. [Figure 6] 4 is a graph illustrating the relationship between the amount of sagging at the bottom end of the second ring and the average oil film thickness in FIG. 3. [Figure 7] 5 is a graph illustrating the relationship between the amount of sagging of the oil ring in FIG. 4 and the average oil film thickness. [Figure 8] FIG. 2 is a Stribeck diagram showing a schematic characteristic of a friction coefficient of a sliding mechanism according to an embodiment. [Figure 9] FIG. 11 is a cross-sectional view of a top ring according to a modified example. [Figure 10] FIG. 11 is a cross-sectional view of a second ring according to a modified example. [Figure 11] FIG. 11 is a cross-sectional view of a second ring according to another modified example. [Figure 12] 1A is an enlarged cross-sectional view of the outer circumferential surface of an oil ring according to a modified example, and FIG. 1B is an enlarged cross-sectional view of the outer circumferential surface of an oil ring according to another modified example. [Figure 13] 12 is a graph illustrating the average oil film thickness of the oil ring of FIG. 11. [Figure 14] 2 is a graph illustrating the relationship between the total surface pressure of the ring set of the multiple piston rings of FIG. 1 and the average oil film thickness. [Figure 15] 2 is a graph illustrating the relationship between the composite roughness of the outer peripheral surfaces of the multiple piston rings and the inner peripheral surface of the cylinder in FIG. 1 and the friction mean effective pressure of the ring set. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the following description, the same or corresponding elements will be designated by the same reference numerals, and duplicated description will be omitted. In the following description, the "upper side" corresponds to the top dead center side of the piston (the combustion chamber side of the engine), and the "lower side" corresponds to the bottom dead center side of the piston (the crank chamber side of the engine).

[0020] The sliding mechanism according to the present disclosure is a sliding mechanism between an engine piston and an inner peripheral surface of a cylinder, and is applied to an engine mounted on, for example, an automobile. The engine here is configured to be automatically paused while the automobile is in use. The engine is, for example, an engine with an idling stop function and an engine mounted on a hybrid vehicle. The engine speed is proportional to the piston sliding speed. Compared with the piston sliding speed during operation of the engine at or above a predetermined rotation speed (for example, 1500 rpm or above), the piston sliding speed during temporary stopping and starting of the engine is low. The piston sliding speed means the speed of reciprocating motion of the piston against the inner peripheral surface of the cylinder.

[0021] The engine lubricating oil contains an organic molybdenum friction modifier. For example, MoDTC (molybdenum dialkyldithiocarbamate) or MoDTP (molybdenum dialkyldithiophosphate) can be used as the organic molybdenum friction modifier. In the engine lubricating oil here, for example, the content of MoDTC is 2000 ppm or less. As the engine lubricating oil, a lubricating oil containing MoDTC more than 2000 ppm can also be used. The organic molybdenum friction modifier is not limited to MoDTC and MoDTP, and 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.

[0022] Fig. 1 is a cross-sectional view showing a sliding mechanism according to one embodiment. The cross-sectional view of Fig. 1 is a cross-sectional view taken along the axial direction of a plurality of piston rings 1. Fig. 1 also shows a schematic cross-section of a part of a piston 3 arranged in a cylinder with the piston rings 1 fitted in ring grooves 2. The axial direction of the piston rings 1 is the same as the reciprocating direction of the piston 3.

[0023] 1, a plurality of ring grooves 2 are formed in the piston outer peripheral surface 3a of the piston 3. The plurality of ring grooves 2 here are, in order from the top, a top ring groove 2a, a second ring groove 2b, and an oil ring groove 2c. A plurality of piston rings 1 are respectively assembled in the plurality of ring grooves 2.

[0024] The piston rings 1 include an annular pressure ring and an annular oil ring. The piston rings 1 here are a top ring (pressure ring) 10 fitted in the top ring groove 2a, a second ring (pressure ring) 20 fitted in the second ring groove 2b, and an oil ring 30 fitted in the oil ring groove 2c. That is, the sliding mechanism 100 includes a plurality of piston rings 1 respectively assembled in the plurality of ring grooves 2 of the piston 3. Each piston ring 1 slides against the cylinder inner peripheral surface 4 to which the lubricating oil is attached, and thereby can perform a gas seal function between the combustion chamber side and the crank chamber side, a function of scraping off the lubricating oil, and a function of forming an oil film of the lubricating oil. The piston rings 1 assembled in the plurality of ring grooves 2 slide against the cylinder inner peripheral surface 4 at a speed equal to the piston sliding speed via the oil film of the lubricating oil as the piston reciprocates. The cylinder inner peripheral surface 4 is the inner wall surface of the cylinder bore.

[0025] [Top ring] 2 is a cross-sectional view of the top ring of FIG. 1. As shown in FIGS. 1 and 2, the top ring 10 has an annular main body portion 11 and a joint portion (not shown) formed in a part of the main body portion 11. The main body portion 11 has a pair of side surfaces 12 and 13, an inner circumferential surface 14 and an outer circumferential surface 15. The side surfaces 12 and 13 are, for example, approximately perpendicular to the inner circumferential surface 14. In the following description, the direction connecting the side surfaces 12 and 13 is defined as the width direction of the piston ring 1, and the direction connecting the inner circumferential surface 14 and the outer circumferential surface 15 is defined 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".

[0026] The main body 11 has a generally rectangular cross section with a long side in the thickness direction and a short side in the width direction. The main body 11 is made of, for example, cast iron or steel containing a plurality of metal elements, and is formed with sufficient strength, heat resistance, and elasticity.

[0027] The surface of the main body 11 may be modified to form a hard film. The hard film is, for example, a physical vapor deposition film (PVD film) formed by using a physical vapor deposition method (PVD method). This allows the hard film to be formed with sufficient hardness. The hard film is an ion plating film containing at least one of titanium (Ti) and chromium (Cr) and at least one of carbon (C), nitrogen (N), and oxygen, or a diamond-like carbon (DLC) film. 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, when emphasis is placed on wear resistance and scuff resistance, it is preferable to use a chromium nitride film. The hard film may be a laminate, and may include, for example, a chromium nitride film and a diamond-like carbon film. A hard film (film) containing chromium nitride (CrN) may be formed on the outer peripheral surface 15 of the top ring 10 here.

[0028] The abutment is a portion where part of the main body 11 is separated, and is formed by a pair of abutment ends facing each other. Each of the pair of abutment ends is a free end of the main body 11. The gap at the abutment (abutment gap) is adapted to narrow when, for example, the top ring 10 is heated and thermally expands. The abutment functions as an escape 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 when the top ring 10 is in use.

[0029] A detailed description will be given of the outer circumferential surface 15 of the main body portion 11 of the top ring 10. In the following description, the shape of a line relating to the outer circumferential surface of each piston ring 1 means the shape of the line in a cross-sectional view of the piston ring 1, unless otherwise specified.

[0030] The outer peripheral surface 15 of the top ring 10 is provided with a curved surface 16 having a cross-sectional shape that is convexly curved toward the radially outward direction, for example. The curved surface 16 is, for example, an arc surface having both ends, the radially outer end of the side surface 12 and the radially outer end of the side surface 13. The center of the arc surface of the curved surface 16 may be located, for example, midway between the side surface 12 and the side surface 13. In this case, the outermost part of the arc surface in the radial direction of the top ring 10 is located at the center of the outer peripheral surface 15 in the width direction connecting the side surface 12 and the side surface 13. In other words, a vertex 17, which is the radially outermost point of the curved surface 16 of the top ring 10, is located at a center portion 15M of the outer peripheral surface 15 of the top ring 10 in the width direction (axial direction) of the top ring 10. The vertex 17 is the part of the outer peripheral surface 15 that protrudes most radially outward, and is a point that becomes a sliding contact portion with the cylinder inner peripheral surface 4. The apex 17 forms a ring shape by extending over the entire circumferential direction of the main body 11. The outer peripheral surface 15 of the top ring 10 has a symmetrical barrel shape that is symmetric in the width direction with respect to the apex 17.

[0031] The barrel shape refers to a curved surface that is convexly curved toward the radially outward side of the piston ring 1 and includes the radially outermost part of the piston ring 1. The barrel shape includes a symmetric barrel shape and an eccentric barrel shape. The symmetric barrel shape refers to a curved surface that is barrel-shaped and the radially outermost part (apex) of the piston ring 1 is located at the center in the width direction of the outer circumferential surface of the piston ring 1. The eccentric barrel shape refers to a curved surface that is barrel-shaped and the radially outermost part (apex) of the piston ring 1 is located lower (closer to the crank chamber) than the center in the width direction of the outer circumferential surface of the piston ring 1.

[0032] The size of the arcuate surface of the curved surface 16 can be determined by the amount of sagging (dimension of the drop) in the radial direction of the top ring 10 between a point (e.g., one point 0.3 mm above and below) that is a certain distance away from the apex 17 in the width direction and the position of the apex 17. The amount of sagging of the curved surface 16 may be, for example, 2 μm or more and 10 μm or less. The amount of sagging of the curved surface 16 may be, for example, 3 μm or more and 7 μm or less. The amount of sagging of the curved surface 16 may be, for example, 4 μm or more and 5 μm or less. In other words, the curved surface 16 of the top ring 10 forms an arcuate surface that passes through a pair of points that are 0.3 mm (Lh1 in FIG. 2 is 0.6 mm) away from the apex 17 of the top ring 10 on both sides in the width direction (axial direction) of the top ring 10 and that are located with a drop of 2 μm or more (D1 in FIG. 2) on the radially inner side.

[0033] [Second Ring] Fig. 3 is a cross-sectional view of the second ring of Fig. 1. As shown in Fig. 1 and Fig. 3, the second ring 20 has an annular main body portion 21 and a joint portion (not shown) formed in a part of the main body portion 21. The main body portion 21 has 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, approximately perpendicular to the inner peripheral surface 24. The joint portion is configured similarly to the joint portion of the main body portion 11 of the top ring 10 described above.

[0034] The main body 21 has a generally rectangular cross section with a long side in the thickness direction and a short side in the width direction. The main body 21 is made of, for example, cast iron or steel containing a plurality of metal elements, and is formed with sufficient strength, heat resistance, and elasticity.

[0035] The surface of the main body 21 may be modified to form a hard coating, similar to the main body 11 of the above-described top ring 10. The outer peripheral surface 25 of the second ring 20 here may be formed with a hard coating (coating) containing chromium nitride (CrN).

[0036] The outer peripheral surface 25 of the main body 21 of the second ring 20 will be described in detail. The outer peripheral surface 25 of the second ring 20 includes a tapered surface 26, as an example. The tapered surface 26 here is an inclined surface with a cross-sectional shape that protrudes radially outward as it approaches the lower side. The tapered surface 26 extends, for example, so as to linearly connect the radially outer end of the side surface 22 and the radially outer end of the side surface 23. A chamfered portion 26a that continues to the radially outer end of the side surface 22 is formed at the upper end of the tapered surface 26. A lower end sagging portion 27 that is an arc surface that continues to the radially outer end of the side surface 23 is formed at the lower end of the tapered surface 26. The boundary between the linear portion of the tapered surface 26 and the lower end sagging portion 27 is the outermost vertex 28 of the outer peripheral surface 25 in the radial direction of the second ring 20, and is located on the bottom dead center side of the middle between the side surface 22 and the side surface 23 in the width direction. That is, the outer peripheral surface 25 of the second ring 20 protrudes radially outward, and the apex 28, which is the radially outermost point of the outer peripheral surface 25 of the second ring 20, is located on the bottom dead center side of the piston 3 from the center 25M of the outer peripheral surface 25 of the second ring 20 in the width direction (axial direction) of the second ring 20. The 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 width direction distance between the side surface 22 and the side surface 23) based on the bottom dead center side (side surface 23) of the second ring 20. The apex 28 is the part of the outer peripheral surface 25 that protrudes most radially outward, and is a point that becomes a sliding contact portion with the cylinder inner peripheral surface 4. The apex 28 forms an annular shape by extending over the entire circumferential direction of the main body 21.

[0037] The size of the arc surface of the lower end sagging portion 27 can be determined by the amount of lower end sagging (dimension of the drop) from the position of the apex 28 in the radial direction of the second ring 20 to the radially inner end end of the arc surface. The amount of lower end sagging may be, for example, 2 μm or less. The amount of lower end sagging may be, for example, greater than 0 μm and 1 μm or less. In other words, the arc surface of the lower end sagging portion 27 of the second ring 20 forms an arc surface that passes through a point located at a drop (D2 in FIG. 3) of 2 μm or less radially inward from the position of the apex 28 in the radial direction of the second ring 20. Also, the axial dimension from the side surface 23 to the apex 28 may be, for example, 2 μm or less.

[0038] [Oil ring] FIG. 4(a) is a cross-sectional view of the oil ring of FIG. 1. FIG. 4(b) is an enlarged cross-sectional view of the outer circumferential surface of the oil ring of FIG. 4(a). As shown in FIG. 1, FIG. 4(a), and FIG. 4(b), the oil ring 30 is a three-piece oil control ring including a pair of side rails 31, 32 and a spacer expander 30X disposed between the pair of side rails 31, 32. The outer circumferential surfaces 35, 36 of the side rails 31, 32 contact the cylinder inner circumferential surface 4. The inner circumferential surfaces 31a, 32a of the side rails 31, 32 contact the ears of the spacer expander 30X. A spacer expander having a known configuration can be used as the spacer expander 30X.

[0039] The axial width of the side rails 31, 32 is, for example, about 0.35 mm. The widths of the side rails 31, 32 may be the same or different from each other.

[0040] The side rails 31, 32 are made of, for example, stainless steel or carbon steel. The side rails 31, 32 may have a hard coating (not shown) provided to cover at least the outer circumferential surfaces 35, 36. The hard coating of the side rails 31, 32 may be the same as that of the main body 11 of the top ring 10 described above. A hard coating (coating) containing chromium nitride (CrN) may be formed on the outer circumferential surfaces 35, 36 of the oil ring 30 here. As shown in FIG. 4(a), the side rails 31, 32 have, for example, the same shape. Below, the shape of the side rail 31 will be described, and a description of the shape of the side rail 32 will be omitted.

[0041] The outer peripheral surface 35 of the side rail 31 is provided with a curved surface 37 having a cross-sectional shape that is curved convexly toward the radially outward direction, for example. The curved surface 37 is, for example, an arc surface having both ends, the radially outer end of the side surface 33 and the radially outer end of the side surface 34. The center of the arc surface of the curved surface 37 may be located, for example, in the middle between the side surface 33 and the side surface 34. In this case, the outermost part of the arc surface in the radial direction of the oil ring 30 is located at the center of the outer peripheral surface 35 in the width direction connecting the side surface 33 and the side surface 34. In other words, each of the pair of outer peripheral surfaces 35, 36 of the oil ring 30 includes the curved surface 37 having a cross-sectional shape that is curved convexly toward the radially outward direction. The vertex 38, which is the radially outermost point of the curved surface 37 of the oil ring 30, is located at the center 35M of the outer peripheral surfaces 35, 36 of the oil ring 30 in the width direction (axial direction) of the oil ring 30. The apex 38 is the part of the outer circumferential surface 35 that protrudes most radially outward, and is the point that comes into sliding contact with the cylinder inner circumferential surface 4. The apex 38 forms an annular shape by extending over the entire circumferential direction of the side rail 31. The outer circumferential surface 35 of the oil ring 30 has a symmetrical barrel shape that is symmetrical in the width direction with respect to the apex 38.

[0042] The size of the arc surface of the curved surface 37 can be determined by the amount of sagging (dimension of the drop) in the radial direction of the side rail 31 between a point (e.g., one point 0.075 mm above and below) that is a certain distance away from the apex 38 in the width direction and the position of the apex 38. The amount of sagging of the curved surface 37 may be, for example, 10 μm or more and 30 μm or less. The amount of sagging of the curved surface 37 may be, for example, 15 μm or more and 25 μm or less. The amount of sagging of the curved surface 37 may be, for example, 20 μm or more and 25 μm or less. In other words, the curved surface 37 of the oil ring 30 forms an arc surface that passes through a pair of points that are 0.075 mm away from the apex 38 of the oil ring 30 on both sides in the width direction (axial direction) of the oil ring 30 and are located with a drop of 10 μm or more on the radial inside.

[0043] Regarding the sliding mechanism 100 including the top ring 10, the second ring 20, and the oil ring 30 configured as above, a simulation result in a situation where the piston sliding speed is low will be described with reference to FIGS. The simulation results of Figures 5 to 7 are the results of a simulation performed under motoring operating conditions in which, in the sliding mechanism 100, the width dimension of the top ring 10 is 1.2 mm, the width dimension of the second ring 20 is 1.2 mm, the width dimension of the oil ring 30 is 0.35 mm, a hard 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, 36 of the oil ring 30, the diameter of the cylinder bore is 86 mm, the stroke of the reciprocating motion of the piston 3 is 86 mm, the cylinder inner peripheral surface 4 is made of cast iron (material: FC250), the engine lubricating oil is low viscosity engine oil (SAE 0W-8), the oil temperature of the engine lubricating oil is 80°C, and the engine speed is 1200 rpm.

[0044] 5 is a graph illustrating the relationship between the amount of sagging of the top ring in FIG. 2 and the average oil film thickness. The horizontal axis of FIG. 5 is the amount of sagging 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 cylinder inner peripheral surface 4. As shown in FIG. 5, it can be seen that the greater the amount of sagging of the top ring 10, the smaller 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 be. This tendency corresponds to the fact that the greater the amount of sagging of the top ring 10, the smaller the radius of curvature of the curved surface 16 of the outer peripheral surface 15 of the top ring 10 becomes, and the outer peripheral surface 15 approaches a pointed shape at the curved surface 16, making it easier to scrape off the lubricating oil on the cylinder inner peripheral surface 4 and making the oil film thinner.

[0045] FIG. 6 is a graph illustrating the relationship between the amount of sagging at the bottom end of the second ring in FIG. 3 and the average oil film thickness. The horizontal axis of FIG. 6 is the amount of sagging at the bottom 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 cylinder inner peripheral surface 4. As shown in FIG. 6, it can be seen that the average oil film thickness between the outer peripheral surface 25 of the second ring 20 and the cylinder inner peripheral surface 4 tends to become smaller as the amount of sagging at the bottom end of the second ring 20 becomes smaller within a range of 2 μm or less. This tendency corresponds to the fact that the vicinity of the apex 28 of the outer peripheral surface 25 of the second ring 20 becomes acute-angled (approaching a sharp edge) as the amount of sagging at the bottom end of the second ring 20 becomes smaller within a range of 2 μm or less, so that the lubricating oil on the cylinder inner peripheral surface 4 is easily scraped off and the oil film becomes thinner.

[0046] 7 is a graph illustrating the relationship between the amount of sagging of the oil ring in FIG. 4 and the average oil film thickness. The horizontal axis of FIG. 7 is the amount of sagging of the side rails 31, 32 of the oil ring 30, and the vertical axis is the average oil film thickness between the outer peripheral surfaces 35, 36 of the side rails 31, 32 and the cylinder inner peripheral surface 4. As shown in FIG. 7, it can be seen that the greater the amount of sagging of the side rails 31, 32, the smaller the average oil film thickness between the outer peripheral surfaces 35, 36 of the side rails 31, 32 and the cylinder inner peripheral surface 4 tends to be. This tendency corresponds to the fact that the greater the amount of sagging of the side rails 31, 32, the smaller the radius of curvature of the curved surface 37 of the outer peripheral surfaces 35, 36 of the side rails 31, 32 becomes, and the outer peripheral surfaces 35, 36 approach a pointed shape at the curved surface 37, making it easier to scrape off the lubricating oil on the cylinder inner peripheral surface 4 and the oil film becomes thinner.

[0047] Here, Fig. 8 is a Stribeck diagram showing the schematic characteristics of the friction coefficient of the sliding mechanism according to one embodiment. The horizontal axis of Fig. 8 is a value obtained by dividing the product of the viscosity of the lubricating oil of the engine and the piston sliding speed by the normal load (load) on the cylinder inner circumferential surface 4, and the vertical axis is the friction coefficient through the oil film of the lubricating oil between the multiple piston rings 1 and the cylinder inner circumferential surface 4.

[0048] In FIG. 8, assuming that the viscosity of the engine lubricant and the vertical load are constant, the horizontal axis is proportional to the piston sliding speed. In this case, the higher the engine speed, the faster the piston sliding speed, which corresponds to the points on the right side of the horizontal axis. The lower the engine speed, the slower the piston sliding speed, which corresponds to the points on the left side of the horizontal axis. The vertical dashed lines Lu1 and Lu2 in FIG. 8 are lines that separate the fluid lubrication region, the boundary lubrication region, and the mixed lubrication region of the fluid lubrication region and the boundary lubrication region. The left side of the dashed line Lu1 on the horizontal axis corresponds to the boundary lubrication region. The right side of the dashed line Lu2 on the horizontal axis corresponds to the fluid lubrication region. The part between the dashed lines Lu1 and Lu2 on the horizontal axis corresponds to the mixed lubrication region.

[0049] In FIG. 8, the solid line L1 corresponds to the expected characteristics of the sliding mechanism 100 having the configuration of the above embodiment, using a low-viscosity lubricating oil containing an organic molybdenum friction modifier. The short-dashed line L2 corresponds to the known characteristics of the comparative example, which is different from the configuration of the above embodiment in that the outer peripheral surface shape of the piston rings 1 is returned to a known shape, relative to the solid line L1. The long-dashed line L3 corresponds to the known characteristics of the comparative example, which is different from the comparative example in that a low-viscosity lubricating oil not containing an organic molybdenum friction modifier is used, relative to the short-dashed line L2. The dash-dotted line L4 corresponds to the known characteristics of the comparative example, which is different from the comparative example in that the viscosity of the lubricating oil is high, relative to the long-dashed line L3.

[0050] In FIG. 8, comparing the dotted line L4 and the long dashed line L3 as the conventional technology, the long dashed line L3 (low viscosity lubricant) has a smaller friction coefficient in the fluid lubrication region to the right of the dashed line Lu2 than the dotted line L4, and a larger friction coefficient in the mixed lubrication region and boundary lubrication region to the left of the dashed line Lu2. In this comparison, it can be said that the low viscosity lubricant is more likely to increase the frequency of the lubrication state between the multiple piston rings 1 and the cylinder inner surface 4 becoming boundary lubrication (solid lubrication). In an engine that is not configured to be automatically paused, there are many opportunities to operate at a piston sliding speed that is in the fluid lubrication region, so it can be said that the design concept prioritizes low friction in the fluid lubrication region. In addition, by adding an organic molybdenum-based friction modifier to the low viscosity lubricant, low friction is achieved in the short dashed line L2 than in the long dashed line L3 in the mixed lubrication region and boundary lubrication region to the left of the dashed line Lu2. However, in an engine that can be automatically temporarily stopped, the engine is likely to be temporarily stopped and started more frequently, and the piston sliding speed is likely to be low, so the lubrication state between the outer circumferential surface of the piston ring and the inner circumferential surface of the cylinder is more likely to be boundary lubrication (solid lubrication) than in an engine that is not automatically temporarily stopped. When applied to the example of Figure 8, even the short-dashed line L2 still has room for reducing friction in the boundary lubrication region.

[0051] Therefore, the inventor of the present invention adopts a configuration of the outer circumferential surface of the piston rings 1 that makes the oil film of the lubricating oil on the cylinder inner circumferential surface 4 thin, unlike the conventional design concept that prioritizes low friction in the fluid lubrication region, as the configuration of the sliding mechanism 100. According to this configuration, even in a situation where the piston sliding speed is low, the phenomenon that a film-like molybdenum disulfide or the like is formed from the organic molybdenum friction modifier in the sliding contact portion is easily generated. As a result, as shown by the solid line L1 corresponding to the sliding mechanism 100 in FIG. 8, it is expected that it is possible to achieve low friction in the boundary lubrication region compared to the short dashed line L2.

[0052] As described above, in the sliding mechanism 100, the outer peripheral surface 15, the outer peripheral surface 25, and the outer peripheral surfaces 35, 36 are configured to have a shape that easily scrapes off the lubricating oil on the cylinder inner peripheral surface 4 and easily makes the oil film thin when the piston sliding speed is fast and the ratio of the fluid lubrication region is high during engine operation. As a result, the lubrication state at the sliding contact portion between the outer peripheral surface 15, the outer peripheral surface 25, and the outer peripheral surfaces 35, 36 and the cylinder inner peripheral surface 4 is easily boundary lubrication, and the micro-contact between the outer peripheral surface 15, the outer peripheral surface 25, and the outer peripheral surfaces 35, 36 and the cylinder inner peripheral surface 4 is easily increased. As a result, frictional heat increases at the sliding contact portion, and film-like molybdenum disulfide and the like are easily formed from the organic molybdenum friction modifier. Therefore, in the sliding mechanism 100 between the piston 3 and the cylinder inner peripheral surface 4 applied to an engine that can be automatically temporarily stopped, when an engine lubricating oil containing an organic molybdenum friction modifier is used, it is possible to achieve low friction and low wear even in a situation where the piston sliding speed is low.

[0053] In the sliding mechanism 100, a coating containing chromium is formed on the outer peripheral surface 15, the outer peripheral surface 25, and the outer peripheral surfaces 35 and 36. This makes it easier for the presence of chromium to cause the phenomenon in which a film-like molybdenum disulfide or the like is formed from the organic molybdenum friction modifier in the sliding contact portion.

[0054] [Variations] Although 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 including the above-described embodiments and various modifications and improvements based on the knowledge of those skilled in the art.

[0055] In the above embodiment, the top ring 10 has a symmetrical barrel shape that is symmetrical in the width direction with respect to the apex 17, but may have an eccentric barrel shape. For example, FIG. 9 is a cross-sectional view of a top ring according to a modified example. As an example, the outer peripheral surface 15A of the top ring 10A in FIG. 9 has a curved surface 16A having a cross-sectional shape that is convexly curved radially outward. The curved surface 16A is, for example, a part of a circular arc surface whose center is located on the bottom dead center side of the middle between the side surface 12 and the side surface 13 in the width direction. In other words, the outer peripheral surface 15A of the top ring 10A protrudes radially outward, and the apex 17A, which is the outermost point in the radial direction of the outer peripheral surface 15A of the top ring 10A, is located on the bottom dead center side of the piston in the width direction (axial direction) of the top ring 10A relative to the center part 15M of the outer peripheral surface 15A of the top ring 10A. The outer peripheral surface 15A of the top ring 10A is barrel-shaped, and is a curved surface (eccentric barrel shape) in which the outermost part (vertex 17A) in the radial direction of the top ring 10A is located lower (closer to the crank chamber) than the center part 15M in the width direction of the outer peripheral surface 15A of the top ring 10A. The amount of sagging of the curved surface 16A of the top ring 10A is specified, for example, in the same manner as the amount of sagging of the curved surface 16 of the top ring 10. In this case, a connecting surface 18 having a different radius of curvature from 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 from 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 sagging of the curved surface 16A of the eccentric barrel-shaped top ring 10A may be different between the upper side (top dead center side of the piston) and the lower side (bottom dead center side of the piston) of the apex 17A.

[0056] In the above embodiment, at the lower end of the tapered surface 26 of the second ring 20, the lower end sagging portion 27 is directly connected to the radially outer end of the side surface 23, but a notch may be provided between the lower end sagging portion 27 and the radially outer end of the side surface 23. For example, FIG. 10 is a cross-sectional view of a second ring according to a modified example. As shown in FIG. 10, a notch 29A is formed in the main body portion 21A of the second ring 20A. The notch 29A is a portion where a corner portion formed by the side surface 23A and the outer peripheral surface 25A is cut out, and extends over the entire circumferential direction of the main body portion 21A. The notch 29A is formed by cutting out a part of the main body portion 21A on the side surface 23A side and the outer peripheral surface 25A side over the entire circumference using, for example, a cutting, grinding, or polishing jig. The notch 29A may also be formed by plastic processing the part of the main body portion 21A by rolling, drawing, or the like. The cutout portion 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 substantially parallel to the inner peripheral surface 24, and the second surface 29y extends substantially parallel to the side surfaces 22 and 23A. Therefore, the angle between the first surface 29x and the second surface 29y is substantially a right angle. The lower end sagging portion 27A formed by the second surface 29y and the outer peripheral surface 25A is configured in the same manner as the lower end sagging portion 27 of the second ring 20. The second ring 20A has a so-called scraper cut shape.

[0057] As another example, FIG. 11 is a cross-sectional view of a second ring according to another modified example. As shown in FIG. 11, in 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 portion 21B. The second surface 29z is inclined so as to approach the side surface 23A as it moves radially outward of the second ring 20B. The lower end sagging portion 27B formed by the second surface 29z and the outer peripheral surface 25B is configured similarly to the lower end sagging portion 27 of the second ring 20. The second ring 20B has a so-called napier cut shape. In addition, in the second ring 20B, instead of the chamfered portion 26a, an inclined surface 26b that is inclined more in the width direction than the tapered surface 26 may be provided.

[0058] In the above embodiment, the outer peripheral surfaces 35, 36 of the oil ring 30 have a symmetrical barrel shape that is symmetrical in the width direction with respect to the apex 38, but the present invention is not limited to this example. For example, FIG. 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 a contact surface 37A that is substantially parallel to the width direction and an inclined surface 37B that extends from the upper end of the contact surface 37A toward the combustion chamber side. The inclined surface 37B has a smaller diameter (reduced diameter) toward the combustion chamber side on the combustion chamber side with respect to the contact surface 37A. The outer peripheral surface 35A of the side rail 31A has a so-called tapered rail shape.

[0059] As another example, FIG. 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 has an outer peripheral surface 35B, side surfaces 33 and 34, an inclined surface 35x between the outer peripheral surface 35B and the side surface 33, and an inclined surface 35y between the outer peripheral surface 35B and the side surface 34. As an example, the outer peripheral surface 35B, the inclined surface 35x, and the inclined surface 35y have a shape that is symmetrical up and down in an axial cross section. The outer peripheral surface 35B of the side rail 31B has a so-called bullet shape. For example, the tip of the outer peripheral surface 35B is composed of an arc surface with a predetermined curvature radius. The inclined surface 35x is an inclined surface that connects in a straight line from the end of the arc surface that forms the tip of the outer peripheral surface 35B on the side surface 33 side to the radial outermost part of the side surface 33 in an axial cross section. In an axial cross section, the inclined surface 35y is an inclined surface that connects in a straight line from the end of the arcuate surface forming the tip portion of the outer circumferential surface 35B on the side surface 34 side to the radial outermost portion of the side surface 34. The angle formed by the inclined surfaces 35x and 35y and a plane perpendicular to the axial direction may be, for example, 30 to 50°.

[0060] FIG. 13 is a graph showing the average oil film thickness of the oil ring of FIG. 12. The horizontal axis of FIG. 13 corresponds to the outer peripheral surface shape of the side rails 31A, 31B of the oil rings 30A, 30B of FIG. 12(a) and FIG. 12(b), 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. As shown in FIG. 13, the average oil film thickness is smaller than 0.1 μm in the tapered rail-shaped side rail 31A of FIG. 12(a) and the bullet-shaped side rail 31B of FIG. 12(b). Therefore, since the average oil film thickness is smaller than that in the case where the sagging amount in the oil ring 30 of FIG. 4(b) is 10 μm, the oil rings 30A, 30B of FIG. 12(a) and FIG. 12(b) easily scrape off the lubricating oil on the cylinder inner peripheral surface 4, and the oil film is easily thinned.

[0061] Incidentally, by increasing the total surface pressure of the piston rings 1 against the cylinder inner peripheral surface 4, it may be possible to easily scrape off the lubricating oil on the cylinder inner peripheral surface 4, increase frictional heat at the sliding contact portion, and easily form a film-like molybdenum disulfide or the like from the organic molybdenum friction modifier. The total surface pressure P here is a value obtained by adding up the surface pressures of each piston ring 1 calculated by the following formula (1) for all the piston rings 1. For the oil ring 30, the sum of both the side rails 31 and 32 is used. In the following formula (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.

number

[0062] For example, the total surface pressure P of the piston rings 1 on the cylinder inner peripheral surface 4 may be 0.85 MPa or more and 5.5 MPa or less. The total surface pressure P of the ring set of the piston rings 1 on the cylinder inner peripheral surface 4 may be 1.5 MPa or more and 5.5 MPa or less. FIG. 14 is a graph illustrating the relationship between the total surface pressure P of the ring set of the piston rings in FIG. 1 and the average oil film thickness. The horizontal axis of FIG. 14 is the total surface pressure P of the ring set of the piston rings 1, and the vertical axis is the average oil film thickness between the ring set and the cylinder inner peripheral surface 4. This average oil film thickness of the ring set is a value obtained by further averaging the four average oil film thicknesses for the top ring 10, the second ring 20, and the oil ring 30 (a pair of side rails 31, 32). As shown in FIG. 14, it can be seen that the average oil film thickness between the ring set and the cylinder inner peripheral surface 4 tends to decrease as the total surface pressure P of the ring set increases. This tendency corresponds to the fact that as the total surface pressure P of the ring set increases, the outer peripheral surfaces of the multiple piston rings 1 are pressed against the cylinder inner peripheral surface 4, which makes it easier to scrape off the lubricating oil on the cylinder inner peripheral surface 4 and makes the oil film thinner. For example, when the total surface pressure P of the top ring 10, the second ring 20, and the oil ring 30 against the cylinder inner peripheral surface 4 is 0.85 MPa or more, the oil film on the cylinder inner peripheral surface 4 becomes thinner than when the total surface pressure P is less than 0.85 MPa, and therefore the phenomenon in which a film of molybdenum disulfide or the like is formed from the organic molybdenum friction modifier at the sliding contact portion becomes more likely to occur.

[0063] In addition, by increasing the composite roughness of 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, 36 of the oil ring 30, and the cylinder inner peripheral surface 4, the lubricating oil on the cylinder inner peripheral surface 4 may be easily scraped off, frictional heat may be increased in the sliding contact portion, and film-like molybdenum disulfide or the like may be easily formed from the organic molybdenum friction modifier. The composite roughness Ra here is σ on the left side calculated by the following formula (2), and can be calculated for each piston ring 1. In the following formula (2), σr is the outer peripheral roughness Ra of the piston ring 1, and σl is the inner peripheral roughness Ra of the cylinder inner peripheral surface 4. The composite roughness Ra is calculated as a value of the piston ring 1 in an unused state (a state immediately after processing or a new shipping state).

number

[0064] For example, the composite roughness Ra may be Ra0.1 μm or more and Ra0.5 μm or less. FIG. 15 is a graph illustrating the relationship between the composite roughness of the outer peripheral surface of the plurality of piston rings and the cylinder inner peripheral surface of FIG. 1 and the average oil film thickness. The horizontal axis of FIG. 15 is the composite roughness Ra, and the vertical axis is the FMEP (Friction Mean Effective Pressure) of the ring set of the plurality of piston rings. As shown in FIG. 15, when the composite roughness Ra is Ra0.1 μm or more and Ra0.5 μm or less, the roughness of the outer peripheral surface of the piston ring 1 tends to cause minute contact (projection contact) with the cylinder inner peripheral surface 4, and the FMEP (i.e., friction force) of the outer peripheral surface of the piston ring 1 tends to be high. This makes it easier for frictional heat to be generated, and the phenomenon in which film-like molybdenum disulfide or the like is formed from the organic molybdenum-based friction modifier at the sliding contact portion is more likely to occur.

[0065] In the above embodiment, the top ring 10, the second ring 20, and the oil ring 30 all have the outer peripheral shape that is effective for reducing friction and wear even in a situation where the piston sliding speed is low, but the present invention is not limited to this example. It is sufficient that such an outer peripheral shape is adopted for at least one of the top ring 10, the second ring 20, and the oil ring 30. For example, the outer peripheral shape of the top ring and / or the second ring pressure ring may be adopted as in the above embodiment and modified example, and a known outer peripheral shape may be adopted for the oil ring. Alternatively, the outer peripheral shape of the oil ring may be adopted as in the above embodiment and modified example, and a known outer peripheral shape may be adopted for at least one of the top ring and the second ring pressure ring.

[0066] In the above embodiment, a film containing chromium is formed on the outer peripheral surface of the piston ring 1 for all of the top ring 10, the second ring 20, and the oil ring 30, but this is not limited to this example. A film other than chromium (e.g., DLC film) may be formed on at least one of the top ring 10, the second ring 20, and the oil ring 30. When the shape of the outer peripheral surface of the oil ring 30 is adopted as in the above embodiment and modified example, a film containing chromium may be formed on the outer peripheral surface of the oil ring 30. In this case, the presence of chromium on the outer peripheral surface of the oil ring 30, which tends to have a higher surface pressure against the cylinder inner peripheral surface 4 among the multiple piston rings 1, makes it easier to effectively realize the phenomenon that a film-like molybdenum disulfide or the like is formed from the organic molybdenum friction modifier at the sliding contact portion of the oil ring 30.

[0067] In the above embodiment, for example, in the case of the top ring 10, the curved surface 16 is an arc surface having both ends, for example, the radially outer end of the side surface 12 and the radially outer end of the side surface 13, but the present invention is not limited to this example, and the shape of the outer circumferential surface away from the sliding contact portion may be modified. For example, a connecting surface including an arc surface or a linear tapered surface different from the curved surface 16 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 including an arc surface or a linear tapered surface different from the curved surface 16 may 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.

[0068] In the above embodiment, the oil ring 30 is a three-piece oil control ring, but it may be a two-piece oil control ring.

[0069] In the above embodiment, the side surfaces 22, 23 of the second ring 20 are substantially perpendicular to the inner peripheral surface 24, but this is not limited to the example. For example, the second ring may have an inner cut surface that cuts out the corner where the side surfaces 22, 23 and the inner peripheral surface 24 intersect (a so-called inner bevel shape).

[0070] The side surfaces 12, 13 of the top ring 10 are substantially perpendicular to the inner peripheral surface 14, but are not limited to this example. For example, the top ring may have an inner cut surface (a so-called inner bevel shape) that cuts out a corner where the side surfaces 12, 13 and the inner peripheral surface 14 intersect. The top ring may also have a balance cut surface (a so-called balance cut shape) that cuts out a corner where the side surfaces 12, 13 and the outer peripheral surfaces 15, 15A intersect. [Explanation of symbols]

[0071] 1...piston ring, 2...ring groove, 3...piston, 4...cylinder inner surface, 10,10A...top ring (compression ring), 15,15A,25,25A,25B,35,35A,35B,36...outer surface, 15M,25M,35M...center, 16,16A,37...curved surface, 17,17A,28,38...apex, 20,20A,20B...second ring (compression ring), 30,30A,30B...oil ring, 100...sliding mechanism.

Claims

1. A sliding mechanism between a piston and an inner peripheral surface of a cylinder, which is applied to an engine that can be automatically temporarily stopped, The engine lubricating oil contains an organic molybdenum friction modifier, a plurality of piston rings assembled in the plurality of ring grooves of the piston, respectively; The plurality of piston rings include an annular pressure ring and an annular oil ring, each of a pair of outer peripheral surfaces of the oil ring includes a curved surface having a cross-sectional shape that is convexly curved radially outward; the curved surface of the oil ring is an arcuate surface having two ends, one of which is a radially outer end of one side surface of the oil ring and the other of which is a radially outer end of the other side surface of the oil ring, an apex, which is the radially outermost point of the curved surface of the oil ring, is located at a 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 that are spaced apart from the apex of the oil ring by 0.075 mm on both sides of the axial direction of the oil ring and that are spaced apart by a drop of 15 μm or more radially inward, an outer circumferential surface of the compression ring includes a curved surface having a cross-sectional shape that is convexly curved radially outward; an apex, which is the radially outermost point of the curved surface of the compression ring, is located at a center portion of the outer peripheral surface of the compression ring or at a position closer to a bottom dead center of the piston than the center portion of the compression ring in the axial direction of the compression ring, the curved surface of the compression ring forms an arc surface passing through a pair of points that are 0.3 mm away from the apex of the compression ring on both sides in the axial direction of the compression ring and that are positioned with a drop of 2 μm or more radially inward, a total surface pressure of the plurality of piston rings against the cylinder inner peripheral surface is 0.85 MPa or more and 5.5 MPa or less, a composite roughness of the outer peripheral surface of the compression ring, the outer peripheral surface of the oil ring, and the inner peripheral surface of the cylinder is Ra 0.1 μm or more and Ra 0.5 μm or less (excluding the case where the surface roughness of the oil ring is Ra 0.1 μm or less).

2. 2. The sliding mechanism according to claim 1, wherein a coating containing chromium is formed on the outer circumferential surface of the pressure ring and the outer circumferential surface of the oil ring.

3. 2. The sliding mechanism according to claim 1, wherein a coating containing chromium is formed on the outer peripheral surface of the oil ring.

Images