Spark plug

Abstract: In a spark plug, a center electrode is made of a heat-conductor core cladded by a nickel-alloyed metal. A recess is provided on a front end surface of the nickel-alloyed metal. A columnar tip is made of a precious metal, and fitted in the recess in such a manner that a front end of the tip is protracted from the recess. An outer surface of the tip being welded to an inner surface of the recess. A relationship among A, B, C, D, E, F and G is as follows: 0.3 mm ≦ A ≦ 0.8 mm, 1.2A ≦ B ≦ 3A, 0.1 mm ≦ (C-A)/2 ≦ 0.5 mm, D ≦ (C-A)/2, E ≧ B/4, 0 mm ≦ F ≦ 0.5 mm and A/5 ≦ G ≦ A/2, where A: a diameter of the columnar tip, B: a length of the columnar tip, C: a diameter of a front end of the nickel-alloyed metal, D: a length of a front end of the nickel-alloyed metal, E: a length of the front end of the tip which is protacted from the recess, F: a distance between a rear end of the tip and a front end of the heat-conductor core, G: a distance of a welding penetrated from the outer surface of the tip to the inner surface of the recess.

Description: This invention relates to a spark plug in which an erosion-resistant tip is secured to the front end of a center electrode by means of welding. In a center electrode for a spark plug for use in an internal combustion engine, in order to provide the center electrode with heat-and oxidation-resistant property, the center electrode has a nickel-based metal in which a copper core is embedded as a heat-conductor. Further, a tip which is made of precious metal such as platinum-based alloy is welded to the front end of the center electrode so as to improve spark-erosion resistance. In related prior art, US-A-3,146,370 suggests a center electrode for a spark plug in which a tip is welded to a firing portion of the center electrode in which the tip has a cobalt (Co) core cladded by an iridium (Ir) sheath. JP-A-3/176978 suggests an optimal dimensional relationship between a tip and a recess in which an iridium-based tip is fitted in a recess provided at a front end surface of a center electrode, and the tip is secured to an outer wall of the recess by means of laser or electron beam welding. With high speed and high power requirements of internal combustion engines the front end of the center electrode tends to be exposed to high ambient temperatures. In order to protect the tip against thermal deterioration, it is necessary to prevent the temperature of the tip from rising abnormally. The iridium-made tip, a melting point of which is as high as 2500 °C, has remarkable spark-erosion resistant property. The tip, however, deteriorates due to evaporation when oxidized by being exposed to an ambient temperature of more than 900 °C. In addition, the distance between a rear end of the tip and a front end of the copper core is 1.0 mm or more, and therebetween lies a part of the nickel-alloyed sheath which is relatively poor in thermal conductivity. This can block the flow of a sufficient amount of heat from the tip to the rear of the center electrode by way of the copper core thus deteriorating heat conduction when the tip is exposed to the combustion chamber of the internal combustion engine. For this reason, the temperature of the tip is likely to rise excessively particularly when the engine runs at high speed with high load. When the tip is bonded directly to the front end of the copper core by means of electrical resistance welding, the front end of the copper core is likely to protrude from the nickel-alloyed sheath due to their thermal expansion difference, and be oxidized in the higher temperature atmosphere. Therefore, it is an object of the invention to provide a center electrode for a spark plug which is better capable of preventing a tip from rising abnormally and keeping the tip firmly in place without the tip falling off the recess through thermal damage to the welding portion, and contributing to an extended service life with relatively low cost. In a center electrode for a spark plug, a tip is fitted in a recess provided on a front end surface of the nickel-alloy metal, and the tip is made in such a manner that a front end of the tip protrudes from the recess, and an outer surface of the tip is bonded to an inner surface of the recess by means of laser or electron beam welding. According to the invention, there is provided a spark plug as defined in claim 1. The invention provides for a center electrode in which the relationship between a diameter (A) of the tip and (G) is A/5 ≦ G ≦ A/2 so that the strength of the welding portion is significantly enhanced, where (G) is a distance of a welding portion penetrated from the outer surface of the tip to the inner surface of the recess which is provided on a front end surface of the nickel-alloyed metal when an outer surface of the tip is bonded to an inner surface of the recess by means of laser or electron beam welding. This effectively prevents the tip from falling off the nickel-alloyed metal when the tip is subjected to a thermal stress in a direction in which the heat-conductor core is pushed by the nickel-alloyed metal due to the thermal expansional difference between the heat-conductor core and the nickel-alloyed metal. Further, the rear end of the tip is terminated short of the thermal conductor core within a range from 0.0 mm to 0.5 mm. By way of the heat-conductor, a considerable amount of heat to which the tip is subjected is promptly transmitted to a rear end of the center electrode. The heat transmitted from the center electrode is transferred to a cylinder head through an insulator and a metallic shell, thus keeping the temperature of the tip from abnormally rising so as to secure good heat-dissipating effect. With the employment of an inexpensive iridium-based tip which has a relatively high melting point and good spark-erosion resistance, the good heat-dissipating effect compensates drawback of the iridium-based tip in which the tip is likely to evaporate by oxidation at 900 ∼ 1000 °C. Furthermore, upon preparing the metallic oxide such as oxide of aluminum (Al), magnesium (Mg) or thorium (Th) each of which has a melting point of 2000 °C or more, the tip is made by dispersing the metallic oxide into iridium (Ir), thus making it possible to effectively prevent the evaporation of the iridium-based tip due to oxidation. In this instance, an oxide or oxides of rare earth metal (Y, La, Ce) in less than 15.0% by volume may be dispersed together with iridium (Ir) to form a sintered complex body. The ground electrode is provided to form a spark gap, and the ground electrode has a tip made of a platinum metal, iridium metal, nickel-platinum alloy or nickel-iridium alloy. Moreover, an addition of nickel to the outer electrode makes it possible to diminish the thermal expansional difference between the tip and the outer electrode, thus preventing the tip from falling off the outer electrode due to their thermal expansional difference, and contributing to an extended period of service life. The invention will be further understood from the following description, when taken together with the attached drawings, which are given by way of example only and in which: Fig. 1 is a cross sectional view of a spark plug, but its upper part is broken away; Fig. 2 is an enlarged cross sectional view of a front end of a center electrode according to one embodiment of the invention; Fig. 3 is similar to Fig. 2 to show an oxidation part of a heat-conductor core; Fig. 4 is similar to Fig. 2 according to another embodiment of the invention; Fig. 5 is a graph showing a relationship how a spark gap decrement (mm) changed depending on a distance (F mm) between the tip and a heat-conductive core; Fig. 6a is a cross sectional view of a front end of a center electrode to show an appearance of cracks; Fig. 6b is a graph showing a relationship between an occurence of cracks and permeation distance (G) of a welding portion; Fig. 7 is similar to Fig. 2 to show a drawback when (G) exceeds (A/2); Fig. 8 is a representation of a microscopic photograph showing the front end of the center electrode; Fig. 9a shows a cross sectional view of a front end of a center electrode to show an appearance of cracks; Fig. 9b is a graph showing a relationship between an occurence of cracks (%) and an addition Y₂O₃ (vol %); Figs. 10a and 10b are graphs each showing how the spark gap increment (mm) changes depending upon an addition of Y₂O₃ (vol %) and ; Figs. 11a and 11b show an enlarged cross sectional view of a front end of a center electrode to show modification forms of the tip. Referring to Fig. 1 which substantially shows a lower half portion of a spark plug, the spark plug has a cylindrical metallic shell 2, to a front end of which a L-shaped outer electrode 1 is fixedly attached by means of welding. Within the metallic shell 2, a tubular insulator 3 is placed, an inner space of which serves as an axial bore 31. The insulator 3 has a shoulder 32 which is, by way of a packing 22, received by a stepped portion 21 provided with an inner wall of the metallic shell 2 so as to support the insulator 3 within the metallic shell 2. A rear head 23 of the metallic shell 2 is inturned to engage against an outer surface of the insulator 3 by means of caulking to secure the insulator 3 against removal. Within the axial bore 31 of the insulator 3, is a center electrode 4 placed with its front end portion 4A somewhat diametrically reduced, and extends beyond that of the insulator 3. A rear end 4B of the center electrode 4 is brought into engagement with a stepped shoulder 4C which is provided with an inner wall of the axial bore 31. To a rear end of the center electrode 4, is a middle axis 35 connected by way of a monolithic resistor 34 interposed between glass sealants 33a, 33b. Meanwhile, the outer electrode 1 is made of nickel or nickel-based alloy to which a tip 6 is welded in correspondence with a tip 5 as described hereinafter so as to form a spark gap (Sp) with the tip 5. The tip 6 is made of platinum (Pt), iridium (Ir) or alloy of platinum (Pt) and nickel (Ni), in which a ratio of nickel (Ni) ranges from 10.0 wt% to 40.0 wt%. A shown in Fig. 2, the center electrode 4 is made of a nickel-alloyed metal 41 including 15.0 wt% chromium and 8.0 wt% iron. In the nickel-alloyed metal 41, is a copper or silver core embedded as a heat-conductor core 42 to form a composite structure 40. A recess 43 is provided on an front end surface 41a of the nickel-based metal 41 in such a manner as to reach a front end surface 42a of the heat-conductor core 42. In the recess 43, is a rear portion 51 of the tip 5 fitted in such a manner that a front end portion 53 of the tip 5 is somewhat protruding from the recess 43. In this instance, the rear end surface 52 of the tip 5 is in thermally transferable contact with a front end surface 42a of the heat-conductor core 42. An outer surface 51a of the tip 5 is thermally bonded to an inner surface 43a of the recess 43 by means of laser or electron beam welding as designated at 5A. The welding portion 5A prevents entry of the combustion gas to the heat-conductor core 42, and protects the core 42 against corrosion due to oxidation as shown at 5B in Fig. 3. It is observed that before the laser or electron beam welding is carried out, an electrical resistance welding may be provisionally done between the tip 5 and the inner surface 43a of the recess 43 so as to enhance the strength of the welding portion 5A, and at the same time, strengthening the thermally transferable contact between the tip 5 and the heat-conductor core 42, thus enabling good heat-dissipating effect. As shown in Fig. 4, a dimensional relationship among A, B, C, D, E and F is as follows: 0.3 mm ≦ A ≦ 0.8 mm, 1.2A ≦ B ≦ 3A, 0.1 mm ≦ (C-A)/2 ≦ 0.5 mm, D ≦ (C-A)/2, E ≧ B/4, 0 mm ≦ F ≦ 0.5 mm and A/5 ≦ G ≦ A/2. Where A: a diameter of the columnar tip 5, B: a length of the columnar tip 5, C: a diameter of the front end portion 4A of nickel-alloyed metal 41, D: a length of the front end portion 4A of nickel-alloyed metal 41, E: the protrusion of the front portion 53 of the tip 5 from the recess 43, F: a distance between the rear end surface 52 of the tip 5 and the front end surface 42a of the heat-conductor core 42, G: a distance of a welding portion 5A penetrated from the outer surface 51a of the tip 5 to the inner surface 43a of the recess 43 when the tip 5 is bonded to the inner surface 43a of the recess 43 by means of laser or electron beam welding. Regarding the distance (F), it is necessary that the rear end surface 52 of the tip 5 is in contact with and the front end surface 42a of the heat-conductor core 42 as shown in Fig. 2, otherwise the former is terminated at the latter within the range of 0.5 mm as shown in Fig. 4. Fig. 5 shows how an increment of the spark gap (Sp) changes depending on the distance (F) between the rear end surface 52 of the tip 5 and the front end surface 42a of the heat-conductor core 42. This is obtained after carrying out a spark-erosion resistance experiment at full load and 5500 rpm for 200 hours with the spark plug mounted on a six-cylinder, 2000 cm³. engine. It is found from Fig. 5 that an amount of spark-erosion is least when the distance (F) is less than 0.5 mm which indicates the least increment of the spark gap (Sp). The upper limit of the diameter (A) is 0.8 mm because the diameter (A) exceeding 0.8 mm prevents the compactness of the tip 5, and iridium (Ir) or iridium-based alloy has spark-erosion resistance more superior than platinum-based alloy including 20.0 wt% iridium. The lower limit of the diameter (A) is 0.3 mm because the diamete (A) less than 0.3 mm fails to ensure minimum necessary spark gap. The formula is 1.2A ≦ B ≦ 3A (preferably 1.5 mm ≦ B ≦ 2.0 mm) because it is necessary to obtain the length of the tip 5 protruding from the receess 43 with minimum cost of expensive iridium ensured. The relationship is 0.1 mm ≦ (C-A)/2 ≦ 0.5 mm (preferably 0.1 mm ≦ (C-A)/2 ≦ 0.3 mm) because when (C-A)/2 exceeds 0.5 mm, the enlarged diameter (C) diverts the incidence energy of the laser welding to the front end surface 41a of the nickel-alloyed metal 41, which decreases the formation of the welding portion (Ir - Ni alloyed layer) 5A so as to weaken the firmness between the outer surface 51a of the tip 5 and the inner surface 43a of the recess 43. When (C-A)/2 is less than 0.1 mm, the lessened diameter (C) allows the spark discharge to attack the welding portion (Ir - Ni alloyed layer) 5A so as to weaken the firmness between the outer surface 51a of the tip 5 and the inner surface 43a of the recess 43. The formula is D ≦ (C-A)/2 because a greater length (D) makes it impossible to sufficiently supply the incident energy of the laser welding to the rear end surface 52 of the tip 5 so as to lose the sufficient strength of the welding portion 5A. The protracted length (E) of the tip 5 is E ≧ B/4 mm because it is necessary to prevent the front portion 53 of the tip 5 from being embedded by the welding portion 5A, and to serve the tip 5 for an extended period of time. Fig. 6a shows a longitudinal cross sectional front portion of the center electrode 4 to show cracks 5C. Fig. 6b shows a relationship between an occurrence of cracks (%) and the penetration distance (G) of the welding portion 5A. This is obtained after carrying out a spark-erosion resistance experiment at 5500 rpm by repeatedly running at full load × 1 min. and idling × 1 min. alternately for 100 hours with the spark plug mounted on a six-cylinder, 2000 cm³. engine. It is found from Fig. 6b that the occurrence of cracks abruptly increases when the penetration distance (G) is less than A/5. This is because the welding portion 5A can't sufficiently work as stress relieving layer which absorbs the thermal expansional difference between the tip 5 and the front end portion 4A of the nickel-alloyed metal 41. As a result, the cracks 5C are likely to circumferentially occur due to the thermal expansional difference between the tip 5 and the front end portion 4A of the nickel-alloyed metal 41. On the other hand, the penetration distance (G) exceeding A/2 concentrates the energy of the laser welding into the tip 5 to melt too much of the tip 5 and the front end portion 4A of the nickel-alloyed metal 41 as shown in Fig. 7. The tip 5 is made by sintering a mixture of 95.0 vol% iridium powder and 5.0 vol% yttrium oxide (Y₂O₃) powder (oxide of rate earth metal). The sintered tip 5 forms a Cermet in which the yttrium oxide (darkened area) is dispersed into grain boundary of the iridium (blank area) as shown in the representation of a microscopic photograph (Fig. 8). In this distance, the addition of the yttrium oxide (Y₂O₃) ranges from 0.1 vol% to 15.0 vol%, preferably ranging from 1.0 vol% to 10.0 vol%. Instead of the yttrium oxide (Y₂O₃) it is noted that thorium oxide (ThO₂) or lanthanum oxide (La₂O₃) may be used as an oxide of rare earth metal, otherwise an oxide of Zr, Al or Mg may be used alone or in combination. Fig. 9a shows a longitudinal cross sectional front portion of the center electrode 4 to show cracks (Cr). Fig. 6b shows a relationship between an occurence of cracks (%) and an addition of yttria (Y₂O₃) (vol%) of the tip 5. This is obtained after carrying out a spark-erosion resistance experiment at full load and 5500 rpm for 200 hours with the spark plug mounted on a six-cylinder, 2000 cm³. engine. It is found from Fig. 9b that the occurence of cracks sufficiently decreases when the addition of yttria (Y₂O₃) (vol%) is 0.1 ∼ 15.0. Fig. 10a shows a relationship between an increment of the spark gap (Gp) an occurrence of cracks (%) and an addition of yttria (Y₂O₃) (vol%) of the tip 5. This is obtained after carrying out a spark-erosion resistance experiment at full load and 5500 rpm for 200 hours with the spark plug mounted on a six-cylinder, 2000 cm³. engine in which the tip 5 (5.0 mm in dia.) shown in Figs. 1 and 2 is employed. It is found from Fig. 10a that the evaporation of the tip is effectively prevented when the addition of yttria (Y₂O₃) (vol%) is 0.1 ∼ 15.0. Fig. 10b shows a relationship between an increment of the spark gap (Gp) an occurrence of cracks (%) and an addition of yttria (Y₂O₃) (vol%) of the tip 5. This is obtained after carrying out a spark-erosion resistance experiment with the spark plug activated at 50 mJ and 60 cycles/sec. for 200 hours in which the tip 5 (5.0 mm in dia.) shown in Figs. 1 and 2 is employed. It is also found from Fig. 10b that the least amount of the spark erosion of the tip 5 is achieved when the addition of yttria (Y₂O₃) (vol%) is 0.1 ∼ 15.0. If the tip is made of only iridium, the tip is likely to evaporate because the iridium is oxidized at 900 °C or more, although the iridium has a high melting point. In order to prevent the evaporation of the tip, it is necessary to prepare the oxide having a high melting or boiling point, and disperse the oxide into the iridium when sintering the tip 5. An increased addition of the oxide makes such a structure that the iridium is dispersed into the oxide, and thus concentrating the spark discharge into the iridium to corrode the iridium since the oxide is poor in electrical conductivity. The erosion of the iridium leaves fragile mesh-like structure of the oxide which is consequently attached by the spark discharge so as to furtherenhance the spark-erosion. The tip 5 is bonded to the inner surface 43a of the recess 43 all through their circumferences by means of laser or electron beam welding. The welding portion 5A is mechanically strengthened so as to make it substantially immune to the thermal stress caused from the thermal expansional difference between the tip 5, the heat-conductor core 42 and the front end portion 4A of the nickel-alloyed metal 41. Since a negative high voltage is usually applied to the center electrode 4, heavy anode ions impinge on the tip 5 of the center electrode 4 to attack the tip 5. On the other hand, light-weight electrons impinge on the outer electrode 1, and therefore the outer electrode 1 is eroded less than the center electrode 4. However, the outer electrode 1 is subjected to high temperature from the combustion gas, and the tip 6 is likely to fall off the outer electrode 1 due to the thermal stress caused from the thermal expansional difference between the tip 6 and the outer electrode 1 unless the thermal expansional difference substantially remains. In order to substantially eliminate the thermal expansional difference between the tip 6 and the outer electrode 1, nickel (Ni) is added to the tip 6. The addition of nickel less than 10.0 wt% remains the thermal expansional difference, while the addition of nickel exceeding 40.0 wt% is likely to erode the tip 6 by oxidation. It is noted that pure iridium (Ir) or pure ruthenium (Ru) may be used to the tip 5 instead of the Cermet. Fig. 11a shows a modification form of the heat-conductor core 42 in which a centermost core 44 is cladded by the heat-conductor core 42 which is made of copper. The centermost core 44 is made of pure nickel (Ni) or pure iron (Fe). The provision of the centermost core 44 makes it possible to keep the condition of the welding portion 5A good without sacrificing the heat-dissipating effect of the heat-conductor core 42. Fig. 11b shows another modification form of the tip 5, the front portion 53 of which is diametrically enlarged. In each of the modification forms, the diameter of the tip 5 is 0.8 mm or more so that it is unfavorable that the discharge between the electrodes 1, 4 occurs at lowered voltage, but it is effective in keeping the temperature of the tip 5 under 900 °C and preventing a greater amount of the spark erosion. While, the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by skilled artisan without departing from the scope of the invention as defined in the appended claims.

Claims: A spark plug including a metallic shell having a tubular insulator (3) in which a centre electrode (4) is provided, which by means of a shoulder portion ends in a narrower columnar front end portion (4A), said front end portion defining a spark gap with an outer electrode (6) extending from the metallic shell, wherein: the centre electrode (4) is made of a heat-conductive core (42) cladded by a nickel-alloyed metal (41), whereby the front end portion consists of said metal; a recess (43) is provided on the front end surface (41a) of the front end portion (4A); a substantially columnar tip (5) of a precious metal is provided, a rear end of the tip being fitted into the recess in such a manner that the front end (53) of the tip protrudes from the front end surface (41a); at least part (5A) of the surface (51a) of the columnar tip is bonded to the inner surface of the recess by means of laser or electron beam welding; and, the relationship between the diameter (A) of the columnar tip (5), the length (B) of the columnar tip (5), the diameter (C) of the front end portion (4A), the length (D) of the front end portion (4A), the protrusion (E) of the front end of the tip (5) from the recess, the distance (F) between the rear end surface (52) of the tip and the front end surface (42a) of the heat-conductor core, and the penetration depth (G) of the weld zone (5A) between the tip and the recess taken from said surface (51a) of the tip (5) is as follows: 0.3 mm ≦ A ≦ 0.8 mm, 1.2A ≦ B ≦ 3A, 0.1 mm ≦ (C-A)/2 ≦ 0.5 mm, D ≦ (C-A)/2, E ≧ B/4, 0 mm ≦ F ≦ 0.5 mm and A/5 ≦ G ≦ A/2. A spark plug according to claim 1, wherein the distance between the rear end surface (52) of the tip and front end surface (42a) of the heat-conductor core is between 0.0 mm and 0.5 mm. A spark plug according to claim 1 or 2, wherein the tip (5) is made of iridium or an iridium-based alloy in which iridium is dispersed into a sintered mixture of an oxide of a rare earth metal or an oxide of aluminium, magnesium or thorium, the content of the oxide of the metal or the rare earth metal being less than 15.0% by volume. A spark plug according to claim 3, wherein the oxide has a melting point of more than 2000°C. A spark plug according to any one of the preceding claims, wherein a tip (6) is placed on the outer electrode to correspond to the tip (5) of the centre electrode, the tip being made of a nickel-platinum alloy, an iridium alloy or a nickel-iridium alloy. An internal combustion engine including a spark plug according to any preceding claim.

Inventors: IWATA KAZUYA, OKAYAMA TSUTOMU, OSHIMA TAKAFUMI, IWATA, KAZUYA, OKAYAMA, TSUTOMU, OSHIMA, TAKAFUMI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1992
Main classification: H01T13
IPC codes: H01T13