H0015804

HIGH THREAD GROUND SHIELD

BACKGROUND

This application relates generally to spark plugs for internal combustion engines and, more particularly, to a ground shield for a spark plug having an annular threaded portion for engaging the engine with a spark plug seat that is located between the spark gap and the threaded portion.

Traditional spark plug construction includes an annular metal casing having threads near one end and a ceramic insulator extending from the threaded end through the metal casing and beyond the opposite end. A central electrode is exposed near the threaded end and is electrically connected through the insulator interior to a terminal which extends from the opposite insulator end to which a spark plug ignition wire attaches. A "J" shaped ground electrode extends from one edge of the threaded end of the metal casing into axial alignment with the central electrode to define a spark gap therebetween. The force applied to seal the spark plug in the head is the result of torque transmitted by the threaded metal casing; hence, the threaded portion of the metal casing must be sturdy and of substantial size. A portion of the metal casing is formed to be engaged by a socket tool to provide torque to the threaded portion. The threaded portion is located away from the portion which is engaged by the socket tool.

To facilitate the controlled and efficient exhaust of gases from a combustion chamber, the valves are sometimes increased in size. This may necessitate a decrease in the size of the spark plug, a reduction in the size and sturdiness of the threaded metal casing end, and, in particular, a decrease in the inside diameter of the metal bore of the spark plug and in the combustion chamber wall area available to threadedly receive the spark plug.

The decrease in the inside diameter of the metal bore of the spark plug reduces the ability of the spark plug to resist carbon build up and similar deposits reducing ignition efficiency. Various designs for spark plugs that

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reduce the deleterious effect of reducing the spark plug size by having an insulator with a cylindrical body that surrounds a central electrode are taught in U.S. Pat. Nos. 5,091,672, 5,697,334, 5,918,571, and 6,104,130, the contents for each incorporated herein by reference. In these designs, the cylindrical body is provided with a first diameter section separated from a second diameter section by a shoulder that provides a surface for sealing to the engine cylinder head. A shield that surrounds the second diameter has a base portion that is positioned a fixed distance from the tip to the center electrode by the engagement of a flange on the shield with the shoulder on the cylindrical body. The shield is formed with a ground electrode that integrally extends from the base portion. A shell portion surrounds the first diameter section of the cylindrical body and contains a threaded section positioned higher than the cylinder head seating surface along the cylindrical body. A radial tab extends from an end of the shell and aligns with the flange within the head to establish uniform positioning of the base portion. A separate end or retainer nut extends from the opposing end of the shell to locate and position the spark plug within the combustion chamber.

Particularly suited for high-compression, high-performing engines, these various high-thread sparkplug designs can provide more power by allowing for more space to optimize engine design, a superior cylinder head- seating position, a more compressive seal, improved heat transfer, and a more stable spark plug operating temperature for a more focused ignition, as well as a longer service life and increased corrosion protection. Nevertheless, to maintain the sparking gap between the center electrode and the ground electrode, the ground shield must be manufactured from an expensive, proprietary nickel alloy material.

Accordingly, the inventor herein has recognized that it is desirable to provide a cost effective ground shield for use in a high-thread spark plug structure.

SUMMARY

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Exemplary embodiments of the present invention relate to a spark plug for an internal combustion engine. The sparkplug comprises an elongated center electrode having a center electrode tip at one end and a terminal proximate the other end, an insulator substantially surrounding the center electrode, and a ground shield. The insulator has a substantially cylindrical body with at least a first insulator section and a second insulator section. The first and second insulator sections having first and second diameters respectively and are separated by an insulator shoulder. The ground shield has an elongated base section substantially surrounding the first insulator section, a frustoconical flange protruding from one end of the base section to engage the insulator shoulder, and a ground electrode extending from the other end of the base section to define an axial spark gap with respect to the center electrode tip. The base section and the ground electrode are formed as separate components and secured together to form the ground shield.

Exemplary embodiments of the present invention also relate to a ground shield for a spark plug that has a center electrode and an insulator surrounding the center electrode. The center electrode has a tip at one end and a terminal proximate the other end. The insulator has a substantially cylindrical body with at least a first insulator section and a second insulator section. The first and second insulator sections having first and second diameters respectively and are separated by an insulator shoulder. The ground shield comprises an elongated base section configured to surround the first insulator section, a frustoconical flange protruding from a first end of the base section configured to engage the insulator shoulder, and a ground electrode extending from a second end of the base section configured to define a spark gap with respect to the tip of the center electrode. The base section and the ground electrode are formed as separate components and secured together to form the ground shield.

BRIEF DESCRIPTION OF DRAWINGS: Figure 1 is a cross-sectional view of a spark plug in accordance

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with an exemplary embodiment of the present invention;

Figure 2 is a side view of the exemplary spark plug illustrated in Figure 1;

Figure 3 is a perspective view of the exemplary spark plug illustrated in Figure 1 ;

Figure 4 is a side view of the sparking end of the exemplary spark plug illustrated in Figure 1;

Figure 5 is a partial cross-sectional view of a sparking end of an exemplary embodiment of a spark plug in accordance with the present invention;

Figures 6 and 7 are side views of a ground shield for a spark plug in accordance with an exemplary embodiment of the present invention;

Figure 8 shows various views of a ground electrode of the exemplary ground shield illustrated in Figures 6 and 7; Figure 9 shows various views of a base section of the exemplary ground shield illustrated in Figures 6 and 7; and

Figure 10 is a side view of a ground shield for a spark plug in accordance with an exemplary embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Figures 1-4 illustrate an overall structure of an exemplary embodiment of a high-thread spark plug employing a ground shield in accordance with the present invention. The spark plug 10 is designed for use in internal combustion engines of automotive vehicles. The installation of spark plug 10 into an internal combustion engine is achieved by fitting it so that it protrudes into a combustion chamber (not shown) of the engine through a threaded bore provided in the engine head (not shown). Spark plug 10 includes a cylindrical center electrode 21 extending along the axial length of the spark plug, a ceramic or similarly comprised insulator 41 that concentrically surrounds center electrode 21, and a cylindrical shell shaped ground shield 37 that concentrically surrounds insulator 41.

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In the present exemplary embodiment, center electrode 21 has a cylindrical body with a tip 33 at one end, and is secured concentrically within insulator 41 to be electrically isolated from ground shield 37. The end of center electrode 21 opposing tip 33 is electrically connected to an end of a resistive element 13 through a glass seal 15 that comprises an electrically conductive material. In exemplary embodiments, glass seal 15 can be a fired-in seal (conductive or otherwise) that coaxially surrounds resistive element 13 such that it is located between the inner surface of insulator 41 and the outer surface of the resistive element. The other end of resistive element 13 is electrically connected, through the glass sealing material 15, to an end of a cylindrical terminal stud 23. Glass seal 15 serves as the electrical connection between terminal stud 23 and center electrode 21. Terminal stud 23, in turn, is attached to a terminal nut 17, which is configured to attach to the ignition cable (not shown) that supplies the electric current to the plug when the plug is installed.

In exemplary embodiments, center electrode 21 can comprise a core 49 made of a highly heat conductive metal material such as, for example, copper, covered by a longer than conventional sheath 47 made a highly heat- resistant, corrosion-resistant metal material such as, for example, Inconel, another nickel-based alloy, or other suitable metal or metal alloy. In exemplary embodiments, center electrode 21 can include a noble metal chip 45, such as one made from a gold, palladium, or platinum alloy in any suitable form for enabling proper spark plug functioning such as, for example, flat or finewire, that is joined to center electrode tip 33 to improve heat transfer and maintain the sparking gap. In exemplary embodiments, terminal stud 23 can comprise steel or a steel-based alloy material with a nickel-plated finish.

In the present exemplary embodiment, insulator 41 has an elongated, substantially cylindrical body with first 25, second 27, and third 67 insulator sections having different diameters. First insulator section 25 substantially surrounds center electrode 21. Second insulator section 27 is located intermediate first 25 and third 67 insulator sections and the diameter of

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the second insulator section 27 is greater than that of either of the other two insulator sections. Second insulator section 27 and narrower first insulator section 25 are separated by a shoulder 29, and the second insulator section and narrower third insulator section 67 are separated by a shoulder 69. In exemplary embodiments, insulator 41 can comprise a non-conducting ceramic material such as, for example, alumina ceramic so that it may fixedly retain center electrode 21 while preventing an electrical short between the center electrode and ground shield 37.

Ground shield 37, which surrounds first insulator section 25, includes a frustoconical section 31 at one end that is juxtaposed with insulator shoulder 29, a generally U-shaped ground electrode strap 39 that extends from and diametrically spans the ground shield near the opposing end, and a cylindrical base portion 43 axially extending between frustoconical section 31 and ground electrode strap 39. Base portion 43 concentrically surrounds first insulator section 25. Ground electrode strap 39 includes a free end 55 that faces and is axially spaced from a center electrode tip 33. hi exemplary embodiments, free end 55 can include a noble metal chip 57, such as one made from a gold, palladium, or platinum alloy in finewire form, that is joined to ground electrode strap 39 to improve heat transfer and enhance durability. In exemplary embodiments in which noble metal chips 45, 57 are joined to center electrode tip 33 and ground electrode strap 39 respectively, the noble metal chips define the spark gap and serve as the sparking surfaces of the spark plug. In exemplary embodiments, noble metal chips 45, 57 can be joined to center electrode tip 33 and ground electrode strap 39 respectively by a joining technique such as brazing, laser welding, resistance welding, or plasma welding.

As illustrated in detail in Figure 5, exemplary embodiment of spark plugs in accordance with the present invention can comprise a ground electrode strap 139 that includes a free end 155 facing and axially spaced from a center electrode tip 133. Ground electrode strap 139 thus diametrically surrounds center electrode tip 133 to define an axial spark gap 135

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therebetween, between which an electrical discharge can be passed to ignite a combustible mixture. Center electrode 121 can include a noble metal chip 145, such as one made from a gold, palladium, or platinum alloy in any suitable form for enabling proper spark plug functioning such as, for example, flat or finewire, that is joined to center electrode tip 133 to improve heat transfer and enhance durability.

Referring again to the exemplary embodiment illustrated in Figures 1-4, an annular retainer 59, such as a nut or a castle head jam screw, has a threaded portion 61 surrounding second insulator section 27. Annular retainer 59 extends axially to integrally form a jam nut 56 at one end that surrounds a portion of third insulator section 67. Threaded portion 61 is configured to threadedly engage the threaded portion of a generally cylindrical opening that is in communication with the combustion chamber of an internal combustion engine. With jam nut 56 being formed integrally with annular retainer 59, spark plug 10 can be removed in a helical pattern as the jam nut is unscrewed, resulting in easy, direct removal with negligible tipping. A suitable socket tool such as, for example, a 9/16 socket wrench, can engage jam nut 56 of annular retainer 59 for screwing spark plug 10 into and out of the engine bore.

Annular retainer 59 includes a frustoconical portion 63 that is situated below threaded section 61 and overlaps frustoconical section 31 of ground shield 37 in juxtaposed alignment with insulator shoulder 29. At this juncture, ground shield 37 and retainer 59 are secured together, with the insulator 41 being captured therewithin. Annular retainer 59 also includes a frustoconical portion 71 axially extending between threaded portion 61 and jam nut 56 that engages insulator shoulder 69. Third insulator section 67 protrudes from annular retainer 59 beyond jam nut 56. hi exemplary embodiments, annular retainer 59 can comprise a conductive metal material such as a nickel- plated, low-carbon steel-based alloy. In exemplary embodiments, threaded section 61 can have an outer thread diameter of about 16 mm or less; for example, the threaded section can have an outer diameter of about 10 mm to

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allow for a greater amount of engine space. The shape, size, and particular construction of annular retainer 59 may, of course, vary greatly from one design to another; hence, the dimensional attributes of the annular retainer are provided in Figures 1-3 only as an exemplary embodiment.

When spark plug 10 is threaded into the engine bore, insulator 41 provides a compressive force that transmits a mechanical connection between retainer 59 and ground shield 37 by urging ground shield frustoconical portion 31 into sealing engagement with annular retainer frustoconical portion 63. Frustoconical portion 63 will, in turn, be urged to act as the external motor seat for sealingly engaging a frustoconical sealing seat portion of the engine bore (not shown) and thus establish an electrical ground connection between ground shield 37 and the engine head while at the same time sealing the combustion chamber from the surrounding environment. The assembled annular retainer 59 and ground shield 37 thus function as a unit and may be referred to herein as the ground shield and retainer unit. In exemplary embodiments, frustoconical portion 63 of annular retainer 59 and frustoconical section 31 of ground shield 37 can also be joined to one another using a joining technique such as brazing, laser welding, resistance welding, or plasma welding, to secure the ground shield and the retainer together.

Exemplary embodiments of the present invention employ a ground shield design that may represent a substantial cost savings. As illustrated in the exemplary embodiment of Figures 6-7, ground shield 237, rather than being integrally formed as a unitary piece, is a composite of base portion 243 and ground electrode strap 239, which are formed separately and then secured together. As shown in Figures 8 and 9, ground electrode strap 237 is formed with a pair of legs 275, and base portion 243 is formed with axial extending slots 273 configured to fittingly receive the legs of the U-shaped ground electrode strap 239 at the end proximate to the axial electrode gap. Thus, to assemble ground shield 237, legs 275 of ground electrode strap 239 are fit within slots 273 and joined to otherwise open-ended base portion 243. In

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exemplary embodiments, legs 275 can be joined to slots 273 using a joining technique such as brazing, laser welding, resistance welding, or plasma welding, to secure the ground electrode strap to base portion 243.

Because base portion 243 and ground electrode strap 239 are formed separately, these two portions of ground shield 237 may be made from different materials. Thus, in exemplary embodiments, ground electrode strap 239 can be manufactured from an expensive, proprietary nickel alloy material such as, for example, Iconel to enhance durability between a center electrode tip (such as, for example, center electrode tip 33 depicted in the exemplary embodiment illustrated in Figure 1) and ground electrode 257, while base portion 243 (which can comprise as much as 90% or more of the total size of ground shield 237 in exemplary embodiments) can be made from any low cost, corrosion resistant material such as any suitable metal-based alloy like stainless steel and similar steel-based alloys. Accordingly, by forming ground shield 237 by securing ground electrode strap 239 to otherwise open-ended base portion 243 as described, the need to fabricate the larger base portion from an expensive nickel alloy is avoided, thereby reducing the cost of forming the high-thread ground shield to as little as 10% or less of its former cost in exemplary embodiments.

Furthermore, as shown in the alternative exemplary embodiment of a ground shield 337 illustrated in Figure 10, cost can further be reduced by forming the ground shield as a composite of a base portion 343 and a generally J-shaped ground electrode strap 339 having a free end that is radially aligned with and axially spaced from a center electrode tip to form the spark gap, as illustrated in Figure 10. In such an embodiment, the ground electrode strap will thus be formed with a single leg that is welded to base portion 343 in a single open slot 373. In yet another alternative exemplary embodiment, the ground electrode strap can be formed as a generally U-shaped member having an annular opening within free end in which a center electrode tip ends within or slightly below the annular opening.

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The unique technique for fabricating a spark plug in accordance with exemplary embodiments of the present invention should now be clear. Referring again to the exemplary embodiment illustrated in Figures 1-4, center electrode 21 is axially into passed a bore formed within insulator 41 such that center electrode firing end or tip 33 projects from one end of the insulator, and terminal stud 23 can be passed into glass sealing material 15 of resistive element 13 to axially extend from the opposing end of the insulator. Insulator 41 and its included center electrode 21 are then axially passed into cylindrical shell ground shield 37 such that base portion 43 surrounds smaller diameter first insulator section 25, flared frustoconical section 31 engages insulator shoulder 29, and axial sparking gap 35 is formed between center electrode tip 33 and ground electrode tip 57.

Cylindrical annular retainer 59 is then axially passed over the insulator from the opposite end and its interior frustoconical ledge 71 engages insulator second shoulder 69 such that threaded section 61 surrounds larger diameter second insulator section 27 and jam nut 56 surrounds a portion of third insulator section 67. Frustoconical portion 63 of annular retainer 59 is then radially collapsed about frustoconical section 31 to secure ground shield 37 and annular retainer 59 together with insulator 41 being captured therebetween. In exemplary embodiments, frustoconical portion 63 of annular retainer 59 can be "hot pressed" onto frustoconical section 31 , and jam nut 56 can be joined in a similar fashion onto third insulator section 67.