Abstract:
An internal combustion engine includes an internal combustion engine body formed with a combustion chamber, and an ignition device to ignite an air-fuel mixture in the combustion chamber. Repetitive combustion cycles, including the ignition of the air-fuel mixture by the ignition device and combustion of the air-fuel mixture, are executed. The internal combustion engine further has an electromagnetic (EM) wave-emitting device that emits EM radiation to the combustion chamber; a plurality of receiving antennas located on an outer circumference side of the zoning material that defines the combustion chamber; antenna which resonate with to the EM radiation that is emitted into the combustion chamber from the EM-wave-emitting device; and a control means which controls the EM-wave-emitting device such that the radiating antenna emits EM radiation into the combustion chamber while a flame caused by the ignition of the air-fuel mixture propagates.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an internal combustion engine that promotes combustion of an air-fuel mixture using electromagnetic (EM) radiation. 
       BACKGROUND 
       [0002]    An internal combustion engine that uses EM radiation to promote combustion of an air-fuel mixture is known. For example, JP 2007-113570A1 describes such an internal combustion engine. 
         [0003]    The internal combustion engine described in JP 2007-113570A1 is equipped with an ignition device that generates plasma discharge by emitting microwaves in a combustion chamber before or after ignition of an air-fuel mixture. The ignition device generates local plasma using the discharge from an ignition plug such that plasma is generated in a high-pressure field, and develops this plasma using microwave radiation. The local plasma is generated in a discharge gap between the tip of an anode terminal and a ground terminal. 
         [0004]    In a conventional internal combustion engine, plasma is generated near the ignition plug by microwave radiation emitted following the ignition of an air-fuel mixture. Thus, it was difficult to increase the propagation speed of a flame passing the center portion of the combustion chamber where the ignition plug is located. For example, the flame may not reach the wall face of the combustion chamber when the air-fuel mixture is lean and the propagation speed of the flame is slow, thereby emitting a substantial amount of unburned fuel. 
       SUMMARY OF INVENTION 
       [0005]    The first invention relates to an internal combustion engine including an internal combustion engine body formed with a combustion chamber, and an ignition device to ignite the air-fuel mixture in the combustion chamber. Repetitive combustion cycles, including ignition of the air-fuel mixture by the ignition device and combustion of the air-fuel mixture, are executed. The internal combustion engine comprises: an EM-wave-emitting device that emits EM radiation to the combustion chamber; a plurality of receiving antennas, located at the outer circumference of the zoning material that defines the combustion chamber; an antenna that resonates at the frequency of the EM radiation emitted into the combustion chamber from the EM-wave-emitting device; and a control means which controls the EM-wave-emitting device such that the radiating antenna emits EM radiation into the combustion chamber while the flame caused by ignition of the air-fuel mixture propagates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a longitudinal sectional view of an internal combustion engine according to one embodiment. 
           [0007]      FIG. 2  shows a front view of the ceiling surface of the combustion chamber of the internal combustion engine according to one embodiment. 
           [0008]      FIG. 3  shows a block diagram of an ignition device and an EM-wave-emitting device according to one embodiment. 
           [0009]      FIG. 4  shows a front view of the top surface of a piston according to one embodiment. 
           [0010]      FIG. 5  shows a longitudinal sectional view of a portion of an internal combustion engine with a different structure according to one embodiment. 
           [0011]      FIG. 6  shows a front view of a top surface of a piston of the different structure according to one embodiment. 
           [0012]      FIG. 7  shows a longitudinal sectional view of a portion of an internal combustion engine according to the second modification. 
           [0013]      FIG. 8  shows a longitudinal sectional view of a portion of an internal combustion engine according to the third modification. 
           [0014]      FIG. 9  shows a longitudinal sectional view of a portion of an internal combustion engine according to the fourth modification. 
           [0015]      FIG. 10  shows a longitudinal sectional view of a portion of an internal combustion engine according to the sixth modification. 
           [0016]      FIG. 11  shows a longitudinal sectional view of a portion of an internal combustion engine according to the seventh modification. 
           [0017]      FIG. 12  shows a longitudinal sectional view of a portion of an internal combustion engine according to the eighth modification. 
           [0018]      FIG. 13  shows a longitudinal sectional view of a piston according to the ninth modification. 
           [0019]      FIG. 14  shows a front view of a piston according to the tenth modification. 
           [0020]      FIG. 15  shows a front view of a piston of the different structure according to the tenth modification. 
           [0021]      FIG. 16  shows a front view of a piston according to the eleventh modification. 
           [0022]      FIG. 17  shows a longitudinal sectional view of a piston according to the twelfth modification. 
           [0023]      FIG. 18  shows a longitudinal sectional view of a piston of the different structure according to the twelfth modification. 
           [0024]      FIG. 19  shows a front view of a piston according to the thirteenth modification. 
           [0025]      FIG. 20  shows a front view of a piston according to the fourteenth modification. 
           [0026]      FIG. 21  shows a front view of a piston according to the fifteenth modification. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    The embodiments of the present invention are detailed with reference to the accompanying drawings. The embodiments below are the preferred embodiments of the present invention but they are not intended to limit the scope of invention and application or usage thereof. 
         [0028]    The present embodiment relates to internal combustion engine  10  of the present invention. Internal combustion engine  10  is a reciprocating internal combustion engine where piston  23  reciprocates. Internal combustion engine  10  has internal combustion engine body  11 , ignition device  12 , EM-wave-emitting device  13 , and control device  35 . In internal combustion engine  10 , the combustion cycle is repetitively executed by ignition device  12  to ignite and burn the air-fuel mixture. Internal combustion engine body 
         [0029]    As illustrated in  FIG. 1 , internal combustion engine body  11  has cylinder block  21 , cylinder head  22 , and piston  23 . Multiple cylinders  24 , each having a rounded cross-section, are formed in cylinder block  21 . Reciprocal pistons  23  are located in each cylinder  24 . Pistons  23  are connected to a crankshaft through a connecting rod (not shown in the figure). The rotatable crankshaft is supported on cylinder block  21 . The connecting rod converts reciprocations of pistons  23  to rotation of the crankshaft when pistons  23  reciprocate in each cylinder  24  in the axial direction of cylinders  24 . Cylinder head  22  is located on cylinder block  21  sandwiching gasket  18  in between. Cylinder head  22  forms a circular-sectioned combustion chamber  20  together with cylinders  24 , pistons  23 , and gasket  18 . The diameter of combustion chamber  20  is approximately half the wavelength of the microwave radiation emitted from EM-wave-emitting device  13 . 
         [0030]    A single ignition plug  40 , which is a part of ignition device  12 , is provided for each cylinder  24  of cylinder head  22 . In ignition plug  40 , a front-tip part exposed to combustion chamber  20  is placed at the center part of the ceiling surface  51  of combustion chamber  20 . Surface  51  is exposed to combustion chamber  20  of cylinder head  22 . The circumference of the front-tip part is circular when it is viewed from the axial direction. Center electrode  40   a  and earth electrode  40   b  are formed on the tip of the ignition plug  40 . A discharge gap is formed between the tip of center electrode  40   a  and the tip of earth electrode  40   b.    
         [0031]    Inlet port  25  and outlet port  26  are formed for each cylinder  24  in cylinder head  22 . Inlet port  25  has inlet valve  27  for opening and closing an inlet port opening  25   a  of inlet port  25  and injector  29 , which injects fuel. Outlet port  26  has outlet valve  28  for opening and closing an outlet port opening  26   a  of outlet port  26 . Inlet port  25  is designed so that a strong tumble flow is formed in combustion chamber  20  in internal combustion engine  10 . 
       Ignition Device 
       [0032]    Ignition device  12  is provided for each combustion chamber  20 . As illustrated in  FIG. 3 , each ignition device  12  has ignition coil  14  to output a high-voltage pulse, and ignition plug  40 , which receives the high-voltage pulse outputted from ignition coil  14 . 
         [0033]    Ignition coil  14  is connected to a direct current (DC) power supply (not shown in the figure). Ignition coil  14  boosts the voltage applied from the DC power when an ignition signal is received from control device  35 , and then outputs the amplified high-voltage pulse to center electrode  40   a  of ignition plug  40 . In ignition plug  40 , dielectric breakdown occurs at the discharge gap when a high-voltage pulse is applied to center electrode  40   a . A spark discharge then occurs, and discharge plasma is generated in the discharge channel. A negative voltage is applied as the high-voltage pulse at center electrode  40   a.    
         [0034]    Ignition device  12  may have a plasma-enlarging component, which enlarges the discharge plasma by supplying electrical energy to the discharge plasma. The plasma-enlarging component may, for example, enlarge the spark discharge by supplying energy of high-frequency wave, e.g. microwave radiation to the discharge plasma. The plasma-enlarging component allows for improvements in the stability of the ignition of a lean air-fuel mixture. EM-wave-emitting device  13  may be used as the plasma-enlarging component. 
       Electromagnetic Wave-Emitting Device 
       [0035]    As illustrated in  FIG. 3 , EM-wave-emitting device  13  has EM-wave-generating device  31 , EM-wave-switching device  32  and radiating antenna  16 . One EM-wave-generating device  31  and one EM-switching device  32  are provided for each EM-wave-emitting device  13 . Radiating antennas  16  are provided for each combustion chamber  20 . 
         [0036]    EM-wave-generating device  31  iteratively outputs current pulses at a predetermined duty ratio when an EM-wave-driving signal is received from control device  35 . The EM-wave-driving signal is a pulsed signal. EM-wave-generating device  31  iteratively outputs microwave pulses during the pulse-width time of the driving signal. In EM-wave-generating device  31 , a semiconductor oscillator generates microwave pulses. Other oscillators, such as a magnetron, may also be used instead of a semiconductor oscillator. 
         [0037]    EM-wave-switching device  32  has one input terminal and multiple output terminals provided for each radiation antenna  16 . The input terminal is connected to EM-wave-generating device  31 . Each of the output terminals is connected to the corresponding radiation antenna  16 . EM-wave-switching device  32  is controlled by control device  35  so that the destination of the microwaves outputted from generating device  31  switches between the multiple radiation antennas  16 . 
         [0038]    Radiation antenna  16  is located on ceiling surface  51  of combustion chamber  20 . Radiation antenna  16  is ring-shaped in form when it is viewed from the front side of ceiling  51  of combustion chamber  20 , and it surrounds the tip of ignition plug  40 . Radiation antenna  16  can also be C-shaped when it is viewed from the front side of ceiling  51 . 
         [0039]    Radiation antenna  16  is laminated on ring-shaped insulating layer  19  formed around an installation hole for ignition plug  40  on ceiling surface  51  of combustion chamber  20 . Insulating layer  19  may, for example, be formed by the spraying of an insulating material. Radiation antenna  16  is electrically insulated from cylinder head  22  by insulating layer  19 . The perimeter of radiation antenna  16 , i.e., the perimeter of the centerline between the inner circumference and the outer circumference, is set to half the wavelength of the microwave radiation emitted from radiation antenna  16 . Radiation antenna  16  is electrically connected to the output terminal of EM-wave-switching device  32  via microwave transmission line  33  located in cylinder head  22 . 
         [0040]    In internal combustion engine body  11 , multiple receiving antennas  52   a  and  52   b  resonate with the microwave radiation emitted into combustion chamber  20  from EM-wave-emitting device  13 , and are provided on a zoning material defining combustion chamber  20 . In this embodiment, receiving antennas  52   a  and  52   b  are located close to the outer circumference. Here, “close to the outer circumference” refers to the area outside the mid-point of the center and outer circumference of the top of piston  23 . The period of time when the flame propagates to this area is referred to as the “second half of the flame propagation”. The length L of antenna  52  satisfies Eq. 1, where the wavelength of the microwave radiation is A, and n is a natural number. 
         [0000]        L =( n ×λ)/2  (Eq. 1)
 
         [0041]    Receiving antennas  52   a  and  52   b  are located close to the outer circumference of the top of piston  23 , as shown in  FIGS. 1 and 4 . Here, “close to the outer circumference” refers to the area outside the mid-point of the center and outer circumferences of the top of piston  23 . 
         [0042]    Receiving antennas  52   a  and  52   b  are annular in shape and are concentric with the center axis of piston  23 . The diameters of the two receiving antennas  52   a  and  52   b  are different, and they are located such that a double ring is formed. Receiving antennas  52   a  and  52   b  are arranged in a co-axial fashion. The first receiving antenna  52   a  is located at the outer side and the second receiving antenna  52   b  is located at the inner side. The distance x between antennas  52   a  and  52   b  satisfies Eq. 2, where λ is the wavelength of the microwave radiation emitted from radiation antenna  16  to combustion chamber  20 . 
         [0000]      λ/16≦×≦2λ/3  (Eq. 2)
 
         [0043]    Receiving antennas  52   a  and  52   b  are located on insulating layer  56  formed on the top of piston  23 , i.e., the combustion-chamber-side surface of the zoning material. Receiving antennas  52   a  and  52   b  are electrically insulated from piston  23  using insulating layer  56 , and are provided in an electrically floating state. 
         [0044]    The number of receiving antennas  52  provided on the top of piston  23  as shown in  FIG. 5  may be one. 
         [0045]    Regardless of the number of receiving antennas  52  on piston  23 , the center of antenna  52  may be shifted from the center axis of piston  23 . For example, the center of receiving antenna  52  may be shifted to the exhaust side from the center of piston  23 , as shown in  FIG. 6 . In such a case, the flame front passes the exhaust side and the intake side of receiving antenna  52  almost simultaneously during the microwave radiation period. 
         [0046]    Annular receiving antennas  52   a  and  52   b  do not have to be allocated concentrically. For example, the center of antenna  52   b  located inner side may be shifted toward intake-side opening  25   a . In this case, the distance between the antennas  52   a  and  52   b  becomes shorter as approaching the intake-side opening  25   a . This increases the strength of the electric field at intake-side opening  25   a.    
       Operation of the Control Device 
       [0047]    Here, the operation of control device  35  will be described. Control device  35  executes a first operation directing ignition device  12  to ignite the air-fuel mixture, and a second operation directing EM-wave-emitting device  13  to emit microwaves following the ignition of the air-fuel mixture in one combustion cycle for each combustion chamber  20 . 
         [0048]    In other words, control device  35  executes the first operation immediately prior to piston  23  reaching top dead center (TDC). Controller  35  outputs an ignition signal as the first operation. 
         [0049]    As described above, a spark discharge occurs in the discharge gap of ignition plug  40  in ignition device  12  when an ignition signal is received. The air-fuel mixture is ignited by the spark discharge. When the air-fuel mixture is ignited, a flame grows from the igniting position of the air-fuel mixture in the center part of combustion chamber  20  to the wall face of cylinder  24 . 
         [0050]    Control device  35  executes the second operation after the ignition of the air-fuel mixture, i.e., at the start of the second half of the flame propagation. Control device  35  outputs an EM-wave-driving signal as the second operation. 
         [0051]    EM-wave-emitting device  13  repeatedly outputs microwave pulses from radiating antenna  16  when the EM-wave-driving signal is received. Microwave pulses are emitted repetitively throughout the second half of the flame propagation. 
         [0052]    The microwave pulses resonate in each receiving antenna  52 . In the area close to the outer circumference of combustion chamber  20 , where the two receiving antennas  52  are located, an intense electric field is formed during the second half of the flame propagation. The propagation speed of the flame increases due to absorption of the microwave radiation when the flame passes the intense electric field. 
       Advantage of the Embodiment 
       [0053]    In this embodiment, an intense electric field is formed close to the outer circumference of combustion chamber  20  during flame propagation. This allows for an increase in the propagation speed of the flame close to the outer circumference of combustion chamber  20 . 
       Modification 1 
       [0054]    In the first modification, EM-wave-emitting device  13  is provided such that plasma is generated by microwave radiation emitted from radiation antenna  16 . The energy per unit time of the microwave radiation from EM-wave-generating device  31  is set such that microwave plasma is generated near each receiving antenna  52  via absorption of the microwave radiation emitted from radiation antenna  16 . 
         [0055]    EM-wave-emitting device  13  continuously emits microwave pulses throughout the second half of the flame propagation period. Plasma is generated near each receiving antenna  52  during the second half of the flame propagation period. In the area where the plasma is generated, active species, such as OH radicals, are produced. The propagation speed of the flame thereby increases in this area. 
         [0056]    EM-wave-emitting device  13  may repeatedly emit microwave pulses during the first half of the flame propagation period. In such a case, the microwave plasma is generated by the microwave radiation during the first half of the flame propagation period. The flame propagation speed in the area close to the circumference of combustion chamber  20  increases due to the production of active species in the first half of the flame propagation period. 
         [0057]    Internal combustion engine  10  may have a discharge device so that discharge occurs close to the circumference of combustion chamber  20  in order to reduce the power of the microwave radiation emitted from radiation antenna  16 . For example, the discharge device may cause the discharge by applying a high-voltage pulse between a pair of electrodes. In this case, one electrode (referred to as the first electrode) is located on cylinder head  22  and a second electrode is located on the upper surface of piston  23 . The second electrode is located in the top portion of the convex portion of the top side of piston  23  so that the distance between the first and second electrodes may be reduced. 
       Modification 2 
       [0058]    In the second modification, multiple receiving antennas  52  are located concentrically on the top surface of piston  23 , as shown in  FIG. 7 . Each receiving antenna  52  has different resonance frequencies. EM-wave-generating device  31  varies the frequency of the emitted microwave radiation such that receiving antenna  52  located at inner portion of the ring resonates first during the flame propagation. A strong electric field is sequentially formed in the neighborhood of receiving antennas  52 . The propagation speed of the flame increases near each receiving antenna  52 . 
         [0059]    In the second modification, inner-side insulation layer  56   b  is laminated with second receiving antenna  52   b , and therefore is thicker than outer-side insulation layer  56   a , which is laminated with first receiving antenna  52   a.    
       Modification 3 
       [0060]    In the third modification, receiving antenna  52  is grounded via a diode, as shown in  FIG. 8 . In this embodiment, only second receiving antenna  52   b  is grounded using a diode. However, either only first receiving antenna  52   a  or both antennas  52   a  and  52   b  may be grounded using a diode. 
         [0061]    The third modification allows inducing an ion of polarity opposite to second receiving antenna  52   b , that is in a flame, due to fact the signal in grounded antenna  52   b  may be a DC signal. The propagation speed of the flame is thereby increased. 
       Modification 4 
       [0062]    In the fourth modification, annular receiving antenna  52  is located in the inner part of gasket  18 , as shown in  FIG. 9 .  FIG. 9  shows single annular receiving antenna  52  provided in gasket  18 . Instead, multiple annular antennas  52  may be provided at intervals in the thickness direction of gasket  18 . Receiving antenna  52  may be provided on the top surface of piston  23  in addition to those in gasket  18 . 
       Modification 5 
       [0063]    In the fifth modification, receiving antenna  52  is located on the inner side of a constricted flow area. The microwave plasma generated near receiving antenna  52  thereby moves inside due to the constricted flow. Activated species produced in the plasma area are thereby diffused. 
       Modification 6 
       [0064]    In the sixth modification, receiving antenna  52  is located in insulating layer  56 , as shown in  FIG. 10 . Insulating layer  56  may, for example be formed of a ceramic material. 
         [0065]    In the cross-sectional surface of insulating layer  56 , where receiving antenna  52  is installed, coating layer  56   a  is formed from an insulating material. Receiving antenna  52  and supporting layer  56   b  are also formed from an insulating material and are stacked in sequence from the side of combustion chamber  20 . Supporting layer  56  is laminated on a zoning material, such as pistons  23 . 
         [0066]    In the sixth modification, coating layer  56   a  is thinner than supporting layer  56   b . This prevents a decrease in the electric field at the side of combustion chamber  23  when receiving antenna  52  is protected using the insulating material. 
       Modification 7 
       [0067]    In the seventh modification, two receiving antennas  52  are installed on the top of piston  23 , as shown in  FIG. 11 . The receiving antennas  52  are covered with coating layer  56   a . The thickness of coating layer  56   a  is reduced going from the inside to the outside of combustion chamber  20 . On coating layer  56   a , which coats the receiving antennas  52 , the electric field increases at the outer side compared with the inner side when microwave radiation is emitted into combustion chamber  20 . This allows for an increase in the propagation speed of the flame at the outer side of combustion chamber  20 . 
       Modification 8 
       [0068]    In the eighth modification, insulation layer  56  is located in trench  70  formed on piston  23  (the zoning material) along the circumference of combustion chamber  20 . As shown in  FIG. 12 , receiving antenna  52  is elongated along trench  70  between inner wall  121  and outside wall  122  of trench  70 . When the microwave radiation is emitted from radiation antenna  16 , an electric field is formed in the vertical direction in the inner side and outer side of receiving antenna  52  between antenna  52  and wall face  121  or  122 . This allows for an increase in the propagation speed of the flame via the electric field near receiving antenna  52 . 
         [0069]    In the eighth modification, the distance A between the outer circumference of receiving antenna  52  and outer wall  122  of trench  70  is shorter than the distance B between the inner circumference of receiving antenna  52  and inner wall  121  of trench  70 . This allows for an increase in the propagation speed of the flame front near the wall of combustion chamber  20  because the electric field is stronger at the outer side than the inner side of receiving antenna  52 . 
       Modification 9 
       [0070]    In the ninth modification, two ring-shaped receiving antennas  52  are located in ring-shaped insulation layer  56 , which is laminated on piston  23  (the zoning material) at intervals in the thickness direction of insulation layer  56 , as shown in  FIG. 13 . 
         [0071]    In insulation layer  56 , two receiving antennas  52  are connected to each other, at least at one location, using pressure equalizing conductor  80 , whereby conductor  80  equalizes the pressure at the connection. In the ninth modification, conductor  80  is located between two receiving antennas  52 , at intervals of the quarter wavelength of the microwave radiation in the circumferential direction of receiving antenna  52 . 
         [0072]    Ring-shaped receiving antennas  52  may be allocated in gasket  18  in a multilayer configuration. Receiving antennas  52  are provided in the thickness direction of gasket  18 , which is formed of insulating materials at intervals. Pressure equalizing conductor  80  may be also used in such a case. 
       Modification 10 
       [0073]    In the tenth modification, annular receiving antenna  52  has a different cross-sectional area in the conducting material that constitutes receiving antenna  52  in the circumferential direction. In this modification, convex portion  120  is provided in receiving antenna  52  such that portion  120  protrudes toward piston  23  at regular intervals. The cross-sectional surface area of the conductor varies in convex portion  120 . In receiving antenna  52 , the thickness of convex portion  120  is large compared to the separation between convex portions  120 . The tenth modification allows for a particular electric field distribution to form on receiving antenna  52  when microwave radiation is emitted from radiation antenna  16 . 
         [0074]    The cross-sectional surface area of the conductor may be altered by varying the width of receiving antenna  52 . For example, receiving antenna  52  may be formed in a gear-like fashion when viewed from above. The cross-sectional surface area of the conductor may be varied by allocating disc portion  140  having a diameter larger than the width of adjacent portion  141  in receiving antenna  52 , as shown in  FIG. 15 . The cross-sectional surface area of the conductor constituting antenna  52  may be varied in intake side-opening  25   a.    
       Modification 11 
       [0075]    In the eleventh modification, multiple curved portions  85  are formed on the outer circumference of annular receiving antennas  52  to concentrate the electric field, as shown in  FIG. 16 . The electric field is concentrated at curved portions  85  of receiving antenna  52  when the microwave radiation is emitted from radiation antenna  16 . This allows for the generation of plasma with reduced energy consumption. 
         [0076]    In this modification, curved portions  85  are provided only at the sides of inlet opening  25 . However, curved portions  85  may also be provided at other locations. For example, curved portions  85  may be provided on the inner side of ring shaped receiving antenna  52 . 
       Modification 12 
       [0077]    In the twelfth modification, receiving antenna  52  is provided in ceramic insulation material  90  laminated on the top surface of piston  23 , for example, as shown in  FIG. 17 . Multiple convex parts  92  that engage to concave part  91  formed on the top surface of piston  23  are formed in insulation material  90  at the side of piston  23 . This modification prevents insulation material  90  from peeling off from piston  23 . 
         [0078]    Cushioning layer  95 , which is softer than piston  23 , may be installed between piston  23  and insulation material  90 , as shown in  FIG. 18 . Cushioning layer  95  may be formed of a ductile metal, such as gold. Cushioning layer  95  may prevent damage to insulation material  90  due to knocking. 
       Modification 13 
       [0079]    The annular antenna may be divided into lengths of half the wavelength of the microwave radiation, as shown in  FIG. 19 . 
         [0080]    When the frequency of the EM radiation emitted from radiation antenna  16  is 2.45 GHz, the wavelength (in vacuum) is λ=12.2 cm since the wavelength is obtained by dividing the light speed (3×10 8 ) by the frequency. Thus, the length of receiving antenna  52  should be multiples of 6.1 cm. When receiving antenna  52  is designed as an annular antenna, as shown in  FIG. 4 , the diameter should be a multiple of 1.95 cm. In other words, the sensitivity of the receiving antenna may suffer when the diameter is not a multiple of 1.95 cm. 
         [0081]    Thus, receiving antennas with high sensitivity may be arranged at arbitrary radial locations when receiving antenna  52  is a multiple of half wavelengths of the microwave radiation, as shown in  FIG. 19 . This allows for an intense electric field to be induced at arbitrary radial locations using microwave radiation. 
       Modification 14 
       [0082]    One end of each receiving antenna  52  may be electrically connected to ground via switch  55 , as shown in  FIG. 20 . The length of each receiving antenna  52  should be multiples of half the wavelength of the microwave radiation, and receiving antenna  52  should be insulated from piston  23  using insulation layer  56 , for example, as shown in  FIG. 19 . 
         [0083]    In this example, one end of receiving antenna  52  is connected to the outer wall of piston  23  when switch  55  is closed. Antenna  52  is thereby grounded. In this case, the grounded part becomes the fixed end, and the other side becomes the floating end. In such a configuration, the sensitivity is a maximum when the length of the antenna is an odd multiple of the quarter wavelength. The length of receiving antenna  52  is half the wavelength of the microwave radiation; therefore, the induced current from the microwave radiation emitted from radiating antenna  16  is small in receiving antenna  52 . Receiving antenna  52  is thereby switched off. 
         [0084]    When switch  55  is closed, receiving antenna  52  becomes floating (i.e., electrically insulated from piston  23 ). Both sides of receiving antenna  52  thereby become floating ends. In this case, the receiving sensitivity becomes a maximum when the length of the antenna is a multiple of the half wavelength. Receiving antenna  52  switches on since the length of receiving antenna  52  is the half wavelength of the microwaves. 
         [0085]    Receiving antenna  52  can therefore be switched by opening or closing switch  55 . 
         [0086]    The microwave radiation from antenna  16  is concentrated close to receiving antenna  52 , which is switched on. The electric field therefore increases near the antenna. This allows control over the intensity of the electric field at an arbitrary location in the combustion chamber, and may therefore result in an enlargement of the plasma at an arbitrary position in the combustion chamber. 
       Modification 15 
       [0087]    As shown in  FIG. 21 , receiving antennas  52 , which are of the same length (i.e., half the wavelength of the microwave radiation) may be located in different radial positions on piston  23 . For example, four receiving antennas  52   a  may be arranged at the outer circumference. At the inner side, four receiving antennas  52   b  having the same length as antenna  52   a , but with a smaller radius of curvature, may be arranged. Furthermore, four receiving antennas  52   c  having the same length as antennas  52   a , but with a radius of curvature smaller than antennas  52   b , may be arranged at the inner side. This allows for an intensification of the electric field at the various radial locations in combustion chamber  20 . 
       Other Embodiments 
       [0088]    Other embodiments may be contemplated. 
         [0089]    Center electrode  40   a  of ignition plug  40  may also function as a radiation antenna. Center electrode  40   a  of ignition plug  40  is connected electrically with an output terminal of a mixing circuit. The mixing circuit receives a high-voltage pulse from ignition coil  14  and microwaves from EM-wave switch  32  from separate input terminals, and outputs both the high-voltage pulse and the microwaves from the same output terminal. 
         [0090]    An annular radiation antenna  16  may be provided in gasket  18 . An annular receiving antenna  52  may be provided on top of piston  23 . 
         [0091]    Receiving antenna  52  may be provided on the inner-wall surface of cylinder  24 . 
         [0092]    In the above embodiment, the following steps may be executed in sequence to fix a heat-resistant dielectric substance, such as a ceramic material, on which receiving antenna  52  is provided. (i) Spraying an organic mask onto receiving antenna  52 ; (ii) thermal spraying of aluminum toward the dielectric substance; (iii) peeling this aluminum layer on receiving antenna  52  together with the organic mask; and (iv) fixing the dielectric substance to piston  23  via the aluminum layer. In this case, the planar form of receiving antenna  52  and the dielectric substance may be annular or such a shape whereby the antenna is curved with a small radius of curvature. 
         [0093]    Radiation antenna  16  may be termed the “first antenna” and receiving antenna  52  can be termed the “second antenna”. 
       INDUSTRIAL APPLICABILITY 
       [0094]    As described above, the present invention is useful for an internal combustion engine that promotes the combustion of an air-fuel mixture using EM radiation.