Spark plug for internal combustion engine

Abstract

Spark plug (1) for an internal combustion engine, comprising: a center electrode (3) held inside a cylindrical insulator (2) and projecting from a tip of the insulator (2) to a distal end; a ground electrode (4) provided at a distal end of a housing (H) holding the insulator (2), the ground electrode (4) facing the center electrode (3) in an axial direction (X); and a composite chip (5) formed on the center electrode (3) and / or the ground electrode (4) and protruding in the axial direction (X), wherein the composite chip (5) comprises a core (51) with a holder (511) formed integrally with an electrode base material (3A, 4A), and a cup-shaped surface layer (52) having a discharge portion (521) covering a projection end surface (512) of the core (51) and a side surface coating (522) covering a side surface (513) continuously to the projection end surface (512), the core (51) is formed from a Ni alloy material and the surface layer (52) is formed from a Pt alloy material, and in the surface layer (52), a coating thickness S of the side surface coating (522) in a radial direction (Y), an outer diameter D1 of the discharge portion (521) and a coating length L1 of the side surface coating (522) in the axial direction (X) satisfy a relationship represented by an expression 1: Expression 1 : S ≥ D1 / 20 + L1 / 10 − 0.005 mm .

Description

Cross-reference to related applications

[0001] This application is based on Japanese Patent Application No. 2017-202589 filed on October 19, 2017, and Japanese Patent Application No. 2018-189149 filed on October 4, 2018. Technical area

[0002] The present disclosure relates to a spark plug for an internal combustion engine. General state of the art

[0003] An internal combustion engine, such as an automobile engine, is provided with an ignition device including a spark plug for generating a spark discharge to ignite a gas mixture of fuel gas and air. In recent years, lean combustion has been utilized to improve the fuel economy of internal combustion engines. To improve ignitability for lean combustion, the tips of electrodes forming a spark gap have been designed as chips. For example, a spark plug disclosed in JP 5 545 166 B2 has a needle-shaped chip formed on at least one of its center electrode and ground electrode, and is a composite chip formed of a base material connecting portion and a discharge portion to improve ignitability and reduce costs. The discharge portion is made of a high-density material, such asNoble metal, and covers at least a part of the side surface of the base material connecting portion, and the thickness of the discharge portion decreases toward the electrode base material to reduce the noble metal consumption.

[0004] JP 6 017 027 B2 discloses a spark plug in which at least one of its center electrode and ground electrode is formed from a shaft and an electrode chip bonded to a surface of the shaft. In the shaft, a first core made of a material including copper is coated with a first outer layer having higher corrosion resistance than that of the first core, while in the electrode chip, a second outer layer forming its outer surface and made of a noble metal-containing material covers a second core having higher thermal conductivity than that of the second outer layer. In addition, the first core and the second core are bonded to each other via a diffusion-bonded region, while the first outer layer and the second outer layer are bonded to each other via a laser-melted region.

[0005] WO 2017 / 170273 A1 discloses that a spark plug for an internal combustion engine includes an electrode protrusion protruding from an electrode base material of a ground electrode toward a discharge gap. The electrode protrusion has a base part integrated with the electrode base material and a cover part connected to the base part and facing the discharge gap. The base part has an end surface facing in a protrusion direction of the base part and a side peripheral surface. An outer edge of the end surface has a curved surface. The cover part is formed of a noble metal or a noble metal alloy having a lower linear expansion coefficient than that of a material for forming the base part, and covers at least a part of the side peripheral surface and the end surface of the base part.While the spark plug is mounted on an internal combustion engine and the electrode projection is heated and then cooled, a projection is formed on an outer surface of a portion covering the side peripheral surface of the base part.

[0006] Furthermore, DE 10 2017 104 822 A1 discloses a spark plug comprising an elongated center electrode, a ground electrode, a convex portion, and a precious metal coating layer. The center electrode is contained in a housing. The ground electrode has a tip-end opposite portion located opposite the center electrode. The convex portion extends from the tip-end opposite portion to an opposite portion in an axial direction of the spark plug. A spark gap is formed between a tip-end portion of the center electrode and the convex portion. The precious metal coating layer covers a surface of the convex portion.The noble metal plating layer includes an end surface plating layer covering a protruding end surface of the convex portion, and a side surface plating layer covering at least a portion of a side surface of the convex portion extending from the protruding end surface. A base portion of the side surface plating layer is recessed in the portion opposite the tip end. An extension portion is formed such that at least a portion of the base portion is extended toward an exterior of the spark plug along the opposite portion. Summary of the invention

[0007] Because a lean-burn engine accelerates the flow velocity in each cylinder to promote combustion, a spark discharge generated in the spark gap tends to be blown away by airflow. In such a case, a high-speed airflow can change the discharge path, shifting the spark discharge toward the proximal end of the chip, resulting in chip side surface wear. Furthermore, to suppress spark blowout by changing the discharge path, the ignition energy has become larger than before, which tends to accelerate electrode wear and also increases chip side surface wear.

[0008] In the structure disclosed in JP 5 545 166 B2, since the discharge portion covering the side surface of the base material connecting portion becomes thinner toward the proximal end of the side surface, the base material connecting portion may be exposed with lower wear resistance if the thin portion is rapidly worn. Alternatively, if the thin portion cracks under thermal stress due to the difference in linear expansion coefficients from the base material connecting portion, the base material connecting portion is exposed and prone to further wear. Therefore, it is desirable to further improve the wear resistance of the chip side surface.

[0009] In the structure disclosed in JP 6 017 027 B2, the second outer layer of the electrode chip covers the entire second core and thus contains a larger amount of precious metal. The costs are correspondingly high. Furthermore, the second outer layer is directly bonded and fixed to the first outer layer for the shaft, and if the second outer layer becomes thinner, cracks tend to occur due to the difference in linear expansion coefficients. Furthermore, such different metals make it difficult to increase their bond strength.

[0010] An object of the present disclosure is to provide a spark plug for an internal combustion engine having a long life and excellent ignitability by reducing side surface wear on a composite chip and using a noble metal material.

[0011] One aspect of the present disclosure corresponds to a spark plug for an internal combustion engine, and the spark plug comprises: a center electrode held inside a cylindrical insulator and projecting from the tip of the insulator to a distal end; a ground electrode provided at the distal end of a housing holding the insulator, the ground electrode facing the center electrode in the axial direction; and a composite chip formed on the center electrode and / or the ground electrode and protruding in the axial direction.

[0012] The composite chip includes a core having a holder formed integrally with an electrode base material, and a cup-shaped surface layer having a discharge portion covering the protrusion end surface of the core and a side surface coating covering a side surface continuous to the protrusion end surface.

[0013] The core is made of a Ni alloy material and the surface layer is made of a Pt alloy material.

[0014] For the surface layer, the coating thickness S of the side surface coating in the radial direction, the outer diameter D1 of the discharge section and the coating length L1 of the side surface coating in the axial direction satisfy the relationship represented by an expression 1: Expression 1: S≥D1 / 20+L1 / 10−0.005 mm.

[0015] In the spark plug for an internal combustion engine, the cup-shaped surface layer covering the core of the composite chip is formed so that the outer diameter D1 of the discharge portion and the coating thickness S and the coating length L1 of the side surface coating satisfy the relationship represented by Expression 1. Therefore, the spark plug can prevent cracking in the side surface coating. In particular, thermal stress, which may cause cracking, is generated by the difference in linear expansion coefficients between the Ni alloy material constituting the core and the Pt alloy material constituting the surface layer. Furthermore, it is considered that factors for cracking and deterioration are important.Cracking is caused by both the thermal stress generated in the radial direction due to the outer diameter D1 of the discharge section and the thermal stress generated in the axial direction due to the coating length L1 of the side surface coating. Therefore, appropriately adjusting the coating thickness S of the side surface coating to satisfy Expression 1, which incorporates both factors, can reduce the material consumption of the Pt alloy and also suppress cracking. Suppressing cracking can minimize core exposure to increase the wear resistance of the composite chip.

[0016] According to the aspects described above, a spark plug for an internal combustion engine is provided which has a long life and excellent ignitability by reducing the side surface wear on its composite chip and using a precious metal material. Short description of the figures

[0017] The above and other objects, features and advantages of the present disclosure will become more apparent from the detailed description provided below with reference to the accompanying drawings: Fig. 1 is an enlarged cross-sectional view showing a primary part of a spark plug according to a first embodiment; Fig. 2 is an enlarged cross-sectional view of a main part showing a composite chip portion of the spark plug according to the first embodiment; Fig. 3 is a partially sectioned front view showing the entire structure of the spark plug according to the first embodiment; Fig. 4 is an enlarged cross-sectional view of a main part illustrating a spark discharge in the spark gap of the spark plug according to the first embodiment; Fig. 5 is an enlarged cross-sectional view of a main part showing the structure of the composite chip according to the first embodiment; Fig. 6 shows relationships observed in an evaluation test 1 between the outer diameter D1 of a discharge portion and the coating thickness S of a side surface coating in test examples 1 and 2 (specifically, coating length L1: 0.2 mm); Fig. 7 shows relationships observed in Evaluation Test 1 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 3 and 4 (specifically, coating length L1: 0.3 mm); Fig. 8 shows relationships observed in Evaluation Test 1 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 5 and 6 (specifically, coating length L1: 0.4 mm); Fig. 9 shows relationships observed in Evaluation Test 1 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 7 and 8 (specifically, coating length L1: 0.5 mm); Fig. 10 shows relationships observed in Evaluation Test 1 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating when the side surface coating has coating lengths L1 of 0.2 mm to 0.5 mm; Fig. 11 shows the relationship observed in an evaluation test 2 between wear ratios and D2 / D1 ratios corresponding to ratios of diameters D2 of the smallest diameter parts in the holders to an outer diameter D1 of the discharge sections in test examples 9 to 13 in which the diameters D2 are varied; Fig. 12 shows the relationship between the D2 / D1 ratio and the wear ratio in Evaluation Test 2; Fig. 13 is a photograph showing a state change of the metal surface observed in Evaluation Test 3 after a temperature cycling test of Experimental Example 14; Fig. 14 is a photograph showing a state change of the metal surface observed in Evaluation Test 3 after a temperature cycling test of Experimental Example 15; Fig. 15 is an enlarged cross-sectional view of a composite chip constituting a main part of a spark plug according to a second embodiment; Fig. 16 is an enlarged cross-sectional view of a main part showing the edge of the composite chip according to the second embodiment; Fig. 17 is an enlarged cross-sectional view schematically showing an example of a crack observed in Evaluation Test 3 extending in a surface layer of the composite chip; Fig. 18 is an enlarged cross-sectional view of a composite chip constituting a main part of a spark plug according to a modification example of the second embodiment; Fig. 19 shows relationships observed in an evaluation test 4 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 16 and 17 (specifically, coating length L1: 0.2 mm, maximum thickness differences Q: 0 mm and 0.05 mm); Fig. 20 shows relationships observed in Evaluation Test 4 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 18 and 19 (specifically, coating length L1: 0.3 mm, maximum thickness differences Q: 0 mm and 0.05 mm); Fig. 21 shows relationships observed in Evaluation Test 4 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 20 and 21 (specifically, coating length L1: 0.5 mm, maximum thickness differences Q: 0 mm and 0.05 mm); Fig. 22 shows relationships observed in Evaluation Test 4 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 22 and 23 (specifically, coating length L1: 0.2 mm, maximum thickness differences Q: 0.1 mm and 0.25 mm); Fig. 23 shows relationships observed in Evaluation Test 4 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 24 and 25 (specifically, coating length L1: 0.3 mm, maximum thickness differences Q: 0.1 mm and 0.25 mm); and Fig. 24 shows relationships observed in Evaluation Test 4 between the outer diameter D1 of the discharge portion and the coating thickness S of the side surface coating in Experimental Examples 26 and 27 (specifically, coating length L1: 0.5 mm, maximum thickness differences Q: 0.1 mm and 0.25 mm). Description of the Embodiments (First Embodiment)

[0018] A first embodiment of a spark plug for an internal combustion engine will now be described with reference to the Fig. 1 to 5 described.

[0019] As in Fig. 1, a spark plug 1 includes a center electrode 3 held inside a cylindrical insulator 2, a ground electrode 4 provided at the distal end of a housing H and facing the center electrode 3 in the axial direction X, and a composite chip 5 formed on at least one of the center electrode 3 and the ground electrode 4. The center electrode 3 protrudes from the tip of the insulator 2 toward a distal end, and the housing H holds the insulator 2 therein.

[0020] In the present embodiment, the composite chip 5 is installed on both the center electrode 3 and the ground electrode 4, and the composite chips 5 extend in the axial direction X (that is, the vertical direction in the figure) and face each other. Each of the composite chips 5 on the center electrode 3 and on the ground electrode 4 has the same structure and includes a core 51 and a cup-shaped surface layer 52 covering the core 51. An internal combustion engine using the spark plug 1 corresponds, for example, to a lean-burn engine for an automobile.

[0021] As in Fig. As shown in Fig. 2, in which the composite chip 5 installed on the ground electrode 4 is illustrated as an example, the core 51 has a holder 511 integrally bonded to an electrode base material 4A for the ground electrode 4. The surface layer 52 includes a discharge portion 521 covering a protrusion end surface 512 of the core 51, and a side surface coating 522 covering a side surface 513 continuous with the protrusion end surface 512. The core 51 is formed of a Ni alloy material, and the surface layer 52 is formed of a Pt alloy material.

[0022] The surface layer 52 is formed so that the outer diameter D1 of the discharge portion 521, the coating thickness S of the side surface coating 522 in the radial direction Y (that is, the right-left direction in the figure), and the coating length L1 of the side surface coating 522 in the axial direction X satisfy the relationship represented by Expression 1. Expression 1: S≥D1 / 20+L1 / 10−0.005 mm

[0023] The spark plug 1 according to the present embodiment will be described in detail below.

[0024] As in Fig. 3, the spark plug 1 includes the cylindrical shell H extending in the axial direction X. The shell H has a mounting thread H1 formed on the outer peripheral surface thereof adjacent to the distal end (i.e., the bottom in the figure). The inner peripheral surface of the shell H adjacent to the proximal end (i.e., the upper side in the figure) is stepped to widen toward the proximal end, and the step supports the outer periphery of a central portion 21 formed as a large-diameter part of the insulator 2. The insulator 2 includes a tip 22 that tapers with its diameter reduced toward the distal end and projects from the distal end of the shell H toward the distal end, and has a gap between the insulator 2 and the inner peripheral surface of the shell H.

[0025] Inside the cylindrical insulator 2, the elongated center electrode 3 is provided on the distal end side thereof, and an elongated metal terminal 11 is coaxially provided on the proximal end side thereof. The center electrode 3 is electrically connected to the metal terminal 11 via a resistor 12. The metal terminal 11 has a proximal end protruding from the proximal end of the insulator 2 and connected to an external power supply (not shown), thereby enabling the supply of high voltage for ignition. The resistor 12 is obtained by dispersing a conductive material, such as a carbon material, in a substrate with a glass material and an aggregate. Conductive glass sealing layers 13 and 14 are filled between the resistor 12 and the center electrode 3 or the metal terminal 11, respectively.The housing H is formed, for example, from a metal material such as an iron-based alloy, and the insulator 2 is formed from an electrically insulating ceramic material such as alumina.

[0026] The spark plug 1 is mounted on a cylinder of an internal combustion engine (not shown), with the distal end of the plug exposed within the cylinder. The composite chip 6 at the distal end of the center electrode 3 and the opposing composite chip 5 at the ground electrode 4 form a spark gap G between them. When the center electrode 3 is supplied with a predetermined high voltage from the external power supply at a predetermined time, a spark discharge is generated in the spark gap G, which ignites and combusts an air-fuel mixture supplied into the cylinder.

[0027] In Fig. 1, the ground electrode 4 is integral with the distal end surface of the housing H, extends toward the distal end, and is bent into a substantially L-shape. The extending end of the ground electrode 4 corresponds to a tip 41 pointing toward a tip 31 of the center electrode 3 in the axial direction X. The tip 31 of the center electrode 3 is tapered, with its diameter decreasing toward the distal end, and the composite chip 5 is bonded to its distal end surface, which protrudes from the tip of the insulator 2 toward the distal end. The composite chip 5 is bonded to the tip 41 of the ground electrode 4 on its surface facing the center electrode 3. The composite chip 5 on the center electrode 3 and the composite chip 5 on the ground electrode 4 are coaxially arranged at a predetermined interval on a plug center axis 15, forming the spark gap G between the composite chips 5.

[0028] In Fig. 2, each composite chip 5 is generally substantially columnar and formed as a needle-shaped chip protruding in the axial direction X from the tip 41 of the ground electrode 4. The composite chip 5 holds the core 51 in close contact within the cup-shaped surface layer 52, which forms the outer surface of the composite chip 5. The support 511 of the core 51, exposed from the surface layer 52, is integrally bonded to the tip 41 of the ground electrode 4. The surface layer 52 has the shape of a cylindrical cup having a substantially constant outer diameter and whose bottom corresponds to the end of the chip protrusion, and includes the discharge portion 521 positioned above the protrusion end surface 512 of the core 51 in the axial direction X, and the side surface coating 522 positioned around the side surface 513 adjoining the protrusion end surface 512 in the radial direction Y.

[0029] The surface layer 52 is formed from a Pt-containing alloy material, which corresponds to a high-density material. Pt alloy materials have high melting points and excellent oxidation resistance, thus improving the wear resistance of the surface layer 52. Pt alloy materials are also ductile materials, which advantageously facilitate the forming of the surface layer 52 into the cup shape. In particular, materials in which another noble metal is added to Pt, such as a Pt-Rh alloy, a Pt-Ir alloy, and a Pt-Pd alloy, or materials in which a non-noble metal, for example, Ni, is added to Pt, such as a Pt-Ni alloy, can be used. A Pt-Rh alloy or a Pt-Ni alloy is preferably used as the Pt alloy material.

[0030] If a Pt-Rh alloy is used as the Pt alloy material, the Pt-Rh alloy can have a Rh content in a range of 10 mass% to 30 mass%. In this state, the linear expansion coefficient is in a range of, for example, 9.5 × 10 -6 / °C to 12.0 × 10 -6 / °C (that is, the coefficient of linear expansion at 900°C when a reference temperature is 50°C). Although both Pt and Rh are oxidation-resistant materials, Pt has a relatively low melting point among noble metals (more precisely, 1,770°C). Therefore, the Pt-Rh alloy obtained by adding Rh with a higher melting point (more precisely, 1,960°C) ensures spark wear resistance and oxidation resistance. When the Rh content is less than 10 mass%, a sufficient effect of improving wear resistance can be achieved by increasing the melting point, and when the Rh content exceeds 30 mass%, hardness may increase and cup-shape formability may decrease.

[0031] If a Pt-Ni alloy is used as the Pt alloy material, the Pt-Ni alloy can have a Ni content in a range of 5 mass% to 20 mass%. In this state, the coefficient of linear expansion is in a range of, for example, 10.5 × 10 -6 / °C to 13.0 × 10 -6 / °C (that is, the coefficient of linear expansion at 900°C when a reference temperature is 50°C). Pt is expensive because it is a precious metal, and its price fluctuations can greatly affect costs. The addition of Ni, which is a non-precious metal, can reduce costs. When the Ni content is less than 5 mass%, the cost-reducing effect may be insufficient. When the Ni content exceeds 20 mass%, hardness may increase and cup-shape formability may decrease.

[0032] The core 51 is formed of an alloy material containing Ni, which is equivalent to a low-density material. Ni alloy materials are non-precious metal materials and are cheaper than the Pt alloy material forming the surface layer 52, thus contributing to cost reduction. Furthermore, Ni alloy materials allow a reduction in the amount of the high-density Pt alloy material to avoid problems such as peeling of the composite chip 5 due to its own weight. In particular, a Ni-Cr-based alloy or a Ni-Cr-Fe-based alloy can be suitably used as Ni alloy materials. The Ni content can be in a range of, for example, 50 mass% to 90 mass%. An element other than Cr and Fe, such as Mo, Al, Co, Mn, Si, C, or S, can be added to the Ni.The linear expansion coefficient of such a Ni alloy material is usually higher than that of the Pt alloy material, and can, for example, be in a range of 14.0 × 10. -6 / °C to 17.0 × 10 -6 / °C (that is, the linear expansion coefficient at 900°C when a reference temperature is 50°C). Although iron-based materials are widely used as non-precious metals, the composite chip 5 of the spark plug 1, which is exposed in the combustion chamber of the engine, is exposed to a high-temperature and highly oxidative environment, and therefore, a Ni-based material is suitably used due to its oxidation resistance.

[0033] The core 51 and the surface layer 52 are fixed in close contact with each other, for example, by press-fitting or resistance welding. When fixed in close contact, the core 51 and the surface layer 52 can be heat-treated to improve their connection properties by diffusion bonding. In some cases, the surface layer 52 can be formed into a cup shape while the core 51 is inserted in the same process. Then, the support 511 exposed from the surface layer 52 can be arranged on the tip 41 of the ground electrode 4 and connected by resistance welding or laser welding. The ground electrode 4 (or the electrode base material 4A) can be formed of, for example, a Ni alloy material. Using the same material as the core 51 can reduce thermal stress.

[0034] For example, in Fig. In the holder 511 shown in Figure 2, the end surface in the axial direction is connected to the surface of the ground electrode 4 by resistance welding, and the peripheral surface is connected to the surface of the ground electrode 4 by laser welding and is curved in such a way that it widens slightly toward the base. The laser welding is used to melt and solidify the interface of the holder 511, forming a molten region that ensures connectability. Some of the materials that form the surface layer 52 may be melted into the core 51, resulting in a molten alloyed region. In this case, the material composition of at least a portion of the holder 511 is a Pt-containing Ni alloy, which forms the surface layer 52.

[0035] For the composite chip 5 thus obtained by combining the core 51 and the surface layer 52, by enclosing the core 51 inside the surface layer 52, the use of expensive Pt alloy material can be reduced while maintaining wear resistance, while ensuring the bondability of the holder 511 to the ground electrode 4.

[0036] The composite chip 5 installed on the center electrode 3 may also have the same structure. At the tip 31 of the center electrode 3, the support 511 of the core 51 is integrally formed with an electrode base material 3A (see, for example, Fig. 1) for the center electrode 3, and the surface layer 52 covering the projection of the core 51 is provided.

[0037] The effects of the shape of the composite chip 5, particularly the relationship between the outer diameter D1 of the surface layer 52 and the coating length L1 and the coating thickness S of the side surface coating 522 shown in the above-mentioned Expression 1, will now be described.

[0038] As in Fig. As shown in Figure 4, in a lean-burn engine in which air flows into its cylinder at high speed, the composite chips 5 forming the spark gap G for the spark plug 1 are exposed to a high-speed airflow F. When the two composite chips 5 facing each other across the spark gap G form a spark discharge P generated therebetween, the spark discharge P tends to be blown away by, for example, the lateral airflow F indicated by an arrow in the figure. As the spark discharge P expands laterally in this way (i.e., in the direction of the airflow F), each end of the spark discharge P shifts from the outer periphery to the side surface of the composite chip 5 on the center electrode 3 or from the outer periphery to the side surface of the composite chip 6 on the ground electrode 4.

[0039] In this condition, it was found that the spark discharge P was located on the outer circumference of the Fig. 5, and increases wear there, that is, near an edge 53 from the outer peripheral edge of the discharge portion 521 of the surface layer 52 to the side surface coating 522. In particular, when the spark discharge P is blown and displaced to the side surface by the air flow F, the relatively thin side surface coating 522 wears, and a crack tends to occur under thermal stress. In particular, repeated cycles of heating by the heat of the spark discharge P and cooling by the air flow F lead to thermal stress at the bonding interface between the core 51 formed of a Ni alloy material and the surface layer 52 formed of a Pt alloy material having a lower linear expansion coefficient due to a difference in the linear expansion coefficients.In addition, the thin side surface coating 522 expands, increasing the possibility of cracking. Furthermore, the corrosive, high-temperature atmosphere in the cylinders can cause cracked areas to undergo high-temperature oxidation, resulting in the peeling off of the surface layer 52. If a more wear-prone core 51 is exposed, wear will consequently increase at an accelerated rate, shortening the life of the spark plug 1.

[0040] Therefore, the coating thickness S and the coating length L1 of the side surface coating 522 are determined based on the following expression 1 derived from the evaluation test 1 described later. Expression 1: S≥D1 / 20+L1 / 10−0.005 mm

[0041] The test results have shown that the relationship between the coating thickness S and the coating length L1 thereof and the outer diameter D1 of the discharge portion 521 is important for cracking of the side surface coating 522. Specifically, cracking is affected by both the thermal stress in the axial direction X due to the coating length L1 of the side surface coating 522 and the thermal stress in the radial direction Y due to the outer diameter D1 of the discharge portion 521. Each increases with its thermal stress and the possibility of cracking. Against the thermal stress due to these dimensions, wear resistance can be improved by appropriately determining the coating thickness S to satisfy the relationship represented by Expression 1.

[0042] Suitably, the coating thickness S of the side surface coating 522 is set equal to or smaller than the coating thickness T of the discharge portion 521 in the axial direction X (i.e., T ≥ S). Even more suitably, the coating thickness S may be smaller than the coating thickness T of the discharge portion 521 (i.e., T > S). By setting the thickness not larger than necessary within a range satisfying Expression 1, the amount of expensive precious metal material used in the surface layer 52 can be reduced. The coating thickness T of the discharge portion 521 may be in a range of, for example, 0.15 mm ≤ T ≤ 0.25 mm. This range provides sufficient wear resistance against wear over time and an increasing discharge maintenance voltage with the spark gap G widened by wear.

[0043] Since the composite chip 5 has a structure in which different elements of the core 51 formed of a Ni alloy material having a high linear expansion coefficient and corresponding to a low-density material and the surface layer 52 formed of a Pt alloy material having a low linear expansion coefficient and corresponding to a high-density material are bonded, it appears that thermal stress due to the difference in the linear expansion coefficients causes cracks in the side surface cladding 522. One factor for the occurrence of cracks is the thermal stress applied in the radial direction Y due to the outer diameter D1 of the discharge portion 521, and the larger the outer diameter D1, the greater the thermal stress becomes.Another factor is the thermal stress applied in the axial direction X due to the coating length L1 of the side surface coating 522, and the thermal stress increases in proportion to the coating length L1.

[0044] Considering these thermal stress factors, the coating thickness S sufficient to prevent cracking can be adjusted to improve thermal stress resistance, thereby preventing cracking. These factors are reflected in the first term (i.e., D1 / 20) and the second term (i.e., L1 / 10) of Expression 1, respectively.

[0045] The outer diameter D1 of the discharge portion 521 is appropriately determined within a range of 0.5 mm ≤ D1 ≤ 1.1 mm. As the outer diameter D1 increases, the wear resistance of the discharge portion 521 improves, but more of the thermal energy of the spark discharge P is absorbed by the discharge portion 521, so that flame retardancy is increased. In contrast, as the outer diameter D1 decreases, the flame retardancy decreases and the ignitability improves, but the wear resistance also decreases. Therefore, the outer diameter D1 can be appropriately selected from the above range so that both the ignitability and the wear resistance are maintained.

[0046] The coating length L1 of the side surface coating 522 is set to be in a range of 0.2 mm ≤ L1 ≤ 0.5 mm. A longer coating length L1 increases the wear reduction effect on the side surface by covering an area against displacement of the spark discharge P to the side surface coating 522. However, as the coating length L1 becomes longer, the thermal stress along the axial direction X tends to increase. Therefore, in a typical internal combustion engine, the coating length L1 can be appropriately selected from the above range to prevent thermal stress as well as provide sufficient coverage against the spark discharge P displaced by the airflow F in the cylinder.

[0047] The exposed length L2 of the holder 511 in the axial direction X is appropriately set so that the total length of the composite chip 5 in the axial direction X (i.e., chip length = T + L1 + L2) becomes a defined length. The exposed length L2 can be appropriately set in a range of 0.2 mm ≤ L2 ≤ 0.5 mm.

[0048] Since the outer peripheral surface of the retainer 511 is not coated with the surface layer 52 but is exposed to the atmosphere in the cylinder, heat dissipation can be improved to reduce the thermal expansion of the core 51. However, with a larger exposed length L2, heat dissipation from the core 51 may be accelerated, unnecessarily increasing the flame retardancy. Therefore, the exposed length L2 can be appropriately determined within the above range to achieve good ignitability and prevent cracking due to thermal stress.

[0049] In addition, the ratio of the diameter D2 of the smallest diameter part of the holder 511 exposed from the side surface coating 522 and the outer diameter D1 of the discharge portion 521 (D2 / D1) desirably satisfies the relationship represented by an expression 2 derived from the evaluation test 2 described later. Expression 2:D2 / D1≥0.8

[0050] As the ignition energy increases, the discharge portion 521 tends to wear due to the heat of the spark discharge P. Therefore, it is desirable to properly transfer the heat from the discharge portion 521 to the electrode base material 4A via the core 51. However, if the holder 511 has a small diameter relative to the outer diameter D1 of the discharge portion 521, the heat energy of the spark discharge P may be difficult to transfer. Appropriately, the diameter D2 of the smallest diameter part of the holder 511 and the outer diameter D1 of the discharge portion 521 may be determined, as necessary, so that D2 / D1 satisfies Expression 2 to improve wear resistance. (Evaluation Test 1)

[0051] The spark plug 1 according to the first embodiment was evaluated for the occurrence of cracks in the side surface coating 522 with a variation of the outer diameter D1 of the discharge portion 521 of the composite chip 5 and a variation of the coating thicknesses S and coating lengths L1 of the side surface coating 522.

[0052] As in the Fig. As shown in Figures 6 to 9, various samples with different dimensions were prepared for Test Examples 1 to 8. Each of the samples for Test Examples 1 to 8 corresponded to a composite chip 5 made of alloy materials, the core 51 of which was made of a Ni-Cr-Fe-based alloy (specifically, 72 mass% Ni, 17 mass% Cr, and 10 mass% Fe; linear expansion coefficient: 16.4 × 10 -6 / °C) and whose surface layer 52 consists of a Pt-Rh alloy (in particular 80 mass% Pt and 20 mass% Rh; linear expansion coefficient: 9.9 × 10-6 / °C). It should be noted that the linear expansion coefficient was measured at 900 °C (reference temperature: 50 °C), and the same applies below.

[0053] In Evaluation Test 1, the spark plug 1, equipped with each composite chip 5 with the dimensions specified in each test sample, was placed on a temperature-controlled heating and cooling table, and a temperature cycle was repeated under the conditions described below. In each cycle, the sample mounted in a heating furnace was heated to 950°C and allowed to stand for one minute, and then cooled to 150°C and allowed to stand for one minute. The cycle was repeated 200 times. The sample was then taken out into the room and air-cooled. The results of the 200-cycle durability test are shown in the Fig. 6 to 9, in which each circle indicates that the sample was good (◯) or that no cracks occurred in the side surface coating 522, and each cross indicates that the sample was bad (×) or that cracks occurred in the side surface coating 522.

[0054] In the Fig. In Experimental Examples 1 and 2 shown in Figure 6, the coating length L1 was set to 0.2 mm, while the coating thickness S was varied in increments of 0.01 mm within a range of 0.04 mm to 0.09 mm, and the outer diameter D1 of the discharge portion 521 was varied in increments of 0.2 mm within a range of 0.5 mm to 1.1 mm. In Experimental Example 1, the holder 511 of the core 51 had an exposed length L2 set to 0.5 mm, and the discharge portion 521 of the surface layer 52 had a coating thickness T set to 0.15 mm. In Experimental Example 2, the exposed length L2 was set to 0.2 mm, and the coating thickness T of the discharge portion 521 was set to 0.25 mm. The relationship between crack occurrence and combinations of coating thickness S and outer diameters D1 was evaluated.

[0055] As in the upper and lower part of Fig. As shown in Figure 6, there is a correlation between the coating thicknesses S and the outer diameters D1 that do not cause cracks, and Experimental Examples 1 and 2 obtained equivalent results. Specifically, the expressions of the boundary lines shown in this figure showed that when the coating length L1 was set to 0.2 mm, the combinations satisfying the expression S ≥ D1 / 20 + 0.015 mm did not cause cracks regardless of the coating thickness T of the discharge portion 521 and the exposed length L2 of the holder 511. Any combination satisfying the expression S < D1 / 20 + 0.015 mm caused cracks due to thermal expansion of the core 51.

[0056] In Experimental Examples 3 and 4, the evaluation was conducted in the same manner as in Experimental Example 1, except that the coating length L1 was set to 0.3 mm. Specifically, the coating thickness S was varied in a range from 0.04 mm to 0.09 mm, and the outer diameter D1 of the discharge portion 521 was varied in a range from 0.5 mm to 1.1 mm. In Experimental Example 3, the holder 511 had an exposed length L2 set to 0.5 mm, and the discharge portion 521 had a coating thickness T set to 0.15 mm. In Experimental Example 4, the exposed length L2 was set to 0.2 mm, and the coating thickness T of the discharge portion 521 was set to 0.25 mm. The relationship between the occurrence of cracks and combinations of coating thickness S and outer diameter D1 was evaluated.

[0057] As in the upper and lower part of Fig. As shown in Figure 7, Test Examples 3 and 4 also obtained equivalent results for the coating length L1 set at 0.3 mm. Specifically, the boundary line expressions shown in this figure indicate that the combinations satisfying the expression S ≥ D1 / 20 + 0.025 mm did not cause cracks regardless of the coating thickness T of the discharge portion 521 and the exposed length L2 of the holder 511. The combinations satisfying the expression S < D1 / 20 + 0.025 mm caused cracks due to thermal expansion of the core 51.

[0058] In Experimental Examples 5 and 6, the evaluation was conducted in the same manner as in Experimental Example 1, except that the coating length L1 was set to 0.4 mm. Specifically, the coating thickness S was varied within a range of 0.04 mm to 0.09 mm, and the outer diameter D1 of the discharge portion 521 was varied within a range of 0.5 mm to 1.1 mm. In Experimental Example 5, the holder 511 had an exposed length L2 set to 0.5 mm, and the discharge portion 521 had a coating thickness T set to 0.15 mm. In Experimental Example 6, the exposed length L2 was set to 0.2 mm, and the coating thickness T of the discharge portion 521 was set to 0.25 mm. The relationship between the occurrence of cracks and combinations of coating thickness S and outer diameter D1 was evaluated.

[0059] As in the upper and lower part of Fig. As shown in Figure 8, Test Examples 5 and 6 also obtained equivalent results for the coating length L1 set at 0.4 mm. Specifically, the boundary line expressions shown in this figure showed that the combinations satisfying the expression S ≥ D1 / 20 + 0.035 mm did not cause cracks regardless of the coating thickness T of the discharge portion 521 and the exposed length L2 of the holder 511. The combinations satisfying the expression S < D1 / 20 + 0.035 mm caused cracks due to thermal expansion of the core 51.

[0060] In Experimental Examples 7 and 8, the evaluation was conducted in the same manner as in Experimental Example 1, except that the coating length L1 was set to 0.5 mm. Specifically, the coating thickness S was varied in a range from 0.04 mm to 0.09 mm, and the outer diameter D1 of the discharge portion 521 was varied in a range from 0.5 mm to 1.1 mm. In Experimental Example 7, the holder 511 had an exposed length L2 set to 0.5 mm, and the discharge portion 521 had a coating thickness T set to 0.15 mm. In Experimental Example 8, the exposed length L2 was set to 0.2 mm, and the coating thickness T of the discharge portion 521 was set to 0.25 mm. The relationship between the occurrence of cracks and combinations of coating thickness S and outer diameter D1 was evaluated.

[0061] As in the upper and lower part of Fig. As shown in Figure 9, Test Examples 7 and 8 also obtained equivalent results for the coating length L1 set at 0.5 mm. Specifically, the boundary line expressions shown in this figure showed that the combinations satisfying the expression S ≥ D1 / 20 + 0.045 mm did not cause cracks regardless of the coating thickness T of the discharge portion 521 and the exposed length L2 of the holder 511. The combinations satisfying the expression S < D1 / 20 + 0.045 mm caused cracks due to thermal expansion of the core 51.

[0062] As in Fig. As shown in Figure 10, which summarizes the results of Experimental Examples 1 to 8, the coating thickness S that can prevent cracks varies according to the outer diameter D1 and the coating length L1 of the discharge portion 521. Specifically, the sufficient coating thickness S for a fixed coating length L1 is expressed as a linear function: S ≥ D1 / 20 + α, where D1 has a coefficient of 1 / 20. The value of the constant term α is defined depending on L1. As L1 increases (e.g., in a range of 0.2 mm to 0.5 mm), α also increases (e.g., in a range of 0.005 mm to 0.045 mm), and the sufficient coating thickness S becomes larger.

[0063] This indicates that one factor for cracking is the outer diameter D1, that is, the thermal stress generated at the interface between the core 51 and the side surface coating 522 and applied in the radial direction Y, and another factor for cracking is the coating length L1, that is, the thermal stress generated at the interface between the core 51 and the side surface coating 522 and applied in the axial direction X. In other words, since the Ni-Cr-Fe-based alloy constituting the core 51 has a higher linear expansion coefficient than the linear expansion coefficient of the Pt-Rh alloy constituting the surface layer 52, the thermal stress is applied due to the difference in the linear expansion coefficients in both the radial direction Y and the axial direction X.If the coating thickness S is insufficient, cracking occurs in the side surface coating 522 due to the thermal expansion of the core 51.

[0064] The coating thickness S is therefore desirably determined as a sufficient value based on both the thermal stress applied in the radial direction Y due to the outer diameter D1 of the discharge portion 521 and the thermal stress applied in the axial direction X due to the coating thickness L1. In particular, based on the Fig. 10, the sufficient coating thickness S can be represented as Expression 1 using the outer diameter D1 and the coating thickness L1. Expression 1:S≥D1 / 20+L1 / 10−0.005 mm

[0065] By setting a sufficient coating thickness S so that Expression 1 is satisfied, the strength required for the thermal stress applied in both the radial direction Y and the axial direction X is improved and the occurrence of cracks in the side surface coating 522 is prevented. (Evaluation Test 2)

[0066] Next, the spark plug 1 according to the first embodiment was evaluated for the influence of different diameters D2 of the smallest diameter part of the retainer 511 in the composite chip 5 on the amount of wear at the discharge portion 521. A Ni-Cr-Fe-based alloy and a Pt-Rh alloy, which were the same alloy materials as for the samples in Evaluation Test 1 described above, were used, respectively, as the alloy materials constituting the core 51 and the surface layer 52.

[0067] As in Fig. As shown in Figure 11, the samples in Experimental Examples 9 to 13 correspond to composite chips 5 having their cores 51 and surface layers 52, which have the same dimensions except for the diameter D2 of the smallest diameter part of each holder 511. The diameter D2 of the smallest diameter part was varied so that the ratio of the diameter D2 to the outer diameter D1 of the discharge section 521 (D2 / D1) was in a range of 0.6 to 1.0. The dimensions of each part were as follows: Outer diameter D1 of the discharge section 521: 0.7 mm Coating thickness T of the discharge section 521: 0.25 mm Coating length L1 of the side surface coating 522: 0.4 mm Coating thickness S of the side surface coating 522: 0.08 mm Exposed length L2 of bracket 511: 0.2 mm Diameter D2 of the part with the smallest diameter of the holder 511: 0.42 mm to 0.7 mm

[0068] In Evaluation Test 2, the spark plug 1 provided with each composite chip 5 having the dimensions specified in each test example was installed in an engine cylinder, and the engine was operated under the conditions described below to calculate a wear ratio Q0 after a durability test. Engine: Inline four-cylinder, 2,000 CC Operating condition: 5,600 WOT Operating time: 100 H

[0069] The wear amount at each discharge section 521 in the worn state after the durability test as shown in the lower part of Fig. 11, compared to the factory-new condition before the durability test, as shown in the upper part of Fig. 11 is represented by ΔG. Taking the wear amount in Experimental Example 4 with D2 / D1 of 1.0 as ΔG0, a ratio of the wear amount of ΔG in the sample in each Experimental Example is defined as a wear ratio Q0=ΔG / ΔG0. For the sample in each Experimental Example, the value of D2 / D1 in the as-new state and the calculated wear ratio Q0 are shown in the figure. Furthermore, the relationship between them is shown in Fig. 12 shown.

[0070] The results in Fig. 11 indicate that the wear ratio Q0 in Experimental Example 9 with D2 / D1 of 0.6 is 1.4, but the wear ratio Q0 decreases sharply with increasing D2 / D1, while in Experimental Examples 11 to 13 with D2 / D1 of 0.8 or more, each wear ratio Q0 is 1.0. In this way, when the surface layers 52 of the composite chips 5 have the same shape and the exposed lengths L2 of the supports 511 are constant, the wear amount ΔG of a discharge portion 521 increases or decreases depending on the size of the smallest diameter part of the supports 511. It can be assumed that for a smallest diameter part with a smaller diameter D2, the heat energy of the spark discharge P from the supports 511 cannot be sufficiently transferred to the electrode base material, thus promoting the wear of the discharge portion 521. As shown in Fig. As shown in Fig. 12, which summarizes the results, as the diameter D2 of the smallest diameter part increases, the wear at the discharge portion 521 is reduced, and this effect is substantially constant when D2 / D1 is 0.8 or more.

[0071] Therefore, to reduce wear on the discharge portion 521 of the surface layer 52, the composite chip 5 may be appropriately formed so that D2 / D1 is 0.8 or more. In this way, cracking in the side surface coating 522 caused by thermal stress is prevented and wear on the discharge portion 52 caused by high temperature is suppressed, further improving the wear resistance of the composite chip 5, and thus extending the life of the spark plug 1. (Evaluation Test 3)

[0072] The spark plug 1 according to the first embodiment was evaluated for wear resistance by varying alloy materials constituting the surface layers 52 of the composite chips 5 and subjecting it to a thermal cycling test conducted in the same manner as in Evaluation Test 1 described above. In each cycle of the thermal cycling test, the sample was heated to 1,050°C and allowed to stand for six minutes, then cooled to 150°C and allowed to stand for six minutes. The appearance after 200 cycles was observed to evaluate wear.

[0073] As in Fig. 13, the sample in Experimental Example 14 used a Pt-Ni alloy (specifically, 90 mass% Pt and 10 mass% Ni; linear expansion coefficient: 11.4 × 10 -6 / °C) as a feed material for the surface layer 52.

[0074] The Ni-Cr-Fe-based alloy (specifically, 72 mass% Ni, 17 mass% Cr, and 10 mass% Fe), which was the same material as for the samples of Evaluation Test 1 described above, was used as an insert material for the core 51.

[0075] For comparison, the same temperature cycling test was carried out on a sample in Test Example 15, as in Fig. 14, while changing an insert material of the core 51 to an Fe-based alloy (specifically, 85Fe-11Cr-3Si-0.5C; coefficient of linear expansion: 13.2 × 10 -6 °C). A Pt-Ni alloy, which was the same material as for Example 14, was used as an insert material for the surface layer 52.

[0076] The dimensions of each part in both Examples 14 and 15 were the same as those of the samples of Evaluation Test 2 described above, and as follows: Outer diameter D1 of the discharge section 521: 0.7 mm Coating thickness T of the discharge section 521: 0.25 mm Coating length L1 of the side surface coating 522: 0.4 mm Coating thickness S of the side surface coating 522: 0.08 mm Exposed length L2 of bracket 511: 0.2 mm Diameter D2 of the smallest diameter part of the bracket 511: 0.6 mm

[0077] In Experimental Example 14, the appearance of the pattern before the temperature cycles was shown in the left part of Fig. 12, with the appearance of the pattern after the temperature cycles, as shown in the right part of Fig. 12. Although wear was observed on the surface layer 52 and the support 511, which form the outer surface of the composite chip 5, in the sample after the temperature cycles, the appearance changed only slightly and showed good wear resistance.

[0078] In contrast, the pattern in Experimental Example 15 was obtained from the appearance before the temperature cycles, as shown in the left part of Fig. 13, to the appearance after the temperature cycles, as in the right part of Fig. 13, significantly changed. The support 511 bulged near the interface adjacent to the surface layer 52 due to high-temperature oxidation, and the support 511 exposed from the surface layer 52 was severely worn.

[0079] As shown in these results, by using a Ni alloy material having good oxidation resistance for an insert material of the core 51, high-temperature oxidation can be reduced to improve wear resistance, and the service life of the spark plug 1 can be extended. (Second embodiment)

[0080] A second embodiment of a spark plug for an internal combustion engine will now be described with reference to Fig. 15 and Fig. 16 described.

[0081] In this embodiment, too, a spark plug 1 and composite chips 5 formed on a center electrode 3 and a ground electrode 4 have the same basic structures as in the first embodiment, so their description will be omitted. In the present embodiment, as shown in Fig. As shown in Figure 15, the core 51 has a different outer peripheral shape along an edge 53 of the composite chip 5, and the surface layer 52 covering the core 51 has a different inner peripheral shape. Differences will now be mainly described.

[0082] Among the reference numerals used in the second and subsequent embodiments, the same reference numerals as in a previous embodiment denote the same or corresponding components as in the previous embodiment unless otherwise specified.

[0083] In Fig. 15, the core 51 is substantially columnar and has a tapered portion 514 with an R-shaped taper along the junction between a circular and flat protrusion end surface 512 and a cylindrical side surface 513. The surface layer 52 covering the surface of the core 51 except for the support 511 has an outer shape of a can with a substantially constant diameter, and includes a discharge portion 521 covering the protrusion end surface 512 of the core 51, a side surface coating 522 covering the side surface 513, and a thick portion 523 adjacent to and covering the tapered portion 514.

[0084] In this arrangement, the surface layer 52 at the thick portion 523 covering the tapered area 514 has a coating thickness in the radial direction Y that is equal to or greater than the coating thickness S of the side surface coating 522 covering the side surface 513.

[0085] The thick portion 523 becomes thinner (the coating thickness decreases in the radial direction Y) as it approaches the side surface 513 of the core 51, and becomes thicker as it approaches the protrusion end surface 512 of the core 51. The coating thicknesses have a maximum difference Q depending on the tapered shape of the tapered portion 513.

[0086] In particular, as in Fig. 16, on the core 51, the connecting portion between the side surface 513 and the outer peripheral edge of the protrusion end surface 512 is chamfered in an R-shape to form the chamfered portion 514 that protrudes outward and has an outer periphery in a substantially quadrant arc shape. In the surface layer 52, the thick portion 523 coating the chamfered portion 514 has an inner surface with a substantially quadrant arc shape corresponding to the chamfered portion 514. The thick portion 523 has a maximum coating thickness S1 at the connecting portion with the discharge portion 521, and the maximum coating thickness S1 is greater than the coating thickness S of the side surface coating 522. The thick portion 523 has the minimum thickness at the connecting portion with the side surface coating 522, and the minimum thickness is equal to the coating thickness S of the side surface coating 522.

[0087] Therefore, the maximum difference Q of the coating thickness in the radial direction Y (hereinafter sometimes referred to as the maximum thickness difference) corresponds to the difference between the maximum coating thickness S1 of the thick portion 523 and the coating thickness S of the side surface coating, and is represented by Expression 3: Expression 3: Q=S1−S

[0088] Also in this configuration, the relationship between the outer diameter D1 of the discharge portion 521 and the coating thickness S and the coating length L1 of the side surface coating 522 can be set in a manner so as to satisfy the above-described Expression 1. It is appropriate to satisfy the relationship of the following Expression 1A, which is created by adding the term expressing the maximum thickness difference Q to Expression 1. Expression 1A is derived from an evaluation test 4 described later. Expression 1A: S≥D1 / 20+L1 / 10−Q / 10−0.005 mm

[0089] The maximum thickness difference Q can be suitably determined in a range of e.g. 0 mm < Q ≤ 0.25 mm.

[0090] As in Fig. As shown in Fig. 17, the results of the durability test in the first embodiment revealed that if the chamfered portion 514 of the core 51 is not formed, cracks are likely to occur in the surface layer 52 from the inner periphery of the edge 53, indicated by part A in the figure, as the starting point. Therefore, in order to improve the strength of a portion corresponding to part A, the thick portion 523 is formed in the joint portion between the discharge portion 521 and the side surface coating 522. More specifically, the chamfered portion 514 is formed in the joint portion corresponding to the thick portion 523 between the side surface 513 and the protrusion end surface 512 of the core 51 and coated with the cup-shaped surface layer 52.In this way, the thick portion 523 can be formed adjacent to the tapered portion 514 to reduce stress concentration and improve strength.

[0091] As in Fig. As shown in FIG. 18 as a modification example, the tapered portion 514 of the core 51 may be tapered in a C-shape rather than an R-shape. In this case, the outer peripheral surface to form the tapered portion 514 is tapered in a C-shape into a flat surface sloping downward from the outer peripheral edge of the protrusion end surface 512 to the side surface 513. The inner peripheral surface of the thick portion 523 covering the tapered portion 514 also corresponds to a tapered flat surface corresponding to the tapered portion 514.

[0092] Also in this configuration, the surface layer 52 has a maximum coating thickness S1 in the radial direction Y at the connecting portion between the thick portion 523 and the discharge portion 521. Similarly, by setting each part to satisfy the above-described Expression 3 for the maximum thickness difference Q (= S1 - S), stress concentration can be reduced and strength can be improved.

[0093] The maximum thickness difference Q corresponds to the chamfer length of the chamfered portion 514 in the radial direction Y.

[0094] The tapered portion 514 can have any inclination angle. For example, at 45°, the thick portion 523 has a length Q1 in the axial direction X that is equal to the maximum thickness difference Q. At an inclination angle greater than 45°, the thick portion 523 has a length Q1 in the axial direction X that is less than the maximum thickness difference Q. (Evaluation Test 4)

[0095] Next, the spark plug 1 according to the second embodiment was evaluated for the occurrence of cracks in the side surface coating 522 by varying the outer diameters D1 of the discharge portion 521 of the composite chip 5, varying the coating thicknesses S and the coating lengths L1 of the side surface coating 522, and further varying the maximum thickness differences Q at the thick portion 523.

[0096] As in the Fig. As shown in Figures 19 to 24, various samples with different dimensions were prepared for Test Examples 16 to 27. Each of the samples was subjected to a thermal cycling test in the same manner as in Evaluation Test 1 described above, and the results were compared. In Test Examples 16, 18, and 20, the maximum thickness difference Q was 0 mm. In other words, the configurations in these Test Examples were the same as those in the first embodiment.

[0097] In Experimental Examples 16 to 27, the support 511 of the core 51 had an exposed length L2 set to 0.2 mm, and the discharge portion 521 of the surface layer 52 had a coating thickness T set to 0.15 mm. For the core 51 and the surface layer 52, respectively, a Ni-Cr-Fe-based alloy and a Pt-Rh alloy were used, which were the same alloy materials as those used for the samples in Evaluation Test 1 described above.

[0098] In test examples 16 and 17 in Fig. 19, the coating length L1 was set to 0.2 mm, while the coating thickness S was varied in increments of 0.01 mm within a range of 0.03 mm to 0.09 mm, and the outer diameter D1 of the discharge portion 521 was varied in increments of 0.2 mm within a range of 0.5 mm to 1.3 mm. In Experimental Example 16, the maximum thickness difference Q was 0 mm, and in Experimental Example 17, the maximum thickness difference Q was 0.05 mm. Then, the relationship between the thick portion 523 and the occurrence of cracks was evaluated.

[0099] As in the upper and lower part of Fig. 19, it was found that, for the same outer diameter D1, the lower limit of the coating thickness S which gives good results (O), that is, no cracks, is smaller in Experimental Example 17 with the thick portion 523 compared to Experimental Example 16 without the thick portion 523.

[0100] In particular, the boundary line expressions shown in this figure indicate that in Test Example 16, the combinations satisfying the expression S ≥ D1 / 20 + 0.2 / 10 - 0.005 mm did not cause cracks, while in Test Example 17, the combinations satisfying the expression S ≥ D1 / 20 + 0.2 / 10 - 0.05 / 10 - 0.005 mm did not cause cracks. Any combinations that did not satisfy these expressions caused cracks due to thermal expansion of the core 51.

[0101] In test examples 18 and 19, which are Fig. 20, the thermal cycling test was conducted in the same manner as in Test Examples 16 and 17, except that the coating length L1 was set to 0.3 mm. In Test Examples 20 and 21 shown in Fig. 21, the thermal cycling test was conducted in the same manner as in Experimental Examples 16 and 17, except that the coating length L1 was set to 0.5 mm. The test results are shown in the figures.

[0102] As in the upper and lower part of the Fig. 20 and Fig. 21, Test Examples 19 and 21 with the thick portion 523 achieved similar results to Test Examples 18 and 20 without the thick portion 523.

[0103] Specifically, the boundary line expressions shown in each figure showed that in Test Example 18, the combinations satisfying the expression S ≥ D1 / 20 + 0.3 / 10 - 0.005 mm did not cause cracks, while in Test Example 19, the combinations satisfying the expression S ≥ D1 / 20 + 0.3 / 10 - 0.05 / 10 - 0.005 mm did not cause cracks. In Test Example 20, the combinations satisfying the expression S ≥ D1 / 20 + 0.5 / 10 - 0.005 mm did not cause cracks. In Test Example 21, the combinations satisfying the expression S ≥ D1 / 20 + 0.5 / 10 - 0.05 / 10 - 0.005 mm did not cause cracks. Any combination that did not satisfy the expressions caused cracks due to thermal expansion of core 51.

[0104] These results have shown that, when the coating length L1 is constant, the coating thickness S sufficient to prevent cracks increases with the increase in the outer diameter D1, while reducing the term of the maximum thickness difference Q shifts the expressions of the boundary lines in the direction in which the value of the coating thickness S decreases. That is, forming the thick portion 523 can reduce the coating thickness S required to prevent cracks.

[0105] In addition, in Experimental Examples 22 to 27, the relationship between the thick portion 523 and the occurrence of cracks with different maximum thickness differences Q was evaluated.

[0106] In test examples 22 and 23, which are Fig. 22, the coating length L1 was set to 0.2 mm. In Test Example 22, the maximum thickness difference Q was 0.1 mm, and in Test Example 23, the maximum thickness difference Q was 0.25 mm. The same thermal cycling test was performed with these dimensions. In Test Examples 24 and 25, shown in Fig. 23, the coating length L1 was set to 0.3 mm. In Test Example 24, the maximum thickness difference Q was 0.1 mm, and in Test Example 25, the maximum thickness difference Q was 0.25 mm. The same thermal cycling test was conducted with these dimensions. In addition, in Test Examples 26 and 27, shown in Fig. 24, the coating length L1 was set to 0.5 mm. In Test Example 26, the maximum thickness difference Q was 0.1 mm, and in Test Example 27, the maximum thickness difference Q was 0.25 mm. The same thermal cycling test was performed with these dimensions. The results are shown in the figures.

[0107] As in the upper and lower parts of the Fig. 22 to 24, in Experimental Examples 22, 24 and 26 in which the thick portion 523 had the maximum thickness difference Q of 0.1 mm, the expressions of the boundary lines shown shifted in the direction in which the value of the coating thickness S decreases, compared with Experimental Examples 23, 25 and 27 in which the thick portion 523 had the maximum thickness difference Q of 0.25 mm.

[0108] In particular, the combinations that do not cause cracks are obtained from the following expressions of the boundary lines shown in the figures. Test example 22: S ≥ D1 / 20 + 0.2 / 10 - 0.1 / 10 - 0.005 mm Test example 23: S ≥ D1 / 20 + 0.2 / 10 - 0.25 / 10 - 0.005 mm Test example 24: S ≥ D1 / 20 + 0.3 / 10 - 0.1 / 10 - 0.005 mm Test example 25: S ≥ D1 / 20 + 0.3 / 10 - 0.25 / 10 - 0.005 mm Test example 26: S ≥ D1 / 20 + 0.5 / 10 - 0.1 / 10 - 0.005 mm Test example 27: S ≥ D1 / 20 + 0.5 / 10 - 0.25 / 10 - 0.005 mm

[0109] Based on the relationship of these expressions, the coating length L1 and the maximum thickness difference Q can be used to create Expression 1A. Expression 1A: S≥D1 / 20+L1 / 10−Q / 10−0.005 mm

[0110] Then, by adjusting the coating thickness S to satisfy Expression 1A according to the maximum thickness difference Q, an improvement in the required strength against the thermal stress applied in both the radial direction Y and the axial direction X can be achieved, which suppresses the occurrence of cracks in the side surface coating 522.

[0111] Although in the above embodiments, the composite chips 5 are installed on both the center electrode 3 and the ground electrode 4 of the spark plug 1, one composite chip 5 may be installed on the center electrode 3 and / or the ground electrode 4.

[0112] The present disclosure is not limited to the above-described embodiments, but is applicable to various embodiments without departing from the essence thereof. In the above embodiments, for example, the spark plug 1 is described as being installed in a lean-burn engine, but it may be applied to any internal combustion engine other than a lean-burn engine. The components of the spark plug 1 may be those described in Fig. 3 can also be modified as appropriate.

Claims

[1] Spark plug (1) for an internal combustion engine, comprising: a center electrode (3) held inside a cylindrical insulator (2) and projecting from a tip of the insulator (2) to a distal end; a ground electrode (4) provided at a distal end of a housing (H) holding the insulator (2), the ground electrode (4) facing the center electrode (3) in an axial direction (X); and a composite chip (5) formed on the center electrode (3) and / or the ground electrode (4) and protruding in the axial direction (X), wherein the composite chip (5) comprises a core (51) with a holder (511) formed integrally with an electrode base material (3A, 4A), and a cup-shaped surface layer (52) having a discharge portion (521) covering a projection end surface (512) of the core (51) and a side surface coating (522) covering a side surface (513) continuously to the projection end surface (512), the core (51) is formed from a Ni alloy material and the surface layer (52) is formed from a Pt alloy material, and in the surface layer (52), a coating thickness S of the side surface coating (522) in a radial direction (Y), an outer diameter D1 of the discharge portion (521) and a coating length L1 of the side surface coating (522) in the axial direction (X) satisfy a relationship represented by an expression 1: Expression 1: S≥D1 / 20+L1 / 10−0.005 mm. [2] Spark plug (1) for the internal combustion engine according to claim 1, wherein the surface layer (52) comprises a thick portion (523) along a connecting region between the discharge portion (521) and the side surface coating (522), wherein the thick portion (523) has a maximum coating thickness S1 in the radial direction (Y) that is greater than the coating thickness S of the side surface coating (522), and the coating thickness S of the side surface coating (522), the outer diameter D1 of the discharge section (521), the coating length L1 of the side surface coating (522) in the axial direction (X), and a maximum coating thickness difference Q corresponding to a difference between the maximum coating thickness S1 and the coating thickness S of the side surface coating (522) satisfy a relationship represented by an expression 1A: Expression 1A: S≥D1 / 20+L1 / 10−Q / 10−0.005 mm. [3] The spark plug (1) for the internal combustion engine according to claim 2, wherein the core (51) includes a tapered portion (514) along a connecting portion between the protrusion end surface (512) and the side surface (513), the thick portion (523) being adjacent to the tapered portion (514) in the radial direction (Y), and the maximum coating thickness difference Q being in a range of 0 mm < Q ≤ 0.25 mm. [4] Spark plug (1) for the internal combustion engine according to one of claims 1 to 3, wherein the Pt alloy material for the surface layer (52) corresponds to a Pt-Rh alloy, a Pt-Ni alloy, a Pt-Ir alloy or a Pt-Pd alloy. [5] Spark plug (1) for the internal combustion engine according to one of claims 1 to 4, wherein the Ni alloy material for the core (51) corresponds to a Ni-Cr-based alloy or a Ni-Cr-Fe-based alloy. [6] Spark plug (1) for the internal combustion engine according to one of claims 1 to 5, wherein the holder exposed from the side surface coating (522) has a smallest diameter part with a diameter D2, and the diameter D2 and the outer diameter D1 of the discharge section (521) satisfy a relationship represented by an expression 2: Expression 2:D2 / D1≥0.

8. [7] The spark plug (1) for the internal combustion engine according to any one of claims 1 to 6, wherein the side surface coating (522) in the axial direction (X) has a coating length L1 within a range of 0.2 mm ≤ L1 ≤ 0.5 mm, and the support exposed from the side surface coating (522) has an exposed length L2 within a range of 0.2 mm ≤ L2 ≤ 0.5 mm. [8] The spark plug (1) for the internal combustion engine according to any one of claims 1 to 7, wherein the discharge portion (521) has an outer diameter D1 within a range of 0.5 mm ≤ D1 ≤ 1.1 mm, and the discharge portion (521) has a coating thickness T in the axial direction (X) within a range of 0.15 mm ≤ T ≤ 0.25 mm. [9] Spark plug (1) for the internal combustion engine according to one of claims 1 to 8, wherein the coating thickness S of the side surface coating (522) and the coating thickness T of the discharge portion (521) have a relationship S ≤ T. [10] Spark plug (1) for the internal combustion engine according to one of claims 1 to 9, wherein the holder is connected to the electrode base material and is formed from a Ni alloy or a noble metal-containing Ni alloy.

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