Pistons for combustion engines

Abstract

Piston (7) for internal combustion engines, comprising: a piston body (9) adapted to reciprocate relative to a wall (6a) of a cylinder bore (6); a lower portion (10, 11) depending from the piston body (9); a piston pin boss portion (14, 15) for holding a piston pin (16); a side wall connected to the base section (10, 11); and a resin coating film layer (39) provided on a surface opposite to the wall (6a) of the cylinder bore (6), wherein the lower section (10, 11) comprises: a central lower section (37) having a central portion with respect to a direction of the central axis (C) of the piston body (9), the central portion having a largest outer diameter; an upper bottom portion (36) curved with an outer diameter that gradually decreases from an upper boundary of the middle bottom portion (37) toward the central axis (C) as the upper bottom portion (36) extends upward from the upper boundary; and a lower base portion (38) curved with an outer diameter that gradually decreases from a lower boundary of the central base portion (37) toward the central axis (C) as the lower base portion (38) extends downward from the lower boundary, wherein the lower section (10, 11) is formed with a curvature which increases in the circumferential direction from a central circumferential section (10c, 11c) of the lower section (10, 11) towards a connecting section to a side wall section (12, 13), a set of recesses (41; 42; 61; 62; 63; 64), each formed to hold oil, having an outer edge at the resin coating film layer (39) and a bottom as a non-coating area free from the resin coating film layer (39), wherein the recesses (41; 42; 61; 62; 63; 64) each comprise a large-area portion (41A; 42A; 61A; 62A; 63A; 64A) and a pair of small-area portions (41B, 41C; 42B, 42C; 61B, 61C; 62B, 62C; 63B, 63C; 64B, 64C) which protrude from the large-area portion (41A; 42A; 61A; 62A; 63A; 64A) such that the large-area portion (41A; 42A; 61A; 62A; 63A; 64A) is inserted therebetween, and which have a smaller area than the large-area portion (41A; 42A; 61A; 62A; 63A; 64A).

Description

BACKGROUND OF THE INVENTIONTechnical field

[0001] This invention relates to pistons for internal combustion engines, and more particularly to pistons for internal combustion engines including a base portion adapted to slide relative to a wall of a cylinder bore with a resin coating therebetween. State of the art

[0002] As a piston configured to reciprocate relative to a wall of a cylinder bore in an internal combustion engine, one disclosed in Japanese Patent JP 4 749 398 B2 is known. This piston includes a bottom body having a pair of bottom portions depending therefrom, a pair of sidewall portions connecting the paired bottom portions, and a pair of piston pin boss portions provided on the paired sidewall portions for supporting a piston pin.

[0003] The bottom sections are combined in a barrel shape with an axially central section having a largest outer diameter, taking into account the thermal expansion effect. A resin coating film layer is formed on each bottom section. The resin coating film layer has oil-accumulating recesses arranged at predetermined intervals across its entire surface to retain oil in the recesses, thereby reducing sliding resistance.

[0004] US 2009 / 0025549 A1 discloses a piston for an internal combustion engine equipped with a skirt whose sliding surface is coated with an oil-repellent resin layer. A solid lubricating resin layer and the oil-repellent resin layer are exposed on the sliding surface. The oil-repellent resin layer extends inclined on the sliding surface with respect to the centerline of the sliding surface in the circumferential direction of the piston.

[0005] DE 10 2014 204 463 A1 discloses a piston for an internal combustion engine which can reduce a sliding resistance of the piston by lubricating oil when a load applied in an axial direction of the piston is small.

[0006] DE 197 08 252 A1 discloses a piston for an internal combustion engine, which has an upper ring land with piston ring recesses and a lower piston skirt with a piston bore. A slot at the bottom of a piston ring recess establishes a fluid connection between the inside and outside of the piston.

[0007] JP 2014-092124 A discloses a piston for an internal combustion engine. The piston comprises a bottom portion with an outer peripheral surface having an elliptical shape. Resin coating layers are formed on the outer peripheral surface of the bottom portion, some of which have a smooth surface and some of which contain recesses.

[0008] JP H11-269508 A discloses a multilayer sintered sliding element for improving wear resistance in a bearing. On the surface of a flat, plate-like steel backing plate, projections and continuous depressions are formed, allowing lubricant to flow evenly over the surface. SUMMARY OF THE INVENTION

[0009] Thus, in the past, the piston for internal combustion engines includes a bottom portion formed in a barrel shape having clearances to a wall of a cylinder bore that are small in a portion having a central portion with respect to an axial direction of a piston body, the central portion having a largest outer diameter, and that are large in portions located above and below the central portion with respect to the axial direction of the piston body.

[0010] Therefore, when the piston moves back and forth, it is subjected to a portion of the combustion pressures by the piston pin, and the bottom portion in contact with the cylinder bore wall has on one surface those areas that absorb high surface pressures and those areas that absorb low surface pressures.

[0011] When depressions are formed, their depths trap oil in areas that receive high surface pressures, failing to supply sufficient oil to the cylinder bore wall, potentially causing lubrication problems.

[0012] Particularly in areas that include small gaps between the bottom portion and the cylinder bore wall, there is a possibility that the bottom portion and the cylinder bore wall have a small amount of oil interposed therebetween.

[0013] Furthermore, in areas associated with high contact pressures between the base section and the cylinder bore wall, there is a possibility that high compressive forces are present at the base section and the cylinder bore wall, forcing oil out of spaces between them, resulting in even more oil missing.

[0014] As a result, there is a concern that lubrication between the bottom portion and the cylinder bore wall is insufficient, causing an increase in frictional forces between the bottom portion and the cylinder bore wall, thereby deteriorating fuel economy.

[0015] This invention was designed with such problems in mind.

[0016] The object of this invention is to provide a piston for internal combustion engines which enables improved lubrication between a piston and a wall of a cylinder bore.

[0017] To achieve the object, according to aspects of this invention, a piston for internal combustion engines is provided, comprising: a piston body configured to reciprocate relative to a cylinder bore wall, a bottom portion suspended from the piston body, a piston pin boss portion for supporting a piston pin, a side wall connected to the bottom portion, and a resin coating film layer provided on a surface opposite to the cylinder bore wall, wherein the bottom portion comprises: a middle bottom portion having a middle portion with respect to a direction of a center axis of the piston body, the middle portion having a largest outer diameter, an upper bottom portion curved with an outer diameter that gradually decreases from an upper boundary of the middle bottom portion toward the center axis,when the upper bottom portion extends upward from the upper boundary, and a lower bottom portion curved with an outer diameter that gradually decreases from a lower boundary of the middle bottom portion toward the central axis when the lower bottom portion extends downward from the lower boundary, the bottom portion being formed to have a curvature that increases in the circumferential direction from a middle circumferential portion of the bottom portion toward a connecting portion to the side wall portion, a set of recesses each configured to hold oil, having an outer edge at the resin coating film layer and a bottom as a non-coating area free from the resin coating film layer, the recesses each comprising a large-area portion and a pair of small-area portions protruding from the large-area portion,that the large-area section is inserted therebetween, and which are formed with a smaller area than the large-area section.,

[0018] According to aspects of this invention, improved lubrication between a piston and a wall of a cylinder bore can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of an internal combustion engine as a figure showing a piston for internal combustion engines according to a first embodiment of this invention. Fig. 2 is a left-side sectional view of an essential portion of a cylinder bore with a piston fitted therein as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 3 is a rear view (ie, a view from the side of a driver's seat in a subject vehicle) of the piston as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 4 is a bottom view of the piston as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 5 is a sectional view along a Fig. 3 is a sectional view of an essential portion of the cylinder bore with the piston fitted therein, shown in arrowed section plane IV-IV (i.e., a right-hand side sectional view), as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 6 is a rear view of the piston combined with a diagram showing positional relationships of curved planes at each bottom portion of the piston, as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 7 is an outline diagram of a vertically elongated recess formed at a bottom portion of the piston in a resin coating layer as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 8 is a rear view of the piston combined with a clearance distribution map showing positional relationships of various clearances that the piston has at each bottom portion relative to a corresponding wall surface of the cylinder bore, as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 9 is a rear view of the piston combined with a pressure distribution map showing positional relationships of various contact pressures that the piston experiences at each bottom portion with an associated wall surface of the cylinder bore, as a figure showing a piston for internal combustion engines according to the first embodiment of this invention. Fig. 10 is a graph illustrating a level of dynamic pressure of oil generated from a vertically elongated groove and a level of dynamic pressure of oil generated from a transversely elongated groove, as a figure showing a combination of operating characteristics of pistons for internal combustion engines according to the first embodiment of this invention and a modification thereof. Fig. 11 is a graph illustrating a relationship between an oil film thickness and a ratio of a radius of a large circle portion to a center-to-center distance between the large circle portion and a small circle portion, as a figure showing operating characteristics of a piston for internal combustion engines according to the first embodiment of this invention. Fig. 12 is a graph comparing a relationship of a rotational speed of an engine to a frictional force between a wall of a cylinder bore and bottom portions of the piston having recesses with that of a conventional piston having round grooves, as a figure showing operating characteristics of a piston for internal combustion engines according to the first embodiment of this invention. Fig. 13 is a rear view of a piston having resin coating film layers with laterally elongated recesses formed therein, as a figure showing a piston for internal combustion engines according to a first modification of the first embodiment of this invention. Fig. 14 is an outline diagram of a transversely elongated recess formed in a resin coating layer at a bottom portion of a piston, as a figure showing a piston for internal combustion engines according to the first modification of the first embodiment of this invention. Fig. 15 is a rear view of a piston including resin coating film layers each having a combination of vertically elongated recesses and transversely elongated recesses formed therein, as a figure showing a piston for internal combustion engines according to a second modification of the first embodiment of this invention. Fig. 16 is a rear view of a piston including resin coating film layers each having a combination of vertically elongated recesses and vertical grooves formed therein, as a figure showing a piston for internal combustion engines according to a second embodiment of this invention. Fig. 17 is a diagram showing clearances between a bottom portion and a wall of a cylinder bore and positional relationships between recesses and vertical recesses, as a figure showing a piston for internal combustion engines according to the second embodiment of this invention. Fig. 18 is a diagram showing flows of oil when the piston is moved upward, as a figure showing a piston for internal combustion engines according to the second embodiment of this invention. Fig. 19 is a diagram showing flows of oil when the piston is moved downward, as a figure showing a piston for internal combustion engines according to the second embodiment of this invention. Fig. 20 is an external view of a resin coating film layer as a figure showing a piston for internal combustion engines according to another embodiment of this invention. Fig. 21 is an external view of a resin coating film layer as a figure showing a piston for internal combustion engines according to another embodiment of this invention. Fig. 22 is an external view of a resin coating film layer as a figure showing a piston for internal combustion engines according to another embodiment of this invention. Fig. 23 is an external view of a resin coating film layer as a figure showing a piston for internal combustion engines according to another embodiment of this invention. DESCRIPTION OF EMBODIMENTS

[0019] Pistons for internal combustion engines according to embodiments of this invention will be described with reference to the drawings. It should be noted that the Fig. 1 to 6, Fig. 8, Fig. 9, Fig. 13 and the Fig. 15 to 19 in the drawings each include a combination of forward, backward, rightward and / or upward directions marked with arrows indicating respective directions of the longitudinal direction of the vehicle, the transverse direction of the vehicle and the vertical direction of the vehicle, which are indicated from a viewpoint of a driver's seat in a respective vehicle. First embodiment

[0020] The Fig. 1 to 15 illustrate characteristics of a piston according to a first embodiment of this invention.

[0021] The configuration of a corresponding combustion engine is now described.

[0022] In Fig. 1 shows an engine 1 as the associated internal combustion engine installed in a vehicle of interest. The engine 1 is composed of a cylinder block 3 having a crankcase 2 integrated therewith, and a cylinder head 4 provided at an upper portion of the cylinder block 3.

[0023] The cylinder block 3 includes a set of cylinder bores 6 arranged in a transverse direction of the vehicle, each having a piston 7 received therein. The piston 7 is made of an aluminum alloy or the like. The piston 7 is designed to reciprocate vertically relative to a corresponding cylinder bore 6.

[0024] The entire pistons 7, which are accommodated in the cylinder bores 6, are connected to a common crankshaft 5 by means of one-to-one corresponding connecting rods 8 in order to achieve a conversion of reciprocating movements of the pistons 7 by the connecting rods 8 into rotary movements of the crankshaft 5.

[0025] Here, the engine 1 has a predetermined number of cylinders, which corresponds in number to the cylinder bores 6. Assuming that the number of cylinders is four, the engine 1 has four cylinder bores 6. According to the embodiments described here, the engine 1 is assumed to have four cylinders, but the number of cylinders of the engine 1 is not limited to four. Furthermore, the engine 1 may also consist of another type of engine, such as a gasoline engine or a diesel engine. Furthermore, the type of the engine 1 is not limited to these.

[0026] As in Fig. 2, which is a sectional view from the left side of any cylinder bore 6, or in Fig. 5, which is a sectional view from the right side thereof, each piston 7 has a piston head portion 9 (see also the Fig. 3 and Fig. 4) configured to reciprocate vertically relative to an entire circumference of a wall 6a, which is an inner peripheral wall of the cylinder bore 6, and a pair of front and rear skirt portions 11 and 10 depending from the piston head portion 9. Here, the piston head portion 9, according to embodiments described herein, constitutes a piston body according to this invention.

[0027] As in Fig. As shown in Figure 4, which is a bottom view of any piston 7, each piston 7 includes a pair of left and right side wall portions 13 and 12 continuously connected to respective left and right sections 11a and 10a and right sections 11b and 10b of the front and rear bottom sections 11 and 10, respectively. Furthermore, each piston 7 includes a pair of left and right piston pin boss portions 15 and 14 provided at respective middle sections of the paired left and right side wall portions 13 and 12 to support a piston pin 16 extending in a transverse direction of the vehicle (see Fig. 1) to be held together in such a way that it can rotate around a central axis C1 (see also the Fig. 2 and Fig. 3).

[0028] As from Fig. 1, the piston pin 16 is formed in a cylindrical shape, and as shown in the Fig. 3, Fig. 4 or Fig. 5, the central axis C1 of the piston pin 16 extends in a direction perpendicular to a central axis C of the piston head portion 9.

[0029] Here, the left sections 11a and 10a of the front and rear base sections 11 and 10 according to embodiments described herein each correspond to one side in a circumferential direction of a base section according to this invention, and the right sections 11b and 10b of the front and rear base sections 11 and 10 each correspond to the other side (i.e., a side opposite to one side) in the circumferential direction of the base section according to this invention. It should be noted that the left sections 11a and 10a, as well as the right sections 11b and 10b, each have a predetermined circumferential length or width from a corresponding one of the left ends or the right ends of the front and rear base sections 11 and 10, respectively, as shown in FIGS. Fig. 2, Fig. 4 or Fig. 5 shown.

[0030] As in the Fig. 2, Fig. 4 or Fig. 5, the left and right piston pin boss portions 15 and 14, respectively, have left and right piston pin mounting holes 15A and 14A for the piston pin 16 to be fitted therein. The piston pin 16 is inserted through the left and right piston pin mounting holes 15A and 14A and is held by the left and right piston pin boss portions 15 and 14.

[0031] As from Fig. As can be seen in Figure 1, each piston 16 on the engine 1 is operatively connected to a small-diameter portion 8A of a corresponding connecting rod 8, and a large-diameter portion 8B of the connecting rod 8 is operatively connected to the crankshaft 5. Reciprocating movements of the piston 7 are thereby converted into rotational movements of the crankshaft 5.

[0032] As from Fig. As can be seen in Figure 1, a set of intake ports 21 is formed in the cylinder head 4 on the engine 1 for individual fluid communication with the cylinder bores 6. Each cylinder bore 6 has a combustion chamber 18 defined between an upper portion of a wall 6a and an associated piston 7. Intake air streams can be introduced into the combustion chamber 18 through an associated intake port 12.

[0033] A set of exhaust ports 22 is formed in the cylinder head 4 of the engine 1 for individual fluid communication with the cylinder bores 6. Exhaust gas streams generated in the combustion chamber 18 and discharged therefrom through an associated exhaust port 22 may be present at each cylinder bore 6.

[0034] As from Fig. 1, the cylinder head 4 on the engine 1 is formed with a combination of an intake camshaft 23 carrying a set of intake cams 23A, and an exhaust camshaft 24 carrying a set of exhaust cams 24A. Furthermore, each cylinder bore 6 on the cylinder head 4 is provided with a combination of an intake valve 25 actuated by means of an associated intake cam 23A to establish or break fluid communication with the combustion chamber 18, and an exhaust valve 26 actuated by means of an associated exhaust cam 23A to establish or break fluid communication with the combustion chamber 18.

[0035] As in the Fig. 2, Fig. 3 or Fig. 5, in the piston head portion 9 on each piston 7, three ring grooves are formed at an outer edge region, which are a first piston ring groove 31, a second piston ring groove 32 and an oil ring groove 33 in this order from the top.

[0036] An unillustrated annular first and second piston rings are fitted into the first piston ring groove 31 and the second piston ring groove 32 on each piston 7, and an unillustrated annular oil ring is fitted into the oil ring groove 33 as a piston ring.

[0037] The first and second piston rings each have a function of forming contact with a portion of a wall 6a of an associated cylinder bore 6, whereby the combustion chamber 18 is well sealed.

[0038] The oil ring has a function of forming contact with a portion of the wall 6a of the cylinder bore 6, moving in accordance with a reciprocating movement of the piston 7, thereby scraping off oil adhering to the portion of the wall 6a.

[0039] As further stated in the Fig. 2 or Fig. 3, the oil ring groove 33 on each piston 7 has paired sets of oil return holes 34 formed in a (radially inward) bottom. In particular, one set of oil return holes 34 (four locations, see Fig. 3) on each of a pressure side and a counterpressure side (ie from one and the other side pressure direction before and after a top dead center, in particular from the front and the back in Fig. 2) of the piston head portion 9, thus forming a total of two sets (four x 2 = eight locations). The oil return holes 34 each have open ends at the bottom of the oil ring groove 33a and an inner peripheral region of the piston head portion 9 for fluid communication therebetween.

[0040] Here, the term 'pressure side' refers to a side portion of the piston 7 to which force is exerted in a stroke that goes downwards in the cylinder bore 6 from the top dead center, so that due to a torque of the crankshaft 5, forces are present that act in a pressure direction perpendicular to an axial direction of the crankshaft 5 on an associated side region of the wall 6a of the cylinder bore 6.

[0041] Furthermore, the term 'counter-pressure side' means an opposite side portion of the piston 7 to which force is exerted in a stroke that goes upwards in the cylinder bore 6 in the direction of the dead center, so that due to a torque of the crankshaft, forces are present that act in a pressure direction that is opposite to the above pressure direction on an associated opposite side region of the wall 6a of the cylinder bore 6.

[0042] As from Fig. 1, each cylinder bore 6 has a spatial area defined by the wall 6a and an associated piston 7 and between them, into which oil is supplied from an oil jet hole 8a provided through a large diameter portion 8B of an associated connecting rod 8. As a result, as shown in Fig. 2, the cylinder bore 6 has an oil film 35 formed over an entire circumference of the wall 6a for outer peripheral areas of the piston 7 (particularly the piston head portion 9) to be brought into contact therewith.

[0043] Such oil supply serves to cool the piston 7 and to lubricate between outer peripheral portions of the piston 7 and the wall 6a of the cylinder bore 6. It should be noted that elements other than the oil jet holes 8a may also be provided which are used for such oil introduction.

[0044] The engine 1 includes an unillustrated oil pan provided in a lower portion of the crankcase 2, the oil pan communicating with each cylinder bore 6. When a corresponding piston 7 reciprocates vertically in the cylinder bore 6, the oil ring fitted in the oil ring groove 33 of the piston 7 scrapes oil adhering to the wall 6a of the cylinder bore 6. Flows of scraped oil can be guided through oil return holes 34 at the bottom of the oil ring groove 33 and discharged to inner peripheral regions in the piston head portion 9 of the piston 7, thereby returning to the oil pan through spaces between the front and rear skirt portions 11 and 10 of the piston 7.

[0045] As from Fig. 2, the front and rear base sections 11 and 10 on each piston 7 are designed to come into contact with an associated cylinder bore 6 (in particular, with the wall 6a) on the pressure side or the counter-pressure side when the piston 7 reciprocates, thereby having a function of preventing oscillatory movements of the piston 7.

[0046] As in Fig. 3, which is a rear view of any one piston 7, the rear bottom portion 10 on each piston 7 is formed substantially in a rectangular shape (specifically, a rectangular elongated shape) having a combination of two narrow sides extending parallel to the central axis C of the piston head portion 9 and two long sides extending parallel to a direction (e.g., a direction in which the axis C1 of the piston pin 16 extends) perpendicular to the central axis C of the piston head portion 9.

[0047] In addition, the front bottom portion 11 of the piston 7 is formed in a substantially rectangular shape (specifically, a rectangular elongated shape) similar to that of the rear bottom portion 10, the shape having a combination of two narrow sides and two long sides.

[0048] As in the Fig. 5 or Fig. 6, the front and rear bottom sections 11 and 10 on each piston 7 have, with respect to a direction in which the central axis C (i.e., an axial direction) of the piston head section 9 extends, intermediate portions that collectively form a bottom center section 37 of the piston 7. This bottom center section 37 has a maximum outer diameter. The bottom center section 37 has a combination of arcuate outer peripheral portions (corresponding to those of the front and rear bottom sections 11 and 10) that partially form a right cylindrical shape having an axis extending parallel to the central axis C of the piston head section 9. Here, the central axis C of the piston head section 9 overlaps with a central axis of a combination of the front and rear bottom sections 11 and 10 (i.e., they are located on an identical straight line).

[0049] Furthermore, the front and rear bottom sections 11 and 10 on each piston 7 have upper portions located at levels above an upper boundary (specifically, an imaginary upper boundary plane) 37a of the central bottom section 37, and together form an upper bottom section 36 of the piston 7. This upper bottom section 36 has a combination of arcuate outer peripheral portions (corresponding to those of the front and rear bottom sections 11 and 10) partially forming an inverted bowl shape curved with an outer diameter that gradually decreases in an extension direction from the upper boundary 37a upward from an outer periphery of the upper boundary 37a of the central bottom section 37 toward the central axis C of the piston head section 9.

[0050] It should be noted that the upper boundary 37a of the middle bottom portion 37 is located at a level which is below the center axis C1 of the piston pin 16 with respect to a direction in which the center axis C of the piston head portion 9 extends.

[0051] Furthermore, the front and rear bottom portions 11 and 10 of each piston 7 have lower sections located at levels below a lower boundary (specifically, an imaginary lower boundary plane) 37b of the central bottom portion 37, and collectively form a lower bottom portion 38 of the piston 7. This lower bottom portion 38 has a combination of arcuate outer peripheral sections (corresponding to those of the front and rear bottom portions 11 and 10) partially forming a bowl shape curved with an outer diameter gradually decreasing in an extension direction from the upper boundary 37a upward from an outer periphery of the upper boundary 37a of the central bottom portion 37 toward the center axis C of the piston head portion 9.

[0052] Fig. 6 shows, in a graphical representation on the right, a profile of a barrel shape defined by a combination of three bottom sections, described as the upper bottom section 36, the middle bottom section 37, and the lower bottom section 38. In this graphical representation, the horizontal axis represents a decrease in the diameter of the upper or lower bottom section 36 or 38 relative to the middle bottom section 37, and the vertical axis represents a distance in the vertical direction from a lower end of the combination of the bottom sections 36, 37, and 38 (specifically, from a lower bottom of the barrel shape).

[0053] In other words, each piston 7 has a combination of a front and a rear bottom section 11 and 10, which form a barrel-shaped section of the piston 7.

[0054] It should be noted that the front and rear lower sections 11 and 10 in the Fig. 5 and Fig. 6 are shown as being significantly curved for the sake of simplicity of description.

[0055] As in Fig. 4, the front and rear bottom sections 11 and 10 on each piston 7 have greater curvatures in a circumferential extension direction from their central circumferential sections 11c and 10c toward their end sections (specifically, their left sections 11a and 10a and their right sections 11b and 10b) connected to the left and right side wall sections 13 and 12, respectively.

[0056] Here, the central circumferential portions 11c and 10c form crests on circumferences (i.e., radially most protruding portions) of the front and rear bottom sections 11 and 10, respectively connecting a front or rear end portion of the left side wall section 13 and a front or rear end portion of the right side wall section 12. Accordingly, on the barrel-shaped portion, which is a combination of the front and rear bottom sections 11 and 10, the outer periphery has a smallest clearance in a circumferential extension direction at the central circumferential portions 11c and 10c relative to the wall 6a of a corresponding cylinder bore 6.

[0057] As in the Fig. 2, Fig. 3 or Fig. As shown in Fig. 4, the front and rear bottom sections 11 and 10 of each piston 7 have front and rear resin coating film layers 39 formed on their surfaces with a predetermined thickness, for example, by a screening method. The resin coating film layers 39 have low frictional resistance and high heat resistance. The front and rear resin coating film layers 39 are formed over respective front and rear surface areas of the upper bottom section 36, the middle bottom section 37, and the lower bottom section 38, which face the wall 6a of a corresponding cylinder bore 6.

[0058] The front and rear resin coating film layers 39 each include an upper end 39a as an upper edge extending along an upper edge of the upper base portion 36, and a lower end 39b as a lower edge extending along a lower edge of the lower base portion 38.

[0059] As in Fig. As shown in Figure 3, the front and rear resin coating film layers 39 on each piston 7 each have a set of recesses 41 formed as holes therein so as to be straight in the depth direction. Each recess 41 has a depth set within a range between 5 µm or more and 20 µm or less so as to be equal to the thickness of the front and rear resin coating film layers 39. As shown in Fig. As shown in Figure 7, each recess 41 is composed of a central large circular portion 41A and a pair of upper and lower small circular portions 41B and 41C. The circular portions 41A, 41B, and 41C are defined by their circular arcuate edges.

[0060] As in Fig. 7, the central large circular portion 41A of each recess 41 has a recess wall defined by a pair of left and right arc segments of a circle described by a radius R. The upper and lower small circular portions 41B and 41C have recess walls each defined by an upper and lower arc segment of a circle described by a radius smaller than the radius R. The small circular portions 41B and 41C have a cross-sectional area smaller than a cross-sectional area of ​​the large circular portion 41A. The upper and lower arc segments defining the upper and lower small circular portions 41B and 41C have chords each overlapping with an upper and lower chord each connecting the left and right arc segments defining the large circular portion 41A.The small circular sections 41 B and 41 C are arranged parallel to the central axis C of an associated piston head section 9.

[0061] As a result, the recess walls of the small circular sections 41B and 41C, which are defined by the upper and lower arc segments, enter Fig. 7 protrude vertically (i.e., parallel to the central axis C of the piston head portion 9) in mutually opposite directions from the recess wall of the large circular portion 41A defined by the left and right arc segments, having spatial regions in the three recess walls that communicate with each other. Here, the central large circular portion 41A, according to embodiments described herein, forms a large surface portion according to this invention, and the upper and lower small circular portions 41B and 41C form a pair of small surface portions according to this invention.

[0062] The upper and lower small circular portions 41B and 41C at each recess 41 have centers C3 and C4 (corresponding to inscribed circle centers of the small surface portions according to this invention) spaced from a center C2 of the central large circular portion 41A (corresponding to an inscribed circle center of the large surface portion according to this invention) by a distance L within a range of 0.5 times or more to 0.875 times or less of the radius R of the large circular portion 41A. That is, the centers C3 and C4 of the small circular portions 41B and 41C are spaced from the center C2 of the large circular portion 41A by a distance L set to be smaller than the radius R of the large circular portion 41A.

[0063] As in Fig. 8, on each piston 7, the front and rear bottom sections 11 and 10, which form a barrel-shaped section, on the other hand, have clearances to the wall 6a of an associated cylinder bore 6, which include most enlarged clearances at respective combinations of four corner regions which are the upper left and upper right end sections and the lower left and lower right end sections of the front and rear bottom sections 11 and 10, and most reduced clearances at respective middle regions which are middle sections with respect to the vertical direction and the transverse direction of the front and rear bottom sections 11 and 10.

[0064] The front and rear lower sections 11 and 10 each have, via an intermediate region which is an intermediate portion extending between a central region and a combination of four corner regions, a distribution of gaps to the wall 6a of the cylinder bore 6 which are set to be larger than gaps in the central region and smaller than gaps in the combination of four corner regions.

[0065] In other words, as shown by a gap distribution pattern as a map in the lower part of the Fig. 8, the front and rear base sections 11 and 10 each consist of a large gap region 51 which is a connecting combination of four corner regions thereof, a small gap region 52 which is a middle connecting region thereof, and a middle gap region 53 which is an intermediate connecting region extending between the middle region and the combination of four corner regions.

[0066] It should be noted that the map in Fig. 8 is created as an overlapping rear view of the front and rear lower sections 11 and 10 from the driver's seat. In this map, the front lower section 11 has left and right edges, depicted as end sections 11L and 11R, which respectively overlap with left and right edges, depicted as end sections 10L and 10R, of the right lower section 10.

[0067] As in Fig. 3, the rear base section 10 on each piston 7 has a set of recesses 41 arranged in the central peripheral section 10c of the base section 10, parallel to the central axis C of the piston head section 9.

[0068] Each recess 41 includes a combination of a central large circular portion 41A and upper and lower small circular portions 41B and 41C arranged in one direction. Defining this arrangement direction as an elongated direction of the recess 41, each recess 41 is arranged to have the elongated direction parallel to the center axis C of the piston head portion 9 of the piston 7, which extends in the vertical direction of the vehicle in the figure.

[0069] Namely, on each piston 7, each recess 41 formed in the rear bottom portion 10 represents, according to embodiments described herein, a vertically elongated arrangement type having an elongated direction parallel to the central axis C of the piston head portion 9.

[0070] It should be noted that a set of recesses 41 is also formed in the front base portion 11, similarly to the rear base portion 10. Accordingly, a description will now be given of recesses 41 formed in the rear base portion 10.

[0071] The rear bottom portion 10 on each piston 7 has a density of recesses, as the number of recesses 41 per unit area, which is set to be greater at the middle bottom portion 37 than at the upper bottom portion 36 or the lower bottom portion 38, and particularly to be greatest near the center axis C of the piston head portion 9 in the middle bottom portion 37. As a result, the bottom portion 10 has recesses 41 concentrated in the small-space area 52.

[0072] The bottom section 10 has gradually decreasing densities of recesses in an extension direction further away from the central circumferential section 10c (i.e., in an extension direction toward the left and right sidewall sections 13 and 12). According to embodiments described herein, the bottom section 10 has sharper curvatures in a circumferential extension direction from the central circumferential section 10c toward connecting sections to the left and right sidewall sections 13 and 12.

[0073] As a result, the lower section 10 has gradually reduced densities of recesses in an extension direction from the region 52 with small gaps to the region 51 with large gaps. As shown in Fig. 6, the bottom portion 10 further includes a set of recesses 41 arranged in an inhomogeneous matrix including seven rows in the transverse direction and seven columns in the vertical direction (specifically, in a symplectic matrix of five rows by five columns except for elements at four vertices). Those recesses 41 arranged in the transverse rows are arranged, in a rear view of the bottom portion 10, so as to avoid overlapping with the central axis C1 of the piston pin 16 or the upper boundary 37a or the lower boundary 37b of the central bottom portion 37. In other words, those recesses 41 located near the central axis C1 of the piston pin 16 are arranged so that the central axis C1 of the piston pin 16, the upper boundary 37a, and the lower boundary 37b are interposed therebetween.

[0074] The front and rear resin coating layers 39 on each piston 7 are formed by applying a paint coating containing PAI (polyamideimide) and molybdenum disulfide as main components to the front and rear base sections 11 and 10 using, for example, a screen printing device, while a masking process is performed on the front and rear base sections 11 and 10 using a set of masks identical in shape to the respective recesses 41, each including a large circle section 41A and small circle sections 41B and 41C, so as to provide, on surfaces of the front and rear base sections 11 and 10, a combination of a front and a rear set of recesses 41 designed to hold oil flows.As a result, each recess 41 has a wall defined by a resin coating film and a bottom defined by an uncoated portion without a resin coating film layer.

[0075] Now follows a description of frictional forces that develop between each piston 7 and the wall 6a of an associated cylinder bore 6.

[0076] The front and rear skirt portions 11 and 10 of each piston 7, when viewed from behind, have circumferential areas that overlap with the left and right piston pin boss portions 15 and 14 or extensions thereof. Such areas exhibit increased contact pressures with the wall 6a of a corresponding cylinder bore 6. The reason for this is as follows.

[0077] On the piston 7, which moves back and forth in the cylinder bore 6, a part of the combustion pressures is applied through the connecting rod 8 and the piston pin 16, which then, when the front bottom section 11 or the rear bottom section 10 comes into contact with the wall 6a of the cylinder bore 6, act to make contact between portions between the piston pin 16 and the left and right piston pin boss sections 15 and 14 and to press the bottom section 11 or 10 against the wall 6a of the cylinder bore 6, with compressive forces being introduced by the contacting portions as input points.

[0078] Accompanying reciprocating movements of the piston 7, frictional forces develop between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6 when the front and rear bottom sections 11 and 10 move in the vertical direction under the exertion of compressive forces due to combustion pressures in the wall 6a of the cylinder bore 6.

[0079] Fig. 9 shows in a map at the bottom a distribution pattern of contact pressures generated between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6.

[0080] According to embodiments described herein, the front and rear bottom sections 11 and 10 are manufactured to form a barrel shape so that they are large at the central circumferential portions 11c and 10c of the front and rear bottom sections 11 and 10 in directions perpendicular to the center axis C1 of the piston pin 16 and have greater curvatures in their circumferential extension direction from the central circumferential portions 11c and 10c toward the left and right side wall portions 13 and 12, respectively.

[0081] As a result, the middle lower section 37 has the most reduced clearances to the wall 6a of the cylinder bore 6, and the front and rear lower sections 11 and 10 point in an extension direction from the middle circumferential sections 11c and 10c in the circumferential direction outwards (in Fig. 4 in particular in the circumferential direction to the left and to the right of the central circumferential sections 11c and 10c) in areas which extend outside the central lower section 37 with respect to a direction in which the central axis C of the piston head section 9 extends (in Fig. 6 (in particular upwards from the upper limit 37a of the central lower section 37 and downwards from the lower limit 37b of the central lower section 37) reduced contact pressures to the wall 6a of the cylinder bore 6.

[0082] Accordingly, the piston 7 has a combination of a front and a rear first outer region, which are accompanied by relatively small gaps formed between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6, as well as relatively high contact pressures generated between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6, and has a combination of a front and a rear second outer region, which are accompanied by relatively large gaps formed between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6, as well as relatively low contact pressures generated between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6.The combination of the first outer areas is subject to more stringent lubrication conditions than the combination of the second outer areas.

[0083] According to the embodiments described herein, the pistons 7 are each designed for preferential lubrication to be performed under such stringent lubrication conditions. A specific method for lubricating each piston 7 is described below.

[0084] It should be noted that the front and rear base sections 11 and 10 on a respective piston 7 are identical in configuration and performance, and a description will be given of the rear base section 10. Further, with reference to a direction of movement of that piston 7, 'ahead or forward' and 'behind or rearward' are sometimes referred to as 'upstream' and 'downstream', respectively.

[0085] According to embodiments described herein, the rear bottom portion 10 on the above-mentioned piston 7 has the set of recesses 41 formed in the resin coating film layer 29 at a rear first outer region of the piston 7, which is associated with relatively small clearances formed between the bottom portion 10 and the wall 6a of the cylinder bore 6 and with relatively high contact pressures generated between the bottom portion 10 and the wall 6a of the cylinder bore 6.

[0086] In this set of recesses 41, each recess 41 is composed of a central large circular section 41A and a pair of upper and lower small circular sections 41B and 41C, which are smaller in cross-sectional area than the large circular section 41A, as shown in Fig. 7. The large circular portion 41A is inserted between the small circular portions 41B and 41C in a direction parallel to the central axis C of the piston head portion 9. This allows the recesses 41 to cause dynamic oil pressures when the piston 7 moves in the vertical direction. In view of this, each recess 41 according to embodiments described herein is composed of the large circular portion 41A having a relatively large cross-sectional area and the small circular portions 41B and 41C having a relatively small cross-sectional area. The small circular portions 41B and 41C protrude from the large circular portion 41A in the vertical direction opposite to each other, with the large circular portion 41A being inserted between them.

[0087] Fig. 10 is a graph showing a variation of a dynamic oil pressure (shown by solid lines in the figure) generated by a depression 41 having small circular segments 41B and 41C arranged vertically relative to a large circular segment 41A. The dynamic oil pressure (represented by the vertical axis of this graph) increases (i.e., becomes higher) as a piston stroke (represented by the horizontal axis of this graph) changes from downstream to upstream. Furthermore, this dynamic pressure increases as the rotational speed of the engine 1 increases.

[0088] In other words, as the piston 7 moves upward, oil flows are introduced from the upstream side (the side containing the combustion chamber 18) into the gaps between the upper bottom portion 26 and the wall 6a of the cylinder bore 6. Respective recesses 41 may each have such oil flows introduced through an upper small circular portion 41B (located upstream) into a central large circular portion 41A (located downstream of the small circular portion 41B), where oil then accumulates. If the accumulated oil is increased, oil flows may be introduced from the large circular portion 41A into a lower small circular portion 41C (located downstream of the large circular portion 41A). This process is accompanied by a significant decrease in oil flow, generating high dynamic pressures.

[0089] In particular, friction boundaries (contact surfaces of two bodies) help oil flows to generate dynamic pressures with varying magnitudes depending on the respective inner diameters of the circles, which exhibit increased film thicknesses with increasing magnitudes.

[0090] As a result, the piston 7, when moving upwards, absorbs pressures exerted thereon in directions that force the lower section 10 to move away from the wall 6a of the cylinder bore, whereby oil flows are introduced and films of increasing thickness are deposited between the wall 6a of the cylinder bore 6 and an outer region of the central lower section 37 (which is associated with relatively small gaps with the wall 6a of the cylinder bore 6 and relatively high contact pressures with the wall 6a of the cylinder bore 6).

[0091] On the other hand, as the piston 7 moves downward, oil flows are introduced from the upstream side (the side containing the crankshaft 5) into the gaps between the lower base portion 38 and the wall 6a of the cylinder bore 6. Respective recesses 41 may each have such oil flows introduced through a lower small circular portion 41C (located upstream) into a central large circular portion 41A (located downstream of the small circular portion 41C), where oil then accumulates. If the accumulated oil is increased, oil flows may be introduced from the large circular portion 41A into an upper small circular portion 41B (located downstream of the large circular portion 41A). This process is accompanied by a significant decrease in oil flow, generating high dynamic pressures.

[0092] As a result, the piston 7 has oil flows during a downward movement, which are applied with increasing thickness as a film between an outer region of the central lower section 37 and the wall 6a of the cylinder bore 6.

[0093] It is also possible to use sets of transversely elongated recesses each having a longitudinal direction (which is a direction of arrangement from small circle segments to a large circle segment) perpendicular to the central axis C of the piston head portion 9. In such a case, as shown in the graphic representation of Fig. 10 by alternating long and short dashed lines, dynamic pressures generated by a transversely elongated depression become smaller over a whole range of variation thereof than dynamic pressures generated by a vertically elongated depression 41 shown in the graphical representation of Fig. 10 is marked by solid lines.

[0094] This is because a piston stroke varying from downstream to upstream, crossing rows of transversely elongated recesses having oil flows passing through a combination of a large circular segment, a small circular segment, and a large circular segment, is subject to smaller variations in cross-sectional area than when passing through a vertically elongated recess 41.

[0095] According to embodiments described herein, a set of recesses 41 for holding oil is formed in the piston 7 in the resin coating film layer 39 on the base portion 10. The recesses 41 each consist of a large circular portion 41A and a pair of small circular portions 41B and 41C projecting from the large circular portion 41A with the large circular portion 41A sandwiched therebetween. The small circular portions 41B and 41C each have a smaller cross-sectional area than that of the large circular portion 41A.

[0096] As a result, the piston 7 can have an oil film of greater thickness between the wall 6a of the cylinder bore 6 and an outer region of the central lower section 37 (which includes relatively small gaps to the wall 6a of the cylinder bore 6 and relatively high pressures for contact formation with the wall 6a of the cylinder bore 6), thereby enabling better lubrication between the central lower section 37 and the wall 6a of the cylinder bore 6.

[0097] Furthermore, according to embodiments described herein, the small circular portions 41B and 41C in each recess 41 on the piston 7 have centers C3 and C4 set at a distance L from the center C2 of the large circular portion 41A within a range of distances between 0.5 times or more and 0.875 times or less of the radius R of the large circular portion 41A.

[0098] As a result, the piston 7 can have oil films of greater thickness between the wall 6a of the cylinder bore 6 and the front and rear lower sections 11 and 10. Fig. 11 shows, for each recess 41, a relationship between an oil film thickness and a ratio (R / L) of the radius R of the large circular portion 41A to a center-to-center distance between the center C2 of the large circular portion 41A and the centers C3 and C4 of the small circular portions 41B and 41C.

[0099] As well as from Fig. 11, the centers C3 and C4 of the small circular sections 41B and 41C may be set at a distance L from the center C2 of the large circular section 41A within a range between 0.5 times or more and 0.875 times or less of the radius of the large circular section 41A so that there is a greater oil film thickness between the front and rear bottom sections 11 and 10 and the wall 6a of the cylinder bore 6 than in the case of deviating from the range.

[0100] Furthermore, the centers C3 and C4 of the small circular segments 41B and 41C may be set at a distance L from the center C2 of the large circular segment 41A within a range between 0.5 times or more and 0.875 times or less of the radius R of the large circular segment 41A so that the centers C3 and C4 of the small circular segments 41B and 41C are closer to the center C2 of the large circular segment 41A.

[0101] This allows the sets of recesses 41 of the piston 7 to be arranged so that the dynamic pressure performance of each recess 41 (composed of a large circular segment 41A and a pair of small circular segments 41B and 41C) is not degraded, allowing individual recesses 41 to be mounted within reduced mounting areas, enabling a higher density of recesses (i.e., the number of recesses 41 per unit area). Therefore, they can generate high dynamic pressures even in a low rotational speed range of the engine 1.

[0102] Furthermore, each piston 7 according to embodiments described herein has sets of recesses 41 arranged on respective central circumferential portions 11c and 10c of the front and rear base sections 11 and 10 parallel to the central axis C of the piston head section 9.

[0103] The central circumferential sections 11c and 10c of the front and rear base sections 11 and 10 are those piston sections which are designed to always come into contact with a wall 6a of the cylinder bore 6 at relatively high contact pressures when the piston 7 moves vertically in an associated cylinder bore 6, with relatively small gaps from the wall 6a of the cylinder bore 6.

[0104] In such piston sections, a set of recesses 41 is formed in each case, which have dynamic pressures acting thereon, which allows oil films between the wall 6a of the cylinder bore 6 and the central circumferential sections 11c and 10c of the front and rear bottom sections 11 and 10 to become thicker, which enables a reduction of frictional resistance during a movement of the piston 7 in the vertical direction.

[0105] In particular, each recess 41 on each piston 7 according to embodiments described herein has a longitudinal direction parallel to the central axis C of the piston head portion 9, so that the recess 41 becomes elongated in the vertical direction, thereby forming dynamic oil pressures higher than in the case of a transversely elongated recess, which enables the thicker oil films between the wall 6a of the cylinder bore 6 and the central circumferential portions 11c and 10c of the front and rear bottom portions 11 and 10.

[0106] Furthermore, the front and rear bottom sections 11 and 10 on each piston 7 according to embodiments described herein have a density of recesses (which is the number of recesses 41 per unit area) that varies so as to be greater at the middle bottom section 37 than at each of the upper bottom section 36 and the lower bottom section 38, and in particular, is greatest in portions of outer regions of the middle bottom section 37 near locations that overlap with the central axis C of the piston head section 9 in a rear view.

[0107] This allows each set of recesses 41 to have recesses 41 concentrated in a region 52 with small gaps, allowing a thicker oil film between the wall 6a of the cylinder bore 6 and an outer region of the central base portion 27 (associated with relatively small gaps with the wall 6a of the cylinder bore 6 and relatively high contact pressures with the wall 6a of the cylinder bore 6). Therefore, the piston 7 can effectively exhibit reduced frictional resistance when moving in the vertical direction.

[0108] Furthermore, according to embodiments described here, the piston 7 has, in a rear view, rows of recesses 41 arranged such that the center axis C1 of the piston pin 16 and the upper limit 37a and the lower limit 37b of the central bottom portion 37 are separately inserted therebetween.

[0109] As a result, dynamic oil pressures can be increased by recesses 41 between the wall 6a of the cylinder bore 6 and those portions of outer regions of the front and rear lower sections 11 and 10 which, in a rear view, overlap with the center axis C1 of the piston pin 16 (which have relatively high contact pressures with the wall 6a of the cylinder bore 6).

[0110] Accordingly, thicker oil films can be present between the wall 6a of the cylinder bore 6 and those portions that overlap with the center axis C1 of the piston pin 16, enabling reduced frictional resistances between the wall 6a of the cylinder bore 6 and the front and rear bottom sections 11 and 10.

[0111] Furthermore, according to embodiments of the invention, the front and rear bottom sections 11 and 10 have densities of recesses that gradually decrease in a circumferential extension direction away from the central circumferential sections 11c and 10c.

[0112] This allows the front and rear base sections 11 and 10 to have reduced oil viscosity resistance. Specifically, each set of recesses 41 can have a function of generating high dynamic pressures to increase oil film thicknesses.

[0113] Furthermore, the front and rear bottom sections 11 and 10 have greater curvatures in an extension direction from the central peripheral sections 11c and 10c in the circumferential direction to connecting sections to the left and right side wall sections 13 and 12, and have increased clearances between the wall 6a of the cylinder bore 6 and the front and rear bottom sections 11 and 10 in an extension direction from the central peripheral sections 11c and 10c in the circumferential direction to the left and right side wall sections 13 and 12.

[0114] As a result, the front and rear lower sections 11 and 10 exhibit increased oil flows between the wall 6a of the cylinder bore 6 and the left sections 11a and 10a and the right sections 11b and 10b of the front and rear lower sections 11 and 10, which are accompanied by relatively large gaps. Therefore, for given arrangements of vertically elongated recesses 41, the left sections 11a and 10a and the right sections 11b and 10b, which are accompanied by relatively large gaps, continue to exhibit increased oil flows, causing the front and rear lower sections 11 and 10 to exhibit increased oil viscosity resistances.

[0115] Therefore, the front and rear bottom sections 11 and 10 can have gradually reduced densities of recesses in an extension direction away from the central circumferential sections 11c and 10c, thereby preventing excessive oil flows between the wall 6a of the cylinder bore 6 and the left sections 11a and 10a and the right sections 11b and 10b of the front and rear bottom sections 11 and 10, which involve relatively large clearances. As a result, the front and rear bottom sections 11 and 10 can achieve reduced oil viscosity resistances.

[0116] At Fig. 12 is data from experiments involving measuring frictional forces between a wall of a cylinder bore and base portions using resin coating film layers having uniformly formed round conventional grooves or using resin coating film layers 39 having recesses 41 formed therein according to embodiments described herein.

[0117] These experimental data include results of varying the speed of an engine to actuate a piston, using a device for appropriate evaluation of frictional forces acting on the piston. A result was obtained for a piston 7 using resin coating film layers 39 according to embodiments described herein, which includes data regarding frictional forces between a cylinder bore wall and base portions, ensuring a reduction of approximately 20% on average compared to a piston using conventional resin coating film layers.

[0118] It should be noted that each piston 7 in the engine 1 according to embodiments described herein, as shown in Fig. 13, may also use sets of transversely elongated recesses 42 (instead of the sets of vertically elongated recesses 41). In other words, each recess 42, as shown in Fig. 14, may also be composed of a central large circular portion 42A and a pair of left and right small circular portions 42B and 42C. In this case, the recess 42 has a longitudinal direction (as a direction in which a pair of small circular portions are arranged to form a large circular portion) which is shown in a rear view of the piston 8 shown in Fig. 13, is oriented perpendicular to a central axis C of the piston head portion 9, so as to form the recess 42 in a transversely elongated shape. For each set of recesses 42, the respective recesses 42 are arranged in a matrix similar to a corresponding set of vertically elongated recesses 41. When rotated 90 degrees at an arrangement position, each transversely elongated recess 42 overlaps with a corresponding vertically elongated recess 41. Transversely elongated recesses 42 are each formed in a resin coating film layer 39, similar to a corresponding vertically elongated recess 41.

[0119] When moving upwards, the Fig. 13, the piston 7 has oil flows introduced from the upstream side (the side containing the combustion chamber 18) into spaces between the upper bottom portion 36 and the wall 6a of the cylinder bore 6.

[0120] A respective row may be present that includes a focused depression 42 with such oil flows that are introduced through a depression 42 (included in a large circular section 42A) located in an upstream row of the respective row, into a central large circular section 42A and left and right small circular sections 42B and 42C of the focused depression 42, where oil then accumulates. If the accumulated oil is in an increased amount, oil flows from the large circular section 41A and the small sections 42B and 42C of the focused depression 42 may be introduced into a depression 42 (included in a large circular section 42A) located in a downstream row of the respective row. This process involves a sudden decrease in the oil flow, which generates relatively high dynamic pressures.

[0121] If this is the case, the piston 7 can have a thicker oil film between the wall 6a of the cylinder bore 6 and an outer region of the central lower section 37 (which is associated with relatively small gaps to the wall 6a of the cylinder bore 6 and relatively high pressures for contact with the wall 6a of the cylinder bore 6), thereby enabling better lubrication between the central lower section 37 and the wall 6a of the cylinder bore 6.

[0122] It should be noted that the centers C3 and C4 of the small circular portions 42B and 42C at each recess 42 on the piston 7 are also set at a distance L from a center C2 of the large circular portion 42A within a range between 0.5 times or more and 0.875 times or less of a radius R of the large circular portion 42A.

[0123] As from Fig. 10, each transversely elongated recess 42 on the piston 7 is designed to reduce oil flow from upstream of the recess 42 to downstream at a smaller rate of reduction than a corresponding vertically elongated recess 41. Accordingly, each transversely elongated recess 42 is designed to generate a lower dynamic pressure than a corresponding vertically elongated recess 41.

[0124] This property can be utilized. The thickness of an oil film can also be optimized by combining vertically elongated recesses 41 and transversely elongated recesses 42 on a piston 7.

[0125] For example, the lower section 10 in the Fig. 15 has vertically elongated recesses 41 formed on the central peripheral portion 10c (associated with small gaps between the base portion 10 and the wall 6a of the cylinder bore 6) and in regions of the upper base portion 36, the central base portion 37 and the lower base portion 38, which regions extend near the central peripheral portion 10c.

[0126] Furthermore, the lower section 10 of the piston 7 has transversely elongated recesses 42 formed in regions located away from the central peripheral section 10c in the direction of the left section 10a and the right section 10b, the regions comprising large gaps between the lower section 10 and the wall 6a of the cylinder bore 6.

[0127] Friction boundaries help oil flows generate dynamic pressures of varying magnitudes depending on the respective inner diameters, with film thicknesses increasing with increasing magnitudes, as described. Therefore, the piston 7 may have vertically elongated recesses 41 arranged to ensure favorable oil film thicknesses at locations (associated with high contact pressures between the base portion 10 and the wall 6a of the cylinder bore 6) in the small clearance region 52 (associated with small clearances between the base portion 10 and the wall 6a of the cylinder bore 6) to reduce frictional resistance between the base portion 10 and the wall 6a of the cylinder bore 6.

[0128] On the other hand, the piston 7 has a large amount of oil at friction boundaries in the large-gap region 51 (associated with large gaps between the base portion 10 and the wall 6a of the cylinder bore 6). When oil films at friction boundaries have excessive thicknesses, the piston 7 experiences oil viscosity resistance. Therefore, it is preferable to generate lower dynamic pressures in a region associated with small gaps between the base portion 10 and the wall 6a of the cylinder bore 6 than when vertically elongated recesses 41 are provided.

[0129] In this regard, the piston 7 may have a combination of vertically elongated recesses 41 and transversely elongated recesses 42 arranged in the intermediate clearance region 53 (associated with intermediate clearances between the base portion 10 and the wall 6a of the cylinder bore 6) to generate adequate dynamic pressures to prevent excessive thicknesses of oil films.

[0130] Furthermore, transversely elongated high-capacity recesses 42, each composed of a large circular section 42A and small circular sections 42B and 42C, may be used to introduce excess oil from friction boundaries. As a result, the base section 10 can achieve reduced oil viscosity resistance.

[0131] If this is the case, a combination of vertically elongated recesses 41 and transversely elongated recesses 42 can be arranged on the base section 10 and varied in distribution and / or proportion according to local variations of gaps between the base section 10 and the wall 6a of the cylinder bore 6 in order to control frictional resistances and viscous resistances between the base section 10 and the wall 6a of the cylinder bore 6.

[0132] When the engine 1 is cooled, the piston 7 has enlarged gaps between the lower section 10 and the wall 6a of the cylinder bore 6 compared to warming up of the engine 1, which is accompanied by thermal expansions of the upper lower section 36 and the lower lower section 38.

[0133] According to embodiments described herein, the piston 7 is designed to provide transversely elongated recesses 42 to provide an operating behavior for generating dynamic pressure combined with an operating behavior for discharging accumulated excess oil in the recesses 42 for removal when required.

[0134] This allows the piston 7 to have transversely elongated recesses 42 provided for reducing viscous drags during cooling of the engine 1 and arranged at optimal locations on the base portion 10, thereby implementing a scheme for reducing oil viscous drags under conditions associated with large gaps during cooling of the engine 1. Second embodiment

[0135] The Fig. 16, Fig. 17, Fig. 18 and Fig. 19 show a piston for internal combustion engines according to a second embodiment of this invention. With reference to the first embodiment, like components are designated by like reference numerals, and redundant descriptions are omitted. Note that the front and rear bottom portions 11 and 10 on each piston 7 have identical resin coating film layers 39 in configuration. Accordingly, a description will sometimes be given simply of a resin coating film layer 39 formed on the rear bottom portion 10.

[0136] As in Fig. As shown in Fig. 16, vertical grooves 45, 46A, 46B, 47A, and 47B are formed in the resin coating film layer 39, which are adjacent in the circumferential directions of the base portion 10. The vertical grooves 45, 46A, 46B, 47A, and 47B are formed with depths within a range between 5 µm or more and 20 µm or less so as to be identical to a thickness of the resin coating film layer 39.

[0137] The vertical grooves 45, 46A, 46B, 47A and 47B extend parallel to the center axis C of the piston head portion 9. The vertical grooves 45, 46A, 46B, 47A and 47B are within a range (in directions in which the left and right piston pin boss portions 15 and 14 extend) defined in the Fig. 18 and Fig. 19 is defined by imaginary lines, arranged in a direction (ie, a transverse direction of the vehicle) perpendicular to the central axis C of the piston head portion 9.

[0138] The vertical grooves 45, 46A, 46B, 47A and 47B have lengths which include the shortest lengths of the upper left and right and the lower left and right vertical grooves 47A and 47B on the sides including the central circumferential sections 11c and 10c of the front and rear bottom sections 11 and 10, and which include the longest lengths of the left and right vertical grooves 45 and 45 on the sides including the left and right side wall sections 13 and 12. The upper left and right and lower left and right vertical grooves 46A and 46B located between the left and right vertical grooves 45 and 45 and the upper left and right and lower left and right vertical grooves 47A and 47B are shorter than the left and right vertical grooves 45 and 45 and longer than the upper left and right and lower left and right vertical grooves 47A and 47B.

[0139] The vertical grooves 46A, 46B, 47A, and 47B are formed in regions on the resin coating film layers 39 corresponding to the upper base portion 36 and the lower base portion 38. Regarding the vertical grooves 46A, 46B, 47A, and 47B, no vertical grooves are formed in regions on the resin coating film layers 39 corresponding to the central base portion located on the sides including the central circumferential portions 11c and 10c of the front and rear base portions 11 and 10.

[0140] Specifically, the vertical grooves 46A and 46B on the upper bottom portion 36 extend from the upper portions 39a of the resin coating film layers 39 to just above the upper boundary 37a, which is a boundary between the upper bottom portion 36 and the middle bottom portion 37.

[0141] The vertical grooves 47A and 47B extend on the lower bottom portion 38 from the lower portions 39b of the resin coating film layers 39 to just below the lower boundary 37b between the lower bottom portion 38 and the middle bottom portion 38. Furthermore, the vertical grooves 47A and 47B are farther away from the upper boundary 37a and the lower boundary 37b than the vertical grooves 46A and 46B in a direction in which the center axis C of the piston head portion 9 extends.

[0142] The vertical grooves 45 and 45 extend over the upper lower section 36, the middle lower section 37 and the lower lower section 38.

[0143] As in Fig. 17, the vertical grooves 45, 46A, 46B, 47A and 47B are formed in a region of the resin coating film layer 39 corresponding to the large gap region 51 of the base portion 10.

[0144] Fig. 17 shows positional relationships on the base portion 10 between a clearance between the base portion 10 and the wall 6 of the cylinder bore 6 and a combination of a set of vertically elongated recesses 41 and the vertical grooves 45, 46A, 46B, 47A and 47B.

[0145] The vertical grooves 47A and 47B are formed in areas accompanied by a smallest clearance between the base portion 10 and the wall 6a of the cylinder bore 6 in a circumferential direction of the base portion 10, in the large clearance area 51 of the base portion 10.

[0146] The vertical grooves 45 and 45 are formed in areas accompanied by a largest clearance between the base portion 10 and the cylinder bore wall 6a in a circumferential direction of the base portion 10, in the large clearance area 51 of the base portion 10.

[0147] A description will now be given of configurations of the vertical grooves 45, 46A, 46B, 47A, and 47B. It should be noted that the front and rear bases have similar operating characteristics, and sometimes a description will simply be given of the rear base section 10.

[0148] The piston pin 16 and the left and right piston boss portions 15 and 14 have contacting portions therebetween, which form input points for compressive forces when the base portion 10 is pressed against the wall 6a of the cylinder bore 6, as described. Therefore, the base portion 10 has contact pressures applied to the wall 6a of the cylinder bore 6, which become high in circumferential areas of the front and rear base portions 11 and 10, in the same areas as the directions in which the left and right piston pin boss portions 15 and 14 extend.

[0149] According to embodiments described herein, the vertical grooves 45, 46A, 46B, 47A and 47B of the piston 7 are arranged in the large clearance areas 51 of the front and rear base sections 11 and 10, thereby allowing a large amount of oil to be evenly received between the wall 6a of the cylinder bore 6 and the large clearance areas 51 of the base sections 11 and 10 when the piston reciprocates.

[0150] As in Fig. 18, the piston 7, when moving upward, has oil flows introduced from upstream into spaces between the upper base portion 36 and the wall 6a of the cylinder bore 6.

[0151] Oil flows may be introduced into gaps between the upper base portion 36 and the wall 6a of the cylinder bore 6, and may be partially introduced (as oil O1 indicated by dashed lines) into the vertical grooves 46A and 47A in the large gap area 51 in the same areas as the directions in which the left and right piston pin boss portions 15 and 14 extend.

[0152] Oil flows O1 can be introduced into the vertical grooves 46A and 47A and guided to the upper boundary 37a within a range of directions in which the left and right piston pin boss portions 15 and 14 extend, and thereafter guided to portions of the region 53 having intermediate spaces between the wall 6a of the cylinder bore 6 and the intermediate bottom portion 37 in which the intermediate portion has a maximum outer diameter in the direction in which the central axis C of the piston head portion 9 extends.

[0153] The bottom section 10 has, in an extension direction from a left end 10L toward the central circumferential section 10c, reduced gaps between the wall 6a of the cylinder bore 6 and the bottom sections 11 and 10, thereby allowing oil flows O1 to be introduced into the intermediate gap region 53 and transported as oil O2, indicated by dashed lines, toward the central circumferential section 10c of the bottom section 10 while being introduced into vertically elongated recesses 41 in the course of this transport.

[0154] As a result, as in the first embodiment, high dynamic pressures can be generated by recesses 41, which enables lubrication in the small gap area 52 and the medium gap area 53 of the base section 10 under a strict lubrication condition and on the wall 6a of the cylinder bore 6.

[0155] Furthermore, there may be plenty of oil in gaps between the large gap area 51 and the wall 6a of the cylinder bore 6, so that these may be places that receive oil resistance.

[0156] According to embodiments described herein, oil flows O1 on the piston 7 can be introduced into gaps between the large-gap region 51 and the wall 6a of the cylinder bore 6, and guided from the vertical grooves 45, 46A, and 47A as upstream ends to the vertical grooves 45, 46B, and 47B as downstream ends. This allows excess oil flows from the vertical grooves 45, 46B, and 47B to be smoothly discharged downstream, enabling a reduction in oil drag resistance on the piston 7.

[0157] On the other hand, the piston 7, as shown in Fig. 19, during a downward movement, oil flows occur which are introduced from upstream into spaces between the lower base section 38 and the wall 6a of the cylinder bore 6.

[0158] Oil flows may be introduced into gaps between the lower skirt portion 36 and the wall 6a of the cylinder bore 6, and may be partially introduced (as oil O3 indicated by dashed lines) into the vertical grooves 46B and 47B in the large gap area 51 in the same areas as the directions in which the left and right piston pin boss portions 15 and 14 extend.

[0159] Oil flows O3 can be introduced into the vertical grooves 46B and 47B and can be guided to the lower boundary 37b within a range of directions in which the left and right piston pin boss portions 15 and 14 extend, and thereafter guided to partial portions of the region 53 having intermediate spaces between the cylinder bore wall 6a and the central bottom portion 37.

[0160] Oil flows O3 can be introduced into the intermediate gap region 53 and transported as oil O4, indicated by dashed lines, toward the central circumferential portion 10c of the base section 10, while being introduced into vertically elongated recesses 41 during this transport.

[0161] As a result, high dynamic pressures can be generated by recesses 41 as in the first embodiment, which enables lubrication in the small gap region 52 and the medium gap region 53 of the base section 10 under a strict lubrication condition and on the wall 6a of the cylinder bore 6.

[0162] Furthermore, oil flows O1 can be introduced into gaps between the large-gap region 51 and the wall 6a of the cylinder bore 6, and guided from the vertical grooves 45, 46B, and 47B as upstream ends to the vertical grooves 45, 46A, and 47A as downstream ends. This allows excess oil flows from the vertical grooves 45, 46A, and 47A to be smoothly discharged downstream, enabling a reduction in oil drag resistance on the piston 7.

[0163] In particular, the piston 7 according to embodiments described herein has the vertical grooves 45 extending continuously from the upper base portion 36 to the lower base portion 38 in areas that are below areas in the large gap area 51 with largest gaps between the base portion 10 and the wall 6a of the cylinder bore 6.

[0164] As a result, the piston 7 is adapted, in the course of the reciprocating movement (when handling oil flows introduced into gaps in areas accompanying areas in the large gap area 51 with largest gaps between the base portion 10 and the wall 6a of the cylinder bore 6), to use the vertical grooves 45 for discharging such oil to downstream ends, thereby enabling further oil to be discharged to downstream ends.

[0165] On the other hand, the piston 7 cooperates with the wall 6 of the cylinder bore 6 to introduce therebetween an amount of oil proportional to a speed of the piston 7. Therefore, the amount of oil is reduced in a low speed range of the engine 1, with concerns about deteriorated lubrication between the wall 6 of the cylinder bore and the small clearance area 52 and the medium clearance area 53.

[0166] Furthermore, in a high speed range of the engine 1, the piston 7 has an increased amount of oil introduced between the large clearance area 51 and the wall 6 of the cylinder bore 6, which causes an increased oil drag resistance on the piston 7.

[0167] According to embodiments described herein, in the low speed range of the engine 1, the piston 7 is designed such that a sufficient amount of oil is introduced from the vertical grooves 46A, 46B, 47A and 47B into spaces between the wall 6 of the cylinder bore 6 and the small gap region 52 and the medium gap region 53, which enables improved lubrication between the wall 6 of the cylinder bore 6 and the small gap region 52 and the medium gap region 53.

[0168] Furthermore, in the high-speed range of the engine 1, the piston 7 is designed so that a lot of oil is introduced into gaps between the large-gap region 51 and the wall 6 of the cylinder bore 6 and discharged to downstream ends through the vertical grooves 45, 46A, 46B, 47A, and 47B, enabling reduced oil drag on the piston 7. In this case, the piston 7 is designed so that lubrication for the base portion 10 becomes compatible with a reduction in drag on the piston 7 regardless of whether the speed of the engine 1 is increased or decreased.

[0169] It should be noted that the piston 7 according to embodiments described herein, which is described as using vertically elongated recesses 41, is not limited thereto and may also use transversely elongated recesses 42 or a mixture of vertically elongated recesses 41 and transversely elongated recesses 42.

[0170] Furthermore, it is preferable that in each described embodiment, there are recesses 41 or 42 or vertical grooves 45, 46A, 46B, 47A and 47B formed with depths within a range between 5 µm or more and 20 µm or less.

[0171] For vertical grooves 45, 46A, 46B, 47A, and 47B with given depths of 5 µm or less, it may be difficult to conduct oil along vertical grooves 45, 46A, 46B, 47A, and 47B, so they are undesirable. Furthermore, vertical grooves 45, 46A, 46B, 47A, and 47B with given depths of 20 µm or more may involve the problem of increased thicknesses of resin coating film layers 39, leading to stagnation of excess oil, so they are undesirable.

[0172] Furthermore, the piston 7 according to embodiments described here, which uses recesses 41 or 42 each consisting of circles, is not limited to these.

[0173] For example in Fig. 20, there may also be a piston 7 using recesses 61 each composed of a rectangular large-area portion 61A and semicircular small-area portions 61B and 61C which protrude from the large-area portion 61A with the large-area portion 61A sandwiched therebetween and which are formed with a smaller area than the large-area portion 61A.

[0174] As further stated in Fig. 21, there may also be a piston 7 using recesses 62 each composed of a rectangular large-area portion 62A and triangular small-area portions 62B and 62C which protrude from the large-area portion 62A with the large-area portion 62A sandwiched therebetween and which are formed with a smaller area than the large-area portion 62A.

[0175] As further stated in Fig. 22, there may also be a piston 7 using recesses 63 in the shape of a rhombus, each composed of a rectangular large-area portion 63A and triangular small-area portions 63B and 63C, which protrude from the large-area portion 63A so that the large-area portion 63A is sandwiched therebetween, and which are formed with a smaller area than the large-area portion 63A.

[0176] As further stated in Fig. 23, there may also be a piston 7 using recesses 64 each composed of an elongated circular large-area portion 64A and semicircular small-area portions 64B and 64C which protrude from the large-area portion 64A with the large-area portion 64A sandwiched therebetween and which are formed with a smaller area than the large-area portion 64A.

[0177] In other words, there may also be a piston 7 which uses recesses each composed of a large-area portion having an arbitrary shape and small-area portions having an arbitrary shape, which protrude from the large-area portion with the large-area portion sandwiched therebetween, and which are formed with a smaller area than the large-area portion.

[0178] Although embodiments of this invention have been described, it will be apparent that those skilled in the art may make changes without departing from the scope of this invention. Any and all such modifications and equivalents are intended to be included in the appended claims. List of reference symbols 1 engine (as an internal combustion engine) 6 cylinder bore 6a Wall 7 pistons 8 connecting rod 8A small diameter section 9 Piston head section (as a piston body) 10 rear lower section 11 front lower section 10a, 11a left section (as one circumferential side of the lower section) 10b, 11b right section (as opposite circumferential side of the lower section) 10c, 11c middle circumference section 12 right side wall section 13 left side wall section 14 right piston pin boss section 15 left piston pin boss section 16 piston pins 36 upper lower section 37 middle lower section 37a upper limit 37b lower limit 38 lower lower section 39 Resin coating film layer 41, 42, 61, 62 Deepening 41A, 42A large circular section (as a large-area section) 41B, 41C, 42B, 42C small circular section (as a small-area section) 61A, 62A large-scale section 61B, 62B, 61C, 62C small-area section C Central axis (as a central axis of the piston body) C1 Center axis (as a center axis of the piston pin)

Claims

[1] Piston (7) for internal combustion engines, comprising: a piston body (9) adapted to reciprocate relative to a wall (6a) of a cylinder bore (6); a lower portion (10, 11) depending from the piston body (9); a piston pin boss portion (14, 15) for holding a piston pin (16); a side wall connected to the base section (10, 11); and a resin coating film layer (39) provided on a surface opposite to the wall (6a) of the cylinder bore (6), wherein the lower section (10, 11) comprises: a central lower section (37) having a central portion with respect to a direction of the central axis (C) of the piston body (9), the central portion having a largest outer diameter; an upper bottom portion (36) curved with an outer diameter that gradually decreases from an upper boundary of the middle bottom portion (37) toward the central axis (C) as the upper bottom portion (36) extends upward from the upper boundary; and a lower base portion (38) curved with an outer diameter that gradually decreases from a lower boundary of the central base portion (37) toward the central axis (C) as the lower base portion (38) extends downward from the lower boundary, wherein the lower section (10, 11) is formed with a curvature which increases in the circumferential direction from a central circumferential section (10c, 11c) of the lower section (10, 11) towards a connecting section to a side wall section (12, 13), a set of recesses (41; 42; 61; 62; 63; 64), each formed to hold oil, having an outer edge at the resin coating film layer (39) and a bottom as a non-coating area free from the resin coating film layer (39), wherein the recesses (41; 42; 61; 62; 63; 64) each comprise a large-area portion (41A; 42A; 61A; 62A; 63A; 64A) and a pair of small-area portions (41B, 41C; 42B, 42C; 61B, 61C; 62B, 62C; 63B, 63C; 64B, 64C) which protrude from the large-area portion (41A; 42A; 61A; 62A; 63A; 64A) such that the large-area portion (41A; 42A; 61A; 62A; 63A; 64A) is inserted therebetween, and which have a smaller area than the large-area portion (41A; 42A; 61A; 62A; 63A; 64A). [2] Pistons for internal combustion engines according to claim 1, wherein the large-area portion (41A; 42A) and the small-area portions (41B, 41C; 42B, 42C) have arcuate outer edges. [3] Pistons for internal combustion engines according to claim 2, wherein inscribed circles of the small-area portions (41B, 41C; 42B, 42C) have centers set within a range between 0.5 times or more and 0.875 times or less of a radius of an inscribed circle of the large-area portion (41A; 42A). [4] Pistons for internal combustion engines according to claim 1, wherein the set of recesses (41; 42; 61; 62; 63; 64) comprises recesses (41; 42; 61; 62; 63; 64) arranged along the central axis (C) of the piston body (9) in a central circumferential section (10c, 11c) of the base portion (10, 11). [5] Pistons for internal combustion engines according to claim 1, wherein the bottom portion (10, 11) has a density of recesses (41; 42; 61; 62; 63; 64) as a number of recesses (41; 42; 61; 62; 63; 64) per unit area, which is set to be larger in the middle bottom portion (37) than each of the upper bottom portion (36) and the lower bottom portion (38) and to be largest near the center axis (C) of the piston body (9) in the bottom portion (10, 11). [6] Piston for internal combustion engines according to claim 1, wherein the piston pin (16) has a central axis (C1) perpendicular to the central axis (C) of the piston body (9) and the recesses (41; 42; 61; 62; 63; 64) are arranged so that the central axis (C1) of the piston pin (16) is inserted therebetween. [7] Pistons for internal combustion engines according to claim 1, wherein the recesses (41; 42; 61; 62; 63; 64) each have a longitudinal direction as a direction in which the small-area portions (41B, 41C; 42B, 42C; 61B, 61C; 62B, 62C; 63B, 63C; 64B, 64C) are arranged to the large-area portion (41A; 42A; 61A; 62A; 63A; 64A), and the set of recesses (41; 42; 61; 62; 63; 64) comprises recesses (41; 61; 62; 63) formed on the central peripheral portion (10c, 11c) of the base portion (10, 11) and the central lower section (37), wherein the longitudinal direction of each recess (41; 61; 62; 63) runs parallel to the central axis (C) of the piston body (9). [8] Pistons for internal combustion engines according to claim 1, wherein the recesses (41; 42; 61; 62; 63; 64) each have a longitudinal direction as a direction in which the small-area portions (41B, 41C; 42B, 42C; 61B, 61C; 62B, 62C; 63B, 63C; 64B, 64C) are arranged to the large-area portion (41A; 42A; 61A; 62A; 63A; 64A), and the set of recesses (41; 42; 61; 62; 63; 64) comprises recesses (41; 61; 62; 63) arranged circumferentially on one side and the other side of the bottom portion (10, 11) with respect to the central circumferential portion (10c, 11c) of the lower section (10, 11), wherein the longitudinal direction of each recess (41; 61; 62; 63) runs parallel to the central axis (C) of the piston body (9). [9] Pistons for internal combustion engines according to claim 1, wherein the bottom portion (10, 11) has a density of recesses (41; 42; 61; 62; 63; 64) which gradually decreases in an extension direction of the bottom portion (10, 11) away from the central peripheral portion (10c, 11c).

Images