Patent Publication Number: US-2023143967-A1

Title: Spark plug

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 17/304,637 titled “SPARK PLUG,” filed Jun. 23, 2021, the entire disclosure of which is expressly incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a spark plug and, for example, to a spark plug for a spark-ignition (SI) engine. 
     BACKGROUND 
     An internal combustion engine powers a machine by converting chemical energy stored in fuel (e.g., gasoline, compressed natural gas (CNG), methanol, ethanol, bioethanol, or another type of fuel) into mechanical work. In such an engine, air is mixed with the fuel to form an air-fuel mixture. Some engines utilize a spark plug, which typically includes a central electrode and one or more outer electrodes. The spark plug may transmit an electric current along the central electrode into a chamber that is fluidly connected to or inside of a cylinder. A piston is movably mounted within the cylinder to travel in a cycle between a top dead center (TDC) position and a bottom dead center (BDC) position. In some embodiments, as the piston reaches the TDC position, a spark resulting from the electric current jumps a gap between the central electrode and the one or more outer electrodes, causing the air-fuel mixture to combust. A force of the combustion drives the piston down towards the BDC position, and the cycle repeats. Because the piston is connected to a drivetrain of the machine, continued movement of the piston propels and/or powers the machine. 
     While gaseous fuel (e.g., CNG, methanol, ethanol, bioethanol, and/or the like) is known to provide a relatively low power density, such fuel is also known to emit relatively low emissions. Thus, manufacturers have sought to produce engines that efficiently utilize such fuel. For example, to compensate for the relatively low power density provided by natural gas, manufacturers have developed CNG engines that operate under high compression ratios. Because of the high compression ratios, however, the combustion of the air-fuel mixture exposes certain engine components, such as a spark plug, to high temperatures and/or significant stress. As a result, the spark plug may be susceptible to premature wear, which may lead to increased costs associated with repair, replacement, and/or machine downtime. Furthermore, in some cases, the electrodes may wear unevenly, leading to a widening of a spark gap between the electrodes which prevents the electric current from bridging the spark gap. In such a case, in addition to the above-described costs, valuable material may also be wasted. 
     U.S. Pat. No. 10,145,292 discloses a spark plug including a pre-chamber for an engine. The spark plug includes a first cylindrical structure having a wall defining a bore. An electrode is positioned inside the bore such that the electrode is spaced apart from the wall to define at least one electrode spark gap. The spark plug further includes a second cylindrical structure configured to receive the first cylindrical structure. The second cylindrical structure has one or more access apertures configured to facilitate access to the wall of the first cylindrical structure. 
     The spark plug of the present disclosure solves one or more of the problems set forth above and/or other problems in the art. 
     SUMMARY 
     In some implementations, a spark plug includes a central electrode member that includes a base and a plurality of electrode prongs extending from the base, wherein the base is substantially centered on a longitudinal axis that extends through a geometric center of a first reference circle and a second reference circle, wherein the first reference circle has a first diameter, and the second reference circle has a second diameter that is greater than the first diameter by a gap length, an electrode prong, of the plurality of electrode prongs, includes an axial portion and a radial portion, wherein the axial portion includes an outer surface that partially defines the first reference circle, wherein the axial portion extends in an axial direction that is substantially parallel to the longitudinal axis, and axial portion has a width along a circumferential direction of the first reference circle and a thickness along a radial direction that is perpendicular to the axial direction, and the radial portion connects the axial portion to the base; and an outer electrode member that includes an interior surface that defines the second reference circle, and wherein 
     
       
         
           
             P 
             = 
             
               
                 
                   w 
                   2 
                 
                 ⁢ 
                 
                   l 
                 
               
               
                 t 
                 2.5 
               
             
           
         
       
     
     where P is a parameter having a value in a range of approximately 1.5 to approximately 7.5, w is the width in millimeters, l is the gap length in millimeters, and t is the thickness in millimeters. 
     In some implementations, a spark plug includes a central electrode member that includes: a central base, and six electrode prongs extending radially and axially from the central base; and an outer electrode member that is concentric with and surrounds the central electrode member, wherein the outer electrode member includes a wall that is radially spaced from the six electrode prongs to allow a series of electric arcs to form between the wall and the six electrode prongs; wherein the outer electrode member and the central electrode member are sized and positioned relative to one another such that a first rate of wear of the outer electrode member, along a longitudinal axis of the spark plug, is substantially equal to a second rate of wear of the central electrode member along the longitudinal axis. 
     In some implementations, a method includes activating a power system that includes a spark plug attached to a cylinder, the spark plug including: a central electrode member extending an initial length along a longitudinal axis, and an outer electrode member that is concentric with and surrounds the central electrode member, wherein the outer electrode member includes a wall that is radially spaced from the central electrode member to define a gap between the wall and the central electrode member; transmitting a pulse of electric current along the central electrode member to generate a spark in the gap between the central electrode member and the outer electrode member, wherein the spark causes an air-fuel mixture to combust within the cylinder, the central electrode member to shorten from the initial length along the longitudinal axis, and a concavity to develop in the wall of the outer electrode member; and repeating the transmitting until the central electrode member has shortened from the initial length by at least 1.5 millimeters to a reduced length. 
     In some implementations, a spark plug includes a housing defining a longitudinal axis, a first electrode having an electrode surface extending circumferentially around the longitudinal axis, and a second electrode including an electrode prong spaced from the electrode surface to form a spark gap between the first electrode and the second electrode. The electrode prong has a thickness t in a radial direction, a width w in a circumferential direction, and is spaced from the electrode surface a gap length l of the spark gap in a radial direction. Further, t, w, and l together define a parameter P having a value according to the equation 
     
       
         
           
             P 
             = 
             
               
                 
                   w 
                   2 
                 
                 ⁢ 
                 
                   
                     l 
                     1 
                   
                 
               
               
                 t 
                 2.5 
               
             
           
         
       
     
     from approximately 1.5 to approximately 7.5. 
     In some implementations, a prechamber spark plug includes a housing having formed therein a combustion prechamber and a flow passage from the combustion prechamber. The spark plug further includes a first electrode, and a second electrode having a plurality of electrode prongs. A plurality of spark gaps are defined between each one of the plurality of electrode prongs and the first electrode. Each of the plurality of electrode prongs has a thickness t and a width w and is spaced from the first electrode a gap length at a respective one of the plurality of spark gaps. Further, t, w, and l together define a parameter P according to the equation 
     
       
         
           
             P 
             = 
             
               
                 
                   w 
                   2 
                 
                 ⁢ 
                 
                   
                     l 
                     1 
                   
                 
               
               
                 t 
                 2.5 
               
             
           
         
       
     
     having a value from approximately 1.5 to approximately 7.5. 
     In some implementations, a spark electrode assembly includes a first electrode having an electrode surface extending circumferentially around a longitudinal axis, and a second electrode including an electrode prong supported at a fixed location relative to the electrode surface. A spark gap is formed between the electrode surface and the electrode prong. The electrode prong has a size defined by a thickness t and a width w, and is positioned at a gap length l of the spark gap from the electrode surface that is based on a direct exponential relation to t and an inverse exponential relation to w, such that axial wear rates of the electrode surface and the electrode prong are substantially equal. 
     In some implementations, a method of making a spark plug includes placing a first electrode and a second electrode at a fixed position and orientation relative to one another, and forming, by way of the placing a first electrode and a second electrode, a spark gap between an electrode surface of the first electrode extending circumferentially around a longitudinal axis and an electrode prong of the second electrode. The method further includes establishing, by way of the forming a spark gap, a gap length of the spark gap in inverse relation to a width of the electrode prong, and in direct relation to a thickness of the electrode prong. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an example power system. 
         FIG.  2    is a side view of an example spark plug of the engine system. 
         FIG.  3    is a cross-sectional view of the spark plug in an initial state, taken along lines A-A of  FIG.  2   . 
         FIG.  4    is a cross-sectional view of the spark plug in the initial state, taken along lines B-B of  FIG.  2   . 
         FIG.  5    is a cross-sectional view of the spark plug in a final state, taken along lines A-A of  FIG.  2   . 
         FIG.  6    is a cross-sectional view of the spark plug in the final state, taken along lines B-B of  FIG.  2   . 
         FIG.  7    is a cross-sectional view of a part of a spark plug, according to another embodiment. 
         FIG.  8    is another cross-sectional view of the spark plug as in  FIG.  7   . 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to a spark plug, which is applicable to spark-ignition (SI) engines (e.g., a compressed natural gas (CNG)-powered engine, a methanol-powered engine, an ethanol-powered engine, a bioethanol-powered engine, a gasoline-powered engine, a gaseous hydrogen-powered engine, or another type of SI engine employing any of a variety of liquid fuels or gaseous fuels including blends) and/or systems including SI engines. Such engines and/or engine systems may be implemented in a machine, such as a generator, a movable machine (e.g., a motor vehicle, a railed vehicle, a watercraft, an aircraft), or another type of machine. 
     To simplify the explanation below, the same reference numbers may be used to denote like features. The drawings may not be to scale. 
       FIG.  1    depicts a power system  100 . The power system  100  includes an air inlet  102 , a fuel tank  104 , an ignition system  106 , an engine  108 , and an exhaust system  110 . The air inlet  102  is a structure that is configured to receive and route air toward the engine  108 . The fuel tank  104  is a structure that is configured to receive and distribute fuel (e.g., CNG, methanol, ethanol, bioethanol, gasoline, or another type of fuel) toward the engine  108  to mix with the air to form an air-fuel mixture. The ignition system  106  is a system that is configured to initiate a combustion of the air-fuel mixture in the engine  108 . The ignition system  106  includes an electrical energy source  112 , such as an ignition coil, that is electrically coupled to the engine  108 . In some implementations, the ignition system  106  may further include one or more other electrical devices that are configured to control and/or communicate with the engine  108 , such as an electronic control unit. 
     The engine  108  is a device that is configured to convert chemical energy stored in the fuel into mechanical work (e.g., by driving a crankshaft). The engine  108  includes an engine block  114 , at least one inlet valve  116 , a piston  118 , a spark plug  120 , and at least one outlet valve  122 . The engine block  114 , which includes at least one cylinder  124  and a cylinder head  126 , houses the inlet valve  116 , the piston  118 , the spark plug  120 , and the at least one outlet valve  122 . The at least one inlet valve  116  is a mechanism that is configured to selectively permit the air-fuel mixture to enter the cylinder  124 , which drives the piston  118  downward toward a bottom dead center (BDC) position. The piston  118  is a device that is movable within the cylinder  124  in a continuous cycle between the BDC position and a top dead center (TDC) position to propel and/or power a machine. During such movement, the piston  118  compresses the air-fuel mixture. The spark plug  120 , which is mounted to a bore  128  within the cylinder head  126  above the cylinder  124 , is a device that is configured to transmit an electric current from the electrical energy source  112  to cause the compressed air-fuel mixture to combust. A force of the combustion drives the piston  118  back down toward the BDC position. The at least one outlet valve  122  is a mechanism that is configured to selectively permit exhaust gas, resulting from combustion, to be expelled from the cylinder  124  as the piston  118  moves back to the TDC position. 
     The exhaust system  110  is a system, positioned downstream of the engine  108 , that is configured to reduce or remove emission compounds (e.g., nitrous oxides (NOx), particulate matter, and/or hydrocarbons) from the exhaust gas to satisfy emission standards. For example, the exhaust system  110  may include a diesel particulate filter (DPF) (e.g., to treat the particulate matter), a selective catalytic reduction (SCR) module (e.g., to treat the NOx), and/or a diesel oxidation catalyst (DOC) (e.g., to treat the hydrocarbons). 
     As indicated above,  FIG.  1    is provided as an example. Other examples may differ from what is described with regard to  FIG.  1   . For example, the number and arrangement of components (e.g., the air inlet  102 , the fuel tank  104 , the ignition system  106 , the engine  108 , and/or the exhaust system  110 ) may differ from that shown in  FIG.  1   . Thus, there may be additional components, fewer components, different components, differently shaped components, differently sized components, and/or differently arranged components than those shown in  FIG.  1   . 
       FIGS.  2 - 6    depict the spark plug  120 . The spark plug  120  may include a prechamber spark plug as further discussed herein. As will also be explained below,  FIGS.  3 - 6    depict internal components of the spark plug  120  in different states of wear. In particular,  FIGS.  3 - 4    depict the internal components of the spark plug  120  in an initial (e.g., unworn) state.  FIGS.  5 - 6    depict the internal components of the spark plug  120  in a final (e.g., substantially worn) state. 
     The spark plug  120  includes a body  202  and a nozzle assembly  204  secured thereto. The body  202  includes an insulator  206  and a central conductor  208 . The insulator  206 , which may be made of ceramic or another type of electrically-insulating material, is configured to electrically isolate the central conductor  208  and maintain structural integrity of the spark plug  120  in a high temperature environment. The insulator  206  includes an upper end surface  210 , a lower end surface  302 , and an exterior surface  212  that connects the upper end surface  210  to the lower end surface  302 . The upper end surface  210  includes an upper opening  214 , and the lower end surface  302  includes a lower opening  304  that communicates with the upper opening  214  to define a through hole  306 . The exterior surface  212 , which may be substantially cylindrical in shape, includes a plurality of annular ribs  216  and a flange  308 . The plurality of annular ribs  216  are arranged at a location proximate to the upper end surface  210  and are configured to mitigate grounding of the electric current traveling through the spark plug  120 . The flange  308 , which is arranged at a location proximate to the lower end surface  302 , is shaped and sized to facilitate attachment of the insulator  206  to the nozzle assembly  204 . 
     The central conductor  208  is a series of electrical conductors which are sequentially arranged along a longitudinal axis  218  of the spark plug  120  and are together electrically connected to transmit the electric current from the electric energy source  112  into the nozzle assembly  204 . The series of electrical conductors include a terminal connector  220 , a terminal pin  310 , and a central electrode member  312 . The terminal connector  220  is a conductive component that is mounted to the upper end surface  210  of the insulator  206  and is configured to be connected to a wire extending from the electrical energy source  112 . The terminal connector  220  may be, for example, made of a nickel alloy. The terminal pin  310  is an elongated conductive element that is received in and extends along the through hole  306  of the insulator  206  to connect the terminal connector  220  to the central electrode member  312 . The terminal pin  310  may be, for example, made of steel. 
     The central electrode member  312  is a conductive component that is sized and arranged to interact with an outer electrode member  222  (described below) to generate an electric arc or spark within the nozzle assembly  204  to cause the air-fuel mixture to combust within the cylinder  124 . The central electrode member  312 , which may be made of a material such as an iridium alloy or a platinum alloy, includes a central base  314  and a plurality of electrode prongs  316  extending therefrom. The central base  314  is secured within the through hole  306  and protrudes from the lower opening  301  of the insulator  206 . As shown in  FIG.  4   , the central base  314  is substantially centered on the longitudinal axis  218 , which extends through a geometric center of a first reference circle  402 . The plurality of electrode prongs  316 , which may be substantially identical to one another, may include six electrode prongs, five electrode prongs, four electrode prongs, or another quantity of electrode prongs. Other arrangements of the plurality of electrode prongs  316  are contemplated. For example, the plurality of electrode prongs  316  may form an equiangular arrangement. 
     Each of the plurality of electrode prongs  316  (hereinafter referred to as the electrode prong  316 ) includes an axial portion  318  and a radial portion  320  that connects the axial portion  318  to the central base  314 . The axial portion  318  extends in an axial direction and includes an outer surface  322  that defines a width w of the electrode prong  316  and partially defines the first reference circle  402 . In other words, the outer surfaces  322  of the axial portions  318  lie on the first reference circle  402 . The axial direction is substantially parallel to the longitudinal axis  218 . The radial portion  320  extends in a radial direction that is substantially perpendicular to the longitudinal axis  218 . In some implementations, at least a portion of the radial portion  320  may be curved and thus extend at an acute angle relative to the radial direction. As will be described below, the electrode prong  316  is sized and positioned, relative to the nozzle assembly  204 , in such a way that extends a service life of the spark plug  120 . For reference, the electrode prong  316  further includes a thickness t that is substantially perpendicular to the width w. 
     The nozzle assembly  204  includes a housing  224 , a gasket  226 , and an outer electrode member  222 . The housing  224 , which may be made of carbon steel, is configured to be secured to the exterior surface  212  of the insulator  206 . The housing  224  includes a first protruding segment  228 , a second protruding segment  230 , and a connection segment  232  therebetween. The first protruding segment  228  includes a first upper surface  234 , a first lower surface  236 , a first outer surface  238 , and a first inner surface  330 . The first upper surface  234  is opposite to the first lower surface  236 . The first outer surface  238 , which is opposite to the first inner surface  324 , includes an engagement portion  240  that is configured to be engaged by a tool or otherwise engaged to facilitate attachment of the spark plug  120  to the cylinder head  126 . For example, the engagement portion  240  may include a hex protrusion that is configured to be rotated by a wrench. The first inner surface  324  is configured to be secured to the flange  308  of the insulator  206 . 
     The second protruding segment  230  includes a second upper surface  242 , a second lower surface  244 , a second outer surface  246 , and a second inner surface  326 . The second upper surface  242  faces the first lower surface  236  and is opposite to the second lower surface  244 . The second outer surface  246  includes external threads to facilitate threadably attaching the spark plug  120  to the bore  128  within the cylinder head  126  to position the outer electrode member  222  within the cylinder  124 . The connection segment  232  is sized to improve sealing of the bore  128 . For example, the connection segment  232  may have a relatively increased length in a range of approximately 5 millimeters (mm) to approximately 6 mm. 
     The gasket  226  is an annular sealing component that is configured to be secured to the first lower surface  236  of the housing  224  to seal the bore  128  of the cylinder head  126 . To resist creep, the gasket  226  may be made of INCONEL® or a similar type of material. In other words, the gasket  226  may be configured to mitigate the potential of deformation due to exposure to mechanical stresses associated with the combustion process. 
     The outer electrode member  222  is a conductive component that is configured to interact with the central electrode member  312  to generate the electric arc therebetween. When attached to the housing  224  of the spark plug  120 , as described below, the outer electrode member  222  is concentric with and surrounds the central electrode member  312 . The outer electrode member  222 , which may be made of a nickel alloy, a platinum alloy, or an iridium alloy, includes a side wall  248  and a bottom wall  250 . The side wall  248  includes an exterior surface  252  and an interior surface  328  that is opposite to the exterior surface  252 . The exterior surface  252  includes a first exterior axial portion  330 , a second exterior axial portion  254 , and a radial portion  332  extending therebetween. The first exterior axial portion  330  is configured to be attached (e.g., via welding, soldering, and/or the like) to the second inner surface  326  of the housing  224 . The second exterior axial portion  254 , which has a diameter that is substantially equal to a diameter of the second outer surface  246  of the housing  224 , includes a plurality of exterior openings  256 . The radial portion  332  is configured to be attached (e.g., via welding, soldering, and/or the like) to the second lower surface  244  of the housing  224 . 
     The interior surface  328  of the outer electrode member  222  is configured to be radially spaced from the outer surfaces  322  of the axial portions  318  of the plurality of electrode prongs  316 . The interior surface  328  includes a first interior axial portion  334  and a second interior axial portion  336 , which may be substantially cylindrical in the initial state of the spark plug  120 . The first interior axial portion  334  is opposite to the first exterior axial portion  330  of the side wall  248 . The second interior axial portion  336 , which is opposite to the second exterior axial portion  332  and of side wall  248 , includes a plurality of interior openings  338  that fluidly communicate with the plurality of exterior openings  256  to define a respective plurality of side wall flow passages  258 . 
     When the spark plus  120  is in the initial state, the interior surface  328  of the outer electrode member  222  defines a second reference circle  404 . In other words, when the spark plug  120  is unworn, both the first interior axial portion  334  and the second interior axial portion  336  lie on the second reference circle  404 . The second reference circle  404  has a diameter that is greater than a diameter of the first reference circle  402  by an initial length l 1  of a gap  340 , across which the electric current extends to form the electric arc. 
     The bottom wall  250  of the includes an upper surface  342 , which has an upper opening  344 , and a lower surface  260 , which has a lower opening  346 . The lower opening  346  fluidly communicates with the upper opening  344  to define a bottom wall flow passage  348 . Together with the plurality of side wall flow passages  258 , the bottom wall flow passage  348  is configured to permit the air-fuel mixture to flow into a combustion prechamber or “pre-combustion chamber”  350  formed by a combination of the insulator  206 , the housing  224 , and the outer electrode member  222 . 
     As implemented within the power system  100 , the spark plug  120  has a limited service life due to erosion of the central electrode member  312  and the outer electrode member  222 . Based on activating the power system  100 , the air-fuel mixture may flow into the pre-combustion chamber  350  through the plurality of side wall flow passages  258  and the bottom wall flow passage  348  as the piston  118  travels upward toward the TDC position to compress the air-fuel mixture. The electrical energy source  112  transmits a pulse of electric current, which travels along the central conductor  208  and enters the pre-combustion chamber  350  as the piston  118  approaches a desired position. Because the voltage of the electric current exceeds a dielectric strength of the air-fuel mixture, the electric current bridges the gap  340  between the central electrode member  312  and the outer electrode member  222 . With the air-fuel mixture ionized by the electric current, a spark is generated to ignite an ignition charge of fuel and air in the pre-combustion chamber  350  that triggers ignition of a main charge of fuel and air within the cylinder  124 . As the engine  108  continues to operate, the spark plug  120  will continue to generate sparks between the central electrode member  312  and the outer electrode member  222 , which exposes the central electrode member  312  and the outer electrode member  222  to extreme temperatures and pressures within pre-combustion chamber  350 . Due at least in part to such exposure, the plurality of electrode prongs  316  of the central electrode member  312  experience particle ejection and surface oxidation, which causes the plurality of electrode prongs  316  to gradually shorten along the longitudinal axis  218  until reaching the final state shown in  FIGS.  5 - 6   . At the same time, the first interior axial portion  334  of the interior surface  328  likewise experiences particle ejection and surface oxidation, which causes a concavity  502  to develop in the interior axial portion  334  and thus increases a length of the gap  340 . As the plurality of electrode prongs  316  shorten, the concavity  502  correspondingly elongates along the longitudinal axis  218  until likewise reaching the final state. When the spark plug  120  is in the final state, which marks an end of the service life of the spark plug  120 , the pulses of electric current are no longer able to bridge the gap  340 , which has increased in size from the initial length l 1  (shown in  FIGS.  3 - 4   ) to a final length l 2  (as shown in  FIGS.  5 - 6   ). In the final state, the plurality of electrode prongs  316  may have a reduced length that is less than an initial length of the plurality of electrode prongs  316  by at least 1.5 mm. 
     In order to function as described above, the central electrode member  312  and the outer electrode member  222  are sized and positioned relative to one another such that a rate of shortening or “wear rate” of the plurality of electrode prongs  316  is substantially equal to a rate of elongation of the concavity  502 . In other words, based on the series of electric arcs extending through the air-fuel mixture within the pre-combustion chamber  350 , the central electrode member  312  and the outer electrode member  222  are configured to wear at a substantially uniform rate along the longitudinal axis  218 . To achieve this substantially uniform rate of wear, the central electrode member  312  and the outer electrode member  222  are sized and arranged such that there is an inverse relationship between the width w of the electrode prong  316  and the initial length l 1  of the gap  340 . In some implementations, such a relationship may be represented by the Equation 1: 
     
       
         
           
             P 
             = 
             
               
                 
                   w 
                   2 
                 
                 ⁢ 
                 
                   
                     l 
                     1 
                   
                 
               
               
                 t 
                 2.5 
               
             
           
         
       
     
     where P is a parameter having a value in a range of approximately 1.5 to approximately 7.5, w is the width of an electrode prong  316  in mm, l 1  is the initial length of the gap  340  in mm, and t is the thickness of the electrode prong  316  in mm. In some implementations, the value of the parameter P may be in a range of approximately 2.25 to approximately 2.75. In some implementations, the value of the parameter P may be in a range of approximately 4.5 to approximately 5.5. Other values are herein contemplated. 
     As indicated above,  FIGS.  2 - 6    are provided as an example. Other examples may differ from what is described with regard to  FIGS.  2 - 6   . For example, the number and arrangement of components may differ from that shown in  FIGS.  2 - 6   . Thus, there may be additional components, fewer components, different components, differently shaped components, differently sized components, and/or differently arranged components than those shown in  FIGS.  2 - 6   . For example, the outer electrode member  222  may include a different arrangement and/or quantity of flow passages (e.g., one flow passage, two flow passages, or another quantity). 
     Turning now to  FIGS.  7  and  8   , there are shown views of portions of a spark plug  420  according to one embodiment. The spark plug  420  has similarities to other embodiments described herein and, absent explanation to the contrary, can be understood to function generally analogously to such other embodiments in an ignition system in an internal combustion engine system. For instance, the components shown in  FIGS.  7  and  8    are part of a spark plug including a nozzle assembly  424  that could be swapped for some or all of a nozzle assembly as disclosed in connection with other embodiments. Moreover, except where otherwise indicated or apparent from the context description and discussion herein of any feature and/or functionality of any one embodiment can be understood to refer to features and/or functionality of any other embodiment. 
     The spark plug  420  includes a housing  422  defining a longitudinal axis  426 . The housing  422  includes a tip piece  430  forming a pre-combustion chamber or combustion prechamber and one or more openings  434  from the combustion prechamber  428  functionally analogous to openings described elsewhere herein in connection with other embodiments. It will be noted the openings  434  may be angularly oriented relative to the longitudinal axis  426 . The spark plug  420  might include a set of angularly oriented openings  434  spaced circumferentially around the longitudinal axis  426  and additionally or alternatively also include a centrally located end opening as depicted in the other embodiments and/or radially oriented openings. The present disclosure contemplates at least one opening, and any number of openings in any suitable arrangement, to fluidly connect a combustion prechamber to a cylinder in an engine. 
     The spark plug  420  may include a nozzle assembly  424  formed by the tip piece  430 , a housing body piece  436 , and electrode components to be described, again generally analogous to foregoing embodiments. A spark electrode assembly  438  is located in the housing  422  and includes a first electrode  440  having an electrode surface  442  extending circumferentially around the longitudinal axis  426 . The first electrode  440  may include an inner electrode having a base  464  supported in an insulator  468  in some embodiments. The first electrode  440  extends into the prechamber  428  to a terminal electrode tip  462 . As illustrated, the first electrode  440  may be centrally located in the spark plug  420  and generally extends axially along the longitudinal axis  426 . Embodiments where an inner electrode is offset in a radial direction from a longitudinal axis defined by a spark plug housing would nevertheless fall within the scope of the present disclosure. 
     The spark electrode assembly  438  further includes a second electrode  444 . Second electrode  444  may be electrically connected to the housing  422 , such as by direct physical attachment or unattached but direct physical and electrical contact with tip piece  430 . The second electrode  444  may be one of a plurality of second electrodes each including an electrode prong  446 . The electrode prongs  446  may be spaced circumferentially around the longitudinal axis  426 . The electrode prongs  446  may also be understood as spaced circumferentially around the first electrode  440  and the electrode surface  442 . The electrode prongs  446  may be substantially identical to one another, and thus description and discussion herein of a second electrode or an electrode prong in the singular will be understood by way of analogy to refer to any of a plurality of second electrodes and/or electrode prongs. 
     The electrode prong  446  is spaced from the electrode surface  442  to form a spark gap  448  between the first electrode  440  and the second electrode  444 . It will be appreciated a plurality of spark gaps are formed between the first electrode  440  and the second electrode  444 /electrode prongs  446 . In an embodiment, a number of the electrode prongs  446  is 4 or greater. In some implementations, a number of the electrode prongs  446  is 5 or greater, and potentially 6 or greater as in the depiction of  FIGS.  7  and  8    where a total of 8 electrode prongs of the second electrode  444  are provided. 
     It will be appreciated that in the embodiment of  FIGS.  7  and  8    the second electrode  444  includes an outer electrode that is radially outward of the first electrode  440 . The tip piece  430  includes an inner surface  431  forming the prechamber  428  and extending circumferentially around the longitudinal axis  426  at a location radially outward of the first electrode  440  and the second electrode  444 . The inner surface  431  may extend continuously circumferentially around the longitudinal axis  426 . The electrode surface  442  may likewise extend continuously circumferentially around the longitudinal axis  426 . The electrode prong  446  may also include a radial portion  454  extending radially inward from the tip piece  430  in a direction of the first electrode  440  and generally along a radius of a circle defined by the longitudinal axis  426 . The electrode prong  446  may also include an axial portion  456  extending generally along the electrode surface  442  in an axial direction. A bend section  458  of the electrode prong  446  connects between the radial portion  454  and the axial portion  456 . The axial portion  456  is radially inward of the radial portion  454  and extends to a terminal end or electrode tip  452 . The electrode tip  452  may be spaced an offset distance  460  in an axial direction from the tip  462  of the first electrode  440 . Put differently, the first electrode  440  may extend in an axially outward direction along the longitudinal axis  426  a greater distance than does the electrode prong  446 . 
     As noted above, the spark plug  420  has similarities to the other embodiments discussed herein. It will be recalled the central electrode in the embodiments of  FIGS.  2 - 6    includes electrode prongs that form spark gaps together with an electrode surface of an outer electrode member. In the case of the embodiment of  FIGS.  7  and  8   , the first, inner electrode  440  is surrounded by the electrode prongs  446  of the second, outer electrode  444 . The geometry of the electrode prongs  446  may nevertheless be similar or substantially identical in certain respects to that of the electrode prongs of the spark plug  120 . 
     The electrode prong  446  has a thickness tin a radial direction, a width w in a circumferential direction, and is spaced from the electrode surface  442  a gap length b of the spark gap  448  in the radial direction. Moreover, t, w, and b together define a parameter P having a value from approximately 1.5 to approximately 7.5 according to Equation 1 set forth above. In some implementations, P has a value from approximately 4.5 to approximately 5.5. In other implementations, P has a value from approximately 2.25 to approximately 2.75. The value of P may thus be from approximately 2.25 to approximately 5.5 in some embodiments, and having a value in a range having as a lower limit approximately 2.5 and/or having as an upper limit approximately 5.5. In some implementations w is greater than or equal to t. The electrode prong  446  may have a substantially uniform width and substantially uniform thickness throughout, and typically at least within axial portion  456 . 
     The spark plug  420  can be operated with either of a first polarity or a second polarity. In one implementation the first electrode  440  is a cathode and the second electrode  440  is an anode, with the electrical energy source  112  structured to energize a spark control circuit such that charge flows from the first electrode  440  to the second electrode  440 . In other implementations, the electrical energy source  112  is structured to energize a spark control circuit such that charge flows from the electrode prongs  446  of the second electrode  440  to the electrode surface  442  of the first electrode  440  with the first electrode  440  being the anode and the second electrode  440  being the cathode. The latter implementation can be associated with further extended service life as compared to certain known strategies. 
     According to the present disclosure, and as will be appreciated from the above Equation 1, the electrode prong  446  has a size defined by the thickness t and the width w, and is positioned at the gap length l 1  of the spark gap  448  from the electrode surface  442  that is based on a direct exponential relation to t and an inverse exponential relation tow, such that axial wear rates of the electrode surface  442  and the electrode prong  446  are substantially equal. Principles of the equal wear rates as applicable to all embodiments of the present disclosure are further discussed herein. 
     INDUSTRIAL APPLICABILITY 
     Referring to the drawings and various embodiments generally, but by exemplary reference to  FIGS.  2 - 6   , the spark plug  120  of the present disclosure is particularly applicable within the engine  108  of the power system  100 . The engine  108  may be configured to utilize fuel (e.g., CNG, methanol, ethanol, bioethanol, gasoline, and/or the like) to power a generator, propel a movable machine (e.g., a motor vehicle, a railed vehicle, a watercraft, an aircraft), and/or the like. In contrast to spark plugs of the prior art, in which electrodes tend to wear unevenly and thus waste material that might otherwise have been utilized to generate additional sparks, the spark plug  120  of the present disclosure is configured such that the central electrode member  312  wears along the longitudinal axis  218  at a rate that is substantially equal to that of the outer electrode member  222 . As a result, the spark plug  120  has an extended service life compared to spark plugs of the prior art, with the central electrode member  312  being configured to shorten by at least 1.5 mm along the longitudinal axis  218  from an initial length to a reduced length. Furthermore, due to the narrower and/or thinner design of the plurality of electrode prongs  316 , more space is available within the pre-combustion chamber  350 . As a result, the central electrode member  312  may include additional electrode prongs  316  which are thus capable of further extending the service life of the spark plug  120 . Because the spark plug  120  has an increased service life relative to other spark plugs, the spark plug  120 , when utilized within the power system  100 , may conserve material and expenses that would otherwise result from repair and/or replacement of the spark plug  120 . 
     The normalization of the wear rates between a first electrode and a second electrode according to the present disclosure produce a material erosion phenomenon that can be understood as a “wicking” progression of wear. The wicking progression of wear proceeds akin to burning of a candle to displace material from an electrode prong in a predictable and relatively uniform manner that better matches a wear rate of the associated other electrode. As a result, neither electrode outpaces the other and a greater amount of the electrode prong can be consumed before a gap distance becomes too large to be practicably bridged. The present disclosure also reflects the insight that a material area of an electrode prong and a length of separation (the gap length l 1 ) can be optimized to promote the normalization of wear rate between the respective electrodes. Whereas as discussed above earlier spark plug and spark electrode assemblies were often observed to wear away only a relatively small portion of a precious metal electrode prong, of iridium or platinum for example, before the spark plug would fail, according to the present disclosure a majority and in some instances nearly an entirety of an axial portion of an electrode prong can be consumed during service. Also differing from prior art spark plugs and spark electrode assemblies generally, according to the present disclosure the gap length l 1  may be established in inverse exponential relation to the electrode prong width w and in direct exponential relation to the electrode prong thickness t as discussed above. Conventional practices commonly establish a larger spark gap length where more electrode material is present, and a shorter spark gap length where less material is present. Thus, the present disclosure also proceeds counter to certain conventional practices in exploiting the discovery that a relatively narrower electrode prong may be advantageously positioned for extended service life relatively further from a second electrode rather than relatively closer. 
     Making spark plugs according to the present disclosure can include placing a first electrode and a second electrode at a fixed position and orientation relative to one another. In the case of the embodiment of  FIGS.  7  and  8   , this step could include installing the tip piece  430  in or on the housing piece  436 . The first electrode  440  may be supported in insulator  468  which is in turn supported in housing piece  436 . The second electrode  446  may be attached to tip piece  430  and secured to housing piece  436  to thus support the first electrode  440  and the second electrode  446  at the fixed position and orientation. 
     Making a spark plug according to the present disclosure can further include forming, by way of the placing a first electrode and a second electrode, a spark gap between an electrode surface of the first electrode extending circumferentially around a longitudinal axis and an electrode prong of the second electrode. In the case of the embodiment of  FIGS.  7  and  8    this step could be part of the final positioning and attachment of the tip piece  430  to the housing piece  436 . By way of the forming of a spark gap, a gap length of the spark gap is established that is in inverse relation to a width of the electrode prong, and in direct relation to a thickness of the electrode prong. As discussed above, the inverse relation may be an exponential relation and the direct relation may be an exponential relation. Establishing a gap length may include setting the gap length in direct relation to a parameter P as defined in the above Equation 1 having a value from approximately 1.5 to approximately 7.5. Making a spark plug can also include as part of the steps discussed above, or separately depending on spark plug design, positioning a spark gap of a spark electrode assembly within a combustion prechamber. 
     The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the implementations to the precise forms disclosed. Modifications and variations may be made in light of the above disclosure or may be acquired from practice of the implementations. Furthermore, any of the implementations described herein may be combined unless the foregoing disclosure expressly provides a reason that one or more implementations cannot be combined. Even though particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure of various implementations. Although each dependent claim listed below may directly depend on only one claim, the disclosure of various implementations includes each dependent claim in combination with every other claim in the claim set. 
     As used herein, “a,” “an,” and a “set” are intended to include one or more items, and may be used interchangeably with “one or more.” Further, as used herein, the article “the” is intended to include one or more items referenced in connection with the article “the” and may be used interchangeably with “the one or more.” Further, as used herein, the terms “comprises,” “comprising,” “having,” “including,” or other variations thereof, are intended to cover non-exclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements, but may include other elements not expressly listed. In addition, in this disclosure, relative terms, such as, for example, “about,” “generally,” “substantially,” and “approximately” are used to indicate a possible variation of ±10% of the stated value, except where otherwise apparent to one of ordinary skill in the art from the context. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. Also, as used herein, the term “or” is intended to be inclusive when used in a series and may be used interchangeably with “and/or,” unless explicitly stated otherwise (e.g., if used in combination with “either” or “only one of”). Further, spatially relative terms, such as “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the apparatus, device, and/or element in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.