piston rings

Abstract

A piston ring for an internal combustion engine using hydrogen fuel, wherein the piston ring satisfies 0.75≤FPS when the flexural index of the piston ring when subjected to a flexural test under specified conditions is set to FP1, the post-corrosion flexural index of the piston ring when subjected to a corrosion test after immersing the piston ring in a 1% nitric acid aqueous solution for 30 minutes is set to FP2, and the flexural strength maintenance rate FPS of the piston ring is set to FPS=FP2 / FP1.

Description

Technical Field

[0001] The present invention relates to a piston ring for an internal combustion engine using hydrogen fuel. Background Art

[0002] In recent years, there have been demands to reduce CO2 emissions and fossil fuel usage. From the perspective of environmental burden, research is underway on a technology involving internal combustion engines that use hydrogen or a fuel mixed with other fuels (hereinafter, both will be collectively referred to as hydrogen fuel) as fuel.

[0003] In connection with this, for example, Patent Document 1 discloses a component used in a hydrogen engine, which is provided with a coating made of stainless steel in order to suppress hydrogen embrittlement fracture.

[0004] Patent Document 2 discloses a hydrogen engine component that has been subjected to physical and chemical treatments to prevent rusting of the component caused by water vapor generated by combustion of hydrogen.

[0005] Furthermore, Patent Document 3 shows that in a conventional hydrogen engine, cylinder cooling based on condensation of water vapor is not performed, and discloses a hydrogen engine for controlling condensation of water vapor in a cylinder forming a combustion chamber.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Utility Model Application Laid-Open No. 51-137004

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 51-081203

[0010] Patent Document 3: Japanese Patent Application Laid-Open No. 2003-013765 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] Unlike gasoline, hydrogen fuel produces a lot of H2O and NO during combustion. x , H2O backflow occurs within the cylinder, potentially causing corrosive wear on the upper and lower surfaces of the piston ring due to condensed water. Furthermore, if the corrosion resistance of the upper and lower surfaces of the piston ring is insufficient, corrosion can lead to reduction in the piston ring's width, roughening of the upper and lower surfaces, and pitting, potentially causing the piston ring to break during operation. The present invention aims to provide a piston ring with sufficient breakage resistance for use in internal combustion engines fueled by hydrogen.

[0013] Solutions for solving problems

[0014] The present inventors conducted research to solve the above-mentioned problems and found that the above-mentioned problems can be solved by the following piston ring, thereby completing the present invention: the piston ring is a piston ring for an internal combustion engine using hydrogen fuel, and the flexural strength maintenance rate of the piston ring when undergoing a specified flexural test meets the specified conditions.

[0015] The present invention is a piston ring for an internal combustion engine using hydrogen fuel, wherein:

[0016] The bending resistance index of the piston ring when the piston ring is subjected to a bending resistance test under prescribed conditions is denoted as FP1.

[0017] The post-corrosion flexural index of the piston ring obtained by performing the flexural test after the piston ring was immersed in a 1% nitric acid aqueous solution for 30 minutes was denoted as FP2.

[0018] When the flexural strength maintenance rate FPS of the piston ring is set to FPS=FP2 / FP1,

[0019] The piston ring satisfies 0.75≤FPS.

[0020] Furthermore, preferably, when the arithmetic mean roughness of the upper surface of the piston ring after the corrosion test is defined as Ra (μm) and the first strength maintenance coefficient KDA is defined as KDA=FPS / Ra,

[0021] The piston ring satisfies 0.2≤KDA.

[0022] Furthermore, when the maximum valley depth of the upper surface of the piston ring after the corrosion test is defined as Rv (μm) and the second strength maintenance coefficient KDV is defined as KDV=FPS / Rv,

[0023] The piston ring satisfies 0.06≤KDV.

[0024] Furthermore, preferably, the internal combustion engine is a spark ignition engine.

[0025] Effects of the Invention

[0026] According to the present invention, it is possible to provide a piston ring having sufficient breakage resistance in an internal combustion engine using hydrogen fuel. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of the fixing fixture used in the flexural test. Figure 1 (a) is a cross-sectional view. Figure 1 (b) is a schematic top view.

[0028] Figure 2 It is a schematic diagram (top view) for explaining the force point length dg in the bending test.

[0029] Figure 3 It is a schematic diagram for explaining the shape of the indenter 23 used in the bending test. DETAILED DESCRIPTION

[0030] One embodiment of the present invention is a piston ring for an internal combustion engine using hydrogen fuel. A specific embodiment thereof will be described below. However, unless otherwise specified, the configurations described in the following embodiments are not intended to limit the technical scope of the invention to these configurations.

[0031] When in use, the piston ring of this embodiment is attached to a piston ring groove formed in a piston, and reciprocates while sliding on the inner peripheral surface of a cylinder bore by the reciprocating motion of the piston.

[0032] The piston ring of the present embodiment may be a so-called compression ring such as a top ring or a second ring, or may be an oil ring.

[0033] In this specification, the "upper surface" of the piston ring refers to the surface on the combustion chamber side when the piston ring is mounted on the piston, and the "lower surface" refers to the surface on the crank chamber side.

[0034] In this specification, hydrogen fuel refers to a fuel composed solely of hydrogen (excluding impurities) or a fuel mixed with other fuels. The proportion of hydrogen in the total fuel is 50% or more, and may be 60% or more, or 70% or more.

[0035] The piston ring of the present embodiment is a piston ring for an internal combustion engine using hydrogen fuel, wherein, when the flexural index of the piston ring when the piston ring is subjected to a flexural test under prescribed conditions is set to FP1, when the flexural index of the piston ring when the flexural test is performed after the piston ring is subjected to a corrosion test in which the piston ring is immersed in a 1% nitric acid aqueous solution for 30 minutes is set to FP2, and when the flexural strength maintenance rate FPS of the piston ring is set to FPS=FP2 / FP1, the piston ring satisfies 0.75≤FPS.

[0036] This means that the greater the flexural strength maintenance rate FPS, the smaller the reduction rate of the flexural index after corrosion.

[0037] If the FPS is less than 0.75, the flexural strength will be significantly reduced due to corrosion, and the piston ring may be damaged during operation when using hydrogen fuel.

[0038] An FPS of 0.75 or higher ensures sufficient breakage resistance in hydrogen-fueled internal combustion engines, even in environments prone to corrosion caused by condensed water. Meeting these FPS requirements can be achieved by appropriately selecting the piston ring material and performing surface treatment.

[0039] The flexural test is as follows.

[0040] Fix the piston ring to Figure 1 The fixing fixture shown is used to perform the flexural test using a flexural testing machine (universal testing machine). Figure 1 In the figure, (a) is a schematic cross-sectional view of the fixing fixture, and (b) is a schematic top view of the fixing fixture. In the bending test, the piston ring 21 is fixed with the fastener 22 so that the distance L from the force point P to the fulcrum is 15 mm. A load is applied to the piston ring 21 from the upper surface along the axial direction of the piston ring to perform the bending test. Figure 2 As shown, the force point length dg (mm) is calculated from the length of the chord of the piston ring 21 relative to the distance L from the force point P to the fulcrum. Note that the chord length is calculated using the following formula based on the outer diameter d (nominal diameter) of the piston ring when attached to the cylinder bore.

[0041] θa=sin －1 (L / (d / 2))dg=d×sin(θa / 2)

[0042] The speed of the test load is set to 1 mm / min. Figure 3 As shown, the opening angle θ of the tip of the indenter 23 was set to 60 degrees, the indenter width PW was set to 10 mm, and the tip shape was set to a rounded R shape with a radius of 0.5 mm. The test temperature was set to room temperature, the environment was set to atmospheric air, and the load was detected by an attached load cell.

[0043] Here, the maximum axial width of the piston ring is set to h11 (mm), and the maximum radial thickness is set to a11 (mm). If the piston ring is damaged during the bending test, the test is terminated at that time, and the maximum load up to that time is set to the maximum load F1 (N). If the piston ring is not damaged when the indenter 23 is lowered to 10 mm, the maximum load up to that time is set to the maximum load F1 (N). The section modulus Z1 (mm 3 ) is set as Z1 = (a11 × h11 × h11) / 6, and the flexural index FP1 (N / mm 2 ) is set as FP1=(F1×dg) / Z1.

[0044] When a test is performed using a piston ring that has undergone a corrosion test described later, the piston ring is fixed so that a portion corroded by the corrosion test serves as a fulcrum.

[0045] The maximum axial width of the piston ring after the corrosion test was h12 (mm), the maximum radial thickness after the corrosion was a12 (mm), the maximum load in the bending test after the corrosion test was F2 (N), and the section modulus after the corrosion was Z2 (mm 3 ) is set as Z2 = (a12 × h12 × h12) / 6, and the flexural index FP2 (N / mm 2 ) is set as FP2=(F2×dg) / Z2.

[0046] The corrosion test was as follows.

[0047] The nitric acid aqueous solution used in the corrosion test was prepared by mixing 8 g of nitric acid (1.38) (standard content 60% to 61%) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. and 492 g of distilled water to give a nitric acid concentration of 1%.

[0048] Immerse a beaker filled with nitric acid solution in a water bath. Once the temperature of the nitric acid solution in the beaker reaches 80°C, immerse the piston ring in the nitric acid solution. While maintaining the temperature at 80°C, stir the solution every 5 minutes for 30 minutes. After 30 minutes, remove the piston ring and rinse it with distilled water.

[0049] In addition, for the piston ring of this embodiment, it is preferable that the arithmetic mean roughness of the upper surface of the piston ring after the corrosion test is Ra (μm), the first strength maintenance coefficient KDA (μm) -1 ) is set to KDA=FPS / Ra, and 0.2≤KDA is satisfied.

[0050] If KDA is less than 0.2, the bending strength will be reduced due to surface roughness such as pitting caused by corrosion, which may cause the piston ring to break during operation. On the other hand, by setting KDA to 0.2 or above, the bending strength retention rate after corrosion is increased, and surface roughness (breaking starting point) is reduced, which can prevent piston ring breakage.

[0051] If KDA is to meet the above conditions, it can be achieved by appropriately selecting the piston ring material, processing conditions such as heat treatment, and surface treatment to increase the FPS value or reduce Ra.

[0052] In addition, for the piston ring of this embodiment, it is preferable that the maximum valley depth of the upper surface of the piston ring after the corrosion test is Rv (μm), the second strength maintenance coefficient KDV (μm) -1) is set to KDV=FPS / Rv, and 0.06≤KDV is satisfied.

[0053] If KDV is less than 0.06, the reduction in flexural strength due to corrosion is significant, and there is a risk that the piston ring may break during operation due to corrosion. On the other hand, by setting KDV to 0.06 or higher, the flexural strength retention rate after corrosion is increased, and surface roughness (the starting point of breakage) is reduced, thus preventing breakage of the piston ring.

[0054] If KDV is to meet the above conditions, it can be achieved by appropriately selecting the material and surface treatment of the piston ring to increase the FPS value or reduce Rv.

[0055] For Ra and Rv, the roughness of the piston ring upper surface was measured at three locations with an evaluation length of 4 mm (sampling length 0.8 mm) using a stylus surface roughness tester in accordance with ISO 4287, and the average value of the three measured values ​​was used.

[0056] The arithmetic mean roughness Ra (μm) of the upper surface of the piston ring according to the present embodiment after the corrosion test may be 5 μm or less, or 4 μm or less.

[0057] Known methods can be applied to the surface treatment of piston rings to meet the above-mentioned preferred surface property parameters. Examples include surface treatments such as PVD (Physical Vapor Deposition) and DLC (Diamond-Like Carbon), chemical conversion treatments, and nitriding treatments.

[0058] The maximum valley depth Rv (μm) of the upper surface of the piston ring according to the present embodiment after the corrosion test may be 14 μm or less, or 12 μm or less.

[0059] The cross-sectional shape of the piston ring of this embodiment relative to the circumferential direction is not particularly limited and may be rectangular, undercut, or inner cut, or may be keystone.

[0060] The size of the opening gap of the piston ring of the present embodiment is not particularly limited, but is, as an example, within a range of 0.1 mm to 0.8 mm.

[0061] The axial width of the piston ring of this embodiment is not particularly limited, but is usually 0.8 mm or more and 4.0 mm or less. In addition, the outer diameter d is usually 50 mm or more and 220 mm or less.

[0062] The piston ring of this embodiment may also have a coating on its upper and lower surfaces. When the piston ring has a coating on its upper and lower surfaces, for example, it may be any single coating such as a PVD coating, a DLC coating, a hard chrome plating coating, a nitriding coating, a ferroferric oxide coating, a phosphate coating, a manganese-based phosphate coating, or a resin coating, or it may be a laminate of two or more of these. Furthermore, the coating may be applied only to the upper or lower surface, to both surfaces, or to a portion of the upper and lower surfaces rather than the entire surface.

[0063] In addition, when the upper surface has a soft coating such as a resin coating, the above-mentioned corrosion test is performed in a state where the lower soft coating is peeled off.

[0064] When the piston ring of this embodiment has coatings on the upper and lower surfaces, the thickness of the coating is not particularly limited, but is usually 0.001 mm or more, preferably 0.005 mm or more, and usually 0.1 mm or less, preferably 0.05 mm or less.

[0065] It should be noted that, when the coating includes a DLC coating, the DLC coating may be a DLC coating containing hydrogen or a so-called hydrogen-free DLC coating.

[0066] The material of the piston ring base material needs to be appropriately selected to satisfy the above-mentioned FPS. Examples of high-alloy steel include martensitic stainless steel, low-alloy steel include valve spring steel and spring steel, and examples of carbon steel include hard steel wire.

[0067] Example

[0068] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

[0069] Examples 1 to 17, Comparative Examples 1 to 3

[0070] By varying the axial length (width) of the piston ring from 0.78 to 1.52 and the radial thickness from 1.95 to 3.72, piston ring samples with varying section moduli were produced, as shown in Table 1. As shown in Table 1, the samples differed in material and top surface treatment. It should be noted that even with the same type of surface treatment, varying treatment conditions such as temperature and time can produce samples with varying top surface roughness.

[0071] These are referred to as Examples 1 to 17 and Comparative Examples 1 to 3.

[0072] In Table 1, the surface treatments on the piston ring tops are: A: ferrosoferric oxide coating, B: manganese-based phosphate coating, C: nitriding coating, D: phosphate coating, and E: no treatment. Furthermore, the material is: A: low-alloy steel, B: high-alloy steel, and C: cast iron.

[0073] Use in Figure 1 The piston rings produced in each example and comparative example were subjected to a bending test before and after the corrosion test using a bending tester whose outline is shown in FIG. 1 , and FPS, KDA, and KDV were calculated. The results are shown in Table 1.

[0074] The piston rings used in the test all had an outer diameter d (nominal diameter) of 81 mm when attached to the cylinder bore, and a force point length dg obtained by calculating the chord relative to L, dg = 15.3 mm.

[0075] [Table 1]

[0076] Table 1

[0077]

[0078] As described above, when the piston ring specified in the present invention is used in an internal combustion engine using hydrogen fuel, a piston ring having sufficient breakage resistance even in an environment where condensed water generated by combustion of hydrogen fuel is present can be provided.

[0079] Description of reference numerals:

[0080] 21: piston ring;

[0081] 22: Fasteners;

[0082] 23: pressure head;

[0083] P: force point;

[0084] L: distance from the force point P to the fulcrum;

[0085] dg: force point length;

[0086] PW: press head width;

[0087] θ: opening angle.

Claims

1. A piston ring for a spark ignition engine using hydrogen fuel, wherein: The bending resistance index of the piston ring when the piston ring is subjected to a bending resistance test under prescribed conditions is denoted as FP1. The post-corrosion flexural index of the piston ring obtained by performing the flexural test after the piston ring was immersed in a 1% nitric acid aqueous solution for 30 minutes was denoted as FP2. When the flexural strength maintenance rate FPS of the piston ring is set to FPS=FP2 / FP1, The piston ring satisfies 0.75≤FPS, When the arithmetic mean roughness of the upper surface of the piston ring after the corrosion test is defined as Ra, the maximum valley depth is defined as Rv, and the first strength maintenance coefficient KDA is defined as KDA=FPS / Ra, The piston ring satisfies 0.2≤KDA, Ra≤5, Rv≤14, The piston ring has one or more coatings selected from the group consisting of a PVD coating, a DLC coating, a hard chrome plating coating, a nitriding coating, a ferrosoferric oxide coating, a phosphate coating, a manganese-based phosphate coating, and a resin coating. The unit of Ra is μm, and the unit of Rv is μm.

2. The piston ring according to claim 1, wherein: When the second strength maintenance coefficient KDV of the piston ring after the corrosion test is set to KDV=FPS / Rv, The piston ring satisfies 0.06≤KDV.

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