Patent Publication Number: US-2011050069-A1

Title: Spark plug

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/275,042, filed Aug. 25, 2009, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates generally to the field of spark plugs. More specifically, the present invention relates to spark plugs designed to reduce hydrocarbon emissions from internal combustion engines. 
     A spark plug is used with a gasoline-fueled engine to ignite an air and fuel mixture in a combustion chamber of the engine. The spark plug is coupled to the engine by screwing a threaded portion of the spark plug into a cylinder head of the engine such that a firing portion of the spark plug is within a combustion chamber. Other spark plugs may be clamped or otherwise fastened to the engine. An electrical charge is supplied by an ignition armature, ignition coil, magneto, or other source of electricity. Timing of the charge may coincide with piston strokes of a two- or four-stroke engine cycle. The electrical charge travels through an ignition lead wire of the engine to an ignition plug. The ignition plug connects to a terminal connection portion of the spark plug. 
     A high-voltage ignition pulse of electricity (i.e., electrical charge) enters a terminal electrode on the connection portion of the spark plug. The high-voltage pulse travels along a center wire of the spark plug. The wire runs through an axial bore formed within an electrical insulator of the spark plug. The electrical insulator may be formed from a ceramic material and may include ribs to increase the surface area of the spark plug, reducing the likelihood of charge traveling along the surface of the spark plug. The wire additionally passes through an annular shell of the spark plug, and couples to a firing electrode (i.e., central electrode, center electrode, etc.). The shell is typically formed from a conductive metal and may include the threaded portion. The wire is not electrically coupled to the shell, but is instead insulated from the shell via the electrical insulator, which extends between the wire and the shell. 
     The firing electrode extends into the combustion chamber of the engine. A tip of the firing electrode may be coated or formed from a precious metal, such as platinum, intended to reduce wear damage or corrosion. Proximate to the firing electrode, a ground electrode is connected to the shell of the spark plug. A spark arcs between the tip of the firing electrode and the ground electrode. The ground electrode is grounded by the shell coupled to the cylinder head, and the rest of the engine. The spark ignites fuel and air in the combustion chamber to drive the piston and power the engine. 
     An air gap is typically positioned between the electrical insulator and the shell on the firing end of the spark plug. Air has relatively poor thermal conductivity, and the air gap helps to thermally insulate the tip of the electrical insulator, allowing the tip of the electrical insulator to reach a temperature sufficient to prevent carbon deposits from forming on the surface of the tip of the electrical insulator, which may otherwise short the firing electrode with the shell. However, the gap may also provide a shelter for fuel and air to escape ignition during the combustion processes of the engine, allowing unburned fuel through the combustion chamber. 
     SUMMARY 
     One embodiment of the invention relates to a spark plug, which includes a terminal for receiving an electrical charge, a firing electrode on a firing end of the spark plug, and a conductor electrically coupling the terminal and the firing electrode. The spark plug further includes a casing, an electrical insulator, and a ground electrode. The casing is at least partially formed from an electrically conductive material and is configured to be electrically coupled to a ground. The electrical insulator separates the conductor from the casing. The ground electrode is positioned proximate to the firing electrode, but is separated from the firing electrode to allow a spark to jump between the firing and ground electrodes during operation of the spark plug. The ground electrode includes an extension coupled to the casing and projecting from the casing such that the firing electrode is closer to the ground electrode than the firing electrode is to the casing. There is substantially no air gap between the interior of the casing and the electrical insulator from the firing end of the spark plug. 
     Another embodiment of the invention relates to a spark plug, which includes a terminal for receiving an electrical charge, a firing electrode on a firing end of the spark plug, and a conductor electrically coupling the terminal and the firing electrode. The spark plug further includes a casing, an electrical insulator, a ground electrode, and a thermal insulator. The casing is at least partially formed from an electrically conductive material and is configured to be electrically coupled to a ground. The electrical insulator separates the conductor from the casing. The ground electrode is positioned proximate to the firing electrode, but is separated from the firing electrode to allow a spark to jump between the firing and ground electrodes during operation of the spark plug. The thermal insulator extends between the casing and the electrical insulator on the firing end of the spark plug. 
     Yet another embodiment of the invention relates to a spark plug, which includes a terminal for receiving an electrical charge, a firing electrode on a firing end of the spark plug, and a conductor electrically coupling the terminal and the firing electrode. The spark plug further includes a casing, an electrical insulator, and a ground electrode. The casing is at least partially formed from an electrically conductive material and is configured to be electrically coupled to a ground. The electrical insulator separates the conductor from the casing. The ground electrode is positioned proximate to the firing electrode, but is separated from the firing electrode to allow a spark to jump between the firing and ground electrodes during operation of the spark plug. The electrical insulator adjoins the casing along the interior periphery of the casing at the firing end of the spark plug. A tip of the electrical insulator extends longitudinally from the casing on the firing end of the spark plug, such that the electrical insulator provides both a longitudinal separation and a latitudinal separation between the firing electrode and the casing on the firing end of the spark plug. 
     Alternative exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, in which: 
         FIG. 1  is a perspective view of a spark plug. 
         FIG. 2 . is a perspective view of another spark plug. 
         FIG. 3 . is a perspective view of a spark plug according to an exemplary embodiment of the invention. 
         FIG. 4 . is a side view of a spark plug according to another exemplary embodiment of the invention. 
         FIG. 5 . is a sectional view of a spark plug according to yet another exemplary embodiment of the invention. 
         FIG. 6  is a perspective view of a spark plug according to another embodiment of the invention. 
         FIG. 7  is a side view of the spark plug of  FIG. 6 . 
         FIG. 8  is a sectional view of the spark plug of  FIG. 6 , taken along line  8 - 8 . 
         FIG. 9  is a sectional view of the spark plug of  FIG. 8 , taken along line  9 - 9 . 
         FIG. 10  is a sectional view of a spark plug according to another exemplary embodiment of the invention. 
         FIG. 11  is an end view of the spark plug according to yet another exemplary embodiment of the invention. 
         FIG. 12  is a perspective view of a spark plug according to still another exemplary embodiment of the invention. 
         FIG. 13  is a sectional view of the spark plug of  FIG. 12 , taken along line  13 - 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Before turning to the figures, which illustrate the exemplary embodiments in detail, it should be understood that the present application is not limited to the details or methodology set forth in the description or illustrated in the figures. It should also be understood that the terminology is for the purpose of description only and should not be regarded as limiting. 
     Referring to  FIG. 1 , a spark plug  110  includes an outer shell  112  having a hexagonal portion  114 , a screwhead portion  116 , and a hook electrode  118 . The hexagonal portion  114  allows for a wrench to be used to turn the screwhead portion  116  into a cylinder head of a combustion engine. The hook electrode  118  serves as a ground electrode for the spark plug  110 . The spark plug  110  further includes an electrical insulator  124  having an axial bore through which a center wire (see, e.g., carbon rod  534  as shown in  FIG. 5 ) extends. The center wire terminates in a firing electrode  122 . During operation of the spark plug  110 , at particularly-timed intervals, sparks arc over a spark gap between the firing electrode  122  and the hook electrode  118 . 
     Still referring to  FIG. 1 , the spark plug  110  further includes an air gap  120  (i.e., annular crevice) between the electrical insulator  124  and the shell  112 . The air gap  120  thermally isolates the firing electrode  122  from the shell  112 , allowing the firing electrode  122  and electrical insulator  124  to heat to a temperature hot enough to burn off oil or other deposits that might otherwise foul (or short) the spark plug  110 , inhibiting the ability to form sparks. As mentioned, drawback of the air gap  120  is that unburned fuel and air may enter the air gap  120  and not be burned during the combustion stroke. Unburned fuel and air then exits the combustion chamber, resulting in increased hydrocarbon emissions. 
     Referring to  FIG. 2 , a spark plug  210  includes an outer shell  212  having a hexagonal portion  214 , a threaded portion  216 , and a ground electrode  218 , but no hook electrode (e.g., hook electrode  118  as shown in  FIG. 1 ). The spark plug  210  further includes an electrical insulator  220  and a firing electrode  222 . During operation of the spark plug  210 , sparks arc laterally over a spark gap  226  between the firing electrode  222  and the ground electrode  218 . The spark plug  210  may be used with a fast-running engine, such as an outboard engine for a motor boat, or a high-output engine (i.e., runs at high load). 
     The spark plug  210  does not include an air gap (e.g., air gap  120  as shown in  FIG. 1 ) between the electrical insulator  220  and the ground electrode  218 . During operation, conduction heat transfer occurs between the insulator  220  and the shell  212 , which cools the insulator  220  more than the design of the spark plug  110 . The cooler electrical insulator  220  may be unable to burn off oil or other deposits that may foul the firing electrode  222 . The cooler firing electrode  222  or electrical insulator  220  of the spark plug  210  may not be a problem in hotter or faster-running engines, such as two-stroke engines and air-cooled small engines, because the firing electrode  222  or electrical insulator  220  may get hot enough to prevent fouling. 
     The spark gap  226  of the spark plug  210  is wider than the spark gap of the spark plug  110 . As such, the wider spark gap  226  requires a greater electrical charge to initiate a longer arc between the firing electrode  222  and the ground electrode  218 . For example, in a small engine with an ignition system not using a battery, the spark plug  210  may require an engine speed of approximately 200-300 revolutions per minute (rpm) to initiate a spark, while the spark plug  110  may generate sparks at an engine speed of approximately 150 rpm. As such, an engine with the spark plug  210  may be more difficult to start (e.g., with a recoil starter) than an engine with the spark plug  110 . 
     Additionally, a spark of the spark plug  210  may occur further from the center of a corresponding combustion chamber than a spark of the spark plug  110 , because the hook electrode  118  is directed into the combustion chamber, which orients the corresponding spark toward the center of the combustion chamber. Furthermore, a spark of the spark plug  110 , with the hook electrode  118 , is surrounded by fewer surfaces than a spark of the spark plug  210 . The open space and closer-to-center location of a spark from the spark plug  110  may allow for a more efficient burn, as the flame propagates through the combustion chamber. A more efficient burn increases engine performance and may reduce hydrocarbon emissions. 
     Referring to  FIG. 3 , a spark plug  310  includes an outer shell  312  having a hexagonal portion  314 , a threaded portion  316 , and a hook electrode  318  that is integrally connected to (e.g. welded to) the threaded portion  316  of the shell  312 . The spark plug  310  further includes an electrical insulator  320  and a firing electrode  322 , but does not include an air gap between the electrical insulator  320  and the firing electrode  322 , 210 . During operation, sparks arc over a spark gap between the firing electrode  322  and the hook electrode  318 . The spark gap of the spark plug  310  may be narrower than the spark gap  226  of the spark plug  210 , which, for an engine with an ignition system not using a battery, allows for a slower engine speed (rpm) to produce a spark, improving start-ability of the engine. Additionally, the lack of an air gap reduces hydrocarbon emissions of an engine using the spark plug  310  by preventing the opportunity for unburned fuel and air to be caught in the air gap (e.g., air gap  120  as shown in  FIG. 1 ). However, similar to the spark plug  210 , the spark plug  310  may have a lower-temperature firing electrode  322 , which may be susceptible to fouling. Increased chances of misfiring due to spark plug  310  fouling may increase hydrocarbon emissions. 
     Referring to  FIG. 4 , a spark plug  410  includes a terminal connection portion  412  having a terminal electrode, a ceramic electrical insulator  414  having an axial bore through which extends a center wire. The electrical insulator  414  is fastened to a shell  416  (i.e., jacket, casing, etc.), which includes a hexagonal surface  418 , a screwhead  420 , and a hook electrode  422 . Extending from within the shell  416 , a firing electrode  424  includes a bulbous tip  426  (e.g., terminus, end) and a narrower rod  428  (e.g., neck) that connects to the center wire or carbon rod. 
     Similar to the spark plugs  210 ,  310 , the spark plug  410  includes no air gap (e.g., air gap  120  as shown in  FIG. 1 ) between the electrical insulator  414  and the firing electrode  424 . Additionally the firing electrode  424  is designed to reduce heat transfer away from the tip  426 . Increased surface area of the tip  426  is intended to increase the rate of heat flux into the tip  426 , while the narrow cross-section of the rod  428  reduces the ability of the heat to transfer away from the tip  426 . The hotter tip  426  temperature may be able to reduce the chances of spark plug  410  fouling by increasing the ability of the spark plug  410  to burn off oil or other deposits on the tip  426 , when compared to the spark plugs  210 ,  310 . In other embodiments, the tip of the spark plug  410  may be shapes other than a bulb, such as diamond-shaped, box-shaped, etc., having an increased cross-sectional area relative to the rod  428 . 
     Referring to  FIG. 5 , a spark plug  510  includes a terminal connection portion  512  having a terminal electrode  530 , a porcelain electrical insulator  514  having an axial bore  532  through which extends a carbon rod  534  with increased electrical resistance to reduce RF interference (see also conductor  620  in  FIG. 8 , which includes a carbon pellet). The electrical insulator  514  extends within a shell  516 , which includes shoulders  536 ,  538  to hold the electrical insulator  514 . The rod  534  terminates in a firing electrode  524  from which sparks arc to a ground electrode  522  coupled to the shell  516 . In other embodiments, capacitive or inductive elements are used in place of a resistive element. 
     The spark plug  510  further includes an annular air gap  540  that is sealed off from the combustion chamber by a thermally-insulating washer  542  positioned between the porcelain electrical insulator  514  and the shell  516 . The washer  542  is designed to compliment the air gap  540 , reducing heat transfer between the porcelain electrical insulator  514  and the shell  516 . As such, the firing electrode  524  and porcelain electrical insulator  514  becomes hot enough to reduce the chance of spark plug  510  fouling. The thermally-insulating washer  542  may be formed from a commercially-available thermally-insulating material having a low thermal conductivity (e.g., cement, fiberglass). 
     Referring to  FIG. 6 , a spark plug  610  includes a terminal end  612  and a firing end  614 . The terminal end  612  includes a terminal  616  configured to receive an electrical charge, such as from an ignition armature of an engine. On the firing end  614 , the spark plug  610  includes a firing electrode  618  (e.g., positive electrode, cathode, central electrode) in electrical communication with the terminal  616  by way of a conductor  620  ( FIG. 8 ) extending through an insulator  622 . 
     The spark plug  610  further includes a casing  624  (e.g., shell) at least partially formed from an electrically conductive material allowing the casing to be electrically coupled to a ground, such as a cylinder head of the engine. A ground electrode  628  (e.g., negative electrode, anode, side electrode) is coupled to the casing  624 . In some embodiments, the casing  624  includes threading  626  that is designed to fasten the spark plug  610  with the cylinder head of the engine, so that the firing end  614  is in communication with fuel in the combustion chamber. According to a preferred embodiment, there is substantially no air gap between the casing  624  and the insulator  622  on the firing end  614  of the spark plug  610 . 
     In some embodiments, the spark plug  610  includes a washer  630  configured for sealing and securing the casing  624  to the cylinder head of the engine. In some such embodiments, the washer  630  is a trifold or other form of compressible washer. Compression of the washer  630  helps to control the torque between the spark plug  610  and the cylinder head. In some embodiments, the washer  630  may be formed from a thermally-insulating material. 
     Referring to  FIG. 7 , the ground electrode  628  is coupled to and projects from a portion of the casing  624 , toward the firing electrode  618  and locating the ground electrode  628  proximate to the firing electrode  618 . However, the spark plug  610  includes a narrow space longitudinally positioned between the firing and ground electrodes  618 ,  628  (see also  FIG. 9  with space shown as a rectangle between the electrodes  618 ,  628 ), which in some embodiments may be less than about an eighth of an inch. During operation of the spark plug  610 , sparks jump between the firing and ground electrodes  618 ,  628 . In some embodiments, the ground electrode  628  is hook shaped and extends in front of the firing electrode  618  such that a spark jumps longitudinally between the firing and ground electrodes  618 ,  628 . 
     According to an exemplary embodiment, a tip  632  of the electrical insulator  622  extends longitudinally beyond the casing  624  on the firing end  614  of the spark plug  610  by a distance (see also  FIG. 9  showing the distance that the insulator  632  extends beyond the casing  624  in the longitudinal direction). As such, the firing electrode  618  is separated from the casing  624  longitudinally as well as latitudinally, helping to prevent shorting by providing a longer surface path between the firing electrode  618  and the casing  624 . Additionally, extending the tip  632  of the electrical insulator  622  beyond the casing  624  increases the surface area of the tip  632  exposed to heat from the combustion chamber, increasing heat flux into the electrical insulator  622 , and in turn helping to prevent and/or remove carbon deposits from the tip  632  of the electrical insulator  622 . In some embodiments, the tip  632  extends more than a sixteenth of an inch beyond the casing  624  on the firing end  614  of the spark plug  610 , such as about an eighth of an inch. 
     Referring to  FIG. 8-9 , according to an exemplary embodiment, the electrical insulator  622  is formed from ceramic material (e.g., porcelain) and the casing  624  is formed from metal (e.g., ferric metal). In some embodiments, the electrical insulator  622  extends through and electrically separates the conductor  620  from the casing  624 . In some embodiments, seals  638  (e.g., washer, gasket, adhesives) are used between the electrical insulator  622  and the casing  624  to prevent pressurized gases in the combustion chamber from passing through the spark plug  610  to escape the combustion chamber. 
     Referring specifically to  FIG. 9 , the electrical insulator  622  adjoins the interior periphery  636  of the casing  624  on the firing end  614  of the spark plug  610 , preventing fuel-carrying air (e.g., fuel and air mixture, fuel-enriched air, fuel vapor) in the combustion chamber from being shielded from ignition between the electrical insulator  622  and the casing  624 . In some embodiments, the electrical insulator  622  fully contacts the interior periphery  636  of the casing  624 . In other embodiments, the electrical insulator  622  is slightly separated from the casing  624  by an additional thermal insulator, such as a thermally-insulating washer, gasket, coating, etc. (see, e.g., thermally-insulating washer as shown in  FIG. 5 ). 
     Referring to  FIGS. 10-11 , in some exemplary embodiments a tip  710 ,  810  of the electrical insulator  622  includes ridges  712  ( FIG. 10 ), waves  812  ( FIG. 11 ), spikes, spines, and/or other contours (e.g., surface curvature) designed to increase the exterior surface area of the tip  710 ,  810 . Increased surface area exposed to the heat of the combustion chamber increases heat flux into the tip  710 ,  810 , helping to burn off and/or prevent carbon deposits on the exterior surface of the tip  710 ,  810  of the electrical insulator  622 . Prevention and/or removal of the carbon deposits helps to prevent shorting of the spark plug  610 , and in turn, to prevent fouling the spark plug  610 , wasting fuel, and production of hydrocarbon emissions. 
     Referring to  FIGS. 12-13 , manufacturing the spark plug  610  with narrow tolerances between the electrical insulator  622  and casing  624  on the firing end  614  may be inefficient with regard to time, effort, or cost. Instead, providing a curvature  912  (e.g., widening curvature, conical curvature) to the tip  910  of the electrical insulator  622  and/or a curvature  914  (e.g., bevel) to the interior periphery  636  of the casing  624  allows for a broader tolerances when manufacturing the spark plug  610 , while still substantially providing no gap for fuel-carrying air to avoid combustion between the electrical insulator  622  and the casing  624 . Instead, only a small groove  916  is formed, which is believed to effectively provide the same improved emissions benefit provided by the spark plug  610  with narrower tolerances (see, e.g.,  FIG. 6 ). 
     In contemplated embodiments, a spark plug includes an electrical insulator formed from a material that is also thermally insulating (e.g., cement including fine-grain quartz), which is intended to keep a higher surface temperature by retaining heat while also electrically separating the conductor and the casing. In such embodiments, the electrical insulator (and also thermal insulator) adjoins the casing to prevent fuel-carrying air from avoiding ignition. The electrical insulator may extend longitudinally beyond the casing to improve heat retention of the tip of the electrical insulator, and to improve electrical isolation of the firing electrode from the casing. In such contemplated embodiments, the spark plug may also include a hook-shaped ground electrode. 
     In other contemplated embodiments, a ground electrode may not extend toward the firing electrode, but may project from the casing as a straight rod. In such contemplated embodiments, the ground electrode may extend substantially in parallel with the firing electrode, such that the firing electrode is closer to the ground electrode than the firing electrode is to the casing. A spark would jump horizontally between the firing electrode and such a ground electrode. Use of a straight ground electrode may expose the tip of the insulator to a greater amount of heat from the combustion chamber than use of a hook-shaped electrode, which may partially shield the tip of the insulator. 
     The construction and arrangements of the spark plug, as shown in the various exemplary embodiments, are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Some elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process, logical algorithm, or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention.