Patent Publication Number: US-8536770-B2

Title: Plasma jet spark plug

Description:
FIELD OF THE INVENTION 
     The present invention relates to a plasma jet spark plug used for generating plasma and igniting an air-fuel mixture in an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Conventionally, an internal combustion engine for an automobile engine uses a spark plug for igniting an air-fuel mixture by means of spark discharge. In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, a plasma jet spark plug capable of providing quick propagation of combustion and reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio has been developed. 
     Such a plasma jet spark plug has a structure in which an insulator formed of ceramics or the like surrounds a spark discharge gap between a center electrode and a ground electrode, thereby forming a small-volume discharge space (cavity). Taking an example of an ignition system of a plasma jet spark plug, in igniting an air-fuel mixture, first, a high voltage is applied between the center electrode and the ground electrode so as to perform spark discharge. By virtue of associated occurrence of dielectric breakdown, current can flow therebetween at a relatively low voltage. Thus, through transition of a discharge state effected by further supply of electric, plasma is generated within the cavity. The generated plasma is ejected through a hole (i.e., orifice), thereby igniting the air-fuel mixture. 
     In the conventional plasma jet spark plug described in Japanese Patent Application Laid-Open (kokai) No. 2007-287666, an inner surface of an insulator has a stepped portion so as to form a reduced space in the cavity, whereby sufficient ignitability is achieved with about 50-200 mJ of energy. Further, in order to extend a plasma ejected distance and improve ignitability, a plasma jet spark plug according to Japanese Patent Application Laid-Open (kokai) No. 2006-294257 has a cavity of 10 mm 3  or less in volume and a ratio of a length to a diameter of the cavity with 2 or more. Further, a distance between a center electrode and a ground electrode is 3 mm or less. 
     However, since the plasma jet spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. 2007-287666 includes an insulator having a stepped inner wall therein, channeling (a channel formed due to electric discharge or the like) phenomenon tends to occur and significant deterioration in ignitability is likely to result. 
     Further, Japanese Patent Application Laid-Open (kokai) No. 2006-294257 discloses a plasma jet spark plug in which a distance between a center electrode and a ground electrode is 3 mm or less, and the center electrode has φ1.5 mm or less. Thus, since the center electrode assumes a long and thin shape, the heat conduction (magnitude of heat decline) of a front end of the center electrode deteriorates, resulting in lowering durability of the center electrode. When the front end of the center electrode has poor heat conduction, the front end of the center electrode is likely to be abraded and eroded, which is prone to cause an oxidization of the front end during the spark discharge at high temperature. 
     Therefore, the present invention has been achieved in order to solve the above-mentioned problems. An advantage of the invention is a plasma jet spark plug capable of preventing channeling progress and having excellent heat conduction. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in order to solve at least a part of above-mentioned problems, and the following mode and aspects can be realized. 
     Aspect 1 
     In accordance with a first aspect of the present invention, there is provided a plasma jet spark plug comprising: a cylindrical insulator having an axial bore in an axis direction. A rod-like center electrode is accommodated in the axial bore of the insulator. A plate-like ground electrode is disposed on a front end of the insulator. The center electrode has a body portion, and a front end portion having an outer diameter smaller than that of the body portion. The front end portion is located on a front end side with respect to the body portion. A frontmost portion has an outer diameter smaller than that of the front end portion and is located on a front end side with respect to the front end portion. A portion of the insulator where the axial bore is formed has an accommodating portion having an inner diameter smaller than the outer diameter of the body portion of the center electrode. The accommodating portion accommodates therein at least the front end portion of the center electrode. A small diameter portion having an inner diameter smaller than the outer diameter of the front end portion of the center electrode and smaller than the inner diameter of the accommodating portion. The small diameter portion located on the front end side with respect to the accommodating portion and accommodating therein at least the frontmost portion of the center electrode. A front end of the center electrode is located on a rear side with respect to a front end of the insulator in the small diameter portion, and the front end of the center electrode forms a cavity with an inner circumference of the small diameter portion. The ground electrode has an opening for use in communicating the cavity and an outside air. The small diameter portion in the axis direction assumes a linear shape. 
     Aspect 2 
     In accordance with a second aspect of the present invention, there is provided a plasma jet spark plug as described above, further comprising: wherein the portion of the insulator where the axial bore is formed further includes a first step portion located between the accommodating portion and the small diameter portion, and wherein the following relationship is satisfied: 0.2≦(2c)/(a+b)≦4, where “a” represents a distance between a first intersection and a second intersection. The first intersection is an intersection of a first straight line drawn from an inner circumference of the opening of the ground electrode in the axis direction and the front end of the insulator. The second intersection is an intersection of the first straight line and the first step portion of the insulator, when the inner diameter of the opening of the ground electrode is smaller than the outer diameter of the front end portion of the center electrode and is larger than the inner diameter of the small diameter portion. The letter “a” represents a length of an inner circumference of the small diameter portion in the axis direction when the inner diameter of the opening of the ground electrode is smaller than the outer diameter of the front end portion of the center electrode and is smaller than the inner diameter of the small diameter portion. The letter “b” represents a distance between a third intersection and a fourth intersection. The third intersection is an intersection of a second straight line drawn from the outer circumference of the front end portion of the center electrode in the axis direction and the first step portion of the insulator, and the fourth intersection is an intersection of the second straight line and the front end of the insulator. The letter “c” represents an overlapping area of the ground electrode with the front end portion of the center electrode when the ground electrode and the front end portion of the center electrode are projected in the axis direction. 
     Aspect 3 
     In accordance with a third aspect of the present invention, there is provided a plasma jet spark plug according to aspect 1 or 2, wherein the inner diameter of the opening of the ground electrode falls within a range from 75% to 120% of the inner diameter of the front end of the small diameter portion of the insulator. 
     Aspect 4 
     In accordance with a fourth aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 3, wherein the center electrode serves as a negative electrode. 
     Aspect 5 
     In accordance with a fifth aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 4, wherein an overlapping-amount of the small diameter portion with the frontmost portion in the axis direction falls within the range from 0.5 to 3 mm. 
     Aspect 6 
     In accordance with a sixth aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 5, wherein the center electrode further includes a second step portion between the front end portion and the frontmost portion, and wherein angles θ 1  and θ 2  satisfies the following relationship:
 
θ1&lt;θ2,
 
     where θ 1  represents an angle formed by the first step portion and the accommodating portion, and where θ 2  represents an angle formed by the second step portion and the front end portion. 
     Aspect 7 
     In accordance with a seventh aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 6, wherein the following relationships are satisfied:
 
 R≦ 2.5 mm 3 , and  S/N≧ 0.3,
 
     where “R” represents a volume of the cavity, where “S” represents a length of the cavity in the axis direction, and where “N” represents an inner diameter of the small diameter portion. 
     Aspect 8 
     In accordance with an eighth aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 7, wherein a gap is provided between the first step portion and the second step portion. 
     Aspect 9 
     In accordance with a ninth aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 8, wherein at least a tip end of the center electrode is made of pure metal or an alloy with a melting point of 2400 degrees C. or more. 
     Aspect 10 
     In accordance with a tenth aspect of the present invention, there is provided a plasma jet spark plug according to one of aspects 1 to 9, wherein at least the tip end of the center electrode is made of tungsten or a tungsten alloy. 
     The above-mentioned various modes and aspects may be appropriately combined or partially omitted. 
     In the plasma jet spark plug according to aspect 1, the small diameter portion of the insulator assumes a linear shape in the axis direction. Thus, a discharge path in the cavity also assumes a linear shape, whereby electrical field intensity inside of the insulator can be weakened, as compared to the case where a discharge path assumes a curved shape or an “L” shape. As a result, progress of the channeling phenomenon can be prevented. 
     Moreover, since the center electrode has the outer diameter increasing in the order of the frontmost portion, the front end portion and the body portion, the heat received at the tip end of the center electrode can be efficiently conducted from the frontmost portion to the body portion. Thus, the heat conduction of the center electrode can be improved. As a result, the durability of the center electrode can be secured. 
     In the plasma jet spark plug according to aspect 2, a portion surrounding the cavity is formed so that the distances “a”, “b” and the area “c” satisfy the following relationship: 0.2≦(2c)/(a+b)≦4. 
     In this way, a portion of the insulator which is sandwiched between the center electrode and the ground electrode (i.e., the portion surrounding the cavity) has a suitable value of an electrostatic capacity. Therefore, so-called “plasma current absence” can be prevented. 
     In the plasma jet spark plug according to aspect 3, the inner diameter of the opening of the ground electrode falls within a range from 75 to 120% of the inner diameter of the front end of the small diameter portion of the insulator. In this way, channeling progress occurs almost uniformly on the inner circumference of the small diameter portion in the cavity even if the channeling phenomenon is generated. Since the channeling progress is consistent, the durability of the plasma jet spark plug can be improved, and excellent ignitability thereof is also achievable. 
     In the plasma jet spark plug according to aspect  4 , the center electrode serves as the negative electrode. In this way, the front end portion of the small diameter portion of the insulator is unlikely to be eroded by the channeling phenomenon. In the inner circumference of the small diameter portion, a portion close to the front end of the center electrode tends to be eroded. In such a case, the discharge path first goes to the outer circumference side and then to the inner circumference side in the radial direction, resulting in preventing deterioration in ignitability due to channeling progress. 
     In the plasma jet spark plug according to aspect 5, the overlapping amount “d” of the frontmost portion of the center electrode with the small diameter portion of the insulator is within the range from 0.5 to 3 mm. In this way, a form of electric discharge becomes a creeping discharge, which results in preventing an increase in spark discharge voltage. 
     In the plasma jet spark plug according to aspect 6, the angles θ 1  and θ 2  have the relationship of θ 1 &lt;θ 2 , where θ 1  is the angle formed by the first step portion and the accommodating portion, and where θ 2  is the angle formed by the second step portion and the front end portion. Thus, the erosion of the first step portion and the small diameter portion can be prevented, and deterioration in heat conduction of the center electrode is also prevented. Further, a combustion gas is unlikely to be pooled between the step portions. 
     In the plasma jet spark plug according to aspect  7 , the volume R of the cavity is defined as R≦2.5 mm 3 , and the ratio of the length S of the cavity to the inner diameter N of the small diameter portion is defined as S/N≧0.3. In this way, the excellent ignitability is achievable by defining the shape of the cavity. 
     In the plasma jet spark plug according to aspect 8, the gap is provided between the first step portion of the insulator and the second step portion of the center electrode. In this way, the heat in the end portion of the insulator is unlikely conducted to the center electrode, whereby an increase in temperature of the tip end of the center electrode can be prevented. 
     In the plasma jet spark plug according to aspect 9, at least the tip end of the center electrode is made of pure metal or an alloy with a melting point of 2400 degrees C. or more. In this way, when plasma current is fed to the plasma jet spark plug, the tip end of the center electrode is unlikely to melt. 
     In the plasma jet spark plug according to aspect 10, at least the tip end of the center electrode is made of tungsten or a tungsten alloy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a plasma jet spark plug  100  according to an embodiment of the present invention. 
         FIG. 2  is an enlarged sectional view showing around a center of the end portion of the plasma jet spark plug  100  of  FIG. 1 . 
         FIG. 3  is a block diagram showing an overview of an ignition unit used for operating the plasma jet spark plug of  FIG. 1 . 
         FIG. 4  is an enlarged sectional view showing a portion of a ceramic insulator  10  sandwiched between the center electrode  20  and the ground electrode  30  in  FIG. 2 . 
         FIG. 5  is a waveform chart showing a waveform of a voltage of the center electrode against the ground electrode before and after electric discharge. 
         FIG. 6  is an explanatory view showing an example of the plasma current absence evaluation result according to an embodiment of the present invention. 
         FIG. 7  is an enlarged sectional view showing an opening  31  of the ground electrode  30  and a small diameter portion  15  front end of the ceramic insulator  10  in  FIG. 2 . 
         FIG. 8  is an explanatory view showing an example of the ignitability evaluation result according to an embodiment of the present invention. 
         FIG. 9  is an explanatory view showing the opening  31  of the ground electrode  30  in  FIG. 2  and the small diameter portion  15  front end of the ceramic insulator  10  when viewed from the front end side of the plasma jet spark plug  100 . 
         FIG. 10  is an explanatory view showing a difference in channeling progress due to difference in polarity of the center electrode  20  in  FIG. 2 . 
         FIG. 11  is an explanatory view showing a difference in ignitability level durable time due to difference in polarity of the center electrode  20 . 
         FIG. 12  is an explanatory view showing an overlapping condition of the small diameter portion  15  of the ceramic insulator  10  and the frontmost portion  23  of the center electrode  20  in  FIG. 2 . 
         FIG. 13  is an explanatory view showing the spark discharge voltage increase rate due to difference in overlapping amount of the small diameter portion  15  of the ceramic insulator  10  with the frontmost portion  23  of the center electrode  20 . 
         FIG. 14  is an enlarged sectional view showing around the step portion  16  of the ceramic insulator  10  and the step portion  24  of the center electrode  20  in  FIG. 2 . 
         FIG. 15  is an enlarged sectional view showing a cavity  60  in  FIG. 2 . 
         FIG. 16  is an explanatory view showing evaluation results of the ignitability in relation to a different shape of the cavity  60  in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described based on an embodiment in the following order. 
     A. Configuration of plasma jet spark plug 
     B. Operation of plasma jet spark plug 
     C. Features of an embodiment 
     C-1. Shape of Ceramic Insulator Small Diameter Portion 
     C-2. Shape of Center Electrode 
     C-3. Electrostatic capacity around Cavity 
     C-4. Ground Electrode and Inner Diameter of Small Diameter Portion 
     C-5. Polarity of Center Electrode 
     C-6. Overlapping of Ceramic Insulator Small Diameter Portion with a Center Electrode Frontmost portion 
     C-7. Angle of a step portion 
     C-8. Shape of a cavity 
     C-9. Gap between step portions 
     C-10. Material of a center electrode front end 
     D. Modification 
     A. Configuration of Plasma Jet Spark Plug 
       FIG. 1  is a sectional view of a plasma jet spark plug  100  according to an embodiment of the present invention.  FIG. 2  is an enlarged sectional view of the end portion of the plasma jet spark plug  100  of  FIG. 1 . In addition, the direction of the axis O of the plasma jet spark plug  100  in  FIG. 1  is referred to as the vertical direction, and the lower side of the plasma jet spark plug  100  in  FIG. 1  is referred to as the front end side of the plasma jet spark plug  100 , and the upper side as the rear end side of the plasma jet spark plug  100 . 
     As shown in  FIGS. 1 and 2 , the plasma jet spark plug  100  has a cylindrical ceramic insulator  10  which has an axial bore  12  in the axial O direction. A center electrode  20  is disposed in the axial bore  12  of the ceramic insulator  10 . A plate-like ground electrode  30  is disposed at a front end of the ceramic insulator  10 . A terminal fitting  40  is disposed at a rear end of the ceramic insulator  10 , and a metal shell  50  holds the ceramic insulator  10  therein. 
     As is well known, the ceramic insulator  10  is an insulative member which is made of sintered alumina or the like and has a dielectric constant between 8 to 11. In the outer appearance of the ceramic insulator  10 , a flange portion is provided at the generally center in the axial O direction. The flange portion serves as a border of the rear end side and the front end side of the ceramic insulator  10 . The front end side of the ceramic insulator  10  with respect to the flange portion assumes a step-like shape, and a front end portion of the ceramic insulator has a further reduced diameter. 
     The center electrode  20  is a rod-like electrode and made of a nickel system alloy, such as INCONEL 600 or 601 (trade name). The center electrode  20  has a core metal (not shown) made of highly thermal conductive copper or the like that is embedded therein. Further, a disc-like electrode tip (not shown) made of tungsten or a tungsten alloy is welded to the front end of the center electrode  20 . As best seen in  FIG. 2 , the center electrode  20  has a body portion  21 , a front end portion  22  located in the front end side with respect to the body portion  21 , a frontmost portion  23  located in the front end side with respect to the front end portion  22 , and a step portion  24  located between the front end portion  22  and the frontmost portion  23 . An outer diameter of the front end portion  22  is smaller than that of the body portion  21 , and an outer diameter of the frontmost portion  23  is smaller than that of the front end portion  22 . A portion between the body portion  21  and the front end portion  22  assumes a flange shape, and this flange portion comes into contact with the step portion of the ceramic insulator  10  in the axial bore  12  so as to position the center electrode  20  within the axial bore  12 . 
     Referring now to  FIG. 4  that shows a portion of the axial bore  12  of the ceramic insulator  10 , the ceramic insulator  10  is comprised of: an accommodating portion  14  for accommodating the front end portion  22  of the center electrode  20 . The accommodating portion  14  is provided in the front end side with respect to the above-mentioned step portion. Ceramic insulator  10  further comprises a small diameter portion  15  where the frontmost portion  23  of the center electrode  20  is disposed, and a step portion  16  located between the accommodating portion  14  and the small diameter portion  15 . The inner diameter of the small diameter portion  15  is smaller than that of the accommodating portion  14  and smaller than the outer diameter of the front end portion  22  of the center electrode  20 . In the small diameter portion  15  of the ceramic insulator  10 , the front end of the center electrode  20  is positioned. As shown in the drawing, the front end of the center electrode  20  is disposed near the rear end side of small diameter portion  15  with respect to the front end of the ceramic insulator  10 . The front end of the center electrode  20  forms a small space (a cavity  60 , best seen in  FIG. 2 ) with the front end of the center electrode  20  and the inner circumference of the small diameter portion  15 . The cavity  60  serves as a discharge space. 
     Further, the ground electrode  30  is made of a metal excellent in anti-spark erosion, such as an Ir system alloy. The ground electrode  30  assumes a disk plate shape with 0.3 to 1 mm in thickness. In the center of the ground electrode  30 , an opening  31  is formed so that the cavity  60  can communicate with outside air. The ground electrode  30  is engaged with a engagement portion  58  formed in the inner circumferential face of the front end of the metal shell  50 , while being in contact with the front end of the ceramic insulator  10  (as best seen in  FIG. 1 ). The ground electrode  30  is integrally fixed to the metal shell  50  by laser welding an outer circumferential edge of the ground electrode  30 . 
     The center electrode  20  is electrically connected to the terminal fitting  40  located on the rear end side of plasma jet spark plug  100  through a conductive seal material  4  which is made of metal-glass composition and disposed in the axial bore  12 . The center electrode  20  and the terminal fitting  40  are fixed and electrically conductive through the seal material  4  in the axial bore  12 . In addition, the seal material  4  is disposed in the position as far as possible from the front end portion of the center electrode  20  so as not to melt by heat. Moreover, a high voltage cable (not shown) is connected to the terminal fitting  40  through a plug cap (not shown). 
     The metal shell  50  is a cylindrical metal shell for fixing the plasma jet spark plug  100  to an engine head (not illustrated) of an internal combustion engine. The metal shell  50  holds therein and surrounds the insulator  10 . The metal shell  50  is formed of an iron-based material and has a tool engagement portion  51 , with which a plug wrench (not illustrated) is engaged, and a threaded portion  52  on which are formed external threads that are dimensioned to engage with a mounting hole (not shown) of the engine head. 
     The caulking portion  53  is formed in the rear end side with respect to the tool engagement portion  51  of the metal shell  50 . Annular ring members  6  and  7  intervene between a portion of the metal shell  50  which extends from the tool engagement portion  51  to the caulking portion  53 , and the rear end side of the insulator  10 . A space between the annular ring members  6  and  7  is filled with a powder of talc  9 . By means of caulking of the caulking portion  53 , the insulator  10  is pressed frontward in the metal shell  50  via the ring members  6  and  7  and talc  9 . By this procedure, the step portion of the insulator  10  is supported, via an annular packing  80  (best seen in  FIG. 1 ), on a catching portion  56  of the metal shell  50  which is formed on the inner circumferential face of the metal shell  50 , whereby the metal shell  50  and the insulator  10  are united together. At this time, the packing  80  provides a gas-tight seal between the metal shell  50  and the insulator  10 , thereby preventing outflow of combustion gas. Moreover, a flange portion  54  is formed between the tool engagement portion  51  and the threaded portion  52 , and a gasket  5  is fitted in the proximity of the rear end side of the threaded portion  52 , i.e., to a seat face  55  of the flange  54 . 
     B. Operation of Plasma Jet Spark Plug 
       FIG. 3  is a block diagram showing an overview of an ignition unit used for operating the plasma jet spark plug of  FIG. 1 . 
     The plasma jet spark plug  100  is connected to an ignition unit  200  through the above-mentioned high voltage cable. The ignition unit  200  is equipped with a trigger power source  210  and a plasma power source  220  which are different systems. A plasma power source  220  having a capacity to supply 10 to 120 mJ of energy is used. 
     When a predetermined electric power from the trigger power source  210  is supplied to the plasma jet spark plug  100  through a coil  212  and the high voltage cable, in the plasma jet spark plug  100 , the electric power is supplied to the center electrode  20  through the seal material  4  from the terminal fitting  40  to which the above-mentioned high voltage cable is connected. Thereby, spark discharge (breakdown) occurs in the spark discharge gap between the center electrode  20  and the ground electrode  30 , and the spark discharge passes through the space or the wall of a cavity  60 . In this way, when a dielectric breakdown occurs due to spark discharge, a discharge sustaining voltage serving as a voltage of the center electrode  20  falls immediately after the dielectric breakdown. At this time, plasma current is fed from the plasma power source  220  which is another system, and such energy induces plasma in the cavity  60 . The thus-induced plasma is ejected from the opening  31  of the ground electrode  30  to ignite an air-fuel mixture in an internal combustion engine. In addition, in  FIG. 3 , “C 1 ” indicates electrostatic capacity which the plasma jet spark plug  100  generally holds. “C 2 ” will be described later. 
     C. Features of Embodiment 
     C-1. Shape of Small Diameter Portion of Ceramic Insulator 
     As shown in  FIG. 2 , in this embodiment, the small diameter portion  15  of the ceramic insulator  10  assumes a linear shape in the axis O direction. 
     Since the small diameter portion  15  in the axis O direction assumes the linear shape, the discharge path in the cavity  60  also assumes a linear shape. Thus, electrical field intensity inside of the ceramic insulator  10  can be weakened, as compared to the case where a discharge path assumes a curved shape or an “L” shape. As a result, progress of the channeling phenomenon can be prevented. 
     C-2. Shape of Center Electrode 
     As shown in  FIG. 2 , in this embodiment, the center electrode  20  has the outer diameter increasing in the order of the frontmost portion  23 , the step portion  24 , the front end portion  22  and the body portion  21 . Therefore, the heat received at the tip end of the center electrode  20  can be efficiently conducted from the frontmost portion  23  to the body portion  21 , and the heat conduction of the center electrode  20  can be improved. As a result, the durability of the center electrode  20  can be secured. 
     C-3. Electrostatic Capacity around Cavity 
     In this embodiment, relationships are defined with respect to the shape of a portion of the ceramic insulator  10  that is sandwiched between the center electrode  20  and the ground electrode  30 , namely, a portion surrounding the cavity  60  (such as the small diameter portion  15  and the step portion  16 ) and the positional relationship between the center electrode  20  and the ground electrode  30  are such that the portion of the ceramic insulator  10  has a suitable electrostatic capacity “C 2 ”. However, as mentioned above, the dielectric constant of the ceramic insulator  10  falls within the range from 8 to 11. 
       FIG. 4  is an enlarged sectional view showing a portion of a ceramic insulator  10  sandwiched between the center electrode  20  and the ground electrode  30  in  FIG. 2 . As shown in  FIG. 4 , when an inner diameter of the opening  31  of the ground electrode  30  is smaller than the outer diameter of the front end portion  22  of the center electrode  20  and is larger than an inner diameter of the small diameter portion  15  of the ceramic insulator  10 , “a” represents a distance between an intersection P 1  and an intersection P 2 . The intersection P 1  is an intersection of a straight line K 1  that is drawn from the inner circumference of the opening  31  of the ground electrode  30  in the axis O direction and the front end of the ceramic insulator  10 . The intersection P 2  is an intersection of the straight line K 1  and the step portion  16  of the ceramic insulator  10 . Further, “b” represents a distance between an intersection P 3  and an intersection P 4 . The intersection P 3  is an intersection of a straight line K 2  that is drawn from the outer circumference of the front end portion  22  of the center electrode  20  in the axis O direction and the step portion  16  of the ceramic insulator  10 . The intersection P 4  is an intersection of the straight line K 2  and the front end of the ceramic insulator  10 . Furthermore, “c” represents an overlapping area of the ground electrode  30  with the front end portion  22  of the center electrode  20  when the ground electrode  30  and the front end portion  22  of the center electrode  20  are projected in the axis O direction. A portion surrounding the cavity  60  is formed so that the thus-defined distances “a”, “b” and the area “c” satisfy the following relationship:
 
0.2≦(2 c )/( a+b )≦4.
 
     In addition, when the inner diameter of the opening  31  of the ground electrode  30  is smaller than the outer diameter of the front end portion  22  of the center electrode  20  and is smaller than the inner diameter of the small diameter portion  15  of the ceramic insulator  10 , the distance “a” represents a length of the inner circumference of the small diameter portion  15  in the axis O direction. Moreover, when the inner diameter of the opening  31  of the ground electrode  30  varies depending on the position in an inner circumference direction (e.g., a plurality of projections is provided in the inner circumference of the opening  31  as shown  FIG. 9  ( d ) which will be mentioned later), the above-mentioned distance “a” is defined in each position and the average value thereof is applied to the above-mentioned relationship. 
     In this way, the portion (the small diameter portion  15 , the step portion  16 , or the like) surrounding the cavity  60  can have a suitable value of an electrostatic capacity C 2 , resulting in preventing, so-called “plasma current absence.” 
     Using  FIGS. 3 and 5 , the principle of the plasma current absence prevention shall now be described. 
     In  FIG. 3 , when the plasma current is fed to the plasma jet spark plug  100  from the plasma power source  220  of the ignition unit  200 , as mentioned above, generally the electric discharge phenomenon occurs at first between the center electrode  20  and the ground electrode  30  (between the plug gap) by the trigger power source  210 . In a state that the center electrode  20  and the ground electrode  30  are electrically conductive, the discharge-sustaining voltage, that serves as a voltage of the center electrode  20  to the ground electrode  30 , becomes, for example, −500V or more. In this way, an electric charge that is accumulated in a capacitor (not shown) can be at once fed as plasma current from the plasma power source  220 . In addition, in  FIG. 3 , “C 2 ” indicates an electrostatic capacity of the portion surrounding the cavity  60  as it mentioned above. 
       FIG. 5  is a waveform chart showing a waveform of a voltage of the center electrode against the ground electrode before and after electric discharge.  FIG. 5(   a ) is a waveform of a conventional plasma jet spark plug.  FIG. 5(   b ) is a waveform of the plasma jet spark plug  100  according to an embodiment.  FIG. 5(   c ) is a waveform of an electrostatic capacity C 2  of the portion surrounding the cavity  60 , the waveform showing an excessive value. 
     As shown in  FIG. 5(   a ), a conventional plasma jet spark plug has a tendency wherein a discharge-sustaining voltage sometimes becomes too high under a certain operating condition. In such a case, plasma current cannot be fed to a plasma jet spark plug, thereby causing a plasma current absence. On the other hand, the plasma jet spark plug  100  according to the embodiment can reduce the discharge-sustaining voltage as shown in  FIG. 5(   b ), because the electrostatic capacity “C 2 ” of the portion surrounding the cavity  60  has a suitable value as mentioned above. As a result, plasma current can be easily fed to the plasma jet spark plug  100 . Moreover, the electric charge discharged at the time of spark discharge (breakdown) is again accumulated in the electrostatic capacity “C 2 ” of the portion surrounding the cavity  60 . As a result, since the voltage of the center electrode  20  varies greatly to an opposite direction (plus side), a feeding of plasma current is facilitated. 
     In the case where the distances “a” and “b” and the area “c” neither satisfy the relationships of a&lt;b nor 0.2&lt;=(2c)/(a+b)&lt;=4, and in the case where the electrostatic capacity “C 2 ” of the portion surrounding the cavity  60  is too high, a discharge-sustaining voltage serving as a voltage of the center electrode  20  becomes −500V or less, because an inductive current flows in from an inductor of the coil  212  of the trigger power source  210  as shown in  FIG. 5(   c ). As a result, a feeding of plasma current cannot be conducted. 
     Referring now to  FIG. 6 , an example of the plasma current absence evaluation results according to the embodiment will next be described.  FIG. 6  is an explanatory view showing an example of the plasma current absence evaluation results according to the present embodiment. In  FIG. 6 , a vertical axis shows an occurrence rate of plasma current absence (%), and a horizontal axis shows a value of (2c)/(a+b) based on the above-defined distances “a”, “b” and the area “c”. In addition, the evaluation was conducted under 1.0 MPa chamber pressure, and a judgment line of the occurrence rate of plasma current absence was 3%. 
     As shown in  FIG. 6 , when the value was 2c/(a+b)≧0.2, the occurrence rate of the plasma current absence was 3% or less, whereby the number of the plasma current absence occurrences was sharply reduced. Further, when the value was within 1.0≦(2c)/(a+b)≦2.0, the occurrence rate of the plasma current absence was 0%, which was an excellent rate. When the value was (2c)/(a+b)&gt;4, the plasma current absence occurred because the electrostatic capacity “C2” of the portion surrounding the cavity  60  had a large value, causing an increase in the discharge-sustaining voltage after the spark discharge (breakdown). 
     C-4. Ground Electrode and Inner diameter of Small Diameter Portion 
       FIG. 7  is an enlarged sectional view showing the opening  31  of the ground electrode  30  and the front end of small diameter portion  15  of the ceramic insulator  10  in  FIG. 2 . In this embodiment, the inner diameter “m” of the opening  31  of the ground electrode  30  shown in  FIG. 7  falls within a range from 75% to 120% of an inner diameter “n” of the front end of the small diameter portion  15  of the ceramic insulator  10 . 
     Thus, when a ratio of the inner diameter “m” of the opening  31  to the inner diameter “n” of the front end of the small diameter portion  15 , i.e., an inner diameter of the cavity  60 , falls within the above range, channeling progress occurs almost uniformly on the inner circumference of the small diameter portion  15  in the cavity  60  even if the channeling is generated. Therefore, since the channeling progress is consistent, the durability of the plasma jet spark plug  100  can be improved, and excellent ignitability thereof is achievable. 
     On the other hand, when the ratio of the inner diameter of the opening  31  to the inner diameter of the cavity  60  is beyond the above range, and when the inner diameter of the opening  31  is too small against the inner diameter of the cavity  60 , the plasma ejected from the opening  31  is intercepted, which causes deterioration in ignitability. On the other hand, when the inner diameter of the opening  31  is too large against the inner diameter of the cavity  60 , a discharge path assumes an “L” shape, which causes the channeling progress. A location where the channeling phenomenon occurs has a reduced distance between the ground electrode  30  and the center electrode  20 , whereby the discharge path is concentrated on the location. As a result, the ignitability deteriorates. 
     An example of the ignitability evaluation results in this embodiment will be described with reference to  FIG. 8 .  FIG. 8  is an explanatory view showing an example of the ignitability evaluation results. In  FIG. 8 , a vertical axis shows an A/F (air/fuel) value at the misfire occurrence rate of 1% when an internal combustion engine is operated at no load (N/L) (820 rpm). A horizontal axis shows a ratio (m/n) (%) of the inner diameter “m” of the opening  31  to the inner diameter “n” of the front end of the small diameter portion  15  (i.e., the inner diameter of the cavity  60 ). The evaluation was conducted such that a sample having an A/F value beyond an ignition limit line (A/F=15) was determined acceptable. Unused (new) products and products subjected to 1,000 hours channeling durable process (1,000 Hr durable process) were used for the evaluation test. The 1,000 Hr channeling durable process was conducted in a 0.4 MPa pressure chamber at the frequency of 60 Hz so as to cause spark discharge (trigger discharge). 
     As shown clearly in  FIG. 8 , excellent ignitability is exhibited when the ratio of the inner diameter of the opening  31  to the inner diameter of the cavity  60  is 75% or more. However, as for the product subjected to 1,000 hours channeling durable process, the ignitability falls sharply as the ratio is larger than 120%. Therefore, excellent ignitability can be exhibited when the ratio falls within a range from 75% to 120%. 
     In this embodiment, the opening  31  of the ground electrode  30  can take various forms with respect to the cavity  60  as shown in  FIGS. 9(   a )- 9 ( d ).  FIGS. 9(   a )- 9 ( d ) are views showing various embodiments of opening  31  of the ground electrodes  30  in  FIG. 2  and the front end of the small diameter portion  15  of the ceramic insulator  10  when viewed from the front end side of the plasma jet spark plug  100 . 
     In  FIG. 9(   a ), the inner diameter “m” of the opening  31  of a ground electrode  30  is made larger than the inner diameter “n” of the front end of the small diameter portion  15  of the ceramic insulator  10  (i.e., the inner diameter of the cavity  60 ). On the contrary,  FIG. 9(   b ) shows the inner diameter “m” of the opening  31  of a ground electrode  30  made smaller than the inner diameter “n” of the front end of the small diameter portion  15  of the ceramic insulator  10 . Further,  FIG. 9(   c ) shows a ground electrode  30  where a part of the inner circumference of the opening  31  is exposed and connected to the outer circumference of the ground electrode  30 . Furthermore,  FIG. 9(   d ) shows a ground electrode  30  having a plurality of protrusions along the inner circumference of the opening  31  of the ground electrode  30 . 
     C-5. Polarity of Center Electrode 
     In this embodiment, the center electrode  20  serves as a negative electrode to the ground electrode  30 . 
       FIGS. 10(   a ) and  10 ( b ) are explanatory views showing a difference in channeling progress resulting from a difference in polarity of the center electrode  20  in  FIG. 2 .  FIG. 10(   a ) shows a case where the center electrode  20  serves as a negative electrode, and  FIG. 10(   b ) shows a case where the center electrode  20  serves as a positive electrode. 
     Generally, as shown in  FIG. 10(   b ), since the channeling progress is greater at the negative electrode, the front end of the small diameter portion  15  of the ceramic insulator  10  tends to be eroded due to channeling phenomenon when the center electrode  20  serves as the positive electrode to the ground electrode  30 . Thus, a discharge path is established towards a bottom surface side of the ground electrode  30 . This causes deterioration in ignitability. On the other hand, as shown in  FIG. 10(   a ), the front end of the small diameter portion  15  is not eroded when the center electrode  20  serves as the negative electrode to the ground electrode  30 . However, in the inner circumference of the small diameter portion  15 , a portion close to the front end of the center electrode  20  tends to be eroded. In such a case, the discharge path first goes to the outer circumference side and then to the inner circumference side in the radial direction. This results in preventing deterioration in ignitability due to channeling progress. 
     Referring now to  FIG. 11 , difference in ignitability level sustaining time according to a polarity of the center electrode  20  will be described.  FIG. 11  is an explanatory view showing a difference in ignitability level sustaining time in relation to the polarity of the center electrode  20 . In  FIG. 11 , a vertical axis shows a durable time under channeling durable conditions. More particularly, this durable time is defined as a period of time until an A/F (air/fuel) value at the misfire occurrence rate of 1% becomes less than 15, when an internal combustion engine is operated at no load (N/L) (820 rpm). Similar to  FIG. 8 , the channeling durable process was conducted in a 0.4 MPa pressure chamber at the frequency of 60 Hz so as to cause spark discharge (trigger discharge). 
     It is clear from  FIG. 11  that the ignitability level sustaining time is remarkably improved when the center electrode  20  serves as a negative electrode to the ground electrode  30 . 
     C-6. Overlapping of Ceramic Insulator Small Diameter Portion with Center Electrode Frontmost Portion 
       FIG. 12  is an explanatory view showing an overlapping status of the small diameter portion  15  of the ceramic insulator  10  and the frontmost portion  23  of the center electrode  20  in  FIG. 2 . As shown in  FIG. 12 , an overlapping amount “d” of the small diameter portion  15  of the ceramic insulator  10  with the frontmost portion  23  of the center electrode  20  in the axis O direction falls within the range from 0.5 to 3 mm. 
       FIG. 13  is an explanatory view showing the spark discharge voltage increase rate due to difference in overlapping amount of the small diameter portion  15  of the ceramic insulator  10  with the frontmost portion  23  of the center electrode  20 . In  FIG. 13 , a vertical axis shows an electric discharge voltage increase rate (%) after the plasma durability test, and horizontal axis shows the overlapping amount “d” of the small diameter portion  15  of the ceramic insulator  10  with the frontmost portion  23  of the center electrode  20 . In addition, the plasma durability test was conducted at the frequency of 60 Hz for 100 hours (60 Hz×100 Hr) with 118 mJ, and the chamber pressure power of 0.4 mPa. The judgment line of the spark discharge voltage increase rate was 50%. 
     When the overlapping-amount “d” is smaller than 0.5 mm, and when the frontmost portion  23  of the center electrode  20  has an electrode-erosion, such electrode-erosion progresses to a portion which is not overlapped with the small diameter portion  15  of the ceramic insulator  10 . This causes an aerial discharge and a creeping discharge, thereby resulting in a sharp increase in spark discharge voltage as shown in  FIG. 13 . 
     When the overlapping amount “d” is larger than 3 mm, the heat conduction of the center electrode  20  deteriorates, and an oxidation of the center electrode  20  remarkably advances. As a result, a sharp increase in spark discharge voltage occurs. 
     On the other hand, when the overlapping amount “d” is within the range from 0.5 to 3 mm, a form of electric discharge becomes a creeping discharge, which results in preventing an increase in spark discharge voltage. 
     C-7. Angle of Step Portion 
       FIG. 14  is an enlarged sectional view showing around the step portion  16  of the ceramic insulator  10  and the step portion  24  of the center electrode  20  in  FIG. 2 . In this embodiment, as shown in  FIG. 14(   a ), angles θ 1  and θ 2  satisfies the following relationship: θ 1 &lt;θ 2 , where θ 1  represents an angle formed by the step portion  16  and the accommodating portion  14  of the ceramic insulator  10 , and where θ 2  represents an angle formed by the step portion  24  and the front end portion  22  of the center electrode  20 . 
     On the other hand, as shown in  FIG. 14(   b ), when the relationship between the θ 1 , which is formed by the step portion  16  and the accommodating portion  14 , and the θ 2 , which is formed by the step portion  24  and the front end portion  22 , is θ 1 &gt;θ 2  under the conditions that a length of the frontmost portion  23  of the center electrode  20  in the axis O direction is the same as in the case of  FIG. 14(   a ). In these conditions, an electrical field concentrates on a point “e” when the frontmost portion  23  of the center electrode  20  is eroded. Thus, the point “e” tends to be a starting point of electric discharge between the center electrode  20  and the ground electrodes  30 . Therefore, the step portion  16  and the small diameter portion  15  of the ceramic insulator  10  tend to be eroded. 
     Moreover, as shown in  FIG. 14(   c ), when the relationship between the angle θ 1  and the angle θ 2  is θ 1 &gt;θ 2 , as well as the length of the frontmost portion  23  of the center electrode  20  in the axis O direction is extended longer than that of the case in  FIG. 14(   a ), an influence by the erosion of the frontmost portion  23  of the center electrode  20  is ameliorated. However, the heat conduction of the front end of the center electrode  20  deteriorates because the length of the frontmost portion  23  becomes long. Thus, the front end of the center electrode  20  tends to be eroded, thereby widening a space between the step portion  16  and the step portion  24 . As a result, a combustion gas is likely to be pooled in this space. 
     C-8. Shape of Cavity 
       FIG. 15  is an enlarged sectional view showing the cavity  60  in  FIG. 2 . In this embodiment, as shown in  FIG. 15 , the following relationships are satisfied: R≦2.5 mm 3 , and S/N≧0.3, where “R” represents a volume of the cavity  60 , “S” represents a length of the cavity  60  in the axis O direction, and “N” represents an inner diameter of the cavity  60  (i.e., the inner diameter of the small diameter portion  15  of the ceramic insulator  10 ). 
     By defining the volume R, the length S, and the inner diameter N of the cavity  60  in the respective range, the excellent ignitability is achievable. 
       FIG. 16  is an explanatory view showing evaluation results of the ignitability in relation to difference in shape of the cavity  60  in  FIG. 2 . In  FIG. 16 , similar to the case of  FIG. 8 , a vertical axis shows an A/F (air/fuel) value at the misfire occurrence rate of 1% when an internal combustion engine is operated at no load (N/L) (820 rpm), and a horizontal axis shows the volume R of the cavity  60  (mm 3 ). The graph shows five cases where the inner diameter N (mm) of the cavity  60  is φ0.5, φ1.0, φ1.3, φ1.5, and φ2.0, respectively. In addition, similar to the case of  FIG. 8 , the evaluation was conducted such that a sample having an A/F value beyond an ignition limit line (A/F=15) was determined acceptable. 
     As is clear from  FIG. 16 , the ignitability dramatically deteriorates when the volume R of the cavity  60  exceeds 2.5 mm 3 . Further, when the inner diameter N of the cavity  60  is φ2.0 mm, and the volume R thereof is 2 mm 3  or less, the ignitability also deteriorates. This is because the ratio of the inner diameter N to the length S varies as the inner diameter N of the cavity  60  is large. 
     As for all the data in  FIG. 16 , the samples having low ignitability shows that the ratio of the length S to the inner diameter N of the cavity  60  is 0.25 or less when the volume R of the cavity  60  is 2.5 mm 3  or less. However, the samples having the ratio of 0.3 or more, the ignitability does not deteriorate. 
     Therefore, excellent ignitability is achievable when the volume R of the cavity  60  is 2.5 mm 3  or less and the ratio of the length S to the inner diameter N of the cavity  60  is 0.3 or more. 
     C-9. Gap between Step Portions 
     In the embodiment shown in  FIG. 2 , a gap is provided between the step portion  16  of the ceramic insulator  10  and the step portion  24  of the center electrode  20 . In this way, the heat in the end portion of the ceramic insulator  10  is unlikely conducted to the center electrode  20 , whereby an increase in temperature of the tip end of the center electrode  20  can be prevented. 
     C-10. Material of Tip End of Center Electrode 
     In this embodiment, the tip end of the center electrode  20  is made of tungsten or a tungsten alloy as mentioned above. Thus, when plasma current is fed to the plasma jet spark plug  100 , the tip end of the center electrode is unlikely melt. Further, the tip end of the center electrode  20  may be made of pure metal or an alloy with a melting point of 2400 degrees C. or more in addition to tungsten or a tungsten alloy. 
     D. Modification 
     The present invention is not particularly limited to the embodiments described above but may be modified in various ways within the scope of the invention. 
     In the above-mentioned embodiment, although the features C-1 to C-10 are mentioned, the present invention is not limited to the above embodiment. That is, the present invention can have at least features C-1 and C-2, and it is not necessary to have other features. Furthermore, when other features are included in the present invention, the combination thereof can be arbitrarily selected.