Patent Publication Number: US-8973562-B2

Title: Ignition device and ignition method for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ignition device and an ignition method for an internal combustion engine for igniting a combustible mixture in a combustion chamber of the internal combustion engine. 
     2. Description of the Related Art 
     In recent years, there have been posed problems such as the environmental conservation and the fuel exhaustion, and a response to those problems is an urgent matter in the automobile industry. As measures to these problems, the ultra-lean combustion (stratified lean combustion) operation of an engine using a stratified mixture, for example, is known. However, a distribution of a combustible mixture may vary in the stratified lean combustion, and an ignition device capable of absorbing this variation is required. 
     In order to satisfy the above-mentioned requirement, there is proposed an ignition device, which includes an ignition plug for generating a spark discharge in a combustion chamber and a microwave generation device for feeding energy to the spark discharge of the ignition plug (refer to Japanese Patent Application Laid-open No. 2010-96128, for example). 
     This ignition device can form large discharge plasma, increase spatial ignition opportunities, and absorb the variation in distribution of the combustible mixture. Therefore, the ignition device can satisfy the requirement for the stratified lean combustion. 
     However, the related art has the following problems. 
     The ignition device described in Japanese Patent Application Laid-open No. 2010-96128 can form large discharge plasma, and hence can prevent a misfire to restrain a variation in generated torque, but a passage for supplying the microwave into the combustion chamber is required in addition to the ignition plug. Therefore, there is a problem in that it is difficult to apply the ignition device according to Japanese Patent Application Laid-open No. 2010-96128 to an existing engine. 
     Moreover, in the combustion chamber, a large change in pressure is repeated by a reciprocal motion of a piston, and the formation and an extinction of plasma are repeated by the discharge and combustion, which leads to a very unstable state. Therefore, there is a problem in that a stable supply of high frequency energy such as the microwave to the combustion chamber is very difficult in impedance matching and the like technically and in terms of matching individual products. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems, and therefore has an object to provide an ignition device and an ignition method for an internal combustion engine, which are capable of easily forming large discharge plasma with a simple configuration. 
     According to an exemplary embodiment of the present invention, there is provided an ignition device for an internal combustion engine, including: an ignition plug including a first electrode and a second electrode opposed to each other via a predetermined gap, for generating in the predetermined gap a spark discharge for igniting a combustible mixture in a combustion chamber of the internal combustion engine; an ignition coil including a primary coil and a secondary coil, for generating a high voltage in the secondary coil by supplying or stopping a primary current flowing through the primary coil, and then applying the generated high voltage to the first electrode; and a control unit for driving the ignition coil for a plurality of times within a single ignition process, and changing a primary voltage for driving the ignition coil within the single ignition process. 
     According to an exemplary embodiment of the present invention, there is also provided an ignition method for an internal combustion engine, the internal combustion engine including: an ignition plug including a first electrode and a second electrode opposed to each other via a predetermined gap, for generating in the predetermined gap a spark discharge for igniting a combustible mixture in a combustion chamber of the internal combustion engine; and an ignition coil including a primary coil and a secondary coil, for generating a high voltage in the secondary coil by supplying or stopping a primary current flowing through the primary coil, and then applying the generated high voltage to the first electrode, the ignition method including driving the ignition coil for a plurality of times within a single ignition process, and changing a primary voltage for driving the ignition coil within the single ignition process. 
     According to the ignition device and the ignition method for the internal combustion engine according to the present invention, the control unit (control step) drives the ignition coil for a plurality of times within a single ignition process, and changes the primary voltage for driving the ignition coil within the single ignition process. 
     Therefore, large discharge plasma can easily be formed with the simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a configuration diagram illustrating an ignition device for an internal combustion engine according to a first embodiment of the present invention; 
         FIG. 2  is a timing chart illustrating an operation of the ignition device for the internal combustion engine according to the first embodiment of the present invention; and 
         FIG. 3  is a timing chart illustrating an operation of an ignition device for an internal combustion engine according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description is now given of preferred embodiments of an ignition device and an ignition method for an internal combustion engine according to the present invention, referring to the drawings. The same or corresponding components are denoted by the same reference symbols throughout the drawings. 
     The ignition device and the ignition method for the internal combustion engine according to the present invention can be installed on a motor vehicle, a motor cycle, an outboard motor, and other special machines which use an internal combustion engine, and surely ignite a fuel so that the internal combustion engine can be efficiently operated. Therefore, the ignition device and the ignition method for the internal combustion engine according to the present invention are useful for the environmental conservation and the fuel exhaustion problem. 
     First Embodiment 
       FIG. 1  is a configuration diagram illustrating an ignition device  100  for an internal combustion engine according to a first embodiment of the present invention. In  FIG. 1 , the ignition device  100  includes an ignition plug  110  for generating a spark discharge for igniting a combustible mixture in a combustion chamber (not shown) of the internal combustion engine, an ignition coil  120  for applying a predetermined high voltage to the ignition coil  110  and feeding a current to the ignition coil  110 , and a control unit  130  for controlling an operation of the ignition coil  120 . 
     A description is now given of a configuration and a function of each component of this ignition device  100 . 
     The ignition plug  110  includes a high voltage electrode  111  as a first electrode and an outer electrode  112  as a second electrode, which is opposed to the high voltage electrode  111  via a predetermined gap (hereinafter, referred to as “main plug gap”). 
     The ignition coil  120  includes a primary coil  121 , a secondary coil  122 , and an iron core  123  for magnetically coupling the primary coil  121  and the secondary coil  122 . One end of the secondary coil  122  is connected to the high voltage electrode  111  of the ignition plug  110 , and the other end thereof is connected to a ground (GND). 
     The control unit  130  includes a first switching element  131 , an ignition capacitor  132 , a first rectifier diode  133 , an inductor  134 , a first power supply  135 , a second rectifier diode  136 , a second power supply  137 , a second switching element  138 , and a signal generation unit  139 . 
     The ignition capacitor  132  is connected between both ends of the primary coil  121  via the first switching element  131  constituted by an insulated gate bipolar transistor (IGBT). To a positive electrode side of the ignition capacitor  132 , the first power supply  135  is connected via the first rectifier diode  133  and the inductor  134 , and the second power supply  137  is connected via the second rectifier diode  136 . 
     A negative electrode side of the ignition capacitor  132  is connected to the GND via the second switching element  138  constituted by an IGBT. Moreover, a base of the first switching element  131  and a base of the second switching element  138  are connected to the signal generation unit  139 . 
     On this occasion, the second power supply  137  is a power supply which can apply a voltage twice or more as high as a voltage which the first power supply  135  can apply. For example, the first power supply  135  and the second power supply  137  are selected as a 100 V power supply and a 1,000 V power supply, respectively, according to the first embodiment of the present invention. 
     Switching of the first switching element  131  and the second switching element  138  is controlled respectively by a first control signal SH and a second control signal SL from the signal generation unit  139  constituted by a microprocessor (micro-processing unit: MPU). The signal generation unit  139  sets the number and timings of operations of the ignition coil  120  in accordance with an operation state of the internal combustion engine, thereby generating the first control signal SH and the second control signal SL. 
     Note that, the signal generation unit  139 , the first switching element  131 , and the second switching element  138  constitute a capacitive current supply unit for supplying the primary side of the ignition coil  120  with a capacitive current by means of a charge accumulated in the ignition capacitor  132 , and the capacitive current supply unit constitutes a part of the control unit  130  for controlling the operation of the ignition coil  120 . 
     On this occasion, a primary current I 1  flowing through the primary coil  121  of the ignition coil  120  is constituted by a capacitive current from the ignition capacitor  132  which flows on a discharge path which starts from the positive electrode side of the ignition capacitor  132 , passes through the primary coil  121  and a collector and an emitter of the first switching element  131 , and returns to the negative electrode side of the ignition capacitor  132 . 
     Therefore, as the electric charge accumulated in the ignition capacitor  132  increases and the voltage for charging the ignition capacitor  132  increases, the value of the primary current I 1  increases and the secondary voltage generated on the secondary side of the ignition coil  120  increases. Therefore, “a large current” can be supplied, and “a high voltage” can be applied by setting the electrostatic capacity C of the ignition capacitor  132  and the charge voltage to appropriate values. 
     On this occasion, the ignition capacitor  132  is charged through a charge path which starts from the first power supply  135 , the first rectifier diode  133 , and the inductor  134 , or starts from the second power supply  137  and the second rectifier diode  136 , passes through the positive electrode side of the ignition capacitor  132 , the negative electrode side of the ignition capacitor  132 , a collector and an emitter of the second switching element  138 , and reaches the GND. 
     Moreover, the ignition capacitor  132  is connected to the first power supply  135  via the inductor  134 , and hence the charge current flowing from the first power supply  135  to the ignition capacitor  132  is amplified at a cycle of a so-called LC resonance determined by the electrostatic capacity C of the ignition capacitor  132  and the inductance L of the inductor  134 . 
     In other words, the ignition capacitor  132  can be charged very quickly to a voltage higher than the voltage 100 V of the first power supply  135 , approximately 200 V, for example, by setting the electrostatic capacity C of the ignition capacitor  132  and the inductance L of the inductor  134  to appropriate values. 
     Moreover, the ignition capacitor  132  is connected to a voltage higher than the voltage charged by means of the LC resonance from the first power supply  135 , namely, the second power supply  137  at 1,000 V according to the first embodiment. Therefore, though the charge takes time, the ignition capacitor  132  can be charged to the voltage higher than the voltage brought about by the charge from the first power supply  135  via the first rectifier diode  133  and the inductor  134 . 
     A description is now given of a method of forming the discharge plasma in this ignition device  100 . 
     It is necessary to supply the main plug gap of the ignition plug  110  with “a large current” “repeatedly in a short period” in order to form large discharge plasma in the main plug gap. For example, as the current supplied to the main plug gap increases, more plasma is formed. 
     However, the plasma concentrates around the discharge path, and hence discharge plasma in a desired volume is not formed by simply increasing the discharge current. The discharge needs to be generated for a plurality of times, namely, a multi-ignition is necessary in order to distribute the formed plasma in a spatially wide area. 
     Specifically, the plasma is generated in the main plug gap of the ignition plug  110  by the discharge generated in the main plug gap. On this occasion, when the discharge is discontinued, the plasma takes various forms such that a part of the plasma is diffused by its own heat, another part thereof flows along with the combustible mixture in the combustion chamber of the internal combustion engine, and still another part thereof disappears. 
     On this occasion, when the discharge is discontinued and a predetermined high voltage is applied to the main plug gap in order to generate again the discharge in the main plug gap, the charge is resumed on a path lower in impedance in the main plug gap. 
     This path lower in impedance varies and may be a path high in plasma density, or may be a path of the shortest distance in the main plug gap. Therefore, a probability that a discharge is generated again on a path different from a previous discharge path is increased by the multi-ignition. 
     In other words, the so-called multi-ignition, which simply repeats the ignition, cannot form sufficient plasma by a single discharge, and hence cannot form large discharge plasma as a whole. Further, a simple increase in the discharge current results in a narrow supply range of the plasma, and cannot form large discharge plasma. 
     In contrast, according to the first embodiment of the present invention, the discharge current which can form sufficient plasma can be supplied and plasma is formed repeatedly in a wide area from spatially different locations by the multi-ignition, resulting in formation of large discharge plasma. 
     In view of the above, the signal generation unit  139  controls the first switching element  131  and the second switching element  138  so that the discharge is started again in an interval that is shorter than that in which the plasma formed in the main plug gap of the ignition plug  110  entirely disappears, and that allows the formed plasma to be appropriately diffused. 
     Referring to a timing chart in  FIG. 2 , a description is now given of an operation of the ignition device  100  for the internal combustion engine according to the first embodiment of the present invention. 
     In  FIG. 2 , part (a) illustrates the second control signal SL output to the base of the second switching element  138 , part (b) illustrates the first control signal SH output to the base of the first switching element  131 , part (c) illustrates a potential difference between the both ends of the ignition capacitor  132 , part (d) illustrates the primary current I 1  flowing through the primary coil  121  of the ignition coil  120 , part (e 1 ) illustrates the voltage applied to the high voltage electrode  111  of the ignition plug  110 , and part (f 1 ) illustrates a waveform of a discharge current I 2  flowing through the main plug gap. 
     When the second control signal SL from the signal generation unit  139  reaches the H level at a timing corresponding to a time T 0  of  FIG. 2 , the second switching element  138  is brought into the ON state. On this occasion, the first control signal SH from the signal generation unit  139  is at the L level, and the first switching element  131  is thus in the OFF state. 
     When the second switching element  138  is brought into the ON state, the ignition capacitor  132  is quickly charged from the first power supply  135  up to approximately 200 V, which is approximately twice as high as the voltage of the first power supply  135 , in a very short period by the LC resonance via the above-mentioned charge path as illustrated in part (c) of  FIG. 2 . 
     Further, the ignition capacitor  132  is slowly charged up to approximately 1,000 V, which is the voltage of the second power supply  137 , from the second power supply  137 . Note that, the charge by the second power supply  137  is slow, and hence a sufficient period is set for a charge period (period from the time T 0  to a time T 1 ). 
     Moreover, the first control signal SH and the second control signal SL are output from the signal generation unit  139  so that, when one of the first control signal SH and the second control signal SL is at the H level, the other of the first control signal SH and the second control signal SL is at the L level in an ignition operation at the time T 0  and thereafter. As a result, switching control is provided for the first switching element  131  and the second switching element  138  so that, when one of the first switching element  131  and the second switching element  138  is in the ON state, the other of the first switching element  131  and the second switching element  138  is in the OFF state. 
     When the first control signal SH from the signal generation unit  139  reaches the H level at a timing corresponding to the time T 1  of  FIG. 2 , the first switching element  131  is brought into the ON state. On this occasion, the second control signal SL from the signal generation unit  139  reaches the L level, and the second switching element  138  is thus brought into the OFF state. 
     When the first switching element  131  is brought into the ON state, the capacitive current of the ignition capacitor  132  charged to approximately 1,000 V flows as the primary current I 1  through the ignition coil  120  on the above-mentioned path. 
     On this occasion, the primary current I 1  is caused to quickly flow in accordance with the charged voltage 1,000V, which is higher than the ordinary voltage 200 V brought about by the first power supply  135 , and hence a secondary voltage, which is higher than an ordinary voltage, is generated on the secondary side of the ignition coil  120 . 
     For example, in a case where the ignition capacitor  132  is charged to 200 V and the primary current I 1  is caused to flow, if the secondary voltage generated on the secondary side of the ignition coil  120  is approximately 10 kV, the secondary voltage of approximately 50 kV can be generated on the secondary side of the ignition coil  120  when the ignition capacitor  132  is charged to 1,000 V and the primary current I 1  is caused to flow. 
     Moreover, a configuration in which a negative high voltage is generated on the high voltage electrode  111  of the ignition plug  110  at the time T 1  is provided. In other words, in order to surely generate a dielectric breakdown in the main plug gap at the time T 1 , attention is paid so that the negative high voltage, which more easily generates the dielectric breakdown, is applied to the high voltage electrode  111 . As a result, the dielectric breakdown can be surely generated in the main plug gap at the time T 1 . 
     When an attempt is made to increase the secondary current (discharge current I 2 ) flowing through the secondary coil  122  of the ignition coil  120 , the secondary voltage generated on the secondary coil  122  decreases, and hence dielectric breakdown may not be generated in the main plug gap of the ignition plug  110 . As a result, a misfire state may be brought about. 
     However, as described in the first embodiment of the present invention, the dielectric breakdown can surely be generated by supplying the primary current I 1  in accordance with the voltage higher than the ordinary voltage in the initial period of the multi-ignition. 
     Therefore, even if the ignition coil  120  is configured by a current-oriented specification, such as a specification in which a turn ratio between the primary coil  121  and the secondary coil  122  is equal to or less than 80, instead of a conventional voltage-oriented type specification, a dielectric breakdown can surely be generated in the main plug gap, and a large discharge current I 2  can be caused to flow. 
     Next, when the first control signal SH reaches the L level at a timing corresponding to the time  12  of  FIG. 2 , the first switching element  131  is brought into the OFF state. On this occasion, the primary current I 1  from the ignition capacitor  132  is stopped, and the second control signal SL simultaneously reaches the H level. As a result, the second switching element  138  is brought into the ON state. 
     When the second switching element  138  is brought into the ON state, the ignition capacitor  132  is quickly charged from the first power supply  135  up to approximately 200 V, which is approximately twice as high as the voltage of the first power supply  135 , in a very short period by the LC resonance via the above-mentioned charge path. 
     A period between the time T 2  and a time T 3  of  FIG. 2  is short for charging the ignition capacitor  132  by the second power supply  137 , and the charged voltage of the ignition capacitor  132  hardly increases in this period. In other words, the charged voltage remains approximately 200 V at the time T 3 . 
     Moreover, the dielectric breakdown has already been generated in the main plug gap between the high voltage electrode  111  and the outer electrode  112  of the ignition plug  110  at the time T 1 , thereby forming a discharge path. Therefore, subsequently, generation of a high secondary voltage is no longer necessary unless the discharge is discontinued for a while, and the discharge current I 2  can be cause to flow through the discharge path in the main plug gap by a voltage of approximately 500 V, for example. 
     At the timing corresponding to the time T 2  and thereafter of  FIG. 2 , the first control signal SH and the second control signal SL are alternately switched between the H level and the L level in a short period at the time T 3  and a time T 4 . As a result, the conduction states of the first switching element  131  and the second switching element  138  alternately change as described above, and the primary current I 1  flows repeatedly in the each short period in the ignition coil  120 . 
     In the ignition device  100  for the internal combustion engine according to the first embodiment of the present invention illustrated in  FIG. 1 , the first switching element  131  repeats the ON state and the OFF state, and the secondary current (discharge current I 2 ) flowing through the secondary side of the ignition coil  120  thus flows as an alternate current as illustrated in part (f 1 ) of  FIG. 2 . 
     As described above, according to the first embodiment, the control unit drives the ignition coil a plurality of times within a single ignition process, and changes the primary voltage for driving the ignition coil within the single ignition process. Therefore, large discharge plasma can be easily formed with the simple configuration. 
     Moreover, as described above, the high secondary voltage is generated to form the discharge path in the main plug gap by causing the primary current to flow in accordance with the voltage higher than the ordinary voltage at the initial period of the multi-ignition. Subsequently, the primary current is caused to flow at the voltage lower than the voltage at the initial period of the multi-ignition, and hence it is possible to continuously feed the large discharge current though the discharge path in the main plug gap. 
     Therefore, the large discharge plasma can be efficiently formed, and a large amount of plasma can be fed to a wide area in the combustion chamber of the internal combustion engine, thereby facilitating the combustion reaction. As a result, a limit region and the like of the lean combustion or the diluted combustion can be extended. 
     In other words, the large alternate discharge current can be supplied between the electrodes of the ignition plug in an early period, and hence the large plasma can be formed with the simple configuration, resulting in the stable lean combustion. As a result, the fuel used for the operation of the internal combustion engine can be significantly reduced, thereby largely reducing the quantity of emission of CO 2 , and contributing to the environmental conservation. 
     Second Embodiment 
     According to the first embodiment, by increasing the charged voltage of the ignition capacitor  132  in the initial period of the multi-ignition operation, the primary current I 1  is caused to quickly flow on the primary side of the ignition coil  120 , thereby supplying the high voltage electrode  111  of the ignition plug  110  with the secondary voltage, as the negative high voltage, which is generated by so called “magnetic excitation” when the current flows in. As a result, the dielectric breakdown is generated in the main plug gap, and the discharge path is formed. 
     On this occasion, “release of magnetic flux” has an opposite meaning of “magnetic excitation”. Moreover, it is known that a higher secondary voltage is more easily generated at the time of the “release of magnetic flux”. In other words, the electromotive force of the coil is proportional to a quantity of a temporal change in the magnetic flux. Moreover, the numbers of turns of the coils of the ignition coil  120  are not variable elements, and hence it can also be rephrased that the electromotive force of the coil is proportional to a quantity of temporal change in current. Moreover, the coil has inductance, and it is thus difficult to instantaneously flow a required current but it is easy to stop a flowing current. 
     Considering these points, a high voltage can be more efficiently generated by employing the “release of magnetic flux”, and hence an ignition coil  120  having a small turn ratio can be employed. As a result, a larger discharge current I 2  can be caused to flow in the discharge path in the main plug gap. 
     Referring to a timing chart in  FIG. 3 , a description is now given of an operation of the ignition device  100  for the internal combustion engine according to the second embodiment of the present invention. Note that, the configuration of the ignition device  100  of the internal combustion engine according to the second embodiment of the present invention is the same as that of the first embodiment described above, and a description thereof is therefore omitted. 
     In  FIG. 3 , part (a) illustrates the second control signal SL output to the base of the second switching element  138 , part (b) illustrates the first control signal SH output to the base of the first switching element  131 , part (c) illustrates a potential difference between the both ends of the ignition capacitor  132 , part (d) illustrates the primary current I 1  flowing through the primary coil  121  of the ignition coil  120 , part (e 2 ) illustrates the voltage applied to the high voltage electrode  111  of the ignition plug  110 , and part (f 2 ) illustrates a waveform of a discharge current I 2  flowing through the main plug gap. 
     When the second control signal SL from the signal generation unit  139  reaches the H level at a timing corresponding to a time T 0  of  FIG. 3 , the second switching element  138  is brought into the ON state. On this occasion, the first control signal SH from the signal generation unit  139  is at the L level, and the first switching element  131  is thus in the OFF state. 
     When the second switching element  138  is brought into the ON state, the ignition capacitor  132  is quickly charged from the first power supply  135  up to approximately 200 V, which is approximately twice as high as the voltage of the first power supply  135 , in a very short period by the LC resonance via the above-mentioned charge path illustrated in part (c) of  FIG. 3 . 
     Further, the ignition capacitor  132  is slowly charged up to approximately 1,000 V, which is the voltage of the second power supply  137 , from the second power supply  137 . Note that, the charge by the second power supply  137  is slow, and hence a sufficient period is set for a charge period (period from the time T 0  to a time T 1 ). 
     Moreover, the first control signal SH and the second control signal SL are output from the signal generation unit  139  so that, when one of the first control signal SH and the second control signal SL is at the H level, the other of the first control signal SH and the second control signal SL is at the L level in an ignition operation at the time T 0  and thereafter. As a result, switching control is provided for the first switching element  131  and the second switching element  138  so that, when one of the first switching element  131  and the second switching element  138  is in the ON state, the other of the first switching element  131  and the second switching element  138  is in the OFF state. 
     When the first control signal SH from the signal generation unit  139  reaches the H level at a timing corresponding to the time T 1  of  FIG. 3 , the first switching element  131  is brought into the ON state. On this occasion, the second control signal SL from the signal generation unit  139  reaches the L level, and the second switching element  138  is thus brought into the OFF state. 
     When the first switching element  131  is brought into the ON state, the capacitive current of the ignition capacitor  132  charged to approximately 1,000 V flows as the primary current I 1  through the ignition coil  120  on the above-mentioned path to generate a secondary voltage on the secondary side of the ignition coil  120 . 
     The circuit is configured so that the secondary voltage generated by this “magnetic excitation” is applied as a positive high voltage to the high voltage electrode  111  of the ignition plug  110 . On this occasion, if a dielectric breakdown is not generated in the main plug gap at the time T 1 , more magnetic flux is accumulated in the iron core  123  of the ignition coil  120 . 
     Next, when the first control signal SH reaches the L level at a timing corresponding to the time T 2  of  FIG. 3 , the first switching element  131  is brought into the OFF state. On this occasion, the primary current I 1  from the ignition capacitor  132  is stopped, and the second control signal SL simultaneously reaches the H level. As a result, the second switching element  138  is bought into the ON state. 
     When the second switching element  138  is brought into the ON state, the ignition capacitor  132  is quickly charged from the first power supply  135  up to approximately 200 V, which is approximately twice as high as the voltage of the first power supply  135 , in a very short period by the LC resonance via the above-mentioned charge path. 
     On this occasion, the time T 2  is set to a timing in which the primary current I 1  flowing on the primary coil side  121  of the ignition coil  120  reaches near a peak. This setting can bring about the maximum quantity of change in magnetic flux, resulting in generation of a higher secondary voltage on the secondary side of the ignition coil  120 . 
     Even if the dielectric breakdown is not generated at the time T 1 , a higher negative voltage can be applied to the high voltage electrode  111  of the ignition plug  110  at the time T 2 , which is slightly later, and hence a dielectric breakdown is surely generated in the main plug gap, resulting in formation of the discharge path. Therefore, an ignition coil  120 , which is small in turn ratio and can thus supply a larger secondary current, can be selected. 
     The discharge path is surely formed in the main plug gap at the time T 2 , and subsequently, generation of a high secondary voltage is no longer necessary unless the discharge is discontinued for a while. Therefore, the discharge current I 2  can be cause to flow through the discharge path in the main plug gap between the high voltage electrode  111  and the outer electrode  112  of the ignition plug  110  by a voltage of approximately 500 V, for example. 
     An operation is the same as that of the above-mentioned first embodiment at and after a timing corresponding to the time T 2  of  FIG. 3 , though the polarities of the voltage and the current are opposite, and a detailed description thereof is therefore omitted. 
     As described above, according to the second embodiment, the dielectric breakdown can be more efficiently generated in the main plug gap, and the discharge path is formed. Accordingly, an ignition coil small in turn ratio can be employed, and a larger discharge current can continuously be fed via the discharge path in the main plug gap. 
     Therefore, the large discharge plasma can be efficiently formed, and a large amount of plasma can be fed to a wide area in the combustion chamber of the internal combustion engine, thereby facilitating the combustion reaction. As a result, a limit region and the like of the lean combustion or the diluted combustion can be extended.