Spark plug

Abstract: In a spark plug in which a tubular insulator has a stepped portion which rests on a shoulder portion provided on the metallic shell, the insulator has a rear half made of aluminium oxide and a front half made of aluminium nitride. The rear half has a protrusion, while the front half has a recess, and the protrusion is integrally joined to the housing. The protrusion extends past the shoulder portion.

Description: This invention relates to a spark plug in which an insulator is supported in a metallic shell by resting its stepped portion on a shoulder portion of the metallic shell. In a spark plug for an internal combustion engine, an insulator has generally been made of aluminium oxide (Al₂O₃) which is inferior in heat-conductivity. With the high speed and high power requirements of the internal combustion engine, a front end of the insulator tends to be exposed to higher ambient temperatures which may cause preignition. In order to protect the spark plug against the preignition, it has been suggested that the insulator, in an integral type of the spark plug, may be integrally made of aluminium nitride (AlN) which has a higher heat-conductivity so as to conduct heat away from the front end of the spark plug to prevent preignition within an extensive range of running conditions of the internal combustion engine. In the integral type of the spark plug, a shortage of withstand voltage of the insulator may cause a spark discharge to penetrate across the insulator so as to cause a misfire when a high tension is applied across the spark plug. Because aluminium nitride (AlN) is inferior to aluminium oxide (Al₂O₃) in withstanding high voltage, this is more likely if the insulator is made of aluminium nitride (AlN). It is very often that the spark discharge penetrates across the portion of the insulator located substantially adjacent to the shoulder portion of the insulator, since the stepped portion of the insulator rests on the shoulder portion. Provisional Japanese Patent Application No. 2-183985 filed on January 6, 1989 and its equivalent EP-A-0,349,183 disclose a spark plug for producing a spark at a front end and comprising a metallic shell having a shoulder portion within the metallic shell, and a tubular insulator having a stepped portion which rests on the shoulder portion within the metallic shell, and a center electrode placed within the insulator, wherein the insulator has a rear half made of aluminium oxide and has a front half made of aluminium nitride, the rear half has a central protrusion, the front half has a recess, the protrusion being in the recess and being integrally joined thereto in an air-tight manner by means of a glass sealant. In this split-type insulator, the spark discharge frequently penetrates across the front half of the insulator for the reason that the portion of the insulator which is adjacent to the shoulder portion of the metallic shell is entirely made of aluminium nitride. Therefore, it is an object of the invention to provide a spark plug with a relatively simple structure which is capable of effectively preventing a spark discharge from penetrating across the insulator. According to the invention, there is provided a spark plug for producing a spark at a front end and comprising a metallic shell having a shoulder portion within the metallic shell, and a tubular insulator having a stepped portion which rests on the shoulder portion within the metallic shell, and a center electrode placed within the insulator, wherein the insulator has a rear half made of aluminium oxide and has a front half made of aluminium nitride, the rear half has a central protrusion, the front half has a recess, the protrusion being in the recess and being integrally joined thereto in an air-tight manner by means of a glass sealant or a heat-resistant adhesive, and the front end of the protrusion extends past the shoulder portion of the metallic shell. The structure is such that a spark discharge is more effectively prevented from penetrating across the insulator by increasing the thickness of the protrusion of the rear half, which is superior in withstanding a high voltage. The protrusion of the rear half is joined in an air-tight manner to the recess by means of glass sealant or heat-resistant adhesive so that it is possible to avoid a voltage leakage through a joining surface between the protrusion and the housing. The invention may be more fully understood from the following description, given by way of example only, with reference to the drawings, in which: Fig. 1 is a longitudinal view of a spark plug, where part of the upper part is shown cut away; Fig. 2 is a longitudinal view of a spark plug according to one embodiment of the invention, where part of the upper part is shown cut away; Fig. 3 is a longitudinal view of a spark plug that is employed for the purpose of carrying out endurance test, where part of the upper part is shown cut away; and Fig. 4a - 4f are longitudinal cross sectional views of various spark plugs which show examples in which a spark discharges across an insulator. Referring to Figs. 1 and 2 which substantially shows a spark plug 100, the spark plug 100 has a cylindrical metallic shell 2, to a front end of which an L-shaped ground electrode 1 is fixed and attached by means of welding. Within the metallic shell 2, is a tubular insulator 3, an inner space of which serves as an axial bore 31. Concentrically within the bore 31, is a centre electrode 4. The outer surface of the metallic shell 2 has a diameter-increased hexagonal head 21, a barrel portion 22 and a threaded portion 23 which is to be attached to a cylinder block of an internal combustion engine. Each inner diameter of the hexagonal head 21 and the barrel portion 22 are larger, while an inner diameter of the threaded portion 23 is smaller. An inner wall of the region of the metallic shell 2 that is inside the threaded portion 23 has a shoulder portion 24 on which a stepped portion 3a rests by way of a gasket 25. The stepped portion 3a rests by way of a gasket 25. The stepped portion 3a is provided on the insulator 3 to support the insulator 3 within the metallic shell 2. The shoulder portion 24 includes an upper shoulder and lower shoulder as indicated at (a), (b) in Fig. 1. The insulator 3 has a tapered leg portion 32, a diameter-increased portion 33 and a head 35 whose outer surface has a corrugation 34. Between the diameter-increased portion 33 and the leg portion 32, is a middle portion 36 which is diametrically somewhat larger than the leg portion 32. The insulator 3 has a rear half 6 made of aluminium oxide (Al₂O₃) and a front half 5 made of aluminium nitride (AlN) so as to form a split-type insulator. The front half 5 includes the middle portion 36. In the rear of the front half 5 is a cylindrical recess 51 which extends into the leg portion 32 in such a manner that the rear portion of the front half 5 forms a housing 51a around the recess 51. The rear half 6 includes the diameter-increased portion 33 whose front end integrally has a protrusion 61. The protrusion 61 is air-tightly joined to the housing 51a by means of a heat-resistant adhesive (a silicon resin) or a glass sealant 7. In this instance, an overlapping portion (Op) of the protrusion 61 and the housing 51a is at least partly located in coplanar relationship with the upper and lower shoulders (a), (b) as apparent from Figs. 1, 2. Meanwhile, the center electrode 4 is inserted into the axial bore 31 whose inner wall has a ledge portion 31a. The center electrode has a flange head 4B which rests on the ledge portion 31a to bring the rear half 6 into a tight engagement with an inner wall of the housing 51a. To the rear end of the center electrode 4, is a terminal 83 and a middle axis 8 connected by way of a monolithic resistor 82 interposed between conductive glass seals 81a, 81b. Meanwhile, a front end of the spark plug 100 is, in use, projected into a combustion chamber on an internal combustion engine. When a high voltage is applied across the electrodes, a spark discharge is likely to occur between the center electrode 4 and the metallic shell 2 across the portion of the insulator where the shoulder portion directly meets the insulator, perhaps from the upper and lower shoulders (a), (b) or from the corners of the shoulder portion 24, if the insulator is poor in withstanding the high voltage. In order to avoid the occurrence of such a spark discharge, the Al₂O₃-made protrusion 61, which is superior in withstanding the high voltage, is of such a length as to extend past the coplanar line which the lower shoulder (b) forms i.e.(d) is below (b) in Fig. 1. Around the protrusion 61 the housing 51a is located, which is made of aluminium nitride (AlN), which is superior in heat conductivity to aluminium oxide (Al₂O₃) of which the rear half 6 is made. With this structure, the heat developed in the front end of 4A of the spark plug 100 may be more favourably dissipated to the metallic shell 2, by way of the front half 5, the gasket 25, the stepped portion 31a and the shoulder portion 24, than if the insulator is made entirely of aluminium oxide (Al₂O₃). The insulator is more capable of withstanding a high voltage than if the insulator is made entirely of aluminium nitride (AlN), because the protrusion extends past the shoulder portion 24 to which a spark discharge is likely to occur. Even if the thickness of the housing is reduced, it is possible to protect against a spark discharge by increasing the diameter of the protrusion 61. The adhesive 7 used at the joining surface between the protrusion 61 and the housing 51a is a glass sealant or a synthetic resin which also resists the high voltage. The glass sealant is very fine, and has a good wetting relationship with the protrusion 61 and the housing 51a. The coefficient of thermal expansion of the glass sealant is preferably 5 ∼ 8 x 10⁻⁶ K⁻¹ which is intermediate between that of aluminium nitride (AlN) and that of aluminium oxide (Al₂O₃) . When using a glass sealant which has a high softening point, the protrusion 61 is generally joined to the housing 51a by means of the glass sealant, prior to thermally encapsulating the conductive glass seals 81a, 81b, the monolithic resistor 82 and the terminal 83 into the insulator 3 (prior joint system). When using the prior joint system, it is necessary to use a heat-resistant glass sealant so that it is not melted when thermally encapsulating or hot pressing the conductive glass seals 81a, 81b, the monolithic resistor 82 and the terminal 83. When using the glass sealant which has an intermediate softening point, the protrusion 61 is generally joined to the housing 51a by means of the glass sealant, and simultaneously thermally encapsulating the conductive glass seals 81a. 81b, the monolithic resistor 82 and the terminal 83 into the insulator 3 (simultaneous joint system). When using the simultaneous joint system, it is necessary to use a heat-resistant glass sealant having a softening point similar to the glass seals. Upon using the glass sealant which has a low softening point, the protrusion 61 is joined to the housing 51a by means of the glass sealant after thermally encapsulating the conductive glass seals 81a, 81b, the monolithic resistor 82 and the terminal 83 into the insulator 3 (post joint system). The post joint system enables the use of inexpensive resin when the joint portion is used in a sufficiently low temperature ambience. The adhesive needs high voltage resistance and high insulation properties. A spark-endurance experiment was carried out at full load and 5000 rpm for 100 hours with the spark plug mounted on a four-cylinder, 2000 cc engine the joint system, the adhesive and the spark plug dimensions. Results of the experiment are shown in the Table. Figs. 4a ∼ 4f shows counterpart spark plugs as examples how a spark discharge occurs across the insulator. According to specimens (A) and (B) of the invention, the protrusion 61 is fitted into the recess 51 deeply as shown by a distance between (c) and (d) in Fig. 1. In this instance, the overlapping portion of the protrusion 61 and the housing 51a is exposed to a high temperature ambience, a glass sealant having a high or intermediate softening point is used as an adhesive. In the case of specimens (C), (D) and (F) of the invention the protrusion 61 is not fitted so deeply into the recess 51, the overlapping portion of the protrusion 61 and the housing 51a is not exposed to as high a temperature ambience and a glass sealant having a synthetic resin or a low softening point is used as an adhesive. In the case of specimen (E) of the invention, the protrusion 61 is not fitted so deeply into the recess 51 and the simultaneous system is adopted. It is found in any specimens (A) to (E) that no spark discharge penetrates across the insulator during the 100 hour-experiment. In the case of counterpart specimen (G) in which the insulator is integrally made of aluminium nitride as shown in Fig. 4a, it is found that a spark discharge penetrates across the insulator a few hours after the experiment has begun. In the case of counterpart specimen (H) in which the rear half and front half of the insulator are made of aluminium nitride respectively as shown in Fig. 4b, it is found that a spark discharge penetrates across the insulator in a few hours because the adhesive can not withstand the high voltage. In the case of counterpart specimen (I) in which a front end of the protrusion 61 is flush with the lower shoulder (b) as shown in Fig. 4c, it is found that a spark discharge penetrates across the insulator earlier than the specimen (G). In the case of counterpart specimen (J) in which the overlapping portion of the protrusion 61 and the housing 51a is above the level of the portion 3a as shown in Fig. 4d, it is found that a spark discharge penetrates across the insulator in a few hours. In the case of counterpart specimen (K) in which the rear half 5 has a recess, and the front half 6 has a protrusion as shown in Fig. 4e, the shoulder portion 3a directly meets the front half 6 so that a spark discharge penetrates across the insulator in a few hours. In the case of counterpart specimen (L) in which the insulator 3 is similar to the specimen (A), but no adhesive is used between the protrusion 61 and the housing 51a as shown in Fig. 4f, it is found that a spark discharge penetrates across the insulator because of a lack of tightness between protrusion 61 and the housing 51a. In any case of the counterpart specimens (G) to (K), it is found that a spark discharge penetrates across the insulator to cause a misfire because of the lack of an insulating resistance and withstand voltage. As understood from the foregoing description, the front half 5 made of aluminium nitride which is inexpensive and superior in heat-conductivity in order to quickly transmit the heat of the front end of the spark plug, while the rear half is made of aluminium oxide which is superior in withstanding high voltage. Further, the heat-resistant adhesive 7 which is superior in insulation, is used as an adhesive between the protrusion 61 and the housing 51a. This enables to provide an economic and reliable spark plug which is capable of preventing a spark discharge from penetrating across the insulator. A male thread may be made on an outer surface of the protrusion 61 and a female thread made on an inner wall of the housing 51a so that the protrusion 61 may be screwed into the housing 51a in order to integrate the protrusion 61 into the housing 51a. A glass sealant may be used instead of the heat-resistant adhesive.

Claims: A spark plug for producing a spark at a front end and comprising a metallic shell (2) having a shoulder portion (24) within the metallic shell (2), and a tubular insulator (3) having a stepped portion (3a) which rests on the shoulder portion (24) within the metallic shell (2), and a center electrode (4) placed within the insulator (3), wherein the insulator has a rear half (6) made of aluminium oxide and has a front half (5) made of aluminium nitride, the rear half (6) has a central protrusion (61), the front half (5) has a recess (51), the protrusion (61) being in the recess (51) and being integrally joined thereto in an air-tight manner by means of a glass sealant or a heat-resistant adhesive, and the front end of the protrusion extends past the shoulder portion (24) of the metallic shell (2). A spark plug as claimed in claim 1, wherein the glass sealant or heat-resistant adhesive has a coefficient of thermal expansion intermediate between that of the rear half (6) and that of the front half (5) of the insulator (3). A spark plug as claimed in claim 1 or 2, wherein a coefficient of thermal expansion of the glass sealant or heat resistant adhesive is in the range from 5 x 10⁻⁶K⁻¹ to 8 x 10⁻⁶K⁻¹. A spark plug as claimed in any preceding claim wherein the surface of the protrusion (61) and the surface of the recess (51) are both threaded so as to allow the protrusion (61) to be screwed into the recess (51).

Inventors: KONISHI MASAHIRO, MUSASA MAMORU, SUGIMOTO MAKOTA, TANABE HIROYUKI, KONISHI, MASAHIRO, MUSASA, MAMORU, SUGIMOTO, MAKOTA, TANABE, HIROYUKI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1993
Main classification: H01T13
IPC codes: H01T13