Sliding structure of an internal combustion engine

Abstract

To provide a cylinder which reduces friction resistance and improves fuel consumption efficiency or reduces oil consumption.SOLUTION: The present invention relates to a cylinder in which a piston including a piston ring slides on an inner wall surface. A plurality of recesses are formed in a process center region of the inner wall surface and on a side face of the recess, a sagging region which is defined as follows is formed. A lower reference height line is defined by offsetting to a recess bottom side by 0.50 μm from an upper reference height line offset to the recess bottom side by 0.15 μm from an inner wall surface reference height line transformed into a single straight line using the least square method regarding a plurality of inner wall surfaces. A line segment connecting an upper pint where a cross-sectional curve first crosses the upper reference height line in the vicinity of a boundary with the inner wall surface and a lower point where the cross-sectional curve first crosses the lower reference height line is defined as a virtual sagging cross section of the sagging region. An absolute value of a gradient with an inner wall surface of the virtual sagging cross section defined as a reference is set within a range from 0.00625 to 0.0167.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a sliding structure of an internal combustion engine having a cylinder and a piston. [Background technology]

[0002] Conventionally, in internal combustion engines having a cylinder and a piston, efforts have been made to reduce the sliding resistance (frictional force) between the cylinder and the piston in order to improve fuel efficiency and reduce oil consumption. The applicant of the present invention has developed a so-called dimple liner as a method for reducing the frictional force between the piston ring and the cylinder (see, for example, Patent Document 1), which reduces the sliding resistance during operation by forming multiple recesses in the stroke center region of the inner wall surface of the cylinder. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5155924 Summary of the Invention [Problem to be solved by the invention]

[0004] Although this was not publicly known at the time of filing this application, further research by the inventors has revealed that there is still room for this dimple liner technology to achieve improved fuel efficiency, etc. by further reducing frictional resistance.

[0005] In view of the above circumstances, the present invention aims to achieve further improvement in fuel efficiency and reduction in oil consumption with respect to dimple liners. [Means for solving the problem]

[0006] The present invention for achieving the above object is a cylinder in which a piston equipped with piston rings slides on an inner wall surface, and a plurality of recesses are formed in a stroke center region that is the whole or part of the inner wall surface between a lower surface position of a ring groove of a lowermost piston ring at the top dead center of the piston and an upper surface position of a ring groove of a topmost piston ring at the bottom dead center of the piston, and a sag region defined by the following region conditions is formed on the side surface of the recesses, and (region conditions) the shapes of at least two of the recesses arranged in the cylinder axial direction are measured using a stylus type surface roughness tester (JIS B 0651:2001), the cross-sectional curve obtained by measuring with a measuring needle tip radius of 2 μm and a cutoff value (wavelength) λs for the cross-sectional curve set to 2.5 μm is used as the reference: A line obtained by extracting multiple inner wall surfaces and converting them into a single straight line using the least squares method is defined as the inner wall surface reference height line: A line offset by 0.15 μm from the inner wall surface reference height line to the bottom side of the recess is defined as the upper reference height line that indicates the upper edge of the sagging area: A line offset by 0.50 μm from the upper reference height line to the bottom side of the recess The line drawn is defined as a lower reference height line meaning the lower edge of the sagging region: the line segment connecting the upper point where the cross-sectional curve extending from the inner wall surface toward the bottom surface of the recess first intersects with the upper reference height line and the lower point where the cross-sectional curve first intersects with the lower reference height line near the boundary with the inner wall surface is defined as a virtual sagging cross section of the sagging region: the absolute value of the gradient of the virtual sagging cross section in the sagging region based on the inner wall surface is set within the range of 0.00625 to 0.0167.

[0007] In relation to the above cylinder, at least a part of the surface in contact with the piston ring in the stroke central region is characterized in that a central low roughness region is formed in which the arithmetic mean roughness Ra of a profile curve measured by a stylus type surface roughness tester is 0.140 μm or less.

[0008] In relation to the above cylinder, the recess having the sagging region is formed within the range of the central bottom roughness region.

[0009] The cylinder is characterized in that the arithmetic mean height Sa of the contour curved surface of the central low roughness region measured by a non-contact surface roughness measuring device is 0.20 μm or less.

[0010] The cylinder is characterized in that the protruding valley depth Svk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.41 μm or less.

[0011] The cylinder is characterized in that the height Spk of the protruding peaks of the central low roughness region measured by a non-contact surface roughness measuring device is 0.16 μm or less.

[0012] The cylinder is characterized in that the level difference Sk of the core portion of the central low roughness region measured by a non-contact surface roughness measuring device is 0.53 μm or less.

[0013] In relation to the above cylinder, when the height of the protruding peaks measured by the non-contact surface roughness measuring device in the central low roughness region is E (Spk) and the depth of the protruding valleys is I (Svk), the ratio I / E is 2.6 or less.

[0014] The cylinder is characterized in that the arithmetic mean roughness Ra of the profile curve of the central low roughness region measured by a stylus type surface roughness measuring instrument is 0.120 μm or less.

[0015] The present invention for achieving the above object is a sliding structure for an internal combustion engine having a cylinder and a piston, wherein the cylinder has a plurality of recesses formed in an inner wall surface in a stroke center region that is the whole or part of the region between a lower surface position of a ring groove of a lowest piston ring at the top dead center of the piston and an upper surface position of a ring groove of a highest piston ring at the bottom dead center of the piston, and a sag region defined by the following region conditions is formed on the side surface of the recesses, and (region conditions) the shapes of at least two of the recesses arranged in the cylinder axial direction are measured using a stylus type surface roughness tester (JIS B 0651:2001), the cross-sectional curve obtained by measuring with a measuring needle tip radius of 2 μm and a cutoff value (wavelength) λs for the cross-sectional curve set to 2.5 μm is used as the reference: A line obtained by extracting a plurality of the inner wall surfaces and converting them into a single straight line using the least squares method is defined as the inner wall surface reference height line: A line offset by 0.15 μm from the inner wall surface reference height line to the bottom side of the recess is defined as the upper reference height line indicating the upper edge of the sagging area: A line offset by 0.50 μm from the upper reference height line to the bottom side of the recess A line is defined as a lower reference height line meaning the lower edge of the sag region: a line segment connecting an upper point where the cross-sectional curve extending from the inner wall surface toward the bottom surface of the recess first intersects with the upper reference height line near the boundary with the inner wall surface and a lower point where the cross-sectional curve first intersects with the lower reference height line is defined as a virtual sag cross section of the sag region: the absolute value of the gradient of the virtual sag cross section in the sag region based on the inner wall surface is set within the range of 0.00625 to 0.0167.

[0016] In relation to the sliding structure of the internal combustion engine, a central low roughness region is formed in at least a part of the surface of the cylinder that comes into contact with the piston ring in the stroke central region, and the central low roughness region has an arithmetic mean roughness Ra of a profile curve measured by a stylus type surface roughness tester of 0.140 μm or less.

[0017] The sliding structure of the internal combustion engine is characterized in that the recess having the sagging region is formed within the range of the central bottom roughness region.

[0018] The sliding structure of the internal combustion engine is characterized in that the arithmetic mean height Sa of the contour curved surface of the central low roughness region measured by a non-contact surface roughness measuring device is 0.20 μm or less.

[0019] The sliding structure of the internal combustion engine is characterized in that the protruding valley depth Svk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.41 μm or less.

[0020] The sliding structure of the internal combustion engine is characterized in that the protruding peak height Spk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.16 μm or less.

[0021] The sliding structure of the internal combustion engine is characterized in that the level difference Sk of the core portion of the central low roughness region measured by a non-contact surface roughness measuring device is 0.53 μm or less.

[0022] In relation to the sliding structure of the internal combustion engine, when the height of the protruding peaks in the central low roughness region measured by the non-contact surface roughness measuring device is E(Spk) and the depth of the protruding valleys is I(Svk), the ratio I / E is 2.6 or less.

[0023] The sliding structure of the internal combustion engine is characterized in that the arithmetic mean roughness Ra of the profile curve of the central low roughness region measured by a stylus type surface roughness measuring instrument is 0.120 μm or less.

[0024] In relation to the sliding structure of the internal combustion engine, the central low roughness region includes the vicinity of the upper end edge and the vicinity of the lower end edge in the stroke central region.

[0025] In relation to the sliding structure of the internal combustion engine, the entire stroke central region is characterized by being the central low roughness region.

[0026] The sliding structure of the internal combustion engine may have the following configuration.

[0027] For example, when the kinetic viscosity (dynamic viscosity coefficient) in the central low roughness region is μ, the relative velocity with respect to the piston is Q, the contact load on the piston is W, and the friction coefficient with the piston is f, and the evaluation parameter of the Stribeck diagram is defined as A = μ × Q / W, the minimum value fmin of the friction coefficient f in the central low roughness region can be achieved when the evaluation parameter A is in the range of 0.0003 or less.

[0028] For example, the minimum value fmin can be achieved when the evaluation parameter A is in a range of 0.0001 or more.

[0029] For example, when the kinetic viscosity (dynamic viscosity coefficient) in the central low roughness region is μ, the relative velocity with respect to the piston is Q, the contact load on the piston is W, and the friction coefficient with the piston is f, and the evaluation parameter of the Stribeck diagram is defined as A = μ × Q / W, the friction coefficient f in the central low roughness region can be characterized as being 0.07 or less anywhere within a range where the evaluation parameter A is 0.0003 or less.

[0030] For example, when the kinetic viscosity (dynamic viscosity coefficient) in the central low roughness region is μ, the relative velocity with respect to the piston is Q, the contact load on the piston is W, and the friction coefficient with respect to the piston is f, and the evaluation parameter of the Stribeck diagram is defined as A = μ × Q / W, it can be characterized in that the piston and the cylinder in the central low roughness region are in a fluid lubrication state anywhere within a range where the evaluation parameter A is 0.0003 or less.

[0031] For example, in a friction test in a non-combustion state using a top ring, a second ring, and an oil ring corresponding to the piston ring, when the rotation speed of the internal combustion engine is N (r / min) and the friction loss mean effective pressure (FMEP) between the piston ring and the central low roughness region is T (kPa), the minimum value Tmin of the friction loss mean effective pressure T in the central low roughness region is achieved when the rotation speed N is in a range of 700 or less.

[0032] For example, in a friction test in a non-combustion state using a top ring, a second ring, and an oil ring corresponding to the piston ring, when the rotation speed of the internal combustion engine is N (r / min) and the friction loss mean effective pressure (FMEP) between the piston ring and the central low roughness region is T (kPa), the friction loss mean effective pressure T in the central low roughness region is 14 kPa or less anywhere within a range where the rotation speed N is 700 or less.

[0033] For example, in a non-combustion friction test using a top ring, a second ring, and an oil ring corresponding to the piston ring, when the rotation speed of the internal combustion engine is N (r / min) and the friction loss mean effective pressure (FMEP) between the piston ring in the central low roughness region is T (kPa), the piston and the cylinder in the central low roughness region are in a fluid lubrication state anywhere within a range where the rotation speed N is 700 or less. Effect of the Invention

[0034] According to the present invention, it is possible to obtain the excellent effects of reducing frictional resistance, improving fuel efficiency, and reducing oil consumption. [Brief description of the drawings]

[0035] [Figure 1] 1 is a cross-sectional view taken along an axial direction of a cylinder liner that is applied to a sliding structure for an internal combustion engine according to a first embodiment of the present invention. [Diagram 2] 4(A) and 4(B) are development views showing the inner peripheral wall of the cylinder liner developed in the circumferential direction. [Diagram 3] 2 is a cross-sectional view of the inner peripheral wall of the cylinder liner taken along a direction perpendicular to the axis. FIG. [Figure 4] 4(A) and (B) are cross-sectional views showing the cross-sectional curves of the inner peripheral wall and the recessed portion of the cylinder liner. [Diagram 5]1A is a side view showing a piston and a piston ring applied to the sliding structure of the internal combustion engine, FIG. 1B is a partially enlarged cross-sectional view showing the piston and the piston ring, FIG. 1C is a partially enlarged cross-sectional view of a top ring, and FIG. 1D is a partially enlarged cross-sectional view of a second ring. [Figure 6] 1A is a cross-sectional view of a two-piece type oil ring, and 1B is a cross-sectional view of a three-piece type oil ring. [Figure 7] FIG. 1 shows (A) a Stribeck diagram and (B) an FMEP diagram relating to sliding in a typical internal combustion engine. [Figure 8] 4(A) and 4(B) are graphs showing the results of measuring the sagging area of ​​the cylinder liner. [Figure 9] 6A and 6B are graphs showing the results of measuring the sagging area of ​​a cylinder liner according to a comparative example. [Figure 10] FIG. 1 is a cross-sectional view showing a typical friction unit measuring device for measuring the sliding state of an internal combustion engine. [Figure 11] FIG. 1A is a graph showing the results of measuring the frictional forces of the first to third cylinder liners according to the embodiment, and FIG. 1B is a development view showing the inner circumferential walls of the first to third cylinder liners developed in the circumferential direction. [Figure 12] FIG. 2A is a Stribeck diagram using a verification cylinder liner for analyzing the sliding structure of the internal combustion engine of the first embodiment, and FIG. 2B is an FMEP diagram using the same verification cylinder liner. [Figure 13] FIG. 2 is a side view showing a sliding stroke of a cylinder liner and a piston ring of the internal combustion engine. [Figure 14] FIG. 1A is a cross-sectional view along the axial direction of a cylinder liner applied to a sliding structure of an internal combustion engine related to a second embodiment of the present invention, and FIG. 1B is a side view showing the sliding stroke of the same cylinder liner and a piston ring. [Figure 15] 1 is a cross-sectional view along the axial direction of a cylinder liner showing an example of a cylinder liner to which microtexture technology is applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. First, a sliding structure of an internal combustion engine according to a first embodiment of the present invention will be described in detail. Note that in this first embodiment, a case where the internal combustion engine is a diesel engine is illustrated as an example, but the present invention is not limited thereto and can be applied to other types of internal combustion engines such as a gasoline engine.

[0037] <Cylinder liner>

[0038] As shown in FIG. 1, a plurality of recesses 14 are formed on the inner wall surface 12 of the cylinder liner 10 according to the first embodiment of the internal combustion engine. The recesses 14 are formed in a stroke mid-region 20 on the inner wall surface 12. The stroke mid-region 20 is the entire or partial region of the maximum range from the lower surface position of the ring groove of the lowest piston ring at the top dead center T of the piston 30 to the upper surface position of the ring groove of the highest piston ring at the bottom dead center U of the piston 30 (here, the entire range is the stroke mid-region 20, and the recesses 14 are formed over the entire region). If the region outside the stroke mid-region 20 is defined as an outer region 25, the outer region 25 is composed of an upper outer region 25A adjacent to the top dead center side of the stroke mid-region 20 and a lower outer region 25B adjacent to the bottom dead center side of the stroke mid-region 20. When the piston 30 reciprocates in the cylinder liner 10, it repeatedly passes through the upper external region 25A, the stroke central region 20, the lower external region 25B, the stroke central region 20, and the upper external region 25A in this order. The boundary between the upper external region 25A and the stroke central region 20 is defined as the upper boundary 27A, and the boundary between the lower external region 25B and the stroke central region 20 is defined as the lower boundary 27B.

[0039] Of course, it is possible to form multiple recesses 14 beyond the mid-stroke region 20, but from the standpoint of oil consumption (LOC), it is preferable to form the recesses 14 limited to within the mid-stroke region 20.

[0040] <Dimples formed on cylinder liners>

[0041] The recesses 14 are arranged so that at least one recess 14 is present in any cross section taken in the axis-perpendicular direction at any location on the inner wall surface 12 of the stroke mid-region 20. That is, the recesses 14 are arranged so as to overlap in the axial direction. As a result, the outer circumferential surface of the piston ring passing through the stroke mid-region 20 always faces at least one recess 14. On the other hand, no recesses 14 are formed in the upper outer region 25A and the lower outer region 25B.

[0042] The shape of the recess 14 is a quadrangle (square or rectangle) arranged diagonally with respect to the axial direction, and as a result, the plurality of recesses 14 are arranged in a diagonal lattice pattern. In this way, when focusing on a particular recess 14, as shown in the development view of FIG. 2(A), the axial lowest point 14b of the recess 14 is located axially lower than the axial highest point 14a of the other recesses 14. In this way, since the plurality of recesses 14 overlap in the axial direction, the recesses 14 can always be present in the cross section perpendicular to the axis at every location (for example, arrow A, arrow B, and arrow C) in the stroke central region 20. Here, the plurality of recesses 14 having the same area are uniformly arranged in the surface direction (axial and circumferential directions) in the stroke central region 20.

[0043] 2B, multiple recesses 14 having the same area may be arranged non-uniformly in the planar direction. In this case, the area occupied by multiple recesses 14 is small in a circumferential band region 20P at the axial end of the stroke central region 20, and the area occupied by multiple recesses 14 is large in a circumferential band region 20Q at the axial center of the stroke central region 20.

[0044] The dimensions and shape of the recesses 14 are not particularly limited, but are appropriately selected according to the dimensions and purpose of the cylinder and the piston ring. For example, the recesses 14 can be formed in a slit or band shape so as to penetrate (or extend) through the stroke center region 20 in the cylinder axial direction. On the other hand, from the viewpoint of airtightness of the cylinder, it is preferable that the maximum average length J (see FIG. 2(A)) of the recesses 14 in the cylinder axial direction is equal to or less than the length (width) of the piston ring (top ring) located at the top of the piston in the cylinder axial direction, specifically, about 5 to 100% of that. The average length J of the recesses 14 means the average value when there is variation in the maximum axial dimensions of the multiple recesses 14.

[0045] The maximum average length S of the recesses 14 in the cylinder circumferential direction is preferably in the range of 0.1 mm to 15 mm, and more preferably in the range of 0.3 mm to 5 mm. If it is smaller than these ranges, the effect of reducing the sliding area by the recesses 14 themselves may not be sufficiently obtained. On the other hand, if it is larger than these ranges, a part of the piston ring may easily enter the recess, causing problems such as deformation of the piston ring.

[0046] As shown in FIG. 3(A), the maximum average length R (maximum average depth R) of the recesses 14 in the cylinder diameter direction is preferably in the range of 0.1 μm to 1000 μm, more preferably in the range of 0.1 μm to 500 μm. More preferably, it is set to 0.1 μm to 50 μm. If the maximum average length R of the recesses 14 in the cylinder diameter direction is smaller than these ranges, the effect of reducing the sliding area of ​​the recesses 14 themselves may not be sufficiently obtained. On the other hand, if it is attempted to be made larger than these ranges, processing becomes difficult and problems such as the need to increase the thickness of the cylinder may occur. For convenience of explanation, the recesses 14 in FIG. 3 are drawn with the maximum average length R direction greatly exaggerated with respect to the maximum average length J direction.

[0047] Returning to Fig. 2, the average value of the minimum distance Hc in the cylinder circumferential direction between the recesses 14 adjacent in the circumferential direction at the same axial position is preferably in the range of 0.05 mm to 15 mm, and particularly preferably in the range of 0.1 mm to 5.0 mm. If it is smaller than these ranges, the contact area (sliding area) between the piston ring and the cylinder liner may be too small to allow stable sliding. On the other hand, if it is larger than these ranges, the effect of reducing the sliding area of ​​the recesses 14 itself may not be sufficient.

[0048] The average value of the minimum distance Ha in the cylinder axial direction between the recesses 14 adjacent in the axial direction at the same circumferential position is preferably in the range of 0.05 mm to 15 mm, and particularly preferably in the range of 0.1 mm to 5.0 mm. If it is smaller than these ranges, the contact area (sliding area) between the piston ring and the cylinder liner may be too small to allow stable sliding. On the other hand, if it is larger than these ranges, the effect of reducing the sliding area of ​​the recesses 14 itself may not be sufficient.

[0049] Furthermore, regardless of the direction, the average value of the minimum distance Hm between adjacent recesses 14 is preferably in the range of 0.001 mm to 15 mm, and particularly preferably in the range of 0.001 mm to 5.0 mm. If it is greater than these ranges, the effect of reducing the sliding area of ​​the recesses 14 themselves may not be sufficiently obtained.

[0050] These intervals Hc, Ha, and Hm are, in other words, synonymous with the minimum width in each direction of the inner wall surface 12 remaining between adjacent recesses 14. Accordingly, although details will be described later with reference to Fig. 4(B), these intervals Hc, Ha, and Hm refer to the distance between the upper point GS that first intersects with the upper reference height line G1 of one recess 14 and the upper point GS that first intersects with the upper reference height line G1 of the other recess 14 adjacent thereto.

[0051] In this embodiment, it is preferable that the average value of the interval Ha in the cylinder axial direction is in the range of 0.05 mm to 15 mm, and more preferably in the range of 0.1 mm to 5.0 mm. When expanding the range of the fluid lubrication region 114 of the piston ring 40 sliding in the cylinder axial direction shown in FIG. 7(A) with respect to the recess 14 in which the sagging region 200 described later is formed and the inner wall surface 12 around it, it is possible to suppress localized surface pressure fluctuation acting on the piston ring 40 by ensuring the interval Ha of the inner wall surface 12 in the cylinder axial direction.

[0052] <Dimple sagging area>

[0053] As shown enlarged in FIG. 3B, a so-called sagging region 200 is formed near the edge of the side surface 300 of the recess 14 that approaches the inner wall surface 12. This sagging region 200 is a small inclined surface that starts to drop into the recess 14 with the inner wall surface 12 as the reference surface. The sagging region 200 can be regarded as a partial region of the side surface 300 of the recess 14, and in particular, is a region close to the inner wall surface 12. The sagging region 200 can also be regarded as a partial region formed at the end of the inner wall surface 12. In any case, the sagging region 200 is formed locally near the boundary between the inner wall surface 12 and the recess 14.

[0054] The sag region 200 is formed by honing the inner wall surface 12 of the cylinder liner 10 in which the recesses 14 are formed using a honing machine and polishing the inner wall surface 12 after the honing. It is preferable to form the smooth sag region 200 by performing the polishing process particularly carefully.

[0055] In this embodiment, in order to objectively evaluate the shape of the sagging region 200, the sagging region 200 is defined as follows.

[0056] As shown in Fig. 4(A), the shapes of at least two recesses 14 that are at the same position in the cylinder circumferential direction and aligned in the cylinder axial direction are measured by a stylus surface roughness tester (JIS B 0651:2001) while passing through the location where the minimum interval Ha is between them and including the adjacent inner wall surfaces 12. The measurement is performed by adopting a standard tip radius of 2 μm for the measurement needle and selecting a cutoff value (wavelength) λs for the cross-sectional curve as 2.5 μm. The cross-sectional curve DK to be measured should include at least the two adjacent recesses 14, the pair of inner wall surfaces 12A, 12C that extend on both outer sides of the two recesses 14 in the cylinder axial direction, and the inner wall surface 12B that exists between the two recesses 14.

[0057] In this cross-sectional curve DK, only the range of three inner wall surfaces 12A-12C is extracted, and these wall surface groups 12A-12C are converted into a single straight line 250 using the least squares method. This straight line 250 corresponds to the position where the inner wall surface 12 is smoothed, and is defined as the inner wall surface reference height line GK.

[0058] Next, as shown in FIG. 4(B), the sagging region 200 to be specified is enlarged and extracted from the total of four sagging regions 200 included in the two recesses 14. The actual wall surfaces 12A to 12C include fine irregularities corresponding to surface roughness. Since the amplitude of the irregularities is approximately 0.3 μm, in order to exclude the influence of the surface roughness when defining the sagging region 200, a line offset from the inner wall surface reference height line GK by 0.15 μm toward the base side of the recess 14 is defined as the upper reference height line G1. This upper reference height line G1 means the virtual start position (start height) of the sagging region 200 in the depth direction. Furthermore, a line offset from the upper reference height line G1 by 0.50 μm toward the base side of the recess 14 is defined as the lower reference height line G2. This lower reference height line G2 means the virtual end position (end height) of the sagging region 200. As a result, the length (sag height) DH of the sag region 200 in the cylinder radial direction is defined as 0.50 μm.

[0059] An upper point GS where a cross-sectional curve DK extending from the inner wall surfaces 12A-12C toward the bottom surface (depth direction) of the recess 14 first intersects with an upper reference height line G1 and a lower point GE where a cross-sectional curve DK extending from the inner wall surfaces 12A-12C first intersects with a lower reference height line G2 in the vicinity of the boundary between the inner wall surfaces 12A-12C are selected. A line segment GG connecting the upper point GS and the lower point GE with a straight line is defined as a virtual sag cross section GG of the sag region 200. The length of the virtual sag cross section GG in a direction parallel to the straight line 250 (here, the cylinder axial direction) is defined as a sag width DW of the sag region 200. The absolute value of the gradient DH / DW of this virtual sag cross section GG (hereinafter, this is defined as a sag gradient DA) is calculated based on the straight line 250. When the sag gradient DA is small, it means that the gentle sag region 200 is correctly formed, and when the sag gradient DA is large, it means that the sag region 200 is incomplete.

[0060] Note that the cross-sectional curve DK contains minute irregularities and the like, so there is variation in each measurement. Therefore, for each sag region 200, measurements are performed 60 times using a stylus-type surface roughness measuring instrument (JIS B 0651:2001), and 15 pieces from the maximum side and 15 pieces from the minimum side of the calculated 60 sag gradients DA are cut, and an average value is calculated from the middle 30 pieces of data. This average value is the evaluation sag gradient DA of the sag region 200. Note that, although an example is given here of a case in which the sag gradient DA is used to evaluate the formation state of the sag region 200, the evaluation is equivalent to using the sag width DW as it is.

[0061] In this embodiment, the sag gradient DA for evaluation of the sag region 200 is set in the range of 0.00625 to 0.0167 (the sag width DW for evaluation is set in the range of 0.030 to 0.080).

[0062] By the way, unlike the dimple liner in which multiple recesses are arranged to overlap in the axial direction, there is a microtexture technology that forms the same type of recesses, so we will briefly explain this technology. As shown in FIG. 15, the microtexture is a theory that, along the cylinder axial direction of the inner wall surface of the cylinder liner, an area V where recesses are formed and an area Z where no recesses exist are alternately repeated without overlapping in the axial direction, and every time the piston ring moves along this inner wall surface, engine oil flows in and out of the recesses, and the dynamic pressure thickens the oil film and reduces friction. The present invention is also applicable to such microtexture technology. In other words, the present invention is effectively applicable to any structure in which multiple recesses are formed in the stroke center area to reduce the contact area with the piston ring.

[0063] <Central low roughness area formed on cylinder liner>

[0064] In the inner wall surface 12 of the cylinder liner 10, at least a part of the stroke central region 20 has a surface roughness (arithmetic mean roughness Ra (JIS B 0601:2013) of a profile curve) measured by a stylus surface roughness tester (JIS B 0651:2001) of 0.140 (μm) or less, preferably a central low roughness region 22 having a surface roughness Ra of 0.120 (μm) or less. Specifically, the surface of the inner circumferential surface 12 that may come into contact with the piston ring 40, i.e., at least a part of the surface excluding the recesses 14 of the inner circumferential surface 12, is processed to have a surface roughness Ra of 0.140 (μm) or less, more preferably 0.120 (μm) or less, to form the central low roughness region 22. In this embodiment, the surface roughness measured by the stylus surface roughness measuring instrument (arithmetic mean roughness of the contour curve) is represented as Ra, and the three-dimensional surface roughness measured by the non-contact surface roughness measuring instrument described later (arithmetic mean height of the contour curve (JIS B 0681-2:2018, ISO 25178-2:2012)) is represented as Sa.

[0065] More preferably, the surface roughness Ra of the central low-roughness region 22 is set to 0.090 (μm) or less, specifically 0.083 (μm).

[0066] The three-dimensional surface roughness values ​​(JIS B 0681-2:2018, ISO 25178-2:2012) of this central low-roughness region 22 when measured using a non-contact surface roughness measuring device (measurement magnification 1080x, field of view size 259.4 μm x 259.4 μm, no cutoff, height direction (Z direction) measurement pitch 0.06 μm) using a laser microscope in accordance with JIS B 0681-6:2014 (ISO 25178-6:2010) are shown below. Arithmetic mean height Sa (μm): 0.192 or less, preferably 0.163 or less, and more preferably 0.120 or less (specifically, set to 0.110). Protruding peak height Spk (μm): 0.159 or less, preferably 0.144 or less, and more preferably 0.121 or less (specifically, set to 0.116). Core level difference Sk (μm): 0.521 or less, preferably 0.449 or less, and more preferably 0.340 or less (specifically, set to 0.315). Protruding valley depth Svk (μm): 0.409 or less, preferably 0.342 or less, and more preferably 0.241 or less (specifically, set to 0.218).

[0067] In particular, in this embodiment, not only the height of the protruding peaks is reduced, but also the depth of the protruding valleys is actively reduced, thereby reducing the frictional force during sliding. Incidentally, in the conventional cylinder liner, in order to ensure the retention of the lubricating oil, it is necessary to increase the depth of the protruding valleys to a certain extent, and as a result, it is difficult to reduce the height of the protruding peaks, and there is a limit to the reduction of the frictional force during sliding. On the other hand, in this embodiment, since the lubricating oil is sufficiently retained in the adjacent recesses 14, even if the contact surface itself with the piston ring 40 has a low retention of the lubricating oil, it is possible to form a sufficient oil film. With this in mind, when the protruding peak height is E and the protruding valley depth is I, in the central low roughness region 22 of this embodiment, the value of I / E is preferably set to 2.6 or less, more preferably 2.4 or less, and even more preferably 2.0 or less.

[0068] In this embodiment, the entire range of the surface in the stroke central region 20 that may come into contact with the piston ring 40 is the central low roughness region 22. As a result, the central low roughness region 22 includes the vicinity of the upper end edge and the vicinity of the lower end edge of the stroke central region 20 where the recess 14 is formed. Furthermore, an upper low roughness region 23A having a surface roughness Ra of 0.120 (μm) or less is formed in the upper outer region 25A adjacent to the top dead center side of the stroke central region 20, and a lower low roughness region 23B is formed in the lower outer region 25B adjacent to the bottom dead center side of the stroke central region 20. The upper low roughness region 23A, the central low roughness region 22, and the lower low roughness region 23B are completely connected with a uniform surface roughness state, and form an integrated continuous surface as a whole.

[0069] Near the upper and lower edges of the stroke central region 20, the relative speed Q between the cylinder liner 10 and the piston 30 decreases, so that the region is likely to transition from the fluid lubrication region to the boundary lubrication region. However, the presence of this central low roughness region 22 allows the fluid lubrication region to be predominantly expressed. Since the so-called dimple liner technology can exert its effect in the fluid lubrication region, the advantages of the dimple liner technology can also be obtained near the upper and lower edges of the stroke central region 20. Although it is possible to form the central low roughness region 22 only near the upper and / or lower edges of the stroke central region 20, it is preferable to form the central low roughness region 22 over the entire stroke central region 20 as in this embodiment. When the relative speed Q between the cylinder liner 10 and the piston 30 becomes lower, the boundary lubrication region will approach the center of the stroke central region 20, but even in that case, it is possible to expand the range of the fluid lubrication region.

[0070] The central low-roughness region 22 of the inner wall surface 12 of the cylinder liner 10 is formed by honing using a honing machine. The grain size of the honing stone used here is preferably an abrasive grain finer than, for example, F500 or #800 (JIS R 6001-2:2017, ISO8486-2:2007).

[0071] Furthermore, after the central low-roughness region 22 is formed by this honing process, it is preferable not to perform a chemical conversion treatment on the surface. For example, if a phosphate coating or the like that is generally used in the manufacturing process of the cylinder liner 10 is performed, the surface properties of the central low-roughness region 22 will vary depending on the coating. On the other hand, if the coating has a small variation in the surface properties, it is also preferable to add a chemical conversion treatment to the central low-roughness region 22.

[0072] <Pistons and piston rings>

[0073] 5A) and 5B) show a piston 30 and piston rings 40 (top ring 50, second ring 60, oil ring 70) installed in the ring groove of the piston 30. The piston ring 40 reciprocates in the cylinder axial direction with its outer circumferential surface 42 facing the inner wall surface 12 of the cylinder liner 10. The top ring 50 eliminates the gap between the piston 30 and the cylinder liner 10, and prevents the phenomenon (blow-by) in which compressed gas escapes from the combustion chamber to the crankcase. The second ring 60, like the top ring 50, has the role of eliminating the gap between the piston 30 and the cylinder liner 10 and the role of scraping off excess engine oil adhering to the inner wall surface 12 of the cylinder liner 10. The oil ring 70 scrapes off excess engine oil adhering to the inner wall surface 12 of the cylinder liner 10 and forms an appropriate oil film, thereby preventing the piston 30 from seizing.

[0074] As shown in an enlarged view in FIG. 5(C), the top ring 50 is a single annular member, and when the outer peripheral surface 52 is viewed in cross section, it has a so-called barrel shape that is convex radially outward. Specifically, both outer edges of the outer peripheral surface 52 in the cylinder axial direction are inclined in a direction away from the inner wall surface 12 toward the outside in the cylinder axial direction. It is preferable that the contact width f of the outer peripheral surface 52 with the inner wall surface 12 of the cylinder liner 10 is formed to be, for example, 0.3 mm or less. In addition, it is preferable that the surface roughness (arithmetic mean roughness Ra (JIS B 0601:2013) of the profile curve) of the outer peripheral surface 52 measured by a stylus surface roughness measuring instrument (JIS B 0651:2001) is 0.250 (μm) or less.

[0075] As shown in an enlarged view in FIG. 5(D), the second ring 60 is a single annular member, and the vicinity of its outer periphery is tapered in a shape that expands in diameter from the upper end in the cylinder axial direction toward the lower end in the cylinder axial direction. The outer peripheral surface 62, which is located at the outermost end of this tapered shape and contacts the inner wall surface 12 of the cylinder liner 10, is flat in cross section. The contact width f of the outer peripheral surface 62 with the inner wall surface 12 of the cylinder liner 10 is preferably formed to be, for example, 0.3 mm or less. The surface roughness (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) of the outer peripheral surface 62 measured by a stylus surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.250 (μm) or less.

[0076] The tension of the top ring 50 and the second ring 60 is set to a relatively low value, and the surface pressure acting on the contact surfaces of the outer circumferential surfaces 52, 62 is, for example, 0.5 MPa or less, and preferably 0.3 MPa or less. As a result, the top ring 50 and the second ring 60 often slide in the fluid lubrication region, except near the top dead center and the bottom dead center.

[0077] The oil ring 70 shown in an enlarged view in FIG. 6A is a two-piece type oil ring, and includes a ring body 72 and a coil expander 76 in the form of a coil spring. The ring body 72 includes a pair of annular rails 73, 73 arranged at both axial ends, and an annular columnar portion 75 arranged between the pair of rails 73, 73 and connecting them. The cross-sectional shape of the pair of rails 73, 73 and the columnar portion 75 is substantially I-shaped or H-shaped, and by utilizing this shape, an inner peripheral groove 79 having a semicircular cross-sectional shape for accommodating the coil expander 76 is formed on the inner peripheral surface side. In addition, annular protrusions 74, 74 are formed on the pair of rails 73, 73, respectively, protruding radially outward from the columnar portion 75. An outer peripheral surface 82, 82 formed at the tip of the annular protrusion 74, 74 abuts against the inner wall surface 12 of the cylinder liner 10. The coil expander 76 is housed in the inner circumferential groove 79, thereby pressing and biasing the ring body 72 radially outward. Note that a plurality of oil return holes 77 are formed in the column portion 75 of the ring body 72 in the circumferential direction.

[0078] The contact width of each of the pair of outer peripheral surfaces 82, 82 in FIG. 6(A) is preferably 0.02 mm to 0.30 mm, and is set to, for example, 0.15 mm. The surface pressure acting on the contact surface of the outer peripheral surface 82 of the oil ring 70 is, for example, 1.0 MPa to 2.0 MPa, for example, about 1.75 MPa. Therefore, when the engine speed is high, the oil ring 70 often slides in the hydrodynamic lubrication region, but when the engine speed is low, the oil ring 70 often slides in the boundary lubrication region. In FIG. 6(A), the radial cross-sectional shape of the outer peripheral surfaces 82, 82 is illustrated as a simple trapezoid, but the present invention is not limited to this, and the corners of the outer peripheral surface 82 of the upper rail 73 and the outer peripheral surface 82 of the lower rail 73 facing each other (the coil expander 76 side) may be cut out in a step shape (so-called step land shape). Furthermore, the surface roughness (arithmetic mean roughness Ra (JIS B 0601:2013) of the outer circumferential surface 82 measured by a stylus surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.450 (μm) or less.

[0079] The oil ring 70 is not limited to a two-piece type, and may be, for example, a three-piece type oil ring 70 as shown in Fig. 6(B). This oil ring 70 has annular side rails 73a, 73b separated into upper and lower parts, and a spacer expander 76s disposed between the side rails 73a, 73b.

[0080] The spacer expander 76s is formed by plastic processing a steel material into a wave shape with repeated concaves and convexes in the cylinder axial direction. Using this wave shape, an upper support surface 78a and a lower support surface 78b are formed, and a pair of side rails 73a, 73b are supported in the axial direction. The inner peripheral end of the spacer expander 76s has ears 74m that are erected in an arch shape toward the outside in the axial direction. The ears 74m abut the inner peripheral surfaces of the side rails 73a, 73b. The spacer expander 76s is assembled into the ring groove of the piston 30 in a circumferentially contracted state with the joints butted together. As a result, the ears 74m press and urge the side rails 73a, 73b radially outward due to the restoring force of the spacer expander 76s.

[0081] It is preferable that the contact width f of each of the outer circumferential surfaces 82, 82 of the side rails 73a, 73b in FIG. 6(B) is set to 0.02 mm to 0.40 mm.

[0082] <Friction between cylinder liner and piston ring>

[0083] Next, the frictional behavior of the cylinder liner and the piston ring will be described. The change in the friction coefficient during general sliding is expressed as a Stribeck diagram shown in FIG. 7(A). In this Stribeck diagram, the frictional behavior is divided into the frictional behavior of the solid contact region 110 where the pistons slide in direct contact with each other, the frictional behavior of the boundary lubrication region 112 where the pistons slide through an oil film, and the frictional behavior of the fluid lubrication region 114 where the pistons slide through a viscous lubricant oil film. In addition, between the boundary lubrication region 112 and the fluid lubrication region 114, there is a frictional behavior of the mixed lubrication region 113 where both states are mixed. In addition, in this Stribeck diagram, the horizontal axis is a logarithmic representation of "kinetic viscosity (kinetic viscosity coefficient) μ" × "velocity Q" / "contact load W", and the vertical axis is the frictional coefficient (f). Therefore, the frictional force can be smallest in the fluid lubrication region 114 or the mixed lubrication region 113, and effective use of these regions 114 and 113 is effective for reducing friction, that is, reducing fuel consumption. On the other hand, if the speed Q increases but it is not possible to transition from the boundary lubrication region 112 to the fluid lubrication region 114, the boundary lubrication region 112 will continue into the high speed region as shown by the dotted line.

[0084] Incidentally, most of the frictional force in the fluid lubrication region 114 is the shear resistance of the oil, and this shear resistance is defined as (viscosity) x (velocity) x (area) / (oil film thickness). As a result, reducing the shear cross-sectional area directly leads to a reduction in frictional force.

[0085] Therefore, in this embodiment, oil is actively introduced into the contact surface of the outer circumferential surface 42 of the piston ring 40, quickly transitioning to the fluid lubrication region 114 and achieving low friction. At the same time, by applying the so-called dimple liner technology to the cylinder liner 10, recesses 14 are formed in the stroke center region 20 of the cylinder liner 10, and the effective area where the shear resistance of the oil occurs is reduced, thereby achieving a more efficient reduction in frictional force.

[0086] In addition, the Stribeck diagram in FIG. 7(A) shows the dynamic change of the friction coefficient (f) during one stroke of the piston 40, but another index for evaluating the friction mode is the friction loss mean effective pressure (FMEP). This friction loss mean effective pressure indicates a value obtained by dividing the friction work per cycle by the stroke volume. A diagram of this friction loss mean effective pressure (FMEP diagram) is shown in FIG. 7(B). In the FMEP diagram, the horizontal axis is the rotation speed (N) and the vertical axis is the friction loss mean effective pressure (kPa). The higher the rotation speed (N), the higher the proportion of the fluid lubrication region 114 in one stroke. On the other hand, when the rotation speed (N) is lower, the proportion of the fluid lubrication region 114 in one stroke decreases, and the proportion of the mixed lubrication region 113 (or boundary lubrication region 112) increases. Therefore, the shape of the FMEP diagram in FIG. 7(B) is relatively similar to the shapes of the hydrodynamic lubrication region 114 and the mixed lubrication region 113 in the Stribeck diagram in FIG. 7(A).

[0087] <Measurement results of sagging area>

[0088] Next, the measurement results of the sagging region 200 formed in the cylinder liner 10 of the first embodiment will be described with reference to Fig. 8. Fig. 8(A) and (B) show that the maximum average length R (maximum average depth R) of the recess 14 in the cylinder diameter direction is set to 3.0 µm, and the sagging region 200 is actively formed on the edge of the recess 14 by polishing.

[0089] FIG. 8(A) shows the frequency of sagging width DW when the cross-sectional curve of sagging region 200 is measured 60 times. Although there is some variation in sagging width DW, the average value is 0.036 mm. This average sagging width DW can be converted into a sagging gradient DA of 0.0139. FIG. 8(B) shows the frequency of the median sagging width DW from the 60 pieces of data in FIG. 8(A), excluding the top 15 points and bottom 15 points. The average value of sagging width DW is 0.030 mm. This average sagging width DW can be converted into a sagging gradient DA of 0.0167.

[0090] Fig. 9 shows the results of actual measurements of the sagging region 200 formed in the cylinder liner 10 as a comparative example. In Fig. 9(A) and (B), the maximum average length R (maximum average depth R) of the recess 14 in the cylinder diameter direction is set to 3.0 μm, and the sagging region 200 is not actively formed on the edge of the recess 14.

[0091] FIG. 9(A) shows the frequency of sagging width DW when the cross-sectional curve of sagging region 200 is measured 60 times. Although there is some variation in sagging width DW, the average value is 0.017 mm. This average sagging width DW can be converted into a sagging gradient DA, which is 0.0294. FIG. 9(B) shows the frequency of the median sagging width DW from the 60 pieces of data in FIG. 9(A), excluding the top 15 points and bottom 15 points. The average value of sagging width DW is 0.016 mm. This average sagging width DW can be converted into a sagging gradient DA, which is 0.0313.

[0092] <Effect of sagging area>

[0093] According to the cylinder liner 10 of this embodiment, the sag region 200 is formed near the edge that is the boundary between the recess 14 and the inner wall surface 12, so that the interference between the piston ring 40 and the sag region 200 is reduced. As a result, when the fluid lubrication region 114 or the mixed lubrication region 113 is formed between the piston ring 40 and the recess 14 and the inner wall surface 12 around it, an oil film is stably formed and the friction force is reduced. In particular, in the cylinder liner 10 of this embodiment, the inner circumferential wall 12 around the recess 14 is made into the central low roughness region 22, so that the fluid lubrication region 114 is expressed as predominantly as possible even when the relative speed Q between the cylinder liner 10 and the piston 30 becomes lower. In this embodiment, by forming the sag region 200 in the recess 14 formed in the range overlapping with the central bottom roughness region 22, it is possible to further expand the range of the fluid lubrication region 114.

[0094] In addition, although actual measurement and analysis are difficult, it is speculated that forming the sag region 200 in the recess 14 will smooth the flow of lubricating oil in and out of the recess 14. As a result, it is speculated that the lubricating oil in the recess 14 will be smoothly guided to the surrounding inner wall surface 12, which will stabilize the formation of an oil film on the inner wall surface 12.

[0095] For example, if the sagging region 200 is not sufficiently formed in the recess 14, interference may occur between the piston ring 40 and the edge of the recess 14. This interference directly leads to an increase in sliding resistance, and at the same time, induces an unstable state in the oil film formation on the surrounding inner wall surface 12, resulting in a narrowing of the range of the fluid lubrication region 114. Furthermore, if interference occurs between the piston ring 40 and the edge of the recess 14 and the piston ring is damaged, the damaged piece may form a hairline-like scratch on the inner wall surface 12. This scratch also becomes a factor that destabilizes the oil film formation on the inner wall surface 12, making it difficult to maintain the fluid lubrication region 114 or the mixed lubrication region 113, leading to an increase in friction force. In particular, when the inner peripheral wall 12 around the recess 14 is made into the central low roughness region 22, the adverse effects of scratches are likely to become apparent.

[0096] Next, a method for measuring the friction state between the cylinder liner 10 and the piston ring 40 in the first embodiment will be described. Since the fixed positions of the top ring 50, second ring 60, and oil ring 70 mounted on the piston 30 are relatively different in the cylinder axial direction, the friction state between the cylinder liner 10 and each piston ring is slightly different strictly speaking, but here, the position of the second ring 60 is used as the reference position for the piston ring 40 for the description. Note that, only at the fastest passing point C, the top ring 50 is used as the reference.

[0097] <Method of measuring friction behavior (non-combustion friction test)>

[0098] FIG. 10 shows a friction unit measuring device 500 for measuring the friction state between the cylinder liner 10 and the piston ring 50 employed in the first embodiment. The friction unit measuring device 500 measures the friction state between the two by fixing the piston ring 40 side and moving the cylinder liner 10 side up and down. That is, the measurement of this friction state is a friction test in a state where no combustion occurs in the internal combustion engine (friction test during non-combustion). The friction unit measuring device 500 holds a virtual piston 510 on which the piston rings 40 (three rings: top ring, second ring, and oil ring) are set by a fixed shaft 514 via a load cell 512. The load cell 512 measures the external force (friction force) acting on the piston ring 40 in the vertical direction.

[0099] The cylinder liner 10 is held by a movable sleeve 530 on its outer wall side. The lower end of the movable sleeve 530 is held by a driving piston 540. This driving piston 540 is held by a connecting rod 550 that moves up and down by a crankshaft (not shown). As a result, the cylinder liner 10 moves back and forth in the vertical direction. A fixed sleeve 560 is disposed on the outer periphery of the movable sleeve 530. The fixed sleeve 560 is fixed to a base 570. The fixed shaft 514 is fixed to a cover member 562 at the upper end of the fixed sleeve 560. The outer periphery of the movable sleeve 530 and the inner periphery of the fixed sleeve 560 are slidable. A temperature control jacket 565 is provided inside the fixed sleeve 560, and the temperature of the fixed sleeve 560 can be controlled by circulating hot or cold water inside the temperature control jacket 565.

[0100] For example, the conditions for measuring the friction mode using friction unit measuring device 500 may be such that the lubricant specification is 10W-30, the oil temperature is set to 60 degrees, and the rotation speed of the crankshaft is changed from 215 rpm to 2154 rpm.

[0101] For example, the top ring 50 has a height (width) of 2.5 mm, a surface roughness Ra of the outer peripheral surface 52 of 0.180 (μm), and a tension of 16.7 N. The second ring 60 has a height (width) of 2.0 mm, a surface roughness Ra of the outer peripheral surface 62 of 0.180 (μm), and a tension of 12.3 N. The oil ring 70 has a height (width) of 3.0 mm, a surface roughness Ra of the outer peripheral surface 82 of 0.330 (μm), and a tension of 22.6 N.

[0102] <Example: Friction force measurement results (friction test when not burning)>

[0103] As an example, the first to third cylinder liners 10 employed in the first embodiment are 01 , 10 02 , 10 03 The results of measuring the frictional force of each of the first to third cylinder liners 10 using the friction unit measuring device 500 shown in FIG. 10 are shown in FIG. 11(A). 01 , 10 02 , 10 03 The contour shape of the recess 14 is a perfect circle as shown in FIG. 11(B), the diameter of the perfect circle is about 0.50 mm, the maximum average length (maximum average depth) of the recess 14 in the cylinder diameter direction is about 5 μm, and a sagging area 200 is actively formed on the edge of the recess 14 by polishing, so that a subsequent chemical conversion treatment is not performed. The area occupancy rate of the recess 14 is set to 40% of the total area of ​​the inner wall surface 12 and the recess 14. The sagging width DW in the cylinder axial direction is about 100 mm. 01 is 0.030 mm (sag gradient DA is 0.0167), and the second cylinder liner 10 02 is 0.050 mm (sag gradient DA is 0.0100), and the third cylinder liner 10 03 was set to 0.080 mm (sag gradient DA was 0.0063).

[0104] In addition, the cylinder liner 10 as a comparative example 99 The sag width DW is 0.019 mm (sag gradient DA is 0.0263), and other specifications are 10 for the first to third cylinder liners. 01 , 10 02 , 1003 I produced something similar to the above.

[0105] In the graph of FIG. 11(A), the cylinder liner 10 99 When the friction force of the first to third cylinder liners 10 is set to 100%, 01 , 10 02 , 10 03 As can be seen from this result, the larger the sag width DW is (the smaller the sag gradient DA is), the lower the friction force is.

[0106] <Changes in friction behavior due to formation of sagging areas>

[0107] In the cylinder liner 10 of this embodiment, the reduction in frictional force due to the presence of the sagging region 200 can be confirmed by measuring the friction state using the friction unit measuring device 500. In particular, in the cylinder liner 10 of this embodiment, the presence of the sagging region 200 in the recess 14 formed in the range overlapping with the central bottom roughness region 22 makes it possible to reduce the frictional force even when, for example, the relative speed Q between the cylinder liner 10 and the piston 30 becomes slower.

[0108] <Changes in friction behavior due to the presence or absence of a central low roughness region>

[0109] In the first embodiment, the case where both the sagging region 200 and the central low roughness region 22 are formed in the cylinder liner 10 is exemplified, but from the viewpoint of verifying the action in detail, it is important to clarify the influence on the friction force of only the central low roughness region 22. Therefore, it is preferable to measure the friction mode by the friction unit measuring device 500 also for a verification cylinder liner having the central low roughness region 22 but not having the sagging region 200 formed in the recess 14. The actual measurement results are shown below.

[0110] <Friction behavior of a verification cylinder liner having only a central low roughness region and a piston ring>

[0111] (About the Stribeck diagram)

[0112] In order to reveal the frictional behavior of the central low roughness region in this first embodiment, a verification cylinder liner 10-A in which the surface roughness Ra of the central low roughness region 22 is 0.120 (μm) without forming the sagging region 200, and a verification cylinder liner 10-B in which the surface roughness Ra is 0.083 (μm) were prepared, and the frictional behavior was measured using a friction unit measuring device 500. For reference, a verification cylinder liner X in which the surface roughness Ra is 0.160 (μm) with the same shape as the verification cylinder liner 10 (without the sagging region 200) and the recess 14 itself are not formed, and a verification cylinder liner Y in which the surface roughness Ra is 0.160 (μm) were prepared, and the other conditions were set to the same, and the frictional behavior was measured using the friction unit measuring device 500. Note that no chemical conversion treatment was performed on all the verification cylinder liners 10-A, 10-B, X, and Y. The measurement results of the Stribeck diagram are shown in Fig. 12(A), and the measurement results of the FMEP diagram are shown in Fig. 12(B). Fig. 12(A) and (B) also show estimated values ​​for a hypothetical verification cylinder liner K in which the surface roughness Ra of the central low-roughness region 22 is set to Ra0.140 (μm) without forming the sagging region 200.

[0113] First, the surface roughness of the verification cylinder liners 10-A, 10-B, and K is listed in Table 1 below. The arithmetic mean roughness Ra is a value measured by a stylus-type surface roughness measuring instrument (JIS B 0651:2001), and the arithmetic mean height Sa (μm), protruding peak height Spk (μm), protruding valley depth Svk (μm), and core level difference: Sk (μm) are values ​​measured by a non-contact surface roughness measuring instrument using a laser microscope as already explained. [Table 1]

[0114] The test cylinder liners 10-A and 10-B and the test cylinder liner X have recesses 14 formed in the entire stroke central region 20. As shown in FIG. 13, the change in the friction coefficient between the test cylinder liner and the piston ring 40 when the piston 30 slides from the top dead center T to the bottom dead center U of the test cylinder liner depends on the relative speed between them. This relative speed is uniquely determined with respect to the engine speed (rpm). The piston 30 descends from a state where the speed at the top dead center T of the test cylinder liner is zero, passes through stroke A, and reaches a maximum speed C on the way. After that, when it reaches the bottom dead center U via stroke B, the speed becomes zero. During this time, the friction coefficient constantly changes along the Stribeck diagram of FIG. 12(A).

[0115] In the Stribeck diagram of FIG. 12(A), the kinetic viscosity (dynamic viscosity coefficient) in the central low roughness region 22 is defined as μ, the relative velocity with respect to the piston 30 (piston ring 40) is defined as Q, the contact load with respect to the piston 30 is defined as W, and the friction coefficient with respect to the piston 30 is defined as f (vertical axis of the graph), and the evaluation parameter is defined as A=μ×Q / W (horizontal axis of the graph (logarithmic notation)).

[0116] In the test cylinder liners 10-A and 10-B, the minimum value fmin of the friction coefficient f in the central low roughness region 20 is located within a range in which the evaluation parameter A is 0.0003 or less. More preferably, the minimum value fmin of the friction coefficient f is located within a range in which the evaluation parameter A is 0.0002 or less. On the other hand, the minimum value fmin of the friction coefficient f in the central low roughness region 20 is located within a range in which the evaluation parameter A is 0.0001 or more.

[0117] Furthermore, in the test cylinder liners 10-A and 10-B, the friction coefficient f in the central low roughness region 20 is 0.07 or less within a range in which the evaluation parameter A is 0.0003 or less. Desirably, the friction coefficient f is 0.06 or less.

[0118] As can be seen by comparing with the test cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses), in the test cylinder liners 10-A and 10-B, the range where the friction coefficient is 0.07 or less spreads to the left of the graph where the evaluation parameter A is 0.0003 or less. This means that even in low-speed sliding, the range of the fluid lubrication region 114 and the mixed lubrication region 113 spreads, resulting in a sliding mode with an extremely low friction coefficient f.

[0119] As can be seen from the test results of these test cylinder liners, in addition to the above-mentioned low friction coefficient state created by the central low-roughness region 22, forming a sagging region 200 in the recess 14 leads to the low friction coefficient being maintained for a long period of time.

[0120] For reference, in the case of the verification cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses), when the evaluation parameter A exceeds 0.0003, the effect of the recesses 14 appears, the rate of increase of the friction coefficient f decreases, and the friction coefficient f becomes smaller than that of the verification cylinder liner Y (surface roughness Ra 0.160 (μm) / without recesses). On the other hand, when the evaluation parameter A is in the range of 0.0003 to 0.0005, the graph of the verification cylinder liner X (surface roughness Ra0.160 (μm) / with recesses) intersects with the graph of the verification cylinder liner Y (surface roughness Ra0.160 (μm) / without recesses), and the evaluation parameter A becomes 0.0003 or less, the friction coefficient f of the verification cylinder liner X (surface roughness Ra0.160 (μm) / with recesses) increases in the range exceeding 0.07 and exceeds the friction coefficient f of the verification cylinder liner Y (surface roughness Ra0.160 (μm) / without recesses). That is, in the case of the verification cylinder liner X (surface roughness Ra0.160 (μm) / with recesses), when the evaluation parameter A is in the range exceeding 0.0003, the recesses 14 act to reduce the friction coefficient f, but when the evaluation parameter A is in the range of 0.0003 or less, the recesses 14 increase the friction coefficient f. According to the inventors' undisclosed consideration, in the case of the verification cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses), the actual contact area between the cylinder liner and the piston ring 40 is reduced due to the presence of the recesses 14, so it is presumed that insufficient lubrication by the lubricant is likely to occur in the low speed region, and the boundary lubrication region is likely to occur. The sagging region 200 of the cylinder liner 10 of this embodiment can exert an effect of reducing the lack of lubrication (increase in frictional force) caused by the recesses 14.

[0121] In the verification cylinder liners 10-A and 10-B, the surface roughness Ra of the central low roughness region 20 is set to 0.120 (μm) or less, so that even if the evaluation parameter A is in a low speed region of 0.0003 or less and the actual contact area with the piston ring 40 is small due to the recess 14, it is easy to maintain the fluid lubrication region 114 or the mixed lubrication region 113. Even if the boundary lubrication state is reached, the friction coefficient is kept small because the surface roughness is small. Generally, if the surface roughness Ra of the inner surface of the cylinder liner is set to 0.120 (μm) or less, the retention force of the lubricant on the contact surface decreases and the lubricant is likely to be insufficient, but in this embodiment, the recess 14 formed overlapping the central low roughness region 22 functions as a lubricant storage portion for the central low roughness region 20, so that a synergistic effect is obtained in which the lubricant is less likely to be insufficient in the central low roughness region 20.

[0122] In this case, it is presumed that if a sagging region 200 is further formed in the recess 14, the lubricating oil will flow smoothly in and out of the recess 14. As a result, it is presumed that the lubricating oil will be smoothly guided from the recess 14, which serves as a reservoir for the lubricating oil, to the surrounding central low-roughness region 22, thereby stabilizing the formation of an oil film in the central low-roughness region 22.

[0123] Furthermore, in the test cylinder liners 10-A and 10-B, the friction coefficient f in the range where the evaluation parameter A exceeds 0.0003 (high speed region) is close to or smaller than the friction coefficient f of the test cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses). In other words, in the test cylinder liners 10-A and 10-B, the central low-roughness region 20 contributes to reducing the friction coefficient f even in the high speed region.

[0124] 12(A) shows an estimated friction coefficient of the verification cylinder liner K, which has the same configuration as the verification cylinder liner 10-A and has a surface roughness Ra of 0.140 (μm) in the central low roughness region 20, although it is not an actual measurement value. The friction coefficient of the verification cylinder liner K can be estimated to be close to the intermediate value between the verification cylinder liners X and Y, which have a surface roughness Ra of 0.160 (μm), and the verification cylinder liner 10-A, which has a surface roughness Ra of 0.120 (μm). Even with this verification cylinder liner K, the friction coefficient f in the central low roughness region 20 is 0.07 or less, preferably 0.06 or less, in any state within the range in which the evaluation parameter A is 0.0003 or less. Furthermore, in the test cylinder liner K, the friction coefficient f in the range where the evaluation parameter A exceeds 0.0003 (high speed region) is close to or smaller than the friction coefficient f of the test cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses).

[0125] (About the FMEP diagram)

[0126] In the FMEP diagram of FIG. 12(B), the friction loss mean effective pressure (FMEP) between the central low roughness region 22 and the piston ring 40 is defined as T (kPa) (vertical axis), and the rotation speed of the internal combustion engine is defined as N (r / min) (horizontal axis).

[0127] In the test cylinder liners 10-A and 10-B, the minimum value Tmin of the friction loss mean effective pressure T in the central low roughness region 20 is located within a range of the rotation speed N of 700 or less. More preferably, the minimum value Tmin of the friction loss mean effective pressure T is located within a range of the rotation speed N of 600 or less.

[0128] Furthermore, in the test cylinder liners 10-A and 10-B, the friction loss mean effective pressure T in the central low roughness region 20 is set to be 14 kPa or less when the rotation speed N is within a range of 700 or less.

[0129] As can be seen by comparing with the test cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses), the range in which the friction loss mean effective pressure T falls to 14 kPa or less is expanded to the rotation speed N of 700 or less (left side of the graph) in the test cylinder liners 10-A and 10-B. As a result, even at low rotation speeds, the sliding mode has extremely low friction loss.

[0130] For reference, in the case of the test cylinder liner X (surface roughness Ra0.160 (μm) / with recesses), when the rotation speed N reaches a high rotation range exceeding 700, the effect of the recesses 14 appears and the rate of increase in friction loss decreases, and the friction loss is particularly smaller than that of the test cylinder liner X (surface roughness Ra0.160 (μm) / without recesses) when the rotation speed N exceeds 1000. On the other hand, when the rotation speed N is in the range of 1000 to 1300, the graph of the test cylinder liner X (surface roughness Ra0.160 (μm) / with recesses) and the graph of the test cylinder liner Y (surface roughness Ra0.160 (μm) / without recesses) intersect. When the rotation speed N becomes 1000 or less, the friction loss exceeds that of the test cylinder liner Y (surface roughness Ra0.160 (μm) / without recesses), and when the rotation speed N becomes 700 or less, the friction loss increases in a range exceeding 14 kPa. That is, in the case of the verification cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses), when the rotation speed N exceeds 1000, the recesses 14 act to reduce friction loss, but when the rotation speed N is in the range of 1000 or less, the recesses 14 act to increase friction loss. According to the undisclosed consideration of the present inventors, in the case of the verification cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses), the actual contact area between the cylinder liner and the piston ring 40 is reduced due to the presence of the recesses 14, so it was inferred that insufficient lubrication by the lubricating oil is likely to occur in the low rotation range, and the ratio occupied by the boundary lubrication region is likely to increase. The sagging region 200 of the cylinder liner 10 of this embodiment can exert an effect of suppressing the increase in friction loss due to the recesses 14.

[0131] In the test cylinder liners 10-A and 10-B, the surface roughness Ra of the central low roughness region 20 is set to 0.120 (μm) or less, so that even in a low rotation region where the rotation speed N is 700 or less, friction loss is suppressed by maintaining the fluid lubrication region 114 or the mixed lubrication region 113. Even if a boundary lubrication state is reached, friction loss is suppressed because the surface roughness is small. Specifically, when the rotation speed N is 700 or less, a reduction effect of about 2.0 kPa is obtained for the test cylinder liner 10-A and about 4.0 kPa is obtained for the test cylinder liner 10-B, based on the test cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses). Generally, when the surface roughness Ra of the inner surface of the cylinder liner is 0.120 (μm) or less, the lubricating oil retention ability decreases and the lubricating oil becomes insufficient. However, in this embodiment, the recess 14 formed overlapping the central low-roughness region 22 functions as a lubricating oil storage area for the central low-roughness region 20, thereby achieving a synergistic effect of making it less likely for lubricating oil shortage to occur in the central low-roughness region 20.

[0132] In the above-mentioned low friction loss state created by the central low roughness region 22, if the sagging region 200 is further formed in the recess 14, this low friction loss state can be maintained for a long period of time.

[0133] Furthermore, in the test cylinder liners 10-A and 10-B, the friction loss in the high rotation region where the rotation speed N exceeds 700 is close to or smaller than that of the test cylinder liner X (surface roughness Ra 0.160 (μm) / with recesses). In other words, in the test cylinder liners 10-A and 10-B, the central low-roughness region 20 does not adversely affect the friction loss even in the high rotation region.

[0134] Incidentally, this FMEP diagram is based on friction tests in a non-combustion state. Therefore, the FMEP value of an actual internal combustion engine where actual combustion occurs will be higher than this because of the effect of combustion pressure.

[0135] The FMEP diagram in FIG. 12(B) shows the estimated FMEP of the test cylinder liner K, which has the same configuration as the test cylinder liner 10-A and has a surface roughness Ra of 0.140 (μm) in the central low roughness region 20, although it is not an actual measurement value. The FMEP diagram of the test cylinder liner K is estimated to be close to the intermediate value between the FMEP diagrams of the test cylinder liners X and Y, which have a surface roughness Ra of 0.160 (μm), and the FMEP diagram of the test cylinder liner 10-A, which has a surface roughness Ra of 0.120 (μm). The range in which the friction loss mean effective pressure T becomes smaller than 14 kPa for this test cylinder liner K also expands to the range where the rotation speed N is 700 or less (left side of the graph). As a result, even at low rotation speeds, the sliding mode with extremely low friction loss is achieved.

[0136] That is, in the verification cylinder liner K, even in a low rotation region where the rotation speed N is 700 or less, friction loss is suppressed by maintaining the fluid lubrication region 114 or the mixed lubrication region 113. Also, even if a boundary lubrication state is reached, friction loss is suppressed because the surface roughness is small. Generally, when the surface roughness Ra of the inner peripheral surface of the cylinder liner is 0.140 (μm) or less, the lubricant retention power decreases and the lubricant is easily insufficient. However, in the verification cylinder liner, the recess 14 formed overlapping the central low roughness region 22 functions as a lubricant storage portion for the central low roughness region 20, so a synergistic effect is obtained in which the lubricant is less likely to be insufficient in the central low roughness region 20.

[0137] <Oil consumption>

[0138] In the case of the sliding structure of the cylinder liner 10 and the piston 30 of this embodiment, the amount of oil consumption is also suppressed. This is due to the fact that the absolute amount of the lubricating oil film formed in the central low roughness region 22 is reduced. Even if the absolute amount of the oil film is reduced, the recess 14 having the sagging region 200 is formed in an overlapping manner, so that the lubricating oil in the recess 14 is smoothly guided to the surrounding inner wall surface 12, and there is no lack of lubrication. In other words, in this embodiment, it is possible to rationally solve both the reduction of oil consumption and sufficient lubrication effect.

[0139] <Cylinder liner according to the second embodiment>

[0140] Next, a sliding structure for an internal combustion engine according to a second embodiment of the present invention will be described with reference to the accompanying drawings.

[0141] As shown in Fig. 14(A), a plurality of recesses 14 are formed on the inner wall surface 12 of the cylinder liner 10 according to the internal combustion engine of the second embodiment. The recesses 14 are formed with a sagging region 200 similar to that of the first embodiment. The recesses 14 are formed only in a stroke center region 20 on the inner wall surface 12. The stroke center region 20 is a part of the entire range (hereinafter referred to as a reference stroke region 19) from a lower surface position 19A (hereinafter also referred to as a top dead center side edge) of the ring groove of the lowest piston ring at the top dead center T of the piston 30 to an upper surface position 19B (hereinafter also referred to as a bottom dead center side edge) of the ring groove of the uppermost piston ring at the bottom dead center U of the piston 30.

[0142] In this embodiment, the stroke mid-region 20 is located below the top dead center edge 19A of the reference stroke region 19. As a result, a smooth upper smooth region 130A having no recesses is formed in the entire area between the top dead center edge 19A of the reference stroke region 19 and the top dead center edge 27A of the stroke mid-region 20. The upper smooth region 130A is the region through which the piston ring 40 passes.

[0143] In addition, the stroke mid-region 20 of this embodiment is located above the bottom dead center edge 19B of the reference stroke region 19. As a result, a smooth lower smooth region 130B having no recesses is formed in the entire area between the bottom dead center edge 19B of the reference stroke region 19 and the bottom dead center edge 27B of the stroke mid-region 20. The lower smooth region 130B is the region through which the piston ring 40 passes.

[0144] In this embodiment, the edge 27A on the top dead center side of the stroke mid-region 20 may be referred to as the "upper boundary" meaning the boundary between the place where the recess 14 is formed and the place where the recess 14 is not formed, and the edge 27B on the bottom dead center side of the stroke mid-region 20 may be referred to as the "lower boundary" meaning the boundary between the place where the recess 14 is formed and the place where the recess 14 is not formed. The edge (lower boundary) 27B on the bottom dead center side of the stroke mid-region 20 may be aligned with the bottom dead center side edge 19B of the reference stroke region 19 or may be extended below that.

[0145] If the area outside the stroke mid-region 20 is defined as the outer region 25, this outer region 25 is composed of an upper outer region 25A adjacent to the top dead center side of the stroke mid-region 20, and a lower outer region 25B adjacent to the bottom dead center side of the stroke mid-region 20. Note that a portion of the upper outer region 25A includes the upper smooth region 130A, and a portion of the lower outer region 23B includes the lower smooth region 130B.

[0146] In this embodiment, a central low-roughness region 22 is formed in at least a portion of the stroke central region 20, in which the surface roughness (arithmetic mean roughness) Ra measured by a stylus-type surface roughness measuring device is 0.140 (μm) or less, and preferably the surface roughness Ra is 0.120 (μm) or less. Here, the entire stroke central region 20 is defined as the central low-roughness region 22. As a result, the central low-roughness region 22 includes the vicinity of the upper end edge and the vicinity of the lower end edge of the stroke central region 20 where the recess 14 is formed.

[0147] The surface roughness values ​​of this central low-roughness region 22 when measured by a non-contact surface roughness measuring device using a laser microscope are shown below. Arithmetic mean height Sa (μm): 0.192 or less, preferably 0.163 or less, and more preferably 0.120 or less (specifically, set to 0.110). Protruding peak height Spk (μm): 0.159 or less, preferably 0.144 or less, and more preferably 0.121 or less (specifically, set to 0.116). Core level difference Sk (μm): 0.521 or less, preferably 0.449 or less, and more preferably 0.340 or less (specifically, set to 0.315). Protruding valley depth Svk (μm): 0.409 or less, preferably 0.342 or less, and more preferably 0.241 or less (specifically, set to 0.218).

[0148] Furthermore, in the upper smooth region 130A adjacent to the upper side of the stroke central region 20, an upper low roughness region 23A is formed in which the surface roughness (arithmetic mean roughness) Ra is 0.140 (μm) or less, preferably 0.120 (μm) or less, as measured by a stylus surface roughness measuring device. The upper low roughness region 23A of this embodiment is formed to a range that overlaps the upper smooth region 130A and extends further upward. In addition, in the lower smooth region 130 adjacent to the lower side of the stroke central region 20, a lower low roughness region 23B is formed in which the surface roughness (arithmetic mean roughness) Ra is 0.140 (μm) or less, preferably 0.120 (μm) or less. The lower low roughness region 23B of this embodiment is formed to a range that overlaps the lower smooth region 130B and extends further downward. The upper low-roughness region 23A, the central low-roughness region 22, and the lower low-roughness region 23B are completely connected with a uniform surface roughness, and as a whole form an integral, continuous flat surface.

[0149] The surface roughness values ​​of the upper low-roughness region 23A and / or the lower low-roughness region 23B when measured by a non-contact surface roughness measuring device using a laser microscope are shown below. Arithmetic mean height Sa (μm): 0.192 or less, preferably 0.163 or less, and more preferably 0.120 or less (specifically, set to 0.110). Protruding peak height Spk (μm): 0.159 or less, preferably 0.144 or less, and more preferably 0.121 or less (specifically, set to 0.116). Core level difference Sk (μm): 0.521 or less, preferably 0.449 or less, and more preferably 0.340 or less (specifically, set to 0.315). Protruding valley depth Svk (μm): 0.409 or less, preferably 0.342 or less, and more preferably 0.241 or less (specifically, set to 0.218).

[0150] As the piston 30 reciprocates within the cylinder liner 10, it repeatedly passes through the upper outer region 25A (upper low roughness region 23A), the stroke central region 20 (central low roughness region 22), the lower outer region 25B (lower low roughness region 23B), the stroke central region 20 (central low roughness region 22), and the upper outer region 25A (upper low roughness region 23A) in this order.

[0151] The stroke direction distance of the upper smooth region 130A is preferably set to 30% or more of the total stroke direction distance of the reference stroke region 19. In addition, the stroke direction center point 20M of the stroke mid-region 20 is located closer to the bottom dead center U of the piston than the stroke direction center point 19M of the reference stroke region.

[0152] If the position where the uppermost piston ring (top ring 50 described later) passes over the inner wall surface 12 at the highest speed is defined as the fastest passing point C, the edge (upper boundary) 27A on the top dead center side of the stroke central region 20 is set to be lower than the fastest passing point C. In this embodiment, the edge 27A on the top dead center side and the fastest passing point C are set to coincide with each other.

[0153] The central low-roughness region 22, the upper low-roughness region 23A, and the lower low-roughness region 23B of the cylinder liner 10 are formed by honing using a honing machine. The grain size of the honing stone used in this process is preferably finer than F500 or #800 (JIS R 6001-2:2017, ISO8486-2:2007). After the central low-roughness region 22, the upper low-roughness region 23A, and the lower low-roughness region 23B are formed by this honing process, it is preferable not to perform a chemical conversion treatment on the surface. For example, if a phosphate coating or the like that is generally used in the manufacturing process of the cylinder liner 10 is performed, the surface properties of the central low-roughness region 22, the upper low-roughness region 23A, and the lower low-roughness region 23B will vary depending on the coating. On the other hand, if the coating has a small variation in surface properties, it is also desirable to apply a chemical conversion treatment to the central low roughness region 22, the upper low roughness region 23A, and the lower low roughness region 23B.

[0154] <Effect of sagging area>

[0155] According to the cylinder liner 10 of the second embodiment, a sag region 200 is formed near the edge that is the boundary between the recess 14 and the inner wall surface 12, thereby reducing interference between the piston ring 40 and the sag region 200. As a result, when the fluid lubrication region 114 or the mixed lubrication region 113 is formed between the piston ring 40 and the recess 14 and the inner wall surface 12 around it, an oil film is stably formed and frictional force is reduced.

[0156] In addition, although actual measurement and analysis are difficult, it is speculated that forming the sag region 200 in the recess 14 will smooth the flow of lubricating oil in and out of the recess 14. As a result, it is speculated that the lubricating oil in the recess 14 will be smoothly guided to the surrounding inner wall surface 12, which will stabilize the formation of an oil film on the inner wall surface 12.

[0157] Furthermore, the presence of the sagging region 200 makes it difficult for interference to occur between the piston ring 40 and the edge of the recess 14, thereby suppressing damage to the edge of the recess 14 and the piston ring. As a result, it is possible to reduce the occurrence of hairline scratches formed in the upper low roughness region 23A by chipped pieces.

[0158] <Significance of existence of upper low roughness region 23A>

[0159] As already described, in this embodiment, the upper low roughness region 23A, where no recesses are formed, is provided on the top dead center side of the central low roughness region 22 where the recesses are formed in an overlapping manner. The significance of this upper low roughness region 23A is as follows. The top dead center side of the piston 30 is a high temperature environment because of the presence of a combustion chamber. Therefore, if a recess is formed on the top dead center side of the cylinder liner 10 and engine oil is allowed to remain in the recess, the engine oil will become hot and vaporize, increasing the oil consumption. Therefore, the upper low roughness region 23A is not formed with a recess, thereby suppressing the oil consumption. On the other hand, if the upper low roughness region 23A is made low rough as in this embodiment, there is a possibility that lubrication shortage will occur, or the recess 14 formed in an overlapping manner with the central low roughness region 22 adjacent to the lower side functions as a lubricating oil reservoir, and lubricating oil is actively supplied to the upper low roughness region 23A through this recess 14, so a synergistic effect is obtained in which lubrication shortage is unlikely to occur.

[0160] In addition, the viscosity of the engine oil is reduced due to the high temperature environment on the side of the top dead center T of the piston 30, so that an oil film is difficult to form, but by making the surface roughness Ra 0.120 (μm) or less, preferably 0.100 (μm) or less by the upper low roughness region 23A, even if the oil film is small, it is made into a fluid lubrication region or a mixed lubrication region. Even if it becomes a boundary lubrication region, the roughness is low, so the friction coefficient is small. Furthermore, since the lubricating oil can be supplied to the upper low roughness region 23A by the lubricating oil stored in the recess 14 formed overlapping the central low roughness region 22, there is an advantage that the lubricating oil shortage in the upper low roughness region 23A is unlikely to occur.

[0161] <Sliding structure of cylinder liner and piston ring according to the second embodiment>

[0162] 14(B) shows the stroke of the piston ring 40 moving relatively through the cylinder liner 10 from top dead center T toward bottom dead center U. While the piston ring 40 moves relatively through the upper low roughness region 23A, the stroke lines are A and L. Then, when the piston ring 40 passes through the central low roughness region 22, the stroke line is M. Thereafter, while the piston ring 40 moves relatively through the lower low roughness region 23B of the cylinder liner 10 toward the bottom dead center, the stroke lines are N and B.

[0163] According to the sliding structure for the internal combustion engine of the second embodiment, as in the first embodiment, the friction coefficient during low-speed movement can be reduced by the central low-roughness region 22. Also, it is possible to reduce friction loss during low rotation. Furthermore, the oil consumption can be suppressed by the upper low-roughness region 23A through which the piston ring 40 passes.

[0164] Incidentally, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0165] 10 Cylinder liner 12 Inner wall surface 14 Recess 20 Center area of ​​the journey 22 Central low roughness area 23A Upper low roughness area 23B Lower low roughness area 25 External Area 25A Upper external area 25B Lower external area 30 Piston 40 Piston ring 110 Solid contact area 112 Boundary lubrication region 113 Mixed lubrication area 114 Fluid lubrication area 200 Sagging area

Claims

1. A cylinder in which a piston having a piston ring slides on an inner wall surface, a plurality of recesses are formed in the inner wall surface in a stroke center region that corresponds to the whole or a part of a region between a lower surface position of a ring groove of a lowest piston ring at a top dead center of the piston and an upper surface position of a ring groove of a top piston ring at a bottom dead center of the piston, A sagging region defined by the following regional conditions is formed on the side surface of the recess, (Area condition) The shapes of at least two of the recesses arranged in the cylinder axial direction, including the adjacent inner wall surfaces, are measured with a stylus-type surface roughness tester (JIS B 0651:2001) with a stylus tip radius of 2 μm and a cutoff value (wavelength) λs for the profile curve set to 2.5 μm, and the profile curve is obtained as a reference: A line obtained by extracting the multiple inner wall surfaces and converting them into a single straight line using the least squares method is defined as the inner wall surface reference height line: A line offset by 0.15 μm from the inner wall surface reference height line toward the bottom side of the recess is defined as an upper reference height line, which means the upper edge of the sagging region: A line offset by 0.50 μm from the upper reference height line toward the bottom side of the recess is defined as a lower reference height line, which means the lower edge of the sagging region: The line segment connecting the upper point where the cross-sectional curve extending from the inner wall surface toward the bottom surface of the recess first intersects with the upper reference height line and the lower point where the cross-sectional curve first intersects with the lower reference height line in the vicinity of the boundary with the inner wall surface is defined as a virtual sag cross section of the sag region: When the width of the virtual sag cross section in the sag region in the cylinder axial direction is defined as a sag width DW, and the height of the virtual sag cross section in the cylinder radial direction is defined as a sag height DH, the absolute value of the gradient DH / DW based on the inner wall surface is set within the range of 0.00625 to 0.0167. A cylinder characterized in that

2. At least a part of the surface in contact with the piston ring in the stroke central region is provided with a central low roughness region having an arithmetic mean roughness Ra of a profile curve measured by a stylus type surface roughness tester of 0.140 μm or less.

2. The cylinder of claim 1.

3. The recess having the sagging region is formed within the range of the central low roughness region.

3. The cylinder according to claim 2.

4. The arithmetic mean height Sa of the contour curved surface of the central low roughness region measured by a non-contact surface roughness measuring device is 0.20 μm or less. A cylinder according to claim 2 or 3.

5. The protruding valley depth Svk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.41 μm or less. A cylinder according to any one of claims 2 to 4.

6. The central low roughness region has a protruding peak height (Spk) of 0.16 μm or less as measured by a non-contact surface roughness measuring device. A cylinder according to any one of claims 2 to 5.

7. The level difference Sk of the core portion of the central low roughness region measured by a non-contact surface roughness measuring device is 0.53 μm or less. A cylinder according to any one of claims 2 to 6.

8. The present invention is characterized in that, when the height of the protruding peaks in the central low roughness region measured by a non-contact surface roughness measuring device is E (Spk) and the depth of the protruding valleys is I (Svk), the ratio I / E is 2.6 or less. A cylinder according to any one of claims 2 to 7.

9. The arithmetic mean roughness Ra of the profile curve of the central low roughness region measured by a stylus type surface roughness measuring instrument is 0.120 μm or less. A cylinder according to any one of claims 2 to 8.

10. A sliding structure of an internal combustion engine having a cylinder and a piston, The cylinder is a plurality of recesses are formed in an inner wall surface of the piston in a stroke center region that corresponds to the whole or part of a region between a lower surface position of a ring groove of a lowest piston ring at a top dead center of the piston and an upper surface position of a ring groove of a top piston ring at a bottom dead center of the piston; A sagging region defined by the following regional conditions is formed on the side surface of the recess, (Area condition) The shapes of at least two of the recesses arranged in the cylinder axial direction, including the adjacent inner wall surfaces, are measured with a stylus-type surface roughness tester (JIS B 0651:2001) with a stylus tip radius of 2 μm and a cutoff value (wavelength) λs for the profile curve set to 2.5 μm, and the profile curve is obtained as a reference: A line obtained by extracting the multiple inner wall surfaces and converting them into a single straight line using the least squares method is defined as the inner wall surface reference height line: A line offset by 0.15 μm from the inner wall surface reference height line toward the bottom side of the recess is defined as an upper reference height line, which means the upper edge of the sagging region: A line offset by 0.50 μm from the upper reference height line toward the bottom side of the recess is defined as a lower reference height line, which means the lower edge of the sagging region: The line segment connecting the upper point where the cross-sectional curve extending from the inner wall surface toward the bottom surface of the recess first intersects with the upper reference height line and the lower point where the cross-sectional curve first intersects with the lower reference height line in the vicinity of the boundary with the inner wall surface is defined as a virtual sag cross section of the sag region: When defining the width of the virtual sag section in the sag region in the cylinder axial direction as the sag width DW and the height of the virtual sag section in the cylinder radial direction as the sag height DH, the absolute value of the gradient DH / DW based on the inner wall surface is set within the range of 0.00625 to 0.0167. Sliding structure of an internal combustion engine.

11. At least a part of the surface of the cylinder in the stroke central region that comes into contact with the piston ring is provided with a central low roughness region having an arithmetic mean roughness Ra of a profile curve measured by a stylus-type surface roughness tester of 0.140 μm or less. The sliding structure of an internal combustion engine according to claim 10.

12. The recess having the sagging region is formed within the range of the central low roughness region. The sliding structure of an internal combustion engine according to claim 11.

13. The arithmetic mean height Sa of the contour curved surface of the central low roughness region measured by a non-contact surface roughness measuring device is 0.20 μm or less. The sliding structure of an internal combustion engine according to claim 11 or 12.

14. The protruding valley depth Svk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.41 μm or less. The sliding structure for an internal combustion engine according to any one of claims 11 to 13.

15. The central low roughness region has a protruding peak height (Spk) of 0.16 μm or less as measured by a non-contact surface roughness measuring device. The sliding structure for an internal combustion engine according to any one of claims 11 to 14.

16. The level difference Sk of the core portion of the central low roughness region measured by a non-contact surface roughness measuring device is 0.53 μm or less. The sliding structure for an internal combustion engine according to any one of claims 11 to 15.

17. The present invention is characterized in that, when the height of the protruding peaks in the central low roughness region measured by a non-contact surface roughness measuring device is E (Spk) and the depth of the protruding valleys is I (Svk), the ratio I / E is 2.6 or less. The sliding structure for an internal combustion engine according to any one of claims 11 to 16.

18. The arithmetic mean roughness Ra of the profile curve of the central low roughness region measured by a stylus type surface roughness measuring instrument is 0.120 μm or less. The sliding structure for an internal combustion engine according to any one of claims 11 to 17.

19. The central low roughness region includes a vicinity of an upper end edge and a vicinity of a lower end edge in the stroke central region. The sliding structure for an internal combustion engine according to any one of claims 11 to 18.

20. The entire stroke central region is the central low roughness region. The sliding structure for an internal combustion engine according to any one of claims 11 to 19.

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