Cylinder and method for machining recess in inner wall surface of cylinder

Abstract

To reduce frictional resistance, improve fuel consumption, or reduce oil consumption.SOLUTION: Provided is a cylinder in which a piston equipped with a piston ring slides on an inner wall surface, a plurality of recesses 14 being formed by laser processing in the stroke center region of the inner wall surface, wherein the surface roughness of the bottom surface 14A of the recess 14 satisfies at least one of the following states: first state - the core level difference Rk after polishing the bottom surface is less than 1.6 μm; second state - the protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm; third state - the core level difference Rk after chemical conversion treatment and polishing the bottom surface is less than 1.1 μm; and fourth state - the protruding valley depth Rvk after chemical conversion treatment and polishing the bottom surface is less than 0.9 μm.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] In the past, in internal combustion engines having cylinders and pistons, efforts have been made to reduce the sliding resistance (frictional force) between the 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, Patent Document 1), 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. [Prior art documents] [Patent documents]

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

[0004] 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.

[0005] 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. [Means for solving the problem]

[0006] To achieve the above object, the present invention provides 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 a stroke center region of the inner wall surface, which region corresponds to all or part of the area 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, and the surface roughness of the bottom surfaces of the recesses satisfies at least one of the following first to fourth conditions. First state: The core level difference Rk after polishing the bottom surface is less than 1.6 μm Second state: The protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm Third state: The core level difference Rk after the bottom surface is chemically treated and polished is less than 1.1 μm Fourth state: The protruding valley depth Rvk after the bottom surface is subjected to chemical conversion treatment and polishing is less than 0.9 μm

[0007] In relation to the cylinder, the surface roughness of the bottom surface of the recess may be characterized by satisfying either the fifth or sixth condition below. Fifth state: 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 state: The sum of the core level difference Rk and the protruding valley depth Rvk after the bottom surface is subjected to chemical conversion treatment and polishing is less than 2.0 μm

[0008] In relation to the cylinder, the depth of the recess may be 0.1 μm to 1000 μm.

[0009] In relation to the cylinder, the recess may be machined using a laser having a pulse width of 100 picoseconds or less.

[0010] In relation to the above cylinder, the periphery of the concave portion may be a tapered surface based on an increase in the number of overlapping irradiations of the laser spot in the laser processing, from the outside to the inside of the concave portion.

[0011] In relation to the above cylinder, when defining the maximum dimension in the axial direction of the cylinder of the concave portion as WJ, the amount of axial displacement Z at the axial center position between a pair of adjacent concave portions in the circumferential direction of the inner wall surface is set such that 0 < Z < WJ.

[0012] The present invention for achieving the above object is a method for processing a concave portion on the inner wall surface of a cylindrical cylinder on which a piston provided with a piston ring slides on the inner wall surface, by irradiating a laser spot to form a concave portion having a size larger than the spot diameter of the laser spot. The method includes a first irradiation step of irradiating a laser spot of a laser having a pulse width of less than 100 picoseconds along a linear first movement path to perform concave processing on a first strip-shaped region for a concave portion target region where the concave portion is to be formed, and after the first irradiation step, irradiating the laser spot overlapping at least a part of the first strip-shaped region 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 perform concave processing on a second strip-shaped region overlapping the first strip-shaped region.

[0013] In relation to the above method for processing a concave portion, a plurality of the concave portion target regions are present at intervals along the circumferential direction of the inner wall surface. A first irradiation group step of irradiating the laser spot along the circumferential first movement path to perform the first irradiation step on each of the plurality of concave portion target regions, and after the first irradiation group step, irradiating the laser spot along the circumferential second movement path to perform the second irradiation step on each of the plurality of concave portion target regions may be provided.

[0014] In the recess processing method, the scanning path of the laser spot in the first irradiation group step may be along a spiral path.

[0015] In relation to the recess processing method, the recesses formed on the inner wall surface may be arranged along the spiral path.

[0016] 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 state: The core level difference Rk after polishing the bottom surface is less than 1.6 μm Second state: The protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm Third state: The core level difference Rk after the bottom surface is chemically treated and polished is less than 1.1 μm Fourth state: The protruding valley depth Rvk after the bottom surface is subjected to chemical conversion treatment and polishing is less than 0.9 μm

[0017] In relation to the above 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.

[0018] 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. [Effects of the Invention]

[0019] According to the present invention, it is possible to obtain the excellent effects of reducing frictional resistance, improving fuel efficiency, or reducing oil consumption. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a cross-sectional view taken along the axial direction of a cylinder liner that is applied to an internal combustion engine according to an embodiment of the present invention. [Figure 2]4A and 4B are developments showing the inner peripheral wall of the cylinder liner developed in the circumferential direction. [Figure 3] 1A and 1B are cross-sectional views of the inner peripheral wall of the cylinder liner taken along a direction perpendicular to the axis. [Figure 4] 4A and 4B are cross-sectional views showing the cross-sectional curves of the inner peripheral wall and recess of the cylinder liner. [Figure 5] 1A is a side view showing a piston and a piston ring applied to the internal combustion engine, FIG. 1B is a partially enlarged cross-sectional view showing the piston and the piston ring, FIG. 1C is a partially enlarged cross-sectional view of a top ring, and FIG. 1D is a partially enlarged cross-sectional view of a second ring. [Figure 6] (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 7] (A) Stribeck diagram and (B) FMEP diagram of sliding in a typical internal combustion engine. [Figure 8] FIG. 2 is a perspective view showing the recess processing device for the cylinder liner. [Figure 9] 10(A) to 10(D) are diagrams showing the procedure for machining recesses in the cylinder liner, and 10(E) and 10(F) are diagrams showing modified examples of the laser scanning method. [Figure 10] 1A and 1B are cross-sectional views taken in a direction perpendicular to the scanning direction, showing the machining procedure for the recesses of the cylinder liner. [Figure 11] 10(A) to 10(D) are diagrams showing a procedure for simultaneously machining a plurality of recesses in the cylinder liner. [Figure 12] Graphs (A) to (C) show the surface roughness of the bottom surface of the recess of the cylinder liner. [Figure 13] FIG. 10A is a front view showing a modified example of the recess processing device. [Figure 14] 5(A) to 5(D) are perspective views conceptually showing the scanning path of a laser in the recess processing device. [Figure 15] FIG. 2 is a development view showing the inner peripheral wall of the cylinder liner developed in the circumferential direction. [Figure 16]1 is a cross-sectional view along the axial direction of a cylinder liner showing an example of a cylinder liner to which microtexture technology is applied. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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.

[0022] <Cylinder liner>

[0023] As shown in Fig. 1, a plurality of recesses 14 are formed in an inner wall surface 12 of a 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 (here, the entire region is the mid-stroke region 20, and the recesses 14 are formed throughout this region as an example). 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.

[0024] 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.

[0025] <Dimples formed on the cylinder liner>

[0026] 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.

[0027] The recesses 14 are shaped like polygons (squares, rectangles, hexagons) obliquely arranged relative to the cylinder axial direction; here, a hexagon is used. As shown in the developed view of FIG. 2(A), when focusing on a specific recess 14, the axial lowest point 14b of that recess 14 is located axially lower than the axial highest points 14a of the other recesses 14. Since multiple recesses 14 overlap in the cylinder axial direction, recesses 14 can always be present in the 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 directions (cylinder axial direction and cylinder circumferential direction) in the stroke center region 20.

[0028] Specifically, as shown in the development view of FIG. 2(B), 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.

[0029] The dimensions and shape of the recesses 14 are not particularly limited and are appropriately selected depending on the dimensions and purpose of the cylinder and piston ring. For example, the recesses 14 can be formed in a slit or strip shape so as to penetrate (or extend) through the mid-stroke region 20 in the axial direction of the cylinder. From the viewpoint of airtightness of the cylinder, the maximum average length J of the recesses 14 in the axial direction (see FIG. 2(A)) 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 the maximum axial dimensions of the multiple recesses 14 vary. 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 the average length J 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 to 15 mm, more preferably 0.1 to 5 mm, and even more preferably 0.1 to 1.0 mm.

[0030] The maximum average length S of the recesses 14 in the cylinder circumferential direction is preferably in the range of 0.1 mm to 15 mm, 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 recesses 14 themselves may not be sufficiently obtained. On the other hand, if it is larger than these ranges, a part of the piston ring may easily enter the recess, causing deformation of the piston ring or, if the recesses 14 are shallow, contact with the bottom surface of the recess.

[0031] As shown in FIG. 3(A), 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.

[0032] 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.

[0033] Returning to FIG. 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 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 sufficient.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] <Detailed shape definition inside the dimple>

[0038] As shown enlarged in FIG. 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 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.

[0039] 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.

[0040] 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.

[0041] (Measurement of the primary curve) As shown in Figure 4(A), the shapes of at least two recesses 14 that are aligned circumferentially and axially aligned are measured using a stylus surface roughness tester (JIS B 0651:2001), passing through the location where the minimum distance Ha is between them and including adjacent inner wall surfaces 12. The standard stylus tip radius is 2 μm, and the 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.

[0042] (Definition of the base) 4(A), the range of 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.

[0043] The actual wall surfaces 12A to 12C contain minute irregularities that correspond to surface roughness. These minute irregularities can be evaluated using Ra (arithmetic mean roughness). Therefore, 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 to 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 to 12C, whichever is larger.

[0044] This upper reference height line G1 means a virtual starting point position in the depth direction of the side surface 300. Therefore, the last point where the cross-sectional curve DK 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 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 the range of 1 / 4GL on both sides outside the recess center GM (a total range of 1 / 2GL) is defined as the reference bottom range 14G on the bottom surface 14A.

[0045] (Definition of maximum average depth of recess) 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. Alternatively, as an alternative calculation method, the cross-sectional curve DK may be enlarged 5,000 times vertically and 50 times horizontally, the third highest peak of the cross-sectional curve DK extracted, and the third lowest valley bottom of the cross-sectional curve DK extracted. The height midway between the peak and the valley bottom may then 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.

[0046] (parameter of the surface roughness of the 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.

[0047] 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 of scratches occurring 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 are, the less likely the flow of lubricating oil entering and exiting the recess 14 is obstructed.

[0048] When measuring the surface roughness of the reference bottom surface range 14G, the following two conditions can exist. One is the surface roughness after laser processing of the recesses 14 and subsequent polishing (hereinafter referred to as "surface roughness after laser polishing"). The other is the surface roughness after laser processing of the recesses 14, further chemical conversion treatment (coating treatment), and then polishing (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 in the reference bottom surface range 14G, resulting in a corresponding reduction in surface roughness.

[0049] 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.

[0050] Furthermore, the surface roughness after laser polishing must be less than 2.9 μm, which is the sum of the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk).The surface roughness after laser coating and polishing must be less than 2.0 μm, which is the sum of the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk).

[0051] 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.

[0052] (Definition of side inclination angle) Next, the definition of the inclination angle of the side surface 300 will be explained. As shown in FIG. 4(B), 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.

[0053] The point where the cross-sectional curve DK first intersects with the side lower reference height line G3 is defined as the end point SE for side surface inclination evaluation, and the line segment SS connecting the evaluation start point GS of the side surface 300 and the end point SE for side surface inclination evaluation 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.

[0054] Note that the cross-sectional curve DK contains minute irregularities, resulting in variations in each measurement. Therefore, each side surface 300 is targeted and measurements are performed 60 times using a stylus-type surface roughness measuring instrument (JIS B 0651:2001). Of the 60 calculated side surface gradient SA values, 15 are cut from the maximum and 15 from the minimum, and the average value is calculated from the middle 30 data points. This average value is used as the evaluation side surface gradient SA for side surface 300.

[0055] (Definition of sagging area) As shown in Figure 4(B), a cross-sectional curve DK of the side surface 300 of the recess 14 is enlarged and extracted. A line offset 0.50 µm from the upper reference height line G1 toward the base of the recess 14 is defined as the sag lower reference height line G2. This sag lower reference height line G2 represents the virtual end position (end height) of the sag region 200 on the side surface 300. As a result, the length (sag height) DH of the sag region 200 in the cylinder radial direction is defined as 0.50 µm.

[0056] 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 on 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.

[0057] Note that the cross-sectional curve DK contains minute irregularities, resulting in variations 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). Of the 60 calculated sag gradients DA, 15 are removed from the maximum side and 15 from the minimum side, 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 to evaluate the formation status of the sag region 200, evaluation using the sag width DW as is is equivalent.

[0058] While different from dimple liners, in which multiple recesses are arranged so as to overlap in the cylinder axial direction, microtexture technology exists as a method for forming similar recesses, and we will briefly explain this technology. Microtexture, 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 without 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 microtexture 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.

[0059] <Central low-roughness area formed on cylinder liner>

[0060] 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 center 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 on 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 contour curve) is represented as Ra, and the three-dimensional surface roughness measured by the non-contact surface roughness measuring instrument described later (arithmetic mean height of the contour curve (JIS B 0681-2:2018, ISO 25178-2:2012)) is represented as Sa.

[0061] 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).

[0062] 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 device (measurement magnification 1080x, field of view size 259.4 μm × 259.4 μm, no cutoff, measurement line pitch in the height direction (Z direction) 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).

[0063] In particular, in this embodiment, not only is the height of the protruding peaks on the inner wall surface 12 reduced, but the depth of the protruding valleys on the inner wall surface 12 is also actively reduced, thereby reducing the frictional force during sliding. 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, the 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 move 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.

[0064] With this in mind, if 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 2.4 or less, and even more preferably 2.0 or less.

[0065] 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 integrated, continuous surface as a whole.

[0066] 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 be effective 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.

[0067] 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 abrasive grains finer than, for example, F500 or #800 (JIS R 6001-2:2017, ISO8486-2:2007).

[0068] <About the chemical conversion treatment process>

[0069] Furthermore, when forming the central low-roughness region 22 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.

[0070] 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. In the first procedure, the recesses 14 are formed on the inner wall surface 12 of the cylinder liner 10 using a laser, followed by a chemical conversion treatment and subsequent polishing. In the first procedure, a coating is formed on the bottom surface 14A of the laser-machined recesses 14, and the coating conforms 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 the chemical conversion treatment, the recesses 14 must be formed deeper by 0.5 μm using a laser.

[0071] 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.

[0072] <Pistons and piston rings>

[0073] 5A and 5B show a piston 30 and the piston rings 40 (top ring 50, second ring 60, and oil ring 70) installed in the ring groove of the piston 30. The piston ring 40 reciprocates in the axial direction of the cylinder with its outer 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 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.

[0074] As shown enlarged in Figure 5(C), the top ring 50 is a single annular member, and when viewed in cross section, the outer peripheral surface 52 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 in a direction away from the inner wall surface 12 toward the outside in the cylinder axial direction. Note that the contact width f of the outer peripheral surface 52 with the inner wall surface 12 of the cylinder liner 10 is preferably set to, for example, 0.3 mm or less. Furthermore, the surface roughness of the outer peripheral surface 52 measured with a stylus-type surface roughness tester (JIS B 0651:2001) (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) is preferably 0.250 (µm) or less.

[0075] 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 set to, for example, 0.3 mm or less. The surface roughness of the outer peripheral surface 62 measured with a stylus-type surface roughness tester (JIS B 0651:2001) (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) is preferably 0.250 μm or less.

[0076] The tension of the top ring 50 and the second ring 60 is set to a relatively low value, and the surface pressure acting on the contact surfaces of the outer 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.

[0077] The oil ring 70, shown enlarged in FIG. 6(A), 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 includes 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 combined 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, annular protrusions 74, 74 are formed on each of the pair of rails 73, 73, protruding radially outward from the pillar portion 75. 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 pillar portion 75 of the ring body 72 in the circumferential direction.

[0078] The contact width of each of the pair of outer peripheral surfaces 82, 82 in FIG. 6(A) is preferably 0.02 mm to 0.30 mm, and is set to, for example, 0.15 mm. The surface pressure acting on the contact surface of the outer peripheral surface 82 of the oil ring 70 is, for example, 1.0 MPa to 2.0 MPa, 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, the oil ring 70 often slides in the boundary lubrication region. While FIG. 6(A) 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 surface 82 of the upper rail 73 and the outer peripheral surface 82 of 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 (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.

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

[0080] 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 and 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 against the inner peripheral surfaces of the side rails 73a and 73b. The spacer expander 76s is fitted into the ring groove of the piston 30 in a circumferentially contracted state with the butt joints butted together. As a result, the restoring force of the spacer expander 76s causes the ears 74m to press and urge the side rails 73a and 73b radially outward.

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

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

[0083] Next, we will explain the friction behavior between a cylinder liner and a piston ring. The change in the friction coefficient during typical sliding is expressed as a Stribeck diagram, as shown in Figure 7(A). This Stribeck diagram distinguishes between the friction behavior in the solid contact region 110 (direct contact), the friction behavior in the boundary lubrication region 112 (sliding through an oil film), and the friction behavior in 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 friction behavior in the mixed lubrication region 113, where both conditions coexist. In this Stribeck diagram, the horizontal axis represents the logarithmic representation of kinematic viscosity 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 has the smallest frictional force. Effective utilization of these regions 114 and 113 is effective for reducing friction and, therefore, fuel consumption. 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.

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

[0085] Therefore, in this embodiment, oil is actively introduced into the contact surface of the outer circumferential surface 42 of the piston ring 40, quickly transitioning to the fluid lubrication region 114 and achieving low friction. At the same time, by applying 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.

[0086] The Stribeck diagram in Figure 7(A) 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. Figure 7(B) shows a diagram of this friction mean effective pressure (FMEP diagram). In the FMEP diagram, the horizontal axis represents the rotational speed (N) and the vertical axis represents the friction mean effective pressure (kPa). As the rotational speed (N) increases, the proportion of the hydrodynamic lubrication region 114 during one stroke increases. 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. 7(B) is relatively similar to the shapes of the hydrodynamic lubrication region 114 and the mixed lubrication region 113 in the Stribeck diagram in FIG. 7(A).

[0087] <Recess processing equipment>

[0088] Next, the machining of the recesses 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.

[0089] 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.

[0090] The control device 570 is a computer having a central processing unit (CPU), memory, 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.

[0091] <Method for machining the recessed portion alone>

[0092] Next, detailed control of the recess processing apparatus 500 by the control device 570 will be described. First, a method for processing each recess 14 will be described. As shown in FIG. 9(A), recess target areas 614 where recesses 14 are to be formed exist on the inner wall surface 12. The recess processing apparatus 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.

[0093] When the first irradiation step is completed, the heat accumulated at the arbitrary point X of the first strip-shaped region 621 is diffused or released to the outside.

[0094] After the first irradiation step, as shown in FIG. 9(B), 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-shaped 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-shaped region 622 that overlaps the first band-shaped 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 outline of the recess target region 614. In this case, the scanning direction of the first movement path 611 (the movement direction of the laser spot 590S) and the scanning direction of the second movement path 612 are the same direction (parallel).

[0095] 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 that OS≦C / 2. In this way, it becomes possible to overlap the strip-shaped regions on the bottom surface 14A of the recess 14.

[0096] When the second irradiation step is completed, the heat accumulated in the second strip-shaped region 622 is diffused or released to the outside.

[0097] 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 recesses 14.

[0098] 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.

[0099] FIG. 10A shows a schematic cross section of a cylinder liner 10 along a first movement path 611 during processing by a laser 590. For ease of explanation, the upper part of the figure illustrates the first movement path 611 of a 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-shaped region 621 in the cylinder liner 10. A movement distance 592 between adjacent pulses of the laser spot 590S (inter-pulse movement distance) is shorter than a 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 one 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, 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 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 terminal region 621e of the first belt-like region 621, the number of overlapping irradiations of the laser spot 590S decreases stepwise toward the terminal end, forming a tapered surface (side surface 300). This is also true for the starting end region of the first belt-like region 621.

[0100] (Number of overlapping spots in the strip area) The number of overlapping irradiations of the laser spot 590S 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 overlap number) is preferably set to 2 or more, more preferably 3 or more, and more preferably 5 or more. On the other hand, the band-shaped region spot overlap number is preferably 50 or less, more preferably 30 or less, more preferably 20 or less, and even more preferably 10 or less. Similarly, the calculated virtual overlapping irradiation number (spot diameter / inter-pulse travel distance, hereinafter referred to as the virtual band-shaped region spot overlap number) that can be calculated from the processing conditions is preferably set to 2 or more, more preferably 3 or more, and more preferably 5 or more. On the other hand, the virtual band-shaped region spot overlap number is preferably 50 or less, more preferably 30 or less, more preferably 20 or less, and even more preferably 10 or less.

[0101] (Number of overlapping spots in a strip area per unit depth) More specifically, the maximum engraving amount (engraving depth) of each strip-shaped region processed by one scan can be adjusted as needed depending on the purpose. For example, if you want to increase the maximum engraving amount per scan, you can increase the number of overlapping strip-shaped region spots or increase the energy of the laser spot 590S 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 melting the processed surface, resulting in increased unevenness.

[0102] Conversely, if you want to reduce the maximum engraving depth per scan, you can either set a smaller number of overlapping spots in the strip-shaped areas or reduce the energy of the laser spot 590S 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 deteriorate dramatically, and in the end, processing may not be performed at all.

[0103] In order to standardize the above idea, the inventors define the unit engraving depth as 1 μm, and the number of spot overlaps that achieves this unit engraving depth as the number of spot overlaps (number of times / μm) for a band-shaped area per 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 area per unit depth is within a range of 5 to 500. It is also 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 area per unit depth is within a range of 5 to 250, and 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 area per unit depth is within a range of 5 to 50.

[0104] (Number of overlapping bands) FIG. 10(B) shows a cross section of the cylinder liner 10 taken along a direction perpendicular to the scanning direction of the laser 590 (the cylinder axial direction). The cylinder liner 10 is formed with a plurality of band-like recessed groove regions extending, for example, in the depth direction of the paper in FIG. 10(B). Specifically, first to sixth band-like regions 621 to 626 are formed in this order, arranged side by side in the band width direction (the left-right direction of the paper in FIG. 10(B)). After forming the first band-like region 621, the laser spot 590S moves axially by a predetermined line pitch 596 to form the second band-like region 622, then moves axially by the predetermined line pitch 596 to form the third band-like region 623, then moves axially by the predetermined line pitch 596 to form the fourth band-like region 624, then moves axially by the predetermined line pitch 596 to form the fifth band-like region 625, and then moves axially by the predetermined line pitch 596 to form the sixth band-like region 626.

[0105] The line pitch 596 is shorter than the spot diameter 594 of the laser spot 590S. For example, when the line pitch 596 is 0.03 mm and the spot diameter 594 is 0.09 mm, three overlapping strip regions (third to fifth strip 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 strip regions 620 decreases in a stepped manner toward the terminal, so that a stepped tapered surface (side surface 300) is formed. The same applies to the axial starting region 14s.

[0106] The number of overlapping band regions with respect to the deepest point X of 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.

[0107] In FIG. 11, for the sake of convenience, the depth dimension of the recess 14 is greatly exaggerated.

[0108] (Total number of overlapping spots) The number of times that laser spots 590S are applied to reference bottom area 14G or recess center GM of recess 14 completed by laser processing (hereinafter referred to as the total number of overlapping spots) 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), preferably 50 or less (the product of 10 overlapping strip area spots and 5 overlapping strip area spots).

[0109] (Total number of overlapping spots per unit depth) The total number of overlapping spots 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 overlapping spots divided by the maximum average depth R is defined as the total number of overlapping spots 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 overlapping spots 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 overlapping spots 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 overlapping spots per unit depth (number of times / μm) is within the range of 5 to 50.

[0110] <Method for machining 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. 11(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 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.

[0111] 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.

[0112] 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.

[0113] As shown in Fig. 11(C), a third irradiation group step is carried out in the same manner as the first irradiation group step and 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, etc. are repeated in the same manner. As a result, a plurality of recesses 14 are formed as shown in Fig. 11(D).

[0114] <Example>

[0115] 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 first to fourth examples. As a comparative example, recesses 14 were processed in the cylinder liner 10 by conventional blasting.

[0116] 12(A) and 12(B), the surface roughness of the bottom surface 14A of the recess 14 in Examples 1, 2, 3, and 4 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 of the bottom surface 14A of the recess 14 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.

[0117] On the other hand, in the comparative example where recesses were formed by blasting, the surface roughness after laser polishing was a core level difference Rk of 1.6 μm or more and a protruding valley depth Rvk of 1.3 μm or more.Furthermore, in the comparative example, the surface roughness after laser coating and polishing was a core level difference Rk of 1.1 μm or more and a protruding valley depth Rvk of 0.9 μm or more.

[0118] 12(C), for the bottom surfaces 14A of the recesses 14 in the first, second, third, and fourth examples, the surface roughness after laser polishing, i.e., the sum of the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk), was less than 2.9 μm. The surface roughness after laser coating and polishing, i.e., the sum of the core level difference Rk and the protruding valley depth Rvk (Rk+Rvk), was less than 2.0 μm.

[0119] On the other hand, for the comparative example in which recesses were formed by blasting, the surface roughness after laser polishing was the sum of the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk) of 2.9 μm or more.Furthermore, for the comparative example in which recesses were formed by blasting, the surface roughness after laser coating and polishing was the sum of the core level difference Rk and the protruding valley depth Rvk (Rk + Rvk) of 2.0 μm or more.

[0120] <Oil consumption>

[0121] 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.

[0122] <Modification of recess processing device> Although the recess machining apparatus 500 in Fig. 8 is a galvanometer mirror type, the present invention is not limited to this. 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 provided in the laser oscillator 810 and through which the laser 890 passes, a reflection mirror 820 provided in the holding cylinder 815 and reflecting the laser 890 at a 90-degree angle, an axial movement device 830 that moves the laser oscillator 810 and the cylinder liner 10 relatively 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 rotation movement device 840 that rotates the cylinder liner 10 and the laser 890 relatively in the cylinder circumferential direction, and a control device 870 that controls the entire apparatus.

[0123] 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, although the axial direction moving device 830 moves the laser oscillator 810 here as an example, the cylinder liner 10 may also be moved in the cylinder axial direction. Furthermore, although the rotational moving device 840 rotates the cylinder liner 10 as an example, the entire laser oscillator 810 or the reflecting mirror 820 (retaining tube 815) may also be rotated.

[0124] 14(A), for example, the recess machining apparatus 800 can repeat the following operations: scanning the laser 890 360 degrees in a first direction in the circumferential direction of the cylinder, then moving the laser 890 by a predetermined line pitch 596 in the axial direction of the cylinder, scanning the laser 890 again 360 degrees in the first direction in the circumferential direction of the cylinder, and then moving the laser 890 by the predetermined line pitch 596 in the axial direction of the cylinder. Alternatively, for example, as shown in the scanning path of FIG. 14(B), the recess machining apparatus 800 can repeat the following operations: scanning the laser 890 360 degrees in the first direction in the circumferential direction of the cylinder, then moving the laser 890 by the predetermined line pitch 596 in the axial direction of the cylinder, scanning the laser 890 360 degrees in a second direction in the circumferential direction of the cylinder (the direction opposite to the first direction), and then moving the laser 890 by the predetermined line pitch 596 in the axial direction of the cylinder. Furthermore, for example, as shown in the scanning path of FIG. 14(C), the laser 890 can be spirally scanned in the circumferential and axial directions of the cylinder, rotating 360 degrees and simultaneously moving the laser 890 by the 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.

[0125] <Arrangement of recesses when operating in a spiral> Figure 15 shows the arrangement of multiple recesses 14 formed on the inner wall surface 12 of a cylinder liner 10 by combining the spiral laser scanning path of Figure 14(C) with the multiple recess processing method 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 lined up 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.

[0126] 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: β=tan -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.

[0127] As a result, the recess centers GM of the multiple recesses 14 arranged at intervals in the circumferential direction are gradually displaced in the axial direction of the cylinder along the torsion angle β. That is, the multiple recesses 14 are arranged along a spiral scanning path. Furthermore, the difference in axial distance KK between 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, and preferably set to be equal to or smaller than J / 2.

[0128] 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. [Explanation of symbols]

[0129] 10 Cylinder liner 12 Inner 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 Ring 200 Sagging area 500, 800 recess processing equipment 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 Route 612 Second Movement Route 613 Third Migration Route 614 Recessed target area 620 Band Area 621 First Belt 621e Termination area 622 Second Belt Region 623 Second Belt Region 623 Third Belt 624 Fourth Belt 625 Fifth Belt 626 Sixth Belt

Claims

1. A cylinder in which a piston equipped with a piston ring slides on an inner wall surface, a plurality of recesses are formed by laser processing in a stroke center region of the inner wall surface, the region being the whole or a part of the area between a lower surface position of a ring groove of a lowest piston ring at the top dead center of the piston and an upper surface position of a ring groove of an uppermost piston ring at the bottom dead center of the piston; A cylinder characterized in that the surface roughness of the bottom surface of the recess satisfies at least one of the following first to fourth conditions. First state: The core level difference Rk after polishing the bottom surface is less than 1.6 μm Second state: The protruding valley depth Rvk after polishing the bottom surface is less than 1.3 μm Third state: The core level difference Rk after the bottom surface is subjected to chemical conversion treatment and polishing is less than 1.1 μm Fourth state: The protruding valley depth Rvk after the bottom surface is subjected to chemical conversion treatment and polishing is less than 0.9 μm

2. 2. The cylinder according to claim 1, wherein the surface roughness of the bottom surface of the recess satisfies either the fifth or sixth condition below. Fifth state: 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 state: The sum of the core level difference Rk and the protruding valley depth Rvk after the bottom surface is subjected to chemical conversion treatment and polishing is less than 2.0 μm

3. The depth of the recess is 0.1 μm to 1000 μm.

3. A cylinder according to claim 1 or 2.

4. The recess is processed by a laser having a pulse width of 100 picoseconds or less.

3. A cylinder according to claim 1 or 2.

5. The periphery of the recessed portion is a tapered surface formed from the outside to the inside of the recessed portion due to an increase in the number of overlapping irradiations of the laser spot in the laser processing.

3. A cylinder according to claim 1 or 2.

6. When defining a maximum dimension of the recess in the axial direction of the cylinder as WJ, an axial deviation amount Z of center positions in the axial direction of a pair of the recesses adjacent in the circumferential direction of the inner wall surface is set to 0<Z<WJ.

3. A cylinder according to claim 1 or 2.

7. 1. A recess processing method comprising: irradiating an inner wall surface of a cylindrical cylinder, on which a piston having a piston ring slides, with a laser spot to form a recess having a size larger than a spot diameter of the laser spot, a first irradiation step of irradiating a recess target region 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 region; a second irradiation step after the first irradiation step, in which the laser spot overlapping at least a portion of the first band-shaped region is irradiated onto the recess target region 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, thereby recessing the second band-shaped region that overlaps with the first band-shaped region; A method for machining recesses on an inner wall surface of a cylinder, comprising:

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

9. The scanning path of the laser spot in the first irradiation group step is along a spiral path. The method for machining a recess in a cylinder inner wall surface according to claim 8.

10. The arrangement of the plurality of recesses formed on the inner wall surface is along the spiral path. The method for machining a recess in an inner wall surface of a cylinder according to claim 9.

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

12. When a spot diameter of the laser spot is defined as C and an offset amount of a center line between the first movement path and the second movement path is defined as OS, OS≦C / 2 is satisfied. The method for machining recesses in a cylinder inner wall surface according to any one of claims 7 to 11.

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

Images