SPARK PLUG

Abstract: A spark plug comprising a first electrode (4) to which a high potential is to be applied when a spark is to be generated and a second electrode (6) to be connected to ground. An intermediate insulated electrode (5) is positioned between the first (4) and second (6) electrodes such that when a high potential is applied to the first electrode (4) two sparks are formed, one between the first electrode (4) and the intermediate electrode (5) and the other between the second electrode (6) and the intermediate electrode (5).

Description: SPARK PLUG The present invention relates to spark plugs. Spark-ignited internal combustion engines such as those used on conventional petrol-powered vehicles comprise a spark plug located in each cylinder of the engine which is "fired" at a time appropriate to the movement of a piston within the cylinder. An ignition system is provided which applies a high potential to an electrode of the spark plug so that a spark is generated between that electrode and an earth portion of the spark plug. The spark ignites any fuel mixture within the cylinder. Thus the conventional spark-ignited internal combustion engine relies upon the use of "one-shot" spark plugs which generate single sparks timed to occur at a critical point during the upward compression stroke of the engine piston. Typically ignition is timed to occur 10° before top dead centre. The spark plug then fires and ignites the fuel mixture to convert it into useful thermal energy in the following downward power stroke of the piston. The amount of useful energy produced at this time is governed by many complex factors, but the nature of the ignition spark has a most critical role to play in the energy conversion process. The conventional one-shot spark-ignition systems represent a basic compromise between the ease with which the spark can be generated and the type of spark which would be generated in ideal conditions. Thus the potential generating spark fluctuates considerably as a function of engine speed with a resultant adverse affect on engine efficiency. The spark temperature also varies and although the fuel mixture is still generally ignited the ignition conditions are in many operating conditions not ideal. It has been known for a considerable period that spark-ignited internal combustion engines would perform better if it were possible to "fire" the spark plug in each cylinder twice during the ignition period. If it was possible to do this a greater amount of energy would be released from a given fuel mixture than is possible with a single shot spark plug, a lot less choke would be needed for cold starting, faster warm up would be achieved, and generally a greater level of overall efficiency would be obtainable. It is an object of the present invention to provide a spark plug which is capable of generating two sparks. According to the present invention there is provided a spark plug comprising a first electrode to which a high potential is to be applied when a spark is to be generated, a second electrode to be connected to ground, and an intermediate insulated electrode positioned between the first and second electrodes such that when a high potential is applied to the first electrode two sparks are formed one between the first electrode and the intermediate electrode and the other between the second electrode and the intermediate electrode. The spark plug may comprise a ceramic insulator similar to that of conventional single spark plugs through which the first electrode extends to a point which is located within the cylinder of the engine when the spark plug is installed. The second electrode can be secured to a conductive threaded portion which is screwed into the cylinder head again in a conventional manner. The intermediate electrode may be supported between the first and second electrodes in any convenient manner. For example the intermediate electrode may be mounted on the ceramic insulator, or on an insulating support secured to the threaded portion of the spark plug tip which is engaged inside the cylinder head, or on an insulating pin extending from the first or second electrode. The gaps between the first and second electrodes and the intermediate member may be the same or one may be slightly greater than the other. For example the gap between the first electrode and the intermediate member may be 25 thousandths of an inch and the gap between the second electrode and the intermediate electrode may be 28 thousandths of an inch. When high tension potential is applied to the central first electrode a first spark is generated on one side of the intermediate electrode followed by the formation of a second spark on the other side of the intermediate electrode. This effectively achieves the desired double spark which it has been acknowledged in the past improves engine efficiency. In addition the total duration of the spark and its effective extent is increased, further improving efficiency. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which : Fig. 1 is an axial section through a first embodiment of the present invention; Figs. 2 to 6 illustrate various alternative electrode structures to that of Fig. 1 incorporated in an adaptor intended to receive a modified conventioal spark plug; Figs. 7 to 11 illustrate modifications of the spark plugs illustrated in Figs. 1 to 6; Figs. 12 and 13 illustrate further alternative embodiments of the invention in which the intermediate electrode is not supported on the main insulator of the spark plug; and Figs. 14 to 18 illustrate a further embodiment of the invention in which the electrodes are spaced apart in a direction transverse to the spark plug body axis. Referring to Fig. 1, the illustrated spark plug comprises an electrically conductive metal body 1 defining threaded portion 2 which, in use, is screwed into a conventional cylinder head. The body 1 encases a ceramic insulator 3 which supports a first electrode 4 and an intermediate electrode 5. The tip 6 of the body 1 defines a second electrode. A high tension conductor 7 extends into and is retained within an extension 8 of the insulator 3. An insert 9 also retained within the extension 8 electrically connects the core 10 of the high tension conductor 7 to the end of the first electrode 4. When a high tension potential is applied to the core 10 of the conductor 7, a spark is first generated between the tip of the first electrode 4 and the intermediate electrode 5. The electrode 5 is supported entirely by the ceramic body 3 and is therefore insulated from its surroundings. Accordingly a second spark is formed between the intermediate electrode 5 and the second electrode formed by the tip 6 of the body 1. Thus two sparks are formed with the formation of the sparks being separated by a short interval. Referring now to Fig. 2, in the illustrated arrangement the first electrode 4, the intermediate electrode 5 and the second electrode 6 are all component parts of an adaptor which is intended to be screwed into a conventional plug socket in the cylinder head of an engine. The body 1 of the adaptor defines a threaded socket into which a spark plug 11 can be screwed. The spark plug 11 is entirely conventional except for the fact that its earth electrode has been removed. Accordingly the main axial electrode 12 of the spark plug 11 contacts the first electrode 4. The operation of the combination illustrated in Fig. 2 is exactly the same as that of the embodiment of the invention illustrated in Fig. 1. It will be noted however that the earthed second electrode 6 is defined by an extension of the tip of the body 1. Figs. 3 to 6 illustrate alternative electrode structures in adaptors of the type illustrated in Fig. 2. Again the first electrode 4 is intended to be contacted by the axial electrode of a conventional spark plug from which the earth terminal has been removed. In the case of Fig. 3 the earthed second electrode is again formed simply by the tip of the body 1 rather ..than by an extension. In the case of Fig. 4 the first electrode 4 and the intermediate electrode 5 both have bent tips. In the case of Fig. 5 the earthed second electrode 6 is formed by a separate member supported within the ceramic body 3 but connected to the body 1 by a conductive element 13. In the case of Fig. 6 the structure of the first and intermediate electrodes corresponds closely with that of the embodiment of Fig. 1. The basic structure of the embodiments illustrated in Figs. 7 to 11 correspond respectively to the structures of the embodiments illustrated in Figs. 1, 3, 4, 5 and 6 although in each case in the embodiments of Figs. 7 to 11 a dished recess 14 is defined in the insulator 3 around the tip of the first electrode 4. It has been found that the provision of such a dish improves fuel ignition in the immediate vicinity of the spark. Referring now to Fig. 12, the same reference numerals are used as in earlier figures for equivalent components. The first electrode 4 extends through the ceramic insulator 3 from a terminal 15 to which a plug lead can be connected. The threaded portion 2 of the conductive body 1 supports the earthed second electrode 6. The intermediate electrode 5 is supported on an insulating mass 16. As with previously described embodiments when high tension is applied to the first electrode 4 two sparks are generated one between the electrode 4 and the intermediate electrode 5 and the other between the electrode 6 and the intermediate electrode 5. It will be' appreciated that the insulated intermediate electrode 5 may be supported from the body 1 in a variety of ways other than that illustrated. For example, the body 1 may be formed from an insulating material, with an electrically conductive member extending therethrough and connected to the earthed second electrode 6 so as to provide an earth path for the high tension supply. The intermediate electrode 5 could then be directly supported by the body 1. Alternatively the body 1 could be made from two half shells one of which is electrically conducting and supports the electrode 6 and the other of which is electrically insulating and supports the intermediate electrode 5. Referring now to Fig. 13, the illustrated arrangement is similar to that of Fig. 12 except for the fact that the intermediate electrode 5 is in the form of an electrically conducting body .17 supported on a threaded rod 18 received in a threaded axial bore in the tip of the first electrode 4. The threaded rod 18 is of an insulating material so that the body 17 is electrically isolated from both the first electrode 4 and the second electrode 6. The principle of operation is exactly the same as in the case of Fig. 12. Referring now to Figs. 14 to 18, a further embodiment of the invention is described in greater detail. The same reference numerals are used however for equivalent components as in the case of Figs. 1 to 13. Fig. 15 is a view on the lines 15-15 of Fig. 14, Fig. 16 is a sectional view through the insulator 3 of Fig. 14 and Figs. 17 and 18 are end views taken on lines 17-17 and 18-18 of Fig. 16. Referring in detail to Fig. 14, the first electrode 4 contacts a copper compression disc 19 which in turn contacts a copper electrode 20 extending along the length of the insulator 3. The intermediate electrode 5 is supported from the end of the insulator 3 and a sealing washer 21 is sandwiched between the insulator 3 and the metallic body 1. The earthed sec—id electrode is defined by an extended portion of tne body 1. The operation of the embodiment according to Figs. 14 to 18 is exactly the same as that of the previously described embodiments of the invention. It will be noted that a variety of configurations for the first, second and intermediate electrodes is possible. In the case of Figs. 12 and 13 the three electrodes are spaced apart along the axis of the spark plug whereas in the case of Fig. 14 the three electrodes are spaced apart transversely with respect to the spark plug axis. In the case of Fig. 1 both the first and second electrodes are on the same side of the intermediate electrode whereas in the case of, for example. Fig. 5 the three electrodes are arranged to be spaced apart in a direction transverse to the plug axis. It will thus be appreciated that any convenient configuration for the three electrodes can be used. The materials selected for the components of the illustrated spark plugs may be those currently used on conventional spark plugs where conventional spark plugs have equivalent components. For example, the first electrode 4 may be fabricated from a special nickel alloy and the same material may be used for the intermediate electrode. Any convenient material may be used for the insulator.

Claims: CLAIMS : 1. A spark plug comprising a first electrode to which a high potential is to be applied when a spark 5 is to be generated, a second electrode to be connected to ground, and an intermediate insulated electrode positioned between the first and second electrodes such that when a high potential is applied to the first electrode two sparks are formed one 0 between the first electrode and the intermediate electrode and the other between the second electrode and the intermediate electrode. 2. A spark plug according to claim 1, comprising a ceramic insulator through which the 5 first electrode extends, to a point which is located with the cylinder of the engine when the spark plug is installed, and a conductive threaded portion which in use is screwed into the cylinder head and supports the second electrode, the intermediate electrode o being supported by the ceramic insulator. 3. A spark plug according to claim 1, comprising a ceramic insulator through which the first electrode extends to a point which is located within the cylinder of the engine when the spark plug 5 is installed, a conductive threaded portion which in use is screwed into the cylinder head and supports the second electrode, and an insulating support extending from the threaded portion and supporting the intermediate electrode. 0 4. A spark plug according to claim 1, comprising a ceramic insulator through which the first electrode extends to a point which is located with the cylinder of the engine when the spark plug is installed, a conductive threaded portion which in 5 use is screwed into the cylinder head and supports the second electrode, and an insulating support extending from the first electrode and supports the intermediate electrode. 5. A spark plug according to any preceding claim, wherein the first, second and intermediate electrodes are spaced apart in a direction parallel to the direction of insertion of the spark plug into the cylinder head of an engine. 6. A spark plug according to any one of claims 1 to 4, wherein the first, second and intermediate electrodes are spaced apart in a direction transverse to the direction of insertion of the spark plug into the cylinder head of an engine. 7. A spark plug according to any one of claims 1 to 4, wherein the first and second electrodes are located on the same side of the intermediate electrode. 8. A spark plug according to any preceding cla.im, wherein the first, second and intermediate electrodes are supported in an adaptor which can be screwed into the cylinder head of an engine, the adaptor defining a socket into which a conventional spark plug can be inserted after removal of its earth electrode so that the high tension electrode of the conventional electrode contacts the first electrode of the adaptor. 9. A spark plug substantially as hereinbefore described with reference to Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13 or Figs. 14 to 18 of the accompanying drawings.

Inventors: GIBSON-DAWSON JOSEPH, GIBSON-DAWSON, JOSEPH
Applicants: DAWSON ROYALTIES LTD, DAWSON ROYALTIES LIMITED
Application year: 1985
Main classification: H01T13
IPC codes: H01T13