Spark plug electrode including a profiled noble-metal part

The present invention relates to a spark plug electrode, at whose spark exit or spark entrance area a burn-resistant noble-metal is applied.

The use of nobel-metal platinum alloy results in a substantially longer service life of he electrodes, which are exposed to very great corrosion and erosion stresses in the combustion chamber because of the conditions prevailing there. For cost reasons, no provision has been made to date to produce the electrode of a spark plug completely from noble metal. However, varied possibilities have already been found to ennoble a spark plug electrode. German Patent No. 3 727 526 describes a method of producing a spark plug for internal combustion engines, in which a noble-metal platelet on the end face of the central or ground electrode is joined by laser welding to the material of the electrode.

The use of ennobled electrodes for a spark plug is always taken into consideration when one would like to achieve very long service live and longer maintenance intervals for a spark plug. In this connection, it is known to attach these noble-metal parts to the spark plug electrodes by laser welding and resistance welding. To compensate for different expansion coefficients, it is also conventional to provide a compensating layer between the electrode and the noble-metal part, to thus produce a better and more durable bond.

In accordance with the present invention, the use of a noble-metal part, which has a smooth surface on one side and a profiled surface on the other, for welding onto the spark plug electrode has the advantage that the profiled surface, because of the smaller contact area, exhibits a high transition resistance between the noble-metal part and the electrode during the welding process. The means a correspondingly high heat energy develops in the welding zone between the noble-metal part and the spark plug electrode. In turn, the provision of a smooth surface for the noble-metal part on the side facing the welding electrode has the advantage that a small transition resistance is present here, and less heat develops at this location than at the welding position. The present construction of the welded connection largely prevents a rupture and the corrosion of the welded connection between the noble-metal part and the electrode associated with such a rupture.

Profiling in the manner that, at the beginning of the welding process, the contact areas between the noble-metal part and the central electrode exist only in the edge area of the noble-metal part offers the advantage that the heating of the noble-metal part to be welded increases starting from the edge area toward the center, and because of the star-shaped structure, the air still present between the noble-metal part and the electrode can escape via radial channels. The welding temperature has a relatively uniform characteristic over the entire cross-section, and particularly in the critical edge area, can be brought to the desired high values. In this manner, the durability of the welded connection is substantially increased, which in turn influences the service life of the spark plug in a positive manner.

FIG. 1 shows a spark plug electrode and a noble-metal part of the present invention prior to welding.

FIG. 2 shows a plain view of a profiled side of the noble-metal part.

FIGS. 3 a , 3 b , 4 a , 4 b , 5 a and 5 b show individual method steps for producing the profiled noble-metal part and its welding to the spark plug electrode.

FIG. 1 shows a spark plug electrode 10 and a noble-metal part 11 which is placed on the end face of spark plug electrode 10 . This noble-metal part 11 has a smooth end face 12 and a profiled end face 13 . Noble-metal part 11 is arranged in such a way that it rests with its profiled side 13 on spark plug electrode 10 . Welding electrode 14 is shown schematically on the side of noble-metal part 11 having smooth surface 12 and facing away from spark plug electrode 10 .

FIG. 2 illustrates the design of profiled surface 13 of noble-metal part 11 . In FIG. 2, components which have already been explained with respect to FIG. 1 are provided with the same reference numerals. Noble-metal part 11 is profiled in such a way that, between spark plug electrode 10 and noble-metal part 11 , contact areas 20 are located at the outer edge of noble-metal part 11 . As can be seen in FIG. 2, a star-shaped profiling was selected here which ensures that only the outer regions initially form a contact area 20 with spark plug electrode 10 .

The production of the profiled noble-metal part according to the present invention and its welding to the central electrode shall now be clarified with reference to FIGS. 3 a and 3 b , FIGS. 4 a and 4 b , as well as FIGS. 5 a and 5 b . Since all the Figures relate to the same subject matter, the same reference numeral shall be used in each case for the same components, the reference numerals then not being completely explained again for the respective Figures.

In FIG. 3 a , a noble-metal wire 30 , e.g. a platinum wire, is guided through a retaining part 31 , such that noble-metal wire 30 projects from this retainer 31 on the other side in a controlled manner. The length of the noble-metal part can be determined by a setting or else a preselection of the projection of the noble-metal part. The portion of noble-metal wire 30 projecting from retaining part 31 is now sheared off by a shearing tool 32 . In FIG. 3 b , a noble-metal cropping 40 has already been sheared off from noble-metal wire 30 . Even given a relatively small wire diameter, it is possible to realize quite a great length of noble-metal cropping 40 using is this shearing method. The ratio of length to diameter of the noble-metal cropping is advantageously greater than 1.

In FIG. 4, noble-metal cropping 40 is disposed in an embossing die, between two stamps 41 and 42 and is compressed for enlarging the diameter, i.e., reducing the length. In this setting process, a first stamp 41 is provided with a smooth surface and a second stamp 42 is provided with a profiled surface. Because of this arrangement, noble-metal cropping 40 is embossed during the setting process shown in FIG. 4 b into the form already explained with respect to FIG. 1, i.e., one surface has a smooth or even flat shape, and the opposite surface of noble-metal part 11 is profiled.

As shown in FIG. 5 a , noble-metal part 11 thus formed is now mounted on spark plug electrode 10 , e.g. a central electrode, in the manner that the side having smooth surface 12 faces welding electrode 14 . Consequently, a good contact area with low transition resistance is present between noble-metal part 11 to be welded and welding electrode 14 . Profiled end face 13 of noble-metal part 11 is arranged opposite the spark plug electrode, so that a high transition resistance, advantageous for the welding process, exists between the spark plug electrode and noble-metal part 11 . The welding process itself may be carried out in two steps. In a first step, the profile tips are welded at contact areas 20 with a relatively low current intensity. In this manner, uniform starting conditions are produced for the second welding step. In the second welding step, the noble metal is welded with a higher current intensity up to the point of an all-over bond with the electrode.

FIG. 5 b shows the bond between spark plug electrode 10 and noble metal part 11 after the welding process. This welded bond can now be fed to further processing steps. Thus, for example, given a use as central electrode, it is possible to shear off this bond in its edge area to ensure that, at the farthest, the noble-metal part reaches up to the opposite electrode of the spark plug.

The described method permits a largely free formation of the noble-metal part on the spark plug electrode.

In principle, widely varying profilings of the noble-metal part are conceivable. Possible. However, in each case the contact points between the noble-metal part and the electrode should be situated at the edge area of the noble-metal part. During heating, the necessary welding temperature is thus reached first at this location in the critical edge area, then spreads to the middle of the noble-metal part, and thus assures a good, planar welded connection and a cleanly welded edge area.