Spark ignition device with bridging ground electrode and method of construction thereof

A spark ignition device and method of construction is provided. The device includes a ceramic insulator and a metal shell surrounding at least a portion of the ceramic insulator. The metal shell extends along a central axis between an upper terminal end and a lower fastening end. The fastening end has a pair of projections diametrically opposite one another extending axially to free ends. A center electrode assembly is received at least in part in the ceramic insulator. In addition, the device includes an elongate ground electrode having opposite sides extending along a length of the ground electrode between opposite ends. The ground electrode has opposite faces with a sparking surface attached to one of the faces, wherein the face with the sparking surface attached thereto is sunk axially into the free ends of the projections with at least a portion of the opposite sides being surrounded by the projections.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to spark ignition devices, including spark plugs for internal combustion engines, and more particularly to their ground electrode sparking surfaces and methods of construction thereof.

2. Related Art

Spark plugs used for automotive, industrial and/or marine internal combustion engine applications typically have a center electrode terminating at a sparking tip that is spaced opposite a ground electrode sparking tip across a spark gap. The sparking tips are commonly subject to relative torsional and axial movement, electrical erosion and chemical corrosion due to their construction and operating environment. As the tips move and/or wear, the sparking gap can change and the performance of the spark plug can deteriorate over time.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a spark ignition device is provided. The device includes a ceramic insulator and a metal shell surrounding at least a portion of the ceramic insulator. The metal shell extends along a central axis between an upper terminal end and a lower fastening end. The fastening end has a substantially planar surface extending transversely to the central axis and a pair of projections diametrically opposite one another extending axially from the substantially planar surface away from the terminal end to free ends. Further, a center electrode assembly is received at least in part in the ceramic insulator. In addition, the device includes an elongate ground electrode having opposite sides extending along a length of the ground electrode between opposite ends. The ground electrode has opposite faces with a sparking surface attached to one of the faces, wherein the one face with the sparking surface attached thereto is sunk axially into the free ends of the projections with at least a portion of the opposite sides being surrounded by the projections.

In accordance with another aspect of the invention, a method of constructing a spark ignition device is provided. The method includes providing a ceramic insulator; disposing a center electrode assembly having a sparking surface at least in part in the ceramic insulator; providing an annular metal shell having a central axis extending between an upper terminal end and a lower fastening end and forming a pair of projections extending axially from a substantially planar surface adjacent the fastening end to free ends; providing an elongate ground electrode having opposite sides extending along a length between opposite ends and having opposite faces with a sparking surface attached to one of the faces; and sinking the ground electrode into the free ends of the projections in a welding process and bringing the sparking surface of the ground electrode into a predetermined spaced relation with the sparking surface of the center electrode to form a spark gap therebetween.

In accordance with another aspect of the invention, the ground electrode is welded to the ends of the projections using a resistance welding process.

In accordance with another aspect of the invention, opposite ends are recessed in pockets formed in the ends of the shell projections during the welding process.

In accordance with another aspect of the invention, recessed pockets are formed in the shell projections for receipt of the ground electrode prior to attaching the ground electrode to the projections.

The ground electrode is maintained in a desired fixed position relative to the center electrode with the ends of the ground electrode being at least partially surrounded by projection material. As such, the spark ignition device provides a spark gap having a precise and uniform axial width that is maintained over an extended useful life. Accordingly, the device constructed in accordance with the invention exhibits a long and useful life.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

Referring in more detail to the drawings,FIG. 1illustrates a spark ignition device, represented as an internal combustion engine spark plug assembly10, constructed in accordance with one presently preferred aspect of the invention. The assembly10has an annular ceramic insulator12and an annular metal outer shell14surrounding at least a portion of the ceramic insulator12. A center electrode assembly16is received and extends a least partially through the insulator12coaxially along a central longitudinal axis17of the assembly10from a proximal terminal stud18to a distal sparking end, also referred to as sparking surface20(FIG. 2). A ground electrode22has opposite ends24,25attached to diametrically opposite sides27,29of the shell14, unlike a typical cantilevered configuration, with a ground sparking surface, also referred to as sparking surface26, being fixed to the ground electrode22in axially spaced and centered or substantially centered relation with the central axis17from the sparking surface20of the center electrode assembly16to provide a spark gap28. With the ground electrode22being fixed at least partially about its opposite ends24,25, an enhanced heat flow path from the ground electrode22to the shell14is provided, thereby reducing the potential for electrical erosion and chemical corrosion of the ground electrode sparking surface26, and further, the potential for movement of ground electrode22and sparking surface26during use is reduced, thus, maximizing the potential for optimal sparking between the ground and center electrode sparking surfaces20,26.

The electrically conductive metal outer shell14may be made from any suitable metal, including various coated and uncoated steel alloys. The shell14has a generally tubular body with a generally annular outer surface30extending between an upper terminal end31and a lower fastening end32. The fastening end32has an external threaded region34configured for threaded attachment within a combustion chamber opening of a cylinder head (not shown). The shell14also has an annular flange36extending radially outwardly from the outer surface30to provide an annular, generally planar sealing seat38for sealing engagement with an upper surface of the cylinder head with the threaded region34depending therefrom. The sealing seat38may be paired with a gasket39to facilitate a hot gas seal of the space between the outer surface of the shell14and the threaded bore in the combustion chamber opening. Alternately, the sealing seat38may be configured as a tapered seat located along the lower portion of the shell12to provide a close tolerance and a self-sealing installation in a cylinder head which is also designed with a mating taper for this style of spark plug seat. The shell14also has an attachment portion41on an upper portion, such as a tool receiving hexagon or other feature for removal and installation of the spark plug10in a combustion chamber opening.

The tubular shell body of the outer shell14has an inner wall or surface40providing an open cavity42extending through the length of the shell between the terminal and fastening ends31,32. An internal lower flange44extends radially inwardly from the inner surface40adjacent the fastening end32to provide a lower stop surface46. The inner surface40is represented as having an enlarged diameter region48adjacent the terminal end31to accommodate the insulator12. Accordingly, an annular upper flange or shoulder50extends radially inwardly from the enlarged diameter region48to a reduced diameter region52of the cavity42. The enlarged diameter region48extends upwardly from the shoulder50to an annular turnover51that extends radially inwardly to retain the insulator12. The shell14may also include a deformable buckle zone53which is designed and adapted to collapse axially and radially outwardly in response to heating of buckle zone53and associated application of an overwhelming axial compressive force subsequent to the deformation of the turnover51in order to hold the shell14in a fixed axial position with respect to the insulator12and form a gas-tight seal between insulator12and the shell14. Gaskets, cement, or other packing or sealing compounds can also be interposed between the insulator12and the shell14to perfect a gas-tight seal and to improve the structural integrity of the spark plug assembly10.

The fastening end32of the shell14has a pair of legs or projections54extending axially generally parallel to the central axis17. The projections54extend from the diametrically opposite sides27,29of the shell14, with recessed surfaces56extending between the projections54. The projections54are shown having a width (WP) slightly greater than the width (WG) of the ground electrode22and extending a predetermined length axially from the recessed surfaces56to established the desired axial parallel width, also referred to as uniform width, of the spark gap28both before and after attaching the ground electrode22to distal free ends58of the projections54. In addition, outer sides55of the projections54can be flush with or preferably, as shown, configured to extend radially outwardly of the ends24,25of the ground electrode22. Accordingly, the distance or length (L) between the outer sides55is preferably slightly greater than the overall length (l) of the ground electrode22.

The projections54are formed as monolithic extensions of the shell material, and can be formed in a machining process wherein the recessed surfaces56are machined into an end face of the shell material, thereby leaving the projections54to extend from the resulting recessed surfaces56. Of course, it should be recognized that other processes could be used to form the projections54, including laser cutting or forging, for example.

The insulator12, which may include aluminum oxide or another suitable electrically insulating material having a specified dielectric strength, high mechanical strength, high thermal conductivity, and excellent resistance to thermal shock, may be press molded from a ceramic powder in a green state and then sintered at a high temperature sufficient to densify and sinter the ceramic powder. The insulator12has an elongate body with an annular outer surface60extending between an upper terminal or proximal end62and a lower distal end64. The insulator12is of generally tubular or annular construction, including a central bore or passage66, extending longitudinally between an upper mast portion68proximate the terminal or proximal end62and a lower nose portion70proximate the distal end64. The central passage66is of varying cross-sectional diameter, generally greatest at or adjacent the terminal end62and smallest at or adjacent the nose portion70, thereby generally having a continuous series of tubular sections of varying diameter. These sections include a first insulator section which surrounds a connector extension of the terminal stud18of the center electrode assembly16. The first insulator section transitions to an uppermost first insulator shoulder72which is in pressing engagement with the turn-over51of the shell14and in turn transitions to a second insulator section having a diameter which is greater than the diameter of the first insulator section and is housed within the barrel portion of the shell14. The second section transitions to a third insulator section via a second shoulder74. The third insulator section preferably has a diameter less than the diameter of the second insulator section, and generally less than the diameter of the first insulator section. The third insulator section transitions to the nose portion70via a third insulator shoulder76that is configured for abutment with the lower stop surface46of the shell14.

The center electrode assembly16includes a center electrode78that may have any suitable shape, and is represented here, by way of example and without limitation, as having a body with a generally cylindrical outer surface extending generally between an upper terminal end79and a lower firing end80, and having a radially outward arcuate flair or taper to an increased diameter annular head at the terminal end79. The annular head facilitates seating and sealing the terminal end79within the insulator12, while the firing end80generally extends axially out of nose portion70. The center electrode78is constructed from any suitable conductor material, as is well-known in the field of sparkplug manufacture, such as various Ni and Ni-based alloys, for example, and may also include such materials clad over a Cu or Cu-based alloy core. The center electrode assembly16is also shown having a glass seal82immediately adjacent the head, an intermediate spring84and a resistor/suppressor86adjacent the terminal stud18.

The ground electrode22is attached to the projections54to establish the predetermined fixed spark gap28. As such, prior to attaching the ground electrode22to the projections54, the ground electrode sparking surface26is attached to the midsection of the ground electrode22midway between the opposite ends24,25, as shown inFIG. 3. The ground electrode22is constructed as a straight, rectangular member, such as from Inconel 600, having opposite faces88,89facing oppositely from one another along the direction of the axis17and opposite sides90,91facing oppositely from one another generally transverse to the axis17. The faces88,89and sides90,91meet at square shaped edges that extend lengthwise between the ends24,25. The length l of the ground electrode22is constructed to be equal to or preferably slightly less than the distance L between the outer sides55of the projections54. As such, upon attaching the ground electrode22in centered relation between the outer sides55and at least partially sunken in the free ends58of the projections, the outer sides55of the projections54extend radially outwardly of the ends24,25of the ground electrode22to surround at least a portion of the ends24,25. Accordingly, the material of the projections54radially outward of the ground electrode22facilitates maintaining the ground electrode22in its fixed position by resisting movement of the ground electrode22in a lateral direction relative to the center axis17. In addition, as noted above, with the width WG of the ground electrode22preferably being slightly less than the width WP of the projections54, upon attaching the ground electrode22to the projections54, material of the projections54extends partially along and outwardly from the sides90,91of the ground electrode22. As such, at least some material of the projections54abuts and confronts the ground electrode22along a portion of the sides90,91, and thus, the ground electrode22is prevented from rotating about the central axis17relative to the projections54, thus, further maintaining the sparking surfaces20,26in their axially spaced and coaxially aligned relation with one another.

The ground electrode sparking surface26is constructed having a maximum sparking area facing the center electrode sparking surface20. This results in large part from having the center electrode fixed at both its opposite ends24,25to the shell14, which provides an enhance heat sink for the heat generated at the sparking surface26. Further enhancement of the heat sink is provided by the ends24,25of the ground electrode22being at least partially surrounded by the material of the projections54. Accordingly, the increased heat generated by the maximized surface area of the sparking surface26is able to be dissipated without concern of electrical erosion and chemical erosion of the ground electrode sparking surface26. In addition, the enhanced heat sink provided by the shortened heat flow paths between the ground electrode22and the shell14provides assurance that the sparking surface26will remain fixed to the ground electrode22. The maximum surface area of the sparking surface26is bounded in one aspect by the width WG of the ground electrode22. Accordingly, the width of the sparking surface26is equal to or preferably slightly less than the width WG of the ground electrode. By being slightly less than the width WG of the ground electrode, a continuous bond, e.g. weld joint, can extend completely about the sparking surface26. The length of the sparking surface26is bounded by the length L of the ground electrode22, however, it is contemplated that a round sparking surface material, e.g. iridium or alloys thereof, including 1.7-2.3% Rh, 0.2-0.4% W, 0.01-0.03% Zr by mass, and the balance Ir, be used. As such, the diameter of the wire is selected to be slightly less than the width WG of the ground electrode22. For example, if the ground electrode22has a width WG of about 3.8 mm, then sparking surface26can be provided having a diameter of about 3.75 mm. The sparking surface26is attached to one of the faces88of the ground electrode22using any suitable process of attachment, such as resistance and/or laser welding. The resistance welding can penetrate the sparking surface26into the face88of the ground electrode22, such as between about 0.004-0.008″ using a weld current between about 6200 to 6600 amps, and then the laser welding can be used to form a circumferential weld joint about the entire circumference of the sparking surface26. During attachment, the sparking surface26is maintained centered between the ends24,25and the sides90,91of the ground electrode22, and the sparking surface26is fixed in substantially parallel relation to the faces88,89.

Upon completing the construction of the ground electrode assembly, including attaching the sparking surface26to the ground electrode22, the ground electrode assembly can be attached to the shell14. The sparking surface26of the ground electrode22is oriented to face the sparking surface20of the center electrode assembly16and the associated face88of the ground electrode22is brought into abutment with the free ends58of the projections54. Given the dimensional relations of the ground electrode22(WG, l) and the projections54(WP, L) discussed above, upon centering the face88of the ground electrode22over and between the projection free ends58, a portion of the free ends58extends outwardly in abutment with the sides90,91and the ends24,25of the ground electrode22. Then, in accordance with one presently preferred method of attaching the ground electrode assembly to the projections54, a portion of the ground electrode22is sunk axially into the projections54using a resistance welding process. As shown inFIGS. 2 and 2B, during the resistance welding step, the ground electrode22penetrates axially into the free ends58of the projections54to a predetermined axial depth along the central axis17between about 0.001″-0.003″ under an axially applied force between about 150 lbs-250 lbs using a resistance weld current between about 6000-7000 amps, though the depth could be increased, if desired. As such, during the resistance welding step, the center and ground electrode sparking surfaces20,26are brought into a predetermined, closely controlled axially aligned and axially spaced relation with one another to provide the desired finished uniform spark gap28. Further, the ground electrode22is fixed against rotation and axial translation relative to the axis17by both the material of the projections54bounding the ends24,25and the sides90,91of the ground electrode22, as well as by the weld joint itself.

It should be recognized that other methods of attachment of the ground electrode22to the projections54are contemplated. For example, rather than causing the ground electrode22to become recessed into the free ends58of the projections54solely via a resistance welding process, generally U-shaped pockets, also referred to as recesses93, can be preformed, such as by machining or coining, for example, in the free ends58and the ground electrode22can be placed and fixed in the recesses93using other mechanism of attachment, such as laser and/or tack welding, by way of example and without limitation. As such, the preformed recesses93can act to self locate the ground electrode22and its associated sparking surface26in centered and pre-gapped relation with the sparking surface20of the center electrode78prior to fixing the ground electrode22to the projections54. Further, to ensure the gap28is precisely established, a gapping gage can be inserted between the respective center and ground sparking surfaces20,26prior to and while forming the laser weld joint. As such, upon forming the laser weld joint and removing the gapping gage, the spark gap28is precisely formed as intended, without need for further processes to establish the desired width of the gap28. Of course, a combination of pre-forming the recesses93and sinking the ground electrode22into the projections54can be employed as well.