WO2012017317A1

WO2012017317A1 - Piston of internal combustion engine and manufacturing method of the piston

Info

Publication number: WO2012017317A1
Application number: PCT/IB2011/002052
Authority: WO
Inventors: Takashi Koyama; Noriyuki Tokoro; Katsuhiko Aoyama; Motoichi Murakami; Hisashi Ohki; Hirofumi Arabiki; Ichiro Tsukada
Original assignee: Toyota Jidosha Kabushiki Kaisha; Art Metal Mfg, Co., Ltd.
Priority date: 2010-08-02
Filing date: 2011-08-02
Publication date: 2012-02-09
Also published as: JP2012031814A; WO2012017317A9; CN103052787A

Abstract

The present invention relates to a piston of an internal combustion engine, including a piston body (11), a pair of skirt sections (12A, 12B), a pair of side wall sections (13A, 13B), and pin hole sections (14A, 14B), and a cavity is formed by the bottom wall surface of the piston body, inner peripheral wall surfaces of the skirt sections, and inner wall surfaces of the side wall sections. In the present invention, a bulge section (20) that extends in a direction form a region of an outer wall surface of a side wall section adjacent to the piston body and a skirt section (AR1) toward a region of the outer wall surface of the side wall section adjacent to a lateral part of a pin hole section (AR3) is provided in the outer wall surface of the side wall section.

Description

PISTON OF INTERNAL COMBUSTION ENGINE AND MANUFACTURING

METHOD OF THE PISTON

BACKGROUND OF THE INVENTION

1 . Field of the Invention

[0001] The present invention relates to a piston of an internal combustion engine and a manufacturi ng method of the piston.

2. Description of Related Art

[0002] Japanese Utility Model Application Publication No. 2-132834 (JP-U-2- 132S34) discloses the piston of the internal combustion engine. The piston includes a cylindrical piston body, a pair of skirt sections extending downward from the piston body, a pair of side wall sections connecting the skirt sections each other, and a pair of pin hole sections provided in each of the side wall sections. The side wall section of the piston has tlie thickness becoming thinner from the lower part toward the upper part. Therefore, the piston has lower rigidity in the upper part of the side wall section and higher rigidity in the lower part of the side wall section. The rigidity of the side wall section affects the rigidity of the skirt section, such that the rigidity in a part of the skirt section close to a part of the side wall section with low rigidity is low, and the rigidity in a part of the skirt section close to a part of the side wall section with high rigidity is high. Accordingly, the piston disclosed in JP-U-2- 132834 has low rigidity in the upper part of the skirt section and high rigidity in the lower part of the skirt section as a result. Therefore, the following effects can be achieved.

[0003] That is, when the piston is disposed within a cylinder bore of the internal combustion engine, the internal combustion engine is operated, and fuel is burnt within a combustion chamber in an expansion stroke, a top wall surface of the piston body is subjected to high combustion pressure. In general, there are some clearances between an outer peripheral wall surface of the piston body and an inner peripheral wall surface of the cylinder bore and between an outer peripheral wall surface of skirt section and an inner peripheral wall surface of the cylinder bore. Therefore in this case, when the top wall surface of the piston body is subjected to high combustion pressure, the piston rotates about the piston pin (that is, the piston pin inserted into the pin hole formed .with the pin hole section) as the central axis such that a piston central axis inclines with respect to a cylinder bore central axis. When the piston rotates as described above, the upper part of the skirt section on one side and the lower part of the skirt section on the other side strongly collide with the inner peripheral wall surface of the cylinder bore.

[0004] On the other hand, the pressure in the combustion chamber decreases in the exhaust stroke following the expansion stroke, and therefore the pressure applied on the top wall surface of the piston body decreases. At this time, the piston rotates about the piston pin such that the piston central axis corresponds to the cylinder bore central axis. When the piston rotates as described above, the upper part of the skirl section on one side and the lower part of the skirt section on the other side strongly colliding with the inner peripheral wall surface of the cylinder bore move away from the inner peripheral wall surface of the cylinder bore.

[0005] That is, during engine operation (namely, during operation of the internal combustion engine), the upper part of the skirt section on one side and the lower part of the skirt section on the other side strongly collide with or move away from the inner peripheral wall surface of the cylinder bore.

[0006] If the rigidity in the upper part of the skirt section is high, the part is hardly deformed. Thus, when the upper part of the skirt section strongly collides with the inner peripheral wall surface of the cylinder bore, so-called slap noise or oil film shortage occurs between the upper part of the skirt section and the inner peripheral wall surface of the cylinder bore. In order to reduce the occurrence of the slap noise or oil film shortage, the rigidity of the upper part of the skirt section is preferably low such that the upper part of the skirt section is easily deformed when the upper part collides with the inner peripheral wall surface of the cylinder bore. As described above, the piston disclosed in JP-U-2- 132834 has low rigidity in the upper part of the skirt section and therefore can achieve the effect that the occurrence of the slap noise or oil film shortage can be reduced. [0007] If the rigidity in the lower part of the skirt section is low, the part is easily deformed. Thus, when the lower part of the skirt section strongly collides with the inner peripheral wall surface of the cylinder bore, the lower part of the skirt section is greatly deformed, and therefore the inclination of the piston central axis with respect to the cylinder bore central axis increases. As a result, the upper part of the skirt section further strongly collides with the inner peripheral wall surface of the cylinder bore, and thus the slap noise or oil film shortage easily occurs. As a matter of course, when the lower part of the skirt section strongly collides with the inner peripheral wall surface of the cylinder bore, the lower part of the skirt section is greatly deformed, and therefore the bearing point of the deformation in the lower part of the skirt section easily gets fatigue degradation. In order to prevent sucli the fatigue degradation, the rigidity in the lower part of the skirt section is preferably high such that the lower part of the skirt section is hardly deformed when the lower part of the skirt section collides with the inner peripheral wall surface of the cylinder bore. As described above, the piston disclosed in JP-U-2- 132834 has high rigidity in the lower part of the skirt section, and therefore the slap noise or oil film shortage hardly occurs, and the bearing point of the deformation in the lower part of the skirt section hardly gets fatigue degradation when the lower part of the skirt section collides with the inner peripheral wall surface of the cylinder bore.

[0008] Accordingly, various effects can be achieved by changing the thickness of the side wall section in each section, instead of maintaining constant thickness over the entire side wall section.

[0009] When the piston is disposed in the cylinder bore of the internal combustion engine, the piston pin is inserted into the pin hole of the piston, the piston pin is connected with one end of a connecting rod, and the other end of tlie connecting oci is connected with a crank shaft. When the internal combustion engine is operated and fuel is burnt within a combustion chamber in an expansion stroke, the top wall surface of the piston body is subjected to large load from the combustion pressure (hereinafter, the load is referred to as "combustion pressure load "). The combustion pressure load is transmitted from the piston to the connecting rod by means of the piston pin and then transmitted from the connecting rod to the crank shaft. Accordingly, when the top wall surface of the piston body is subjected to the combustion pressure load, the piston bears the combustion pressure load at the pin hole section. Therefore, due to the large combustion pressure load, an inner peripheral wall surface of an upper part of the pin hole section (hereinafter, referred to as a "pin hole upper part") is strongly pressed against an outer peripheral wall surface of an upper part of the piston pin (hereinafter, referred to as a "piston pin upper part"). Because the contact surface between the inner peripheral wall surface of the pin hole upper part and the outer peripheral wall surface of the piston pin upper part is small, contact pressure between the wall surfaces becomes high. As a result, both the pin hole upper part and the piston pin deform such that curvature radii of the inner peripheral wall surface of the pin hole upper part and the outer peripheral wall surface of the piston pin upper part become greater than original radii of the pin hole and the piston pin. Hereby, a bend occurs in the pin hole upper part, and tensile stress thus occurs in a part at the side of the inner peripheral wall surface of the pin hole upper part. On the other hand, the pressure in the combustion chamber decreases in the exhaust stroke and the intake stroke following the expansion stroke, and therefore the tensile stress applied to the part at the side of the inner peripheral wall surface of the pin hole upper part disappears. During engine operation, the expansion stroke, the exhaust stroke, and the intake stroke are repeated, and the pin hole upper part thus gets fatigue degradation.

[0010] Reduction of the piston weight or reduction of the friction related to the piston is required for the field of the piston. The diameter of the pin hole may be decreased in order to achieve the requirement. However, if the diameter of the pin hole is decreased, the curvature radii of the inner peripheral wall surface of the pin hole upper part and the outer peripheral wall surface of the piston pin upper part decrease.

Accordingly, because the wall surfaces are strongly pressed against each other at smaller contact surface, the contact pressure between the wall surfaces becomes high. As a result, tensile stress occurring in the pin hole upper part becomes higher, and the fatigue degradation of the pin hole upper part is thus promoted. [0011] When the piston having a cavity inside is manufactured with a mold, the cavity of the piston is generally formed by using a core. Specifically, a piston material (that is, a material from which the piston is made) is disposed around the core and solidified, and the cavity is thus formed. After the formation of the cavity, the core is removed from the cavity.

[0012] As described above, the side wall section of the piston disclosed in JP-U- 2-132834 has the thickness of being thinner from the lower part toward the upper part for a specific purpose (that is, for the purpose of decreasing the rigidity in the upper part of the side wall section and increasing the rigidity in the lower part of the side wall section). Here, the skirt section may have the thickness that is reduced from the lower part toward the upper part for some specific purpose, for example. When the piston with such the skirt section has the cavity inside and the piston is manufactured with a mold, if the cavity is formed by using a general core as described above, the lower part of the skirt section makes it difficult for the core to be removed from the cavity after the formation of the cavity, and therefore the core may not be removed from the cavity.

SUMMARY OF THE INVENTION

[0013] The present invention provides a piston that can prevent the fatigue degradation of the pin hole upper par t and that can reduce the diameter of the pin hole.

[0014] The present invention provides a manufactur ing method of the piston that has the skirt section whose thickness is thicker in a lower part and thinner in an upper part.

[0015] A first aspect of the present invention relates to a piston of an internal combustion engine, including: a cylindrical piston body; a pair of generally partially annular skirt sections that extends from a bottom wall surface of the piston body to a lower side in parallel with a central axis of the piston body; and a pair of flat shaped side wall sections that extends from the bottom wall surface of the piston body to the lower side in parallel with the central axis of the piston body and that connects the skirt sections each other, and the side wall sections have annular pin hole sections that include central axes perpendicular to extending planes of the side wall sections, and a cavity is formed by the bottom wall surface of the piston body, inner peripheral wall surfaces of the skirt sections, and inner wall surfaces of the side wall sections. In the piston of the internal combustion engine according to the present invention, a bulge section that extends in a direction form a region of an outer wall surface of a side wall section adjacent to the piston body and a skirt section toward a region of the outer wall surface of the side wall section adjacent to a lateral part of a pin hole section is provided in the. side wall section.

[0016] According to the aspect of the present invention, the piston that can prevent the fatigue degradation of the pin hole upper part and that can reduce the diameter of the pin hole is provided. That is, when the piston is disposed within a cylinder bore, the internal combustion engine is operated, and fuel is burnt within a combustion chamber in an expansion stroke, a top wall surface of the piston body is subjected to large combustion pressure load (that is, large load from the combustion pressure). Due to the large combustion pressure load, the inner peripheral wall surface of the pin hole upper part (that is, the upper part of the pin hole) is pressed against the outer peripheral wall surface of the piston pin upper part (that is, the upper part of the piston pin inserted into the pin hole). As a result, both the pin hole upper part and the piston pin deform such that curvature radii of the inner peripheral wall surface of the pin hole upper part and the outer peripheral wall surface of the piston pin upper part become greater than original radii of the pin hole and the piston pin. Hereby, tensile stress occurs in a part at the side of the inner peripheral wall surface of the pin hole upper part. On the other hand, the pressure in the combustion chamber decreases in the exhaust stroke and the intake stroke following the expansion stroke, and therefore the tensile stress applied to the part at the side of the inner peripheral wall surface of the pin hole upper part disappears. During engine operation, the expansion stroke, the exhaust stroke, and the intake stroke are repeated, and the pin hole upper part thus gets fatigue degradation. In the present invention, the bulge sections are provided in the side wall sections. The bulge section extends in the side wall section in a direction form a region of the outer wall surface of the side wall section adjacent to the piston body and the skirt section toward a region of the outer wail surface of the side wall section adjacent to a lateral part of the pin hole section. Therefore, the bulge sections transmit the force of deformation of the piston body by the combustion pressure load to the pin hole lateral part. By the force transmitted to the pin hole lateral part, the pin hole section is subjected to the force in the direction for preventing the bend of the pin hole upper part caused by the combustion pressure load in the expansion stroke. Accordingly, the bend of the pin hole upper part is prevented. As a result, occurrence of tensile stress in the pin hole upper part is prevented. Therefore, even if the diameter of the pin hole is formed small, the fatigue degradation of the pin hole upper part is prevented. According to the present invention, the piston that can prevent the fatigue degradation of the pin hole upper part and that can reduce the diameter of the pin hole is provided.

[0017] The groove that extends along the bulge section may be fonned in the wall surface of the side wall section opposite to a projecting side of the bulge section.

[0018] According to the present invention, since the groove is fonned in the wall surface of the side wall section, the weight of the piston is reduced.

[0019] When a plane that includes a pin hole central axis and a piston central axis is referred to as a pin hole vertical plane, a part of the pin hole section in vicinity of the pin hole vertical plane as well as a part at a side of the piston body with respect to the pin hole central axis is referred to as a pin hole upper part, a plane that includes the pin hole central axis and is perpendicular to the piston central axis is referred to as a pin hole lateral plane, a part of the pin hole section in the vicinity of the pin hole lateral plane is referred to as a pin hole lateral part, and a part of the pin hole section that is located in a generally middle part between the pin hole upper part and the pinhole lateral part is referred to as a pin hole oblique upper part, the bulge section may extend generally straight from a region of an outer wall surface of the side wall section adjacent to the piston body and the skirt section toward a region of the outer wall surface of the side wall section adjacent to the pin hole oblique upper part.

[0020] According to the aspect of the present invention, even if the diameter of the pin hole is formed small, the fatigue degradation of the pin hole upper part is securely prevented. That is, when the lop wall surface of the piston body is subjected to large combustion pressure load, as described with reference to the first aspect of the invention, tensile stress occurs in a part of the pin hole upper part through the large combustion pressure load. In this regard, the inventors of the present application found through the studies that when a force is applied from the region of the outer wall surface of the side wall section adjacent to the piston body and the skirt section to the pin hole oblique upper part, the_. bend of the pin hole upper part is prevented well, and accordingly the occurrence of the tensile stress in the pin hole upper part is prevented well. In the present invention, the bulge section extends generally straight from a region of an outer wall surface of the side wall section adjacent to the piston body and the skirt section toward a region of the outer wall surface of the side wall section adjacent to the pin hole oblique upper part. Therefore, the force of deformation of the piston body by the combustion pressure load is applied form a region of the outer wall surface of the side wall section adjacent to the piston body and the skirt section to the pin hole oblique upper part. Accordingly, even if the diameter of the pin hole is formed small, the fatigue degradation of the pin hole upper part is securely prevented.

[0021] A second aspect of the present invention relates to a method of manufacturing a piston with a mold, the piston including: a cylindrical piston body; a pair of generally partially annular skirt sections that extends from a bottom wall surface of the piston body to a lower side in parallel with a central axis of the piston body; and a pair of flat shaped side wall sections that extends from the bottom wall surface of the piston body to the lower side in parallel with the central axis of the piston body and that connects the skirt sections each other, a cavity is formed by the bottom wall surface of the piston body, inner peripheral wall surfaces of the skirt sections, and inner wall surfaces of the side wall sections, and thickness of a lower part of the skiit section is thicker than thickness of an upper part of the skirt section. In the method according to the present invention, the cavity is formed in the piston by using: a first core that defines an inner wall surface of a first side wall section, a portion of the bottom wall surface of the piston body in the vicinity of the inner wall surface, and a portion of the inner peripheral wall surfaces of both skill sections in the vicinity of the inner wall surface of the first side wall section; a second core that defines an inner wall surface of a second side wall section, a portion of the bottom wall surface of the piston body in the vicinity of the inner wall surface, and a portion of the inner peripheral wall surfaces of both skirt sections in the vicinity of the inner wall surface of the second side wall section; a third core that defines a portion of the bottom wall surface of the piston body that is not defined by the first core and the second core, and a portion of the inner peripheral wall surfaces of the first skirt section that is not defined by the first core and the second core, the third core being arranged between the first core and the second core; a fourth core that defines a portion of the bottom wall surface of the piston body that is not defined by the first core and the second core, and a portion of the inner peripheral wall surfaces of the second skirt section that is not defined by the first core and the second core, the fourth core being arranged between the first core and the second core; and a fifth core that defines a portion of the bottom wall surface of the piston body that is not defined by the first core, the second core, the third core, and the fourth core, the fifth core being an anged among the first core, the second core, the third core, and the fourth core.

[0022] According to the second aspect of the invention, the wall surface constituted by the outer wall surfaces of the first core through the fifth core defines the bottom wall surface of the piston body, inner peripheral wall surfaces of the skirt sections, and inner wall surfaces of the side wall sections, and hereby a cavity is formed. Each core occupies only a part of the region in the cavity of the piston, and therefore all cores can be removed from the cavity of the piston.

[0023] After the cavity is formed in the piston by using the first core, the second core, the third core, the fourth core, and the fifth core, the fifth core may be removed first, then the third core and the fourth core may be removed, and subsequently the first core and the second core may be removed.

[0024] According to the aspect of the present invention, all cores can easily be removed from the cavity of the piston. [0025] A third aspect of the present invention relates to a piston of an internal combustion engine, including: a piston body; a first skirt section that is provided in a lower part of the piston body; a second skirt section that is opposed to the first skirt section; a first side wall section that is provided in a lower part of the piston body and between the first skirt section and the second skirt section; and a second side wall section that is provided in a lower part of the piston body and between the first skirt section and the second skirt section and that is opposed to the first side wall section. The first and the second side wail sections include: a pin hole section; a first bulge section that is provided between a comer of the piston body and the first skirt section and in the pin hole section; and a second bulge section that is provided between a comer of the piston body and the second skirt section and in the pin hole section.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 A is a side view of a piston according to a first embodiment of the present invention, and FIG. 1 B is a bottom plan view of the piston;

FIG. 2A is a side view of a piston according to the first embodiment, and FIG. 2B is a bottom plan view of the piston, as shown in FIGs. 1 A and 1 B;

FIG. 3 is a cross-sectional view showing a bulge section of the piston according to the first embodiment, taken along a line X-X in FIG. 2A;

FIG. 4 is a side view similar to FIG. 1 A that shows a piston having a different bulge section from the bulge section according to the first embodiment;

FIG. 5 shows a similar view to FIG. 1 A that indicates, plural planes along which the cross sections of the side wall section and the skirt section of the piston according to the first embodiment are taken;

FIG. 6 shows a similar view to FIG. 5 that illustrates the plane indicated in FIG. 5; FIGs. 7A through 7G respectively show the cross sections of the side wall section and the skirt section of the piston according to the first embodiment taken along the planes A through G in FIG. 5, and FIG. 7H shows a bottom end face of the side wal l section and the skirt section of the piston according to the first embodiment;

FIG. 8 shows a similar view to FIG. 1 A that indicates plural planes along which the cross sections of the side wall section and the skirt section of the piston according to the first embodiment are taken;

FIG. 9 shows a simitar view to FIG. 8 that illustrates the plane indicated in FIG. 8;

FIG. 10 shows a similar view to FIG. 1 A that indicates plural planes along which the cross sections of the side wall section and the skirt section of the piston according to the first embodiment are taken;

FIG. 1 1 shows a similar view, to FIG. 10 that illustrates the plane indicated in FIG.

10;

FIG. 12 shows a similar view to FIG. 1 A that indicates plural planes along which the cross sections of the side wall section and the skirt section of the piston according to the first embodiment are taken;

FIG. 13 shows a similar view to FIG. 12 that illustrates the plane indicated in FIG.

12;

FIG. 14A is a vertical cross-sectional view in the vicinity of the skirt section of the piston according to the first embodiment; FIG. 14B is a vertical cross-sectional view in the vicinity of the skirt section of a different piston from the embodiment of the present invention; FIG. 14C is a vertical cross-sectional view in the vicinity of the skirt section of the piston of a modification according to the first embodiment;

FIG. 15A is a side view similar to FIG. 1 A that shows a piston according to a second embodiment, and FIG. 1 5B is a vertical cross-sectional view of the side wall section of the piston taken along a line Y-Y in FIG. 15A;

FIG. 16A shows a similar view to FIG. 15B that illustrates a storing state of oil in a recess of the piston when the piston according to the second embodiment is disposed in the cylinder bore, and FIG. 16B shows a similar view to FIG. 15B that illustrates a diffusing state of oil in the recess of the piston when the piston is disposed in the cylinder bore;

FIG. 1 7 is a bottom plan view similar to FIG. I B that shows a piston according to a third embodiment;

FIG. 18 is a bottom plan view similar to FIG. I B that shows a piston according to a fourth embodiment;

FIG. 19A is a cross-sectional view similar to FIG. 3 that shows a bulge section provided in a first side wall section of the piston according to a fifth embodiment, and FIG. 19B is a cross-sectional view similar to FIG. 3 that shows a bulge section provided in a second side wall section of the piston;

FIG. 20 shows the piston according to the embodiment of the present invention seen from the bottom end face, illustrating a core applied to form a cavity of the piston;

FIG. 21 is a side view similar to FIG. t A that shows a piston according to a sixth or seventh embodiment;

FIG. 22 A is a transverse cross-sectional view of the skirt section of the piston according to the sixth embodiment, taken along a line Zl -Zl in FIG. 21 , and FIG . 22B is a transverse cross-sectional view of the skirt section of the piston, taken along a line Z2- Z2 in FIG. 21 ; and

FIG. 23A is a transverse cross-sectional view of the skirt section of the piston according to the seventh embodiment, taken along a line Zl -Zl in FIG. 21 , and FIG. 23B is a transverse cross-sectional view of the skirt section of the piston, taken along a line Z2-Z2 in FIG. 21.

DETAILED DESCRIPTION OF EMBODIMENTS

[0027] FIG. 1 A is a side view of a piston according to a first embodiment of the present invention, and FIG. I B is a bottom plan view of the piston. As shown in FIGs. 1 A and I B, the piston 10 has a body section (hereinafter, referred to as a "piston body") 1 1 , a pair of skirt sections 12A and I2B, a pair of side wall sections 13 A and 13B, and a pair of pin hole sections 14A and 14B. [0028] Here, in the following descriptions, the term "upper" denotes an "upper" part in the drawing of FIG. I A (for example, the direction toward the side where the piston body 1 1 is located when the pin hole sections 14A and 14B are assumed to be references). The term "lower" denotes a "lower" pail in the drawing of FIG. 1 A (that is, the opposite direction of the "upper" direction). The term "lateral" denotes the direction perpendicular to the direction from the lower pail to the upper part. The term "outer" denotes the direction from the inside of the piston 10 to the outside of the piston. The term "inner" denotes the direction from the outside of the piston 10 to the inside of the piston (that is, tlie opposite direction of tlie "outer" direction). Furthermore, the term "bottom end face" denotes an end face on the bottom.

[0029] The piston body 1 1 is a cylindrical section that is centered around a piston central axis C I . The piston body 1 1 has a wall surface 1 1 1 that forms a circular plane centered around the piston central axis C l and faces upward (hereinafter, the wall surface is referred to as a "piston body top wall surface"), a wall surface 1 12 that forms a circular plane centered around tlie piston central axis C l and faces downward (hereinafter, the wall surface is referred to as a "piston body bottom wall surface"), and an outer peripheral wall surface 1 13 that forms a cylindrical surface centered around the piston central axis C l and faces outward (hereinafter, the wall surface is referred to as a "piston body outer peripheral wall surface").

|0030] The piston body outer peripheral wall surface 1 1 3 connects an outer peripheral end of the piston body top wall surface 1 1 1 to an outer peripheral end of the piston body bottom wall surface 1 12. Jn addition, the piston body outer peripheral wall surface ] 13 is formed with a number of annular grooves 1 14 that are centered around the piston central axis Cl . The grooves ] 14 accommodate annular oil rings (not shown) respectively. The piston body top wall surface 1 1 1 is formed with a cavity 1 15.

[0031 ] Each of the skirt sections 12A and 12B is a generally partially annular section that is centered around the piston central axis C l . Each of the skirt sections 12A and 12B has an outer peripheral wall surface 121 that forms a generally partially cylindrical surface centered around the piston central axis C l and faces outward (hereinafter, the wall surface is referred to as a "skirt outer peripheral wall surface"), and an inner peripheral wall surface 122 that forms a generally partially cylindrical surface centered around the piston central axis C I and faces inward (hereinafter, the wall surface is referred to as a "skirt inner peripheral wall surface"). The skirt sections 12A and 1 2B are symmetrically arranged in the opposite side to each other with respect to the piston central axis C I . In other words, (he skirt sections 12A and 12B extend from a partially annular outer peripheral region of the piston body bottom wall surface 1 12 disposed in the opposite side from each other with respect to the piston central axis C I to the lower part in parallel with the piston central axis C I . Each skirt outer peripheral wall surface 121 is generally flush with the piston body outer peripheral wall surface 1 1 3.

[0032] Each of the side wall sections 13 A and 13B is a flat shaped part Each of the side wall sections 13A and 1 3B has an outer wall surface 131 that forms a flat surface and faces outward (hereinafter, the wall surface is referred to as an "outer side wall surface"), and an inner wall surface 132 that forms a flat surface and faces inward (hereinafter, the wall surface is referred to as an "inner side wall surface"). The side wall sections 13A and 13B are symmetrically arranged in the opposite side to each other with respect to the piston central axis C I . In other words, the side wal l sections 13A and 13B extend from a rectangular region of the piston body bottom wall surface 1 12 disposed in the opposite side from each other with respect to the piston central axis CI to the lower part in parallel with the piston central axis CI . Lateral sides of each of the side wall sections 13A and 13B are connected to corresponding lateral sides of the skirt section 1 . Accordingly, each of the side wall sections 1 3A and 1 3B is disposed between two skirt sections 12A and 12B and connects the two skirt sections 12A and 12B with each other.

[0033] Each of the pin hole sections 14A and 1 B is an annular part. Therefore, each of the pin hole sections 14A and 14B is formed with a tlirough hole 141 (hereinafter, referred to as a "pin hole"). One common piston pin (not shown) for connecting the piston 10 to a connecting rod (not shown) is inserted into the pin hole 141 . Each of the pin hole sections 1 A and 14B is disposed such that its central axis C2 (that is, the central axis of the pin hole 1 1 , and hereinafter the central axis is referred to as a "pin hole central axis") is perpendicular to an extending plane of the side wail sections 13A and I 3B and passes through an approximate center part of the side wall sections 13A and 1 3B. Accordingly, one end of the pin hole sections 14A and 14B in the direction parallel to the pin hole central axis C2 is projecting outward from the outer side wall surface 13 1 , and the other end of .the pin hole sections 14A and 14B in the direction parallel to the pin hole centra! axis C2 is projecting inward from the inner side wall surface 132.

[0034] The pin hole sections 14Λ and 14B are arranged such that the central axis C2 of one pin hole 141 corresponds to the central axis C2 of the other pin hole 141. The pin hole sections 14 A and 14B are respectively disposed in the side wall sections 13 A and 1 3B, and the side wall sections 13 A and 13B are connected to the piston body bottom wall surface 1 12. Therefore, the side wall sections 13A and 13B are considered as connecting parts that connect the piston body 1 1 to the pin hole sections 14A and 14B each other.

[0035] The upper part of the pin hole sections 14A and 14B of FIG. 1 is referred to as a "pin hole upper part 143," and the lateral part of the pin hole sections 14A and 14B of FIG. 1 is referred to as a "pin hole lateral part 142".

[0036] A lib 30 is disposed in the outer side wall surface 131 between one of the pin hole sections 14A and 14B and the piston body 1 1 . The lib 30 extends in parallel to the piston central axis CI and connects the pin hole upper part 143 to the piston body bottom wall surface 1 12.

[0037] A cavity 101 (hereinafter, referred to as a "piston cavity") is formed inside the piston 10. The piston cavity 101 is generally defined by the piston body bottom wall surface 1 12, the skirt inner peripheral wall surface 122, and the inner side wall surface 132.

[0038j Oil passages (not shown) that passes oil for cooling the piston body 1 1 are formed inside the piston body 1 1.

[0039] As shown in FIG. I B, an oil introducing passage defining wall 103 is disposed in the inner wall surface 132 of the side wall section 13A (hereinafter, referred to as a "first side wall section") between the pin hole section 14A (hereinafter, referred to as a "first pin hole section") and the skirt section 12A (hereinafter, referred to as a "first skirt section"). This oil introducing passage defining wall 103 extends generally upward from the region close to the bottom end face of the first side wall section 13 A through the region close to the first pin hole section 14A to the piston body bottom wall surface 1 12. The oil introducing passage defining wall 103 defines an oil introducing passage 102 for introducing oil to the oil passage that is formed inside the piston body 1 1 . The oil introducing passage 102 is connected to the oil passage in the piston body bottom wall surface 1 12.

[0040] An oil discharging passage defining wall 105 is disposed in the inner wall surface 132 of the other side wall section 13B (hereinafter, referred to as a "second side wall section") between the other pin hole section 14B (hereinafter, referred to as a "second pin hole section") and the other skirt section 12B (hereinafter, referred to as a "second skirt section"). This oi l discharging passage defining wall 105 extends generally upward from the region close to the bottom end face of the second side wall section 13B through the region close to the second pin hole section 14B to the piston body bottom wall surface 1 12. The oil discharging passage defining wall 105 defines an oil discharging passage 104 for discharging oil in the oil passage that is formed inside the piston body 1 1. The oil discharging passage 104 is connected to the oil passage in the piston. body bottom wall surface 1 12.

[0041] Accordingly, these defining walls 103 and 105 are disposed in corresponding inner side wall surfaces 1 32 respectively in a symmetric manner with respect to the piston central axis Cl .

[0042] In the following descriptions, a lateral end of the first side wall section 13A close to the oil introducing passage defining wall 103 is referred to as a "first lateral end", and a lateral end of the first side wall section 13A far from the oil introducing passage defining wall 103 is referred to as a "second lateral end". A lateral end of the second side wall section 13B close to the oil discharging passage defining wall 105 is referred to as a "first lateral end", and a lateral end of the second side wall section 13 B far from the oil discharging passage defining wall 105 is referred to as a "second lateral end". Furthermore, in the following descriptions, a lateral end of the first skirt section 1 2A that is connected to the first lateral end of the first side wall section 13 A is referred to as a "first lateral end", and a lateral end of the first skirt section 12A that is connected to the second lateral end of the second side wall section 1 B is referred to as a ''second lateral end". A lateral end of the second skirt section 12B that is connected to the first lateral end of the second side wall section 13B is referred to as a "first lateral end", and a lateral end of the second skirt section 12B that is connected to the second lateral end of the first side wall section 13A is referred to as a "second lateral end".

[0043] The first side wall section 13 A is provided with two bulge sections 20, and the second side wall section 13B is provided with two bulge sections 20. The bulge sections 20 are part of the side wall sections 13A and 13B and portions projecting outward in comparison with the other parts of the side wall sections 13A and 13B. Accordingly, the outer side wall surface 131 in the region corresponding to the bulge section 20 is projecting outward in comparison with the outer side wall surface 13 1 in the region other than described above.

[0044] Next, the bulge sections 20 will be described in detail.

[ 0045] Upper comer regions AR 1 as shown in FIG. 2 A are regions in the outer side wall surfaces 13 1 in the vicinity of upper ends of connecting portions between the side wall sections 13A and 1 3B and the skirt sections 12A and 12B, respectively (that is, regions in the outer side wall surfaces 131 in the vicinity of connecting portions between the skirt sections 12A and 12B and the piston body 1 1 ). Lower corner regions AR2 as shown in FIG. 2A are regions in the outer side wall surfaces 131 in the vicinity of lower ends of connecting portions between the side wall sections 1 3 A and 13B and the skirt sections 12A and 12B, respectively. Pin hole lateral regions AR3 as shown in FIG. 2A are regions in the outer side wall surfaces 131 in the vicinity of the pin hole lateral parts 142.

[0046] Each of the outer side wail surfaces 131 has two upper corner regions AR1 , two lower corner regions AR2, and two pin hole lateral regions AR3. The bulge sections 20 are respectively provided in the side wall sections 13A and 1 3B so as to extend from the upper comer regions ARl to the pin hole lateral regions AR3 close to the upper corner regions ARl . More specifically, each bulge section 20 extends generally straight toward a generally middle part between an upper part of the pin hole section 14B with respect to the pin hole central axis in view of FIG. 2A and a lateral part of the pin hole section 14B with respect to the pin hole central axis in view of FIG. 2 A (hereinafter, the part is referred to as a "pin hole oblique upper part") from the upper comer region AR l to the pin hole lateral region AR3. In other words, when a plane that includes the pin hole central axis and the piston central axis is referred to as^'a pin hole vertical plane, a plane that includes the pin hole central axis and that is perpendicular to the piston central axis is referred to as a pin hole lateral plane, and a part of the pin hole section 14B in the vicinity of the pin hole lateral plane is referred to as a pin hole lateral part, the pin hole upper part 143 is a part of the pin hole section in the vicinity of the pin hole vertical plane and at the side of the piston body 1 1 with respect to the pin hole central axis, the pin hole oblique upper part is a pari of the pin hole section 14B that is located in the generally middle part between the pin hole upper part 143 and the pinhole lateral part, and the bulge section 20 extends generally straight from the upper corner region ARl to the pin hole lateral region AR3 adjacent to the pin hole oblique upper part. An outer wall surface of each bulge section 20 is projecting as shown in FIG. 3 so as to form a generally partially cylindrical surface where a line along the extending direction of the bulge section 20 is assumed to be a generatrix. The diickness of the bulge section 20 may be the same as that of the side wall sections 13A and 13B in the periphery. The thickness of the bulge section 20 may be uniform. It is preferable that the rigidity of the bulge section 20 is higher than its periphery; however, the weight increase of the piston may be prevented if the rigidity is increased by causing the side wall sections 13A and 13B to bulge without the thickness of the bulge section 20 being increased.

[0047] An inner wall surface of each bulge section 20 (that is, the inner side wall surface 132 corresponding to the bulge section 20) is recessed as shown in FIG. 3 so as to form a generally partially cylindrical surface where a line along tiie extending direction of the bulge section 20 is assumed to be a generatrix. In other words, the inner side wall surface 132 is recessed along the bulge section 20 in the region corresponding to the bulge section 20. Therefore, a groove 21 that extends from the upper corner region AR 1 to the pin hole lateral region AR3 close to the upper corner region AR l is formed in the inner side wall surface 132.

[0048] Disposing the bulge sections 20 in the side wall sections 13A and 13B can achieve the following effects.

[0049] That is, when the piston 10 is disposed in the cylinder bore of the internal combustion engine, the piston body top wall surface 1 1 1 , the inner peripheral wall surface of the cylinder bore (not shown), and the bottom wall surface of a cylinder head (not shown) form a combustion chamber (not shown). A piston pin (not shown) is inserted into the pin hole 141 . The piston 10 is connected to a connecting rod (not shown) by means of the piston pin.

[0050] When the internal combustion engine is operated and air-fuel mixture is burnt within tlie combustion chamber in the expansion stroke, the piston body top wall surface 1 1 1 is subjected to large combustion pressure load (that is, the load from the combustion pressure). Due to the large combustion pressure load, the inner peripheral wall surface of the pin hole upper part 143 is pressed against the outer peripheral wall surface of the piston pin upper part (that is, the upper part of the piston pin). As a result, bot h the pin hole upper part 143 and the piston pin deform such that curvature radii of the inner peripheral wall surface of the pin hole upper part 143 and the outer peripheral wall surface of the piston pin upper part become greater than original radii of the pin hole 141 and (he piston pin. Hereby, tensile stress occurs in a part at the side of the inner peripheral wall surface of the pin hole upper part 1 3. On the other hand, the pressure in the combustion chamber decreases in the exhaust stroke and the intake stroke following the expansion stroke, and therefore the tensile stress applied to the part at the side of the inner peripheral wall surface of the pin hole upper part 143 disappears.

[0051 ] During engine operation (that is, during the operation of the internal combustion engine), the expansion stroke, the exhaust stroke, and the intake stroke are repeated, and the pin hole upper part 143 thus easily degrades by fatigue (hereinafter, the degradation by the fatigue is referred to as "fatigue degradation").

[0052] The piston according to the first embodiment is provided with the bulge sections 20 in the side wall sections 13A and 1 3B. The bulge sections 20 extend generally straight in the side wall sections 13A and 13B toward the pin hole oblique upper part from the upper corner region AR 1 to the pin hole lateral region AR3.

Therefore, the bulge sections 20 transmit the force of deformation of the piston body 1 1 by the combustion pressure load to the pin hole lateral part 142. By the force transmitted to the pin hole lateral part 142, the pin hole section 14B is subjected to the force in the direction that prevents the bend of the pin hole upper part 143 by the combustion pressure load in the expansion stroke. Accordingly, the bend of the pin hole upper part 143 is prevented. As a result, occurrence of tensile stress in the pin hole upper part 143 is prevented. Therefore, even if the diameter of the pin hole is formed small, the fatigue degradation of the pin hole upper part 143 is prevented. Therefore, according to the first embodiment, the fatigue degradation of the pin hole upper part 143 can be prevented, and the diameter of the pin hole can be formed small.

[0053] Incidentally, the bulge section 20 according to the first embodiment is an example of the bulge section according to the present invention. That is, the bulge section according to the present invention includes any bulge sections extending in the direction from the upper corner region AR1 toward the pin hole lateral region AR3. In other words, the bulge section according to the present invention includes any bulge sections extending in the direction from the upper corner region AR1 toward the pin hole lateral region AR3 along the path in which a compression force applied to the side wall sections 1 3A and 13B by the displacement of the piston body outer peripheral section 1 16 resulting from the deformation of the pin body 1 1 is transmitted through the side wall sections I A and 1 B. That is to say, the bulge section according to the present invention includes any bulge sections extending in the direction from the upper corner region AR1 toward the pin hole lateral region AR3 so as to prevent the deformation of the piston body 1 1 when the piston body top wall surface 1 1 1 is subjected to the combustion pressure load.

[0054] Therefore, the bulge section according to the present invention also includes the bulge section 20 in the shape as shown in FIG. 4. That is, each bulge section 20 shown in FIG. 4 extends generally in the shape of an arc that the middle part is convex toward the lower corner region AR2 in the extending direction. In other words, each bulge section 20 extends downward from the upper corner region AR 1 to the middle part, gradually changes the extending direction to the direction toward the pin hole lateral part 142 in the middle part, and extends toward the pin hole lateral part 142 after crossing the middle part.

[0055J In the first embodiment, the bulge sections 20 extending from the upper comer regions AR1 to the pin hole lateral regions AR3 are provided to the side wall sections 13A and 1 3B. Alternatively, the bulge sections 20 extending in a part of the regions from the upper corner regions AR1 to the pin hole lateral regions AR3 may be provided to the side wall sections 13A and 1 3 B.

[0056] In the first embodiment, the bulge sections 20 extending continuously are provided to the side wall sections 1 3 A and 1 3B. Alternatively, the bulge sections 20 extended while being divided into multiple parts may be provided to the side wall sections 13A and 13B.

[0057} In the first embodiment, the bulge sections 20 projecting outward are provided to the side wall sections 13A and I 3B. Alternatively, the bulge sections 20 projecting inward may be provided to the side wall sections 13A and 13B. In this case, in the first embodiment, a groove 21 extending along the bulge section 20 is formed in the inner side wall surface 132. Alternatively, a groove extending along the bulge section is formed in the outer side wall surface 131 .

[0058] As described above, a groove 21 is formed along the bulge section 20 in the inner side wall surface 132 corresponding to the bulge section 20. When the groove 21 is formed as described above, the effect of weight reduction of the piston 10 can be achieved. Incidentally, when such the effect is not required, or when it is preferable that the groove 21 is not provided in the inner side wall surface 132, the groove 21 may not be formed in the inner side wall surface 132.

[0059] Next, a connecting configuration of the side wall sections 13A and 13B and the skirt sections 12A and 12B will be described.

[0060] In the following description, the side wall sections 13 A and 13B and the skirt sections 12A and 12B are referred to col lectively as a "piston lower wall". A cross section of the piston lower wail taken along a specific plane is referred to as a "piston lower wall cross section". A bottom end face of the piston lower wall is referred to as a "piston lower wall bottom end face". The connecting portion between the side wall section and the skirt section in the piston lower wall cross section or the piston lower wall bottom end face is referred to as a "piston lower wall connecting portion". A part of the side wall section in the vicinity of the piston lower wall connecting portion is referred to as a "side wall connecting portion". A part of the skirt section in the vicinity of the piston lower wall connecting portion is referred to as a "skirt connecting portion". The direction that the side wall connecting portion extends toward the piston lower wall connecting portion in the piston lower wall cross section or the piston lower wail bottom end face is referred to as a "side wall extending direction". The direction that the skirt connecting portion extends toward the piston lower wall connecting portion in the piston lower wall cross section or the piston lower wall bottom end face is referred to as a "skirt extending direction". An angle at the intersection of the side wall extending direction and the skirt extending direction is referred to as a "piston lower wall intersection angle".

[0061 ] As shown in FIG. 5, plural planes A through G for taking the piston lower wall cross sections are set. Here, as shown in FIG. 6, when a plane P I

perpendicular to the piston central axis C I is referred to as a "horizontal plane" and a distance D l between the horizontal plane P I and the pin hole central axis C2 is referred to as a "horizontal plane distance", each of the planes A through G shown in FIG. 5 is the horizontal plane P I that has a different horizontal plane distance D l .

[0062] In an example shown in FIG. 5, the horizontal plane distance D l of the plane D is zero. With respect to the plane D, the planes A and G, the planes B and F, and the planes C and E are symmetric. The horizontal plane -distances D l of the planes A and G are set to be the largest, the horizontal plane distances D l of the planes B and F are set to be the second largest, and the horizontal plane distances Dl of the planes C and E are set to be the third largest.

[0063) The piston lower wall cross sections in the case that the piston lower wall is cut along the planes A through G and viewed from the lower side are respectively shown in FIGs. 7A through 7G. The piston lower wall bottom end face is shown in FIG. 7H. ^"

[0064] As seen with reference to FIGs. 7 A through 7H, in the piston 10, the piston lower wall intersection angle AN gradually increases from the lower side to the upper side in the piston lower wall. That is to say, the angle between a plane perpendicular to the axis of the pin hole 141 and the side wall section 13A or 13B decreases toward the lower side of the piston.

[0065] Therefore, the piston lower wall intersection angle in the upper part of the skirt section 12A or 12B is relatively large. Accordingly, the following effects can be achieved.

[0066] That is, when the piston 10 is disposed within a cylinder bore and the internal combustion engine is operated, the skirt sections 12A and 12B are subjected to so-called thrust force from the inner peripheral wall surface of the cylinder bore. During the engine operation, the temperature of the upper part of the skirt section 12A or 12B (hereinafter, referred to as a "skirt upper part") is higher than that of the middle part of the skirt section (hereinafter, referred to as a "skirt middle part") and the lower part of the skirt section (hereinafter, referred to as a "skirt lower part"). Accordingly, the degree of thermal expansion in the skirt upper part is greater than that in the skirt middle part and the skirt lower part. Therefore, if the thrust resistance (that is, resistance to deformation by the thrust force) of the skirt upper part is high, there is a nigh possibility that the skirt upper part is pressed relatively strongly against the inner peripheral wall surface of the cylinder bore or so-called "interference fit" occurs. On the other hand, if the thrust resistance of the skirt upper part is low, there is a low possibility of the interference fit, because the skirt upper part can deform radially inward with respect to the piston central axis C 1 when the skirt upper part is thermally expa ded. Accordingly, in order to prevent the interference fit, the thrust resistance of the skirt upper part is preferably reduced.

[0067] Here, as the piston lower wall intersection angle is larger, the thrust resistance of the skirt section 12A or I2B is reduced. In the piston 10, because the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall, the piston lower wall intersection angle of the skirt upper part is relatively large. Accordingly, since the thrust resistance of the skirt upper part is reduced, the interference fit of the skirt upper part is prevented.

[0068] In the piston 10, the piston lower wall intersection angle of the skirt lower part is relatively small. Accordingly, the following effects can be achieved.

[0069] That is, when the piston 10 is disposed within a cylinder bore and the internal combustion engine is operated, the piston 10 is subjected to the force that displaces the piston 10 such that the piston central axis C l inclines with respect to the cylinder bore central axis. Thereby, the skirt lower part is pressed against the inner peripheral wall surface of the cylinder bore. At this time, if the rigidity of the skirt lower part is low, the skirt lower part can easily be deformed inward. Therefore, when the skirt lower part is pressed against the inner peripheral wall surface of the cylinder bore, the skirt lower pari is deformed inward. On the other hand, if the rigidity of the skirt lower part is high, the skirt lower part is not deformed inward, in spite of being pressed against the inner peripheral walJ surface of the cylinder bore. Accordingly, in order to prevent the inward deformation of the skirt lower part, the rigidity of the skirt lower part is preferably increased.

[0070] Here, as the piston lower wall intersection angle is small, the rigidity of the skirt section 12A or 12B increases. In the piston 10, because the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall, the piston lower wall intersection angle of the skirt lower part is relatively small. Accordingly, since the rigidity of the skirt lower part increases, the inward deformation of the skirt lower part is prevented.

[0071 ] As described above, in the piston 10, because the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall, the prevention of the interference fit of the skirt upper part and the prevention of the inward deformation of the skirt lower part is concurrently achieved.

[0072] In the piston 1 0, when plural piston lower wall cross sections are obtained as described below, and the piston lower wall cross sections and the piston lower wall intersection angles in the piston lower wall cross sections are compared with each other, the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall.

[0073] Thai is, as shown in FIG. 8, plural planes A through G for taking the piston lower wall cross sections are set. As shown in FIG. 9, when a plane P2 including the pin hole central axis C2 and the piston central axis Cl is referred to as a "reference plane", a pair of planes P3 and P4, in which that are symmetric with respect to the reference plane P2 the plane P3 extends in one side of the reference plane P2 and the plane P4 extends in the other side of.the reference plane P2, and in which the intersecting line of the planes P3 and P4 corresponds to the pin hole central axis C2 are referred to as "paired planes", and an angle AN 1 between the paired planes P3 and P4 is referred to as a "plane-io-plane angle", the planes A through G as shown in FIG. 8 are the paired planes P3 and P4 that have different plane-to-plane angles AN 1 .

[0074] In an example shown in FIG. 8, the plane-to-plane angle AN 1 is set to be larger in the order from the paired plane A to the paired plane G.

[0075j In the case that the piston lower wall intersection angles in the piston lower wall cross sections are compared with each other when the piston lower walls are taken along the paired planes A through G, the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall.

[0076] In the piston 10, when plural piston lower wall cross sections are obtained as described below, and the piston lower wall cross sections and the piston lower wall intersection angles in the piston lower wall cross sections are compared with each other, the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall.

[0077] As shown in FIG. 10, plural planes A through G for taking the piston lower wall cross sections are set. As shown in FIG. 1 1 , when a plane P2 including the pin hole central axis C2 and the piston central axis C I is referred to as a "reference plane", a pair of planes P5 and P6 in which the plane P5 extends in one side of the reference plane P2 and the plane P6 extends in the other side of the reference plane P2 and which are symmetric with respect to the reference plane P2 are referred to as "paired planes", an angle ΛΝ2 between the paired planes P5 and P6 is referred to as a "plane-to- plane angle", and a distance D2 between the intersecting line of the planes P5 and P6 and the pin hole central axis C2 is referred to as a "plane distance", the planes A through G as shown in FIG. 10 are the paired planes P5 and P6 that have different plane-to-plane angles AN2 and different plane distances D2.

[0078] In an example shown in FIG. 10, the plane-to-plane angle of the paired plane D is 1 80°. With respect to the paired plane D, the paired planes A to C are arranged in the upper side, and the paired planes E to G are arranged in the lower side. With respect to the plane D, the paired planes A and G, the paired planes B and F, and the paired planes C and E are symmetric, respectively. The plane distances D2 of the paired planes A and G are set to be the largest, the plane distances D2 of the paired planes B and F ar e set to be the second largest, and the plane distances D2 of the paired planes C and E are set to be the third largest. The plane-to-plane angle AN2 is set to be larger in the order from the paired plane A to the paired plane C and from the paired plane E to the paired plane G.

(0079] In the case that the piston lower wall intersection angles in the piston lower wall cross sections and the piston lower wall intersection angles in the piston lower wall bottom end faces are compared with each other when the piston lower walls are taken along the paired planes A through G, the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall. [0080] In the piston 10, when plural piston lower wall cross sections are obtained as described below, and the piston lower wall cross sections and the piston lower wall intersection angles in the piston lower wall cross sections are compared with each other, the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall.

[0081 ] As shown in FIG. 12, plural planes A through G for taking the piston lower wall cross sections are set. As shown in FIG. 13, when a plane P2 including the pin hole central axis C2 and the piston central axis C I is referred to as a "reference plane", a cylindrical'^' plane P7 centered around the central axis on the reference plane P2 is merely referred to as a "cylindrical plane", and a distance D3 between the intersecting line of the cylindrical plane P7 and the reference plane P2 and the pin hole central axis C2 is referred to as a "cylindrical plane distance", the planes A to C and E to G shown in FIG. 12 are respectively the cylindrical plane P7 with different curvature radii and cylindrical plane distances D3, and the plane D shown in FIG. 12 is the plane that is perpendicular to the reference plane P2 and that includes the pin hole central axis C2.

[0082] In an example shown in FIG. 12, with respect to the plane D, the cylindrical planes A to C are arranged on the upper side, and the cylindrical planes E to G are arranged on the lower side. With respect to the plane D, the cylindrical planes A and G, the cylindrical planes B and F, and the cylindrical planes C and E are symmetric, respectively. The cylindrical plane distances D3 of the cylindrical planes A and G are set to be the largest, the cylindrical plane distances D3 of the cylindrical planes B and F are set to be the second largest, and the cylindrical plane distances D3 of the cylindrical planes C and E are set to be the third largest. The curvature radii of the cylindrical planes A and G are set to be the smallest, the curvature radii of the cylindrical planes B and F are set to be the second smallest, and the curvature radii of the cylindrical planes C and E are set to be the third smallest.

[0083] In the case that the piston lower wall intersection angles in the piston lower wall cross sections and the piston lower wall intersection angles in the piston lower wall bottom end faces are compared with each other when the piston lower walls are taken along the cylindrical planes A to C and E to G and the plane D-D, the piston lower wall intersection angle gradually increases from the lower side to the upper side in the piston lower wall.

[0084] Here, the characteristics of the piston lower wall intersection angle described with reference to FIGs. 5 tlixough 13 will be comprehensively described. In the piston 10, when the piston lower wall intersection angles in at least two piston lower wall cross sections that are not crossing each other (or, the piston lower wall intersection angles in one piston lower wall cross section and the piston lower wall bottom end face) are compared with each other, the piston lower wall intersection angle in the piston lower wall cross section that is located on the upper side is greater than the piston lower wall intersection angle in the piston lower wall cross section (or the piston lower wall bottom end face) that is located on the lower side.

[0085] As seen with reference to FIGs. 7A through 7H, in the piston 10, the side wall connecting portion (that is, a part of the side wall section 13 A or 13B in the vicinity of the piston lower wall connecting portion) is at least bent. The curvature radius of the side wall connecting portion (hereinafter, referred to as a "side wall curvature radius") gradually decreases from the lower side to the upper side in the side wall section.

[0086] In the piston 10, the side wall connecting portion in the piston lower wall bottom end face may extend linearly, and the side wall connecting portion in the piston lower wall cross section that is located in the lower side and the piston lower wall bottom end face may extend linearly.

[0087] As described above, in the piston 10, the side wall curvature radius gradually decreases from the lower side to the upper side in the side wall sections 13Λ and 13 B. Accordingly, the side wall curvature radius in the side wall upper part is relatively small. Accordingly, the following effects can be achieved.

[0088] In other words, as described above, in order to prevent the interference fit, the thrust resistance of the skirt upper part is preferably reduced. As the side wall curvature radius is smaller, the thrust resistance of the skirt section is reduced.

Accordingly, since in the piston 10, the side wall curvature radius gradually decreases from lhe lower side to the upper side in the side wall sections 13A and 13B, the thrust resistance of the skirt upper part is reduced, and the interference fit of the skirt upper part is prevented.

[0089] As described above, in the piston 10, the side wall curvature radius gradually decreases from the lower side to the upper side in the side wall sections 13 A and 13B. Accordingly, the side wall curvature radius in the side wall lower part is relatively large. Accordingly, the following effects can be achieved.

[0090] In other words, as described above, in order to prevent the inward deformation of the skirt lower part, the rigidity of the skirt lower part is preferably increased. As the side wall curvature radius is larger, the rigidity of the skirt section increases. Accordingly, since in the piston 10, the side wall curvature radius gradually decreases from the lower side to the upper side in the side wall sections 13A and 13B, the rigidity of the skirt lower part increases, and the inward deformation of the skirt lower part is prevented.

[0091 ] As described above, in the piston 10, because the side wall curvature radius gradually decreases from the lower side to the upper side in the side wall sections 13A and 13B, the prevention of the interference fit of the skirt upper part and the prevention of the inward deformation of the skirt lower part is concurrently achieved.

[0092] Next, the thickness of the skirt sections 12A and 12B when it is measured in a vertical direction with respect to the piston central axis C I will be described.

[0093] In the piston 10, the skirt section 12A or 12B has a thickness that gradually increases from the upper side to the lower side in the skirt section as shown in FIG. I A (hereinafter, the thickness of the skirt section is referred to as a "skirt thickness"). Accordingly, the following effects can be achieved.

[0094] That is, the skirt section I 2A or I 2B is connected to the piston body 1 1 having a high rigidity in an upper end (hereinafter, referred to as a "skirt upper end"). Therefore, if the skirt sections 12A and I 2B have a constant thickness over all, then the thrust resistance (that is, the capability to withstand the deformation by the thrust force) of (he skirt section tends to be high from the lower end of the skirt section (hereinafter, referred to as a "skirt lower end") toward the skirt upper end.

[0095] Accordingly, as shown in FIG. 14B for example, if the skirt sections 12A and 12B have the thickness that gradually decreases from the skirt upper end toward the skirt lower end, the thickness of the part of the middle region of the skirl section (hereinafter, referred to as a "skirt middle part") and the thickness of the part of the lower region of the skirt section (hereinafter, referred to as a "skirt lower part") relatively decreases, and thus the thrust resistance of those parts is significantly reduced. In this case, when the thrust force is applied to the skirt section 12A or! 2B, at least the skirt middle part gets dented. In this case, an acute comer is formed in a boundary between the dented part of the skirt middle part and an undented part of the skirt section. When such a corner is formed in the skirt section 12A or 12B, the friction between the skirt section and the inner peripheral wall surface of the cylinder bore becomes high in the corner.

[0096] If the skirt sections 12A and 12B have the thickness that gradually increases from the skirt upper end toward the skirt lower end as the piston 10, the thicknesses of the skirt middle part and the skirt lower part relatively increase, and thus the thrust resistance of those parts relatively increases. Incidentally, although the thick ness of the part of the upper region of the skirt sections 12A and 12B (hereinafter, the part is refeired to as a "skirt upper part") is relatively reduced, the part is close to the piston body 1 1 , and thus the thrust resistance of those parts relatively increases.

[0097] As described above, since the thrust resistance of the whole skirt sections 12A and 12B relatively increases in the piston 10, the skirt middle part does not get dented by the thrust force (or, even if the skirt middle part deforms to dent, the deformation amount is very small, and the area of the dented part is also very small). TTierefore, (he friction between the skirt section 1 2A or 12B and the inner peripheral wall surface of the cylinder bore is prevented from being high.

[0098] Incidentally, as described above, if the thrust resistance of the skirt upper part is high, there is a high possibility that the interference fit of the skirt upper part occurs. Therefore, in order to prevent the interference fit of the skirt upper part, the thrust resistance of the skin upper part is preferably reduced.

[0099] As described above, the skirt sections 12A and 12B have the thickness that gradually increases from the skirt lower end toward the skirt upper end in the piston 10, the thickness of the skirt upper part is relatively reduced. Accordingly, since the thrust resistance of the skirt upper part is reduced, the interference fit of the skirt upper part is prevented.

[0100] As shown in FIG. 14A, when the skirt outer peripheral wail surface 121 is a partially cylindrical surface generally centered around the piston central axis CI , but the part where the diameter with respect to the piston central axis C I is large (hereinafter, the part is referred to as a "large diameter part") is located in the skirt outer peripheral wall surface 121 , the large diameter part is subjected to the large thrust force. Therefore, the large diameter part tends to get dented by the thrust force. As described above, in order to prevent the friction between the skirt section 12A or 12B and the inner peripheral wall surface of the cylinder bore from being high, it is preferable to prevent the skirt outer peripheral wall surface 121 from being dented. When the diameter for each part of the skirt outer peripheral wall surface 121 with respect to the piston central axis C I is different, the thickness of each part may be increased in proportion to the diameter of the part . Accordingly, even if the skirt outer peripheral wall surface 121 has a large diameter part, the friction between the skirt section 12A or 12B and the inner peripheral wall surface of the cylinder bore is prevented from being high.

[0101] In such a case that the thickness of each part of the skirt section 12Λ or 12B is increased in proportion to the diameter of the part, when the large diameter part is located in the middle region of the skirt outer peripheral wall surface 121, the thickness of the skirt upper part is eventually reduced. Accordingly, since the thrust resistance of the skirt upper part is reduced, the interference fit of the skirt upper part is prevented.

[0102) When the large diameter part is located in the middle region of the skirt outer peripheral wall surface 121 , in order to prevent the skirt outer peripheral wall surface 121 from being dented radially inward with respect to the piston central axis C l and to prevent the interference fit of the skirt upper part, at least the thickness of the skirt middle part may relatively be increased and the thickness of the skirt upper part may relatively be reduced. Therefore, in this case, the thickness of the skirt lower part may be relatively thin. Accordingly, when the large diameter part is located in the middle region of the skirt outer peripheral wall surface 121 , the thickness of the skirt middle part may relatively be increased and the thickness of the skirt upper part and the skirt lower part may relatively be reduced as shown in FIG. 14C.

[0103] In this case, since the thickness of the skirt lower part is relatively reduced, the effect of weight reduction of the piston 10 can be achieved.

[0104] As described above, when the piston body 1 1 deforms, the side wall section 13A or 13B also deforms at the pin hole upper part 143 at the bearing point. As described above, the pin hole upper part 143 becomes the bearing point of the deformation of the side wall section 13 A or 1 3B. Therefore, the temperature in the pin hole upper part 143 and its vicinity tends to be higher than that in the other part.

Accordingly, in order to prevent the fatigue, degradation of the pin hole upper part 143 and its vicinity, it is desirable to efficiently cool those parts.

[0105] In the piston 10, the pin hole upper part 143 and its vicinity may be constituted as shown in FIGs. 1 5 A and 15B. That is, in the embodiment as shown in FIGs. ] 5 A and 15B (hereinafter, referred to as a "second embodiment"), recesses 31 (shaded areas in FIG. 15A) are disposed on the outer side wall surface 131 in the vicinity of the pin hole upper part 143 at the both sides of the rib 30. The wall surface defining each recess 31 has at least a wall surface 32 that extends inward from the region adjacent to the pin hole upper part 143 to the inside of the side wall section 13 A or 1 3B and obliquely upward and faces generally upward (hereinafter, the wall surface is referred to as a "recessed oblique surface"), and a wall surface 33 that extends outward from an inner end of the recessed oblique surface 32 and generally perpendicular to the piston central axis CI . [0106] As the shown in second embodiment, the recess 3 1 is disposed iu the outer side wall surface 131 in the vicinity of the pin hole upper part 143, and therefore the following effects can be achieved.

[0107] That is, when the piston 10 according to the second embodiment is disposed within a cylinder bore, and the internal combustion engine is operated, cooling and lubricating oil is blown up from the lower side to the space between the outer side wall surface 13 1 and the inner peripheral wall surface of the cylinder bore. The blown-up oil arrives in the pin hole upper part 143 and its vicinity through the space between the outer side wall surface 131 and the inner peripheral wall surface of the cylinder bore.

[0108] If the recessed oblique surface 32 is not disposed at the portion in the vicinity of the pin hole upper part 143, the oil arrived at the portion flows out from the portion sooner rather than later. That is, the time that the oil stays around the portion in the vicinity of the pin hole upper part 143 is short.

[0109] On the other hand, as the second embodiment, if the recessed oblique surface 32 is disposed at the portion in the vicinity of the pin hole upper part 143, the time that the oil stays around the portion in the vicinity of the pin hole upper part gets longer because of the following reason. That is, as shown in FIG. 16A, when the piston 10 is disponed within the cylinder bore 50, the piston central axis C I is generally in parallel to the vertical direction. Therefore, when the piston is disposed within the cylinder bore, the recessed oblique surface 32 is obliquely disposed with respect to the vertical direction. Accordingly, the oil arrived at the portion in the vicinity of the pin hole upper part stays on the recessed oblique surface 32 as shown in FIG. 16A. Thus, if the recessed oblique surface 32 is disposed at the portion in the vicinity of the pin hole upper part, the time that the oil stays around the portion in the vicinity of the pin hole upper part gets longer.

[0110] According to the second embodiment, because the time that the oil stays around the portion in the vicinity of the pin hoie upper part 143 gets longer, the parts of the piston around the portion in the vicinity of the pin hole upper part (that is, the pin hole upper part 143 and its vicinity) are efficiently cooled. [0111 ] As shown in the second embodiment, i f the recessed oblique surface 32 is disposed at the portion in the vicinity of the pin hole upper part 143, the time that the oil stays around the portion in the vicinity of the pin hole upper part gets further longer because of the following reason. That is, as shown in FIG. 16B, when the piston 10 is disposed within the cylinder bore 50, the recessed oblique surface 32 is obliquely disposed with respect to the vertical direction. For this reason, the recessed oblique surface 32 is inclined with respect to the diffusion direction of incoming oil . Therefore, as shown with an arrow A in FIG. 16B, the recessed oblique surface 32 can splash the incoming oil back to the upper side. For this reason, the oil arrived at the recessed oblique surface 32 diffuses in the recess 3 1. That is, the oil-arrived at the portion in the vicinity of the pin hole upper part 143 stays around the portion in the vicinity of the pin hole upper part. For this reason, as the second embodiment, if the recessed oblique surface 32 is disposed at the portion in the vicinity of the pin hole upper part 143, the time that the oil stays around the portion in the vicinity of the pin hole upper part gets further longer.

[01 12 ] According to the second embodiment, because the time that the oil stays around the portion in the vicinity of the pin bole upper part 143 gets longer, the paits of the piston around the portion in the vicinity of the pin hole upper part (that is, the pin hole upper part 243 and its vicinity) are further efficiently cooled.

[0113] Incidentally, the recess 3 1 according to the second embodiment is an example of the recess according to the present invention. That is, the recess according to the present invention includes any recesses that can keep the oil in the portion in the vicinity of the pin hole upper part.

[0114] Therefore, in the piton according to the second embodiment, the recess 31 is disposed in the outer side wall surface 131 on the both sides of the rib 30.

Alternatively, the recess may be disposed only in the outer side wall surface 131 on one side of the rib 30.

[0115] The rib 30 has the effect of increasing the rigidity of the side wall section 13A or 13B between the pin hole upper part 143 and the piston body 1 1. However, if the effect of keeping a quantity of oil in the portion in the vicinity of the pin hole upper part 143 is prioritized over the effect described above, the recess similar to the recess 3 1 of the piston 10 according to the second embodiment may be provided in the outer wall surface of the rib 30 in the vicinity of the pin hole upper part 143, in addition to the recess 31 in the outer side wall surface 1 31 on the both sides of the rib 30 in the piston according to the second embodiment.

[0116] As a matter of course, instead of providing the recess 3 1 in the outer side wall surface 131 on the both sides of the rib 30 in the piston according to the second embodiment, the recess similar to the recess 31 of the piston 10 according to the second embodiment may be provided in the outer wall surface of the rib 30 in the vicinity of the pin hole upper part 143.

[0117] The recessed oblique surface 32 according to the second embodiment is an example of the recessed oblique surface according to the present invention. That is, the recessed oblique surface according to the present invention includes any wall surfaces that can keep the oil arriving at the portion in the vicinity of the pin hole upper part when the piston is disposed witliin the cylinder bore. The recessed oblique surface according to the present invention includes any wall surfaces that can splash the oil, back to the upper side, arriving at the portion in the vicinity of the pin hole upper part when the piston is disposed witliin the cylinder bore.

[0118] Accordingly, as shown in the second embodiment, the recessed oblique surface 32 extending obliquely upward with respect to the vertical direction from the outer side wail surface 13 1 is provided in the outer side wall surface ] 31 in the vicinity of the pin hole upper part 143. Alternatively, the wall surface extending obliquely downward with respect to the piston central axis CI from the region in the vicinity of the pin hole upper part 143 towaid the inside of the side wall section 13A or 13B may be provided in the outer side wall surface 131 in the vicinity of the pin hole upper part 143, or the wall surface extending in the vertical direction with respect to the piston central axis C 1 from the region in the vicinity of the pin hole upper part 143 towaid the inside of the side wall section 13A or 13B may be provided in (he outer side wall surface 131 in the vicinity of the pin hole upper part 1 3.

[01 19] As shown in FIG. 1 5A, in the second embodiment, the bulge section 20 similar to the bulge section 20 of the first embodiment is provided in the outer side wall surface 131 . Accordingly, because of the fol lowing reason, the portion in the vicinity of the pin hole upper pari 143 is further efficiently cooled by oil.

[0120] That is, the bulge sections 20 according to the second embodiment are provided to the side wall sections 13Λ and 13B, in the similar manner to the bulge sections according to the first embodiment, so as to extend from the pin hole lateral regions AR3 to the upper comer regions AR l . In other words, the bulge sections 20 extend in the outer side wall surface 131 from the region in the vicinity of the recesses 31 (that is, in the vicinity of the recessed oblique surfaces 32) and the region of the outer side wall surface 131 in the vicinity of the pin hole section 14A or 14B to the direction away from the pin hole sections and obliquely upward. In yet other words, the bulge sections 20 extend from the region in the vicinity of the recesses 31 (that is. in the vicinity of the recessed oblique surfaces 32) and the region of the outer side wall surface 13 1 in the vicinity of the pin hole section 14 A or 14B to the region of the outer side wall surface 131 adjacent to the piston body 1 i in the direction away from the pin hole sections and obliquely upward.

[0121 ] Therefore, when the piston 10 is disposed within the cylinder bore such that the piston central axis C l becomes parallel to the vertical direction, the outer wall surface of the upper region of the bulge section 20 (that is, the region located on the upper side when the outer wall surface of the bulge section 20 is divided into two regions by the vertical surface with respect to the outer side wall surface 131 along the extending direction) is inclined at least with respect to the vertical direction. Accordingly, the outer wall surface can trap and keep the oil flown out from the recess 31 and also trap and keep the incoming oil to the outer side wall surface 13 1 that is located in the upper side of the bulge section 20. That is, the buige section 20 can keep the oil in the portion in the vicinity of the pin hole upper part 143 and in its peripheral portion. By using the kept oil, the portion in the vicinity of the pin hole upper part 143 and its peripheral portion are cooled. Therefore, in tlie second embodiment, the portion in the vicinity of the pin hole upper part 143 and its peripheral portion are further efficiently cooled by the oil.

[0122] The recess 3 1 according to the embodiment as described above can keep a sufficient quantity of the oi l irrespective of the viscosity of the oil. However, as the viscosity of the oil is higher, the recess 3 1 can keep the oil more securely. The bulge section 20 according to the embodiment as described above can keep a sufficient quantity of the oil irrespective of the viscosity of the oil. However, as the viscosity of the oil is higher, the bulge section 20 can keep the oil more securely.

[0123] As shown in FIG . I B, in the embodiment as described above, the oil introducing passage defining wall 103 is disposed in the inner wall surface of the side wall section 13 A. and the oil discharging passage defining wall 105 is disposed in the inner wall surface of the other side wall section 13B. Next, the defining walls 103 and 1 05 will be described in detail.

[0124] When the oil introducing passage defining wall 103 is provided as shown in FIG. 1 B, the oil introducing passage defining wall 103 increases the rigidity of the portion in the vicinity of the first lateral end of the first side wall section 1 3A. As a result, the rigidity of the portion in the vicinity of the first lateral end of the fust skirt section 1 2 A connected to the first lateral end of the first side wall sectionl3A is increased.

However, the portion in the vicinity of the second lateral end of the second side wall section 1 3B is not provided with a wall such as the oil introducing passage defining wall 103 that increases the rigidity of the portion. Therefore,^' the rigidity of the portion in the · vicinity of the second lateral end of the first skirt section 12A connected to the second lateral end of the second side wall section l 3B is not increased. Accordingly, the rigidity of the portion in the vicinity of the first lateral end of the first skirt section 12A is higher than the rigidity of the portion in (he vicinity of the second lateral end of the first skirt section 12A.

[0125] When the piston is disposed within a cylinder^'bore and the internal combustion engine is operated, the skirt sections 12A and 12B are subjected to the thrust force from the inner peripheral wall surface of the cylinder bore. The thrust force increases or decreases. When the thrust force increases, the thrust force deforms at least part of the skirt sections 12A and 12B. Then, when the thrust force decreases, the shape of the portion of the skirt sections 12A and 12B that are deformed recovers to the original shape.

[0126] When the rigidity of the portion in the vicinity of the first lateral end of the first skirt section 1 2A is higher than the rigidity of the portion in the vicinity of the second lateral end of the first skirt section 12A by means of the oil introducing passage defining wall 103, the degree of the deformation of the portion at the first lateral end side of the first skirt section 1 2A by the thrust force is smaller than the degree of the deformation of the portion at the second lateral end side of the first skirt section 12A by the thrust force. That is, in the first skirt section 12A, the degree of the deformation of the portion at the first lateral end side by the thrust force and the degree of the deformation of the portion at the second lateral end side by the thrust force differ from each other. If the degree of the deformation differs from each other, when the first skirt section 12A receives the thmst force to deform, large stress is generated in a portion of the first skirt section 12A. Then, if the tltrust force applied to the first skirt section 12A decreases, the shape of the deformed portion of the first skirt section 12A recovers to the original shape, and the large stress generated in a portion of the first skirt section 12A disappears. Due to the generation or the disappearance of large stress in the first skirt section 12 A, the first skirt section 12A degrades by fatigue.

[0127] In order to prevent such fatigue degradation of the first skirt section 12A, as shown in FIG. 1 7, without providing the oil introducing passage defining wall 103, an oil introducing port 104 that introduces oil into the oil passage inside the piston body 1 1 may be provided in a portion of the piston body bottom wall surface 1 12 in the vicinity of the upper end of the connecting portion between the first lateral end of the first skirt section 12A and the first lateral end of the first side wall section 13 A.

[0128] In the embodiment shown in FIG. 17 (hereinafter, referred to as a "third embodiment"), since the rigidity of the first skirt section 12A is uniform over all, large stress is prevented from being generated in a portion of the first skirt section I 2A.

Therefore, the fatigue degradation of the first skirl section 12A is prevented.

[0129] Similarly,, when the rigidity of the portion in the vicinity of the first lateral end of the second skirt section 1 2B is higher than the rigidity of the portion in the vicinity of the second lateral end of the second skirt section 12B by means of the oil discharging passage defining wall 105, the degree of the deformation of the portion at the first lateral end side of the second skirt section 12B by the thrust force is smaller than the degree of the deformation of the portion at the second lateral end side of the second skirt section 12B by the thrust force. That is, in the second skirl section 12B, the degree of the deformation of the portion at the first lateral end side by the thrust force and the degree of the deformation of the portion at the second lateral end side by the thrust force differ from each other. If the degree of the deformation differs from each other, when the second skirt section 12B receives the thrust force to deform, large stress is generated in a portion of the second skirt section 12B. Then, if the thr ust force applied to the second skirt section 12B decreases, the shape of the deformed portion of the second skirt section 12B recovers to the original shape, and the large stress generated in a portion of the second skirt section 12B disappears. Due to the generation or the disappearance of large stress in the second skirt section 12B, the second skirt section 12B degrades by fatigue.

[0130] In order to prevent such fatigue degradation of the second skirt section 12B, as shown in FJG. 17, without providing the oil discharging passage defining wall 105, an oil discharging port 105 that discharges oil into the oil passage inside the piston body 1 1 may be provided in a portion of the piston body bottom wall surface 1 12 in the vicinity of the upper end of the connecting portion between the first lateral end of the second skirt section J 2B and the first lateral end of the second side wall section 1 3B.

[0131] In tlie third embodiment shown in FIG. 17, since the rigidity of the second skirt section 12B is uniform over all, large stress is prevented from being generated in a portion of the second skirt section 12B. Therefore, the fatigue degradation of the second skirt section 12B is prevented. [0132] In the embodiment described above, when the piston is disposed within the cylinder bore, the oil is blown from the. lower side of a side wall bottom end face to the oil introducing port 104, and the blown oil flows into the oil introducing port 104. Therefore, as shown in the third embodiment, when the oil introducing port 104 is disposed in the piston body bottom wall surface 1 12, the oil does not flow efficiently into, the oil introducing port 104.

[0133] In order to efficiently flow the oil into the oil introducing port 104 while preventing the fatigue degradation of the skirt section 12A or 12B due to non-uniform rigidity of the skirt section described above, provided that the second skirl section 12B is disposed on a thrust side, as shown FIG. 1 8, the oil introducing passage defining wall 103 that extends from the bottom end face of the first side wall section 13A to the piston body bottom wall surface 1 12 may be provided in the inner wall surface 132 of the first side wall section 1 3 A between the first pin hole section 14A and the first lateral end of the first skirt section I 2A to connect the oil introducing passage that is defined by the oil introducing passage defining wail 103 to the oil passage inside the piston 10. In addition, the oil discharging port 105 may be provided in a portion of the piston body bottom wall surface 1 12 in the vicinity of the upper end of the connecting portion between the first lateral end of the second skirt section 12B and the first lateral end of the second side wall section 13B. That is, the oil discharging passage defining wall 105 is not provided in the inner wall surface of the second side wall section 1 B between the second pin hole section I4B and the first lateral end of the second skirt section 12B.

[0134] In the embodiment shown in FIG. 18 (hereinafter, referred to as a "fourth embodiment"), since the second skirt section 12B is disposed on the thrust side, the thrust force applied to the first skirt section 12A is smaller than the thrust force applied to the second skirt section 12B during the engine operation. Therefore, the oil introducing passage defining wall 103 is provided in the inner wal) surface 132 of the first side wall section 13A between the first pin hole section 14A and the first lateral end of the first skirt section 12 A. As a result, even if the rigidity of the portion on the first lateral end side of the first skirt section 12A and the rigidity of the portion on the second lateral end side of the first skirt section 12A differ from each other, because tlie thrust force applied to the first skirt section 12A is relatively small, large stress is not generated in a portion of the first skirt section 12 A. Therefore, the fatigue degradation of the first skirt section 12A is prevented.

[0135] In the fourth embodiment, since the oil introducing passage defining wall 103 extends from the bottom end face of the first side wall section 13A to the piston body bottom wall surface 1 12, the oil introducing port 104 is formed in the vicinity of the bottom end face of the first side wall section 13A. Therefore, the oil is efficiently flown into the oil introducing port 104.

[0136] On the other hand, in the fourth embodiment/since the oil discharging passage defining wall 105 is not provided in the inner wall surface 132 of the second side wall section 13B between the second pin hole section 14B and the first lateral end of the second skirt section 12B, the rigidity of the second skirt section 12B is uniform over all. Accordingly, even if the second skirt section 12B is disposed on the thrust side and the thrust force applied to the second skirt section 12B is relatively large, large stress is nol generated in a portion of the second skirt section 12B. Therefore, the fatigue degradation of the second skirt section 12B is prevented.

[0137] In the embodiment described above, the combustion chamber formed with the piston body top wall surface 1 1 1 and the inner peripheral wall surface of the cylinder bore is generally connected to an intake port that introduces air into the combustion chamber and to an exhaust port that discharges an exhaust gas from the combustion chamber. Here, the temperature of the exhaust gas discharged from the combustion chamber is higher than that of the air introduced into the combustion chamber. Accordingly, the temperature of the region inside the combustion chamber close to the exhaust port is higher than that of the region inside the combustion chamber close to the intake port. Consequently, the temperature of tlie part of the inner peripheral wall surface of the cylinder bore close to the exhaust port is higher than that of the part of the inner peripheral wall surface of the cylinder bore close to the intake port. Therefore, when the piston according to the embodiment described above is disposed within a cylinder bore and the internal combustion engine is operated, the temperature of the part of the piston close to the exhaust port becomes higher than that of the part of the piston close to the intake port.

[0138] On the other hand, in the embodiment described above, the cooling effect by the oil in the part of the piston close to the oil introducing port 104 is higher than the cooling effect by the oil in the part of the piston away from the oil introducing port 104.

[0139] Therefore, the piston according to the embodiment described above is disposed within the cylinder bore such that the skirt section close to the oil introducing port 104 (that is, in the embodiment described above, the first skirt section 12A) is disposed close to the exhaust port and such that the skirt section away from the oil introducing port ] 04 (that is, in the embodiment described above, the second skirt section 12B) is disposed close to the intake port. By this arrangement, entire piston can be efficiently cooled by the oil .

[0140] In the first embodiment, the oil introducing passage defining wall 103 and the oil discharging passage defining wall 105 are provided in the inner side wall surface 132 for forming the oil introducing passage 102 and the oil discharging passage 104. When the defining walls 103 and 105 are provided in the inner side wall surface 132 as described above, the weight of the piston increases as compared to the case that the defining walls 103 and 105 are not provided in the inner side wall surface 132. On the other hand, in the field of the piston, the weight reduction of the piston is demanded. Accordingiy, even if the oil introducing passage 102 and the oil discharging passage 104 are formed, there is a case that the weight reduction of the piston as much as possible is demanded,

(0141 ] In the first embodiment, in order to reduce the weight of the piston as much as possible even if the oil introducing passage 102 and the oil discharging passage 104 are formed, the passages 102 and 104 may be formed as shown in FIGs. 1 A and 19B. [0142] That is, in the embodiment shown in FIGs. 19A and 19B (hereinafter, referred to as a "fifth embodiment"), as shown in FIG. 19A, the oil introducing passage 102 is formed by covering, with a wall 22, the groove 21 that is formed along the bulge section 20 in the inner wall surface of the bulge section 20 provided in the first side wall section 13A between the first pin hole section 14A and the first lateral end of the first skirt section 12A. That is, the bulge section 20 is utilized as a part of the oil introducing passage defining wall 103.

[0143] When the bulge section 20 is utilized as a part of the oil introducing passage defining wall 103, the weight of the piston is reduced as compared to the case that the bulge section 20 is not utilized as a part of the oil introducing passage defining wall 103.

[0144] in the fifth embodiment, as shown in FIG. 19B, the oil discharging passage is formed by covering with a wall 23 the groove 21 that is formed along the bulge section 20 in the inner wall surface of the bulge section 20 provided in the second side wall section 13B between the second pin hole section 14B and the first lateral end of the second skirl section 1 2B. That is, the bulge section 20 is utilized as a part of the oil discharging passage defining wall 1 05.

[0145] When the bulge section 20 is utilized as a part of the oil discharging passage defining wall 105, the weight of the piston is reduced as compared to the case that the bulge section 20 is not utilized as a part of the oil discharging passage defining wall 105.

[0146] The concept of the fifth embodiment that the bulge section 20 is utilized as a part of the oil introducing passage defining wall 103 is applicable to the fourth embodiment that the oil discharging passage defining wall 105 is not provided but only the oil introducing passage defining wall 103 is provided.

[0147] The piston according to the first embodiment includes the piston cavity 101 defined by the inner side wall surface 132, the skirt inner peripheral wall suiface 122, and the piston body bottom wall surface 1 12. When such the piston is manufactured with a mold, the piston cavity 101 is formed by using the core in the shape corresponding to the piston cavity 101. Specifically, the piston cavity 101 is formed by solidifying a piston-forming material (hereinafter, referred to as a "piston material") in a state that the piston material is placed around the core.

[0148] When the piston cavity 101 is formed by solidifying the piston material in a state that the piston material is placed around the core, the core is required to be removed from the piston cavity 101 after the formation of the piston cavity 101 . In contrast to the piston according to the embodiment described above, when the skirt inner peripheral wall surface is a partially cylindrical surface centered around the piston central axis, or when the skirt inner peripheral wall surface is a partially conical surface extending from the skirt upper end to the skirt lower end with respect to the piston central axis (for example, as shown in FIG. 14B), the core can be easily removed from the piston cavity after the formation of the piston cavity.

[0149J However, as described with reference to FIGs. 14A and 14C, as a result of increasing the thickness of a part of the skirt section 12A or 12B thicker than the other part, when the part of the skirt inner peripheral wali surface 122 is projecting inward, it is very hard to remove the core from the piston cavity after the formation of the piston cavity 1 01.

[0150] When the piston according to the first embodiment is manufactured witha mold, the piston cavity 101 is formed as follows, and the core is removed from the piston cavity after the formation of the piston cavity.

[0151 ] That is, in the embodiment of the present invention, the core shown in FIG. 20 is used in order to form the piston cavity 101 in the piston.

[0152] Specifically, the core including the following is used: (1) a first core 41 that defines "entire inner wall surface of the first side wall section 13 A", "a portion of the inner peripheral wall surface of the first skirt section 12A in the vicinity of the first lateral end of the first skirt section 12A", "a portion of the inner peripheral wall surface of the second skirt section 12B in the vicinity of the second lateral end of the second skirt section 12B", and "a portion of the piston body bottom wal l surface 1 12 in the vicinity of the upper end of the first side wall section 13 A"; (2) a second core 42 that defines "entire inner wall surface of the second side wall section 13B", "a portion of the inner peripheral wall surface of the first skirt section 12A in the vicinity of the second lateral end of the first skirt section 12A", "a portion of the inner peripheral wall surface of the second skirt section 1 2B in the vicinity of the first lateral end of the second skirt section 12B", and "a portion of the piston body bottom wall surface 1 12 in the vicinity of the upper end of the second side wall section 13B"; (3) a third core 43 that defines "the other portion of the inner peripheral wall surface of the first skirt section 12 A" and "a portion of the piston body bottom wall surface 1 12 in (he vicinity of the middle part of the upper end of the first skirt section 12 A"; (4) a fourth core 44 that defines "the other portion of the inner peripheral wall surface of the second skirt section 12B" and "a portion of (he piston body bottom wall surface 1 12 in the vicinity of the middle part of the upper end of the second skirt section 12B"; and (5) a fifth core 45 that defines "the other portion of the piston body bottom wall surface 1 12".

[0153] When the piston cavity 101 is formed, the third core 43 and the fourth core 44 are arranged between the first core 41 and the second core 42 so as to respectively contact with the first core 41 and the second core 42, and the fifth core 45 is arranged between the first core 41 and the second core 42 and between the third core 43 and the fourth core 44 so as to contact with the first core 41 through ⁽he fourth core 44. The geometry of the first core 41 through the fifth core 45 is made such that when the cores 1 through 45 are arranged as described above, the geometry formed by the cores corresponds with the shape of the wall surface that defines the piston cavity 101 .

[0154] After the piston cavity 101 is formed by the first core 41 through the fifth core 45, if the cores are removed from the piston cavity 101 in the order of the fifth core 45, the fourth core 44, the third core 43, the second core 42, and the first core 41 , the cores 41 through 45 can be easily removed from the piston cavity 101.

[0155] The described concept of the formation of the piston cavity 101 in the case that the piston according to the first embodiment is manufactured with a mold is applicable to the formation of the piston cavity in the case that the piston with a projecting part that projects toward the piston cavity more in the lower part of the skirt inner peripheral wall surface than in the upper part of the skill inner peripheral wall surface is manufactured with a mold.

[0156] When the piston according to the first embodiment is manufactured with a mold transverse cross-sectional shapes of the skirt sections 12A and 12B may be formed in the shapes shown in FIGs. 22A and 22B, respectively, in order to easily remove the core described above from the piston cavity, (hereinafter, the embodiment shown in FIGs. 22A and 22B is referred to as a "sixth embodiment") or the shapes shown in FIGs. 23 A and 23B, respectively (hereinafter, the embodiment shown in FIGs. 23A and 23B is referred to as a "seventh embodiment").

[0157] That is, FIG. 22A is a transverse cross-sectional view of the second skirt section 12B of the piston according to the sixth embodiment, taken along a line Zl -Zl in FIG. 21 . and FIG. 22B is a transverse cross-sectional view of the second skirt section 12B of the piston according to the sixth embodiment, taken along a line Z2-Z2 in FIG. 21. In the piston 10 according to the sixth embodiment, as shown in FIG. 22 A, the upper part of the second skirt section I 2B has a shape in which the inner peripheral wall surface 12CN of the middle region in a circumferential direction of the second skirt section 12B gets dented more than the inner peripheral wall surface 12LT of the lateral region. Therefore, the inner peripheral wall surface 122 of the upper part of the second skirt section 12B is formed with the strip-shaped groove 123 that extends in the parallel direction to the piston central axis C I in lbe middle region in the circumferential direction of the second skirt section 12B. The projecting part 124 that projects toward the piston central axis C l more than the inner peripheral wall surface 12LT of the lateral region is formed between the middle region of the upper part of the second skirt section 12B where the strip-shaped groove 123 is formed and the lateral region of the upper part.

[0158] In the piston 10 according to the sixth embodiment, as shown in FIG. 22B, the lower part of the second skirl section ] 2B has a constant thickness in the circumferential direction of the second skirt section 12B. That is, the strip-shaped groove 123 and the projecting part 124 that are formed in the inner peripheral wall surface 122 of the upper part of the second skirt section 12B are not formed in the lower part of the second skirt section 12B.

[0159] In the piston 10 according to the sixth embodiment, although it is not shown in drawings, the first skirt section 12A has the same shape as the second skirt section 12B.

[0160] FIG. 23 A is a transverse cross-sectional view of the second skirt section 12B of the piston according to the seventh embodiment, taken along a line Zl -Z l in FIG. 21 , and FIG. 23 B is a transverse cross-sectiona! view of the second skiit section 12B of the piston according to the seventh embodiment, taken along a line Z2-Z2 in FIG. 21 . In the piston 10 according to the seventh embodiment, as shown in FIG. 23 A, the upper part of the second skirt section 12B has a shape that the inner peripheral wall surface 12CN of the middle region in a circumferential direction of the second skirt section 12B gets dented more than the inner peripheral wall surface 12LT of the lateral region. Therefore, the inner peripheral wall surface 122 of the upper part of the second skirt section 12B is formed with the strip-shaped groove 123 that extends in the parallel direction to the piston central axis CI in the middle region in the circumferential direction of the second skirt section 12B. However, the projecting part that projects toward the piston central axis C 1 more than the inner peripheral wall surface J 2LT of the lateral region is not formed between the middle region of the upper part of the second skirt section 12B where the strip-shaped groove 123 is formed and the lateral region of the upper part.

[0161 ] In the piston 10 according to the seventh embodiment, as shown in FIG. 23B, the lower part of the second skirt section 12B has a constant thickness in the circumferential direction of the second skirt section 12B. That is, the strip-shaped groove 123 that is formed in the inner peripheral wall surface 122 of the upper part of the second skirt section 12B is not formed in the lower part of the second skirt section 12B.

[0162] In the piston 10 according to the seventh embodiment, although it is not shown in drawings, the first skirt section 12A has the same shape as the second skirt section 12B. [0163] The piston according to the embodiment described above has a pair of pin hole sections. However, the piston according to the embodiment described above may have one generally annular pin hole section. In this case, the pin hole section is provided so as to pass through the side wall sections 1 3A and I 3B. The central axis of the pin hole section is perpendicular to the extending plane of the side wall sections 13A and 13B.

Claims

1 . A piston of an internal combustion engine, comprising:

a cylindrical piston body;

a pair of generally partially annular skirt sections that extend from a bottom wall surface of the piston body to a lower side in parallel with a central axis of the piston body;

a pair of flat shaped side wall sections that extends from the bottom wall surface of the piston body to the lower side in parallel with the central axis of the piston body and that connects the skirt sections each other;

the side wall sections having annular pin hole sections that include central axes perpendicular to extending planes of the side wall sections; and

a cavity formed by the bottom wall surface of the piston body, inner peripheral wall surfaces of the skirt sections, and inner wall surfaces of the side wall sections, wherein a bulge section that extends in a direction form a region of an outer wall surface of a side wail section adjacent to the piston body and a skirt section toward a region of the outer wall surface of the side wall section adjacent to a lateral pail of a pin hole section is provided in the side wall section.

2. The piston according to claim 1 , wherein

a groove that extends along the bulge section is formed in a portion of an inner wall surface of the side wall section corresponding to the bulge section.

3. The piston according to claim 1 or 2, wherein

when a plane that includes a pin hole central axis and a piston central axis is referred to as a pin hole vertical plane,

a part of the pin hole section in vicinity of the pin hole vertical plane and a part at a side of the piston body with respect to the pin hole central axis is referred to as a pin hole upper part,

a plane that includes the pin hole central axis and is perpendicular to the piston central axis is referred to as a pin hole lateral plane, a part of the pin hole section in the vicinity of the pin hole lateral plane is referred to as a pin hole lateral part, and

a part of the pin hole section that is located in a generally middle part between the pin hole upper part and the pinhole lateral part is refened to as a pin hole oblique upper part, the bulge section extends generally straight from a region of an outer wall surface of the side wall section adjacent to the piston body and the skirt section toward a region of the outer wall surface of the side wall section adjacent to the pin hole oblique upper part.

4. A method of manufacturing a piston with a mold, the piston comprising:

a cylindrical piston body;

a pair of generally partially annular skirt sections that extends from a bottom wall surface of the piston body to a lower side in parallel with a central axis of the piston body; and a pair of flat shaped side wall sections that extends from the bottom wall surface of the piston body to the lower side in parallel widi the central axis of the piston body and that connects the skirt sections each other;

a cavity being formed by the bottom wall surface of the piston body, inner peripheral wall surfaces of the skirt sections, and inner wall surfaces of the side wall sections, and thickness of a lower part of the skirt section being thicker than thickness of an upper part of the skirt section,

wherein the cavity is formed in the piston by using:

a first core that defines an inner wall surface of a first side wall section, a portion of the bottom wall surface of the piston body in the vicinity of the inner wall surface, and a portion of the inner peripheral wall sui faces of both skirt sections in the vicinity of the inner wall surface of the first side wall section;

a second core that defines an inner wall surface of a second side wall section, a portion of the bottom wall surface of the piston body in the vicinity of the inner wall surface, and a portion of the inner peripheral wall surfaces of both skirt sections in the vicinity of the inner wall surface of the second side wall section; a third core that defines a portion of the bottom wall surface of the piston body that is not defined by the first core and the second core, and a portion of the inner peripheral wall surfaces of the first skirt section that is not defined by the first core and the second core, the third core being arranged between the first core and the second core;

a fourth core that defines a portion of the bottom wall surface of the piston body that is not defined by the first core and the second core, and a portion of the inner peripheral wall surfaces of the second skirt section that is not defined by the first core and the second core, the fourth core being arranged between the first core and the second core; and

a fifth core that defines a portion of the bottom wall surface of the piston body that is not defined by the first core, the second core, the third core, and the fourth core, the fifth core being airanged among the first core, the second core, the third core, and the fourth core.

5. The method according to cJajm 4, wherein

after the cavity is formed in the piston by using the first core, the second core, the third core, the fourth core, and the fifth core, first the fifth core is removed, then the third core and the fourth core are removed, and substantially the first core and the second core are removed.

6. A piston of an internal combustion engine, comprising;

a piston body;

a first skirt section that is provided in a lower part of the piston body;

a second skirt section that is opposed to the first skirt section;

a first side wall section that is provided in a lower part of the piston body and between the first skirt section and the second skirt section; and

a second side wall section that is provided in a lower part of the piston body and between the first skirt section and the second skirt section and that is opposed to the first side wall section,

wherein the first and the second side wall sections include:

a pin hole section;

a first bulge section that is provided between a corner of the piston body and the first skirt section and in the pin hole section; and

a second bulge section that is provided between a corner of the piston body and the second skirt section and in the pin hole section.

7. The piston according to claim 6, wherein

the first and the second bulge sections have a linear shape.

8. The piston according to claim 7, wherein

the first and the second bulge sections have a straight line shape.

9. The piston according to any one of claims 6 to 8, wherein

the first and the second bulge sections are raised outward of the piston.

10. The piston according to any one of claims 6 to 9, wherein

thicknesses of the first and the second bulge sections are the same as the thicknesses of the peripheries thereof.

1 1. The piston according to any one of claims 6 to 10, wherein

thicknesses of the first and the second bulge sections arc uniform.

12. The piston according to any one of claims 6 to 1 1 , wherein

respective angles between a vertical plane with respect to an axis of the pin hole section and the first and the second side wall sections are smaller in a lower part of the piston.

13. The piston according to any one of claims 6 to 12, wherein thicknesses of a middle part in a height direction of the first and the second skirt sections are thicker than thicknesses of an upper part and a lower part in the height direction.

14. The piston according to any one of claims 6 to 13, wherein

a portion surrounded by the first and the second side wall sections, the piston body, and the pin hole section is recessed.