A multi-gap type spark plug for an internal combustion engine

Abstract: In a multi-gap type spark plug for an internal combustion engine, the plug has a cylindrical metallic shell into which a tubular ceramic insulator is enclosed. The insulator has a tapered front leg portion, a front end of which somewhat extends beyond that of the metallic shell. A centre electrode is enclosed into the insulator, a front end of the centre electrode extending beyond that of the insulator to work as a firing tip A plurality of L-shaped outer electrodes each having a vertical piece and lateral piece, the vertical piece depending from the front end of the metallic shell to surround the front end of the insulator, while the lateral piece having an inner surface arranged in parallel with a front end surface of the insulator and having an end tip terminated to oppose to an outer surface of the firing tip through a spark gap established therebetween. A vertical distance between the front end surface of the insulator and the inner surface of the lateral piece of each outer electrode is determined to be within a dimension ranging from 0.3 mm to 1.2 mm.

Description: This invention relates to a multi-gap type spark plug in which a plurality of outer electrodes are arranged to oppose a centre electrode, and improved electrode gap relationships. In a multi-gap spark plug in which an insulator and a centre electrode are in turn enclosed in a metallic shell, three outer electrodes are provided opposing the centre electrode, as shown in Japanese Patent Provisional Publications 51-95540 and 53-95443. In the first of these a main gap is dimensionally determined to be less than the sum of a secondary gap and a surface-creeping gap so as to improve the ignition of a lean fuel gas mixture. In the second a first spark gap is dimensionally determined to be greater than a second spark gap, so that a voltage needed for discharge at the first spark gap is greater than that for the second spark gap. In both the references, concern is directed to ignition performance which tends to be worsened in comparison with a single-gap type spark plug because the outer electrodes reduce the opportunity for the fuel-gas mixture to pass through the spark gap when it is introduced into the engine cylinder. Similar considerations apply to US 4514657 which shows a spark plug with the features of the precharacterising portions of claims 1 and 2. In order to prevent the ignition performance from being impaired, resort has been made to adjusting the distance by which the front end of leg portion of the insulator extends beyond that of the metallic shell. The leg portion of the insulator is the lower half portion which is tapered towards its front end. It has been required to shorten the leg portion by 0.5 mm to 2.0 mm so as to ensure a heat-resistant property comparable to that which an ordinary spark plug, which has an L-shaped outer electrode, can achieve. As the front end of the insulator extends beyond that of the metallic shell, the distance between the front end of the insulator and the outer electrode is reduced thus causing semi-creeping discharge or channelling, although the extended front end of the insulator is more effectively cooled by the intake fuel gas mixture. On the other hand, as the overall length of the leg portion is shortened to reduce the distance by which the leg portion extends beyond the metallic shell, the chances of discharge spark between the electrodes running along a fouled surface of the front end of the insulator are reduced thus hindering the self-cleaning action, although the decreased heat capacity of the leg portion improves its heat dissipation. Nowadays resort is made to dimensionally decreasing the extent by which the leg portion extends beyond the metallic shell thus sacrificing the self-cleaning action with the result that the front end of the leg portion is vulnerable to fouling due to the deposit of particulate carbon produced when the fuel gas mixture is burned on ignition. According to one aspect of the present invention, there is provided a multi-gap type spark plug for an internal combustion engine comprising; a cylindrical metallic shell enclosing a tubular ceramic insulator; the insulator having a tapered front leg portion, the front end of which extends beyond that of the metallic shell; a centre electrode enclosed in the insulator, the front end of the centre electrode extending beyond the front end surface of the insulator as a firing tip; and, a plurality of L-shaped outer electrodes each having an axial portion and a radial portion, the axial portion extending from the front end of the metallic shell, the radial portion having an inner surface arranged substantially parallel with the front end surface of the insulator, and having an end tip adapted to oppose a lateral surface of the firing tip across a spark gap to be established therebetween, characterised by: the axial distances (b) between the front end surface of the insulator and the inner surface of the radial portion of each of the outer electrodes being in the range 0.3 mm to 1.2 mm inclusively; and, in that the end tip of the lateral portion of each outer electrode extends beyond a cornered portion of the front end surface of the insulator so as to overlap it partially, a relationship among dimensions (a), (b) and (c) being: (a/2) < b < (3a/2), and c > a, where a is the spark gap between the lateral surface of the firing tip and the end tip of the radial portion of each outer electrode. b is the axial distance between the front end surface of the insulator and the inner surface of the radial portion of each outer electrode, and c is the radial distance between the lateral surface of the front leg portion of the insulator and the inner surface of the axial portion of each outer electrode. According to a second aspect of the present invention, there is provided for an internal combustion engine comprising; a cylindrical metallic shell enclosing a tubular ceramic insulator; the insulator having a tapered front leg portion, the front end of which extends beyond that of the metallic shell; a centre electrode enclosed in the insulator, the front end of the centre electrode extending beyond the front end surface of the insulator as a firing tip; and, a plurality of L-shaped outer electrodes each having an axial portion and a radial portion, the axial portion extending from the front end of the metallic shell, the radial portion having an inner surface arranged substantially parallel with the front end surface of the insulator, and having an end tip adapted to oppose a lateral surface of the firing tip across a spark gap to be established therebetween, characterised by: the axial distances (b) between the front end surface of the insulator and the inner surface of the radial portion of each of the outer electrodes being in the range 0.3 mm to 1.2 mm inclusively; and in that the end tip of the radial portion of each outer electrode terminates short of a cornered portion of the front end surface of the insulator a relationship among dimensions (a), (d) and (c) being: (a/2) < d < (3a/2) and c > a, where a is the spark gap defined between the lateral surface of the firing tip and the end tip of the radial portion of each outer electrode, d is the minimum distance between the front end surface of the insulator and the inner surface of the radial portion of each outer electrode, and c is the radial distance between the lateral surface of the front leg portion of the insulator and the inner surface of the axial portion of each outer electrode. The extension of the front end of the insulator makes it possible to enlarge its outer surface area to improve its heat-resistance because it is more effectively cooled each time fuel gas mixture is introduced into the engine cylinder. This substantially reduces the need to decrease the length of the leg portion. Otherwise, it is sufficient only slightly to decrease the length of the leg portion if at all. Further, when fouling decreases the insulating resistance between the electrodes, a spark discharge runs along the front end surface to remove any particulate carbon deposit so as to effect a self-cleaning action. An axial distance (b) of less than 0.3 mm often causes semi-creeping discharge and channelling on an outer surface of the insulator, while an axial distance (b) in excess of 1.2 mm reduces the cooling and self-cleaning effects. When the front end surface of the insulator is free from the particulate carbon deposit, a voltage necessary to cause a spark discharge between the front end surface of the insulator and the outer electrode is 1/2 to 3/4 times greater again than that between the firing tip of the centre electrode and the end tip of the outer electrode. Therefore, it is necessary to arrange (a/2) ≦ (b)so as to cause a discharge to occur through the spark gap between the firing tip of the centre electrode and the end tip of the outer electrode. When the front end surface of the insulator is fouled, its front end surface becomes equivalent to an electrical conductor, leading to a theoretical relationship (b) ≦ (a) and (c) > (a). In this instance, taking positional errors between the insulator and the electrodes into consideration, the relationship between (a), (b) and (c) may be determined to be (a/2) ≦ b ≦ (3a/2) so as to cause the spark discharge to creep between the front end surface of the insulator and the inner side of the radial portion of the outer electrode to effect the self-cleaning action. When the front end surface of the insulator is free from the particulate carbon deposit, a voltage necessary to cause a spark discharge between the front end surface of the insulator and the outer electrode may be 1/2 to 3/4 times greater again than that between the firing tip of the centre electrode and the end tip of the outer electrode. Therefore, it is necessary to arrange (a/2) ≦ (d) so as to cause discharge through the spark gap between the firing tip of the centre electrode and the end tip of the outer electrode. When the front end surface of the insulator is fouled, its front end surface becomes equivalent to an electrical conductor leading to a theoretical relationship (d) ≦ (a) and (c) > (a). In this instance, taking positional errors between the insulator and the electrodes into consideration, the relationship among (a), (d) and (c) may be determined to be (a/2) ≦ d ≦ (3a/2) so as to run the spark discharge between the front end surface of the insulator and the inner side of the radial portion of the outer electrode to effect the self-cleaning action. With the invention the previously mentioned drawbacks may be reduced on the basis that a minimum distance between the outer electrode and a front end surface of the insulator is found not to be so strictly necessary. The invention provides a multi-gap type spark plug which allows a lengthened front end of the leg portion without diminishing the leg portion thus dissipating heat from the leg portion, and at the same time achieving an improved self-cleaning action so as to protect the front end of the leg portion from fouling. The invention will be further understood from the following description, when taken with the accompanying drawings, which are given by way of example only, and in which: Fig. 1 is an enlarged view of a main part of a multi-gap type spark plug according to a first embodiment of the invention; Fig.2 is an elevational view of a multi-gap type spark plug; Fig. 3 is a bottom plan view of Fig. 2; Fig. 4 is an explanatory graph obtained at the time of carrying out a pre-delivery test; Fig. 5 is a graph showing results of the pre-delivery test; and Fig. 6 is a view similar to Fig. 1 according to a second embodiment of the invention. Referring to Fig. 1, there are shown electrodes of a multi-gap type spark plug (A) depicted in Fig. 2 which is incorporated into a cylinder head of an internal combustion engine (not shown) according to a first embodiment of the invention. The spark plug (A) has a cylindrical metallic shell 1 made of a low carbon steel, and comprising a male thread portion 12 (JIS M14 X 1.25), a hexagonal nut portion 13 and a middle portion 14 which is 19.5 mm in diameter. The hexagonal nut portion 13 works to expedite an instalment when the plug (A) is to be secured to the cylinder head by using a tool such as, for example, a wrench. Within the metallic shell 1, a tubular insulator 2 is concentrically placed, an inner space of which serves as an axial bore 22. The insulator 2 is made of a sintered ceramic material with alumina as a main component, and integrally having a tapered leg portion 21 at a lower half portion of the insulator 2 as indicated by a length (1) in Fig. 2 which extends from point (k) to the front end of the insulator 2. The front end of the insulator 2 extends beyond that of the metallic shell 1 by 2.5 mm as indicated at (m) in Fig. 2, while the leg portion 21 is determined to be 14 mm in length, and a front end surface 23 of the leg portion 21 determined to be 5.1 mm in diameter. Within the axial bore 22 of the insulator 2, a centre electrode 3 is concentrically placed which is made of nickel-based alloy, and determined to be 2.5 mm in diameter. A front end of the centre electrode 3 extends beyond that of the insulator 2 to work as firing tip 31. Numeral 4 designates each of three outer electrodes, each of which is dimensionally similar, and made of nickel-based alloy. The outer electrode 4 comprises an axial piece 43 and a radial piece 4b to generally form a L-shape configuration. The axial piece 43 is depended from the front end 11 of the metallic shell 1 to circumferentially surround the front end of the insulator 2 at regular intervals of 120 degrees. The axial piece 43 of the outer electrode 4 integrally connects the radial piece 4b which has an inner surface 42 arranged in parallel with the front end surface 23 of the insulator 2. An end tip 41 of the radial piece 4b extends beyond a cornered portion 25 of the front end surface 23 toward the centre of the insulator 2 so as to partially overlap therewith, and the end tip 41 is located to oppose an outer surface 31a of the firing tip 31 through a spark gap (Gp), a dimension of which is determined in detail hereinafter. As shown in Fig. 1 in which a dimensional relationship is shown somewhat exaggerated for clarity, a vertical distance (b) between the inner surface 42 of the radial piece 4b of the outer electrode 4 and the front end surface 23 of the insulator 2, is determined to be 0.7 mm, for example, which falls within a dimension ranging from 0.3 mm to 1.2 mm both inclusive. A radial distance (c) between the lateral surface of the front leg portion of the insulator and the inner surface 4a of the axial portion 43 of the outer electrode 4, is determined to be 1.5 mm. Further, a minimum distance (a) between the lateral surface 31a of the firing tip 31 and the end tip 41 of the radial portion 4b, is determined to be 0.8 mm, a width distance which is equivalent to that of the spark gap (Gp). In this instance, the axial distance (b) is determined to be 0.7 mm in order to fall within a dimension ranging from 0.3 mm to 1.2 mm both inclusive. The dimensional relationship among the distance (a), (b) and (c) is arranged to satisfy expressions (a/2) ≦ (b) ≦ (3a/2) and (c) > (a). Now, Figs. 4 and 5 show results of pre-delivery test carried out in connection with spark plug (A). Three spark plugs with axial distances (b) 1.2 mm, 0.7 mm and 0.3 mm respectively gave results as shown at numerals 51, 52 and 53 in Fig. 5. As a result is shown at numeral 50 in Fig. 5, a counterpart spark plug is prepared in which an axial distance (b) is measured to be 2 mm, while an extended length (m) of a front end of the insulator is to be 1.5 mm. These spark plugs were separately secured to an internal combustion engine and operated for ten cycles as shown in Fig. 4 as a single cycle under a cold zone simulation in winter season. The results obtained from the above test are as follows: It is found that the counterpart spark plug fails to restart the engine at six cycles. On the other hand, the spark plugs designated at numerals 51, 52 and 53 in Fig. 5 each discharged a spark through the spark gap (Gp), the front end surface 23 of the insulator 2 being free from the particulate carbon deposit. When carbon is deposited on the front end surface 23 of the insulator the insulating resistance between the electrodes decreased to the extent that a spark discharged between the front end surface 23 and the inner surface 42 of the outer electrode, so that the carbon deposit was burned and thus removed from the front end surface 23 a self-cleaning action. According to the invention, it is also found that the spark plugs according to the invention allow restarting of the engine at any stage in the operating cycle. The front end of the leg portion 21 of the insulator 2 extends beyond that of the metallic shell 1 by 2.5 mm, so that the front end of the leg portion 21 is better cooled by the intake fuel gas mixture, leading to heat-resistance properties equivalent to those of a single-gap type spark plug. According to an endurance test separately carried out although not shown herein in detail, it was found that the spark plug of the invention is 1.7 times as durable as a single-gap type spark plug in terms of spark erosion resistance of the centre electrode, and thus contributing to long service life. Referring to Fig. 6 which shows a spark plug (B) according to a second embodiment of the invention, the insulator 2 is somewhat reduced at its diametrical dimension for the purpose of realizing a compact spark plug as a whole. In this second embodiment, like reference numerals in Fig. 1 are identical to those in Fig. 6. In the spark plug (B), the end tip 41 of the radial portion 4b terminates somewhat short of the cornered portion 25 of the front end surface 23 of the leg portion 21. In this instance, as shown by the lines depicted in Fig. 6, a minimum distance (d) between the inner surface 42 of the radial portion 4b of the outer electrode 4 and the front end surface 23 of the insulator 2, is determined to be 0.7 mm, for example. On the other hand, the radial shortest distance (c) between the outer surface 24 of the front leg portion of the insulator 2 and the inner surface 4a of the axial portion 43 of the outer electrode 4, is determined to be 1.5 mm. Further, the gap distance (a) between the lateral surface 31a of the firing tip 31 and the end tip 41 of the radial portion 4b, is determined to be 0.8 mm, equivalent to the spark gap (Gp). In this situation, the axial distance (b) between the inner surface 42 of the radial portion 4b of the outer electrode 4 and the front end surface 23 of the insulator 2 is determined to be approximately 0.7 mm (more precisely 0.65 mm) so as to fall within a dimension ranging from 0.3 mm to 1.2 mm both inclusive. As mentioned above, the axial distance (b) is determined to be approximately 0.7 mm to fall within a dimension ranging from 0.3 mm to 1.2 mm both inclusive. In addition, the dimensional relationship among the distances (a), (d) and (c) is arranged to satisfy expressions of (a/2) ≦ (d) ≦ (3a/2) and (c) > (a). It is noted that instead of 0.7 mm the distances (b), (d) are substantially freely arranged so long as these distances are within a dimension ranging from 0.3 mm to 1.2 mm both inclusive. Further, it is appreciated that the invention is applicable not only to triple-gap type spark plugs but also to dual-gap type spark plugs. It is noted that by calculating an arithmetical mean from maximum and minimum distances, an average distance may be adopted instead of the radial distance between the lateral surface 24 of the front leg portion of the insulator 2 and an inner surface 4a of the axial portion 43 of the outer electrode 4. Furthermore, the material of the centre electrode and the outer electrode is not confined only to nickel-based alloy. Carbon nitride and silicon nitride may be added to the alumina when the insulator 2 is made. It is further appreciated that the outer electrodes may be integrally depended from the front end of the metallic shell.

Claims: A multi-gap type spark plug for an internal combustion engine comprising; a cylindrical metallic shell (1) enclosing a tubular ceramic insulator (2); the insulator (2) having a tapered front leg portion (21), the front end (23) of which extends beyond that of the metallic shell (1); a centre electrode (3) enclosed in the insulator (2), the front end of the centre electrode (3) extending beyond the front end surface of the insulator as a firing tip (31); and, a plurality of L-shaped outer electrodes (4) each having an axial portion (43) and a radial portion (4b), the axial portion (43) extending from the front end (11) of the metallic shell (1), the radial portion (4b) having an inner surface (42) arranged substantially parallel with the front end surface (23) of the insulator (2), and having an end tip (41) adapted to oppose a lateral surface (31a) of the firing tip (31) across a spark gap to be established therebetween, characterised by: the axial distance (b) between the front end surface (23) of the insulator (2) and the inner surface (42) of the radial portion (4b) of each of the outer electrodes (4) being in the range 0.3 mm to 1.2 mm inclusively; and, in that the end tip (41) of the radial portion (4b) of each outer electrode (4) extends beyond a cornered portion (25) of the front end surface (23) of the insulator (2) so as to overlap it partially, a relationship among dimensions (a), (b) and (c) being: (a/2) < b < (3a/2), and c > a, where a is the spark gap between the lateral surface (31a) of the firing tip (31) and the end tip (41) of the radial portion (4b) of each outer electrode (4), b is the axial distance between the front end surface (23) of the insulator (2) and the inner surface (42) of the radial portion (4b) of each outer electrode (4), and c is the distance between the lateral surface (24) of the front leg portion (21) of the insulator (2) and the inner surface (4a) of the axial portion (43) of each outer electrode (4). A multi-gap type spark plug for an internal combustion engine comprising; a cylindrical metallic shell (1) enclosing a tubular ceramic insulator (2); the insulator (2) having a tapered front leg portion (21), the front end (23) of which extends beyond that of the metallic shell (1); a centre electrode (3) enclosed in the insulator (2), the front end of the centre electrode (3) extending beyond the front end surface (23) of the insulator (2) as a firing tip (31); and, a plurality of L-shaped outer electrodes (4) each having an axial portion (43) and a radial portion (4b), the axial portion (43) extending from the front end (11) of the metallic shell (1), the radial portion (4b) having an inner surface (42) arranged substantially parallel with the front end surface (23) of the insulator (2), and having an end tip (41) adapted to oppose a lateral surface (31a) of the firing tip (31) across a spark gap (Gp) to be established therebetween, characterised by: the axial distances (b) between the front end surface (23) of the insulator (2) and the inner surface (42) of the radial portion (4b) of each of the outer electrodes (4) being in the range 0.3 mm to 1.2 mm inclusively; and in that the end tip (41) of the radial portion of each outer electrode (4) terminates short of a cornered portion (25) of the front end surface (23) of the insulator (2) a relationship among dimensions (a), (d) and (c) being: (a/2) < d < (3a/2) and c > a, where a is the spark gap (Gp) defined between the lateral surface (31a) of the firing tip (31) and the end tip (41) of the radial portion (4b) of each outer electrode (4), d is the minimum distance between the front end surface of the insulator (2) and the inner surface (42) of the radial portion (4b) of each outer electrode (4), and c is the radial distance between the lateral surface (24) of the front leg portion (21) of the insulator (2) and an inner surface (4a) of the axial portion (43) of each outer electrode (4). A multi-gap type spark plug for an internal combustion engine according to any one of the preceding claims, wherein the front end (23) of the insulator (2) extends beyond that of the metallic shell (1) by 2.5 mm, while the length of the leg portion of the insulator (2) is determined to be 14 mm. An internal combustion engine comprising a spark plug according to any one of the preceding claims. A vehicle comprising an internal combustion engine according to claim 4.

Inventors: KAGAWA JUNICHI, MURASE MASAAKI, NAKAMURA SHINICHI, KAGAWA, JUNICHI, MURASE, MASAAKI, NAKAMURA, SHINICHI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1991
Main classification: H01T13
IPC codes: H01T13