COLD FOUL RESISTANT SPARK PLUG

BACKGROUND OF THE INVENTION

Exemplary embodiments of the present invention related to spark plugs and, more particularly, to spark plugs having a ceramic tip configured to resist cold fouling.

In general, spark plugs include an insulative sleeve having a central axial bore through which a center electrode extends. The insulating sleeve is positioned within, and secured to, a metal shell that serves as a mounting platform and interface to an internal combustion engine. The metal sleeve also supports a ground electrode that is positioned in a particular spaced relationship relative to the center electrode so as to generate a spark gap. The insulating sleeve includes a shaped tip portion that resides in a recessed end portion of the metal shell. The shaped tip portion is configured to protect the electrode from engine heat and products of combustion. The spark plug is typically mounted to an engine cylinder head and selectively activated to ignite a fuel/air mixture in an associated engine cylinder.

Over time, products of combustion or combustion deposits build up around the center electrode and particularly the shaped tip portion. This build up of combustion product inhibits spark formation across the spark gap. A significant build up of combustion products may foul the spark plug and resulting in ignition failure, i.e., the combustion products completely block the spark from forming between the center and ground electrodes. Combustion deposit build up is particularly problematic during cold starts. During cold starts, complete combustion of the air/fuel mixture is seldom achieved which results in an increased generation of combustion product. As a result of continuous cold starts, combustion product builds up on the center and ground electrodes causing spark plug to become fouled, thereby preventing or weakening the formation of a spark across the spark gap.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one exemplary embodiment, a spark plug is provided including a metal shell having a first end, a second end and an intermediate portion that collectively define a central cavity. In addition, the spark further includes an insulator that extends through the central cavity of the metal shell. The insulator includes a first end portion extending through the intermediate portion of the metal shell to a stepped tip portion having a first length. The stepped tip portion includes a first section, and a substantially straight second section having a second length. The substantially straight section includes a segment that extends beyond the second end of the metal shell to a third length. The second length is based upon at least one of the first length and the third length.

The above described spark plug configuration has been advantageously found to resist cold start fouling. The length proportion of the substantially straight section of the tip relative to the overall length of the stepped tip and/or the length of the substantially straight section of the tip extending beyond the second end of the metal shell inhibits build up of combustion products that would otherwise inhibit spark development. In addition, it has been found that the particular length proportion allows rapid combustion product "burn off" while still providing necessary mechanical and thermal electrode protection. Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an exemplary embodiment of a spark plug;

FIG. 2 is a cross-sectional view along lines 2-2 of FIG. 1;

FIG. 3 is an end view of the spark plug of FIG. 2;

FIG. 4 is a cross-sectional view of an exemplary embodiment of a spark plug mounted to an internal combustion engine; and

FIG. 5 is an enlarged cross-sectional view of an end portion of an exemplary embodiment of a spark plug.

DETAILED DESCRIPTION OF THE INVENTION

Reference is also made to U.S. provisional patent application serial No. 61/024,045 filed January 28, 2008 and U.S. Patent Application No. 12/361,028, filed January 28, 2009, the contents each of which are incorporated herein by reference thereto. With initial reference to FIGS. 1-3, an exemplary embodiment of a spark plug 2 is shown. The spark plug 2 includes an electrically insulating sleeve or insulator 4 having a first end portion 6, a second end portion 7 and an intermediate portion 8. Insulator 4 includes an axial bore 15 that extends between first and second end portions 6 and 7. As best shown in FIG. 2, spark plug 2 includes a first electrode 17 arranged in an upper portion (not separately labeled) of axial bore 15. First electrode 17 includes a first end or tip section 18 located outside and beyond first end portion 6 and a second end section 19 located within and at a first ending of axial bore 15. The first and second end section are joined together through intermediate section 20.

The spark plug 2 also includes a second electrode 24 located within and at a second end of axial bore 15. The second electrode 24 includes a first end section 26 and a second end section 27. The first and second end sections being joined together through an intermediate section 28. Second electrode 24 includes a tip portion 30 provided at second end section 27. First and second electrodes 17 and 24 are electrically connected through a resistor 32. In one embodiment, electrodes 17 and 24 are electrically connected to resistor 32 through a conductive glass 33 or equivalents thereof.

Insulator 4 extends, at least in part, into a metal sleeve or shell 39. Metal shell 39 includes a main body 40 having a first end 42, a second end 43 and an intermediate section 44 that collectively define an interior or central cavity 47. Metal shell 39 also includes a ground electrode 50. Ground electrode 50 includes a first segment 51 that extends from second end 43 of main body 40, a second or curved segment 52 and a

third segment 53. Third segment 53 is spaced from, and extends generally perpendicular to tip portion 30 so as to define a spark gap 'G'.

In a manner known in the art, metal shell 39 provides a robust mounting arrangement for securing spark plug 2 to an engine cylinder head 55. As best shown in FIG. 4, cylinder head 55 includes a spark plug receiving portion 57 having a first end portion 58, an intermediate portion 59 (a portion of which is threaded) and a second end portion 60. Second end portion 60 opens to a recessed area 61 that defines an upper portion of a combustion chamber.

In accordance with one exemplary embodiment of the present invention, intermediate portion 8 of insulator 4 is formed with a series of steps. More specifically, intermediate portion 8 includes a first stepped segment 63 that leads to a second stepped segment 64 which extends to a third stepped segment 65 and terminates in a fourth stepped segment or stepped tip portion 66. A portion of fourth stepped segment projects beyond second end 43 of metal shell 39. Stepped tip portion 66 is spaced from an interior wall portion (not separately labeled) of central cavity 47 so as to define a recess or cup portion 68.

In prior art arrangements, the cup portion would fill with combustion deposits and foul the spark plug. Combustion product build up is particularly prevalent during cold starts. During cold starts, air/fuel ratios are generally rich i.e., there is insufficient air for complete combustion of all the fuel. Over time, the unburned fuel builds up in the cup portion and inhibits a spark from passing across the spark gap.

In accordance with another exemplary embodiment of the present invention, stepped tip portion 66 includes a first section 69 that extends from third stepped segment 65 to a second substantially straight section 70. As will be discussed more fully below, the particular construction of substantially straight section 70 inhibits combustion products from building up within cup portion 68. In this manner, substantially straight portion 70 advantageously resists cold fouling.

As best shown in FIG. 5, stepped tip portion 66 includes an overall length "T" with substantially straight portion 70 having a length "X". As further shown, a

portion of substantially straight portion 70 extends beyond second end 43 of metal shell 39 and has a length "S". Length "X" of substantially straight portion 70 is a function of overall length "T" and length "S". More specifically, in accordance with an exemplary embodiment of the present invention, the dimensional relationship between the different components of tip portion 66 can be expressed by:

X= Cl * (T - S * C2)

Where: 0 <= CK=I; and 0 <= C2 <=l.

When Cl is set at approximately 0.25 and C2 is established at about 1, "T" is preferably in a range of between approximately 0.050 and 1.000 inches (1.27 - 25.4 mm). More preferably, "T" is in a range of between approximately 0.500 and 0.900 inches (12.7 - 22.86 mm). Most preferably, "T" is in a range of between approximately 0.600 and 0.800 inches (15.24 - 20.32 mm). "S" is preferably in a range of between approximately 0.000 and 0.500 inches (0.000 - 12.7 mm). More preferably, "S" is in a range between approximately 0.100 and 0.200 inches (2.54 - 5.08 mm). Most preferably, "S" is in a range between approximately 0.130 and 0.140 inches (3.302 - 3.556 mm). It being understood that in one exemplary embodiment Cl and C2 are of course greater than zero.

It should be appreciated that the above expressed dimensional relationship and configuration of substantially straight portion 70 resists combustion deposit build up and cold fouling. The length proportions of stepped tip portion inhibits build up of combustion products that would otherwise inhibit spark development. In addition, it has been found that the length proportion of stepped tip portion 66 also allows rapid combustion product "burn off while still providing necessary mechanical and thermal electrode protection.

It should be understood that although described with reference to exemplary embodiments of the present invention, it should be readily understood that various changes and/or modifications can be made to the exemplary embodiment without departing from the spirit thereof. For instance, the particular stepped arrangement of the intermediate portion of the insulating sleeve can be varied. In addition, the

number and arrangement of the ground electrode could also be varied. Furthermore, the above described ranges should be understood to be associated with an exemplary embodiment of the invention and can vary without departing from the scope of the invention.