Marine internal combustion engine

Abstract

To provide a piston capable of preventing scuffing from occurring to an inner peripheral surface of a cylinder, and provide a marine internal combustion engine.SOLUTION: A piston includes: a piston body reciprocally moving inside a cylinder where a combustion chamber of a marine internal combustion engine is formed; and a plurality of piston-rings which are provided for an outer peripheral part of the piston body so that it is arranged in a reciprocal movement direction of the piston body and is in slide contact with the inner peripheral surface of the cylinder. A lowest stage piston-ring provided on a lowest side of the plurality of piston-rings has an arc-shaped slide surface in slide contact with the inner peripheral surface of the cylinder by making a projecting arc-shape on the inner peripheral surface side of the cylinder. The arc-shaped slide surface is an eccentric surface having an apex below a central position in a vertical direction of the lowest stage piston-ring.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This invention relates to pistons and marine internal combustion engines. [Background technology]

[0002] Conventionally, marine internal combustion engines installed on ships are equipped with multiple cylinders that house pistons capable of reciprocating motion. The reciprocating motion of the pistons inside each of these cylinders is converted into rotational motion of a crankshaft, causing it to rotate. Generally, in marine internal combustion engines, a combustion chamber is formed inside the cylinder. The piston is housed inside the cylinder with its top facing the combustion chamber. In addition, a scavenging port is formed at the bottom of the cylinder to introduce new combustion gas into the combustion chamber. The piston reciprocates between the combustion chamber side (upper side) and the scavenging port side (lower side) of the cylinder, utilizing the combustion energy from the fuel supplied to the combustion chamber and the compressed combustion gas.

[0003] As described above, the outer circumference of the piston has multiple annular grooves arranged in the direction of the piston's reciprocating motion, and a piston ring is provided in each of these annular grooves. For example, the piston ring has a cut portion called a gap, which gives it elasticity that allows it to expand and contract in the radial direction of the piston. The piston ring also has a barrel-face-shaped outer surface that is convex outward in the radial direction. Each of the multiple piston rings provided on the outer circumference of the piston slides against the inner surface of the cylinder as the piston reciprocates, with the barrel-face-shaped outer surface sliding against the inner surface of the cylinder via a film of lubricating oil. In many cases, the outer surface (sliding surface) of such a piston ring is an arc-shaped surface (hereinafter referred to as the central barrel face surface) with its apex at the center of the piston ring in the vertical direction (direction perpendicular to the radial direction).

[0004] Furthermore, Patent Document 1 discloses a piston in which, among a plurality of piston rings provided on the outer circumference of the piston, the uppermost piston ring, located closest to the combustion chamber, has a piston ring whose outer surface apex is eccentrically offset downwards from the piston, and the lowermost piston ring, located closest to the piston, has a piston ring whose outer surface apex is eccentrically offset upwards (towards the combustion chamber) from the piston. Note that when the outer surface of the piston ring has a apex eccentrically offset downwards from the center position, it is called a lower eccentric barrel face surface, and when the apex eccentrically offset upwards from the center position, it is called an upper eccentric barrel face surface. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2021-162153 [Overview of the project] [Problems that the invention aims to solve]

[0006] As the piston ring described above slides along the inner surface of the cylinder in conjunction with the reciprocating motion of the piston, the outer surface of the piston ring applies surface pressure while sliding against the inner surface of the cylinder via an oil film. Generally, the surface pressure applied from the outer surface of the piston ring to the inner surface of the cylinder (hereinafter abbreviated as piston ring surface pressure) is determined by the pressure difference between the pressure on the inner surface of the piston ring (hereinafter referred to as back pressure) and the pressure on the outer surface (hereinafter referred to as counter pressure). Back pressure is the pressure in the direction that presses the outer surface of the piston ring against the inner surface of the cylinder, and is applied to the inner surface of the piston ring from the upper space of the piston ring within the cylinder. Counter pressure is the pressure that opposes the back pressure, and is applied to the outer surface of the piston ring from the upper and lower spaces of the piston ring within the cylinder, respectively. In each of the multiple piston rings described above, the pressure in the upper space is higher than the pressure in the lower space.

[0007] For example, in the lowest piston ring, the pressure in the upper space is the pressure between it and the piston ring on the combustion chamber side, and the pressure in the lower space is the pressure in the space leading to the cylinder's scavenging port (i.e., a pressure close to atmospheric pressure). Therefore, the pressure difference between the back pressure of the lowest piston ring and the opposing pressure, i.e., the surface pressure of the lowest piston ring, is relatively high among the multiple piston rings mentioned above. It is preferable to reduce the surface pressure of the lowest piston ring so that it does not become excessively high, thereby allowing the outer surface of the lowest piston ring and the inner surface of the cylinder to slide against each other with appropriate surface pressure.

[0008] However, with the conventional pistons described above, it is difficult to reduce the pressure difference between the back pressure of the bottom piston ring and the opposing pressure, and therefore, there is a risk that the surface pressure of the bottom piston ring will become excessively high. As a result, the bottom piston ring scrapes away and thins the oil film on the inner surface of the cylinder, and wear between these bottom piston rings and the cylinder increases, which can lead to problems such as scuffing on the inner surface of the cylinder.

[0009] In particular, since the outer circumferential surface of the bottom piston ring described in Patent Document 1 above is an upper eccentric barrel face surface, the outer circumferential surface area that receives pressure from the upper space is smaller than the outer circumferential surface area that receives pressure from the lower space. For this reason, it is even more difficult to reduce the differential pressure between the back pressure of the bottom piston ring and the opposing pressure. Also, when the outer circumferential surface of the bottom piston ring is an upper eccentric barrel face surface, it is difficult to spread the lubricating oil applied to the inner circumferential surface of the cylinder to the underside of the inner circumferential surface of the cylinder, so the oil film on the inner circumferential surface of the cylinder tends to become even thinner. Therefore, the above problems become more pronounced in bottom piston rings whose outer circumferential surface is an upper eccentric barrel face surface.

[0010] The present invention has been made in view of the above circumstances, and aims to provide a piston and a marine internal combustion engine that can prevent scuffing from occurring on the inner surface of the cylinder. [Means for solving the problem]

[0011] To solve the above-mentioned problems and achieve the objective, the piston according to the present invention comprises a piston body that reciprocates inside a cylinder in which a combustion chamber of a marine internal combustion engine is formed, and a plurality of piston rings provided on the outer circumference of the piston body so as to be aligned in the direction of reciprocating motion of the piston body and sliding against the inner surface of the cylinder, wherein the lowest piston ring provided at the bottom of the plurality of piston rings has an arc-shaped sliding surface that is convex toward the inner surface of the cylinder and sliding against the inner surface of the cylinder, and the arc-shaped sliding surface is an eccentric surface whose apex is located below the vertical center position of the lowest piston ring.

[0012] Furthermore, the piston according to the present invention is characterized in that, in the above invention, the position of the apex of the arc-shaped sliding surface is set based on the ratio of the upper distance, which is the distance between the upper end of the arc-shaped sliding surface and the inner circumferential surface of the cylinder, and the lower distance, which is the distance between the lower end of the arc-shaped sliding surface and the inner circumferential surface of the cylinder.

[0013] Furthermore, the piston according to the present invention is characterized in that, in the above invention, the ratio of the upper spacing to the lower spacing is 1.1 or more and 8.0 or less.

[0014] Furthermore, the piston according to the present invention is characterized in that, in the above invention, the arc-shaped sliding surface has an upper arc-shaped surface and a lower arc-shaped surface separated by the position of the apex, and the radii of curvature of the upper arc-shaped surface and the lower arc-shaped surface are the same as those of the upper arc-shaped surface.

[0015] Furthermore, the piston according to the present invention is characterized in that, in the above invention, the uppermost piston ring, which is provided at the uppermost position among the plurality of piston rings, has the same arc-shaped sliding surface as the lowermost piston ring.

[0016] Furthermore, the marine internal combustion engine according to the present invention is characterized by comprising a piston described in any one of the above inventions and a cylinder that houses the piston so as to be able to reciprocate.

Advantages of the Invention

[0017] According to the present invention, it is possible to prevent the occurrence of scuffing on the inner peripheral surface of the cylinder, which has the effect of preventing such a situation.

Brief Description of the Drawings

[0018] [Figure 1] FIG. 1 is a schematic diagram showing an example of the schematic configuration of a marine internal combustion engine to which a piston according to an embodiment of the present invention is applied. [Figure 2] FIG. 2 is a schematic diagram showing an example of a configuration of a piston according to an embodiment of the present invention. [Figure 3] FIG. 3 is a cross-sectional schematic diagram showing an example of the cross-sectional structure of a piston according to an embodiment of the present invention. [Figure 4] FIG. 4 is a diagram for explaining the vertex position of the arcuate sliding surface of the lowermost piston ring in an embodiment of the present invention. [Figure 5] FIG. 5 is a diagram for explaining the reduction of the surface pressure of the lowermost piston ring in an embodiment of the present invention. [Figure 6] FIG. 6 is a diagram for explaining the ease of fitting of the arcuate sliding surface of the lowermost piston ring to the inner peripheral surface of the cylinder liner.

Modes for Carrying Out the Invention

[0019] Hereinafter, with reference to the accompanying drawings, preferred embodiments of the piston and marine internal combustion engine according to the present invention will be described in detail. Note that the present invention is not limited by this embodiment. Also, the drawings are schematic, and it should be noted that the dimensional relationships between elements, the ratios of each element, etc. may be different from the actual ones. There may also be parts where the dimensional relationships and ratios between the drawings are different. Also, in each drawing, the same reference numerals are assigned to the same components.

[0020] (Marine Internal Combustion Engine) First, a marine internal combustion engine to which a piston according to an embodiment of the present invention is applied will be described. Figure 1 is a schematic diagram showing an example of the general configuration of a marine internal combustion engine to which a piston according to an embodiment of the present invention is applied. The marine internal combustion engine 10 is an example of a crosshead type internal combustion engine mounted on a ship, and is a propulsion engine (main engine) that rotates and drives the ship's propulsion propellers (not shown) via a propeller shaft. For example, the marine internal combustion engine 10 is a two-stroke diesel engine such as a uniflow scavenging crosshead type diesel engine. As shown in Figure 1, the marine internal combustion engine 10 includes a cylinder 1 provided in a cylinder jacket 4, and a piston 5 provided inside the cylinder 1 so as to be able to reciprocate.

[0021] Cylinder 1 is an example of multiple cylinders provided in a marine internal combustion engine, and as shown in Figure 1, it is composed of a cylinder liner 2 and a cylinder cover 3. The cylinder liner 2 is a cylindrical structure and is supported by a cylinder jacket 4, as shown in Figure 1. Cylinder 1 houses a piston 5 inside the cylinder liner 2 so that it can reciprocate. The cylinder cover 3 is fixed to the upper part of the cylinder liner 2. The combustion chamber 7 of the marine internal combustion engine 10 is formed (compartmentalized) inside cylinder 1 by these cylinder liner 2, cylinder cover 3 and piston 5.

[0022] As shown in Figure 1, the piston 5 is configured to reciprocate along the inner surface of the cylinder liner 2 while facing the combustion chamber 7. More specifically, the piston 5 is subjected to the pressure of the combustion chamber 7 (in-cylinder pressure) and moves to top dead center A in the cylinder 1. t and bottom dead center A b It moves back and forth between and . The stroke length S of piston 5 during its reciprocating motion is when it is at top dead center A. t and bottom dead center A b The distance between, for example, as shown in Figure 1, top dead center A t The upper end surface of piston 5 located at bottom dead center A bThis is the distance between the upper end surface of the piston 5 located at [location]. Also, as shown in Figure 1, one end of the piston rod 6 is rotatably connected to the piston 5. Although not specifically shown, the other end of the piston rod 6 is rotatably connected to the crosshead, and one end of the connecting rod is rotatably connected to this crosshead. The other end of this connecting rod is rotatably connected to the crank of the crankshaft.

[0023] Furthermore, as shown in Figure 1, a scavenging port 8 is formed in the lower part of the cylinder liner 2. The scavenging port 8 is located when the piston 5 is at bottom dead center A b When positioned, it opens, connecting the scavenging chamber (not shown) of the marine internal combustion engine 10 with the combustion chamber 7. Combustion gas is supplied to the combustion chamber 7 in the cylinder liner 2 through the scavenging port 8, etc. Also, as shown in Figure 1, the cylinder cover 3 is provided with an exhaust valve 11. The exhaust valve 11 is a valve that can open and close the exhaust port of the exhaust pipe 12 that leads to the combustion chamber 7 in the cylinder 1. Exhaust gas is discharged from the combustion chamber 7 to the exhaust pipe 12 of the marine internal combustion engine 10 through this exhaust port.

[0024] In the marine internal combustion engine 10, each time the piston 5 reciprocates inside the cylinder 1, the following processes occur sequentially: introduction of combustion gas into the combustion chamber 7, compression of this combustion gas, combustion of fuel supplied to the combustion chamber 7 from a fuel injection valve (not shown), and discharge of exhaust gas from the combustion chamber 7. This reciprocating motion of the piston 5 is converted into rotational motion of the crankshaft via the piston rod 6, etc. As a result, the crankshaft rotates, and together with the propeller shaft, the propeller for the ship's propulsion rotates.

[0025] Furthermore, as shown in Figure 1, the cylinder 1 is provided with an oiling rod 9 for injecting lubricating oil into the inner circumferential surface of the cylinder liner 2. For example, multiple oiling rods 9 are arranged at a predetermined distance H from the top dead center At of the piston 5 to the lower side (bottom dead center Ab side) of the cylinder liner 2 at position Ax. This distance H is, for example, 15% to 35% of the stroke length S of the piston 5. Although not specifically shown, an oil film of lubricating oil injected from the oiling rod 9 is formed on the inner circumferential surface of the cylinder liner 2.

[0026] (piston) Next, the configuration of the piston 5 according to an embodiment of the present invention will be described. Figure 2 is a schematic diagram showing one example of the configuration of a piston according to an embodiment of the present invention. Figure 2 shows an enlarged view of the piston 5 of the marine internal combustion engine 10 shown in Figure 1. Figure 3 is a schematic cross-sectional diagram showing an example of the cross-sectional structure of the piston according to an embodiment of the present invention. Figure 3 shows an enlarged view of the cross-sectional structure of region Z of the piston 5 shown in Figure 2.

[0027] As shown in Figures 2 and 3, the piston 5 comprises a piston body 51 that reciprocates inside the cylinder 1, and a plurality of piston rings provided on the outer circumference of the piston body 51. In this embodiment, the piston 5 includes, as an example of these plurality of piston rings, three piston rings: an uppermost piston ring 53, an intermediate piston ring 54, and a lowermost piston ring 55.

[0028] The piston body 51 is configured to reciprocate inside the cylinder 1 in which the combustion chamber 7 of the marine internal combustion engine 10 (see Figure 1) described above is formed. More specifically, as shown in Figure 2, the piston body 51 has a circular top portion 51a when viewed from the axial direction (longitudinal direction) of the piston shaft 5a, and an outer circumference portion 51b around the axis of the piston shaft 5a. The piston body 51 is housed inside the cylinder 1, more specifically inside the cylinder liner 2 that constitutes the cylindrical part of the cylinder 1, so as to be able to reciprocate, with the top portion 51a facing the combustion chamber 7. As shown in Figure 2, this combustion chamber 7 is the space enclosed by the inner circumferential surface 2a of the cylinder liner 2, the inner wall surface of the cylinder cover 3, and the top portion 51a of the piston body 51. Inside the cylinder liner 2, the piston body 51 reciprocates in the axial direction of the piston shaft 5a between the combustion chamber 7 and the lower end of the cylinder liner 2, while maintaining the state in which the top portion 51a faces the combustion chamber 7.

[0029] The piston shaft 5a is an axis (central axis) that passes through the center of the circular top portion 51a and is perpendicular to the radial direction of the piston body 51. The inner circumferential surface of the cylinder 1 is the inner circumferential surface 2a of the cylinder liner 2 that constitutes the cylindrical portion of the cylinder 1.

[0030] Furthermore, as shown in Figure 2, the piston body 51 has a skirt portion 52 that is integral with the outer circumference 51b and extends on the opposite side from the top portion 51a. The skirt portion 52 contacts the inner circumferential surface 2a of the cylinder liner 2, thereby restricting the direction of the reciprocating motion of the piston 5 inside the cylinder liner 2. The piston rod 6 described above is connected to the piston body 51 so as to extend from the skirt portion 52.

[0031] The uppermost piston ring 53, the intermediate piston ring 54, and the lowermost piston ring 55 are each provided on the outer circumference 51b of the piston body 51 and configured to slide against the inner circumferential surface 2a of the cylinder 1. Hereinafter, in this embodiment, the "uppermost piston ring 53, the intermediate piston ring 54, and the lowermost piston ring 55" may be collectively referred to as "multiple piston rings."

[0032] More specifically, as shown in Figure 2, the outer circumference 51b of the piston body 51 has multiple (three in this embodiment) annular grooves 56-58 that extend circumferentially around the axis of the piston shaft 5a, arranged in the axial direction of the piston shaft 5a. The uppermost piston ring 53 is mounted in the uppermost annular groove 56, which is closest to the top 51a of the piston body 51, among these annular grooves 56-58. The intermediate piston ring 54 is mounted in the intermediate annular groove 57, which is located between the uppermost annular groove 56 and the lowermost annular groove 58, among these annular grooves 56-58. The lowermost piston ring 55 is mounted in the lowermost annular groove 58, which is furthest from the top 51a of the piston body 51, among these annular grooves 56-58. In this way, the uppermost piston ring 53, the intermediate piston ring 54, and the lowermost piston ring 55 are provided on the outer circumference 51b of the piston body 51 so as to be aligned in the direction of the reciprocating motion of the piston body 51.

[0033] Furthermore, the uppermost piston ring 53, the intermediate piston ring 54, and the lowermost piston ring 55 each have a gap (not shown) which is a cut portion of the ring. This structure allows each of these multiple piston rings to have elasticity that allows them to expand and contract in the radial direction of the piston body 51. When the piston body 51 is housed inside the cylinder 1 so as to be able to reciprocate, each of these multiple piston rings contacts the inner circumferential surface 2a of the cylinder 1 while applying surface pressure to the inner circumferential surface 2a, and slides along the inner circumferential surface 2a as the piston body 51 reciprocates.

[0034] Although not specifically shown in the diagram, the outer surfaces of the uppermost piston ring 53, the intermediate piston ring 54, and the lowermost piston ring 55 are each coated to improve wear resistance. Examples of these coatings include thermal spray coatings and plated coatings. A thermal spray coating is a coating formed by spraying material particles (fine particles) that have been melted or softened by heating onto the surface of a substrate. Examples of material particles for thermal spray coatings include molybdenum (Mo), chromium carbide (Cr-C), and nickel-chromium alloy (NiCr). A plated coating is a coating formed by plating a hard material with wear resistance onto the surface of a substrate. Examples of hard materials for plated coatings include chromium (Cr), chromium ceramic, chromium nitride (CrN), and diamond-like carbon (DLC).

[0035] Furthermore, as shown in Figure 3, the uppermost piston ring 53, which is located on the uppermost side (combustion chamber 7 side) of the multiple piston rings, has an arc-shaped sliding surface 53a and an inner circumferential surface 53b. The arc-shaped sliding surface 53a is a surface included in the outer circumferential surface of the uppermost piston ring 53 and is formed to form a convex arc towards the inner circumferential surface 2a of the cylinder 1. That is, the arc-shaped sliding surface 53a has a convex apex 53p towards the inner circumferential surface 2a. More specifically, as shown in Figure 3, the arc-shaped sliding surface 53a is an eccentric surface having its apex 53p below the vertical center position 53c of the uppermost piston ring 53, i.e., a lower eccentric barrel face surface. The arc-shaped sliding surface 53a slides against the inner circumferential surface 2a of the cylinder liner 2 via a lubricating oil film (not shown) at and near the apex 53p. The uppermost piston ring 53 presses its arc-shaped sliding surface 53a against the inner circumferential surface 2a of the cylinder liner 2 with a surface pressure P11. As the piston body 51 reciprocates inside the cylinder 1, the uppermost piston ring 53 slides against the inner circumferential surface 2a of the cylinder liner 2 with a surface pressure P11, while the arc-shaped sliding surface 53a is in contact with the inner circumferential surface 2a of the cylinder liner 2.

[0036] The vertical direction of the uppermost piston ring 53 is perpendicular to the radial direction of the uppermost piston ring 53 and is the same direction as the axial direction of the piston shaft 5a shown in Figure 2. In this vertical direction, the upper side is the combustion chamber 7 side and the lower side is the lower side of the cylinder 1 (the scavenging port 8 side shown in Figure 1). The definition of this vertical direction is the same for each of the multiple piston rings.

[0037] The surface pressure P11 of the uppermost piston ring 53 corresponds to the differential pressure (back pressure - counter pressure) between the back pressure applied to the inner circumferential surface 53b of the uppermost piston ring 53 and the counter pressure applied to the arc-shaped sliding surface 53a of the uppermost piston ring 53. The back pressure on the inner circumferential surface 53b is the same as the pressure P1 in the space above the uppermost piston ring 53. The counter pressure on the arc-shaped sliding surface 53a is divided into an upper counter pressure and a lower counter pressure, with the apex position 53d of the arc-shaped sliding surface 53a as the boundary. The upper counter pressure is the same as the pressure P1 in the space above the uppermost piston ring 53, and the lower counter pressure is the same as the pressure P2 in the space below the uppermost piston ring 53. The apex position 53d is the position of the apex 53p in the vertical direction of the uppermost piston ring 53. Furthermore, as shown in Figure 3, pressure P1 is the pressure in the space between the inner circumferential surface 2a of the cylinder liner 2 and the outer circumferential portion 51b of the piston body 51 (hereinafter referred to as the annular space), specifically the space that leads to the combustion chamber 7 (see Figure 2) inside the cylinder 1. In other words, pressure P1 is the same as the in-cylinder pressure of the combustion chamber 7. On the other hand, pressure P2 is the pressure in the space between the uppermost piston ring 53 and the intermediate piston ring 54 within the annular space. The surface pressure P11, based on the differential pressure between these pressures P1 and P2, is the highest pressure among the surface pressures of the multiple piston rings.

[0038] Furthermore, as shown in Figure 3, the lowest piston ring 55, which is located at the bottom (scavenging port 8 side) of the multiple piston rings, has an arc-shaped sliding surface 55a and an inner circumferential surface 55b. The arc-shaped sliding surface 55a is a surface included in the outer circumferential surface of the lowest piston ring 55 and is formed to form a convex arc towards the inner circumferential surface 2a of the cylinder 1. That is, the arc-shaped sliding surface 55a has a convex apex 55p towards the inner circumferential surface 2a. In detail, as shown in Figure 3, the arc-shaped sliding surface 55a is an eccentric surface (lower eccentric barrel face surface) whose apex 55p is located below the vertical center position 55c of the lowest piston ring 55. The arc-shaped sliding surface 55a slides against the inner circumferential surface 2a of the cylinder liner 2 via an oil film of lubricating oil at, for example, the apex 55p and its vicinity. The lowest piston ring 55 presses its arc-shaped sliding surface 55a against the inner circumferential surface 2a of the cylinder liner 2 with a surface pressure P13. As the piston body 51 reciprocates as described above, the lowest piston ring 55 slides against the inner circumferential surface 2a of the cylinder liner 2 with its arc-shaped sliding surface 55a, while maintaining contact with the inner circumferential surface 2a with a surface pressure P13.

[0039] The surface pressure P13 of the lowest piston ring 55 corresponds to the differential pressure between the back pressure of the inner circumferential surface 55b and the opposing pressure of the arc-shaped sliding surface 55a, similar to the case of the uppermost piston ring 53 described above. More specifically, the back pressure of the inner circumferential surface 55b is the same as the pressure P3 in the space above the lowest piston ring 55. The opposing pressure of the arc-shaped sliding surface 55a is divided into an upper opposing pressure and a lower opposing pressure, with the apex position 55d of the arc-shaped sliding surface 55a as the boundary. The upper opposing pressure is the same as the pressure P3 in the space above the lowest piston ring 55, and the lower opposing pressure is the same as the pressure P4 in the space below the lowest piston ring 55. The apex position 55d is the position of the apex 55p in the vertical direction of the lowest piston ring 55. Furthermore, as shown in Figure 3, pressure P3 is the pressure in the space between the intermediate piston ring 54 and the lowest piston ring 55 within the annular space between the cylinder liner 2 and the piston body 51. On the other hand, pressure P4 is the pressure in the space within the annular space that leads to the scavenging port 8 of the cylinder 1 (see Figure 1). That is, pressure P4 is approximately equivalent to atmospheric pressure. The surface pressure P13, based on the differential pressure of these pressures P3 and P4, is the second highest among the surface pressures of the multiple piston rings, after the surface pressure P11 of the uppermost piston ring 53 mentioned above.

[0040] Furthermore, as shown in Figure 3, among the multiple piston rings, the intermediate piston ring 54 provided between the uppermost piston ring 53 and the lowermost piston ring 55 has an arc-shaped sliding surface 54a and an inner circumferential surface 54b. The arc-shaped sliding surface 54a is a surface included in the outer circumferential surface of the intermediate piston ring 54 and is formed to form a convex arc toward the inner circumferential surface 2a of the cylinder 1. That is, the arc-shaped sliding surface 54a has a convex apex 54p toward the inner circumferential surface 2a. In detail, as shown in Figure 3, the arc-shaped sliding surface 54a is a surface having its apex 54p at the vertical center position 54c of the intermediate piston ring 54, i.e., the central barrel face surface. The arc-shaped sliding surface 54a slides against the inner circumferential surface 2a of the cylinder liner 2 via an oil film of lubricating oil, for example, at the apex 54p and its vicinity. The intermediate piston ring 54 presses its arc-shaped sliding surface 54a against the inner circumferential surface 2a of the cylinder liner 2 with a surface pressure P12. As the piston body 51 reciprocates as described above, the intermediate piston ring 54 slides against the inner circumferential surface 2a of the cylinder liner 2 with its arc-shaped sliding surface 54a, while maintaining contact with the inner circumferential surface 2a with a surface pressure P12.

[0041] The surface pressure P12 of the intermediate piston ring 54 corresponds to the differential pressure between the back pressure of the inner circumferential surface 54b and the opposing pressure of the arc-shaped sliding surface 54a, similar to the uppermost piston ring 53 described above. More specifically, the back pressure of the inner circumferential surface 54b is the same as the pressure P2 in the upper space of the intermediate piston ring 54. The opposing pressure of the arc-shaped sliding surface 54a is divided into an upper opposing pressure and a lower opposing pressure, with the apex position 54d of the arc-shaped sliding surface 54a (the same position as the center position 54c) as the boundary. The upper opposing pressure is the same as the pressure in the upper space of the intermediate piston ring 54 (pressure P2 described above), and the lower opposing pressure is the same as the pressure in the lower space of the intermediate piston ring 54 (pressure P3 described above). The surface pressure P12, based on the differential pressure of these pressures P2 and P3, is the lowest among the surface pressures of the multiple piston rings.

[0042] Here, as described above, the surface pressure P11 of the uppermost piston ring 53 is higher than the surface pressures P12 and P13 of the intermediate piston ring 54 and the lowermost piston ring 55, respectively. The arc-shaped sliding surface 53a of the uppermost piston ring 53, which has the highest surface pressure P11, has a downward eccentric barrel face surface that is effective in reducing the surface pressure P11, in order to avoid a situation where the surface pressure applied to the inner circumferential surface 2a of the cylinder liner 2 becomes excessively high. Also, as described above, the surface pressure P13 of the lowermost piston ring 55 is the second highest after the surface pressure P11 of the uppermost piston ring 53. The arc-shaped sliding surface 55a of the lowermost piston ring 55, which has a high surface pressure P13, has a downward eccentric barrel face surface that is effective in reducing the surface pressure P13, for the same purpose as the uppermost piston ring 53. In other words, the uppermost piston ring 53 has an arc-shaped sliding surface 53a that is a downward eccentric barrel face surface, similar to the lowermost piston ring 55.

[0043] Furthermore, from the standpoint of manufacturing costs for the uppermost piston ring 53 and the lowermost piston ring 55, it is preferable that the respective arc-shaped sliding surfaces 53a and 55a of the uppermost piston ring 53 and the lowermost piston ring 55 be the same downward eccentric barrel face surface. This is because by making these arc-shaped sliding surfaces 53a and 55a the same, it becomes possible to mass-produce the uppermost piston ring 53 and the lowermost piston ring 55 in the same manufacturing lot, and as a result, the manufacturing costs of the uppermost piston ring 53 and the lowermost piston ring 55 can be reduced.

[0044] On the other hand, as mentioned above, the surface pressure P12 of the intermediate piston ring is the lowest among the multiple piston rings. The arc-shaped sliding surface 54a of the intermediate piston ring, which has such a low surface pressure P12, has a central barrel face surface where the apex 54p is not eccentric, taking into account the balance between the back pressure of the inner circumferential surface 54b and the counterpressure of the arc-shaped sliding surface 54a.

[0045] (Vertex position of the arc-shaped sliding surface) Next, the vertex positions of the arc-shaped sliding surfaces in each of the plurality of piston rings in the embodiment of the present invention will be described. The vertex positions of the arc-shaped sliding surfaces are set in consideration of reducing the surface pressure of the arc-shaped sliding surface that slidably contacts the inner peripheral surface 2a of the cylinder liner 2, the ease of conforming the arc-shaped sliding surface to the inner peripheral surface 2a, and the ease of forming an oil film on the inner peripheral surface 2a. Note that the ease of conforming means the ease of forming a worn surface due to the sliding of the outer peripheral surface (arc-shaped sliding surface) of the piston ring on the inner peripheral surface 2a of the cylinder liner 2. Hereinafter, as a representative of the plurality of piston rings, the vertex position 55d of the arc-shaped sliding surface 55a of the lowermost piston ring 55 will be described.

[0046] FIG. 4 is a diagram for explaining the vertex position of the arc-shaped sliding surface of the lowermost piston ring in the embodiment of the present invention. The annular groove portion 58 (see FIGS. 2 and 3) provided with the lowermost piston ring 55 may be thermally deformed and tilted due to the high temperature of the piston body 51. In this case, as shown in FIG. 4, the lowermost piston ring 55 is tilted from the original state (shown by the broken line) to the tilted state (shown by the solid line) along with the thermal deformation. The vertex position 55d of the arc-shaped sliding surface 55a of the lowermost piston ring 55 is set in consideration of such tilting of the lowermost piston ring 55.

[0047] Specifically, as shown in FIG. 4, the lowermost piston ring 55 has an arc-shaped sliding surface 55a on its outer peripheral surface (the peripheral surface on the opposite side of the inner peripheral surface 55b). The arc-shaped sliding surface 55a is the surface of the outer peripheral surface of the lowermost piston ring 55 excluding the upper edge surface 55e and the lower edge surface 55f. The upper edge surface 55e is an edge surface located between the upper end surface of the lowermost piston ring 55 and the arc-shaped sliding surface 55a, and as shown in FIG. 4, it is formed in an arc shape with a radius of curvature R a The lower edge surface 55f is an edge surface located between the lower end surface of the lowermost piston ring 55 and the arc-shaped sliding surface 55a, and as shown in FIG. 4, it is formed in an arc shape with a radius of curvature R b The arc-shaped sliding surface 55a has a vertex 55p that protrudes toward the inner peripheral surface 2a side of the cylinder liner 2, and has a radius of curvature R

[0048] S SIt is formed in an arc shape. For example, radius of curvature R S This is set considering the degree of the gap that occurs between the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2 when the arc-shaped sliding surface 55a and the inner circumferential surface 2a come into contact. Between the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2, there is a gap that increases sequentially as the displacement moves from the apex 55p toward the upper edge surface 55e (hereinafter referred to as the upper gap), and a gap that increases sequentially as the displacement moves from the apex 55p toward the lower edge surface 55f (hereinafter referred to as the lower gap).

[0049] The apex position 55d of the arc-shaped sliding surface 55a is set based on the distances of the upper and lower gaps described above. More specifically, the apex position 55d of the arc-shaped sliding surface 55a is set based on the ratio of the upper gap L1 to the lower gap L2 between the inner circumferential surface 2a of the cylinder liner 2 and the arc-shaped sliding surface 55a. As shown in Figure 4, the upper gap L1 is the distance between the upper end B1 of the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2. The upper end B1 is the boundary (intersection) between the arc-shaped sliding surface 55a and the upper edge surface 55e. The lower gap L2 is the distance between the lower end B2 of the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2. The lower end B2 is the boundary (intersection) between the arc-shaped sliding surface 55a and the lower edge surface 55f. The apex position 55d of the arc-shaped sliding surface 55a is set such that the ratio (L1 / L2) of the upper spacing L1 and the lower spacing L2 falls within a predetermined range. The ratio (L1 / L2) is, for example, between 1.1 and 8.0. By setting the apex position 55d in this way, the apex 55p of the arc-shaped sliding surface 55a is set to a position eccentrically downward from the center position 55c, as shown in Figure 4. In other words, the arc-shaped sliding surface 55a becomes a downwardly eccentric barrel face surface.

[0050] Furthermore, by setting the vertex position 55d such that the ratio (L1 / L2) is smaller within the range of 1.1 to 8.0, the eccentricity of vertex 55p from the center position 55c downwards (hereinafter referred to as the downward eccentricity) becomes smaller. When the ratio (L1 / L2) is 1.1, the downward eccentricity of vertex 55p is at its minimum value. On the other hand, by setting the vertex position 55d such that the ratio (L1 / L2) is larger within the range of 1.1 to 8.0, the downward eccentricity of vertex 55p becomes larger. When the ratio (L1 / L2) is 8.0, the downward eccentricity of vertex 55p is at its maximum value.

[0051] For example, to further reduce the surface pressure of the arc-shaped sliding surface 55a against the inner circumferential surface 2a of the cylinder liner 2 (surface pressure P13 of the lowest piston ring 55 shown in Figure 3), it is preferable to set the apex position 55d so that the ratio (L1 / L2) becomes larger, with an upper limit of 8.0. To further improve the ease with which the arc-shaped sliding surface 55a conforms to the inner circumferential surface 2a of the cylinder liner 2, it is preferable to set the apex position 55d so that the ratio (L1 / L2) becomes smaller, with a lower limit of 1.1. To improve the ease with which an oil film is formed on the inner circumferential surface 2a of the cylinder liner 2, it is preferable to set the apex position 55d so that the upper limit of the ratio (L1 / L2) is 8.0, resulting in a thicker oil film.

[0052] Furthermore, the apex position 53d (see Figure 3) of the arc-shaped sliding surface 53a of the uppermost piston ring 53 is preferably set below the center position 53c of the uppermost piston ring 53, based on the same theory as the setting of the apex position 55d of the lowermost piston ring 55 described above. On the other hand, the arc-shaped sliding surface 54a of the intermediate piston ring 54 is the central barrel face surface as described above. In this case, the apex position 54d of the arc-shaped sliding surface 54a is preferably set such that the ratio of the upper gap L1 to the lower gap L2 (L1 / L2) shown in Figure 4 is 1.0.

[0053] (Reduction of surface pressure) Next, the reduction of the surface pressure P13 of the lowest piston ring 55 will be explained. Figure 5 is a diagram illustrating the reduction of the surface pressure of the lowest piston ring in an embodiment of the present invention. As described above, the surface pressure P13 of the lowest piston ring 55 is the pressure (pressing force) that presses the arc-shaped sliding surface 55a against the inner circumferential surface 2a of the cylinder liner 2, and is determined by the differential pressure between the back pressure of the inner circumferential surface 55b of the lowest piston ring 55 and the opposing pressure of the arc-shaped sliding surface 55a.

[0054] More specifically, as shown in Figure 5, the inner circumferential surface 55b of the lowest piston ring 55 receives the pressure P3 from the space above the lowest piston ring 55. That is, the back pressure of the inner circumferential surface 55b is this pressure P3. Also, as shown in Figure 5, the arc-shaped sliding surface 55a of the lowest piston ring 55 has an upper arc-shaped surface 55aa and a lower arc-shaped surface 55ab. For example, the arc-shaped sliding surface 55a can be divided into an upper arc-shaped surface and a lower arc-shaped surface with the apex position 55d as the boundary. The upper arc-shaped surface 55aa is the upper arc-shaped surface, and the lower arc-shaped surface 55ab is the lower arc-shaped surface. The radii of curvature of these upper arc-shaped surface 55aa and lower arc-shaped surface 55ab are the same (radius of curvature R shown in Figure 4). s As shown in Figure 5, the arc-shaped sliding surface 55a receives pressure P3 at the upper arc-shaped surface 55aa and pressure P4 at the lower arc-shaped surface 55ab. In other words, the counterpressure on the arc-shaped sliding surface 55a is the sum of the pressure P3 at the upper arc-shaped surface 55aa and the pressure P4 at the lower arc-shaped surface 55ab.

[0055] As mentioned above, pressure P3 is the pressure in the space above the lowest piston ring 55 and is equal to the back pressure of the lowest piston ring 55. On the other hand, pressure P4 is the pressure in the space below the lowest piston ring 55 and is approximately equivalent to atmospheric pressure as mentioned above. Such pressure P4 is extremely low compared to the above pressure P3 (P4< <P3)。

[0056] Here, the apex 55p of the arc-shaped sliding surface 55a is eccentrically positioned downwards, as shown in Figure 5. Therefore, the area of ​​the upper arc-shaped surface 55aa is larger than the area of ​​the lower arc-shaped surface 55ab. As a result, the arc-shaped sliding surface 55a can receive a pressure P3 equivalent to the back pressure mentioned above as one of the counteracting pressures, compared to a pressure P4 lower than the back pressure. Consequently, the differential pressure between the back pressure of the inner circumferential surface 55b and the counteracting pressure of the arc-shaped sliding surface 55a can be reduced, thereby reducing the surface pressure P13 of the lowest piston ring 55 by the amount of the reduction in the differential pressure.

[0057] Furthermore, the uppermost piston ring 53, as described above, has an arc-shaped sliding surface 53a that is the lower eccentric barrel face surface, similar to the lowermost piston ring 55. Therefore, the surface pressure P11 (see Figure 3) of the uppermost piston ring 53 can also be reduced in the same way as the lowermost piston ring 55 shown in Figure 5.

[0058] (Ease of conforming to the curved sliding surface) Next, the ease with which the arc-shaped sliding surface 55a of the lowest piston ring 55 conforms to the inner circumferential surface 2a of the cylinder liner 2 will be explained. Figure 6 is a diagram illustrating the ease with which the arc-shaped sliding surface of the lowest piston ring conforms to the inner circumferential surface of the cylinder liner. As shown in Figure 6, the lowest piston ring 55 is provided in the annular groove 58 of the piston body 51, and its arc-shaped sliding surface 55a is brought into sliding contact with the inner circumferential surface 2a of the cylinder liner 2. In this state, as the piston body 51 reciprocates, the lowest piston ring 55 slides along the inner circumferential surface 2a of the cylinder liner 2 at its apex 55p and its vicinity. Although not shown in Figure 6, a lubricating oil film is interposed between the inner circumferential surface 2a of the cylinder liner 2 and the arc-shaped sliding surface 55a of the lowest piston ring 55.

[0059] As the piston ring 55 moves back and forth, its arc-shaped sliding surface 55a repeatedly rubs against the inner circumferential surface 2a of the cylinder liner 2 in the direction of the reciprocating motion (up and down). As a result, the arc-shaped sliding surface 55a wears down from its apex 55p towards both the upper arc-shaped surface 55aa and the lower arc-shaped surface 55ab.

[0060] Here, the arc-shaped sliding surface 55a is a downwardly eccentric barrel face surface, as shown in Figure 6, with its apex 55p eccentrically positioned downwards. Therefore, the gap between the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2 is biased between the upper and lower sides of the apex position 55d. That is, when comparing the distance of the gap at the same displacement from the apex position 55d, the distance between the upper arc-shaped surface 55aa of the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2 is greater than the distance between the lower arc-shaped surface 55ab of the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder liner 2. Wear on such an arc-shaped sliding surface 55a is less likely to progress to the upper arc-shaped surface 55aa than to the lower arc-shaped surface 55ab. However, as shown in Figure 4, the apex position 55d of the arc-shaped sliding surface 55a is set within the upper and lower limits of the ratio (L1 / L2) of the upper spacing L1 to the lower spacing L2, so it is not excessively eccentric downwards. This mitigates the difficulty of wear progressing toward the upper arc-shaped surface 55aa on the arc-shaped sliding surface 55aa. Therefore, wear on the arc-shaped sliding surface 55a can progress moderately toward both the upper arc-shaped surface 55aa and the lower arc-shaped surface 55ab.

[0061] In the lowest piston ring 55 having the arc-shaped sliding surface 55a described above, after the reciprocating motion of the piston body 51 (operation of the marine internal combustion engine 10) continues for a target period, a sealing surface 55h is formed on the arc-shaped sliding surface 55a, as shown in Figure 6. The sealing surface 55h is a wear surface formed by the repeated sliding of the arc-shaped sliding surface 55a on the inner circumferential surface 2a of the cylinder liner 2 during the target period. By sliding the sealing surface 55h with the inner circumferential surface 2a of the cylinder liner 2 with a predetermined sealing length SL, the sealing performance between the inner circumferential surface 2a and the outer circumferential surface of the lowest piston ring 55 can be ensured, and the smoothness with the inner circumferential surface 2a can be improved. For this reason, once the sealing surface 55h is formed on the arc-shaped sliding surface 55a, the amount of lubricating oil supplied from the lubrication rod 9 (see Figure 1) to the inner circumferential surface 2a of the cylinder liner 2 can be reduced.

[0062] Furthermore, as shown in Figure 6, the gap between the lower arc-shaped surface 55ab and the inner circumferential surface 2a of the cylinder liner 2 is smaller on the arc-shaped sliding surface 55a than on the upper arc-shaped surface 55aa. Therefore, the lowest piston ring 55 can more easily spread lubricating oil to the lower side of the arc-shaped sliding surface 55a than to the upper side. Consequently, as the piston body 51 reciprocates, the lowest piston ring 55 spreads the lubricating oil injected from the lubrication rod 9 to the lower side of the inner circumferential surface 2a of the cylinder liner 2, as well as to the upper side. As a result, a sufficient oil film is formed on the inner circumferential surface 2a to ensure lubrication with the outer circumferential surfaces of the multiple piston rings, including the lowest piston ring 55.

[0063] As described above, in the embodiment of the present invention, among the multiple piston rings provided on the outer circumference 51b of the piston body 51 that reciprocates inside the cylinder 1, the lowest piston ring 55 has an arc-shaped sliding surface 55a that is convex toward the inner circumferential surface of the cylinder 1 and slides against the inner circumferential surface 2a of the cylinder 1, and this arc-shaped sliding surface 55a is an eccentric surface whose apex 55p is located below the vertical center position 55c of the lowest piston ring 55.

[0064] Therefore, the upper surface of the arc-shaped sliding surface 55a that receives the same pressure P3 as the back pressure of the lowest piston ring 55 can be made larger than the lower surface that receives a lower pressure P4, thereby reducing the differential pressure between the back pressure of the lowest piston ring 55 and the opposing pressure. As a result, the surface pressure P13 of the lowest piston ring 55 against the inner circumferential surface 2a of the cylinder 1 can be reduced, thereby avoiding a situation where the oil film on the inner circumferential surface 2a becomes excessively thin, ensuring lubrication between the arc-shaped sliding surface 55a and the inner circumferential surface 2a, and preventing scuffing from occurring on the inner circumferential surface 2a. In addition, the peeling of the coating film (wear-resistant film, etc.) formed on the outer circumferential surface of the lowest piston ring 55 can be suppressed, thereby suppressing a decrease in sealing performance between the arc-shaped sliding surface 55a of the lowest piston ring 55 and the inner circumferential surface 2a of the cylinder 1.

[0065] Furthermore, in the embodiment of the present invention, the marine internal combustion engine 10 comprises a piston 5 provided with a plurality of piston rings, including the lowermost piston ring 55 described above, and a cylinder 1 that houses the piston 5 so as to be able to reciprocate. Therefore, the effects of the lowermost piston ring 55 described above can be enjoyed, thereby extending the lifespan of the cylinder 1 and piston 5 and reducing the frequency of their replacement.

[0066] Furthermore, in the embodiment of the present invention, the apex position 55d of the arc-shaped sliding surface 55a of the lowest piston ring 55 is set based on the ratio (L1 / L2) of the upper gap L1 to the lower gap L2 between the arc-shaped sliding surface 55a and the inner circumferential surface 2a of the cylinder 1. Therefore, the arc-shaped sliding surface 55a of the lowest piston ring 55 can be easily and reproducibly set as an eccentric surface (lower eccentric barrel face surface) which is effective in reducing the surface pressure of the arc-shaped sliding surface 55a on the inner circumferential surface 2a of the cylinder 1, improving ease of wear, and facilitating the formation of an oil film.

[0067] Furthermore, in the embodiment of the present invention, the ratio (L1 / L2) of the upper spacing L1 to the lower spacing L2 for setting the apex position 55d of the arc-shaped sliding surface 55a of the lowest piston ring 55 is set to a range of 1.1 to 8.0. Therefore, the apex position 55d of the arc-shaped sliding surface 55a can be set within the upper and lower limits of the apex position of the eccentric surface, which is effective in reducing the surface pressure of the arc-shaped sliding surface 55a on the inner circumferential surface 2a of the cylinder 1, improving ease of wear, and facilitating the formation of an oil film.

[0068] Furthermore, in the embodiment of the present invention, the arc-shaped sliding surface 53a of the uppermost piston ring 53 among the multiple piston rings is made the same eccentric surface as the arc-shaped sliding surface 55a of the lowermost piston ring 55 described above. Therefore, the uppermost piston ring 53 can also enjoy the same effects as the lowermost piston ring 55 described above, thereby more reliably preventing scuffing on the inner circumferential surface 2a of the cylinder 1. Moreover, the uppermost piston ring 53 and the lowermost piston ring 55 can be mass-produced in the same manufacturing lot, and as a result, the manufacturing costs required for each of the uppermost piston ring 53 and the lowermost piston ring 55 can be reduced.

[0069] In the embodiments described above, three piston rings were given as an example of multiple piston rings provided on the outer circumference of the piston body, but the present invention is not limited thereto. For example, two piston rings may be provided on the outer circumference of the piston body so as to be aligned in the direction of the reciprocating motion of the piston body, or three or more piston rings may be provided.

[0070] Furthermore, in the above-described embodiment, the outer circumferential surface of the uppermost piston ring was a lower eccentric barrel face surface similar to that of the lowermost piston ring, and the outer circumferential surface of the intermediate piston ring was a central barrel face surface. However, the present invention is not limited to this. For example, while it is more preferable that the outer circumferential surface of the uppermost piston ring be a lower eccentric barrel face surface similar to that of the lowermost piston ring, it may also be a lower eccentric barrel face surface with a different apex position than that of the lowermost piston ring, a central barrel face surface, or an upper eccentric barrel face surface. Also, the outer circumferential surface of the intermediate piston ring may be the central barrel face surface described above, a lower eccentric barrel face surface, or an upper eccentric barrel face surface.

[0071] Furthermore, although the embodiments described above illustrate multiple piston rings in which the joint is the cut portion of the ring, the present invention is not limited thereto. For example, each of the multiple piston rings (especially the uppermost and lowermost piston rings) may be a piston ring having a gastight joint, as exemplified by a nested joint structure, a so-called gastight ring.

[0072] Furthermore, the present invention is not limited by the embodiments described above, and configurations formed by appropriately combining the above-described components are also included in the present invention. In addition, all other embodiments, examples, and operational techniques made by those skilled in the art based on the embodiments described above are also included in the scope of the present invention. [Explanation of Symbols]

[0073] 1 cylinder 2 Cylinder Liners 2a Inner surface 3 Cylinder cover 4 Cylinder Jackets 5 pistons 5a Piston shaft 6 Piston rod 7 Combustion chamber 8 scavenging ports 9 Lubrication rod 10 Marine internal combustion engines 11 Exhaust valve 12 Exhaust pipes 51 Piston body 51a Top 51b Outer periphery 52 Skirt section 53 Top piston ring 53a, 54a, 55a Arc-shaped sliding surface 53b, 54b, 55b Inner surface 53c, 54c, 55c center position Vertex positions 53d, 54d, 55d 53p, 54p, 55p vertices 54 Intermediate piston ring 55 Bottom piston ring 55aa upper arcuate surface 55ab Lower arcuate surface 55e Upper edge surface 55f Lower edge surface 55h sealing surface 56, 57, 58 Annular groove section A t Top dead center A b bottom dead center A x position B1 Upper end B2 Lower end Z area

Claims

1. A marine internal combustion engine, A cylinder in which the combustion chamber of the aforementioned marine internal combustion engine is formed, A piston body that reciprocates inside the cylinder, Multiple piston rings are provided on the outer circumference of the piston body so as to be aligned in the direction of reciprocating motion of the piston body, and which slide against the inner surface of the cylinder. Equipped with, The lowest piston ring, which is located at the bottom of the plurality of piston rings, has an arc-shaped sliding surface that is convex toward the inner circumferential surface of the cylinder and slides against the inner circumferential surface of the cylinder. The arc-shaped sliding surface is an eccentric surface whose apex is located below the vertical center position of the lowest piston ring. The ratio of the upper distance, which is the gap between the upper end of the arc-shaped sliding surface and the inner circumferential surface of the cylinder, to the lower distance, which is the gap between the lower end of the arc-shaped sliding surface and the inner circumferential surface of the cylinder, is 1.1 or more and 8.0 or less. A marine internal combustion engine characterized by the following features.

2. The arc-shaped sliding surface has an upper arc-shaped surface and a lower arc-shaped surface separated by the position of the vertex. The radii of curvature of the upper and lower arc-shaped surfaces are the same. The marine internal combustion engine according to feature 1.

3. The uppermost piston ring, which is located at the top of the plurality of piston rings, has the same arc-shaped sliding surface as the lowermost piston ring. A marine internal combustion engine according to claim 1 or 2.

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