GB2361264A - Surface discharge spark plug for i.c. engines - Google Patents

Abstract

The spark plug includes a central high voltage electrode (3) extending within an insulator body (2) and having an exposed portion (3a) protruding from a lower end (2a) of the insulator (2). A ground electrode (4) is mounted on the insulating body with a small longitudinal spacing (12) from the lower end of the insulator (2) such that a spark gap is defined between the two electrodes with a longitudinal component alongside a lower end region (2a) of the insulating body. In operation, a spark produced between the two electrodes runs along a surface area of the lower end region (2a) to ignite the fuel-air mixture. The spark plug has enhanced performance, particularly in ignitability, anti-fouling performance and extended life. Since the spark plug exhibits reduced capacitance so that its associated ignition coil assembly can be of reduced external dimensions, it is possible to locate more of the ignition coil assembly, or the whole of it, within the bore formed in the cylinder head of the engine, so as to provide a more compact arrangement for the ignition system of the engine.

Description

2361264 IMPROVED SPARK PLUG This invention relates to a spark plug for an

internal combustion engine.

Known spark plugs generally comprise three essential 5 elements, that is an insulating body of cylindrical shape, a central high voltage electrode mounted within the insulating body and projecting from a lower end thereof and one or more ground electrodes mounted on the insulating body to define, along with the high voltage electrode, a spark gap, which is eithet an air or a surface gap. By surface gap is meant that the spark between the high voltage and ground electrode is directed across a surface of the spark plug insulator body. A shell mounted on the insulator body is provided with an external screwthread, which is screwthreadedly received in a respective screwthreaded bore provided in the cylinder head. By means of the interengaging screwthreads the spark plug may be tightened to cause a collar on the shell to engage a sealing gasket on the spark plug with a sciat formed in the cylinder head, to provide an effective seal between the spark plug and the cylinder head of the internal combustion engine. Normally, the or edsch ground electrode is attached to the lower end of the spark plug shell by a welded connection. Alternatively, it can form a lower end part of the shell. The shell provides an electrical conduction path between the cylinder head and the ground electrode (s) of the spark plug.

2 In operation, a high voltage is applied to the central electrode by the ignition system of the internal combustion engine, to produce a spark across the air or surface gap at precisely timed intervals to ignite the fuel-air mixture in the combustion chamber during each cycle of operation (two revolutions of the crank shaft for a 4-stroke engine).

Generally speaking, known spark plugs fall into three main categories. The first and commonest is a called single electrode design. Figure 1 which is an axial sectional lower end portion of such a spark plug electrode 100 defines, together with centre electrode 101, an axial design offers good cold starting soWith reference to view through the single ground high voltage gap 102. This and combustion performance and reasonable working life, and it is widely used in cars and commercial vehicles.

The second category of spark plugs is multi-electrode designs, in which the ground electrode consists of multiple ground electrodes are positionEd about the 1 centre electrode to define multiple radialair gaps for sparking. In operation, each time ignition current is applied to the spark plug, a single spar.k is produced by whichever ground electrode offers the smallest resistance to dielectric breakdown. However, when that electrode becomes fouled or worn (through corrosion or erosion), the ignition current will automatically reroute to the next most accessible ground electrode. Therefore, multi-electrode designs provide the advantage of extended service life, though they may not offer the same levels of performance as single- 3 electrode designs and they are generally more expensive to manufacture.

The third category is radial surface and semi-surface 5 gap designs, such as shown in Figure 2 and Figures 3, 3A, respectively, where Figures 2 and 3 are axial sectional views through the lower portion of the spark plugs and Figure 3A is an underneath elevational view of the spark plug of Figure 3. In these Figures, in which the central high voltage electrode 104/108 and ground-electrode 103/107 together define a radial spark gap 105/109 and the central electrode protrudes only slightly below the tip of the spark plug insulator 106/110 such that the spark is guided across the surface of the insulator tip. In surface gap designs, the ground electrode 103 encircles the central high voltage electrode 104 whereas in semi-surface designs (see Figure 3A in particular), two or more short electrodes 107' constituting the ground electrode 107 are positioned alongside the central electrode at equal angular spacings about its longitudinal axis. Such spark plugs offer the advantage of goo combustion initiation characteristics, since the po.sitioning of ground electrode(s) avoids interference with the flame front produced in the combustion chamber.; Surface gap spark plugs are used in high performance racing engines, but are not suitable for other applications owing to their poor cold starting performance... Semisurface designs enable the spark to burn off deposits 3A that have formed on the insulator surface, preventing fouling and improving cold starting and are suitable not only for racing applications but also for less high 4 performance uses, engines.

such as in car and commercial vehicle Critical factors to be designing spark plugs taken into account when include cold starting and combustion performance, fouling considerations and service life. No one spark plug is capable of optimum performance in all areas. Any given spark plug will perform well in certain respects, but less well in others. Therefore, known spark plug designs vary according to particular performance characteristics for which the spark plug is designed and/or for particular applications. Conventional thinking is to select from one of the three categories mentioned above, i.e.

single electrode, multi-electrode and radial surface or semi-surface gap designs, in order to satisfy the particular requirements of any given application or specific performance characteristics.

The present inventors have sought to improve generally the performance of conventional single electrode designs. They have discovered that applyng a surface gap arrangement such that the spark has a component in the longitudinal axial direction of the spark plug, or is directed substantially in that direction alone, enhanced performance can be achieved in different respects.

According to the invention, there is provided a spark 30 plug comprising an insulating body, a central high voltage electrode extending within the body along a longitudinal axis of the spark plug and having an exposed portion protruding from a lower end of the insulating body, and a ground electrode mounted on the body with a longitudinal spacing from its lower end such that the ground electrode and exposed end portion of the central electrode together define a spark gap with a longitudinal gap component alongside a lower end region of said insulating body, along a surface area of which the spark produced between the two electrodes runs in operation of the spark plug.

Since the electrode arrangement results in a spark gap that has a longitudinal component, a longer spark gap, such as a 2 mm or larger gap, can be accommodated within a conventional internal combustion engine design. By contrast, the size of the spark gap in a radial spark gap or surface gap plug is restricted by design considerations such as the number of inlet and exhaust valves for each cylinder and the arrangement of the water cooling passages in the engine. Furthermore, in the inventive spark plug, the spark location is near the region of the combustion chamber where the fuel/air mixture is likely to be most combustible. As a result, improved ignitability is achieved. In addition, due to the sparking taking place along the surface area of the lower end region of the insulating body, sparking across a relatively large spark gap can be produced without the need for an unduly large breakdown (ignition) voltage.

In devising the invention as defined by the appended 330 claims, the present inventors have addressed the following considerations.

6 Starting with general considerations, it is the case that, under racing conditions, maintaining adequate strength of the ground electrode in a single or multiple electrode air gap design concomitant with minimum intrusion into the combustion chamber can be a concern in this extreme environment, due to the welded connection of the ground electrode and the desirability of this electrode having a relatively small crosssectional area. The radial gap, surface discharge design offers a viable alternative in such circumstances. In addition to being a substitute for the vulnerable overhead (single electrode) or sidegapped (multiple-electrode) design, the radial gap, surface discharge design can offer significant combustion advantages for suitable engines. In most circumstances however, this will be at the expense of cold- running operation (fouling).

As for combustion considerations, minimising spark-plug protrusion into the combustion chamber ensures the minimum effect on engine volumetric efficiency. In the highest rated engines, this is of paramoun importance, taking precedence over most other factors, Moving the site of combustion initiation (the spark-gap) away from the thermodynamically ideal geometric centre of the chamber, often results efficiency when the engine is operating at a part load condition. For racing engines this is acceptable but for lesser performance engines which spend a considerable amount of time operating away from maximum load, it may be intolerable.

in questionable combustion 7 The chief drawbacks arising from positioning the spark gap close to the combustion chamber wall are: (i) placing the spark gap in the relatively quiescent wall boundary-layer reduces the likelihood of ignitable mixture being within the gap at time of discharge; (ii) developing flame-fronts must be larger than for the cases with a centrally placed gap, increasing heat loss, reducing thermal efficiency and thereby extending the overall burn period (which itself may pose a potential limit on maximum engine speed and output although this will generally be a smaller effect than and (iii) with similar reduction in volumetric efficiency); increasing heat loss to the chamber walls results.

The charge storage effects and lowered gap resistance apparent with surface discharge as opposed to air-gap discharge facilitates longer gap lengths for a given available voltage from the ignition resulting increase of spark-surface area fuel mixture can increase the potential initiation, in particular cold starting. practice, the non- optimum positioning of outweighs this advantage.

system. The exposed to the for combustion in automotive the gap often The presence of a ground electrode within the developing flame kernal, during and immediately after the discharge, can be shown to reduce the rate of kernal growth. During these initial stages of' flame development, the kernal is highly susceptible to turbulent stretching and quench. Both phenomena are capable of preventing full combustion, usually manifesting a partial of complete misfire. As such, 8 modifying the design such as to reduce the effect of heat-sink on the ground-electrode can in some circumstances increase combustion stability.

Moving on to fouling considerations, the chief defence against fouling is operation of all surfaces exposed to combustion at temperatures above about 350'C. At around this temperature and above, deposited carbon will be sufficiently oxidised to prevent the formation of a low electrical resistance carbon shunt-path. if such a shunt is formed and its overall resistance to ground is lower than that of the spark-gap, the coil discharge energy may be conducted through the path to ground, preventing the occurrence of a spark. Similarly, under maximum thermal loadings of the sparkplug typical of rated engine output, it is absolutely essential to maintain component temperatures below about 9000C to prevent the possibility of pre-ignition.

These two factors determine the safe operational temperature envelope of the spark plug.

The spark plug itself has given fixed geometry and is made of given materials, which in effect define a fixed heat transfer rate. If the engine has a particularly wide range of operation i.e. a wide range of heat supply to the plug, it is likely the plug temperature may stray outside its envelope of safe operation. Fouling is undesirable, but pre-ignition is-highly destructive. Therefore, in the design of a spark plug, consideration should primarily be given to safeguard against excessively high temperature operation even if this is at the expense of fouling protection.

9 These considerations equally apply to the spark plugs of automobile and racing engine. In the case of racing engines, fouling is not usually an issue except during cold starting and warm-up since at all other times the engine should be operating close to maximum output. With automotive engines the problem is significant enough to warrant all automobile manufacturers to standardise a cold-foul evaluation methodology.

it is important to note that any factor which potentially downgrades combustion efficiency can, as a consequence of reduced heat supply to the plug, increase the fouling potential of a particular plug design.

Various solutions available. approaches continuous 20 extension to the problem of fouling are Most if not all focus on two principal that is (i) preventing the deposition of a shunt layer by means of shielding or of surfaces likely to be fouled and (ii) employing the action of the spark to vaporise incipient shunt layers. Relating these considerations to both air gap and surface discharge spark-plugs', the former approach will often compromise spark-gap size (thus 25 downgrading its combustion performance) to ensure gap resistance is not excessive when compare( with any fouling-layer shunt path. Providing the carbon layer builds-up slowly (usually true in port-injected engine designs), such measures will usually provide sufficient 30 protection before the plug reaches its selfcleaning temperature once more. With careful design, such as can be seen in the semi-surface multiple ground electrode plugs, a further extension of the plug's anti-foul performance can be gained. Here, the scouring action of the spark is used to remove carbon from the insulator corenose tip and break the shunt path to ground. In the case of radial surface discharge plugs, increasing the potential shunt-path length is not feasible as this directly affects the plug breakdown voltage and spark plug dimensions. The insulator surface can be operated hot to ensure self cleaning, but only if this does not compromise the maximum operating temperature at high engine outputs. The final and most useful recourse in this case is to use the spark to clean the surface. This technique can be improved by the use of high energy, high secondary current, rapid rise-time coils (or so-called capacitive discharge ignition systems), but only at the expense of rapid electrode erosion and a possible degradation of combustion performance resulting from the reduction in spark-duration that this often entails.

Even with the most energetic coils however, the known (radial) surf acedischarge technique is significantly more prone to fouling than the equivalent air-gapped design since the shunt path across the surface is comparatively much shorter and very exposed to the combustion process. Indeed, only a few consecutive misfiring cycles with a cold plug is often enough for such a plug to be irretrievably fouled.

For given electrode materials, an increase of operating 330 temperature will generally result in a rapid increase in the rate of electrode corrosion. This can of course be countered by ensuring the temperature of the electrode remains within limits (such as by including 11 copper coring within the electrodes) or increasing the volume of material available for sacrificial corrosion. Limitations arise in the overall plug design of course, and the resulting compromises Must be carefully balanced with the features of ignitability (combustion performance) and anti-fouling. Improved materials are available, but in general are expensive and difficult to handle.

An increase of arc-phase current during the discharge will result in a very significant increase in electrode erosion. Similarly, increases in spark-energy increase electrode erosion as the energy increase often results in an increase in peak secondary current. The ability to reduce arc-phase current is limited, since if this current is too small, the arc is extinguished. Using improved materials helps, but often at the expense of increased work-function, which in turn increases breakdown voltage. As for corrosion reduction, improved materials are usually expensive (Pt, Au alloys) or difficult to work (Ir alloys). It is mentioned here that capacitive discharge ignition menioned above increases secondary current by a few orders of magnitude over a typical inductive ignition system.

All the above considerations were taken into account when devising the subject-matter of appended claim 1 and the other included claims.

In a preferred embodiment, the outer surface of the lower end portion of the insulating body is a longitudinally extending surface. As a result, the spark produced between the ground and high voltage 12 electrodes will be directed by the extending surface substantially in the direction of the spark plug, so as to improvement in ignitability without particularly high ignition voltage.

longitudinally longitudinal maximise the needing a However, it will be appreciated that an adequate improvement can be obtained if the electrode configuration defines a spark gap having both longitudinal and radial components, such as in a spark plug in which the lower end portion of the insulating body tapers towards its lower end at an angle not exceeding 300 to the longitudinal axis of the spark plug.

Preferably, the ground electrode is in the form of a sleeve and the exposed end portion of the central high voltage electrode comprises a disc which is arranged coaxially with the sleeve. The relatively large electrode surface areas offered by the lower end surface of the sleeve and the peripheral or upwardly facing surface of the disc (depending on whether or not the disc extends radially beyond the side edges of the lower tip of the insulating body) provides an unrestricted site for the sparking, thereby minimising the effect of corrosion and erosion. Therefore, the service life of the plug is extended. Furthermore, the spark will be established at the location of lowest electrical breakdown resistance, i. e., where fouling deposits are formed, thereby oxidising the carbon fouling. In this way the anti-foul performance of the j30 plug is improved.

In the embodiment in which the outer surface of the lower end portion of the insulating body is a 13 longitudinally extending surface, suitably the diameter of the disc constituting the exposed end portion of the central high voltage electrode is substantially the same as the internal diameter of the sleeve forming the ground electrode. This arrangement helps to guarantee that the spark between the electrodes will be orientated substantially in the longitudinal direction only. Preferably also, the lower end portion of the insulating body is cylindrical and the diameter of the disc is the same as that of the cylindrical lower end portion. This arrangement helps to avoid channelling of the ceramic material from which the insulator body is made, due to the action of the spark.

The sleeve may be provided on its lower edge with a plurality of projections pointing towards the disc, so as to increase the dielectric stress in the gaps between the sleeve and disc at the locations of the proj ections, thereby reducing the breakdown (or ignition) voltage. Such projections may be in the form of castellations or sharp points.

Conveniently, the ground electrode may be in electrical connection with a conductive retainer sleeve which is fitted around the insulating body and externally screwthreaded, for securing the spark plug within the cylinder head of an internal combustion engine. The retainer sleeve and ground electrode may be separate components, but manufacturing cost can be reduced by making them of unitary construction.

The spark plug defined above in general terms and according to certain preferred features offers a 14 further advantage in that its internal components can be re-configured, specifically to reduce the length of the high voltage electrode, to provide a more compact construction for the spark plug. Specifically, improved fouling performance means that the plug can be operated at a comparatively cool temperature (say approximately 500'C) within the possible range, which contributes to extending the life of the spark plug through reduced corrosion. Furthermore, in view of the insulator body extending to a much lower position in the combustion chamber than a radial surface design would, the necessary insulator nose length (the distance between the firing tip of the insulator and the point where it is seated inside the external retainer shell which locates the spark plug in the engine cylinder head) for maintaining the electrodes and insulator tip at the desired, relatively cool, operating temperature, and therefore also the length of the central high voltage electrode, can both be made relatively short. This in turn results in the spark plug having reduced capacitance, since its capacitance is, in part, dependent on the length of the high voltage electrode. This contributes to prblonged spark plug life, since the spark produced is less aggressive for a lower capacitance plug.

Another important advantage of the reduced capacitance of the spark plug is that it only needs an ignition coil assembly of correspondingly lower rating, wh ich in turn means a coil assembly whose external physical dimensions can be small.

The cylinder head of modern internal combustion engines generally holds each spark plug in a lower position in a bore or tunnel formed in the cylinder head. The associated ignition coil assembly is typically a cigarshaped unit, secured in an upper position in the same tunnel in electrical connection with the high voltage electrode of the spark plug. The upper end portion of the coil assembly projects above the outer surface of the cylinder head.

The trend in modern engine design is towards compactness as engines for any given power output become smaller and mechanical and electrical/electronic complexity increases. However, since coils and spark plugs are secondary considerations for engine manufacturers, the emerging trend for these components is towards decreased tunnel diameter tunnel length. In principle, and increased this provides an opportunity to accommodate the entire ignition coil unit within the tunnel, so that it does not project above the surface of the cylinder head, thereby resulting in a more compact arrangement. Such a benefit is not obtainable with known spark plugs because, to a large extent, their size is dictated by their rating, which has to be matched to the (relatively large) capacitance exhibited by the spark plug. On the other hand, since the spark plug disclosed herein exhibits lower capacitance than a conventional plug with an axial air gap or radial air gaps, the external dimensions of its associated ignition coil assembly can be reduced, be more readily accommodated in the tunnel, preferably without projecting so that it can cylinder head above the outer 16 surface of the cylinder head. Therefore, the spark plug assembly can contribute to a more compact construction for the assembled cylinder head and spark plugs/ignition coils, and also an integrated design for accommodating the ignition coils and spark plugs within the internal combustion engine.

According then to a particularly preferred embodiment, there is provided a spark plug assembly comprising a spark plug as described above and an ignition coil assembly for the spark plug, the ignition coil assembly being located remotely from the lower end of the spark plug and in electrical connection with the high voltage electrode.

In one arrangement, the ignition coil assembly includes an insulating outer housing mounted on the insulating body in an upper end region thereof remote from its lower end. Alternatively, the ignition coil assembly includes an insulating outer housing, the housing and insulating body being of integral construction. In the former case, preferably the spark plug is provided with a high voltage terminal, which is in electrical connection with the high voltage electrode and located at the base of a recess, below the upper end of the insulating body of the spark plug, and the ignition coil assembly includes a coiled contact spring providing electrical connection between the ignition coil of the ignition coil assembly and the high voltage terminal of the spark plug, and a labyrinth seal having a cylindrical portion fitted around the contact spring and extending to a position adjacent the high voltage terminal. As a result, the flash-over length for the 17 high voltage terminal, that is to say the length of the potential discharge path to earth, is large, so as to ensure high dielectric integrity for the spark plug and coil assembly.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is an longitudinal sectional view through the lower end portion of a known spark plug of single electrode design, providing an axial spark gap; Figure 2 is a corresponding view of a known spark plug of radial surface gap design; Figure 3 is a corresponding view of a known spark plug of semi-surface gap design; Figure 3A is an underneath elevational view of the spark plug of Figure 3; Figure 4 is a longitudinal sectional view through a 25 spark plug forming a first embodiment of the invention; Figure 4A is an underneath perspective view of - one preferred form of ground electrode that can be, used in the spark plug of Figure 4; Figure 4B shows an alternative construction for the ground electrode; 18 Figure 5 is a longitudinal sectional view through a second embodiment of the invention; Figure 6 is a corresponding view through a still 5 further embodiment having no retainer shell; Figure 7 is a longitudinal sectional view through an ignition coil assembly for fitting to the spark plug shown in Figure 6; Figure 8 is a longitudinal sectional view through a spark plug unit comprising the ignition coil assembly mounted on the spark plug and located in a tunnel or bore formed in the cylinder head of an internal combustion engine; and Figure 9 is a corresponding view of a spark plug unit constituting a fourth embodiment, in which the spark plug and ignition coil together form a unitary 20 assembly.

Referring to Figure 4, the spark plug, denoted by reference numeral 1, according to the embodiment comprises essentially a central electrode-and-terminal assembly 6 extending within a central passage in an insulating body 2 along the longitudinal axis of the spark plug, a re tainer sleeve or shell 7 disposed about the central and lower portion of the insulator for securing the spark plug in the cylinder head of an internal combustion engine 2, and a ground electrode 4 in the form of a sleeve positioned around the insulating body 2 adjacent its lower end and arranged in electrical connection with the retainer shell 7.

19 The central electrode-and-terminal assembly 6 comprises a central terminal 8 located in an upper region of the spark plug, to which the high voltage connector of an ignition coil assembly is to be connected, a resistor 9 formed from fused carbon and glass particles to control the peak current delivered to the spark plug from central terminal 8, a separate mass of fused copper and glass particles 10 forming good electrical connection between terminal 8 and resistor 9, a central, high voltage, electrode 3 in a bottom region of the plug, and another separate mass of fused copper and glass particles 11 forming good electrical contact between resistor 9 and high voltage electrode 3. This electrode, which can serve as a cathode or anode. has an exposed portion 3a protruding from the lower end or tip 2a of the insulator 2, the exposed portion being in the form of a circular disc. This disc is suitably of relatively large thickness as shown, to increase its robustness and extend operating life.

The ground electrode 4 is in the shape of a sleeve fitted around the lower end 2a of the insulator body 2 with a longitudinal spacing 12 between itstransverse bottom end face 4a and the upper face of high voltage electrode disc 3a. Accordingly, the ground and high voltage electrodes together define a spark gap o.f a size equal to the spacing 12 and extending substantially in the longitudinal direction of the 33 0 spark plug only. The upper part of the ground electrode is in the form of a flange 4b, which is secured in a corresponding annular groove formed within the inner wall of the shell 7, thereby maintaining the ground electrode 4 and shell in good electrical connection with one another. The shell, which suitably is made from steel, is externally threaded as indicated at 7a to be engaged with a complementary internal screwthread formed in a bore or tunnel in the cylinder head of an internal combustion engine. The shell has an external flange 13 providing a seat to be sealed against the cylinder head by a gasket (not shown) positioned immediately beneath the flange and above the external screwthread 7a. An external hexagonal nut 14 formed as an integral part of the retainer shell 7 can be gripped by a spanner for securing the spark plug in the cylinder head of the internal combustion engine with the spark gap projecting into the associated combustion chamber.

It i s necessary that there be good thermal conduction between the shell 7 and insulator 2, which is made of ceramic, suitably alumina, glazed over the surface area which is exposed in service. To achieve the good thermal connection necessary to achieve the desired heat sink, the insulator and shell are; held under pressure either by shrink fitting or by! a so-called sillament, which is a very fine alumina powder compressed between the body and shell.

i. _ Similarly to the case of a conventional spark plug having a single electrode and an axial air gap,.a spark is produced in the present spark plug in the following 3 way. Initially, the associated ignition coil causes the voltage on the central electrode to rise over a short time interval, of a few microseconds, to several kilovolts (the pre-breakdown phase), then the 21 resistance of the fuel-air mixture in the gap breaks down (the breakdown phase) and the voltage drops (to tens of volts, for example), after which an arc is produced with the voltage rising (to a few hundred volts) before entering the glow phase. During the prebreakdown phase, energy is stored in the spark plug depending on the capacitance of the plug, which depends on the length of the high voltage electrode 3 and the radial clearance between the centre electrode (3) and the bore of the shell (7). Since the so-called insulator nose length, which is the distance between the firing tip of the insulator (2a) and the point where it is seated in thermal connection with the inside of the shell 7, is comparatively short, the spark plug can run relatively "cool" (say around 500'C) and the high voltage electrode can be made relatively short, thereby reducing the capacitance of the spark plug and accordingly erosion, due to the less aggressive spark produced.

It will be noted that in operation, sparking will take place between the ground and high voltage electrodes in the longitudinal direction, but at an unrestricted site about the longitudinal axis of the spark plug. In other words, the spark will take place each time at a site corresponding to the location of minimum breakdown resistance. Therefore, the sparking site will change when a new site offers lower resistance, such as when the previous site becomes worn (corroded or eroded).

Furthermore, it will be appreciated that the spark will run along the exposed outer surface of the insulating body as it passes between the two electrodes.

22 In view of the relatively large surface area of each electrode facing the other one to allow unrestricted sites for the sparking to take place, the effects of corrosion and erosion are minimised, so as to prolong 5 the life of the spark plug.

In addition, since the spark gap will typically be positioned near the location within the combustion chamber where the fuel-air mixture is likely to be most combustible, ignitability and cold starting are assisted. Furthermore, since the spark is directed in the longitudinal direction, a larger spark gap can be used than in the case of a radial discharge plug (multiple electrode or surface discharge), thereby improving cold starting. Still further, the voltage which needs to be applied to produce the spark is lower than would be the case for an axial air gap (without an adjacent insulator surface), which reduces the breakdown voltage for any given spark gap.

Since the diameter of the exposed high voltage electrode disc is equal to the external diameter of the protruding insulator tip, ceramic channelling can be substantially prevented.

Another advantage of the design is that the ceramic insulator and shell are relatively long, concentric components, which lend themselves readily to accurate coaxial positioning during manufacture.

Still further, the spark plug has desirable antifouling performance since carbon deposits on the electrodes will promote local sparking by reducing 23 electrical resistance, the spark then serving for oxidising the carbon and removing the fouling.

Yet another advantage is that in view of the absence of an annular space between the lower end region of the insulator body and the surrounding ground electrode (contrary to the conventional spark plugs such as shown in Figures 1 to 3), it is expected that harmful hydrocarbon emissions can be reduced over conventional designs since the plug construction does not provide an opportunity for crevices in the spark plug to harbour unburned hydrocarbons.

Figures 4A and 4B show two alternative forms for the ground electrode 4, which in the embodiment just described has an annular bottom edge 4a. In Figure 4A, the lower edge of the sleeve forming the ground electrode is provided with castellations 4c. A modification is shown in Figure 4B in which sharp points 4d are used instead of castellations. In each case, the sharp edges increase the dielectric stress, to promote sparking at a lower breakdown;.voltage than in the case of a plain annulus.

Figure 5 shows a modified design in which the shell and -1 ground electrode are of single-part construcon. This avoids the need to fabricate separate components and reduces the manufacturing cost of the spark plug.

Another modification, also shown in Figure 5, which may be applied to the spark plug according to Figure 4 in addition to or instead of using a unitary construction for the shell 7 and ground electrode 4, is to taper the 24 insulator body towards its lower between the two electrodes. As a will run along the surface of the the insulator body and the spark gap 5 gap component in addition to a tip in the region result, the spark tapered portion of will have a radial longitudinal gap component. Particularly if the insulating body tapers towards its lower end at an angle not exceeding 30' to the longitudinal axis of the spark plug, the spark plug will offer similar advantages to the one described with reference to Figure 4.

Turning now to Figure 6, there is shown another embodiment of the invention in which the spark plug differs fundamentally in that it does not have a retainer shell for retaining the spark plug in the cylinder head of the combustion engine. In this embodiment, the ground electrode 4 is again in the form of a sleeve but the flange at its upper end is in the form of an inverted frustro-conical seating flange 4'.

The insulating body 2 has a central cylindrical portion 2b of slightly larger external diameter than that of the seating flange 4'. The insulating bod 2 is formed with an annual shoulder 2c forming a transition between the central cylindrical portion 2b and an upper cylindrical portion 2d of reduced diameter. The central terminal 8 is disposed entirely within a central bore formed in the insulating body, the head of the central terminal. 8 being located within the'bore at a certain distance from the opening at the top 'of the jM reduced diameter upper portion 2d into the central bore. A securing device in the form of a clamping ring 5 is positioned around the reduced diameter upper portion 2d against the shoulder 2c. The clamping ring is formed with an external screwthread 5a.

Referring now to Figure 7, there is shown an ignition coil assembly 18 for the spark plug. The ignition coil of this assembly is housed within an upper region of an insulating outer housing 20, within a lower portion of which is secured a cylindrical sleeve member 21. A labyrinth seal 22 of insulating material provides a seal between the cylindrical sleeve member 21 and housing 20, as well as a downwardly depending cylindrical portion 28. A coiled contact spring 25 is located in the cylindrical portion 28 of the labyrinth seal 22, projecting from its lower end, and is held captive on the end of a terminal 26, passing through a transverse wall 27 of the coil assembly housing and connected to the ignition coil of the coil assembly 18.

As shown in Figure 8, the ignition coil assembly 18 is designed to be fitted on to the upper cylindrical portion 2d of the spark plug. Then, the cylindrical sealing portion 28 is positioned within the central passage in the upper cylindrical portion 2d, and two axially-separated sealing members 23, 24 located in corresponding annular grooves within the.inner wall of the cylindrical sleeve member 21 seal against the outer surface of the upper portion 2d of the insulator 2. In this position, the spring 25 is compressed and presses against the central terminal 8 of the spark plug. so as to establish electrical connection between the ignition coil of the ignition coil assembly 18 and the high voltage electrode 3 of the spark plug.

26 It is important to note that since the upper end of high voltage terminal 8 is recessed below the upper end of insulator 2 and since the cylindrical sealing portion 28 of labyrinth seal 22 extends downwardly, around contact spring 25, almost to the high voltage electrode terminal 8, the flash-over length (i.e. the potential discharge path from the high voltage terminal to earth, in this case afforded by clamping ring 5) is significantly larger than if the terminal 8 were to be positioned in the region of the upper end of insulating body 2. In this way, the spark plug and coil assembly exhibit high dielectric integrity.

To assemble the spark plug and its ignition coil 15 assembly in an internal combustion engine, firstly the spark plug alone is inserted through the open mouth of a stepped bore 56 in the cylinder head, shown at 29, and lowered through the larger diameter, upper section of the stepped bore to its smaller diameter, lower section. The clamping ring 5, fitted over the upper cylindrical portion 2d of the plug, is then tightened up, so that the seating flange 4' of the spark plug is held pressed against a corresponding seating surface 29' formed in the lower bore adjacent the combustion chamber 27, by the clamping action of the clamping ring 5 whose external screwthread 5a is engaged with a corresponding screwthread 56' in the base of a widened upper section of the bore 56. Lastly, the ignition coil assembly 18 is inserted through the mouth of bore 6 and located on the spark plug 1 and clamped in place. The clamp is not shown since it can assume various forms, dependent on the engine design, and the form 27 adopted is of no relevance to the subject-matter for which protection is sought in the present application.

Since the spark plug has low capacitance due in part to its relatively short high voltage electrode resulting from the spark plug having a comparatively short insulator nose length, the associated ignition coil assembly needs to have only a correspondingly lowered rating. This in turn means that the external dimensions of the coil assembly can be reduced. As a result, the ignition coil assembly can be accommodated to a large extent, or as a whole, within the upper portion of the bore or tunnel in the cylinder head. The upper section of the bore may need to be enlarged to accommodate the ignition coil assembly, as indicated in Figure 8, but this is not disadvantageous since the section of the cylinder head in which the bore diameter needs to be as small as possible is the region adjacent the combustion chamber, which needs to accommodate 20r multiple valves and cooling passages of the internal combustion engine.

Referring now to the fourth embodiment;according to Figure 9, the insulating housing of the ignition coil assembly 18 and the insulating body 2 of the spark plug 1 are of single- part construction, as shown. In this embodiment, the clamping ring 5 is received by means of its external screwthread 5a in a screwthreaded bore 56' in the mouth of the wider, upper section of the bore 56 and presses downwardly against the upper end of the insulating body 2 to hold the seating flange 4' of the spark plug against the seating surface 29' provided near the bottom end of the bore 56. Forming the 28 insulating body of the spark plug and housing of the ignition coil assembly as a single unit reduces manufacturing cost, particularly since no special seals are then required to provide -the above-described sealing when the ignition coil assembly and insulator are fitted together. Furthermore, no coiled contact spring is needed since there is a permanent electrical connection between the ignition coil of the coil assembly and the central electrode.

The sleeve constituting the ground electrode of the third and fourth embodiments can be modified as shown 1 in Figures 4A and 4B, so as to include castellations or sharp points on the lower edge of the sleeve.

It will be appreciated that the third and fourth embodiments are very compact in design, allowing effective packaging within the internal combustion engine, and allowing more freedom in the design of the water jacket and the general cylinder head layout. if required, the spark plug unit comprising the spark plug and its associated ignition coil assembly may be positioned so as to be submerged in' engine oil, allowing high integration potential.

It will be further noted that the advantage of width reduction of the spark plug through not using the conventional external retainer shell can be achieved without reduction in dielectric integrity of the spark plug. The coil-to-plug labyrinth seal used in the third embodiment ensures a long flash-over length allowing close coupling of the coil and spark plug while maintaining excellent oil-tightness.

29 In view of the described advantage of increased electrode life resulting from lower levels of corrosion and erosion, the service life of the spark plug according to all four embodiments will be extended. In the case of suitable design, the spark associated ignition coil assembly, may "fit-for-life", i.e. they may not need changing during the anticipated working life of the eng-ine. The improved performance and life, and the smaller space requirement, of the spark plug provides impetus to incorporate the spark plug unit, comprising spark plug and ignition coil assembly, into the engine design.

plug, and its be regarded as 1 1 i i,-? Federal Mogul Ignition (UK) Limited

Claims (16)

1. CLAIMS:

   G13487 r5/mp 1. A spark plug comprising an insulating body, a central high voltage electrode extending within the body along a longitudinal axis of the spark plug and having an exposed portion protruding from a lower end of the insulating body, and a ground electrode mounted on the body with a longitudinal spacing from its lower end such that the ground electrode and exposed end portion of the central electrode together define a spark gap with a longitudinal gap component alongside a lower end region of said insulating body, 'along a surface area of which the spark produced between the two electrodes runs in operation of the spark plug.
2. 2. A spark plug according to claim 1, wherein the outer surface of said lower end portion of said insulating body is a longitudinally extending surface.
3. 3. A spark plug according to claim 1, wherein said lower end portion of said insulating body tapers towards its lower end at an angle not exdeeding 30' to the longitudinal axis of the spark plug.
4. 4. A spark plug according to any precdaing claim, wherein the ground electrode is in the form of a sleeve and the exposed end portion of the central high voltage electrode comprises a disc which is arranged coaxially with said sleeve.

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5. 5. A spark plug according to claim 4 as appended to claim 2, wherein the diameter of said disc is substantially the same as the internal diameter of said sleeve.
6. 6. A spark plug according to claim 4 as appended to claim 3 or according to cl aim 5, wherein the lower end portion of the insulating body is cylindrical and the diameter of the disc is the same as that of the cylindrical lower end portion.
7. 7. A spark plug according to claim 4, 5 or 6, wherein said sleeve is provided on a lower edge thereof with a plurality of projections pointing towards said disc.
8. 8. A spark plug according to claim 7, wherein said projections are in the form of castellations.
9. 9. A spark plug according to claim 7, wherein said projections are in the form of sharp points.
10. 10. A spark plug according to any prepeding claim, wherein the ground electrode is i:n electrical connection with a conductive retainer sleeve which is fitted around said insulating body and externally screwthreaded for securing the spark plug within the cylinder head of an internal combustion engine, the retai ner sleeve and ground electrode being of unitary construction.
11. 11. A spark plug substantially as hereinbefore described with reference to Figure 4, 5, 6 to 8, or 9 32 of the accompanying drawings, or the modifications of Figures 4A and 4B.
12. 12. A spark plug unit comprising a spark plug according to any one of claims 1 to 10 and an ignition coil assembly therefor, the ignition coil assembly being located remotely from the lower end of the spark plug and in electrical connection with the high voltage electrode.
13. 13. A spark plug unit according to claim 12, wherein the ignition coil assembly includes an insulating outer 1 housing mounted on the insulating body in an upper end region thereof remote from its lower end.
14. 14. A spark plug unit according to claim 13, wherein the spark plug is provided with a high voltage terminal, which is in electrical connection with the high voltage electrode and located at the base of a recess below the upper end of the insulating body of the spark plug, and wherein the ignition coil assembly includes a coiled contact spring providing electrical connection between the ignition coil of ithe ignition coil assembly and the high voltage terminal of the spark plug, and a labyrinth seal having a cylindrical portion fitted around the contact spring and extending to adjacent the high voltage terminal.
15. 15. A spark plug assembly according to claim 12, wherein the ignition coil assembly includes an insulating outer housing, said housing and insulating body being of integral construction.

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16. 16. A cylinder head for an internal combustion engine, having a bore in which the spark plug and coil assembly of a spark plug unit according to any one of claims 11 to 13 are both secured such that the whole coil assembly is located entirely within said bore.

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