Patent Application
20160211653
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.
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.
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.
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.
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.
The following detailed description of certain embodiments and best mode will be set forth with reference to the accompanying drawings, in which:
Referring to
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
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
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
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.
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.
The conducting cover 32 defines a cover inner diameter 34 (
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
As shown in
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.
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.
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.
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 62104403 | Jan 2015 | US |
