Abstract:
An electrically conductive cover which in assembly is received over at least part of a spark plug boot and at least part of a nut portion of a metal body of a spark plug received in the boot. The cover may have an opening through which an arm portion of the boot extends which arm portion receives an electric wire for supplying a current at a high potential voltage to the spark plug.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application claims the benefit under 35 U.S.C. §119(e) of the earlier filed provisional patent application, Ser. No. 62/104,403, filed under 35 U.S.C. §111(b) on Jan. 16, 2015, which is incorporated herein in its entirety by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to spark plug boot covers. More particularly, the disclosure relates to covers for spark plug boots that inhibit the propagation of electromagnetic interference. 
       BACKGROUND 
       [0003]    Gasoline powered spark ignition internal combustion engines and particularly light duty and small engines are used on a large variety of products including handheld, lawn and garden, marine, snowmobile and other home and commercial products. These engines are typically two-cycle or four-cycle engines with one or more cylinders and have a spark plug for each cylinder which in use initiates combustion of a fuel-and-air mixture in the cylinder. The spark plug is typically threaded or otherwise secured in a bore in a metal cylinder head or cylinder of the engine which provides a ground for a metal shell or body of the spark plug which has an electrical ground electrode adjacent one end and for installing or removing the spark plug a non-circular and typically hexagonal nut portion adjacent its other end. An electrically conductive center electrode typically with a copper core extends through the metal body with one end spaced by a gap from the ground electrode and is received in a typically ceramic insulator which projects from the other end of the body and carries an electrically conductive terminal connected to the center electrode. 
         [0004]    In use, though an insulated wire with an end clip removably connected to the terminal, a high potential voltage current is supplied to the center electrode to produce an arc or spark in the gap. Typically, an electrically insulating boot is generally coaxially received over the terminal and an exposed portion of the insulator of the spark plug and terminates short of or adjacent the upper end of the spark plug shell or body. Typically, the boot has an integral arm portion through which the insulated electric wire extends and this arm portion typically is inclined at an acute included angle usually of about 90° or 45° to the longitudinal axis of the main body of the boot and the spark plug. In many small engine applications, in use the high potential voltage is supplied to this wire by a so-called switch or module controlling the ignition timing which is typically part of an electromagneto capacitive discharge ignition system. 
         [0005]    When in use in an operating engine, the arcing or spark produced by the spark plug creates electromagnetic interference (EMI) which may adversely affect the circuitry of the module controlling ignition timing and/or other engine operations which adversely affects engine performance or it may adversely affect other electronic circuitry of the product on which the engine is used or in some instances other devices or products in the vicinity in which the engine is operating. 
       SUMMARY 
       [0006]    In at least some implementations, an electrically conductive cover is configured to be received over at least a significant portion of the main body of a spark plug boot and to extend over at least part of the nut portion of the metal body of the spark plug. The cover may have a firm friction fit such as an interference fit with the nut portion of the spark plug body. The cover may have a closed end and/or an opening thorough which an arm of the boot extends for receiving a portion of an electric wire for supplying power to a spark plug. The cover may be made of an electrically conductive synthetic rubber material with a surface resistance of less than 8,000 ohms per square and/or a volume resistance of less than 85,000 ohms-centimeter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The following detailed description of certain embodiments and best mode will be set forth with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a an exploded perspective view of one embodiment of the invention with a portion of an electric circuit inserted therein and a spark plug. 
           [0009]      FIG. 2  is an end view of one embodiment of the invention. 
           [0010]      FIG. 3  is a cross-sectional view taken along lines A-A of  FIG. 2 . 
           [0011]      FIG. 4  is a side view of one embodiment of the invention. 
           [0012]      FIG. 5  is an enlarged sectional view of one embodiment of the invention received over a spark plug boot assembled on a spark plug. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , one embodiment of the invention is generally indicated at  10 . The invention  10  includes a conductive cover assembly  10  for a spark plug, generally shown at  12 . The spark plug  12  includes a terminal  14  that is electrically connected to an electrical circuit, represented by wire  16  and switch  18 . The spark plug  12  includes a ceramic body  20 , a ground electrode  22  and a nut surface  24 . The nut surface  24  is disposed between the ceramic body  20  and the ground electrode  22  and is the surface about which a manual tool is used to tighten the spark plug  12  onto an internal combustion engine (not shown). 
         [0014]    The electrical circuit  16  also includes a connector (not shown), which is a spring electrode that is forced over the terminal  14  to complete the electrical connection between the switch  18  and the spark plug  12 . This spring electrode is covered by a spark plug boot  26  (only partially shown in  FIG. 1 ). 
         [0015]    Referring now to all of the Figures, the conductive cover assembly  10  includes a base  28 . The base  28  defines an inner base diameter  30  (identified in  FIG. 3 ). The inner base diameter  30  complements the nut surface  24  of the spark plug  12 . More specifically, the inner base diameter  30  is of a dimension that allows it to be forced over the nut surface  24  of the spark plug  12  in a manner in which the conductive cover assembly  10  is friction fit to the nut surface  24  with enough force to keep the conductive cover assembly  10  in place. 
         [0016]    The conductive cover assembly  10  includes an conducting cover  32  that extends up from the base  28 . The conducting cover  32  covers the spark plug  12  and the spark plug boot  26 . More specifically, the conducting cover  32  covers the portion of the spark plug  12  that is exposed outside the internal combustion engine and the portion of the spark plug boot  26  that is coaxial with the spark plug  12 . As is shown in  FIG. 1 , a portion of the spark plug boot  26  extends out from the conducting cover  32  (discussed in greater detail subsequently). 
         [0017]    The conductive cover assembly  10  is fabricated from ethylene propylene diene monomer (EPDM) rubber. The EPDM rubber is infused with graphite such that the EPDM rubber is conductive. The EPDM rubber infused with graphite within the conductive cover assembly  10  creates a grounding shield to protect the area disposed immediately adjacent the spark plug electrode  14  from electromagnetic interference (EMI). In an alternative embodiment, the EPDM rubber may be infused with carbon black instead of graphite. In a preferred embodiment, the conductive cover assembly  10  has a surface resistivity less than or equal to 108 Ωcm and a volume resistivity of less than or equal to 109 Ωcm. 
         [0018]    The EPDM rubber has been tested to have a hardness of 55 Shore A at a temperature of 350° Fahrenheit after ten minutes of heating. The tensile strength of the EPDM rubber is 1,819 psi at the same temperature for the same time. 
         [0019]    The conducting cover  32  defines a cover inner diameter  34  ( FIG. 3 ) that is less than the inner base diameter  30 . A relief surface  36  extends between an inner base surface  38  and an inner cover surface  40 . The cover inner diameter  34  may be large enough to create an air gap  42  between the inner cover surface  40  on the one hand and spark plug  12  and the spark plug boot  26  on the other hand. The air gap  42  provides a level of insulation, which adds to the insulation provided by the spark plug boot  26 . Any EMI that passes through these layers is dissipated by the conductive properties of the EPDM rubber and the conductive cover assembly  10 , which act as a grounded material. 
         [0020]    The conducting cover  32  includes a closed distal end  44  and a circuit opening  46 . The circuit opening  46  is disposed between the closed distal end  44  and the base  28  of the conductive cover assembly  10 . The circuit opening  46  provides access allowing a portion of the spark plug boot  26  shown in  FIG. 1  to extend out from the conductive cover assembly  10  and allow the electrical circuit  16  to extend into the conductive cover assembly  10 . 
         [0021]    As shown in  FIG. 5 , in assembly and in use, the separate cover assembly  10  is received over the main body  50  of the boot  26  which is disposed on the spark plug  12 . The main body  50  of the boot terminates short of or bears on the upper end of the spark plug metal shell or body  52 , and an arm  54  of the boot projects outwardly through the opening  46  in the cover assembly. The lower base portion  28  of the boot desirably, but not necessarily, extends over the entire axial extent of the nut portion  56  of the spark plug  12  preferably with a slight interference fit and preferably is slightly stretched within its elastic limit to provide a firm interference fit with the nut portion  156  over substantially the entire axial and circumferential extent of the peripheral nut surface  24  to thereby provide a good electrical connection or grounding between the cover assembly  10  and the spark plug shell  52 . This frictional engagement with the shell may releasably retain the cover on the boot and the spark plug. Desirably, there may also be a slight interference fit between at least a portion of the cover sidewall inner surface  40  and the main body portion  50  of the boot  26  to releasably retain the cover assembly  10  on the boot  26 . 
         [0022]    Preferably, the base portion  28  of the cover assembly  10  carries axially downwardly beyond the nut portion  56  and over at least as much of the cylindrical portion  58  of the shell  52  of the spark plug and terminates closely adjacent to the cylinder head or cylinder body on which the spark plug is assembled when in use. 
         [0023]    Desirably, the cover assembly  10  is made of an at least somewhat resilient and flexible synthetic rubber which has been doped or infused with graphite, carbon black, or other conductive material so that the cover assembly is conductive and preferably has a surface resistance of less than 8,000 ohms per square and a volume resistance of less than 85,000 ohms-cm as measured and determined in accordance with ASTM Standard D257. Suitable synthetic rubber materials include EPDM, silicone, thermoplastic elastomers (TPE), and the like. Desirably, the synthetic rubber has good heat, ozone and weather resistance. Preferably, the synthetic rubber has a relatively high temperature resistance of at least about 250° F. and preferably 350° F. Preferably, the synthetic rubber has a durometer on the Shore A scale in the range of about 50 to 60 at a temperature of 350° F. 
         [0024]    For ease of assembly, preferably the cover  10  is assembled over the boot  26  before the boot and cover are assembled over the spark plug  12 . Even if the high tension insulated wire  16  is assembled in the boot  26  before the cover  10 , the cover may be readily assembled over the boot by inserting the body portion  50  of the boot through the opening  46  in the resilient cover assembly  10  and into the interior of the cover. This installation of the separate cover assembly  10  is advantageous both when the cover is assembled during original equipment manufacture (OEM) of the spark plug boot  26 , before or after insertion of the high tension wire  16  into the boot  26 , before or after the high tension wire  16  is attached to an ignition module  18 , before or after manufacture of the spark ignition engine, after the engine has been assembled into the product it powers, and even in the aftermarket after the engine or the end or product it powers has been distributed or sold. The manufacture or use of a separate cover is also less expensive than overmolding the conductive cover on an electrically insulative boot during manufacture of the boot. 
         [0025]    While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is understood that the terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.