Spark plug having a center electrode with improved thermal properties

Abstract

A spark plug is described as including an insulator which is situated in a spark plug housing and which has a longitudinal axis and an opening along the longitudinal axis. Furthermore, a center electrode is situated in the opening of the insulator. Furthermore, at least one ground electrode is provided at the spark plug housing. The insulator has a first insulator section and a second insulator section, the first insulator section extending in the direction of the longitudinal axis over a first insulator section length having a first inner diameter. The second insulator section extends in the direction of the longitudinal axis over a second insulator section length having a second inner diameter. Here, the first inner diameter is smaller than the second inner diameter, the first insulator section merging directly into the second insulator section. The center electrode has a center electrode section which extends in the direction of the longitudinal axis over a center electrode section length having a center electrode outer diameter at least over the entire insulator section length of the first insulator section and of the second insulator section.

Claims

1. A spark plug, comprising: a housing; an insulator situated in the housing, the insulator including a longitudinal axis and an opening along the longitudinal axis; a central electrode situated in the opening of the insulator; and at least one ground electrode provided at the housing, wherein: the insulator includes a first insulator section and a second insulator section; the first insulator section extends in a direction of the longitudinal axis over a first insulator section length having a first inner diameter; the second insulator section extends in the direction of the longitudinal axis over a second insulator section length having a second inner diameter; the first inner diameter is smaller than the second inner diameter; the first insulator section merges directly into the second insulator section in a transition; the center electrode includes a center electrode section that extends in the direction of the longitudinal axis over a center electrode section length having a center electrode outer diameter; the center electrode section extends at least over an entire insulator section length of the first insulator section and of the second insulator section; and one or more of the following five features: (1) the center electrode in the first insulator section is connected at least sectionally to the insulator by a glass melt that (a) is arranged between the center electrode and the insulator at least at an end face of the housing at which the at least one ground electrode is provided and/or (b) fills a gap, no greater than 120 m, that separates the center electrode from the insulator; (2) the center electrode has at least sectionally along a longitudinal extension of the center electrode a center electrode core of a first material that extends from inside the insulator to out of the insulator and is embedded in a center electrode sheath of a second material that has a lower thermal conductivity than the first material and that extends from within the insulator to outside of the insulator through the opening to beyond an end of the center electrode core; (3) the second insulator section has at least sectionally a second outer diameter that is greater than a first outer diameter of the first insulator section; and (4) on a combustion chamber side of the second insulator section, the second insulator section merges into a third insulator section having an inner diameter that is different than the second inner diameter.

2. The spark plug as recited in claim 1, wherein in the first insulator section, the center electrode has a radial distance of at least one of at least 40 m and maximally 120 m from the insulator.

3. The spark plug as recited in claim 1, wherein the center electrode in the first insulator section is connected at least sectionally to the insulator by the glass melt that (a) is arranged between the center electrode and the insulator at least at the end face of the housing at which the at least one ground electrode is provided and/or (b) fills the gap, no greater than 120 m, that separates the center electrode from the insulator.

4. The spark plug as recited in claim 1, wherein in the second insulator section, the center electrode has a radial distance of at least one of at least 100 m and maximally 500 m from the insulator.

5. The spark plug as recited in claim 1, wherein the second insulator section length of the second insulator section is at least one of at least 0.5 mm and maximally 10 mm.

6. The spark plug as recited in claim 1, wherein the first insulator section is situated on a side of the second insulator section that faces away from a combustion chamber.

7. The spark plug as recited in claim 1, wherein the second insulator section has at least sectionally the second outer diameter that is greater than the first outer diameter of the first insulator section.

8. The spark plug as recited in claim 1, wherein the center electrode section extends beyond the second insulator section on the combustion chamber side.

9. The spark plug as recited in claim 1, wherein the center electrode has at least sectionally along the longitudinal extension of the center electrode the center electrode core of the first material that is surrounded by the center electrode sheath of the second material that has the lower thermal conductivity than the first material, and the center electrode core has a portion of a cross sectional area of the center electrode of up to 60% at the point of the transition from the first insulator section to the second insulator section.

10. The spark plug as recited in claim 9, wherein the center electrode core has a portion of the cross section of the center electrode of up to 70% at a distance of 5 mm from the end of the center electrode section facing a combustion chamber.

11. The spark plug as recited in claim 9, wherein a distance of the center electrode core from an end of the center electrode section facing a combustion chamber is maximally 2.25 mm.

12. The spark plug as recited in claim 1, wherein: the insulator has a front side on the combustion chamber side, and the second insulator section extends up to the front side of the insulator.

13. The spark plug as recited in claim 1, wherein the center electrode has, at least sectionally along the longitudinal extension of the center electrode, the center electrode core of the first material that extends from inside the insulator to out of the insulator and is embedded in the center electrode sheath of the second material that has the lower thermal conductivity than the first material and that extends from within the insulator to outside of the insulator through the opening to beyond the end of the center electrode core.

14. The spark plug as recited in claim 1, wherein, at a mouth of the opening, an interior wall of the insulator forming the opening is conical.

15. The spark plug as recited in claim 1, wherein, on the combustion chamber side of the second insulator section, the second insulator section merges into the third insulator section having the inner diameter that is different than the second inner diameter.

16. The spark plug as recited in claim 3, wherein the glass melt is arranged between the center electrode and the insulator at least at the end face of the housing at which the at least one ground electrode is provided.

17. The spark plug as recited in claim 3, wherein the glass melt fills the gap, no greater than 120 m, that separates the center electrode from the insulator.

18. The spark plug as recited in claim 15, wherein the opening of the insulator is conical at least sectionally in the third insulator section.

19. The spark plug as recited in claim 18, wherein the third insulator section extends in the direction of the longitudinal axis over a third insulator section length.

20. The spark plug as recited in claim 19, wherein the center electrode section of the center electrode extends at least over an entire first insulator section length of the first insulator section, over an entire second insulator section length of the second insulator section, and over an entire third insulator section length of the third insulator section.

21. The spark plug as recited in claim 19, wherein the center electrode section extends over and beyond the third insulator section on the combustion chamber side.

22. The spark plug as recited in claim 19, wherein the third insulator section extends up to a front side of the insulator.

23. The spark plug as recited in claim 19, wherein a wall thickness of the insulator does not vary within the third insulator section by more than 10%, and the wall thickness is a radial wall thickness of the third insulator section in a plane perpendicular to a longitudinal axis of the spark plug.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic representation of a spark plug known from the related art;

(2) FIG. 2 shows a schematic representation of the combustion chamber side of a spark plug according to a first specific embodiment of the present invention;

(3) FIG. 3 shows a schematic representation of the combustion chamber side of a spark plug according to a second specific embodiment of the present invention;

(4) FIG. 4 shows a schematic representation of the combustion chamber side of a spark plug according to another specific embodiment of the present invention.

DETAILED DESCRIPTION

(5) According to the exemplary embodiments of the present invention, all figures are merely schematic representations of the devices or their components according to the present invention. In particular, distances and relations of magnitude are not represented every time true to scale in the figures. Corresponding elements are provided with identical reference numerals in the different figures.

DETAILED DESCRIPTION OF THE INVENTION

(6) In FIG. 1, a schematic representation of a spark plug 10 known from the related art is shown including an insulator 11, a spark plug housing 21, a longitudinal axis 12, an opening 13 of insulator 11, a center electrode 14 including an inner center electrode core 16, and an outer center electrode sheath 24 as well as at least one ground electrode 15.

(7) Here, insulator 11 is situated in spark plug housing 21. Opening 13 is provided in insulator 11 along longitudinal axis 12. Center electrode 14 is situated in the longitudinal direction of insulator 11 in essentially cylindrical opening 13 of insulator 11 in such a way that center electrode 14 is electrically insulated against spark plug housing 21 by insulator 11 and a thermal conduction from center electrode 14 into spark plug housing 21 is possible via insulator 11. Center electrode 14 includes inner center electrode core 16, the material of center electrode core 16 having a higher thermal conductivity than the material of center electrode sheath 24 which surrounds center electrode core 16. A favorable thermal conduction is thus made possible along center electrode 14.

(8) Ground electrode 15 is situated at an end of spark plug 10 on combustion chamber side 18 and connected to spark plug housing 21.

(9) The ignition energy is induced into spark plug 10 via a connection side 19 of spark plug 10. The applied high voltage generates an electric spark, which is suitable to ignite the air/fuel mixture present in the combustion chamber, at the end of spark plug 10 on combustion chamber side 18 between center electrode 14 and ground electrode 15.

(10) In addition, for a long service life of spark plug 10, favorable heat dissipation is necessary from center electrode 14 into insulator 11 and from ground electrode 15 and from insulator 11 into spark plug housing 21. The temperatures reached at center electrode 14 and ground electrode 15 determine the service life of spark plug 10 to a considerable extent. A higher temperature at center electrode 14 and ground electrode 15 results in increased wear at center electrode 14 and ground electrode 15 and thus in a reduced service life of spark plug 10.

(11) The schematic representation shown in FIG. 2 shows the section on combustion chamber side 18 of a first specific embodiment of the present invention. Identical elements with reference to FIG. 1 are identified with identical reference numerals and are not elucidated in greater detail.

(12) According to the present invention, opening 13 is implemented in the longitudinal direction of insulator 11 in an essentially cylindrical first insulator section 31 which has a first insulator section length 61 having a first essentially constant inner diameter 51 and in an essentially cylindrical second insulator section 32 which is directly adjacent thereto and has a second insulator section length 62 having an essentially constant second inner diameter 52. First inner diameter 51 of opening 13 of insulator 11 is smaller than second inner diameter 52 of opening 13 of insulator 11.

(13) First insulator section 31 merges directly into second insulator section 32. The expansion of a transition 25 in the longitudinal direction of the insulator from first insulator section 31 into second insulator section 32 is in this case considerably smaller than second insulator section length 62 of second insulator section 32 in the longitudinal direction of the insulator. The inner diameter of insulator 11 in the area of transition 25 is not smaller than first inner diameter 51 of first insulator section 31 and not greater than second inner diameter 52 of second insulator section 32. Second insulator section 32 extends on combustion chamber side 18 of insulator 11 on an insulator section length 62 up to a front side 20 of insulator 11.

(14) Furthermore, a center electrode 14 is situated in opening 13 of insulator 11. Center electrode 14 includes a center electrode section 44 having a center electrode section length 64 in the longitudinal direction having a center electrode outer diameter 54 of center electrode 14, center electrode section 44 extending at least over the entire length of first insulator section length 61 of first insulator section 31 and of second insulator section length 62 of second insulator section 32.

(15) Center electrode core 16 is embedded in a center electrode sheath 24. In the present exemplary embodiment, center electrode core 24 has a portion of the cross sectional area of 40% at the point of transition 25 from first insulator section 31 to second insulator section 32. The portion of center electrode core 24 of the cross sectional area of center electrode 14 may reach up to 60% at the point of transition 25 from first insulator section 31 to second insulator section 32. In the present exemplary embodiment, center electrode core 24 has a portion of the cross sectional area of 20% at a distance of 5 mm from end of center electrode section 44 facing combustion chamber 23. The portion of center electrode core 24 of the cross sectional area of center electrode 14 may reach up to 70% at a distance of 5 mm from the end of center electrode section 44 facing combustion chamber 23. The distance of center electrode core 16 from the end of center electrode section 44 facing the combustion chamber is maximally 2.25 mm.

(16) In first insulator section 31 of opening 13 of insulator 11, center electrode 14 is joined by a narrow gap 22 and connected at least partially via a glass melt 17. The width of this gap 22 is at least 40 m and/or maximally 120 m. This connection between insulator 11 and center electrode 14 via glass melt 17 is also used to dissipate the heat from center electrode 14 via insulator 11 into spark plug housing 21.

(17) In order to reduce the heat absorption of center electrode 14 from the part of insulator 11 on the combustion chamber side, opening 13 of insulator 11 is expanded in insulator section 32 by an additional bore. This results in a gap in insulator section 32 between insulator 11 and center electrode 14 which is implemented in center electrode section 44 having an essentially constant diameter. The width of this gap is at least 100 m and/or maximally 500 m. The length of this gap is at least 0.5 mm and/or maximally 10 mm in the longitudinal direction of the center electrode.

(18) FIG. 3 shows in a schematic representation the section 18 on the combustion chamber side in a second specific embodiment of the present invention which is different from the first specific embodiment essentially in that the expansion of opening 13 in insulator section 32 is fully or partially compensated for by an enlarged insulator outer diameter 53. Identical elements with reference to FIGS. 1 and 2 are identified with identical reference numerals and are not elucidated in greater detail.

(19) According to the present invention, the expansion of insulator outer diameter 53 of insulator 11 in insulator section 32 is essentially implemented in such a way that the expansion of opening 13 in insulator section 32 is compensated for.

(20) The schematic representation of the third specific embodiment of the present invention shown in FIG. 4 shows an expansion of opening 13 of insulator 11 which has an at least partially conical design in a third insulator section 33 which is directly adjacent to the second insulator section and extends up to front side 20 of insulator 11.

(21) Identical elements with reference to FIGS. 1, 2, and 3 are identified with identical reference numerals and are not elucidated in greater detail.

(22) The at least partially conical expansion of opening 13 leads to a gap, which changes over a section length 63, between insulator 11 and center electrode 14. The distance from center electrode 14 is thus enlarged even more at the particularly hot end of insulator 11 and a further temperature reduction at the firing tip of center electrode 14 is achieved.