Patent Publication Number: US-9890757-B2

Title: Electronic control of a spark plug for an internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an internal-combustion-engine electronic control system and particularly to an electronic control system that controls an internal-combustion-engine ignition device. 
     Description of the Related Art 
     As is well known, in an internal-combustion-engine ignition device, for example, in the case where an ignition signal is inputted for a time longer than a predetermined time, in the case where an electronic component included in the ignition device overheats, or in the case where an excessive current flowing in an ignition coil is detected, it is required to forcibly shut off the primary current of the ignition coil at a timing different from the regular ignition timing so as to protect the ignition device. 
     However, in the case where the primary current of an ignition coil is forcibly shut off at a timing different from the regular ignition timing, it is required to softly shut off the primary current of an ignition coil in order to prevent a high voltage, which is high enough to cause a spark discharge in the ignition plug, from being generated across the secondary winding of the ignition coil; additionally, in order to suppress heat generation in an electronic component as much as possible, the soft shutoff operation needs to be implemented in a minimum time. 
     To date, there has been disclosed an internal-combustion-engine ignition device where there are included a current limiting circuit that limits a primary current flowing in an ignition coil, a timer circuit that serves as an abnormality detection means, and a detection circuit that detects abnormal heating, and when any one of these circuits detects an abnormality, the primary current of the ignition coil is softly shut off in a time between 17 [ms] and 135 [ms] (for example, refer to Patent Document 1). 
     Patent Document 1: JP-A-2008-45514 
     The conventional system disclosed in Patent Document 1 is provided with a circuit that softly shuts off the primary current of an ignition coil in a time between 17 [ms] and 135 [ms] when an abnormality in an ignition device is detected; however, neither the inductance and the impedance of an ignition coil nor the characteristics of an insulated-gate bipolar transistor is taken into account. As a result, there has been a problem that the time of soft shutoff varies depending on these values, the characteristics, the limiting value for the primary current of an ignition coil, or the like. 
     SUMMARY OF THE INVENTION 
     The present invention has been implemented in order to solve the problem in the foregoing conventional system; the objective thereof is to provide an internal-combustion-engine electronic control system capable of softly shutting off the primary current of an ignition coil in an optimum time. 
     An internal-combustion-engine electronic control system according to the present invention is provided with a power switching device that applies or shuts off a primary current of an ignition coil of an internal combustion engine so that at the secondary side of the ignition coil, there is generated a voltage for making an ignition plug of the internal combustion engine produce a spark discharge; and a control unit that turns on or off the power switching device. The internal-combustion-engine electronic control system is characterized in that the control unit is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state; and based on a change in the conduction state of the power switching device, the characteristics of the soft shutoff is changed halfway through the shutoff operation. 
     In the present invention, the description that “at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state” means a state where the continuous conduction duration of the power switching device exceeds a predetermined time, a state where the value of the primary current of the ignition coil exceeds a predetermined value, a state where the temperature of at least part of constituent elements included in the control unit, the ignition coil, the ignition plug, the power switching device, or the like exceeds a predetermined value, or a state similar to each of the foregoing states. 
     An internal-combustion-engine electronic control system according to the present invention is provided with a power switching device that applies or shuts off a primary current of an ignition coil of an internal combustion engine so that at the secondary side of the ignition coil, there is generated a voltage for making an ignition plug of the internal combustion engine produce a spark discharge; and a control unit that turns on or off the power switching device. The internal-combustion-engine electronic control system is characterized in that the control unit is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state; in that the circuit unit includes a capacitor, a first slow-discharge circuit that makes electric charge in the capacitor discharge with a predetermined time constant, and a second slow-discharge circuit that makes electric charge in the capacitor discharge with a time constant that is smaller than the time constant of the first slow-discharge circuit  15 ; and in that based on the predetermined state, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit and makes the capacitor discharge electric charge so as to perform the soft shutoff. 
     In the present invention, the description that “at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state” means a state where the continuous conduction duration of the power switching device exceeds a predetermined time, a state where the value of the primary current of the ignition coil exceeds a predetermined value, a state where the temperature of at least part of constituent elements included in the control unit, the ignition coil, the ignition plug, the power switching device, or the like exceeds a predetermined value, or a state similar to each of the foregoing states. 
     In the internal-combustion-engine electronic control system according to the present invention, the control unit, which turns on or off the power switching device, is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state; and based on a change in the conduction state of the power switching device, the characteristics of the soft shutoff is changed halfway through the shutoff operation. Therefore, in the case where the primary current of the ignition coil is forcibly shut off at a timing different from the regular ignition timing, the soft shutoff operation can be performed at an optimum timing, regardless of the characteristics of the ignition coil itself or the characteristics of the power switching device. 
     Moreover, the internal-combustion-engine electronic control system according to the present invention is configured in such a way that the control unit, which turns on or off the power switching device, is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state; in such a way that the circuit unit includes a capacitor, a first slow-discharge circuit that makes electric charge in the capacitor discharge with a predetermined time constant, and a second slow-discharge circuit that makes electric charge in the capacitor discharge with a time constant that is smaller than the time constant of the first slow-discharge circuit  15 ; and in such a way that based on the predetermined state, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit and makes the capacitor discharge electric charge so as to perform the soft shutoff. Therefore, in the case where the primary current of the ignition coil is forcibly shut off at a timing different from the regular ignition timing, the soft shutoff operation can be performed at an optimum timing, regardless of the characteristics of the ignition coil itself or the characteristics of the power switching device. 
     The foregoing and other object, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an internal-combustion-engine electronic control system according to Embodiment 1 of the present invention; 
         FIG. 2  is a timing chart for explaining the operation of an internal-combustion-engine electronic control system according to Embodiment 1 of the present invention; 
         FIG. 3  is an explanatory graph for comparing the operation of a conventional system with the operation of an internal-combustion-engine electronic control system according to Embodiment 1 of the present invention; 
         FIG. 4  is a block diagram illustrating a specific circuit for an internal-combustion-engine electronic control system according to Embodiment 1 of the present invention; 
         FIG. 5  is a block diagram of an internal-combustion-engine electronic control system according to Embodiment 2 of the present invention; 
         FIG. 6  is a timing chart for explaining the operation of an internal-combustion-engine electronic control system according to Embodiment 2 of the present invention; and 
         FIG. 7  is a block diagram of an internal-combustion-engine electronic control system according to Embodiment 3 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a block diagram of an internal-combustion-engine electronic control system according to Embodiment 1 of the present invention. In  FIG. 1 , an internal-combustion-engine electronic control system (referred to as an ECU, hereinafter)  1  controls a current that flows in the primary winding of an ignition coil  2 . An ignition plug  3  produces a spark discharge by use of a high voltage induced across the secondary winding of the ignition coil  2  so as to ignite a fuel in an unillustrated combustion chamber of an internal combustion engine. A battery  4  supplies electric power to ECU  1  and the ignition coil  2 . 
     Next, the configuration of ECU  1  will be explained. In ECU  1 , the output terminal of a calculation device (referred to as CPU, hereinafter)  11  is connected with the base of an NPN-type transistor  102  by way of a resistor  114 . The collector of the transistor  102  is connected with the base or the gate of a power switching device  103  by way of a resistor  112 . 
     The collector of the NPN-type transistor  102  is connected with the collector of a PNP-type transistor  101  by way of a resistor  115 . The emitter of the PNP-type transistor  101  is connected with the positive electrode of the battery  4 . A timer circuit  12  is connected with CPU  11  and a resistor  113  connected with the base of the PNP-type transistor  101 . The emitter of the power switching device  103  is grounded by way of a resistor  111 ; the collector thereof is connected with the primary winding of the ignition coil  2 . 
     A first slow-discharge circuit  15  is connected between a capacitor  17  and the connection point between the collector of the PNP-type transistor  101  and the resistor  115 ; a second slow-discharge circuit  16  is connected between the collector of the PNP-type transistor  101  and the collector of the power switching device  103 . 
     In ECU  1 , the constituent components excluding the power switching device  103  configure a control device for turning on or off the power switching device  103 . 
     The first slow-discharge circuit  15  and the second slow-discharge circuit  16  configure a circuit unit that softly shuts off the power switching device  103  in order to prevent the ignition plug  3  from producing a spark discharge in the case where at least one of the power switching device  103  and the foregoing control device is in a predetermined state, described later. 
     A current detection circuit  14  is connected between the connection point between the emitter of the power switching device  103  and the resistor  111  and the connection point between the base of the PNP-type transistor  101  and the resistor  113 . An overheating detection circuit  13  is connected with the base of the PNP-type transistor  101  and the current detection circuit  14 . 
     An ignition signal outputted from CPU  11  is inputted to the base of the NPN-type transistor  102  by way of the resistor  114  and then is transferred to the base of the power switching device  103  by way of the resistor  112 . The ignition signal transferred to the base of the power switching device  103  becomes HIGH level or LOW level so as to turn on or off the power switching device  103 , so that energization/de-energization control of the primary winding of the ignition coil  2  is performed. 
     The timer circuit  12  counts the energization duration in which the ignition signal outputted from CPU  11  is HIGH level; in the case where the energization duration is shorter than a predetermined time, the timer circuit  12  outputs a LOW-level signal and inputs it to the base of the PNP-type transistor  101  by way of the resistor  113 ; in the case where the energization duration is longer than the predetermined time, the timer circuit  12  outputs a HIGH-level signal and inputs it to the base of the PNP-type transistor  101  by way of the resistor  113 . The CPU  11  outputs an energization permission signal or an energization prohibition signal for the ignition coil  2  and inputs it to the base of the PNP-type transistor  101  by way of the resistor  113 . 
     When detecting abnormal heating in the overheating detection circuit  13 , the power switching device  103  changes the level of its output signal from LOW to HIGH, and then inputs the HIGH-level signal to the base of the PNP-type transistor  101 . When determining, based on the voltage across the resistor  111 , that an excessive current is flowing in the power switching device  103 , the current detection circuit  14  changes the level of its output signal from LOW to HIGH, and then inputs the HIGH-level signal to the base of the PNP-type transistor  101 . 
     ECU  1  is configured in such a way as described above; therefore, in the case where any one of CPU  11 , the timer circuit  12 , the overheating detection circuit  13 , and the current detection circuit  14  outputs a HIGH-level signal, the PNP-type transistor  101  turns off, so that the primary current of the ignition coil is softly shut off, as described later. In this situation, the case where any one of the timer circuit  12 , the overheating detection circuit  13 , and the current detection circuit  14  outputs a HIGH-level signal corresponds to the foregoing case where at least one of the power switching device  103  and the control device is in a predetermined state 
     Next, there will be explained the operation of the internal-combustion-engine electronic control system, according to Embodiment 1 of the present invention, that is configured as described above.  FIG. 2  is a timing chart for explaining the operation of the internal-combustion-engine electronic control system according to Embodiment 1 of the present invention.  FIG. 2(A)  represents the waveform of the base voltage or the gate voltage of the power switching device  103 ;  FIG. 2(B)  represents the waveform of the collector voltage or the drain voltage of the power switching device  103 ;  FIG. 2(C)  represents the waveform of the primary current flowing in the primary winding of the ignition coil  2 ;  FIG. 2(D)  represents the waveform of the secondary voltage induced across the secondary winding of the ignition coil  2 . 
     In  FIG. 2 , in the duration ( 1 ) from a time point t 1  to a time point t 2 , based on the ignition signal outputted from CPU  11 , a base voltage A or a gate voltage A of a predetermined HIGH level is applied to the base or the gate of the power switching device  103 . As a result, the power switching device  103  turns on; the collector voltage B or the drain voltage B thereof becomes an electric potential of a predetermined LOW level, whereby the primary current flowing in the primary winding of the ignition coil  2  gradually increases. In this situation, no voltage is induced across the ignition coil  2 . 
     At the time point t 2  when the duration ( 1 ) terminates, the base voltage A or the gate voltage A of the power switching device  103  is shut out, thereby changing to the LOW level. As a result, the power switching device  103  turns off; the collector voltage B or the drain voltage B thereof instantaneously changes from the LOW level to a high voltage because the primary current C of the ignition coil  2  is shut off, and then becomes a predetermined HIGH level. Because the primary current C of the ignition coil  2  is shut off at the time point t 2 , a secondary voltage D, which is a negative high voltage, is induced across the secondary winding of the ignition coil  2 . An ignition plug  3  produces a spark discharge by use of the secondary voltage, which is a negative high voltage induced across the secondary winding of the ignition coil  2 , so as to ignite a fuel in an unillustrated combustion chamber of the internal combustion engine. 
     Next, at a time point t 3 , based on the ignition signal outputted from CPU  11 , the base voltage A or the gate voltage A of the predetermined HIGH level is applied again to the base or the gate of the power switching device  103 . As a result, the power switching device  103  turns on; the collector voltage B or the drain voltage B thereof becomes an electric potential of a predetermined LOW level, whereby the primary current flowing in the primary winding of the ignition coil  2  gradually increases. In this situation, no voltage is induced across the ignition coil  2 . 
     In this case, for example, when at a time point t 4 , the duration where the ignition signal from CPU  11  is HIGH-level exceeds a predetermined time, the level of the output signal of the timer circuit  12  changes from the LOW level to the HIGH level, whereby the state of the PNP-type transistor  101  changes from “ON” to “OFF”. Alternatively, when at the time point t 4 , the overheating detection circuit  13  detects the fact that an electronic component included in the ignition device has overheated, a signal of HIGH level from the overheating detection circuit  13  is applied to the base of the PNP-type transistor  101 ; therefore, the state of the PNP-type transistor  101  changes from “ON” to “OFF”. Alternatively, when at the time point t 4 , the current detection circuit  14  detects the fact that a current flowing in the power switching device  103  is the same as or larger than a predetermined value, a signal of HIGH level from the current detection circuit  14  is applied to the base of the PNP-type transistor  101 ; therefore, the state of the PNP-type transistor  101  changes from “ON” to “OFF”. 
     When the PNP-type transistor  101  turns off at the time point t 4 , the electric charge stored in the capacitor  17  is slowly discharged by way of the first slow-discharge circuit  15  and the second slow-discharge circuit  16 . The discharge time constant of the second slow-discharge circuit  16  is set to be smaller than that of the first slow-discharge circuit  15 ; thus, the electric charge in the capacitor  17  is discharged with the small time constant through the second slow-discharge circuit  16 , and hence the base voltage A or the gate voltage A of the power switching device  103  rapidly lowers as represented in the duration ( 2 ) in  FIG. 2 . As a result, the power switching device  103  moves from the saturation region to the active region, so that the conduction state of the power switching device  103  changes. 
     During the duration ( 2 ) in  FIG. 2 , the primary current of the ignition coil  2  slightly increases; however, because the duration ( 2 ) is only several tens micro-seconds, the primary current does not considerably increase. Even after the power switching device  103  has moved to the active region, the base voltage A or the gate voltage A continues to lower; in contrast, the collector voltage B or the drain voltage B of the power switching device  103  starts to rise. 
     Next, in the case where at a time point t 5 , the base voltage A or the gate voltage A of the power switching device  103  and the collector voltage B or the drain voltage B of the power switching device  103  balance with each other, the discharging path for the capacitor  17  through the second slow-discharge circuit  16  is cut off, and hence the discharging operation moves into the mode where discharge is implemented only through the first slow-discharge circuit  15 . Because as described above, the time constant of the first slow-discharge circuit  15  is set to be larger than that of the second slow-discharge circuit  16 , the base voltage A or the gate voltage A of the power switching device  103  lowers further slowly, as represented in the duration ( 3 ) in  FIG. 2 , so that the power switching device  103  performs soft shutoff operation. When the base voltage A or the gate voltage A of the power switching device  103  and the collector voltage B or the drain voltage B of the power switching device  103  balance with each other, the conduction state of the power switching device  103  changes. 
     The time of soft shutoff by the power switching device  103  is a time in which the secondary voltage D induced, through the soft shutoff, across the secondary winding of the ignition coil  2  becomes a voltage value with which no spark discharge is produced in the ignition plug. As described above, by shutting off the power switching device  103  at a timing different from the regular ignition timing, the primary current of the ignition coil is forcibly shut off; however, because at this moment, the ignition plug  3  produces no ignition spark, the ignition device can be protected. 
       FIG. 3  is an explanatory graph for comparing the operation of a conventional system with the operation of the internal-combustion-engine electronic control system according to Embodiment 1 of the present invention; the ordinate denotes the base (gate) voltage of the power switching device  103 , and the abscissa denotes time. In  FIG. 3 , the dashed line X represents the waveform of soft shutoff operation by a conventional system; the solid line Y represents the waveform of soft shutoff operation by the internal-combustion-engine electronic control system according to Embodiment 1 of the present invention. The hatched area Z denotes a region where the power switching device  103  is completely “ON”, i.e., in the saturation state; because providing no effect to soft shutoff of the primary current of the ignition coil  2 , the slow discharge operation in the area Z only dissipates time. 
     As represented in  FIG. 3 , in the case of the soft shutoff operation by the conventional system, the base voltage or the gate voltage of the power switching device  103  starts to slowly lower at a time point t 11 , as represented by the waveform X; however, until a time point t 14  when the base voltage or the gate voltage of the power switching device  103  leaves the area Z, soft shutoff is not started, and during the time between the time point t 14  and a time point t 15 , soft shutoff is performed. Accordingly, in the time between the time point t 11  and the time point t 14 , soft shutoff is not performed, whereby electric power is dissipated wastefully. 
     In contrast, in the soft shutoff operation by the internal-combustion-engine electronic control system according to Embodiment 1 of the present invention, as represented by the waveform Y, at the time point t 11  at first, slow discharge by the second slow-discharge circuit  16 , which is relatively rapid, is started, and then at a time point t 12 , the soft discharge leaves the area Z; after the time point t 12 , slow discharge by the first slow-discharge circuit  15 , which is relatively slow, is performed. As a result, the soft shutoff operation by the power switching device  103  is performed in the time between the time point t 12  and the time point t 13 . Accordingly, in the internal-combustion-engine electronic control system according to Embodiment 1 of the present invention, soft shutoff operation is started earlier than in the conventional system; thus, compared with the conventional system, wasteful dissipation of electric power can be suppressed as much as possible. 
     In addition, adjustment of the respective time constants of the first slow-discharge circuit  15  and the second slow-discharge circuit  16  makes it possible to adjust the starting time point of soft shutoff or the duration of soft shutoff operation. 
       FIG. 4  is a block diagram illustrating a specific circuit for the internal-combustion-engine electronic control system according to Embodiment 1 of the present invention. In  FIG. 4 , the capacitor  17  for slow discharge and a resistor  121  connected with the capacitor  17  configure the first slow-discharge circuit in  FIG. 1 . A resistor  123 , a diode  124 , and a diode  125  configure the second slow-discharge circuit  16  in  FIG. 1 . 
     That is to say, when at the time point t 4  in  FIG. 2 , the PNP-type transistor  101  turns off, the charge stored in the capacitor  17  is discharged through the diode  124 , the resistor  123  and the diode  125  that configure the second slow-discharge circuit  16  having a small time constant, the diode  125 , and the power switching device  103 . As a result, the base voltage A or the gate voltage A of the power switching device  103  rapidly lowers, as represented in the duration ( 2 ) in  FIG. 2 ; the power switching device  103  moves from the saturation region to the active region; thus, the conduction state of the power switching device  103  changes. 
     Next, in the case where at a time point t 5 , the base voltage A or the gate voltage A of the power switching device  103  and the collector voltage B or the drain voltage B of the power switching device  103  balance with each other, the discharging path for the capacitor  17  through the diode  124 , the resistor  123 , and the diode  125  that configure the second slow-discharge circuit  16  is cut off, and hence the discharging operation moves into the mode where discharge is implemented only through the discharging path consisting of the resistor  121  that forms the first slow-discharge circuit  15 . Because as described above, the time constant of the first slow-discharge circuit  15  is set to be larger than that of the second slow-discharge circuit  16 , the base voltage A or the gate voltage A of the power switching device  103  lowers further slowly, as represented in the duration ( 3 ) in  FIG. 2 , so that the power switching device  103  performs soft shutoff operation. 
     In the circuitry illustrated in  FIG. 4 , the resistors  115  and  121 , a resistor  122 , and the capacitor  17  configure a circuit for suppressing the secondary voltage of the ignition coil  2  from making the ignition plug  3  produce a spark discharge. In other words, when the energization of the ignition coil  2  is started, the capacitor  17  is charged with a charging time constant of a circuit consisting of the resistors  115 ,  122 , and  121 . Accordingly, when the power switching device  103  turns on, the base voltage or the gate voltage in the active region is suppressed from rapidly rising and hence a current is prevented from steeply flowing into the primary coil of the ignition coil  2 ; thus, it is made possible to make an adjustment for preventing the secondary voltage of the ignition coil  2  from making the ignition plug produce a spark discharge. 
     The internal-combustion-engine electronic control system according to Embodiment 1 of the present invention, described heretofore, is provided with characteristics set forth below: 
     (1) The internal-combustion-engine electronic control system is provided with a power switching device that applies or shuts off a primary current of an ignition coil of an internal combustion engine so that at the secondary side of the ignition coil, there is generated a voltage for making an ignition plug of the internal combustion engine produce a spark discharge; and a control unit that turns on or off the power switching device. The internal-combustion-engine electronic control system is characterized in that the control unit is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state; and in that based on a change in the conduction state of the power switching device, the characteristics of the soft shutoff is changed halfway through the shutoff operation. 
     (2) The internal-combustion-engine electronic control system is characterized in that the circuit unit includes a capacitor, a first slow-discharge circuit that makes electric charge in the capacitor discharge with a predetermined time constant, and a second slow-discharge circuit that makes electric charge in the capacitor discharge with a time constant that is smaller than the time constant of the first slow-discharge circuit; and in that when the soft shutoff is started, the control unit makes electric charge in the capacitor discharge by use of the second slow-discharge circuit so as to make the power switching device perform soft shutoff operation, and then makes electric charge in the capacitor discharge by use of the first slow-discharge circuit so as to make the power switching device perform soft shutoff operation. 
     (3) In the control unit, the circuit unit is provided with a function of suppressing a secondary voltage from making the ignition plug produce a spark discharge when energization of the ignition coil with the primary current is started. 
     (4) The control unit is provided with a current detection circuit that detects the primary current flowing in the ignition coil, and when the current detection circuit detects the fact that the primary current is the same as or larger than a predetermined value, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     (5) The control unit is provided with an overheating detection circuit that detects overheating in at least one of the constituent elements included in the control unit, and when the overheating detection circuit detects the overheating, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     (6) The control unit is provided with a timer circuit that detects a continuous energization duration of the primary current flowing in the ignition coil, and when the timer circuit detects the fact that the continuous energization duration is the same as or longer than a predetermined value, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     Embodiment 2 
     Next, there will be explained an internal-combustion-engine electronic control system according to Embodiment 2 of the present invention. In an internal-combustion-engine electronic control system according to Embodiment 2 of the present invention is characterized in that by use of a timer circuit that determines, based on a change in the engine rotation speed, whether or not energization is being abnormally implemented, by measuring the energization duration of the primary current of an ignition coil, a current detection circuit that detects the primary current flowing in the ignition coil, and a heat detection circuit that detects abnormal heating, the operation status of the power switching device is recognized, the time constant for capacitor slow discharge operation is selected, and then the power switching device is softly shut off. 
       FIG. 5  is a block diagram of an internal-combustion-engine electronic control system according to Embodiment 2 of the present invention. In  FIG. 5 , a third slow-discharge circuit  21  is configured in such a way as to be capable of selecting the discharge time constant of the capacitor  17 , based on the output signals from CPU  11 , the timer circuit  12 , and the overheating detection circuit  13 . A fourth slow-discharge circuit  22  is configured in such a way as to be capable of adjusting the discharge time constant of the capacitor  17 , based on the output signal from the current detection circuit  14 . The other configurations are the same as those in Embodiment 1 described above. 
     When any one of CPU  11 , the timer circuit  12 , and the overheating detection circuit  13  detects an abnormality, the level of the output signal thereof changes from a LOW level to a HIGH level, and the output signal is inputted to the base of the PNP-type transistor  101  and the third slow-discharge circuit  21 . When the current detection circuit  14  detects an abnormality, the level of the output signal thereof changes from a LOW level to a HIGH level, and the output signal is inputted to the base of the PNP-type transistor  101  and the fourth slow-discharge circuit  22 . 
     Therefore, in the case where any one of CPU  11 , the timer circuit  12 , the overheating detection circuit  13 , and the current detection circuit  14  outputs a HIGH-level output signal to be inputted to the base of the PNP-type transistor  101 , the PNP-type transistor  101  turns off, so that the primary current of the ignition coil  2  is softly shut off, as described later. 
       FIG. 6  is a timing chart for explaining the operation of the internal-combustion-engine electronic control system according to Embodiment 2 of the present invention.  FIG. 6(A)  represents the waveform of the base voltage or the gate voltage of the power switching device  103 ;  FIG. 6(B)  represents the waveform of the collector voltage or the drain voltage of the power switching device  103 ;  FIG. 6(C)  represents the waveform of the primary current flowing in the primary winding of the ignition coil  2 ;  FIG. 6(D)  represents the waveform of the secondary voltage induced across the secondary winding of the ignition coil  2 . 
     In  FIG. 6 , in the duration ( 1 ) from a time point t 1  to a time point t 2 , based on the ignition signal outputted from CPU  11 , a base voltage A or a gate voltage A of a predetermined HIGH level is applied to the base or the gate of the power switching device  103 . As a result, the power switching device  103  turns on; the collector voltage B or the drain voltage B thereof becomes an electric potential of a predetermined LOW level, whereby the primary current flowing in the primary winding of the ignition coil  2  gradually increases. In this situation, no voltage is induced across the ignition coil  2 . 
     At the time point t 2  when the duration ( 1 ) terminates, the base voltage A or the gate voltage A of the power switching device  103  is shut out, thereby changing to the LOW level. As a result, the power switching device  103  turns off; the collector voltage B or the drain voltage B thereof instantaneously changes from the LOW level to a high voltage because the primary current C of the ignition coil  2  is shut off, and then becomes a predetermined HIGH level. Because the primary current C of the ignition coil  2  is shut off at the time point t 2 , a secondary voltage D, which is a high voltage, is induced across the secondary winding of the ignition coil  2 . An ignition plug  3  produces a spark discharge by use of the secondary voltage D, which is a high voltage induced across the secondary winding of the ignition coil  2  so as to ignite a fuel in an unillustrated combustion chamber of an internal combustion engine. 
     Next, at a time point t 3 , based on the ignition signal outputted from CPU  11 , the base voltage A or the gate voltage A of the predetermined HIGH level is applied again to the base or the gate of the power switching device  103 . As a result, the power switching device  103  turns on; the collector voltage B or the drain voltage B thereof becomes an electric potential of a predetermined LOW level, whereby the primary current flowing in the primary winding of the ignition coil  2  gradually increases. In this situation, no voltage is induced across the ignition coil  2 . 
     In this case, for example, when at a time point t 4 , the duration where the ignition signal from CPU  11  is HIGH-level exceeds a predetermined time, the level of the output signal of the timer circuit  12  changes from the LOW level to the HIGH level, and the HIGH-level output signal is inputted to the base of the PNP-type transistor and the third slow-discharge circuit  21 . Alternatively, when at the time point t 4 , the overheating detection circuit  13  detects the fact that an electronic component included in the ignition device has overheated, a HIGH-level output signal from the overheating detection circuit  13  is applied to the base of the PNP-type transistor  101  and the third slow-discharge circuit  21 . Because the HIGH-level signal is inputted to the base of the PNP-type transistor, the state of the PNP-type transistor  101  changes from “ON” to “OFF”. 
     When the PNP-type transistor  101  turns off at the time point t 4 , the electric charge stored in the capacitor  17  is slowly discharged by way of the third slow-discharge circuit  21 . The duration of this slow discharge is represented as the duration ( 2 ) in  FIG. 6 . Even after the power switching device  103  has moved to the active region, the base voltage A or the gate voltage A continues to lower; in contrast, the collector voltage B or the drain voltage B of the power switching device  103  rises. By setting the discharge time constant of the third slow-discharge circuit  21  in such a way that the value of the secondary voltage D of the ignition coil  2  becomes a value that is as small as possible and with which the ignition plug does not produce any spark discharge, it is made possible that the power switching device  103  is softly shut off in a relatively short time from the time point t 4  to a time point t 5  and hence the ignition plug  3  does not produce any spark discharge. 
     Next, at a time point t 6 , based on the ignition signal outputted from CPU  11 , the base voltage A or the gate voltage A of the predetermined HIGH level is applied again to the base or the gate of the power switching device  103 . As a result, the power switching device  103  turns on; the collector voltage B or the drain voltage B thereof becomes an electric potential of a predetermined LOW level, whereby the primary current flowing in the primary winding of the ignition coil  2  gradually increases. In this situation, no voltage is induced across the ignition coil  2 . 
     Next, when at a time point t 7 , the current detection circuit  14  detects the fact that a current flowing in the power switching device  103  is excessive current, a HIGH-level output signal from the current detection circuit  14  is inputted to the base of the PNP-type transistor  101  and the fourth slow-discharge circuit  22 . Because the HIGH-level signal is inputted to the base of the PNP-type transistor, the state of the PNP-type transistor  101  changes from “ON” to “OFF”. 
     When the PNP-type transistor  101  turns off at the time point t 7 , the electric charge stored in the capacitor  17  is slowly discharged by way of the fourth slow-discharge circuit  22 , as represented in the duration ( 3 ) in  FIG. 6 . By setting the discharge time constant of the fourth slow-discharge circuit  22  in such a way as to be as large as possible, it is made possible that even in the case where a fluctuation in the engine rotation speed prolongs the energization duration, the power switching device  103  is instantaneously turned off at a regular ignition timing t 8  when CPU  11  issues a command so that the primary current of the ignition coil  2  is shut off, a high-voltage secondary voltage is induced across the secondary winding of the ignition coil  2 , and then the ignition plug  3  produces a spark discharge. 
     The foregoing operation in the duration ( 3 ), which is performed because the current detection circuit  14  detects an excessive current, is similar to the operation in the case where a current limiting circuit is added; in the internal-combustion-engine electronic control system according to Embodiment 2 of the present invention, addition of a simple circuit makes it possible to obtain an effect the same as that obtained in the case where a current limiting circuit is provided. 
     The internal-combustion-engine electronic control system according to Embodiment 2 of the present invention, described heretofore, is provided with characteristics set forth below: 
     (1) The internal-combustion-engine electronic control system is provided with a power switching device that applies or shuts off a primary current of an ignition coil of an internal combustion engine so that at the secondary side of the ignition coil, there is generated a voltage for making an ignition plug of the internal combustion engine produce a spark discharge; and a control unit that turns on or off the power switching device. The internal-combustion-engine electronic control system is characterized in that the control unit is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device, the control unit, the ignition coil, and the ignition plug is in a predetermined state; in that the circuit unit includes a capacitor, a first slow-discharge circuit that makes electric charge in the capacitor discharge with a predetermined time constant, and a second slow-discharge circuit that makes electric charge in the capacitor discharge with a time constant that is smaller than the time constant of the first slow-discharge circuit  15 ; and in that based on the predetermined state, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit and makes the capacitor discharge electric charge so as to perform the soft shutoff. 
     (2) In the control unit, the circuit unit is provided with a function of suppressing a secondary voltage from making the ignition plug produce a spark discharge when energization of the ignition coil with the primary current is started. 
     (3) The control unit is provided with a current detection circuit that detects the primary current flowing in the ignition coil, and when the current detection circuit detects the fact that the primary current is the same as or larger than a predetermined value, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     (4) The control unit is provided with an overheating detection circuit that detects overheating in at least one of the constituent elements included in the control unit, and when the overheating detection circuit detects the overheating, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     (5) The control unit is provided with a timer circuit that detects a continuous energization duration of the primary current flowing in the ignition coil, and when the timer circuit detects the fact that the continuous energization duration is the same as or longer than a predetermined value, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     Embodiment 3 
     Next, there will be explained an internal-combustion-engine electronic control system according to Embodiment 3 of the present invention. In Embodiment 3, by combining a slow-discharge circuit with a current limiting circuit, for limiting the primary current flowing in the ignition coil, that is a circuit for suppressing the secondary voltage from making the ignition plug produce a spark discharge when energization of the ignition coil is started, the signal outputted from the power switching device is suppressed from oscillating when the primary current is limited. 
       FIG. 7  is a block diagram of an internal-combustion-engine electronic control system according to Embodiment 3 of the present invention. 
     The current detection resistor  111  connected with the emitter or the source of the power switching device  103  converts the primary current of the ignition coil  2  into a voltage; when this voltage exceeds a reference voltage (Vth)  132  of an operational amplifier  131 , the level of the output of the operational amplifier  131  changes from a LOW level to a HIGH level, and the output is inputted to the base of the PNP-type transistor  101  by way of a resistor  133 . 
     When receiving a HIGH-level signal from the operational amplifier  131 , the state of the PNP-type transistor  101  changes from “ON” to “OFF”. When the PNP-type transistor  101  turns off, the charge stored in the capacitor  17  is slowly discharged through the resistor  121  included in the first slow-discharge circuit that is configured in the same manner as the first slow-discharge circuit in Embodiment 1, the resistors  115 ,  122 , and  123  that configure the second slow-discharge circuit, and the diodes  124  and  125 . 
     As a result, as is the case with Embodiment 1, because the base voltage of the gate voltage of the power switching device  103  slowly lowers, the primary current of the ignition coil also decreased slowly. When the primary current of the ignition coil slowly decreases and then the voltage generated across the resistor  111  becomes lower than the reference voltage  132 , the level of the output of the operational amplifier  131  changes from the HIGH level to the LOW level; then, the LOW-level signal is inputted to the base of the transistor  101  by way of the resistor  133 . Accordingly, the state of the PNP-type transistor  101  changes from “OFF” to “ON”, whereby the power switching device  103  turns on. 
     When the power switching device  103  turns on and hence energization of the primary winding of the ignition coil  2  is started, the base voltage or the gate voltage of the power switching device  103  slowly rises due to the circuit for suppressing the secondary voltage of the ignition coil  2  from making the ignition plug produce a spark discharge; therefore, the primary current of the ignition coil  2  also rises slowly. While the current is limited, the foregoing operation is repeated so that the amount of the primary current of the ignition coil  2  is slowly controlled; thus, there is demonstrated an effect that a signal outputted from the power switching device  103  is suppressed from oscillating. 
     In Embodiment 3 illustrated in  FIG. 7 , the timer circuit and the overheating detection circuit  13  provided in Embodiment 1 are not provided; however, it goes without saying that the timer circuit  12  and the overheating detection circuit  13  similar to those in Embodiment 1 may be provided. 
     The internal-combustion-engine electronic control system according to Embodiment 3 of the present invention, described heretofore, is provided with characteristics set forth below: 
     (1) The internal-combustion-engine electronic control system is provided with a power switching device that applies or shuts off a primary current of an ignition coil of an internal combustion engine so that at the secondary side of the ignition coil, there is generated a voltage for making an ignition plug of the internal combustion engine produce a spark discharge; and a control unit that turns on or off the power switching device. The control unit is provided with a circuit unit that makes the power switching device softly shut off so as to prevent the ignition plug from producing the spark discharge, in the case where at least one of the power switching device and the control unit is in a predetermined state; and based on a change in the conduction state of the power switching device, the characteristics of the soft shutoff is changed halfway through the shutoff operation. 
     (2) The circuit unit includes a capacitor, a first slow-discharge circuit that makes electric charge in the capacitor discharge with a predetermined time constant, and a second slow-discharge circuit that makes electric charge in the capacitor discharge with a time constant that is smaller than the time constant of the first slow-discharge circuit; and when the soft shutoff is started, the control unit makes electric charge in the capacitor discharge by use of the second slow-discharge circuit so as to make the power switching device perform soft shutoff operation, and then makes electric charge in the capacitor discharge by use of the first slow-discharge circuit so as to make the power switching device perform soft shutoff operation. 
     (3) In the control unit, the circuit unit is provided with a function of suppressing a secondary voltage from making the ignition plug produce a spark discharge when energization of the ignition coil with the primary current is started. 
     (4) The internal-combustion-engine electronic control system control unit is provided with a current limiting circuit that limits the primary current flowing in the ignition coil. 
     (5) The control unit is provided with a current detection circuit that detects the primary current flowing in the ignition coil, and when the current detection circuit detects the fact that the primary current is the same as or larger than a predetermined value, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     (6) The control unit is provided with an overheating detection circuit that detects overheating in at least one of the constituent elements included in the control unit, and when the overheating detection circuit detects the overheating, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     (7) The control unit is provided with a timer circuit that detects a continuous energization duration of the primary current flowing in the ignition coil, and when the timer circuit detects the fact that the continuous energization duration is the same as or longer than a predetermined value, the control unit selects one of the first slow-discharge circuit and the second slow-discharge circuit so as to perform the soft shutoff. 
     In addition, in each of Embodiments 1 through 3, the slow-discharge circuit is formed of a time constant circuit consisting of a capacitor and a resistor; however, there is obtained the same effect by controlling the base voltage or the gate voltage of the power switching device by use of a constant current circuit or the like. 
     Moreover, in each of Embodiments 1 through 3, there are provided both the first slow-discharge circuit and the second slow-discharge circuit; however, even if two or more slow-discharge circuits are utilized, the same effect can be demonstrated. 
     Still moreover, it goes without saying that with regard to the present invention, by combining and utilizing the circuits of the foregoing embodiments, the respective effects thereof can be demonstrated. 
     Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this is not limited to the illustrative embodiments set forth herein.