Ignition System for Internal Combustion Engine

This invention relates to improvements in the structure of ignition systems for internal combustion engines. More particularly, the present invention relates to an improved ignition system for an internal combustion engine in which circuits for detecting the level of spark energy supplied to the spark plugs in the engine and controlling the value of primary current of the ignition coil on the basis of the result of energy level detection are additionally provided so that spark energy of predetermined level can be supplied to the spark plugs in the engine.

It is known that, in a prior art ignition system for an internal combustion engine, the duration of secondary current flowing through the ignition coil is detected to control the value of primary current supplied to the ignition coil so that the secondary current duration can be controlled or maintained at a predetermined length of time. It is also known that, in another prior art ignition system having a concern with the aforementioned one, the detected value of secondary current is integrated to calculate the area, and the value of primary current is controlled on the basis of the calculated area so that the integrated amount of secondary current can be controlled or maintained at a predetermined setting. However, in both of these prior art ignition systems, the spark energy cannot always be accurately detected since the secondary current is only detected. It has therefore been difficult for these prior art ignition systems to strictly meet the operation requirements of the engine when an excess or shortage of spark energy results from, for example, a variation of the secondary voltage.

It is therefore a primary object of the present invention to provide an improved ignition system for an internal combustion engine in which the values of primary voltage and secondary current in the ignition coil are detected to calculate the arc energy or the relevant characteristic of an electrical circuit including a resistor connected to the secondary winding of the ignition coil thereby controlling the value of primary current supplied to the ignition coil so that spark energy of optimum level can be supplied to the spark plugs without regard to the type and individuality of the engine.

FIG. 1 is an electrical circuit diagram showing in detail the structure of a first embodiment of the ignition system according to the present invention.

FIG. 2 shows voltage and current waveforms appearing at various parts in FIG. 1 to illustrate the operation of the first embodiment of the present invention.

FIG. 3 shows also voltage and current waveforms appearing at various parts in FIG. 1 to illustrate the operation of the first embodiment of the present invention.

FIG. 4 is an electrical circuit diagram showing in detail the structure of a reference signal compensating circuit which may be employed in place of the reference signal source in the ignition system shown in FIG. 1.

FIG. 5 is an electrical circuit diagram showing in detail the structure of a second embodiment of the ignition system according to the present invention.

FIG. 6 shows voltage and current waveforms appearing at various parts in FIG. 5 to illustrate the operation of the second embodiment of the present invention.

FIG. 7 is an electrical circuit diagram showing in detail the structure of one form of the reference signal compensating circuit which may be employed in the ignition system shown in FIG. 5.

FIG. 8 is an electrical circuit diagram showing in detail the structure of an ignition monitoring circuit which may be employed in the first and second embodiments of the present invention.

FIG. 9 is an electrical circuit diagram showing in detail the structure of a third embodiment of the ignition system according to the present invention.

FIG. 10 shows voltage and current waveforms appearing at various parts in FIG. 9 to illustrate the operation of the third embodiment of the present invention.

FIG. 11 is an electrical circuit diagram showing in detail the structure of a fourth embodiment of the ignition system according to the present invention.

FIG. 12A is a graph showing of the stream of the air-fuel mixture and the critical spark energy in an internal combustion engine.

FIG. 12B is a schematic view to illustrate the state of the arc channel extending between the electrodes of the spark plug.

FIG. 13 shows voltage and current waveforms appearing at various parts in FIG. 11 to illustrate the operation of the fourth embodiment of the present invention.

FIG. 14 is an electrical circuit diagram showing in detail the structure of a fifth embodiment of the ignition system according to the present invention.

FIG. 15 is an electrical circuit diagram showing in detail the structure of one form of the dwell angle control circuit in the ignition system shown in FIG. 14.

FIG. 16 shows voltage and current waveforms appearing at various parts in FIG. 15 to illustrate the operation of the dwell angle control circuit.

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Referring first to FIG. 1 showing a first embodiment of the present invention applied to an internal combustion engine, the ignition system includes a battery 1, an ignition switch 2, an ignition coil 3 including a primary winding 3a and a secondary winding 3b, a spark plug 4, and a primary current detecting resistor 6 detecting the value of primary current supplied to the primary winding 3a of the ignition coil 3. A known ignition signal generator 14 generates an ignition signal and includes, for example, an ignition timing detector employing an electromagnetic pickup or an ignition timing signal generator employing an electronic advancer. The ignition signal from the ignition signal generator 14 is applied to a known current-supply duration (dwell angle) control circuit 19 which controls the duration of primary current supplied to the primary winding 3a of the ignition coil 3 in response to the application of the ignition signal. A power transistor unit 5 includes a pair of Darlington-connected power transistors or a field effect power transistor, and a Zener diode 7 protects the power transistor unit 5 against breakdown. A current value control circuit 12 controls the primary current supplied to the primary winding 3a of the ignition coil 3 so that the peak value of primary current can be stabilized or maintained at a predetermined setting. A secondary current detecting resistor 8 detects the spark current flowing through the secondary winding 3b of the ignition coil 3, and a capacitor 9 is connected in parallel with this resistor 8.

The primary voltage V.sub.1, secondary voltage V.sub.2, secondary current I.sub.2 and primary current I.sub.1 have waveforms as shown in (a), (b), (c) and (d) of FIG. 2 respectively. The spark energy E.sub.2 supplied from the ignition coil 3 is given by E.sub.2 =.intg.V.sub.2 .multidot.I.sub.2 dt. In the waveform of the secondary voltage V.sub.2 shown in (b) of FIG. 2, the hatched portion is available for providing the spark energy, and this portion corresponds to the hatched portion in the waveform of the primary voltage V.sub.1 shown in (a) of FIG. 2. Therefore, the spark energy E.sub.2 is expressed as E.sub.2 =C.intg.V.sub.1 .multidot.I.sub.2 dt where C is a constant. Thus, the signal corresponding to the spark energy can be basically obtained by applying the primary voltage V.sub.1 and the secondary current I.sub.2 to a multiplying circuit 10. In the present invention, the secondary voltage V.sub.2 is not used for the calculation of the spark energy E.sub.2 for the reason that measurement of the secondary voltage V.sub.2 is difficult, and the cost of the system increases, when measuring means is additionally provided.

The output from the multiplying circuit 10 is applied to a constant-current value compensating signal output circuit 11 to be integrated into a predetermined level signal, and this latter signal is applied from the circuit 11 to the current value control circuit 12 to be used for compensating the value of primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3, so that the peak value of primary current I.sub.1 can be limited to within the range of I.sub.min and I.sub.max shown in (d) of FIG. 2 thereby maintaining the spark energy E.sub.2 at a substantially predetermined level. Therefore, the spark energy of predetermined level can be supplied to the spark plug 4 without regard to the type and individuality of the engine by virtue of the above structure, and because of this advantage, the ignition performance of the engine can be improved, and the level of the spark energy can be optimized.

The multiplying circuit 10 carrying out the multiplication for calculating the spark energy includes a multiplier 110a which is, for example, that of Model No. 4214 manufactured by the Analog Device Corporation in USA. This multiplier 110a has four inputs X.sub.1, X.sub.2, Y.sub.1 and Y.sub.2 to produce an output E.sub.o which is given by E.sub.o .varies.(X.sub.1 -X.sub.2).multidot.(Y.sub.1 -Y.sub.2). The input X.sub.1 is provided by a voltage signal V.sub.1 ' obtained by dividing the primary voltage V.sub.1 by voltage-dividing resistors 102 and 103, and the input X.sub.2 is provided by a voltage signal V.sub.B ' obtained by dividing the power supply voltage (the battery voltage) V.sub.B by voltage-dividing resistors 101 and 104. The input Y.sub.1 is provided by a current signal I.sub.2 ' obtained by supplying the secondary current I.sub.2 through a buffer 110b, and the input Y.sub.2 is grounded. Consequently, the output E.sub.o from the multiplier 110a is given by E.sub.o .varies.(V.sub.1 '-V.sub.B ')I.sub.2 ', and when E.sub.o is selected to satisfy the relation E.sub.o =E.sub.2, a voltage signal proportional to the spark energy E.sub.2 can be obtained. A monostable circuit 100 is triggered in response to the fall time of the ignition current (the secondary current I.sub.2) to turn on an analog switch 109 which is maintained in its on-state for a predetermined period of time. During the on-time of the analog switch 109, the spark energy output signal E.sub.o from the multiplier 110a is applied through the analog switch 109 and a buffer 112 to a capacitor 117 in the circuit 11 to be integrated therein. The integrated output is applied to an operational amplifier 118b to be compared with a reference signal r applied from a reference signal source 116, and the resultant signal is then amplified in the operational amplifier 118b. The output signal .xi. from the operational amplifier 118b is applied through a resistor 119c to the input gate of a field effect transistor (FET) 120 in the circuit 12, so that the primary current I.sub.1 can be so controlled as to provide the predetermined spark energy. A monostable circuit 113 in the circuit 11 generates a short pulse signal B which is applied to an analog switch 118a so that the charge stored in the capacitor 117 is discharged at the spark ignition timing of the ignition timing signal, that is, at the appearing timing of the signal turning off the power transistor unit 5. This is done by turning on the analog switch 118a by the signal .beta..

The current value control circuit 12 includes an operational amplifier 128. The voltage signal appearing across the primary current detecting resistor 6 is divided by voltage-dividing resistors 124 and 127 to provide a primary current signal corresponding to the detected value of the primary current I.sub.1, and this primary current signal is applied to one of the input terminals of the operational amplifier 128. The output signal from the FET 120 is applied to the other input terminal of the operational amplifier 128 after being divided by voltage-dividing resistors 122 and 126. The former signal is compared with the latter signal in the operational amplifier 128. When the level of the former signal is higher than that of the latter signal a drive transistor 129a provided for driving the power transistor unit 5 is turned on to decrease the current supplied through a resistor 129b to the base of the power transistor unit 5 so that the peak value of the primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3 can be stabilized or maintained at the predetermined value. The output from the other output terminal of the operational amplifier 128 is applied to the dwell angle control circuit 19 as a signal which acts to limit the stabilized period of time T.sub.S of the primary current I.sub.1 shown in (d) of FIG. 2 to within the predetermined range.

FIG. 3 shows voltage and current waveforms appearing at various parts in FIG. 1. FIG. 3 shows in (a) the ignition signal .alpha., in (b) the spark energy signal c applied to the circuit 11 by passing the output from the multiplier 110a through the analog switch 109, in (c) the spark energy signal .eta. integrated and held in the capacitor 117, in (d) the difference signal .sctn. obtained as the result of comparison between the spark energy signal .eta. and the reference signal r in the operational amplifier 118b to provide the compensating signal applied to the current value control circuit 12, in (e) the reset signal .beta. applied to the analog switch 118a from the monostable circuit 113, and in (f) the primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3.

It will be seen that, by the functions of the circuits above described, the value of the primary current I.sub.1 can be limited to within the predetermined range so that the spark energy can be controlled or maintained at the predetermined level. In FIG. 1, a Zener diode 125 acts as a limiter limiting the upper limit of the primary current value, and the voltage-dividing resistors 122 and 126 constitute a lower limit limiter. A resistor 121 and a Zener diode 123 constitute a constant-voltage circuit. The system further includes resistors 106, 107, 108, 114, 115, 119a and 119b.

FIG. 4 shows the structure of a simple form of a reference signal compensating circuit 15 which may be employed in place of the reference signal source 116 in the first embodiment of the present invention shown in FIG. 1. Signals indicative of the engine parameters including the temperature of engine cooling water, rotation speed of the engine, EGR (exhaust gas recirculation) and pressure in the intake manifold are applied to this circuit 15 to provide a reference signal indicative of a predetermined function of these parameters. Referring to FIG. 4, a pair of transistors Q.sub.1 and Q.sub.2 constitute a current mirror circuit, and the reference output r to be applied to the operational amplifier or comparator 118b appears at the connection point between resistors R.sub.2 and R.sub.3. A plurality of switches SW.sub.1, SW.sub.2 and SW.sub.3 are selectively turned on and off to simply add the selected engine parameters. For example, when EGR is being done, the switch SW.sub.1 is turned on to increase the reference signal level. A resistor R.sub.0 is provided to establish an initial reference level, and adding resistors R.sub.4, R.sub.5 and R.sub.6 connected to the switches SW.sub.1, SW.sub.2 and SW.sub.3 respectively. Although three switches are shown in FIG. 4, by way of example, the number of switches may be more or less than three. A Zener diode ZD.sub.1 and a resistor R.sub.1 constitute a constant-voltage circuit for producing a reference voltage.

FIG. 5 shows a second embodiment of the present invention in which the same or like reference numerals are used to denote the same or like parts appearing in FIG. 1. Referring to FIG. 5, the output signal indicative of the spark energy level from the multiplying circuit 10 is applied to a current-supply duration compensating signal output circuit 11A, and the output from this circuit 11A is applied to a dwell angle control circuit 13 as a current-supply duration compensating signal thereby controlling the duration of the primary current I.sub.1 to lie within the range of T.sub.ON.min and T.sub.ON.max shown in (e) of FIG. 2 so that the spark energy can be controlled or maintained at the predetermined level.

The structure of this current-supply duration compensating signal output circuit 11A is generally the same as that of the constant-current value compensating signal output circuit 11 shown in FIG. 1, except for the provision of resistors 115a and 115b. The output signal from the operational amplifier 118b is connected to one end of a voltage-dividing resistor 138a which is connected at the other end thereof to one of the input terminals of a comparator 131a. By controlling the voltage applied to this input terminal of the comparator 131a, the current-supply duration (the dwell angle) is controlled so that the spark energy can be controlled or maintaned at the predetermined level.

The structure of the dwell angle control circuit 13 will then be described in detail. The ignition signal generator 14 including an electromagnetic pickup generates an AC signal a having a waveform as shown in (a) of FIG. 6, and this AC signal a is applied through a diode 135 and a resistor 138c to a capacitor 134a to charge it with the voltage proportional to the rotation speed of the engine. In the meantime, a comparator 131b generates an output signal b having a waveform as shown in (b) of FIG. 6, and a monostable multivibrator 137 is triggered in response to the fall time of the pulse b to generate an output signal c having a waveform as shown in (c) of FIG. 6. The signal b is also applied to the base of a transistor 130h to provide a positive feedback signal applied to the comparator 131b. A transistor 130b is turned on by the signal c and maintained in its on-state during the monostable period of time T.sub.o of the multivibrator 137, and a capacitor 134b is charged through a resistor 138e. As a result, the potential d of this capacitor 134b increases as shown by the solid curve in (d) of FIG. 6. Upon termination of the monostable period of time T.sub.o, the transistor 130b is turned off, and thereafter, the capacitor 134b discharges through a resistor 138d and transistors 130g, 130a, 130c, 130e and 130d at a rate dependent upon the rotation speed of the engine. As soon as the charged voltage of the capacitor 134b becomes lower than the voltage appearing at the connection point between voltage-dividing resistors 138a and 138b, that is, the voltage e shown by the broken curve in (d) of FIG. 6, an output voltage f having a waveform as shown in (e) of FIG. 6 appears from the comparator 131a to be applied through an OR circuit 132 to an AND circuit 133. As a result, the charge stored in the capacitor 134b is discharged quickly through the OR circuit 132, AND circuit 133, resistor 138f and transistor 130f. An output signal h having a waveform as shown in (f) of FIG. 6 appears thus from the AND circuit 133 to drive the power transistor unit 5 thereby supplying the spark energy to the spark plug 4. The current-supply duration compensating signal output circuit 11A generates an output signal k having a waveform as shown in (j) of FIG. 6 for compensating the current-supply duration on the basis of the result of measurement of the spark energy. The potential e at the connection point between the voltage-dividing resistors 138a and 138b is controlled by the output signal k applied from the circuit 11A through the resistor 138a so that the spark energy can be controlled or maintained at the predetermined level.

The output signal b, shown in (b) of FIG. 6, from the comparator 131b is also applied to the other input terminal of the OR circuit 132. When the engine is rotating in its low rotation speed range, the pulse width of the output signal f from the comparator 131a is smaller than that of the output signal b from the comparator 131b, and the output signal b from the comparator 131b appears from the OR circuit 132. Therefore, in a low rotation speed range lower than a predetermined value, the output signal b from the comparator 131b controls the dwell angle of the primary current I.sub.1 at a predetermined duty ratio.

The output signal c, shown in (c) of FIG. 6, from the monostable multivibrator 137 is applied to the other input terminal of the AND circuit 133. When the engine is rotating in its high rotation speed range, the pulse width of the output signal f from the comparator 131a is larger than that of the output signal c from the monostable multivibrator 137, and the output signal c from the monostable multivibrator 137 appears from the AND circuit 133. Therefore, in a high rotation speed range higher than a predetermined value, too, the dwell angle of the primary current I.sub.1 is controlled by the output signal c from the monostable multivibrator 137 so that the constant coil off-time T.sub.o can be always ensured.

A Zener diode 136b is provided to act as an upper limit limiter, and another Zener diode 136a acts as a constant-voltage circuit. FIG. 6 shows the waveforms of the primary current I.sub.1 and secondary current I.sub.2 in the ignition coil 3 in (g) and (h) respectively. The solid curve and broken curve in (i) of FIG. 6 show the waveforms the output signal j from the buffer 112 and the reference signal r from the reference signal source 116 respectively.

FIG. 7 shows the structure of another form of the reference signal compensating circuit 15 which may be employed in place of the reference signal source 116 in the second embodiment of the present invention. Referring to FIG. 7, switches SW.sub.11 and SW.sub.12 are turned on and off by signals from sensors sensing the engine parameters, for example, EGR (exhaust gas recirculation) and the temperature of engine cooling water respectively, and a suitable combination of resistors R.sub.15, R.sub.16, R.sub.17 and R.sub.18 varies the level of the reference voltage output r. Transistors Q.sub.11, Q.sub.12, Q.sub.13 and Q.sub.14 constitute a constitute a constant-voltage power supply. The circuit 15 further includes resistors R.sub.11 to R.sub.14 and R.sub.19 to R.sub.23, transistors Q.sub.15 to Q.sub.18, diodes Q.sub.19, Q.sub.20, and a Zener diode Q.sub.21.

In the first and second embodiments of the present invention described hereinbefore, the primary current value and the current-supply duration are compensated to maintain the predetermined spark energy level in each ignition cycle. However, they may be compensated every two revolutions of the engine crankshaft or every n ignition cycles (where n is a suitable positive integer). Further, the control of the spark energy for the individual engine cyclinders can also be attained by providing an engine cylinder identifying circuit.

The reference signal compensating circuit 15 which is constructed to make analog compensation of the level of the reference voltage r as shown in FIG. 4 or FIG. 7 may be replaced by another circuit in which necessary compensation data corresponding to the engine parameters such as the intake manifold vacuum and engine rotation speed are previously stored in a digital memory. In such a circuit, those corresponding to the sensed engine parameters are read out from the digital memory, and the read-out compensation data are then subjected to then subjected to digital-analog conversion to provide the desired reference voltage r.

In the aforementioned first and second embodiments of the present invention, the multiplying circuit 10 operates as a spark energy calculating circuit. However, the values of secondary current, primary voltage and necessary engine parameters may be converted into a digital signal which is used as an address input for reading out a corresponding one of spark energy data stored previously in a digital memory, and this read-out spark energy data may be directly used for the digital control of the primary current or may be converted into an analog signal to be used for the analog control of the primary current.

A partial modification of the first and second embodiments of the present invention is shown in FIG. 8. Referring to FIG. 8, a parallel circuit of a light-emitting diode D and a resistor R is inserted between the secondary winding 3b of the ignition coil 3 and the secondary current detecting resistor 8, so that the state of ignition can be monitored by the light emitted from the light-emitting diode D.

FIG. 9 shows a third embodiment of the present invention in which the same or like reference numerals are used to denote the same or like parts appearing in FIG. 1. FIG. 10 shows voltage and current waveforms appearing at various parts in FIG. 9. The structure and operation of this third embodiment will be described in detail with reference to FIGS. 9 and 10.

The power supply voltage V.sub.B of the battery 1 is applied across voltage-dividing resistors 10d and 10e and also across a Zener diode 10k. The voltage appearing at the connection point between these voltage-dividing resistors 10d and 10e is applied to one of the input terminals of a measured voltage amplifier 10b which is, for example, that of Model No. 3660 manufactured by the Analog Device Corporation. FIG. 10 shows in (c) the waveform of the primary voltage V.sub.1 applied across the primary winding 3a of the ignition coil 3. This primary voltage V.sub.1 is applied across voltage-dividing resistors 10f and 10g and also across a Zener diode 10l. The voltage appearing at the connection point between the voltage-dividing resistors 10f and 10g is applied to the other input terminal of the measured voltage amplifier 10b. This amplifier 10b calculates the voltage corresponding to (V.sub.1 -V.sub.B), and the resultant output indicative of (V.sub.1 -V.sub.B) is applied from the amplifier 10b to the Z input of a divider 10a which is, for example, that of Model No. 4205 manufactured by the Analog Device Corporation.

FIG. 10 shows in (b) the waveform of the secondary current I.sub.2 detected by the secondary current detecting resistor 8. The voltage corresponding to this secondary current I.sub.2 is applied across voltage-dividing resistors 10i, 10j and also across Zener diodes 10m, 10n. The voltage appearing at the connection point between these voltage-dividing resistors 10i and 10j is applied through a resistor 10o to the X.sub.1 input of the divider 10a. The divider 10a calculates the value corresponding to (V.sub.1 -V.sub.B)/I.sub.2, and the resultant signal B having a waveform as shown in (d) of FIG. 10 is applied from the divider 10a to an analog switch 11c. Thus, this signal B is indicative of the secondary resistance of the ignition coil 3.

A monostable multivibrator 10c is triggered in response to the rise time of the secondary current I.sub.2 and generates a pulse output having a predetermined pulse width. This pulse output from the monostable multivibrator 10c is applied through an AND circuit 11y to the analog switch 11c to turn on the same, and the output B from the divider 10a passes through this analog switch 11c to provide a signal C having a waveform as shown in (e) of FIG. 10. This signal C is applied through resistors 11i, 11f and 11e to one of the input terminals or inverting input terminal of an operational amplifier 11a. The non-inverting input terminal of this operational amplifier 11a is grounded through a resistor 11d, and a capacitor 11g is connected between the inverting input terminal and the output terminal of this operational amplifier 11a. Therefore, during the operating period of time of the monostable multivibrator 10c, the capacitor 11g is charged with the signal C as shown by the solid curve D in (f) of FIG. 10. In other words, the capacitor 11g is charged up to a voltage level D of the value corresponding to the integrated value or means value of the secondary resistance of the ignition coil 3 during the on-time of the monostable multivibrator 10c.

A monostable multivibrator 11z is triggered in response to the fall time, indicating the ignition timing S, of the ignition signal A generated from the ignition signal generator 14 and generates a pulse output which turns on an analog switch 11h momentarily. The voltage D charged in the capacitor 11g is discharged in response to the turning-on of the analog switch 11h. This voltage signal D is applied through a resistor 11l to the plus input terminal of a comparator 11b. A reference voltage r as shown by the one-dot chain curve in (f) of FIG. 10 is applied through a resistor 11k to the minus input terminal of the comparator 11b from a reference signal source 16. Therefore, the comparator 11b generates an output signal E having a waveform as shown in (g) of FIG. 10 when the level of the voltage D applied from the capacitor 11g exceeds that of the reference voltage r applied from the reference signal source 16, that is, when the secondary resistance of the ignition coil 3 exceeds the critical value at which sooting occurs in the spark plug 4.

The output signal E from the comparator 11b and the ignition signal A shown in (a) of FIG. 10 are applied to an AND circuit 13p, and an output signal F having a waveform as shown in (h) of FIG. 10 appears from this AND circuit 13p to be applied to a timer circuit 13e. In response to the rise time of this signal F, the timer circuit 13e generates an output signal G which maintains its "1" level during a predetermined number of ignition cycles or during a predetermined period of time T.sub.TIMER as shown in (i) of FIG. 10. The output signal G appearing from this timer circuit 13e is applied through an inverter 11x to the AND circuit 11y so as to turn off the analog switch 11c and to maintain the analog switch 11c in the off-state during the period of time in which the signal G appears from the timer circuit 13e.

The output signal G from the timer circuit 13e and the signal obtained by inverting the ignition signal A from the ignition signal generator 14 by an inverter 13c are applied to an AND circuit 13d, and an output signal H having a waveform as shown in (j) of FIG. 10 appears from the AND circuit 13d. An astable multivibration 131 oscillates continuously while this signal H maintains its "1" level. This astable multivibrator 131 is composed of a capacitor 13g resistors 13f, 13h, NAND circuits 13i, 13j, 13k and diodes 13l, 13m. The output signal from the NAND circuit 13i in the astable multivibrator 131 is inverted by an inverter 13r to provide a signal K having a waveform as shown in (l) of FIG. 10. A monostable multivibrator 13q is triggered in response to the fall time, indicating the ignition timing S, of the ignition signal A from the ignition signal generator 14 and generates a signal J which maintains its "1" level for a predetermined period of time T.sub.OFF as shown in (k) of FIG. 10. The signal obtained by inverting this signal J by an inverter 13S and the signal from the inverter 13r are applied to an AND circuit 13t, and an output signal L having a waveform as shown in (m) of FIG. 10 appears from this AND circuit 13t. This signal L and the ignition signal A from the ignition signal generator 14 are applied to an OR circuit 13a, and the output signal from this OR circuit 13a is applied to the power transistor unit 5 to drive the same thereby periodically interrupting the primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3. Consequently, the primary current I.sub.1 is interrupted in a manner as shown in (m) of FIG. 10. Thus, when sooting occurs in the spark plug 4, the power transistor unit 5 is turned on-off at a high frequency at the ignition timing thereby continuously applying a high voltage of high frequency to the spark plug 4 so that, during the exhaust or suction stroke of the piston, carbon accummulating on the surface of the insulator in the vicinity of the electrodes of the spark plug 4 thereby sooting the spark plug 4 can be burnt away to be completely removed.

A practical form of the reference signal source 16 in the third embodiment of the present invention may include a combination of a resistor and a Zener diode connected between the ignition switch 2 and ground, and the reference voltage r is derived from the connection point therebetween. However, the reference signal source 16 is in no way limited to such a fixed voltage source and may be such that its reference voltage output r is variable depending on some of the engine parameters, for example, the temperature of engine cooling water and the temperature of engine intake air.

In the third embodiment of the present invention shown in FIG. 9, the multiple ignition circuit 13 is inserted between the ignition signal generator 14 and the power transistor unit 5. However, a DC-DC converter for the multiple ignition purpose may be independently provided in lieu of the circuit 13.

FIG. 11 shows a fourth embodiment of the present invention in which the same or like reference numerals are used to denote the same or like parts appearing in FIG. 9.

Referring to FIG. 11, a current compensating circuit 11A compensates the constant-current control value of a primary current stabilizing circuit 17 thereby controlling the base current of the power transistor unit 5 when the level of the signal B corresponding to the secondary resistance of the ignition coil 3 calculated by the divider 10a exceeds that of a reference signal generated from a reference signal source 16A. Consequently, the peak value of the primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3 is controlled to lie within the range of I.sub.min and I.sub.max shown in (d) of FIG. 2, and an increase in the spark resistance due to the increase in the secondary resistance of the ignition coil 3 is detected so as to control the spark energy. A dwell angle control circuit 18 controls the on-time of the power transistor unit 5 turned on in response to the ignition signal generated from the ignition signal generator 14. The primary current detecting resistor 7 detects the primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3.

FIG. 12A is a graph showing the relation between the critical ignition energy and the velocity of the stream of the air-fuel mixture in the vicinity of the electrodes of the spark plug 4. It will be seen in FIG. 12A that the critical ignition energy increases with the increase in the velocity of the stream of the air-fuel mixture. The reason will be explained with reference to FIG. 12B. FIG. 12B shows that the air-fuel mixture is flowing in a direction as shown by the arrow a, and an arc jumps across the electrodes 41 and 42 of the spark plug 4. When the velocity of the stream of the air-fuel mixture flowing in the direction a is relatively high, the arc channel b is bent, and the path l of the arc is elongated resulting in an increased spark resistance. Therefore, the ignition performance can be improved when the spark resistance is detected as being indicative of the secondary resistance of the ignition coil 3 used to control the primary current I.sub.1 supplied to the ignition coil 3 thereby controlling the spark energy, as described with reference to FIG. 11 showing the fourth embodiment of the present invention. Similarly, an increase in the spark gap between the electrodes 41 and 42 of the spark plug 4 due to, for example, wear of the spark electrodes 41 and 42 results in an elongation of the arc channel b, hence, an increased spark resistance. According to the fourth embodiment of the present invention, therefore, the spark energy can also be controlled depending on the length of the spark gap of the spark plug 4 thereby further improving the ignition performance.

The detailed structure of the ignition system shown in FIG. 11 will now be described. The current compensating circuit 11A shown in FIG. 11 is generally similar to the comparing circuit 11 shown in FIG. 9 except that a feedback resistor 11m is connected between the plus input terminal and the output terminal of the comparator 11b, and the output signal from the comparator 11b is applied to the control input terminal of an analog switch 11j after being inverted by an inverter 11n. The input terminal of this analog switch 11j is connected to the output terminal of the operational amplifier 11a, and a resistor 11o is connected to the output terminal of the analog switch 11j. The output terminal of the operational amplifier 11a is connected to the minus input terminal of the comparator 11b, and the reference signal source 16A is connected to the plus input terminal of the comparator 11b.

The reference voltage generated from this reference signal source 16A is set at the level corresponding to the secondary resistance value 50 k.OMEGA. of the ignition coil 3. When the level of the output voltage from the capacitor 11g exceeds that of the reference voltage from the reference signal source 16A, that is, when the value of the secondary resistance of the ignition coil 3 exceeds 50 k.OMEGA., the output signal from the comparator 11b turns into its "0" level, and this signal is inverted by the inverter 11n to turn on the analog switch 11j so as to apply the output voltage from the capacitor 11g to the primary current stabilizing circuit 17. This primary current stabilizing circuit 17 includes a voltage-current converter composed of an operational amplifier 17a connected at its plus input terminal to the output terminal of the analog switch 11j, a transistor 17b, a diode 17c and a resistor 17d. The circuit 17 further includes resistors 17e to 17k, a Zener diode 17l connected to the power supply terminal V- so that the potential at the connection point between the resistors 17f and 17g may not rise above a predetermined level, an operational amplifier 17m, and a Zener diode 17n functioning as a constant-voltage element.

The operation of the fourth embodiment having the above structure will now be described. The analog switch 11j is in its off-state when the secondary resistance of the ignition coil 3 is sufficiently low, and the divided constant voltage appearing at the connection point between the resistor 17f and the series resistors 17g, 17e is applied to the plus input terminal of the operational amplifier 17m. When the terminal voltage of the primary current detecting resistor 7 proportional to the primary current I.sub.1 becomes higher than the voltage appearing at the voltage dividing point above described, the potential level of the output from the operational amplifier 17m is lowered to decrease the base current of the power transistor unit 5. Consequently, the power transistor unit 5 operates in its unsaturated region thereby limiting the peak value of the primary current I.sub.1 to I.sub.min shown in (d) of FIG. 2. The output from the other output terminal of the operational amplifier 17m is applied to the dwell angle control circuit 18 which controls the starting time of turning on the power transistor unit 5 so as to minimize the period of time T.sub.S in (d) of FIG. 2 during which time the primary current I.sub.1 is stabilized to be constant.

On the other hand, when the value of the secondary resistance of the ignition coil 3 exceeds the predetermined value, the analog switch 11j is turned on, with the result that the current corresponding to the charged voltage of the capacitor 11g is supplied through the operational amplifier 17a, transistor 17b and resistor 17d to the resistor 17e, and the potential appearing at the connection point between the resistors 17g and 17f rises to the level corresponding to the charged voltage of the capacitor 11g. Thus, with the increase in the secondary resistance of the ignition coil 3, the level of the input signal applied to the plus input terminal of the operational amplifier 17m is correspondingly increased to increase the peak value of the primary current I.sub.1 between I.sub.min and I.sub.max shown in (d) of FIG. 2 thereby increasing the spark energy.

FIG. 13 shows voltage and current waveforms appearing at various parts of the ignition system shown in FIG. 11. The waveform of the ignition signal A generated from the ignition signal generator 14 is shown in (a) of FIG. 13. The waveform of the primary voltage V.sub.1 applied across the primary winding 3a of the ignition coil 3 is shown in (b) in FIG. 13. The waveform of the secondary current I.sub.2 flowing through the secondary winding 3b of the ignition coil 3 is shown in (c) of FIG. 13. The waveform of the output signal B from the divider 10a is shown in (d) of FIG. 13. The waveform of the output signal C from the monostable multivibrator 10c is shown in (e) of FIG. 13. The waveform of the output signal D from the analog switch 11c is shown in (f) of FIG. 13. The charged voltage E of the capacitor 11g and the reference voltage r generated from the reference signal source 16A are shown in (g) of FIG. 13. The waveform of the output signal F from the monostable multivibrator 11z is shown in (h) of FIG. 13. The waveform of the voltage G appearing at the connection point between the resistors 17f and 17g is shown in (i) of FIG. 13. The waveform of the primary current I.sub.1 is shown in (j) of FIG. 13.

FIG. 14 shows a fifth embodiment of the present invention in which the same or like reference numerals are used to denote the same or like parts appearing in FIG. 11. In this fifth embodiment, the dwell angle control circuit 18 is controlled so that the current-supply duration of the primary current I.sub.1 is variable between T.sub.ON.min and T.sub.ON.max shown in (e) of FIG. 2 depending on the output signal from the current compensating circuit 11A, whereby the spark energy can be controlled depending on the value of the secondary resistance of the ignition coil 3.

FIG. 15 shows a practical form of the dwell angle control circuit 18 preferably employed in the ignition system shown in FIG. 14. Referring to FIG. 15, the dwell angle control circuit 18 is composed of a constant-current charging circuit 18a, an analog switch 18b turned on and off in response to the output from the ignition signal generator 14, a capacitor 18c charged with the current from the charging circuit 18a during the on-time of the analog switch 18b, a constant-current discharging circuit 18d providing a discharging path pf the stored charge of the capacitor 18c discharging the voltage at a constant current value, a voltage-current converter 18e generating an output current corresponding to the output from the current compensating circuit 11A, a resistor 18f and a Zener diode 18g constituting a constant-voltage circuit, voltage-dividing resistors 18h, 18i, 18j, a comparator 18k, a Zener diode 18l connected to the power supply terminal V- so that the level of the input applied to the plus input terminal of the comparator 18k may not exceed a predetermined level, an inverter 18m inverting the ignition signal from the ignition signal generator 14, and an AND circuit 18n receiving the outputs from the inverter 18m and comparator 18k as its inputs.

The operation of the dwell angle control circuit 18 shown in FIG. 15 will be described with reference to a waveform diagram shown in FIG. 16. Subsequent to the ignition timing S, the ignition signal generated from the ignition signal generator 14 turns into its "1" level as shown in (a) of FIG. 16, and the analog switch 18b is turned on. The ignition signal maintains its "1" level during a predetermined period of time corresponding to a predetermined crank angle .alpha.. During this period of time, the capacitor 18c is charged with the current from the constant-current charging circuit 18a as shown by I.sub..alpha. in (b) of FIG. 16. Then, when the ignition signal generated from the ignition signal generator 14 turns into its "0" level to turn off the analog switch 18b, the charge stored in the capacitor 18c is discharged by the constant-current discharging circuit 18d as shown by I.sub..beta. in (b) of FIG. 16 during a predetermined period of time corresponding to a predetermined crank angle .beta., that is, during the period of time in which the ignition signal generated from the ignition signal generator 14 remains in its "0" level. The output from the comparator 18k turns into its "1" level as shown in (e) of FIG. 16 when the charged voltage of the capacitor 18c decreases to a level lower than the potential level at the connection point between the resistors 18i and 18h, which level is shown by the one-dot chain line in (b) of FIG. 16. In response to the application of the outputs from the comparator 18k and inverter 18m to the AND circuit 18n, an output signal having a waveform as shown in (d) of FIG. 16 appears from the AND circuit 18n to turn on-off the power transistor unit 5.

The potential at the connection point between the resistors 18i and 18h has a constant level determined by the voltage dividing ratio between the resistors 18i, 18j and the resistor 18h, since the analog switch 11j in the current compensating circuit 11A is in its off-state when the secondary resistance of the ignition coil 3 is sufficiently low. When the secondary resistance of the ignition coil 3 exceeds a predetermined value, the analog switch 11j in the current compensating circuit 11A is turned on, and the current corresponding to the charged voltage of the capacitor 11g is supplied through the voltage-current converter 18e to the resistor 18j thereby increasing the potential at the connection point between the resistors 18i and 18h. Consequently, the dwell angle of the primary current I.sub.1 supplied to the ignition coil 3 is increased between T.sub.ON min and T.sub.ON max shown in (e) of FIG. 2 due to the increase in the secondary resistance of the ignition coil 3. FIG. 16 shows in (c) the waveform of the primary current I.sub.1 supplied to the primary winding 3a of the ignition coil 3.

Although, in the aforementioned third, fourth and fifth embodiments of the present invention, the voltage corresponding to the integrated value (the mean value) of the secondary resistance of the ignition coil 3 during the monostable time of the monostable multivibrator 10c is obtained for the purpose of comparison, the time-based mean of the integrated value may be obtained by dividing the integrated value by the monostable time of the monostable multivibrator 10c.

Further, the aforementioned third, fourth and fifth embodiments of the present invention may include a cylinder identifying circuit so as to attain the spark energy control for each of the engine cylinders.

Further, although the secondary resistance of the ignition coil 3 is detected in each ignition cycle in the third, fourth and fifth embodiments of the present invention, it may be detected every two revolutions of the engine crankshaft or every n ignition cycles (where n is a suitable positive integer).

Furthermore, although the analog control elements are employed in the third, fourth and fifth embodiments of the present invention for the analog control of the spark ignition energy, a digital divider and an up-down counter may be employed for the digital control of the spark ignition energy. Moreover, although the secondary resistance computing circuit including the divider is provided to calculate the secondary resistance of the ignition coil 3, the primary voltage V.sub.1, the power supply voltage V.sub.B and the secondary current I.sub.2 may be converted into corresponding digital signals which may be applied as an address input for reading out one of the secondary resistance data previously stored in a digital memory.