Patent Publication Number: US-8528531-B2

Title: Ignition apparatus of plasma jet ignition plug

Description:
TECHNICAL FIELD 
     Apparatuses consistent with the present disclosure relates to an ignition apparatus of a plasma jet ignition plug for an internal combustion engine performing the ignition of mixed gas by forming plasma. 
     BACKGROUND ART 
     In the related art, a spark plug performing a firing of mixed gas by spark discharge (simply referred to as ‘discharge’) is used for an igniting plug of an engine, for example, an internal combustion engine of automobiles. In recent years, a plasma jet ignition plug in which the expansion of combustion is speedy and which can be surely fired for lean mix gas having high igniting limit fuel efficiency has been used as an ignition plug since a high output and low fuel efficiency of an internal combustion engine are required. 
     In such a plasma jet ignition plug, a spark discharge gap between a center electrode and a ground electrode is formed when connected to an electric source. A plasma jet ignition plug surrounds a vicinity of this spark ignition gap using an insulator such as ceramics, and thus has a structure forming a discharge space of small volume called a cavity. A plasma jet ignition plug using an overlap type electric source is described with reference to one example (e.g., see patent document 1). In an ignition of mixed gas, first, a high voltage is applied between a center electrode and a ground electrode, and then a spark discharge (also called ‘trigger discharge) is produced. Due to dielectric breakdown produced at the time of spark discharge, a current can flows between both electrodes through a relatively low voltage. Thus, more energy is provided, thereby transiting a discharge status, and therefore plasma is formed in the cavity. Further, the formed plasma is extruded through a communicating hole, so that an ignition of nixed gas is performed. One effusion of plasma corresponds to each stroke. 
     In such a plasma jet ignition plug, a higher current than typically used to spark discharge in a general spark plug is required to flow through the spark discharge gap when plasma is formed. In order to increase a flowing current, it is necessary to lower the electric resistance value of the circuit through which the current flows, and normally a resistor is no provided in the circuit of an ignition apparatus or inside the plasma jet ignition plug (e.g., see patent document 2). 
     RELATED ART DOCUMENT 
     Patent Document 
     
         
         [Patent document 1] Japanese patent publication 2002-327672-A 
         [Patent document 2] Japanese patent publication SHO 57-28869-A 
       
    
     DISCLOSURE OF THE INVENTION 
     Problem that the Invention is to Solve 
     However, since in a plasma jet ignition plug, high current flows in a short time, fluctuations in a current value per unit time are significant. Therefore, there can be a problem a large amount of electrical noise being easily (in this specification, noise, such as electric waves radiating outside from an appliance, is sometimes called ‘electrical noise’. When a high frequency current flows within an electronic appliance, this electric noise is radiated, and affects external appliances or other signals with interference) attributable to stray capacity (the stray capacity is formed at a high voltage line positioned between a plasma jet ignition plug and a discharge voltage application unit applying a voltage to that plasma jet ignition plug). To suppress the occurrence of such electric noise, a resistor may be provided inside the plasma jet ignition plug or on a circuit of an ignition apparatus to suppress energy accumulated as stray capacity, but if a resistor is provided directly, a discharge current producing capacity is smaller, thus causing uncertainty whether an energy capacity discharge of sufficient size to form plasma will be obtained. 
     The present invention has been made considering the above facts, and it is an object of exemplary embodiments to provide an ignition apparatus of a plasma jet ignition plug that produces a current of sufficient size to form plasma but suppressing the occurrence of electric noise, at the ignition of the plasma jet ignition plug. 
     Means for Solving the Problem 
     To achieve the above-described objective, an ignition apparatus of a plasma jet ignition plug of the invention is characterized by the following (1)˜(4). 
     (1) An ignition apparatus of a plasma jet ignition plug, comprising: 
     a plasma jet ignition plug including an insulator having an axis hole that extends in an axial direction and a center electrode in the axis hole, a metal shell having a cylindrical shape and holding the insulator, and a ground electrode having a plate shape and having a communication hole that communicates along the axial direction in its center; 
     a discharge voltage application unit applying a voltage for providing a spark discharge in a spark discharge gap, which is formed between the center electrode and the ground electrode, to the plasma jet ignition plug; and 
     a diode disposed between the plasma jet ignition plug and the discharge voltage application unit; 
     wherein 
     the ignition apparatus further includes: 
     a resistor, wherein one end of the resistor is connected to the diode and the other end of the resistor is electrically connected to the center electrode of the plasma jet ignition plug; and 
     a capacitor functional component, wherein one end of the capacitor functional component is electrically connected to the center electrode of the plasma jet ignition plug and the other end of the capacitor functional component is grounded. 
     (2) The ignition apparatus according to the above (1), 
     wherein 
     the capacity functional component includes: 
     a terminal fitting electrically connected to the center electrode of the plasma jet ignition plug, and 
     a metal cask having a hollow cylindrical shape and receiving the terminal fitting in an inside of the metal cask. 
     (3) The ignition apparatus according to the above (2), 
     wherein 
     the capacitor functional component has a dielectric filing a gap between the terminal fitting which is received in the metal cask and the metal cask. 
     (4) The ignition apparatus according to the above (2) or (3), 
     wherein 
     the other end of the resistor is disposed inside the metal cask, 
     wherein electrostatic capacity is accumulated on an area sandwiched between a cross-section of the metal cask, where the other end of the resister is included, perpendicular to an axis center direction of the metal cask and a cross-section of the metal cask containing an end face of the metal cask, 
     wherein the terminal fitting is disposed inside the area, and 
     wherein the electrostatic capacity in the area is more than 1 pF and less than 100 pF. 
     According to an ignition apparatus of a plasma jet ignition plug of the above (1) construction, at the ignition of a plasma jet ignition plug, a current of sufficient size for forming plasma flows at a spark discharge gap and also occurrence of electric noise is reduced. 
     According to an ignition apparatus of a plasma jet ignition plug of the above (2) construction, because with respect to electric noise attributable to a newly provided capacitor, a metal cask forming the capacitor acts to shield, and the propagation of electric noise producing from the capacitor may be suppressed. 
     According to an ignition apparatus of a plasma jet ignition plug of the above (3) construction, by using dielectric of a desired dielectric constant, the miniaturization of this capacitor functional component may be realized. 
     According to an ignition apparatus of a plasma jet ignition plug of the above (4) construction, at the ignition of a plasma jet ignition plug, a current of sufficient size for forming plasma effectively flows in a spark discharge gap and also occurrence of electric noise is more effectively prevented. 
     Effect of the Invention 
     According to an ignition apparatus of a plasma jet ignition plug of the present invention, at the ignition of a plasma jet ignition plug, a current of sufficient size for forming plasma flows at a spark discharge gap and also occurrence of electric noise is prevented. 
     As described above, the present invention has been concisely described. Also, by carefully reading the following described preferred embodiments in reference to the annexed drawings, the details of the invention will be further elucidated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view showing a basic structure of a plasma jet ignition plug; 
         FIG. 2  is a partial cross-sectional view showing an exemplary structure of a plasma jet ignition plug used in the invention; 
         FIG. 3  is an electric circuit view showing an exemplary structure of an ignition apparatus using a plasma jet ignition plug indicated in  FIG. 2 ; and 
         FIG. 4  is a waveform view showing a specific example of waveforms of a discharge voltage and a discharge current applied to a plasma jet ignition plug. 
     
    
    
     EXEMPLARY EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Hereinafter, one embodiment of an ignition apparatus for a plasma jet ignition plug according to the present invention will be described with reference to drawings. First, a plasma jet ignition plug  100  will be described. A basic structure of a plasma jet ignition plug  100  usable in an ignition apparatus  200  of the present invention is shown in  FIG. 1 . In  FIG. 1  an axis direction O of the plasma jet ignition plug  100  is defined as an upward/downward direction in the figure, the lower side is described as a frond end side of the plasma jet ignition plug  100 , and the upper side is described as a back end side. 
     The plasma jet ignition plug  100  shown in  FIG. 1  is formed from an insulating member obtained by sintering alumina using a well known method, and has an insulator  10  of cylindrical shape formed with a shaft hole  12  extending in an axis direction O. The insulator  10  has a center body part  19 , which has a biggest outer diameter and is centered in the axis direction O. At the back end side of this center body part  19 , a back end side body part  18  having a diameter smaller than the center body part  19  is formed. Also, a front end side body part  17  with a smaller outer diameter than the back end side body part  18  is formed in a more front end side than the center body part  19 . Also, a long leg section  13  with a smaller outer diameter than the front end side body part  17  is formed in a more front end side of the front end side body part  17 . Further a part corresponding to an inner-rim of the long leg section  13  of the shaft hole  12  of the insulator  10  is as a smaller diameter than other parts of the shaft hole  12  and is formed to be an electrode reception part  15 . An inner-rim of this electrode reception part  15  is contiguous to a front end surface  16  of the insulator  10 , and forms an opening part  14  of a later-described cavity  60 . 
     At the inner part of the electrode reception part  15 , a bar-type center electrode  20 , which uses Cu or Cu alloy as a center material and has Ni alloy as a shell is provided. A structure formed by bonding a discus-type electrode tip  25 , which is formed of an alloy having noble metal or W as a main ingredient, at the front end of the center electrode  20  or is formed integrally with the center electrode  20  may also be used (in the present embodiment, a formation of the center electrode  20  integrated with the electrode tip  25  is also called the ‘center electrode’). Herein, a low volume discharge space is enclosingly formed at an inner-rim of the electrode reception part  15  of the shaft hole  12 , and the front end surface of the center electrode  20  (or, a front end surface of an electrode tip  25  integrally bonded to the center electrode  20 ). In the present embodiment, this discharge space is called a cavity  60 . Also, the center electrode  20  extends toward a back end side in the shaft bore  12 , and electrically connected to a terminal fitting  40  provided at the back end side of the shaft bore  12  via a conductive sealed body formed of mixture of metal and glass. The terminal fitting  40  is connected to a high-voltage cable (not shown) through a plug cap (not shown), and constructed to receive a high-voltage from a later-described ignition apparatus  200  (see  FIG. 3 ). 
     Also, a part of the insulator  10  from a portion of the back end side body section  18  to the long leg part  13  is supported by a metal shell  50  that is formed in a tubular shape using ferrous material, and the insulator  10  is supported by the crimping portion of the metal shell  50 . The metal shell  50  has a screw attachment part  52  having a screw thread for screw connection with an attachment hole  301  of the engine head  300 . Further, a proximal end side of the attachment screw part  52  is surrounded by a ring-shape gasket  5  to prevent a leakage of air tightness inside the engine through the attachment hole, when the plasma jet ignition plug  100  is attached to the attachment hole of the engine head  300 . 
     A front end of the metal shell  50  is provided with a disk shaped ground electrode  30  formed using Ni alloy having a superior spark-proof composition such as INCONEL™ 600 or 601. The ground electrode  30  is integrally bonded with the metal shell  50 , in a state that the ground electrode  30  contacts the front end surface  16  of the insulator  10  with its thickness direction oriented in a line with the axis direction O. The center of the ground electrode  30  is formed with a communication hole  31 , axially aligned with the opening part  14  of the cavity  60 , and through this communication hole  31  the inner part of the cavity  60  communicates with the open air. Between the ground electrode  30  and the center electrode  20  is formed as a spark discharge gap, and the cavity  60  is formed to enclose at least this part. When a spark discharge is performed in this spark discharge gap, energy is supplied so that plasma is formed in the cavity  60 , and this plasma is extruded from the opening part  14  through the communication hole  31 . 
     In addition to components shown in  FIG. 1 , a plasma jet ignition plug  100  used in the ignition apparatus  200  of the present embodiment also has components corresponding to a capacitor C 1  shown in  FIG. 3  For example, as shown in  FIG. 2 , the plasma jet ignition plug  100  also has a tubular electrode  111  and a dielectric  112 , and the tubular electrode  111  and the dielectric  112  form the capacitor C 1 . 
     The tubular electrode  111  is comprised of conductive metal material, and formed to have a hollow structure with a tubular shape. The inner diameter of the tubular electrode  111  is greater than a diameter of the back end side body part  18  of the insulator  10 . The tubular electrode  111  is disposed to enclose and surround the back end side body part  18  of the insulator  10  and disposed the center axis of the tubular electrode  111  aligned with the center axis of the back end side body part  18 . As illustrated in  FIG. 2 , a length of the axis direction of the tubular electrode  111  is about two times a length of the back end side body part  18 . A lower half side region of the tubular electrode  111  encloses the outer-rim of the back end side body part  18 . Thus, the tubular electrode  111  extends further up toward the upper part of the plasma jet ignition plug than the back end side body part  18 . 
     A space between either the insulator  10  or the terminal fitting  40  and the tubular electrode  111  is filled with dielectrics  112 . The dielectric  112  is comprised of electric insulators having dielectric properties. The dielectric  112  is comprised of higher dielectric material than the insulator  10 . 
     The terminal fitting  40  of the conductor and the tubular electrode  111  enclosing the conductor face each other with the dielectric interposed there between, thereby which electrostatic capacity, i.e. a circuit component corresponding to the capacitor C 1  shown in  FIG. 3 , is formed. It is preferable that the size of the electrostatic capacity of the capacitor C 1  formed in the plasma jet ignition plug  100  be more than 1 pF and less than 100 pF. However, even though in an embodiment of the present invention, a structure filled with dielectric  112  in a space between the terminal fitting  40  and the tubular electrode  111  is described, it is not necessary that the structure be filled with the dielectric  112 . It is required only that a size of electrostatic capacity of the capacitor C 1  is more than 1 pF and less than 100 pF. 
     Originally, for example, a member, such as the terminal fitting  40 , and a ground side, such as the ground electrode  30 , is formed with a relatively small stray capacity (not shown) there between. The stray capacity greatly affects a discharge action in the plasma jet ignition plug  100 . In the present embodiment, in order to provide a more secured discharge action, the tubular electrode  111  and the dielectric  112  are provided. Thus, by providing the tubular electrode  111  and the dielectric  112 , a capacitor C 1  having a larger value than the stray capacitor is formed in the vicinity of the center electrode  20 . 
     As illustrated in  FIG. 2 , the plasma jet ignition plug  100  is installed to fit (screw connection) into the engine head  300  of an internal combustion engine with the attachment screw part  52  formed with metal. The tubular electrode  111  provided at the plasma jet ignition plug  100  is electrically connected to the engine head  300  by a wiring  113 , and connected through the engine head  300  to the ground. 
     As illustrated in  FIG. 2 , the terminal fitting  40  is connected with the other end of a resistor R 1  and one end (anode) of a diode D 2 . In an example shown in  FIG. 2 , the resistor R 1  and the diode D 2  are disposed within a space of the inner side of the tubular electrode  111 . Further, the other end P of that resistor R 1  (the other end of the resistor R 1  is an end part opposite to a resistor R 1  connected to the diode D 1 ; This end part is called nexus point P in some cases) is also disposed in a space of the inner part of the tubular electrode  111 . On the one hand,  FIG. 3  recites that one end of a resistor R 1  is connected to an anode of a diode disposed between the plasma jet ignition plug  100  and a later-described high-voltage generation circuit  210  (also, called a discharge voltage application unit in some cases), but the present invention is not restricted to one end of a resistor R 1  connected to an anode. According to a circuit construction of the ignition apparatus  200 , it may alternatively be properly connected to a cathode. 
     When an electric discharge occurs at the plasma jet ignition plug  100 , high-frequency current flows through the circuit including the terminal fitting  40 , in a short time and electrical noise caused by the high-frequency current, that is electric waves, radiates out of the plasma jet ignition plug  100  to affect the surrounding electronic devices. However, with respect to noise occurring nearby the terminal fitting  40 , which surrounded and covered with a grounded tubular electrode  111 , the radiation of the noise toward the outside is steeply suppressed by an electrostatic shield effect. 
     The plasma jet ignition plug  100  having such a structure forms plasma in the cavity  60  by connecting to the ignition apparatus  200  shown in  FIG. 3  and being provided with energy, and extrudes plasma from the opening part  14  and ignites the mixed gas. Hereinafter, referring to  FIG. 3 , the ignition apparatus  200  of the plasma jet ignition plug  100  will be explained. 
     As shown in  FIG. 3 , the ignition apparatus  200  has two high-voltage generation circuits  210 ,  220 . The high-voltage generation circuit  210  on one side (called a discharge voltage application unit in some cases) is an electric source for performing a spark discharge between the center electrode  20  and the ground electrode  30  of the plasma jet ignition plug  100 , and can temporarily output about several tens of kV of high voltage. A high-voltage generation circuit  220  on the other side is an electric source for supplying electric energy needed for producing of plasma to the plasma jet ignition plug  100  after a spark discharge has occurred, and outputs about 500 V of high-voltage. With the electric power supplied from the high-voltage generation circuit  210  and power supplied from the high-voltage generation circuit  220 , plasma from the opening part  14  of the plasma jet ignition plug  100  extrudes towards an inner side space of the engine head  300 , and by this plasma, an ignition to mixed gas is performed. 
     The high-voltage generation circuit  210  shown in  FIG. 3  has an ignition coil  211  and a transistor Q 1 . The ignition coil  211  is a high-voltage transformer having a primary coil L 1  and a secondary coil L 2 . In the primary coil L 1  of the ignition coil  211 , one end is connected to a plus terminal of a DC electric source  230  (including batteries, etc.), and the other end is connected to a collector terminal of the transistor Q 1 . The minus terminal of the DC source  230  is connected to the ground. 
     A control terminal of the transistor Q 1 , that is a base electrode, is applied with an ignition coil electric current signal from a control circuit that is not shown. The ignition coil electric current signal is a signal having one pulse signal per 1 discharge cycle in the plasma jet ignition plug  100 , and used in a switching control of the transistor Q 1 . 
     Thus, when the ignition coil current signal becomes a high-level, the transistor Q 1  conducts, and current flows to the primary coil L 1  of the ignition coil  211  from power supplied by the DC source  230 . Also, when the ignition coil current signal becomes a low-level, the transistor Q 1  does not conduct, and the current flowing through the primary coil L 1  of the ignition coil  211  is abruptly blocked. 
     When a current starts to flow through the primary coil L 1  of the ignition coil  211 , and when the current of the primary coil L 1  of the ignition coil  211  is blocked, a high-voltage is produced at the secondary coil L 2 . The voltage producing at the secondary coil L 2  is determined by a ratio of the winding number of the primary coil L 1  and the secondary coil L 2 . 
     As shown in  FIG. 3 , an output terminal  210   a  of the high-voltage generation circuit  210  is connected to the cathode terminal, that is one end of the diode D 1 , and the anode, that is the other end of the diode D 1 , connects to one end of the resistor R 1 , and the other end of the resistor R 1  is electrically connected to the terminal fitting  40  of the plasma jet ignition plug  100 . The Diode is provided to prevent a counter-flow of current. That is, in order that current at a spark discharge flows only in a direction from the terminal fitting  40  to the secondary coil L 2  by a negative voltage, the diode D 1  controls a polarity. A resistor value of more than 100Ω is preferred for the resistor R 1 . Like the structure of a normal plasma jet ignition plug, the plasma jet ignition plug  100  according to an exemplary embodiment does not have a special resistor inside. 
     A capacitor C 2  is connected between an output terminal and a ground terminal of the high-voltage generation circuit  220 . Also, the output terminal of the high-voltage generation circuit  220  is connected with one end of a coil L 3 , and the other end of the coil L 3  is connected with one end of the diode D 2 , that is a cathode terminal, and the other end of the diode D 2 , that is an anode terminal, is electrically connected to the terminal fitting  40  of the plasma jet ignition plug  100 . The diode D 2  is provided to prevent the counter-flow of a current. That is, in order to ensure current at plasma discharge flows only in a direction from the terminal fitting  40  towards the output side of the high-voltage generation circuit  220  by a negative voltage, the diode D 2  controls polarity. Further, a DC resistive value such as the coil L 3  in a wiring connecting a capacitor C 2  to the terminal fitting  40  of the plasma jet ignition plug  100  is less than 1Ω. 
     A specific example of waveforms of a discharge current flowing at discharge and a discharge voltage applied to the plasma jet ignition plug  100  during one cycle of a discharge action are shown in  FIG. 4 . Discharge voltage V 11  and discharge current I 11  shown in  FIG. 4  indicate a case of having the resistor R 1  and the capacitor C 1  as illustrated in  FIG. 3 , and discharge voltage V 12  and discharge current I 12  indicate a waveform not existing with the resistor R 1  and the capacitor C 1  as illustrated in  FIG. 3 . 
     When a discharge is started, first in order to produce a spark discharge (referred to as a trigger discharge), a high-voltage from the high-voltage generation circuit  210  is supplied to the plasma jet ignition plug  100 . That is, the transistor Q 1  shown in  FIG. 3  converts from a conducting state to a non-conducting state, the secondary coil L 2  of the ignition coil  211  instantly has a high-voltage, this high-voltage comes up at the output  210   a  of the high-voltage generation circuit  210  as a negative voltage against the ground potential, and this high-voltage is applied to the terminal fitting  40  of the plasma jet ignition plug  100  through the diode D 1  and the resistor R 1 . 
     Electrostatic capacity other than the capacitor C 1 , i.e. stray capacity, exists between electrodes inside the plasma jet ignition plug  100 , between the ground and a high-voltage cable (a wiring including D 1 , R 1 ) connecting the high-voltage generation circuit  210  and the plasma jet ignition plug  100 , and between the secondary coil L 2  of the ignition coil  211  and the ground. 
     When the output  210   a  of the high-voltage generation circuit  210  instantly has a high-voltage, electric charge is accumulated at each point of stray capacity or the capacitor C 1  by the high-voltage. In an initial stage of the plasma jet ignition plug  100  (a timing of ‘capacity discharge’ shown in  FIG. 4 : about several nanoseconds), a dielectric break down in the cavity  60  is induced by a high voltage and a spark discharge occurs, but by the emitting of electric charge accumulated at each point of each stray capacity or the capacitor C 1 , electric energy is supplied into the plasma jet ignition plug  100 . Also, after the electric charge of each stray capacitor or the capacitor C 1  is emitted (‘a timing of induced discharge’ shown in  FIG. 4 : about several μsec), energy accumulated on inductance of the secondary coil L 2  of the ignition coil  211  is emitted and continues the discharge. 
     Meanwhile, to produce plasma by discharge, it is necessary to supply high electric energy into the plasma jet ignition plug  100 . Because a current that can be supplied from the high-voltage generation circuit  210  to the plasma jet ignition plug  100  is relatively small, energy to produce plasma is supplied from the high-voltage generation circuit  220  of a separate circuit. Actually, power outputted by the high-voltage generation circuit  220  is accumulated on the capacitor C 2 , and then electric charge of the capacitor C 2  is supplied through the diode D 2  and the coil L 3  into the plasma jet ignition plug  100 . When performing a plasma discharge subsequent to the spark discharge, an electric discharge production becomes easy by a dielectric breakdown occurring at the spark discharge, so that a discharge can continue even at a relatively low voltage. 
     When a negative voltage applied to the terminal fitting  40  of the plasma jet ignition plug  100  from the high-voltage generation circuit  210  side is smaller than a negative voltage appearing at terminals of the capacitor C 2  connected to the high-voltage generation circuit  220 , the diode D 2  conducts, and electric charge accumulated on the capacitor C 2  is supplied into the plasma jet ignition plug  100  through the diode D 2  and the coil L 3 . That is, a current (also called plasma current) flowing by plasma occurring in the cavity  60  of the plasma jet ignition plug  100  dribbles into the capacitor C 2  via the diode D 2  and the coil L 3  from the terminal fitting  40 . 
     Thus, as the waveform shown in the discharge current I 11  of  FIG. 4 , a plasma current starts to flow in the middle of a timing of ‘capacity discharge’, and proportionate to an amount of electric charge accumulated on the capacitor C 2 , the plasma current continuously flows. 
     However, during the timing of ‘capacity discharge’, a high-amplitude high-frequency current electric charge caused by of a high-voltage appears for a very short time on a waveform of a discharge current. When noise such as electric waves radiates from the high-frequency current, the radiated noise adversely affects electronic devices surrounding the ignition apparatus  200 . Therefore, noise radiated from the ignition apparatus  200  needs to be prevented. 
     In the ignition apparatus  200  having a construction as shown in  FIG. 3 , as the stray capacitance and the discharge voltage increase, current at ‘capacitor discharge’ rises, and radiated noise also increases. Also, as a DC resistor existing in a path through which a current at ‘capacity discharge’ flows becomes smaller, a current at ‘capacitor discharge’ rises, and radiated noise also increases. 
     In the meantime, when a current at ‘capacity discharge’ is small, it is difficult for plasma current to flow into the plasma jet ignition plug  100  at a plasma discharge. That is, when a current of ‘capacity discharge’ is small, a time needed to emit electric charge accumulated on the points of stray capacity gets longer, and a time in which a negative high-voltage applied to the plasma jet ignition plug  100  from the high-voltage generation circuit  210  side decays becomes longer. If this high-voltage has not sufficiently decayed, the diode D 2  does not conduct, so that it is impossible to supply electric charge of the capacitor C 2  to the plasma jet ignition plug  100  for plasma discharge. 
     Also, for a plasma current through line (a current path having the diode D 2 , the coil L 3 , etc.), it is desirable to keep a DC resistor small. By this, a peak value of the plasma current becomes larger, so that a production efficiency of plasma is improved. 
     A resistor R 1  inserted between the output of the high-voltage generation circuit  210  and the terminal fitting  40  of the plasma jet generation plug  100  suppresses an amplitude of a high-frequency current flowing at ‘capacity discharge’ by a high-voltage cable connecting the high-voltage generation circuit  210  and the plasma jet ignition plug  100  and electric charge accumulated on stray capacitance of the secondary coil L 2  of the ignition coil  211 , thereby having an effect of saving the above-described noise. 
     However, by providing the resistor R 1 , a current at ‘capacity discharge’ becomes smaller, a time until when a high-voltage applied to the terminal fitting  40  decays as described above becomes longer, and it is difficult to flow a current of the capacitor C 2  into the plasma jet ignition plug  100  at a plasma discharge. 
     As illustrated in  FIG. 3 , by connecting the capacitor C 1  to the plasma jet ignition plug  100 , even in a case the resistor R 1  exists, it is easy to flow a current of the capacity C 2  at plasma discharge into the plasma jet ignition plug  100 . That is, by an addition of stray capacitance existing in the plasma jet ignition plug  100  itself and capacitance of the capacitor C 1 , increasing the current at capacity discharge, it is easy to flow a current of the capacitor C 2  at plasma discharge into the plasma jet ignition plug  100 . Further after a high-voltage from an output of the high-voltage generation circuit  210  is applied to the plasma jet ignition plug  100 , electric discharge accumulated on the capacitor C 1  is swiftly emitted through a path with a small resistor value (between electrodes of the plasma jet ignition plug  100 ), a high-voltage applied to the terminal fitting  40  decays fast, and in a short time the diode D 2  conducts and thus a plasma current starts to flow. 
     Therefore, the capacitor C 1  needs to be disposed at a position near the terminal fitting  40  rather than the resistor R 1 . In addition, since a very high voltage is applied from the high-voltage generation circuit  210 , a high internal pressure (several tens of kV) is required for the capacitor C 1 . Due to this, it is difficult to use a general capacitor selling on the market as the capacitor C 1 . Thus, as shown in  FIG. 2 , by disposing the tubular electrode  111  near the terminal fitting  40  and filling dielectric  112  between them, a capacitor C 1  having a sufficiently high capacitance is constructed. 
     For electrostatic capacity of the capacitor C 1 , more than 1 pF and less than 100 pF is suitable. In particular, as shown in  FIG. 2 , when a point P, that is the other end of the resistor R 1 , is disposed in an inner space of the tubular electrode  111 , the point P, the other end of that resistor R 1 , is contained, and electrostatic capacity accumulated on a sandwiched area between a cross-section S 1  of the tubular electrode  111  perpendicular to an axis center direction of the tubular electrode  111  and a cross-section S 2  of the tubular electrode  111  containing a cross-section of the tubular electrode  111  has preferably more than 1 pF and less than 100 pF. Note the cross-section of the tubular electrode  111  is a surface in which an end part of the tubular electrode  111  places on the same plane, and have a side to which the plasma jet ignition plug  100  is inserted and penetrated and a side not inserted and penetrated. Also note this area is sandwiched by the cross-section S 2  of the tubular electrode  111  containing the cross-section of the tubular electrode  111  placed at the side inserted and penetrated by the plasma jet ignition plug  100 , and its inside has the terminal fitting  40 . The electrostatic capacity accumulated on an area sandwiched between the above-described cross-section S 1  and the cross section S 2  is equivalent to electrostatic capacity accumulated on the capacitor C 1  of  FIG. 3 , and electric energy by discharge of electric discharge accumulated on this area is supplied to the plasma jet ignition plug  100 . 
     Meanwhile electric charge is also accumulated in an area sandwiched between a cross-section S 0  of the tubular electrode  111  containing the cross-section of the tubular electrode  111  (a cross-section containing a cross-section of the tubular electrode  111  at the side to which the plasma jet ignition plug  100  is not inserted and penetrated, positioned opposite to the above-described cross-section S 2 ) and the above-described cross-section S 1  of the tubular electrode  111 . However, electric energy by discharge of this electric charge is supplied to the resistor R 1  placed in this area, but not supplied to the plasma jet ignition plug  100 . Because of this, in order to make it easy for a current of the capacitor C 2  to flow at plasma discharge into the plasma jet ignition plug  100 , it is significantly necessary to take action by emitting electric charge accumulated in an area sandwiched between the above-described cross-section S 1  and the cross-section S 2 ), and thus, it is proper that electrostatic capacity accumulated in the area is more than 1 pF and less than 100 pF. 
     Electrostatic capacity accumulated in an area between the above-described cross-section S 1  and the cross-section S 2  may be, for example, specified by cutting a part from the cross-section S 0  to the cross-section S 1  of the tubular electrode  111  and measuring electrostatic capacity of the remaining tubular electrode  111  by an LCR meter. On the one hand, a method of specifying electrostatic capacity is not limited to this. It may be theoretically calculated in reference to a shape and its inner dielectric of the tubular electrode  111 , a position of the other end of the resistor R 1  inside the tubular electrode  111 , and a shape and its dielectric of the plasma jet ignition plug  100  placed inside the tubular electrode  111 . 
     In a case electrostatic capacity of the capacitor C 1  is less than 1 pF, an effect of the capacitor C 1  is not sufficiently obtained, but it is difficult for a current of the capacitor C 2  to flow at plasma discharge into the plasma jet ignition plug  100 . Specifically, in a case electrostatic capacity of the capacitor C 1  is less than 1 pF, a probability that plasma is formed in the cavity  60  (plasma occurrence probability) is lowered to 70˜80%. Also, in a case electrostatic capacity of the capacitor C 1  exceeds 100 pF, when a high-voltage from an output of the high-voltage generation circuit  210  is applied to the terminal fitting  40 , a rising velocity of a voltage slows by a relaxation time of R 1 , C 1 , and it may not be possible for capacity discharge. Specifically, in a case electrostatic capacity of the capacitor C 1  exceeds 100 pF, a probability that capacity discharge occurs by the plasma jet ignition plug  100  (discharge probability) lowers to 70-80%. On the one hand, when electrostatic capacity of the capacitor C 1  is more than 1 pF and less than 100 pF, both a plasma occurrence probability and a discharge probability can be a high-level, that is 80-100%. 
     For an electrode constructing the capacitor C 1 , it is not limited to a cylindrical-shape such as the tubular electrode  111 , but for example, an electrode of metal having a plane shape is also possible. However, in a case of using an electrode enclosing the surrounding of the terminal fitting  40  such as the tubular electrode  111  and grounding this, an effect of electrostatic shield can be obtained. That is, because an electric potential of the tubular electrode  111  is constant, noise occurring by a high-frequency current flowing through a part such as the terminal fitting  40  may not radiate to the outside of the tubular electrode  111 . Noise occurring at ‘capacity discharge’ by stray capacitance existing in the secondary coil L 2  of the ignition coil  211  or a high-pressure cable, because a current is suppressed by an effect of the resistor R 1 , becomes relatively smaller. It is suitable that a resistive value of the resistor R 1  is more than 100Ω. 
     To sufficiently lower noise by an effect of the resistor R 1 , it is desirable that a length of a current path through which a high-frequency current flows that can cause noise may possibly shorten. Specifically, for a wiring distance from a fitting part of the plasma jet ignition plug  100  and the engine head  300  to the resistor R 1 , and a wiring distance from the fitting part to the diode D 2 , each may be less than 30 cm. Also, a wiring distance from the resistor R 1  to the diode D 2  may be less than 10 cm. For example, as illustrated in  FIG. 2 , when the resistor R 1  and the diode D 2  at the inner space of the tubular electrode  111  are disposed, a nearly overall of current path through which a high-frequency current flows that can be the cause of noise becomes electrostatic shielded, so that an effect of noise lowering gets very high. 
     As to a position connected with the capacitor C 1 , it is better to be on a path between a connecting point Q of the resistor R 1  and the diode D 2  and an electrode of the plasma jet ignition plug  100  (the terminal fitting  40 , etc.). 
     Further, electrostatic capacity of the capacitor C 2  is set in order that a sum of energy at plasma formation, which is energy content supplied by stray capacitance or the capacitor C 1  at trigger discharge, for a spark discharge gap, and energy content supplied from the capacitor C 2 , equals a supplied amount by performing one time plasma extrusion (e.g., 150 mJ). By this energy, plasma of a fire pillar shape (frame shape) can be extruded from the opening part  14 , and an ignition of mixed gas by plasma can be performed. 
     While embodiments of the present invention has been described in detail and also in reference to a specific embodiment, it is clear to those skilled in the art that several modifications and changes can be without departing from the scope and spirit of the present invention. 
     This application is based on Japanese Patent Application 2009-035107 filed on Feb. 18, 2009, the content of which is hereby encompassed in reference. 
     DESCRIPTION OF REFERENCE NUMERALS AND SIGNS 
     
         
           4 : seal body 
           5 : gasket 
           10 : insulator 
           12 : axis hole 
           13 : long leg part 
           14 : opening part 
           15 : electrode reception part 
           16 : front end part 
           17 : front end side body part 
           18 : back end side body part 
           19 : center body part 
           20 : center electrode 
           25 : electrode tip 
           30 : ground electrode 
           31 : communication hole 
           40 : terminal fitting 
           50 : metal shell 
           52 : attachment screw part 
           60 : cavity 
           100 : plasma jet ignition plug 
           111 : tubular electrode 
           112 : dielectric 
           113 : wiring 
           200 : ignition apparatus 
           210 ,  220 : high-voltage generation circuit 
           211 : ignition coil
         230 : DC source     
           300 : engine head