Patent Publication Number: US-7714488-B2

Title: Plasma jet spark plug and ignition system for the same

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a plasma jet spark plug for an internal combustion engine which generates plasma to ignite an air-fuel mixture and to an ignition system for the plasma jet spark plug. 
   Conventionally, when an internal combustion engine such as automobile engine runs at low load (hereinafter referred to as “low load operation”), such as while starting or during idling, accidental firing due to unstable combustion tends to occur. In response, lowering the mixture ratio of air and fuel (hereinafter referred to as “the A/F ratio”) is performed to facilitate smooth ignition and prevent stalling. However, such an adjustment causes excessive fuel consumption. Therefore, improvement in the ignition characteristics of a spark plug, which achieves secure ignition and a stable combustion of the air-fuel mixture despite a high A/F ratio has been demanded. 
   A plasma jet spark plug is known as a spark plug with high ignitability as disclosed in Laid Open Japanese Patent Application Publication No. S56-98570. As used herein, “ignitability” refers to the ability of a spark plug or plasma jet spark plug to ignite the air-fuel mixture in the cylinder of an internal combustion engine. Such a plasma jet spark plug (igniter plug) includes a small electric discharge space and a circumferential face of a spark discharge gap between a center electrode and a ground electrode which is surrounded by an insulating material such as ceramic. High voltage is applied between the center electrode and the ground electrode in order to generate a spark discharge. The dielectric breakdown caused by the spark discharge causes a current flow at relatively low voltage. Further, the spark discharge transits and generates plasma in the spark discharge space to ignite the air-fuel mixture by supplying energy. 
   Plasma has a high ignitability and provides stable combustion at low load operation. However, plasma tends to cause an increase in temperature of a spark plug due to its high energy, thereby resulting in a significant wearing of the electrode of the spark plug. Japanese Patent Publication No. S56-98570 also discloses that plasma is generated to ignite the air-fuel mixture at low load operation. On the contrary, only the spark discharge is performed at the time of high load operation (hereinafter referred to as “high load operation”), such as at high speed running of an internal combustion engine, to prevent wearing out of the electrode as well as to improve the ignitability. 
   However, since a plasma jet spark plug according to the above-noted Japanese patent application has a construction in which a spark discharge gap is surrounded by a face made of an insulating material, a spark discharge ignites an air-fuel mixture, which is included in the spark discharge gap, at high load operation where only an ignition by the spark discharge is performed. Thus, poor ignitability and slow combustion may occur because a flame core cannot be formed in a flow of the air-fuel mixture in a combustion chamber. 
   The present invention is accomplished in view of the foregoing problems of the prior art and an object of the present invention is to provide a plasma jet spark plug which can improve the ignitability and durability thereof by forming a part of a spark discharge gap in the outside of the electric discharge space which generates plasma. An ignition system for the plasma jet spark plug is also provided. 
   SUMMARY OF THE INVENTION 
   A plasma jet spark plug according to a first aspect of the invention comprises: a center electrode, an insulator having a bore extending in an axial direction of the center electrode, accommodating a front end of the center electrode therein and holding the center electrode, a metal shell surrounding the insulator in a radial direction so as to hold the insulator therein, a ground electrode including one end bonded to a front end face of the metal shell and the other end bent towards a front end of the insulator and forming a spark discharge gap with the center electrode, and a cavity forming a discharge space surrounded by an inner circumferential face of said axial bore which extends from an opening portion at a front end of the bore and a front end face of the center electrode, wherein plasma formed in the discharge space is shot out from the opening portion when a spark discharge occurs in the spark discharge gap. 
   In addition to the construction according to the first aspect of the invention, a plasma jet spark plug according to a second aspect of the invention includes a spark discharge gap comprising: an aerial discharge gap in which a spark is discharged between the other end of the ground electrode and a surface of a front end portion of the insulator, an outer creeping discharge gap in which a spark is discharged between an originating point of the aerial discharge gap on the surface of the front end portion of the insulator and the opening portion along the surface of the insulator and an inner creeping discharge gap in which a spark is discharged between the opening portion and the center electrode along an inner circumferential face of the cavity. 
   In addition to the construction according to the first or the second aspect of the invention, a plasma jet spark plug according to a third aspect of the invention includes a spark discharge cavity in which the length of the cavity in the axial direction is greater than the inner diameter of the cavity. 
   Finally, a fourth aspect of the invention is an ignition system which applies voltage to a plasma jet spark plug according to any one of aspects one, two or three, wherein the ignition system comprises: a spark discharge voltage applying means in which voltage is applied to the plasma jet spark plug so as to generate a spark discharge in the spark discharge gap due to a dielectric breakdown, a capacitor which stores energy and supplies energy to the spark discharge gap so that plasma may be formed along with the spark discharge generated by said spark discharge voltage applying means, charging means which charges the capacitor so that plasma may be formed at the time of the spark discharge, switching means which switches on and off an electric connection between the capacitor and the charging means, and control means which controls the switching means, wherein the charging means does not charge the capacitor when the spark discharge voltage applying means generates only the spark discharge and the charging means charges the capacitor when the spark discharge voltage applying means generates spark discharge and the capacitor supplies energy to said spark discharge gap. 
   Since a plasma jet spark plug according to the first aspect of the invention has a construction such that one end of the ground electrode is bent towards a front end portion of the insulator in which a cavity is included so that plasma may be formed and shot out from an opening portion, a spark may be discharged outside the cavity in a spark discharge gap formed between the ground electrode and a center electrode. That is, since the air-fuel mixture in a combustion chamber can be ignited not only inside the cavity but also outside the cavity, ignitability may be improved compared to the case where the ignition is performed inside the cavity, despite the fact that the ignition is caused by only the spark discharge without plasma. Therefore, in the situation where high ignitability is required, such as while starting an internal combustion engine or while idling, the ignition can be performed by shooting out plasma. On the other hand, in the situation where high ignitability is not required, such as during high speed running of an internal combustion engine, the ignition can be performed by only the spark discharge. 
   The high energy of a plasma is likely to cause significant overheating and wearing out of an electrode of a plasma jet spark plug. However, when an ignition method is properly used according to the operational status, i.e., low or high speed operation, of an internal combustion engine as mentioned above, the degree of electrode consumption may be minimized, thereby resulting in improved durability of the plasma jet spark plug. Further, because the number of times it is necessary to utilize high energy for forming plasma is reduced, it leads to less consumption of energy resources, such as a battery and an improvement of fuel consumption. 
   When a spark discharge gap comprises an aerial discharge gap, an outer creeping discharge gap and an inner creeping discharge gap according to the second aspect of the invention, effective ignition of an air-fuel mixture may be achieved by the spark discharged in the aerial discharge gap and the outer creeping discharge gap without forming plasma. Further, despite the fact that a plasma jet spark plug is fouled, the plasma jet spark plug of the present invention can clean the surface of the front end portion of the insulator because high energy plasma may shoot out. 
   In order to securely form such plasma, the length of the cavity in the axial direction is preferably greater than the inner diameter of the cavity as mentioned in the third aspect of the invention. When the inner diameter of the cavity is equal to or greater than the length (depth) thereof, the shape of the plasma may not be formed like a column of flame, i.e., a flame-like shape. In order to improve ignition, the plasma preferably ignites the air-fuel mixture in a location distant from the insulator or the ground electrode which both cause a flame inhibiting action. For that purpose, plasma is preferably shot out with a flame-like shape. 
   Further, with an ignition system according to the fourth aspect of the invention, the plasma jet spark plug according to any one of aspects one through three of the invention can be properly and effectively used according to the operational status of the internal combustion engines. Therefore, the durability of the electrode of a plasma jet spark plug may be improved. Furthermore, it is possible to reduce the consumption of energy resources, such as a battery and improve the fuel consumption. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view in half-section of a plasma jet spark plug according to the present invention; 
       FIG. 2  is a fragmentary, full sectional view of an enlarged front end portion of a plasma jet spark plug according to the present invention; and 
       FIG. 3  is a schematic view of an electrical circuit configuration of an ignition system according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of a plasma jet spark plug embodying the present invention and an ignition system for the plasma jet spark plug will now be described with reference to the drawings. First, referring to  FIGS. 1 and 2 , a construction of a plasma jet spark plug  100  according to the present invention will be explained. In  FIG. 1 , the direction of axis “O” of the plasma jet spark plug  100  is regarded as the top-to-bottom direction in the drawing. A lower portion of the drawing is regarded as a front end of the plasma jet spark plug  100  and an upper portion of the drawing is regarded as a back end of the plasma jet spark plug  100 . 
   As shown in  FIG. 1 , the plasma jet spark plug  100  includes an insulator  10 , a metal shell  50  holding the insulator  10  therein, a center electrode  20  held in the insulator  10  in the direction of the axis “O”, two pieces of ground electrode  30  each having a base portion  32  welded to a front end face  57  of the metal shell  50 , wherein a front end portion  31  of the ground electrode is bent towards a peripheral face of a front end portion  11  of the insulator  10  and a terminal metal shell  40  is provided at a back end portion of the insulator  10 . 
   The insulator  10  is a tubular insulating member including an axial hole or bore  12  in the axis “O” direction, which is formed by sintering alumina or the like as is commonly known. A flange portion  19  having the largest outer diameter is formed almost at the center in the axis “O” direction and a back end body portion  18  is formed at the back end therefrom. A front end body portion  17  having a smaller outer diameter than that of the back end body portion  18  is formed near the front end from the flange portion  19 . A long leg portion  13  having a smaller outside diameter than that of the front end body portion  17  is formed nearer the front end from the front end body portion  17 . The diameter of the long leg portion  13  gradually becomes smaller toward the front end, and the long leg portion  13  is exposed to the combustion chamber when the plasma jet spark plug  100  is assembled in an internal combustion engine (not shown). An area formed between the long leg portion  13  and the front end body portion  17  assumes a step form. 
   As shown in  FIG. 2 , the axial hole or bore  12  of the insulator  10  is formed so as to have a reduced diameter portion  15  at the long leg portion  13  and hold the center electrode  20  therein. A part of the axial hole  12 , which extends to an opening portion  14  of the front end of the axial hole  12 , has a further reduced diameter than that of the reduced diameter portion  15 . In this part, a discharge space defined by an inner circumferential face of the axial hole or bore  12  (serving as an inner circumferential face  61  of a cavity  60  later described) and a front end face of the front end portion  21  of the center electrode  20 , i.e., a front end face  26  of an electrode tip  25  which is integrally bonded to the center electrode  20  at the front end portion  21  of the center electrode  20 , is provided. This space serves as a cavity  60  where plasma is formed and shot out from the opening portion  14 . The cavity  60  is formed so that the depth thereof, i.e., the length in the axis “O” direction (length “e” shown in  FIG. 2 ) may be longer than the inner diameter of the cavity  60  (inner diameter “d” shown in  FIG. 2 ). 
   The center electrode  20  is a rod-shaped electrode comprising nickel-system alloys or the like such as Inconel®  600  or  601  in which a metal core  23  comprising copper or the like with excellent thermal conductivity is provided. Inconel is a registered trademark of Huntington Alloys Corporation of Huntington, W. Va. A disk-shaped electrode tip  25  comprising a noble metal is welded to the front end portion  21  so as to integrate it with the center electrode  20 . Suitable noble metals include platinum, rhodium and tantalum. As mentioned above, the center electrode  20  is accommodated in the reduced diameter portion  15  of the axial hole or bore  12  while exposing the electrode tip  25  to the cavity  60 . The diameter of the back end of the center electrode  20  is expanded like a flange shape, and this flange portion is located in contact with a step portion that extends to the reduced diameter portion  15  of the axial hole or bore  12 . 
   As shown in  FIG. 1 , the center electrode  20  is electrically connected to a terminal or metal fitting  40  at the back end through a conductive sealing body  4  provided inside the axial hole or bore  12  which is made from a mixture of metal and glass. The sealing body  4  is employed to electrically connect the center electrode  20  and the terminal or metal fitting  40  and fix them in the axial hole or bore  12 . A high tension cable (not shown) is connected to the terminal or metal fitting  40  through a plug cap (not shown), to which high voltage is applied by an ignition system  200  (illustrated in  FIG. 3 ) which will be described subsequently. 
   Next, the ground electrode  30  shown in  FIG. 2  comprises a metal having excellent corrosion resistance. As one of the examples, a nickel-system alloy such as Inconel®  600  or  601  is used. The ground electrode  30  has a generally rectangular cross-section in its longitudinal direction and one end (base portion  32 ) is welded to the front end face  57  of the metal shell  50 . The other end (front end portion  31 ) of the ground electrode  30  is bent towards the front end portion  11  of the insulator  10 . According to this embodiment, two ground electrodes  30  are provided and are disposed in the symmetrical position centering on the position of axis “O.” An electrode tip  33  comprising a noble metal is bonded to the front end portion  31  of the ground electrodes  30 , respectively, so as to be integrated therewith. 
   The metal shell  50  shown in  FIG. 1  is a tubular metal fitting which surrounds and holds the insulator  10  to fix the plasma jet spark plug  100  to an engine head of the internal combustion engine. The metal shell  50  comprises an iron system material and includes a tool engagement flats  51  to which a plasma jet spark plug wrench (not shown) is fit and a screw or threaded portion  52  which screws into a cylinder head of the internal combustion engine. 
   Annular ring members  6 ,  7  are interposed between the tool engagement flats  51  and a caulking portion  53  of the metal shell  50  and the back end body portions  18  of the insulator  10 . Further, talc powder  9  is filled between both ring members  6 ,  7 . The caulking portion  53  is formed at the back end of the tool engagement flats  51 , and the insulator  10  is pushed toward the front end in the metal shell  50  through the ring members  6 ,  7  and the talc  9  by caulking the caulking portion  53 . Thus, a step portion between the front end body portion  17  and the long leg portion  13  is supported by a step portion  56  formed in the inner periphery of the metal shell  50  through an annular packing  80 . As a result, the metal shell  50  and the insulator  10  are integrated. Airtightness between the metal shell  50  and the insulator  10  is maintained by the packing  80 , which prevents combustion gas from flowing past. A flange portion  54  is formed between the tool engagement flats  51  and the screw portion  52 , and a gasket  5  is inserted and fitted in the vicinity of the back end of the screw portion  52 , that is, on a seat surface  55  of the flange portion  54 . 
   In the plasma jet spark plug  100  according to this embodiment, a spark discharge gap formed between the ground electrode  30  and the center electrode  20  includes three discharge gaps, i.e., an aerial discharge gap, an outer creeping discharge gap and an inner creeping discharge gap. The aerial discharge gap is located where a dielectric breakdown occurs between the electrode tip  33  of the front end portion  31  of the ground electrode  30  and the front end portion  11  of the insulator  10 , which is indicated by an arrow “A” in  FIG. 2 . A spark is discharged from an originating point of the aerial discharge gap at the insulator  10  side, i.e., a location on an outer circumferential face of the front end portion  11  where the spark discharge occurs between the front end portion  31  of the ground electrode  30  and the center electrode  20  through the opening portion  14  along the surface of the insulator  10 . The outer creeping discharge gap is the location where the spark is discharged outside the cavity  60 , that is, along the outer surface of the front end portion  11  of the insulator  10  (referred to as arrow “B” in  FIG. 2 ). The inner creeping discharge gap is the location where the spark is discharged along the inner circumferential face  61  of the cavity  60  (referred to as arrow “C” in  FIG. 2 ). 
   Next, with reference to  FIG. 3 , one example of the construction of the ignition system  200  that generates and controls the application of high voltage to the plasma jet spark plug  100  according to the above embodiment will be described. 
   The ignition system  200  includes a spark discharge circuit portion  210  which comprises a capacitive discharge ignition or CDI type power supply circuit. The spark discharge circuit portion  210  is electrically connected to the center electrode  20  of the plasma jet spark plug  100  through a diode  201  for preventing reverse current flow. The spark discharge circuit portion  210  is controlled by a controlling circuit portion  220  connected to an ECU (electronic control unit) in an automobile or other motor vehicle. The spark discharge circuit portion  210  is a power circuit portion for performing a so-called “trigger discharge” which causes a dielectric breakdown by applying a high voltage (e.g., −20 kV) to the spark discharge gap and produces a spark discharge. In this embodiment, the direction of potential and the direction of the diode  201  in the spark discharge circuit portion  210  are established so that current may flow into the center electrode  20  from the ground electrode  30  during the trigger discharge. The spark discharge circuit portion  210  is equivalent to a “spark discharge voltage applying means” in the present invention. 
   Further, the ignition system  200  includes a plasma discharge circuit portion  230  which is controlled by a controlling circuit portion  240  connected to the ECU (electronic control unit) of an automobile. The plasma discharge circuit portion  230  is also connected to the center electrode  20  of the plasma jet spark plug  100  through a diode  202  for preventing current backflow. The plasma discharge circuit portion  230  is a power circuit portion for supplying high energy to the spark discharge gap where the dielectric breakdown is caused by the trigger electric discharge performed by the spark discharge circuit portion  210  and producing the plasma. 
   The plasma discharge circuit portion  230  includes a capacitor  231  for storing electric charge. One end of the capacitor  231  is grounded and the other end is electrically connected to the center electrode  20  through the diode  202 . Further, a high voltage generation circuit  233  which generates the high voltage (e.g., −500V) of negative polarity is connected to the other end of capacitor  231  so that electric charge may be stored by the capacitor  231 . Further, the high voltage generation circuit  233  is connected to the controlling circuit portion  240  so as to be able to control the output electric power based on a signal from the controlling circuit portion part  240 . Similarly to the above, in this embodiment, when the energy for generating plasma is supplied to the spark discharge gap from the capacitor  231 , the direction of potential and the direction of the diode  202  in the high voltage generation circuit  233  are established so that current may flow into the center electrode  20  from the ground electrode  30 . It is noted that the controlling circuit portion part  240  is equivalent to a “switching means control means” in the present invention and the high voltage generation circuit  233  which switches output electric power based on the signal from the controlling circuit portion part  240  is equivalent to a “switching means” in the present invention. Furthermore, the high voltage generation circuit  233  charges the capacitor  231  according to the output electric power, and is equivalent to a “charging means” in the present invention. 
   In addition, the ground electrode  30  of the plasma jet spark plug  100  is grounded through the metal shell  50  as shown in  FIG. 1 . 
   Next, operation of the plasma jet spark plug  100  connected to the ignition system  200  for igniting the air-fuel mixture will be explained. The ignition system  200  controls the discharge operation of the plasma jet spark plug  100 . For example, at high load operation, such as at high speed operation of the internal combustion engine, only a spark discharge generated by a trigger electric discharge is implemented in the spark discharge gap. On the other hand, at low load operation, such as during starting of the internal combustion engine or during idling operation, the plasma, which is formed along with the trigger discharge, is shot out. 
   When the controlling circuit portion  240  shown in  FIG. 3  receives the operational information from the ECU, which indicates the low load operation, the high voltage generation circuit  233  outputs the power. Before achieving dielectric breakdown in the spark discharge gap, the capacitor  231  is charged by a closed loop formed by the capacitor  231  and the high voltage generation circuit  233  because current backflow is prevented by the diodes  201 ,  202 . 
   When the controlling circuit portion  220  receives the information, which indicates ignition timing, from the ECU, the controlling circuit portion  220  controls the spark discharge circuit portion  210  so that the high voltage may be applied to the plasma jet spark plug  100 . With this operation, the insulation between the ground electrode  30  and the center electrode  20  is destroyed, thereby generating the trigger discharge. As shown in  FIG. 2 , the spark discharge generated at this time destroys the insulation produced by the air between the front end portion  31  of the ground electrode  30  (the electrode tip  33 ) and the front end portion  11  of the insulator  10  (the aerial discharge gap A). Then, the spark is discharged towards the cavity  60  along the outer surface of the front end portion  11  from the originating point of electric discharge at the front end portion  11  (the outer creeping discharge gap B). Subsequently, the spark is discharged towards the front end portion  21  of the center electrode  20  (the electrode tip  25 ) along the inner circumferential face  61  of the cavity  60  (the inner creeping discharge gap C). 
   When, the insulation of the spark discharge gap is destroyed by the trigger discharge, current can be fed to the spark discharge gap with a relatively low voltage. Therefore, the energy stored in the capacitor  231  is released and supplied to the spark discharge gap. Thus, plasma with high energy is generated in the small space cavity  60  surrounded by the wall. Because the inner diameter “d” of the cavity  60  is shorter than the length “e” of the cavity  60 , the shape of the plasma is like a column of flame, i.e., a flame-like shape. The flame shoots out from the opening portion  14  of the front end portion  11  of the insulator  10  towards the outside, i.e., towards the combustion chamber. Then, the flame ignites the air-fuel mixture in the combustion chamber and the flame core grows therein so as to achieve combustion. 
   When the diameter “d” of the cavity  60  is equal to or longer than the length “e” of the cavity  60 , the plasma may not be shaped like a flame. In order to improve the ignition, the plasma preferably assumes the flame shape and ignites the air-fuel mixture in a location distant from the insulator  10  or the ground electrode  30  which both cause a flame inhibiting action. For that purpose, the diameter “d” of the cavity  60  is preferably less than the length “e” of the cavity  60 . 
   On the other hand, when the controlling circuit portion  240  shown in  FIG. 3  receives the operational information, which indicates the high load operation, from the ECU, no output is sent from the high voltage generation circuit  233 . Because the capacitor  231  is not charged, only the trigger discharge will be performed at the above-mentioned ignition timing. As mentioned above, although this spark discharge runs through the aerial discharge gap A, the outer creeping discharge gap B and the inner creeping discharge gap C, the air-fuel mixture present about the circumference of the front end portion  11  of the insulator  10  is ignited by the spark discharge, thereby being capable of combusting the air-fuel mixture. 
   It goes without saying that all kinds of modifications are possible in the present invention. For example, although the spark discharge circuit portion  210  employs a publicly known capacity electric discharge type (CDI) ignition circuit, other ignition methods, such as a full transistor type or a point type can also be employed. 
   For convenience, although the controlling circuit portion  220  and the controlling circuit portion  240  are constituted as an individual body, they may be integrated and the communication to the ECU may also be united. Alternatively, the ECU can directly control the spark discharge circuit portion  210  and the plasma discharge circuit portion  230 . 
   Further, although two pieces of ground electrodes  30  are provided in this embodiment, the number of ground electrodes  30  may be only one or may be three or more. 
   Furthermore, current flows into the center electrode  20  from the ground electrode  30  in the present invention, however, the power supply or the circuit composition can be constituted such that current flows into the ground electrode  30  from the center electrode  20  by reversing the polarity. In detail, the high voltage generated from the high voltage generation circuit  233  is treated as a positive terminal, and the orientation of the diodes  201 ,  202  may be reversed. It is noted that the electrode tip  25  bonded to the center electrode  20  is relatively smaller than the electrode tip  33  of the ground electrode  30  in the construction. Therefore, current preferably flows into the ground electrode  30  from the center electrode  20  when considering the wearing out of the electrode of the center electrode  20  side. 
   The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent, however, that devices incorporating modifications and variations will be obvious to one skilled in the art of plasma jet spark plugs and ignition systems. Inasmuch as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the instant invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.