Cylinder

Abstract

Provided is a cylinder having grooving in a cylinder inner circumferential surface. The grooving includes a honed groove and a recessed groove having a larger groove width and a larger groove depth than the honed groove. The honed groove and the recessed groove extend in a cylinder circumferential direction and have an inclination angle in relation to a direction perpendicular to the cylinder axial direction. In a sliding region of an oil ring when a crank angle is from 50º to 140º, the cylinder inner circumferential surface is such that, in the cylinder axial direction, a length of 4 mm in which the recessed groove is present in four or more places can be selected.

Description

[Technical Field]

[0001] The present invention relates to a cylinder for use in an internal combustion engine. [Background technology]

[0002] The cylinders of internal combustion engines are mainly made of cast iron on the inner circumferential surface, and examples include a structure in which a cast iron cylinder liner is cast into an aluminum alloy block, a cast iron monoblock structure, and a structure in which a cast iron dry liner or a cast iron wet liner is inserted. On the other hand, there are also cylinders in which the inner circumferential surface of the cylinder is coated with a coating such as thermal spraying or plating that has excellent wear resistance, or in which an aluminum alloy is used for direct sliding.

[0003] In all of these structures, there is a demand for reducing CO2 emissions and fossil fuel consumption, and therefore technologies for reducing friction by controlling the roughness and structure of the cylinder inner surface are being investigated.

[0004] For example, Patent Document 1 discloses a cylinder for an internal combustion engine that can reduce the frictional mean effective pressure without complex machining, in which the inner wall surface is divided into an upper region, a lower region, and a mid-stroke region, and the surface roughness of the mid-stroke region is greater than the surface roughness of the upper region and the lower region. Patent Document 2 discloses that the inner peripheral surface of a honed cylinder liner has axial undulations, which maintains a good lubrication state between the piston and the cylinder liner. Patent Document 3 also discloses that the piston ring sliding surface of the cylinder liner is formed with a wavy uneven surface along the sliding direction of the piston ring, and the waves are formed deepest near the center of the sliding stroke of the first piston ring, thereby improving the lubrication performance of the cylinder liner. Furthermore, Patent Document 4 discloses that recesses are formed in the stroke center region of the cylinder inner wall surface, the total area of ​​all recesses is 1 to 80% of the area of ​​the stroke center region, and no recesses are formed in regions other than the stroke center region, thereby reducing reciprocating friction between the piston ring and the cylinder inner wall surface. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-78267 [Patent Document 2] Japanese Utility Model Application Publication No. 4-106560 [Patent Document 3] Special Publication No. 60-001504 [Patent Document 4] Patent No. 5513945 Summary of the Invention [Problem to be solved by the invention]

[0006] The above-mentioned Patent Document 1 states that the influence of oil shear resistance on friction loss is significant in the mid-stroke region, where the sliding speed between the inner wall surface of the cylinder liner and the piston ring is high. Friction loss in an internal combustion engine is particularly pronounced near the mid-stroke, where the amount of work is large. Therefore, reducing friction near the mid-stroke without degrading oil consumption is effective in reducing CO2 emissions, and the above-mentioned Patent Document 1 proposes increasing the surface roughness of the mid-stroke region. However, the surface roughness of the plateau surface formed by plateau machining after honing using an adjusted grinding stone does not sufficiently reduce friction, leaving room for improvement. Furthermore, as in Patent Document 2, when there are simply undulations on the inner peripheral surface of the cylinder, the axial undulation pitch is 10 to 20 mm, and the undulation depth is 1 to 5 μm, the behavior of the piston ring may be affected, and the effect of reducing friction cannot be sufficiently obtained, leaving room for improvement. Furthermore, as in Patent Document 3, if the wave depth on the inner surface of the cylinder liner is 0.005 to 0.1 mm, there is a risk that oil consumption performance and friction characteristics may deteriorate. Furthermore, as in Patent Document 4, when the maximum height Ry of the surface of the formed recesses is 0.1 μm or more and 30 μm or less, the effect of reducing friction is not sufficiently obtained, and there is room for improvement. An object of the present invention is to provide a cylinder that can achieve a higher friction reduction effect by a method different from those already proposed. [Means for solving the problem]

[0007] The inventors have conducted further research to solve the above problems, and have discovered that a high friction reduction effect can be achieved by forming honed grooves in the stroke center region of the inner circumferential surface of the cylinder and by performing surface processing so that there are a certain number of recessed grooves that are wider and deeper than the honed grooves, and have thus completed the present invention.

[0008] The present invention relates to a cylinder having grooves on its inner circumferential surface, The groove processing includes a honing groove and a recessed groove having a groove width and a groove depth larger than those of the honing groove, the honing groove and the recessed groove extend in the cylinder circumferential direction and have an inclination angle with respect to a direction perpendicular to the cylinder axial direction, The cylinder inner surface is a cylinder in which a length of 4 mm can be selected in the cylinder axial direction in which the recessed grooves exist in four or more locations in the sliding area of ​​the oil ring when the crank angle is 50° or more and 140° or less.

[0009] The honed groove and the recessed groove preferably have a portion where they cross each other in the sliding region of the oil ring when the crank angle is 50° or more and 140° or less. The groove width is preferably 30 μm or more.

[0010] Furthermore, it is preferable that the selected length of 4 mm further satisfies at least one of the following parameters (i) to (vi). (i) The average depth W of the undulation motif of the envelope undulation curve is 0.28 μm or more; (ii) (average depth W of the undulation motif of the envelope curve) × (average length AW of the undulation motif) is 80 μm 2 More than 300μm 2 Below is the (iii) (Level difference Rke of the core part of the envelope waviness curve) / (Level difference Rk of the core part of the roughness curve) is 0.6 or less, (iv) (average depth of the protruding valleys of the envelope waviness curve Rvke) / (average depth of the protruding valleys of the roughness curve Rvk) is 0.9 or less; (v) (the average height Rpke of the protruding peaks of the envelope waviness curve + the level difference Rke of the core portion + the average depth Rvke of the protruding valleys) / (the average height Rpk of the protruding peaks of the envelope waviness curve + the level difference Rk of the core portion + the average depth Rvk of the protruding valleys) is 0.9 or less, (vi) (average depth Rvke of the protruding valleys of the envelope swell curve) / (average depth W of the swell motifs of the envelope swell curve) is 1.0 or less.

[0011] Furthermore, it is preferable that the inner peripheral surface of the cylinder has a configuration in which it is not possible to select a length of 4 mm in which the recessed grooves are present at four or more locations in the axial direction of the cylinder in the sliding region of the oil ring when the crank angle, including the top dead center, is 0° or more and less than 50°. [Effects of the Invention]

[0012] By forming grooves in addition to honed grooves in the center of stroke region as in the present invention, it is possible to provide a cylinder that can reduce friction on the sliding surface below conventional levels and improve fuel economy without significantly deteriorating oil consumption performance. In addition, as a preferred embodiment, by reducing or eliminating the grooves near the top dead center, it is possible to achieve both good oil consumption performance and good fuel economy. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a cross-sectional schematic view of the cylinder liner according to the embodiment. [Figure 2] FIG. 2 is an enlarged schematic view showing a portion of the inner circumferential surface of the cylinder liner according to the embodiment. [Figure 3] FIG. 2 is a cross-sectional view of a test machine used in a friction test conducted in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] One embodiment of the present invention is a cylinder having grooves on its inner circumferential surface, the grooves including honed grooves and recessed grooves having a width and depth greater than those of the honed grooves, the honed grooves and the recessed grooves extending circumferentially of the cylinder and having an inclination angle with respect to a direction perpendicular to the cylinder axial direction, and the cylinder inner circumferential surface having a length of 4 mm along the cylinder axial direction in the sliding region (hereinafter also referred to as the central region) of an oil ring when the crank angle is between 50° and 140°. This embodiment will be described using FIG.

[0015] Figure 1 is a cross-sectional view of a cylinder liner. The cylinder may have a structure in which a cast iron cylinder liner is cast into an aluminum alloy block, a cast iron monoblock structure, or a structure in which a cast iron dry liner or a cast iron wet liner is inserted, but the following description will be given using a cylinder liner. The cylinder liner 10 is typically made of cast iron, but may also be made of an aluminum alloy or a copper alloy, and the material is not particularly limited.

[0016] The cylinder liner 10 is installed in the cylinder block of an internal combustion engine, and a piston slides inside it in the vertical direction (the axial direction of the cylinder liner) as shown in Fig. 1. In Fig. 1, the combustion chamber side is "upper" and the crank chamber side is "lower". In FIG. 1, the dashed dotted line 1 indicates the top dead center of the oil ring, and the dashed dotted line 2 indicates the bottom dead center of the oil ring.

[0017] The inner peripheral surface of the cylinder liner 10 in FIG. 1 has grooves, which include honed grooves and recessed grooves that are wider and deeper than the honed grooves. The honed grooves may be grooves obtained by honing using a known method, and there are no particular limitations on the number of honing operations or the shape, type, and grain size of the grindstone used in the honing. Also, crosshatching may be formed on the inner peripheral surface of the cylinder liner by honing. When crosshatching is formed, the angle (acute angle) is preferably 2° or more, may be 5° or more, or may be 10° or more. Furthermore, it is usually 60° or less, may be 45° or less, may be 30° or less, or may be 15° or less.

[0018] The recessed grooves, which are wider and deeper than the honing grooves, are formed by a method other than honing, and can be formed, for example, by grinding the cylinder liner surface, plastic processing, etching, laser processing, etc. In this embodiment, by forming recessed grooves that can become oil pockets, the contact area between the piston and piston ring during sliding can be reduced, and the shear resistance of the oil can be reduced. The size of the recessed groove is not particularly limited as long as the groove width and groove depth are larger than those of the honed groove. However, from the viewpoint of obtaining the effect of reducing oil shear resistance, the lower limit of the groove width is preferably 30 μm or more, more preferably 50 μm or more, even more preferably 70 μm or more, and particularly preferably 100 μm or more. On the other hand, if the groove width of the recessed groove is too wide, the surface pressure of the piston ring increases, making the piston ring more susceptible to wear and making the recessed groove difficult to manufacture. Therefore, the upper limit of the groove width is preferably 860 μm or less, and may be 800 μm or less, 500 μm or less, or 300 μm or less. The groove depth is usually 1.0 to 2.0 μm, and may be 0.5 μm or more, 0.7 μm or more, 10.0 μm or less, or 9.0 μm or less. In one example, the groove width is about 200 μm and the depth is about 1.0 μm. The groove width of the honed groove and recessed groove can be measured, for example, using a contact roughness meter (SURFCORDER SE600 manufactured by Kosaka Laboratory Co., Ltd.) at a magnification of 5000x in the radial direction and 100x in the axial direction. Measurements are taken at 12 locations in total, including three axial locations and four circumferential locations at 90° intervals from each axial measurement location. The groove width is determined as the average value of the maximum opening length in the axial direction for each profile curve. Note that the measurement magnification may be changed as appropriate depending on the minimum groove width to be measured. For example, when measuring a honed groove, measurements may be taken at a magnification of 5000x in the radial direction and 500x in the axial direction. A non-contact laser measuring device (for example, VK-X1000 manufactured by Keyence Corporation) may be used as a measuring device for measuring the groove width.

[0019] In this embodiment, in order to reduce friction near the center of the stroke where the workload is large, the above-mentioned groove is provided in the central region 3 of the inner circumferential surface of the cylinder liner 10, which is the sliding region of the oil ring when the crank angle is between 50° and 140°. This will be explained using Figure 2.

[0020] FIG. 2 is an enlarged schematic diagram of the dashed line region 4 in the central region 3 of FIG. The dashed line region 4 has honed grooves represented by thin lines and recessed grooves represented by thick lines. The honed grooves and recessed grooves extend in the circumferential direction of the cylinder and have an inclination angle with respect to a direction perpendicular to the cylinder axial direction. The inclination angle of the honed grooves is not particularly limited. On the other hand, the inclination angle of the recessed groove is not particularly limited, and may be 2° or more, 5° or more, or 10° or more. It is usually 60° or less, 45° or less, 30° or less, or 15° or less. It is preferable that the recessed grooves cross each other in the sliding region of the oil ring when the crank angle is 50° or more and 140° or less, because this makes the sliding environment more uniform.

[0021] In this embodiment, the length 41 can be selected to have four or more grooves in the axial direction of the cylinder. In FIG. 2, there are five grooves along the length 41. In this way, rather than simply having grooves on the inner peripheral surface of the cylinder, the grooves are formed in the central region of the cylinder in the axial direction of the cylinder so that a length of 4 mm can be selected to have four or more grooves. This reduces the contact area between the piston and piston ring when they slide, and reduces the shear resistance of the oil. The number of grooves present in the 4 mm length 41 is not particularly limited as long as it is four or more, and the 4 mm length 41 may be selected so that there are five or more grooves, or there may be six or more grooves. On the other hand, if there are too many grooves, oil consumption performance may deteriorate, so the upper limit of the number of grooves present in the 4 mm length is preferably 60 or less, and may be 40 or less, or may be 20 or less.

[0022] Furthermore, when measuring the roundness of the cylinder's inner peripheral surface, the grooves are detected as recessed toward the outer peripheral side, so the presence of grooves can also be determined by measuring the roundness of the cylinder's inner peripheral surface.

[0023] It is preferable that the selected length of 4 mm further satisfies at least one of the following parameters (i) to (vi). (i) The average depth W of the undulation motif of the envelope undulation curve is 0.28 μm or more; (ii) (average depth W of the undulation motif of the envelope curve) × (average length AW of the undulation motif) is 80 μm 2 More than 300μm 2 Below is the (iii) (Level difference Rke of the core part of the envelope waviness curve) / (Level difference Rk of the core part of the roughness curve) is 0.6 or less, (iv) (average depth of the protruding valleys of the envelope waviness curve Rvke) / (average depth of the protruding valleys of the roughness curve Rvk) is 0.9 or less; (v) (the average height Rpke of the protruding peaks of the envelope waviness curve + the level difference Rke of the core portion + the average depth Rvke of the protruding valleys) / (the average height Rpk of the protruding peaks of the envelope waviness curve + the level difference Rk of the core portion + the average depth Rvk of the protruding valleys) is 0.9 or less, (vi) (average depth Rvke of the protruding valleys of the envelope swell curve) / (average depth W of the swell motifs of the envelope swell curve) is 1.0 or less.

[0024] (i) By satisfying the average depth W, oil retention can be improved, which is preferable. The average depth W is preferably 0.4 or more, and although there is no particular upper limit, it is usually 1.0 or less. By satisfying the value of W×AW in (ii), a further friction-reducing effect can be obtained, which is preferable. The value of W×AW is preferably 90 or more and 270 or less. By satisfying the Rke / Rk value in (iii), a further friction-reducing effect can be obtained, which is preferable. The Rke / Rk value is preferably 0.5 or less, and although there is no particular lower limit, it is usually 0.1 or more. By satisfying the value of Rvke / Rvk in (iv), the shear resistance of the oil can be reduced, which is preferable. The value of Rvke / Rvk is preferably 0.7 or less, and although there is no particular lower limit, it is usually 0.05 or more. By satisfying the value of (Rpke+Rke+Rvke) / (Rpk+Rk+Rvk) in (v), the shape of the recessed grooves on the cylinder surface becomes favorable, which is preferable. The value of (Rpke+Rke+Rvke) / (Rpk+Rk+Rvk) is preferably 0.75 or less, and although there is no particular lower limit, it is usually 0.1 or more. It is preferable to satisfy the value of Rvke / W in (vi), that is, to make the value of Rvke smaller relative to W than that of the cylinder inner circumferential surface that has been subjected to conventional honing, because this can provide a friction reduction effect. Rvke / W is preferably 0.75 or less, and although there is no particular lower limit, it is usually 0.1 or more.

[0025] Measurement of motif parameters including Rke, Rvke, Rpke, W, and AW is performed in accordance with ISO12085, and the average of values ​​measured at four locations around the cylinder circumference is used. The evaluation length specified by ISO is 3.2 mm, and measurement can be performed at any 3.2 mm of the 4 mm measurement target. The measurement conditions for roughness parameters including Rk, Rvk, and Rpk are in accordance with ISO 4287, and the average of values ​​measured at four locations around the cylinder circumference is used. The evaluation length specified by ISO is 4.0 mm.

[0026] The inner peripheral surface of the cylinder liner according to this embodiment is not particularly limited as long as the honed grooves and recessed grooves satisfy specific requirements in the central region 3. The entire inner peripheral surface may be honed, or may have a coating treatment such as a thermal spray coating.

[0027] Among these, from the viewpoint of not significantly deteriorating oil consumption performance and friction characteristics, it is preferable that the inner peripheral surface of the cylinder has four or more grooves in the sliding region of the oil ring when the crank angle, including top dead center, is between 0° and 50° (hereinafter also referred to as the top dead center region; in Figure 1, this is the region indicated by reference numeral 5), so that no length of 4 mm can be selected. Additionally, in the sliding region of the oil ring when the crank angle, including bottom dead center, is greater than 140° and less than or equal to 180° (hereinafter also referred to as the bottom dead center region; in Figure 1, this is the region indicated by reference numeral 6), it is preferable that the grooves exist in four or more locations in the axial direction of the cylinder, and that a length of 4 mm cannot be selected. Furthermore, if grooves exist in the bottom dead center region, they may impede the generation of oil pressure, thereby deteriorating the friction characteristics, so it is more preferable that no grooves exist. In addition, in the central region 3, particularly in the sliding region of the oil ring when the crank angle is 80° to 110°, it is preferable to be able to select a length of 4 mm in which the recessed grooves are present in four or more locations in the cylinder axial direction, from the viewpoint of obtaining a further friction reduction effect.

[0028] The sliding area of ​​the oil ring on the inner surface of the cylinder varies depending on the engine design, but generally, the sliding area of ​​the oil ring is the area from 1 / 5 of the way through to 3 / 4 of the way through the entire length, starting from the top end of the cylinder liner, and the groove exists in all or part of this range. As an example, if the engine bore diameter is φ100 mm, the total length of the cylinder liner will be about 194 mm, and the sliding area of ​​the oil ring will be the range from 39 mm to 146 mm from the top end of the cylinder liner.

[0029] In forming the inner peripheral surface of the cylinder of this embodiment, means for forming honed grooves and recessed grooves may be appropriately applied so as to satisfy the above requirements. Even in the case of a cylinder block that does not have a cylinder liner, the inner peripheral surface of the cylinder block can be machined in the same manner as the inner peripheral surface of the cylinder liner.

[0030] The bore diameter of the cylinder is not particularly limited, and the above groove shape can be applied to cylinders of various bore diameters, but from a manufacturing standpoint, the bore diameter is preferably φ50 mm or more, more preferably φ60 mm or more, and is preferably φ250 mm or less, more preferably φ200 mm or less. [Example]

[0031] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0032] Cylinder liners were prepared using cast iron with an inner diameter (bore diameter) of 83 mm and a thickness of 1.5 mm. The inner peripheral surface of the cylinder liners was finished by honing (crosshatch: acute angle 20°) to have an Rvk of 0.17 to 2.50 μm (Examples 1 to 4), 0.88 to 1.22 μm (Examples 5 to 7), 0.50 to 0.75 μm (Examples 8 and 9), 0.41 to 0.86 μm (Examples 10 to 13), or 0.38 to 0.46 μm (Examples 14 to 16).

[0033] (Examples 1 to 16) Next, with the top dead center of the oil ring of a piston fitted with a piston ring in a cylinder liner at a crank angle of 0° and the bottom dead center of the oil ring at a crank angle of 180°, the sliding area of ​​the inner circumferential surface of the cylinder liner at a crank angle of 80° to 110° was honed to reduce the roughness in Examples 1, 5 to 7, and 14 to 16, to increase the roughness in Examples 2 to 4, and to normal roughness in Examples 8 to 13, after which grooves with a width and depth greater than those of the honed grooves were formed. Note that the depth of the grooves was changed as appropriate in Examples 5 to 13, and the depth of the grooves was uniform in Examples 1 to 4 and 14 to 16. The grooves had an average inclination angle of 2.5° relative to the direction perpendicular to the cylinder liner axial direction. Furthermore, by the above-described processing, it was possible to select a length of 4 mm in which four or more grooves were present in the cylinder axial direction in the crank angle range of 80° or more and 110° or less for the cylinder liners of Examples 1 to 16. On the other hand, it was not possible to select a length of 4 mm in which four or more grooves were present in the cylinder axial direction in the crank angle range of 0° or more and less than 50°, and in the crank angle range of more than 140° and 180° or less for the cylinder liners of Examples 1 to 16.

[0034] For the selected length where there were four or more grooves, the profile curve of the cylinder liner inner surface was obtained, and the values ​​of Rk, Rpk, Rvk, Rke, Rpke, Rvke, W, and AW were calculated. The results are shown in Table 1.

[0035] (Comparative Examples 1 to 6) The inner peripheral surface of the cylinder liner was machined by honing the surface to the roughness shown in Table 2. Furthermore, in Comparative Example 6, grooves were formed that were wider and deeper than the honed grooves. Note that in none of the regions on the surface of the cylinder liners according to Comparative Examples 1 to 6 was it possible to select a length of 4 mm in which grooves were present in four or more locations in the axial direction of the cylinder.

[0036] The groove widths of the grooves formed in the cylinder liners of Examples 1 to 16 and Comparative Examples 1 to 6 obtained above were measured using a contact roughness meter (SURFCORDER SE600 manufactured by Kosaka Laboratory Co., Ltd.) at a magnification of 5000x in the radial direction and 100x in the axial direction. Measurements were taken at 12 locations in total, including three axial locations and four circumferential locations at 90° intervals from each axial measurement location. The average value of the maximum axial opening length in each profile curve was taken as the groove width. For Comparative Examples 1 to 5, which did not have grooves, the groove widths of the honed grooves were measured at a magnification of 5000x in the radial direction and 500x in the axial direction. The results are shown in Tables 1 and 2.

[0037] The cylinder liners according to Examples 1 to 16 and Comparative Examples 1 to 6 obtained above were subjected to the following friction test and oil consumption test using an actual machine. <Friction test> The friction test was conducted using a single-cylinder floating liner testing machine (a testing machine that captures changes in friction between the piston and piston ring during one cycle) in an open-air motoring evaluation. For the friction test, a crank-type single-cylinder motoring testing machine 20 (floating liner type) with a bore diameter of φ83 mm and a stroke of 86 mm was used. Figure 3 shows a schematic cross-sectional view of the crank-type single-cylinder motoring testing machine used for the friction test. The cylinder liner 21 is restricted in its radial movement by a stopper 23, allowing it to move only in the axial direction. A sensor 24 attached to the cylinder liner 21 detects the axial sliding friction force acting on the cylinder liner 21. The friction torque per cycle of this sliding friction force was divided by the displacement to obtain the friction mean effective pressure (FMEP). The test conditions were a coolant temperature of 80°C, an engine oil temperature of 80°C, engine oil 10W-30 (viscosity classification: SAE J300), and measurements were taken at evaluation speeds between 600 rpm and 2000 rpm. The FMEP was classified according to the following criteria based on the relative value when the value of Comparative Example 3 (BM) was set to 100%, with A and B being accepted levels. The results are shown in Tables 1 and 2. ·80% or less :A ·More than 80% and below 90% :B ·More than 90% and less than 100%: C ·Over 100% :D

[0038] <Oil consumption test> An 8L diesel engine was used for the oil consumption test. The engine was operated at 2000 rpm and full load for a set period of time with fresh oil, and the difference in oil weight before and after operation was compared using a sampling method. The oil consumption was classified according to the following criteria based on the relative value when the value of Comparative Example 3 (BM) was set to 100%, and α and β were set as acceptable levels. The results are shown in Tables 1 and 2. ·More than 100% and below 110%: α ·More than 110% and below 130%: β ·130% or more :γ These results show that by appropriately forming grooves in the relevant area, friction can be reduced without significantly deteriorating oil consumption performance.

[0039] [Table 1]

[0040] [Table 2] [Explanation of symbols]

[0041] 10 Cylinder liner 1 Oil ring top dead center 2 Bottom dead center of oil ring 3 Central area 4 Dashed area 41 Length 5 Top dead center area 6 Bottom dead center area 20 Crank type single cylinder motoring test machine 21 Cylinder liner 23 Stopper 24 sensors

Claims

1. A cylinder having grooves on its inner circumferential surface, The groove processing includes a honing groove and a recessed groove having a groove width and a groove depth larger than those of the honing groove, the honing groove and the recessed groove extend in the cylinder circumferential direction and have an inclination angle with respect to a direction perpendicular to the cylinder axial direction, The cylinder inner circumferential surface has, in a sliding region of the oil ring when the crank angle is 50° or more and 140° or less, The groove width of the recessed groove is 30 μm or more, The recessed grooves have portions where they cross each other, In the sliding region, a length of 4 mm can be selected in which the recessed grooves exist at four or more locations in the cylinder axial direction, A cylinder in which, within the selected 4 mm length, (average depth Rvke of the protruding valleys of the envelope wavy curve) / (average depth W of the wavy motifs of the envelope wavy curve) is 1.0 or less.

2. 2. The cylinder of claim 1, wherein the mean depth W of the undulation motifs of the envelope undulation curve is 0.28 μm or greater over the selected 4 mm length.

3. In the selected length of 4 mm, (average depth W of the undulation motif of the envelope undulation curve) × (average length AW of the undulation motif) is 80 μm ２ 300 μm or more ２ 3. A cylinder according to claim 1 or 2, wherein:

4. 3. The cylinder according to claim 1, wherein (level difference Rke of the core portion of the envelope waviness curve) / (level difference Rk of the core portion of the roughness curve) is 0.6 or less in the selected 4 mm length.

5. 3. The cylinder according to claim 1, wherein the ratio (average depth of the protruding valleys of the envelope waviness curve Rvke) / (average depth of the protruding valleys of the roughness curve Rvk) is 0.9 or less in the selected 4 mm length.

6. 3. The cylinder according to claim 1, wherein, in the selected 4 mm length, (the average height Rpke of the peaks of the envelope waviness curve + the level difference Rke of the core portion + the average depth Rvke of the valleys of the peaks of the envelope waviness curve) / (the average height Rpk of the peaks of the envelope waviness curve + the level difference Rk of the core portion + the average depth Rvk of the valleys of the peaks of the roughness curve) is 0.9 or less.

7. 3. The cylinder according to claim 1, wherein the inner peripheral surface of the cylinder cannot select a length of 4 mm in which the recessed grooves exist at four or more locations in the cylinder axial direction in a sliding region of the oil ring when the crank angle, including the top dead center, is 0° or more and less than 50°.

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