Patent Publication Number: US-6655367-B2

Title: Plug-hole-installed ignition coil unit for internal combustion engines

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a plug-hole-installed ignition coil unit for internal combustion engines, particularly to an ignition coil unit installed in a hole opened through the engine cylinder head for direct ignition which houses a circuit board on which an ionic-current detection circuit and succeeding processing circuits for misfiring detection are formed. 
     2. Description of the Related Art 
     There have recently been proposed plug-hole-installed ignition coil units which are installed in a hole opened through engine cylinder head (so-called “plug hole”). Since such an ignition coil unit is installed in the plug hole, the coil unit is almost free from environmental influence such as moisture, oil, or any other foreign objects and can exhibit stable ignition performance. An example of the plug-hole-installed ignition coil unit is disclosed in Japanese Laid-Open Patent Application No. Sho 63 (1988)-120,863 in which a power transistor for switching operation of the primary coil is integrally housed so as to effect stable ignition performance in a reduced size. 
     In a gasoline or other spark-ignition internal combustion engine, a high voltage generated by the ignition coil is applied to spark plugs installed in the individual cylinders. The spark discharge that the high voltages produces across the gap between the spark plug electrodes ignites the air-fuel mixture, causing combustion. However, when certain causes are present during the engine ignition/combustion stroke, the combustion of the air-fuel mixture does not proceed normally, i.e., misfiring occurs. 
     Causes of misfiring fall in two classes, those attributable to the fuel supply system and those attributable to the ignition system. Misfiring attributable to the fuel supply system is the result of either excessively lean or excessively rich air-fuel mixture. In this case, a spark discharge is produced across the gap of the spark plug but the air-fuel mixture does not ignite. Misfiring attributable to the ignition system is the result of spark plug fouling (smoldering) caused by deposits of soot, etc., or of a problem in the ignition circuit that prevents normal spark discharge (mis-sparking). 
     When the air-fuel mixture burns normally, the combustion is accompanied by ionization of the air-fuel mixture (more precisely the combustion gas produced by normal burning of the air-fuel mixture) that gives rise to ionic-current. When misfiring occurs and the air-fuel mixture does not burn, the air-fuel mixture does not ionize and no ionic-current arises. A widely used method of detecting misfiring has therefore been to detect the ionic-current occurring during the combustion stroke using the spark plug as a probe for ionic-current detection and comparing the detected value with a prescribed value. 
     When conducting the misfire detection using this plug-hole-installed ignition coil unit, the ignition coil unit would normally be separated from the ionic-current detection circuit and other processing circuit and be connected by a cable or wire. That would degrade the accuracy of ionic-current detection and misfire detection. To be more specific, since the ionic-current resulting from combustion can be present for a extremely small period of time after ignition, the detection must be implemented at a fixed timing in a prescribed period of time. 
     However, if the ignition coil unit is connected to the circuits by a cable or wire, the detection accuracy of ionic-current and misfiring could be degraded by environmental influence. Specifically, the resistance in the cable or wire could be varied with the change in environmental temperature. The stray capacity of the wire or cable may be affected by moisture, oil or foreign conductive materials deposited thereon. The capacity in the circuit capacitors may also be varied by these factors. When these happen, it makes difficult to conduct the detection at a fixed timing in a prescribed period of time. 
     Moreover, when the ignition coil unit and the circuits are connected by a wire or cable, since the flowing current could rise up to 30 kV, it may become necessary to take some countermeasures against this high voltage by, for example, using a high-tension code. In addition, this high voltage current flow may generate noise in other electronic units. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to overcome the aforesaid problems and to provide a plug-hole-installed ignition coil unit having an improved detection accuracy of ionic-current detection and misfiring. 
     A second object of the present invention is to provide a plug-hole-installed ignition coil unit which does no longer require to take high-voltage countermeasurement and can avoid generation of noise in other electronic units. 
     For achieving the first object, this invention provides a n ignition coil unit to be installed in a hole opened through a cylinder head of an engine, comprising: a case; an ignition coil installed in the case and comprising a primary coil connected to a power source and a secondary coil connected to a spark plug which is installed in a combustion chamber of a cylinder of the engine for producing spark discharge to ignite air-fuel mixture to generate combustion thereof in the combustion chamber; and a circuit board on which an ionic-current detection circuit, connected to the secondary coil, which detects ionic-current that flows during the combustion of the air-fuel mixture; and a processing circuit, connected to the ionic-current detection circuit, which generates an output indicative of the detected ionic-current, are formed; wherein the improvement comprises: the circuit board is installed in the case with the ignition coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will be made more apparent with reference to the following description and drawings, in which: 
     FIG. 1 is a sectional view of a plug-hole-type ignition coil unit according to an embodiment of the invention; 
     FIG. 2 is a cross-sectional view taken along lines II—II in FIG. 1; 
     FIG. 3 is a circuit diagram showing the configuration of circuits formed on a circuit board of the ignition coil unit illustrated in FIG. 1; 
     FIG. 4 is an experimental data showing output of the circuit on the circuit board illustrated in FIG. 3; and 
     FIG. 5 is a view, similar to FIG. 4, but showing an experimental data obtained in a configuration where an ignition coil unit is assumed to be separated from the circuit board and to be connected by a wire or cable. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A plug-hole-installed ignition coil unit according to an embodiment of the present invention will now be explained with reference to the attached drawings. 
     FIG. 1 is a sectional view of a plug-hole-type ignition coil unit according to the embodiment. 
     In the figure, reference numeral  10  indicates the plug-hole-installed ignition coil unit (hereinafter referred simply to the “ignition coil unit”), which has a spark plug  12  (shown by phantom lines). A cylinder head (partly shown)  14  of a spark ignition internal combustion engine (not completely shown) mounted on a vehicle (not shown) is opened by a cylindrical plug hole (hereinafter referred simply to the “hole”)  16 . 
     The ignition coil unit  10  is inserted in the hole  16  and is installed there in such a way that electrodes  18  of the spark plug  12  is located in a combustion chamber (partly shown)  20  of the engine. In the figure, the ignition coil unit  10  is housed in the hole  16  with a gap. However, this is for ease of understanding and in fact, the ignition coil unit  10  is tight-fitted in the hole  16 . Thus, the ignition coil unit  10  is a plug-hole-installed ignition coil unit for internal combustion engine for direct ignition. 
     The ignition coil unit  10  includes a case or housing  22  which houses an ignition coil comprising a primary coil  24  and a secondary coil  26 , a circuit board or unit  28  and an igniter  30 . On the circuit board  28 , there are mounted an ionic-current detection circuit for ionic-current detection and a processing circuit connected to the ionic-current detection circuit  28   a  for inputting output therefrom to generate an output indicative of the ionic-current. The igniter  30  supplies a signal to the primary coil  24  to generate a high voltage. Thus, the ignition coil unit  10  has the circuit board  28  and the igniter  30  which are installed or housed in the case  22  integrally with the ignition coil. 
     Continuing explanation of the ignition coil unit  10  referring sometimes to FIG. 2 which is a cross-sectional view taken along line II—II in FIG. 1, the case  22  is made of a hard plastic material such as PBT (Polybutylenetherephthalate) and as shown in FIG. 2, it has a rectangular section  22   a  which is in the shape of rectangle when viewed from top and a cylindrical section  22   b  which is in an elongated narrow cylindrical shape and extends continually from the rectangular section  22   a.    
     The case rectangular section  22   a  houses the circuit board  28  and the igniter  30 . At one end, the case rectangular section  22   a  is provided with a connector  34  that has a plurality of pins  32 . The pins  32  are connected, at one end, to the circuit board  28  and the igniter  30  and are connected, at the other end, to an ECU (Electronic Control Unit) and a power source (on board battery.) 
     Although the illustration of the ECU (and the power source) is omitted in FIGS. 1 and 2, the ECU comprises a microcomputer having a CPU, a ROM, a RAM and input/output circuits, etc. It is inputted with the outputs of a group of sensors, including a crank angle sensor that is installed near the crankshaft or camshaft of the engine and outputs a signal representing the TDC (Top Dead Center) of the individual cylinders and unit crank angles (obtained by dividing the interval between the TDCs), a manifold absolute pressure sensor that outputs a signal representing the absolute pressure (PBA) in the air intake pipe, and other sensors. The ECU outputs an ignition signal to the circuit board  28  and the igniter  30  at a certain crank angular position. It also inputs the output indicative of the ionic-current generated by the circuit board  28  and based thereon, detects the misfiring occurred in the engine. 
     The ignition coil is stored in the case cylindrical section  22   b . The ignition coil comprises a rounded iron core  36 , the aforesaid primary coil  24  wound around on the iron core  36  and the aforesaid secondary coil  26  wound around on the primary coil  24 . The primary coil  24  is connected, at one end, to the power source through the igniter  30  and the connector pin  32  and is grounded at the other end. The secondary coil  26  is connected, at one end, to a shaft  38  made of conductive material and is connected, at the other end, to the circuit board  28  (more precisely, to the ionic-current detection circuit). The shaft  38  is connected, at its distal end, to the aforesaid spark plug  12  through a spring  40  similarly made of conductive material. 
     As mentioned above, the spark plug  12  is located in the combustion chamber  20  and ignites the air-fuel mixture in the combustion chamber  20  when supplied with a high voltage from the primary coil  26 . Ionic-current resulting from the air-fuel mixture combustion is detected by the circuits mounted on the circuit board  28  through the spark plug  12  (more specifically the electrodes  18 ), the spring  40 , the shaft  38  and the primary coil  26 . 
     At the side of the case rectangular section  22   a , there is provided a flange  44  which is in the shape of triangle when viewed from top as illustrated in FIG.  2 . The flange  44  has a threaded hole  46  opened therethrough. The cylindrical section  22   b  is inserted in the hole  16  such that the electrodes  18  of the spark plug  12  face into the combustion chamber  20 , and the rectangular section  22   a  is fastened on the cylinder head  14  by putting screws (not shown) in the threaded hole  46  and mated threaded hole formed at the cylinder head  14  and by tightening them. Thus, the ignition coil unit  10  is fixed on the cylinder head  14 . A gap  42   a  in the case rectangular section  22   a  and a gap  42   b  in the case cylindrical section  22   b  are molded or filled with resin such as epoxy resin such that the inside of the case  22  including the circuit board  28  is shut out from the exterior. 
     The interconnection between the components and the configuration of the circuit board  28  will now be explained with reference to FIG.  3 . 
     As illustrated in the figure, one end of the primary coil  24  is connected to a power source  48  and the other end is grounded. The igniter  30  including a power transistor (not shown) is connected in the current path between the one end of the primary coil  24  and the power source  48  or in the current path between the other end of the primary coil  24  and the ground. The igniter  30  is connected to the ECU (now assigned with reference numeral  50 ) through the connector pin  32  (not shown in FIG.  3 ). 
     One end of the secondary coil  26  is connected to the spark plug  12 , more precisely the center electrode of the electrodes  18  and is then grounded. The other end of the secondary coil  26  is connected to the circuit board  28 , more specifically an ionic-current detection circuit  28   a  mounted on the circuit board  28 . The ionic-current detection circuit  28   a  includes an ionic-current detection capacitor  28   a   1  to be charged in the illustrated polarity by discharge current and a Zener diode  28   a   2  that regulates the charging voltage of the ionic-current detection capacitor  28   a   1 , a diode  28   a   3  that prevents reverse current flow through which the Zener diode  28   a   2  is grounded, and a detection resistor  28   a   4  through which the ionic-current detection capacitor  28   a   1  is grounded 
     On the circuit board  28 , a processing circuit  28   b  is connected to the ionic-current detection circuit  28   a  at a junction between the capacitor  28   a   1  and the resistor  28   a   4 . The processing circuit  28   b  has an integrating capacitor (not shown) that integrates the ionic-current inputted from the junction. The processing circuit  28   b  is connected to the ECU  50  through the connector pin  32  (not shown in FIG. 3) and sends the output indicative of the integrated value of the detected ionic-current. Since detailed configuration of the processing circuit  28   b  itself does not have a direct relationship with the gist of the present invention, no more explanation will be made. 
     Thus, in the ignition coil unit  10  according to the embodiment, since the circuit board  28  (on which ionic-current detection circuit  28   a  and the processing circuit  28   b  are mounted and formed) is installed in the case  12 , more precisely in the rectangular section  12   a  of the case  12  integrally with the secondary coil  26  and molded by resin, no exterior wire or cable is needed to connect the secondary coil  26  with the ionic-current detection circuit  28   a , and to connect the ionic-current detection circuit  28   a  with the processing circuit  28   b , With this, it becomes possible to reduce the influence on the change in the resistance and stray capacity caused by the aforesaid environmental factors to a minimum extent, thereby enhancing the detection accuracy of ionic-current and misfiring. 
     Moreover, since the igniter  30  is installed in the case  12 , more precisely in the rectangular section  12   a  of the case  12  integrally with the primary coil  24  and molded by resin, no exterior wire or case is needed to connect the igniter  30  with the primary coil  24 . With this, it becomes possible to obtain stable ignition performance, thereby further enhancing the detection accuracy of ionic-current and misfiring. 
     The detection of ionic-current and misfiring will then be explained with reference to the same figure. 
     The flow of current from the power source  48  through the primary coil  24  is switched (turned ON and OFF) by the power transistor of the igniter  30  in response to the ignition command pulse sent from the ECU  50 . 
     When the ignition command pulse is made from ON to OFF, i.e., when the current flow through the primary coil  24  is stopped by switching of the power transistor from ON to OFF, a high voltage of negative polarity is concurrently produced in the secondary coil  26 . Discharge current therefore flows through the path of the spark plug  12 →secondary coil  26 →ionic-current detection capacitor  28   a   1  (or Zener diode  28   a   2 )→diode  28   a   3  produces a spark discharge across the gap of the spark plug  12  (between the center electrode and ground electrode) that ignites or fires the air-fuel mixture in the cylinder combustion chamber  20  and causes combustion thereof. 
     The discharge current charges the capacitor  28   a   1  to the polarity as illustrated in the figure. When charged, this capacitor  28   a   1  functions as a current detection power source with a bias voltage for detecting ionic-current. 
     During the combustion of the air-fuel mixture set off by the spark discharge at the spark plug  12 , the air-fuel mixture (more precisely the combustion gas produced by normal burning of the air-fuel mixture) ionizes. The ions produced migrate owing to the effect of the bias voltage of the ionic-current detection capacitor  28   a   1  and their resulting presence between the electrodes of the spark plug  12  lowers the electrical resistance between the electrodes. As a result, ionic-current flows through the path of the ionic-current detection capacitor  28   a   1 →secondary coil  26 →spark plug  12 . The ionic-current occurring at this time causes the voltage drop across the detection resistor  28  to change. The ionic-current detection circuit  28   a  outputs this voltage change to the processing circuit  28   b  which generates the output indicative of integrated value of the ionic-current and sends the same to the ECU  50 . The ECU  50  compares the detected output with a threshold value and detects whether misfiring has occurred in the engine. The integrating capacitor is discharged by the next ignition command pulse such that the output of the processing circuit  28   b  is resetted for next detection. 
     Thus, the ionic-current detection circuit  28   a  has the ionic-current detection capacitor  28   a   1  which plays a significant role in the ionic-current detection. Although not shown, the processing circuit  28   b  has a plurality of capacitors. As mentioned above, if the stray capacity in these capacitors varies, it becomes difficult to conduct the detection at a fixed timing in a prescribed period of time and this degrades the detection accuracy of ionic-current and misfiring. Since, however, the circuit board  28  including these circuits is housed within the case rectangular section  12   a  and the gap  42   a  therebetween is filled with resin, the stray capacity in the capacitors in the circuits can remain unchanged. 
     FIG. 4 is an experimental data showing detected output (aforesaid integrated value) of the processing circuit  28   b  illustrated in FIG. 3, and FIG. 5 is data similarly showing an experimental data obtained in a configuration where the ignition coil unit assumed to be separated from the circuit board  28  and to be connected by a wire or cable. The data illustrated in FIG. 5 is based on the condition that the wire or cable is wet. The data illustrated in FIGS. 4 and 5 are based on a condition in which misfiring occurred at engine speed of 1000 rpm under normal temperature. 
     Since misfiring occurred in the engine, the detected output must be zero as illustrated in FIG.  4 . However, in the data in FIG. 5, the detected output is raised above zero due to the change in the stray capacity and resistance. If the raised output exceeds a threshold value for misfiring detection, it could be judged that the ionic-current is generated normally and no misfiring occurs. This will require to change the threshold value in response to the change in environment when the ignition coil unit and the circuit board are connected by a cable or wire. 
     Since, however, in the embodiment, the circuit board  28  is housed within the case rectangular section  12   a  and the gap  42   a  therebetween is filled with resin such that the stray capacity in the capacitors in the circuits can accordingly remain unchanged, it can improve the detection accuracy. In addition, this configuration can eliminate the countermeasure against high voltage and can avoid generation of noise in other electronic units. 
     Having been described in the foregoing, the embodiment is configured to have an ignition coil unit  10  to be installed in a hole  16  opened through a cylinder head  14  of an engine, comprising: a case  12 ; an ignition coil installed in the case and comprising a primary coil  24  connected to a power source and a secondary coil  26  connected to a spark plug  12  which is installed in a combustion chamber of a cylinder  20  of the engine for producing spark discharge to ignite air-fuel mixture to generate combustion thereof in the combustion chamber; and a circuit board  28  on which an ionic-current detection circuit  28   a , connected to the secondary coil, which detects ionic-current that flows during the combustion of the air-fuel mixture; and a processing circuit  28   b , connected to the ionic-current detection circuit, which generates an output indicative of the detected ionic-current, are formed. In the unit, the circuit board  28  is installed in the case  12  with the ignition coil. 
     The ignition coil unit  10  further includes; an igniter  30 , connected to the primary coil, which outputs a signal to generate the spark discharge in the spark plug; and wherein the igniter is installed in the case with the ionic-current circuit. a gap  42   a  between the circuit board and the case is filled with resin, and a gap  42   a  between the igniter and the case is filled with the case. Further, a gap  42   b  formed around the ignition coil is filled with resin. 
     The entire disclosure of Japanese Patent Application No. 2001-202592 filed on Jul. 3, 2001, including specification, claims, drawings and summary, is incorporated herein in reference in its entirety. 
     While the invention has thus been shown and described with reference to specific embodiments, it should be noted that the invention is in no way limited to the details of the described arrangements but changes and modifications may be made without departing from the scope of the appended claims.