Piston ring set for internal combustion engine using hydrogen fuel, piston ring, and method for suppressing deposit formation in internal combustion engine using hydrogen fuel

Abstract

This piston ring set for an internal combustion engine using hydrogen fuel includes a pressure ring and an oil control ring respectively assembled to a plurality of ring grooves of a piston for the internal combustion engine using hydrogen fuel. In the piston ring set, the contact pressure of the outer circumferential surface of the pressure ring against the inner circumferential surface of a cylinder of the internal combustion engine is set to 0.22-0.69 MPa inclusive, and the contact pressure of the outer circumferential surface of the oil control ring against the inner circumferential surface of the cylinder is set to 0.89-3.87 MPa inclusive. Thus, deposits in the land part and the ring groove of the piston resulting from the combustion of the hydrogen fuel are minimized.

Description

Piston ring set and piston ring for hydrogen-fueled internal combustion engine, and method for inhibiting deposit formation in hydrogen-fueled internal combustion engine

[0001] The present disclosure relates to a piston ring set and piston ring for a hydrogen-fueled internal combustion engine, and a method for inhibiting deposit formation in a hydrogen-fueled internal combustion engine.

[0002] BACKGROUND ART Conventionally, there is known a technique for suppressing sticking of piston rings, which are attached to pistons of internal combustion engines, due to carbon sludge (for example, see Patent Document 1).

[0003] Japanese Utility Model Application Laid-Open Publication No. 04-101067

[0004] Carbon sludge, a deposit containing carbon components, forms in conventional diesel and gasoline-fueled internal combustion engines. With diesel fuel, carbon sludge forms on the piston's top land, which is closer to the combustion chamber, particularly in the top ring groove. With gasoline fuel, carbon sludge forms on the piston's oil ring groove, which is farther from the combustion chamber. The inventors discovered that deposits form on each piston land and ring groove in hydrogen-fueled internal combustion engines. The inventors investigated the composition of the deposits. The deposits formed in hydrogen-fueled internal combustion engines were found to be free of carbon, unlike the carbon sludge formed in hydrocarbon-fueled internal combustion engines, such as diesel and gasoline. The deposits formed in hydrogen-fueled internal combustion engines were compounds of additives containing metal elements (e.g., Ca, S, etc.) in the engine oil. In the following description, deposits formed in hydrogen-fueled internal combustion engines will be referred to simply as "deposits" to distinguish them from carbon sludge. In internal combustion engines that use hydrogen fuel, if deposits accumulate on the piston land and increase in thickness, the deposits may slide against the cylinder inner surface, accelerating wear on the cylinder inner surface. Furthermore, the narrowing of the gap between the piston land and the cylinder inner surface reduces the amount of blow-by caused by combustion gases, increasing the amount of backflow gas and raising concerns about increased engine oil uptake. Furthermore, if deposits accumulate on piston rings, they may impede their proper movement. As a result, the sealing between the piston rings and the cylinder inner surface may be impaired, potentially significantly increasing engine oil consumption.

[0005] The present disclosure aims to suppress the formation of deposits on the land portion and inside the ring groove of a piston that accompanies the combustion of hydrogen fuel in an internal combustion engine that uses hydrogen fuel.

[0006] To solve the above problems, the present inventors conducted extensive research. As a result, the present inventors discovered that in an internal combustion engine using hydrogen fuel, deposits form in the piston land and ring groove due to a reaction between hydrogen gas leaking from the piston top side toward the crankcase through a compression ring and engine oil rising from the crankcase toward the piston land through an oil control ring. If the surface pressure of the outer surface of the piston ring against the inner surface of the cylinder is too high, the outer surface of the piston ring and the inner surface of the cylinder tend to wear, which reduces the sealing performance between the piston ring and the inner surface of the cylinder. As a result, hydrogen gas and engine oil are more likely to reach the land. If the surface pressure of the outer surface of the piston ring against the inner surface of the cylinder is too low, engine oil is more likely to rise. As a result, engine oil is more likely to reach the land. Taking these trade-offs into consideration, the present inventors discovered a surface pressure of the outer surface of the piston ring against the inner surface of the cylinder that can suppress the formation of deposits in the piston land and ring groove due to the combustion of hydrogen fuel.

[0007] One aspect of the present disclosure is a piston ring set including a compression ring and an oil control ring that are respectively assembled into multiple ring grooves of a piston of an internal combustion engine that uses hydrogen fuel, wherein the surface pressure of the outer surface of the compression ring against the inner surface of the cylinder of the internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less, and the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder is set to 0.89 MPa or more and 3.87 MPa or less, thereby suppressing the formation of deposits on the land portion of the piston and in the ring groove due to the combustion of hydrogen fuel.

[0008] In a piston ring set according to one aspect of the present disclosure, the surface pressure of the outer surface of the compression ring against the inner surface of the cylinder of an internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less. The surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder is set to 0.89 MPa or more and 3.87 MPa or less. This prevents wear between the outer surface of the piston ring and the inner surface of the cylinder, which would be caused by excessively high surface pressure. This prevents an increase in engine oil leaking due to excessively low surface pressure. As a result, this prevents a reaction between hydrogen gas leaking from the top surface side of the piston toward the crankcase through the compression ring and engine oil leaking from the crankcase toward the land of the piston through the oil control ring. Therefore, in an internal combustion engine that uses hydrogen fuel, it is possible to prevent deposits from forming on the land and ring groove of the piston due to combustion of hydrogen fuel.

[0009] In one embodiment, the multiple ring grooves include, in order from the top surface side of the piston, a pressure ring groove in which a pressure ring is assembled and an oil ring groove in which an oil control ring is assembled, and deposits may be generated in a land portion between the pressure ring groove and the oil ring groove.

[0010] Another aspect of the present disclosure is a piston ring including a compression ring assembled in a ring groove of a piston in an internal combustion engine that uses hydrogen fuel, wherein the surface pressure of the outer surface of the compression ring against the inner surface of the cylinder of the internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less, thereby suppressing the formation of deposits in the land portion of the piston and in the ring groove that are caused by the combustion of hydrogen fuel.Alternatively, another aspect of the present disclosure is a piston ring including an oil control ring assembled in a ring groove of a piston in an internal combustion engine that uses hydrogen fuel, wherein the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder of the internal combustion engine is set to 0.89 MPa or more and 3.87 MPa or less, thereby suppressing the formation of deposits in the land portion of the piston and in the ring groove that are caused by the combustion of hydrogen fuel.

[0011] In a piston ring according to another aspect of the present disclosure, the surface pressure of the outer surface of the compression ring against the inner surface of a cylinder of an internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less. Alternatively, the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder is set to 0.89 MPa or more and 3.87 MPa or less. This prevents wear between the outer surface of the piston ring and the inner surface of the cylinder, which would be caused by excessively high surface pressure. Alternatively, it prevents an increase in engine oil leaking due to excessively low surface pressure. As a result, it prevents a reaction between hydrogen gas leaking from the top surface of the piston toward the crankcase through the compression ring and engine oil leaking from the crankcase toward the land of the piston through the oil control ring. Therefore, in an internal combustion engine that uses hydrogen fuel, it is possible to suppress the formation of deposits on the land and ring groove of the piston due to the combustion of hydrogen fuel.

[0012] Yet another aspect of the present disclosure is a method for suppressing deposit formation in an internal combustion engine that uses hydrogen fuel, comprising: a preparation step of preparing a piston ring set including a pressure ring and an oil control ring for an internal combustion engine that uses hydrogen fuel; and an assembly step of assembling the pressure ring and the oil control ring into multiple ring grooves of a piston of the internal combustion engine, wherein the preparation step includes at least one of a step of setting the surface pressure of the outer surface of the pressure ring against the inner surface of the cylinder of the internal combustion engine to be 0.22 MPa or more and 0.69 MPa or less, and a step of setting the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder to be 0.89 MPa or more and 3.87 MPa or less, thereby suppressing the formation of deposits in the land portion and ring groove of the piston that accompanies combustion of hydrogen fuel.

[0013] In a method for suppressing deposit formation according to yet another aspect of the present disclosure, in the preparation step, the surface pressure of the outer surface of the compression ring against the inner surface of the cylinder of an internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less, or the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder is set to 0.89 MPa or more and 3.87 MPa or less. This suppresses wear between the outer surface of the piston ring and the inner surface of the cylinder, which would be caused by excessively high surface pressure. Alternatively, it suppresses an increase in engine oil leaking due to excessively low surface pressure. As a result, reaction between hydrogen gas leaking from the top surface of the piston toward the crankcase through the compression ring and engine oil leaking from the crankcase toward the land of the piston through the oil control ring is suppressed. Therefore, in an internal combustion engine that uses hydrogen fuel, it is possible to suppress the formation of deposits on the land and ring groove of the piston due to combustion of hydrogen fuel.

[0014] According to the present disclosure, in an internal combustion engine that uses hydrogen fuel, it is possible to suppress the formation of deposits on the land portion and inside the ring groove of the piston that are caused by the combustion of hydrogen fuel.

[0015] 1 is a cross-sectional view schematically showing a piston ring set according to one embodiment. FIG. 1 is a cross-sectional view of the top ring of FIG. 1. FIG. 2 is a cross-sectional view of the second ring of FIG. 1. FIG. 3 is a perspective view of the oil control ring of FIG. 1. FIG. 4 is an exploded perspective view of the oil control ring of FIG. 4. FIG. 5 is a plan view of the oil ring main body of FIG. 5. FIG. 6 is a cross-sectional view taken along line VI-VI of FIG. 7. FIG. 8 is a flow chart illustrating a method for suppressing deposit formation in an internal combustion engine using hydrogen fuel according to one embodiment. FIG. 9 is a graph of a simulation result illustrating the relationship between the surface pressure of the top ring and the accumulation rate of deposits. FIG. 10 is a graph of a simulation result illustrating the relationship between the surface pressure of the oil control ring and the accumulation rate of deposits.

[0016] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following description, the same or equivalent elements will be designated by the same reference numerals, and duplicated descriptions will be omitted. In the following description, the "upper side" corresponds to the top side of the piston (top dead center side, combustion chamber side), and the "lower side" corresponds to the skirt side of the piston (bottom dead center side, crankcase side). In the following description, the cross-sectional shape refers to the cross-sectional shape along a cross section including the center axis of the piston ring.

[0017] The piston ring set according to the present disclosure is applied to an internal combustion engine that uses hydrogen fuel. The internal combustion engine in this case is, for example, a four-stroke reciprocating engine mounted on a vehicle or the like. The engine oil used in this internal combustion engine contains additives including metal elements. Examples of metal elements contained in the engine oil include Ca, S, P, and Zn.

[0018] Fig. 1 is a cross-sectional view that schematically shows a piston ring set according to one embodiment. The cross-sectional view of Fig. 1 is a cross-sectional view along the axial direction of a plurality of piston rings 1 that constitute a piston ring set 100. Fig. 1 also shows a schematic cross-section of a part of a piston 3 that is disposed in a cylinder with the piston rings 1 fitted in ring grooves 2. The axial direction of the piston rings 1 is the same as the reciprocating direction of the piston 3. The piston ring set 100 is a piston ring set for an internal combustion engine that uses hydrogen fuel.

[0019] As shown in FIG. 1 , the plurality of piston rings 1 includes an annular compression ring and an annular oil control ring. The piston rings 1 are for an internal combustion engine that uses hydrogen fuel. The plurality of piston rings 1 here include a top ring (compression ring) 10, a second ring (compression ring) 20, and an oil control ring 30. That is, the piston ring set 100 includes the top ring 10, the second ring 20, and the oil control ring 30. The top ring 10, the second ring 20, and the oil control ring 30 are respectively assembled into a plurality of ring grooves 2 of a piston 3 of the internal combustion engine that uses hydrogen fuel.

[0020] A plurality of ring grooves 2 are formed in the outer circumferential surface 3a of the piston 3. The plurality of ring grooves 2 here are, in order from the top surface 3b side of the piston, a top ring groove (compression ring groove) 2a in which the top ring 10 is assembled, a second ring groove (compression ring groove) 2b in which the second ring 20 is assembled, and an oil ring groove 2c in which the oil control ring 30 is assembled. A plurality of piston rings 1 are assembled in the plurality of ring grooves 2, respectively.

[0021] Here, the piston outer peripheral surface 3a has a top land 3c, a second land 3d, and a third land 3e as land portions extending in a band shape in the circumferential direction. The top land 3c is the region of the piston outer peripheral surface 3a from the piston top surface 3b to the top ring groove 2a. The second land 3d is the region of the piston outer peripheral surface 3a from the top ring groove 2a to the second ring groove 2b. The third land 3e is the region of the piston outer peripheral surface 3a from the second ring groove 2b to the oil ring groove 2c.

[0022] Each piston ring 1 slides against the cylinder inner peripheral surface 4, which is covered with engine oil, while abutting against the cylinder inner peripheral surface 4 with a predetermined surface pressure. This allows each piston ring 1 to perform functions such as sealing gas between the combustion chamber and crank chamber, scraping off engine oil, and forming an oil film of engine oil. The cylinder inner peripheral surface 4 refers to the inner wall surface of the cylinder bore.

[0023] [Top Ring] Figure 2 is a cross-sectional view of the top ring of Figure 1. As shown in Figures 1 and 2, the top ring 10 has an annular main body portion 11 and a joint portion (not shown) formed in a part of the main body portion 11. The main body portion 11 has a pair of side surfaces 12 and 13. The main body portion 11 has an inner circumferential surface 14 and an outer circumferential surface 15. The side surfaces 12 and 13 are, for example, approximately perpendicular to the inner circumferential surface 14. In the following description, the direction connecting the side surfaces 12 and 13 is defined as the width direction of the piston ring 1, and the direction connecting the inner circumferential surface 14 and the outer circumferential surface 15 is defined as the thickness direction of the piston ring 1. The width direction of the piston ring 1 corresponds to the "up-down direction" and the "axial direction."

[0024] The main body 11 has a generally rectangular cross section with the longer side in the thickness direction and the shorter side in the width direction. The main body 11 is made of, for example, cast iron or steel containing multiple metal elements, and is formed with sufficient strength, heat resistance, and elasticity.

[0025] The surface of the main body 11 may be modified to form a hard coating. The hard coating may be, for example, a physical vapor deposition (PVD) film formed using a physical vapor deposition (PVD) method. This allows the hard coating to be formed with sufficient hardness. The hard coating may be an ion plating film containing at least one of titanium (Ti) and chromium (Cr) and at least one of carbon (C), nitrogen (N), and oxygen, or a diamond-like carbon (DLC) film. Specific examples of the hard coating include a titanium nitride film, a chromium nitride film, a titanium carbonitride film, a chromium carbonitride film, a chromium oxynitride film, a chromium film, or a titanium film. Among these, a chromium nitride film may be used when wear resistance and scuff resistance are important. The hard coating may be a laminate, and may include, for example, a chromium nitride film and a diamond-like carbon film. Here, a hard film (film) containing chromium nitride (CrN) may be formed on the outer peripheral surface 15 of the top ring 10 .

[0026] The abutment is a portion of the main body 11 that is separated, and is formed by a pair of opposing abutment ends. Each of the pair of abutment ends is a free end of the main body 11. The gap at the abutment (abutment gap) narrows when, for example, the top ring 10 is heated and thermally expands. When the top ring 10 is in use, the abutment functions as a relief for thermal expansion of the main body 11 caused by the temperature difference between the top ring 10 and the cylinder inner surface 4.

[0027] The outer peripheral surface 15 of the main body 11 of the top ring 10 is provided with a curved surface 16, for example, having a cross-sectional shape that is convexly curved radially outward. The curved surface 16 is, for example, an arcuate surface having the radially outer end of the side surface 12 and the radially outer end of the side surface 13 as its two ends. The vertex 17, which is the radially outermost point of the curved surface 16, is located, for example, midway between the side surfaces 12 and 13. The vertex 17 is located at the center of the outer peripheral surface 15 in the width direction connecting the side surfaces 12 and 13. The vertex 17 is the portion of the outer peripheral surface 15 that protrudes most radially outward. The vertex 17 is a point that forms a sliding contact with the cylinder inner peripheral surface 4. The outer peripheral surface 15 of the top ring 10 has a barrel shape (BF shape) that is symmetrical in the width direction across the vertex 17.

[0028] The size of the arcuate surface of the curved surface 16 can be determined by the barrel amount (dimension of the drop) in the radial direction of the top ring 10 between a point spaced a certain distance in the width direction from the apex 17 and the position of the apex 17. The points spaced a certain distance in the width direction from the apex 17 are, for example, points spaced 0.8 mm above and below the apex 17 in the width direction. The width between the points spaced a certain distance in the width direction from the apex 17 is referred to as the measurement width of the barrel amount. In this example, the measurement width of the barrel amount is 1.6 mm. The barrel amount of the curved surface 16 may be, for example, 0.008 mm, 0.016 mm, or the like. The outer diameter (nominal diameter) of the top ring 10 is, for example, 112 mm. Each dimension of the top ring 10 can be measured using a contact or non-contact shape measuring device.

[0029] The tension of the top ring 10 may be equal to or greater than 10 N and may be up to 30 N. The tension of the top ring 10 can be measured in accordance with the international standard ISO 6621-2:2020.

[0030] The surface pressure of the outer peripheral surface 15 of the top ring 10 against the inner peripheral surface 4 of the cylinder (hereinafter also referred to as "surface pressure of the top ring 10") may be 0.22 MPa or more and 0.69 MPa or less. The surface pressure of the top ring 10 may be 0.22 MPa or more and 0.67 MPa or less. The surface pressure of the top ring 10 may be 0.22 MPa or more and 0.49 MPa or less. The surface pressure of the top ring 10 may be 0.22 MPa or more and 0.33 MPa or less. The surface pressure of the top ring 10 may be 0.33 MPa or more and 0.69 MPa or less. The surface pressure of the top ring 10 may be 0.49 MPa or more and 0.69 MPa or less. The surface pressure of the top ring 10 may be 0.67 MPa or more and 0.69 MPa or less. The surface pressure of the top ring 10 may be, for example, 0.49 MPa.

[0031] The surface pressure P of each piston ring 1 can be calculated by the following formula (1). In the formula (1), P is the surface pressure of the outer surface of the piston ring 1 against the inner peripheral surface 4 of the cylinder. Ft is the tension of the piston ring 1. D is the nominal diameter of the piston ring 1. W is the contact width of the piston ring 1 against the inner peripheral surface 4 of the cylinder.

[0032] The nominal diameter of the piston ring 1 is a dimension corresponding to the diameter of the cylinder bore. The nominal diameter of the piston ring 1 is not particularly limited, but may be, for example, 112 mm.

[0033] The contact width with respect to the cylinder inner peripheral surface 4 may be an actual measured value or an estimated value. For example, in the case of a top ring 10 having a symmetrical barrel shape, the contact width can be estimated by assuming that the outer peripheral surface 15 is a parabola. The contact width can be estimated by applying the barrel amount, the measured width of the barrel amount, and the assumed radial wear amount corresponding to the contact width to a quadratic function whose apex is located at vertex 17. The estimated contact width of the top ring 10 may be, for example, 0.5 mm or more and 2.5 mm or less.

[0034] [Second Ring] Figure 3 is a cross-sectional view of the second ring of Figure 1. As shown in Figures 1 and 3, the second ring 20 has an annular main body portion 21 and a joint portion (not shown) formed in a part of the main body portion 21. The main body portion 21 has a pair of side surfaces 22 and 23. The main body portion 21 has an inner circumferential surface 24 and an outer circumferential surface 25. The side surfaces 22 and 23 are, for example, approximately perpendicular to the inner circumferential surface 24. The joint portion has a structure similar to that of the main body portion 11 of the top ring 10 described above.

[0035] The main body 21 has a generally rectangular cross section with the longer side in the thickness direction and the shorter side in the width direction. The main body 21 is made of, for example, cast iron or steel containing multiple metal elements, and is formed with sufficient strength, heat resistance, and elasticity.

[0036] The surface of the main body 21 may be subjected to surface modification to form a hard coating, similar to the main body 11 of the top ring 10 described above.

[0037] The outer peripheral surface 25 of the main body 21 of the second ring 20 includes, as an example, a tapered surface 26. Here, the tapered surface 26 is an inclined surface having a cross-sectional shape that protrudes radially outward as it extends downward. The tapered surface 26 extends, for example, so as to linearly connect the radially outer end of the side surface 22 and the radially outer end of the side surface 23. A vertex 28, which is the radially outermost point of the outer peripheral surface 25 of the second ring 20, is located on the bottom dead center side of the outer peripheral surface 25 in the width direction (axial direction) of the second ring 20. The vertex 28 is the portion of the outer peripheral surface 25 that protrudes most radially outward. The vertex 28 is the point that becomes a sliding contact portion with the cylinder inner peripheral surface 4.

[0038] The outer diameter (nominal diameter) of the second ring 20 is, for example, 112 mm. Each dimension of the second ring 20 can be measured using a contact or non-contact shape measuring device.

[0039] The tension of the second ring 20 may be equal to or greater than 10 N and equal to or less than 30 N. The tension of the second ring 20, like the tension of the top ring 10, can be measured in accordance with the international standard ISO 6621-2:2020.

[0040] The surface pressure of the outer peripheral surface 25 of the second ring 20 against the inner peripheral surface 4 of the cylinder (hereinafter also referred to as the "surface pressure of the second ring 20") may be, for example, 0.92 MPa.

[0041] [Oil Control Ring] Fig. 4 is a perspective view of the oil control ring of Fig. 1. Fig. 5 is an exploded perspective view of the oil control ring of Fig. 4. Fig. 6 is a plan view of the oil ring main body of Fig. 5. Fig. 7 is a cross-sectional view taken along line VI-VI of Fig. 6.

[0042] 4, 5, and 6, the oil control ring 30 includes an annular main body 32 and an annular coil expander 33 attached along the inner circumferential surface 32a of the main body 32. The oil control ring 30 is a so-called two-piece oil ring. The main body 32 has a joint 34 formed therein.

[0043] The main body 32 contacts the inner peripheral surface of the cylinder at a substantially constant surface pressure due to the tangential tension of the coil expander 33. The coil expander 33 is a spring-like component formed in an annular shape, as shown in Fig. 5. The coil expander 33 is formed from a wire rod such as oil-hardened spring steel.

[0044] The main body 32 is formed of, for example, cast iron or steel containing multiple metal elements so as to have the strength, heat resistance, and elasticity required for the application. The outer peripheral surface 32b of the main body 32 is surface-modified with, for example, a hard chrome plating layer, a chrome nitride layer, a PVD layer, or an iron nitride layer, thereby improving the wear resistance of the main body 32. The outer peripheral surface 32b of the main body 32 refers to the outer peripheral surface of the first rail portion 37 and the outer peripheral surface of the second rail portion 38.

[0045] 4, 5, and 7, the main body 32 has a pair of first and second rail portions 37, 38. The main body 32 has a pillar portion 39 connecting the first and second rail portions 37, 38.

[0046] As shown in FIG. 7 , the inner circumferential surface 32a of the main body 32 has a curved shape recessed toward the column portion 39 so as to accommodate the coil expander 33. The main body 32 has a pair of side surfaces 32c (one side surface) and 32d (the other side surface). The pair of side surfaces 32c and 32d are substantially perpendicular to a pair of end surfaces (inner circumferential surfaces) 32e located radially inward of the inner circumferential surface 32a. In the following description, the direction connecting the inner circumferential surface 32a and the outer circumferential surface 32b is defined as the thickness direction of the oil control ring 30. The direction connecting the side surfaces 32c and 32d is defined as the width direction of the oil control ring 30. The radial direction of the oil control ring 30 coincides with the thickness direction.

[0047] The outer diameter (nominal diameter d1, see FIG. 6 ) of the main body 32 is, for example, 112 mm. The dimensions of the main body 32 can be measured using a contact or non-contact shape measuring device. The shape measuring device includes a surface roughness measuring device.

[0048] The pair of first and second rail portions 37, 38 face each other in the axial direction of the oil control ring 30, sandwiching a pillar portion 39 therebetween. The pillar portion 39 connects the thickness-wise central portions of the first and second rail portions 37, 38. The first rail portion 37 is disposed on the side surface 32c of the pillar portion 39. The first rail portion 37 protrudes radially inward and outward from the pillar portion 39. The second rail portion 38 is disposed on the side surface 32d of the pillar portion 39. The second rail portion 38 protrudes radially inward and outward from the pillar portion 39. The pillar portion 39 is formed thinner in the thickness direction than the first and second rail portions 37, 38. In this way, the main body portion 32 has a substantially H-shaped cross section. The first and second rail portions 37, 38 are integrally formed with the pillar portion 39.

[0049] As shown in Figures 4 and 5, a plurality of oil passage holes 39a are provided in the widthwise center of the pillar portion 39. The plurality of oil passage holes 39a are aligned along the circumferential direction of the oil control ring 30. The cross-sectional shape of each of the plurality of oil passage holes 39a along the circumferential direction is, for example, a substantially elliptical shape. As shown in Figure 7, the oil passage holes 39a are provided in the axial center of the pillar portion 39. The oil passage holes 39a penetrate the pillar portion 39 in the radial direction.

[0050] The oil holes 39a are formed by, for example, cutting or laser drilling. Engine oil for lubricating the cylinder inner peripheral surface 4 passes through these oil holes 39a from the inner peripheral side to the outer peripheral side of the pillar portion 39, thereby being supplied to the cylinder inner peripheral surface 4. The engine oil on the cylinder inner peripheral surface 4 passes through these oil holes 39a from the outer peripheral side to the inner peripheral side of the pillar portion 39, thereby being returned to, for example, an oil pan.

[0051] 4 to 6, the abutment 34 is a section of the main body 32. The abutment 34 is formed by a pair of opposing abutment ends 5 and 6. When the oil control ring 30 is in use, the abutment 34 functions as a relief for thermal expansion of the main body 32 caused by the temperature difference between the oil control ring 30 and the cylinder.

[0052] With this oil control ring 30, for example, when the piston 3 moves toward the top dead center, the first rail portion 37 and the second rail portion 38 apply engine oil to the cylinder inner circumferential surface 4, forming an oil film. For example, when the piston 3 moves toward the bottom dead center, the oil control ring 30 scrapes off excess oil from the cylinder inner circumferential surface 4 with the first rail portion 37 and the second rail portion 38. This allows an oil film of an appropriate thickness to be formed on the cylinder inner circumferential surface 4.

[0053] 7, the outer peripheral surface 32b of the main body 32 includes an outer peripheral surface 37a of a first rail portion 37 and an outer peripheral surface 38a of a second rail portion 38. The second rail portion 38 here is axially symmetrical to the first rail portion 37 with respect to the center of the main body 32 in the width direction. Therefore, the configuration of the first rail portion 37 will be described as a representative, and redundant description of the second rail portion 38 may be omitted.

[0054] The outer peripheral surface 37a of the first rail portion 37 has a so-called straight shape and includes a contact surface 37b. A connecting surface 37c is provided as an inclined portion connecting the side surface 32c and the contact surface 37b. A connecting surface 37d is provided as an inclined portion connecting the contact surface 37b and the column portion 39. The outer peripheral surface 38a of the second rail portion 38 has a so-called straight shape and includes the contact surface 38b.

[0055] The contact surfaces 37b and 38b are the surfaces of the outer peripheral surface 32b that protrude most radially. When the oil control ring 30 is installed in the oil ring groove 2c of the piston 3, the contact surfaces 37b and 38b slide against the cylinder inner peripheral surface 4. The contact surfaces 37b and 38b extend over the entire circumferential direction of the main body 32. The contact surfaces 37b and 38b are substantially perpendicular to the side surfaces 32c and 32d and substantially parallel to the end surface (inner peripheral surface) 32e. The widths of the contact surfaces 37b and 38b are referred to as land widths. The land width may be, for example, 0.3 mm, 0.5 mm, or the like. For example, if the land width of the outer peripheral surface 37a of the first rail portion 37 and the land width of the outer peripheral surface 38a of the second rail portion 38 are equal to each other, the width (contact width) at which the outer peripheral surfaces 37a and 38a contact the cylinder inner peripheral surface 4 will be twice the land width, such as 0.6 mm, 1.0 mm, etc.

[0056] The tension of the oil control ring 30 may be equal to or greater than 30 N and equal to or less than 110 N. The tension of the oil control ring 30, like the tension of the top ring 10, can be measured in accordance with the international standard ISO 6621-2:2020.

[0057] The surface pressure of the contact surfaces (outer peripheral surfaces) 37b, 38b of the oil control ring 30 against the cylinder inner peripheral surface 4 (hereinafter also referred to as the "surface pressure of the oil control ring 30") may be 0.89 MPa or more and 3.87 MPa or less. The surface pressure of the oil control ring 30 may be 0.89 MPa or more and 3.27 MPa or less. The surface pressure of the oil control ring 30 may be 0.89 MPa or more and 2.68 MPa or less. The surface pressure of the oil control ring 30 may be 0.89 MPa or more and 2.08 MPa or less. The surface pressure of the oil control ring 30 may be 0.89 MPa or more and 1.49 MPa or less. The surface pressure of the oil control ring 30 may be 0.89 MPa or more and 1.25 MPa or less. The surface pressure of the oil control ring 30 may be 1.25 MPa or more and 3.87 MPa or less. The surface pressure of the oil control ring 30 may be 1.49 MPa or more and 3.87 MPa or less. The surface pressure of the oil control ring 30 may be 2.08 MPa or more and 3.87 MPa or less. The surface pressure of the oil control ring 30 may be 2.68 MPa or more and 3.87 MPa or less. The surface pressure of the oil control ring 30 may be 3.27 MPa or more and 3.87 MPa or less. The surface pressure of the oil control ring 30 may be, for example, 2.08 MPa.

[0058] By configuring piston ring set 100 as described above, it is possible to suppress the formation of deposits in an internal combustion engine that uses hydrogen fuel. Figure 8 is a flowchart illustrating a method for suppressing the formation of deposits in an internal combustion engine that uses hydrogen fuel according to one embodiment.

[0059] 8 , the method for suppressing deposit formation in an internal combustion engine powered by hydrogen fuel includes a preparation step (S01) of preparing a piston ring set including a pressure ring and an oil control ring for an internal combustion engine powered by hydrogen fuel. The preparation step includes, for example, preparing a top ring 10 as the pressure ring. The preparation step may also include preparing a second ring 20 as the pressure ring. The preparation step includes preparing an oil control ring 30 as the oil control ring. In other words, the preparation step includes preparing a piston ring set 100 including the top ring 10, the second ring 20, and the oil control ring 30.

[0060] The preparatory step (S01) here includes a step of adjusting the surface pressure of the outer surface 15 of the top ring 10 against the cylinder inner surface 4 to 0.22 MPa or more and 0.69 MPa or less. This step can also be described as a step of adjusting the surface pressure of the top ring 10 to 0.22 MPa or more and 0.69 MPa or less so that the deposition rate ratio of the top ring 10 after a predetermined test is, for example, 0.2 or less. The preparatory step (S01) here includes a step of adjusting the surface pressure of the contact surfaces 37b and 38b of the oil control ring 30 against the cylinder inner surface 4 to 0.89 MPa or more and 3.87 MPa or less. This step can also be described as a step of adjusting the surface pressure of the oil control ring 30 to 0.89 MPa or more and 3.87 MPa or less so that the deposition rate ratio of the oil control ring 30 after a predetermined test is, for example, 0.2 or less. This preparatory step suppresses the formation of deposits on the top land 3c, second land 3d, and third land 3e of the piston 3 due to the combustion of hydrogen fuel. The formation of deposits in the top ring groove 2a, the second ring groove 2b, and the oil ring groove 2c of the piston 3 due to the combustion of hydrogen fuel is suppressed.

[0061] The method for suppressing deposit formation in an internal combustion engine that uses hydrogen fuel includes an assembling step (S02) of assembling a pressure ring and an oil control ring into multiple ring grooves of a piston of the internal combustion engine, respectively. In this assembling step, for example, the top ring 10 is assembled into the top ring groove (pressure ring groove) 2a, the second ring 20 is assembled into the second ring groove (pressure ring groove) 2b, and the oil control ring 30 is assembled into the oil ring groove 2c. These assembling steps enable the operation of an internal combustion engine that uses hydrogen fuel using a piston ring set that includes a pressure ring and an oil control ring that suppress deposit formation associated with the combustion of hydrogen fuel.

[0062] In the piston ring set 100 and the method for suppressing deposit formation described above, the surface pressure of the outer surface 15 of the top ring 10 against the cylinder inner surface 4 of the internal combustion engine is set to be 0.22 MPa or more and 0.69 MPa or less. The surface pressure of the contact surfaces 37b and 38b of the oil control ring 30 against the cylinder inner surface 4 is set to be 0.89 MPa or more and 3.87 MPa or less. This prevents wear between the outer surface of the piston ring 1 and the cylinder inner surface 4, which would be caused by excessively high surface pressure. This prevents an increase in engine oil leaking from the top surface 3b side of the piston 3 toward the crankcase, and engine oil leaking from the crankcase side toward the top land 3c, second land 3d, and third land 3e of the piston 3 through the oil control ring 30, from reacting. Therefore, in an internal combustion engine that uses hydrogen fuel, it is possible to suppress the formation of deposits on the top land 3c, second land 3d, and third land 3e of the piston 3 that are caused by the combustion of hydrogen fuel, and it is also possible to suppress the formation of deposits in the top ring groove 2a, second ring groove 2b, and oil ring groove 2c of the piston 3 that are caused by the combustion of hydrogen fuel.

[0063] The multiple ring grooves include, in order from the top surface 3b side of the piston 3, a top ring groove 2a in which the top ring 10 is installed, a second ring groove 2b in which the second ring 20 is installed, and an oil ring groove 2c in which the oil control ring 30 is installed. Deposits may be formed on at least one of a second land 3d and a third land 3e between the top ring groove 2a and the oil ring groove 2c, or on the third land 3e between the second ring groove 2b and the oil ring groove 2c. In this case, the formation of deposits on at least one of the second land 3d and the third land 3e of the piston 3 due to the combustion of hydrogen fuel is suppressed. As a result, deposits that have accumulated and increased in thickness on at least one of the second land 3d and the third land 3e are prevented from sliding against the cylinder inner circumferential surface 4, thereby suppressing increased wear of the cylinder inner circumferential surface 4.

[0064] [Modifications of Piston Ring] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. The present disclosure can be embodied in various forms including the above-described embodiments and various modifications and improvements based on the knowledge of those skilled in the art.

[0065] In the above embodiment, the surface pressure of the top ring 10 is set to 0.22 MPa or more and 0.69 MPa or less, and the surface pressure of the oil control ring 30 is set to 0.89 MPa or more and 3.87 MPa or less. However, this is not limited to this example. For example, only one of the surface pressures of the top ring 10 may be set to 0.22 MPa or more and 0.69 MPa or less, or the surface pressure of the oil control ring 30 may be set to 0.89 MPa or more and 3.87 MPa or less. Even in this case, wear of the outer peripheral surface of the piston ring 1 and the inner peripheral surface 4 of the cylinder, which would be caused by excessively high surface pressure, is suppressed. Alternatively, an increase in engine oil leaking from the top surface 3b of the piston 3 toward the crankcase is suppressed. As a result, reaction between hydrogen gas leaking from the top surface 3b side of the piston 3 toward the crankcase through the top ring 10 and engine oil leaking from the crankcase side toward the second land 3d and third land 3e of the piston 3 through the oil control ring 30 is suppressed. Therefore, in an internal combustion engine that uses hydrogen fuel, it is possible to suppress the formation of deposits on the top land 3c, second land 3d, and third land 3e of the piston 3 that are caused by the combustion of hydrogen fuel, and it is also possible to suppress the formation of deposits in the top ring groove 2a, second ring groove 2b, and oil ring groove 2c of the piston 3 that are caused by the combustion of hydrogen fuel.

[0066] The present disclosure will be explained in more detail by the following examples, but the present disclosure is not limited to these examples.

[0067] Top rings for Examples 1 to 5 and Comparative Examples 1 to 4 were produced as shown in Table 1 below. First, top rings with a symmetrical barrel-shaped outer circumferential surface were produced using a wire material equivalent to JIS-SUS440. The nominal diameter of the top ring was set to 112 mm. The measurement width of the barrel volume of the symmetrical barrel shape was set to 1.6 mm. The barrel volume and tension of the symmetrical barrel shape were set as shown in Table 1.

[0068] The contact widths of the top rings with respect to the cylinder inner peripheral surface 4 in Examples 1 to 5 and Comparative Examples 1 to 4 were estimated by assuming that the outer peripheral surface of the top ring was parabolic. Using the barrel amounts and measured widths of the barrel amounts in Table 1, the contact widths were estimated by assuming that the assumed radial wear amount corresponding to the contact width was 2 μm. Regarding the contact widths with respect to the cylinder inner peripheral surface 4, the contact width in Example 1 was 0.800 mm. The contact width in Example 2 was 0.566 mm. The contact width in Example 3 was 0.800 mm. The contact width in Example 4 was 0.800 mm. The contact width in Example 5 was 0.800 mm. The contact width in Comparative Example 1 was 0.800 mm. The contact width in Comparative Example 2 was 2.263 mm. The contact width in Comparative Example 3 was 0.566 mm. The contact width in Comparative Example 4 was 2.263 mm.

[0069] The surface pressure of the top ring in Examples 1 to 5 and Comparative Examples 1 to 4 was calculated using the above-mentioned formula (1). With regard to the surface pressure of the top ring, the surface pressure in Example 1 was 0.49 MPa. The surface pressure in Example 2 was 0.69 MPa. The surface pressure in Example 3 was 0.22 MPa. The surface pressure in Example 4 was 0.33 MPa. The surface pressure in Example 5 was 0.67 MPa. The surface pressure in Comparative Example 1 was 0.89 MPa. The surface pressure in Comparative Example 2 was 0.08 MPa. The surface pressure in Comparative Example 3 was 1.26 MPa. The surface pressure in Comparative Example 4 was 0.17 MPa.

[0070] As shown in Table 2 below, oil control rings for Examples 11 to 17 and Comparative Examples 11 and 12 were fabricated. First, a special alloy steel was used as the wire material to fabricate first and second rail portions having outer peripheral surfaces including straight contact surfaces. The nominal diameter of the oil control ring was set to 112 mm. The land width and tension were set as shown in Table 2.

[0071] The contact width of the oil control rings in Examples 11 to 17 and Comparative Examples 11 and 12 against the cylinder inner peripheral surface 4 was set to twice the land width, as shown in Table 2. The contact width against the cylinder inner peripheral surface 4 in Example 11 was 0.6 mm. The contact width in Example 12 was 1.0 mm. The contact width in Example 13 was 0.6 mm. The contact width in Example 14 was 0.6 mm. The contact width in Example 15 was 0.6 mm. The contact width in Example 16 was 0.6 mm. The contact width in Example 17 was 0.6 mm. The contact width in Comparative Example 11 was 0.6 mm. The contact width in Comparative Example 12 was 1.0 mm. The surface pressure of the oil control rings in Examples 11 to 17 and Comparative Examples 11 and 12 was calculated using the above-mentioned formula (1). The surface pressure of the oil control ring in Example 11 was 2.08 MPa. The surface pressure in Example 12 was 1.25 MPa. The surface pressure in Example 13 was 0.89 MPa. The surface pressure in Example 14 was 1.49 MPa. The surface pressure in Example 15 was 2.68 MPa. The surface pressure in Example 16 was 3.27 MPa. The surface pressure in Example 17 was 3.87 MPa. The surface pressure in Comparative Example 11 was 4.46 MPa. The surface pressure in Comparative Example 12 was 0.54 MPa.

[0072] Simulation results of the effects of deposit formation in a hydrogen-fueled internal combustion engine are described with reference to Figures 9 and 10. In Figures 9 and 10, the hydrogen-fueled internal combustion engine was assumed to be operating on a bench with a radiator outlet water temperature of 85±2°C, an intercooler outlet water temperature of 40±5°C, and oil temperature left unchanged. The simulation simulated the deposit accumulation rate and piston ring sticking state when the engine was operated at a predetermined partial load for a predetermined time at an engine speed of 1000 rpm. The predetermined operating time for the deposit accumulation rate was 100 hours, and the predetermined operating time for the piston ring sticking state was 3000 hours.

[0073] Figure 9 shows the results of a simulation of the relationship between the deposit accumulation ratio and the state of piston ring sticking. The simulation in Figure 9 corresponds to a case in which a top ring from Examples 1 to 5 and Comparative Examples 1 to 4 in Table 1 was used, a second ring with a tapered outer periphery and a contact pressure of 0.92 MPa, and an oil control ring with a straight outer periphery and a contact pressure of 2.08 MPa were used. The plots of black circles correspond to the top rings from Examples 1 to 5 and Comparative Examples 1 to 4 in Table 1. The plots of white circles correspond to the results of a conventional internal combustion engine using hydrocarbon fuel. For comparison, the deposition ratios of deposits other than carbon sludge in a conventional internal combustion engine are plotted as white circles in Figures 9 and 10.

[0074] As shown in FIG. 9 , for Examples 1 to 5 in Table 1, the deposition amount ratio was estimated to be 0.2 or less, and for Comparative Examples 1 to 4 in Table 1, the deposition amount ratio was estimated to be greater than 0.5. Thus, by setting the surface pressure of the top ring to 0.22 MPa or more and 0.69 MPa or less, it was estimated that the deposition amount ratio of the top ring after the predetermined test could be reduced to 0.2 or less. Furthermore, for the top rings of Examples 2 and 3, it was estimated that the piston rings would not be stuck even after 3,000 hours of operation. In contrast, for the top rings of Comparative Examples 1 and 4, it was estimated that the piston rings would be stuck after 3,000 hours of operation.

[0075] Figure 10 shows the results of a simulation of the relationship between the ratio of deposit accumulation and the state of piston ring adhesion. The simulation in Figure 10 corresponds to a case in which a top ring with a symmetrical barrel-shaped outer periphery and a contact pressure of 0.49 MPa was used, a second ring with a tapered outer periphery and a contact pressure of 0.92 MPa was used, and the oil control rings of Examples 11 to 17 and Comparative Examples 11 and 12 in Table 2 were used.

[0076] As shown in Figure 10, for Examples 11 to 17 in Table 2, it was estimated that the deposition ratio would be 0.2 or less, and for Comparative Examples 11 and 12 in Table 2, it was estimated that the deposition ratio would exceed 0.5. Thus, by setting the contact pressure of the oil control ring to 0.89 MPa or more and 3.87 MPa or less, it was estimated that the deposition ratio of the oil control ring after the specified test could be reduced to 0.2 or less. Furthermore, it was estimated that the piston rings of Examples 13 and 17 would not be stuck even after 3,000 hours of operation. In contrast, it was estimated that the piston rings of Comparative Examples 11 and 12 would be stuck after 3,000 hours of operation.

[0077] 1...piston ring, 2...ring groove, 2a...top ring groove (pressure ring groove), 2b...second ring groove (pressure ring groove), 2c...oil ring groove, 3...piston, 3c...top land (land portion), 3d...second land (land portion), 3e...third land (land portion), 4...cylinder inner surface, 10...top ring (pressure ring), 15, 25, 37a, 38a...outer surface, 20...second ring (pressure ring), 30...oil control ring, 37b, 38b...contact surface (outer surface), 100...piston ring set.

Claims

1. A piston ring set for an internal combustion engine that uses hydrogen fuel, comprising a pressure ring and an oil control ring that are respectively assembled into multiple ring grooves of a piston of the internal combustion engine that uses hydrogen fuel, wherein the surface pressure of the outer surface of the pressure ring against the inner surface of the cylinder of the internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less, and the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder is set to 0.89 MPa or more and 3.87 MPa or less, thereby suppressing the formation of deposits on the land portion of the piston and in the ring grooves that accompany the combustion of the hydrogen fuel.

2. A piston ring set for an internal combustion engine that uses hydrogen fuel as described in claim 1, wherein the plurality of ring grooves include, in order from the top surface side of the piston, a pressure ring groove in which the pressure ring is assembled and an oil ring groove in which the oil control ring is assembled, and the deposits are generated in the land portion between the pressure ring groove and the oil ring groove.

3. A piston ring for an internal combustion engine that uses hydrogen fuel, including a compression ring that is assembled into the ring groove of a piston of the internal combustion engine that uses hydrogen fuel, wherein the surface pressure of the outer surface of the compression ring against the inner surface of the cylinder of the internal combustion engine is set to 0.22 MPa or more and 0.69 MPa or less, thereby suppressing the formation of deposits on the land portion of the piston and in the ring groove that accompanies the combustion of the hydrogen fuel.

4. A piston ring for an internal combustion engine that uses hydrogen fuel, including an oil control ring that is assembled into the ring groove of a piston of the internal combustion engine that uses hydrogen fuel, wherein the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder of the internal combustion engine is set to 0.89 MPa or more and 3.87 MPa or less, thereby suppressing the formation of deposits on the land portion of the piston and in the ring groove that accompanies the combustion of the hydrogen fuel.

5. A method for suppressing deposit formation in an internal combustion engine that uses hydrogen fuel, comprising: a preparation step of preparing a piston ring set including a pressure ring and an oil control ring for the internal combustion engine that uses hydrogen fuel; and an assembly step of assembling the pressure ring and the oil control ring into a plurality of ring grooves of a piston of the internal combustion engine, wherein the preparation step includes at least one of the steps of: setting the surface pressure of the outer surface of the pressure ring against the inner surface of the cylinder of the internal combustion engine to be 0.22 MPa or more and 0.69 MPa or less; and setting the surface pressure of the outer surface of the oil control ring against the inner surface of the cylinder to be 0.89 MPa or more and 3.87 MPa or less, thereby suppressing the formation of deposits on the land portion of the piston and in the ring grooves that accompany combustion of the hydrogen fuel.

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