JP7593932B2 - Sliding structure of an internal combustion engine - Google Patents

Description

The present invention relates to a sliding structure of an internal combustion engine having a cylinder and a piston.

Conventionally, 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 Publication No. 5155924), which reduces the sliding resistance during operation by forming multiple recesses in the stroke center region of the inner wall surface of the cylinder.

Although this was not publicly known at the time of filing this application, further research by the inventors has revealed that there is still room for further improvements in fuel efficiency, etc. with this dimple liner technology.

In light of this current situation, the present invention aims to achieve further improvements in fuel efficiency and reductions in oil consumption with dimple liners.

The present invention, which achieves the above-mentioned object, is a sliding structure for an internal combustion engine having a cylinder and a piston, wherein the cylinder has a plurality of recesses formed in its inner wall surface in a mid-stroke region that is all or part of the area between the lower surface position of the ring groove of the lowest piston ring at the top dead center of the piston and the upper surface position of the ring groove of the highest piston ring at the bottom dead center of the piston, and at least a portion of the surface that comes into contact with the piston ring in the mid-stroke region is formed with a central low-roughness region in which the arithmetic mean roughness Ra of the profile curve measured by a stylus-type surface roughness tester is 0.140 or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, it is characterized in that the arithmetic mean height Sa of the contour curved surface of the central low roughness region measured by a non-contact surface roughness measuring device is 0.20 μm or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, it is characterized in that the protruding valley depth Svk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.41 μm or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, it is characterized in that the protruding peak height Spk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.16 μm or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, it is characterized in that the level difference Sk of the core portion of the central low roughness region measured by a non-contact surface roughness measuring device is 0.53 μm or less.

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

In relation to the sliding structure of the above-mentioned internal combustion engine, it is characterized in that the arithmetic mean roughness Ra of the contour curve of the central low roughness region measured by a stylus-type surface roughness measuring instrument is 0.120 μm or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, the central low roughness region is characterized by including the vicinity of the upper end edge and the vicinity of the lower end edge in the stroke central region.

In relation to the sliding structure of the above-mentioned internal combustion engine, it is characterized in that the entire stroke central region becomes the central low roughness region.

In relation to the sliding structure of the above-mentioned internal combustion engine, the arithmetic mean roughness Ra of the profile curve measured by a stylus-type surface roughness measuring device in the central low roughness region may be 0.090 μm or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, when the viscosity in the central low roughness region is defined as μ, the relative speed with respect to the piston is defined as U, the contact load on the piston is defined as W, and the friction coefficient with respect to the piston is defined as f, and an evaluation parameter of a Stribeck diagram is defined as A = μ × U/W, the minimum value fmin of the friction coefficient f in the central low roughness region is achieved when the evaluation parameter A is in a range of 0.0003 or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, the minimum value fmin is characterized in that the evaluation parameter A is achieved within a range of 0.0001 or more.

In relation to the sliding structure of the above-mentioned internal combustion engine, when the viscosity in the central low roughness region is μ, the relative speed with respect to the piston is U, the contact load on the piston is W, and the friction coefficient with respect to the piston is f, and the evaluation parameter of the Stribeck diagram is defined as A = μ × U/W, the friction coefficient f in the central low roughness region is 0.07 or less anywhere within a range where the evaluation parameter A is 0.0003 or less.

In relation to the sliding structure of the above-mentioned internal combustion engine, when the viscosity in the central low roughness region is μ, the relative speed with respect to the piston is U, the contact load on the piston is W, and the friction coefficient with respect to the piston is f, and the evaluation parameter of the Stribeck diagram is defined as A = μ x U/W, the piston and the cylinder in the central low roughness region are in a fluid lubrication state anywhere within a range where the evaluation parameter A is 0.0003 or less.

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

In relation to the sliding structure of the above-mentioned internal combustion engine, the minimum value Tmin may be achieved within a range in which the rotation speed N is 600 or less.

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

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

In relation to the sliding structure of the above-mentioned internal combustion engine, an upper low-roughness region is formed on the inner wall surface above the upper edge of the stroke central region, and the arithmetic mean roughness Ra of the contour curve measured by a stylus-type surface roughness measuring instrument is 0.140 μm or less, and the upper low-roughness region and the central low-roughness region may be continuous.

In relation to the sliding structure of the above-mentioned internal combustion engine, a lower low-roughness region is formed on the inner wall surface below the lower edge of the stroke central region, and the lower low-roughness region and the central low-roughness region are continuous, so that the lower low-roughness region and the central low-roughness region have an arithmetic mean roughness Ra of 0.140 μm or less as measured by a stylus-type surface roughness measuring instrument.

In relation to the sliding structure of the above-mentioned internal combustion engine, it may be characterized in that the surface of the central low roughness region is in an uncoated state.

The present invention has the excellent effect of improving fuel efficiency and reducing oil consumption.

1 is a cross-sectional view taken along an axial direction of a cylinder liner that is applied to a sliding structure for an internal combustion engine according to a first embodiment of the present invention. 4A and 4B are development views showing the inner peripheral wall of the cylinder liner developed in the circumferential direction. 2 is a cross-sectional view of the inner peripheral wall of the cylinder liner taken along a direction perpendicular to the axis. FIG. FIG. 1A is a side view showing a piston and a piston ring applied to the sliding structure of the internal combustion engine, FIG. 1B is a partially enlarged cross-sectional view showing the piston and the piston ring, FIG. 1C is a partially enlarged cross-sectional view of a top ring, and FIG. 1D is a partially enlarged cross-sectional view of a second ring. 1A is a cross-sectional view of a two-piece type oil ring, and FIG. 1B is a cross-sectional view of a three-piece type oil ring. 1A is a Stribeck diagram and FIG. 1B is an FMEP diagram relating to sliding in a typical internal combustion engine. FIG. 1 is a cross-sectional view showing a typical friction unit measuring device for measuring the sliding state of an internal combustion engine. 3A is a Stribeck diagram for explaining a sliding structure of an internal combustion engine of a first embodiment, and FIG. 3B is an FMEP diagram of the same sliding structure. FIG. 2 is a side view showing a sliding stroke of a cylinder liner and a piston ring of the internal combustion engine. 4A and 4B are graphs showing the fluctuation of the friction force between a cylinder liner and a piston ring during one stroke of the same internal combustion engine. 1A is a cross-sectional view along the axial direction of a cylinder liner applied to a sliding structure of an internal combustion engine related to a second embodiment of the present invention, and FIG. 1B is a side view showing the sliding stroke of the same cylinder liner and a piston ring. 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.

Embodiments of the present invention will now be described with reference to the accompanying drawings. First, a detailed description will be given of the sliding structure of an internal combustion engine according to a first embodiment of the present invention. Note that, in this first embodiment, an example is shown in which the internal combustion engine is a diesel engine, but the present invention is not limited thereto and can be applied to other types of internal combustion engines, such as a gasoline engine.

<Cylinder liner>

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

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

<Dimples formed on the cylinder liner>

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

The shape of the recess 14 is a square (square or rectangle) arranged diagonally with respect to the axial direction, and as a result, the multiple recesses 14 are arranged in a diagonal lattice pattern. In this way, as shown in the development 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 point 14a of the other recesses 14. In this way, since the multiple recesses 14 overlap in the axial direction, the recesses 14 can always be present in the axis-perpendicular cross section at every location (e.g., arrow view A, arrow view B, arrow view C) in the stroke central region 20. Here, multiple recesses 14 with the same area are uniformly arranged in the surface direction (axial and circumferential directions) in the stroke central region 20.

As shown in the development view of Fig. 2(B), multiple recesses 14 of the same area may be arranged non-uniformly in the surface direction. Here, the area occupied by multiple recesses 14 is small in the circumferential band region 20P at the axial end of the stroke central region 20, and the area occupied by multiple recesses 14 is large in the circumferential band region 20Q at the axial center of the stroke central region 20.

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

The maximum average length S of the recess 14 in the cylinder circumferential direction is preferably in the range of 0.1 mm to 15 mm, and preferably in the range of 0.3 mm to 5 mm. If it is smaller than these ranges, the effect of reducing the sliding area by the recess 14 itself may not be sufficient. On the other hand, if it is larger than these ranges, part of the piston ring may easily get into the recess, causing problems such as deformation of the piston ring.

As shown in Figure 3, the maximum average length R (maximum average depth R) of the recesses 14 in the cylinder diameter direction is preferably in the range of 0.1 µm to 1000 µm, and more preferably in the range of 0.1 µm to 500 µm. More preferably, it is set to 0.1 µm to 50 µm. If the maximum average length R of the recesses 14 in the cylinder diameter direction is smaller than these ranges, the effect of reducing the sliding area of the recesses 14 themselves may not be sufficiently obtained. On the other hand, if it is attempted to be made larger than these ranges, processing becomes difficult and problems such as the need to increase the thickness of the cylinder may occur.

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

Incidentally, although it is different from a dimple liner in which multiple recesses are arranged to overlap in the axial direction, there is a microtexture technology that forms a similar type of recess, so we will briefly explain this. As shown in Figure 12, microtexture is a theory in which regions V where recesses are formed and regions Z where no recesses exist are repeated alternately along the cylinder axial direction of the inner wall surface of the cylinder liner without overlapping in the axial direction, and every time the piston ring moves along this inner wall surface, engine oil flows in and out of the recesses, and the dynamic pressure thickens the oil film and reduces friction .

<Central low roughness area formed on cylinder liner>

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

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).

The three-dimensional surface roughness value (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, height direction (Z direction) measurement pitch 0.06 μm) using a laser microscope in accordance with JIS B 0681-6:2014 (ISO 25178-6:2010) is 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).

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

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

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

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

Furthermore, after the central low-roughness region 22 is formed by this honing process, it is preferable not to perform a coating process 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.

<Pistons and piston rings>

4(A) and 4(B) show the piston 30 and the piston rings 40 (top ring 50, second ring 60, oil ring 70) installed in the ring groove of the piston 30. The piston ring 40 reciprocates in the cylinder axial direction with the outer peripheral surface 42 facing the inner wall surface 12 of the cylinder liner 10. The top ring 50 eliminates the gap between the piston 30 and the cylinder liner 10, and prevents the phenomenon (blow-by) in which compressed gas escapes from the combustion chamber to the crankcase side. The second ring 60, like the top ring 50, has the role of eliminating the gap between the piston 30 and the cylinder liner 10 and the role of scraping off excess engine oil adhering to the inner wall surface 12 of the cylinder liner 10. The oil ring 70 scrapes off excess engine oil on the inner wall surface 12 of the cylinder liner 10 and forms an appropriate oil film, thereby preventing the piston 30 from burning.

As shown enlarged in FIG. 4(C), the top ring 50 is a single annular member, and when the outer peripheral surface 52 is viewed in cross section, it has a so-called barrel shape that is convex radially outward. Specifically, both outer edges of the outer peripheral surface 52 in the cylinder axial direction are inclined in a direction away from the inner wall surface 12 toward the outside in the cylinder axial direction. It is preferable that the contact width f of the outer peripheral surface 52 with respect to the inner wall surface 12 of the cylinder liner 10 is formed to be, for example, 0.3 mm or less. In addition, the surface roughness (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) of the outer peripheral surface 52 measured by a stylus surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.250 (μm) or less.

As shown enlarged in FIG. 4(D), the second ring 60 is a single annular member, and the vicinity of its outer periphery has a tapered shape that expands in diameter from the upper end in the cylinder axial direction toward the lower end in the cylinder axial direction. The outer peripheral surface 62, which is located at the outermost end of this tapered shape and contacts the inner wall surface 12 of the cylinder liner 10, has a flat shape in a cross-sectional view. It is preferable that the contact width f of the outer peripheral surface 62 with the inner wall surface 12 of the cylinder liner 10 is formed to be, for example, 0.3 mm or less. In addition, the surface roughness (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) of the outer peripheral surface 62 measured by a stylus surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.250 (μm) or less.

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 surface of the outer circumferential surfaces 52, 62 is, for example, 0.5 MPa or less, and preferably 0.3 MPa or less. As a result, the top ring 50 and the second ring 60 often slide in the fluid lubrication region, except near the top dead center and the bottom dead center.

The oil ring 70 shown in an enlarged view in FIG. 5A is a two-piece type, and has a ring body 72 and a coil spring-like coil expander 76. The ring body 72 has a pair of annular rails 73, 73 arranged at both axial ends, and an annular column portion 75 arranged between the pair of rails 73, 73 and connecting them. The cross-sectional shape of the pair of rails 73, 73 and the column portion 75 is approximately I-shaped or H-shaped, and by utilizing this shape, an inner peripheral groove 79 having a semicircular cross-sectional shape for accommodating the coil expander 76 is formed on the inner peripheral surface side. In addition, the pair of rails 73, 73 each have annular protrusions 74, 74 that protrude radially outward from the column portion 75. The outer peripheral surfaces 82, 82 formed at the tips of the annular protrusions 74, 74 abut against the inner wall surface 12 of the cylinder liner 10. The coil expander 76 is housed in the inner circumferential groove 79, thereby pressing and biasing the ring body 72 radially outward. Note that a plurality of oil return holes 77 are formed in the column portion 75 of the ring body 72 in the circumferential direction.

The contact width of each of the pair of outer peripheral surfaces 82, 82 in FIG. 5(A) is preferably formed to be 0.02 mm to 0.30 mm, and is set to, for example, 0.15 mm. The surface pressure acting on the contact surface of the outer peripheral surface 82 of the oil ring 70 is, for example, 1.0 MPa to 2.0 MPa, for example, about 1.75 MPa. Therefore, when the engine speed is high, the oil ring 70 often slides in the fluid lubrication region, but when the engine speed decreases, it often slides in the boundary lubrication region. In FIG. 5(A), the radial cross-sectional shape of the outer peripheral surfaces 82, 82 is illustrated as a simple trapezoid, but the present invention is not limited to this, and the corners of the outer peripheral surface 82 of the upper rail 73 and the outer peripheral surface 82 of the lower rail 73 facing each other (the coil expander 76 side) may be cut out in a step shape (so-called step land shape). Furthermore, the surface roughness (arithmetic mean roughness Ra of the profile curve (JIS B 0601:2013)) of the outer peripheral surface 82 measured by a stylus type surface roughness measuring instrument (JIS B 0651:2001) is preferably 0.450 (μm) or less.

The oil ring 70 is not limited to a two-piece type, and may be, for example, a three-piece type oil ring 70 as shown in Fig. 5(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.

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

In addition, it is preferable that the contact width f of each of the outer peripheral surfaces 82, 82 of the side rails 73a, 73b in Figure 5 (B) be formed to be 0.02 mm to 0.40 mm.

<Friction between cylinder liner and piston ring>

Next, the frictional behavior of the cylinder liner and the piston ring will be described. The change in the friction coefficient during general sliding is expressed as a Stribeck diagram shown in FIG. 6(A). In this Stribeck diagram, the frictional behavior is divided into the frictional behavior of the solid contact region 110 where the pistons slide in direct contact with each other, the frictional behavior of the boundary lubrication region 112 where the pistons slide through an oily film, and the frictional behavior of the fluid lubrication region 114 where the pistons slide through a viscous lubricant film. In addition, between the boundary lubrication region 112 and the fluid lubrication region 114, there is a frictional behavior of the mixed lubrication region 113 where both states are mixed. In addition, in this Stribeck diagram, the horizontal axis is a logarithmic representation of " viscosity μ" × "speed U" / "contact load W", and the vertical axis is the frictional coefficient (f). Therefore, the frictional force can be smallest in the fluid lubrication region 114 or the mixed lubrication region 113, and effective use of these regions 114 and 113 is effective for reducing friction, that is, reducing fuel consumption. On the other hand, if the speed U increases but it is not possible to transition from the boundary lubrication region 112 to the hydrodynamic lubrication region 114, the boundary lubrication region 112 will continue into the high speed region, as shown by the dotted line.

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

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 stroke center 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.

The Stribeck diagram in FIG. 6(A) shows the dynamic change in the friction coefficient (f) during one stroke of the piston 40, but another index for evaluating the friction state is the friction loss mean effective pressure (FMEP). This friction loss mean effective pressure indicates the value obtained by dividing the friction work per cycle by the stroke volume. A diagram of this friction loss mean effective pressure (FMEP diagram) is shown in FIG. 6(B). In the FMEP diagram, the horizontal axis is the rotation speed (N) and the vertical axis is the friction loss mean effective pressure (kPa). The higher the rotation speed (N), the greater the proportion of the fluid lubrication region 114 in one stroke. On the other hand, when the rotation speed (N) becomes lower, the proportion of the fluid lubrication region 114 in one stroke decreases, and the proportion of the mixed lubrication region 113 (or boundary lubrication region 112) increases. Therefore, the shape of the FMEP diagram in FIG. 6B is relatively similar to the shapes of the hydrodynamic lubrication region 114 and the mixed lubrication region 113 in the Stribeck diagram in FIG. 6A.

Next, the actual friction state between the cylinder liner 10 and the piston ring 40 of this first embodiment will be described. Note that the fixed positions of the top ring 50, second ring 60, and oil ring 70 installed on the piston 30 are relatively different in the cylinder axial direction, so strictly speaking, there are slight differences in the friction state between the cylinder liner 10 and each piston ring, but here, the position of the second ring 60 will be described as the reference position of the piston ring 40. Note that, for the fastest passing point C only, the top ring 50 is used as the reference.

<Method for measuring friction characteristics (non-combustion friction test)>

Figure 7 shows the friction unit measuring device 500 used in the first embodiment to measure the friction state between the cylinder liner 10 and the piston ring 50. The friction unit measuring device 500 measures the friction state between the two by fixing the piston ring 40 side and moving the cylinder liner 10 side up and down. In other words, the measurement of this friction state is a friction test in a state where no combustion occurs in the internal combustion engine (non-combustion friction test). The friction unit measuring device 500 holds the virtual piston 510 on which the piston rings 40 (top ring, second ring, and oil ring) are set by a fixed shaft 514 via a load cell 512. The load cell 512 measures the external force (friction force) acting on the piston ring 40 in the vertical direction.

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

In this embodiment, the measurement conditions for the friction behavior using the friction unit measuring device 500 were as follows: the lubricating oil standard was 10W-30, the oil temperature was set to 60 degrees, and the crankshaft rotation speed was changed from 215 rpm to 2154 rpm.

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

<Friction between cylinder liner and piston ring (Stribeck diagram)>

In order to analyze the friction behavior of the first embodiment, a cylinder liner 10-A with a surface roughness Ra of 0.120 (μm) in the central low roughness region 22 and a cylinder liner 10-B with a surface roughness Ra of 0.083 (μm) were prepared, and the friction behavior was measured using the friction unit measuring device 500. As a comparative example for reference, a cylinder liner X with the same shape as the cylinder liner 10 and a surface roughness Ra of 0.160 (μm) and a cylinder liner Y with no recess 14 and a surface roughness Ra of 0.160 (μm) were prepared, and the other conditions were set to the same, and the friction behavior was measured using the friction unit measuring device 500. The measurement results of the Stribeck diagram are shown in FIG. 8(A), and the measurement results of the FMEP diagram are shown in FIG. 8(B). In addition, FIG. 8(A) and (B) also show estimated values for a hypothetical cylinder liner K (surface roughness Ra 0.140 (μm)) corresponding to the first embodiment.

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

In the cylinder liners 10-A and 10-B of this embodiment and the cylinder liner X of the comparative example, the recesses 14 are formed in the entire stroke central region 20. As shown in FIG. 9, the change in the friction coefficient between the cylinder liner and the piston ring 40 when the piston 30 slides from the top dead center T to the bottom dead center U of the cylinder liner depends on the relative speed between them. This relative speed is uniquely determined by the engine speed (rpm). The piston 30 descends from a state where the speed of the cylinder liner 10 at the top dead center T is zero, passes through stroke A, and reaches a maximum speed C on the way. After that, when it reaches the bottom dead center U via stroke B, the speed becomes zero. During this time, the friction coefficient constantly changes along the Stribeck diagram of FIG. 8 (A).

In the Stribeck diagram of FIG. 8(A), the viscosity in the central low roughness region 22 is defined as μ, the relative velocity with respect to the piston 30 (piston ring 40) is defined as U, the contact load with respect to the piston 30 is defined as W, and the friction coefficient with respect to the piston 30 is defined as f (vertical axis of the graph), and the evaluation parameter is defined as A=μ×U/W (horizontal axis of the graph (logarithmic notation)).

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

Furthermore, in the cylinder liners 10-A and 10-B of this embodiment, the friction coefficient f in the central low-roughness region 20 is 0.07 or less when the evaluation parameter A is within a range of 0.0003 or less. Desirably, the friction coefficient f is 0.06 or less.

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

For reference, in the case of cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses), when evaluation parameter A exceeds 0.0003, the effect of recesses 14 appears, the rate of increase in friction coefficient f decreases, and the friction coefficient f becomes smaller than that of cylinder liner Y (surface roughness Ra 0.160 (μm)/without recesses). On the other hand, when the evaluation parameter A is in the range of 0.0003 to 0.0005, the graph of the cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses) intersects with the graph of the cylinder liner Y (surface roughness Ra 0.160 (μm)/without recesses), and when the evaluation parameter A is 0.0003 or less, the friction coefficient f of the cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses) rises in the range exceeding 0.07 and exceeds the friction coefficient f of the cylinder liner Y (surface roughness Ra 0.160 (μm)/without recesses). That is, in the case of the conventional cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses), when the evaluation parameter A is in the range exceeding 0.0003, the recesses 14 act to reduce the friction coefficient f, but when the evaluation parameter A is in the range of 0.0003 or less, the recesses 14 increase the friction coefficient f. According to undisclosed considerations by the present inventors, in the case of the conventional cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses), the presence of the recesses 14 reduces the actual contact area between the cylinder liner and the piston ring 40, so that it is likely that insufficient lubrication by the lubricating oil occurs in the low speed range, and the cylinder liner is likely to fall into the boundary lubrication range.

In the cylinder liners 10-A and 10-B of this embodiment, the surface roughness Ra of the central low roughness region 20 is set to 0.120 (μm) or less, so that even if the evaluation parameter A is in a low speed region of 0.0003 or less and the actual contact area with the piston ring 40 is small due to the recess 14, it is easy to maintain the fluid lubrication region 114 or the mixed lubrication region 113. In addition, even if a boundary lubrication state is reached, the friction coefficient is maintained small because the surface roughness is small. Generally, if the surface roughness Ra of the inner surface of the cylinder liner is set to 0.120 (μm) or less, the retention force of the lubricating oil on the contact surface decreases and it is easy to fall into a lubricating oil shortage, but in this embodiment, the recess 14 formed overlapping the central low roughness region 22 functions as a lubricating oil reservoir for the central low roughness region 20, so a synergistic effect is obtained in which the lubricating oil shortage is unlikely to occur in the central low roughness region 20.

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

The Stribeck diagram in FIG. 8(A) shows an estimated friction coefficient for cylinder liner K, which has the same configuration as cylinder liner 10-A and has a surface roughness Ra of 0.140 (μm) in the central low-roughness region 20, although this is not an actual measurement. The friction coefficient of cylinder liner K can be estimated to be close to the intermediate value between the state of conventional cylinder liners X and Y, which has a surface roughness Ra of 0.160 (μm), and cylinder liner 10-A, which has a surface roughness Ra of 0.120 (μm). Even for this cylinder liner K, the friction coefficient f in the central low-roughness region 20 is 0.07 or less, preferably 0.06 or less, within a range in which evaluation parameter A is 0.0003 or less. Furthermore, in cylinder liner K, the friction coefficient f in the range where evaluation parameter A exceeds 0.0003 (high speed region) is close to or smaller than the friction coefficient f of cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses).

<Friction between cylinder liner and piston ring (FMEP diagram)>

In the FMEP diagram of Figure 8 (B), the friction loss mean effective pressure (FMEP) between the central low roughness region 22 and the piston ring 40 is defined as T (kPa) (vertical axis), and the internal combustion engine speed is defined as N (r/min) (horizontal axis).

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

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

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

For reference, in the case of cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses), when the rotation speed N reaches a high rotation range exceeding 700, the effect of the recesses 14 appears and the rate of increase in friction loss decreases, and in particular when the rotation speed N exceeds 1000, the friction loss becomes smaller than that of cylinder liner Y (surface roughness Ra 0.160 (μm)/without recesses). On the other hand, when the rotation speed N is in the range of 1000 to 1300, the graphs for cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses) and cylinder liner Y (surface roughness Ra 0.160 (μm)/without recesses) intersect. When the rotation speed N is 1000 or less, the friction loss exceeds that of cylinder liner Y (surface roughness Ra 0.160 (μm)/without recesses), and when the rotation speed N is 700 or less, the friction loss increases to a range exceeding 14 kPa. That is, in the case of the conventional cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses), when the rotation speed N exceeds 1000, the recesses 14 act to reduce friction loss, but when the rotation speed N is in the range of 1000 or less, the recesses 14 act to increase friction loss. According to undisclosed considerations by the present inventors, in the case of the conventional cylinder liner X (surface roughness Ra 0.160 (μm)/with recesses), the actual contact area between the cylinder liner and the piston ring 40 is reduced due to the presence of the recesses 14, so it was presumed that insufficient lubrication by the lubricating oil is likely to occur in the low rotation speed region, and the proportion of the boundary lubrication region is likely to increase.

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

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

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

The FMEP diagram in FIG. 8(B) shows the estimated FMEP of cylinder liner K, which has the same configuration as cylinder liner 10-A and has a surface roughness Ra of 0.140 (μm) in the central low-roughness region 20, although it is not an actual measurement value. It can be assumed that the FMEP diagram of cylinder liner K is close to the intermediate value between the FMEP diagrams of conventional cylinder liners X and Y, which have a surface roughness Ra of 0.160 (μm), and the FMEP diagram of cylinder liner 10-A, which has a surface roughness Ra of 0.120 (μm). This cylinder liner K also has a range where the friction loss mean effective pressure T is 14 kPa or less, which expands to a rotation speed N of 700 or less (left side of the graph). As a result, even at low rotation speeds, the sliding mode has extremely low friction loss.

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

<Changes in frictional force during one stroke>

Figure 10 (A) shows the variation in frictional force during the stroke of cylinder liner X and cylinder liner 10-B when the rotation speed N is 646 (low rotation). Figure 10 (B) shows the variation in frictional force during the stroke of cylinder liner X and cylinder liner 10-B when the rotation speed N is 2154 (high rotation). The horizontal axis of these graphs is the phase (angle) of the connecting rod. It can be seen that there is a large difference in frictional force between cylinder liner 10-B and cylinder liner X during low rotation operation in Figure 10 (A). In particular, it can be seen that the difference in frictional force is large in the phase range of 45 degrees to 135 degrees in the stroke moving toward the top dead center, and in the phase range of 225 degrees to 315 degrees in the stroke moving toward the bottom dead center. In particular, the frictional force is significantly smaller overall in the range of 180 degrees to 360 degrees in the stroke moving toward the bottom dead center. At low revolutions, it is presumed that the cylinder liner X slides in a manner close to the boundary lubrication region, whereas the cylinder liner 10-B slides in a manner close to the hydrodynamic lubrication region (or mixed lubrication region).

During high speed operation in Figure 10 (B), the frictional force of cylinder liner 10-B is generally smaller than the frictional force of cylinder liner X. During high speed operation, both cylinder liner 10-B and cylinder liner X are in the fluid lubrication region, but even in this fluid lubrication region, it can be seen that the frictional force of cylinder liner 10-B is always smaller. At the same time, it can be seen that the frictional force is significantly smaller near phases 0 degrees and 360 degrees, which are bottom dead center and near phase 180 degrees, which is top dead center, which are prone to becoming boundary lubrication regions.

<Oil consumption>

In the case of the sliding structure of the cylinder liner 10 and piston 30 of this embodiment, oil consumption is also suppressed. 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, there is no lack of lubrication because the recesses 14 are formed in an overlapping manner. In other words, this embodiment makes it possible to rationally achieve both reduced oil consumption and sufficient lubrication effect. According to simulations by the inventors, in the case of a diesel engine, it is estimated that the LOC ratio is reduced by approximately 10% with cylinder liner 10-B compared to cylinder liner X.

<Cylinder liner of the second embodiment>

Next, the sliding structure of an internal combustion engine relating to the second embodiment of the present invention will be explained with reference to the attached drawings.

11A, a plurality of recesses 14 are formed in the inner wall surface 12 of the cylinder liner 10 of the internal combustion engine of the second embodiment. The recesses 14 are formed only in the stroke center region 20 of the inner wall surface 12. This stroke center region 20 is part of the entire range (hereinafter referred to as the reference stroke region 19) from the lower surface position 19A (hereinafter also referred to as the top dead center side edge) of the ring groove of the lowest piston ring at the top dead center T of the piston 30 to the upper surface position 19B (hereinafter also referred to as the bottom dead center side edge) of the ring groove of the uppermost piston ring at the bottom dead center U of the piston 30.

In this embodiment, the stroke mid-region 20 is positioned below the top dead center edge 19A of the reference stroke region 19. As a result, a smooth upper smooth region 130A without recesses is formed in the entire area between the top dead center edge 19A of the reference stroke region 19 and the top dead center edge 27A of the stroke mid-region 20. The upper smooth region 130A is the region through which the piston ring 40 passes.

In addition, the stroke mid-region 20 in this embodiment is positioned above the bottom dead center edge 19B of the reference stroke region 19. As a result, a smooth lower smooth region 130B without recesses is formed in the entire area between the bottom dead center edge 19B of the reference stroke region 19 and the bottom dead center edge 27B of the stroke mid-region 20. The lower smooth region 130B is the region through which the piston ring 40 passes.

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

If the area outside the mid-stroke region 20 is defined as the outer region 25, then 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. A portion of the upper outer region 25A includes the upper smooth region 130A, and a portion of the lower outer region 23B includes the lower smooth region 130B.

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

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

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

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

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

The stroke direction distance of the upper smooth region 130 is preferably set to 30% or more of the total stroke direction distance of the reference stroke region 19. In addition, the stroke direction center point 20M of the stroke center region 20 is located on the bottom dead center U side of the piston compared to the stroke direction center point 19M of the reference stroke region.

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

The central low-roughness region 22, the upper low-roughness region 23A, and the lower low-roughness region 23B of the cylinder liner 10 are formed by honing using a honing machine. It is preferable to use a honing stone with a finer abrasive grain than F500 or #800 (JIS R 6001-2: 2017, ISO8486-2: 2007). After the central low-roughness region 22, the upper low-roughness region 23A, and the lower low-roughness region 23B are formed by this honing process, it is preferable not to perform a coating 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, the upper low-roughness region 23A, and the lower low-roughness region 23B will vary depending on the coating.

<The significance of the existence of the upper low-roughness region 23A>

As already mentioned, in this embodiment, the upper low-roughness region 23A, in which no recess is formed, is provided on the top dead center side of the central low-roughness region 22 where the recess is overlapped. The significance of this upper low-roughness region 23A is as follows. The top dead center side of the piston 30 is a high-temperature environment because of the presence of a combustion chamber. Therefore, if a recess is formed on the top dead center side of the cylinder liner 10 and engine oil is allowed to remain in the recess, the engine oil will become hot and vaporize, increasing the amount of oil consumed. Therefore, by not forming a recess in the upper low-roughness region 23A, the amount of oil consumed is suppressed. On the other hand, if the upper low-roughness region 23A is made low-rough as in this embodiment, there is a possibility that lubrication shortage will occur, or the recess 14 overlapping the central low-roughness region 22 adjacent to the lower side functions as a lubricating oil reservoir, and lubricating oil is actively supplied to the upper low-roughness region 23A through this recess 14, so a synergistic effect is obtained in which lubrication shortage is less likely to occur.

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

<Sliding structure of cylinder liner and piston ring in the second embodiment>

Figure 11 (B) shows the stroke of the piston ring 40 moving relatively through the cylinder liner 10 from top dead center T towards bottom dead center U. While the piston ring 40 is moving relatively through the upper low roughness region 23A, it follows stroke lines A and L. Then, when the piston ring 40 passes through the central low roughness region 22, it follows stroke line M. Thereafter, while the piston ring 40 is moving relatively through the lower low roughness region 23B of the cylinder liner 10 towards the bottom dead center, it follows stroke lines N and B.

According to the sliding structure for the internal combustion engine of the second embodiment, as in the first embodiment, the central low-roughness region 22 can reduce the coefficient of friction during low-speed movement. It is also possible to reduce friction loss during low rotation. Furthermore, the upper low-roughness region 23A through which the piston ring 40 passes can suppress oil consumption.

The present invention is not limited to the above-described embodiment, and various modifications can of course be made without departing from the spirit and scope of the present invention.

REFERENCE SIGNS LIST 10 Cylinder liner 12 Inner wall surface 14 Recess 20 Stroke center region 22 Central low roughness region 23A Upper low roughness region 23B Lower low roughness region 25 Outer region 25A Upper outer region 25B Lower outer region 30 Piston 40 Piston ring 110 Solid contact region 112 Boundary lubrication region 113 Mixed lubrication region 114 Hydrodynamic lubrication region

Claims (15)

A sliding structure of an internal combustion engine having a cylinder and a piston,
The cylinder is
a plurality of recesses are formed in a stroke center region of the inner wall surface, the stroke center region being the whole or a part of a region between a lower surface position of a ring groove of a lowest piston ring at a top dead center of the piston and an upper surface position of a ring groove of a top piston ring at a bottom dead center of the piston;
In the stroke center region, at least one of the recesses is disposed in any cross section perpendicular to the axis,
a central low roughness region is formed in at least a portion of the surface in contact with the piston ring in the stroke central region, the central low roughness region having an arithmetic mean roughness Ra of a profile curve measured by a stylus surface roughness tester of 0.140 μm or less;
The viscosity in the central low roughness region is μ, the relative velocity with respect to the piston is U, the contact load with respect to the piston is W, and the friction coefficient with respect to the piston is f,
When the evaluation parameter of the Stribeck diagram is defined as A = μ × U/W,
The minimum value fmin of the friction coefficient f in the central low roughness region is achieved within a range where the evaluation parameter A is 0.0003 or less.
Sliding structure of an internal combustion engine. A sliding structure of an internal combustion engine having a cylinder and a piston,
The cylinder is
a plurality of recesses are formed in a stroke center region of the inner wall surface, the stroke center region being the whole or a part of a region between a lower surface position of a ring groove of a lowest piston ring at a top dead center of the piston and an upper surface position of a ring groove of a top piston ring at a bottom dead center of the piston;
In the stroke center region, at least one of the recesses is disposed in any cross section perpendicular to the axis,
a central low roughness region is formed in at least a portion of the surface in contact with the piston ring in the stroke central region, the central low roughness region having an arithmetic mean roughness Ra of a profile curve measured by a stylus surface roughness tester of 0.140 μm or less;
In a friction test in a non-combustion state using a top ring, a second ring, and an oil ring corresponding to the piston ring, the rotation speed of the internal combustion engine is N (r/min) and the friction loss mean effective pressure (FMEP) between the piston ring and the central low roughness region is T (kPa),
The minimum value Tmin of the friction loss mean effective pressure T in the central low roughness region is achieved within a range in which the rotation speed N is 700 or less.
Sliding structure of an internal combustion engine. The arithmetic mean height Sa of the contour curved surface of the central low roughness region measured by a non-contact surface roughness measuring device is 0.20 μm or less.
3. The sliding structure of an internal combustion engine according to claim 1 or 2. The protruding valley depth Svk of the central low roughness region measured by a non-contact surface roughness measuring device is 0.41 μm or less.
The sliding structure for an internal combustion engine according to any one of claims 1 to 3. The central low roughness region has a protruding peak height (Spk) of 0.16 μm or less as measured by a non-contact surface roughness measuring device.
The sliding structure for an internal combustion engine according to any one of claims 1 to 4. The level difference Sk of the core portion of the central low roughness region measured by a non-contact surface roughness measuring device is 0.53 μm or less.
The sliding structure for an internal combustion engine according to any one of claims 1 to 5. The present invention is characterized in that, when the height of the protruding peaks in the central low roughness region measured by a non-contact surface roughness measuring device is E (Spk) and the depth of the protruding valleys is I (Svk), the ratio I/E is 2.6 or less.
The sliding structure for an internal combustion engine according to any one of claims 1 to 6. The arithmetic mean roughness Ra of the profile curve of the central low roughness region measured by a stylus type surface roughness measuring instrument is 0.120 μm or less.
The sliding structure for an internal combustion engine according to any one of claims 1 to 7. The central low roughness region includes a vicinity of an upper end edge and a vicinity of a lower end edge in the stroke central region.
The sliding structure for an internal combustion engine according to any one of claims 1 to 8. The entire stroke central region is the central low roughness region.
The sliding structure of an internal combustion engine according to any one of claims 1 to 9 . The minimum value fmin is achieved when the evaluation parameter A is in a range of 0.0001 or more.
The sliding structure of an internal combustion engine according to claim 1. The friction coefficient f in the central low roughness region is 0.07 or less in any range in which the evaluation parameter A is 0.0003 or less.
The sliding structure of an internal combustion engine according to claim 1. The piston and the cylinder in the central low roughness region are in a fluid lubrication state within a range in which the evaluation parameter A is 0.0003 or less.
The sliding structure of an internal combustion engine according to claim 1. The friction loss mean effective pressure T in the central low roughness region is 14 kPa or less when the rotation speed N is within a range of 700 or less.
The sliding structure of an internal combustion engine according to claim 2. The piston and the cylinder in the central low roughness region are in a fluid lubrication state at any one of the rotational speeds N within a range of 700 or less.
The sliding structure of an internal combustion engine according to claim 2.

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