INTERNAL COMBUSTION ENGINE AND TRANSPORTATION DEVICE

Abstract

 An internal combustion engine (100) includes a piston (40) formed of an aluminum alloy, the piston including a piston head (43) and a piston skirt (44) extending from an outer circumferential portion of the piston head; and a cylinder block (10) formed of an aluminum alloy, the cylinder block including a cylinder wall (12) including a sliding surface (12a), along which the piston is slidable. The aluminum alloy is exposed to the sliding surface of the cylinder wall. The piston skirt includes a skirt substrate (b1) formed of an aluminum alloy, the skirt substrate including a plurality of streak grooves (sg) formed in an outer circumferential surface thereof, the piston skirt further including a resin layer (rl) formed on at least a part of the outer circumferential surface of the skirt substrate. The outer circumferential surface of the skirt substrate has a ten-point average surface roughness Rz_JIS of 20 µm or larger.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an internal combustion engine, and specifically to an internal combustion engine including a piston formed of an aluminum alloy and a cylinder block formed of an aluminum alloy, and also relates to a transportation vehicle including such an internal combustion engine.

BACKGROUND ART

[0002] Recently, an aluminum alloy has been in a wide use as a material of a piston and a cylinder block for an internal combustion engine. In the case where a piston formed of an aluminum alloy and a cylinder block formed of an aluminum alloy are used in combination, a sleeve (liner) formed of cast iron is located in a cylinder bore or a surface of a cylinder wall is plated in order to guarantee a certain level of wear resistance.

[0003] Apart from the above, a technology of forming streak grooves in a surface of a skirt portion of a piston and forming a resin layer thereon is known (e.g., Patent Document No. 1). The formation of the streak grooves increases the adhesiveness between a substrate of the skirt portion and the resin layer. A sliding motion of the piston shaves the resin layer to decrease the surface roughness thereof. As a result, a sliding loss is decreased and the initial conformability is improved.

[0004] When the internal combustion engine is operated for a certain period time, the resin layer of the skirt portion is worn away. In the case where a sleeve formed of cast iron is located in the cylinder bore, or in the case where the surface of the cylinder wall is plated, the aluminum alloy forming the piston and the aluminum alloy forming the cylinder block do not contact each other even if the resin layer is worn away. Therefore, a problem of seizure or the like is not caused easily.

CITATION LIST

PATENT LITERATURE

[0005] Patent Document No. 1: Japanese Patent No. 6485952

SUMMARY OF INVENTION

TECHNICAL PROBLEM

[0006] It has been proposed to use a high-silicon aluminum alloy containing a high content of silicon (having a hyper eutectic composition) as a material of the cylinder block. Formation of a cylinder block of a high-silicon aluminum alloy makes it unnecessary to use a cast iron sleeve or to perform plating, and therefore, may reduce the weight of the internal combustion engine and simplify the production process thereof. It should be noted that, however, in the case where a cast iron sleeve is not used or plating is omitted, when the resin layer of the skirt portion of the piston is worn away, the aluminum alloy forming the piston and the aluminum alloy forming the cylinder block contact each other. This causes a risk of seizure.

[0007] An embodiment of the present invention made in light of the above-described problem has an object of, for an internal combustion engine including a cylinder block formed of an aluminum alloy and a piston formed of an aluminum alloy, realizing a structure that may suppress seizure after a resin layer is worn away even if a cast iron sleeve is not used or plating is omitted.

SOLUTION TO PROBLEM

[0008] This specification discloses the internal combustion engine and the transportation vehicle described in the following items.

[Item 1]

[0009] An internal combustion engine, including:

a piston formed of an aluminum alloy, the piston including a piston head and a piston skirt extending from an outer circumferential portion of the piston head; and

a cylinder block formed of an aluminum alloy, the cylinder block including a cylinder wall including a sliding surface, along which the piston is slidable;

wherein the aluminum alloy is exposed to the sliding surface of the cylinder wall,

wherein the piston skirt includes a skirt substrate formed of an aluminum alloy, the skirt substrate including a plurality of streak grooves formed in an outer circumferential surface thereof, the piston skirt further including a resin layer formed on at least a part of the outer circumferential surface of the skirt substrate, and

wherein the outer circumferential surface of the skirt substrate has a ten-point average surface roughness Rz_JIS of 20 µm or larger.

[0010] In the internal combustion engine according to an embodiment of the present invention, the piston skirt includes the skirt substrate formed of an aluminum alloy and including the plurality of streak grooves formed in the outer circumferential surface thereof, and the resin layer formed on at least a part of the outer circumferential surface of the skirt substrate. The resin layer formed on the outer circumferential surface of the skirt substrate decreases the sliding loss and improves the initial conformability of the piston to the cylinder block. In the internal combustion engine according to the present invention, the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate is 20 µm or larger. This indicates that the streak grooves formed in the outer circumferential surface of the skirt substrate are relatively deep. During the formation of the resin layer on the outer circumferential surface of the skirt substrate, the resin material necessarily goes into the streak grooves. Thus, in the case where the steak grooves are deep, after the resin layer is worn away, a state where a portion of the surface of the piston skirt where the aluminum alloy is exposed and a portion of the surface of the piston skirt where the resin material remains are mixed may be kept for a long period of time. Therefore, even after the resin layer is worn away, the seizure of the piston formed of an aluminum alloy and the cylinder block formed of an aluminum alloy may be suppressed.

[Item 2]

[0011] The internal combustion engine of item 1,
wherein the plurality of streak grooves extend in a circumferential direction of the piston.

[0012] The streak grooves may be formed by, for example, a turning process performed by use of a cutting tool (blade). In this case, the streak grooves extend in the circumferential direction of the piston.

[Item 3]

[0013] The internal combustion engine of item 1 or 2, wherein the outer circumferential surface of the skirt substrate has an average peak-trough interval Sm of 100 µm or longer and 500 µm or shorter.

[0014] The average peak-trough interval Sm (corresponding to the pitch of the streak grooves) of the outer circumferential surface of the skirt substrate is preferably 100 µm or longer and 500 µm or shorter. In the case where the average peak-trough interval Sm is shorter than 100 µm, there is a risk that the productivity is decreased, or that the production of the blade is made difficult. In the case where the average peak-trough interval Sm is longer than 500 µm, there is a risk that the processing precision of the piston is decreased. In addition, the width of the portion where the aluminum alloy is exposed after the resin layer is worn away is increased, which causes a risk that the seizure resistance is decreased by a certain degree.

[Item 4]

[0015] The internal combustion engine of any one of items 1 through 3, wherein the resin layer has a thickness of 10 µm or greater and 50 µm or less.

[0016] From the point of view of keeping the resin layer for a long period of time, the thickness of the resin layer is preferably 10 µm or greater. From the point of view of ease of production, the thickness of the resin layer is preferably 50 µm or less.

[Item 5]

[0017] The internal combustion engine of any one of items 1 through 4, wherein the resin layer includes a solid lubricant agent and hard particles.

[0018] In the case where the resin layer includes hard particles, the occurrence of the state where the resin layer is worn away may be delayed.

[Item 6]

[0019] The internal combustion engine of any one of items 1 through 5, wherein the skirt substrate includes, at the outer circumferential surface thereof, an anodic oxide film formed by alumite treatment with a phosphoric acid.

[0020] In the case where the skirt substrate includes, at the outer circumferential surface thereof, the anodic oxide film formed by alumite treatment with a phosphoric acid, the adhesiveness of the resin layer to the skirt substrate may be improved.

[Item 7]

[0021] The internal combustion engine of any one of items 1 through 6, wherein the cylinder block is formed of an aluminum alloy containing silicon.

[0022] In the case where an aluminum alloy containing silicon is used as a material of the cylinder block, deposited silicon crystal grains are exposed to the sliding surface. As a result, the seizure resistance and the wear resistance may be improved.

[Item 8]

[0023] A transportation vehicle, including the internal combustion engine having any of the above-described structures.

[0024] The internal combustion engine according to an embodiment of the present invention is preferably usable in any of various types of transportation vehicles.

ADVANTAGEOUS EFFECTS OF INVENTION

[0025] According to an embodiment of the present invention, a structure, for an internal combustion engine including a cylinder block formed of an aluminum alloy and a piston formed of an aluminum alloy, may be realized that may suppress seizure after a resin layer is worn away even if a cast iron sleeve is not used or plating is omitted may be realized.

BRIEF DESCRIPTION OF DRAWINGS

[0026] 

FIG. 1 is a cross-sectional view schematically showing an engine (internal combustion engine) 100 according to an embodiment of the present invention.

FIG. 2 is a perspective view schematically showing a cylinder block 10 included in the engine 100.

FIG. 3 is an enlarged plan view of a sliding surface 12a of a cylinder wall 12.

FIG. 4A is a side view schematically showing a piston 40 included in the engine 100.

FIG. 4B is a side view schematically showing the piston 40.

FIG. 5 is a cross-sectional view schematically showing a piston skirt 44 of the piston 40.

FIG. 6 shows how wearing proceeds in a piston skirt 44' in a comparative example.

FIG. 7 shows how wearing proceeds in the piston skirt 44.

FIG. 8 is a graph showing, regarding examples 1, 2 and 3 and comparative examples 1 and 2, the relationship between the remaining ratio of a resin layer rl represented by the vertical axis and the test time period from the start represented by the horizontal axis.

FIG. 9 shows binarized photographs of an outer circumferential surface of the piston skirt 44 in each of comparative example 1 and example 3 at the elapse of specific test time periods from the start.

FIG. 10 is a cross-sectional view showing another example of structure of the piston skirt 44.

FIG. 11 is a cross-sectional view schematically showing piston rings 42 of the piston 40.

FIG. 12 is a side view schematically showing an automatic two-wheeled vehicle 300 including the engine 100.

DESCRIPTION OF EMBODIMENTS

[0027] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. While a water-cooled engine will be described as an example below, the engine according to an embodiment of the present invention is not limited to being of a water-cooled type and may be of an air-cooled type. While a single-cylinder engine will be described as an example below, there is no specific limitation on the number of the cylinders in the engine.

[Structure of the engine]

[0028] FIG. 1 shows an engine (internal combustion engine) 100 according to an embodiment of the present invention. FIG. 1 is a cross-sectional view schematically showing the engine 100.

[0029] As shown in FIG. 1, the engine 100 includes a cylinder block 10, a cylinder head 20, and a crankcase 30. The engine 100 further includes a piston 40, a crankshaft 50, and a con rod (connecting rod) 60. The following description will be made with settings that a direction from the cylinder block 10 toward the cylinder head 20 is an "upward direction" and a direction from the cylinder block 10 toward the crankcase 30 is a "downward direction".

[0030] The cylinder block (may also be referred to as a "cylinder body") 10 includes a cylinder wall 12 and an outer wall 13. The cylinder wall 12 is formed to define a cylinder bore 11. The outer wall 13 surrounds the cylinder wall 12 and forms an outer enclosure of the cylinder block 10. A water jacket 14 holding cooling water is provided between the cylinder wall 12 and the outer wall 13.

[0031] The cylinder head 20 is provided above the cylinder block 10. The cylinder head 20 defines a combustion chamber 70 together with the cylinder wall 12 and the piston 40. The cylinder head 20 includes an intake port 21, through which fuel is to be introduced into the combustion chamber 70, and an exhaust port 22, through which exhaust gas is to be discharged from the combustion chamber 70. An intake valve 23 is provided in the intake port 21, and an exhaust valve 24 is provided in the exhaust port 22.

[0032] The crankcase 30 is provided below the cylinder block 10. Namely, the crankcase 30 is located so as to be on the side opposite to the cylinder head 20 with the cylinder block 10 being located therebetween. The crank case 30 may be separate from, or may be integrally formed with, the cylinder block 10.

[0033] The piston 40 is accommodated in the cylinder bore 11. In this embodiment, no cylinder sleeve is fit into the cylinder bore 11. Therefore, the piston 40 moves up and down in a reciprocating manner in the cylinder bore 11 while being in contact with an inner circumferential surface (cylinder bore 11-side surface) 12a of the cylinder wall 12. Namely, the inner circumferential surface 12a of the cylinder wall 12 is a sliding surface along which the piston 40 is slidable.

[0034] The crankshaft 50 is accommodated in the crankcase 30. The crankshaft 50 includes a crankpin 51 and a crank arm 52.

[0035] The con rod 60 includes a rod main body 61 having a rod-like shape, a small end portion 62 provided at one end of the rod main body 61, and a large end portion 63 provided at the other end of the rod main body 61. The con rod 60 connects the piston 40 and the crankshaft 50 to each other. Specifically, a piston pin 48 of the piston 40 is inserted into a through-hole (piston pin hole) of the small end portion 62, and the crankpin 51 of the crankshaft 50 is inserted into a through-hole (crankpin hole) of the large end portion 63. This structure connects the piston 40 and the crankshaft 50 to each other. A bearing 66 is provided between an inner circumferential surface of the large end portion 63 and the crankpin 51.

[0036] FIG. 2 is a perspective view schematically showing the cylinder block 10 of the engine 100. As described above, the cylinder block 10 includes the cylinder wall 12 including the sliding surface 12a, and the outer wall 13. The water jacket 14 is provided between the cylinder wall 12 and the outer wall 13. In this embodiment, the cylinder block 10 is formed of an aluminum alloy containing silicon. More specifically, the cylinder block 10 is formed of an aluminum-silicon-based alloy having a hyper eutectic composition. In this embodiment, the inner circumferential surface 12a of the cylinder wall 12 is not plated. Therefore, the aluminum alloy is exposed to the inner circumferential surface (sliding surface) 12a of the cylinder wall 12.

[0037] FIG. 3 is an enlarged plan view of the sliding surface 12a of the cylinder wall 12. The cylinder wall 12 of the cylinder block 10 includes an aluminum-containing solid-solution matrix (alloy substrate) 1 and a plurality of primary-crystal silicon grains 2 dispersed in the matrix 1. Some of the primary-crystal silicon grains 2 are exposed to the sliding surface 12a. Namely, the cylinder block 10 includes the primary-crystal silicon grains 2 at the sliding surface 12a.

[0038] Although not shown, the cylinder wall 12 further includes a plurality of eutectic silicon grains dispersed in the matrix 1. Therefore, the cylinder block 10 may further include the eutectic silicon grains at the sliding surface 12a. When a molten aluminum-silicon-based alloy having a hyper eutectic composition is cooled, relatively large silicon crystal grains are deposited first and then relatively small silicon crystal grains are deposited. The relatively large silicon crystal grains are the "primary-crystal silicon grains", and the relatively small silicon crystal grains are the "eutectic silicon grains".

[0039] FIG. 4A and FIG. 4B are side views schematically showing the piston 40 of the engine 100. FIG. 4A is a view of the piston 40 as seen in an axial direction of the piston pin 48 (see FIG. 1) (hereinafter, will be referred to as a "piston pin axial direction"), whereas FIG. 4B is a view of the piston 40 as seen in a direction perpendicular to the piston pin axial direction.

[0040] In this embodiment, the piston 40 (more specifically, a piston main body 41 described below) is formed of an aluminum alloy. The piston 40 may be formed by forging or casting.

[0041] As shown in FIG. 4A and FIG. 4B, the piston 40 includes the piston main body 41 and a plurality of piston rings 42. The piston main body 41 includes a piston head 43 and a piston skirt 44.

[0042] The piston head 43 is located at a top end of the piston 40. Ring grooves holding the piston rings 42 are formed in an outer circumferential portion of the piston head 43.

[0043] The piston skirt 44 extends downward from the outer circumferential portion of the piston head 43. The piston skirt 44 includes two portions 44a and 44b (referred to as a "first skirt portion" and a "second skirt portion") located so as to sandwich, in a radial direction, a central axis (cylinder axis line) of the cylinder bore 11.

[0044] The piston main body 41 includes a pair of piston pin bosses 45 having a piston pin hole 45a into which the piston pin 48 (see FIG. 1) is insertable, and ribs 46 connecting the piston pin bosses 45 and the piston skirt 44 to each other.

[0045] The piston rings 42 are attached to an outer circumferential portion of the piston main body 41, more specifically, to the outer circumferential portion of the piston head 43. In this embodiment, the piston 40 includes three piston rings 42. The number of the piston rings 42 is not limited to three. Among the three piston rings 42, the piston rings at a top position and at a central position (a top ring and a second ring) 42a and 42b, for example, are compression rings that keep the combustion chamber 70 in an airtight state. The piston ring at a bottom position (third ring) 42c is an oil ring that scrapes off extra oil attached to the cylinder wall 12. The piston rings 42 are formed of a metal material (e.g., steel).

[0046] The piston skirt 44 includes a resin layer rl formed on at least a part of an outer circumferential surface thereof. In the example shown in FIG. 4A and FIG. 4B, the resin layer rl is formed on generally the entirety of the outer circumferential surface thereof.

[0047] FIG. 5 shows an example of cross-sectional structure of the piston skirt 44. As shown in FIG. 5, the piston skirt 44 includes a skirt substrate b1 formed of an aluminum alloy and the resin layer rl formed on at least a part of an outer circumferential surface of the skirt substrate b1.

[0048] A plurality of streak grooves sg are formed in the outer circumferential surface of the skirt substrate b1. The streak grooves sg are striped grooves. The streak grooves sg may be formed by, for example, a turning process performed by use of a cutting tool (blade). In this case, the streak grooves sg extend in a circumferential direction of the piston 40. In the example shown in FIG. 5, each of the streak grooves sg has a generally triangular cross-section in a direction perpendicular to the circumferential direction. The cross-section of the streak groove sg is not limited to having such a shape, and may be, for example, generally bow-shaped. In the example shown in FIG. 5, the plurality of streak grooves sg are aligned with almost no gap. Alternatively, a flat portion may be present between adjacent streak grooves sg.

[0049] The resin layer rl includes, for example, a polymer matrix and solid lubricant particles (solid lubricant agent) dispersed in the polymer matrix. As a material of the polymer matrix, thermosetting polyamideimide, for example, is preferably usable. Needless to say, the material of the polymer matrix is not limited to this. As the solid lubricant particles, any of various known types of solid lubricant particles may be used. For example, graphite particles and molybdenum disulfide particles are preferably usable. The resin layer rl may be formed by, for example, applying a liquid resin material to the substrate b1 by a spray method or any of various printing methods (a screen printing method, a pad printing method or the like).

[0050] In this embodiment, the piston 40 is formed such that the outer circumferential surface of the skirt substrate b1 has a ten-point average surface roughness Rz_JIS in a predetermined range. Specifically, the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 is 20 µm or larger in generally the entirety of the outer circumferential surface.

[0051] As represented by the following expression, the ten-point average surface roughness Rz_JIS is of a certain reference length of a profile curve, and is a difference between an average value of elevations of the highest peaks through the fifth highest peaks R1, R3, R5, R7 and R9, and an average value of elevations of the lowest trough through the fifth lowest trough R2, R4, R6, R8 and R10. The ten-point average surface roughness Rz_JIS may be measured by use of a surface roughness meter (e.g., Surfcom 1400D produced by Tokyo Seimitsu Co., Ltd.).

[0052] As described above, in the engine 100 in this embodiment, the piston skirt 44 includes the skirt substrate b1 formed of an aluminum alloy and including the plurality of streak grooves sg formed in the outer circumferential surface thereof, and the resin layer rl formed on at least a part of the outer circumferential surface of the skirt substrate b1. The resin layer rl formed on the outer circumferential surface of the skirt substrate b1 decreases the sliding loss and improves the initial conformability of the piston 40 to the cylinder block 10.

[0053] In the engine 100 in this embodiment, the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 is 20 µm or larger. This indicates that the streak grooves sg formed in the outer circumferential surface of the skirt substrate b1 are relatively deep. During the formation of the resin layer rl on the outer circumferential surface of the skirt substrate b1, the resin material necessarily goes into the streak grooves sg. Thus, in the case where the steak grooves sg are deep, after the resin layer rl is worn away, a state where a portion of the surface of the piston skirt 44 where the aluminum alloy is exposed and a portion of the surface of the piston skirt 44 where the resin material remains are mixed may be kept for a long period of time. Therefore, even after the resin layer rl is worn away, seizure of the piston 40 formed of an aluminum alloy and the cylinder block 10 formed of an aluminum alloy may be suppressed. The ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 may be adjusted by, for example, changing the cutting tool to be used to apply a turning process to the skirt substrate b1.

[0054] Now, reasons why the seizure may be suppressed will be described with reference to FIG. 6 and FIG. 7.

[0055] FIG. 6 and FIG. 7 show how the wearing proceeds in a piston skirt 44' in a comparative example having relatively shallow streak grooves sg (having a ten-point average surface roughness Rz_JIS of an outer circumferential surface of, for example, about 1.6 to about 3.2 µm) and in the piston skirt 44 in this embodiment. FIG. 6 and FIG. 7 each show an initial state, a state where the resin layer rl is worn away (i.e., a state where the skirt substrate b1 starts to be exposed), and a state where the wearing further proceeds thereafter.

[0056] As can be seen from FIG. 6, in the piston skirt 44' in the comparative example, when the wearing proceeds after the resin layer rl is worn away, the ratio of the aluminum alloy exposed to the outer circumferential surface is rapidly increased. Therefore, the seizure occurs quickly.

[0057] By contrast, in the piston skirt 44 in this embodiment, as can be seen from FIG. 7, even when the wearing proceeds after the resin layer rl is worn away, the ratio of the aluminum alloy exposed to the outer circumferential surface is increased merely slowly. Therefore, occurrence of the seizure may be delayed.

[0058] Samples of the piston 40, the outer circumferential surface of the skirt substrate b1 of which had a ten-point average surface roughness Rz_JIS of 20 µm or larger, were produced (examples 1, 2 and 3), and the effect of suppressing (delaying) the occurrence of the seizure was investigated by a sliding test. The results will be described below. For the investigation, pistons in examples 1, 2 and 3 were compared against pistons in comparative examples 1 and 2. The ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 was 24.8 µm, 66.7 µm and 67.8 µm in examples 1, 2 and 3 respectively, and was 1.9 µm and 11.6 µm in comparative examples 1 and 2 respectively. The resin layer rl had a thickness of 3 to 5 µm in examples 1, 2 and 3 and comparative examples 1 and 2 (as described below, the thickness of the resin layer rl may be 10 µm or greater, but in this test, the thickness of the resin layer rl was set to of a relatively small value in order to shorten the test time).

[0059] FIG. 8 and FIG. 9 show the results of the sliding test. The sliding test was performed under the conditions that the load was 980 N, the rotation rate was 600 rpm, and the temperature was 140°C. FIG. 8 is a graph showing, regarding examples 1, 2 and 3 and comparative example 1 and 2, the relationship between the remaining ratio of the resin layer rl (the ratio occupied by the area size of the remaining resin layer rl with respect to the area size of a region of the outer circumferential surface of the piston skirt 44 where the resin layer rl was formed) represented by the vertical axis, and the test time period from the start represented by the horizontal axis. FIG. 9 shows, in a top part, binarized photographs of the outer circumferential surface of the piston skirt 44' in comparative example 1 at the elapse of specific test time periods from the start. FIG. 9 shows, in a bottom part, such binarized photographs of the piston skirt 44 in example 3. In each of the photographs shown in FIG. 9, black portions are portions where the aluminum alloy of the skirt substrate b1 is exposed, and gray portions are portions where the resin layer rl remains. Each of the photographs in FIG. 9 is accompanied by the remaining ratio of the resin layer rl.

[0060] As shown in FIG. 8, in comparative examples 1 and 2, the remaining ratio of the resin layer rl was rapidly decreased as the test time period from the start was extended. In comparative examples 1 and 2, the seizure occurred at the elapse of the test time periods from the start of 3060 seconds and 3600 seconds, respectively. By contrast, in examples 1, 2 and 3, the remaining ratio of the resin layer rl was decreased slowly as the test time period from the start was extended. The seizure did not occur even when the test time period from the start exceeded 7000 seconds.

[0061] It is also seen from a comparison between the top part and the bottom part of FIG. 9 that in example 3, the remaining ratio of the resin layer rl is decreased more slowly than in comparative example 1 and that a state where the seizure does not occur (state where a relatively large amount of the resin layer rl remains) may be kept for a long period of time in example 3.

[0062] From the above-described results of the investigation, it is confirmed that the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 of 20 µm or larger may suppress the seizure of the piston 40 formed of an aluminum alloy and the cylinder block 10 formed of an aluminum alloy for a long period of time.

[0063] There is no specific limitation on the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1. In the case where the ten-point average surface roughness Rz_JIS is too large, an adverse effect may be exerted on the processing precision of the piston. Therefore, from the point of view of the processing precision of the piston, the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 is preferably 50 µm or smaller.

[0064] The outer circumferential surface of the skirt substrate b1 preferably has an average peak-trough interval Sm (corresponding to the pitch of the streak grooves sg) of 100 µm or longer and 500 µm or shorter. In the case where the average peak-trough interval Sm is shorter than 100 µm, there is a risk that the productivity is decreased, or that the production of the blade used to turn the skirt substrate b1 is made difficult. In the case where the average peak-trough interval Sm is longer than 500 µm, there is a risk that the processing precision of the piston is decreased. In addition, the width of the portion where the aluminum alloy is exposed after the resin layer rl is worn away is increased, which causes a risk that the seizure resistance is decreased by a certain degree. The average peak-trough interval Sm may be measured by a surface roughness meter like the ten-point average surface roughness Rz_JIS.

[0065] FIG. 4A and FIG. 4B each show an example in which the resin layer rl is formed on generally the entirety of the outer circumferential surface of the piston skirt 44. The resin layer rl may be formed only on a part of the outer circumferential surface. It should be noted that, however, from the point of view of enhancing the effects of, for example, decreasing the sliding loss and improving the initial conformability, it is preferred that the resin layer rl is formed on a maximum possible area of the outer circumferential surface of the piston skirt 44. For example, with respect to the outer circumferential surface of the piston skirt 44, the resin layer rl has an area size occupying a ratio of preferably 500 or higher, more preferably 70% or higher, and still more preferably 90% or higher (namely, the resin layer rl is formed on generally the entirety of the outer circumferential surface of the piston skirt 44).

[0066] In the structure described above, the resin layer rl includes solid lubricant particles. The resin layer rl may include hard particles in addition to the solid lubricant agent. In the case where the resin layer rl includes hard particles, the occurrence of the state where the resin layer rl is worn away may be delayed. As the hard particles, for example, metal oxide particles may be used. The amount, the particle diameter and the like of the hard particles are appropriately adjusted in accordance with the type of the hard particles used.

[0067] The resin layer rl has a thickness t1 (see FIG. 5) that is not specifically limited. From the point of view of keeping the resin layer rl for a long period of time, the thickness t1 of the resin layer rl is preferably 10 µm or greater. From the point of view of ease of production, the thickness t1 of the resin layer rl is preferably 50 µm or less. As can be seen from FIG. 5, the thickness t1 of the resin layer rl does not include a thickness of the portion of the resin material in the streak grooves sg. The expression "the resin layer rl is worn away" indicates that the resin material is made non-existent except for the resin material in the streak grooves sg, and does not indicate that the resin material in the streak grooves sg is also made non-existent.

[0068] FIG. 10 shows another example of structure of the piston skirt 44. In the example shown in FIG. 10, the skirt substrate b1 includes, at the outer circumferential surface thereof, an anodic oxide film b1a formed by alumite treatment with a phosphoric acid (phosphoric acid-alumite film b1a). In the case where the skirt substrate b1 includes the phosphoric acid-alumite film b1a at the outer circumferential surface thereof, the adhesiveness of the resin layer rl to the skirt substrate b1 may be improved. The phosphoric acid-alumite film b1a has a thickness t2 of, for example, 30 µm or greater and 200 µm or less.

[0069] FIG. 11 is a cross-sectional view showing an example of structure of the piston ring 42 of the piston 40. In the example shown in FIG. 11, a diamond-like carbon layer (hereinafter, referred to as a "DLC layer") 42D is formed on an outer circumferential portion (outer circumferential surface) of the piston ring 42. The outer circumferential portion of the piston ring 42 is a portion to be in contact with the cylinder wall 12. The piston ring 42 does not need to include the DLC layer 42D. However, the DLC layer 42D formed on the outer circumferential surface of each of the piston rings 42 may prevent, with more certainty, the cylinder wall 12 from being scuffed by the piston rings 42.

[0070] The DLC layer 42D is preferably formed by a deposition method (e.g., a CVD method or a PVD method) . The DLC layer 42D may have any composition or a thickness with no specific limitation. From the point of view of preventing the scuffing with more certainty, the thickness of the DLC layer 42D is preferably 2 µm or greater. From the point of view of the adhesiveness, the thickness of the DLC layer 42D is preferably 20 µm or less.

[0071] In the case where an aluminum alloy containing silicon as described in this embodiment as an example is used as a material of the cylinder block 10, deposited silicon crystal grains (primary-crystal silicon grains 2) may be exposed to the sliding surface 12a. As a result, the seizure resistance and the wear resistance may be improved.

[0072] From the point of view of sufficiently improving the wear resistance and the strength of the cylinder block 10, the aluminum alloy as the material of the cylinder block 10 preferably contains silicon at a content of 15% by mass or higher and 25% by mass or lower. In the case where the silicon content is 15% by mass or higher, a sufficiently large amount of the primary-crystal silicon grains 2 may be deposited, which may sufficiently improve the wear resistance of the cylinder block 10. In the case where the silicon content is 25% by mass or lower, the strength of the cylinder block 10 may be kept sufficiently high. The aluminum alloy contains aluminum at a content of, for example, 73.4% by mass or higher and 79.6% by mass or lower. The aluminum alloy may contain copper. In this case, the aluminum alloy contains copper at a content of, for example, 2.0% by mass or higher and 5.0% by mass or lower.

[0073] The primary-crystal silicon grains 2 have an average grain diameter in the range of 8 µm or longer and 50 µm or shorter. In this case, the wear resistance of the cylinder block 10 may be further improved. In the case where the average grain diameter of the primary-crystal silicon grains 2 is longer than 50 µm, the number of the primary-crystal silicon grains 2 per unit area size of the sliding surface 12a is small. Therefore, a large load is applied to each of the primary-crystal silicon grains 2 while the engine 100 is operated, and the primary-crystal silicon grains 2 may possibly be crushed. The crushed pieces of the primary-crystal silicon grains 2 act undesirably as polishing particles, which causes a risk that the sliding surface 12a is significantly worn. In the case where the average grain diameter of the primary-crystal silicon grains 2 is shorter than 8 µm, merely a small part of the primary-crystal silicon grains 2 is embedded in the matrix 1. Therefore, the primary-crystal silicon grains 2 easily fall while the engine 100 is operated. The primary-crystal silicon grains 2 that have fallen act undesirably as polishing particles, which causes a risk that the sliding surface 12a is significantly worn.

[0074] By contrast, in the case where the average grain diameter of the primary-crystal silicon grains 2 is 8 µm or longer and 50 µm or shorter (more preferably 12 µm or longer and 50 µm or shorter), the primary-crystal silicon grains 2 are present in a sufficient number per unit area size of the sliding surface 12a. Therefore, the load applied to each of the primary-crystal silicon grains 2 while the engine 100 is operated is relatively small, which suppresses the crushing of the primary-crystal silicon grains 2. Since the part of the primary-crystal silicon grains 2 that is embedded in the matrix 1 is sufficiently large, the fall of the primary-crystal silicon grains 2 is suppressed. Therefore, the wear of the sliding surface 12a by the primary-crystal silicon grains 2 that have fallen is suppressed.

[0075] The eutectic silicon grains have an average grain diameter shorter than that of the primary-crystal silicon grains 2. The average grain diameter of the eutectic silicon grains is, for example, 7.5 µm or shorter.

[0076] The average grain diameters of the primary-crystal silicon grains 2 and the eutectic silicon grains may be measured as follows by image processing performed on an image of the sliding surface 12a. First, a diameter (equivalent diameter) of each of the silicon crystal grains with an assumption that the silicon crystal grains are of a true circle is calculated based on an area size of each silicon crystal grain obtained by the image processing. As a result, the number (frequency) and the diameters of the silicon crystal grains are specified. Tiny crystal grains each having a diameter shorter than 1 µm are not counted as silicon crystal grains. Based on the calculated number (frequency) and the calculated diameters of the silicon crystal grains, a grain size distribution of the silicon crystal grains is obtained. The obtained grain size distribution (histogram) includes two peaks. The grain size distribution is divided into two regions with the threshold being a diameter of a portion forming a trough between the two peaks. The region corresponding to longer diameters is set as the grain size distribution of the primary-crystal silicon grains, and the region corresponding to shorter diameters is set as the grain size distribution of the eutectic silicon grains. Based on each of the grain size distributions, the average crystal diameter of the primary-crystal silicon grains and the average crystal diameter of the eutectic silicon grains may be calculated.

[Transportation vehicle]

[0077] The engine 100 according to an embodiment of the present invention is preferably usable for various types of transportation vehicles. FIG. 12 shows an example of automatic two-wheeled vehicle including the engine 100 according to an embodiment of the present invention.

[0078] In an automatic two-wheeled vehicle 300 shown in FIG. 12, a head pipe 302 is provided at a front end of a main body frame 301. A front fork 303 is attached to the head pipe 302 so as to be swingable in a left-right direction of the vehicle. A front wheel 304 is rotatably supported at a bottom end of the front fork 303.

[0079] A seat rail 306 is attached so as to extend rearward from a top portion of a rear end of the main body frame 301. A fuel tank 307 is provided on the main body frame 301, and a main seat 308a and a tandem seat 308b are provided on the seat rail 306.

[0080] A rear arm 309 extending rearward is attached to the rear end of the main body frame 301. A rear wheel 310 is rotatably supported at a rear end of the rear arm 309.

[0081] The engine 100 is held on a central portion of the main body frame 301. A radiator 311 is provided to the front of the engine 100. An exhaust pipe 312 is connected to an exhaust port of the engine 100, and a muffler 313 is attached to a rear end of the exhaust pipe 312.

[0082] A transmission 315 is coupled with the engine 100. A drive sprocket 317 is attached to an output shaft 316 of the transmission 315. The drive sprocket 317 is coupled with a rear wheel sprocket 319 of the rear wheel 310 via a chain 318. The transmission 315 and the chain 318 act as a transmission mechanism that transmits power generated by the engine 100 to the driving wheel.

[0083] In this embodiment, the automatic two-wheeled vehicle is shown as an example of the transportation vehicle. The engine according to an embodiment of the present invention is not limited to being used for an automatic two-wheeled vehicle, and is also preferably usable for any other transportation vehicle such as an automatic four-wheeled vehicle, an automatic three-wheeled vehicle, a seacraft or the like.

[0084] As described above, the internal combustion engine 100 according to an embodiment of the present invention includes the piston 40 formed of an aluminum alloy, the piston 40 including the piston head 43 and the piston skirt 44 extending from the outer circumferential portion of the piston head 43; and the cylinder block 10 formed of an aluminum alloy, the cylinder block 10 including the cylinder wall 12 including the sliding surface 12a, along which the piston 40 is slidable. The aluminum alloy is exposed to the sliding surface 12a of the cylinder wall 12. The piston skirt 44 includes the skirt substrate b1 formed of an aluminum alloy, the skirt substrate b1 including the plurality of streak grooves sg formed in the outer circumferential surface thereof, the piston skirt 44 further including the resin layer rl formed on at least a part of the outer circumferential surface of the skirt substrate b1. The outer circumferential surface of the skirt substrate b1 has a ten-point average surface roughness Rz_JIS of 20 µm or larger.

[0085] In the internal combustion engine 100 according to an embodiment of the present invention, the piston skirt 44 includes the skirt substrate b1 formed of an aluminum alloy and including the plurality of streak grooves sg formed in the outer circumferential surface thereof, and the resin layer rl formed on at least a part of the outer circumferential surface of the skirt substrate b1. The resin layer rl formed on the outer circumferential surface of the skirt substrate b1 decreases the sliding loss and improves the initial conformability of the piston 40 to the cylinder block 10. In the internal combustion engine 100 according to the present invention, the ten-point average surface roughness Rz_JIS of the outer circumferential surface of the skirt substrate b1 is 20 µm or larger. This indicates that the streak grooves sg formed in the outer circumferential surface of the skirt substrate b1 are relatively deep. During the formation of the resin layer rl on the outer circumferential surface of the skirt substrate b1, the resin material necessarily goes into the streak grooves sg. Thus, in the case where the steak grooves sg are deep, after the resin layer rl is worn away, a state where a portion of the surface of the piston skirt 44 where the aluminum alloy is exposed and a portion of the surface of the piston skirt 44 where the resin material remains are mixed may be kept for a long period of time. Therefore, even after the resin layer rl is worn away, the seizure of the piston 40 formed of an aluminum alloy and the cylinder block 10 formed of an aluminum alloy may be suppressed.

[0086] In an embodiment, the plurality of streak grooves sg extend in the circumferential direction of the piston 40.

[0087] The streak grooves sg may be formed by, for example, a turning process performed by use of a cutting tool (blade). In this case, the streak grooves sg extend in the circumferential direction of the piston 40.

[0088] In an embodiment, the outer circumferential surface of the skirt substrate b1 has an average peak-trough interval Sm of 100 µm or longer and 500 µm or shorter.

[0089] The average peak-trough interval Sm (corresponding to the pitch of the streak grooves sg) of the outer circumferential surface of the skirt substrate b1 is preferably 100 µm or longer and 500 µm or shorter. In the case where the average peak-trough interval Sm is shorter than 100 µm, there is a risk that the productivity is decreased, or that the production of the blade is made difficult. In the case where the average peak-trough interval Sm is longer than 500 µm, there is a risk that the processing precision of the piston is decreased. In addition, the width of the portion where the aluminum alloy is exposed after the resin layer rl is worn away is increased, which causes a risk that the seizure resistance is decreased by a certain degree.

[0090] In an embodiment, the resin layer rl has the thickness t1 of 10 µm or greater and 50 µm or less.

[0091] From the point of view of keeping the resin layer rl for a long period of time, the thickness t1 of the resin layer rl is preferably 10 µm or greater. From the point of view of ease of production, the thickness t1 of the resin layer rl is preferably 50 µm or less.

[0092] In an embodiment, the resin layer rl includes a solid lubricant agent and hard particles.

[0093] In the case where the resin layer rl includes hard particles, the occurrence of the state where the resin layer rl is worn away may be delayed.

[0094] In an embodiment, the skirt substrate b1 includes, at the outer circumferential surface thereof, the anodic oxide film b1a formed by alumite treatment with a phosphoric acid.

[0095] In the case where the skirt substrate b1 includes, at the outer circumferential surface thereof, the anodic oxide film b1a formed by alumite treatment with a phosphoric acid, the adhesiveness of the resin layer rl to the skirt substrate b1 may be improved.

[0096] In an embodiment, the cylinder block 10 is formed of an aluminum alloy containing silicon.

[0097] In the case where an aluminum alloy containing silicon is used as a material of the cylinder block 10, deposited silicon crystal grains (primary-crystal silicon grains 2) may be exposed to the sliding surface 12a. As a result, the seizure resistance and the wear resistance may be improved.

[0098] A transportation vehicle according to an embodiment of the present invention includes the internal combustion engine 100 having any of the above-described structures.

[0099] The internal combustion engine 100 according to an embodiment of the present invention is preferably usable in any of various types of transportation vehicles.

INDUSTRIAL APPLICABILITY

[0100] According to an embodiment of the present invention, a structure, for an internal combustion engine including a cylinder block formed of an aluminum alloy and a piston formed of an aluminum alloy, may be realized that may suppress the seizure after a resin layer is worn away even if a cast iron sleeve is not used or plating is omitted. The internal combustion engine according to an embodiment of the present invention is preferably usable in any of various types of transportation vehicles including an automatic two-wheeled vehicle.

REFERENCE SIGNS LIST

[0101] 1: matrix (alloy substrate); 2: primary-crystal silicon grain; 10: cylinder block; 11: cylinder bore; 12: cylinder wall; 12a: sliding surface (inner circumferential surface of the cylinder wall); 13: outer wall; 14: water jacket; 20: cylinder head; 21: intake port; 22: exhaust port; 23: intake valve; 24: exhaust valve; 30: crankcase; 40: piston; 41: piston main body; 42: piston ring; 42a: top ring; 42b: second ring; 42c: third ring; 42D: diamond-like carbon layer; 43: piston head; 44: piston skirt; 44a: first skirt portion; 44b: second skirt portion; 45: piston pin boss; 45a: piston pin hole; 46: rib; 48: piston pin; 50: crankshaft; 51: crankpin; 52: crank arm; 60: con rod; 61: rod main body; 62: small end portion; 63: large end portion; 70: combustion chamber; 100: engine (internal combustion engine); 300: automatic two-wheeled vehicle; b1: skirt substrate; b1a: anodic oxide film (phosphoric acid-alumite film); rl: resin layer; sg: streak groove

Claims

1. An internal combustion engine, comprising:

a piston formed of an aluminum alloy, the piston including a piston head and a piston skirt extending from an outer circumferential portion of the piston head; and

a cylinder block formed of an aluminum alloy, the cylinder block including a cylinder wall including a sliding surface, along which the piston is slidable;

wherein the aluminum alloy is exposed to the sliding surface of the cylinder wall,

wherein the piston skirt includes a skirt substrate formed of an aluminum alloy, the skirt substrate including a plurality of streak grooves formed in an outer circumferential surface thereof, the piston skirt further including a resin layer formed on at least a part of the outer circumferential surface of the skirt substrate, and

wherein the outer circumferential surface of the skirt substrate has a ten-point average surface roughness Rz_JIS of 20 µm or larger.

2. The internal combustion engine of claim 1,
wherein the plurality of streak grooves extend in a circumferential direction of the piston.

3. The internal combustion engine of claim 1 or 2, wherein the outer circumferential surface of the skirt substrate has an average peak-trough interval Sm of 100 µm or longer and 500 µm or shorter.

4. The internal combustion engine of any one of claims 1 through 3, wherein the resin layer has a thickness of 10 µm or greater and 50 µm or less.

5. The internal combustion engine of any one of claims 1 through 4, wherein the resin layer includes a solid lubricant agent and hard particles.

6. The internal combustion engine of any one of claims 1 through 5, wherein the skirt substrate includes, at the outer circumferential surface thereof, an anodic oxide film formed by alumite treatment with a phosphoric acid.

7. The internal combustion engine of any one of claims 1 through 6, wherein the cylinder block is formed of an aluminum alloy containing silicon.

8. A transportation vehicle, comprising the internal combustion engine of any one of claims 1 through 7.

Drawing