EP4224004A1

EP4224004A1 - Internal combustion engine and transport equipment

Info

Publication number: EP4224004A1
Application number: EP21957735.0A
Authority: EP
Inventors: Keita Watanabe; Hirotaka Kurita
Original assignee: Yamaha Motor Co Ltd
Current assignee: Yamaha Motor Co Ltd
Priority date: 2021-12-14
Filing date: 2021-12-14
Publication date: 2023-08-09
Also published as: EP4224004A4; WO2023112123A1

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) including a cylinder wall (12) including a sliding surface (12a), along which the piston is slidable. The cylinder block is formed of an aluminum alloy containing silicon, and includes a plurality of primary-crystal silicon grains (2) at the sliding surface. The piston skirt includes a resin layer (rl) formed on at least a part of an outer circumferential surface thereof.

Description

TECHNICAL FIELD

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

BACKGROUND ART

Recently, for the purpose of reducing the weight of an engine (internal combustion engine), the material of a cylinder block is progressively shifted from cast iron to an aluminum alloy. Especially, use of a high-silicon aluminum alloy containing a high content of silicon (i.e., having a hyper eutectic composition) makes it unnecessary to use a sleeve to be fit into a cylinder bore. This may further reduce the weight of the engine and also reduce the size of the engine because of a shorter inter-cylinder distance realized.
A cylinder block formed of a high-silicon aluminum alloy contains silicon crystal grains standing out at a sliding surface, and therefore, has an improved wear resistance. In addition, since oil is held at a step portion between an aluminum alloy substrate and the silicon crystal grains, the cylinder block has an improved seizure resistance and an improved scuff resistance. It should be noted that, however, if the silicon crystal grains at the sliding surface are crushed, the wear resistance is declined. Therefore, a processing method has been proposed to realize a state where such crushed crystal grains are prevented, to a maximum possible degree, from being present at the sliding surface (e.g., Patent Document No. 1).

CITATION LIST

PATENT LITERATURE

Patent Document No. 1: Japanese Laid-Open Patent Publication No. Hei 7-88711

SUMMARY OF INVENTION

TECHNICAL PROBLEM

However, when the internal combustion engine is operated for the first time, the silicon crystal grains present at the sliding surface partially fall and damage a surface of a cylinder wall, which causes oil to be consumed in a deteriorated manner.
An embodiment of the present invention made in light of the above-described problem has an object of, in an internal combustion engine including a cylinder block formed of an aluminum alloy containing silicon, suppressing damage of a surface of a cylinder wall when the internal combustion engine is operated for the first time and thus suppressing oil consumption made in a deteriorated manner.

SOLUTION TO PROBLEM

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

[Item 1]

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 including a cylinder wall including a sliding surface, along which the piston is slidable;
wherein the cylinder block is formed of an aluminum alloy containing silicon, and includes a plurality of primary-crystal silicon grains at the sliding surface; and
wherein the piston skirt includes a resin layer formed on at least a part of an outer circumferential surface thereof.

In the internal combustion engine according to an embodiment of the present invention, the piston skirt includes the resin layer formed on at least a part of the outer circumferential surface thereof. The resin layer is softer than an aluminum alloy substrate (matrix) of the cylinder block. Therefore, even if the primary-crystal silicon grains at the sliding surface of the cylinder wall fall, the primary-crystal silicon grains that have fallen are pushed into the resin layer, and thus damage of the surface of the cylinder wall is suppressed (namely, the cylinder wall is protected). On the piston side, piston rings protrude to an outermost position. Therefore, there is no problem even if a surface of the resin layer is damaged. Since the damage of the surface of the cylinder wall is suppressed, oil (lubricant oil) attached to the cylinder wall is scraped off appropriately by the piston rings. Therefore, oil consumption made in a deteriorated manner may be suppressed.
The resin layer of the piston skirt prevents heat transfer from the piston skirt to the cylinder wall. Therefore, after the engine is started, the piston is warmed promptly, and an appropriate size of gap is made between the piston skirt and the cylinder wall. This may quickly suppress noise generated immediately after the engine is started, and may also suppress wearing and adhesion of the piston and the cylinder wall.
After the resin layer of the piston skirt disappears as a result of the engine being operated for a certain period of time, a bare surface of the piston skirt (portion covered with the resin layer) and the cylinder wall contact each other. In the engine according to this embodiment, the primary-crystal silicon grains are present at the sliding surface of the cylinder wall, and silicon has a heat conductivity lower than that of aluminum. Therefore, the heat is not easily transferred from the piston skirt to the cylinder block. For this reason, even after the resin layer disappears, the piston is warmed promptly, which may suppress noise generated immediately after the engine is started, and may also suppress wearing and adhesion of the piston and the cylinder wall.

[Item 2]

The internal combustion engine of item 1,

wherein the piston includes a plurality of piston rings attached to the outer circumferential portion of the piston head, and
wherein each of the plurality of piston rings includes a diamond-like carbon layer on an outer circumferential surface thereof.

In the case where each of the piston rings includes the diamond-like carbon layer on the outer circumferential surface thereof, the heat of the piston is prevented from being transferred easily to the cylinder block. This makes it easier to warm the piston after the engine is started.

[Item 3]

The internal combustion engine of item 1 or 2, wherein the resin layer includes a solid lubricant agent and hard particles.
In the case where the resin layer includes hard particles, wearing of the resin layer may be delayed.

[Item 4]

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.
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]

The internal combustion engine of any one of items 1 through 4, wherein the cylinder block is formed of an aluminum alloy containing silicon at a content of 15% by mass or higher and 25% by mass or lower.
From the point of view of sufficiently improving the wear resistance and the strength of the cylinder block, the aluminum alloy as the material of the cylinder block 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 may be deposited, which may sufficiently improve the wear resistance of the cylinder block. In the case where the silicon content is 25% by mass or lower, the strength of the cylinder block may be kept sufficiently high.

[Item 6]

The internal combustion engine of any one of items 1 through 5, wherein the plurality of primary-crystal silicon grains have an average grain diameter of 8 µm or longer and 50 µm or shorter.
In the case where the primary-crystal silicon grains have an average grain diameter in the range of 8 µm or longer and 50 µm or shorter, the damage of the surface of the cylinder wall may be suppressed with more certainty.
In the case where the average grain diameter of the primary-crystal silicon grains is longer than 50 µm, the number of the primary-crystal silicon grains per unit area size of the sliding surface is small. Therefore, a large load is applied to each of the primary-crystal silicon grains while the engine is operated, and the primary-crystal silicon grains may possibly be crushed. The crushed pieces of the primary-crystal silicon grains act undesirably as polishing particles, which causes a risk that the surface of the cylinder wall is damaged.
In the case where the average grain diameter of the primary-crystal silicon grains is shorter than 8 µm, merely a small part of the primary-crystal silicon grains is embedded in the matrix. Therefore, the primary-crystal silicon grains easily fall while the engine is operated. The primary-crystal silicon grains that have fallen act undesirably as polishing particles, which causes a risk that the surface of the cylinder wall is damaged.
By contrast, in the case where the average grain diameter of the primary-crystal silicon grains is 8 µm or longer and 50 µm or shorter, the primary-crystal silicon grains are present in a sufficient number per unit area size of the sliding surface. Therefore, the load applied to each of the primary-crystal silicon grains while the engine is operated is relatively small, which suppresses the crushing of the primary-crystal silicon grains. Since the part of the primary-crystal silicon grains that is embedded in the matrix is sufficiently large, the fall of the primary-crystal silicon grains is suppressed. Therefore, the damage of the surface of the cylinder by the primary-crystal silicon grains that have fallen is suppressed.

[Item 7]

The internal combustion engine of any one of items 1 through 6, wherein the plurality of primary-crystal silicon grains have an area size occupying a ratio of 8% or higher of the sliding surface.
In the case where the area size of the primary-crystal silicon grains occupies a ratio of 8% or higher of the sliding surface, the surface pressure applied to the alloy substrate is decreased. Therefore, the primary-crystal silicon grains do not easily fall, which may suppress the damage of the surface of the cylinder wall with more certainty. In addition, the piston is easily warmed.

[Item 8]

The internal combustion engine of any one of items 1 through 7, wherein where the sliding surface is divided into a plurality of grids each having a size of 0.1 mm × 0.1 mm and the ratio of the number of grids where no primary-crystal silicon grain is present with respect to the total number of the grids is referred to as a "blank ratio", the blank ratio is 55.5% or lower.
The "blank ratio" is an index indicating how the primary-crystal silicon grains are dispersed. A lower blank ratio indicates that the primary-crystal silicon grains are better dispersed. In the case where the blank ratio of the sliding surface is 55.5% or lower, the surface pressure applied to the alloy substrate is sufficiently decreased. Therefore, the primary-crystal silicon grains do not easily fall, which may suppress the damage of the surface of the cylinder wall with more certainty. In addition, the piston is easily warmed.

[Item 9]

The internal combustion engine of any one of items 1 through 8, wherein the plurality of primary-crystal silicon grains have a crushing ratio of 20% or lower at the sliding surface.
In the case where the crushing ratio of the primary-crystal silicon grains at the sliding surface of the cylinder wall is 20% or lower, a large number of the primary-crystal silicon grains that are not crushed (that may be referred to as "healthy") are exposed at the sliding surface. Therefore, the surface pressure applied to the alloy substrate is sufficiently decreased. For this reason, the primary-crystal silicon grains do not easily fall, which may suppress the damage of the surface of the cylinder wall with more certainty.

[Item 10]

A transportation vehicle, including the internal combustion engine of any one of items 1 through 9.
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

According to an embodiment of the present invention, in an internal combustion engine including a cylinder block formed of an aluminum alloy containing silicon, when the internal combustion engine is operated for the first time, damage of a surface of a cylinder wall is suppressed, and thus oil consumption made in a deteriorated manner is suppressed.

BRIEF DESCRIPTION OF DRAWINGS

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 is a cross-sectional view schematically showing a piston ring 42 of the piston 40.
FIG. 7 shows an example of image of the sliding surface 12a.
FIG. 8 is a view provided to illustrate the definition of a blank ratio of the sliding surface 12a.
FIG. 9 is a side view schematically showing an automatic two-wheeled vehicle including the engine 100.

DESCRIPTION OF EMBODIMENTS

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]

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.
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".
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.
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.
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.
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.
The crankshaft 50 is accommodated in the crankcase 30. The crankshaft 50 includes a crankpin 51 and a crank arm 52.
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.
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.
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.
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".
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.
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.
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.
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.
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.
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.
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).
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.
FIG. 5 shows a cross-sectional structure of the piston skirt 44. FIG. 5 is a cross-sectional view taken along line 5A-5A' in FIG. 4B . As shown in FIG. 5 , the resin layer rl is provided on a substrate b1 formed of an aluminum alloy. 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).
As described above, in the engine 100 according to this embodiment, the piston skirt 44 includes the resin layer rl formed on at least a part of the outer circumferential surface thereof. The resin layer rl is softer than the aluminum alloy substrate (matrix) 1 of the cylinder block 10. Therefore, even if the primary-crystal silicon grains 2 at the sliding surface 12a of the cylinder wall 12 fall, the primary-crystal silicon grains 2 that have fallen are pushed into the resin layer rl, and thus damage of the surface of the cylinder wall 12 is suppressed (namely, the cylinder wall 12 is protected). On the piston 40 side, the piston rings 42 protrude to an outermost position. Therefore, there is no problem even if a surface of the resin layer rl is damaged. Since the damage of the surface of the cylinder wall 12 is suppressed, the oil (lubricant oil) attached to the cylinder wall 12 is scraped off appropriately by the piston rings 42. Therefore, oil consumption made in a deteriorated manner is suppressed. "Wear" of the surface of the cylinder wall 12 occurs at the entirety of the sliding surface 12a along with the sliding motion of the piston 40, whereas the "damage" is a defect occurring locally in a much smaller range than the "wear".
The resin layer rl of the piston skirt 44 prevents heat transfer from the piston skirt 44 to the cylinder wall 12. Therefore, after the engine 100 is started, the piston 40 is warmed promptly, and an appropriate size of gap is made between the piston skirt 44 and the cylinder wall 12. This may quickly suppress noise generated immediately after the engine 100 is started, and may also suppress wearing and adhesion of the piston 40 and the cylinder wall 12.
After the resin layer rl of the piston skirt 44 disappears as a result of the engine 100 being operated for a certain period of time, a bare surface of the piston skirt 44 (portion covered with the resin layer rl) and the cylinder wall 12 contact each other. In the engine 100 according to this embodiment, the primary-crystal silicon grains 2 are present at the sliding surface 12a of the cylinder wall 12, and silicon has a heat conductivity lower than that of aluminum. Therefore, the heat is not easily transferred from the piston skirt 44 to the cylinder block 10. For this reason, even after the resin layer rl disappears, the piston 40 is warmed promptly, which may suppress noise generated immediately after the engine 100 is started, and may also suppress wearing and adhesion of the piston 40 and the cylinder wall 12.
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 above-described effect, 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 50% 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).
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, wearing of the resin layer rl 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.
The resin layer rl has a thickness t (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 t of the resin layer rl is preferably 10 µm or greater. From the point of view of ease of production, the thickness t of the resin layer rl is preferably 50 µm or less.
FIG. 6 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. 6 , 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, in the case where each of the piston rings 42 includes the DLC layer 42D on the outer circumferential surface thereof, the heat of the piston 40 is prevented from being transferred easily to the cylinder block 10. This makes it easier to warm the piston 40 after the engine 100 is started.
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 enhancing the effect that the heat of the piston 40 is prevented from being transferred easily to the cylinder block 10, 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.
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.
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 damage of the surface of the cylinder wall 12 may be suppressed with more certainty.
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 surface of the cylinder wall 12 is damaged. 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 surface of the cylinder wall 12 is damaged.
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, 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 damage of the surface of the cylinder 12 by the primary-crystal silicon grains 2 that have fallen is suppressed.
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.
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.
The primary-crystal silicon grains 2 are crushed at the sliding surface 12a at a crushing ratio of, preferably, 20% or lower. The crushing ratio of the primary-crystal silicon grains 2 is a ratio, represented by percentage, of the area size of the crushed part of the primary-crystal silicon grains 2 with respect to the area size of the primary-crystal silicon grains 2 at the sliding surface 12a.
In the case where the crushing ratio of the primary-crystal silicon grains 2 at the sliding surface 12a of the cylinder wall 12 is 20% or lower, a large number of the primary-crystal silicon grains 2 that are not crushed (that may be referred to as "healthy") are exposed at the sliding surface 12a. Therefore, the surface pressure applied to the alloy substrate 1 is sufficiently decreased. For this reason, the primary-crystal silicon grains 2 does not easily fall, which may suppress the damage of the surface of the cylinder wall 12 with more certainty.
The crushing ratio of the primary-crystal silicon grains 2 may be measured as follows, for example.
First, an image of the sliding surface 12a is captured by use of a bore scope. FIG. 7 shows an example of the image of the sliding surface 12a. As shown in FIG. 7 , crushed parts 2a of the primary-crystal silicon grains 2 and non-crushed parts 2b of the primary-crystal silicon grains 2 are present at the sliding surface 12a. Next, an area size S1 of the crushed parts 2a of the primary-crystal silicon grains 2 is found by binarization using image analysis software. The crushed parts 2a have a black external appearance, and thus may be distinguished by binarization from the non-crushed parts 2b and the alloy substrate 1. Next, an area size S2 of the primary-crystal silicon grains 2 (including both of the crushed parts 2a and the non-crushed parts 2b) is found by binarization using the image analysis software. Then, the crushing ratio of the primary-crystal silicon grains 2 is calculated based on the following expression from the found area sizes S1 and S2. $\begin{array}{l} Crushing ratio [%] of the ptimary-crystal silicon \\ grains \\ 2 = (S 1 / S 2) \times 100 \end{array}$
It is preferred that the primary-crystal silicon grains 2 have an area size occupying a ratio of 8% or higher of the sliding surface 12a. In the case where the area size of the primary-crystal silicon grains 2 occupies a ratio of 8% or higher of the sliding surface 12a, the surface pressure applied to the alloy substrate 1 is decreased. Therefore, the primary-crystal silicon grains 2 do not easily fall, which may suppress the damage of the surface of the cylinder wall 12 with more certainty. In addition, the piston 40 is easily warmed.
The ratio of the area size occupied by the primary-crystal silicon grains 2 with respect to the area size of the sliding surface 12a may be measured as follows, for example. First, an image of the sliding surface 12a is captured by use of the bore scope. Next, the area size S2 of the primary-crystal silicon grains 2 is found by binarization using the image analysis software. Then, the ratio of the area size occupied by the primary-crystal silicon grains 2 may be calculated based on the following expression from the found area size S2 and an area size S3 of the entire measurement field of view. $\begin{array}{l} Ratio [%] of the area size occupied by the primary- \\ crystal silicon grains with respect to the area size of the \\ sliding surface = (S 2 / S 3) \times 100 \end{array}$
The sliding surface 12a may also be evaluated by a "blank ratio". FIG. 8 is a view provided to illustrate the definition of the "blank ratio". As shown in FIG. 8 , the sliding surface 12a is divided into a plurality of grids Sq each having a size of 0.1 mm × 0.1 mm. These grids Sq naturally include grids Sq1, where the primary-crystal silicon grains 2 are present, and grids Sq2, where the primary-crystal silicon grains 2 are not present. The "blank ratio" is the ratio (percentage) of the number of the grids Sq2 with no primary-crystal silicon grains 2 with respect to the total number of the grids Sq.
The "blank ratio" may be considered as an index indicating how the primary-crystal silicon grains 2 are dispersed. A lower blank ratio indicates that the primary-crystal silicon grains 2 are better dispersed. In the case where the blank ratio of the sliding surface 12a is 55.5% or lower, the surface pressure applied to the alloy substrate 1 is sufficiently decreased. Therefore, the primary-crystal silicon grains 2 do not easily fall, which may suppress the damage of the surface of the cylinder wall 12 with more certainty. In addition, the piston 40 is easily warmed.

[Transportation vehicle]

The engine 100 according to an embodiment of the present invention is preferably usable for various types of transportation vehicles. FIG. 9 shows an example of automatic two-wheeled vehicle including the engine 100 according to an embodiment of the present invention.
In an automatic two-wheeled vehicle 300 shown in FIG. 9 , 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.
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.
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.
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.
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.
The automatic two-wheeled vehicle 300 includes the engine 100 according to an embodiment of the present invention, and therefore, may suppress oil consumption made in a deteriorated manner, and may also suppress noise generated immediately after the engine 100 is started.
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.
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 including the cylinder wall 12 including the sliding surface 12a, along which the piston 40 is slidable. The cylinder block 10 is formed of an aluminum alloy containing silicon, and includes the plurality of primary-crystal silicon grains 2 at the sliding surface 12a. The piston skirt 44 includes the resin layer rl formed on at least a part of the outer circumferential surface thereof.
In the internal combustion engine 100 according to an embodiment of the present invention, the piston skirt 44 includes the resin layer rl formed on at least a part of the outer circumferential surface thereof. The resin layer rl is softer than the aluminum alloy substrate (matrix) 1 of the cylinder block 10. Therefore, even if the primary-crystal silicon grains 2 at the sliding surface 12a of the cylinder wall 12 fall, the primary-crystal silicon grains 2 that have fallen are pushed into the resin layer rl, and thus the damage of the surface of the cylinder wall 12 is suppressed (namely, the cylinder wall 12 is protected) . On the piston 40 side, the piston rings 42 protrude to the outermost position. Therefore, there is no problem even if the surface of the resin layer rl is damaged. Since the damage of the surface of the cylinder wall 12 is suppressed, the oil (lubricant oil) attached to the cylinder wall 12 is scraped off appropriately by the piston rings 42. Therefore, oil consumption made in a deteriorated manner may be suppressed.
The resin layer rl of the piston skirt 44 prevents heat transfer from the piston skirt 44 to the cylinder wall 12. Therefore, after the engine 100 is started, the piston 40 is warmed promptly, and an appropriate size of gap is made between the piston skirt 44 and the cylinder wall 12. This may quickly suppress noise generated immediately after the engine 100 is started, and may also suppress wearing and adhesion of the piston 40 and the cylinder wall 12.
After the resin layer rl of the piston skirt 44 disappears as a result of the engine 100 being operated for a certain period of time, a bare surface of the piston skirt 44 (portion covered with the resin layer rl) and the cylinder wall 12 contact each other. In the engine 100 according to this embodiment, the primary-crystal silicon grains 2 are present at the sliding surface 12a of the cylinder wall 12, and silicon has a heat conductivity lower than that of aluminum. Therefore, the heat is not easily transferred from the piston skirt 44 to the cylinder block 10. For this reason, even after the resin layer rl disappears, the piston 40 is warmed promptly, which may suppress noise generated immediately after the engine 100 is started, and may also suppress wearing and adhesion of the piston 40 and the cylinder wall 12.
In an embodiment, the piston 40 includes the plurality of piston rings 42 attached to the outer circumferential portion of the piston head 43. Each of the plurality of piston rings 42 includes the diamond-like carbon layer 42D on the outer circumferential surface thereof.
In the case where each of the piston rings 42 includes the DLC layer 42D on the outer circumferential surface thereof, the heat of the piston 40 is prevented from being transferred easily to the cylinder block 10. This makes it easier to warm the piston 40 after the engine 100 is started.
In an embodiment, the resin layer rl includes a solid lubricant agent and hard particles.
In the case where the resin layer rl includes hard particles, wearing of the resin layer rl may be delayed.
In an embodiment, the resin layer rl has a thickness t of 10 µm or greater and 50 µm or less.
From the point of view of keeping the resin layer rl for a long period of time, the thickness t of the resin layer rl is preferably 10 µm or greater. From the point of view of ease of production, the thickness t of the resin layer rl is preferably 50 µm or less.
In an embodiment, the cylinder block 10 is formed of an aluminum alloy containing silicon at a content of 15% by mass or higher and 25% by mass or lower.
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.
In an embodiment, the plurality of primary-crystal silicon grains 2 have an average grain diameter of 8 µm or longer and 50 µm or shorter.
In the case where the primary-crystal silicon grains 2 have an average grain diameter in the range of 8 µm or longer and 50 µm or shorter, the damage of the surface of the cylinder wall 12 may be suppressed with more certainty.
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 surface of the cylinder wall 12 is damaged.
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 surface of the cylinder wall 12 is damaged.
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, 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 damage of the surface of the cylinder 12 by the primary-crystal silicon grains 2 that have fallen is suppressed.
In an embodiment, the plurality of primary-crystal silicon grains 2 have an area size occupying a ratio of 8% or higher of the sliding surface 12a.
In the case where the area size of the primary-crystal silicon grains 2 occupies a ratio of 8% or higher of the sliding surface 12a, the surface pressure applied to the alloy substrate 1 is decreased. Therefore, the primary-crystal silicon grains 2 do not easily fall, which may suppress the damage of the surface of the cylinder wall 12 with more certainty. In addition, the piston 40 is easily warmed.
In an embodiment, where the sliding surface 12a is divided into a plurality of grids each having a size of 0.1 mm × 0.1 mm and the ratio of the number of grids where no primary-crystal silicon grain is present with respect to the total number of the grids is referred to as a "blank ratio", the blank ratio is 55.5% or lower.
The "blank ratio" is an index indicating how the primary-crystal silicon grains 2 are dispersed. A lower blank ratio indicates that the primary-crystal silicon grains 2 are better dispersed. In the case where the blank ratio of the sliding surface 12a is 55.5% or lower, the surface pressure applied to the alloy substrate 1 is sufficiently decreased. Therefore, the primary-crystal silicon grains 2 do not easily fall, which may suppress the damage of the surface of the cylinder wall 12 with more certainty. In addition, the piston 40 is easily warmed.
In an embodiment, the plurality of primary-crystal silicon grains 2 have a crushing ratio of 20% or lower at the sliding surface 12a.
In the case where the crushing ratio of the primary-crystal silicon grains 2 at the sliding surface 12a of the cylinder wall 12 is 20% or lower, a large number of the primary-crystal silicon grains 2 that are not crushed (that may be referred to as "healthy") are exposed at the sliding surface 12a. Therefore, the surface pressure applied to the alloy substrate 1 is sufficiently decreased. For this reason, the primary-crystal silicon grains 2 do not easily fall, which may suppress the damage of the surface of the cylinder wall 12 with more certainty.
A transportation vehicle according to an embodiment of the present invention includes the internal combustion engine 100 having any of the above-described structures.
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

According to an embodiment of the present invention, in an internal combustion engine including a cylinder block formed of an aluminum alloy containing silicon, when the internal combustion engine is operated for the first time, damage of a surface of a cylinder wall may be suppressed and thus oil consumption made in a deteriorated manner may be suppressed. 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

1: matrix (alloy substrate); 2: primary-crystal silicon grain; 2a: crushed part of the primary-crystal silicon grain; 2b: non-crushed part of the 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; Sq: grid; Sq1: grid where the primary-crystal silicon grains are present; Sq2: grid where the primary-crystal silicon grains are not present; b1: substrate; rl: resin layer

Claims

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 including a cylinder wall including a sliding surface, along which the piston is slidable;

wherein the cylinder block is formed of an aluminum alloy containing silicon, and includes a plurality of primary-crystal silicon grains at the sliding surface; and

wherein the piston skirt includes a resin layer formed on at least a part of an outer circumferential surface thereof.
The internal combustion engine of claim 1,
wherein the piston includes a plurality of piston rings attached to the outer circumferential portion of the piston head, and

wherein each of the plurality of piston rings includes a diamond-like carbon layer on an outer circumferential surface thereof.
The internal combustion engine of claim 1 or 2, wherein the resin layer includes a solid lubricant agent and hard particles.
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.
The internal combustion engine of any one of claims 1 through 4, wherein the cylinder block is formed of an aluminum alloy containing silicon at a content of 15% by mass or higher and 25% by mass or lower.
The internal combustion engine of any one of claims 1 through 5, wherein the plurality of primary-crystal silicon grains have an average grain diameter of 8 µm or longer and 50 µm or shorter.
The internal combustion engine of any one of claims 1 through 6, wherein the plurality of primary-crystal silicon grains have an area size occupying a ratio of 8% or higher of the sliding surface.
The internal combustion engine of any one of claims 1 through 7, wherein where the sliding surface is divided into a plurality of grids each having a size of 0.1 mm × 0.1 mm and the ratio of the number of grids where no primary-crystal silicon grain is present with respect to the total number of the grids is referred to as a "blank ratio", the blank ratio is 55.5% or lower.
The internal combustion engine of any one of claims 1 through 8, wherein the plurality of primary-crystal silicon grains have a crushing ratio of 20% or lower at the sliding surface.
A transportation vehicle, comprising the internal combustion engine of any one of claims 1 through 9.