SPARK PLUG

Abstract: Spark plug (1) comprising at least two insulated electrodes (5), and (6), an earthed electrode (7) and a chamber (4) so arranged with respect to one another that when a voltage is applied arcs are struck in series between electrodes (7) and (6) and (6) and (5) to provide a continuous plasma jet expelled from the chamber (4) at low energies (< 100mJ) and which can be used for combustion of a lean air/fuel mixture.

Description: Spark Plug TECHNICAL FIELD OF INVENTION The invention relates to a spark plug, particularly though not exclusively for use in an internal combustion engine. BACKGROUND ART In proposed spark plugs, combustion is initiated from a plasma generated by striking an arc between two electrodes. With typical spark energies of 30 to 40 mJ. these devices are not capable of igniting mixtures with an air/fuel ratio significantly greater than stoichiometric. Published work indicates that a jet of plasma is much more efficient at igniting mixtures with a high air fuel ratio (see example SAE Technical Papers 770355 and 800042). In further spark plugs to generate jets of plasma at a high air/fuel ratio, a high energy ( 1 Joule) is also required to be dissipated in the arc. DISCLOSURE OF INVENTION It is an object of the invention to seek to mitigate this disadvantage of proposed spark plugs. A spark plug according to the invention is characterised by at least two insulated electrodes and an earth electrode so arranged with respect to one another that arcs are struck in series between successive electrodes. Spark plugs embodying the invention are hereinafter described, by way of example, with reference to the accompanying drawings. OMPI BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is schematic longitudinal sectional view of one proposed or traditional spark plug; Fig. 2 is a schematic longitudinal sectional view of another proposed spark plug; Fig. 3 is a schematic longitudinal sectional view of a spark plug according to the invention; Fig. 4 is a schematic longitudinal sectional view of a second embodiment of spark plug according to the invention; Figs. 5 to 8 are schematic longitudinal sectional views respectively of different electrode configurations of spark plugs according to the invention; Fig. 9 is schematic longitudinal sectional view of a further spark plug according to the invention; and Fig. 10 is schematic lay-out of a driver unit for a spark plug according to the invention. Referring to the drawings, Fig. 1 shows a spark plug in which combustion is initiated from a plasma generated by striking an arc between electrodes A and B. With typical spark energies of 30 to 40mJ. such a plug is not capable of igniting mixtures with an air/fuel ratio significantly greater than stoichiometric. Fig. 2 shows a cross section of a relevant part of another proposed spark plug which can generate a plasma jet for igniting combustible mixtures with a high air/fuel ratio. An arc is struck between end electrode A and centre insulated electrode B, within cavity C in ceramic body D. Providing sufficient energy is dissipated in the arc (~> l Joule) a high level of ionisation is produced in the cavity C. The energy dissipated also heats the gas to expand rapidly. Consequently, ionised gas is ejected from the cavity C as a plasma jet E. Spark plugs embodying the invention are illustrated in Figs 3-9 , in which like reference symbols are used for like parts. Referring to Fig. 3 there is shown a spark plug 10 embodying the invention, which is essentially a twin gap spark plug for producing a plasma jet at E. The relevant, spark forming part of the spark plug 1 has an insulator in the form of a ceramic body 2 terminating at a surface 3, the insulator encompassing a cavity 4 in which there is a (central) insulated electrode 5« There is also an insulated electrode 6 and an earth electrode 7- The electrodes are spaced apart and form sequential pairs 7-6 and 6-5. The spark plug 1 embodying the invention operates as follows:- An arc is struck simultaneously between electrodes 7 and 6, and 6 and 5 which arcs are in series. The heat generated by the arc between electrodes 6 and 5 causes the gas in the cavity 4 to expand and be ejected from the cavity 4« As the expanding gas escapes from the cavity 4 it has to pass through the arc between the electrodes 7 and 6, and is in consequence ionised. Hence a jet of plasma is generated at E. This system of plasma jet generation is very efficient and will work with spark energies of lOOrπ , for example in the range of about 50mJ to lOOmJ. The operation of the device is self-stabilising as a consequence of negative feedback inherent in the design. The arcs 7-6, 6-5 are in series. Any tendancy for the arc 7-6 to be extinguished by the expanding gas passing through it will reduce the arc current. This will also reduce the current through arc 6-5, hence reducing the heat dissipation within cavity 4 • The gas will therefore expand OMPI more slowly, reducing the tendancy to extinguish arc 7-6. Thus two spark gaps in series are used. One spark gap is inside the cavity to heat and expand the gas. The other is across the orifice of the cavity to ionize the expanding gas as it is e ected f om the ' cavity. It is possible to generate a plasma heat with less than lOOmJ of energy. Referring now to Fig 4 > there is shown a longitudinal sectional view of a part of a spark plug 1, according to the invention, which can generate a plasma jet for igniting combustible mixtures with a high air/fuel ratio. The relevant, spark forming part of the spark plug 1 has an insulator in the form of a cermaic body 2 terminating at a surface 3. the insulator encompassing a cavity 4 in which there is a (central) insulated electrode 5- There is also an insulated electrode 6 and an earth electrode J . The electrodes 6 and 7 are each extended upwardly, as viewed, above or away from the surface 3 of the body 2. In the embodiment shown, the respective extensions 8 and 9 of the electrodes 6 and 7 diverge upwardly as viewed. In use, an arc is struck simultaneously between electrodes 7 and 6, and 6 and 5 which arcs are in series. The gas expanding out of the cavity 4 produces a sheet kind of discharge as indicated at 10, which sheet discharge effect cannot be obtained by striking an arc between the electrodes 7 and 6 only there being in this latter case a normal narrow spark discharge only. The spark plug 1 according to the invention thus produces a sheet discharge which provides for ionisation of the hot gas expanding in and being ejected fom the cavity 4 so producing a jet of plasma as indicated by arrow 'E' . The jet E then provides a reliable combustion of lean air/fuel mixture. The extensions 8 and 9 of the electrodes to provide a sheet discharge means that the spark plug gap has in effect been extended, but the voltage does not have to be increased to achieve combustion. PI Further, the spark for ionising the gas is self-stabilising because any tendency for the spark between electrodes 6 and 7 to be extinguished by the gas expanding out of the cavity 4 will tend to reduce the arc current hence reducing the heat dissipation- in the cavity 4 which will result in the gas expanding more slowly, reducing the tendency to extinguish the spark between electrodes 6 and 7 and so maintaining the sheet discharge 10 and providing comprehensive ionisation of the gas. The net result is that the plasma jet E is maintained. The plasma jet E can be produced with less than lOOmJ of energy in such a plug. It will be understood that the spark plug shown in the drawing and above described may be modified. For example, the parts 8 and 9 may converge or may be substantially parallel. Referring now to Figs 5 to 8, different electrode configurations of spark plugs according to the invention are illustrated. In Fig 5 . electrodes 6 and 7 are flush with the surface 3 while in Fig 6 there is shown electrodes 6 and 7 which are proud of the surface 3« Fig 7 shows electrodes in which the surface 3 itself is inclined to provide a generally frusto-conical configuration, the electrodes 6 and 7 being shaped to provide inclined electrodes which protrude above the surface 3 as in Fig 4 * The motion of the jet in use in this embodiment draws fresh charge into the active region adjacent the plug thus encouraging mixing and improving combustion. Similarly in Fig 8, the electrodes 6 and 7 have extensions 8 and 9 respectively, the electrodes being laid on the surface of the insulator rather than being buried in it leading to maximum exposure of the fuel charge to the arc. In Figs 5 to 8, there is shown a metal body 11 of the plug 1 with part of a screw thread by which the plug 1 may be screwed into an internal combustion engine. There is a central rod electrode 5 which is sheathed in a plastics material 12 such * as polytetrafluorethylene (PTFE) except on the face exposed to the interior of the cavity 4- Fig 9 shows another embodiment of spark plug electrode 6 comprising two parts 6A and 6B which are linked electrically as shown. The series arcs between electrodes 7-6A and 6B-5 are shown, there being provided a sheet discharge between extensions 8 and 9 n use. In Figs 5 to 9, the exit from the cavity 4 is narrower than the cavity itself. In Fig 9 the exit may be 1mm in width, the volume of the cavity 4 being 28mm , the extensions being 3πm "long" - in other words this is the distance from the free ends of the extensions to the surface 3 , measured vertically as viewed. In the embodiments shown in Figs 4, 8 and 9 ? for example, the extensions 8 and 9 may be formed by wires secured to the electrodes 6,7* by a tube secured to those electrodes, or by integrally forming the electrodes to form the required shape of exteπsion(s) . These extensions modify the shape of the arc/plasma jet to provide a continuous combustion of lean mixtures. The net result stated another way, in each embodiment is the provision of a stable sheet discharge which can be generated at low energies and which can be used for the ignition of lean mixtures of fuel in internal combustion engines. Fig. 10 shows schematically a driver unit which can be used for driving a spark plug 1 embodying the invention. The plug 1 is connected in a circuit including a battery.12 (+350 V supply), an electronic switch 13, a capacitor 14 and ignition coil 15. The capacitor may have a capacitance of 1 LF. The stored energy is then: CV = 60mJ Using such a circuit with the switch closed for 0.5 m seconds a peak spark current of about 75m falling exponentially to 20m peak at the end of the 0.5ms period is achieved, the circuit "ringing" at about 4KHZ. The volume of the cavity 4 may be varied by making the position of the electrode 5 relative to the body 2 adjustable. The space between facing surfaces of electrodes 6 and 7 may have a width/diameter of O.508πm which is also equivalent to the lateral extent of the cavity. The distance between the base (as viewed of the electrode 6 and the top (as viewed) of the electrode 5 may be 0.38lmm. It will be understood that a spark plug embodying the invention has many applications, and is not just for internal combustion engines, for example:- 1. Other types of engine where the plug could be used are: a) Gas Turbines b) Gas Engines c) Spark Associated Diesels d) Any other type of engine requiring or benefitting from a spark to initiate combustion. f OMPI 2. The plug could be used as an ignitor in static applications: a) Boilers b) Heaters c) Process Plant d) Gas Jets e) Blow Lamps f) Torches. 3- The plug could also be used anywhere a pulsed source of ioas is suitable: a) Cleanig by Ion Bombardment b) Surface Treatment c) Inducing Chemical Changes in Surfaces d) Triggering Electrical Discharges e) Displays f) Inducing Chemical Reactions. In every embodiment, the use of two arcs in series utilises the available energy more efficiently, a larger proportion thereof being dissipated in the arcs and less in the (external) driving circuitry.

Claims: CLAIMS 1. A spark plus characterised by at least two insulated electrodes (5) and (6) and an electrode (4) so arranged with respect to one another that arcs are struck in series between successive electrodes (7,6) and (6,5)- 2. A spark plug according to Claims 1, characterised by the electrodes (6), (7) being flush with a surface (3) of the plug (1) . 3. A spark plug according to Claim 1, characterised by the electrodes (6) , (7) being proud of a surface (3) of the plug (1) . 4- A spark plug according to Claim 1, characterised by an extension (8) and (9) of the electrodes (6), (7) projecting away from the electrode (5) externally of the surface (3). 5. A spark plug according to Claim 4 > characterised by the extensions comprising wires (8), (9) secured to the respective electrodes (6), (7). 6. A spark plug according to Claim 4 . characterised by the extension (8), (9) comprising a tubular extension secured to the electrodes (6), (7) . 7. A spark plug according to any of Claims 4 to 6, characterised by the free end of the extension (8), (9) being spaced further apart than the end adjacent the surface (3) • 8. A spark plug according to any preceding claim, characterised by the surface (3) being of generally frusto-conical configuration. 9. A spark plug according to any preceding claim, characterised in that the electrode (5) is within a plasma production chamber (4). 10. A spark plug according to Claim 9* characterised in that the volume of the chamber (4) is adjustable. O

Inventors: LEE ANTHONY JOHN JAMES, LEE, ANTHONY, JOHN, JAMES
Applicants: LEE ANTHONY JOHN JAMES, LEE, ANTHONY, JOHN, JAMES
Application year: 1983
Main classification: H01T13
IPC codes: H01T13