Cylinder

Abstract

The present invention provides a cylinder or the like having a high-precision recess that can reduce frictional resistance, improve fuel efficiency, or reduce oil consumption. [Solution] A cylinder 10 in which a piston equipped with piston rings slides along an inner wall surface 12, wherein a plurality of recesses 14 formed by laser processing are formed in the stroke center region of the inner wall surface, which is all or part of the region from the lower surface position of the ring groove of the lowest piston ring at the piston's top dead center to the upper surface position of the ring groove of the highest piston ring at the piston's bottom dead center, and the absolute load gradient of the first essential region defined by the following procedure in the recesses 14 is 500 or more.

Description

Technical Field

[0001] The present invention relates to a cylinder having a recess formed in an inner wall surface.

Background Art

[0002] Conventionally, in an internal combustion engine 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 present applicant 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), and by forming a plurality of recesses in the central region of the stroke of the inner wall surface of the cylinder, etc., the sliding resistance during operation is reduced. These recesses are generally formed by so-called blasting.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When forming a recess by blasting, it is difficult to stabilize the recess shape, the roughness of the recess bottom surface, etc. due to various factors such as grid variations and wear of the grid discharge nozzles. Specifically, due to the mixing of grids of sizes outside the specification, local irregularities are likely to be formed on the side surfaces and bottom surfaces of the recesses. When a local protrusion is formed on the bottom surface, the tip thereof contacts the piston and damages the piston. In addition, it is assumed that this local irregularity hinders the flow of lubricating oil entering and leaving the inside and outside of the recess.

[0005] In view of such circumstances, the present invention aims to provide a cylinder having highly accurate recesses and the like.

Means for Solving the Problems

[0006] The present invention, which achieves the above objective, is a cylinder in which a piston equipped with piston rings slides along an inner wall surface, wherein a plurality of recesses are formed by laser processing in the stroke center region of the inner wall surface, which is all or part of the region from the lower surface position of the ring groove of the lowest piston ring at the piston's top dead center to the upper surface position of the ring groove of the highest piston ring at the piston's bottom dead center, and the absolute load gradient of the first essential region defined by the following procedure in the recesses is 500 or more. (Definition of the first essential region) - Measure a cross-sectional curve that includes four consecutive recesses in the evaluation length. - For the cross-sectional curve, calculate a load absolute value curve with the cut level (μm) indicating the depth direction of the recess on the vertical axis and the load length (μm) on the horizontal axis. - For the load absolute value curve, define the range of the cut level ratio (%) from 25% to 65% as the first essential region on the side surface of the recesses. (Definition of absolute load gradient) - Extract the partial curve of the first essential region of the absolute load value curve, and calculate the cut level height difference PH1 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW1 (μm), which is the horizontal axis distance of the partial curve. - Calculate the absolute load gradient PW1 / PH1 of the first essential region.

[0007] In relation to the cylinder described above, the absolute load gradient of the second essential region defined in the recess by the following procedure may be 500 or more. (Definition of the second essential region) · With respect to the absolute load curve, the range of cut level ratio (%) from 25% to 60% is defined as the second essential region on the side surface of the recess. (Definition of the absolute load gradient) · Extract the partial curve of the second essential region of the absolute load curve, and calculate the cut level height difference PH2 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW2 (μm), which is the horizontal axis distance of the partial curve. · Calculate the absolute load gradient PW2 / PH2 of the second essential region.

[0008] In relation to the cylinder described above, the absolute load gradient of the third essential region defined in the recess by the following procedure may be 500 or more. (Definition of the third essential region) The third essential region on the side surface of the recess is defined as the range of the cut level ratio (%) from 25% to 55% for the absolute load curve. (Definition of the absolute load gradient) A partial curve of the third essential region of the absolute load curve is extracted, and the cut level height difference PH3 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW3 (μm), which is the horizontal axis distance of the partial curve, are calculated. The absolute load gradient PW3 / PH3 of the third essential region is calculated.

[0009] In relation to the cylinder described above, the absolute load gradient of the fourth essential region defined in the recess by the following procedure may be 500 or more. (Definition of the fourth essential region) The fourth essential region on the side surface of the recess is defined as the range of the cut level ratio (%) from 25% to 50% for the absolute load curve. (Definition of the absolute load gradient) A partial curve of the fourth essential region of the absolute load curve is extracted, and the cut level height difference PH4 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW4 (μm), which is the horizontal axis distance of the partial curve, are calculated. The absolute load gradient PW4 / PH4 of the second essential region is calculated.

[0010] In relation to the cylinder described above, the recess may be characterized by being processed by a laser having a pulse width of 100 picoseconds or less.

[0011] In relation to the cylinder described above, the first main portion region of the recess may be characterized by having a tapered surface from the outside to the inside of the recess, based on an increase in the number of superimposed irradiations of the laser spot in the laser processing. [Effects of the Invention]

[0012] According to the present invention, excellent effects such as reducing frictional resistance, improving fuel efficiency, or reducing oil consumption can be achieved. [Brief explanation of the drawing]

[0013] [Figure 1] This is a cross-sectional view along the axial direction of a cylinder liner applied to an internal combustion engine according to an embodiment of the present invention. [Figure 2] (A) and (B) are unfolded diagrams showing the inner circumferential wall of the cylinder liner in a circumferentially unfolded state. [Figure 3] (A) and (B) are cross-sectional views of the inner circumferential wall of the cylinder liner perpendicular to its axis. [Figure 4] (A) and (B) are cross-sectional views showing the cross-sectional curves of the inner circumferential wall and recess of the cylinder liner. [Figure 5] (A) is a cross-sectional view showing the cross-sectional curve of the inner circumferential wall and recess of the cylinder liner, (B) is a load ratio curve calculated based on the same cross-sectional curve, and (C) is a load absolute value curve calculated based on the same cross-sectional curve. [Figure 6] This figure shows an enlarged view of the absolute load curve in Figure 5(C). [Figure 7] (A) is a side view showing the piston and piston rings applied to the internal combustion engine, (B) is a partially enlarged cross-sectional view showing the piston and piston rings, (C) is a partially enlarged cross-sectional view of the top ring, and (D) is a partially enlarged cross-sectional view of the second ring. [Figure 8] (A) is a cross-sectional view of a two-piece type oil ring, and (B) is a cross-sectional view of a three-piece type oil ring. [Figure 9] (A) Stribeck diagram and (B) FMEP diagram relating to the sliding of a typical internal combustion engine. [Figure 10] This is a perspective view showing the recess machining equipment for the cylinder liner. [Figure 11] Figures (A) to (D) show the machining procedure for the recess of the cylinder liner, and figures (E) and (F) show modified examples of the laser scanning method. [Figure 12] (A) is a cross-sectional view in the scanning direction, and (B) is a cross-sectional view in the direction perpendicular to the scanning direction, showing the machining procedure for the recess in the cylinder liner. [Figure 13] Figures (A) to (D) show the procedure for machining multiple recesses in the cylinder liner together. [Figure 14] (A) is the absolute load value curve of the cylinder liner of the first embodiment, and (B) is the absolute load value curve of the cylinder liner of the first comparative example. [Figure 15] (A) is the absolute load value curve of the cylinder liner of the second embodiment, and (B) is the absolute load value curve of the cylinder liner of the second comparative example. [Figure 16] It is a chart showing the absolute load gradients of the first to fourth main part regions of the first embodiment, the second embodiment, the first comparative example, and the second comparative example. [Figure 17] (A) to (C) are graphs showing the surface roughness of the bottom surface of the recess of the same cylinder liner. [Figure 18] (A) is a front view showing a modified example of the recess processing device. [Figure 19] (A) to (D) are perspective views conceptually showing the laser scanning paths in the same recess processing device. [Figure 20] [[ID=2)0]]It is a development view showing the state in which the inner peripheral wall of the same cylinder liner is developed in the circumferential direction. [Figure 21] It 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.

MODE FOR CARRYING OUT THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. First, the cylinder of the internal combustion engine according to the embodiment of the present invention will be described in detail. The cylinder of the present embodiment is exemplified by a cylinder liner that is fitted into an engine block when the internal combustion engine is a diesel engine, but the present invention is not limited to this and can be applied to cylinders of other types of internal combustion engines such as gasoline engines.

[0015] <Cylinder liner>

[0016] As shown in Figure 1, a plurality of recesses 14 are formed on the inner wall surface 12 of the cylinder liner 10 in the internal combustion engine of this embodiment. The recesses 14 are formed in the stroke center region 20 on the inner wall surface 12. This stroke center region 20 is defined as the area 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, and is all or part of this area (here, the case where the entire area is the stroke center region 20 and recesses 14 are formed throughout it is illustrated). If the area outside the stroke center region 20 is defined as the external region 25, this external region 25 consists of an upper external region 25A adjacent to the top dead center side of the stroke center region 20 and a lower external region 25B adjacent to the bottom dead center side of the stroke center region 20. As the piston 30 reciprocates within the cylinder liner 10, it repeatedly passes through the upper outer region 25A, the stroke center region 20, the lower outer region 25B, the stroke center region 20, and the upper outer region 25A in this order. The boundary between the upper outer region 25A and the stroke center region 20 is defined as the upper boundary 27A, and the boundary between the lower outer region 25B and the stroke center region 20 is defined as the lower boundary 27B.

[0017] Of course, it is possible to form multiple recesses 14 beyond the central stroke region 20, but from the viewpoint of oil consumption (LOC), it is preferable to form the recesses 14 exclusively within the central stroke region 20.

[0018] <Dimples formed in the cylinder liner>

[0019] The recesses 14 are arranged such that, regardless of the location of the cross-section perpendicular to the axis on the inner wall surface 12 of the stroke central region 20, at least one recess 14 exists in that cross-section. That is, the recesses 14 are arranged to overlap in the direction of the cylinder axis. As a result, the outer circumferential surface of the piston ring passing through the stroke central 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.

[0020] The shape of the recess 14 is a polygon (square, rectangle, or hexagon) positioned diagonally with respect to the cylinder axis direction, and a hexagon is used here. In this case, as shown in the unfolded view in Figure 2(A), when focusing on a particular recess 14, the lowest point 14b in the axial direction of that recess 14 is located axially lower than the highest point 14a in the cylinder axis direction of other recesses 14. In this way, multiple recesses 14 overlap in the cylinder axis direction, so that recesses 14 can always be present in the cross section perpendicular to the axis at any location in the stroke central region 20 (for example, in the direction of arrow A, arrow B, and arrow C). Here, multiple recesses 14 with the same area are uniformly arranged in the surface direction (in the cylinder axis direction and the cylinder circumferential direction) in the stroke central region 20.

[0021] Specifically, as shown in the exploded view of Figure 2(B), multiple recesses 14 are always present in the band-shaped regions 20P, 20Q, and 20R where the piston ring contacts the central stroke region 20.

[0022] The dimensions and shape of the recess 14 are not particularly limited, but are appropriately selected according to the dimensions and purpose of the cylinder and piston ring. For example, the recess 14 can be formed in a slit shape or a strip shape so as to penetrate (or extend) through the cylinder axial direction in the central stroke region 20. On the other hand, from the viewpoint of the airtightness of the cylinder, it is preferable that the maximum average length J of the recess 14 in the cylinder axial direction (see Figure 2(A)) be less than or equal to the cylinder axial length (width) of the piston ring (top ring) located at the uppermost position of the piston, specifically about 5 to 100% thereof. The average length J of the recess 14 means the average value if there is variation in the maximum dimensions in the cylinder axial direction of multiple recesses 14. Also, if the lengths of the recesses 14 are mixed, it is preferable that the maximum dimension is used as the average length J and that it does not exceed the cylinder axial length (width) of the piston ring (top ring) located at the uppermost position of the piston. The maximum average length J of the recess 14 in the cylinder axial direction is preferably in the range of 0.1 mm to 15 mm, and more preferably in the range of 0.1 mm to 5 mm. More preferably it is 0.1 mm to 1.0 mm.

[0023] The maximum average length S of the recess 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 0.1 mm to 5 mm. More preferably, it is 0.1 mm to 1.0 mm. If it is smaller than these ranges, the sliding surface reduction effect of the recess 14 itself 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, which may cause problems such as deformation of the piston ring or contact with the bottom surface of the recess if the depth of the recess 14 is shallow.

[0024] As shown in Figure 3(A), the maximum average length R (maximum average depth R) of the recess 14 in the cylinder radial direction is preferably in the range of 0.1 μm to 1000 μm, and more preferably in the range of 0.1 μm to 500 μm. More preferably, it is set to 0.1 μm to 50 μm, and even more preferably to 0.1 μm to 10 μm. Even more preferably, the recess 14 is machined so that its maximum average depth R is 0.5 μm or more. In this embodiment, the maximum average depth R is set to 1.5 μm to 2.5 μm. If the maximum average length R of the recess 14 in the cylinder radial direction is smaller than these ranges, the sliding surface reduction effect of the recess 14 itself may not be sufficiently obtained, or the bottom surface 14A of the recess 14 may come into contact with the piston ring. The definition of the maximum average depth R will be described later.

[0025] On the other hand, if the maximum average depth R is made larger than these ranges, the machining time will be too long, or the thickness of the cylinder wall will need to be increased. Note that, for the sake of explanation, the recess 14 in Figure 3 is drawn with a significant exaggeration in the direction of the maximum average length R relative to the direction of the maximum average length J.

[0026] Returning to Figure 2, the average value of the minimum circumferential distance Hc between adjacent recesses 14 in the cylinder circumferential direction at the same position in the cylinder axial direction 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. More preferably, it should be 0.1 mm to 1.0 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, making stable sliding impossible. On the other hand, if it is larger than these ranges, the sliding area reduction effect of the recess 14 itself may not be sufficiently obtained.

[0027] The average minimum distance Ha in the cylinder axial direction between adjacent recesses 14 located at the same position in the cylinder circumferential direction 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. More preferably, it is 0.1 mm to 1.0 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, making stable sliding impossible. On the other hand, if it is larger than these ranges, the sliding area reduction effect of the recess 14 itself may not be sufficiently obtained.

[0028] Furthermore, regardless of direction, the average 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 sliding surface area reduction effect of the recesses 14 themselves may not be sufficiently obtained.

[0029] These intervals Hc, Ha, and Hm are, in other words, equivalent to the minimum width in each direction of the inner wall surface 12 remaining between adjacent recesses 14. Therefore, as will be described in detail in Figure 4(B), these intervals Hc, Ha, and Hm represent the distance between the evaluation starting point GS that first intersects the upper reference height line G1 of one recess 14 and the evaluation starting point GS that first intersects the upper reference height line G1 of the other recess 14 adjacent to it.

[0030] <Detailed shape definition within the dimple>

[0031] As shown in an enlarged view in Figure 3(B), the recess 14 has a bottom surface 14A and a side surface 300. Furthermore, a so-called sagging region 200 is formed near the edge of the side surface 300 that approaches the inner wall surface 12. This sagging region 200 is a gently sloping surface that begins to drop into the recess 14, with the inner wall surface 12 as the reference plane. The sagging region 200 can be considered as a part of the side surface 300 of the recess 14, and is particularly close to the inner wall surface 12. Alternatively, this sagging region 200 can be considered as a part 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.

[0032] The bottom surface 14A and the sagging area 200 are formed by further polishing the recess 14, which is formed by laser processing as described later. By performing the polishing process particularly carefully, the bottom surface 14A of the recess 14 is made even smoother, and at the same time, a smooth sagging area 200 is formed.

[0033] In this embodiment, in order to objectively evaluate the shape of the bottom surface 14A (maximum average depth and bottom surface roughness) and the side surface 300 (including the sagging region 200), the bottom surface 14A and the side surface 300 are defined as follows.

[0034] (Measurement of cross-sectional curve) As shown in Figure 4(A), the shape of at least two recesses 14 located at the same position in the circumferential direction of the cylinder and aligned in the axial direction of the cylinder is measured using a stylus-type surface roughness measuring instrument (JIS B 0651:2001), passing through the location where the minimum distance Ha between the recesses is reached and including the adjacent inner wall surface 12. The tip radius of the measuring needle is a standard 2 μm, and the cutoff value (wavelength) λs for the cross-sectional curve is selected to 2.5 μm for measurement. The measured cross-sectional curve DK includes at least the two adjacent recesses 14, the pair of inner wall surfaces 12A and 12C extending outward from both sides of the two recesses 14 in the axial direction of the cylinder, and the inner wall surface 12B located between the two recesses 14.

[0035] (Definition of base) As shown in Figure 4(A), in the cross-sectional curve DK, the range of three inner wall surfaces 12A to 12C (two recesses 14) is extracted, and these inner wall surfaces 12A to 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.

[0036] The actual wall surfaces 12A to 12C contain fine irregularities corresponding to surface roughness. Since these fine irregularities can be evaluated using Ra (arithmetic mean roughness), when defining the starting point of the side surface 300, in order to exclude the influence of these irregularities, a line offset by an amount equivalent to Ra from the inner wall surface reference height line GK toward the base side of the recess 14 is defined as the upper reference height line G1. However, even if Ra is small, offsetting by at least 0.2 μm excludes the influence of irregularities in the boundary (edge) shape between the wall surfaces 12A to 12C and the side surface 300. In other words, the offset amount of the upper reference height line G1 is selected from the larger of 0.2 μm or the Ra (arithmetic mean roughness) of the wall surfaces 12A to 12C.

[0037] This upper reference height line G1 represents a virtual starting point in the depth direction on the side surface 300. Therefore, the point where the cross-sectional curve DK last intersects the upper reference height line G1 from the wall surfaces 12A to 12C towards the interior of the recess 14 is defined as the evaluation starting point GS on the side surface 300. The length GL connecting the two evaluation starting points GS, GS on the pair of side surfaces 300 that define the recess 14 is the axial outer dimension of the recess 14. The midpoint of this axial outer dimension GL is defined as the recess center GM, and the range of 1 / 4 GL on both sides (a total range of 1 / 2 GL) relative to the recess center GM is defined as the reference bottom surface range 14G on the bottom surface 14A.

[0038] (Definition of the maximum average depth of the recess) For the section curve DK of the reference base range 14G, it is converted into a single straight line using the least squares method. This single straight line is defined as the base reference height line BK. Alternatively, as a calculation method, the section curve DK may be enlarged 5,000 times vertically and 50 times horizontally, the third highest peak of the section curve DK may be extracted, the third lowest valley bottom of the section curve DK may be extracted, and the height midway between the peak and the valley bottom may be defined as the base reference height BK. The maximum average depth R of the recess 14 is the average of the distances between the inner wall reference height line GK and the base reference height line BK of a total of 20 recesses 14.

[0039] (Parameters for the surface roughness of the bottom surface) Furthermore, for the reference bottom surface range 14G of the total of 20 recesses 14 defined above, the core level difference Rk and the protruding valley depth Rvk (JIS B 0671-2:2002) are calculated and averaged, and these are defined as the core level difference Rk and the protruding valley depth Rvk of the bottom surface 14 of the recess 14.

[0040] The reasons for using the core level difference Rk and the protruding valley depth Rvk when evaluating the surface roughness of the reference bottom surface range 14G are as follows: A large core level difference Rk increases the risk of the piston ring contacting the reference bottom surface range 14G. If the piston ring does contact the reference bottom surface range 14G, the risk of scratches occurring during sliding increases. A large protruding valley depth Rvk increases the volume of excess oil reservoir in the recess 14, increasing oil consumption. Furthermore, the smaller both the core level difference Rk and the protruding valley depth Rvk are, the less the flow of lubricating oil moving in and out of the recess 14 is obstructed.

[0041] When measuring the surface roughness of the reference bottom surface range 14G, the following two conditions may exist. One is the surface roughness after the recesses 14 are processed with a laser and then polished (hereinafter referred to as "surface roughness after laser polishing"), and the other is the surface roughness after the recesses 14 are processed with a laser, then chemical conversion treatment (coating treatment) is performed, and then polishing is performed (hereinafter referred to as "surface roughness after laser coating and polishing"). Whether or not to perform coating treatment depends on the required specifications of the cylinder liner 10. When coating treatment is performed, the coating components penetrate into the fine irregularities of the reference bottom surface range 14G, resulting in a reduction in surface roughness.

[0042] In this embodiment, the surface roughness after laser polishing is such that the core level difference Rk is less than 1.6 μm and the protruding valley depth Rvk is less than 1.3 μm. In this embodiment, the surface roughness after laser coating and polishing is such that the core level difference Rk is less than 1.1 μm and the protruding valley depth Rvk is less than 0.9 μm.

[0043] Furthermore, the surface roughness after laser polishing shall be less than 2.9 μm, calculated by adding the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk). The surface roughness after laser coating and polishing shall be less than 2.0 μm, calculated by adding the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk).

[0044] In this embodiment, by setting the core level difference Rk of the reference bottom surface range 14G to be small as described above, the depth of the recess 14 becomes shallower, and even if the piston ring comes into contact with the bottom surface 14A, contact scratches and other damage are less likely to be formed between them. Furthermore, by actively reducing the protruding valley depth Rvk of the reference bottom surface range 14G, the fluidity of the lubricating oil in the recess 14 can be increased.

[0045] (Definition of lateral inclination angle) Next, we will explain the definition of the inclination angle of the side surface 300. As shown in Figure 4(B), the cross-sectional curve DK of the side surface 300 of the recess 14 is enlarged and extracted. The line obtained by offsetting R / 2 above the recess 14 from the bottom surface reference height line BK is defined as the lower side surface reference height line G3. This offset of the lower side surface reference height line G3 reduces the influence of the irregularities of the bottom surface 14A. The length in the cylinder radial direction (height for evaluating side surface inclination) SH when evaluating the inclination angle of the side surface 300 is defined as the distance between the upper reference height line G1 and the lower side surface reference height line G3.

[0046] The point where the cross-sectional curve DK first intersects the lower side reference height line G3 is defined as the endpoint SE for side slope evaluation. The line segment SS connecting the evaluation starting point GS of side 300 and the endpoint SE for side slope evaluation is defined as the virtual side cross-section SS of side 300. The length of the virtual side cross-section SS in the direction parallel to the line 250 (in this case, the cylinder axis direction) is defined as the width SW for side slope evaluation of side 300. Using the line 250 as a reference, the absolute value of the gradient SH / SW of this virtual side cross-section SS (hereinafter defined as the side slope SA) is calculated.

[0047] Furthermore, since the cross-sectional curve DK contains minute irregularities, variations occur with each measurement. Therefore, 60 measurements are performed on each side 300 using a stylus-type surface roughness measuring instrument (JIS B 0651:2001). The 15 data points from the maximum and 15 data points from the minimum of the 60 calculated side slopes SA are then discarded, and the average value is calculated from the remaining 30 data points. This average value is used as the evaluation side slope SA for side 300.

[0048] (Definition of the Dare domain) As shown in Figure 4(B), the cross-sectional curve DK of the side surface 300 of the recess 14 is enlarged and extracted. A line offset by 0.50 μm from the upper reference height line G1 toward the base side of the recess 14 is defined as the lower reference height line G2. This lower reference height line G2 represents the virtual end position (end height) of the sag region 200 on the side surface 300. Thus, the length DH of the sag region 200 in the cylinder radial direction (sag height) is defined as 0.50 μm.

[0049] The point where the section curve DK first intersects the lower reference height line G2 is defined as the sag endpoint GE, and the line segment GG connecting the evaluation starting point GS on the side 300 and the sag endpoint GE is defined as the virtual sag section GG of the sag region 200. The length of the virtual sag section GG in the direction parallel to the straight line 250 (here, in the direction of the cylinder axis) is defined as the sag width DW of the sag region 200. Using the straight line 250 as a reference, the absolute value of the gradient DH / DW of this virtual sag section GG (hereinafter defined as the sag gradient DA) is calculated.

[0050] Note that the cross-sectional curve DK contains fine irregularities, resulting in variations between measurements. Therefore, 60 measurements are performed using a stylus-type surface roughness measuring instrument (JIS B 0651:2001) targeting each sag region 200. The 15 data points from the maximum and 15 from the minimum of the 60 calculated sag gradients DA are then discarded, and the average value is calculated from the remaining 30 data points. This average value is used as the evaluation sag gradient DA for the sag region 200. Here, we illustrate the use of the sag gradient DA when evaluating the formation of the sag region 200, but the same result can be obtained by using the sag width DW directly for evaluation.

[0051] Incidentally, although it differs from dimple liners, which have multiple recesses arranged to overlap in the cylinder axis direction, microtexture technology exists to form similar recesses, so we will briefly explain it. Microtexture, as shown in Figure 21, is a theory that alternating regions V in which recesses are formed and regions ZX in which no recesses are present at all are repeated along the cylinder axis direction on the inner wall surface of the cylinder liner, without overlapping in the cylinder axis direction. Each time the piston ring moves along this inner wall surface, engine oil flows in and out of the recesses, and the resulting 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 can be effectively applied to any structure that reduces the contact area with the piston ring by forming multiple recesses in the central region of the stroke.

[0052] <Definition of concave shape and side slope based on load curve> Next, other definitions regarding the overall shape of the recess 14 and the gradient of the side surface 300 will be explained. For example, if the maximum average depth R of the recess 14 becomes small enough to be 10 μm or less, and furthermore, if the inclination angle of the virtual side cross section SS of the side surface 300 becomes gentler, surface roughness (fine irregularities) that are machining errors will be superimposed on the original irregular shape of the recess 14, becoming noise, and it may become difficult to separate the wall surfaces 12A~12C, the bottom surface 14A, and the side surface 300 from the geometric shape of the cross section curve DK. Therefore, in this embodiment, the load ratio curve and the absolute load value curve of the cross section curve DK are used to define the division of the overall shape of the recess 14 and the gradient of the side surface 300.

[0053] As shown in Figure 5(A), the shape of at least four recesses 14 located at the same position in the circumferential direction of the cylinder and aligned in the axial direction of the cylinder is measured using a stylus-type surface roughness measuring instrument (JIS B 0651:2001) as the cross-sectional curve DK, including the adjacent inner wall surfaces 12. The tip radius of the measuring needle is a standard 2 μm, and the cutoff value (wavelength) λs for the cross-sectional curve is selected to be 2.5 μm. The upper and lower limits of the notch treatment are set to a load length ratio of 0 to 98% and a removal length of 5 μm. The measured cross-sectional curve DK includes at least the four adjacent recesses 14 and a total of five inner wall surfaces 12A, 12B, 12C, 12D, and 12E extending outward on both sides of the four recesses 14 in the axial direction of the cylinder. If the length of the horizontal axis to be evaluated in the cross-sectional curve DK is defined as the evaluation length ln, then there are four recesses 14 and a total of five inner wall surfaces 12A, 12B, 12C, 12D, and 12E within this evaluation length ln. Specifically, the evaluation length ln is four times the pitch between recesses. The total length from the maximum height position (Hp) to the minimum height position (Lp) in the cross-sectional curve DK is defined as the maximum height difference HLp. In this embodiment, a total of 20 recesses 14 are evaluated by measuring five cross-sectional curves DK.

[0054] (Explanation of the load ratio curve) Figure 5(B) shows the load ratio curve FKr. This load ratio curve FKr is the average value of the load ratio curves calculated from each of the five cross section curves DK. In generating the load ratio curve FKr, as shown in Figure 5(A), first, the cut line CT extending horizontally from the cross section curve DK is displaced in the height direction (depth direction) over the entire range of the maximum height difference HLp. The total length of the actual portion m1 of the cross section curve DK on the cut line CT at each height position (load length Ml) is calculated, and the ratio of this load length Ml to the evaluation length ln is defined as "load length ratio: (Ml / ln) × 100 (%)" and is shown on the horizontal axis. The vertical axis of the load ratio curve FKr is the ratio of the distance c (see Figure 5(A)) from the maximum height position (Hp) to the height position of the cut level to the maximum height difference HLp (cut level ratio: (c / HLp) × 100 (%)).

[0055] Furthermore, if the cut line CT is at the maximum height position (Hp) of the cross-sectional curve DK, the load length ratio will be 0%, and if the cut line CT is at the minimum height position (Lp) of the cross-sectional curve DK, the load length ratio will be 100%.

[0056] (Explanation of the absolute load curve) Figure 5(C) shows the absolute value curve (absolute load curve FKa) obtained by converting the vertical and horizontal axes of the load ratio curve FKr shown in Figure 5(B) to absolute values. The vertical axis of the absolute load curve FKa is the distance c from the maximum height position (Hp) to the cut level (see Figure 5(A)). The horizontal axis of the absolute load curve FKa is the total length (load length Ml) of the actual portion ml of the cross section curve DK on the cut line CT at each height position. For the sake of explanation, a second vertical axis is placed to the right of the absolute load curve FKa for reference, and the cut level ratio (%) is also shown. In this embodiment, a cross section curve DK containing a total of four recesses 14 is adopted, so a total of eight sides 300 are inherent. In other words, this load length Ml is the sum of the individual load lengths of the eight inclined surfaces 300. Therefore, the value Ml / 8 can be defined as the unit load length at a unit inclined surface 300. Furthermore, since the absolute load curve FKa is the average value of the absolute load curves FKa obtained from five cross-sectional curves DK, a total of 40 side surfaces 300 are evaluated.

[0057] (Functional classification of concave areas based on load ratio curve FKr and absolute load curve FKa) The load ratio curve FKr and absolute load curve FKa function as characteristic information regarding the overall shape of the four recesses 14 that contain microscopic surface noise (surface roughness) caused by machining. For example, as shown in Figures 5(B) and 5(C), by extracting the curves in the load ratio curve FKr and absolute load curve FKa where the cut level ratio (%) is within a predetermined range, it becomes possible to classify the function of the recesses 14.

[0058] For example, extracting a curve in the range C1 where the cut level ratio (%) is from 0% to 15% primarily functions as characteristic information of the surface roughness of the wall surfaces 12A to 12E. Similarly, extracting a curve in the range C2 where the cut level ratio (%) is from 15% to 25% primarily functions as characteristic information of the sagging region 200 in the recess 14. Furthermore, extracting a curve in the range C3 where the cut level ratio (%) is from 25% to 65% primarily functions as characteristic information of the inclined shape of the key area of ​​the side surface 300 of the recess 14. Finally, extracting a curve in the range C4 where the cut level ratio (%) is from 65% to 100% primarily functions as characteristic information of the area from the bottom surface 14A to the vicinity of the boundary between the bottom surface 14A and the side surface 300 (hereinafter referred to as the bottom surface region). The reason why the area where the cut level ratio (%) exceeds 65% is defined as the bottom surface area is that, as can be seen from the cross-sectional curve DK in Figure 5(A), processing marks from laser processing tend to remain as uneven shapes on the bottom surface 14A, and this uneven shape noise is reflected in the 65% to 100% range.

[0059] As described above, the load ratio curve FKr and the absolute load curve FKa are calculated, and the sagging region 200 of the recess 14, the essential region of the side surface 300, and the bottom region are demarcated according to a predetermined numerical range of the cut level ratio (%). This allows for objective demarcation compared to defining the region from the individual geometric shapes of the cross-sectional curve DK of the recess 14.

[0060] (Definition of absolute load gradient) As shown in Figure 5(C), in the absolute load curve FKa, a portion of the curve PP is extracted, and the height difference of the cut levels of the ends PS and PE is defined as PH (μm), and the relative difference of the load lengths Ml of the ends PS and PE is defined as PW (μm). The gradient of the line segment connecting this portion of the PP with a straight line is defined as PW / PH. If the inclination angle of the line segment connecting the portion of the PP with a straight line is defined as θ with respect to the depth direction (cut level direction) of the recess 14, then this gradient PW / PH is synonymous with tan(θ). In this embodiment, this gradient PW / PH is referred to as the "absolute load gradient". The larger the value of the absolute load gradient, the gentler the inclination of the side surface 300 of the recess 14 with respect to the inner wall surface (the smaller the inclination angle with respect to the inner wall surface).

[0061] (Ranking of key areas) As shown in Figure 6, the key areas are further classified into four stages. First, the entire key area where the cut-off level (%) is between 25% and 65% is defined as the first key area Y1. Furthermore, the curve in the range of 25% to 60% cut-off level (%) that is part of the first key area Y1 is defined as the second key area Y2, the curve in the range of 25% to 55% cut-off level (%) that is part of the second key area Y2 is defined as the third key area Y3, and the curve in the range of 25% to 50% cut-off level (%) that is part of the third key area Y3 is defined as the fourth key area Y4. As already mentioned, in the range where the cut-off level (%) exceeds 65%, surface irregularity noise on the bottom surface 14A side is likely to be inherent. Therefore, the influence of surface irregularity noise decreases in the order of the first key area Y1, second key area Y2, third key area Y3, and fourth key area Y4, as they move further away from the bottom surface area. As a result, the evaluation accuracy of the inclined shape of the side surface 300 improves in the order of the first key area Y1, the second key area Y2, the third key area Y3, and the fourth key area Y4.

[0062] (Absolute load gradient in the key area) As shown in Figure 6, in the absolute load curve FKa, the absolute load gradient of the first key region Y1 is PW1 / PH1, given the difference in height PH1 between the cut levels of PS1 and PS2 at both ends of the first key region Y1 and the relative difference PW1 between the load lengths Ml. Similarly, the absolute load gradient of the second key region Y2 is PW2 / PH2. Similarly, the absolute load gradient of the third key region Y3 is PW3 / PH3. Similarly, the absolute load gradient of the fourth key region Y4 is PW4 / PH4.

[0063] In this embodiment, the absolute load gradient PW1 / PH1 of the first main region Y1 is 500 or more. Furthermore, the absolute load gradient PW2 / PH2 of the second main region Y2 is 500 or more. Furthermore, the absolute load gradient PW3 / PH3 of the third main region Y3 is 500 or more. Furthermore, the absolute load gradient PW4 / PH4 of the fourth main region Y4 is 500 or more. In this way, by making the inclination (angle with respect to the inner wall surface) of the first to fourth main regions Y1 to Y4 gentler, the sliding resistance of the recess 14 against the piston ring can be suppressed. Also in this embodiment, the absolute load gradient PW4 / PH4 of the fourth main region Y4 is larger than the absolute load gradient PW3 / PH3 of the third main region Y3. Furthermore, in this embodiment, the absolute load gradient increases in the order of the first main region Y1, the second main region Y2, the third main region Y3, and the fourth main region Y4. In other words, the inclination of the side surface 300 (angle with respect to the inner wall surface) gradually becomes steeper towards the depth of the recess 14. As a result, a sufficient volume of oil reservoir can be secured within the recess 14.

[0064] <Central low-roughness region formed in the cylinder liner>

[0065] Returning to Figure 1, the inner wall surface 12 of the cylinder liner 10 has a central low-roughness region 22 formed in at least a portion of the stroke central region 20, where the surface roughness (arithmetic mean roughness Ra of the contour curve (JIS B 0601:2013)) measured by a stylus-type surface roughness measuring instrument (JIS B 0651:2001) is 0.140 (μm) or less, and preferably, the surface roughness Ra is 0.120 (μm) or less. Specifically, the central low-roughness region 22 is formed by machining the surface roughness Ra of at least a portion of the surface of the inner wall surface 12 that can come into contact with the piston ring 40, i.e., the surface of the inner wall surface 12 excluding the recess 14, to 0.140 (μm) or less, and more preferably to 0.120 (μm) or less. In this embodiment, the surface roughness measured by a stylus-type surface roughness measuring instrument (arithmetic mean roughness of the contour curve) is denoted as Ra, and the three-dimensional surface roughness measured by a non-contact surface roughness measuring instrument (arithmetic mean height of the contour surface (JIS B 0681-2:2018, ISO 25178-2:2012)) is denoted as Sa.

[0066] The central low-roughness region 22 is more preferably set to a surface roughness Ra of 0.090 (μm) or less, specifically set to 0.083 (μm).

[0067] The three-dimensional surface roughness values ​​(JIS B 0681-2:2018, ISO 25178-2:2012) obtained by measuring this central low-roughness region 22 using a non-contact surface roughness measuring instrument (magnification 1080x, field of view 259.4 μm × 259.4 μm, no cutoff, height direction (Z direction) measurement line pitch 0.06 μm) 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). The height of the protruding peak, Spk (μm), is 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).

[0068] In particular, in this embodiment, not only is the height of the protruding peaks of the inner wall surface 12 reduced, but the depth of the protruding valleys of the inner wall surface 12 is also actively reduced to achieve a reduction in frictional force during sliding. Incidentally, in conventional cylinder liners, in order to ensure the lubricating oil retention force of the inner wall surface 12 itself, it is necessary to make the depth of the protruding valleys somewhat large, and consequently, it is difficult to reduce the height of the protruding peaks, which limits the reduction of frictional force during sliding. On the other hand, in this embodiment, since lubricating oil is sufficiently retained in the recess 14 with high smoothness of the bottom surface 14A, the lubricating oil in the recess 14 can easily move to the surrounding inner wall surface 12, so that the contact surface with the piston ring 40 (inner wall surface 12) can form a sufficient oil film even if the lubricating oil retention force is small.

[0069] In line with this principle, when the height of the protruding peaks of the inner wall surface 12 is Spk and the depth of the protruding valleys of the inner wall surface 12 is Svk, in the central low-roughness region 22 of this embodiment, it is preferable to set the value of Svk / Spk to 2.6 or less, more preferably to 2.4 or less, and even more preferably to 2.0 or less.

[0070] Furthermore, in this embodiment, the entire area of ​​the surface that can come into contact with the piston ring 40 in the 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 edge and the vicinity of the lower edge of the stroke central region 20 where the recess 14 is formed. In addition, an upper low-roughness region 23A with 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, forming a continuous, integrated surface as a whole.

[0071] Near the upper and lower edges of the stroke center region 20, the relative velocity Q between the cylinder liner 10 and the piston 30 decreases, making it easy 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 so-called dimple liner technology can demonstrate its effects in the fluid lubrication region, the advantages of dimple liner technology can also be obtained near the upper and lower edges of the stroke center region 20. It is also possible to form the central low-roughness region 22 only near the upper and / or lower edges of the stroke center region 20, but it is preferable to form the central low-roughness region 22 over the entire stroke center region 20, as in this embodiment. When the relative velocity Q between the cylinder liner 10 and the piston 30 becomes even lower, the boundary lubrication region will encroach on the central side of the stroke center region 20, but even in that case, it is possible to expand the range of the fluid lubrication region.

[0072] 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. In this process, it is preferable to use a finer abrasive grain (JIS R 6001-2:2017, ISO8486-2:2007) than, for example, F500 or #800 for the honing wheel.

[0073] <About the chemical treatment process>

[0074] Furthermore, when forming the central low-roughness region 22 by this honing process, it is preferable not to perform chemical conversion treatment on its surface. For example, if a phosphate coating, which is commonly used in the manufacturing process of the cylinder liner 10, is applied, the surface properties of the central low-roughness region 22 will be altered by the coating.

[0075] On the other hand, if the film has small surface properties, it is also desirable to apply a chemical conversion treatment to the central low-roughness region 22. In this embodiment, when applying a chemical conversion treatment to the inner wall surface 12, the following two procedures are possible. The first procedure involves forming a recess 14 on the inner wall surface 12 of the cylinder liner 10 with a laser, then performing a chemical conversion treatment, and finally performing polishing. In the first procedure, a film is formed on the bottom surface 14A of the laser-processed recess 14, and as the film settles during the polishing process, the fine irregularities on the bottom surface 14A of the recess 14 can be further reduced. On the other hand, since the depth of the recess 14 is reduced by the thickness of the film, it is necessary to pre-process the recess 14 deeper with a laser by an amount equivalent to the film thickness, which increases the processing time. For example, if a 0.5 μm film is formed by the chemical conversion treatment, it is necessary to form the recess 14 deeper by 0.5 μm with a laser.

[0076] The second procedure involves performing a chemical conversion treatment on the inner wall surface 12 of the cylinder liner 10, then forming a recess 14 with a laser, and finally polishing it. In the second procedure, no film is formed on the bottom surface 14A of the recess 14. However, since the bottom surface 14A of the recess 14 formed by the laser has small Rk and Rvk values, there is little need to use a film to ensure proper adhesion. Furthermore, since the recess 14 can be processed with a laser without considering the thickness of the film, there is the advantage of reducing processing time.

[0077] <Pistons and piston rings>

[0078] Figures 7(A) and 7(B) show the piston 30 and the piston rings 40 (top ring 50, second ring 60, oil ring 70) installed in the ring groove of the piston 30. The piston rings 40 reciprocate in the cylinder axial direction with their 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, preventing the phenomenon of compressed gases leaking from the combustion chamber to the crankcase (blow-by). The second ring 60, like the top ring 50, serves to eliminate the gap between the piston 30 and the cylinder liner 10 and also scrapes 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, forming an appropriate oil film and preventing the piston 30 from seizing.

[0079] As shown in an enlarged view in Figure 7(C), the top ring 50 is a single annular member, and when viewed in cross-section, the outer circumferential surface 52 has a so-called barrel shape that is convex radially outward. Specifically, both outer edges of the outer circumferential surface 52 in the cylinder axial direction are inclined toward the outside in the cylinder axial direction, away from the inner wall surface 12. The contact width f of the cylinder liner 10 with respect to the inner wall surface 12 on the outer circumferential surface 52 is preferably formed to be, for example, 0.3 mm or less. Furthermore, the surface roughness (arithmetic mean roughness Ra of the contour curve (JIS B 0601:2013)) of the outer circumferential surface 52, as measured by a stylus-type surface roughness measuring instrument (JIS B 0651:2001), is preferably 0.250 (μm) or less.

[0080] As shown in an enlarged view in Figure 7(D), the second ring 60 is a single annular member, and its outer circumference has a tapered shape that widens from the upper end in the cylinder axial direction towards the lower end in the cylinder axial direction. The outer peripheral surface 62, located at the outermost end of this tapered shape and in contact with the inner wall surface 12 of the cylinder liner 10, has a planar shape in cross-sectional view. The contact width f of the outer peripheral surface 62 with respect to the inner wall surface 12 of the cylinder liner 10 is preferably formed to be, for example, 0.3 mm or less. Furthermore, the surface roughness (arithmetic mean roughness Ra of the contour curve (JIS B 0601:2013)) of the outer peripheral surface 62, as measured by a stylus-type surface roughness measuring instrument (JIS B 0651:2001), is preferably 0.250 (μm) or less.

[0081] Furthermore, the tensions of the top ring 50 and the second ring 60 are set to relatively low values, so that the surface pressure acting on the contact surfaces of the outer circumferential surfaces 52 and 62 is, for example, 0.5 MPa or less, 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.

[0082] The oil ring 70, shown in an enlarged view in Figure 8(A), is a two-piece type and comprises a ring body 72 and a coil spring-shaped coil expander 76. The ring body 72 has a pair of annular rails 73, 73 positioned at both axial ends and an annular column 75 positioned between the pair of rails 73, 73 and connecting them. The combined cross-sectional shape of the pair of rails 73, 73 and the column 75 is approximately I-shaped or H-shaped, and this shape is used to form an inner circumferential groove 79 with a semicircular cross-section for housing the coil expander 76 on the inner circumferential surface. In addition, annular projections 74, 74 are formed on the pair of rails 73, 73, respectively, projecting radially outward with respect to the column 75. The outer circumferential surfaces 82, 82 formed at the tips of these annular projections 74, 74 abut against the inner wall surface 12 of the cylinder liner 10. The coil expander 76 is housed in the inner circumferential groove 79, thereby biasing the ring body 72 radially outward. Multiple oil return holes 77 are formed in the circumferential direction of the column portion 75 of the ring body 72.

[0083] The contact width of each of the pair of outer peripheral surfaces 82, 82 in Figure 8(A) is preferably formed to be 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, and is approximately 1.75 MPa. Therefore, when the engine speed is high, the oil ring 70 often slides in the fluid lubrication region, but when the engine speed decreases, it often slides in the boundary lubrication region. In Figure 8(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 outer peripheral surfaces 82 of the upper rail 73 and the outer peripheral surfaces 82 of the lower rail 73 may have a stepped shape in which the corners on the sides facing each other (coil expander 76 side) are cut out (a so-called step land shape). Furthermore, the surface roughness (arithmetic mean roughness Ra of the contour curve (JIS B 0601:2013)) measured by a stylus-type surface roughness measuring instrument (JIS B 0651:2001) on the outer surface 82 is preferably 0.450 (μm) or less.

[0084] Note that the oil ring 70 is not limited to a two-piece type; for example, it may be a three-piece type oil ring 70 as shown in Figure 8(B). This oil ring 70 has annular side rails 73a and 73b that are separated vertically, and a spacer expander 76s positioned between these side rails 73a and 73b.

[0085] The spacer expander 76s is formed by plastically deforming a steel material into a corrugated shape with repeated ridges and depressions in the direction of the cylinder axis. This corrugated shape is used to form an upper support surface 78a and a lower support surface 78b, which support a pair of side rails 73a and 73b in the axial direction. The inner circumferential end of the spacer expander 76s has an arch-shaped lug 74m that extends outward in the axial direction. This lug 74m abuts against the inner circumferential surfaces of the side rails 73a and 73b. The spacer expander 76s is then fitted into the ring groove of the piston 30 in a contracted state in the circumferential direction with its joints facing each other. As a result, the restoring force of the spacer expander 76s causes the lug 74m to press and bias the side rails 73a and 73b radially outward.

[0086] Furthermore, it is preferable that the contact width f of the outer peripheral surfaces 82, 82 of the side rails 73a, 73b in Figure 8(B) be formed to be between 0.02 mm and 0.40 mm.

[0087] <Friction pattern between cylinder liner and piston ring>

[0088] Next, the friction patterns between the cylinder liner and piston ring will be explained. The change in the coefficient of friction during general sliding is represented by the Stribeck diagram shown in Figure 9(A). In this Stribeck diagram, the friction patterns are divided into the solid contact region 110 where sliding occurs in direct contact, the boundary lubrication region 112 where sliding occurs via an oil film, and the fluid lubrication region 114 where sliding occurs via a viscous lubricating oil film. Furthermore, between the boundary lubrication region 112 and the fluid lubrication region 114, there is a mixed lubrication region 113 where both conditions coexist. In this Stribeck diagram, the horizontal axis is a logarithmic representation of "kinematic viscosity (kinematic viscosity ratio) μ" × "velocity Q" / "contact load W", and the vertical axis is the coefficient of friction (f). Therefore, the fluid lubrication region 114 or the mixed lubrication region 113 can have the smallest friction force, and effectively utilizing these regions 114 and 113 is effective in reducing friction, i.e., in improving fuel efficiency. On the other hand, if the speed Q increases but the transition from the boundary lubrication region 112 to the fluid lubrication region 114 is not possible, the boundary lubrication region 112 will continue as is up to the high-speed region, as shown by the dotted line.

[0089] Incidentally, the majority of the frictional force in the fluid lubrication region 114 is due to the shear resistance of the oil, which is defined as (viscosity) × (velocity) × (area) / (oil film thickness). As a result, reducing the shear area directly leads to a reduction in frictional force.

[0090] Therefore, in this embodiment, oil is actively introduced into the contact surface of the outer circumferential surface 42 of the piston ring 40, allowing for a quick transition to the fluid lubrication region 114 and achieving reduced friction. Simultaneously, by applying so-called dimple liner technology to the cylinder liner 10, recesses 14 are formed in the central stroke region 20 of the cylinder liner 10, reducing the effective area where oil shear resistance occurs, thereby achieving a more efficient reduction in frictional force.

[0091] Furthermore, the Stribeck diagram in Figure 9(A) shows the dynamic change in the friction coefficient (f) of the piston 30 during one stroke. Another indicator for evaluating the friction pattern is the Friction Mean Effective Pressure (FMEP). This Friction Mean Effective Pressure represents the friction work per cycle divided by the stroke volume. The diagram of this Friction Mean Effective Pressure (FMEP diagram) is shown in Figure 9(B). In the FMEP diagram, the horizontal axis represents the rotational speed (N), and the vertical axis represents the Friction Mean Effective Pressure (kPa). The higher the rotational speed (N), the larger the proportion occupied by the fluid lubrication region 114 during one stroke. On the other hand, as the rotational speed (N) decreases, the proportion occupied by the fluid lubrication region 114 during one stroke decreases, and the proportion occupied by the mixed lubrication region 113 (or boundary lubrication region 112) increases. Therefore, the shape of the FMEP diagram in Figure 9(B) is relatively similar to the shapes of the fluid lubrication region 114 and the mixed lubrication region 113 in the Streibeck diagram in Figure 9(A).

[0092] <Recessed Drilling Equipment>

[0093] Next, the machining of the recess 14 in the cylinder liner 10 will be described. As shown in Figure 10, the recess machining apparatus 500 employs a galvanometer mirror system and comprises a laser oscillator 510, an X-axis mirror 704 driven by an X-axis motor 702, a Y-axis mirror 714 driven by a Y-axis motor 712, a terminal mirror 718, and a control device 570. The laser 590 emitted from the laser oscillator 510 is reflected by the X-axis mirror 704, the Y-axis mirror 714, and the terminal mirror 718 and reaches the cylinder liner 10. Therefore, the scanning direction can be freely controlled to a desired direction. This galvanometer mirror system makes it easy to irradiate the laser 590 while freely changing the scanning direction. However, the present invention is not limited to this, and scanning methods such as a polygon mirror system, which scans by reflecting the laser 590 off a polygonal rotating mirror, can also be employed.

[0094] The laser oscillator 510 is a short-pulse laser oscillator. A short pulse is preferably a pulse width of 100 picoseconds or less (100,000 femtoseconds or less), more preferably 20 picoseconds or less (20,000 femtoseconds or less), and even more preferably less than 1 picosecond (less than 1,000 femtoseconds). A device that can be set to a pulse width of less than 1 picosecond (less than 1,000 femtoseconds) is called a femtosecond laser oscillator, and a femtosecond laser oscillator is used in this embodiment. The laser oscillator includes a seed pulse section that emits a weak femtosecond laser (seed pulse), an amplification section that amplifies the seed pulse with an excitation laser, a compensation section that disperses and compensates for the amplified laser, and so on. The laser 590 emitted from the laser oscillator 510 can be set to a pulse width of less than 1 picosecond (in femtosecond units). The laser oscillator 510 is set to a pulse width of, for example, 400 femtoseconds to 20,000 femtoseconds. The laser wavelength can be any wavelength suitable for metal processing, for example, 100 nm to 3000 nm. The laser frequency can be 20 kHz to 2000 kHz. The pulse energy can be up to 2000 μJ. In this embodiment, the laser wavelength is set to 1030 nm, but it is not particularly limited as long as it is a wavelength suitable for metal processing.

[0095] The control device 570 is a computer equipped with a central processing unit (CPU), memory, external storage device, etc. When the recess machining processing program is executed in this control device 570, recess machining by the recess machining device 500 is realized.

[0096] <Processing method for individual recesses>

[0097] Next, the detailed control of the recess processing device 500 by the control device 570 will be described. First, the processing method for each individual recess 14 will be described. As shown in Figure 11(A), there is a recess target area 614 on the inner wall surface 12 where the recess 14 is to be formed. The recess processing device 500 processes the first band-shaped area 621 into a recessed groove shape by irradiating the laser spot 590S of the laser 590 along a linear first movement path 611 to each recess target area 614. This process is defined as the first irradiation process. The ends of the first movement path 611 intersect with the contour of the recess target area 614.

[0098] Once the first irradiation process is complete, the heat accumulated at any point X in the first zone-shaped region 621 is diffused or released to the outside.

[0099] After the first irradiation step, the recess processing apparatus 500 irradiates the recess target area 614 with a laser spot 590S that overlaps with at least a portion of the first strip-shaped area 621, along a linear second movement path 612 that is parallel to the first movement path 611 and offset by a line pitch 596 from the center line of the first movement path 611, as shown in Figure 11(B), thereby processing the second strip-shaped area 622 that overlaps with the first strip-shaped area 621 into a recessed groove. This step is defined as the second irradiation step. Both ends of the second movement path 612 intersect with the contour of the recess target area 614. In this case, the scanning direction in the first movement path 611 (the movement direction of the laser spot 590S) and the scanning direction in the second movement path 612 are the same direction (parallel).

[0100] When defining the offset amount of the line pitch 596 as OS and the spot diameter of the laser spot as C, it is preferable to set OS ≤ C / 2. In this way, it becomes possible to superimpose a band-shaped region on the bottom surface 14A of the recess 14.

[0101] Once the second irradiation process is complete, the heat accumulated in the second zone-shaped region 622 is diffused or released to the outside.

[0102] As shown in Figure 11(C), the third irradiation step is performed using the exact same procedure as the first and second irradiation steps described above, to process the third strip-shaped region 623 along the third movement path 613. The process is then repeated with the fourth irradiation step, fifth irradiation step, and so on. As a result, as shown in Figure 11(D), multiple strip-shaped regions 620 are formed in parallel in the strip width direction, spreading out planarly within the recess target region 614. The centerlines of adjacent strip-shaped regions 620 are offset by a predetermined line pitch 596 and are parallel to each other. This forms the recess 14.

[0103] In the procedure shown in Figures 11(A) to (D), the scanning direction of all lasers is illustrated as being parallel to (or approximately parallel to) the circumferential direction of the cylinder liner 10. However, the present invention is not limited to this, and scanning may also be performed in the axial direction of the cylinder liner 10, or in an oblique direction including both the axial direction and the circumferential direction of the cylinder liner 10. Furthermore, as shown in Figure 11(E), scanning can also be performed in an annular or spiral manner along an annular line that approximates (similar to) the contour of a single recess 14. Furthermore, as shown in Figure 11(F), a scanning path may be adopted in which the first movement path 611, the second movement path 612, and the third movement path 613, which are parallel to each other, continuously meander within a single recess 14.

[0104] Figure 12(A) schematically shows a cross-section of the cylinder liner 10 along the first movement path 611 being processed by the laser 590. For ease of explanation, the first movement path 611 of the laser spot 590S and the ON / OFF timing (pulse waveform) of the laser spot 590S moving along the first movement path 611 are shown at the top of this figure. The laser spot 590S moves along the first movement path 611 while repeatedly switching ON and OFF. As a result, a groove-shaped first band-shaped region 621 is formed in the cylinder liner 10. The distance traveled between adjacent pulses of the laser spot 590S (inter-pulse travel distance) 592 is shorter than the spot diameter 594 of the laser spot 590S. The inter-pulse travel distance 592 is the value obtained by dividing the scanning speed A (mm / s) by the frequency B (kHz) of the laser 590 (A / B). Furthermore, the deepest point X in the first band-shaped region 621 formed in a single scanning step along the first movement path 611 is where the number of superimposed irradiations of the laser spot 590S is maximized. When the spot diameter 594 is defined as C (mm), this maximum number of superimposed irradiations is the value obtained by dividing the spot diameter C by the inter-pulse travel distance (A / B), i.e., C / (A / B). For example, if the scanning speed is 100 mm / s, the frequency is 200 kHz, and the spot diameter 594 is 0.04 mm, the inter-pulse travel distance 592 is 0.0005 mm, and the number of superimposed irradiations of the laser spot 590S at the deepest point X in the first band-shaped region 621 (reference bottom surface range 14G or the center GM of the recess) is 80. In this way, the desired groove is formed by superimposing multiple irradiations of the laser spot 590S at the deepest point X. Furthermore, in the terminal region 621e of the first band-shaped region 621, the number of superimposed irradiations of the laser spot 590S decreases in a stepwise manner towards the end, forming a tapered surface (side surface 300). The same is true in the starting region of the first band-shaped region 621.

[0105] (Number of overlapping spots in a band-shaped area) The number of times the laser spot 590S is superimposed on the deepest point X (reference bottom surface range 14G or recess center GM) when forming each band-shaped region (hereinafter referred to as the band-shaped region spot superimposition count) is preferably 2 or more, more preferably 3 or more, and most preferably 5 or more. On the other hand, the band-shaped region spot superimposition count is preferably 50 or less, more preferably 30 or less, more preferably 20 or less, and most preferably 10 or less. Similarly, the calculated virtual superimposition count that can be calculated from the processing conditions (spot diameter / pulse travel distance, hereinafter referred to as the virtual band-shaped region spot superimposition count) is preferably 2 or more, more preferably 3 or more, and most preferably 5 or more. On the other hand, the virtual band-shaped region spot superimposition count is preferably 50 or less, more preferably 30 or less, more preferably 20 or less, and most preferably 10 or less.

[0106] (Number of overlapping spots in a zone of a unit depth) More specifically, the maximum amount of excavation (excavation depth) of each strip-shaped region processed in a single scan can be adjusted as appropriate depending on the purpose. For example, if you want to increase the maximum amount of excavation in a single scan, you can either set a larger number of overlapping strip-shaped region spots or increase the energy of the laser spot 590S to increase the amount of excavation per laser spot 590S irradiation. However, if the energy of the laser spot 590S is increased too much, excess energy will be absorbed (accumulated) on the processed surface, resulting in energy loss and increased surface irregularities due to melting of the processed surface.

[0107] Conversely, if you want to reduce the maximum depth of digging per scan, you can either set a smaller number of overlapping zone spots or reduce the energy of the laser spot 590S to reduce the depth of digging per pass. However, if you reduce the energy of the laser spot 590S too much, the processing efficiency will deteriorate drastically, and in some cases, processing may not even occur.

[0108] To standardize the above concept, the inventors define a unit depth of 1 μm and define the number of spot overlaps that achieve this unit depth as the number of spot overlaps per unit depth zone (number of times / μm). It is preferable to adjust the laser irradiation conditions and scanning conditions so that this number of spot overlaps per unit depth zone (number of times / μm) is within the range of 5 to 500. Furthermore, it is preferable to adjust the laser irradiation conditions and scanning conditions so that the number of spot overlaps per unit depth zone (number of times / μm) is within the range of 5 to 250, and even more preferably, to adjust the laser irradiation conditions and scanning conditions so that the number of spot overlaps per unit depth zone (number of times / μm) is within the range of 5 to 50.

[0109] (Number of overlapping zones) Figure 12(B) shows a cross-section of the cylinder liner 10 along the direction perpendicular to the scanning direction of the laser 590 (the cylinder axis direction). The cylinder liner 10 has multiple groove-like band-shaped regions formed on it, for example, extending in the depth direction of the paper in Figure 12(B). Specifically, the first to sixth band-shaped regions 621 to 626 are formed in parallel in the band width direction (the left-right direction of the paper in Figure 12(B)) in this order. After the laser spot 590S forms the first band-shaped region 621, it moves axially by a predetermined line pitch 596 to form the second band-shaped region 622, then moves axially by a predetermined line pitch 596 to form the third band-shaped region 623, then moves axially by a predetermined line pitch 596 to form the fourth band-shaped region 624, then moves axially by a predetermined line pitch 596 to form the fifth band-shaped region 625, and then moves axially by a predetermined line pitch 596 to form the sixth band-shaped region 626.

[0110] The line pitch 596 is shorter than the spot diameter 594 of the laser spot 590S. For example, if the line pitch 596 is 0.03 mm and the spot diameter 594 is 0.09 mm, three band-shaped regions (third to fifth band-shaped regions 623 to 625) are superimposed at the deepest point X of the recess 14. In addition, in the axial end region 14e of the recess 14, the number of superimposed band-shaped regions 620 decreases stepwise towards the end, so a stepped tapered surface (side surface 300) is formed. The same is true in the axial start region 14s.

[0111] Furthermore, the number of overlapping band-shaped regions with respect to the deepest point X of the recess 14 (hereinafter referred to as the number of overlapping band-shaped regions) is preferably 2 or more, and more preferably set to 3 or more. On the other hand, the number of overlapping band-shaped regions is preferably 10 or less, more preferably 8 or less, and even more preferably 5 or less. Similarly, the number of overlapping band-shaped regions calculated from the processing conditions (spot diameter / line pitch, hereinafter referred to as the number of overlapping virtual band-shaped regions) is preferably 2 or more, and more preferably set to 3 or more. On the other hand, the number of overlapping virtual band-shaped regions is preferably 10 or less, more preferably 8 or less, and even more preferably 5 or less.

[0112] Note that in Figure 13, for the sake of explanation, the depth dimension of the recess 14 is greatly exaggerated.

[0113] (Total number of spot superimposed areas) The number of times the laser spot 590S is superimposed on the reference bottom surface range 14G or the center GM of the recess 14 completed by laser processing (hereinafter referred to as the total number of spot superpositions) is the product of the number of spot superpositions in the band-shaped area and the number of spot superpositions in the band-shaped area. As a result, the total number of spot superpositions is preferably 4 times (product of 2 spot superpositions in the band-shaped area and 2 spot superpositions in the band-shaped area), preferably 6 times (product of 2 spot superpositions in the band-shaped area and 3 spot superpositions in the band-shaped area), preferably 9 times (product of 3 spot superpositions in the band-shaped area and 3 spot superpositions in the band-shaped area), preferably 10 times (product of 5 spot superpositions in the band-shaped area and 2 spot superpositions in the band-shaped area), and preferably 15 times (product of 5 spot superpositions in the band-shaped area and 3 spot superpositions in the band-shaped area). On the other hand, the total number of spot overlaps is preferably 500 times (product of 50 spot overlaps in the band-shaped area and 10 spot overlaps in the band-shaped area) or less, preferably 400 times (product of 50 spot overlaps in the band-shaped area and 8 spot overlaps in the band-shaped area) or less, preferably 300 times (product of 30 spot overlaps in the band-shaped area and 10 spot overlaps in the band-shaped area) or less, preferably 200 times (product of 20 spot overlaps in the band-shaped area and 10 spot overlaps in the band-shaped area) or less, preferably 160 times (product of 20 spot overlaps in the band-shaped area and 8 spot overlaps in the band-shaped area) or less, preferably 100 times (product of 10 spot overlaps in the band-shaped area and 10 spot overlaps in the band-shaped area) or less, preferably 80 times (product of 10 spot overlaps in the band-shaped area and 8 spot overlaps in the band-shaped area) or less, and preferably 50 times (product of 10 spot overlaps in the band-shaped area and 5 spot overlaps in the band-shaped area) or less.

[0114] (Total number of overlapping spots per unit depth) The total number of spot overlaps can be adjusted by the maximum average depth R (in μm) of the recess 14. Therefore, a depth of 1 μm is defined as the unit recess depth, and the value obtained by dividing the total number of spot overlaps by the maximum average depth R is defined as the total number of spot overlaps per unit depth (number of times / μm). It is preferable to adjust the laser irradiation conditions and scanning conditions so that the total number of spot overlaps per unit depth (number of times / μm) is within the range of 5 to 500. Furthermore, it is preferable to adjust the laser irradiation conditions and scanning conditions so that the total number of spot overlaps per unit depth (number of times / μm) is within the range of 5 to 250, and even more preferably, to adjust the laser irradiation conditions and scanning conditions so that the total number of spot overlaps per unit depth (number of times / μm) is within the range of 5 to 50.

[0115] <Method for processing multiple recesses> Next, a processing method will be described for cases where multiple recessed target areas 614 exist in the scanning direction (circumferential or axial direction). As shown in Figure 13(A), a laser spot 590S is irradiated along a linear first movement path 611 in the circumferential or axial direction. This first movement path 611 is set to span multiple recessed target areas 614. The first irradiation step described above is performed sequentially for each of the multiple recessed target areas 614, from upstream to downstream along the first movement path 611. This step is defined as the first irradiation group step. As a result, a groove-shaped first band-shaped area 621 is formed in each of the multiple recessed target areas 614. These are defined as the first band-shaped area group.

[0116] After the first irradiation group process, as shown in Figure 13(B), a laser spot 590S is irradiated along a linear second movement path 612 having a center line offset by a predetermined line pitch 596 from the center line of the first movement path 611. This second movement path 612 is set to span multiple recessed target areas 614. The second irradiation process is performed sequentially for each of the multiple recessed target areas 614, from upstream to downstream along the second movement path 612. This process is defined as the second irradiation group process. As a result, a recessed groove-shaped second band-shaped area 622 is formed in each of the multiple recessed target areas 614. These are defined as the second band-shaped area group.

[0117] Focusing on each recessed area 614, the recess processing device 500 processes other recessed areas 614 from immediately after the first irradiation process until immediately before the second irradiation process. As a result, each recessed area 614 has the advantage of being able to secure sufficient heat dissipation time immediately after processing in the first and second irradiation processes.

[0118] As shown in Figure 13(C), the third irradiation group step is performed in the same manner as the first and second irradiation group steps described above, forming a plurality of third zone-shaped regions 623 along the third movement path 613. The same process is repeated thereafter for the fourth irradiation group step, the fifth irradiation group step, and so on. As a result, a plurality of recesses 14 are formed, as shown in Figure 13(D).

[0119] <Examples> Next, an example of the cylinder liner 10 of this embodiment will be described. Using the recess processing device 500, recesses 14 were processed in two cylinder liners 10 representing the first and second embodiments. In addition, as first and second comparative examples, recesses 14 were processed in the cylinder liner 10 by conventional blasting. In the first embodiment, the cylinder liner 10 was processed with a laser to create the recesses 14, and then polished. In the second embodiment, the cylinder liner 10 was processed with a laser to create the recesses 14, and then subjected to chemical conversion treatment (coating treatment) before being polished. In the first comparative example, the cylinder liner was processed with blasting to create the recesses, and then subjected to polishing. In the second comparative example, the cylinder liner was processed with blasting to create the recesses, and then subjected to chemical conversion treatment (coating treatment) before being polished.

[0120] For the cylinder liners of the first and second embodiments and the first and second comparative examples, five cross-sectional curves DK were measured using a stylus-type surface roughness measuring instrument (JIS B 0651:2001) in a range including at least four recesses 14 aligned in the cylinder axial direction. A standard tip radius of 2 μm was used for the measuring needle, and the cutoff value (wavelength) λs for the cross-sectional curve was selected to 2.5 mm for measurement. The evaluation length was set to 2.800 mm, including the four recesses 14. The absolute load curve FKa was calculated using each of the five obtained cross-sectional curves DK.

[0121] Figure 14(A) shows the absolute load curve FKa of the cylinder liner of the first embodiment, and Figure 14(B) shows the absolute load curve FKa of the cylinder liner of the first comparative example. Furthermore, Figure 15(A) shows the absolute load curve FKa of the cylinder liner of the second embodiment, and Figure 15(B) shows the absolute load curve FKa of the cylinder liner of the second comparative example. In addition, Figure 16 shows the absolute load gradients of the first key area Y1, second key area Y2, third key area Y3, and fourth key area Y4 for the first and second embodiments and the first and second comparative examples.

[0122] In the first embodiment shown in Figure 14(A), the absolute load gradient in the first key region Y1 was PW1 / PH1 = 721.53 μm / 1.18 μm = 611. The absolute load gradient in the second key region Y2 was PW2 / PH2 = 659.44 μm / 1.03 μm = 638. The absolute load gradient in the third key region Y3 was PW3 / PH3 = 601.66 μm / 0.89 μm = 679. Similarly, the absolute load gradient in the fourth key region Y4 was PW4 / PH4 = 532.48 μm / 0.74 μm = 721.

[0123] In the first comparative example shown in Figure 14(B), the absolute load gradient in the first key region Y1 was PW1 / PH1 = 683.04 μm / 1.54 μm = 444. The absolute load gradient in the second key region Y2 was PW2 / PH2 = 528.23 μm / 1.35 μm = 392. The absolute load gradient in the third key region Y3 was PW3 / PH3 = 408.68 μm / 1.15 μm = 354. Similarly, the absolute load gradient in the fourth key region Y4 was PW4 / PH4 = 311.03 μm / 0.96 μm = 324.

[0124] In the second embodiment shown in Figure 15(A), the absolute load gradient in the first main region Y1 was PW1 / PH1 = 918.73 μm / 1.69 μm = 545. The absolute load gradient in the second main region Y2 was PW2 / PH2 = 765.06 μm / 1.48 μm = 518. The absolute load gradient in the third main region Y3 was PW3 / PH3 = 656.66 μm / 1.26 μm = 518. Similarly, the absolute load gradient in the fourth main region Y4 was PW4 / PH4 = 555.35 μm / 1.05 μm = 527.

[0125] In the second comparative example shown in Figure 15(B), the absolute load gradient in the first key region Y1 was PW1 / PH1 = 743.73 μm / 1.53 μm = 488. The absolute load gradient in the second key region Y2 was PW2 / PH2 = 511.26 μm / 1.33 μm = 383. The absolute load gradient in the third key region Y3 was PW3 / PH3 = 360.39 μm / 1.14 μm = 315. Similarly, the absolute load gradient in the fourth key region Y4 was PW4 / PH4 = 276.76 μm / 0.95 μm = 290.

[0126] In the first and second embodiments, the absolute load gradient was 500 or more in all of the first, second, third, and fourth main area Y1, second, third, and fourth main area Y4. On the other hand, in the first and second comparative examples, the absolute load gradient was less than 500 in all of the first, second, third, and fourth main area Y1, second, third, and fourth main area Y4. In other words, the recess 14 in the first and second embodiments had a gentler slope on the side surface 300 compared to the first and second comparative examples.

[0127] In the first and second embodiments, the recess 14 is laser-processed, so as shown in Figure 12, the number of superimposed irradiations of the laser spot 590S decreases in a stepwise manner towards the edge of the recess 14, which is presumed to have resulted in the formation of a gently tapered surface (side surface 300). On the other hand, in the case of blast treatment in the first and second comparative examples, the recess 14 is formed by impacting the abrasive material with the inner wall surface from a direction perpendicular to it, which is presumed to have resulted in a steeper slope of the side surface 300.

[0128] Furthermore, in the first and second embodiments, the absolute load gradient of the fourth main region Y4 was larger than that of the third main region Y3. This means that on the opening side (inlet side) of the side surface 300 of the recess 14, the slope of the side surface 300 gradually became steeper in the depth direction. A gentler slope on the opening side (inlet side) of the recess 14 has the advantage of reducing the sliding resistance with the piston ring.

[0129] On the other hand, in the first and second comparative examples, the absolute load gradient of the fourth main area Y4 was smaller than that of the third main area Y3. This means that on the opening side of the side surface 300 of the recess 14, the slope gradually became gentler towards the depth direction. In the case of the blast treatment in the first and second comparative examples, the recess 14 is formed by impacting it with abrasive material, so it is presumed that the slope of the side surface 300 on the opening side (inlet side) of the recess 14, where the abrasive material is most likely to impact, became steeper, and the slope of the side surface 300 gradually became gentler towards the bottom surface 14A. A steeper slope on the opening side (inlet side) of the recess 14 makes it easier to achieve greater sliding resistance with the piston ring.

[0130] Furthermore, in the first embodiment, the absolute load gradient increased in the order of the first main region Y1, the second main region Y2, the third main region Y3, and the fourth main region Y4. In other words, the partial curve of the first main region Y1 is convex, which means that the slope gradually became steeper towards the depth direction along the entire side surface 300 of the recess 14. On the other hand, in the first and second comparative examples, the absolute load gradient decreased in the order of the first main region Y1, the second main region Y2, the third main region Y3, and the fourth main region Y4. In other words, the partial curve of the first main region Y1 is concave, which means that the slope gradually became gentler towards the depth direction along the entire side surface 300 of the recess 14.

[0131] <Example of a reference case> Next, a reference embodiment of the cylinder liner 10 of this embodiment will be described. Using the recess processing device 500, recesses 14 were processed into four cylinder liners 10, which represent the first to fourth reference embodiments. In addition, as a reference comparative example, recesses 14 were processed into the cylinder liner 10 by conventional blasting.

[0132] As shown in Figures 17(A) and (B), for the bottom surface 14A of the recess 14 in the first, second, third, and fourth reference embodiments, the surface roughness after laser polishing was such that the core level difference Rk was less than 1.6 μm and the protruding valley depth Rvk was less than 1.3 μm. In this embodiment, the surface roughness after laser coating and polishing was such that the core level difference Rk was less than 1.1 μm and the protruding valley depth Rvk was less than 0.9 μm.

[0133] On the other hand, in the comparative example where recesses were formed by blasting, the surface roughness after laser polishing showed a core level difference Rk of 1.6 μm or more, and a protruding valley depth Rvk of 1.3 μm or more. In the reference comparative example, the surface roughness after laser coating and polishing showed a core level difference Rk of 1.1 μm or more, and a protruding valley depth Rvk of 0.9 μm or more.

[0134] As shown in Figure 17(C), for the bottom surface 14A of the recess 14 in the first, second, third, and fourth reference embodiments, the surface roughness after laser polishing, calculated by adding the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk), was less than 2.9 μm. For the laser coating and polishing, the surface roughness after laser coating and polishing, calculated by adding the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk), was less than 2.0 μm.

[0135] On the other hand, in the reference comparative example where recesses were formed by blast treatment, the surface roughness after laser polishing, calculated as the sum of the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk), was 2.9 μm or more. In addition, in the reference comparative example, the surface roughness after laser coating and polishing, calculated as the sum of the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk), was 2.0 μm or more.

[0136] <Oil Consumption>

[0137] In the case of the cylinder liner 10 of this embodiment, oil consumption is also suppressed. This is because the absolute amount of lubricating oil film formed in the central low-roughness region 22 is reduced. Even if the absolute amount of oil film decreases, since recesses 14 having a smooth bottom surface 14A are formed in superimposed, the lubricating oil in the recesses 14 is smoothly guided to the surrounding inner wall surface 12, and lubrication deficiency does not occur. In other words, in this embodiment, it is possible to rationally solve both the reduction of oil consumption and sufficient lubrication effect.

[0138] <Modified example of a recess processing device> Although the recess processing apparatus 500 in Figure 10 illustrates a galvanometer mirror system, the present invention is not limited thereto. For example, the recess processing apparatus 800 shown in Figure 18 includes a laser oscillator 810 that emits a laser 890, a holding cylinder 815 provided in the laser oscillator 810 through which the laser 890 passes, a reflective mirror 820 provided inside the holding cylinder 815 that reflects the laser 890 at a 90-degree angle, an axial movement device 830 that moves the laser oscillator 810 and the cylinder liner 10 relative to each other in the direction of the cylinder axis, a base 850 that holds the cylinder liner 10, a chuck 852 that fixes the cylinder liner 10 to the base 850, a rotational movement device 840 that rotates the cylinder liner 10 and the laser 890 relative to each other in the circumferential direction of the cylinder, and a control device 870 that controls the entire apparatus.

[0139] The central axis of the retaining cylinder 815 extends parallel to the cylinder axis direction of the cylinder liner 10. The retaining cylinder 815 is inserted into the inside of the cylinder liner 10. The laser 890 passing through the inside of the retaining cylinder 815 is bent (reflected) by the reflecting mirror 820 and travels in the cylinder diameter direction of the cylinder liner 10, irradiating the inner wall surface 12 approximately perpendicularly. Here, the example shows the case where the axial movement device 830 moves the laser oscillator 810, but the cylinder liner 10 may also be moved in the cylinder axis direction. Similarly, the example shows the case where the rotational movement device 840 rotates the cylinder liner 10, but the entire laser oscillator 810 or the reflecting mirror 820 (retaining device 815) may also be rotated.

[0140] The recess processing device 800 can, for example, as shown in the scanning path of Figure 19(A), scan the laser 890 360 degrees in the first direction circumferentially of the cylinder, then move it by a predetermined line pitch 596 in the direction axial of the cylinder, scan the laser 890 360 degrees again in the first direction circumferentially of the cylinder, and then repeat the operation of moving it by a predetermined line pitch 596 in the direction axial of the cylinder. Alternatively, as shown in the scanning path of Figure 19(B), scan the laser 890 360 degrees in the first direction circumferentially of the cylinder, then move it by a predetermined line pitch 596 in the direction axial of the cylinder, scan the laser 890 360 degrees in the second direction circumferentially of the cylinder (opposite to the first direction), and then repeat the operation of moving it by a predetermined line pitch 596 in the direction axial of the cylinder. Furthermore, as shown in the scanning path of Figure 19(C), scan the laser 890 in a spiral (helical) manner in the circumferential and axial directions of the cylinder so as to rotate 360 ​​degrees and move by a predetermined line pitch 596 in the direction axial of the cylinder. Furthermore, as shown in the scanning path of Figure 19(D), for example, the laser 890 can be scanned for a predetermined distance in the first direction along the cylinder axis, then moved by a predetermined line pitch of 596 in the circumferential direction of the cylinder, and the operation of scanning for a predetermined distance in the second direction along the cylinder axis (opposite to the first direction) can be repeated.

[0141] <Arrangement of recesses when operating in a spiral motion> Figure 20 shows the arrangement of multiple recesses 14 formed on the inner wall surface 12 of the cylinder liner 10 by combining the spiral laser scanning path in Figure 19(C) with the processing method for multiple recesses in Figure 13. Note that Figure 20 shows the inner wall surface 12 unfolded in the circumferential direction. The center of the recess 14 in the coaxial direction at the maximum average length J in the cylinder axial direction is defined as the recess center GM. The scanning path (spiral path) of the laser 590 on the inner wall surface 12 is parallel to the line segment connecting the recess centers GM arranged in the circumferential direction. The angle at which the scanning path (spiral direction) intersects with the circumferential direction of the cylinder liner is defined as the twist angle β.

[0142] This twist angle β is set to be greater than 0 degrees and less than 45 degrees. Preferably, the twist angle β is 20 degrees or less, more preferably 10 degrees or less, even more preferably 5 degrees or less, and even more preferably 1 degree or less. Preferably, the twist angle β is determined by the line pitch 596. If the circumference of the cylinder liner 10 is defined as KS and the line pitch 596 as LP, then the twist angle β is given by β = tan -1 It can be defined as (LP / KS). For example, if the circumference of the cylinder liner 10 is defined as 314 mm and the line pitch as 596 mm is defined as 0.010 mm, then β is 1.825 × 10 -4 It becomes a degree.

[0143] As a result, the central GM of the multiple recesses 14, which are spaced apart in the circumferential direction, are arranged to be gradually displaced in the cylinder axis direction along the twist angle β. In other words, the multiple recesses 14 are arranged to follow a helical scanning path. Furthermore, the difference in distance KK between the central GM of the recesses of a pair of adjacent recesses 14 in the cylinder axis direction is set to be smaller than the maximum average length J of the recesses 14, preferably to J / 2 or less.

[0144] It should be noted that the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. [Explanation of Symbols]

[0145] 10 Cylinder Liners 12 Interior wall surface 14 recess 14A Bottom 14a Highest point 14b Lowest point 14e Termination area 14s Start area 20 Center area of ​​the journey 30 pistons 40 piston rings 70 Oil Rings 200 Dare area 500, 800 recess machining equipment 510 Laser Oscillator 570, 870 control unit 590, 890 lasers 590S Laser Spot 592 pulse-to-pulse distance 594 Spot diameter 596 Line Pitch 611 First migration route 612 Second migration route 613 Third migration route 614 Recessed area 620 Band-shaped region 621 First zone region 621e Termination area 622 Second zonal region 623 Second zonal region 623 Third Zone Region 624 Fourth Zone Region 625 Fifth zoning region 626 Sixth Zing Region

Claims

1. A cylinder in which a piston equipped with a piston ring slides against the inner wall surface, Multiple recesses are formed by laser processing in the central stroke region of the inner wall surface, which is all or part of the region from the lower surface position of the ring groove of the lowest piston ring at the top dead center of the piston to the upper surface position of the ring groove of the highest piston ring at the bottom dead center of the piston. A cylinder characterized in that the absolute load gradient of the first essential region defined by the following procedure in the recess is 500 or more. (Definition of the first essential area) - Measure the cross-sectional curve that includes the four consecutive recesses in the evaluation length. - For the cross-sectional curve, calculate the absolute load curve with the cut level (μm) indicating the depth direction of the recess on the vertical axis and the load length (μm) on the horizontal axis. Regarding the absolute load curve, the range of the cut-off level ratio (%) from 25% to 65% is defined as the first essential region on the side surface of the recess. (Definition of absolute load gradient) - Extract the partial curve of the first essential region of the absolute load curve, and calculate the cut level height difference PH1 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW1 (μm), which is the horizontal axis distance of the partial curve. - Calculate the absolute load gradient PW1 / PH1 in the first main region.

2. The cylinder according to claim 1, characterized in that the absolute load gradient of the second main region defined by the following procedure in the recess is 500 or more. (Definition of the second essential area) Regarding the absolute load curve, the range of the cut-off level ratio (%) from 25% to 60% is defined as the second essential region on the side surface of the recess. (Definition of absolute load gradient) - Extract the partial curve of the second essential region of the absolute load curve, and calculate the cut level height difference PH2 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW2 (μm), which is the horizontal axis distance of the partial curve. - Calculate the absolute load gradient PW2 / PH2 in the second main region.

3. The cylinder according to claim 1, characterized in that the absolute load gradient of the third essential region defined in the recess by the following procedure is 500 or more. (Definition of the third essential area) Regarding the absolute load curve, the range of the cut-off level ratio (%) from 25% to 55% is defined as the third essential region on the side surface of the recess. (Definition of absolute load gradient) - Extract the partial curve of the third essential region of the absolute load curve, and calculate the cut level height difference PH3 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW3 (μm), which is the horizontal axis distance of the partial curve. - Calculate the absolute load gradient PW3 / PH3 in the third essential region.

4. The cylinder according to claim 1, characterized in that the absolute load gradient of the fourth essential region defined in the recess by the following procedure is 500 or more. (Definition of the fourth essential area) Regarding the absolute load curve, the range of the cut-off level ratio (%) from 25% to 50% is defined as the fourth essential region on the side surface of the recess. (Definition of absolute load gradient) - Extract the partial curve of the fourth essential region of the load absolute value curve, and calculate the cut level height difference PH4 (μm), which is the vertical axis distance of the partial curve, and the load length difference PW4 (μm), which is the horizontal axis distance of the partial curve. - Calculate the absolute load gradient PW4 / PH4 in the second main region.

5. The recess is characterized by being processed by a laser with a pulse width of 100 picoseconds or less. The cylinder according to claim 1.

6. The first main region of the recess is characterized in that it becomes a tapered surface from the outside to the inside of the recess based on an increase in the number of superimposed irradiations of the laser spot in the laser processing. The cylinder according to claim 1.

Images