Piston cooling structure

Abstract

 A piston cooling structure includes a cylinder block (1) and a cooling medium supplying device (20). The cylinder block (1) includes a cylinder bore (1a) and an inter-bore wall (2) at least partially forming the cylinder bore (1a). The cooling medium supplying device (20) is mounted to the inter-bore wall (2) to supply a cooling medium into the cylinder bore (1a). The inter-bore wall (2) has a bearing support portion (2b), a through opening (15) and an internal passageway (16). The through opening (15) communicates with the cylinder bore (1a) and extending in a direction parallel a rotational axis of the crankshaft (6). The internal passageway (16) extends from the bearing support portion (2b) to the through opening (15) in a direction parallel to a center reciprocation axis of the cylinder bore (1a). The cooling medium supplying device (20) is fluidly connected to the internal passageway (16) with an injection part (20a and 20b) being arranged with respect to the through opening (15) to supply the cooling medium from the injection part (20a and 20b) to the cylinder bore (1a).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to Japanese Patent Application No. 2008-154465, filed on June 12, 2008 and Japanese Patent Application No. 2008-320118 filed on December 16, 2008. The entire disclosures of Japanese Patent Application Nos. 2008-154465 and 2008-320118 are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

[0002] The present invention relates to a piston cooling structure configured and arranged to cool and lubricate a piston by spraying lubricating oil toward the piston. Background Information

[0003] Examined Japanese Utility Model Publication No. 6-14005 discloses an example of a piston cooling structure in which a piston is cooled and lubricated by spraying oil from an oil jet part toward a rearward side of the piston.

[0004] In the piston cooling structure disclosed in the above-mentioned reference, the oil jet part simply includes an oil spraying hole that passes from an internal surface of a crank bearing portion of a cylinder block to an internal wall surface of a cylinder bore. Consequently, the spray direction can be selected with relatively few restrictions and there is a high degree of design freedom regarding the lubrication position. Moreover, if oil spraying holes are formed to be aimed at both of two adjacent cylinder bores, then oil can be sprayed toward two adjacent pistons simultaneously.

SUMMARY OF THE INVENTION

[0005] However, with the piston cooling structure disclosed in the above-mentioned publication, it is necessary to machine the oil spraying holes diagonally toward the cylinder bore, which is not easily accomplished in some cases. The machining is even more difficult when oil spraying holes are formed aiming toward both of two adjacent cylinder bores.

[0006] Accordingly, one object of the present invention is to relatively easily secure a piston cooling structure whereby a cooling medium can be sprayed onto a piston.

[0007] In order to achieve at least a portion of the above object, a piston cooling structure includes a cylinder block and a cooling medium. The cylinder block includes a first cylinder bore and a first inter-bore wall at least partially forming the first cylinder bore. The cooling medium supplying device is mounted to the first inter-bore wall and arranged to supply a cooling medium into the first cylinder bore. The first inter-bore wall has a bearing support portion configured to rotatably support a crankshaft, a through opening and an internal passageway. The through opening communicates with the first cylinder bore and extending in a direction parallel a rotational axis of the crankshaft to be supported on the bearing support portion. The internal passageway extends from the bearing support portion to the through opening in a direction parallel to a center reciprocation axis of the first cylinder bore. The cooling medium supplying device is fluidly connected to the internal passageway with an injection part of the cooling medium supplying device being arranged with respect to the through opening to supply the cooling medium from the injection part to the first cylinder bore.

[0008] These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Referring now to the attached drawings which form a part of this original disclosure:

[0010] Figure 1 is a schematic partial cross sectional view of a cylinder block of an internal combustion engine as taken along a direction parallel to center reciprocation axes of cylinder bores and an alignment direction of the cylinder bores according to an embodiment of the present invention;

[0011] Figure 2 is an enlarged partial cross sectional view showing a portion of an inter-bore wall of the cylinder block illustrated in Figure 1 according to the embodiment of the present invention;

[0012] Figure 3 is a schematic partial perspective view showing a crankshaft and a bearing member that are coupled to the cylinder block according to the embodiment of the present invention; and

[0013] Figure 4 is an enlarged schematic partial cross sectional view of the cylinder block showing the inter-bore wall with the bearing member being installed in a bearing support portion of the inter-bore wall as taken along a direction parallel to the center reciprocation axes of the cylinder bores and a direction perpendicular to the alignment direction of the cylinder bores according to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] A selected embodiment of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following description of the embodiment of the present invention is provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

[0015] Referring initially to Figure 1, a piston cooling structure provided in a cylinder block 1 of an internal combustion engine is illustrated in accordance with the embodiment of the present invention. Figure 1 is a schematic cross sectional view of the cylinder block 1. In the illustrated embodiment, a four-cylinder engine is used as an example of the internal combustion engine that is provided with the piston cooling structure. As used herein to describe the illustrated embodiment, the directional terms such as "bottom", "top", "downward", "upward", "vertical", "horizontal", "above" and "below" as well as any other similar directional terms refer to those directions of the engine when the engine is oriented as illustrated in Figure 1.

[0016] As shown in Figure 1, a crankshaft 6 is rotatably arranged inside the cylinder block 1. A flywheel 12 is provided on the crankshaft 6 on the outside of the cylinder block 1. In this embodiment, the cylinder block 1 defines a first cylinder bore 1a, a second cylinder bore 1b, a third cylinder bore 1c, and a fourth cylinder bore 1d that are arranged along an axial direction of the crankshaft 6 in order as listed from left to right (in Figure 1). A first piston 7a, a second piston 7b, a third piston 7c, and a fourth piston 7d are each connected to the crankshaft 6 with connecting rods 8a, 8b, 8c, and 8d, respectively, and arranged inside each of the first to fourth cylinder bores 1a, 1b, 1c, and 1d, respectively, such that the first to fourth pistons 7a to 7d can move reciprocally in the first to fourth cylinder bores 1a to 1d.

[0017] As shown in Figure 1, the cylinder block 1 includes a plurality of inter-bore walls 2, 3 and 4 with the inter-bore wall 2 being disposed between the first and second cylinder bores 1a and 1b, the second inter-bore wall 3 being disposed between the second and third cylinder bores 1b and 1c, and the third inter-bore wall 4 being disposed between the third and fourth cylinder bores 1c and 1d. In other words, the first inter-bore wall 2 partially forms the first and second cylinder bores 1a and 1b, the second inter-bore wall 3 partially forms the second and third cylinder bores 1b and 1c, and the third inter-bore wall 4 partially forms the third and fourth cylinder bores 1c and 1d.

[0018] Figure 2 is an enlarged partial cross sectional view showing a portion of the first or third inter-bore wall 2 or 4 of the cylinder block 1. The first and third inter-bore walls 2 and 4 have the same structure as illustrate in Figure 2.

[0019] In this embodiment, an oil spraying device 20 (an example of a cooling medium supplying device) is provided in each of the first and third inter-bore walls 2 and 4. Moreover, each of the first to third inter-bore walls 2, 3 and 4 has a thicker-walled base portion 2a, 3a, and 4a, respectively, at a bottom end portion thereof (e.g., a portion closer to the crankshaft 6), as shown in Figures 1 and 2. Each of the inter-bore walls 2, 3 and 4 includes a through opening 15 extending along the axial direction of the crankshaft 6. As shown in Figures 1 and 2, the through opening 15 passes through an upper portion of each of the base portions 2a, 3a, and 4a so as to allow communication among the first to fourth cylinder bores 1a to 1d. The through opening 15 at least partially opens at a sliding surface 1' of the cylinder bores 1a to 1d to suppress a pressure increase from occurring inside a crank chamber S when the first to fourth pistons 7a to 7d move downward in the cylinder bores 1a to 1d. Thus, the through opening 15 serves as a ventilation hole for suppressing an increase in the pressure inside the crank chamber S that occurs when the first to fourth pistons 7a to 7d descend.

[0020] The inter-bore walls 2, 3 and 4 of the cylinder block 1 further include generally semicircular bearing support portions 2b, 3b and 4b, respectively, formed in lower end portions of the base portions 2a, 3a, and 4a. A journal portion 6a of the crankshaft 6 is rotatably supported on each of the bearing support portions 2b, 3b, and 4b through a bearing member 5 .

[0021] More specifically, a bearing cap 13 is fitted onto a bottom edge of each of the bearing members 5 from below. Among the bearing support portions 2b, 3b, and 4b, the bearing support portions 2b and 4b each has an internal passageway 16 extending to the through opening 15 in a direction parallel to center reciprocation axes of the cylinder bores 1a to 1d.

[0022] As shown in Figure 1, an oil pan 9 is provided in a bottom portion of the cylinder block 1 and a strainer 10 is arranged inside the oil pan 9. The engine is also coupled to an oil pump 11 that is configured and arranged to draw oil from inside the oil pan 9 through the strainer 10 and to supply the oil to the oil spraying devices 20 via the bearing members 5. Oil is sprayed from the oil spraying devices 20 toward the pistons 7a, 7b, 7c, and 7d.

[0023] Each of the oil spraying devices 20 is housed inside the internal passageway 16 formed in the inter-bore walls 2 and 4 as shown in Figures 1 and 2. As shown in Figure 2, each of the oil splaying devices 20 includes an upper member 21 and a lower member 22 attached to the upper member 21. The upper member 21 and the lower member 22 together define an internal space 200. The oil spraying device 20 further includes a spring member 24 and a ball member 23 housed in the internal space 200. The upper member 21 has a protruding portion 21a that protrudes upward and that preferably has a generally pyramidal or conical shape as shown in Figure 2. The protruding portion 21a forms first and second oil spraying holes 20a and 20b (examples of an injection part of the oil splaying device 20) passing through the protruding portion 21a diagonally upward to the left and right, respectively, when viewed from the perspective of Figure 2. A small hole 22a is formed through a bottom portion of the lower member 22 so as to enable communication between the internal space 200 and the internal passageway 16. The first oil spraying hole 20a and the second oil spraying hole 20b are arranged to spray the oil toward the first piston 7a and the second piston 7b simultaneously (in the case of the oil spraying device 20 disposed in the internal passageway 16 of the inter-bore wall 2) or toward the third piston 7c and the fourth piston 7d simultaneously (in the case of the oil spraying device 20 disposed in the internal passageway 16 of the inter-bore wall 4).

[0024] Figure 3 is a schematic partial perspective view showing a lubricating oil passage formed in the crankshaft 6 and the bearing member 5. Figure 4 is an enlarged schematic partial cross sectional view of the cylinder block 1 as taken along a direction parallel to the center reciprocation axes of the cylinder bores 1a to 1d and a direction perpendicular to an alignment direction of the cylinder bores 1a to 1d showing an oil supply circuit for supplying oil to one of the oil spraying devices 20.

[0025] As shown in Figure 3, the bearing member 5 includes an upper bearing member 5a and a lower bearing member 5b, and internal circumference surfaces of the upper bearing member 5a and the lower bearing member 5b form a bearing surface P. Each of the upper and lower bearing members 5a and 5b has protruding engagement flange pieces 51a and 51b formed integrally onto both axially-facing edges of the exterior circumference thereof. The upper bearing member 5a is configured and arranged to be attached such that the base portion 2a, 3a, or 4a of the inter-bore wall 2, 3, or 4 fits between the flange pieces 51a and 51b as shown in Figure 2. Therefore, the upper bearing member 5a is prevented from being displaced from the base portion 2a, 3a, or 4a of the inter-bore wall 2, 3, or 4 even if the upper bearing member 5a receives a thrust force in an axial direction of the crankshaft 6. As shown in Figures 2 to 4, the bearing surface P of the upper bearing member 5a includes a recessed oil groove 52 oriented in a circumferential direction thereof in a middle position relative to the axial direction of the crankshaft 6. Moreover, as shown in Figures 3 and 4, the upper bearing member 5a defines a first supply passage 53 and a second supply passage 54 that pass radially through the upper bearing member 5a at two locations along the groove 52.

[0026] As shown in Figure 4, an oil passage 18 is formed in the base portion 2a, 3a, or 4a of the inter-bore wall 2, 3, or 4 in a position corresponding to the first supply passage 53 when the upper bearing member 5a is attached to the bearing support portion 2b, 3b, or 4b of the base portion 2a, 3a, or 4a of the inter-bore wall 2, 3, or 4. The oil passage 18 communicates with a main gallery 17 that extends in the axial direction of the crankshaft 6. Oil is supplied to the main gallery 17 from the oil pump 11. Also, as described above, the internal passageway 16 is formed in the base portion 2a or 4a in a position corresponding to the second supply passage 54.

[0027] As shown in Figure 3, a through opening 6d is formed in the journal portion 6a of the crankshaft 6, and a through opening 6d is also formed in a crankpin 6b of the crankshaft 6 (onto an outer circumferential surface of which a bearing of the connecting rod 8a, 8b, 8c, or 8d of the piston 7a, 7b, 7c, or 7d is fitted). A lubricating oil passage 6e is formed inside the crankshaft 6 such that communication is established between the opening 6d of the journal portion 6a and the opening 6d of the crankpin 6b.

[0028] As shown Figures 1 and 3, a counterweight 6c is provided between the journal portion 6a and the crankpin 6b.

[0029] With the piston cooling structure according to the illustrated embodiment, when the engine is started, oil is drawn from inside the oil pan 9 by the operation of the oil pump 11. The oil passes through the main gallery 17 to the oil passages 18 and passes through the first supply passages 53 into the oil groove 52 of each of the upper bearing members 5a. The oil inside the oil groove 52 lubricates the area between the bearing surface P of the bearing member 5 and the journal portion 6a of the crankshaft 6. The oil also passes from the openings 6d into the lubricating oil passages 6e inside the crankshaft 6 and flows out of the openings 6d of the crankpins 6b, thereby lubricating between the outer circumferential surface of the crankpins 6b and the bearings (not shown) of the connecting rods 8a, 8b, 8c, and 8d of the pistons 7a, 7b, 7c, and 7d.

[0030] Additionally, oil that flows into the oil grooves 52 of the upper bearing members 5a flows from the second supply passage 54 into the internal passageways 16 of the base portions 2a and 4a of the inter-bore walls 2 and 4. Oil that flows into the internal passageways 16 passes through the small hole 22a in the lower member 22 of the oil spraying device 20 and oil pressure acts on the ball member 23 housed inside the internal space 200 of the oil spraying device 20. When the pressure acting on the ball member 23 reaches a prescribed pressure, the oil that has flowed into the small hole 22a pushes the ball member 23 upward against the spring force of the spring member 24 housed in the internal space 200 and flows into the internal space 200. The oil then flows through the first oil spraying hole 20a and the second oil spraying hole 20b formed in the protruding portion 21a of the upper member 21 and swiftly sprays diagonally upward into the first to fourth cylinder bores 1a to 1d. Thus, oil is sprayed toward a rear surface of each of the pistons 7a, 7b, 7c, and 7d. In this way, the pistons 7a, 7b, 7c, and 7d are cooled and lubricated at the same time.

[0031] Also, the oil can be used efficiently because the oil spraying devices 20 do not operate until the oil reaches a prescribed oil pressure. In other words, with the piston cooling structure of the illustrated embodiment, the oil is only sprayed when the engine load is high and cooling of the pistons 7a, 7b, 7c, and 7d is necessary. Therefore, the oil can be used more efficiently because it is not sprayed when the engine is operating with a low load and cooling of the pistons 7a, 7b, 7c, and 7d is not particularly necessary.

[0032] In this embodiment, the internal passageway 16 that passes from the bearing support portion 2b, 4b to the through opening 15 in a direction parallel to center reciprocation axes of the cylinder bores 1a to 1d is formed in each of the base portion 2a and 4a of the inter-bore wall 2 and 4, and the oil spraying device 20 that sprays oil diagonally upward is arranged inside the internal passageway 16. Therefore, with the piston cooling structure according to this embodiment, it is not necessary to machine oil spraying holes oriented in a diagonal direction from the bearing portion toward the cylinder bore in the conventional manner, and a structure for spraying oil to cool and lubricate the pistons 7a, 7b, 7c, and 7d can be obtained more easily. Since a ventilation hole formed to suppress a pumping loss that occurs when each of the pistons 7a, 7b, 7c and 7d descends is used as the through opening 15, it is not necessary to form a separate through opening for oil spraying purpose.

[0033] Since each of the oil spraying devices 20 has two oil spraying holes 20a and 20b and the two oil spraying holes 20a and 20b are arranged in the internal passageway 16 such that they aim toward to the two adjacent pistons 7a and 7b or the two adjacent pistons 7c and 7d, the two pistons 7a and 7b or 7c and 7d can be cooled and lubricated simultaneously with one oil spraying device 20. As a result, the number of parts used for the piston cooling structure can be reduced.

[0034] In four-cylinder engines, such as the engine presented in this embodiment, there are cases in which the counterweight 6c needs to be arranged on the crankshaft 6 between the second cylinder bore 1b and the third cylinder bore 1c. In such cases, the counterweight 6c would prevent the cooling medium from reaching the pistons 7b and 7c moving inside the second cylinder bore 1b and the third cylinder bore 1c if the oil spraying device 20 was arranged in an internal passageway that was formed in the inter-bore wall 3 between the second cylinder bore 1b and the third cylinder bore 1c and that pass from the bearing support portion 3b to the through opening 15. This problem can be avoided by arranging the oil spraying device 20 in the internal passageway 16 formed in the inter-bore wall 2 between the first cylinder bore 1a and the second cylinder bore 1b and arranging another oil spraying device 20 in the internal passageway 16 formed in the inter-bore wall 4 between the third cylinder bore 1c and the fourth cylinder bore 1d. Thus, in the four-cylinder engine as in the illustrated embodiment, the pistons 7a to 7d can be cooled reliably using a minimum number of the oil spraying devices 20.

[0035] Moreover, with the piston cooling structure of the illustrated embodiment, the crankshaft 6 has the journal portion 6a that is rotatably supported on the bearing support portion 2b or 4b of the inter-bore wall 2 or 4 through the bearing member 5 and the lubricating oil passage 6e configured to supply lubricating oil to the bearing surface P formed between the bearing member 5 and the journal portion 6a. The bearing member 5 has the supply passage 54 through which the lubricating oil supplied to the bearing surface P can be supplied to the oil spraying device 20 through the internal passageway 16 so that the oil spraying device 20 can spray lubricating oil supplied thereto from the supply passage 54 and the internal passageway 16 toward the pistons 7a to 7d. Therefore, a supply circuit for supplying a cooling medium (lubricating oil) to the oil spraying device 20 can be secured easily.

[0036] Although in the illustrated embodiment two oil spraying holes, i.e., the first oil spraying hole 20a and the second oil spraying hole 20b, are provided in the protruding portion 21a of the upper member 21 of the oil spraying device 20, it is also acceptable to provide three, four, or more oil spraying holes.

[0037] In the illustrated embodiment, each of the oil spraying devices 20 includes the upper member 21, the lower member 22 attached to the upper member 21, the internal space 200 formed by the upper member 21 and the lower member 22, and the spring member 24 and the ball member 23 housed in the internal space 200, and the oil spraying device 20 does not spray oil until the oil pressure acting on the ball member 23 reaches a prescribed pressure. However, the spring member 24 and the ball member 23 may be omitted from the oil spraying device 20. More specifically, it is acceptable for the oil spraying device 20 to be a generally cylindrical member having a protruding portion in which two oil spraying holes are formed and having a bottom communication hole that extends upward from a bottom surface and communicates with the two oil spraying holes. Of course, as described above, the number of the oil spraying holes is not limited to two.

[0038] In the illustrated embodiment, the internal passageways 16 communicate with the through opening 15 that serves as a ventilation hole for suppressing an increase in pressure that occurs inside the crank chamber S when the first piston 7a, the second piston 7b, the third piston 7c, and the fourth piston 7d descend. However, it is acceptable to form a through hole in each of the inter-bore walls 2 and 4 that communicates with the internal passageway 16 separately from the through opening 15 used as a ventilation hole.

[0039] Although in the illustrated embodiment the oil spraying device 20 is not arranged in the inter-bore wall 3 between the second cylinder bore 1b and the third cylinder bore 1c, it is acceptable to form another internal passageway 16 in the inter-bore wall 3 between the second cylinder bore 1b and the third cylinder bore 1c and arrange another oil spraying device 20 therein. In such a case, it is preferable to adopt an arrangement in which the first piston 7a is cooled and lubricated by the oil spraying device 20 arranged in the inter-bore wall 2 between the first cylinder bore 1a and the second cylinder bore 1b, the second piston 7b and the third piston 7c are cooled and lubricated by the oil spraying device 20 arranged in the inter-bore wall 3 between the second cylinder bore 1b and the third cylinder bore 1c, and the fourth piston 7d is cooled and lubricated by the oil spraying device 20 arranged in the inter-bore wall 4 between the third cylinder bore 1c and the fourth cylinder bore 1d.

[0040] Although in the illustrated embodiment the oil groove 52 is formed in the upper bearing member 5a of the bearing member 5, it is also acceptable to form an oil groove in the journal portion 6a of the crankshaft 6 instead of or in addition to the oil groove 52 of the upper bearing member 5a.

[0041] Although in the illustrated embodiment oil is supplied to the oil spraying devices 20 via a path that passes from the main gallery 17 through the lubricating oil passage 6e inside the crankshaft 6 and through the oil groove 52 and second supply passage 54 of the upper bearing member 5a, it is also acceptable to configure an oil supply passage that, for example, supplies the oil directly from the main gallery 17 to the oil spraying device 20. Various other configurations are also feasible.

[0042] Although in the embodiment the engine is a four-cylinder engine, the invention can be applied to engines having any number of cylinders, including single-cylinder engines, two-cylinder engines, three-cylinder engines, five-cylinder engines, and other conventional engines. Moreover, the piston cooling structure according to the invention can be applied to various types of engines, e.g., in-line engines, V-type engines, horizontally-opposed engines, etc. When the invention is applied to V-type engines and horizontally-opposed engines, the oil spraying device can be arranged in the internal passageway extending from the bearing support portion to the through opening in a direction parallel to a center reciprocation axis of the cylinder bore.

GENERAL INTERPRETATION OF TERMS

[0043] In understanding the scope of the present invention, the term "comprising" and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, "including", "having" and their derivatives. Also, the terms "part," "section," "portion," "member" or "element" when used in the singular can have the dual meaning of a single part or a plurality of parts.

[0044] While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

Claims

1. A piston cooling structure comprising:

a cylinder block (1) including a first cylinder bore (1a) and a first inter-bore wall (2) at least partially forming the first cylinder bore (1a); and

a cooling medium supplying device (20) mounted to the first inter-bore wall (2) and arranged to supply a cooling medium into the first cylinder bore (1a),

the first inter-bore wall (2) having a bearing support portion (2b) configured to rotatably support a crankshaft (6), a through opening (15) and an internal passageway (16), with the through opening (15) communicating with the first cylinder bore (1a) and extending in a direction parallel a rotational axis of the crankshaft (6) to be supported on the bearing support portion (2b), with the internal passageway (16) extending from the bearing support portion (2b) to the through opening (15) in a direction parallel to a center reciprocation axis of the first cylinder bore (1a), and with the cooling medium supplying device (20) being fluidly connected to the internal passageway (16) with an injection part (20a and 20b) of the cooling medium supplying device (20) being arranged with respect to the through opening (15) to supply the cooling medium from the injection part (20a and 20b) to the first cylinder bore (1a).

2. The piston cooling structure as recited in claim 1, wherein
the through opening (15) at least partially opens at a sliding surface (1') of the first cylinder bore (1a) to suppress a pressure increase from occurring inside a crank chamber (S) when a piston (7a) moves downward in the first cylinder bore (1a).

3. The piston cooling structure as recited in claim 1 or 2, wherein
the cylinder block (1) further includes a second cylinder bore (1b) at least partially formed by the first inter-bore wall (2) and with the through opening (15) communicating with both of the first and second cylinder bores (1a and 1b), and
the injection part (20a and 20b) of the cooling medium supplying device (20) is arranged with respect to the through opening (15) to supply the cooling medium to the first and second cylinder bores (1a and 1b).

4. The piston cooling structure as recited in claim 3, wherein
the cylinder block (1) further includes a third cylinder bore (1c) and a fourth cylinder bore (1d) with a second inter-bore wall (4) at least partially forming the third and fourth cylinder bores (1c and 1d), and the first, second, third and fourth cylinder bores (1a, 1b, 1c and 1d) being arranged sequentially along the rotational axis of the crankshaft (6).

5. The piston cooling structure as recited in claim 4, further comprising
an additional cooling medium supplying device (20) is arranged in the second inter-bore wall (4), with the second inter-bore wall (4) having an additional bearing support portion (4b) configured to rotatably support the crankshaft (6), an additional through opening (15) and an additional internal passageway (16), with the additional through opening (15) communicating with the third and fourth cylinder bores (1c and 1d) and the additional internal passageway (16) extending from the additional bearing support portion (4b) to the additional through opening (15) in a direction parallel to center reciprocation axes of the third and fourth cylinder bores (1c and 1d) such that the additional cooling medium supplying device (20) is fluidly connected to the additional internal passageway (16) with an injection part (20a and 20b) of the additional cooling medium supplying device (20) being arranged with respect to the additional through opening (15) to supply the cooling medium to the third and fourth cylinder bores (1c and 1d).

6. The piston cooling structure as recited in any one of claims 1 to 5, further comprising
a bearing member (5) disposed on the bearing support portion (2b) to rotatably support a journal portion (6a) of the crankshaft (6), the bearing member (5) having a supply passage (54) fluidly communicating with the cooling medium supplying device (20) via the internal passageway (16), with the supply passage (54) of the bearing member (5) being arranged to receive a lubricating oil as the cooling medium from a bearing surface (P) formed between the bearing member (5) and the journal portion (6a) that receives the lubricating oil from a lubricating oil passage (6e) of the crankshaft (6).

7. The piston cooling structure as recited in claim 5, further comprising
a bearing member (5) disposed on the bearing support portion (2b) to rotatably support a journal portion (6a) of the crankshaft (6), the bearing member (5) having a supply passage (54) fluidly communicating with the cooling medium supplying device (20) via the internal passageway (16), with the supply passage (54) of the bearing member (5) being arranged to receive a lubricating oil as the cooling medium from a bearing surface (P) formed between the bearing member (5) and the journal portion (6a) that receives the lubricating oil from a lubricating oil passage (6e) of the crankshaft (6), and
an additional bearing member (5) disposed on the additional bearing support portion (4b) to rotatably support an additional journal portion (6a) of the crankshaft (6), the additional bearing member (5) having an additional supply passage (54) fluidly communicating with the additional cooling medium supplying device (20) via the additional internal passageway (16), with the additional supply passage (54) of the additional bearing member (5) being arranged to receive the lubricating oil as the cooling medium from a bearing surface (P) formed between the additional bearing member (5) and the additional journal portion (6a) that receives the lubricating oil from an additional lubricating oil passage (6e) of the crankshaft (6).

Drawing