GB2027797A

GB2027797A - Spark plug

Info

Publication number: GB2027797A
Application number: GB7918522A
Authority: GB
Original assignee: NGK Spark Plug Co Ltd
Current assignee: Niterra Co Ltd
Priority date: 1978-07-28
Filing date: 1979-05-29
Publication date: 1980-02-27
Also published as: IT7949225A0; FR2432232A1; BR7904108A; DE2922305A1; FR2432232B1; DE2922305C2; IT1117363B; GB2027797B

Abstract

A spark plug has a central electrode projecting from an insulator 2, and an L-shaped outer electrode 1. The outer end surface of the central electrode has a groove which extends in the direction of extent of the outer electrode 1 so as to define projections 4 and 5 which are disposed opposite respective side edges 11 and 12 of the outer electrode. Such an arrangement ensures that sparking will occur at the edges of the electrodes so as to reduce the quenching effect of the electrodes on the flame nuclei. Projection opposite the projections 4 and 5 may be provided on the outer electrode. <IMAGE>

Description

SPECIFICATION Spark plug This invention relates to a spark plug.

Recently developed automotive engines adopt a lean air/fuel combustion mixture for clean exhaust gases, and an increased spark advance to reduce the fuel consumption, particularly during idling. For this reason, the idle does not spoil engine power. The present inventor has found that, when the spark advance is increased, a spark discharge occurs during an engine compression stroke, and flame nuclei produced by the spark discharge as shown in Fig. 1, propogate from a position A adjacent the outer electrode to a position A' adjacent the centre electrode D of a spark plug C in compliance with movement of piston B. However, a strong quenching effect is applied to the flame by the centre electrode D and this quenching effect is the main cause of misfire during idle low speed running of the engine.A spark plug having a groove in the centre electrode or in the outer electrode in order to enhance ignitability has been previously proposed. However, since such a spark plug having a groove is made irrespective of the above-noted fact, the enhancement of the ignitability is still deficient.

Many spark plug designs having various cuts and holes. in the electrodes have been proposed. Typical are annular discharge portions shown in the inner and outer electrode surfaces of U.S. Patent 4015160, an annular ring and channel arrangement of U.S. Patent 4023058 and a V-type earthing electrode with a channel shown in U.S. Patent 222641 5. Additionally, U.S. Patent 3970885 discloses, in addition to various types of groove, a projection provided on the earth electrode. As described in U.S. Patent 3970885, the projection is disposed in the area of the spark discharge confronting the tip surface of the centre electrode. The flame nuclei produced by the spark are rapidly spread out over the projection to facilitate growth of flame nuclei and easy propogation of flame.The hallmark of these previously proposed devices is the modification of the earth electrode to limit the area of the earthed surface. However, it has been found that many deficiencies remain, in actual use, so these spark plugs do not satisfactorily perform, especially in an idle speed engine condition.

According to the present invention there is provided a spark plug including an insulator, a centre electrode disposed in said insulator and projecting therefrom, and an outer electrode at least one of the electrodes having a groove which extends in the direction of extent of the outer electrode so as to define a pair of projections on opposite sides of said groove whereby spark gaps are formed between said projections and the other electrode.

In the accompanying drawing: Figure 1 shows a schematic view of a typical combustion chamber in an internal combustion engine, with a spark plug in position; Figure 2 shows a side view of the tip of a spark plug according to the present invention; Figure 3 shows a front view of the spark plug tip shown in Fig. 2; Figure 4 shows a plan view of the spark plug tip shown in Figs. 2 and 3; Figure 5 shows a side view of a conventional spark plug tip which has eroded in service; Figures 6 and 7show the tip and centre electrodes, respectively, of other embodiments of spark plug according to the present invention; Figures 8 and 9 are graphs in which the number of misfires during idle speed in a three minute period are plotted against the carbon monoxide content of the exhaust gas;; Figure 10 shows a front view of the tip of another embodiment of spark plug according to the present invention; Figure 11 is a graph similar to the graphs of Figs. 8 and 9; Figures 12 to 16 show the tips of further embodiments of spark plug of the present invention; and Figure 1 7 shows a front view in greater detail of the tip of a spark plug of the present invention.

Referring now to Figs. 2 to 4, the spark plug has an L-shaped outer or earth electrode 1 anchored to an externally threaded, metal sleeve portion 1 0. The cross-section of the electrode 1 forms a closed region and, in this embodiment, is rectangular. The spark plug has an electrical insulator 2 which projects from the sleeve portion 1 0. A centre electrode 3 is coaxially disposed in insulator 2 and projects from the outer end thereof. The outer end surface of the centre electrode 3 is provided with projections 4 and 5 extending away from the insulator 2 and towards the electrode 1. These projections are formed by grinding the outer end surface of the centre electrode 3 and are disposed opposite respective sides 11 and 1 2 of the outer electrode 1.

in the above-described spark plug, spark discharges are generated in regions E and F (Fig. 3) defined between end surfaces of the projections 4 and 5 and the outer electrode 1.

The spark discharges seldom occur in regions G and H (see Figs. 2 and 4). The previously mentioned quenching effect seldom occurs to flame nuclei produced by the spark in the regions E and F which are far from the outer surfaces of the centre electrode 3 and the outer electrode 1. The same effect is obtained when the flame nuclei produced by the spark are moved toward the centre electrode.

In contrast, in the region G of the conventional spark plug as shown in Fig. 5, the flame nuclei tend to contact the outer electrode 1 having a large contact area which enhances the quenching effect. Region H has a large contact area with the centre electrode if an end surface 1 3 of the outer electrode 1 is, shown in Fig. 5, located adjacent the central axis of the centre electrode, i.e. if it is displaced to a region H' defined by a surface 13' due to an assembly error due to erosion of the electrode 1 in service. In this case, the flame nuclei are moved to the centre electrode in compliance with the movement of the piston during the compression stroke, as mentioned above, enhancing the quenching effect.

The preferred height h (Fig. 3) of the projections 4 and 5 is 0.1 to 2.5mm and the preferred maximum width W thereof is 0.3 to 2.0 mm with a view to preventing misfire and obtaining a good spark plug durablilty.

In the spark plug as shown in Figs. 2 to 4, since the ignitability is enhanced and the projections 4 and 5 are formed on an electrode which generally has electrically negative characteristics in use, an effect can be obtained where the discharge voltage can be reduced. Additionally, the projections 4 and 5 can be simply formed by cutting or grinding the groove in the central electrode, during the final cutting process where the end surface of the centre electrode is finished to determining the final dimensions thereof. Any additional complicated process such as the welding on of additional parts is not necessary. Furthermore, the outer and centre electrodes have sufficient durability due to the reduced wear rate of the electrodes and, therefore a spark plug of this invention has high practical value.

Referring now to Fig. 6, where parts similar to the parts of the spark plug of Figs. 2 to 4 are accorded the same reference numeral, projections 14 and 1 5 project from the outer electrode and are disposed opposite the projections 4 and 5, respectively, to enhance further the possibility of the spark discharge in the regions E and F. In an alternative embodiment (not shown), the projections are defined by forming a central groove in the surface of the electrode 1 which faces the central electrode 3. The groove between the projections in or on the centre and/or the outer electrode may be of U-section, V-section, rectangular section or hemispherical section (see Fig. 7 where the shape of the groove of the central electrode only is shown). The projections and associated grooves may be slanted toward both sides of the outer electrode.However, irrespective of the configuration used, it is noted that two projections and a channel or groove therebetween are provided.

Experimental data on the operating characteristics of a spark plug according the present invention will be hereinafter described. A fourcycle four-cyclinder engine having a total piston displacement volume 2000ml was used for obtaining the data. Fig. 8 shows experimental results obtained from a conventional spark plug (curve 1) with a centre electrode having an outer diameter of 2.4mm and no groove, a spark plug (curve ll) with a rectangular groove having a depth of 0.5mm and a width of 1.0mum formed in the outer electrode, and a spark plug (curve Ill) having a Vshaped groove having a depth of 0.5mm and width of 1 .Omm thereby to form projections in the centre electrode in the direction of the outer electrode according to the present invention.The results show numbers of misfires per three minutes with ignition at 17 BTOC at an idle speed of 650 RPM, standardized as concentrations of CO contained in the exhaust gas, which corresponds to the air/fuel ratio. It is obvious from the results that with a spark plug according to the present invention there is a significant reduction of misfires and the ignitability is excellent by use of a markedly lean air/fuel ratio during idle speed. Fig. 9 shows other experimental results of the spark plug producing (curve III). Curve Ill(a) designates a spark plug wherein the direction of the groove is the same as that of the outer electrode.Curves Ill(b), Ill(c) and Ill(d) relate to spark plugs wherein the groove is intersected by the outer electrode at an angle of 30 , 60 and 90 , respectively. The numbers of the misfires were measured in the same manner. From the results, it is obvious that the plug producing curve Ill(a) has the smallest number of misfires where the groove is in the same direction of the outer electrode while the plug producing curve Ill(d) has the largest number of misfires where the groove is normal to the outer electrode.The effect of the present invention due to the fact that the projections are positioned in the centre electrode toward both sides of the outer electrode is therefore substantiated.

As mentioned herein, the parts of the centre electrode positioned facing the outer sides of the outer electrode are projected so that the spark discharge is selectively generated between the prejections and the outer electrode.

Therefore, the flame nuclei generated therebetween are out of the influence of the quenching effect of the centre and outer electrodes. Accordingly, misfire can be prevented during low engine speed conditions such as at idling and the engine can be efficiently driven using a lean air/fuel mixture.

Referring now to Fig. 10, the spark plug illustrated therein has outer electrode 1 formed with a cylindrically concave surface 41 facing the central electrode 3 so as to define wedge-shaped projections 42 and 43 at opposite sides thereof. The central electrode 3 has a flat end surface 30. In this spark plug, because of the electric concentration phenomenon of the end of the wedge-shaped projections 42 and 43 and because the spark dis charge distance thereof is short, a non-uniform electric field is generated and the strength of the electric field becomes large.

Therefore, most of spark discharge occurs between the projections 42 and 43 and edge portions 31 and 32 opposite the respective projections 42 and 43. Flame nuclei are produced in regions a and b positioned on both sides of the spark gap. The flame nuclei thus produced in regions a and b at opposite sides of the spark gap are not susceptible to the quenching effect of the walls of the centre and outer electrodes on the flame nuclei during the growing period of the flame nuclei.

Accordingly, ignition is positively achieved to prevent misfires in the internal combustion engine.

In contrast, in case where a spark discharge occurs in a central region d of the spark gap, a large quenching effect by the end surface 30 of the centre electrode and the surface of the outer electrode on the centre electrode is applied to the flame nuclei so that misfires are liable to result, especially under the engine running conditions such as idling where difficulty of ignition is most noticed due to low engine RPM.

The above-described projections 42 and 43 must be sharpened at their tips as wedges and at the same time, the tips must be positioned at extreme edges of the surface 41. In this embodiment, distance A in the axial direction of the centre electrode from the centre of the surface 41 to the tip of each projection 42, 43 is less than about 0.3mm.

Angle B between a tangent to the surface 41 at the tip of the wedge-shaped projection 42 or 43 and the end surface of the centre electrode 3 is in a range from 5" to 15 .

Angle C of the tip portion of the projection 42, 43 is 80 to 90 . The surface 41 is not planar between the wedge-shaped projections 42 and 43. The above-described numerical limitations are determined or desired in view of the durability of the spark plug. For enhancement of the ignitability, desirably, the diameter of the centre electrode is the same as the width of the outer electrode or the difference therebetween is less than 1 .0mum.

The reason is that if the difference deviates, increasing from the range, a spark is generated in the central portion, on the electrode side, where the spark discharge is large so that a strong quenching effect occurs.

Experimental data showing differences between the spark plug as shown in Fig. 10 and that of the prior art in the ignitability will be described.

Referring now to Fig. 11, curve (I) is that produced by a spark plug in which the spark gap between a centre electrode and an outer electrode is formed by plane surfaces. Curve (II) is that produced by a spark plug in which a longitudinal groove is formed in the centre electrode-facing surface of the outer electrode.

Curve (III) is that produced by the spark plug of Fig. 10. The experiment was carried out under substantially the same conditions as those of the preceding experiment. The engine used was a four-stroke, four-cylinder engine, having a total piston displacement volume of 1 600ml, the engine having been improved for exhaust emissions. As will be aparent from Fig. 11, with a spark plug according to the present invention, it is possible to operate the engine with a lean air/fuel ratio (low CO concentrations and at the same time, the number of the misfires is small.

Referring now to Fig. 12, the spark plug illustrated therein has wedge-shaped projections 51 and 52 at the outer end face 30 of the centre electrode 3. These projections 51 and 52 are in alignment with side portions 1 2 and 11, respectively of the outer electrode 1.

Also, in the spark plug of this embodiment, the spark discharge is generated in regions a and b at opposite sides of the spark gap. As in the preceding embodiments an enhancement of the ignitability is achieved.

In Fig. 13, wedge-shaped projections 51, 52 and 42, 43 are formed on the outer end face 30 of the centre electrode 3 and the inner surface 41 of the outer electrode 1, respectively. Also in this embodiment, a spark discharge occurs in region a and b on both sides of the spark gap.

In Fig. 1 4 the central electrode-facing surface of the outer electrode 1 is V-shaped in a concave manner and, as a result, both sides 71 and 72 become wedge-shaped projections.

Figs. 1 5 and 1 6 show combinations of an outer electrode 1 having a wedge-shaped projection in both sides of its central electrodefacing surface and rectangular, and V-shaped grooves 8 and 9, respectively in the central electrode. These modifications have the same spark discharge generated position and ignitability enhancement as the preceding embodiments.

Fig. 1 7 shows a practical embodiment of spark plug according to the present invention in which the apices of the wedge-shaped projections are rounded by machining and the maximum depth of concave portion of the outer electrode 1 is very small. A V-shaped groove 9 is formed in the centre electrode.

Dimensions in nm used in Fig. 1 7 are only for reference.

As mentioned above, a spark plug of the present invention is constructed so that at least wedge-shaped projections are formed on both side surfaces, on the centre electrode, of the outer electrode or both side portions, on both sides of the outer electrode, of the end surface of the centre electrode, as a result of which the spark discharge is positively generated in the both side regions of the spark gap.

Therefore, the quench effect of the centre electrode and the outer electrode is very small to thereby enhance the ignitability.

Claims

CLAIMS (29 May 1979)

1. A spark plug including an insulator, a centre electrode disposed in said insulator and projecting therefrom, and an outer electrode, at least one of the electrodes having a groove which extends in the direction of extent of the outer electrode so as to define a pair of projections on opposite sides of said groove whereby spark gaps are formed between said projections and the other electrode.

2. A spark plug as claimed in Claim 1 wherein said groove is formed in the centre electiode.

3. A spark plug as claimed in Claim 2, wherein a further groove is provided in a central electrode-facing surface of the outer electrode so as to define a pair of wedgeshaped edge projections, said further groove extending in the direction of extent of the outer electrode, thereby forming spark gaps with the opposite projections of the centre electrode.

4. A spark plug as claimed in Claim 2, wherein the height of said projections from the centre of the end surface of the centre electrode is in the range of 0.1 to 2.5mm.

5. A spark plug as claimed in Claim 2 to 4, wherein the width of said projections in a direction perpendicular to the direction of extent of said groove is in the range of 0.3 to 2.0mm.

6. A spark plug as claimed in claim 2, 4 or 5, wherein said groove is semicircular.

7. A spark plug as claimed in claim 2, 4 or 5, wherein said groove is V-shaped.

8. A spark plug as claimed in claim 2, 4 or 5, wherein said first groove is rectangular.

9. A spark plug as claimed in claim 3, wherein the height from the centre of said inner surface of the top of each wedge-shaped projection is less than 0.3mm.

10. A spark plug as claimed in claim 3, wherein the angle formed by a tangent to said central electrode-facing surface and an end surface of the centre electrode is in the range of 5" to 15'.

11. A spark plug as claimed in claim 1, 2 or 3, wherein said centre electrode is circular and has a diameter substantially equal to the width of said outer electrode.

1 2. A spark plug as claimed in claim 1 or 2, wherein a central electrode-facing surface of the outer electrode is flat.

1 3. A spark plug substantially as hereinbefore described with reference to Figs. 2 to 4 or Fig. 6 or Fig. 7 or Fig. 10 or Fig. 12 or Fig. 13 or Fig. 14 or Fig. 15 or Fig. 16 or Fig. 1 7 of the accompanying drawings.

CLAIMS (24 Oct 1979)

1. A spark plug including an insulator, a centre electrode disposed in said insulator and projecting therefrom, and an outer electrode, at least the centre electrode having a groove which extends in the direction of extent of the outer electrode so as to define a pair of projections on opposite sides of said groove whereby spark gaps are formed between said projections and the outer electrode.