A spark plug for use in internal combustion engine

Abstract: In a spark plug which includes a metallic shell within which a tubular insulator is placed, and having a center electrode provided within the insulator to form a spark gap between a front end of the center electrode and an outer electrode extended from the metallic shell; first and second stepped portions are provided with a front portion of the center electrode to form a clearance between an outer wall of the center electrode and an inner wall of the insulator. the stepped portions are arranged to progressively decrease their diameter as they position toward the front end of the center electrode. A tapered portion is provided at an inner wall of the insulator by bevelling a front end of the insulator.

Description: This invention relates to a spark plug for use in internal combustion engines and particularly concerns a spark plug which may remove wet or dry carbon deposits by self-cleaning action with a relatively simple structure. In a spark plug for use in internal combustion engines, there is provided a tubular insulator within a metallic shell. Within the insulator, a center electrode is provided so that the center electrode is thermally sealed by a sealant, and supported by a shoulder portion which is provided at an inner wall of the insulator. Then the center electrode's outer surface is tight engaged with an inner surface of the insulator so as to be firmly supported by the insulator. This is to protect the center electrode against impacts caused by combustion of the air-fuel mixture in the combustion chamber since the front portion of the centre electrode is exposed to the combustion chamber. During the operation of the spark plug, a very slight clearance unavoidably occurs between the outer surface of the center electrode and the inner surface of the insulator. This clearance may entrain the air-fuel mixture by capillary action and retain it as droplets of liquid fuel. These droplets, thus retained, remain without being replaced by the swirl which accompanies combustion. The liquid fuel absorbs particulate carbon thus reducing the insulating resistance between the insulator and the center electrode thus causing misfire. On the other hand, a deposit of dry carbon placed between the outer surface of the center electrode and the inner surface of the insulator also reduces the insulation resistance between the insulator and the center electrode and also occasions misfire. EP-A-0287080 shows a spark plug with a stepped central electrode and a parallel-sided or inwardly extending insulator about the stepped end of the electrode. The precharacterising portion of claim 1 is based on this disclosure. According to the present invention, there is provided a spark plug comprising a metallic shell, a tubular insulator provided within the metallic shell, a center electrode provided within the insulator and defining a spark gap between the front end of the center electrode, and an outer electrode extending from the metallic shell, first and second stepped portions provided at the front portion of the center electrode so as to form a plurality of clearances between the outer wall of the center electrode and the inner wall of the insulator, successive stepped portions being of reduced diameter towards the front end of the center electrode, the second stepped portion forming the front end, characterised in that a tapered portion of increasing diameter towards the front end of the spark plug is provided on the inner wall at the front end of the insulator, and in that the dimensional relationships between L1, L2, L3, L4, L5 and L6 are as follows: L1 ≧ 0.4 mm, 1.0 mm ≧ L2 ≧ 0.3 mm, L3 ≧ 0.2 mm, L4 ≧ 0.3 mm, L5 ≧ 0.1 mm, L6 ≧ 0.1 mm, where L1 = the radial distance between the outermost periphery of the tapered portion and the lateral outer surface of the second stepped portion; L2 = the axial length of the first stepped portion; L3 = the radial distance between the outermost and innermost peripheries of the tapered portion; L4 = the axial distance between the front outer surface of the first stepped portion and the innermost periphery of the tapered portion; L5 = the difference in radius between the first and second stepped portions; and L6 = the difference in radius between the first stepped portion and the main body of the center electrode. The plurality of stepped portions and the tapered portion work to take in flares of combustion swirl thus removing droplets of liquid fuel which may be retained between the center electrode and the insulator, and thus preventing particulate carbon from being caught between the center electrode and the insulator, thus resisting decreases in insulation resistance. The stepped portions make use of sparks between the center electrode and the outer electrode to facilitate a self-cleaning action, and removing dry carbon deposits between the center electrode and the insulator. With the invention, a spark plug may be provided which can reduce the particulate carbon and dry carbon deposits and prevent the insulation resistance from unfavorably decreasing, thus contributing to an extended period of service life with a relatively simple structure. The invention will be more clearly understood from the following description when taken together with the accompanying drawings, which are given by way of example only, and in which: Fig. 1 is a plan view of an embodiment of the invention, with the left of the spark plug shown in section. Fig. 2 is an enlarged sectional view of the spark plug of Figure 1. Fig. 3 is a view similar to Fig. 2 of a second embodiment of the invention; and Fig. 4 is a sectional view of a spark plug showing the dimensions L1, L2, L3, L4, L5 and L6. Referring to Fig. 1 which shows a spark plug 1 for use in an internal combustion engine. The spark plug 1 has a cylindrical metallic shell 8 which has a male thread 10 to mount the spark plug 1 on a cylinder head (not shown) of the internal combustion engine. Within the metallic shell 8, a tubular insulator 2 is concentrically placed with its inner space as an axial bore 7. Within the insulator 2, a terminal electrode 6 and a center electrode 3 are respectively placed in concentric and thermally sealing relationship with the insulator 2. The terminal electrode 6 is rigidly connected in series with the center electrode 3 through an electrically conductive sealant 4a, a resistor 5 and an electrically conductive sealant 4b. A front portion of the center electrode 3 projects from the front end of the insulator 2 to form a firing tip 3a at a front end of the center electrode 3. The firing tip 3a somewhat extends beyond a front end of the insulator 2 to form a spark gap (Gp) between the firing tip 3a and an outer electrode 9 extended downward from the metallic shell 8. With the front portion of the center electrode 3, a first stepped portion 13a and a second stepped portion 13b are provided as a plurality of stepped portions to form a clearance 14 between an outer wall of the center electrode 3 and an inner wall of the insulator 2 as shown in Fig. 2. In this instance, those stepped portions 13a, 13b are arranged to be of progressively decreased diameter towards the front end of the center electrode 3. At the front open end 11 of the insulator 2, a tapered portion 12 is provided on the inner wall of the insulator 2 by bevelling the front open end 11 of the insulator 2. It is noted that the tapered portion 12 may be provided as a diameter-increased annular recess 15 as shown in Fig. 3. As shown in Fig. 4, the dimensional relationship among L1, L2, L3, L4, L5 and L6 are determined to be as follows: L1 ≧ 0.4 mm, 1.0 mm ≧ L2 ≧ 0.3 mm, L3 ≧ 0.2 mm, L4 ≧ 0.3 mm, L5 ≧ 0.1 mm, L6 ≧ 0.1 mm, where L1 = the radial distance between the outermost periphery of the tapered portion 12 and the lateral outer surface of the second stepped portion 13b; L2 = the length of the first stepped portion 13a; L3 = the radial distance between the outermost and innermost peripheries of the tapered portion 12; L4 = the axial distance between the front outer surface of the first stepped portion 13a and the innermost periphery of the tapered portion 12; L5 = the difference in radius between the first stepped portion 13a and the second stepped portion 13b; and L6 = the difference in radius between the first stepped portion 13a and the main body of the center electrode 3. With the structure thus described, the relationship L1 ≧ 0.4 mm allows the tapered portion 12 to introduce flares of combustion swirl into the clearance 14 to remove droplets of liquid fuel retained between the insulator 2 and the center electrode 3, thus preventing the insulation resistance therebetween from unfavourably decreasing. Otherwise, the droplets of liquid fuel absorb particulate carbon and reduce the insulation resistance. Concerning the first and second stepped portions 13a, 13b, the particularly determined relationship L4 ≧ 0.3 mm makes it possible to establish flares of spark along the tapered portion 12 between the front end of the metallic shell 8 and the second stepped portion 13b when a pile of carbon deposit is placed on the front end of the insulator 2. This allows the removal by burning of dry carbon deposits between the insulator 2 and the center electrode 3 so as to facilitate self-cleaning action and positively maintain the initial insulation resistance of the insulator 2. With the length of the first stepped portion 13a determined to fall within the dimensional range 1.0 mm ≧ L2 ≧0.3 mm, the dimensional determination allows the heat accumulated at the front end of the center electrode 3 to be effectively dissipated directly or through the insulator 2, and thus improves the heat-dissipating effect compared to the prior art in which only the second stepped portion is provided with the center electrode. In order to prove how the subject invention is improved compared with the prior art device in which only the second stepped portion is provided with the center electrode, and no tapered portion is provided with the open front end of the insulator, fouling experiments were carried out with L1 = 0.9 mm, L2 = 0.5 mm, L3 = 0.5 mm, L4 = 0.1 mm, L5 = 0.3 mm and L6 = 0.1 mm determined respectively. In order to carry out fouling experiment due to the particulate carbon, the spark plug 1 was mounted on 78.5 cc, two-stroke engine. With the ambient temperature 0°C and choke 3/4 closed, the operation of the engine was alternately repeated by idling at 1800 rpm for 10 sec. and racing at 1800 ≈ 4500 rpm for 10 sec. with these idling and racing as a single cycle. The relationship between the number of cycles and the insulation resistance is shown in Table 1 which indicates how the insulation resistance was maintained compared to the prior art device. In order to carry out fouling experiment due to the dry carbon deposit, the spark plug 1 was mounted on 256 cc, four-stroke engine. With the normal temperature and choke 3/4 closed, the operation of the engine was alternately repeated by idling at 1750 rpm for 3 min. and cessation for 1 min. with these idling and cessation as a single cycle. The relationship between the number of cycles and the insulation resistance is shown in Table 2 which indicates how the insulation resistance was maintained compared to the prior art device. As shown in Table 1 which depicts a relationship between the insulation resistance (MΩ) and the number of cycles by dotting circles (o) and crosses (x) according to the spark plug 1 of the subject invention and the counterpart device in turn, the result shows that the tapered portion 12 allows removal of the particulate carbon to protect the insulation resistance against deterioration, as opposed to the prior art device in which the insulation resistance gradually deteriorates to cause starting failure at 10 cycles. Table 2, which depicts the same relationship as Table 1, shows that although the insulation resistance gradually deteriorates until completing 6 cycles, thereafter the dimensional arrangement L4 ≧0.3 mm makes it possible to establish flares of spark along the tapered surface 12 between the front end of the metallic shell 8 and the first and second stepped portions 13a, 13b, and thus removing the dry carbon deposit by burning and recovering the insulation resistance so as to avoid misfire. This shows how the antifouling effect is improved compared to the prior art device in which the insulation resistance gradually deteriorates to cause starting failure at 8 cycles. As understood from the foregoing description, the tapered portion is provided at the front end of the insulator, and at the same time, the stepped portions are provided at the front portion of the center electrode. The tapered portion causes droplets of liquid fuel retained between the center electrode and the insulator to be removed, and thus prevents particulate carbon from being caught between the center electrode and the insulator so as to resist against decrease in insulation resistance. The stepped portions allow removal of the dry carbon deposit by burning between the center electrode and the insulator and prevent spark plug misfire, thus avoiding starting failure and contributing to savings in fuel consumption. It is noted that the first and second stepped portions may be respectively bevelled. Moreover, it is appreciated that the stepped portions may be made separately so that the stepped portions may be fixedly soldered or brazed to the front end of the center electrode.

Claims: A spark plug (1) comprising a metallic shell (8), a tubular insulator (2) provided within the metallic shell (8), a center electrode (3) provided within the insulator (2) and defining a spark gap (Gp) between the front end (3a) of the center electrode (3) and an outer electrode (9) extending from the metallic shell (8), first and second stepped portions (13a, 13b) provided at the front portion of the center electrode (3) so as to form a plurality of clearances (14) between the outer wall of the center electrode and the inner wall of the insulator, successive stepped portions being of reduced diameter towards the front end of the center electrode (3), the second stepped portion (13b) forming the front end, characterised in that a tapered portion (12) of increasing diameter towards the front end of the spark plug is provided on the inner wall at the front end (11) of the insulator, and in that the dimensional relationships between L1, L2, L3, L4, L5 and L6 are as follows: L1 ≧ 0.4 mm, 1.0 mm ≧ L2 ≧ 0.3 mm, L3 ≧ 0.2 mm, L4 ≧ 0.3 mm, L5 ≧ 0.1 mm, L6 ≧ 0.1 mm, where L1 = the radial distance between the outermost periphery of the tapered portion (12) and the lateral outer surface of the second stepped portion (13b); L2 = the axial length of the first stepped portion (13a); L3 = the radial distance between the outermost and innermost peripheries of the tapered portion (12); L4 = the axial distance between the front outer surface of the first stepped portion (13a) and the innermost periphery of the tapered portion (12); L5 = the difference in radius between the first and second stepped portions (13a, 13b); and L6 = the difference in radius between the first stepped portion (13a) and the main body of the center electrode (3). A spark plug according to claim 1, wherein the tapered portion (12) of the insulator (2) is produced by bevelling. A spark plug according to claim 1, wherein the tapered portion (12) of the insulator (2) is provided by at least one internal annular step. A spark plug according to claim 3, wherein the tapered portion (12) of the insulator is in the form of two or more internal annular steps, of successively increasing diameter towards the front of the spark plug. An internal combustion engine including a spark plug according to any preceding claim.

Inventors: ENOMOTO TUNEKAZU, MORIYA TORU, YOSHIDA MITSUTAKA, ENOMOTO, TUNEKAZU, MORIYA, TORU, YOSHIDA, MITSUTAKA
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1991
Main classification: H01T13
IPC codes: H01T13