Patent Application
20070290594
These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:
Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a spark plug according to the subject invention is generally shown at 10 in
The insulator 12 is of generally tubular construction, including a central passage 28 extending longitudinally between the upper terminal end 24 and the lower nose end 22. The central passage 28 is of varying cross-sectional area, generally greatest at or adjacent the terminal end 24 and smallest at or adjacent the nose end 22.
A conductive, preferably metallic, shell is generally indicated at 30. The shell 30 surrounds the lower regions of the insulator 12 and includes at least one ground electrode 32. While the ground electrode 32 is depicted in the traditional single J-shaped style, it will be appreciated that multiple ground electrodes, or an annular ground electrode, or any other known configuration can be substituted depending upon the intended application for the spark plug 10.
The shell 30 is generally tubular in its body section, and includes an internal lower compression flange 34 adapted to bear in pressing contact against the lower seat 19 of the insulator 12. The shell 30 further includes an upper compression flange 36 which is crimped or deformed over during the assembly operation to bear in pressing contact against the upper seat 17 of the insulator 12. A buckle zone 38 collapses under the influence of an overwhelming compressive force during or subsequent to the deformation of the upper compression flange 36, to hold the shell 30 in a fixed position with respect to the insulator 12. Gaskets, cement or other sealing compounds can be interposed between the insulator 12 and shell 30 at the points of engagement to perfect a gas tight seal and improve the structural integrity of the assembled spark plug 10. Accordingly, after assembly, the shell 30 is held in tension between the upper 36 and lower 34 compression flanges, whereas the insulator 12 is held in compression between the upper seat 17 and the lower seat 19. This results in a secure, gas-tight, permanent fixation between the insulator 12 and the shell 30. Although the type of seal described and depicted in
The shell 30 further includes a tool receiving hexagon 40 for removal and installation purposes. The hex size complies with industry standards for the related application. A threaded section 42 is formed at the lower portion of the metallic shell 30, immediately below a seat 44. The seat 44 may either be tapered to provide a close tolerance installation in a cylinder head which is designated for this style of spark plug, or may be provided with a gasket (not shown) to provide a suitable interface against which the spark plug seats in the cylinder head.
A conductive terminal stud 46 is partially disposed in the central passage 28 of the insulator 12 and extends longitudinally from an exposed top post 48 to a bottom end 50 embedded part way down the central passage 28. The top post 48 connects to an ignition wire (not shown) and receives timed discharges of high voltage electricity required to fire the spark plug 10.
The bottom end 50 of the terminal stud 46 is embedded within a conductive glass seal 52 forming the top layer of a composite suppressor-seal pack or assembly, generally indicated at 54. To ensure adequate clearance for glass flow during hot pressing, a radial clearance of about 0.005″ is provided around the insulator wall. The conductive glass seal 52 functions to seal the bottom end 50 of the terminal stud 46 within the central passage 28, while conducting electricity from the terminal stud 46 to a resistor layer 56. This resistor layer 56, which comprises the center layer of the 3-tier suppressor seal pack 54, can be made from any suitable composition known to reduce electromagnetic interference (EMI). The suppressor glass seal includes glass, fillers, and carbon/carbonaceous materials in such ratios to ensure appropriate resistance when pressed and provide a stable resistance over the anticipated service life. Depending upon the recommended installation and the type of ignition system used, such resistor layers 56 may be designed to function as a more traditional resistor suppressor, or in the alternative as an inductive suppressor. Immediately below the resistor layer 56, another conductive glass seal 58 establishes the bottom, or lower layer of the suppressor seal pack 54. The conductive glass can be made from a mixture of glass and copper metal powder at approximately 1:1 ratio by mass, as is well-known in the industry. Accordingly, electricity travels from the bottom end 50 of the terminal stud 46, through the top layer conductive glass seal 52, through the resistor layer 56 and into the lower conductive glass seal layer 58.
A conductive center electrode 60 is partially disposed in the central passage 28 and extends longitudinally between a head 62 encased in the lower glass seal layer 58 to an exposed sparking tip 64 proximate the ground electrode 32. Thus, the head 62 of the center electrode 60 is longitudinally spaced from the bottom end 50 of the terminal stud 46, within the central passage 28. The suppressor seal pack 54 electrically interconnects the terminal stud 46 and the center electrode 60, while simultaneously sealing the central passage 28 from combustion gas leakage and also suppressing radio frequency noise emissions from the spark plug 10. As shown, the center electrode 60 is preferably a one-piece, unitary structure extending continuously and uninterrupted between its head 62 embedded in the glass seal 58 and its sparking tip 64 opposite the center electrode. The sparking tip 64 may or may not be fitted with a precious or noble metal end which is known to enhance service life. One advantage of this invention is that the center electrode 60 does not need to be made entirely of a homogenous precious metal as is required in comparable prior art designs.
Referring now to
Once these granular materials have been loaded into the central passage 28, the terminal stud 46 is forced down the central passage 28, cold-compressing the granular materials as shown in
Referring now to
A head clearance HC may be defined as the radial clearance space between the outer cylindrical wall of the head 62 and the surrounding portion of the central passage 28. Typically, the head clearance HC will be sized to promote good flow and fill of the lower glass seal layer 58 during the hot press operation as shown in
Other significant dimensions may be keyed to external features of the insulator 12. For example, the large shoulder 16 may be located in the longitudinal direction by the theoretical intersection 68 between the mast portion 14 and the angled surface of the upper seat 17 forming an upper limit and the filleted transition 26 forming its lower limit. Specifically, the filleted transition 26 is defined at the theoretical intersection 70 of that outer surface tapering inwardly from the large shoulder 16 and that generally straight, shank-like portion of the outer surface forming the small shoulder 18. The small shoulder 18 is thus located between the filleted transition reference point 70 and the theoretical intersection 72 between the tapered portion of the lower seat 19 and the nose section 20. Hence, a large shoulder section LS (which represents the length of the large shoulder 16) is defined as the longitudinal region between reference points 68 and 70, whereas a small shoulder section SS (which represents the length of the small shoulder 18) is the longitudinal region between reference points 70 and 72.
The center electrode head 62 is seated at its bottom edge on an internal ledge 74 in the central passage 28. The internal ledge 74 establishes a transition to a smaller cross-sectional diameter which is generally equivalent to the straight, cylindrical length of the center electrode 60 plus a moderate clearance. This internal ledge 74 also coincides with the lowermost reaches, or base, of the suppressor seal pack 54. The internal ledge 74 can be shaped with a convex or radiused profile to engage a correspondingly shaped undersurface of the head 62 and thereby perfect a tight sealing seat without introducing excessive stresses into the material of the insulator 12 during the cold press operation (
An “A” dimension is defined as the longitudinal measure between the small shoulder reference point 72 and the internal ledge 74 where the bottom of the center electrode head 62 seats. A positive “A” dimension (+A) occurs when the center electrode head 62 is disposed longitudinally between the small shoulder reference point 72 and the filleted transition reference point 70. A negative “A” dimension (−A) results when the internal ledge 74 is located between the small shoulder reference point 72 and the nose end 22 of the insulator 12. As shown in
The subject suppressor seal pack 54 is of the tapered variety, which, as best shown in
The reducing taper 80 may take various geometric configurations, but is shown in the preferred embodiment having a straight, conical sidewall. Mindful of the expansionary forces imposed upon the central passage 28 during the cold pressing operation (
In addition to maximizing the insulator 12 strength and dielectric properties, the tapered suppressor seal pack 54 also enhances the gas-tight qualities of the seal established around the center electrode head 62. More specifically, during the hot press operation as depicted in
Another advantage of the subject tapered suppressor seal pack 54 arises out of its enabling use of larger diameter, and hence more robust, terminal studs 46. In many applications, including small engine applications, there is a tendency toward the use of so-called “coil-on-plug” designs, wherein a heavy ignition coil is supported directly on top of the spark plug 10. These heavy designs impose significantly greater torsional stresses on the terminal stud 46, which stresses can be better withstood through the use of larger diameter materials. Small engine applications, such as used in lawn and garden power tools, are notorious for producing high-frequency vibrations which can be better resisted through the more robust terminal stud 46. The subject tapered suppressor seal pack 54 enables the use of such larger diameter terminal studs 46 without compromising the structural integrity and dielectric properties of the insulator 12 in its more vulnerable, small shoulder section SS and nose section 20. The larger diameter terminal stud 46 also is less prone to buckling during hot pressing operations. Prior art style small diameter terminal studs, by contrast, tend to soften and buckle during hot pressing, thus reducing load transfer to the glass pack and stressing the insulator.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
