US3691418A

US3691418A - High voltage sparkplug

Info

Publication number: US3691418A
Application number: US165151A
Authority: US
Inventors: Otto Beesch
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1969-03-18
Filing date: 1971-07-22
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12
Also published as: GB1260455A; JPS4844702B1; ES156885U; FR2039027A5; ES156885Y; DE1913582C3; DE1913582B2; DE1913582A1

Abstract

A high-voltage sparkplug has an electrically conductive housing from one end of which a ground electrode projects. An elongated center electrode extends through the housing and has a first end portion projecting from the one end and defining a spark gap with the ground electrode and a second end portion projecting from the opposite end of the housing and having a free terminal. An electrical insulator insulates the center electrode from the housing and includes an elongated insulator portion which surrounds the second end portion of the center electrode intermediate the housing and the terminal and is provided in its outer circumferential surface with a plurality of axially spaced grooves which serve to retard current leakage between the terminal and the housing by increasing the sparkover potential between the two. The grooves are provided at least in that part of the insulator portion which is nearer to the free terminal and are separated from each other by ribs having flanks which make with a plane normal to the axis of the insulator an angle of about 10*.

Description

United States Patent.

Beesch 51 Sept. 12, 1972 [541 HIGH VOLTAGE SPARKPLUG [63] Continuation-in-part of Ser. No. 20,275, March 17, 1970, abandoned,

Stuttgart,

[30] Foreign Application Priority Data March 18, 1969 Germany ..P 19 13 582.1

[52] US. Cl. ..313/13l, 313/118, 313/137 [51] Int. Cl. ..H0lt 13/20 [58] Field of Search ..3l3/118, 130, 131,137, 325;

' Primary Examiner-Roy Lake Assistant Examiner-Darwin R. l-lostetter Attomey--Michael S. Striker [57] ABSTRACT A high-voltage sparkplug has an electrically conductive housing from one end of which aground electrode projects. An elongated center electrode extends through the housing and has a first end portion projecting from the one end and defining a spark gap with the ground electrode and a second end portion projecting from the opposite end of the housing and having a free terminal. An electrical insulator insulates the center electrode from the housing and includes-an elongated insulator portion which surrounds the second end portion of the center electrode intermediate the housing and the terminal and is provided in its outer circumferential surface with a plurality of axially spaced grooves which serve to retard current leakage between the terminal and the housing by increasing the sparkover potential between the two. The grooves are provided at least in that part of the insulator portion which is nearer to the free terminal and are separated from each other by ribs having flanks which make with a plane normal to the axis of the insulator an angle of about 10.

[56] References Cited UNITED STATES PATENTS 2,723,364 11/1955 Cipriani et a1. ..315/58 FOREIGN PATENTS 0R APPLICATIONS 893,137 4/1962 Great Britain ..174/212 1 E E R l PATENTEDsmzmn 3.691.418

SHEET 2 OF 2 Fig. 3 5 #299 IN VENTOR 07m BENCH BY wwf fa/M ATTORNEY HIGH VOLTAGE SPARKPLUG CROSS-REFERENCES TO RELATED APPLICATIONS The present application is a continuation-in-part of 5 my previous application, Ser. NO. 20,275, filed on Mar. 17, 1970, now abandoned under the title HIGH- VOLTAGE SPARKPLUG.

BACKGROUND OF THE INVENTION The present invention relates generally to spark plugs, and more particularly to high-voltage sparkplugs. It is known to construct sparkplugs in the outer circumferential surface of that portion of their insulator which is located exteriorly of the sparkplug housing, with circumferential grooves which increase the current leakage path between the center electrode and the housing which is connected to ground, to thereby reduce spark-over. The conventional sparkplugs provided with such grooves are employed under conditions requiring a medium potential, under which circumstances the increas of the current-leakage path afforded by these grooves-which usually are of triangular or near-triangular cross-section--is entirely adequate.

However, increasing developments and refinements in internal combustion engines have brought and are still bringing with them additional problems. Thus, the requirements for increasing the compression of combustion engines, providing pollution control measures and analogous requirements necessitate an increasingly high voltage potential which, in turn, leads to increasing spark-over potential. It has been found that under these circumstances the length of current leakage paths obtainable in the known sparkplugs-always keeping in mind that it is difficult if not impossible to increase the height or length of the sparkplugs because of other requirements which dictate this dimension-is inadequate without changing the length or height, so that these known sparkplugs no longer are capable of preventing spark-over between the terminal of the center electrode at which the latter is connected with a voltage supply, and the housing.

Reference may be has to US. Pat. No. 2,723,364 to Cipriani et al. which discloses an insulator with a pair of grooves remote from the exposed terminal of the center electrode and serving primarily to prolong the useful life of a carbonaceous resistor in an axial bore of the insulator. The grooves are flanked by surfaces which are strongly inclined with reference to a plane normal to the axis of the insulator.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-voltage sparkplug for use in internal combustion engines which is particularly suited to prevent current leakage between the housing or shell of the sparkplug and the exposed part of the center electrode.

A further object of the invention is to provide a sparkplug wherein the spark-over potential is substantially increased over the value know from and achievable with prior-art sparkplugs.

One feature of the invention resides in the provision of a high-voltage sparkplug for use in internal combustion engines which comprises a conductive housing or shell having opposite ends and a ground electrode provided at one of these ends. An elongated center electrode extends through the housing and has a first end portion projecting from the one end and defining a spark gap with the ground electrode. A second end portion of the elongated center electrode projects from the other end of the housing and has an exposed terminal remote from the latter. An electrical insulator insulates the center electrode from the housing and includes an elongated insulator portion surrounding the second end portion intermediate the other end of the housing and the terminal. At least in that part of the insulator portion nearest the exposed terminal of the center electrode the insulator portion is provided with a plurality of circumferential ribs separated from each other by annular current leakage-reducing grooves. These ribs have substantially radially extending flanks which make with respect to a plane that is normal to the axis of the insulator an angle of substantially less than 20, preferably 5-l 5 and most preferably 10.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved sparkplug itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view, partly in section, illustrating a sparkplug which is constructed in accordance with the present invention;

FIG. 2 is a fragmentary sectional detail view, on an enlarged scale, of the electrical insulator of the sparkplug in FIG. 1; and

FIG. 3 is a view similar to that of FIG. 2 but further showing the dimensions of various parts of the insulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing now the drawings in detail, and referring first to FIG. 1, the reference numeral 10 denotes a housing or shell which forms part of my novel sparkplug. This housing 10 consists of steel. Secured to the housing 10 at one end thereof is a ground electrode 11 which is conductively connected to the housing. An insulator 12 consisting of ceramic material capable of providing the requisite electrical insulation extends into the housing 10. The insulator 12 is gas-tightly mounted in the housing 10. Extending through the insulator is a high-voltage electrode 13 which is gastightly mounted in the insulator 12.

The free or terminal end of the electrode 13 is provided with an exterior thread (no reference numeral) which meshes with corresponding interior threads provided on a cap nut 14 WHICH serves to connect to the center electrode 13 a voltage supply cable (not illustrated). The cable can also be connected with the terminal end of the electrode 13 in some other manner, and it is not a part of this invention. The other end of the electrode 13 defines with ground electrode 11 a spark gap.

as shown in FIG. 3, and grooves 15 that are provided in the outer circumferential surface of the exposed portion-exposed with respect to the confines of the housing 10-of the insulator 12.

As shown in FIG. 2, and in great detail in FIG. 3, four circumferential ribs 16 separated from each other by four circumferential grooves 15 are provided in the upper portion of the circumferential surface of insulator 12 as viewed in the drawing. The

flanks

16a and 16b of each rib 16 make an angle of about 10 with a plane A-A which is normal to the axis X-X of the insulator. As can be seen from the dimensions shown in FIG. 3 the average width, as considered in the direction of the axis X-X of the insulator, each rib 16 is approximately the same as the average width of each groove 15. Furthermore, each of the grooves 15 and ribs 16 consist of two halves which are substantially symmetrical with reference to a plane extending through the respective groove or rib in the direction normal to the axis X-X of the insulator. The transition between the

flanks

16a and 16b and the peripheral surfaces 16c of the ribs 16 and the surfaces 15a in the innermost regions of the grooves 15 have been smoothed. These transitions are shown in FIG. 3 with radii of curvature of 0.3 mm and 0.4 mm respectively. It will be noted that the insulator 12 is slightly tapered. At its upper end ithas a diameter of 11.8 mm which increases to a diameter of 12.2 mm at the lowest rib. The diameter of the insulator at the uppermost groove is 10.0 mm which increases to a diameter of 10.3 mm at the lowest groove.

It has been determined that for the purpose of preventing current leakage best results would be achieved if the ribs would have flanks which are paral lel to the plane A-A. However, for convenience in manufacture, a small angle substantially less than 20 has been found satisfactory to achieve the objects of this invention. For example, such angle can be between and 15.

Due to such selection of the angle, the grooves are of a substantially trapezoidal cross-section. It is these grooves, and more particularly their cross-sectional configuration, which retards current leakage between the terminal 13 and the housing by increasing the spark-over potential between the two. The rounded surfaces that make up the transition between the peripheral surfaces of the ribs, the flanks, and the innermost region of the grooves prevent potential peaks and discharges from these junctures as could take place if the junctures were sharp rather than rounded.

It has been found that with a sparkplug constructed in accordance with the present invention and without increasing the number of grooves it is possible to achieve an increase of approximately percent in the spark-over potential between the center electrode 13 and the housing 10. The spark-over potential can be increased in the sparkplug according to the present invention, for example, from 27,000 volts attainable with a conventional sparkplug to 31,000 volts. The grooves are (but need not be) of substantially trapezoidal crosssection and constitute a leakage-barrier which prevents spark-over between the center electrode and the sparkplug housing in the potential range in question.

It is emphasized that by increasing the number of grooves 15 the spark-over potential can be further increased, that is the spark-over will be further retarded.

To my knowledge no sparkplugs were available prior to the invention which were capable of meeting the increased requirements made of them with respect to spark-over potential, thus making it in turn impossible to meet the requirements which are now being made by engine designers in view of anti-polution measures, higher compression ratios, and the like.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A high-voltage sparkplug for use in internal combustion engines, comprising an electrically conductive housing having opposite ends; a ground electrode provided at one of said ends; an elongated center electrode extending through said housing and having a first end portion projecting from said one end and defining with said ground electrode a spark gap, and a second end portion projecting from the other of said ends and having a terminal remote from the latter; and an electrical insulator insulating said center electrode from said housing and including and elongated insulator portion having an outer surface and surrounding said second end portion intermediate said other end and said terminal, said insulator portion having a plurality of circumferential ribs separated from each other by annular current-leakage retarding grooves and said ribs having substantially radially extending flanks making with a plane which is normal to the-axis of said insulator an angle of substantially less than 20.

2. A sparkplug as defined in claim 1, wherein said angle is between 5 and 15.

3. A sparkplug as defined in claim 1, wherein said angle is approximately 10.

4. A sparkplug as defined in claim I, wherein said elongated insulator portion includes a first part nearer to and a second part remote from said terminal, said ribs and said grooves being provided at least in said first part of said insulator portion.

5. A sparkplug as defined in claim 1, wherein the average width of said grooves, as considered in the direction of said axis, at least approximates the average width of said ribs.

6. A sparkplug as defined in claim 1, wherein said insulator portion further comprises annular faces disposed in said grooves between said ribs, there being a smooth transition between each of said annular faces and the adjoining flanks.

7. A sparkplug as defined in claim 1, wherein each of said ribs has an annular peripheral surface, there being a smooth transition between each such peripheral surface and the flanks of the respective rib.

8. A sparkplug as defined in claim 1, wherein said insulator consists of ceramic material, and wherein said ribs define four grooves.

9. A sparkplug as defined in claim 1, wherein the flanks of each of said ribs are substantially mirror symmetrical with reference to a plane halving the respective rib and normal to said axis.

Claims

1. A high-voltage sparkplug for use in internal combustion engines, comprising an electrically conductive housing having opposite ends; a ground electrode provided at one of said ends; an elongated center electrode extending through said housing and having a first end portion projecting from said one end and defining with said ground electrode a spark gap, and a second end portion projecting from the other of said ends and having a terminal remote from the latter; and an electrical insulator insulating said center electrode from said housing and including and elongated insulator portion having an outer surface and surrounding said second end portion intermediate said other end and said terminal, said insulator portion having a plurality of circumferential ribs separated from each other by annular current-leakage retarding grooves and said ribs having substantially radially extending flanks making with a plane which is normal to the axis of said insulator an angle of substantially less than 20*.

2. A sparkplug as defined in claim 1, wherein said angle is between 5* and 15*.

3. A sparkplug as defined in claim 1, wherein said angle is approximately 10*.

4. A sparkplug as defined in claim 1, wherein said elongated insulator portion includes a first part nearer to and a second part remote from said terminal, said ribs and said grooves being provided at least in said first part of said insulator portion.