Abstract:
An igniter having a two-piece center electrode assembly that includes an upper electrode member and an iridium lower electrode member. The two electrode members are assembled together by insertion of an expanded head section of the iridium electrode into a blind hole within the upper electrode. The upper electrode is then swaged over the expanded head section to provide a positive mechanical interlock of the two electrode members together. Thereafter, the two components are brazed together by heating of a brazing compound that is pre-loaded into the blind hole prior to insertion of the expanded head section. This assembly results in both a mechanical and metallurgical bond between the two components without subjecting the iridium electrode to deformation that could otherwise result in axial cracking of the iridium material.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to center electrode assemblies for spark plugs and igniters and, more particularly, to multi-piece center electrode assemblies that utilize an iridium-based firing tip.  
         BACKGROUND OF THE INVENTION  
         [0002]    Over the years, a variety of erosion resistant metals and alloys have been employed in center electrode assemblies to increase the life of igniters and spark plugs used in turbine and piston engines. Metals such as platinum and iridium have proven to exhibit improved consumption resistance and ignitability. Because of their expense, these noble metals are not typically used for the entire center electrode, but rather are used only to form a firing tip at the working end of the center electrode where the spark is produced. In this way, erosion at the firing surfaces can be reduced without the expense of forming the entire center electrode from the more expensive metal. Apart from their cost, most of these metals have other inherent drawbacks that complicate their use as a firing tip. For instance, platinum has a relatively low melting point (1769° C.) and while iridium has a much higher melting temperature (24540° C.), it has poor weld characteristics and is a brittle metal that exhibits axial cracking when subjected to deformation or significant pressure.  
           [0003]    An example of the use of iridium in conjunction with a spark plug center electrode can be seen in U.S. Pat. No. 5,557,158 issued to Kanao et al. The Kanao patent discloses a spark plug center electrode having a cylindrical or ringlike iridium insert mechanically attached to the end of the electrode. The iridium insert is attached to the electrode by welding or by otherwise securing a stopper component to the very tip of the center electrode, thereby securing the iridium insert between the two components and preventing it from being dislodged.  
           [0004]    The poor weld characteristics of iridium make it difficult to metallurgically attach iridium firing tips to other electrode components. Similarly, iridium has not be considered a good candidate for swaging or otherwise mechanically attaching it to the center wire because of the potential for cracking. However, where more traditional (non-iridium) metals have been used, various approaches have been proposed for mechanically connecting center electrode components together. For example, U.S. Pat. No. 1,344,954 to Meyer discloses a two-piece spark plug center electrode assembly wherein a lower electrode member having an expanded head section is received by inwardly biased retaining jaws of an upper electrode member. The axial location at which the retaining jaws contact the expanded head section is variable depending on the axial position of the upper member, which may be adjusted via threads located at its upper end. Thus, the two electrode members are mechanically secured to each other through the radially-inward pressure imparted by the retaining jaws against the expanded head section.  
           [0005]    Similarly, U.S. Pat. No. 1,633,435 to De Alcocer et al. discloses a two-piece spark plug center electrode assembly in which the stem of a lower electrode member is inserted into a receiving element of an upper electrode member. The upper member is threaded at one axial end and terminates into a hollow conical head section for receiving the stem of the lower member at the other axial end. Downward axial movement of the upper member against the lower member, by the tightening of a nut over the threaded portion of the upper member, causes the conical head section to contract over the lower member stem, thereby securing the two members together.  
           [0006]    U.S. Pat. No. 6,069,434 to Clifford discloses a three-piece spark plug center electrode assembly wherein input and emitter electrodes are coupled together via an interconnector. The interconnector is a hollow cylindrical body having a series of indentations located on its surface for gripping the input and emitter electrodes. In assembly, the input and emitter electrodes are inserted into opposite ends of the interconnector, and the interconnector is crimped around the electrodes such that the indentations grasp and retain the input and emitter in place. Because the input and emitter members do not physically contact each other, the interconnector serves as both a mechanical and electrical connector.  
           [0007]    None of these latter patents are directed to the problems associated with use of a multi-piece center electrode that uses an iridium electrode member at the firing end of the center electrode assembly. There is thus a need for a center electrode assembly that provides a connection between the iridium and other electrode component that is both secure from separation and that does not damage the structural integrity of the iridium during the attachment process.  
         SUMMARY OF THE INVENTION  
         [0008]    The above-noted shortcomings of prior art center electrode assemblies are overcome by the present invention which provides a center electrode assembly for use in an ignition device such as a spark plug or igniter. The center electrode assembly includes elongated first and second electrode members. The first electrode member has first and second axial ends with a hole extending into the second axial end. The second electrode member comprises iridium and has third and fourth axial ends and a head section located at the third axial end. The head section is positioned within the hole of the first electrode member such that the second axial end of the first electrode member is swaged over the head section, thereby mechanically securing the first electrode member to the second electrode member. Preferably, the head section comprises a radially expanded head that is not only connected to the first electrode member by swaging of the second axial end over the head section, but also by brazing of the head section to the first electrode member within the hole. The center electrode assembly constructed in this manner can be incorporated into an ignition device that also includes a shell having an axial bore, a ground electrode connected at a lower axial end of the shell, and an insulator secured within the shell&#39;s axial bore. The center electrode assembly is located within a central bore of the insulator at its lower axial end.  
           [0009]    In accordance with another aspect of the invention, there is provided a method for producing a multi-piece center electrode assembly for use in an ignition device. The method comprises the steps of:  
           [0010]    (a) providing an elongated first electrode member having a hole located at an axial end,  
           [0011]    (b) providing an elongated second electrode member having a radially expanded head section located at an axial end,  
           [0012]    (c) introducing brazing compound into the blind hole,  
           [0013]    (d) inserting the head section into the blind hole,  
           [0014]    (e) swaging the axial end of the first electrode member around the head section, and  
           [0015]    (f) heating the brazing compound to a temperature sufficient to braze the first and second electrode members together.  
           [0016]    Preferably, the second electrode member comprises iridium so that, by swaging the non-iridium electrode member over the iridium electrode member and then heating the assembled components to brazing temperatures, the electrodes are both mechanically and metallurgically connected together in a manner that helps maintain the integrity of the braze joint over time while not causing any significant axial cracking of the iridium that could otherwise occur if it were to be deformed.  
           [0017]    Objects, features, and advantages of the invention include providing an iridium tipped multi-piece electrode assembly for an ignition device that allows for the two electrode members to be securely fastened together without compromising the structural and electrical integrity of the lower iridium member. Additionally, the invention enables the center electrode assembly to be designed in a manner that is economical to manufacture and assemble and that can provide a relatively long and useful life in service.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0018]    A preferred exemplary embodiment of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:  
         [0019]    [0019]FIG. 1 is a partially cutaway view of an igniter plug constructed according to the present invention;  
         [0020]    [0020]FIG. 2 is an exploded view of the center electrode assembly shown in FIG. 1; and  
         [0021]    [0021]FIG. 3 depicts the method used to form the center electrode assembly of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    With reference to FIG. 1, there is shown an igniter plug (or, simply, igniter)  10  for use in a turbine engine of an aircraft. Igniter  10  generally includes a shell  12 , insulator  14 , center wire assembly  16 , and ground electrode  18 . As is commonly known in the art, the shell  12  is a generally cylindrical, electrically conductive component having a bore running its entire axial length. Located within the bore of shell  12  is the insulator  14  which also includes its own central bore for receipt of the center wire assembly  16 . The center wire assembly  16  includes an upper terminal end electrode  30  as well as a lower center electrode assembly  32 , and extends the entire axial length of the spark plug assembly  10 . Mechanically and electrically connected to the lower axial end of the shell is ground electrode  18 . The lower tips of the center electrode assembly and ground electrode are located at an equivalent axial position, yet they are radially spaced apart such that they form a spark gap. The ground electrode  18  can include a number of spaced vent holes  20  that open into an annular chamber  22  between the shell  12  and insulator  14 , as is known. A semiconducting material (not shown) can be included at the firing end between the center and ground electrodes  16 ,  18  or as a coating on the insulator  14  to provide reduced required sparking voltage for low and medium tension plugs. Such use of a semiconducting material is known to those skilled in the art. The shell  12 , insulator  14 , ground electrode  18 , and terminal end electrode  30  can be constructed in a conventional fashion using materials and techniques that are well known to those skilled in the art.  
         [0023]    In operation, the terminal end electrode  30  is provided with a high energy pulse from the ignition system which travels down the center wire assembly  16  until it reaches the lower end of the center electrode assembly. Assuming the pulse has sufficient energy to bridge the spark gap, it then travels to the ground electrode  18  which is grounded or otherwise connected to the ignition system&#39;s return path. While the general operation and overall structure of the illustrated igniter is well established within the art, the particular design of the lower center electrode assembly  32  is not.  
         [0024]    Referring now to FIG. 2, there is shown the two-piece center electrode assembly  32  which is divided into an elongated first electrode member  40 , referred to as the intermediate electrode, and an elongated second electrode member  42 , referred to as the firing end electrode. This firing end electrode  42  comprises iridium, meaning that it includes at least enough iridium to provide reduced spark erosion to whatever degree is desired or required for a particular application of igniter  10 . Thus, the iridium electrode  42  can be pure iridium or in the form of an alloy that contains other constituent elements. In order to securely fasten the intermediate and firing end electrodes together, an expanded head section  50  is located on the uppermost end of the firing end electrode  42  and is inserted into a hole  52  located on the lowermost end of the intermediate electrode  40 . Once inserted, the outer circumferential surface of the lower end of the intermediate electrode  40  is swaged, or crimped, around the head section  50  to create a mechanical interlock between the two pieces. Furthermore, a brazing material (usually in a solid or paste form) is inserted into hole  52  along with the head section  50 , and following swaging, the components are heated to a temperature sufficient to create a braze joint  54  between the two center electrode components. In this way, the two electrode members are both mechanical and metallurgically connected together.  
         [0025]    The intermediate electrode  40  is an elongated generally cylindrical conductive component that primary includes an upper axial end  56 , a lower axial end  58 , and hole  52 . In the preferred embodiment, the intermediate electrode is an iron/nickel/cobalt alloy (approximately 54% iron, 29% nickel, 17% cobalt) that can be purchased through Carpenter Technologies Corp. under the trade-name Kovar®. The upper axial end includes a flange  62  which is designed to support the terminal electrode  30 , which seats on top of it, such that an electrical pulse may flow between the two components. This junction also employs measures for strengthened attachment, such as resistance welding, to further secure the components together. Furthermore, flange  62  engages a circumferential interior shoulder (FIG. 1) of the insulator axial bore. By seating the flange on this shoulder, the intermediate electrode  40 , and hence the center electrode assembly  16 , is prevented from becoming dislodged and falling into the combustion chamber. In the preferred embodiment, the outer diameter of the intermediate electrode is approximately 0.125″ and is substantially uniform throughout its axial length, excluding the expanded flange  62 . However, it is envisioned that this diameter could vary along the axis as may be necessary to conform to the interior shape of the axial bore or some other component. As seen more clearly in FIG. 3, hole  52  is a cylindrical blind hole (meaning the hole does completely extend from one axial end to the other) extending a predetermined distance X into lower axial end  58  such that the hole is generally co-axial with the intermediate electrode and is deep enough to sufficiently receive head section  50 , as will be subsequently explained. The distance X may vary depending on the various factors such as the dimensions of the head section, but in the preferred embodiment is approximately 0.225″. The inner diameter of the hole D is substantially uniform along the distance X, however, as previously mentioned in reference to the outer diameter of the intermediate electrode, this could vary as a function of axial position. The lower axial end  58  seen in FIG. 3 and in phantom in FIG. 2, illustrates the shape of the lower axial end  58  before the swaging process takes place, after that process, the outer diameter of the lower axial end will be less.  
         [0026]    Referring back to FIG. 2, the firing end electrode  42  comprises iridium and is also generally cylindrical and elongated, having an upper axial end  70  and a lower axial end  72 . In the preferred embodiment, the firing end electrode comprises approximately 99.9% iridium (remaining constituents being trace elements), and the diameter is approximately 0.061″ (excluding the head section). As previously mentioned, head section  50  is located at the upper axial end  70  of the firing electrode. The head section and firing electrode may be formed from a single stock of material by methods commonly known in the art, such as grinding. Located at the opposite axial end of the firing electrode is the lower axial end  72 . This component, in conjunction with ground electrode  18 , forms a spark gap that, when supplied with sufficient voltage, produces a spark and thus begins the combustion process.  
         [0027]    The center electrode assembly of the present invention addresses the known difficulties in attaching iridium firing tips by utilizing a swaged and brazed connection that forms a mechanical interlock and metallurgical bond between the intermediate and firing end electrodes. In assembly, a suitable brazing compound  64 , such as a nickel braze compound sold under the trade-name Nicro-braze®  130  (92% nickel, 4.5% silicon, 2.9% boron; AMS Specification #  4778 ), is inserted into hole  52  prior to the insertion of the head section  50 . Other braze applications are possible such as feeding the joint from an external feedhole. Once a sufficient quantity of the brazing compound is present within the hole, the upper axial end  70  is inserted under pressure and the brazing compound is dispersed throughout the hole  52 . As previously mentioned, the inner hole diameter D and outer diameter of the head section  50  must complement each other such that the hole  52  is large enough to accommodate the head section, yet not so large that the lower axial end  58  is unable to sufficiently grasp the head section once the swaging process takes place. After the intermediate and firing end electrodes are in place along with the brazing compound  64 , a swaging tool  66  is brought into place and applies pressure all around the circumferential surface of lower axial end  58 . This pressure crimps lower axial end  58  such that the inner surface of hole  52  contacts both an inverted shoulder  60  and the shaft portion of the iridium electrode just below head section  50 , thereby preventing the firing end electrode  42  from backing out of the hole  52 . After this process has occurred, the outer diameter of the swaged section is approximately 0.100″ and the swaged area extends for an axial distance of about 0.154″-0.214″. To further strengthen the bond between these two components, the swaged center electrode assembly  32  is then placed in a brazing apparatus, such as an oven, and subjected to heat sufficient to create a braze joint  54  using the brazing compound  64 . During the firing process, brazing material  64  wicks all throughout the hole and around the head section thereby providing thorough material coverage. Other methods of activating brazing compound  64  may be known to those skilled in the art, such as induction brazing, heating the firing end electrode, and any of these methods can be used without departing from the scope of the invention. As will be appreciated, this combination of swaging and brazing provides both a mechanical interlock and metallurgical bond to keep the center electrode assembly  32  intact when it is exposed to the severity of the combustion environment.  
         [0028]    It will thus be apparent that there has been provided in accordance with the present invention an igniter and center electrode assembly therefor which achieves the aims and advantages specified herein. It will, of course, be understood that the foregoing description is of a preferred exemplary embodiment of the invention and that the invention is not limited to the specific embodiment shown. For instance, the center electrode assembly of the present invention may be employed in other types of ignition devices such as spark plugs used for reciprocating engine. Various changes and modifications will become apparent to those skilled in the art and all such changes and modifications are intended to be within the scope of the present invention.