Patent Publication Number: US-6222368-B1

Title: Ion current detection apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ion current detection apparatus for detecting ion current that flows after spark discharge of a spark plug. 
     Conventionally, in order to detect misfire or knocking of an internal combustion engine, as well as various other operation conditions of the internal combustion engine (e.g., such as air-fuel ratio, lean limit of air-fuel ratio, limit in amount of recirculated exhaust gas), there has been utilized a technique for detecting the ion current which flows due to ions present in the vicinity of electrodes of a spark plug of the engine after spark discharge. 
     That is to say, within a cylinder of an internal combustion engine, ions are generated when combustion (flame propagation) occurs after spark discharge of a spark plug, and the resistance between the electrodes of the spark plug changes in accordance with the number of ions generated, which in turn changes depending on the combustion state or the operation state of the engine. Therefore, changes in the resistance between electrodes of the spark plug (i.e., the changes in operation state) can be detected by a method in which, after application of high voltage for ignition purpose (i.e., after spark discharge of the spark plug), a voltage is externally applied to the spark plug in order to cause a flow of ion current, which is then detected. 
     BACKGROUND OF THE INVENTION 
     An example of such an ion current detection apparatus disclosed in Japanese Patent Application Laid-Open No. 4-191465 will be described. 
     As shown in FIG. 6 of the accompanying drawings, an ignition apparatus  2  to which is applied an ion current detection apparatus  100  includes a spark plug  10  provided for each cylinder (only one cylinder is represented in FIG. 6) of an internal combustion engine, as well as an ignition coil  12  for applying the spark plug  10  with high voltage for ignition purpose. 
     A battery voltage Vb is applied to one end of a primary winding L 1  of the ignition coil  12 , while the other end of the primary winding L 1  is grounded via a power transistor  14 , which is turned on and off in accordance with an ignition signal IG. One end of a secondary winding L 2  of the ignition coil  12  is connected to a center electrode of the spark plug  10 , and the other end of the secondary winding L 2  is connected to the ion current detection apparatus  100 . An outer electrode of the spark plug  10  is grounded. 
     In the ignition apparatus  2 , when the ignition signal IG is at a high level, the power transistor  14  is turned on, so that a current flows through the primary winding L 1  of the ignition coil  12 . When the ignition signal IG subsequently reaches a low level and the power transistor  14  is turned off, a high ignition voltage is generated across the secondary winding L 2  of the ignition coil  12 . This high voltage is applied to the center electrode of the spark plug  10  in order to cause the spark plug  10  to effect spark discharge. The ignition apparatus  2  is designed such that the center electrode of the spark plug  10  attains negative polarity during the spark discharge; therefore, the spark discharge current Isp caused by the spark discharge flows from the spark plug  10  to the secondary winding L 2 . 
     The ion current detection apparatus  100  includes a resistor  20 , one end of which is grounded; a diode  22  which is connected in parallel to the resistor  20  and whose cathode is grounded; a capacitor  24  connected in series to the ungrounded end of the resistor  20  and to the ungrounded end of the diode  22 ; and a Zener diode  26  which is connected in parallel to the circuit comprising the resistor  20 , the diode  22 , and the capacitor  24 . The cathode of the Zener diode  26  is connected to the capacitor  24 , and the anode of the Zener diode  26  is grounded. The connection line between the capacitor  24  and the Zener diode  26  is connected to the secondary winding L 2  of the ignition coil  12 . A voltage generated across the resistor  20  is output as a detection value Vio. 
     In the ion current detection apparatus  100  having the above-described structure, the spark discharge current Isp stemming from spark discharge of the spark plug  10  flows through a current path including the capacitor  24  and the diode  22 , while causing the Zener diode  26  to produce a Zener voltage Vz. Therefore, due to the spark discharge current Isp, the capacitor  24  is charged by a voltage Vc (=Vz−Vf) which is smaller than the Zener voltage Vz of the Zener diode  26  by the forward voltage Vf of the diode  22 . 
     When the high ignition voltage induced in the secondary winding L 2  drops to a level lower than the Zener voltage Vz, the capacitor  24  starts discharging, so that a high detection voltage according to the charged voltage Vc is applied to the spark plug  10  via the secondary winding L 2  of the ignition coil  12 . As a result, an ion current Iio flows in accordance with the number of ions generated between the electrodes of the spark plug  10 . Since the ion current Iio flows through the resistor  20 , the ion current detection apparatus  100  outputs a detection value Vio corresponding to the ion current Iio. 
     However, in the secondary-side circuit of the ignition apparatus  2 , since the inductance of the secondary winding L 2  of the ignition coil  12  and the capacitance between the electrodes of the spark plug  10  form a resonant circuit, voltage damped oscillation is generated after completion of spark discharge of the spark plug. 
     Depending on the operation conditions of the internal combustion engine, the magnitude of the current that flows during that period may reach a value of several to several tens of times the ion current Iio. In addition, the oscillation continues for a relatively long period of time as long as several milliseconds. Therefore, as shown in FIG. 7, the oscillation component is superposed on the ion current Iio, resulting in it being impossible to measure properly the ion current Iio. 
     In order to overcome the above-described problem, the measurement may be performed at a point in time when the voltage damped oscillation has converged. However, since the charge accumulated in the capacitor  24  is consumed by the voltage damped oscillation, when the voltage damped oscillation converges, a high voltage required for detection of the ion current Iio becomes impossible to obtain, resulting in possible failure to detect the ion current Iio. 
     This problem can be mitigated through an increase in the capacitance of the capacitor  24 , which allows a larger amount of charge to be accumulated during spark discharge of the spark plug  10 . However, in this case, if only a small amount of charge is consumed due to flow of the ion current Iio, an undesirable voltage is applied to the spark plug  10  due to the charge remaining in the capacitor  24 . In this case, if particles of deposited carbon and liquid fuel are present on the surface of the insulator of the spark plug  10 , particles are easily moved and aligned between the electrodes by an electric field that is produced through the voltage application. As a result, there arises a new problem that so-called contamination of the spark plug  10 , in which the insulating resistance between the electrodes of the spark plug decreases, occurs quickly. 
     SUMMARY OF THE INVENTION 
     In view of the forgoing problems, an object of the present invention is to provide an ion current detection apparatus which can detect ion current with a high degree of accuracy regardless of the presence of voltage damped oscillation and which does not cause contamination of a spark plug. 
     In order to achieve the above object, an ion current detection apparatus according to a first aspect of the invention includes: a capacitor which forms a closed loop together with a spark plug and a secondary winding of an ignition coil; current detection means for detecting current flowing through the closed loop; and charge means for charging the capacitor to a predetermined high voltage for detection, through use of spark discharge current flowing during spark discharge of the spark plug. A high ignition voltage which is generated in the secondary winding through intermittent supply of primary current to a primary winding of the ignition coil is applied to the spark plug attached to a cylinder of an internal combustion engine in order to cause spark discharge. Subsequently, the capacitor charged by the charge means applies to the secondary winding of the ignition coil and the spark plug a high voltage for detection having a polarity opposite that of the high voltage for ignition. An ion current flowing through the closed loop at this time is detected by the current detection means. The ion current detection apparatus of this aspect further comprises a charge diode, a discharge switch, and switching control means. The charge diode is connected in series to the capacitor such that the forward direction of the charge diode coincides with the flow direction of the spark discharge current and is adapted to prevent discharge of charge accumulated in the capacitor by the charge means. The discharge switch short-circuits opposite ends of the charge diode in order to discharge charge accumulated in the capacitor. The switching control means operates the discharge switch at a timing at which the ion current is to be detected. 
     Thus in the ion current detection apparatus having the above-described structure, at the time of spark discharge, through utilization of the spark discharge current, the charge means charges the capacitor to a predetermined high voltage for detection. Since the spark discharge current is supplied to the capacitor via the charge diode, the charge is not discharged even when the high voltage for ignition becomes lower than the charged voltage of the capacitor (high voltage for detection). That is, even when the high voltage for ignition causes oscillation, the oscillation does not cause leaking out of the charge accumulated in the capacitor. 
     Subsequently, at the timing when ion current is to be detected, the switching control means operates the discharge switch in order to short-circuit opposite ends of the charge diode. Thus, a high voltage for detection having a polarity opposite that of the high voltage for ignition is applied to the secondary winding of the ignition coil and the spark plug. As a result, an ion current flows in a closed loop formed by the ignition coil, the spark plug, the capacitor, and a current detection resistor in an amount corresponding to the resistance between the electrodes of the spark plug. The ion current can be detected by the current detection means. 
     That is, in the ion current detection apparatus of the present invention, charge accumulated in the capacitor is discharged, at only the timing when the ion current is to be detected, to thereby apply to the spark plug a high voltage for detection. 
     Accordingly, in the ion current detection apparatus of the present invention, even when voltage damped oscillation occurs in the secondary-side circuit of the ignition coil after spark discharge, charge accumulated in the capacitor is not wastefully consumed thereby, so that the capacitance of the capacitor can be set to a necessary and sufficient value. In addition, reliable detection of the ion current is possible. 
     Further, the ion current detection can be performed after the voltage damped oscillation has converged to some degree, while the period in which the damped oscillation is large is avoided. Therefore, the ion current detection can be performed with a high degree of accuracy. As a result, the value detected by the ion current detection apparatus of the present invention corresponds substantially to the ion current only, so that a filter or the like for removing noise components from the detection value can be omitted or simplified. 
     Further, even when only a small amount of ion current flows after spark discharge due to misfire of the engine or other cause, and charge remains at the capacitor, the voltage of the capacitor is not applied to the spark plug when the discharge switch is opened. Therefore, contamination of the spark plug can be prevented. 
     The ion current detection apparatus may be further characterized in that the timing at which the switching control means operates the discharge switch is set in accordance with the operation conditions of the engine. Since the operation timing of the discharge switch; i.e., the detection timing of the ion current, can be set in accordance with operation conditions, such as the rotation speed of the engine, that affect the timing of generation of the ion current, more accurate and stable detection can be performed. 
     The ion current detection apparatus of the above first aspect may be further characterized by provision of grounding means for grounding a current path extending from the anode of the charge capacitor to the spark plug during an arbitrary period after the discharge switch is opened but before the next spark discharge is caused. Since charge remaining at the electrode of the spark plug can be reliably removed by the grounding means, contamination of the spark plug can be prevented in a more reliable manner. 
     By the way, the detection of ion current can be properly performed through use of a conventional apparatus as is without provision of the charge diode, the discharge switch, and the switching control means described above, if the damped oscillation appearing after spark discharge is reduced through proper adjustment of the inductance and stray capacitance of the secondary winding of the ignition coil. However, even in such a case, if a sufficient amount of ion current does not flow due to misfire or the like and thus charge remains in the capacitor, undesirable voltage is applied to the electrode of the spark plug, resulting in contamination of the spark plug. 
     In a second aspect of the invention an ion current detection apparatus includes: a capacitor which forms a closed loop together with a spark plug and a secondary winding of an ignition coil; current detection means for detecting current flowing through the closed loop; and charge means for charging the capacitor to a predetermined high voltage for detection, through use of spark discharge current flowing during spark discharge of the spark plug. A high voltage for ignition which is generated in the secondary winding through intermittent supply of primary current to a primary winding of the ignition coi is applied to the spark plug attached to a cylinder of an internal combustion engine in order to cause spark discharge. Subsequently, the capacitor charged by the charge means applies to the second winding of the ignition coil and the spark plug a high voltage for detection having a polarity opposite that of the high voltage for ignition. An ion current flowing through the closed loop at this time is detected by the current detection means. The ion current detection apparatus of this aspect further comprises grounding means for grounding a high voltage side of the capacitor charged by the charge means, during an arbitrary period between detection of the ion current by the current detection means and subsequent spark discharge. 
     In the ion current detection apparatus of this aspect of the present invention the charge remaining at the capacitor after spark discharge is reliably removed by the grounding means. Therefore, it is possible to prevent the phenomenon that application of undesirable voltage to the electrode of the spark plug continues until subsequent spark discharge occurs, so that contamination of the spark plug can be prevented reliably. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram showing the overall structure of an internal combustion engine control system to which an ion current detection apparatus of a first embodiment is applied; 
     FIG. 2 is a flowchart showing ion current detection processing executed by the ECU; 
     FIG. 3 is a wave chart showing signals at respective points in the apparatus of the first embodiment; 
     FIG. 4 is a diagram showing the overall structure of an internal combustion engine control system to which an ion current detection apparatus of a second embodiment is applied; 
     FIG. 5 is a wave chart showing signals at respective points in the apparatus of the second embodiment; 
     FIG. 6 is a diagram showing the overall structure of a conventional apparatus; and 
     FIG. 7 is a wave chart showing signals at respective points in the conventional apparatus. 
    
    
     DESCRIPTION OF SYMBOLS USED IN THE DRAWINGS 
       2  . . . ignition apparatus 
       4  . . . ion current detection apparatus 
       6  . . . ECU 
       8  . . . detection circuit 
       10  . . . spark plug 
       12  . . . ignition coil 
       14  . . . power transistor 
       20  . . . resistor 
       22  . . . diode 
       24  . . . capacitor 
       26  . . . Zener diode 
       28  . . . charge diode 
       30  . . . discharge switch 
       32  . . . transistor 
     L 1  . . . primary winding 
     L 2  . . . secondary winding 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 1 shows a schematic structure of an internal combustion engine control system equipped with a single-electrode distributor-less-type ignition apparatus to which the present invention is applied. 
     As shown in FIG. 1, the internal combustion engine control system includes an ignition apparatus  2 , a battery BT, an ion current detection apparatus  4 , an electronic control unit (hereinafter referred to as an “ECU”)  6  for an internal combustion engine, and a detection circuit  8 . In accordance with an externally input ignition signal IG, the ignition apparatus  2  causes a spark plug  10  provided for each cylinder of the internal combustion engine to discharge sparks. The battery BT supplies power to the ignition apparatus  2 . At the timing of an externally input detection signal, the ion current detection apparatus  4  detects an ion current that flows due to ions generated in the vicinity of the electrodes of the spark plug  10 . The ECU  6  outputs the ignition signal IG to the ignition apparatus  2  and also outputs the detection signal Sd to the ion current detection apparatus  4 . The detection circuit  8  converts an analog output of the ion current detection apparatus  4  into a digital signal suitable or input to the ECU  6 . 
     Although corresponding structural components (other than he ECU  6 ) are provided for each cylinder of the engine, in the interests of facilitating understanding, FIG. 1 shows only the structural components provided for one cylinder. 
     The ignition apparatus  2  has the same structure as the ignition apparatus shown in FIG.  6  and described above, whereas the ion current detection apparatus  4  has the same structure as the conventional ion current detection apparatus  100  except for some portions. Therefore, identical structural portions are denoted by the same symbols, and their descriptions will be omitted. Only those portions that differ from the conventional apparatus will be described. 
     In the ion current detection apparatus  4 , within a closed loop formed by a capacitor  24 , a resistor  20 , and a diode  22  in cooperation with a secondary winding L 2  of an ignition coil  12  and the spark plug  10 , a charge diode  28  is connected in series between the capacitor  24  and the secondary winding L 2  of the ignition coil  12 , such that the forward direction of the diode  28  corresponds to the flow direction of spark discharge current Isp. Further, a discharge switch  30 , which short-circuits the opposite ends of the charge diode  28  in accordance with the detection signal Sd input externally, is connected in parallel to the charge diode  28 . That is, the circuit formed by the capacitor  24 , the resistor  20 , the diode  22 , the charge diode  28 , and the discharge switch  30  is connected in parallel to the Zener diode  26 . 
     Further, a transistor  32  is provided in the ion current detection apparatus  4 . The collector of the transistor  32  is connected to a line for connection with the secondary winding L 2  of the ignition coil  12 , whereas the emitter of the transistor  32  is grounded. The transistor  32  grounds the line connected to the secondary winding L 2  in accordance with a ground signal Sg that is externally input to the base. In the present embodiment, the resistor  20  serves as a current detection means, and the Zener diode  26  serves as a charge means. 
     In the ion current detection apparatus  4  having the above-described structure, when the discharge switch  30  is opened, current can flow only in the direction from the line connected to the second winding L 2  toward the ground. At this time, a current flows in a closed loop including the charge diode  28 , the capacitor  24 , and the diode  22 . At the same time, a current flows through the Zener diode  26  in such a direction as to generate a Zener voltage Vz. Therefore, the capacitor  24  is charged by a voltage Vc=(Vz−2×Vf) which is smaller than the Zener voltage Vz of the Zener diode  26  by the sum of the forward voltages Vf of the charge diode  28  and the diode  22 . 
     When the discharge switch  30  is closed and thus the opposite ends of the charge diode  28  are short-circuited, current is allowed to flow from the grounded side toward the line connected to the secondary winding L 2 . At this time, since a current flows in a closed loop including the resistor  20 , the capacitor  24 , and the discharge switch  30 , the voltage produced across the resistor  20  corresponds to the magnitude of the current. 
     The voltage Vp applied to the spark plug  10  at this time becomes smaller than the charged voltage Vc of the capacitor  24  by the voltage drop at the resistor  20  (Vp=Vc−R×Iio, where R is the resistance of the resistor  20 ). The applied voltage Vp must be set to a level at which the spark plug  10  does not cause spark discharge (e.g., about 1 kV); i.e., the Zener voltage Vz of the Zener diode  26  must be set on the basis of the applied voltage Vp. 
     When the transistor  32  is turned on in response to the ground signal Sg and thus the line connected to the secondary winding L 2  is grounded, the charge remaining at the electrodes of the spark plug  10  is discharged. 
     Next, there will be described an ion current detection processing performed by the ECU  6 . 
     The ECU  6  is provided for performing total control of the ignition timing, fuel injection amount, and idling speed of the internal combustion engine, and therefore performs condition detection processing for detecting various operation conditions such as an intake pipe pressure (or intake air amount), rotational speed, cooling water temperature of the engine, and signal output processing for various kinds of signals required for controlling the engine, such as the above-described ignition signal IG in accordance with the detected operation conditions, as well as ion current detection processing, which will be described below. The signal output processing sets the ignition signal IG to a high level at a predetermined time earlier than an ignition timing of each cylinder that is set in accordance with the operation conditions, and then sets the ignition signal IG to a low level at the ignition timing. 
     As shown in FIG. 2, when the ion current detection processing is started, in step S 110 , the ECU  6  reads in conditions, such as the rotational speed of the engine, that affect the timing of generation of ions between the electrodes of the spark plug  10 , among the operation conditions detected through the separately executed condition detection processing. In subsequent step S 120 , the ECU  6  sets a wait time Tw before actuation of the discharge switch  30  in accordance with the operation conditions read in step silo. 
     The wait time Tw is determined such that the ion current Iio can be detected after the voltage damped oscillation generated in the secondary-side circuit of the ignition coil  12  after spark discharge has converged sufficiently. The wait time Tw may be set through use of ROM. In this case, the experimentally obtained relationship between the operation conditions and the wait time Tw is stored in the ROM in the form of a table, and the wait time Tw is read out from the ROM while the operation conditions are used as reference values. 
     In subsequent step S 130 , a judgement is made as to whether the ignition timing at which the spark plug  10  causes spark discharge has arrived. Specifically, the arrival of the ignition timing is judged based on whether the ignition signal IG has been switched from the high level to the low level by the separately executed signal output processing. The ECU  6  repeatedly performs step S 130  until the ignition timing has arrived. When the ignition timing is judged to have arrived, the ECU proceeds to step S 140 . 
     In step S 140 , judgement is made as to whether the wait time Tw set in step S 120  has elapsed. This judgement is made on the basis of clocking time elapsed after the ignition timing, by use of a timer built into the ECU  6 . If it is judged that the wait time Tw has elapsed, the ECU  6  proceeds to step S 150 . In step S 150 , the ECU  6  brings the detection signal Sd to the high level during a predetermined detection period in order to operate the discharge switch  30  during that period, to thereby short-circuit the opposite ends of the charge diode  28 . The detection period is preferably set such that when the ion current Iio flows properly, the charge of the capacitor  24  is discharged completely. 
     In subsequent step S 160 , during the detection period (during which the detection signal Sd is at the high level), the ECU  6  reads in a detection value Dio from the detection circuit  8 , which is obtained through analog-to-digital conversion of the voltage Vio across the resistor  20 . 
     After completion of the detection period, in step S 170 , the ECU  6  outputs a ground signal Sg in order to turn on the transistor  32  to thereby discharge the charge remaining at the spark plug  10 . Subsequently, the present processing is ended. 
     That is, in the present embodiment, when the ignition signal IG is switched from the high level to the low level yes in S 130 ), the power transistor  14  is turned off, so that the current flowing through the primary winding L 1  of the ignition coil  12  is cut off. As a result, a high ignition voltage (several tens of kilovolts) is induced in the secondary winding L 2  and is applied to the center electrode of the spark plug  10 , so that, as shown in FIG. 3, the spark plug  10  causes spark discharge (time t 1 ). 
     The spark discharge current Isp flowing upon the spark discharge causes the Zener diode  26  to generate a Zener voltage Vz and flows into the capacitor  24  via the charge diode  28  to thereby charge the capacitor  24 . 
     Upon completion of discharge, the high ignition voltage induced in the secondary winding L 2  starts damped oscillation (time t 2 ). However, during the wait period Tw, the detection signal Sd is maintained at the low level and thus the discharge switch  30  is maintained open. Therefore, the charge accumulated in the capacitor  24  is not discharged (time t 2  to t 3 ). 
     When the wait time Tw has elapsed (yes in S 140 ) and the detection signal Sd is switched to the high level (S 150 ), the opposite ends of the charge diode  28  are short-circuited by the discharge switch  30  during the detection period, during which the detection signal Sd is maintained at the high level. Thus, discharge from the capacitor  24  is allowed (time t 3 ). As a result, a high detection voltage is applied to the spark plug  10  via the secondary winding L 2  of the ignition coil  12 , so that an ion current Iio flows in correspondence with the number of ions present between the electrodes of the spark plug  10 . 
     At this time, the detection circuit  8  performs analog-to-digital conversion for the voltage Vio that is produced across the resistor  20  due to the ion current Io flowing therethrough, and outputs the thus-obtained detection value Dio. This detection value Dio is taken into the ECU  6  (S 160 ). 
     The detection value Dio of the ion current Iio taken in to the ECU  6  is used for judgement of the generation of misfire or knocking of the engine as well as for detection of various operation conditions (e.g., air-fuel ratio, lean limit of the air-fuel ratio, and limit of amount of recirculated exhaust gas) of the engine. 
     Subsequently, when the detection signal Sd is switched to the low level after completion of the detection period, the discharge from the capacitor  24  is prevented by means of the charge diode  28  (time t 4 ). Accordingly, the voltage generated at the capacitor  24  is not applied to the electrode of the spark plug  10  even when no ion current Iio flows, due to misfire or the like of the engine, and thus charge remains in the capacitor  24 . 
     Further, at the same time, the ground signal sg is switched to the high level in order to cause the transistor  32  to ground the line of the ion current detection apparatus  4  connected to the secondary winding L 2 . Thus, the charge that remains at the electrodes of the spark plug  10  due to insufficient flow of the ion current Iio in the case of, for example, misfire is reliably discharged (time t 4  to t 5 ). Therefore, the spark plug  10  is not left in a state in which an undesired voltage is applied between the electrodes of the spark plug  10 . 
     The turning-on of the transistor  32  (discharge of the remaining charge of the spark plug  10 ) may be performed at an arbitrary timing between the point in time when the detection signal Sd is switched to the low level and the point in time when subsequent spark discharge is caused (when the ignition signal IG is switched to the low level). Further, the transistor  32  may be disposed at any position in the current path between the anode of the charge diode  28  and the spark plug  10 . 
     As described above, in the ion current detection apparatus  4  of the present embodiment, during only the detection period in which the ion current Iio is to be detected, discharge of charge accumulated in the capacitor  24  is allowed in order to apply a high voltage for detection to the spark plug  10 . 
     Accordingly, in the ion current detection apparatus  4  of the present embodiment, even when voltage damped oscillation occurs in the secondary-side circuit of the ignition coil  12  after spark discharge, charge accumulated in the capacitor  24  is not wastefully consumed thereby, so that the capacitance of the capacitor  24  can be set to a necessary and sufficient value. 
     Further, the ion current detection apparatus  4  of the present embodiment is designed to detect the ion current Iio after passage of the wait time Tw after spark discharge of the spark plug  10 . Accordingly, according to the present embodiment, the ion current Iio can be detected in a state in which the voltage damped oscillation of the secondary-side circuit has converged sufficiently. Thus, the accuracy in detecting the ion current Iio can be increased, and a filter circuit or the like for removing, from the detection value Vio (Dio) of the ion current Iio, noise components stemming from the damped oscillation can be omitted or simplified. 
     Further, in the ion current detection apparatus  4  of the present embodiment, since the wait time Tw before actuation of the discharge switch  30 ; i.e., the detection timing of the ion current Iio, is set in accordance with operation conditions, such as the rotation speed of the engine, that affect the generation of the ion current Iio, accurate detection can be always performed regardless of variations in the operation conditions. 
     Moreover, even when only a small amount of ion current Iio flows after spark discharge due to misfire of the engine or other cause, and charge remains at the capacitor  24  and the spark plug  10 , application of an undesirable voltage to the electrode of the spark plug  10  can be reliably prevented through a simple operation of opening the discharge switch  30  and turning on the transistor  32 , so that contamination of the spark plug  10  is prevented. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. 
     As shown in FIG. 4, an ion current detection apparatus  6  according to the present embodiment is constructed in the same manner as in the ion current detection apparatus  4  of the first embodiment, except that the charge diode  28  and the discharge switch  30  are omitted from the ion current detection apparatus  4 . However, the secondary winding L 2  of the ignition coil  12  is designed to have an inductance and stray capacitance such that damped voltage oscillation that is generated in the circuit on the secondary side of the ignition coil  12  after spark discharge is decreased sufficiently. 
     The ion current detection processing performed by the ECU  6  is the same as that performed in the first embodiment, except that the processing of step S 150  related to the operation of the discharge switch  30  is omitted, and the wait time in step S 140  is set such that the detection value Dio of the ion current is read in during a period between completion of spark discharge Isp and extinction of ion current Iio. 
     Accordingly, in the ion current detection apparatus  6  of the present embodiment, when the ignition signal IG is switched from the high level to the low level (S 110 -S 130 ), a high ignition voltage (several tens of kilovolts) is induced in the secondary winding L 2  of the ignition coil  12 , so that the spark plug  10  causes spark discharge (time t 11 ). Due to the spark discharge current Isp flowing during the spark discharge, the capacitor  24  is charged. The above-described operation is completely identical to that in the first embodiment. 
     When the discharge ends (time t 12 ), and the high voltage for ignition induced in the secondary winding L 2  becomes lower than the Zener voltage Vz, due to discharge of the capacitor  24 , a high detection voltage corresponding to the charged voltage Vc of the capacitor  24  is applied to the spark plug  10  via the secondary winding L 2  of the ignition coil  12 , so that an ion current Iio flows in correspondence with the number of ions present between the electrodes of the spark plug  10 . 
     At this time, the detection circuit  8  performs analog-to-digital conversion for the voltage Vio that is produced across the resistor  20  due to the ion current Iio flowing therethrough, and outputs the thus-obtained detection value Dio. This detection value Dio is taken into the ECU  6  (S 140 , S 160 ). 
     When the ions between the electrodes of the spark plug  10  disappear and the ion current Iio becomes zero (time t 13 ), the voltage across the capacitor  24  is held at a level corresponding the residual charge at that time, so that the voltage across the capacitor  24  is applied to the spark plug  10 . Especially, when the ion current Iio does not flow in a sufficient amount due to misfire or the like, the applied voltage becomes considerably high. 
     However, when the ground signal Sg is switched to the high level to turn on the transistor  32  (time t 14 ), the charge that remains in the capacitor  24  is discharged. Therefore, the spark plug  10  is not left in a state in which an undesired voltage is applied between the electrodes of the spark plug  10 . 
     The turning-on of the transistor  32  (discharge of the remaining charge of the spark plug  10 ) through use of the ground signal Sg may be performed at arbitrary timing between the point in time when the ECU  6  reads in the detection value Dio and the point in time when subsequent spark discharge is caused. However, the transistor  32  is preferably turned on as early as possible. Further, the transistor  32  may be disposed at any position in the current path between the capacitor  24  and the spark plug  10 . 
     As described above, in the ion current detection apparatus  6  of the second embodiment, after detection of the ion current Iio, the transistor  32  is turned on in order to discharge the residual charges of the capacitor  24  and the spark plug  10 . Therefore, it is possible to prevent application of an undesirable voltage to the electrode of the spark plug  10 , which would otherwise occur before subsequent spark discharge, so that contamination of the spark plug  10  is prevented. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.