Ignition system for internal combustion engines
BACKGROUND OF THE INVENTION 
     The present invention relates to an ignition system for internal combustion engines. 
     In one of conventional ignition systems of this type, such as disclosed in Japanese Laid-Open Patent Publication No. 55-66659, an energy is stored in the primary winding of an ignition coil by the battery voltage and a power transistor is turned on and off by a switching signal thus producing a high voltage across the secondary winding of the ignition coil. A spark is thereby produced at a spark plug through a high-voltage rectifier diode connected to the high-voltage side of the secondary winding. 
     Then, the current flowing in the primary winding becomes as shown in FIG. 3A and the voltage induced across the secondary winding becomes as shown in FIG. 3B. In this case, the spark plug discharge at a transistor OFF point b shown in FIG. 3A and at this time the ignition coil secondary voltage becomes as shown in FIG. 3C. Also, at this time the discharge current becomes as shown in FIG. 3D. Primarily, the voltage induced in the ignition coil corresponds to a variation of the primary current. Thus, while the voltage for causing the spark plug to discharge is primarily induced at the transistor OFF point b shown in FIG. 3A, a secondary voltage is also induced at a transistor ON point a. This voltage (about 2 KV) corresponds to the ON voltage shown in FIG. 3B. In the conventional ignition system of the distributor-connected type, an idle spark is produced by the distributor high-voltage distributing operation and thus the spark plug is prevented from being fired by the ON voltage. However, in the case of a direct connected-type ignition coil where a spark plug is directly connected to the secondary winding of the ignition coil, no idle spark is produced and there is ample possibility of causing any firing when the transistor is turned on. This causes an engine trouble. As shown in FIG. 3C, this ON voltage is opposite in polarity to the high voltage primarily required for producing the plug spark discharge. Thus, if the high-voltage rectifier diode is connected just before the spark plug, the ON voltage produced at the cathode of the high-voltage rectifier diode no longer appears at the anode of the highvoltage rectifier diode so that there occurs no discharge due to the turning-on of the transistor. 
     In this type, however, the diode is connected on the high-voltage side between the secondary winding and the spark plug thus giving rise to such problems as breaking down of the high-voltage diode by such capacitive discharge current as shown in FIG. 3D and deterioration of the high-voltage diode due to the discharge producing high voltage. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved ignition system for an internal combustion engine so arranged to prevent the occurrence of the plug firing due to the secondary high voltage induced upon the flow of a primary current through the ignition coil and to avoid otherwise possible destroy of such a diode due to a capacitive discharge current and any deterioration of the diode due to the high voltage. 
     It is another object of the invention to provide an ignition system for an internal combustion engine including switching means adapted to open and close in synchronism with the rotation of the engine, an ignition coil whose primary current is switched on and off in response to the opening and closing of the switching means, a spark plug connected to a secondary winding of the ignition coil such that an ignition spark is produced at the spark plug by a high voltage induced in the secondary winding of the ignition coil when the switching means is opened, and a Zener diode connected to a secondary winding low voltage side of the ignition coil. 
     Thus, due to the connection of the Zener diode to the low voltage side, no ill effect is caused by the capacitance distributed in the secondary winding of the ignition coil and there is no danger of the application of a high voltage to the diode during the period of normal ignition. Moreover, when the open voltage of the spark plug is applied to the Zener diode, the Zener diode is turned on so that the reverse voltage applied to the Zener diode is reduced to a low value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an electric circuit diagram showing an embodiment of an ignition system according to the present invention. 
     FIG. 1B shows an alternate embodiment of the system shown in FIG. 1A. 
     FIGS. 2A-2D shows a plurality of waveforms generated at various points in the system of FIG. 1 when the spark plug fails to cause a spark discharge. 
     FIGS. 3A-3D shows a plurality of waveforms generated at various points in a conventional ignition system. 
     FIG. 4 shows an equivalent circuit diagram of a principal part of the system shown in FIG. 1. 
     FIG. 5A is an electric circuit diagram showing a principal part of another embodiment of the system according to the invention. 
     FIG. 5B shows an alternate embodiment of the FIG. 5A system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1A, numeral 1 designates a battery of the negative-terminal grounding type, and 2 a primary winding of an ignition coil 3 having its one end connected to the positive terminal of the battery 1. Numeral 4 designates a power transistor forming switching means and having its collector connected to the other end of the primary winding 2 and its emitter grounded. Numeral 5 designates a secondary winding of the ignition coil 3 whose high voltage-side end is connected directly and not through a distributor to one end of a spark plug 7 whose other end is grounded. Numeral 8 designates a Zener diode having its anode connected to the low voltage-side end of the secondary winding 5 and its cathode connected to the end of the primary winding 2 which is on the positive terminal side of the battery 1. 
     Numeral 21 designates an ignition timing signal generator for generating an ignition timing signal in synchronism with the rotation of the engine, 22 a waveform reshaping circuit for changing the waveform of the ignition timing signal to a rectangular waveform, and 23 an amplifier circuit responsive to the rectangular waveform output from the waveform reshaping circuit 22 to turn on and off the power transistor 4. 
     With the construction described above, the advantages of this embodiment (or its advantageous features over the conventional type) will now be described. 
     (I) When the spark plug discharges (normal operation). 
     (a) The capacitive discharge current has no ill effect. 
     In the conventional system, the high-voltage rectifier diode is loaded to generate heat by the capacitive discharge current shown in FIG. 3D, but there is practically no such ill effect on the Zener diode 8 in the embodiment. Usually, the capacitive discharge current is caused by the distributed capacitance in the ignition coil secondary winding and it becomes as shown in FIG. 4. (Here, while the secondary distributed capacitance primarily exists between the secondary winding and the primary winding or between the secondary winding and the core, it is equivalently represented as being present between the ground and the secondary winding in FIG. 4. However, the distributed capacitance between the windings is shown as such.) According to the equivalent circuit of FIG. 4, the capacitive discharge current does not flow through the Zener diode 8. (Because it is considered that the energy stored in capacitors 9 discharges through a closed loop through the capacitors, spark plug and the ground on the secondary winding side.) 
     (b) The diode is exposed to no high voltages. 
     In the conventional system, the waveform of a voltage produced by the discharge of the spark plug becomes as shown in FIG. 3C. Thus, the breakover voltage, the arc voltage and the ON voltage are always applied to the high-voltage rectifier diode. However, only the ON voltage is applied to the Zener diode 8 in the embodiment of FIG. 1A. This results in a considerable difference in the rate of deterioration between the Zener diode 8 and the conventional high-voltage rectifier diode. 
     (II) When the spark plug fails to cause spark discharge and is kept open (abnormal operation) 
     (a) The Zener diode load voltage is decreased by a decrease in the generated voltage. 
     Shown in FIGS. 2A, 2B, 2C and 2D, respectively, are the voltage waveforms at the cathode and anode of the conventional high-voltage rectifier diode and the voltage waveforms at points C and D of FIG. 1A. The load voltage of the conventional high-voltage diode is shown in FIG. 2A. Also, at this time the ON voltage is practically equal to the difference between the ON voltages in FIGS. 2D and 2C. As a result, the resulting load voltage of the Zener diode 8 becomes about one half (about 1 KV) that of the conventional high-voltage diode. 
     (b) The prevention of system destroy by virtue of the Zener characteristc. 
     Although the conventional high-voltage rectifier diode also has a Zener characteristic, the reverse-blocking voltage is higher than the induced peak voltage and therefore the Zener characteristic is not utilized well. (if such a diode having a Zener voltage lower than the peak voltage is connected to the high voltage side of the secondary winding, the diode will be destroyed instantly.) 
     In FIG. 1A, however, when the voltage at the point C of FIG. 1A exceeds the Zener voltage as shown in FIG. 2C, a current flows in the reverse direction in Zener diode 8, and the voltage across the Zener diode 8 never exceeds the Zener voltage. As a result, it is only necessary that the Zener voltage of the Zener diode 8 of FIG. 1A has a value (2 to 4 KV) slightly higher than the generated ON voltage (1 to 3 KV). The conventional high-voltage rectifier diode for preventing the ON voltage will be destroyed immediately unless it is such a one that can also withstand the essential spark plug open voltage in the failure of causing the discharge (the current can flow nowhere even it is intended to do so). In contrast it has been confirmed that, even with the Zener diode 8 having a Zener characteristic of several volts and connected to the secondary low voltage side of the ignition coil 3 as in FIG. 1A, such a Zener diode is not destroyed although a current flows when the Zener voltage is exceeded. (However, the ON voltage cannot be blocked thus causing a spark discharge of the spark plug due to the ON voltage, thus the Zener characteristic of 2 to 4 KV is preferred for blocking the ON voltage). 
     It is to be noted that while the Zener diode 8 shown in FIG. 1A has its cathode connected to the positive terminal of the battery 1, it may be connected to the ground or the like if it is of lower voltage level as shown in FIG. 1B. Also, while the spark plug 7 discharges with its ungrounded electrode of negative polarity in FIG. 1A, if the spark plug 7 is adapted to discharge with its ungrounded electrode of positive polarity, it is only necessary to reverse the connection of the Zener diode 8. 
     This invention is particularly effective in an ignition system of the type having a single spark plug per each ignition coil to fire plural cylinders individually and such construction as shown in FIG. 5A will be advantageous if its use with a four-cylinder engine is considered. In this construction, the battery voltage from a battery terminal 11 charges ignition coils 121 to 124 for the respective spark plugs 151 to 154 and transistors 141 to 144 are respectively turned on and off in response to the respective input control signals from input control signal terminals 131 to 134 thereby producing a spark at each of the spark plugs 151 to 154. This circuit features that as compared with a construction in which four of the circuit of FIG. 1A are arranged, it is equivalent to one in which the four Zener diodes 8 are reduced to one and this results in a reduced cost. This circuit construction is applicable to the individual cylinder firing system involving any number of cylinders. FIG. 5B shows this embodiment as used with a lower voltage potential. 
     From the foregoing description, it will be seen that the present invention has a great effect in that although there is the occurrence of an ON voltage of about one half the conventional one, the provision of a Zener diode has the effect of blocking the flow of a current and thereby eliminating the danger of any spark discharge of the spark plug due to the ON voltage. 
     While it is liable to consider that the withstand voltage of the conventional type high-voltage rectifier diode during the spark plug discharge is just practical even if it is of the order just sufficient to block the ON voltage, its load current due to a capacitive discharge current cannot be disregarded. Also, when the spark plug fails to discharge, an excessive load is applied and the high-voltage rectifier diode would be destroyed. On the contrary, the present invention has a great effect that the Zener characteristic of a Zener diode prevents it from being destroyed even if any voltage is applied to it.