Abstract:
The purpose of the present invention is to provide an electromagnetic wave emission device in which the destination to which an electromagnetic wave is supplied can be switched among a plurality of emission antennas, wherein there are no problems such as breakage of a switching element even in a case where a large-power electromagnetic wave is emitted. The present invention is an electromagnetic wave emission device characterized by the following: an electromagnetic wave output from an electromagnetic wave generator is distributed to a plurality of emission antennas, and then from among the plurality of emission antennas, a target antenna having plasma nearby is supplied with an electromagnetic wave distributed to the target antenna and all or part of the electromagnetic waves reflected by the emission antennas other than the target antenna; and the all or part of the electromagnetic waves distributed to the target antenna and all or part of the electromagnetic waves supplied to the target antenna after reflection by the emission antennas other than the target antenna are emitted from the target antenna to the plasma.

Description:
TECHNICAL FIELD 
     The present invention relates to an electromagnetic wave emission device that emits electromagnetic wave to plasma. 
     BACKGROUND ART 
     There is known an electromagnetic wave emission device that emits electromagnetic waves to plasma (see JP 2007-113570A). The electromagnetic wave emission device that is described in JP 2007-113570A equips an ignition device that generates a plasma discharge by emitting microwave to a combustion chamber prior to or after the ignition of air-fuel mixture. The ignition device generates a local plasma using discharge of a spark plug so that the plasma is generated in the high-pressure environment, and enhances this plasma using microwave. The local plasma is generated at a discharge gap between a ground terminal part and a tip part of an anode terminal. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP 2007-113570A 
     THE DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In some cases, multiple emission antennas are connected to a single electromagnetic wave generation device via a distribution device. This distribution device switches the emission antenna for supplying the electromagnetic wave by using switching elements such as diode. For purpose of plasma enlargement, for example, the electromagnetic wave that has a large electric power shall be emitted from the emission antenna when the electromagnetic wave generation device is used for enlarging plasma. However, the switching element may damage because of the large electric power electromagnetic wave that flows into the distribution device. 
     The present invention has been made in view of the above described circumstances, and the objective is to provide an electromagnetic wave generation device that can switch the supply destination of the electromagnetic wave among the multiple emission antennas, and that does not cause malfunction such as the switching element damages even when the electromagnetic wave of large electric power is emitted. 
     Means for Solving the Problems 
     The present disclosure relates to an electromagnetic wave emission device comprising: a plurality of emission antennas that emits the electromagnetic waves outputted from the electromagnetic wave generation device; and a transmission system including an electromagnetic wave transmission line that prolongs from an output terminal of the electromagnetic wave generation device and that branch to a plurality of the branching lines. Each of the plurality of the branching line connects to the emission antenna. The transmission system is configured such that the electromagnetic wave is supplied to a target antenna. The target antenna is an antenna where the plasma exists in the neighbor. To the target antenna, provided are; (i) the electromagnetic wave distributed for the target antenna and (ii) all or part of electromagnetic wave reflected in an antenna that is different from the target antenna. What is emitted from the target antenna to the plasma are; (i) all or part of electromagnetic wave that is distributed to the target antenna; and (ii) all or part of electromagnetic wave that is reflected in the antenna that is different from the target antenna. 
     According to the electromagnetic wave generation device of the present invention, the distributed electromagnetic wave reflects at the emission antenna which plasma does not exist in its neighbor. The distributed electromagnetic wave and the reflected electromagnetic wave are emitted to the plasma by the above-mentioned target antenna where the plasma exists in its neighbor. Therefore, the electromagnetic waves generation device of the present invention can switch the supplying destination of the electromagnetic wave among the plurality of the emission antenna even when the switching element is not equipped. 
     The electromagnetic wave emission device of the present invention is characterized in that the switching element is not connected to the branching line. 
     Preferably, the electromagnetic wave emission device of the present invention has a control part that controls an oscillation frequency of the electromagnetic wave generation device such that the electromagnetic wave energy reflected by the target antenna becomes small. The electromagnetic wave emission device of the present invention can supply energy efficiently to the plasma because the oscillating frequency can be controlled so that the electromagnetic wave energy reflected by the target antenna is reduced. This contributes to a cost reduction. 
     Preferably, the branching line includes a stub for performing an impedance matching. A control part shall be also equipped for controlling the stub included in the branching line connected to the target antenna among the stubs so that the electromagnetic wave energy reflected by the target antenna becomes small. The electromagnetic wave emission device of the present invention can supply energy efficiently to the plasma because the stub can be controlled so that the electromagnetic wave energy reflected by the target antenna is reduced. This contributes to cost reduction. 
     Preferably, in the electromagnetic wave emission device of the present invention, the plurality of the emission antenna is provided in each of the combustion chamber which equips a spark plug. The spark plug shall be arranged such that a spark is generated in the neighbor of the emission antenna. The spark timings of the spark plugs shall be shifted among the plurality of the combustion chamber. The emission antenna of the combustion chamber, where the spark is generated, becomes a target antenna when the electromagnetic wave emission device outputs the electromagnetic wave that synchronizes the spark timing of the spark plug of the combustion chamber, and the target antenna shall be switched when a sparking spark plug is switched. This configuration of the electromagnetic wave emission device of the present invention allows a convenient control of the combustion timing in each combustion chamber when it is applied to an automobile engine. 
     Preferably, in the electromagnetic wave emission device of the present invention, the plurality of the emission antennas is arranged so that the passage timing of the flame during the flame propagation differs mutually in the combustion chamber. The emission antenna, where the flame passes, becomes a target antenna among the plurality of the emission antennas when the electromagnetic wave generation device outputs the electromagnetic wave during the flame propagation, and the target antenna shall be switched conforming to the flame propagation. This configuration of the electromagnetic wave emission device of the present invention can promote the flame propagation in the combustion chambers and can improve the combustion efficiency when it is applied to an automobile engine. 
     Preferably, in the electromagnetic wave emission device of the present invention, the center electrode of the spark plug functions as an emission antenna. The structure inside the spark plug can be simplified because the center electrode functions as an emission antenna. 
     Preferably, L=nλ/4 shall be satisfied; where L stands for a distance between the branching point that branches to the plurality of the branching lines of the transmission system to the emission tip of the electromagnetic wave in the emission antenna; λ stands for the wavelength of the in the branching line; and n stands for natural number. This configuration of the electromagnetic wave emission device of the present invention allows improves emission efficiency from each emission antennas. 
     The electromagnetic wave emission device of the present invention can have a structure such that the distance between the branching point that branches to a plurality of the branching lines of the transmission system and the emission tip of the electromagnetic wave in the emission antenna differs among the plurality of the emission antennas. 
     Advantage of the Invention 
     According to the electromagnetic wave emission device of this disclosure, the supplying destination of the electromagnetic wave can be switched appropriately even when there is no switching element. Thus, use of this electromagnetic wave emission device allows reducing the loss due to damages of the switching elements even when high electric power electromagnetic wave is emitted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view of an internal combustion engine according to a first embodiment. 
         FIG. 2  is a front view of the ceiling surface of the internal combustion engine according to the first embodiment. 
         FIG. 3  is a block diagram of an electromagnetic wave emission device and the ignition device according to the first embodiment. 
         FIG. 4  is a schematic view of the electromagnetic wave emission device according to the first embodiment. 
         FIG. 5  is a front view of the ceiling surface of the internal combustion engine according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, a detailed description will be given by an embodiment of the present invention with reference to the accompanying drawings. It should be noted that the following embodiments are merely preferable examples, and do not limit the scope of the present invention, applied field thereof, or application thereof. 
     First Embodiment 
     The present embodiment relates to internal combustion engine  10  equipping electromagnetic wave emission device  13  of the present invention. Internal combustion engine  10  is a reciprocating type internal combustion engine where piston  23  reciprocates. Internal combustion engine  10  includes internal combustion engine body  11 , ignition device  12 , electromagnetic wave emission device  13 , and control device  35 . The combustion cycle i.e. ignition of air-fuel mixture using ignition device  12  and combustion of the air-fuel mixture are performed repeatedly in internal combustion engine  10 . 
     Internal Combustion Engine Body 
     As shown in  FIG. 1 , internal combustion engine body  11  includes cylinder block  21 , cylinder head  22  and piston  23 . Multiple cylinders  24  having circular cross sections are formed in cylinder block  21 . Piston  23  is formed in each cylinder  24  so as to reciprocate freely. Piston  23  is connected to a crankshaft via a connecting rod (not illustrated). The crankshaft is supported by cylinder block  21  so as to rotate freely. When piston  23  reciprocates in the axial direction of cylinder  24  in each cylinder  24 , the connecting rod converts a reciprocation of piston  23  to a rotation of the crankshaft. 
     Cylinder head  22  is placed on cylinder block  21  so as to sandwich a gasket  18 . Cylinder head  22  constitutes a compartment portion that defines combustion chamber  20  having a circular section together with cylinder  24 , piston  23 , and gasket  18 . The diameter of combustion chamber  20  is, for example, approximately a half of the wavelength of the microwave emitted to combustion chamber  20  from electromagnetic wave emission device  13 . 
     In cylinder head  22 , spark plug  40  constituting ignition device  12  is formed in each cylinder  24 . Tip part of spark plug  40 , exposing to combustion chamber  20 , is located in the central part of ceiling side  51  (a surface of cylinder head  22  that faces combustion chamber  20 ) of combustion chamber  20  as shown in  FIG. 2 . The periphery of the tip part of spark plug  40  is circular when it is viewed from the axial side. Center electrode  40   a  and earth electrode  40   b  are formed in the tip part of spark plug  40 . The discharge gap is formed between the tip of center electrode  40   a  and tip part of earth electrode  40   b.    
     Inlet port  25  and exhaust port  26  are formed for each cylinder  24  in cylinder head  22 . Air intake valve  27  that opens and closes air intake side opening  25   a  of inlet port  25 , and injector  29  that injects fuel are arranged in inlet port  25 . Exhaust valve  28  that opens and closes exhaust gas side opening  26   a  of exhaust port  26  is formed in exhaust port  26 . In internal combustion engine  10 , inlet port  25  is designed so that a strong tumble flow is formed inside combustion chamber  20 . 
     Ignition Device 
     Ignition device  12  is provided for every combustion chamber  20 . As shown in  FIG. 3 , each ignition device  12  includes ignition coil  14  that outputs high-voltage pulse, and spark plug  40  to where high-voltage pulse outputted from ignition coil  14  is supplied. 
     Ignition coil  14  is connected to DC (Direct Current) power supply (not illustrated). Ignition coil  14  amplifies the voltage applied from the DC power supply when an ignition signal is received from control device  35 . Then the amplified high-voltage pulse is outputted to center electrode  40   a  of spark plug  40 . When the high-voltage pulse is applied to center electrode  40   a , the dielectric breakdown occurs in the discharge gap and spark discharge arose in spark plug  40 . Discharge plasma is generated in the discharge pass of the spark discharge. 
     Electromagnetic Wave Emission Device 
     As shown in  FIG. 3 , electromagnetic wave emission device  13  includes electromagnetic wave generation device  31 , transmission system  32 , and multiple emission antennas  16   
     Electromagnetic wave generation device  31  outputs microwave pulses repetitively with a predetermined duty ratio when an electromagnetic wave drive signal is received from control device  35 . Electromagnetic wave drive signal is a pulse signal. Electromagnetic wave generation device  31  outputs microwave pulses repetitively during the pulse width period of the electromagnetic wave drive signal. A semiconductor oscillator generates microwave pulses in electromagnetic wave generation device  31 . Other types of oscillators, such as magnetron can be used instead of a semiconductor oscillator. 
     Emission antenna  16  is formed on ceiling side  51  of combustion chamber  20 . Emission antenna  16  is formed near spark plug  40  on ceiling side  51  of combustion chamber  20 . Each emission antenna  16  is insulated electrically from cylinder head  22 . 
     Emission antenna  16  is electrically connected to branching unit  34  through transmission line  33  of the microwave laid inside cylinder head  22 . 
     Transmission system  32  is configured such that the microwave outputted from electromagnetic wave generation device is transmitted to multiple emission antennas  16 . As shown in  FIG. 3 , transmission line of the microwave is prolonged from the output terminal of an electromagnetic wave generation device and is branched to multiple branching lines  36  corresponding to multiple emission antennas  16 . Each branch lines  36  are connected to emission antenna  16 , i.e. an emission antenna corresponding to the branching line). In transmission system  32 , some transmission lines including the branching point  37  branched to multiple branching line  36  are accommodated in branching unit  34 . In  FIG. 3 , branching unit  34  is indicated with wavy line. 
     Transmission system  32  does not have a distribution device that switches the emission antenna, which will be a supplying destination of microwave, among multiple emission antennas using of a switching element. That is, the switching element is not connected to each branching line. 
     As shown in  FIG. 4 , the total sum value L of the length of branching line  36  and the length of the emission antenna, i.e. the length from branching point  37  to the tip of emission antenna  16 , satisfies eq. 1 shown below. The same antennas are used for all of multiple antennas  16 .
 
 L=λ/ 4* n   (eq. 1)
 
     In eq. 1, λ stands for electric length of microwave in branching line  36  and n stands for natural number. In the present embodiment, the total sum values of the lengths of branching line  36  and emission antenna  16  differs mutually among multiple branching lines  36 . The length of each branching line  36  is set as to increase this total sum such that n increases in increment of one from the shortest branching lines  36  and the longest branching lines  36  (in other words, the total sum value of the lengths of branching line  36  and emission antenna  16  shall be increased in increment of λ/4). 
     The electromagnetic wave generation device  31  has three emission antennas  16 , i.e. first antenna, second antenna, and third antenna in this embodiment, but the number of the emission antennas can be two, or can be equal to or more than four. First branching line is connected to first antenna, second branching line is connected to second antenna, and third branching line is connected to third antenna. Total length L1 of the first branching line and the first antenna, total length L2 of the second branching line and the second antenna, and total length L3 of the third branching line and the third antenna satisfies eq. 2.
 
 L 1+(λ/4)*2= L 2+(λ/4)*2= L 3  (eq. 2)
 
     Operation of the Control Device 
     Operation of control device  35  will be discussed. 
     Control device  35  determines the ignition timing of each combustion chamber  20  from a control map based on the operating condition of internal combustion engine  20 , and controls each ignition device  12  provided for each combustion chamber  20  and electromagnetic wave emission device  13  formed one by one for multiple combustion chambers  20 . The ignition timings are shifted mutually between each combustion chamber  20 . 
     Control device  35  outputs an ignition signal to ignition device  12  provided in combustion chamber  20 , i.e. the combustion chamber where the first antenna is arranged, when the ignition timing is approached. When ignition device  12  receives an ignition signal, the spark discharge occurs in the discharge gap of spark plug  40  as mentioned above. 
     Control device  35  outputs an electromagnetic wave drive signal to electromagnetic wave emission device  13  corresponding to the output timing of the ignition signal. For example, the drive signal is outputted simultaneously with the output timing of the ignition signal, immediately after this output timing, or after a predetermined period from the ignition timing. The output timing of the electromagnetic wave drive signal can be adjusted suitably according to the condition of combustion area. When the electromagnetic wave drive signal is received, electromagnetic wave generation device  31  outputs microwave pulses repetitively during the electromagnetic wave emission period. Control device  35  outputs the electromagnetic wave drive signal to electromagnetic wave emission device  13  corresponding to the output timing of the ignition signal for each of multiple combustion chambers  20 . 
     The microwave emitted from electromagnetic wave generation device  31  branches to multiple emission antennas  16  at branching point  37  of transmission system  32 . Among the multiple emission antennas  16 , in emission antenna  16  where the plasma (spark discharge or flame) exists nearby, microwave is not emitted (this kind of emission antenna  16  is referred to as “target antenna”). On the other hand, in the other emission antenna  16  where the plasma does not exist nearby, microwave is not emitted and reflective wave is seen. 
     Hereafter, a first antenna refers to an antenna where the plasma due to spark discharge exists near the tip part of the antenna, and second and third antennas that are different from the first antenna refers to the antennas where the spark does not exist near the tip part of the antenna. The first, second, and third antennas are located in, for example, in first, second and third cylinder respectively. In this case, the distance between the tip part of the first antenna and the plasma is extremely short compared to the electrical length of the microwave because the plasma exists near the tip part of the first antenna. Therefore, it can be assumed that the plasma which is a load exists near the tip part of the first antenna. 
     The energy can be distributed efficiently to each transmission lines that correspond to first, second and third antennas when the impedance of a power supply and a microwave source, and the impedance of a load circuit are equal. For example, when the impedance of the power supply is 50 ohms, the maximum output can be supplied when the impedance of the load circuit is 50 ohms. The impedance of the load circuit corresponding to the tip of the first antenna is 50 ohms because the plasma exists at the tip of the first antenna and the plasma does not exist at the tips of the second and third antennas. On the contrary, the impedance of the load circuit corresponding to the second and third antennas becomes infinity. Therefore, the energy is supplied to the first antenna while the energy is not supplied to the second antenna and third antenna. As a result, the transmitted microwave is reflected and is supplied to the first antenna. 
     The microwave distributed to the first antenna is absorbed by the plasma because the plasma exists near the first antenna. On the contrary, the plasma does not exist near the tip of the second antenna and third antennas, and the reflected microwave is thus supplied to the first antenna via branching point  37 . As a result, the microwave distributed to the first antenna and the microwave reflected at emission antennas  16  are provided to the first antenna. Thus, the energy of the microwave outputted from electromagnetic wave generation device  31  is concentrated on the first antenna. In this case, the above mentioned equations 1 and 2 are satisfied so that the reflective waves from the second and third antennas are emitted efficiently from the first antenna. 
     The microwave distributed to the first antenna and the microwave supplied to the first antenna, after the reflection at the second antenna and third antenna, are emitted from the first antenna to the plasma. Plasma enlarges by absorbing the microwave energy. A lean air-fuel mixture is ignited by this enlarged plasma. In combustion chamber  20 , the flame spreads from the position where the air-fuel mixture is ignited. 
     When the other combustion chamber  20 , e.g. the second cylinder provided with the second antenna encounters an ignition timing, control device  35  outputs an ignition signal to ignition device  12  located in combustion chamber  20  and outputs electromagnetic wave drive signal to electromagnetic wave emission device  13 . In combustion chamber  20 , spark discharge occurs by ignition device  12 . The microwave is emitted from the second antenna where the plasma such as spark discharge or flame exists in the neighbor among the multiple emission antennas  16 , while the microwave turns to be a reflection wave and is not emitted from the first and third antennas where the plasma does not exist in the neighbor. The microwave energy that is outputted from electromagnetic wave generation device  31  is concentrated on the second antenna. The microwave distributed to the second antenna, and the microwave supplied to the second antenna after the reflection at the emission antennas other than the second antenna, are emitted to plasma from the second antenna. 
     Control device  35  is controlled so that the ignition timing of ignition device  12  can be shifted mutually between each cylinder, and is controlled so that electromagnetic wave is emitted from emission antenna  16  from an adequate cylinder according to the ignition timing. 
     Advantage of the First Embodiment 
     In the present embodiment, the microwave energy outputted from one electromagnetic wave generation device  31  can be concentrated to one emission antenna  16  (target antenna) without use of branching device, and the microwave energy can be supplied to plasma from this emission antenna  16 . That is, when the electromagnetic wave is outputted from electromagnetic wave generation device  31  and is emitted from emission antenna  16 , the electromagnetic wave is absorbed by the plasma and the plasma is enlarged in the cylinder, among all cylinders, where the plasma such as spark discharge and flame exists in combustion chamber  20 . In the cylinder where the plasma does not exists, the electromagnetic waves outputted from electromagnetic wave generation device  31  is not emitted from emission antenna  16  and reflects to branching point  37  as a reflective wave. This reflective wave is emitted from emission antenna  16  of a cylinder where the plasma exists, and can promote the enlargement of this plasma. The electromagnetic wave emission device according to the present embodiment can thus switch the supplying destination of the electromagnetic waves among multiple emission antennas even when a switching element is not equipped. Therefore, the use of the electromagnetic wave emission device of this embodiment avoids a loss due to damage of switching element even when the high power electromagnetic wave is emitted. 
     Second Embodiment 
     The present embodiment relates to internal combustion engine  10  comprising electromagnetic wave emission device  13  of the present invention as well as the first embodiment. In this embodiment multiple emission antennas  16  are provided in a single combustion chamber  20  in contrast to the first embodiment. 
     In combustion chamber  20 , multiple emission antennas  16  are arranged on the ceiling surface  51  of combustion chamber  20  so that the flame front, which is spreading from the neighbor of the spark plug, passes near the antenna at different timing. 
     In this embodiment, nine emission antennas  16  are provided in combustion chamber  20 . The number of emission antennas  16  can be eight or less, or can be ten or more. Emission antenna  16   a  is located near the spark plug. The location or geometry of emission antenna  16   a  shall not be limited specifically as long as this antenna is provided near the spark plug. However, this antenna can be a ring like formed as shown in  FIG. 5  so as to surround the center electrode. Emission antenna  16   b  is located on a concentric circle centered by the spark plug and is located in the inner wall side of combustion chamber  20  compared to emission antenna  16   a . The emission antenna  16   c  is located on the concentric circle centered by the spark plug and is located further in the inner wall side of combustion chamber  20  compared to emission antenna  16   b . The flame front that spreads from the neighbor of the spark plug passes emission antennas  16   a ,  16   b , and  16   c  in this order. 
     Control device  35  outputs an ignition signal to ignition device  12  located in combustion chamber  20  when the ignition timing approaches. In ignition device  12 , the spark discharge occurs in the discharge gap of spark plug  40  when the ignition signal is received. Control device  35  outputs an electromagnetic wave drive signal to electromagnetic wave emission device  13  in accordance with the output timing of the ignition signal (for example, the electromagnetic wave drive signal is output ted simultaneously with the output timing of the ignition signal, immediately after the output timing, or a predetermined time after the output timing). Electromagnetic wave generation device  31  repetitively outputs the microwave pulse when the electromagnetic wave drive signal is received. 
     In the multiple emission antennas  16 , microwave is emitted from emission antenna  16   a , where the plasma (spark discharge) exists near emission antenna  16 , while the microwave is reflected at emission antennas  16   b  and  16   c  where the plasma does not exists in the neighbor. The microwave energy outputted from electromagnetic wave generation device  31  is concentrated on emission antenna  16   a . The microwave distributed to emission antenna  16   a  and the microwave supplied to emission antenna  16   a  after reflection at an emission antennas other than emission antenna  16   a  are emitted from emission antenna  16   a  to plasma. Plasma absorbs microwave energy emitted from emission antenna  16   a  and enlarges. The lean air-fuel mixture is ignited by this enlarged plasma. The flame front spreads from this air-fuel ignition position in combustion chamber  20 . 
     In this embodiment, electromagnetic wave generation device  31  continuously outputs the microwave pulse at least until the flame front finishes passing emission antenna  16   c . The microwave energy outputted from electromagnetic wave generation device  31  thus concentrates on emission antenna  16   b  when the flame passes emission antenna  16   b . In this case, microwave is emitted only from emission antenna  16   b . The flame velocity increases when the flame receives microwave energy from emission antenna  16   b . When the flame passes emission antenna  16   c , the microwave energy outputted from electromagnetic wave generation device  31  is concentrated on emission antenna  16   c , and microwave is emitted only from emission antenna  16   c . The flame velocity increases when the flame receives microwave energy from emission antenna  16   c.    
     The output maintenance time of the microwave from electromagnetic wave generation device  31  can be predetermined or the output of the microwave from electromagnetic wave generation device  31  can be suspended after the flame is detected to be passed the emission antenna  16   c  using the measurement result of microwave reflectance in each emission antenna. For example, electromagnetic wave emission device  13  determines that the flame finished passing emission antenna  16   c  when the reflectance of the microwave in all emission antennas  16  exceeds a predetermined threshold value for a predetermined period based on the measurement result of the microwave reflectivity in each emission antennas  16 , and then suspends the microwave output from electromagnetic wave generation device  31 . 
     The microwave energy (power) outputted from electromagnetic wave generation device  31  can be reduced while the flame front does not pass any of emission antennas  16   b  or  16   c  based on the measurement result of the microwave reflectivity in each emission antenna  16 . For example, electromagnetic wave emission device  13  reduces microwave energy outputted from electromagnetic wave generation device  31 , when the microwave reflectivity exceeds predetermined threshold value. 
     Advantage of the Second Embodiment 
     According to this embodiment, the emission antenna for emitting the microwave is switched automatically in accordance with the movement of flame front. Therefore, the microwave can promote the flame propagation with a convenience structure. 
     Other Embodiment 
     In each of above embodiments, control device  35  can control oscillation frequency of electromagnetic wave generation device  31  so that the reflected electromagnetic wave energy in the target antenna becomes small. In combustion chamber  20 , in which the plasma exists near emission antenna  16 , not all electromagnetic wave that is outputted from electromagnetic wave generation device  31  is not emitted from emission antenna  16  (target antenna), and some of them becomes a reflection wave depending on a condition of the pressure or temperature inside the combustion chamber, or intensity of existing plasma. Control part  35  of electromagnetic wave emission device of this embodiment may detect the reflection wave energy and control the frequency of the electromagnetic waves that is generated from the electromagnetic wave generation device so that the energy of the reflective wave becomes small. In this case, electromagnetic wave generation device  31  is configured so that the oscillating frequency can be adjusted continuously between a first setting value f1 (f1=f−X) in a lower frequency and a second setting value f2 (f2=f+X) in a higher frequency centering frequency f=2.45 GHz. X (Hz) is between several to several tens (Hz). Control device  35  scans the frequency of microwave when electromagnetic wave generation device  31  receives the electromagnetic wave drive signal. Control device  35  sets the oscillating frequency of microwave to a frequency where the reflectance of the microwave becomes the minimum in the target antenna (here, the target antenna refers to an antenna where the plasma exists in the neighbor, and specifically the antenna where the reflectance of microwave becomes the minimum among the multiple emission antennas). Electromagnetic wave emission device  13  can have multiple electromagnetic wave generation devices  31  whose oscillating frequencies are mutually different and the frequency of the microwave emitting to combustion chamber  20  can be adjusted by switching the active electromagnetic wave generation device  31 . 
     In this embodiment, the electromagnetic wave energy can be absorbed efficiently in accordance with the fluctuating plasma when control device  35  controls the oscillation frequency of electromagnetic wave generation device  31  so that electromagnetic waves reflected in the target antenna becomes small. 
     The other embodiments can be contemplated. Center electrode  40   a  of spark plug  40  of the above-mentioned embodiment may serve as the emission antenna of electromagnetic waves. Center electrode  40   a  of the spark plug  40  is connected electrically to the output terminal of a mixing circuit. This mixing circuit receives a high-voltage pulse from ignition coil  14  and an electromagnetic wave from electromagnetic wave generation device  31  from separate input terminals and outputs the high-voltage pulse and the electromagnetic wave from the same output terminal. 
     Emission antenna  16  may be covered by an insulator or a dielectric in the above-mentioned embodiment. Receiving antenna  52  may be covered by the insulator or the dielectric. 
     Multiple emission antennas  16  may be arranged on the piston top or on the gasket. The geometry of emission antenna  16  can be a ring-like form. 
     INDUSTRIAL APPLICABILITY 
     According to the electromagnetic wave emission device of the present invention, the supplying destination of the microwave can be switched among the multiple emission antennas even when no switching element exists. Therefore, the use of this electromagnetic wave emission device can reduce a loss due to damage of the switching element even when the high power electromagnetic wave is emitted. This electromagnetic wave emission device can be used conveniently as an electromagnetic wave emission device which emits electromagnetic waves to plasma in combustion chambers of gasoline engines or diesel engines. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           10  Internal combustion engine 
           11  Internal combustion engine body 
           12  Ignition device 
           13  Electromagnetic wave emission device 
           16  Emission antenna 
           20  Combustion chamber 
           35  Control device