Internal combustion engine pistons

Abstract

Internal combustion engine piston (10) with an upper annular groove (13) in an outer peripheral surface, wherein the internal combustion engine piston (10) consists of a silicon-containing aluminum alloy, and a region of an inner surface (13c) of the upper annular groove (13), which is at least an inner surface on a side facing a second annular groove and which is in contact with an upper ring (30), is provided with an anodic oxide layer (20, 20A, 20B, 21, 22, 26), characterized in that the anodic oxide layer (20, 20A, 20B, 21, 22, 26) (i) is formed by alternating current / direct current superposition electrolysis by growing in random directions so that the anodic oxide layer (20, 20A, 20B, 21, 26) includes silicon (16) contained in the aluminum alloy (20, 20A, 20B, 21, 26) and has no orientation, or (ii) has sealed portions (25) formed by filling voids (23) of the anodic oxide layer (20, 20A, 20B, 22, 26) produced in the surface of the anodic oxide layer (20, 20A, 20B, 22, 26) formed by direct current electrolysis with powder of the machined anodic oxide layer (24) are formed, and the anodic oxide layer (20, 20A, 20B, 21, 22, 26) has a surface roughness Rpk according to JIS B 0671-2 of 1.00 µm or less.

Description

[Field of technology]

[0001] The present invention relates to an internal combustion engine piston, more particularly to an internal combustion engine piston in which an anodic oxide layer is formed on the inner surface of an upper annular groove. [State of the art]

[0002] To maintain the airtightness of a combustion chamber and suppress oil infiltration into the combustion chamber, a piston ring is mounted on the outer peripheral surface of a piston for an internal combustion engine, such as an automobile engine. The piston ring is fitted into a ring groove formed in the outer peripheral surface of the piston. Of the ring grooves, an upper ring groove closest to the piston crown is subject to wear and adhesion between itself and the upper piston ring. Therefore, to improve the wear resistance and anti-aluminum adhesion property of the upper ring groove, anodization of the upper ring groove is performed.

[0003] However, especially when an anodic oxide layer is formed on the top ring groove of a piston made of an aluminum alloy containing silicon (Si) to improve wear resistance and anti-aluminum adhesion, protrusions and depressions are formed on the surface of the anodic oxide layer due to the Si contained in the aluminum alloy. Thus, a large number of tiny gaps are formed between the top ring groove and the upper piston ring during engine operation, leading to, for example, an increase in the flow rate of blow-by gas and a deterioration in oil sealing performance.

[0004] According to the disclosure of JP H09 - 159 022 A, in a piston whose main body is made of an aluminum-silicon alloy type cast material, when anodizing is carried out in the region of the upper ring groove in the outer peripheral portion of the piston main body, it is possible to achieve a smooth surface roughness Ra of this anodic oxide layer of 1.5 µm or less and thus to reduce the flow rate of the blow-by gas during engine operation if the grain size of the Si primary crystal and the eutectic Si crystallized in this cast material is 10 µm or less.

[0005] DE 195 06 656 B4 discloses a method for ceramizing conductive metal surfaces, wherein a coating of cylinder bores of internal combustion engines is achieved using spark discharge in electrolytes and plasma-chemical coating. DE 10 2014 221 363 A1 discloses a method for producing a coated cylinder bore of an internal combustion engine, wherein a coating is created by plasma electrolytic deposition. Furthermore, DE 10 2013 223 011 A1 discloses a method for producing a coated surface of a tribological system, wherein a coating is created by electrolysis. Low surface roughness can be achieved with the methods disclosed in the three aforementioned publications.However, none of these publications teach that the body to be coated is made of an aluminum alloy containing silicon (Si) to improve wear resistance and anti-aluminum adhesion properties. [Summary of the invention][Technical problem]

[0006] In the example disclosed in JP H09-159022 A, the surface roughness Ra of all the anodic oxide layers is in the range of 1.1 to 1.5 µm or less, and in the method described in JP H09-159022 A in which the piston main body is formed of a predetermined aluminum alloy, in which the anodization is performed, the lower limit of the surface roughness Ra of the anodic oxide layer is 1.1 µm, so that it is difficult to achieve a further reduction in the flow rate of the blow-by gas.

[0007] Regarding the surface roughness of the anodic oxide layer formed on the piston's upper ring groove, the present inventors found that there is only a weak correlation between the surface roughness Ra, as measured in JP H09-159022 A, and the blow-by gas flow rate. For example, according to one measurement result, the blow-by gas flow rate is lower for a piston with an anodic oxide layer with a surface roughness Ra of 1.63 µm than for a piston with an anodic oxide layer with a surface roughness Ra of 1.30 µm. Therefore, with regard to the blow-by gas flow rate, an accurate evaluation based on the surface roughness Ra is not possible.

[0008] Furthermore, in addition to the flow rate of the blow-by gas, a reduction in the number of discharged fine particles PN (particle number) is required, and even if the surface roughness Ra of the anodic oxide layer is 1.5 μm or less, it is difficult to achieve a specified guideline value.

[0009] In view of the above-mentioned problems, it is an object of the present invention to provide an internal combustion engine piston which enables a further reduction of the flow rate of the blow-by gas and the number of discharged fine particles, which allows the surface roughness of the anodic oxide layer to be accurately judged with respect to the flow rate of the blow-by gas, and which enables a smooth anodic oxide layer to be reliably formed. [Technical solution]

[0010] To achieve the above object, according to the present invention, there is provided an internal combustion engine piston having an upper ring groove in an outer peripheral surface, the internal combustion engine piston being made of a silicon-containing aluminum alloy, and a portion of an inner surface of the upper ring groove, which is at least an inner surface portion (i.e., a lower surface) on a side facing a second ring groove and which is in contact with an upper ring, being provided with an anodic oxide layer.The anodic oxide layer (i) is formed by growing in random directions so that the anodic oxide layer encloses silicon contained in the aluminum alloy and has no orientation, or (ii) has sealed portions formed by filling voids of the anodic oxide layer created in the surface of the anodic oxide layer with the powder of the machined layer. The surface roughness Rpk of the anodic oxide layer according to JIS B 0671-2 is 1.00 µm or less. [Advantageous effects of the invention]

[0011] According to the present invention, since the correlation with the blowby gas flow rate according to JIS B 0601 is higher than in the case of the conventional characteristic value Ra, it is possible to accurately evaluate the surface roughness of the anodic oxide layer with respect to the blowby gas flow rate. In addition, it is possible to further improve the airtightness between the anodic oxide layer on the underside of the upper ring groove and the upper ring, thereby making it possible to further reduce the blowby gas flow rate during engine operation compared to the prior art. Furthermore, it is possible to reduce the number of discharged fine particles PN to a value not higher than a predetermined guideline.Furthermore, it is possible to reduce the area of the upper ring groove to be machined by anodic oxidation, thereby reducing the required energy consumption and shortening the anodic oxidation processing time. Furthermore, it is possible to reduce fluctuations in the amount of heat generated and further reduce the surface roughness Rpk. [Brief description of the drawings] Fig. 1 is a schematic sectional view of the vicinity of an upper ring groove of an internal combustion engine piston according to an embodiment of the present invention. Fig. 2 is a schematic sectional view of the vicinity of an upper ring groove of an internal combustion engine piston according to another embodiment of the present invention. Fig.Figure 3 is a schematic cross-sectional view of an AC / DC overlay electrolysis layer that can be applied to an internal combustion engine piston according to the present invention. Fig. Figure 4 is a schematic cross-sectional view of a conventional DC overlay electrolysis layer. Fig. Figure 5 is a schematic cross-sectional view of a direct current overlay electrolysis layer that can be applied to an internal combustion engine piston according to the present invention. Fig. 6 is a schematic diagram illustrating a state where the wettability of the anodic oxide layer with oil is low. Fig. Figure 7 is a schematic diagram illustrating a state where the wettability of the anodic oxide layer with oil is high. Fig.Fig. 8 is a schematic sectional view illustrating a method of performing anodization on the periphery of the upper ring groove to form the internal combustion engine piston as shown in Fig. 2 shown. [Description of the embodiments]

[0012] An internal combustion engine piston according to an embodiment of the present invention is described below with reference to the figures. The figures are provided for convenience and are not drawn to scale.

[0013] As in Fig.As shown in Figure 1, an internal combustion engine piston 10 according to the present embodiment has an upper annular groove 13 in an outer peripheral surface 12. Three annular grooves are formed one below the other: starting from the piston crown 11, in this order, the upper annular groove 13, a second annular groove (not shown), and an oil ring groove (not shown). An upper ring 30 is fitted into the upper annular groove 13, a second ring (not shown) is fitted into the second annular groove, and an oil control ring (not shown) is fitted into the oil ring groove.

[0014] The internal combustion engine piston 10 is made of an aluminum alloy. The aluminum alloy contains silicon (Si) as a component, which contributes to wear resistance and anti-aluminum adhesion. In contrast, the rings, such as the upper ring 30, are formed of, for example, high-strength carbon steel or martensitic stainless steel. The rings, such as the upper ring 30, have a substantially C-shaped configuration open at a portion of the circumference. The rings are inserted into the ring grooves, such as the upper ring groove 13 of the piston 10, in a state of being elastically enlarged in diameter, and then reduced in diameter by elastic recovery to be fitted into the ring grooves.

[0015] An outer peripheral surface 33 of the upper ring 30 protrudes beyond the outer peripheral surface 12 of the internal combustion engine piston 10 on the outer peripheral side. Of the outer peripheral surface 12 of the internal combustion engine piston 10, the section between the piston crown 11 and the upper ring groove 13 is referred to as the upper land 12a, the section between the upper ring groove 13 and the second ring groove (not shown) is referred to as the second land 12b, and the section between the second ring groove and the oil ring groove (not shown) is referred to as the third land (not shown). The respective outer peripheral surfaces of the upper ring 30, etc., protrude outward beyond the outer peripheral surface 12 of the internal combustion engine piston 10, so that when the piston 10 with the rings attached thereto, such as the upper ring 30, is inserted into a cylinder 40, the rings, such as the upper ring 30, are elastically reduced in diameter.Thus, in the state in which the internal combustion engine piston 10 is inserted into the cylinder 40, the rings, such as the upper ring 30, are pressed against an inner wall 41 of the cylinder 40 due to their elastic resilience. The upper ring 30 and the second ring serve to maintain the airtightness of the combustion chamber, and the oil control ring serves to control the oil remaining on the inner wall 41 of the cylinder 40.

[0016] Of the inner surface of the upper annular groove 13 formed in the outer peripheral surface 12 of the internal combustion engine piston 10, the side of the inner surface facing the piston crown 11 is referred to as an upper surface 13a, the side of the inner surface facing the second annular groove (not shown) is referred to as a lower surface 13c, and the surface therebetween at the groove bottom is referred to as a groove bottom surface 13b. In the present embodiment, an anodic oxide layer 20A is formed at least in the region of the lower surface 13c of the upper annular groove 13 in contact with the upper ring 30.Depending on the piston design, this area can, assuming that the length (groove width) from the outer peripheral surface 12 of the internal combustion engine piston 10 to the groove base surface 13b of the upper annular groove 13 is 100%, preferably extend over a length of at least 90%, more preferably over a length of at least 80%, and even more preferably over a length of at least 70% from the outer peripheral surface 12.

[0017] In this way, the anodic oxide layer 20A is formed on the lower surface 13c of the upper annular groove 13, as shown in Fig.1, because during the compression and expansion strokes, high pressure prevails in the combustion chamber on the side of the piston crown 11 of the internal combustion engine piston 10, so that the lower surface 32 of the upper ring 30 is brought into firm contact with the lower surface 13c of the upper ring groove 13. In contrast, although not shown, in the internal combustion engine piston 10, during the intake process, the upper surface 31 of the upper ring 30 comes into close contact with the upper surface 13a of the upper ring groove 13. Each time these processes are repeated, the internal combustion engine piston 10 moves between the upper surface 13a and the lower surface 13c of the upper ring groove 13. Thus, the lower surface 13c of the upper ring groove 13 is subject to wear and adhesion between itself and the upper ring 30, and therefore, improvement of the wear resistance and anti-aluminum adhesion property requires the formation of the anodic oxide layer 20A.

[0018] Using the surface roughness Rpk according to JIS B 0671-2 as a characteristic value, the surface roughness of the anodic oxide layer 20A is 1.00 µm or less. The anodic oxide layer 20 with a surface roughness Rpk of 1.00 µm or less can be formed, for example, by the AC / DC superimposed electrolysis process. In AC / DC superimposed electrolysis, a step of applying a positive voltage and a step of removing electrical charge are repeatedly performed on an aluminum alloy body that is the subject of the electrolysis process. As shown in Fig.3, an anodic oxide layer (AC / DC superimposed electrolysis layer) 21 grows in random directions and has no orientation, so that the anodic oxide layer 21 encloses silicon 16 contained in the aluminum alloy body 15 subject to the electrolysis process during growth, whereby it is possible to form the anodic oxide layer 21 having a dense and smooth surface.

[0019] In contrast, it is difficult to produce the anodic oxide layer 20 with a surface roughness Rpk of 1.00 µm or less by using only direct current electrolysis. In direct current electrolysis, anodization is carried out by applying a fixed direct current voltage to an aluminum alloy body that forms the subject of the electrolysis process. As shown in Fig.As shown in Figure 4, an anodic oxide layer (DC electrolysis layer) 22 formed by the DC electrolysis process grows in one direction, so that the growth of the anodic oxide layer 22 is hindered by the silicon 16 contained in the aluminum alloy body 15 subject to the electrolysis process, and a plurality of large voids 23 are generated on the surface. With the anodic oxide layer 22 having a surface with such large protrusions and depressions, it is difficult to achieve a surface roughness Rpk of 1.00 μm or less.

[0020] Against this background, as in Fig.5, to form an anodic oxide layer 26 having a surface roughness Rpk of 1.00 µm or less, using the anodic oxide layer 22 formed by direct current electrolysis, a surface portion 24 of the anodic oxide layer 22 is removed by machining, and the surface of the anodic oxide layer 22 is sealed with the resulting machined layer powder. Thus, the anodic oxide layer 26 has sealed portions 25 formed by filling the voids of the direct current electrolysis layer with the machined layer powder, thereby smoothing the surface and making it possible to achieve a surface roughness Rpk of 1.00 µm or less. Such machining can be performed, for example, by machining with cutting tools such as indexable inserts and cutting edges, or by machining such as grinding with a grinding wheel or barrel polishing.

[0021] The anodic oxide layer 26 obtained by direct current electrolysis and machining is expected to have a surface structure referred to as a plateau structure. The term "plateau structure" is also used in JIS B 0671 and describes a structure whose surface consists of a plateau portion (flat portion) and a trough portion. As standardized in JIS B 0671-2, the surface roughness parameter Rpk can be used to evaluate the surface properties of a plateau structure. In contrast, the surface roughness parameter Ra standardized in JIS B 0601 refers to the arithmetic mean roughness of a contour measurement chart. Therefore, it can be assumed that the surface roughness Rpk can contribute to a more accurate evaluation of the surface roughness of the anodic oxide layer 26 in relation to the blowby gas flow rate than the surface roughness Ra.Blowby gas is a gas that escapes from the combustion chamber through the gap between the piston and the cylinder wall into the crankcase during the compression / expansion stroke. It is closely related to the airtightness between the piston ring and the ring groove. The surface of the anodic oxide layer (AC / DC superimposed electrolysis layer) 21 formed by the AC / DC superimposed electrolysis cannot be considered a plateau structure. Since this surface, on the other hand, is very smooth, it can be assumed that it can also be assessed with sufficient accuracy using the surface roughness parameter Rpk.

[0022] In addition, the anodic oxide layer 21, 26 having a surface roughness Rpk of 1.00 µm or less is formed in a predetermined area of the lower surface 13c of the upper ring groove 13 of the internal combustion engine piston 10 as anodic oxide layer 20A, wherein the airtightness between the upper ring 30 and the anodic oxide layer 20A on the lower surface 13c of the upper ring groove 13 is very high when, as shown in Fig.1, the upper ring 30 is brought into close contact with the lower surface 13c of the upper ring groove 13 during the compression stroke and the expansion stroke, so that it is possible to reduce the flow rate of the blowby gas escaping from the combustion chamber into the crankcase via the gap between the engine piston 10 and the cylinder 40. In particular, by forming the surface of the anodic oxide layer 20A into a plateau structure, it is possible to further improve the airtightness between the anodic oxide layer 20A on the lower surface 13c of the upper ring groove 13 and the upper ring 30 and also to reduce the flow rate of the blowby gas during actual engine operation.

[0023] The surface roughness Rpk of the anodic oxide layer 20A is preferably 0.90 μm or less, and more preferably 0.60 μm or less. While there is no specific limitation on the lower limit of the surface roughness Rpk of the anodic oxide layer 20A, it is preferably 0.01 μm or more, more preferably 0.10 μm or more, and even more preferably 0.20 μm or more. In this way, by reducing the surface roughness Rpk of the anodic oxide layer 20A to 0.90 µm or less, and by further reducing it to 0.60 µm or less, it is possible to further improve the airtightness between the anodic oxide layer 20A on the lower surface 13c of the upper ring groove 13 and the upper ring 30, so that it is possible to further reduce the flow rate of the blow-by gas during actual engine operation.

[0024] While there is no specific limitation on the thickness of the anodic oxide layer 20A, the upper limit is preferably 15 µm or less, and more preferably 10 µm or less. The lower limit is preferably 3 µm or more, and more preferably 5 µm or more.

[0025] For an internal combustion engine piston 10, in addition to reducing the blowby gas flow rate, a reduction in the number of ejected fine particles (PN) is required. While the primary cause of PN is believed to be a phenomenon (referred to as oil lift) in which engine oil enters the combustion chamber through the gap between the internal combustion engine piston 10 and the cylinder 40, the anodic oxide layer 20 is porous, and the contribution of the anodic oxide layer 20 to oil wettability is presumably large.

[0026] As described above, the anodic oxide layer 21 formed by the AC / DC superimposed electrolysis (AC / DC superimposed electrolysis layer) is oriented in random directions so that its surface is dense and oil cannot easily penetrate into the anodic oxide layer. Thus, as shown in Fig. 6, has a low wettability towards oil 50a. In the anodic oxide layer 26, which is produced by direct current electrolysis and machining, the surface of the direct current electrolysis layer is removed and the surface is sealed by the resulting powder, so that it, as shown in Fig. 6, has a low wettability towards the oil 50a. In contrast, in the Fig. 4 shown direct current electrolysis layer 22 with a plurality of large empty spaces 23 on the surface, oil easily enters these empty spaces 23, so that the layer, as in Fig.7, has a high wettability towards the oil 50b.

[0027] The anodic oxide layer 21, 26, thus having a surface low in wettability with respect to oil, is formed as anodic oxide layer 20A in a predetermined region of the lower surface 13c of the upper annular groove 13 of the internal combustion engine piston 10, whereby, when the oil has risen along the outer peripheral surface 12 of the internal combustion engine piston 10 from the crank chamber and permeates into the pores of the porous anodic oxide layer 20A on the lower surface 13c of the upper annular groove 13 from the outer periphery to the inner side, the oil cannot easily spread over the anodic oxide layer 20A and stagnates, so that it is possible to suppress the oil rise and thus reduce the number of discharged fine particles PN to a value below a standard value.

[0028] The area of the lower surface 13c of the upper ring groove 13 that is not in contact with the upper ring 30 is formed as a machined surface of an aluminum alloy with low surface roughness, which can further reduce the penetration of oil into the combustion chamber. Furthermore, anodization is performed only in the area of the lower surface 13c of the upper ring groove 13 that is in contact with the upper ring 30, that is, in the area where improving wear resistance and anti-aluminum adhesion properties is important, thereby making it possible to reduce the machining area of the upper ring groove 13 for anode oxidation, thereby making it possible to shorten the required energy consumption and anodizing processing time.Furthermore, it is possible to reduce fluctuations in the amount of heat generated, making it possible to further reduce the surface roughness Rpk of the anodic oxide layer 20. The anodic oxide layer can be formed not only in the contact area with the upper ring 30, but also on the entire lower surface 13c of the upper ring groove 13.

[0029] The present invention is not limited to the embodiment described above. For example, according to another embodiment described in Fig.2, the internal combustion engine piston 10 is provided with an anodic oxide layer 20B covering the entire inner surface of the upper annular groove 13 (i.e., the upper surface 13a, the groove base surface 13b, and the lower surface 13c) and a portion of the outer peripheral surface 12 of the internal combustion engine piston 10 extending from the upper annular groove 13 to the piston crown 11 (i.e., the upper land 12a), and a portion of the outer peripheral surface extending from the upper annular groove 13 to the second annular groove (not shown) (i.e., the second land 12b). The anodic oxide layer 20B may be formed integrally. The anodic oxide layer 20B may be formed over the entire surface of the upper land 12a and the second land 12b, or on a portion thereof. In the latter case, the anodic oxide layer 20B is, for example,formed so that it extends at least 4 mm and preferably 2 mm from the edge of the upper ring groove 13 towards the side of the piston crown 11 or towards the side of the second ring groove (not shown).

[0030] In this way, the anodic oxide layer 20B is formed on both the upper land 12a and the second land 12b, so that the oil stagnates not only on the inner surface of the upper annular groove 13, but also on the upper land 12a and the second land 12b, whereby the oil rise can be significantly suppressed and the PN-reducing effect can be further enhanced.

[0031] Next, a method for manufacturing the internal combustion engine piston according to the present invention will be described, that is, a method for forming an anodic oxide layer in a predetermined region of the upper annular groove provided in the outer peripheral surface of the internal combustion engine piston.

[0032] As in Fig.8, an anodizing device 60 is first arranged to surround the outer peripheral side of the upper annular groove 13 of the internal combustion engine piston 10. The anodizing device 60 and the internal combustion engine piston 10 are brought into close contact with each other by means of sealing rings 62a and 62b, which are arranged substantially in the center of the upper land 12a and the second land 12b, respectively. While a processing liquid 63 is supplied into the upper annular groove 13 of the internal combustion engine piston 10 and to the parts of the upper land 12a and the second land 12b up to the sealing rings 62a and 62b, an electric current is supplied between the internal combustion engine piston 10 serving as an anode and a cathode electrode 61 on the side of the anodizing device 60, whereby the anodic oxide layer 20B is formed on the surface of the part of the aluminum alloy to which the processing liquid 63 is supplied.

[0033] During this anodizing process, hydrogen bubbles 64 are generated on the side of the cathode electrode 61. When the hydrogen bubbles 64 come into contact with the inner surface of the upper annular groove 13, the anodizing does not continue at this contact point, so that the surface roughness of the formed anodic oxide layer 20B is comparatively high. In view of this, as shown in Fig. 8, the processing liquid 63 is supplied to the upper land 12a and the second land 12b, and most of the generated hydrogen bubbles 64 flow to the upper land 12a and the second land 12b where the pressure is low, so that it is possible to suppress the penetration of the hydrogen bubbles 64 into the upper annular groove 13. In this way, it is possible to obtain a smooth anodic oxide layer 20B with low surface roughness.

[0034] This anodizing process is applicable to both AC / DC superimposed electrolysis and DC electrolysis. Fig. 1, in which the anodic oxide layer 20A is to be formed on a partial area of the lower surface 13c of the inner surface of the upper annular groove 13, the remaining part of the inner surface of the upper annular groove 13 and the surfaces of the upper land 12a and the second land 12b are masked by means of resin, a metal spring, a spray coating or the like, whereby it is possible to form the anodic oxide layer 20A only on the desired area. [Examples]

[0035] Examples of the present invention and comparative examples are described below. (anodization)

[0036] In Examples 1 to 3, an anodic oxide layer was formed on the inner surface of the upper ring groove of an internal combustion engine piston made of an aluminum alloy by the following processing steps. First, the internal combustion engine piston was cast from an aluminum alloy containing 12% Si and subjected to a predetermined heat treatment before machining a ring groove. The machining of the upper ring groove was completed so that the surface roughness Rpk was approximately 0.01 µm. In addition, using sulfuric acid as the processing liquid, anodic oxidation was performed on the inner surface of this upper ring groove by the AC / DC superimposed electrolysis method under the following conditions: a power supply duration for applying a positive voltage of 20 to 30 µs and a frequency of 10 to 12 kHz.Subsequently, the surface roughness Ra and surface roughness Rpk of the anodic oxide layer (AC / DC overlay electrolysis layer) formed on the lower surface of the upper annular groove were measured according to JIS B 0601 and JIS B 0671-2, respectively. The measurement results are shown in Table 1.

[0037] In Examples 4 to 6, an anodic oxide layer was formed on the inner surface of the upper annular groove in the same manner as in Examples 1 to 3, except that direct current electrolysis was used for anodization instead of AC / DC superimposed electrolysis, and the surface of the anodic oxide layer (direct current electrolysis layer) was post-processed with cutting tools. The conditions of the direct current electrolysis process were as follows: electric current density 4 to 10 A / dm 2and processing time 0.2 to 1.5 minutes. The measurement results are shown in Table 1.

[0038] In Comparative Examples 1 to 3, an anodic oxide layer was also formed on the inner surface of the upper annular groove in the same manner as in Examples 1 to 3, except that direct current electrolysis was used for anodization instead of alternating current / direct current superimposed electrolysis. The conditions of the direct current electrolysis process were as follows: electric current density 4 to 10 A / dm 2 and processing time 0.2 to 1.5 minutes. The measurement results are shown in Table 1. (Measurement of the flow rate of the blowby gas)

[0039] Experiments to measure the flow rate of blowby gas in an actual engine were conducted on the internal combustion engine pistons of Examples 1 to 6 and Comparative Examples 1 to 3, on which an anodic oxide film had been formed. The test conditions were as follows: a 1200 cc four-cylinder in-line engine was used, the engine speed at full load was 6400 rpm, the water temperature was 90°C, and the oil temperature was 135°C. The test results are shown in Table 1. The flow rate of blowby gas in Table 1 was determined as follows: the average value of the measured values of the flow rate of blowby gas (unit: L / min) 30 hours after the start of engine operation was calculated. The values shown are standardized, with the average value of Comparative Example 1 being 100. [Table 1] anodic oxide layer Surface treatment Ra [µm] Rpk [µm] Flow rate of blowby gas Example 1 AC / DC overlay electrolysis layer Not executed 1.09 0.68 42 Example 2 AC / DC overlay electrolysis layer Not executed 0.93 0.88 69 Example 3 AC / DC overlay electrolysis layer Not executed 1.01 0.73 44 Example 4 Direct current electrolysis layer Executed 0.81 0.59 33 Example 5 Direct current electrolysis layer Executed 0.92 0.22 14 Example 6 Direct current electrolysis layer Executed 1.14 0.43 28 Comparison example 1 Direct current electrolysis layer Not executed 0.85 1.21 100 Comparison example 2 Direct current electrolysis layer Not executed 1.30 1.35 111 Comparison example 3 Direct current electrolysis layer Not executed 1.13 1.07 89 (Evaluation of wettability with oil)

[0040] Oil wettability tests were conducted on samples in which an anodic oxide film was formed on an aluminum alloy base under the same conditions as in Examples 1, 4, and Comparative Example 1, and on non-anodized aluminum alloy bases. The oil used was 5W-30. Each test sample was placed on a hot plate, and a predetermined amount of oil was dropped at a predetermined temperature, and the maximum width (unit: mm) of the oil film was measured in each case. The measurement results are shown in Table 2. The wetting results shown in Table 2 are standardized, with the oil width on the aluminum alloy base being 1.0 at 30°C. [Table 2] temperature 30°C 80°C 130°C 170°C Base body made of aluminum alloy 1.0 1.3 1.4 1.8 Example 1 0.9 1.1 1.2 1.2 Example 4 0.4 0.4 0.6 0.6 Comparison example 1 1.2 1.6 1.7 1.8

[0041] As can be seen from the results in Table 1, a correlation is observed between the surface roughness Rpk and the blowby gas flow rate, while the surface roughness Ra is in a small range of approximately 0.8 to 1.1 µm and shows no correlation with the blowby gas flow rate. This shows that it is impossible to accurately evaluate the surface roughness of the anodic oxide layer with respect to the blowby gas flow rate using the surface roughness parameter Ra. The blowby gas flow rates of Examples 1 to 6 are reduced by 31% or more compared to Comparative Example 1. Consequently, by providing an anodic oxide layer with a surface roughness Rpk of 1.00 µm or less, it is possible to reliably reduce the blowby gas flow rate.

[0042] As can be seen from the results in Table 2, the width of the oil film increases with temperature. In addition, the direct current electrolysis layer of Comparative Example 1 is porous, so that in the low-temperature range, the width of the oil film thereon is larger than that of the oil film on the aluminum alloy base body. In contrast, the AC / DC superimposed electrolysis layer of Example 1 has a dense surface, so that the width of the oil film is smaller than that of the oil film on the aluminum alloy base body. Moreover, at high temperatures, the increase in the ratio of the oil film width is significantly smaller compared to the case of the aluminum alloy base body. On the direct current electrolysis layer subjected to the surface treatment of Example 4, the width of the oil film is even smaller and maintains a small size even with an increase in temperature.Thus, the anodic oxide layer of Examples 1 and 4 exhibits low oil wettability. Especially in the high-temperature range, the difference between these examples and the aluminum alloy base body and Comparative Example 1 is striking. Consequently, by providing an anodic oxide layer with low oil wettability in the high-temperature range, it is possible to effectively suppress oil rise and thus make a significant contribution to reducing the number of discharged fine particles PN. [List of reference symbols] 10 internal combustion engine pistons 11 Piston crown 12a Upper Bridge 12b Second Bridge 13 Upper ring groove 13a upper surface 13b Groove base area 13c Lower surface of the upper ring groove 15 aluminum alloy component 16 Silicon 20, 20A, 20B, 21, 22, 26 anodic oxide layer 23 white space 24 Surface section 25 Sealed Section 30 Upper (piston) ring 31 upper surface 32 lower surface 33 outer peripheral surface 40 cylinders 41 Interior wall 50, 50a, 50b Oil 60 Anodizing device 61 Cathode electrode 62 sealing ring 63 Process fluid 64 Hydrogen bubble

Claims

[1] Internal combustion engine piston (10) with an upper annular groove (13) in an outer peripheral surface, wherein the internal combustion engine piston (10) consists of a silicon-containing aluminum alloy, and a region of an inner surface (13c) of the upper annular groove (13), which is at least an inner surface on a side facing a second annular groove and which is in contact with an upper ring (30), is provided with an anodic oxide layer (20, 20A, 20B, 21, 22, 26), characterized by , that the anodic oxide layer (20, 20A, 20B, 21, 22, 26) (i) is formed by alternating current / direct current superposition electrolysis by growing in random directions so that the anodic oxide layer (20, 20A, 20B, 21, 26) includes silicon (16) contained in the aluminum alloy (20, 20A, 20B, 21, 26) and has no orientation, or (ii) has sealed portions (25) formed by filling voids (23) of the anodic oxide layer (20, 20A, 20B, 22, 26) produced in the surface of the anodic oxide layer (20, 20A, 20B, 22, 26) formed by direct current electrolysis with powder of the machined anodic oxide layer (24) are formed, and the anodic oxide layer (20, 20A, 20B, 21, 22, 26) has a surface roughness Rpk according to JIS B 0671-2 of 1.00 µm or less. [2] An internal combustion engine piston (10) according to claim 1, wherein the anodic oxide layer (20, 20A, 20B, 21, 22, 26) has a surface roughness Rpk according to JIS B 0671-2 of 0.90 µm or less. [3] An internal combustion engine piston (10) according to claim 1, wherein the anodic oxide layer (20, 20A, 20B, 21, 22, 26) has a surface roughness Rpk according to JIS B 0671-2 of 0.60 µm or less. [4] An internal combustion engine piston (10) according to any one of claims 1 to 3, wherein the anodic oxide layer (20, 20B, 21, 22, 26) is provided on the entire inner surface (13a, 13b, 13c) of the upper annular groove (13) and on a portion of the outer peripheral surface of the internal combustion engine piston (10) extending from the upper annular groove (13) to a piston crown (11) and on a portion of the outer peripheral surface extending from the upper annular groove (13) to the second annular groove.

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