Patent Publication Number: US-7589460-B2

Title: Small diameter/long reach spark plug with rimmed hemispherical sparking tip

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. provisional application entitled 12 mm X-Long Reach Spark Plug having Ser. No. 60/814,818 and filed on Jun. 19, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a spark plug for an internal combustion engine, furnace, or the like and, more particularly, toward a spark plug having improved mechanical and dielectric strength. 
     2. Related Art 
     A spark plug is a device that extends into the combustion chamber of an internal combustion engine, furnace or the like and produces a spark to ignite a mixture of air and fuel. Recent developments in engine technology are driving toward smaller engine displacement. At the same time, intake and exhaust valves are being enlarged for improved efficiency. The physical space reserved for the spark plug is being encroached upon by these changes. Combustion efficiencies are also dictating an increase in voltage requirements for the ignition system. These and other factors are urging the physical dimensions of a spark plug to ever-smaller scales, while demanding greater performance from the spark plug. Current industry demands call for high-performing spark plugs in the 10-12 mm range, with the expectation that these sizes will be further shrunk in the future. 
     A particular consideration when attempting to downsize a spark plug arises from the diminished dielectric capacity of the ceramic insulator in thin sections. Dielectric strength is generally defined as the maximum electric field which can be applied to the material without causing breakdown or electrical puncture. Thin cross-sections of ceramic insulator can therefore result in dielectric puncture between the charged center electrode and the grounded shell. 
     Another concern when attempting to downsize a spark plug is diminished mechanical strength resulting from the thinner cross-sections, especially in the ceramic insulator portion. One area in which reduced mechanical strength can be problematic is evidenced in the spark plug manufacturing processes which imposes large axial loads and mechanical stresses on the components. For example, when seating a fired-in suppressor seal inside an insulator and when crimping a shell to the exterior of the insulator, the ceramic material is placed under large stresses and compressive loads. These and other pre-use activities, including the step of installing a spark plug with high torque into a cylinder head, bring the mechanical stresses exerted on a modern spark plug to its yield limits. During use in an engine application, the spark plug is further subjected to mechanical stresses through engine vibration, combustion forces, and thermal gradients. For these reasons, the scaled reduction of a spark plug can push the stress carrying limits of its components to the failure point. 
     Accordingly, there is a need for an improved spark plug that can address both mechanical and dielectric strength limitations found in current regular, long, and extra-long reach spark plug designs subjected to downsizing efforts. 
     SUMMARY OF THE INVENTION 
     The subject invention overcomes the shortcomings and disadvantages found in prior art systems by providing a spark plug for a spark-ignited combustion event. The spark plug of this invention includes a generally tubular ceramic insulator. A conductive shell surrounds at least a portion of the ceramic insulator. The shell includes a ground electrode. A center electrode is disposed in the ceramic insulator and has a lower sparking end in opposing relation to the ground electrode, such that a spark gap is defined in the space therebetween. The ground electrode extends from an anchored end adjacent the shell to a distal end adjacent the spark gap. A metallic sparking tip is attached to the distal end of the ground electrode. The sparking tip has a convex dome and a rim surrounding the dome. The rim is disposed in surface-to-surface contact with the ground electrode. 
     The flattened rim feature of the metallic sparking tip configuration helps assure that the sparking arc occurs only on the metallic sparking tip feature, with little opportunity for rouge arcs to spark outside the metallic sparking tip which often occurs with prior art configurations. Furthermore, the flattened rim feature provides additional contact surface with the base metal of the ground electrode, thereby improving attachment techniques which may include resistance welding, laser welding, high temperature adhesives, mechanical fixation, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a cross-sectional view of a spark plug according to the subject invention; 
         FIG. 2  is an enlarged, fragmentary view of the spark gap region depicting a rimmed, hemispherical metallic sparking tip affixed to the ground electrode; 
         FIG. 3  is a view as in  FIG. 2 , but showing an alternative embodiment wherein the center electrode is likewise provided with a convex domed second metallic sparking tip; 
         FIGS. 4A-D  depict various prior art spark gap configurations including ground and center electrode features with and without precious metal sparking tip designs; 
         FIG. 5  is a view as in  FIG. 2 , and illustrating a conical sparking zone extending from the precious metal tip of the center electrode to the rimmed hemispherical metallic sparking tip of the ground electrode; 
         FIG. 6  is a view as in  FIG. 3 , depicting a generally linear or columnar sparking zone extending between the opposing rimmed hemispherical sparking tips of the center and ground electrodes; 
         FIG. 7  is an enlarged, realistic cross-sectional view taken generally along lines  7 - 7  in  FIG. 2 , with an optional laser welding machine illustratively depicted in phantom; 
         FIG. 8  is a fragmentary perspective view of the ground electrode including a rimmed hemispherical metallic sparking tip according to the invention; 
         FIG. 9  is a cross-sectional view taken longitudinally through the ceramic insulator of a spark plug according to the subject invention, and identifying various dimensional relationships important to some aspects of the subject invention; 
         FIG. 9A  is an enlarged, fragmentary view of the insulator transition surface highlighting the reference points at which the transition length L(transition) is measured between the rounded and filleted transitions; 
         FIG. 10  is a fragmentary cross-sectional view of the lower half of the ceramic insulator, and identifying further dimensional relationships important to some aspects of the subject invention; 
         FIG. 11  is a cross-sectional view taken generally along lines  11 - 11  of  FIG. 10 ; and 
         FIG. 12  is an enlarged, fragmentary cross-sectional view of the lower sparking end of the spark plug. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the figures, wherein like numerals indicate like or corresponding parts throughout the several views, a spark plug according to the subject invention is generally shown at  10  in  FIG. 1 . The spark plug  10  includes a tubular ceramic insulator, generally indicated at  12 , which is preferably made from aluminum oxide or other suitable material having a specified dielectric strength, high mechanical strength, high thermal conductivity, and excellent resistance to heat shock. The insulator  12  may be molded dry under extreme pressure and then kiln-fired to vitrification at high temperature. The insulator  12  has an outer surface which may include a partially exposed upper mast portion  14  to which a rubber spark plug boot (not shown) surrounds and grips to maintain a connection with the ignition system. The exposed mast portion  14  may include a series of ribs  16  to provide added protection against spark or secondary voltage flash-over and to improve grip with the rubber spark plug boot, or may be smooth as in  FIG. 9 . The insulator  12  is of generally tubular construction, including a central passage  18 , extending longitudinally between an upper terminal end  20  and a lower nose end  22 . The central passage  18  is of varying cross-sectional area, generally greatest at or adjacent the terminal end  20  and smallest at or adjacent the nose end  22 . 
     An electrically conductive, preferably metallic, shell is generally indicated at  24 . The shell  24  surrounds the lower regions of the insulator  12  and includes at least one ground electrode  26 . While the ground electrode  26  is depicted in the traditional single L-shaped style, it will be appreciated that multiple ground electrodes of straight or bent configuration can be substituted depending upon the intended application for the spark plug  10 . 
     The shell  24  is generally tubular in its body section and includes an internal lower compression flange  28  adapted to bear in pressing contact against a small lower shoulder  68  of the insulator  12 . The shell  24  further includes an upper compression flange  30  which is crimped or formed over during the assembly operation to bear in pressing contact against a large upper shoulder  66  of the insulator  12 . A buckle zone  32  collapses under the influence of an overwhelming compressive force during or subsequent to the deformation of the upper compression flange  30  to hold the shell  24  in a fixed position with respect to the insulator  12 . Gaskets, cement, or other sealing compounds can be interposed between the insulator  12  and shell  24  to perfect a gas-tight seal and to improve the structural integrity of the assembled spark plug  10 . 
     The shell  24  is provided with a tool receiving hexagon  34  for removal and installation purposes. The hex size complies with industry standards for the related application. Of course, some applications may call for a tool receiving interface other than hexagon, such as is known in racing spark plug applications and in other environments. A threaded section  36  is formed at the lower portion of the metallic shell  24 , immediately below a seat  38 . The seat  38  may be paired with a gasket  39  to provide a suitable interface against which the spark plug  10  seats in the cylinder head. Alternatively, the seat  38  may be designed with a taper to provide a self-sealing installation in a cylinder head designed for this style of spark plug. 
     An electrically conductive terminal stud  40  is partially disposed in the central passage  18  of the insulator  12  and extends longitudinally from an exposed top post to a bottom end embedded part way down the central passage  18 . The top post connects to an ignition wire (not shown) and receives timed discharges of high voltage electricity required to fire the spark plug  10 . 
     In the example illustrated in  FIG. 1 , the bottom end of the terminal stud  40  is embedded within a conductive glass seal  42 , forming the top layer of a composite suppressor-seal pack. The conductive glass seal  42  functions to seal the bottom end of the terminal stud  40  to a resistor layer  44 . This resistor layer  44 , which comprises the center layer of the 3-tier suppressor-seal pack, can be made from any suitable composition known to reduce electromagnetic interference (“EMI”). Depending upon the recommended installation and the type of ignition system used, such resistor layers  44  may be designed to function as a more traditional resistor-suppressor or, in the alternative, as an inductive-suppressor. Immediately below the resistor layer  44 , another conductive glass seal  46  establishes the bottom or lower layer of the suppressor-seal pack. Accordingly, electricity from the ignition system travels through the bottom end of the terminal stud  40  to the top layer conductive glass seal  42 , through the resistor layer  44 , and into the lower conductive glass seal layer  46 . 
     A conductive center electrode  48  is partially disposed in the central passage  18  and extends longitudinally from its head encased in the lower glass seal layer  46  to its exposed sparking end  50  proximate the ground electrode  26 . The head seats in a necked-down section of the central passage  18 . The suppressor-seal pack electrically interconnects the terminal stud  40  and the center electrode  48 , while simultaneously sealing the central passage  18  from combustion gas leakage and also suppressing radio frequency noise emissions from the spark plug  10 . The suppressor-sealed pack, however, may be substituted with other passive or active features depending upon the requirements of an intended application. As shown, the center electrode  48  is preferably a one-piece structure extending continuously and uninterrupted between its head and its sparking end  50 . However, other design arrangements may be used. 
     A second metallic sparking tip  52  is located at the sparking end  50  of the center electrode  48 . (To avoid any confusion, it is noted that a “first” metallic sparking tip will be introduced and described subsequently in connection with the ground electrode  26 .) The second metallic sparking tip  52  provides a sparking surface for the emission of electrons across a spark gap  54 . The second metallic sparking tip  52  for the center electrode  48  can be made according to any of the known techniques, including the loose piece formation and subsequent detachment of a wire-like or rivet-like construction made from any of the known precious metal or high performance alloys including, but not limited to, platinum, tungsten, rhodium, yttrium, iridium, and alloys thereof. Additional alloying elements may include, but are not limited to, nickel, chromium, iron, carbon, manganese, silicon, copper, aluminum, cobalt, rhenium, and the like. In fact, any material that provides good erosion and corrosion performance in the combustion environment may be suitable for use in the material composition of the second metallic sparking tip  52 . 
     The ground electrode  26  extends from an anchored end adjacent the shell  24  to a distal end adjacent the sparking gap  54 . The ground electrode  26  may be of the typical rectangular cross-section, including an iron-based alloy jacket surrounding a copper core. 
     As perhaps best shown in  FIG. 2 , a (first) metallic sparking tip, generally indicated at  56 , is attached to the distal end of the ground electrode  26 , opposing the sparking end  50  of the center electrode  48 . I.e., the metallic sparking tip  56  is located directly across the spark gap  54 . The metallic sparking tip  56  is intentionally shaped with a rimmed, hemispherical configuration such that it presents a convex dome  58  surrounded by a rim  60 . As viewed in profile like in  FIG. 2 , the shape of the metallic sparking tip  56  can be likened to a fried egg, with the convex dome portion  58  representing the yolk of the analogous egg and the rim portion  60  representing the egg white. Preferably, the rim  60  has a generally annular configuration, although non-annular configurations are also possible. Ideally, although again not necessarily, the convex dome portion  58  and rim  60  are generally aligned with one another along an imaginary central axis intersecting the middle of the spark gap  54 . 
     As with the second metallic sparking tip  52 , the (first) metallic sparking tip  56  for the ground electrode  26  can be made according to any of the known techniques, including the loose piece formation into a button-like construction made from any of the known precious metal or high performance alloys including, but not limited to, platinum, tungsten, rhodium, yttrium, iridium, and alloys thereof. Additional alloying elements may include, but are not limited to, nickel, chromium, iron, carbon, manganese, silicon, copper, aluminum, cobalt, rhenium, and alike. In fact, any material that provides good erosion and corrosion performance in the combustion environment may be suitable for use in the material composition of the metallic sparking tip  56 . 
       FIG. 3  represents an alternative embodiment of the invention, wherein the center electrode  48  is fitted with a second metallic sparking tip  52 ′ having a rimmed hemispherical configuration substantially similar to that of the (first) metallic sparking tip  56  attached to the ground electrode  26 . 
       FIGS. 4A-D  depict various prior art configurations for the spark gap  54  between ground and center electrodes. In each example of the prior art, the ground electrode is represented by the letters “GE,” whereas the center electrode is represented by the letters “CE.”  FIG. 4A  illustrates a typical spark gap  54  configuration, wherein neither the center electrode CE nor ground electrode GE are fitted with metallic sparking tips. In this configuration, electrical potential carried through the center electrode CE arcs through a “zone” of the spark gap  54  to the base material of the ground electrode, which typically comprises a durable, nickel based alloy frequently cored with copper for thermal transmission purposes. In other words, all electrical arcing from the center electrode CE to the ground electrode GE occurs in the spark gap  54 . 
       FIGS. 4B-D  represent various prior art configurations where the ground electrode GE is fitted with a metallic sparking tip of either wide or narrow relative construction. An opposing metallic sparking tip on the center electrode CE may be matched or mismatched in terms of its dimensional attributes to the metallic sparking tip on the ground electrode GE. In all of these circumstances, it is common for electrical arcing to overshoot the precious metal pad of the sparking tip and directly land on the base material of the ground electrode GE. This is illustrated by a rogue electrical arc  62 . Rogue arcs  62  are common in the combustion environment, and result in inconsistent combustion with a measurable drop in combustion efficiency. As a result of this cycle-to-cycle variation in the ignition event, an automobile driver may feel the engine is running rough and/or its performance is perceived as inconsistent. Accordingly, rogue arcs  62  are highly undesirable. 
       FIGS. 5 and 6  illustrate the rimmed hemispherical metallic sparking tip  56  fitted to the ground electrode  26 . Whether the second metallic sparking tip  52  is of the conventional or modified ( 52 ′) design, it is illustrated in these figures how the hemispherical shape encourages the zone of normal spark arcing in the gap  54  to occur at a more consistent location from cycle-to-cycle as a result of the convex domed geometry. More consistent arc location, is of course desirable because it results in more consistent combustion. Lower cycle-to-cycle variation in the ignition event improves engine smoothness and consistency in performance. Rogue arcs  62  are markedly controlled through the flattened, flange-like rim  60  feature. Due to the corner profile represented by the extended outer periphery of the rim  60 , rogue arcs  62  are more readily attracted to the precious metal of the metallic sparking tip  56  with little tendency to overshoot the precious metal pad. Again, this results in more consistent combustion on a cycle-to-cycle basis. 
       FIG. 7  is a substantially enlarged cross-sectional view taken along lines  7 - 7  of  FIG. 2 , directly through a metallic sparking tip  56  and ground electrode  26 . This cross-sectional view illustrates yet another advantage of the rim feature  60 . Specifically, the rim  60  creates additional surface area lying in direct contact with the ground electrode  26 . As a result, better attachment, or fixation, of the metallic sparking tip  56  can be accomplished. Those of skill will readily envision different methods for attaching the metallic sparking tip  56  to the ground electrode  26 . In  FIG. 7 , the crater-like interface between the bottom of the metallic sparking tip  56  and the upper surface of the ground electrode  26  is suggestive of a resistance welding type operation. Resistance welding is one of many possible techniques which are improved through the increased surface-to-surface contact area between the metallic sparking tip  56  and the ground electrode  26 . In phantom, a laser welding device  64  is illustrated. The rim  60  feature has the added benefit of increasing the outer circumferential area of the metallic sparking tip  56 , thus in situations where a laser capping operation is carried out, there is a larger welding interface. Similar advantages are realized through the use of high temperature adhesives, mechanical fastening techniques, and the like. 
       FIG. 8  depicts the metallic sparking tip  56  in perspective form. The unique shape of the metallic sparking tip  56  can be formed in many ways, only a few of the possible ways mentioned here. As one example, a piece of precious metal wire can be severed from a spool, heated and then hot-headed into the characteristic fried egg shape. Alternatively, molten precious metal can be shaped in a rolling operation, casting operation, or in any other satisfactory method. 
     Numerous structural and geometric configurations of the insulator  12  may be used in the combination set forth herein or independently of one another so as to enhance the mechanical and dielectric characteristics of the resulting spark plug design. In addition to changes in the geometric designs and shapes of the insulator  12 , various design changes in the shape of the shell  24 , particularly in the lower nose region of the insulator  12 , further contribute to the improvements of the subject invention. For example, particular advantage can be identified through the relatively shallow transitional taper angle provided immediately below the large upper shoulder  66  of the insulator  12 . This relatively shallow angle reduces the compression stresses and lowers bending moment loads. 
       FIGS. 9 and 9A  depict an especially advantageous geometric configuration for the insulator  12  which enables traditional insulator materials (e.g., ceramics) to be manufactured in small, relatively fragile sizes yet withstand the stresses applied to the insulator during assembly and operation. More specifically, the insulator  12  is shown with its exterior surface presenting a generally circular large upper shoulder  66 , proximate the terminal end  20 , and a generally circular small shoulder  68 , proximate the nose end  22 . During assembly in the shell  24 , the small shoulder  68  seats against the lower compression flange  28 , whereas the large shoulder  66  is pressed by the upper compression flange  30  of the shell  24 . A very large compressive force is thus imposed on the insulator  12  in the regions between its large  66  and small  68  shoulders. Mechanically, it becomes very difficult to secure insulator  12  inside of a shell  24  when the size of the spark plug  10  is reduced to fit in small bore or tight fitting engine spaces. For example, spark plugs in the 10-12 millimeter and smaller ranges require the physical dimensions of its insulator  12  to be shrunk to limits where the column strength of the material simply will not support the compression loads which are required to establish and maintain gas-tight seals within the shell  24 . 
     The applicant has discovered a particularly advantageous geometric relationship that enables spark plugs  10  to be reduced in size without exceeding the mechanical strength of standard insulator materials such as ceramics. This is accomplished by manipulating the transition region defined as that portion of the exterior surface of the insulator  12  wherein the physical exterior dimensions of the insulator are reduced from the large shoulder  66  down to the small shoulder  68 . Again referring to  FIG. 9 , the exterior surface of the insulator  12  is shown including a rounded transition  74 , and spaced therefrom by a transition length L(transition) a filleted transition  76 . The terms “rounded” and “filleted” are borrowed from the well known references in drafting technology “fillets” and “rounds,” i.e., interior and exterior corners respectively. As viewed in profile, the rounded transition  74  and filleted transition  76  form something akin to an ogee profile which is necessary to effectively reduce the diameter of the exterior surface of the insulator  12 . As shown in  FIG. 9 , the rounded transition  74  is defined by a major diameter D 2  representing the maximum, outer diameter of the insulator  12  adjacent the large shoulder  66 . The filleted transition  76 , on the other hand, is defined by a minor diameter D 1  which represents that portion of the insulator  12  exterior leading toward the small shoulder  68 . The transition length L(transition) is a measurement of the longitudinal distance between the rounded  74  and filleted  76  transitions. 
       FIG. 9A  provides an enlarged view of the transition length L(transition), wherein takeoff measurements are located by the theoretical intersection between the transitioning surfaces. A frustaconically sloped transition surface  78  extends between the rounded  74  and filleted  76  transitions. Although a frustaconically tapering geometry is preferred for the transition surface  78 , other gently curving profiles may be tolerated without sacrificing the important features of this invention. 
     A particularly advantageous spatial relationship has been identified which provides the subject insulator  12  with remarkably sturdy mechanical strength so as to withstand the compressive stresses applied to the spark plug  10  during assembly and operation, as well as during handling of the insulator  12  during its formation and firing steps. Specifically, the relationship is established between D 1 , D 2  and the transition length L(transition). Preferably, this relationship is expressed according to the formula: 
     
       
         
           
             0.5 
             ≤ 
             
               
                 ( 
                 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   - 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
                 ) 
               
               
                 L 
                 ⁡ 
                 
                   ( 
                   transition 
                   ) 
                 
               
             
             ≤ 
             3.5 
           
         
       
     
     While acceptable results can be obtained through products made within this range of geometric relationships, the applicants have found that even more preferred results can be obtained by narrowing the ranges to the following formula: 
     
       
         
           
             0.55 
             ≤ 
             
               
                 ( 
                 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   - 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
                 ) 
               
               
                 L 
                 ⁡ 
                 
                   ( 
                   transition 
                   ) 
                 
               
             
             ≤ 
             1.2 
           
         
       
     
     For spark plugs manufactured in accordance with vehicular engine applications, the applicant has even defined a most preferred spatial relationship wherein: 
     
       
         
           
             0.6 
             ≤ 
             
               
                 ( 
                 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   - 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
                 ) 
               
               
                 L 
                 ⁡ 
                 
                   ( 
                   transition 
                   ) 
                 
               
             
             ≤ 
             0.8 
           
         
       
     
     Another improvement is achieved by decreasing the thickness of the nose portion of the insulator  12  so as to increase the air gap between the nose portion and the shell  24 . This increased air gap enhances the dielectric capacity, or dielectric strength, of the spark plug  10  in operation because of the high pressure air in this region during the spark event and during initiation of combustion. Furthermore, by reducing the thickness of the nose portion, a reduction or elimination in the tendency for spark tracking and creation of a secondary spark location is realized. 
     Further and favorable spatial relationships can be obtained through a reference to  FIGS. 10-12 . Here, it is illustrated that the nose portion of the insulator  12  has a base diameter d (base) measured immediately below the small shoulder  68 . The opposite, or distal end of the nose portion has a smaller outer diameter d (tip). Over the longitudinal length of the nose portion, the wall thickness of the insulator  12  tapers from the larger d (base) measure to the smaller d (tip) measure. It has been found that by carefully controlling the dimensional relationship between the outer diameters in this insulator nose region, relative to the inner diameter of the grounded shell ID (shell), advantages can be achieved in the areas of reduced spark tracking (i.e., surface charges which travel up the insulator nose), and increased space created for high-dielectric combustion gases which limit the tendency for arcing in small diameter spark plugs. More specifically, the applicant has identified the following spatial relationship as providing exceptionally beneficial spark plug performance: 
             0.5   ≤       (         d   ⁡     (   base   )       +     d   ⁡     (   tip   )         2     )     ÷     ID   ⁡     (   shell   )         ≤   0.7         
For spark plugs manufactured in accordance with vehicular engine applications, the applicant has even defined a most preferred spatial relationship wherein:
 
     
       
         
           
             0.57 
             ≤ 
             
               
                 ( 
                 
                   
                     
                       d 
                       ⁡ 
                       
                         ( 
                         base 
                         ) 
                       
                     
                     + 
                     
                       d 
                       ⁡ 
                       
                         ( 
                         tip 
                         ) 
                       
                     
                   
                   2 
                 
                 ) 
               
               ÷ 
               
                 ID 
                 ⁡ 
                 
                   ( 
                   shell 
                   ) 
                 
               
             
             ≤ 
             0.66 
           
         
       
     
     Yet another especially advantageous relationship can be achieved by controlling the insulator thickness in the region of the seal t (seal) pack to be as large as possible. This may require reducing the inner diameter 1 D (seal) space to provide greater dielectric capacity in this region. 
     In  FIG. 12 , the region of the lower compression flange  28  of the shell  24  is depicted in its abutment against the small shoulder  68  of the insulator  12 . Here, the lower compression flange  28  has an inner peripheral lip  80 . This lip  80  is spaced from the insulator  12  sufficiently so that combustion gases may occupy the space there between, thus enhancing the dielectric properties of the spark plug  10 . More specifically, it has been discovered that highly compressed combustion gases can exhibit a dielectric capacity which is greater than that of the ceramic insulator  12 . Thus, by enabling combustion gases to occupy this region of the spark plug  10 , wherein the grounded shell  24  is closest to the charge center electrode  48 , except in the spark gap  54 , additional dielectric capacity is highly desirable. 
     All of the features described herein are important and contribute, collectively, to a spark plug  10  to that can be manufactured in smaller geometric proportions without sacrificing mechanical integrity or sparking performance. 
     The subject invention as depicted in the accompanying drawings and described above addresses the mechanical and dielectric strength limitations found in the prior art spark plug designs and addresses the issues which arise with respect to demands placed upon spark plugs by newer engine designs. The subject spark plug reduces mechanical stress risers, increases flash-over distance, and reduces electrical stress fields to the elimination of sharp corners throughout the design. Obviously, many modifications and variations of this invention are possible in light of the above teachings. It is, therefore, to be understood that the invention may be practiced otherwise than as specifically described.