Cylinder and method for forming recessed portion of inner wall surface of cylinder

Abstract

This cylinder has an inner wall surface on which a piston with a piston ring slides, and a plurality of recessed portions (14) formed in a stroke central region of the inner wall surface (12) by laser processing such that the surface roughness of a bottom surface (14A) of each recessed portion (14) satisfies at least one of a first state in which a core-portion level difference (Rk) after polishing the bottom surface is less than 1.6 μm, a second state in which a protruding valley-portion depth (Rvk) after polishing the bottom surface is less than 1.3 μm, a third state in which the core-portion level difference (Rk) after performing a chemical conversion treatment and polishing on the bottom surface is less than 1.1 μm, and a fourth state in which the protruding valley-portion depth (Rvk) after performing the chemical conversion treatment and polishing on the bottom surface is less than 0.9 μm. Consequently, friction resistance is reduced, fuel efficiency is improved, or oil consumption is reduced.

Description

Cylinder and method for machining recesses on cylinder inner wall surface

[0001] The present invention relates to a cylinder or the like having a recess formed on its inner wall surface.

[0002] In the past, efforts have been made to reduce the sliding resistance (frictional force) between the cylinder and piston in internal combustion engines having a cylinder and piston in order to improve fuel efficiency and reduce oil consumption. The 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, Japanese Patent Publication No. 5155924), which reduces sliding resistance during operation by forming multiple recesses in the stroke center region of the inner wall surface of the cylinder. These recesses are generally formed by so-called blasting.

[0003] When forming recesses by blasting, it is difficult to stabilize the recess shape and the roughness of the recess bottom due to various factors such as grid variations and wear of the grid discharge nozzle. Specifically, the presence of grids of irregular sizes can easily cause localized irregularities on the side and bottom surfaces of the recess. If localized protrusions form on the bottom surface, their tips can come into contact with the piston and damage it. Furthermore, these localized irregularities are expected to hinder the flow of lubricating oil entering and exiting the recess.

[0004] SUMMARY OF THE INVENTION In view of the above circumstances, the present invention aims to provide a cylinder or the like having a highly accurate recess.

[0005] To achieve the above object, the present invention provides a cylinder in which a piston equipped with piston rings slides on an inner wall surface, the cylinder having a plurality of recesses formed by laser processing in a stroke center region of the inner wall surface that corresponds to all or part of the region between the lower surface of the ring groove of the lowest piston ring at the top dead center of the piston and the upper surface of the ring groove of the highest piston ring at the bottom dead center of the piston, the surface roughness of the bottom surfaces of the recesses satisfying at least one of the following first to fourth conditions: First condition: A core level difference Rk after polishing the bottom surface is less than 1.6 μm; Second condition: A protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm; Third condition: A core level difference Rk after chemical conversion treatment and polishing the bottom surface is less than 1.1 μm; Fourth condition: A protruding valley depth Rvk after chemical conversion treatment and polishing the bottom surface is less than 0.9 μm.

[0006] In relation to the cylinder, the surface roughness of the bottom surface of the recess may satisfy either the fifth or sixth condition below: Fifth condition: The sum of the core level difference Rk and the protruding valley depth Rvk after the bottom surface is polished is less than 2.9 μm. Sixth condition: The sum of the core level difference Rk and the protruding valley depth Rvk after the bottom surface is chemically treated and polished is less than 2.0 μm.

[0007] In relation to the cylinder, the depth of the recess may be 0.1 μm to 1000 μm.

[0008] In relation to the cylinder, the recess may be machined using a laser having a pulse width of 100 picoseconds or less.

[0009] In relation to the cylinder, the periphery of the recess may be a tapered surface from the outside to the inside of the recess due to an increase in the number of overlapping irradiations of the laser spot in the laser processing.

[0010] In relation to the above cylinder, when the maximum axial dimension of the recess of the cylinder is defined as WJ, the axial deviation amount Z of the axial center positions of a pair of recesses adjacent to each other in the circumferential direction of the inner wall surface may be set to 0 < Z < WJ.

[0011] To achieve the above-mentioned object, the present invention provides a recess processing method for forming a recess of a size larger than the spot diameter of the laser spot by irradiating a recess target area in which the recess is to be formed with a laser spot of a laser having a pulse width of less than 100 picoseconds along a linear first movement path, and recessing a first band-shaped area; and after the first irradiation step, a second irradiation step in which the recess target area is irradiated with the laser spot, which overlaps with at least a portion of the first band-shaped area, along a linear second movement path that is parallel to the first movement path and has a center line offset from the center line of the first movement path, to recess a second band-shaped area that overlaps with the first band-shaped area.

[0012] In relation to the above-mentioned recess processing method, a plurality of recess target areas are present at intervals from each other along the circumferential direction of the inner wall surface, and the method may include a first irradiation group process in which the laser spot is irradiated along the first circumferential movement path to perform the first irradiation process on each of the plurality of recess target areas, and a second irradiation group process in which, after the first irradiation group process, the laser spot is irradiated along the second circumferential movement path to perform the second irradiation process on each of the plurality of recess target areas.

[0013] In the recess processing method, the scanning path of the laser spot in the first irradiation group step may be along a spiral path.

[0014] In relation to the recess processing method, the recesses formed on the inner wall surface may be arranged along the spiral path.

[0015] In relation to the recess processing method, the surface roughness of the bottom surface of the recess may satisfy at least one of the following first to fourth conditions: First condition: A core level difference Rk after polishing the bottom surface is less than 1.6 μm; Second condition: A protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm; Third condition: A core level difference Rk after chemical conversion treatment and polishing the bottom surface is less than 1.1 μm; Fourth condition: A protruding valley depth Rvk after chemical conversion treatment and polishing the bottom surface is less than 0.9 μm.

[0016] In relation to the above-mentioned recess processing method, when the spot diameter of the laser spot is defined as C and the offset amount of the center line between the first movement path and the second movement path is defined as OS, it may be characterized in that OS≦C / 2.

[0017] In relation to the recess machining method, the axial size of the recess may be larger than a contact width between the piston ring and the inner wall surface.

[0018] According to the present invention, it is possible to obtain the excellent effects of reducing frictional resistance, improving fuel efficiency, or reducing oil consumption.

[0019] 1A and 1B are cross-sectional views along the axial direction of a cylinder liner used in an internal combustion engine according to an embodiment of the present invention. (A) and (B) are development views showing the inner peripheral wall of the cylinder liner developed in the circumferential direction. (A) and (B) are cross-sectional views of the inner peripheral wall of the cylinder liner taken perpendicular to the axis. (A) and (B) are cross-sectional views showing the cross-sectional curves of the inner peripheral wall and recesses of the cylinder liner. (A) is a side view of a piston and piston ring used in the internal combustion engine, (B) is a partially enlarged cross-sectional view of the piston and piston ring, (C) is a partially enlarged cross-sectional view of a top ring, and (D) is a partially enlarged cross-sectional view of a second ring. (A) is a cross-sectional view of a two-piece oil ring, and (B) is a cross-sectional view of a three-piece oil ring. (A) is a Stribeck diagram, and (B) is an FMEP diagram, showing sliding in a typical internal combustion engine. (B) is a perspective view showing a recess machining device for the cylinder liner. (A) to (D) are diagrams showing the machining procedure for recesses in the cylinder liner, and (E) and (F) are diagrams showing modified examples of the laser scanning method. (A) is a cross-sectional view in the scanning direction, and (B) is a cross-sectional view in a direction perpendicular to the scanning direction, showing the machining procedure for recesses in the cylinder liner. (A) to (D) are diagrams showing the procedure for machining multiple recesses in the cylinder liner at once. (A) to (C) are graphs showing the surface roughness of the bottom surface of the recesses in the cylinder liner. (A) is a front view showing a modified example of the recess machining device. (A) to (D) are perspective views conceptually showing the laser scanning path in the recess machining device. (A) is a development view showing the inner peripheral wall of the cylinder liner developed in the circumferential direction. (B) is a cross-sectional view along the axial direction of the cylinder liner showing an example of a cylinder liner to which microtexture technology is applied.

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. First, a cylinder of an internal combustion engine according to an embodiment of the present invention will be described in detail. Note that the cylinder in this embodiment is an example of a cylinder liner 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.

[0021] <Cylinder liner>

[0022] As shown in Figure 1, a plurality of recesses 14 are formed in the inner wall surface 12 of the cylinder liner 10 of the internal combustion engine of this embodiment. The recesses 14 are formed in a mid-stroke region 20 of the inner wall surface 12. This mid-stroke region 20 refers to the entire or partial region extending from the bottom surface of the ring groove of the lowest piston ring at top dead center T of the piston 30 to the top surface of the ring groove of the highest piston ring at bottom dead center U of the piston 30 (this example illustrates a case in which the entire region is the mid-stroke region 20 and recesses 14 are formed throughout the entire region). If the region outside the mid-stroke region 20 is defined as an outer region 25, this outer region 25 is composed of an upper outer region 25A adjacent to the top dead center side of the mid-stroke region 20 and a lower outer region 25B adjacent to the bottom dead center side of the mid-stroke region 20. As the piston 30 reciprocates within the cylinder liner 10, it repeatedly passes through the upper outer region 25A, the mid-stroke region 20, the lower outer region 25B, the mid-stroke region 20, and the upper outer region 25A in this order. The boundary between the upper outer region 25A and the mid-stroke region 20 is defined as an upper boundary 27A, and the boundary between the lower outer region 25B and the mid-stroke region 20 is defined as a lower boundary 27B.

[0023] Of course, it is possible to form multiple recesses 14 beyond the mid-stroke region 20, but from the perspective of oil consumption (LOC), it is preferable to form recesses 14 limited to within the mid-stroke region 20.

[0024] <Dimples formed on cylinder liners>

[0025] The recesses 14 are arranged so that at least one recess 14 exists in any cross section taken in the axis-perpendicular direction on the inner wall surface 12 of the mid-stroke region 20. That is, the recesses 14 are arranged so that they overlap in the cylinder axial direction. As a result, the outer circumferential surface of a piston ring passing through the mid-stroke 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.

[0026] The recesses 14 are shaped like polygons (squares, rectangles, or hexagons) obliquely arranged relative to the cylinder axial direction; here, a hexagon is used. As shown in the developed view of FIG. 2A , when focusing on a specific recess 14, the axially lowest point 14b of that recess 14 is axially lower than the axially highest points 14a of the other recesses 14. Because multiple recesses 14 overlap in the cylinder axial direction, recesses 14 can always be present in cross sections perpendicular to the axis at all locations (e.g., arrows A, B, and C) in the stroke center region 20. Here, multiple recesses 14 with the same area are uniformly arranged in the plane direction (the cylinder axial direction and the cylinder circumferential direction) in the stroke center region 20.

[0027] Specifically, as shown in the developed view of FIG. 2B, a plurality of recesses 14 are always present in the band-like regions 20P, 20Q, 20R where the piston ring contacts the mid-stroke region 20.

[0028] The dimensions and shape of the recesses 14 are not particularly limited and are selected appropriately depending on the dimensions and purpose of the cylinder and piston ring. For example, the recesses 14 can be formed in a slit or band shape so as to penetrate (or extend) through the mid-stroke region 20 in the axial direction of the cylinder. From the perspective of airtightness of the cylinder, the maximum average length J (see FIG. 2A ) of the recesses 14 in the axial direction is preferably equal to or less than the axial length (width) of the piston ring (top ring) located at the top of the piston, specifically, approximately 5 to 100% of that length. The average length J of the recesses 14 refers to the average value when there is variation in the maximum axial dimensions of multiple recesses 14. Furthermore, when the lengths of the recesses 14 are mixed, the maximum dimension is used as the average length J, and it is preferable that it does not exceed the axial length (width) of the piston ring (top ring) located at the top of the piston. The maximum average axial length J of the recesses 14 in the axial direction of the cylinder is preferably 0.1 mm to 15 mm, more preferably 0.1 mm to 5 mm, and even more preferably 0.1 mm to 1.0 mm.

[0029] 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, more preferably in the range of 0.1 mm to 5 mm, and even more preferably in the range of 0.1 mm to 1.0 mm. If it is smaller than these ranges, the effect of reducing the sliding area by 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, causing deformation of the piston ring or, if the recess 14 is shallow, contact with the bottom surface of the recess.

[0030] As shown in FIG. 3A, the maximum average length R (maximum average depth R) of the recesses 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. It is more preferably set to 0.1 μm to 50 μm, and even more preferably 0.1 μm to 10 μm. In this embodiment, the maximum average depth R is set to 2.5 μm. If the maximum average length R of the recesses 14 in the cylinder radial direction is smaller than this range, the effect of reducing the sliding area of ​​the recesses 14 itself may not be sufficient, or the bottom surface 14A of the recesses 14 may come into contact with the piston ring. The definition of the maximum average depth R will be described later.

[0031] On the other hand, if the maximum average depth R is to be made larger than these ranges, the processing time will be too long and the cylinder wall thickness will need to be increased. For ease of explanation, the recesses 14 in Fig. 3 are drawn with the maximum average length R direction greatly exaggerated relative to the direction of the maximum average length J.

[0032] Returning to Figure 2, the average value of the minimum distance Hc in the cylinder circumferential direction between adjacent recesses 14 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. It is even more preferably 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, preventing stable sliding. On the other hand, if it is larger than these ranges, the effect of reducing the sliding area of ​​the recesses 14 itself may not be sufficiently obtained.

[0033] The average value of the minimum distance Ha in the cylinder axial direction between recesses 14 that are adjacent in the cylinder axial direction 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. It is more preferably 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, preventing stable sliding. On the other hand, if it is larger than these ranges, the effect of reducing the sliding area of ​​the recesses 14 itself may not be sufficiently obtained.

[0034] Furthermore, regardless of the direction, the average value of the minimum distance Hm between adjacent recesses 14 is preferably in the range of 0.001 mm to 15 mm, and particularly preferably in the range of 0.001 mm to 5.0 mm. If it is greater than these ranges, the effect of reducing the sliding area of ​​the recesses 14 itself may not be sufficiently obtained.

[0035] These intervals Hc, Ha, and Hm are, in other words, synonymous with the minimum width in each direction of the inner wall surface 12 remaining between adjacent recesses 14. Therefore, although details will be described later with reference to Fig. 4(B), these intervals Hc, Ha, and Hm mean the distance between the evaluation starting point GS that first intersects with the upper reference height line G1 of one recess 14 and the evaluation starting point GS that first intersects with the upper reference height line G1 of the other recess 14 adjacent thereto.

[0036] <Detailed shape definition inside the dimple>

[0037] As shown enlarged in FIG. 3B , 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 where it approaches the inner wall surface 12. This sagging region 200 is a small inclined surface that begins to drop into the recess 14, with the inner wall surface 12 as the reference surface. The sagging region 200 can be considered to be a partial region of the side surface 300 of the recess 14, particularly a region close to the inner wall surface 12. Note that this sagging region 200 can also be considered to be a partial region formed at the end of the inner wall surface 12. In either case, the sagging region 200 is formed locally near the boundary between the inner wall surface 12 and the recess 14.

[0038] The bottom surface 14A and the sagging region 200 are formed by further polishing the recess 14 formed by laser processing, which will be described later. By performing the polishing process particularly carefully, the bottom surface 14A of the recess 14 is further smoothed, and at the same time, a smooth sagging region 200 is formed.

[0039] In this embodiment, in order to objectively evaluate the shapes 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.

[0040] (Measurement of the Profile Curve) As shown in Figure 4(A), the shapes of at least two recesses 14 that are aligned in the axial direction and located at the same position in the circumferential direction of the cylinder are measured using a stylus-type surface roughness tester (JIS B 0651:2001) while passing through the location where the minimum distance Ha between them is located and including the adjacent inner wall surfaces 12. The measurement is performed using a standard 2 μm tip radius of the stylus, and a cutoff value (wavelength) λs for the profile curve is set to 2.5 μm. The profile curve DK to be measured should include at least the two adjacent recesses 14, the pair of inner wall surfaces 12A and 12C that extend outward from the two recesses 14 in the axial direction, and the inner wall surface 12B that exists between the two recesses 14.

[0041] 4A, the range of the three inner wall surfaces 12A to 12C (two recesses 14) is extracted from the cross-sectional curve DK, 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.

[0042] The actual wall surfaces 12A-12C contain minute irregularities corresponding to the surface roughness. Because these minute irregularities can be evaluated using Ra (arithmetic mean roughness), in order to eliminate the influence of these irregularities when defining the starting point of the side surface 300, a line offset from the inner wall surface reference height line GK by an amount equivalent to Ra toward the base of the recess 14 is defined as the upper reference height line G1. However, even if Ra is small, an offset of at least 0.2 μm eliminates the influence of irregularities in the boundary (edge) shape between the wall surfaces 12A-12C and the side surface 300. In other words, the offset amount of the upper reference height line G1 is selected to be either 0.2 μm or the Ra (arithmetic mean roughness) of the wall surfaces 12A-12C, whichever is larger.

[0043] This upper reference height line G1 refers to a virtual starting point position in the depth direction on the side surface 300. Therefore, the point where the cross-sectional curve DK finally intersects with the upper reference height line G1 from the wall surfaces 12A to 12C toward the inside of the recess 14 is defined as the evaluation starting point GS of the side surface 300. The length GL connecting the two evaluation starting points GS, GS of 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 a range of 1 / 4 GL on both sides outside the recess center GM (a total range of 1 / 2 GL) is defined as the reference bottom surface range 14G on the bottom surface 14A.

[0044] (Definition of the maximum average depth of recesses) The cross-sectional curve DK of the reference bottom range 14G is converted into a single straight line using the least squares method. This single straight line is defined as the bottom reference height line BK. As another calculation method, the cross-sectional curve DK is enlarged 5,000 times vertically and 50 times horizontally, the third highest peak of the cross-sectional curve DK is extracted, and the third lowest valley bottom of the cross-sectional curve DK is extracted. The height midway between the peak and the valley bottom may be defined as the bottom reference height BK. The maximum average depth R of the recesses 14 is the average value of the distances between the inner wall surface reference height lines GK and the bottom reference height lines BK of a total of 20 recesses 14.

[0045] (Parameters of surface roughness of bottom surface) Furthermore, for the reference bottom surface range 14G of a total of 20 recesses 14 defined as 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.

[0046] 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: If the core level difference Rk is large, there is a higher risk that the piston ring will come into contact with the reference bottom surface range 14G. If the piston ring comes into contact with the reference bottom surface range 14G, there is a higher risk that scratches will occur during sliding. If the protruding valley depth Rvk is large, the excess oil reservoir volume within the recess 14 increases, resulting in increased oil consumption. Furthermore, the smaller the core level difference Rk and the protruding valley depth Rvk, the less likely the flow of lubricating oil entering and exiting the recess 14 is obstructed.

[0047] When measuring the surface roughness of the reference bottom surface range 14G, the following two conditions can exist. One is the surface roughness after processing the recesses 14 with a laser and then polishing (hereinafter referred to as the "surface roughness after laser polishing"). The other is the surface roughness after processing the recesses 14 with a laser, further performing a chemical conversion treatment (coating treatment), and then polishing (hereinafter referred to as the "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, coating components penetrate into the fine irregularities in the reference bottom surface range 14G, resulting in a corresponding decrease in surface roughness.

[0048] In this embodiment, the surface roughness after laser polishing is set to a core level difference Rk of less than 1.6 μm and a protruding valley depth Rvk of less than 1.3 μm. Also, in this embodiment, the surface roughness after laser coating and polishing is set to a core level difference Rk of less than 1.1 μm and a protruding valley depth Rvk of less than 0.9 μm.

[0049] Furthermore, the surface roughness after laser polishing is set to a value (Rk + Rvk) obtained by adding the core level difference Rk and the protruding valley depth Rvk to less than 2.9 μm.The surface roughness after laser coating and polishing is set to a value (Rk + Rvk) obtained by adding the core level difference Rk and the protruding valley depth Rvk to less than 2.0 μm.

[0050] In this embodiment, by setting the core level difference Rk of the reference bottom surface range 14G small as described above, the depth of the recess 14 becomes shallow, and even if the piston ring comes into contact with the bottom surface 14A, contact scratches or the like are unlikely to be formed. 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 improved.

[0051] (Definition of Side Surface Inclination Angle) Next, the definition of the inclination angle of the side surface 300 will be described. As shown in FIG. 4B, the cross-sectional curve DK of the side surface 300 of the recess 14 is enlarged and extracted. A line offset by R / 2 from the bottom surface reference height line BK toward the upper side of the recess 14 is defined as the side surface lower reference height line G3. This offset of the side surface lower reference height line G3 reduces the influence of unevenness on the bottom surface 14A. The length in the cylinder radial direction (side surface inclination evaluation height) 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 side surface lower reference height line G3.

[0052] The point where the cross-sectional curve DK first intersects with the side lower reference height line G3 is defined as the side surface inclination evaluation end point SE, and the line segment SS connecting the evaluation start point GS of the side surface 300 and the side surface inclination evaluation end point SE is defined as the imaginary side surface cross section SS of the side surface 300. The length of the imaginary side surface cross section SS in the direction parallel to the straight line 250 (here, the cylinder axial direction) is defined as the side surface inclination evaluation width SW of the side surface 300. Using the straight line 250 as a reference, the absolute value of the slope SH / SW of this imaginary side surface cross section SS (hereinafter defined as the side surface slope SA) is calculated.

[0053] Note that the cross-sectional curve DK contains minute irregularities, resulting in variations in each measurement. Therefore, each side surface 300 is used as a target and measurements are performed 60 times using a stylus-type surface roughness measuring instrument (JIS B 0651:2001). The maximum and minimum values ​​of the 60 calculated side surface slopes SA are then cut, and the average value is calculated from the middle 30 data points. This average value is used as the evaluation side surface slope SA of the side surface 300.

[0054] (Definition of sagging region) 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 of the recess 14 is defined as the sagging lower reference height line G2. This sagging lower reference height line G2 represents the virtual end position (end height) of the sagging region 200 on the side surface 300. As a result, the length (sagging height) DH of the sagging region 200 in the cylinder radial direction is defined as 0.50 µm.

[0055] The point where the cross-sectional curve DK first intersects with the lower reference height line G2 is defined as the sag end point GE, and the line segment GG connecting the evaluation start point GS of the side surface 300 and the sag end point GE is defined as the imaginary sag cross section GG of the sag region 200. The length of the imaginary sag cross section GG in the direction parallel to the straight line 250 (here, the cylinder axial direction) 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 imaginary sag cross section GG (hereinafter defined as the sag gradient DA) is calculated.

[0056] Note that the cross-sectional curve DK contains minute irregularities, etc., and therefore variations occur between measurements. Therefore, each sag region 200 is targeted and measurements are performed 60 times using a stylus-type surface roughness measuring instrument (JIS B 0651:2001). The 15 largest and 15 smallest sag gradients DA of the 60 calculated values ​​are cut, and the average value is calculated from the middle 30 data points. This average value is used as the evaluation sag gradient DA of the sag region 200. Note that, while an example is given here of using the sag gradient DA when evaluating the formation status of the sag region 200, evaluation using the sag width DW as is is equivalent.

[0057] While different from dimple liners, in which multiple recesses are arranged to overlap in the cylinder axial direction, microtexturing technology exists as a method for forming similar recesses. This technology will be briefly described below. Microtexturing, as shown in Figure 16 , is a theory in which regions V where recesses are formed and regions ZX where no recesses are present are alternately repeated along the cylinder axial direction of the inner wall surface of the cylinder liner, with no overlapping in the cylinder axial direction. As the piston ring moves along this inner wall surface, engine oil flows into and out of the recesses, and the resulting dynamic pressure thickens the oil film, reducing friction. The present invention can also be applied to such microtexturing technology. In other words, the present invention can be effectively applied to any structure in which multiple recesses are formed in the mid-stroke region to reduce the contact area with the piston ring.

[0058] <Central low roughness region formed on cylinder liner>

[0059] 1 , the inner wall surface 12 of the cylinder liner 10 has a central low-roughness region 22 in at least a portion of the stroke mid-region 20, where the surface roughness (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) measured with a stylus surface roughness tester (JIS B 0651:2001) is 0.140 μm or less, preferably 0.120 μm or less. Specifically, the central low-roughness region 22 is formed by processing the surface of the inner wall surface 12 that may come into contact with the piston ring 40, i.e., at least a portion of the surface excluding the recesses 14 of the inner wall surface 12, to a surface roughness Ra of 0.140 μm or less, more preferably 0.120 μm or less. In this embodiment, the surface roughness measured by the stylus surface roughness measuring instrument (arithmetic mean roughness of the profile curve) is denoted as Ra, and the three-dimensional surface roughness measured by the non-contact surface roughness measuring instrument described below (arithmetic mean height of the profile curve (JIS B 0681-2:2018, ISO 25178-2:2012)) is denoted as Sa.

[0060] More preferably, the surface roughness Ra of the central low-roughness region 22 is set to 0.090 (μm) or less, specifically 0.083 (μm).

[0061] The three-dimensional surface roughness values ​​(JIS B 0681-2:2018, ISO 25178-2:2012) of this central low-roughness region 22 measured using a non-contact surface roughness measuring instrument (measurement magnification 1080x, field of view size 259.4 μm × 259.4 μm, no cutoff, height direction (Z direction) measurement line pitch 0.06 μm) using a laser microscope in accordance with JIS B 0681-6:2014 (ISO 25178-6:2010) are shown below. Arithmetic mean height Sa (μm): 0.192 or less, preferably 0.163 or less, and more preferably 0.120 or less (specifically set to 0.110). Protruding peak height Spk (μm): 0.159 or less, preferably 0.144 or less, and more preferably 0.121 or less (specifically set to 0.116). Core level difference Sk (μm): 0.521 or less, preferably 0.449 or less, and more preferably 0.340 or less (specifically set to 0.315). Protruding valley depth Svk (μm): 0.409 or less, preferably 0.342 or less, and more preferably 0.241 or less (specifically set to 0.218).

[0062] In particular, in this embodiment, frictional force during sliding is reduced by not only reducing the height of the protruding peaks on the inner wall surface 12 but also actively reducing the depth of the protruding valleys on the inner wall surface 12. Incidentally, in conventional cylinder liners, the depth of the protruding valleys must be increased to a certain extent to ensure the lubricating oil retention capacity of the inner wall surface 12 itself. This makes it difficult to reduce the height of the protruding peaks, limiting the reduction in frictional force during sliding. In contrast, in this embodiment, lubricating oil is sufficiently retained in the recesses 14, whose bottom surfaces 14A are highly smooth. Therefore, the lubricating oil in the recesses 14 can easily migrate to the surrounding inner wall surface 12. Therefore, even if the contact surface (inner wall surface 12) itself with the piston ring 40 has a low lubricating oil retention capacity, a sufficient oil film can be formed.

[0063] With this in mind, if the height of the protruding peaks on the inner wall surface 12 is Spk and the depth of the protruding valleys on 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 2.4 or less, and even more preferably 2.0 or less.

[0064] In this embodiment, the entire surface area of ​​the mid-stroke region 20 that may come into contact with the piston ring 40 is defined as the central low-roughness region 22. As a result, the central low-roughness region 22 includes the vicinity of the upper and lower edges of the mid-stroke region 20 where the recesses 14 are formed. Furthermore, an upper low-roughness region 23A having a surface roughness Ra of 0.120 μm or less is formed in the upper outer region 25A adjacent to the top dead center side of the mid-stroke 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 mid-stroke 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 uniform surface roughness, forming an integral, continuous surface as a whole.

[0065] Near the upper and lower edges of the mid-stroke region 20, the relative velocity Q between the cylinder liner 10 and the piston 30 decreases, making it easy for the region to transition from the hydrodynamic lubrication region to the boundary lubrication region. However, the presence of this central low-roughness region 22 allows the hydrodynamic lubrication region to be dominant. Since so-called dimple liner technology can exert its effects in the hydrodynamic lubrication region, the benefits of dimple liner technology can also be obtained near the upper and lower edges of the mid-stroke region 20. While it is possible to limit the central low-roughness region 22 to the upper and / or lower edges of the mid-stroke region 20, it is preferable to form the central low-roughness region 22 over the entire mid-stroke region 20, as in this embodiment. When the relative velocity Q between the cylinder liner 10 and the piston 30 becomes slower, the boundary lubrication region approaches the center of the mid-stroke region 20, but even in this case, the range of the hydrodynamic lubrication region can be expanded.

[0066] The central low-roughness region 22 of the inner wall surface 12 of the cylinder liner 10 is formed by honing using a honing machine. The grain size of the honing stone used here is preferably finer than, for example, F500 or #800 (JIS R 6001-2:2017, ISO8486-2:2007).

[0067] <About the chemical conversion treatment process>

[0068] Furthermore, when the central low-roughness region 22 is formed by this honing process, it is preferable not to perform a chemical conversion treatment on the surface. For example, if a phosphate coating, which is commonly used in the manufacturing process of the cylinder liner 10, is performed, the surface properties of the central low-roughness region 22 will vary depending on the coating.

[0069] On the other hand, if the coating has minimal variation in surface texture, it is also desirable to apply a chemical conversion treatment to the central low-roughness region 22. In this embodiment, the following two procedures are possible for applying a chemical conversion treatment to the inner wall surface 12. The first procedure involves forming recesses 14 on the inner wall surface 12 of the cylinder liner 10 using a laser, then performing a chemical conversion treatment, followed by polishing. In the first procedure, a coating is formed on the bottom surface 14A of the laser-machined recesses 14. The coating then blends in during the polishing process, further reducing the fine irregularities on the bottom surface 14A of the recesses 14. However, since the depth of the recesses 14 is reduced by the thickness of the coating, the recesses 14 must first be machined deeper by the laser by an amount equivalent to the thickness of the coating, which increases the processing time. For example, if a 0.5 μm coating is formed by chemical conversion treatment, the recesses 14 must be formed deeper by the laser by 0.5 μm.

[0070] The second procedure involves performing a chemical conversion treatment on the inner wall surface 12 of the cylinder liner 10, forming the recesses 14 with a laser, and then polishing. In the second procedure, no coating is formed on the bottom surface 14A of the recesses 14, but since the bottom surface 14A of the recesses 14 formed by the laser has small Rk and Rvk, there is little need to use a coating to achieve a good fit. Another advantage is that the recesses 14 can be processed with a laser without considering the thickness of the coating, which shortens the processing time.

[0071] <Piston and piston ring>

[0072] 5A and 5B show a piston 30 and piston rings 40 (top ring 50, second ring 60, and 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 peripheral surfaces 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 gas leaking from the combustion chamber to the crankcase (blow-by). The second ring 60, like the top ring 50, serves both to eliminate the gap between the piston 30 and the cylinder liner 10 and to scrape 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 from the inner wall surface 12 of the cylinder liner 10 and forms an appropriate oil film, preventing the piston 30 from seizing.

[0073] As shown enlarged in Figure 5(C), the top ring 50 is a single annular member, and when viewed in cross section from its outer peripheral surface 52, it has a so-called barrel shape that is convex outward in the radial direction. Specifically, both outer edges of the outer peripheral surface 52 in the cylinder axial direction are inclined outward in the cylinder axial direction, away from the inner wall surface 12. It is preferable that the contact width f of the outer peripheral surface 52 with the inner wall surface 12 of the cylinder liner 10 be 0.3 mm or less. It is also preferable that the surface roughness of the outer peripheral surface 52 (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) measured with a stylus-type surface roughness measuring instrument (JIS B 0651:2001) be 0.250 (μm) or less.

[0074] As shown enlarged in Figure 5(D), the second ring 60 is a single annular member, and its outer periphery is tapered from the upper end in the axial direction to the lower end in the 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, is flat in cross section. The contact width f of the outer peripheral surface 62 with the inner wall surface 12 of the cylinder liner 10 is preferably 0.3 mm or less. The surface roughness of the outer peripheral surface 62 (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) measured with a stylus-type surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.250 μm or less.

[0075] The tension of the top ring 50 and the second ring 60 is set to a relatively low value, and the surface pressure acting on the contact surfaces of the outer peripheral surfaces 52, 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.

[0076] The oil ring 70, shown enlarged in FIG. 6A , is a two-piece type oil ring. It includes a ring body 72 and a coil spring-like coil expander 76. The ring body 72 has a pair of annular rails 73, 73 disposed at both axial ends thereof, and an annular pillar portion 75 disposed between and connecting the pair of rails 73, 73. The cross-sectional shape of the pair of rails 73, 73 and the pillar portion 75 is generally I-shaped or H-shaped. Utilizing this shape, an inner circumferential groove 79 with a semicircular cross section for accommodating the coil expander 76 is formed on the inner circumferential surface side. Furthermore, the pair of rails 73, 73 each have annular protrusions 74, 74 that protrude radially outward relative to the pillar portion 75. The outer circumferential surfaces 82, 82 formed at the ends of the annular protrusions 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 pressing and biasing the ring body 72 radially outward. Note that a plurality of oil return holes 77 are formed in the column portion 75 of the ring body 72 in the circumferential direction.

[0077] The contact width of each of the pair of outer peripheral surfaces 82, 82 in FIG. 6A is preferably 0.02 mm to 0.30 mm, and is set to, for example, 0.15 mm. The surface pressure acting on the contact surface of the outer peripheral surface 82 of the oil ring 70 is, for example, 1.0 MPa to 2.0 MPa, e.g., approximately 1.75 MPa. Therefore, when the engine speed is high, the oil ring 70 often slides in the hydrodynamic lubrication region, but as the engine speed decreases, it often slides in the boundary lubrication region. While FIG. 6A illustrates an example in which the radial cross-sectional shape of the outer peripheral surfaces 82, 82 is a simple trapezoid, the present invention is not limited thereto. The outer peripheral surfaces 82 of the upper rail 73 and the lower rail 73 may have corners on the opposing sides (the coil expander 76 side) cut out in a step shape (a so-called step land shape). Furthermore, the surface roughness (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) of the outer peripheral surface 82 measured by a stylus surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.450 (μm) or less.

[0078] The oil ring 70 is not limited to a two-piece type, and may be, for example, a three-piece type oil ring 70 shown in Fig. 6(B) . This oil ring 70 has annular side rails 73a and 73b separated into upper and lower sections, and a spacer expander 76s disposed between the side rails 73a and 73b.

[0079] The spacer expander 76s is formed by plastic working a steel material into a corrugated shape with repeated concave and convex portions in the axial direction of the cylinder. This corrugated shape forms an upper support surface 78a and a lower support surface 78b, which support the pair of side rails 73a, 73b in the axial direction. The inner peripheral end of the spacer expander 76s has ears 74m that are arched outward in the axial direction. These ears 74m abut the inner peripheral surfaces of the side rails 73a, 73b. The spacer expander 76s is fitted into the ring groove of the piston 30 in a circumferentially contracted state with the butt joints abutting against each other. As a result, the restoring force of the spacer expander 76s causes the ears 74m to press and urge the side rails 73a, 73b radially outward.

[0080] It is preferable that the contact width f of each of the outer peripheral surfaces 82, 82 of the side rails 73a, 73b in FIG. 6B is set to 0.02 mm to 0.40 mm.

[0081] <Friction between cylinder liner and piston ring>

[0082] Next, the frictional behavior between a cylinder liner and a piston ring will be described. The change in the friction coefficient during typical sliding is expressed as a Stribeck diagram shown in FIG. 7A. This Stribeck diagram separates the frictional behavior into the solid contact region 110 (direct contact), the boundary lubrication region 112 (sliding through an oil film), and the hydrodynamic lubrication region 114 (sliding through a viscous lubricant film). Between the boundary lubrication region 112 and the hydrodynamic lubrication region 114, there exists a frictional behavior called the mixed lubrication region 113, where both conditions coexist. In this Stribeck diagram, the horizontal axis represents the logarithmic representation of "kinematic viscosity (kinetic viscosity coefficient) μ" × "velocity Q" / "contact load W," and the vertical axis represents the friction coefficient (f). Therefore, the hydrodynamic lubrication region 114 or the mixed lubrication region 113 can provide the smallest frictional force. Effective utilization of these regions 114 and 113 is effective for reducing friction, i.e., improving fuel economy. On the other hand, if the speed Q increases but the boundary lubrication region 112 cannot transition to the fluid lubrication region 114, the boundary lubrication region 112 continues into the high speed region, as shown by the dotted line.

[0083] Incidentally, most of the friction force in the fluid lubrication region 114 is due to the shear resistance of the oil, which is defined as (viscosity) x (velocity) x (area) / (oil film thickness). As a result, reducing the shear area directly leads to a reduction in friction force.

[0084] Therefore, in this embodiment, oil is actively introduced into the contact surface of the outer circumferential surface 42 of the piston ring 40, quickly transitioning to the fluid lubrication region 114 and achieving low friction. At the same time, by applying so-called dimple liner technology to the cylinder liner 10, recesses 14 are formed in the mid-stroke region 20 of the cylinder liner 10, reducing the effective area where shear resistance of the oil occurs, thereby achieving a more efficient reduction in frictional force.

[0085] The Stribeck diagram in FIG. 7A shows the dynamic change in the friction coefficient (f) during one stroke of the piston 30. Another index for evaluating friction behavior is the friction mean effective pressure (FMEP). This friction mean effective pressure is calculated by dividing the friction work per cycle by the stroke volume. A diagram of this friction mean effective pressure (FMEP diagram) is shown in FIG. 7B. 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 greater the proportion of the hydrodynamic lubrication region 114 during one stroke. On the other hand, as the rotational speed (N) decreases, the proportion of the hydrodynamic lubrication region 114 during one stroke decreases, and the proportion of the mixed lubrication region 113 (or boundary lubrication region 112) increases. Therefore, the shape of the FMEP diagram in FIG. 7B is relatively similar to the shapes of the hydrodynamic lubrication region 114 and the mixed lubrication region 113 in the Stribeck diagram in FIG. 7A.

[0086] <Recess processing device>

[0087] Next, the machining of the recess 14 in the cylinder liner 10 will be described. As shown in FIG. 8 , the recess machining device 500 employs a galvanometer mirror system and includes 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, an end mirror 718, and a control device 570. A laser 590 emitted from the laser oscillator 510 is reflected by the X-axis mirror 704, the Y-axis mirror 714, and the end mirror 718 before reaching the cylinder liner 10. Therefore, the scanning direction relative to the cylinder liner 10 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 in which the laser 590 is reflected by a polygonal rotating mirror for scanning can also be employed.

[0088] The laser oscillator 510 is a short-pulse laser oscillator. A short pulse preferably has 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 capable of setting the pulse width to less than 1 picosecond (less than 1,000 femtoseconds) is called a femtosecond laser oscillator, and this embodiment employs a femtosecond laser oscillator. The laser oscillator includes a seed pulse unit that oscillates a weak femtosecond laser (seed pulse), an amplifier unit that amplifies the seed pulse using an excitation laser, and a compensation unit that performs dispersion compensation on the amplified laser. The laser 590 emitted from the laser oscillator 510 can have a pulse width set to less than 1 picosecond (in femtosecond units). The laser oscillator 510 is set to have a pulse width of, for example, 400 femtoseconds to 20,000 femtoseconds. The laser wavelength may be any wavelength suitable for metal material processing, and may be, for example, a wavelength of 100 nm to 3,000 nm. The laser frequency may be 20 kHz to 2,000 kHz. The pulse energy may be, for example, up to 2,000 μJ. In this embodiment, the laser wavelength is set to 1,030 nm, but is not particularly limited as long as it is a wavelength suitable for metal processing.

[0089] The control device 570 is a computer having a central processing unit (CPU), a memory, an external storage device, etc. A recess processing program is executed in this control device 570, whereby recess processing by the recess processing device 500 is realized.

[0090] <Processing method for recessed parts alone>

[0091] Next, detailed control of the recess processing device 500 by the control device 570 will be described. First, a method for processing each recess 14 individually will be described. As shown in FIG. 9A, recess target areas 614 in which recesses 14 are to be formed are present on the inner wall surface 12. The recess processing device 500 processes the first band-shaped area 621 into a recessed groove shape by irradiating each recess target area 614 with a laser spot 590S of the laser 590 along a linear first movement path 611. This process is defined as a first irradiation process. Note that both ends of the first movement path 611 intersect with the contour of the recess target area 614.

[0092] When the first irradiation step is completed, the heat accumulated at the arbitrary point X of the first strip region 621 is diffused or released to the outside.

[0093] After the first irradiation step, as shown in FIG. 9B , the recess processing apparatus 500 irradiates the recess target region 614 with a laser spot 590S that overlaps at least a portion of the first band region 621 along a linear second movement path 612 that is parallel to the first movement path 611 and offset from the center line of the first movement path 611 by a line pitch 596, thereby processing the second band region 622 that overlaps the first band region 621 into a groove shape. This step is defined as a second irradiation step. Note that both ends of the second movement path 612 intersect with the contour of the recess target region 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).

[0094] When the offset amount of the line pitch 596 is defined as OS and the spot diameter of the laser spot is defined as C, it is preferable to set OS≦C / 2. In this way, it becomes possible to overlap the strip-shaped regions on the bottom surface 14A of the recess 14.

[0095] When the second irradiation step is completed, the heat accumulated in the second strip-shaped region 622 is diffused or released to the outside.

[0096] As shown in Figure 9(C), a third irradiation step is performed using exactly the same procedures as the first and second irradiation steps described above, to process a third band-shaped region 623 along a third movement path 613. Thereafter, a fourth irradiation step, a fifth irradiation step, and so on are repeated. As a result, as shown in Figure 9(D), multiple band-shaped regions 620 are formed in parallel in the band width direction, spreading out in a planar manner within the recess target region 614. The center lines of adjacent band-shaped regions 620 are offset from each other by a predetermined line pitch 596 and are parallel to each other. This forms the recess 14.

[0097] 9(A) to 9(D) illustrate an example in which all laser scanning directions are parallel to (or approximately parallel to) the circumferential direction of the cylinder liner 10. However, the present invention is not limited to this. Scanning may be performed in the axial direction of the cylinder liner 10 or in an oblique direction including the axial and circumferential directions. Furthermore, as shown in FIG. 9(E), scanning may be performed in a circular or spiral pattern along a circular line that approximates (is similar to) the contour of a single recess 14. Furthermore, as shown in FIG. 9(F), a scanning path may be adopted in a single recess 14 in which a first movement path 611, a second movement path 612, and a third movement path 613, which are parallel to each other, continuously meander.

[0098] FIG. 10A schematically illustrates a cross section of the cylinder liner 10 along a first movement path 611 during processing by the laser 590. For ease of explanation, the upper part of the figure illustrates the first movement path 611 of the laser spot 590S and the ON / OFF timing (pulse waveform) of the laser spot 590S as it moves along the first movement path 611. The laser spot 590S moves along the first movement path 611 while repeatedly turning ON and OFF. This forms a groove-like first band region 621 in the cylinder liner 10. The movement distance 592 between adjacent pulses of the laser spot 590S (inter-pulse movement distance) is shorter than the spot diameter 594 of the laser spot 590S. The inter-pulse movement distance 592 is calculated 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 process along the first movement path 611 has the greatest number of overlapping irradiations of the laser spot 590S. If the spot diameter 594 is defined as C (mm), this maximum number of overlapping irradiations is the spot diameter C divided by the inter-pulse movement distance (A / B), i.e., C / (A / B). For example, when the scanning speed is 100 mm / s, the frequency is 200 kHz, and the spot diameter 594 is 0.04 mm, the inter-pulse movement distance 592 is 0.0005 mm, and the number of overlapping irradiations of the laser spot 590S at the deepest point X (reference bottom surface range 14G or recess center GM) in the first band-shaped region 621 is 80. In this way, the desired groove is formed by overlapping irradiation of the laser spot 590S multiple times at the deepest point X. In the end region 621e of the first belt-shaped region 621, the number of overlapping irradiations of the laser spot 590S decreases stepwise toward the end, forming a tapered surface (side surface 300). This is also true for the start region of the first belt-shaped region 621.

[0099] (Number of overlapping band-shaped region spots) The number of overlapping irradiations of the laser spot 590S with respect to 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 overlapping number) is preferably set to 2 or more, more preferably 3 or more, and more preferably 5 or more. On the other hand, the number of overlapping band-shaped region spots is preferably set to 50 or less, more preferably 30 or less, more preferably 20 or less, and even more preferably 10 or less. Similarly, the calculated number of overlapping virtual irradiations (spot diameter / inter-pulse travel distance, hereinafter referred to as the virtual band-shaped region spot overlapping number) that can be calculated from the processing conditions is preferably set to 2 or more, more preferably 3 or more, and preferably 5 or more. On the other hand, the number of overlapping virtual band-shaped region spots is preferably set to 50 or less, more preferably 30 or less, more preferably 20 or less, and even more preferably 10 or less.

[0100] (Number of overlapping spots in band-shaped regions per unit depth) To explain in more detail, the maximum engraving amount (engraving depth) of each band-shaped region processed by a single scan can be adjusted appropriately depending on the purpose. For example, if it is desired to increase the maximum engraving amount per scan, the number of overlapping spots in band-shaped regions can be set to be large, or the energy of the laser spot 590S can be increased to increase the engraving amount per irradiation of the laser spot 590S. However, if the energy of the laser spot 590S is set too high, excess energy is absorbed (accumulated) on the processed surface, resulting in energy loss, and the processed surface melts, resulting in increased unevenness.

[0101] Conversely, if it is desired to reduce the maximum engraving depth per scan, the number of overlapping spots in the band-like regions can be set to a small number, or the energy of the laser spot 590S can be reduced to reduce the engraving depth per irradiation of the laser spot 590S. However, if the energy of the laser spot 590S is reduced too much, the processing efficiency will be extremely reduced, and ultimately processing may not be performed at all.

[0102] In order to standardize the above idea, the inventors define the unit engraving depth as 1 μm, and define the number of spot overlaps that achieve this unit engraving depth as the number of spot overlaps (number of times / μm) for a band-shaped region at a unit depth. It is preferable to adjust the laser irradiation conditions and scanning conditions so that this number of spot overlaps (number of times / μm) for a band-shaped region at a unit depth is within a 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 (number of times / μm) for a band-shaped region at a unit depth is within a range of 5 to 250, and even more preferably, the laser irradiation conditions and scanning conditions are adjusted so that the number of spot overlaps (number of times / μm) for a band-shaped region at a unit depth is within a range of 5 to 50.

[0103] (Number of overlapping strip regions) Figure 10(B) shows a cross section of the cylinder liner 10 along the direction perpendicular to the scanning direction of the laser 590 (the cylinder axial direction). The cylinder liner 10 has a plurality of groove-like strip regions extending, for example, in the depth direction of the paper in Figure 10(B). Specifically, first to sixth strip regions 621 to 626 are formed in this order, parallel to each other in the strip width direction (the left-right direction of the paper in Figure 10(B)). After forming a first band region 621, the laser spot 590S moves axially by a predetermined line pitch 596 to form a second band region 622, then moves axially by a predetermined line pitch 596 to form a third band region 623, then moves axially by a predetermined line pitch 596 to form a fourth band region 624, then moves axially by a predetermined line pitch 596 to form a fifth band region 625, and then moves axially by a predetermined line pitch 596 to form a sixth band region 626.

[0104] 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 overlapping band-shaped regions (third to fifth band-shaped regions 623 to 625) are formed at the deepest point X of the recess 14. In the axial terminal region 14e of the recess 14, the number of overlapping band-shaped regions 620 decreases in a stepped manner toward the terminal, forming a stepped tapered surface (side surface 300). The same applies to the axial starting region 14s.

[0105] The number of overlapping band regions with respect to the deepest point X of the recess 14 (hereinafter referred to as the overlapping band region number) is preferably 2 or more, and more preferably 3 or more. On the other hand, the overlapping band region number is preferably 10 or less, and more preferably 8 or less, and even more preferably 5 or less. Similarly, the theoretical overlapping band region number (spot diameter / line pitch, hereinafter referred to as the virtual overlapping band region number) that can be calculated from the processing conditions is preferably 2 or more, and more preferably 3 or more. On the other hand, the virtual overlapping band region number is preferably 10 or less, and more preferably 8 or less, and even more preferably 5 or less.

[0106] In FIG. 11, for the sake of convenience, the depth dimension of the recess 14 is exaggerated.

[0107] (Total Number of Spot Overlaps) The number of times that the laser spot 590S is applied to the reference bottom surface range 14G or the recess center GM of the recess 14 completed by laser processing (hereinafter referred to as the total number of spot overlaps) is the product of the number of overlapping strip-shaped area spots and the number of overlapping strip-shaped areas. As a result, the total number of overlapping spots is preferably 4 times or more (the product of the number of overlapping strip-shaped area spots 2 and the number of overlapping strip-shaped areas 2), preferably 6 times or more (the product of the number of overlapping strip-shaped area spots 2 and the number of overlapping strip-shaped areas 3), preferably 9 times or more (the product of the number of overlapping strip-shaped area spots 3 and the number of overlapping strip-shaped areas 3), preferably 10 times or more (the product of the number of overlapping strip-shaped area spots 5 and the number of overlapping strip-shaped areas 2), and preferably 15 times or more (the product of the number of overlapping strip-shaped area spots 5 and the number of overlapping strip-shaped areas 3). On the other hand, the total number of spot overlaps is preferably 500 or less (the product of 50 overlapping strip area spots and 10 overlapping strip area spots), preferably 400 or less (the product of 50 overlapping strip area spots and 8 overlapping strip area spots), preferably 300 or less (the product of 30 overlapping strip area spots and 10 overlapping strip area spots), preferably 200 or less (the product of 20 overlapping strip area spots and 10 overlapping strip area spots), preferably 160 or less (the product of 20 overlapping strip area spots and 8 overlapping strip area spots), preferably 100 or less (the product of 10 overlapping strip area spots and 10 overlapping strip area spots), preferably 80 or less (the product of 10 overlapping strip area spots and 8 overlapping strip area spots), and preferably 50 or less (the product of 10 overlapping strip area spots and 5 overlapping strip area spots).

[0108] (Total Number of Spot Overlaps per Unit Depth) The total number of spot overlaps can be adjusted by the maximum average depth R (unit: μm) of the recesses 14. Therefore, a depth of 1 μm is defined as the unit recess depth, and the total number of spot overlaps per unit depth (number of times / μm) is defined as the value obtained by dividing the total number of spot overlaps by the maximum average depth R. 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, the laser irradiation conditions and scanning conditions are adjusted so that the total number of spot overlaps per unit depth (number of times / μm) is within the range of 5 to 50.

[0109] <Method for Processing Multiple Recesses> Next, a processing method when multiple recess target regions 614 exist in the scanning direction (circumferential or axial direction) will be described. As shown in FIG. 11A , 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 straddle the multiple recess target regions 614. The first irradiation step is performed on each of the multiple recess target regions 614 in order from upstream to downstream of the first movement path 611. This step is defined as a first irradiation group step. As a result, a groove-shaped first band-shaped region 621 is formed in each of the multiple recess target regions 614. These are defined as a first band-shaped region group.

[0110] After the first irradiation group step, as shown in FIG. 11(B), a laser spot 590S is irradiated along a linear second movement path 612 having a centerline offset by a predetermined line pitch 596 from the centerline of the first movement path 611. This second movement path 612 is set to straddle multiple recess target regions 614. The second irradiation step is performed on each of the multiple recess target regions 614 in order from upstream to downstream of the second movement path 612. This step is defined as the second irradiation group step. As a result, a groove-shaped second band-shaped region 622 is formed in each of the multiple recess target regions 614. These are defined as a second band-shaped region group.

[0111] Focusing on each recess target region 614, from immediately after the first irradiation step to immediately before the second irradiation step, the recess processing device 500 processes other recess target regions 614. As a result, there is an advantage that a sufficient heat dissipation time can be ensured in each recess target region 614 immediately after processing in the first irradiation step or the second irradiation step.

[0112] 11(C), a third irradiation group step is performed in the same manner as the first irradiation group step and the second irradiation group step described above, to form a plurality of third band-shaped regions 623 along the third movement path 613. Thereafter, a fourth irradiation group step, a fifth irradiation group step, and so on are similarly repeated. As a result, a plurality of recesses 14 are formed as shown in FIG. 11(D).

[0113] <Example>

[0114] Next, examples of the cylinder liner 10 of this embodiment will be described. Using the recess processing device 500, recesses 14 were processed in four cylinder liners 10, which are the first to fourth examples. As a comparative example, recesses 14 were processed in the cylinder liner 10 by conventional blasting.

[0115] 12A and 12B, the surface roughness after laser polishing of the bottom surface 14A of the recess 14 in Examples 1, 2, 3, and 4 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.

[0116] On the other hand, in the comparative example in which recesses were formed by blasting, the surface roughness after laser polishing was such that the core level difference Rk was 1.6 μm or more and the protruding valley depth Rvk was 1.3 μm or more.Furthermore, in the comparative example, the surface roughness after laser coating and polishing was such that the core level difference Rk was 1.1 μm or more and the protruding valley depth Rvk was 0.9 μm or more.

[0117] 12C, for the bottom surfaces 14A of the recesses 14 in the first, second, third, and fourth examples, the surface roughness after laser polishing was determined as the sum of the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk) of less than 2.9 μm. The surface roughness after laser coating and polishing was determined as the sum of the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk) of less than 2.0 μm.

[0118] On the other hand, in the comparative example in which recesses were formed by blasting, the surface roughness after laser polishing was such that the sum of the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk) was 2.9 μm or more.Furthermore, in the comparative example, the surface roughness after laser coating and polishing was such that the sum of the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk) was 2.0 μm or more.

[0119] <Oil consumption>

[0120] In the case of the cylinder liner 10 of this embodiment, oil consumption is also reduced. This is due to the fact that 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 is reduced, the recesses 14 having smooth bottom surfaces 14A are formed in addition, so that the lubricating oil in the recesses 14 is smoothly guided to the surrounding inner wall surfaces 12, and there is no shortage of lubrication. In other words, this embodiment makes it possible to rationally achieve both reduced oil consumption and sufficient lubrication.

[0121] 8 illustrates a galvanometer mirror system for the recess machining apparatus 500. However, the present invention is not limited to this system. For example, a recess machining apparatus 800 shown in FIG. 13 includes a laser oscillator 810 that emits a laser 890, a holding cylinder 815 that is provided in the laser oscillator 810 and through which the laser 890 passes, a reflection mirror 820 that is provided in the holding cylinder 815 and 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 cylinder axial direction, 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 cylinder circumferential direction, and a control device 870 that controls the entire system.

[0122] The central axis of the retaining tube 815 extends parallel to the cylinder axial direction of the cylinder liner 10. The retaining tube 815 is inserted inside the cylinder liner 10. The laser 890 passing through the inside of the retaining tube 815 is bent (reflected) by the reflecting mirror 820, travels in the cylinder radial direction of the cylinder liner 10, and is irradiated approximately perpendicularly to the inner wall surface 12. Note that while the axial movement device 830 moves the laser oscillator 810 in the illustrated example, the cylinder liner 10 may also be moved in the cylinder axial direction. Furthermore, while the rotational movement device 840 rotates the cylinder liner 10 in the illustrated example, the entire laser oscillator 810 or the reflecting mirror 820 (retaining etc. 815) may also be rotated.

[0123] For example, as shown in the scanning path of FIG. 14(A), the recess machining device 800 can repeat the following operations: scan the laser 890 360 degrees in a first direction around the cylinder circumferential direction, then move the laser 890 a predetermined line pitch 596 in the axial direction of the cylinder, scan the laser 890 again 360 degrees in the first direction around the cylinder circumferential direction, and then move the laser 890 a predetermined line pitch 596 in the axial direction of the cylinder. Alternatively, as shown in the scanning path of FIG. 14(B), the recess machining device 800 can repeat the following operations: scan the laser 890 360 degrees in a first direction around the cylinder circumferential direction, then move the laser 890 a predetermined line pitch 596 in the axial direction of the cylinder, scan the laser 890 360 degrees in a second direction around the cylinder circumferential direction (the direction opposite to the first direction), and then move the laser 890 a predetermined line pitch 596 in the axial direction of the cylinder. Furthermore, as shown in the scanning path of FIG. 14(C), the laser 890 can be spirally scanned around the cylinder circumferential direction and in the axial direction of the cylinder, rotating 360 degrees and simultaneously moving a predetermined line pitch 596 in the axial direction of the cylinder. Furthermore, for example, as shown in the scanning path of Figure 14 (D), after scanning the laser 890 a predetermined distance in a first direction along the cylinder axis, the laser 890 can be moved line pitch by a predetermined line pitch 596 in the cylinder circumferential direction, and the operation of scanning a predetermined distance in a second direction along the cylinder axis (the direction opposite to the first direction) can be repeated.

[0124] <Arrangement of recesses when operating in a spiral pattern> Figure 15 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 of Figure 14(C) with the method for machining multiple recesses of Figure 11. Note that Figure 15 shows the inner wall surface 12 expanded in the circumferential direction. The center of the recess 14 in the axial 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.

[0125] The twist angle β is set to be greater than 0 degrees and less than 45 degrees. The twist angle β is preferably set to be 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. In detail, the twist angle β is preferably determined by the line pitch 596. When the circumferential length of the cylinder liner 10 is defined as KS and the line pitch 596 is defined as LP, the twist angle β is expressed as follows: －１ For example, if the circumferential length of the cylinder liner 10 is defined as 314 mm and the line pitch 596 is defined as 0.010 mm, β is 1.825×10 －４ It becomes degrees.

[0126] As a result, the recess centers GM of the multiple recesses 14 arranged at intervals in the circumferential direction are gradually displaced in the cylinder axial direction along the helix angle β. That is, the multiple recesses 14 are arranged along a spiral scanning path. Furthermore, the difference KK in the axial direction of the recess centers GM of a pair of circumferentially adjacent recesses 14 is set to be smaller than the maximum average length J of the recesses 14, preferably set to J / 2 or less.

[0127] It should be noted that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0128] 10 Cylinder liner 12 Inner wall surface 14 Recess 14A Bottom surface 14a Uppermost point 14b Lowermost point 14e End region 14s Start region 20 Stroke center region 30 Piston 40 Piston ring 70 Oil ring 200 Sagging region 500, 800 Recess machining device 510 Laser oscillator 570, 870 Control device 590, 890 Laser 590S Laser spot 592 Pulse-to-pulse travel distance 594 Spot diameter 596 Line pitch 611 First movement path 612 Second movement path 613 Third movement path 614 Recess target region 620 Band-shaped region 621 First band-shaped region 621e End region 622 Second band-shaped region 623 Second band-shaped region 623 Third band-shaped region 624 Fourth band area 625 Fifth band area 626 Sixth band area

Claims

1. A cylinder in which a piston equipped with piston rings slides on the inner wall surface, wherein a plurality of recesses are formed by laser processing in a stroke center region of the inner wall surface, which corresponds to all or part of the area between the bottom surface of the ring groove of the lowest piston ring at the top dead center of the piston and the top surface of the ring groove of the highest piston ring at the bottom dead center of the piston, and wherein the surface roughness of the bottom surfaces of the recesses satisfies at least one of the following conditions 1 to 4: Condition 1: The core level difference Rk after polishing the bottom surface is less than 1.6 μm Condition 2: The protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm Condition 3: The core level difference Rk after chemical conversion treatment and polishing the bottom surface is less than 1.1 μm Condition 4: The protruding valley depth Rvk after chemical conversion treatment and polishing the bottom surface is less than 0.9 μm 2. The cylinder according to claim 1, characterized in that the surface roughness of the bottom surface of the recess satisfies either the fifth or sixth condition below: Fifth condition: The sum of the core level difference Rk and the protruding valley depth Rvk after polishing the bottom surface is less than 2.9 μm Sixth condition: The sum of the core level difference Rk and the protruding valley depth Rvk after chemical conversion treatment and polishing the bottom surface is less than 2.0 μm 3. A cylinder as set forth in claim 1 or 2, characterized in that the depth of said recess is 0.1 μm to 1000 μm.

4. A cylinder as set forth in claim 1 or 2, characterized in that the recesses are machined using a laser with a pulse width of 100 picoseconds or less.

5. A cylinder as set forth in claim 1 or 2, characterized in that the periphery of the recessed portion is a tapered surface from the outside to the inside of the recessed portion due to an increase in the number of overlapping irradiations of the laser spot during the laser processing.

6. A cylinder as set forth in claim 1 or 2, wherein when the maximum dimension of said recess in the axial direction of said cylinder is defined as WJ, the amount of deviation Z in the axial direction of the central positions of a pair of said recesses adjacent in the circumferential direction of said inner wall surface is set to 0<Z<WJ.

7. A recess processing method for irradiating the inner wall surface of a cylindrical cylinder, on which a piston equipped with a piston ring slides, with a laser spot to form a recess of a size larger than the spot diameter of the laser spot, comprising: a first irradiation step of irradiating a recess target area where the recess is to be formed with a laser spot of a laser having a pulse width of less than 100 picoseconds along a linear first movement path to recess a first band-shaped area; and a second irradiation step of, after the first irradiation step, irradiating the recess target area with the laser spot overlapping at least a portion of the first band-shaped area along a linear second movement path having a center line parallel to the first movement path and offset from the center line of the first movement path to recess a second band-shaped area overlapping the first band-shaped area.

8. The method for machining recesses on a cylinder inner wall surface according to claim 7, characterized in that a plurality of recess target areas are present at intervals from one another along the circumferential direction of the inner wall surface, comprising: a first irradiation group process of irradiating the laser spot along the first circumferential movement path to perform the first irradiation process on each of the plurality of recess target areas; and a second irradiation group process of irradiating the laser spot along the second circumferential movement path after the first irradiation group process to perform the second irradiation process on each of the plurality of recess target areas.

9. The method for machining recesses on the inner wall surface of a cylinder according to claim 8, characterized in that the scanning path of the laser spot in the first irradiation group step follows a spiral path.

10. A method for machining recesses on the inner wall surface of a cylinder as set forth in claim 9, characterized in that the arrangement of the plurality of recesses formed on the inner wall surface follows the spiral path.

11. The method for machining recesses on the inner wall surface of a cylinder according to claim 7, characterized in that the surface roughness of the bottom surface of the recesses satisfies at least one of the following first to fourth conditions: First condition: The core level difference Rk after polishing the bottom surface is less than 1.6 μm; Second condition: The protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm; Third condition: The core level difference Rk after chemical conversion treatment and polishing the bottom surface is less than 1.1 μm; Fourth condition: The protruding valley depth Rvk after chemical conversion treatment and polishing the bottom surface is less than 0.9 μm.

12. A method for machining recesses on the inner wall surface of a cylinder according to any one of claims 7 to 11, characterized in that, when the spot diameter of the laser spot is defined as C and the offset amount of the center lines of the first movement path and the second movement path is defined as OS, OS≦C / 2.

13. A method for machining a recess in the inner wall surface of a cylinder according to any one of claims 7 to 11, characterized in that the axial size of the recess is larger than the contact width between the piston ring and the inner wall surface.

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