GB2189545A

GB2189545A - Spark plugs

Info

Publication number: GB2189545A
Application number: GB08610279A
Authority: GB
Inventors: John Rendell Conrad Pedersen
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-04-26
Filing date: 1986-04-26
Publication date: 1987-10-28
Anticipated expiration: 2006-04-26
Also published as: GB2189545B; GB8610279D0

Abstract

The spark plug has a central electrode 13 and a side electrode 14 adjacent thereto, adjacent opposing surfaces 15, 18 of the central and side electrodes 13, 14 being of generally parallel elongated, form, and each such surface being flanked by surfaces 16, 17 and 19, 20 sloping away therefrom. The plug provides a high efficiency spark initiation and flame generation pattern enabling lean mixtures to be ignited. <IMAGE>

Description

SPECIFICATION Spark plugs This invention relates to sparking plugs for use in internal combustion engines, to ignite fuel and air mixtures entering the combustion chamber.

A conventional sparking plug has a body, within which is supported an insulated central electrode and on which a side electrode is secured so as to extend over the end of the central electrode.

Conventionally the central and side electrodes have respective opposing flat surfaces, across which the spark is generated, the distance apart of these surfaces being adjusted by bending the side electrode towards or away from the central electrode. The two generally flat surfaces are thus often set at an arbitrary angle so that one portion of the pair of surfaces is closer together than another.

The point or points at which the spark will occur is thus quite random.

Sparking, therefore, often occurs in a zone which is relatively enclosed between the electrodes and to which only a small amount of combustible mixture may have penetrated.

Flame generation may thus, in some cases be relatively slow. This may give rise to incomplete combustion or poor combustion characteristics, in relation to the travel of the piston, or otherwise.

Erosion of the electrodes due to uneven sparking, or sparking in one particular zone is also observable.

The time cycle is of course short but it has been shown that there are two stages of combustion, namely spark initiation, during which there is no pressure rise, and then general flame propogation across the combustion chamber. The boundary between the two stages is indistinct but can be defined as the point in time at which the pressure first departs detectably from the level it would have if no combustion took place. The duration of the first stage is known to depend on the pressure and temperature of the charge just before the spark and also on the mixture strength in and around the spark gap, but not, to any great extent, on turbulence in the combustion chamber. The duration of the second stage depends again on pressure, temperature and mixture strength but also very strongly on turbulence.Engines are usually timed so that, at a usual working speed, the first stage and about half of the second stage are completed by the time the piston reaches top dead centre. Any significant variation from this time results in loss of efficiency.

It is also well known that fuel economy and improved exhaust pollution are improved if the mixture is as lean as possible. The shorter the first stage, the more practical is the use of leaner mixtures.

When conventional sparking plugs are used, the duration of the first stage of ignition increases greatly as mixture strength is reduced.

For example, in publication, "The high speed internal combustion engine", by Sir Harry Ricardo published 1953 (4th edition), results of tests are given as follows: Angle turned by crank during Mixture strength first stage 70 50 80 20 90 100 100 6" 110 5 120 5 (These results were taken at an engine speed of 200 rpm.) Increased spark advance reduces the pressure and temperature at time of the spark. As mixture is weakened or engine speed is increased the tendency of reduced pressure and temperature to increase the first stage duration as ignition is advanced eventually exceeds the advance and no further reduction in strength or increase in speed is possible.

It can be shown that the size of the opposing surfaces of conventional electrodes contributes to the relatively slow completion of the first stage of combustion, since the relatively large areas of metal contribute a cooling effect on the combustible mixture. Indeed it can be calculated that the duration of the first stage approximately matches the time for the flame front to reach the limits of the electrodes.

It is therefore the object of this invention to provide a sparking plug in which the effect of weak mixture on the duration of the first stage is minimised, this enabling the known advantages of lean mixtures to be realised, affording favourable combustion characteristics and fuel economy as well as minimising pollution from unburnt or incompletely burnt gases.

In accordance with the present invention there is provided a sparking plug having a central electrode and a side electrode disposed adjacent to the central electrode, the adjacent opposing surfaces of these electrodes being of generally parallel elongated narrow rail-like form, each such surface being flanked by surfaces sloping away therefrom.

Preferably, one end of the pair of rail-like surfaces is closer together than the other, the end of closest proximity being desirably that nearer to the end at which the side electrode is attached to the body of the sparking plug.

Conveniently, therefore, the rail-like surfaces are arranged lengthwise of the side electrode and across the centre of the central electrode.

The surfaces flanking the rail-like surfaces are advantageously relatively steeply inclined to one another so as to provide substantial clearances between the electrodes at the sides of the rail-like surfaces.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of the operative end of a sparking plug constructed in accordance with the invention; Figure 2 is an enlarged perspective view of the electrodes of the sparking plug; and Figure 3 is a side view of the electrodes: The invention is concerned with a sparking plug of generally conventional construction except for the electrodes, as will be described.

The sparking plug therefore has a body 10 of conventional kind including an externally screw threaded portion 11 and a hexagonal or other portion, by means of which it can be engaged by a spanner to fit or remove it from an engine cylinder head or equivalent part.

Within the body 10 is a ceramic insulator 12 which is positioned within the screw threaded hollow cylindrical end of the body 10. There is a gap around the insulator 12 and its shape may be determined by known cooling or other factors.

Passing through the centre of the insulator 12 is a central electrode 13. This extends right through the insulator and terminates in a screw threaded portion or other portion to which a lead can be connected.

Fixed, as by welding, to the annular end of the body 10 is a side or earth electrode 14.

This is of L-shape having one limb secured to the body 10 and the other limb extending over the central electrode 13.

As seen particularly in Fig. 2 the central electrode 13 is of cylindrical form but its end is shaped to provide a flat narrow rectangular rail-like surface 1 5 flanked by two inclined surfaces 16, 17.

The side electrode 14 has a surface presented towards the central electrode, this surface being shaped to provide a narrow rail-like surface 18 aligned with the surface 15 on the central electrode and of generally the same proportions. The surface 18 on the side electrode 14 is also flanked by inclined surfaces 19, 20. The portion of the surface 18 which is presented to the corresponding surface 15 on the central electrode is parallel sided and of similar width and length to the surface 15 on the central electrode but is then flared so as to merge with the surface of the side electrode as shown.

The inclined surfaces 16, 17, 19, 29 provide substantial clearance between the electrodes other than in the regions of the opposing surfaces 15 and 18.

As seen in Fig. 3 the surfaces 15 and 18 lie at a small acute angle relatively to one another with the narrowest gap between them being at the end at which the side electrode 14 is secured to the body 10 of the sparking plug.

In use the sparking plug is fitted into a combusion chamber of an internal combusion engine in conventional manner and means are provided for applying a high voltage current across the gap between the two electrodes to create a spark. Spark initiation occurs at the point at which the two surfaces 15 and 18 are closest together, that is, at the end towards the junction of the side electrode with the sparking plug body. The magnetic forces produced by the current in the spark and the side electrode causes the spark to travel along the rail-like surfaces 15, 18 towards the free end of the side electrode.

The translation of the arc along the rail-like surfaces will have the effect of supplying heat to and hence igniting a larger quantity of mixture than if the spark were static. This cools the thread of gas which is conducting the electrical current. This increases its electrical resistance and since external resistance, due to the leads and the coil, or its equivalent, is largely fixed, there will tend to be an increase in electrical energy at the arc.

Furthermore, the substantial clearance at either side of the rail-like surfaces 15, 18 enables the flame front to grow in area as it advances, thus rapidly reaching a larger volume of fresh combustible mixture. Cooling effects of the bodies of metal represented by the electrodes are also minimised.

The translation of the arc, extra arc energy, better exposure of combustible mixture and minimised cooling all contribute to a reduction of the duration of the first stage of ignition the greater reduction occurring under the more difficult case of leaner mixture.

It might be supposed that the large area of the conventional electrodes has the effect of prolonging the life of the sparking plug. However, erosion of the electrodes is governed by the amount of heat reaching any particular volume of metal. When, as here described, the arc is deliberately caused to traverse the raillike surfaces, the amount of heat to any particular volume is minimised with ensuing prolongation of the electrode life.

The sparking plug as described therefore provides a high efficiency spark initiation and flame generation pattern giving good combustion characteristics and also enabling lean mixtures to be effectively used, thus enhancing the fuel economy of the engine. Good combustion characteristics also ensure maximum burning of the mixture in the combustion chamber so that exhaust pollution is minimised.

The invention can also be applied to sparking plugs having different types of electrodes.

For example, the side electrode may extend laterally directly towards the side of the central electrode from the adjacent wall of the body, either at a level with the end of the body or proud of it.

Claims

CLAIMS 1. A sparking plug having a central electrode and a side electrode disposed adjacent to and spaced from the central electrode, the adjacent opposing surfaces of these electrodes being of generally parallel elongated, narrow rail-like form, each such surface being flanked by surfaces sloping away therefrom. 2. A sparking plug as claimed in Claim 1 in which one end of the pair of rail-like surfaces is closer together than the other, the end of closest proximity being that nearer to the end at which the side electrode is attached to a body of the sparking plug. 3. A sparking plug as claimed in either Claim 1 or Claim 2 in which the rail-like surfaces are arranged lengthwise of the side electrode and across the centre of the central electrode respectively. 4. A sparking plug as claimed in any one of the preceding claims in which the surfaces flanking the rail-like surfaces are advantageously relatively steeply inclined to one another so as to provide substantial clearances between the electrodes at the sides of the raillike surfaces. 5. A sparking plug substantially as hereinbefore described with reference to and as shown in the accompanying drawings. CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 1 to 4 above have been deleted or textually amended. New or textually amended claims have been filled as follows: Claims 5 above has been re-numbered as 8.

1. A sparking plug having a central electrode and a side electrode having respective sparking surfaces arranged opposite one another and defining a spark gap therebetween, each of the sparking surfaces being elongate and being flanked along its longitudinal edges by oppositely inclined surfaces sloping away from the sparking surface, wherein the two sparking surfaces are inclined relative to one another in the longitudinal direction so that the spark gap widens in the direction in which sparking tends to advance along the gap.

2. A sparking plug according to claim 1, wherein the central electrode has a circular cross-section at a distance from its sparking surface and its sparking surface constitutes an end surface of the central electrode.

3. A sparking plug according to claim 2, wherein the sparking surface of the central electrode has its longitudinal edges symmetrically disposed with respect to a diameter of the central electrode.

4. A sparking plug according to claim 1, 2 or 3, wherein the side electrode is elongate and extends inwardly from one side of the central electrode and its sparking surface extends generally lengthwise of the side electrode.

5. A sparking plug according to claim 4, wherein the sparking surface of the side electrode has its longitudinal edges symmetrically disposed with respect to the direction in which the side electrode extends.

6. A sparking plug according to claim 4 or 5, in which the side electrode is generally Lshaped.

7. A sparking plug according to claim 1, wherein the sparking surface of the central electrode is disposed on one side of the central electrode, and the sparking surface of the side electrode constitutes an end surface of the side electrode which extends inwardly from one side of the central electrode.