SPARK PLUG INCLUDING A GROUND ELECTRODE HAVING A SMALL CROSS SECTION

Abstract

A spark plug, including a housing, an insulator situated in the housing, a center electrode situated in the insulator and a ground electrode situated on the housing, the ground electrode and the center electrode being situated relative to one another in such a way that the ground electrode and the center electrode form a spark gap, and the ground electrode including a core and a sheath surrounding the core, the core being made of a material which has a higher thermal conductivity than the material of the sheath, and a cross sectional area of the ground electrode being not greater than 2.76 mm.sup.2, the sheath having a wall thickness c of not greater than 0.4 mm in a first region of the ground electrode.

Claims

1-10. (canceled)

11. A spark plug, comprising: a housing; an insulator situated in the housing; a center electrode situated in the insulator; and a ground electrode situated on the housing; wherein the ground electrode and the center electrode are situated relative to one another in such a way that the ground electrode and the center electrode form a spark gap, and the ground electrode includes a core and a sheath surrounding the core, the core being made of a material which has a higher thermal conductivity than a material of the sheath, wherein a cross sectional area of the ground electrode is not greater than 2.76 mm.sup.2, and wherein the sheath has a wall thickness of not greater than 0.4 mm in a first region of the ground electrode.

12. The spark plug as recited in claim 11, wherein the sheath and the core each have a constant cross sectional area along their longitudinal extent in the first region of the ground electrode.

13. The spark plug as recited in claim 11, wherein in the first region, the cross sectional area of the core corresponds to at least 20% of the entire cross sectional area of the ground electrode.

14. The spark plug as recited in claim 11, wherein in the first region, the cross sectional area of the core corresponds to a maximum of 65% of the entire cross sectional area of the ground electrode.

15. The spark plug as recited in claim 11, wherein at an end of the ground electrode facing away from the housing, a distance between the end of the ground electrode and an end of the core is not greater than 4 mm.

16. The spark plug as recited in claim 16, wherein the distance is not smaller than 0.2 mm.

17. The spark plug as recited in claim 11, wherein the cross sectional area of the ground electrode has a height and a width.

18. The spark plug as recited in claim 17, wherein at least one of: (i) the height of the cross sectional area of the ground electrode is not greater than 1.2 mm, and (ii) the width of the cross sectional area of the ground electrode is not greater than 2.3 mm.

19. The spark plug as recited in claim 18, wherein the first region is longer than the height of the cross sectional area of the ground electrode.

20. The spark plug as recited in claim 11, wherein the cross sectional area of the core and the cross sectional area of the ground electrode have the same shape.

21. The spark plug as recited in claim 11, wherein the material of the sheath includes a nickel-containing alloy, the alloy including at least 20% by weight of chromium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows an example of a ground electrode according to the present invention.

[0024] FIG. 2 shows a cross section of the ground electrode according to the present invention.

[0025] FIG. 3 shows a section of a bent ground electrode according to the present invention.

[0026] FIG. 4 shows a spark plug including a ground electrode according to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0027] FIG. 1 schematically shows depictions for two exemplary specific embodiments of ground electrode 10 according to the present invention. Ground electrode 10 includes a core 12 and a sheath 11 surrounding core 12. Core 12 is made of a material having a higher thermal conductivity than the material of sheath 11. The material of sheath 11 typically has a higher wear resistance than the material of core 12. Core 12 is made of copper, silver or an alloy of copper and/or silver. An Ni-alloy is preferably used as material for sheath 11, whereby chromium and/or yttrium may be contained in the alloy.

[0028] Due to the manufacturing method, in this case an extrusion method, core 12 of ground electrode 10 shown here in section includes at least one section 15a, in which core 12 has relatively constant dimensions (x.sub.K, y.sub.K) and associated with that, a relatively constant cross sectional area. The term “relatively constant” within the context of this application means that the dimensions or the cross sectional area values vary by a maximum of 5%.

[0029] Dimensions (x.sub.K, y.sub.K) and the cross sectional area of core 12 are reduced within a second section 14a. The at least one first section 15a is situated on a side of ground electrode 10 facing away from a combustion chamber, for example, on the side with which the ground electrode is situated on spark plug housing 2. A second section 14a of core 12 is connected to the at least one first section 15a of core 12 in the direction of the end of ground electrode 10, which protrudes into the combustion chamber when using the ground electrode in a spark plug. In principle, it is conceivable that core 12 includes multiple first sections 15a having a constant cross sectional area, dimensions (x.sub.K, y.sub.K) and the cross sectional area of the individual first sections 15a being different. This is the case, in particular, if ground electrode 10 itself includes multiple regions having different dimensions (x.sub.E, y.sub.E) or cross sectional areas. In the case of multiple first sections 15a in core 12, the next closest section to the combustion chamber is second section 14a with the continuously tapering cross sectional area of core 12.

[0030] Ground electrode 10 depicted in FIG. 1 has constant dimensions (x.sub.E, y.sub.E) along its length and associated with that, a constant cross sectional area. At least three regions 13, 14, 15 may be distinguished in ground electrode 10. In a first region 15, ground electrode 10 includes a core 12 having a constant cross sectional area and a sheath thickness c of not greater than 0.4 mm. In a second region 14a, ground electrode 10 includes a core 12 having a continuously tapering cross sectional area. In a third region 13, ground electrode 10 includes no core.

[0031] Thickness c of the sheath of ground electrode 10 results from half the difference of its dimensions (x.sub.E, y.sub.E) to the core dimensions (x.sub.K, y.sub.K). If ground electrode 10 in first region 15 has a constant cross sectional area, then sheath thickness c in this first region 15 is constant. It is advantageously provided that in this first region 15, sheath thickness c of ground electrode 10 is not smaller than 0.15 mm and, in particular, not greater than 0.4 mm, for example, sheath thickness c is equal to or smaller than 0.25 mm.

[0032] In second region 14, ground electrode may have constant dimensions (x.sub.E, y.sub.E) and a constant cross sectional area, sheath thickness c in this case increasing within region 14 in the direction of the combustion chamber. Sheath thickness c in second region 14 is at least 0.15 mm thick.

[0033] In an alternative specific embodiment not shown here, dimensions (x.sub.E, y.sub.E) or the cross sectional area of ground electrode 10 may also be reduced in second region 14. Applicable to sheath thickness c in this case is that the sheath thickness is preferably in the range of 0.15 mm to 0.4 mm. It may be provided that dimensions (x.sub.E, y.sub.E) or the cross sectional area of ground electrode 10 vary at the same rate as dimensions (x.sub.K, y.sub.K) or the cross sectional area of core 12. This has the advantage that sheath thickness c remains constant in second region 14.

[0034] There is no core in third region 13 of ground electrode 10. Third region 13 preferably has constant dimensions (x.sub.E, y.sub.E) and a constant cross sectional area, which corresponds to dimensions (x.sub.E, y.sub.E) or the cross sectional area of second region 14 in the transition to third region 13.

[0035] Third region 13 of ground electrode 10 has a length b, which extends from an end 17 of core 12 on the combustion chamber side to a front face 16 of ground electrode 10 facing away from the housing. Length b is not greater than 4 mm. If ground electrode 10 is formed with a precious metal-containing ignition surface 19, length b may be designed shorter than ignition surface 19 which does not contain a precious metal, as shown in FIG. 1b. Length b is then in the range of 0.2 mm to 2 mm, for example. If a precious metal-containing ignition surface 19 is dispensed with, as shown in FIG. 1a, then length b is to have a minimum length of 1 mm, so that sheath 11 includes enough material on the end of ground electrode 10 facing away from the housing to enable a sufficiently long service life of ground electrode 10.

[0036] The measure of length b is adjusted to the desired length after the extrusion of ground electrode 10. The excess length of sheath 11 is separated off using a cutting method, a shear method, a stamping method or with the aid of a laser beam. Length b has an influence on the heat dissipation in ground electrode 10. By appropriately selecting length b, it is possible to adjust the heat level of ground electrode 10 on its end 16 facing away from the housing to a desired value, so that end 16 of ground electrode 10 facing away from the housing does not drop below a temperature advantageous for the ignition of the gas mixture in a combustion chamber.

[0037] FIG. 2 shows an example of a cross section of ground electrode 10. In this example, ground electrode 10 and core 13 have a rectangular cross section. The area of the cross section of the ground electrode is not greater than 2.76 mm.sup.2. The cross section has a width x.sub.E of not greater than 2.3 mm and a height y.sub.E of not greater than 1.2 mm.

[0038] The cross section of core 12 is situated in the center of the cross section of ground electrode 10. Thickness c of sheath 11 results from half the difference of height y and width x of ground electrode 10 (y.sub.E, x.sub.E) and of core 12 (y.sub.K, x.sub.K). The cross sections of ground electrode 10 and of core 12 advantageously have the same geometric shape.

[0039] The cross sectional area of core 12 is no less than 20% and, in particular, no more than 65% of the cross sectional area of ground electrode 10 in first region 15 of ground electrode 10. The cross sectional area of the ground electrode is composed of the area of sheath 11 and the area of core 12 in cross section.

[0040] In one exemplary embodiment not shown herein, it may be provided that the shape of the cross section of ground electrode 10 varies over the course of the length of ground electrode 10. Ground electrode 10 may, for example, have a rectangular cross section at its end 16 facing away from housing 2 and a cross section at its end situated on the housing which is adapted to circular front face 21 of housing 2. This means that height y.sub.E of the cross section of ground electrode 10 corresponds to or is smaller than a width of circular front face 21. Accordingly, width x.sub.E at the end of ground electrode 10 facing housing 2 is then longer than end 16 of ground electrode 10 facing away from the housing. The cross section of ground electrode 10 on its end facing housing 2 has a curvature, i.e., the cross section has a banana-shaped profile, corresponding to the curvature of front face 21 of the housing. The shape of the cross section is modified typically with the aid of stamping, in such case either a rectangular cross section of the end of ground electrode 10 facing the housing may be reshaped by stamping into a banana-shaped profile or end 16 of a ground electrode facing away from the housing having a banana-shaped profile is stamped to form a planar surface.

[0041] FIG. 3 shows a section for a ground electrode 10 according to the present invention, which differs from ground electrode 10 shown in FIG. 1 only in that the ground electrode in FIG. 3 is bent. Ground electrode 10 is produced in an initial step by an extrusion method such as, for example, the cup method. Straight ground electrode 10 is welded in a subsequent step to front face 21 of the spark plug housing. Depending on the intended purpose, ground electrode 10 is bent into the desired position in a subsequent step, so that a roof electrode or a side electrode or a bow electrode is formed. In addition, ground electrode 10 may also include an ignition surface 19, which is typically welded to the ground electrode prior to bending.

[0042] Even in the case of bent ground electrode 10, it is possible to distinguish the three regions 13, 14, 15 of ground electrode 10 or the two sections 14a, 15a of core 12. The length of each region is a mean based on the longest and the shortest length of the region, which results along the surface of ground electrode 10.

[0043] FIG. 4 schematically shows a depiction of a spark plug 1 including a ground electrode 10 according to the present invention. Spark plug 1 includes a metal housing 2 having a thread for mounting spark plug 1 in a cylinder head. The thread may have an outer diameter of at least 8 mm, 10 mm or greater. The housing also includes a hexagonal section 9, on which a tool for mounting spark plug 1 in the cylinder head is placed. An insulator 3 is situated inside housing 2. A center electrode 5 and a connection bolt 7 are situated inside insulator 3 and electrically connected via a resistor element 6.

[0044] Center electrode 5 projects out of insulator 3 typically at the end of spark plug 1 facing away from the housing. Center electrode 5 is situated with its electrode head 4 on a seat formed on the inner side of insulator 3. Center electrode 5 and/or ground electrode 10 include an attached ignition surface 19. The center electrode may also include a core surrounded by a sheath, the core being made of a material having a higher thermal conductivity than the material of the sheath.

[0045] At least one ground electrode 10 according to the present invention is situated at the end of housing 2 facing away from the housing which, together with center electrode 5, forms a spark gap. Ground electrode 10 may be designed as a roof electrode, a side electrode or a bow electrode.