Abstract:
An ignition apparatus for an internal combustion engine can suppress radiation noise radiated into air from a discharge current path, thereby preventing the malfunction of electrical equipment mounted on a vehicle. The ignition apparatus includes a transformer having a primary winding and a secondary winding, a conductor for impressing a high voltage generated at a high voltage side of the secondary winding upon interruption of a primary current to the primary winding to a spark plug, a plug boot enclosing the conductor so as to electrically insulate it from the outside and to hold the spark plug, and a metal sleeve arranged to enclose the plug boot and having one end thereof electrically connected directly or indirectly to a low voltage side of the secondary winding, and the other end thereof electrically connected directly or indirectly to the spark plug.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an ignition apparatus for an internal combustion engine which supplies a high voltage to a spark plug for each engine cylinder 
   DESCRIPTION OF THE RELATED ART 
   In the past, there have been known ignition apparatuses for an internal combustion engine which include a transformer having a primary winding and a secondary winding, a conductor for impressing a high voltage, which is generated at a high voltage side of the secondary winding when a primary current to the primary winding is interrupted, to a spark plug, and a plug boot that encloses the conductor so as to electrically insulate it from the outside and to hold the spark plug. 
   In a negative electrode discharge type ignition apparatus among such ignition apparatuses for an internal combustion engine, a discharge current containing high frequency noise generated upon electric discharge in a gap between electrodes of the spark plug is returned to the spark plug received in a plug hole in a cylinder head through the secondary winding, the connector, a vehicle body harness, a cam cover, and the cylinder head. 
   Here, as a relevant prior art reference, there is cited a patent document (Japanese patent application laid-open No. 2002-81360). 
   In this case, however, there is a problem that the noise generated upon electric discharge of the spark plug is radiated into air through the body harness, electrical equipment mounted on a vehicle is caused to malfunction due to this noise. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is intended to obviate the problem as referred to above, and has for its object to obtain an ignition apparatus for an internal combustion engine which is capable of suppressing radiation noise radiated into air from a discharge current path thereby to prevent the malfunction of electric equipment mounted on a vehicle. 
   Bearing the above object in mind, according to the present invention, there is provided an ignition apparatus for an internal combustion engine which includes: a transformer having a primary winding and a secondary winding; a conductor that impresses a high voltage, which is generated at a high voltage side of the secondary winding when a primary current to the primary winding is interrupted, to a spark plug; a plug boot that encloses the conductor so as to electrically insulate it from the outside and to hold the spark plug; and a metal sleeve that is arranged to enclose the plug boot and has one end thereof electrically connected directly or indirectly to a low voltage side of the secondary winding, and the other end thereof electrically connected directly or indirectly to the spark plug. 
   According to the ignition apparatus for an internal combustion engine of the present invention, radiation noise radiated into air from the discharge current path can be suppressed, thereby making it possible to prevent the malfunction of electrical equipment mounted on a vehicle. 
   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 
       FIG. 1  is a cross sectional view showing an ignition apparatus for an internal combustion engine according to a first embodiment of the present invention. 
       FIG. 2  is a cross sectional view showing an ignition apparatus for an internal combustion engine according to a second embodiment of the present invention. 
       FIG. 3  is a cross sectional view showing an ignition apparatus for an internal combustion engine according to a third embodiment of the present invention. 
       FIG. 4  is a cross sectional view showing an ignition apparatus for an internal combustion engine according to a fourth embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now, preferred embodiments of the present invention will be described in detail while referring to the accompanying drawings. Throughout respective figures, the same or corresponding members or parts are identified by the same reference numerals and characters. 
   Embodiment 1 
     FIG. 1  is a cross sectional view that shows an ignition apparatus for an internal combustion engine according to a first embodiment of the present invention. The ignition apparatus for an internal combustion engine illustrated in  FIG. 1  is an ignition apparatus of the negative electrode discharge type with a transformer  2  being received in a case  3 . The transformer  2  has an iron core  1  that is formed of a plurality of pieces of thin sheet steel, and a primary winding  20  and a secondary winding  21  that are wound around the iron core  1 . The case  3  is composed of a case main body  3   a  that receives the transformer  2  insulated and fixedly attached thereto by a cast insulating resin  4 , and a high voltage tower  3   b  that is formed integrally with the case main body  3   a  and has an opening portion  5  placed in communication with the case main body  3   a.    
   In the opening portion  5  of the high voltage tower  3   b , a connector terminal  7  is electrically connected to a high voltage side connecting wire  6  of the secondary winding  21  in a manner to close the opening portion  5 . 
   A plug boot  8  made of rubber is fitted onto a high voltage tower  3   b  of the case  3 . The plug boot  8  is formed with a through hole  9  along the central axis thereof. A spring  11 , being a conductor electrically connected to a spark plug  10 , is arranged in the interior of the high voltage tower  3   b  and in the through hole  9  so as to urge the ignition plug  10  in a direction away from the high voltage tower  3   b  and the plug boot  8 . A metal sleeve  12  is arranged on the outer periphery of the plug boot  8 , and has its one end connected to a low voltage side connecting wire  14  of the secondary winding  21  through a low voltage connector terminal  13 , and its other end connected to the spark plug  10 . 
   This ignition apparatus for an internal combustion engine is fixed attached to a cam cover  25  by means of a bolt  18  with the plug boot  8  being inserted into a plug hole  17 . At this time, the spring  11  presses, at its one or tip end, one end face of the spark plug  10 . 
   In the ignition apparatus for an internal combustion engine of the negative electrode discharge type, for example, a high voltage of −30 kV is impressed to the high voltage connector terminal  7 , and a discharge current containing high frequency noise generated in the gap portion of the spark plug  10  is returned to the spark plug  10  mainly through the spring  11 , the high voltage connector terminal  7  of the secondary winding  21 , the low voltage side connecting wire  14 , the low voltage connector terminal  13 , and the metal sleeve  12 . That is, there is substantially no possibility that the discharge current is returned to the spark plug  10  through a high resistance path comprising a connector  26 , the body harness  27 , the cam cover  25  and the cylinder head  16 , but instead the discharge current is returned to the spark plug  10  mainly through an electric path formed at the side of the metal sleeve  12 . 
   Thus, according to this ignition apparatus for an internal combustion engine, the discharge current including noise is returned mainly through a minimum path including the metal sleeve  12 , so the radiation of noise into air can be suppressed to a minimum, and the malfunction of electrical equipment mounted on a vehicle due to noise can also be suppressed. 
   Embodiment 2 
     FIG. 2  is a cross sectional view that shows an ignition apparatus for an internal combustion engine according to a second embodiment of the present invention. 
   In this embodiment, a transformer  2  is arranged in a plug hole  17 , and is composed of an iron core  1  formed of pieces of laminated thin sheet plates, a primary winding  20  surrounding the iron core  1 , and a secondary winding  21  surrounding the primary winding  20 , with the outer periphery of the transformer  2  being enclosed by a case main body  3   a . On the outer peripheral surface of the case main body  3   a , there is arranged an exterior iron core  22  that forms a path through which passes a magnetic flux generated when a primary current is supplied to the primary winding  20 . The exterior iron core  22  has its one end connected to a low voltage side connecting wire  14  of the secondary winding  21  through a low voltage connector terminal  13 , and has its other end electrically connected to a metal sleeve  12  of a cylindrical shape. The metal sleeve  12  connected at its one end to the exterior iron core  22  is connected at its other end to a spark plug  10 . 
   The construction of this second embodiment other than the above is similar to that of the first embodiment. 
   According to the ignition apparatus for an internal combustion engine of this second embodiment, a discharge current containing high frequency noise generated in a gap portion of the spark plug  10  is returned to the spark plug  10  mainly through the spring  11 , the high voltage connector terminal  7  of the secondary winding  21 , the low voltage side connecting wire  14 , the low voltage connector terminal  13 , the exterior iron core  22 , and the metal sleeve  12 . 
   Thus, in the ignition apparatus for an internal combustion engine having the exterior iron core  22 , the radiation of noise into air can be suppressed to a minimum, and the malfunction of vehicle-mounted electrical equipment due to the noise can also be suppressed, as in the above-mentioned first embodiment. Moreover, the exterior iron core  22  together with the metal sleeve  12  functions as a passage for the discharge current, so the length of the metal sleeve  12  can be shortened, thereby making it possible to reduce the production cost. 
   Embodiment 3 
     FIG. 3  is a cross sectional view that shows an ignition apparatus for an internal combustion engine according to a third embodiment of the present invention. 
   In the ignition apparatus for an internal combustion engine of this embodiment, between an end of the metal sleeve  12  and an end of the spark plug  10 , there is arranged a coil spring  23  that encloses the plug boot  8  and is at the same time in contact with the spark plug  10 . 
   The construction of this third embodiment other than the above is similar to that of the first embodiment. 
   According to the ignition apparatus for an internal combustion engine of this third embodiment, advantageous effects similar to those of the first embodiment can be achieved. In addition, the metal sleeve  12  and the spark plug  10  are electrically connected to each other through the coil spring  23  in a reliable manner, so it is possible to prevent the occurrence of noise due to an electric discharge which would otherwise be generated between the metal sleeve  12  and the spark plug  10  that are spaced from each other. 
   Embodiment 4 
     FIG. 4  is a cross sectional view that shows an ignition apparatus for an internal combustion engine according to a fourth embodiment of the present invention. 
   In this embodiment, the metal sleeve  12  is formed at its end near the spark plug  10  with an enlarged portion  24  that expands in a diametral direction, the enlarged portion  24  being in resilient contact with an inner wall surface of the plug hole  17  that receives the spark plug  10 . 
   The construction of this fourth embodiment other than the above is similar to that of the first embodiment. 
   According to the ignition apparatus for an internal combustion engine of this fourth embodiment, a discharge current containing high frequency noise generated in the gap portion of the spark plug  10  is returned to the spark plug  10  mainly through the spring  11 , the high voltage connector terminal  7  of the secondary winding  21 , the low voltage side connecting wire  14 , the low voltage connector terminal  13 , the enlarged portion  24  of the metal sleeve  12 , and the inner wall surface of the plug hole  17 . 
   According to the ignition apparatus for an internal combustion engine of this fourth embodiment, advantageous effects similar to those of the first embodiment can be achieved, and in addition, the enlarged portion  24  of the metal sleeve  12  has a function as a center locator when the ignition apparatus for an internal combustion engine is inserted into the plug hole  17 , so that upon insertion of the ignition apparatus into the plug hole  17 , the enlarged portion  24  of the metal sleeve  12  is inserted into the plug hole  17  while being in contact with the inner wall surface of the plug hole  17 , as a result of which the ignition apparatus can be smoothly inserted into the plug boot  8 . 
   Moreover, the metal sleeve  12  and the spark plug  10  are electrically connected to each other through the inner wall surface of the plug hole  17  in a reliable manner, so it is possible to prevent the occurrence of noise due to an electric discharge which would otherwise be generated between the metal sleeve  12  and the spark plug  10  that are spaced from each other. 
   Although in the above-mentioned respective embodiments, reference has been made to the ignition apparatuses of the negative electrode discharge type for an internal combustion engine in which a high negative voltage is impressed to the high voltage connector terminal  7 , the present invention can of course be applied to an ignition apparatus of the positive electrode discharge type for an internal combustion engine in which a high positive voltage is impressed to a high voltage connector terminal. 
   In this case, the ignition apparatus of the positive electrode discharge type is identical in the path for a discharge current with, but is opposite in the direction of flow to, the ignition apparatus of the negative electrode discharge type. 
   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.