Spark plug

Abstract: A spark plug comprising a shell, an insulator, a center electrode passing through the insulator with its tip protruding from the latter, and a side electrode having a flat plate portion that faces the tip of said center electrode so as to form a spark gap between said tip and said flat piate portion, the periphery of said flat plate portion being joined to the shell via a plurality of legs, is improved by a plurality of holes which are made in said flat plate portion.

Description: The present invention relates to a spark plug for use in an internal combustion engine. More particularly, the present invention relates to the grounded side electrode of this plug. A spark plug is an important component of the ignition system and is the one that is installed on a combustion chamber for firing an air-fuel mixture to be combusted. The spark plug consists of a shell threaded to fit a hole in the cylinder head, a center and a side electrode which define a spark gap, and an insulator which serves as a bushing for efficiently conducting high voltage from its terminal to the spark gap exposed into the combustion chamber. Usually, the shaped side electrode is made in such a way that side electrode is welded to the shell, and then that it is bonded to be spark gap to face the tip of the center electrode. In order to deal with the possible wear of the electrodes, a multi-pole spark plug fitted with a plurality of L-shaped side electrodes is avaiable. Also known from DE-A-3 619 938 is a spark plug fitted with a flat plate that has a hole in the area facing the tip of the center electrode and which is connected to the shell via a plurality of legs. Fig 3(B) is a bottom view of this prior art plug and Fig 3(A) is a longitudinal section taken along the line A-A of Fig 3(B) for showing the essential part of the tip of the plug. In Fig. 3, the side electrode is generally indicated at 1. Being made of a conductive metal such as a Ni base alloy, the side electrode 1 consists of a flat plate portion 3 with a center hole 3 facing the center electrode 5 and a plurality of legs 4 which are connected to the shell 6 to form a unitary assembly. The spark plug of the type shown in Fig. 3 has been developed in an attempt to improve the durability and anti-vibrational properties of the side electrode, and to attain good firing condition. Good firing condition is attained if the difference between the diameter of the center electrode and that of the hole in the side electrode is no smaller than 0.7 mm. However, a durability test conducted on this spark plug has revealed that a marked increase in discharge voltage occurs as a result of cyclic use. The durability test has also indicated the possibility of cracking of the area between the periphery of the flat plate and the center hole in the side electrode. An object, therefore, of the present invention is to provide a spark plug fitted with a side electrode that has improved durability to ensure good firing condition at all times of use. This object of the present invention can be attained by a spark plug fitted with a side electrode that consists of a flat plate portion with a plurality of holes that faces the tip of a center electrode and the periphery of which is connected to the shell via a plurality of legs to form a unitary assembly. In a preferred embodiment of the present invention, the plurality of holes in the flat plate portion are spaced equidistantly around the center of said flat plate portion and the diameter of each of these holes and the diameter of the circle inscribed among these holes are made smaller than the outside diameter of the tip of the center electrode, with the total area of these holes being at least made larger than the area of said inscribed circle. The side electrode of the spark plug of the present invention described above features a balanced structure in that the flat plate portion of that electrode is securely supported by the plurality of legs and this contributes to improved durability and anti-vibrational properties. In addition the holes in the flat plate portion have dimensions that are adaptive to the outside diameter of the tip of the center electrode and are arranged in such an optimum way that it has been confirmed by experiments that good firing conditions are attained at all times to keep the discharge voltage at satisfactorily low levels. This enables the spark plug of the present invention to be used for a prolonged period with reduced chance of misfiring. Fig. 1(A) is a longitudinal section showing the essential part of the tip of a spark plug according to a first embodiment of the present invention, and Fig. 1(3) is a bottom view of the plug; Fig. 2(A) is a bottom view of a spark plug according to a second embodiment of the present invention; Fig. 2(B) is a bottom view of a spark plug according to a third embodiment of the present invention; Fig. 3(A) is a longitudinal section showing the essential part of the tip of a prior art spark plug, and Fig. 3(B) is a bottom view of the plug; Fig. 4 is a graph showing the discharge voltage vs durability test time of the spark plugs according to the first and third embodiments of the present invention as compared with the prior art plug; and Fig. 5 is a graph showing the number of HC spikes occurring in the spark plug of the first embodiment of the present invention as a function of the total area of the holes made in the side electrode. Fig. 1 shows a spark plug according to a first embodiment of the present invention; (B) is a bottom view and (A) is a longitudinal section of (B) taken along the line A-A for showing the essential part of the tip of the plug. In Fig. 1, the side electrode is generally indicated by 1A and consists of a flat plate portion 3A having three hles 2A spaced equidistantly around its center and three legs 4A extending outwardly from the flat plate portion 3A. The inner surface 4a of each leg 4A is connected to the peripheral side 6a of theshell 6. A spark gap 7 is defined by the tip surface of the center electrode 5 and the inner surface of the side electrode 1A. Fig. 2(A) is a bottom view or a spark plug according to a second embodiment of the present invention, in which four holes 2B are formed in the flat plate portion 3B of a side electrode 1B, with the flat plate portion 3B connected to the shell 6 via four legs 4B. The other components of the spark plug according to the second embodiment are the same as those shown in the first embodiment, so like component are identified by like numerals and a longitudinal section of the plug is omitted. The holes 2A or 2B are formed in such a way that the following conditions are satisfied: d₂<d₁; d₃<d₁; n(d₃)²≧(d₁)²≧(d₂)², where d₁ is the outside diameter of the tip of the center electrode 5, d₂ is the diameter of the circle inscribed along the holes, d₃ is the diameter of each hole, and n is the number of the holes. A spark gap is defined by the side electrode 1A or 1B and the center electrode 5. Fig. 2(B) is a bottom view of a spark plug according to a third embodiment of the present invention. The side electrode 1c of this plug has a flat plate portion in which four holes 2a, 2b, 2 and 2d or different diameter are made, with the first three holes being spaced around the center hole 2d. The holes 2a, 2b and 2c are desirably within the circle that is concentric to the center electrode (= d₁) and whose diameter d₄ is no more than 2.5 d₁. If d₄ is unduly large, the distance from each of the holes 2a, 2b and 2c to the periphery of the flat plate portion is so much decreased as to cause potential overheating. Furthermore, the ability of the plug to retard the increase in discharge voltage is impaired. The holes in the flat plate portion desirably satisfy the following condition: wherein n₁ signifies the number of holes, and d₁ is the diameter of each hole. In order to assure high strength and ease of machining, two adjacent holes are desirably spaced apart by a distance of at least 1 mm The spark plug according to the first embodiment of the present invention and the prior art plug depicted in Fig. 3 were subjected to a durability test with an engine operating on a bench at 5,500 rpm x 4/4 (at full throttle) for a period of 200 hours. Discharge voltage measurements were conducted over the range of engine operation from an idling to a racing condition (acceleration up to 5,000 rpm with no load applied). The results are shown in Fig. 4, in which curve (1) plots the results obtained with the spark plug of the first embodiment under such conditions that n = 3, d₁ = 2.5 mm, d₃ = 1.6 mm and the distance between holes being 1 mm (in this case, d₂ = 1.4 mm), and curve (2) plotting the results obtained with the prior art plug under such conditions that n = 1, d₁ = 2.5 mm, and d₃ = 3.0 mm. Obviously, the discharge voltage was lower with the spark plug of the first embodiment than with the prior art version, and this indicates the improved durability of the former plug. The same test was conducted on the spark plug according to the second embodiment of the present invention and the results were similar to those obtained with the first embodiment, so they are not shown in Fig. 4. The test results with the spark plug according to the third embodiment of the present invention are indicated by curve (3) in Fig. 4. In order to investigate the relationship between the total area of holes in the flat plate portion of the side electrode and the firing condition, the spark plug according to the first embodiment wherein d₁ = 2.5 mm and n = 3 was subjected to an idling test with the engine operating at an air/fuel ratio of 14 with the ratio of n(d₃)² to (d₂)² being adjusted to various values. The number of HC spikes (the number of hydrocarbon peaks) occurring at each ignition was counted and plotted in Fig. 5. Good firing condition requires a minimum number of HC spikes and a standard spark plug such as BCP 6ES performs with 8 HC spikes. If this value is taken as a criterion, Fig. 5 shows that good results are attained by satisfying the condition n(d₃)²/(d₂)²>1 or n(d₃)²≧(d₂)². Being equipped with the improved side electrode, the spark plug of the present invention can be used for a prolonged period with the discharge voltage kept at a comparatively low level. This plug also- ensures good firing condition at all times of use and hence featues high durability. The provision of more than one hole in the flat plate portion of the side electrode contributes to uniform distribution of thermal stresses, thereby preventing the occurrence of cracking in the flat plate. In the prior art spark plug which has only one hole formed in the center of the flat plate portion of the side electrode, it has been necessary to achieve precise alignment between the single hole and the axis of the center electrode for the purpose of stabilizing the discharge voltage and preventing abnormal wear such as uneven wear that might occur in the presence of misalignment. Needless to say, it is desirable that the center of the group of holes in the side electrode of the spark plug of the present invention are in registry with the axis of the center electrode but the tolerance that is required is not as close as what must be observed by the prior art plug. Therefore, the spark plug of the present invention has the additional advantage of being manufactured with relative ease.

Claims: A spark plug comprising a shell (6), an insulator , a center electrode (5) passing through the insulator with its tip protruding from the latter, and a side electrode (1A) having a flat plate portion (3A) that faces the tip of said center electrode so as to form a spark gap between said tip and said flat plate portion, the periphery of said plate portion being joined to the shell via a plurality of legs (4A) characterised in that a plurality of holes (2A,2B,2a,2b,2c) are made in said flat plate portion (3A). A spark plug according to claim 1 wherein the plurality of holes (2A,2B) have the same diameter and are spaced equidistantly around the center of said flat plate portion (3A), with the diameter (d₃) of each of these holes and that (d₂) of the circle inscribed among these holes being smaller than the outside diameter (d₁) of the tip of said center electrode (5), and with the total area of these holes being at least made larger than the area of said inscribed circle. A spark plug according to claim 2 wherein the number of said holes is three. A spark plug according to claim 1 wherein said plurality of holes (2a,2b,2c) have different diameters and are within the circle that is concentric to the center electrode (5) and whose diameter (d₄) is no more than 2.5 times the diameter (d₁) of the center electrode (5).

Inventors: GUGGENMOS JOHANNES, HOLZMANN JOSEF, KAGAWA JUNICHI, KRAPPEL ALFRED, MATSUTANI WATARU, GUGGENMOS, JOHANNES, HOLZMANN, JOSEF, KAGAWA, JUNICHI, KRAPPEL, ALFRED, MATSUTANI, WATARU
Applicants: BAYERISCHE MOTOREN WERKE AG, NGK SPARK PLUG CO, BAYERISCHE MOTOREN WERKE AKTIENGESELLSCHAFT, NGK SPARK PLUG CO. LTD.
Application year: 1988
Main classification: H01T13
IPC codes: H01T13