Spark plugs for internal combustion engines

Abstract

To provide a spark plug for an internal combustion engine capable of improving processability while securing wear resistance of a plug cover.SOLUTION: A spark plug 1 has an insulator 3, a center electrode 4, a housing 2, a ground electrode 6, and a plug cover 5. The plug cover 5 is provided at a tip part of the housing 2 so as to cover a sub-combustion chamber 50 where a discharge gap G is arranged. The plug cover 5 is formed with an injection hole 51 communicating the sub-combustion chamber 50 with the exterior. The plug cover 5 contains Ni as a main component and contains 0.01 mass% or more of Y and Ti, respectively. A total content of Y and Ti in the plug cover 5 is 1.21 mass% or less. A content of Cr in the plug cover 5 is less than 1.0 mass%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a spark plug for an internal combustion engine. [Background technology]

[0002] For example, as disclosed in Patent Document 1, a spark plug with an auxiliary combustion chamber at its tip is known. In this spark plug, a nozzle hole is formed in a plug cover that covers the auxiliary combustion chamber. This allows a flame to be ejected from the auxiliary combustion chamber through the nozzle hole into the main combustion chamber, thereby combusting the air-fuel mixture in the main combustion chamber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-35517 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the spark plug described in Patent Document 1, the plug cover is made of an alloy containing a certain percentage or more of Cr, which tends to make the plug cover less workable. On the other hand, it is also necessary to ensure the wear resistance of the plug cover.

[0005] The present invention has been made in view of the above-mentioned problems, and has an object to provide a spark plug for an internal combustion engine that can improve the workability while ensuring the wear resistance of the plug cover. [Means for solving the problem]

[0006] One aspect of the present invention is a cylindrical insulator (3), a center electrode (4) held on the inner circumferential side of the insulator and protruding from the insulator toward the tip side; a cylindrical housing (2) that holds the insulator on its inner periphery; a ground electrode (6) forming a discharge gap (G) between itself and the center electrode; a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed, The plug cover is formed with a nozzle hole (51) that communicates the auxiliary combustion chamber with the outside, The plug cover contains Ni as a main component and 0.01% by mass or more of Y and Ti, The total content of Y and Ti in the plug cover is 1.21% by mass or less, The plug cover contains Cr. Without , The plug cover has a peripheral wall portion (52) covering an outer periphery side of the auxiliary combustion chamber, a bottom wall portion (53) covering a tip side of the auxiliary combustion chamber, and a corner portion (54) connecting a tip of the peripheral wall portion and an outer periphery of the bottom wall portion in a curved shape, The plug cover has a Vickers hardness of less than 120HV. In a spark plug (1) for an internal combustion engine. Effect of the Invention

[0007] In the above spark plug, the plug cover does not contain Cr (i.e., chromium), and contains 0.01 mass% or more of Y (i.e., yttrium) and 0.01 mass% or more of Ti (i.e., titanium), with the total content of Y and Ti being 1.21 mass% or less. In addition, the plug cover has a Vickers hardness of less than 120 HV. Therefore, the plug cover is easy to machine and is not easily worn away by the flame that is ejected from the auxiliary combustion chamber to the main combustion chamber through the nozzle hole. As a result, the wear resistance of the plug cover is ensured while the machineability is improved.

[0008] As described above, according to the above aspect, it is possible to provide a spark plug for an internal combustion engine that can improve the workability while ensuring the wear resistance of the plug cover. In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate a correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief description of the drawings]

[0009] [Figure 1] 2. FIG. 3 is a cross-sectional view of the spark plug in the axial direction in the vicinity of the tip portion of the spark plug according to the first embodiment, which corresponds to a cross-sectional view taken along line II in FIG. [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Diagram 3] 1 is a cross-sectional view of an internal combustion engine in which a spark plug is installed according to a first embodiment. [Figure 4] 4 is a graph showing the relationship between the Cr content of the Ni alloy and the Vickers hardness in Experimental Example 1. [Diagram 5] 13 is a graph showing the relationship between the Y and Ti contents of the plug cover and the rate of diameter change in Experimental Example 2. [Figure 6] 13 is a graph showing the relationship between the Si content of the plug cover and the operating time of the internal combustion engine until the diameter change rate reaches 5% in Experimental Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] (Embodiment 1) An embodiment of a spark plug for an internal combustion engine will be described with reference to Figs. A spark plug 1 for an internal combustion engine according to this embodiment has a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, a ground electrode 6, and a plug cover 5, as shown in Figs. 1 and 2. The center electrode 4 is held on the inner periphery of the insulator 3 and protrudes from the insulator 3 toward the tip side. The housing 2 holds the insulator 3 on the inner periphery side. The ground electrode 6 forms a discharge gap G between itself and the center electrode 4. The plug cover 5 is provided at the tip of the housing 2 so as to cover the auxiliary combustion chamber 50 in which the discharge gap G is arranged. The plug cover 5 has an injection hole 51 that communicates the auxiliary combustion chamber 50 with the outside.

[0011] The plug cover 5 contains Ni as a main component and 0.01 mass% or more each of Y and Ti. The total content of Y and Ti in the plug cover 5 is 1.21 mass% or less. The Cr content in the plug cover 5 is less than 1.0 mass%.

[0012] In this embodiment, the Cr content in the plug cover 5 is preferably 0.5 mass% or less. The Cr content in the plug cover 5 may be, for example, 0.1 to 0.5 mass%. Furthermore, the plug cover 5 may be, for example, substantially free of Cr.

[0013] The plug cover 5 also contains 0.02 mass % or more each of Y and Ti. Furthermore, in this embodiment, the plug cover 5 also contains 0.3 to 1.5 mass % of Si (that is, silicon).

[0014] The Ni content in the plug cover 5 is, for example, 98 mass % or more.

[0015] The spark plug 1 of this embodiment can be used, for example, as an ignition means in an internal combustion engine of an automobile, etc. The spark plug 1 is attached to an internal combustion engine 10 by screwing a threaded portion 21 formed on the outer circumferential surface of a housing 2 into a female threaded portion of a plug hole 711 of a cylinder head 71, as shown in FIG.

[0016] The internal combustion engine 10 also includes a piston 74 that reciprocates within a cylinder 70. The volume of the main combustion chamber 101 changes due to the reciprocating motion of the piston 74. The internal combustion engine 10 is formed with an intake port 721 and an exhaust port 731, and is provided with an intake valve 72 and an exhaust valve 73, respectively.

[0017] One end of the spark plug 1 in the axial direction Z is disposed in the main combustion chamber 101 of the internal combustion engine 10. In the axial direction Z of the spark plug 1, the side exposed to the main combustion chamber 101 is referred to as the tip side, and the opposite side is referred to as the base side. The axial direction Z of the spark plug 1 is also referred to as the plug axial direction Z as appropriate. As shown in Figs. 1 and 2, the central axis C of the spark plug 1 is also the central axis of the center electrode 4 in this embodiment.

[0018] The plug cover 5 is joined to the tip of the housing 2 by welding or the like. The plug cover 5 is in thermal contact with the housing 2. As shown in Fig. 3, when the spark plug 1 is attached to the internal combustion engine 10, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber 101. The injection hole 51 communicates between the auxiliary combustion chamber 50 and the main combustion chamber 101.

[0019] 1, the auxiliary combustion chamber 50 includes a space on the inner circumferential side of the tip portion of the housing 2 around the center electrode 4 protruding from the insulator 3 toward the tip side. The auxiliary combustion chamber 50 also includes a pocket portion 501, which is an annular space formed between the outer circumferential surface of the insulator 3 and the inner circumferential surface of the housing 2.

[0020] In this embodiment, the plug cover 5 has a peripheral wall portion 52, a bottom wall portion 53, and a corner portion 54. The peripheral wall portion 52 is a generally cylindrical portion that covers part of the outer periphery of the auxiliary combustion chamber 50. The bottom wall portion 53 is a portion that covers the tip side of the auxiliary combustion chamber 50. The corner portion 54 is a portion that connects the tip of the peripheral wall portion 52 and the outer periphery of the bottom wall portion 53 in a curved shape.

[0021] In this embodiment, as shown in Figures 1 and 2, five injection holes 51 are formed in the plug cover 5. Four injection holes 51 are formed in the corner portion 54. One injection hole 51 is formed in the bottom wall portion 53.

[0022] The injection holes 51 formed in the corner portion 54 open at an angle with respect to the plug axial direction Z so as to move radially outward toward the tip side. As shown in Fig. 2, when viewed from the plug axial direction Z, the injection holes 51 formed in the corner portion 54 are formed at equal intervals in the plug circumferential direction. Note that the plug radial direction means the radial direction of a circle centered on the central axis C of the spark plug 1 on a plane perpendicular to the central axis C of the spark plug 1. Also, the plug circumferential direction means the circumferential direction of a circle centered on the central axis C of the spark plug 1 on a plane perpendicular to the central axis C of the spark plug 1.

[0023] Further, the injection hole 51 formed in the bottom wall portion 53 is formed so that its central axis is aligned along the plug axial direction Z.

[0024] 1, the nozzle holes 51 have substantially the same diameter D. Each nozzle hole 51 is formed so that the cross section perpendicular to the central axis thereof is substantially circular.

[0025] Next, a method for manufacturing the plug cover 5 will be described. In this embodiment, a plug cover before forming the injection hole 51 is formed by cold forging a plate-shaped alloy member made of a Ni alloy. The alloy member contains Ni as a main component, as well as Y, Ti, and Si. The alloy member has the same composition as the plug cover 5 described above.

[0026] Next, the plug cover 5 in this embodiment can be manufactured by opening the injection hole 51 in the plug cover after the cold forging process. Then, the plug cover 5 with the injection hole 51 opened therein is joined to the tip end portion of the housing 2 by welding or the like, thereby obtaining the spark plug 1 in this embodiment.

[0027] Next, the effects of this embodiment will be described. In the above spark plug 1, the plug cover 5 has a Cr content of less than 1.0 mass%. The plug cover 5 also contains Y and Ti at 0.01 mass% or more, and the total content of Y and Ti is 1.21 mass% or less. Therefore, the plug cover 5 is easy to process and is not easily worn by the flame ejected from the auxiliary combustion chamber 50 to the main combustion chamber through the injection hole 51. As a result, the processability of the plug cover 5 can be improved while ensuring the wear resistance.

[0028] It is known that a plug cover mainly composed of Ni contains a certain proportion of Cr to improve wear resistance. However, if the Cr content is high, the workability of the plug cover decreases. Therefore, the inventors of the present application conducted extensive research to achieve both wear resistance and workability of the plug cover, and found that the plug cover can be formed with the above composition. First, the problem of wear resistance of the plug cover and the mechanism of wear will be described. In this explanation, it is assumed that the plug cover does not contain any of Cr, Y, and Ti. However, the reference numerals used in Figs. 1 to 3 will be used.

[0029] The spark plug 1 ignites the mixture in the auxiliary combustion chamber 50 by generating a discharge in the discharge gap G, forming a flame. Then, the flame generated in the auxiliary combustion chamber 50 is ejected into the main combustion chamber through the nozzle hole 51. As a result, the flame is propagated into the main combustion chamber to burn the mixture. Here, if the strength or direction of the flame jet ejected from the auxiliary combustion chamber 50 to the main combustion chamber through the nozzle hole 51 changes due to wear of the plug cover 5, the main combustion period of the mixture in the internal combustion engine may change. If the main combustion period is too long, the ignitability of unburned fuel in the main combustion chamber may increase, causing combustion abnormalities that may cause knocking. If the main combustion period is too short, noise may be generated due to a sudden change in pressure in the cylinder. Therefore, it is necessary to set the strength and direction of the flame jet within a predetermined range so that the main combustion period falls within a predetermined period. In this specification, the "main combustion period" refers to, for example, the period during which the mass combustion rate in an internal combustion engine increases from 10% to 90%.

[0030] Here, the strength of the flame jet is greatly affected by the ratio of the opening area S of the nozzle hole 51 to the volume V of the auxiliary combustion chamber 50 (hereinafter, simply referred to as the S / V ratio). Therefore, when the inner circumferential surface 511 of the nozzle hole 51 and its surrounding parts are worn away and the opening area S of the nozzle hole 51 changes, the S / V ratio will change compared to before the wear. And when the S / V ratio changes, the strength of the flame jet may change. Also, if the inner circumferential surface 511 of the nozzle hole 51 and its surrounding parts are worn away too much, the direction of the flame jet may change. Therefore, if the inner circumferential surface 511 of the nozzle hole 51 and its surrounding parts are worn away too much, there is a risk of abnormal combustion or noise causing knocking or the like.

[0031] Furthermore, the spark plug 1 installed in the internal combustion engine ejects a flame jet into the main combustion chamber through the nozzle hole 51, and also introduces a relatively low-temperature mixture from the main combustion chamber into the auxiliary combustion chamber 50 through the nozzle hole 51. The ejection of the flame jet and the introduction of the mixture through the nozzle hole 51 are repeated. In other words, the inner circumferential surface 511 of the nozzle hole 51 and the area nearby are continuously exposed to a high-temperature environment in which heating and cooling are repeated. For this reason, the inner circumferential surface 511 of the nozzle hole 51 and the area nearby are generally also areas that are relatively susceptible to wear due to oxidation.

[0032] More specifically, the plug cover 5 has an oxide film formed on its surface, which prevents the progression of oxidation. However, the inner circumferential surface 511 of the injection hole 51 and its surrounding areas, which are repeatedly heated and cooled, are prone to thermal stress in the oxide film due to the difference in linear expansion coefficient between the oxide film and the Ni alloy inside the oxide film. This may result in cracking and peeling of the oxide film. If peeling of the oxide film occurs, oxidation of the plug cover 5 may progress, and the inner circumferential surface 511 of the injection hole 51 and its surrounding areas may be worn away.

[0033] In addition, the inner circumferential surface 511 of the nozzle hole 51 and the area nearby are easily subject to change in metal structure due to repeated heating and cooling. This may cause impurities to segregate at the grain boundaries, which may accelerate corrosion at the grain boundaries. This may cause the inner circumferential surface 511 of the nozzle hole 51 and the area nearby to wear out.

[0034] To address this problem, it may be considered to improve the oxidation resistance by adding Cr to the Ni that constitutes the plug cover 5. However, as described above, this would result in a new problem of reduced workability.

[0035] Therefore, in the spark plug 1 of this embodiment, the plug cover 5 contains Y and Ti at 0.01 mass % or more. The plug cover 5 contains Y, which can strengthen the adhesion between the oxide film and the Ni alloy inside the oxide film. Therefore, peeling of the oxide film can be suppressed, and the oxidation resistance of the plug cover 5 can be improved. The plug cover 5 contains Ti, which can improve the oxidation resistance, and can suppress corrosion, particularly at the grain boundaries. Therefore, wear of the inner circumferential surface 511 of the nozzle hole 51 and the surrounding area of ​​the plug cover 5 can be suppressed. Therefore, the spark plug 1 installed in the internal combustion engine is less likely to change in strength and direction of the flame jet. Therefore, it is possible to suppress combustion abnormalities and noise that cause knocking. As a result, the life of the spark plug 1 can be extended.

[0036] In addition, by containing both Y, which suppresses peeling of the oxide film, and Ti, which suppresses corrosion at the grain boundaries, the wear resistance of the plug cover 5 can be reliably improved, and the life of the spark plug 1 can be reliably extended.

[0037] On the other hand, if the contents of Y and Ti in the plug cover 5 are too large, the thermal conductivity of the plug cover 5 is likely to decrease. If the thermal conductivity decreases, the plug cover 5 is likely to reach a high temperature, and the effect of Y and Ti in improving the oxidation resistance is likely to decrease. Here, in the spark plug 1 of this embodiment, the plug cover 5 has a total content of Y and Ti of 1.21 mass % or less. Therefore, the thermal conductivity of the plug cover 5 is unlikely to decrease. Therefore, it is possible to prevent the plug cover 5 from reaching a high temperature. As a result, it is possible to reliably ensure the wear resistance of the plug cover 5.

[0038] In addition, in order to manufacture the plug cover efficiently without increasing costs, it is more desirable to manufacture the plug cover by cold forging rather than cutting. As described above, Cr is an element that enhances oxidation resistance, but when Cr is contained in an alloy member for manufacturing the plug cover at a predetermined ratio or more, the alloy member is likely to be hardened. Therefore, when the alloy member contains Cr at a predetermined ratio or more, cracks are likely to occur during the manufacture of the plug cover by cold forging, making it difficult to process. In addition, it is difficult to manufacture the plug cover efficiently by cutting. Therefore, in the spark plug 1 of this embodiment, the plug cover 5 has a Cr content of less than 1.0 mass%. Therefore, the plug cover 5 is easily formed by cold forging. As a result, the workability of the plug cover 5 can be improved.

[0039] More preferably, the Cr content in the plug cover 5 is 0.5 mass % or less. This makes it easier to form the plug cover 5 by cold forging. As a result, the workability of the plug cover 5 can be further improved.

[0040] As described above, the plug cover 5 contains Y and Ti, which improve the oxidation resistance. Therefore, even if the content of Cr, which improves the oxidation resistance, is reduced, the plug cover 5 is not easily oxidized. As a result, the wear resistance of the plug cover 5 can be ensured.

[0041] The plug cover 5 contains 0.02 mass % or more each of Y and Ti, which makes the plug cover 5 even more resistant to wear. As a result, the wear resistance of the plug cover 5 can be effectively ensured.

[0042] The plug cover 5 also contains 0.3 to 1.5 mass % of Si. Si is an element with excellent oxidation resistance. Therefore, the oxidation resistance at high temperatures can be further improved. Therefore, the plug cover 5 can be further prevented from being worn down by the flame jet. As a result, the wear resistance of the plug cover 5 can be more effectively ensured.

[0043] Furthermore, by adding small amounts of Y, Ti, and Si as described above, the oxidation resistance of the plug cover 5 can be improved. Therefore, the oxidation resistance can be improved while suppressing a decrease in the thermal conductivity of the plug cover 5. As a result, the wear resistance of the plug cover 5 can be effectively ensured.

[0044] As described above, according to the present embodiment, it is possible to provide the spark plug 1 for an internal combustion engine, which can ensure the wear resistance of the plug cover 5 while improving the workability.

[0045] (Experimental Example 1) In this example, as shown in the graph of Fig. 4, the relationship between the Cr content and the Vickers hardness was investigated for Ni alloys having the same basic composition as the plug cover in embodiment 1. In this example, samples were prepared from four types of Ni alloys having different Cr contents as shown in Table 1 below. Then, a Vickers hardness test was performed for each sample. For the Vickers hardness test, a Mitutoyo HM-200 (micro Vickers hardness tester) was used.

[0046] [Table 1]

[0047] In the graph of Fig. 4, the circles indicate plots of the measurement results of Samples 1 to 4 in Table 1. An approximation line for these plots is also shown in the graph of Fig. 4. Generally, when a Ni alloy has a Vickers hardness of 120 HV or more, it is prone to cracking when subjected to cold forging, and workability is prone to deteriorate.

[0048] It can be seen from Table 1 and the graph in Figure 4 that the Vickers hardness value decreases as the Cr content decreases. From this result, it is considered that when manufacturing a plug cover from an alloy member mainly composed of Ni by cold forging, the lower the Cr content of the alloy member, the more the workability can be improved.

[0049] In addition, from the approximation line shown in the graph of Fig. 4, it was estimated that the Cr content at which the Vickers hardness is less than 120HV is less than 1.0 mass%. From this result, it is considered that the workability of the alloy part mainly composed of Ni can be improved when the Cr content is less than 1.0 mass%. Furthermore, it is considered that the workability can be sufficiently improved by setting the Cr content to 0.5 mass% or less.

[0050] (Experimental Example 2) In this example, as shown in Table 2, a relationship between the composition of the plug cover and the workability and nozzle hole wear of the plug cover was investigated using a plurality of spark plugs having the same basic structure and basic composition of the plug cover as in embodiment 1. In this example, as shown in Table 2, spark plugs were prepared as Examples 1 to 7 and Comparative Examples 1 to 11, in which the respective contents of Cr, Ti, and Y in the plug cover were different from one another. In addition, in Examples 1 to 7 and Comparative Examples 1 to 11, the balance other than the composition shown in Table 2 was Ni. In Table 2, "-" indicates that the content is less than 0.001 mass%.

[0051] [Table 2]

[0052] In this example, the workability was evaluated based on a Vickers hardness of less than 120 HV as in Experimental Example 1. As shown in Table 2, a Vickers hardness of less than 120 HV was indicated by "◯" and a Vickers hardness of 120 HV or more was indicated by "X".

[0053] As shown in Table 2, when the Cr content was less than 1.0 mass%, regardless of the Ti and Y contents, the Vickers hardness was less than 120 HV. Specifically, when the Cr content was 0.5 mass% or less, the Vickers hardness was less than 120 HV. From this result, it is considered that, regardless of the Ti and Y contents, by setting the Cr content to 0.5 mass% or less, it is possible to reliably improve the workability of the plug cover.

[0054] In this example, the nozzle hole wear was evaluated by measuring the diameter of the nozzle hole before and after the internal combustion engine in which the spark plug was installed was operated for a predetermined time, and the change rate of the diameter after operation relative to before operation (hereinafter simply referred to as the diameter change rate) was calculated and evaluated. The operating conditions of the internal combustion engine in the evaluation of the nozzle hole wear were as follows: the internal combustion engine in which the spark plug was installed was a four-cylinder, four-stroke engine with a displacement of 1.8 L, the throttle was fully open, and the rotation speed was 6500 rpm. The diameter of the nozzle hole was measured using a pin gauge every 20 hours of operation of the internal combustion engine. The diameter change rates of all the nozzle holes were measured for each spark plug, and the average value was used for the evaluation.

[0055] In addition, the evaluation of the nozzle hole wear was based on a standard of a rate of change in the S / V ratio of less than 10%, taking into consideration both the knocking resistance and the noise resistance of the spark plug. In the spark plug of the first embodiment, when the rate of change in diameter is 5%, the rate of change in the S / V ratio is 10%. Therefore, in this example, a composition of a plug cover that provides a rate of change in diameter of less than 5% after the internal combustion engine is operated for a predetermined period of time was determined.

[0056] In addition, when converted from a general driving frequency, the operating time of the internal combustion engine when a passenger car travels 100,000 km is 300 hours. In addition, when converted from a general driving frequency, the operating time of the internal combustion engine when a passenger car travels 100,000 miles is 480 hours. Therefore, as shown in Table 2, the nozzle hole wearability evaluation was given as "◎" when the diameter change rate was less than 5% when the internal combustion engine was operated for 480 hours. In addition, the nozzle hole wearability evaluation was given as "○" when the diameter change rate was less than 5% when the internal combustion engine was operated for 300 hours, and as "×" when the diameter change rate was 5% or more.

[0057] As shown in Table 2, except for Comparative Example 1 in which the Cr content was 1.0% by mass or more, Comparative Examples 2 to 6, 9, and 10 in which the Y and Ti content in the plug cover was less than 0.01% by mass were evaluated as "x" for the nozzle hole wear resistance. From this result, it is considered that even if only one of Y and Ti is contained in the plug cover at 0.01% by mass or more, the wear resistance effect cannot be sufficiently exhibited. In addition, Comparative Examples 5, 7, 8, 10, and 11 in which the total content of Y and Ti exceeded 1.21% by mass were also evaluated as "x". In Comparative Examples 7, 8, and 11 in which both Y and Ti were contained in the plug cover at 0.01% by mass or more, the nozzle hole wear resistance was evaluated as "x" when the total content of Y and Ti exceeded 1.21% by mass. This is considered to be because the thermal conductivity of the plug cover decreased due to the total content of Y and Ti becoming too high. In other words, it is thought that because low thermal conductivity makes the plug cover prone to high temperatures, the oxidation resistance effect of the addition of Y and Ti is reduced.

[0058] On the other hand, in Examples 1 to 7, which contain 0.01 mass% or more of Y and Ti, respectively, and the total content of Y and Ti is 1.21 mass% or less, the evaluation of the nozzle hole wearability was "○" or "◎". In particular, in Examples 4 and 7, which contain 0.02 mass% of Y and Ti, respectively, the evaluation of the nozzle hole wearability was "◎". In other words, as shown in FIG. 5, when the contents of Y and Ti are inside the triangle shown by the dashed line, the evaluation of the nozzle hole wearability is considered to be "◎" or "○". On the other hand, when they are outside the triangle shown by the dashed line, it is considered that sufficient wear resistance cannot be ensured.

[0059] (Experimental Example 3) In this example, similarly to Experimental Example 2, the relationship between the composition of the plug cover and the workability and nozzle hole wear of the plug cover was investigated using a plurality of spark plugs having the same basic structure and basic composition of the plug cover as in Embodiment 1. In this example, spark plugs of Examples 2, 5, and 8 to 17 were prepared, each having a different content of Si, Ti, and Y in the plug cover, as shown in Table 3. The other test conditions were the same as in Experimental Example 2.

[0060] [Table 3]

[0061] Regarding the workability evaluation, regardless of the Si content, when the Cr content was 0.5 mass%, the Vickers hardness was less than 120 HV, as shown in Table 3. From this result, it is considered that, regardless of the Si content, by setting the Cr content to 0.5 mass% or less, it is possible to reliably improve the workability of the plug cover.

[0062] As shown in Table 3, the evaluation of the nozzle hole wear resistance is improved by including a certain percentage of Si. Specifically, when Examples 5 and 11 to 13, in which the respective contents of Cr, Ti, and Y are the same, are compared, Example 5, in which the Si content is less than 0.3 mass%, is evaluated as "good", whereas Examples 11 to 13, in which the Si content is 0.3 to 1.5 mass%, are evaluated as "good". In addition, when Examples 2 and 8 to 10, in which the respective contents of Cr, Ti, and Y are the same, are compared, the same results were obtained. In other words, as shown in the graph of FIG. 6, even if the respective contents of Ti and Y are the same, it is considered that the wear resistance of the plug cover can be further improved by including 0.3 to 1.5 mass% of Si. This is considered to be because the wear resistance of the plug cover is improved due to the oxidation resistance effect of Si.

[0063] On the other hand, as shown in Table 3, in Examples 14 to 17 in which the Si content exceeded 1.5 mass%, the evaluation of injection hole wear was "○". This is believed to be because the thermal conductivity of the plug cover was reduced due to the Si content being too high. In other words, it is believed that the oxidation resistance effect of the addition of Si was reduced because the plug cover is prone to becoming hot when the thermal conductivity is low.

[0064] The present invention is not limited to the above-described embodiment, and can be applied to various embodiments without departing from the gist of the present invention. [Explanation of symbols]

[0065] 1...spark plug, 2...housing, 3...insulator, 4...center electrode, 5...plug cover, 50...auxiliary combustion chamber, 51...nozzle hole, 6...ground electrode, G...discharge gap

Claims

1. A cylindrical insulator (3); a center electrode (4) held on the inner circumferential side of the insulator and protruding from the insulator toward the tip side; a cylindrical housing (2) that holds the insulator on its inner periphery; a ground electrode (6) forming a discharge gap (G) between itself and the center electrode; a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed, The plug cover is formed with a nozzle hole (51) that communicates the auxiliary combustion chamber with the outside, The plug cover contains Ni as a main component and 0.01 mass % or more of Y and Ti, The total content of Y and Ti in the plug cover is 1.21% by mass or less, The plug cover does not contain Cr, The plug cover has a peripheral wall portion (52) covering an outer periphery side of the auxiliary combustion chamber, a bottom wall portion (53) covering a tip side of the auxiliary combustion chamber, and a corner portion (54) connecting a tip of the peripheral wall portion and an outer periphery of the bottom wall portion in a curved shape, The spark plug (1) for an internal combustion engine, wherein the plug cover has a Vickers hardness of less than 120 HV.

2. 2. The spark plug according to claim 1, wherein the plug cover contains Y and Ti in an amount of 0.02% by mass or more each.

3. 3. The spark plug according to claim 1, wherein the plug cover contains 0.3 to 1.5 mass % of Si.

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