Abstract:
A spark plug ( 24 ) is used in an ignition system ( 10 ) of the type for creating a precisely timed spark to ignite an air/fuel mixture in an internal combustion engine. The spark plug ( 24 ) is provided with an integrated capacitor feature to increase the intensity of its spark. The capacitor feature is formed by applying metallic film ( 62, 64 ) to the inner ( 30 ) and outer surfaces of a tubular insulator ( 26 ). The insulator ( 26 ), made from an alumina ceramic material, forms a dielectric and sustains an electrical charge when an electrical differential is established between the inner ( 64 ) and outer ( 62 ) metallic films. The stored electrical charge is discharged with the firing of a spark in the spark gap ( 54 ). The inner ( 64 ) and outer ( 62 ) metallic films can be applied as a paint or ink directly to the surfaces of the insulator ( 26 ), or can be mixed with a glazing compound to form conductive coatings simultaneous with the glazing operation. The metallic film ( 62, 64 ) is specially selected from materials that will not migrate into the porous matrix of the ceramic insulator ( 26 ). The metallic film ( 62, 64 ) is preferably gold, platinum, copper, or a platinum group metal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to an ignition system for a spark-ignited internal combustion engine, and more particularly to a spark plug having high capacitance features.  
         [0003]     2. Related Art  
         [0004]     Ignition systems for spark-ignited internal combustion engines rely on a spark plug to produce a spark of sufficiently robust discharge so as to ignite a compressed air/fuel mixture. Often, more efficient ignition can be achieved by increasing the intensity of the spark.  
         [0005]     The prior art has taught to incorporate a capacitor into the spark plug to increase the intensity of its spark. Various methods and configurations for integrating a capacitor into a spark plug have been proposed. All of the various proposed methods, however, have drawbacks and have failed to meet expectations in real world applications. Some designs of integrated capacitors within the spark plug have failed to increase the spark intensity by any appreciable amount. Other designs are not capable of withstanding the high temperature, corrosive operating environment, and as a result their life is limited. Still an additional limitation of spark plugs having integrated capacitors arises out of their fragility. These have been found not capable to withstand normal assembly operations without succumbing to chemical oxidation or destruction from collateral mechanical forces and abrasions.  
         [0006]     One prior art attempt to achieve a higher capacitance spark plug suggested a metallic silver coating applied to the ID and OD of the alumina ceramic insulator, with the insulator forming an interposed dielectric. While this proposal has certain short term successes, it is subject to failure when used long term at high temperature. The failure mode is a high voltage dielectric failure of the ceramic due to deterioration of the ceramic resulting from migration of the silver into the alumina ceramic and reducing its effectiveness as an electrical insulator. Additionally, this prior design is highly susceptible to chemical oxidation, and the silver coating is not capable of withstanding subsequent assembly operations which include harsh, abrasive contact with machine tools and other elements.  
         [0007]     Accordingly, there exists a need for a higher capacitance spark plug which is inexpensive to manufacture, conducive to existing spark plug manufacturing techniques and machinery, not subject to chemical oxidation or mechanical destruction during assembly operations, will not migrate into the matrix of the ceramic insulator, and which provides acceptable service life without deterioration or failure.  
       SUMMARY OF THE INVENTION  
       [0008]     A spark plug for a spark-ignited internal combustion engine comprises a generally tubular ceramic insulator having an outer surface and an inner surface. A metallic shell surrounds at least a portion of the outer surface of the ceramic insulator. The shell includes at least one ground electrode. A center electrode is disposed in the ceramic insulator, in registry with the inner surface thereof. The center electrode has an upper terminal end and a lower sparking end in opposing relation to the ground electrode, with a spark gap defining the space therebetween. The ceramic insulator includes an outer metallic film disposed over at least a portion of its outer surface and in electrical contact with the shell. An inner metallic film is disposed over at least a portion of the inner surface and in electrical contact with the center electrode. The inner and outer metallic films are electrically separated from one another by the ceramic insulator and are operative to store a charge of electrical energy therebetween in response to an electrical potential between the center electrode and the shell.  
         [0009]     According to another aspect of the invention, an ignition system for a spark-ignited internal combustion engine is provided. The ignition system comprises an electrical source, an ignition coil operatively connected to the electrical source for creating a high-tension voltage, and a switching device operatively connected to the ignition coil for distributing the high tension voltage from the coil in precisely timed intervals. At least one spark plug is electrically connected to the switching device and includes a generally tubular ceramic insulator having an outer surface and an inner surface. A metallic shell surrounds at least a portion of the outer surface of the ceramic insulator. The shell include at least one ground electrode. A center electrode is disposed in the ceramic insulator in registry with the inner surface thereof. The center electrode has an upper terminal and a lower sparking end in opposing relation to the ground electrode with a spark gap defining the space therebetween. The ceramic insulator includes an outer metallic film disposed at least over a portion of its outer surface in electrical contact with the shell. An inner metallic film is disposed over at least a portion of the inner surface in electrical contact with the center electrode. The ceramic insulator forms a dielectric between the inner and outer metallic films and is operative to sustain an electrical field therein for discharge with a spark formed in the spark gap.  
         [0010]     According to yet another aspect of the invention, a method for forming a spark plug is provided. The method comprises the steps of forming a ceramic insulator as a generally tubular body of revolution having an outer surface and an inner surface; surrounding at least a portion of the outer surface of the ceramic insulator with a metallic shell; attaching a ground electrode to the metallic shell; inserting a center electrode having an upper terminal end and a lower sparking end into the ceramic insulator in registry with its inner surface; and orienting the sparking end of the center electrode opposite to the ground electrode to create a spark gap in the space therebetween. The method is characterized by coating at least a portion of the inner and outer surfaces of the ceramic insulator with metallic film so that the ceramic insulator forms a dielectric between the opposing metallic films and is operative to sustain an electric field therein for discharge with a spark formed in the spark gap.  
         [0011]     A spark plug, an ignition system and a method according to the invention result from a spark plug capacitor having a useful service live without deterioration or failure, that will not migrate into the ceramic matrix under high temperature, and is particularly adapted to spark plug assembly operations without succumbing to chemical oxidation or mechanical destruction through abrasion.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:  
         [0013]      FIG. 1  is a simplified schematic view of an exemplary ignition system for a spark-ignited internal combustion engine;  
         [0014]      FIG. 2  is a cross section of a exemplary spark plug incorporating the novel features of the subject invention;  
         [0015]      FIG. 3  is an enlarged view of the spark plug of  FIG. 2 ;  
         [0016]      FIG. 4  is a schematic diagram showing a sequential method of applying metallic film to the ceramic insulator; and  
         [0017]      FIG. 5  is a schematic diagram as in  FIG. 4 , but showing an alternative method for applying the metallic film to the ceramic insulator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, an exemplary ignition system for a spark-ignited internal combustion engine is generally shown at  10  in  FIG. 1 . The ignition system  10  can be of any known type, including the standard ignition system with contact points, a breakerless electronic ignition system, a capacitor discharge ignition system, or any other of the known types. In the example of  FIG. 1 , a computer controlled ignition system is depicted, whose primary purpose is to provide a timed electrical discharge of sufficient energy to ignite a compressed air/fuel mixture in the individual cylinders of an internal combustion engine. The voltage needed to produce this electrical discharge is most often generated by means of an auto-transformer where the current in the primary of an ignition coil  12  is interrupted at the desired time of ignition. This is accomplished by a circuit in which the relatively low voltage in a battery  14  is stepped up to the order of 30 to 40 kilovolts or by means of a self-contained magneto. When an ignition switch  16  is in the “on” or “closed” condition, current flows from the battery  14  to a computer control device  18  which is programmed to determine the exact time when ignition is required and to send a signal to the ignition coil  12  to produce the high voltage needed for firing the spark plugs. Sensors, generally indicated at  20 , provide numerous inputs to the computer control device  18  which allow it to compute precise timing parameters. A distributor  22  acts as a switching device for directing high-tension voltage from the coil  12  in precisely timed intervals to the respective combustion chambers in the engine. Those skilled in the art will appreciate that the specific arrangement, circuitry and components in the ignition system  10  may vary by application and as technology evolves.  
         [0019]     A spark plug is generally shown at  24  in  FIGS. 2 and 3 . The spark plug  24  includes a generally tubular ceramic insulator  26  which is preferably made from an aluminum oxide ceramic material having a specified dielectric strength, high mechanical strength, high thermal conductivity and excellent resistance to heat shock. The insulator  24  may be molded dry under extreme pressure, and then kiln-fired to vitrification at high temperature. The insulator  26  has an outer surface which may include ribs  28  for the purpose of providing added protection against spark or secondary voltage “flash-over” and improve grip of a rubber spark plug boot (not shown). The insulator  26  also includes a central passage extending the length of the insulator  26  and defined by an inner surface  30 .  
         [0020]     A metallic shell  32  surrounds the lower section of the outer surface of the insulator  26 . The metallic shell  32  may be fabricated by a cold-extrusion or other process, and include a tool receiving hexagon  34  for removal and installation purposes. The hex size complies with industry standards for the related application. A threaded section  36  is formed at the lower portion of the metallic shell  32 , immediately below a seat  38 . The seat  38  may either be tapered to provide a close tolerance installation in a cylinder head which is designed for this style of spark plug, or may be provided with a gasket (not shown) to provide a smooth surface against which the spark plug seats in the cylinder head. A ground electrode  40  extends radially inwardly from the bottom of the threaded section  36 . The ground electrode  40  may be fabricated from a material different than that of the metallic shell  32 , so as to resist both sparking erosion and chemical corrosion under normal and extreme operating temperature conditions, and to conduct heat. The ground electrode  40  may have a rectangular cross-section to provide increased gap life, but other shapes and configurations are also possible, including the use of multiple ground electrodes or surface gap type electrodes.  
         [0021]     A center electrode, generally indicated at  42 , is disposed in the central passage of the ceramic insulator  26 , in registry with the inner surface  30 . The center electrode  42  preferably comprises an assembly which, in the example of  FIG. 2 , includes an upper terminal end  44  that can be secured within the central passage of the insulator  26  by threads coupled with an applied cement to provide a permanent, gas-tight connection. A suppressor  46  can be included in-line under the upper terminal end  44  for the purpose of reducing electromagnetic interference in certain situations. The suppressor  46  can be of any known type, including the resistive type or the inductive type, depending in part on the configuration of the ignition system  10 . A spring  48  assures firm contact between the suppressor  46  and the upper terminal end  44 . A lower portion  50  of the center electrode  42  abuts the under side of the spring  48  and extends through the remainder of the central passage in the insulator  26  to emerge at a lower sparking end  52  presented in opposing relation to the ground electrode  40 . A spark gap  54  is defined in the space between the sparking end  52  and the ground electrode  40 . The lower portion  50  of the center electrode  42  may include encapsulated copper  56  to improve heat transfer away from the spark gap  54 . A compacted powder seal  58  may be formed under high pressure between the lower portion  50  of the center electrode  42  and the inner surface  30  of the insulator  26  to provide a permanent assembly and eliminate combustion gas leakage. The powder seal  58  is of the type impervious to heat, oxidation, and corrosion. A similar powder seal  60  may be provided between the metallic shell  32  and the outer surface of the insulator  26 . Those skilled in the art will appreciate that the specific construction and configuration of the center electrode  42  can take many forms and may even evolve with technological advances. It can be inserted into the ceramic insulator  26  as a unit, but more preferably is assembled in situ. The sparking surfaces of the center  42  and ground  40  electrodes can be fitted with precious metals to improve durability.  
         [0022]     The spark plug  24  is fitted with an integrated capacitor for the purpose of increasing the intensity of the spark generated in the spark gap  54 . The integrated capacitor is formed by an outer metallic film  62  applied over at least a portion of the outer surface of the insulator  26  so that it is in contact with the grounded metallic shell  32 . This outer metallic film  62  forms one plate of the capacitor. An inner metallic film  64  is disposed over a corresponding portion of the inner surface  30  of the insulator  26  and is in electrical contact with the center electrode  42 . The inner metallic film  64  forms the other plate of the capacitor configurations. The insulator  26 , positioned between the outer  62  and inner  64  metallic films, forms a dielectric and is operative to sustain a capacitive electrical field therein for discharge with a spark formed in the spark gap  54 . As high tension electricity is applied to the center electrode  42 , the electrical potential between the grounded metallic shell  32  and the center electrode  42 , which are respectively conducted to the outer  62  and inner  64  metallic films, creates an integrated electrical device when the two films  62 ,  64  are electrically insulated from each other by the dielectric insulator  26  and in which capacitance is introduced in the form of stored electrical energy. When a spark forms in the spark gap  54 , the capacitor is discharged, with the effect that the stored electrical energy is transmitted into the spark thereby increasing its intensity and its effectiveness in igniting the air/fuel mixture in the cylinder.  
         [0023]     Preferably, the inner  64  and outer  62  metallic films are applied about the full circumferential measure of the insulator  26  so that, like the tubular insulator  26 , each metallic film  62 ,  64  takes the form of a tube, or body of revolution, concentric about the center electrode  42 . The axial extent to which each metallic film  62 ,  64  covers the insulator  62  can be varied depending upon the spark plug configuration and particular applications. In the examples shown, the outer metallic film  62  extends above the shell  32  and presents an exposed portion visible upon external examination of the finished spark plug  24 . In the other direction, the outer metallic film  62  extends partly down the insulator nose so that some of its surface area is exposed to combustion gasses. Internally, the inner metallic film  64  is generally coextensive in the axial direction with the outer metallic film  62 .  
         [0024]     In order to prevent oxidation of the metallic films  62 ,  64  under high temperature operations, and also to prevent diffusion of an electrically conductive element into the matrix of the insulator  26 , the metallic films  62 ,  64  are preferably made from a noble metal coating of gold or a member of the platinum group which consists of platinum, palladium, iridium, osmium, ruthenium, and rhodium. Metallic films  62 ,  64  may also include various alloys or other combinations of these noble metals. Further, metallic films  62 , 64  may also include these noble metals, or noble metal alloys, in combination with non-noble metals, such as for example, Ni, W, Fe and Cr or combinations thereof. Another possible material for the metallic films  62 ,  64  comprises copper, however to address oxidation issues, the copper may be coated with a protective layer such as a glazing.  
         [0025]     The inner  62  and outer  64  metallic films can be applied as coatings or intermixed with the ceramic glazing material and applied as part of the normal glaze process.  FIG. 4  illustrates an exemplary sequence of events in which the inner  64  and outer  62  metallic films are applied as coatings. Here, operation box  66  represents the stage in which the conductive metal is prepared for application. Generally, this will involve formulating the specific material into a liquid state. It can also involve formulating the material as an ink or paint made from the constituent material. Other possibilities include preparing the conductive metallic material as a powder to be applied in a pre-sintering operation. Decision block  68  queries whether the particular material possesses sufficient high temperature corrosion properties. If not, such as in the example of copper, the conductive metal may be applied to the insulator  26  in a non-corrosive environment such as perhaps a nitrogen or argon atmosphere. This is represented in function block  70 . Following this, a protective glaze or other non-corrosive coating is applied over the metallic film to address high temperature corrosion issues. This step is conducted at function block  72 , followed by a curing operation  74 . If, instead of copper, gold or one of the platinum group metals is chosen for the conductive metal, the conductive metal can be applied directly to the insulator  26  as represented in function block  76 , followed by the curing operation  74 , as corrosion will not be an issue. In the example of the conductive metals being prepared in the form of a liquid ink or paint, application to the insulator  26  can take the form of brushing, dipping, rolling, spraying, screening, or any other known operation for applying a liquid coating to a rigid substrate.  
         [0026]     In some applications, it may be desirable to enhance the capacitance of the spark plug by applying the inner  64  and/or the outer  62  metallic films in multiple layers interlaced with layers of an insulator material such as a glaze or other high dielectric constant material. This has the advantageous effect of increasing the effective surface area of the capacitor without substantially increasing the axial length or the radial diameter of the spark plug  24  beyond specified dimensions. Thus, in  FIG. 4 , the sequence of events may include a query  77  to determine whether enough layers of metallic film have been applied. If the answer is “NO” the procedure may advance to functional block  78  where a dielectric layer is applied, followed by a curing of the dielectric  80  if necessary. The sequence is then repeated to apply another layer of metallic film. This loop is repeated until the query  77  has been answered in the affirmative. From here, final finishing operations can be performed at functional block  82 , with the resulting spark plug  24  according to the subject invention being produced as an end product.  
         [0027]     An alternative application technique is described in connection with  FIG. 5 . Here, an appropriate conductive metal is provided in a container  84 , together with a ceramic glaze material in a container  86 . These constituents are mixed together to form an extremely durable, high temperature conductive coating for the insulator  26 . According to this technique, even a material like copper, which has a propensity toward chemical oxidation under high temperature conditions, is protected from corrosion and from migration into the matrix of the insulator  26 . The specially prepared glaze is then applied to the insulator  26  at function block  90 . The glaze is cured at  92  so that the resulting conductive coating is fully set and operational. Query block  94  determines whether multiple layers of the conductive coating are to be applied. If so, it may be necessary to form another dielectric layer at  96 , and cure that dielectric layer at  98  before applying a new layer of glaze at  90 . However, if only one layer of metallic film is to be applied, or when enough layers have been achieved, the insulator  26  is subjected to further finishing operations  100  to yield a fully finished spark plug  24  according to the subject invention.  
         [0028]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.