GB2327459A - Spark plug for i.c. engines - Google Patents

Abstract

The central (cathode) electrode 48 has a recess eg in the form of a continuous 360 degree groove 50 which faces laterally outwards with respect to the axis of the plug. The ground (anode) electrode has a reduced area portion, eg a sharp knife-edge 56, a projection with a triangular cross-section or a rounded projection, which faces the recess in the central electrode. The reduced area portion may be part of the shell 42 of the anode or may be a ring (70, figs 8,9) made of more erosion resistant material and welded or brazed to the shell. The central electrode may be triangular in plan view, figure 14, the triangular end-piece 114 being recessed to form three emitters having curved faces 116 which are opposite pointed projections 120 forming the anodes. The ground electrode may be an annular ring (52, fig.5).

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 or to keep up with improvements made to the engine per se. 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 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-een 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 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 capacitances as integral features into the spark plug construction as exemplified inter alia by 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-A2 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 hot spark having a high energy content with consistent characteristics is not achieved.

According to 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 cathode forming a central electrode has a generally laterally outwardly facing recess with respect to the spark plug axis, the recess extending at least intermittently around the electrode periphery and a ground anode electrode which has a reduced area portion, as hereinbelow defined, facing said recess.

It should be noted that although the central electrode is 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.

The ground anode is described as having "a reduced area portion" facing the recess. The reduced area portion may be in the form of a relatively sharp, knife-like edge which faces the recess; or, a projection having a substantially triangular cross sectional shape wherein the reduced area portion is formed at the apex thereof and faces the recess; or, may be formed of a projection or projections having an edge or tip which is of rounded cross sectional shape, preferably, of complementary shape to the cathode recess. In the latter case, the effective radius of curvature of the anode end where it faces the recess may be smaller than that of the recess itself.

In one embodiment of the present invention, the recess in the cathode may extend substantially through 3600, i.e. a continuous groove extending around the entire periphery of the cathode transverse to the axis thereof.

In another embodiment of the spark plug according to the present invention, the cathode may be other than circular in cross section in a direction transverse to the spark plug axis and may, for example, be triangular or rectangular, the recess or recesses being formed on faces or corners of the cathode.

The cross sectional form of the recess in a direction parallel to the axis may be circular, oval, frustoconical, vee-grooved or rectangular. However, the recess may preferably be an arc of a circle.

The ground anode electrode may be in the form of an annular ring or projection extending around the interior face of the spark plug metal shell opposite the recess of the cathode. The anode may be formed directly from the metal from which the shell is made or may be in the form of a ring which is attached thereto. The anode portion per se may comprise: an essentially continuous ring of reduced area facing the cathode recess or recesses; or, may exist only in those regions of the periphery which have facing recessed portions; or, may comprise two or more spaced apart arcs of circles facing the cathode recess or recesses; or, may have discrete projections of low area facing a continuous recess or recessed portions.

The spark plug according to the present invention is closely related to the technology disclosed in our copending patent application, GB 9710085.3, wherein the cathode has a recessed bowl formation in the end thereof and which recessed bowl faces an anode which has a pin or needle-like form. In this form of spark plug electrode arrangement, we have found that a spark of very high temperature, intensity and energy content forms at relatively much lower voltages than with conventional spark plugs due, we believe, to the focusing effect generated between the cathode bowl and anode pin. The spark plug according to the present invention also employs the principles and advantages described in our earlier application referenced above but in a novel and inventive electrode geometry arrangement. Effectively, the spark plug of the present invention produces one or more sparks, but of greatly increased energy compared with known prior art multiple ground electrode designs.

Thus, where protection of an expensive catalytic converter is a prime concern, the spark plug of the present invention produces at least one spark of greatly increased temperature and energy at a reduced spark-over voltage of at least 20%.

The gap between the anode and cathode may be permanently set on manufacture where the anode is formed internally of the metal shell or may be adjustable where the anode is formed of prong-like projections.

An important advantage of the present invention in technical terms is that the energy content of the spark may be readily increased by increasing the size of the central electrode and consequently the recess size. The cathode in spark plugs of the present invention forms an "emitter" and the spark energy or intensity is at least partially dependent on the size of the emitter. In the present invention the emitter size can be easily increased and may range from about 2.5mm to about 4.5mm in diameter where the recess extends around the periphery thereof in the form of a continuous groove. Because of the increased working area of the cathode, wear by erosion is greatly reduced thus extending the life of the spark plug and providing further protection for a catalytic converter.

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. II; 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 and misfire. 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 202 lower than for a conventional spark plug at the same nominal gap.

Alternatively, 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; and, 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 emissions11; 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 4KV.

Reference to Figures 17 and 18 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 "G"' 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. In the spark plug according to the present invention, the gap "G" between the end of the reduced area portion of the anode 52 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 48, 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 still maintain the same breakover voltage required for a conventional spark plug as described with reference to Figure 17. Furthermore, even with an apparent 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 18, 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 recess is of circular curvature and the tip of the anode lies at the centre of curvature of the recess, the following formulae define the interrelationships.

The length of the gap G: R2~12 G = 2D The depth of the recess D:

and, the angle of the spark arc 0:

radians Figure 19 shows a graph where the interrelationship between recess depth, D, and nominal gap, G, are given for circular curvature of the recess and for a perfectly focused spark.

Other focusing geometries such as parabolic, hyperbolic and coaxial recess shapes for example may be employed.

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 are fulfilled by the spark plugs of the present invention leading to improved engine operating characteristics.

For a given coil and spark gap, 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. 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 10 to 20 > s earlier than a conventional spark plug due to the lower voltage required to form a spark under given conditions. At 3000 rev/min this equates to firing at about 0.20 of a revolution earlier and hence 0.40 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 by providing hotter sparks of greatly increased area, volume and extended duration. An important consequence of the ease with which the spark's physical characteristics are improved by the spark plug of the present invention is that they have greatly increased resistance to misfiring.

As described in our earlier patent application, GB 9710085.3, the spark plug according to the present invention appears to focus the spark energy from the cathode recess onto the anode and produces a generally conical spark. A spark of greatly increased heat and intensity has been found to form compared with sparks generated by conventional spark plugs. Energy increases of about 100 to 500% may be achieved with spark plugs according to the present invention depending upon dimensions.

The spark plug of the present invention, contrary to the prior art, seeks to maximise the area of the centre electrode with respect to the effective area of the ground electrode, whereas prior art spark plugs appear to either reduce the central cathode to virtually a pin or to provide multiple anodes of apparently ever increasing total area.

We have also found that spark plugs according to the present invention emit far less radio frequency interference (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.

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 for example.

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 cross section of part of a first embodiment of a spark plug according to the present invention; Figures 5 and 6 show schematic representations of the manner of spark formation across the electrodes of a spark plug according to the present invention; Figure 7 shows a view similar to Fig. 4 of a second embodiment; Figure 8 shows a shows a view similar to Fig. 4 of a third embodiment; Figure 9 shows a shows a view similar to Fig. 4 of a fourth embodiment; Figures 10 and 11 show views similar to Figs. 4 and 6 respectively of a fifth embodiment; Figures 12 and 13 show views similar to Figs. 4 and 6 respectively of a sixth embodiment; Figure 14 shows a perspective view of the electrode end of a seventh embodiment of a spark plug according to the present invention together with an enlarged detail view thereof; Figure 15 shows a perspective view of the ceramic moulding and cathode emitter of the spark plug of the seventh embodiment as shown in Fig. 14; Figure 16 shows a partially sectioned side view of part of the spark plug according to the seventh embodiment as shown in Figs. 14 and 15; Figure 17 shows a schematic view of the electrode portion of a conventional spark plug; Figure 18 shows a comparative view to that of Fig. 17 but of a spark plug according to the present invention and including information for calculating spark plug parameters from its dimensions; Figure 19 shows a graph of nominal gap G vs bowl depth for a focused spark; Figure 20 shows a schematic view of electrodes of a further embodiment of a spark plug according to the present invention; and Figures 21 and 22 which show part of a spark plug having the electrode geometry of Fig. 20.

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

In Figure 1 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-shape in the manner shown in Figure 3 causing the energy contained to be somewhat dissipated over a larger volume.

Figure 4 shows a cross section of part of a spark plug 40 according to a first embodiment of the present invention and comprises, as with a conventional spark plug, a metal shell 42 having a screw threaded portion 44 for fixing into the plug hole (not shown) of a cylinder head of an internal combustion engine (both of which not shown), a ceramic insulator 46 for insulating the central cathode 48 from the shell 42 which forms the ground anode electrode. However, in this embodiment of the present invention, the cathode 48 has a groove 50 at one end extending completely around the periphery thereof. The groove 50 is in the form of arc of a circle in vertical cross section. The anode is formed of an annular ring 52 extending around the interior surface 54 of the threaded portion 44, the ring 52 having a knife-like or relatively thin edge 56 directly facing the groove 50 and formed from the metal of the shell 42. Where the groove 50 is of circular or oval cross section, the tip of the edge 56 may lie at or about the centre of curvature or focus thereof. Figures 5 and 6 indicate schematically the electrode arrangement and spark 60 formation. The cathode is connected to a high voltage supply such as a coil 62 and the anode ring 52 is connected to ground 64. The spark 60 is of generally conical form extending from the cathode recess 50 to impinge upon the anode edge 56.

Figure 6 indicates the spark 60 being formed at one position around the periphery of the electrodes. However, a spark may form at more than one position.

Figure 7 shows a second embodiment similar to that of Figure 4 but having the anode formed from a slightly projecting mouth.

The embodiments of Figures 4 and 5 may be modified such that the ring 52 is not continuous but comprises two or more arc portions or points around the cathode.

Figures 8 and 9 show third and fourth embodiments respectively wherein the anode is formed from a ring 70 of metal which is more erosion resistant than the metal from which the shell 42 is made. The ring 70 is welded or brazed into position as indicated by the line 72.

The ring 70 of Figures 8 and 9 may be similarly modified as with the first and second embodiments to provide the edges 56 as intermittent arc portions or points around the cathode groove 50.

The gaps as shown in the first to fourth embodiments will generally be set on manufacture and not require adjustment during the life of the spark plug.

Figures 10 and 11 show a fifth embodiment wherein the anode 80 is formed from separate metal bars welded or otherwise fixed to the lower face 82 of the shell 42. The anodes are pointed 84 to face the cathode groove 50 at two or more spaced apart locations therearound (four are shown). The advantage of this construction is that the gap is easily adjustable and the spark more "exposed" to the fuel/air charge.

Figure 12 and 13 show a sixth embodiment which is similar to that of the fifth embodiment but wherein the points of the anodes are formed from highly erosion resistant metal pins or needles 90 fixed in anode bars 92.

Figures 14 to 16 show a seventh embodiment 100 of a spark plug according to the present invention. The spark plug 100 has the usual metal shell 102 and threaded portion 104. However, the ceramic core 106 which insulates the metal cathode 108 from the shell 102 has a lower portion 110 which is of generally triangular form in transverse cross section and which resides inside the threaded portion 104. The central cathode conductor 112 has a triangular end-piece 114 affixed thereto, the corners of the triangle being cut away and recessed to form three emitters having curved faces 116. The areas 118 of the ceramic body 106 adjacent the faces 116 are cut away so as not to interfere with the spark path. However, in this embodiment the curvature of the faces 116 is concave, i.e. more focusing, so as to intensify the spark energy onto the anodes. In the embodiments described with reference to Figures 4 to 13, the curvature' of the cathode recess 50 is convex in the peripheral direction thus tending to an extent to dissipate some of the available energy. The concave curvature of the faces 116 of the seventh embodiment is in both the transverse and vertical directions and focuses all of the available electrical energy onto the anode thus giving a more intensified spark. As shown in Figures 14 and 16, the anodes are formed from pointed projections 120 fixed to the end face of the shell giving an arrangement somewhat similar to that shown in Figures 10 to 14. However, the anode may be provided in a similar form to that as shown with reference to Figures 4 to 9 or as modified in any of the ways described.

Figure 20 shows the electrodes of a further embodiment wherein the anode 130 has a rounded end 132 rather than a point or sharp edge as shown for example in Figures 11 and 4 respectively. The provision of a rounded end 132 enlarges the spark 134 surface area compared with a sharp point or edge as previously described. An anode having a rounded end is also more easily formed than a pointed anode and improves longevity of the electrode. Figures 21 and 22 show a spark plug having an anode formed of a plurality of projections 140 having rounded, ball-like ends 142 facing a peripheral recess 144.

The anode ring 52 for example as shown in Figure 4 may also be formed with a rounded edge if desired as may any of the anodes of the other embodiments where appropriate.

Claims (17)

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 wherein the cathode forming a central electrode has a generally laterally outwardly facing recess with respect to the spark plug axis, the recess extending at least intermittently around the electrode periphery and a ground anode electrode having a reduced area portion, as hereinbefore defined, facing said recess.

2. A spark plug according to claim 1 wherein the recess in the cathode extends substantially through 3600 forming a continuous groove around the periphery of the cathode.

3. A spark plug according to claim 1 wherein the cathode is triangular or rectangular in plan view, the recess being formed on faces or corners of the cathode cross section.

4. A spark plug according to any one preceding claim wherein the cross sectional form of the recess is one of: circular; oval; frusto-conical; vee-grooved; parabolic; hyperbolic; coaxial; or, rectangular.

5. A spark plug according to any one preceding claim wherein the ground anode electrode is in the form of an annular ring or projection extending around the recessed portion of the cathode.

6. A spark plug according to any one preceding claim wherein the anode is formed directly from the metal from which the spark plug shell is made.

7. A spark plug according to any one preceding claim from 1 to 5 wherein the anode is in the form of a ring which is attached to the metal spark plug shell body.

8. A spark plug according to any one preceding claim wherein the anode portion comprises one of: an essentially continuous ring of reduced area facing the cathode recess; or, a series of two or more arcs of circles facing the cathode recess; or, two or more discrete projections of low area facing a cathode recess.

9. A spark plug according to any one preceding claim wherein a gap between the anode and cathode is permanently set on manufacture.

10. A spark plug according to any one preceding claim wherein the diameter of the cathode is about 2.5mm to about 4.5mm where the recess extends around the periphery thereof in the form of a continuous groove.

11. A spark plug according to claim 10 wherein the cathode portion opposite the anode is of greater diameter than a main cathode conductor portion.

12. A spark plug according to claim 3 wherein the recessed portions on the cathode are concave in both the axial and lateral directions.

13. A spark plug according to either claim 3 or claim 12 wherein the anode is formed by a projection opposite to each recess.

14. A spark plug according to any one of claims 3, 12 or 13 wherein a ceramic insulator of said spark plug has a generally triangular cross sectional shape in the transverse plane at a lower portion thereof.

15. A spark plug according to claim 1 wherein the reduced area portion of the anode is of rounded shape.

16. A spark plug according to claim 1 wherein the dimensional interrelationships are defined by the equations: The length of the gap G: R2- D2 G = R2- D2 The depth of the recess D:

and, the angle of the spark arc 0:

radians 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, o = planar angle subtended by spark arc between anode and recess.

17. A spark plug substantially as hereinbefore described with reference to the accompanying description and Figures 4 to 6; or Figure 7; or Figure 8; or Figure 9; or Figures 10 and 11; or Figures 12 and 13; or Figures 14 to 16; or Figures 18 and 19; or Figure 20; or Figures 21 and 22 of the drawings.