Abstract:
An ignition apparatus for an internal combustion engine is small in size and has high reliability capable of withstanding or enduring a vibration environment of a vehicle to which the ignition apparatus is installed. The ignition apparatus includes a spark plug that has its tip end portion presented in the interior of a cylinder of an internal combustion engine main body, a high voltage generation power supply that serves to apply a high voltage to the spark plug, a microwave oscillation device that has an amplifying element for generating a microwave, and a microwave antenna that is mounted on the spark plug, irradiates the micro wave generated from the microwave oscillation device to the interior of the cylinder, thereby forming a plasma generation region around discharge electrodes of the spark plug. The microwave oscillation device is made into a solid state.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an ignition apparatus for an internal combustion engine in which a plasma generation region is formed around discharge electrodes of a spark plug. 
         [0003]    2. Description of the Related Art 
         [0004]    In the past, there has been known an ignition apparatus for an internal combustion engine which includes spark plugs each of which has its tip end portion presented in the interior of a corresponding cylinder of an internal combustion engine main body, a high voltage generation power supply that serves to apply a high voltage to the spark plugs, a magnetron that generates a microwave, and a microwave antenna that is mounted on each of the spark plugs, irradiates the microwave generated from the magnetron to the interior of a corresponding cylinder, thereby forming a plasma generation region around discharge electrodes of the spark plug (see, for example, a first patent document: Japanese patent application laid-open No. 2007-113570). 
         [0005]    In this ignition apparatus, by irradiating a microwave to a mixture containing air and fuel mixed therein, the time of combustion of the fuel can be shortened to improve the combustion efficiency, thus making it possible to reduce the rate or amount of fuel consumed. 
         [0006]    However, in the microwave oscillation device as described above, the magnetron is used which is a thermionic tube generally used for consumer products such as microwave ovens or the like. 
         [0007]    The principle of operation of the magnetron is that when a magnetic field due to a magnet is applied to electrons coming out from a heated cathode filament to an anode or positive terminal, a Lorentz force is applied to the electrons to curve their trajectory, whereby the electrons thus curved resonate in a small chamber of the anode, which serves as a cavity resonator, thereby generating a microwave in a continuous manner. 
         [0008]    However, in “International Telecommunication Union (ITU)”, the assignment of operation frequencies is defined in order to avoid electrical or radio interference and noise jamming between individual applications and uses. Industrial, Scientific and Medical (ISM) Use bands are designated for “dielectric heating”, and a frequency of 2,450 MHz is approved and allocated for microwave ovens according to the Radio Law in Japan. 
         [0009]    Since in magnetrons for microwave ovens, the dimensions of an anode or positive terminal is set according to this approved microwave frequency, there is little or no room for reducing the sizes of magnetrons as a whole. Thus, the entire volume of each magnetron becomes about 10 cm (width)×about 10 cm (depth)×about 20 cm (height). 
         [0010]    Accordingly, in an internal combustion engine main body composed of a plurality of cylinders, it is necessary to provide a magnetron for each cylinder that requires a large space for installation thereof, and hence there has been a problem that in actuality, it is difficult to install a plurality of such magnetrons on the internal combustion engine main body. 
         [0011]    In addition, in case where such an ignition apparatus is installed on a vehicle, there has been another problem that it is difficult for the magnetrons of large weights to withstand or endure vibrations applied thereto. 
       SUMMARY OF THE INVENTION 
       [0012]    In view of the above, the present invention is intended to obviate the problems as referred to above, and has for its object to provide an ignition apparatus for an internal combustion engine which is small in size and has high reliability capable of withstanding or enduring a vibration environment of a vehicle on which the ignition apparatus is installed. 
         [0013]    Bearing the above object in mind, an ignition apparatus for an internal combustion engine according to the present invention includes: a spark plug that has its tip end portion presented in the interior of a cylinder of an internal combustion engine main body; a high voltage generation power supply that serves to apply a high voltage to the spark plug; a microwave oscillation device that has an amplifying element for generating a microwave; and a microwave antenna that is mounted on the spark plug, irradiates the micro wave generated from the microwave oscillation device to the interior of the cylinder, thereby forming a plasma generation region around discharge electrodes of the spark plug. The microwave oscillation device is made into a solid state. 
         [0014]    According to the ignition apparatus for an internal combustion engine of the present invention, the microwave oscillation device is made into a solid state or is formed of solid state components, so there is obtained an advantageous effect that the microwave oscillation device is able to withstand or endure a vibration environment of a vehicle. 
         [0015]    The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram showing an ignition apparatus for an internal combustion engine according to a first embodiment of the present invention. 
           [0017]      FIG. 2  is an electric circuit diagram of a microwave oscillation device shown in  FIG. 1 . 
           [0018]      FIG. 3  is a cross sectional view showing an ignition apparatus for an internal combustion engine according to a second embodiment of the present invention. 
           [0019]      FIG. 4  is a perspective view showing an example in which the ignition apparatus for an internal combustion engine of  FIG. 2  is installed on an internal combustion engine main body. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Now, preferred embodiments of the present invention will be described in detail while referring to the accompanying drawings. Throughout individual figures, the same or corresponding members or parts are identified by the same reference numerals and characters. 
       Embodiment 1 
       [0021]    Referring to the drawings and first to  FIG. 1 , there is shown a block diagram of an ignition apparatus  42  for an internal combustion engine (hereinafter abbreviated as an ignition apparatus) according to a first embodiment of the present invention.  FIG. 2  is an electric circuit diagram of a microwave oscillation device  21  shown in  FIG. 1 . 
         [0022]    This ignition apparatus  42  is provided for each of cylinders  6 , and includes a spark plug  7  having an ignition and discharge part  5 , a high voltage generation power supply  11  that is connected to the ignition and discharge part  5  through a connection cable  10 , a control unit  13  that is electrically connected the high voltage generation power supply  11  through a control unit line  41   b,  a microwave antenna  1  that is arranged in the vicinity of the ignition and discharge part  5  and irradiates a microwave thereby to form a plasma generation region  2 , a microwave oscillation device  21  that is electrically connected to the microwave antenna  1  through a coaxial cable  3 , an oscillator power supply  22  that is electrically connected to the microwave oscillation device  21 , and the above-mentioned control unit  13  that is electrically connected to the oscillator power supply  22  through a control unit line  41   a.    
         [0023]    The ignition and discharge part  5  has a center discharge electrode  8  that is arranged in a ceiling space of a corresponding cylinder  6  and is located in the plasma generation region  2 , and an L-shaped side discharge electrode  9  that is arranged at one side of the center discharge electrode  8 . 
         [0024]    The microwave oscillation device  21  is made into a solid state, and includes a substrate  25  of a several centimeter square made of alumina or the like, a resonator  23  with its conductor printed on the substrate  25 , an amplifier circuit  50  having a choke coil  27 , a coupling capacitor  28 , and an amplifying element  24  of a height of about a few millimeters, and a casing  26  which receives therein the substrate  25 , the resonator  23  and the amplifier circuit  50 , and serves to shield microwaves. 
         [0025]    The microwave oscillation device  21  takes the form of a small box as a whole, in which the substrate  25  has a rear surface bonded to a bottom surface of the casing  26  by means of a bonding or adhesive material. Specifically, the amplifying element  24  is in the form of a transistor made of silicon (Si). The choke coil  27  is an element that serves to supply electric power to the amplifying element  24  and to interrupt an alternating current component to the oscillator power supply  22 . 
         [0026]    Now, the operation of the ignition apparatus as constructed above will be explained below. 
         [0027]    First of all, when a cylinder  6  with a combustion gas of an air fuel mixture filled therein is to be ignited, a predetermined actuation signal is provided from the control unit  13  to the high voltage generation power supply  11  and the oscillator power supply  22 . The oscillator power supply  22  is driven by the actuation signal, so that electric power is supplied to the amplifying element  24  of the microwave oscillation device  21 . 
         [0028]    The microwave oscillation device  21  is driven to oscillate at the frequency of the resonator  23  while applying a positive feedback by means of the resonator  23  and the amplifying element  24 . An oscillating output of the microwave oscillation device  21  is supplied to the microwave antenna  1  through the coupling capacitor  28  and the coaxial cable  3 . 
         [0029]    A microwave is transmitted or irradiated from the microwave antenna  1 , whereby a low temperature plasma is generated in the plasma generation region  2  around the ignition and discharge part  5 . A large amount of OH radicals can be continuously produced from moisture in the air fuel mixture due to the generation of this low temperature plasma. 
         [0030]    A high voltage, which is generated in a secondary side of a transformer (not shown) under such an atmosphere by the turning-off operation of a switching element (not shown) of the high voltage generation power supply  11 , is supplied to the ignition and discharge part  5 , whereby a spark discharge is generated between the center discharge electrode  8  and the side discharge electrode  9 . With this spark discharge as a starting point, the OH radicals having been previously produced serve to facilitate the combustion of the mixture. 
         [0031]    Here, note that the electric circuit diagram of  FIG. 2  showing the microwave oscillation device  21  is just one example, and other electric circuits can of course be used as long as they are able to oscillate a microwave. 
         [0032]    According to the ignition apparatus  42  of this first embodiment, the microwave oscillation device  21  is made into a solid state, i.e., is composed of solid-state components, so the size or dimensions thereof can be reduced to a substantial extent in comparison with a magnetron, and at the same time the weight thereof can be made lighter to a great extent. As a result, it is possible to install the ignition apparatus  42  on the main body of an internal combustion engine in a vehicle, and it is also possible to improve the vibration resistance of the ignition apparatus  42  to the vibration environment of the vehicle to a great extent. 
         [0033]    In addition, with the microwave oscillation device  21 , the microwave generated therein can be prevented from leaking to the outside by means of the casing  26 . 
       Embodiment 2 
       [0034]      FIG. 3  is a block diagram of an ignition apparatus for an internal combustion engine according to a second embodiment of the present invention. 
         [0035]    In this second embodiment, a high voltage generation power supply  11  has a control circuit part  52 , a primary coil  53 , a iron core  54 , and a secondary coil  55  received in a container  51 . By the use of a coupling member  56 , this high voltage generation power supply  11  is integrally coupled with a microwave oscillation device  21  in which a substrate  25 , a resonator  23 , and an amplifier circuit  50  are received in a casing  26 . The high voltage generation power supply  11  and the microwave oscillation device  21  thus integrally coupled with each other cooperate to constitute a power supply and oscillation main body  60 . This power supply and oscillation main body  60  with a portion of a spark plug  7  exposed from a cylinder  6  is covered with a microwave shield member  46  through an outer frame casing  61 . 
         [0036]    The high voltage generation power supply  11  is connected through a high voltage output line  43  to a spark plug terminal  45  of the spark plug  7 . In addition, the microwave oscillation device  21  is connected through a microwave output line  44  to the spark plug terminal  45  of the spark plug  7 . 
         [0037]    In the high voltage generation power supply  11 , a primary current in the primary coil  53  is interrupted by the operation of the control circuit part  52  in accordance with the ignition timing of the internal combustion engine, whereby a high voltage is generated in the secondary coil  55 . As a result, a high-voltage current thus generated in the secondary coil  55  is sent to the spark plug  7  through the high voltage output line  43 . 
         [0038]    In addition, similar to the above-mentioned first embodiment, the oscillating output generated in the microwave oscillation device  21  is supplied to the microwave antenna  1  through the microwave output line  44 . 
         [0039]    The other construction of this second embodiment including the microwave oscillation device  21  is similar to that of the first embodiment. 
         [0040]    Here, note that the casing  26  and the container  51  can be omitted, and the outer frame casing  61  can be made to serve as the casing  26  of the microwave oscillation device  21  and the container  51  of the high voltage generation power supply  11 , and the high voltage generation power supply  11  and the microwave oscillation device  21  can be made integral with each other. 
         [0041]    In addition, the coupling member  56 , the casing  26 , the container  51 , and the outer frame casing  61  need only be made of resin materials or metal materials such as aluminum. 
         [0042]      FIG. 4  is a perspective view showing an example in which a main body  57  of an ignition apparatus  42  excluding a control unit  13  and an oscillator power supply  22  is installed on a main body  48  of an internal combustion engine. In this example, the internal combustion engine is a four-cylinder engine in which individual ignition apparatus main bodies  57  are directly mounted on individual cylinders  6 , respectively. 
         [0043]    Each of the ignition apparatus main bodies  57  has a mounting flange  81  formed at a side thereof opposite to a connector  83 . The internal combustion engine main body  48  has a plurality of mounting bosses  82  formed thereon in opposition to the mounting flanges  81  of the corresponding ignition apparatus main bodies  57 , respectively. The ignition apparatus main bodies  57  are fixedly secured to the internal combustion engine main body  48  by inserting mounting bolts  80  into and through the corresponding mounting flanges  81 , respectively, and putting them into threaded engagement with the corresponding mounting bosses  82 , respectively. 
         [0044]    According to the ignition apparatus  42  of this second embodiment, the same advantageous effects as those of the first embodiment can be achieved, and at the same time, the high voltage generation power supply  11  and the microwave oscillation device  21  are integrally formed with each other by means of the coupling member  56 , so the number of man hours required for assembling the ignition apparatus  42  can be decreased, thus making it possible to reduce the production cost thereof. 
         [0045]    In addition, since the power supply and oscillation main body  60  is covered with the microwave shield member  46 , it is possible to prevent the microwave generated therein from leaking to the outside in a more reliable manner. 
         [0046]    Moreover, the microwave shield member  46  also serves to cover or enclose the portion of the spark plug  7  exposed from the internal combustion engine main body  48 , whereby the leakage of the microwave from the spark plug terminal  45  can be shielded in a reliable manner. 
         [0047]    Further, the ignition apparatus main body  57  is formed into a single unit, so it is possible to abolish or omit the coaxial cable  3  of the first embodiment, which, if broken or damaged by the vibration environment of the vehicle, might allow leakage of a microwave therethrough. As a result, the reliability of the ignition apparatus  42  can be improved. 
         [0048]    Furthermore, the loss of a microwave due to the coaxial cable  3  of a large length can be eliminated, and even if the output of the microwave oscillation device  21  is decreased, it can be ensured that a microwave is output or transmitted from the microwave antenna  1 , whereby the power consumption of the vehicle can be decreased, thereby making it possible to serve for cost reduction. 
         [0049]    Similarly, the connection cable  10  of the first embodiment with the possibility of being broken or damaged by the vibration environment of the vehicle can also be made unnecessary. 
         [0050]    In the above-mentioned first and second embodiments, the transistor made of silicon (Si) is used as the amplifying element  24 , but an amplifying element in the form of a transistor using gallium arsenide (GaAs) may instead be employed. 
         [0051]    In the case of using such an amplifying element, a microwave oscillation device can be obtained which operates at higher speed, consumes less electric power, and is easier to reduce its size, than the transistor made of silicon (Si), and which is high in reliability and small in size so that it can be distributedly mounted on each cylinder. 
         [0052]    In addition, a nitride semiconductor device made of gallium nitride (GaN) has a bandgap of 3.45 eV (corresponding to about 365 nm in the wavelength of light), and hence has a characteristic which is three times as wide as that of a silicon semiconductor device. Therefore, when applied to a transistor, the nitride semiconductor device has various features that it has a high withstand voltage, operates at high temperatures, can have an increased electric current density, is fast in switching, and is small in on-resistance. 
         [0053]    Accordingly, by using the transistor made of gallium nitride (GaN) as the amplifying element  24  of the microwave oscillation device  21 , it is possible to obtain a highly reliable and small-sized microwave oscillation device which can generate a higher output and can be distributedly mounted on each cylinder. 
         [0054]    Moreover, by using, as a semiconductor material, aluminum gallium nitride (AlGaN) which has a bandgap of about 5 eV much wider than that of gallium nitride (GaN), the withstand voltage (dielectric breakdown voltage) of the amplifying element  24  can be improved several times as high as that of gallium nitride (GaN), thus making it possible to obtain a highly reliable and small-sized microwave oscillation device which can generate a much higher output and can be distributedly mounted on each cylinder. 
         [0055]    While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.