EP0983621A1

EP0983621A1 - Spark plug

Info

Publication number: EP0983621A1
Application number: EP98921648A
Authority: EP
Inventors: Joseph Gibson Dawson
Original assignee: Dawson Royalties Ltd
Current assignee: Dawson Royalties Ltd
Priority date: 1997-05-20
Filing date: 1998-05-19
Publication date: 2000-03-08
Also published as: AU7443098A; JP2001525982A; WO1998053540A1

Abstract

A spark plug for an internal combustion engine is described. The spark plug includes at least a cathode and an anode ground electrode between which a spark is formed wherein the part of the ground electrode on which the spark impinges is in the form of at least one low area projection such as a spike and an end of the cathode adjacent the ground electrode may have a recessed form.

Description

SPARK PLUG

The present invention relates to spark plugs for internal combustion engines.

The spark plug is almost as old as the internal combustion engine itself and over the past decades many attempts have been made to modify and improve the performance of the spark plug so as to achieve a corresponding improvement in the performance of the engine. The term "improved performance" has many facets and may include obtaining more power from the engine by improved combustion of the fuel; easier starting from cold; improved performance with regard to reducing misfires of the engine under operating conditions; and, improved performance with regard to the reduction of levels of harmful emissions, to name but a few.

In addition to the many varied designs of spark plugs per se which have been proposed and tested over the years, devices which fit on the plug or in the high tension supply in order to modify the spark have also been proposed. Such devices include those with secondary gaps, devices providing intermediate energy storage and many more. It is significant in that it would appear that not one of the devices proposed and actually marketed has achieved any lasting credibility with the public and certainly not with companies which design and manufacture engines and ignition systems. The only advantages which can be discerned from these prior art devices is that under some circumstances they may increase the output of an ignition coil by causing it to overwork; provide a temporary improvement where a coil is worn, weak or fatigued; or might temporarily improve the apparent performance of a worn or ill-timed engine. What such devices would seemingly not do is to improve the performance of an engine and ignition system which are in good working order.

As may be expected, spark plug manufacturers have mainly concentrated upon the fundamental design and quality improvements to the spark plug per se. Even some of these so-called improvements to the spark plug have imported features such as, for example, air gaps, resistors and capacitance's as integral features into the spark plug construction as exemplified inter alia in US 4,613,789.

Other approaches use very fine, almost pointed, centre electrodes as in DE-A-42 38 973 or DE-A- 32 30 362 and also in spark plugs commercially available on the market.

Yet another approach is the use of multiple ground electrodes spaced around the centre electrode as in GB-A- 2 291 124 and in DE-A- 195 03 223 to provide multiple paths to ground. The latter approach appears to be directed at ensuring that a spark occurs somewhere so as to guard against misfires. However, these spark plugs employ a lot of metal in the electrodes which can result in excessive heat dissipation giving poor cold starting and misfire during warm-up from cold, contrary to what is intended and which can be extremely damaging to catalytic converters.

In spite of all these apparent advances and improvements, in essence, the vast majority of spark plugs actually in use today and fitted as original equipment on vehicles sold to the public and commercially available as replacements are substantially the same as they have been for about the last 30 years or more.

Modern engines are mechanical devices of extreme precision with ever improving design to increase performance, yet spark plugs in use today are not precision components in the sense that their primary purpose, i.e. to produce a spark having consistent characteristics with respect to timing, spatial position, heat content, kernel shape and duration is not achieved. Spark plugs in use today still produce a spark within a relatively very broad spectrum. In order to produce a spark, it is necessary for the potential across the electrodes to ionise the intervening gas charge, and the provision of multiple ground electrodes makes the production of a repeatable spark more difficult in that in theory the spark could form between the centre electrode and any one or more of the ground electrodes. At high engine speeds the time to fire is very short, but when considered in precise terms and paying due regard to the need for a modern engine to fire at a precise instant at a predetermined position of the piston before top dead centre, the time required for the plug to find a position where ionisation of the fuel charge can occur between the electrodes and to create ionisation is critical and can have a significant effect on the engine performance. Therefore, an ignition system which has to "decide" between two, three or four ground electrodes before firing the spark cannot be said to be precise. Similarly, the provision of a plurality of ground electrodes means that the spark position can vary greatly from one spark to the next. The knowledge that a spark is going to occur in a particular position each time and produce a spark kernel/flame front of a repeatable shape would enable an engine designer to develop a better engine. Modern engines are generally fitted with catalytic converters and the over-riding necessity is for the avoidance of misfires to protect the expensive catalytic converter. However, the provision of multiple ground electrodes has been counterproductive in many areas of engine performance, even in that of preventing misfires.

With a conventional "simple" spark plug having a flat- faced cylindrical central cathode electrode facing a ground anode electrode which is in the form of an essentially flat metal bar bent through about 90°, the spark, under normal conditions, can form at virtually any position between the flat cathode face and the flat anode face and as indicated in Figures 1 to 3. In practice, a standard spark wanders around the edges formed between the centre electrode and ground electrode, potentially occurring from a different point at each firing cycle. The disadvantage of this known spark electrode arrangement is the diffuse nature of the electromagnetic energy and the relatively rapid erosion of the electrode sharp edges at about 1000 miles and which are needed for reliable spark formation. In a conventional spark plug, erosion of the centre electrode causes a steady increase in the voltage needed to initiate a spark ("breakover voltage") until a condition is reached where a misfire occurs .

To some extent the spark energy and duration is limited by the use of the standard ignition coils used in induction based ignition systems. Such coils have been designed to produce sufficient energy and spark duration to ensure ignition under most engine circumstances. However, the amount of energy must be limited by consideration of radio frequency interference (RFI) and the ability to charge the coil especially at high RPM. The spark gap distance is also limited by restrictions on RFI and the possibility of misfire, especially at low RPM. If possible, given the specification of standard coil ignition systems, it would be desirable to make the following improvements:

a) increased spark surface area, size and volume; b) accelerate the spark breakover voltage, to reduce cyclic dispersion and the possibility of misfire; c) increase the energy in the spark; d) increase the ionization; and e) reduce RFI.

The primary object of the present invention therefore, is to provide a spark plug working to the theorectical and industrially adopted conclusions of Rudolph Maly, who states in his publication "Spark Ignition: Its Physics and Effects on the Internal Combustion process", Maly. R. , Chapter 3, that "of the total electrical energy supplied to the spark, only that fraction contained within the outer surface layer of the plasma is available for initiating the flame propagation process". The energy density and temperature gradient in this layer depend on the discharge mode. Highest energy densities and temperature gradients are achieved if the ignition energy is supplied in the shortest time interval.

According to a first aspect of the present invention, there is provided a spark plug for an internal combustion engine, the spark plug including at least a cathode and an anode ground electrode between which a spark is formed wherein the part of the ground electrode on which the spark impinges is in the form of at least one low area projection as hereinbelow defined. In the spark plug according to the present invention, the low area projection working part of the one electrode may be in the form of a spike or a pointed end opposite the face of the other electrode at a predetermined distance therefrom.

A significant increase in the heat of the spark in its outer surface layer and in its volume has been found, with the spark tending to cover most of the area of the other electrode end face and forming a spark of generally conical form impinging on the one electrode.

According to a second aspect of the present invention, there is provided a spark plug for an internal combustion engine, the spark plug including at least a cathode and an anode ground electrode between which a spark is formed wherein one electrode has an end facing the other electrode and which end has a recessed form.

Most spark plugs have a central electrode, generally concentric with the plug body and at least one ground electrode fixed to a metal body member, the ends of the two electrodes defining a gap therebetween.

It should be noted that although nowadays the central electrode is generally referred to as the cathode and the ground electrode as the anode; this refers to vehicles having negative earth systems and for positive earth systems the polarity of the central and ground electrodes may be reversed. Thus, it is the geometrical relationship between the electrodes which is of primary importance rather than their strict polarity. In the interests of clarity the centre electrode will be referred to as the cathode and the ground electrode as the anode hereinafter, though it should be remembered that their strict polarity may be reversed and which is intended to be covered by the spark plug of the present invention.

Preferably, the spark plug according to the present invention comprises a cathode having a recessed end formation and a ground electrode having at least one low area projection generally pointed end on which the spark focuses such as the one electrode according to the spark plug of the first aspect of the present invention.

Preferably, the recess at the end of the cathode is of spherical curvature. However, the form of the recess portion may be rounded, hyperbolic, parabolic, conical, frusto-conical or may be of a generally cylindrical recessed form.

However, it has been found that improvements in spark plug performance utilising only the first aspect have been achieved and which improvements amount to up to about 50% of those of a spark plug utilising the combined first and second aspects in terms of increased spark heat output, spark temperature and spark volume.

As stated above, most preferably, the form of the concave portion is a bowl of a spherical curvature. Preferably, the recess is not hemi-spherical in shape but has a larger radius of curvature wherein the centre of curvature lies outside of the electrode recess. More preferably still, the low area projection or end of the anode ground electrode may coincide with the centre of curvature of the spherically dished cathode end, i.e. geometrically focused. When this latter condition is fulfilled, the gap between the anode point and the cathode end may be in the region from about 0.35mm to about 0.9mm. Spark plug gaps of up to about 1.5mm may be used with benefit in the electrode geometry according to the spark plug of the present invention as the focusing effect reduces the voltage required from the coil to initiate the spark and effectively accelerates the spark breakover voltage.

Preferably, the rim or edge of the recess is not sharply defined but has a smoothly curved form.

Tests carried out with spark plugs according to the present invention compared with known spark plugs have shown that a spark is formed at about 20-22% less voltage for double the spark gap and also typically delivers between 100- 300% more energy. Thus spark plugs according to the present invention provide performance increases which are totally unexpected.

Maximising the difference between the areas of the two electrodes, given that the two electrodes possess substantially focused geometry, in this case between the area of the recess and the area of the electrode projection tip on which the spark is focused increases the so-called "voltage stress" which results in a reduced voltage at which a spark is formed and/or permits a larger spark gap to be employed for no rise in breakover voltage .

Although in the most preferred embodiment of spark plugs according to the present invention, one electrode has a recess of spherical curvature with a centre of curvature lying at a point which allows a gap of up to about 1.5mm to be employed utilising an electrode having a point lying at the centre of curvature, it is stressed that considerable improvements in spark plug performance in terms of heat content, volume and breakover voltage with electrode geometry' s varying considerably from this optimum arrangement. For example, an electrode projection tip lying anywhere within a sphere of about 30% of the recess radius of curvature and centred on the centre of curvature of the recess will achieve a large proportion of the benefits of the optimum electrode arrangement.

Thus, in this specification the term "focusing" of the spark is not to be construed in strictly geometric terms but is to be understood as meaning a spark which extends between a relatively large area recess in an electrode and impinges onto a relatively much lower area tip of a projection on the opposing electrode.

As noted above, prior art spark plugs tend to erode away the outer edges of the centre electrode during use causing the spark to become more diffuse and eventually leading to misfires. It has been found that with spark plugs according to the present invention, the focusing geometry of the electrodes appears to be self-preserving. To illustrate this fact, spark plugs with centre electrodes have been made where the recess in the end was of non-spherical curvature, e.g. frusto-conical . It has been found that operation of the spark plug over time has resulted in the frusto-conical form of the recess being eroded away to a spherical curvature having a centre of curvature coincident with the tip of the opposing electrode. Once the electrodes have achieved this geometry further erosion appears to cease, indeed, the rate of erosion under given conditions as the geometry approaches the optimum declines. Thus, where the performance of prior art spark plugs tends to deteriorate with time, the performance of spark plugs according to the present invention appears to improve. The following two quotations are taken from the "Bosch Automotive Handbook, 4th. Edition, 1996"; "Increased energy in the spark and improved precision in the firing characteristics of the plug, conspire to provide a more stabilised ignition means with a consistently predictable outcome, which then serves to stabilise flame propagation, attenuating intercyclical fluctuation. A reduction in cyclical deviation results in smoother engine performance and lower HC emissions."; and, "Increased spark energy is desirable for many reasons, one of which is that although intense fuel/air mixture turbulence is basically desirable too, it can extinguish the spark, thus leading to incomplete combustion. The more energy stored in the ignition coil and the more energy present in the gap, the more efficient the system.". It has been found with spark plugs according to the present invention that a spark will be formed for coil voltages at least 20% lower than for a conventional spark plug at double the nominal gap. For the same coil voltage, a comparatively much larger gap may be employed than with a conventional plug. In this instance a bigger more energetic spark is produced. Quoting again from the same Bosch Automotive Handbook publication as given above: a "bigger spark without the penalty of increased voltage is a desirable improvement, having again, a positive influence on engine smoothness and HC emissions"; and, "Ignition quality is enhanced by extended spark duration and spark length, i.e. long spark duration and a long spark (large electrode gap)". However, large electrode gaps in prior art spark plugs are avoided as they can increase the voltage required from the coil by 2 to 10KV depending upon R.P.M and also with a consequent increase in RFI. Reference to Figures 9 and 10 show schematic sketches of the electrode gap regions of a conventional spark plug and a spark plug according to the present invention respectively. In the conventional plug, the gap between the anode 20 and cathode 16 is denoted by "G¹" and the diameter of the cathode by "2R". For a conventional plug, the gap G' is typically about 0.65mm and the diameter 2R about 2.5mm to 2.8mm. In the spark plug according to the present invention, the gap "G" between the end of the anode 60 and the edges of the recess is also about

0.65mm, but to this must be added the distance "D" from the recess edges to the base thereof of the cathode 46, which may for example also be about 0.65mm thus, giving a total gap of about 1.3mm. By forming a recess, the effective gap is greatly increased. However, due to the focusing effect of the electrode geometry, there is no consequent rise in the voltage necessary to form a spark.

Indeed, measurements have indicated that the actual gap

(G+D) of spark plugs according to the present invention may be double the conventional gap G' and at the same time reduce the breakover voltage required for a conventional spark plug as described with reference to

Figure 9. Furthermore, with the doubled gap (G+D) of spark plugs according to the present invention, the breakover voltage can still be up to 20% less than that required for a conventional spark plug.

To the nominal gap G as shown in Figure 10, and as would be measured with a feeler gauge for example, must be added the recess depth D to find the actual gap, (G+D) . For a perfectly focused spark where the bowl is of spherical curvature and the tip of the anode lies at the centre of curvature of the bowl, the following formulae define the interrelationships. The length of the gap G:

R D ²

G = 2 D

The depth of the bowl D:

the solid angle of the spark arc θ:

θ = 2 J _R (steradians)

and, the volume of the spark, V:

Figure 11 shows a graph where the interrelationship between bowl depth, D, and nominal gap, G, are given for spherical curvature of the bowl for a perfectly focused spark and a radius R of the cathode of 1.25 mm.

Furthermore, even though the gap may be larger and/or the coil voltage lower with spark plugs according to the present invention, the quantity of energy contained in the spark is between 100 to 150% greater and spark path length increased by 100%. Thus, the desirable criteria expressed in the above quotations of Maly and Bosch are fulfilled by the spark plugs of the present invention leading to improved engine operating characteristics. For a given coil, the spark in the spark plug according to the present invention forms more quickly due to the lower voltage required, i.e. there is less wait due to a shortened voltage rise-time and spark formation is accelerated. In addition to the shorter formation time, there is greatly increased energy in the spark as noted above. Typically, spark plugs according to the present invention will fire about lOμs earlier than a conventional spark plug due to the lower voltage required under given conditions. At 3000 rev/min this equates to firing at about 0.2° of a revolution earlier and hence

0.4° at 6000 rev/min. Thus, more precision is available in the ignition system.

However, even though spark plugs according to the present invention form an enhanced spark at lower coil voltages, they also appear to empty the coil of more of its stored energy thus increasing the spark duration and further tending to ensure more complete combustion of the fuel/air charge.

The spark plug according to the present invention and embodying both the first and second aspects, appears to focus the spark energy from the cathode recess onto the anode and produces a generally conical spark extending substantially over the whole area of the end of the cathode. A spark of greatly increased heat and intensity has been found to form compared with the sparks generated by conventional spark plugs. Furthermore, the spark plug of the present invention forms a spark in precisely the same position each time thus allowing an engine designer certainty in factors relating to the point at which fuel combustion is initiated. The spark plug of the present invention, contrary to the prior art, seeks to maximise the area difference between the two electrodes, whereas prior art spark plugs appear to either reduce the central electrode to virtually a pin or to provide multiple ground electrodes of apparently ever increasing total area.

The diameter of the central cathode electrode is desirably between 2.5mm and 3.5mm. Cathode diameters of about 3mm compared with about 2.5mm which is a commonly used diameter in conventional spark plugs appears to improve the performance of the spark plug of the present invention yet further in terms of further increased spark temperature and energy content for example.

In order to increase the area of the recessed cathode, the end thereof may be in the form of an insert to the spark plug wherein the end of the cathode is in the form of a flanged head having the recessed form therein.

That the spark produced by the spark plug of the present invention is of a different nature from the spark produced by conventional spark plugs is evidenced by a noticeable directional thrust which is associated with the spark. It has been found that combustible particles introduced into the spark are actually accelerated and ejected by the spark in a direction consistent with the spark direction. It is believed that this effect is due to the focusing and consequent "concentration" of the electron flow forming the spark from the cathode to the anode. The evident focusing of the spark from the cathode recess onto the anode appears to be analogous to the focusing of a light beam by a lens onto a point giving greatly increased effect in terms of energy and temperature. Tests conducted by passing strips of thermal paper through a spark gap of both conventional spark plugs and spark plugs according to the present invention under substantially the same conditions have shown greatly increased thermal damage to the paper with spark plugs according to the present invention indicating at least the higher temperature generated. Spark temperature rises of the order of several hundred degrees have been observed with otherwise constant conditions of coil voltage, current and gap. Furthermore, there appears to be a noticeable magnetic field associated with the spark formed in spark plugs according to the present invention. A steel pin suspended in the spark by a fine thread can be seen oscillating vigorously. This does not happen with a spark formed in a conventional spark plug.

A significant advantage of the spark plug of the present invention is that manufacturing costs for many of the embodiments may be largely the same as with conventional single cathode and anode spark plugs since the manufacturing technology involved is essentially the same and also in that there may be few if any additional components to the spark plug of the present invention.

The anode or ground electrode of the present invention may comprise one or more suitably formed flat metal bars as with conventional spark plugs but with the addition or formation of a pointed end or projection which may be integrally formed therewith or added thereto. It is preferred that the anode having the pointed end is formed of a single piece of metal which is, for example, welded to the spark plugs metal body such that the pointed end faces the recess in the central electrode. It is possible to provide an anode having a spike fixed thereto but it has been found that such constructions tend to be less reliable and the thermal discontinuity in the joint region leads to overheating. However, if a joint may be made reliably such constructions could be used.

The materials from which the cathode and anode may be made are well known to those skilled in the spark plug art and may include alloys of iron, nickel, chromium, molybdenum, iridium for example.

A further unexpected characteristic of spark plugs according to the present invention is an apparent self- cleaning or self-forming ability. It has been found that where a cathode bowl has been formed imperfectly and has, for example, pips or lumps of unwanted metal remaining thereon, during operation this unwanted material is removed by the action of the spark and results in a spherical curvature, free of imperfections.

We have also found that spark plugs according to the present invention emit far less RFI than conventional spark plugs. This reduction is due to the ability of spark plugs according to the present invention to produce more powerful sparks at reduced peak coil voltages of, for example, about 20% and since RFI is at least partially dependent upon the peak coil voltage there is a consequent reduction. Thus, less shielding is necessary and/or performance of items such as radios is enhanced.

A spark plug construction having a pointed central electrode and a recess formed in the ground electrode may also be formed and which falls within the scope of the present invention.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which:

Figures 1 to 3 show views of part of a prior art spark plug; a schematic illustration of the electric field searching for a discharge path; and, one example of a conventional spark discharge current, respectively;

Figure 4 shows a perspective view of part of a first embodiment of a spark plug according to the present invention;

Figures 5A to 5C show some alternative ground electrode formations;

Figure 6 shows part of a further embodiment having a modified form of central electrode;

Figure 7 shows another embodiment having a modified form of ground electrode;

Figure 8 shows the cathode and anode portion of a spark plug according to the first aspect of the present invention;

Figure 9 shows a schematic view of the electrode portion of a conventional spark plug;

Figure 10 shows a comparative view to that of Fig. 9 but of a spark plug according to the present invention and including information for calculating spark plug parameters from its dimensions;

Figure 11 shows a graph of nominal gap G vs bowl depth for a focused spark; and Figure 12 which shows an example of a spark plug according to the present invention having a recess formed in the ground electrode.

Referring now to the drawings and where the same features are denoted by common reference numerals.

In Figure 4 part of a prior art spark plug is indicated generally at 10 and includes a body 12 having a threaded portion 14 for screwing into an engine cylinder head (not shown) , a central cathode electrode 16 having an insulator 18 therearound and a ground anode electrode 20. The cathode 16 generally has a flat end face 22 which is in opposition to a flat face 24 on the anode 20. Usually, as a result, of wear during use, the initially flat face 22 becomes slightly rounded. Before producing a spark 28, the electrodes 20 and 16 must determine whereabouts initial ionisation is occurring between the electrodes and potentially, following ionisation a spark can occur at any position indicated by the arrows 30 in Figure 2. The spark is rarely formed over the entire area between the electrodes as indicated at 32 in Figure 3 but usually occurs at one relatively small area as indicated at 28 in Figure 1 and furthermore, the spark seldom occurs consistently in the same position. Where the spark does occur over most of the opposing area between the electrodes as indicated at 32, it tends to be a distributed bow-shaped in the manner shown in Figure 3 causing the energy contained to be somewhat dissipated over a larger volume.

Figure 1 shows a feature where the ground electrode 50 has an undercut relief 322 so as to obviate any spark generation between the recess rim and the body of the ground electrode.

Figure 4 shows a portion of a spark plug 40 according to the present invention. As in the spark plug of Figure 1, the spark plug according to the present invention includes a body portion having a threaded portion 44 for screwing into a cylinder head (not shown) , a central cathode electrode 46 insulated from the body portion by a ceramic insulator 48 and a ground anode 50. The cathode is ultimately connected to a coil (not shown) to supply a high voltage and the anode 50 is connected to ground via the engine cylinder head (not shown) . The shape of the end of the cathode 46 has a recess 56 in the form of a spherical curvature and the anode 50 has an integrally formed pin 60 having a tip 62, the tip 62 being in a position which coincides with the centre of curvature of the cathode end recess 56. The spark formed by the spark plug of the present invention and indicated by the lines 66 extends substantially over the whole of the surface area of the recess 56 and to the tip 62 of the anode pin 50 in a generally conical form. The spark always extends between the cathode end and the tip 62 and thus always occurs in the same predictable position. The spark temperature has been measured using an International Pyrometer Company Ltd. (trade name) high temperature analogue meter and is consistently in the region of 300 to 500°C higher than with a conventional type of spark plug as shown in Figure 1. Measurements reveal a 30% temperature increase but due to unavoidable losses during measurement the actual temperature increase may be higher still. It is believed that the geometry of the cathode/anode arrangement of the present invention focuses the electron energy of the current flow into a specific volume and creates benefits in increased temperature, heat content, shape repeatability and positional repeatability to name but a few .

Figures 5A to 5C show schematic representations of some alternative embodiments of ground electrode constructions. Essentially, each ground electrode 100 includes a pin or spike 102 having a pointed or low area tip 104.

Figure 6 shows part of a spark plug having a modified form of central electrode 110. The electrode comprises an internal conductor portion 112 having a cylindrical blind bore in the end and a further portion having a shank which is screwed or welded into the bore of the portion

112. This enables the cathode 110 to have a recessed end portion 120 which is of greater diameter than would otherwise be the case. The recessed end portion 120 also has a circumferential flanged rim 122 which sits over the end of the insulator 124. Thus a spark 126 is formed between the bowl 128 and the ground electrode spike 130.

Figure 7 shows another modified form of spark plug according to the present invention wherein the ground electrode 140 is formed with a pointed end 142.

Figure 8 shows the cathode 240 and anode 242 portion of a spark plug according to the first aspect of the present invention. The cathode has a substantially flat face 244 as with cathodes of prior art spark plugs but, the anode 242 has a low area projection 246 on which the spark 248 impinges from the cathode face 244. Again, with this embodiment, the spark is consistently formed in a predictable position and the temperature is greatly increased compared with conventional spark plugs as exemplified by the construction shown with reference to Figures 1 to 3.

Figure 12 shows a centre electrode 330 having a pointed end 332 and a ground electrode 334 having a recess 336 formed therein.

Spark plugs having the configuration shown in Figure 4 and having a gap of 1mm between the electrodes and a conventional commercially available plug as described with reference to Figures 1 to 3 and having a gap of 0.7mm were both connected to a common coil and shared a common ground connection. Thus, the spark may be expected to form on one plug before the other, i.e. the plug having the lower spark break-over voltage. The result of this test was that the plug according to the present invention consistently sparked first and as the gap on the plug according to the present invention was gradually increased, the conventional plug did not spark first until the gap on the inventive plug reached 1.2 to 1.25mm.

The results below show the drop in voltage needed for a plug according to the present invention compared with a conventional plug for gaps of 0.7mm and 1mm in both cases :

35kV drops by 7kV = 28kV 25kV drops by 5kV = 20kV 15kV drops by 3kV = 12kV 12kV drops by 2.4kV = 9.6kV The above results are valid for both electronic and conventional ignition systems comprising coil, distributor and points.

Energy content tests have shown that the spark formed in a spark plug according to the present invention is able to cut and perforate normal office type paper and thin plastics material which a spark from a conventional plug is unable to do under the same operating conditions since it does not have nearly as high an energy content.

Further tests have shown that a spark plug according to the present invention gapped at 0.7mm and requiring 20% less voltage than a conventional plug at double the gap produces a spark which is 100% bigger in volume. When the gap is increased to restore the original operating voltage, i.e. to about 1mm, the spark size is about 250% bigger than that of a conventional spark plug. If the gap is further slightly increased, the spark size can be increased to 300% and, in some cases where the cathode bowl diameter is increased to about 3.5mm and the anode focal point is made very sharp, increases to 500% of the size of a conventional spark may be achieved at the same voltage. If the gap of a conventional spark plug is increased, the energy content thereof tends to fall and can even be extinguished by turbulent fuel/air charges.

Clearly, the focused spark produced by spark plugs according to the present invention are better able to deal with lean fuel/air ratios and, in a normal engine at stoichiometric fuel/air ratio, it can better buffer and protect catalytic converters against poisoning by incomplete or erratic combustion.

Claims

1. A spark plug for an internal combustion engine, the spark plug including at least a cathode and an anode ground electrode between which a spark is formed characterised in that the part of the ground electrode on which the spark impinges is in the form of at least one low area projection as hereinbefore defined.

2. A spark plug for an internal combustion engine, the spark plug including at least a cathode and an anode ground electrode between which a spark is formed characterised in that an one of said cathode or anode electrodes has an end facing the other electrode and which end has a recess therein.

3. A spark plug for an internal combustion engine the spark plug including at least a cathode and an anode ground electrode between which a spark is formed characterised in that a part of one of said cathode or anode electrode on which the spark impinges is in the form of at least one low area projection as hereinbefore defined and in that an end of the other electrode adjacent said projection has a recessed form.

4. A spark plug according to any one preceding claim wherein the end of the cathode has a concave portion.

5. A spark plug according to claim 4 wherein the form of the concave portion is selected from rounded, conical, frusto-conical; parabolic; hyperbolic; coaxial; or a generally cylindrical recessed form.

6. A spark plug according to any one preceding claims 1 to 4 wherein the form of the concave portion is of a spherical curvature.

7. A spark plug according to claim 6 wherein a tip of the low area projection on the ground electrode coincides with the centre of curvature of the spherically dished cathode end.

8. A spark plug according to any one preceding claim wherein the gap between the point of impingement of the spark on the ground electrode and the cathode end is from about 0.35mm to about 1.5mm.

9. A spark plug according to claim 8 wherein the gap is between about 0.35mm to about 0.9mm.

10. A spark plug according to any one preceding claim wherein the diameter of the cathode is between 2.5mm and 3.5mm.

11. A spark plug according to any one preceding claim wherein the end of the cathode is in the form of a flanged head having the recessed form therein.

12. A spark plug according to any one preceding claim wherein the ground electrode comprises one or more suitably formed metal bars having the addition or formation of a generally pointed end or projection.

13. A spark plug according to claim 12 wherein the low area projection on the ground electrode is integrally formed therewith.

14. A spark plug according to claim 12 wherein the low area projection is a separate piece fixed to the ground electrode .

15. A spark plug according to any one preceding claim wherein a rim around the recess has a smoothly rounded form.

16. A spark plug according to either claim 2 or claim 3 wherein the recess is formed in a ground electrode, as hereinbefore defined.

17. A spark plug according to claim 3 wherein the recess is hemi-spherical .

18. A spark plug according to any one preceding claim from 3 to 15 wherein the projection is on a ground electrode which is relieved in a region adjacent the projection to prevent the formation of unwanted sparks.

19. A spark plug according to any one preceding claim wherein the projection of the ground electrode comprises a portion having a triangular cross section.

20. A spark plug according to any one of preceding claims 6 to 19 wherein the tip of the low area projection lies within an imaginary sphere having a dimension of 30% of the radius of curvature of the recess, said imaginary sphere being centred on the centre of curvature of the recess.

21. A spark plug according to any one of preceding claims 6 to 15 wherein the dimensional characteristics of the spark plug electrode geometry is substantially described by the formulae: the length o f the gap G :

R ² - D ²

G = 2 D

the depth of the bowl D :

D + R

the solid angle of the spark arc ╬╕:

╬╕ = 2 ╧Ç " R (steradians ) and, the volume of the spark, V:

where: the recess curvature is circular; R = radius of spark plug cathode; G = gap between extremity of anode reduced area portion and recess edges; D = depth of recess; and, ╬╕ = solid angle subtended by spark arc between anode and recess.

22. A spark plug substantially as hereinbefore described with reference to the accompanying description and any of Figure 4; or Figures 5A to 5C; or Figure 6; or Figure 7; or Figure 8; or Figure 10; or Figure 12 of the drawings.