CA1038027A - Capacitor discharge ignition system with controlled spark duration - Google Patents

Abstract

ABSTRACT
A capacitor discharge ignition system for a spark-ignition internal combustion engine. The ignition system includes an ignition coil having first and second primary windings, a secondary winding and a ferromagnetic core about which the windings are wound. A spark plug has electrodes which are spaced apart to form a spark gap which is connected in series with a first capacitor. The series-connected spark gap and first capacitor are connected across the secondary winding. A second capacitor is coupled to the first winding and a DC source of electrical energy is provided. First circuit means charge the second capacitor from the DC source and discharge this capacitor through the first primary winding in timed re-lation to operation of the engine. Second circuit means are provided for producing a fixed frequency oscillatory current in the second primary winding for a predetermined time interval subsequent to each discharge of the second capacitor through the first primary winding. The discharge of the second capacitor through the first primary winding and the subsequent supply of fixed frequency oscillatory current to the second primary winding causes ferroresonant oscillations in the secondary circuit of the ignition coil for at least a portion of the aforementioned predetermined time interval. The spark which occurs between the spark plug electrodes exists during the predetermined time interval and has a duration which may be varied as desired.

Description

~0380;i~7 This invention relates to a capacitor discharge ignition system for a spark-ignition internal combustion engine. More particularly, it relates to a ferroresonant capacitor discharge ignition system which produces a spark discharge in the gap of a spark plug. The spark discharge is of controllable duration and is characterized by alternating current flow in the spark gap and a sustained alternating voltage in the secondary circuit of an ignition coil. This voltage oscillates at a ferroresonant frequency. The present invention is related to our commonly assigned patent appli-cation Serial No. 222,585 filed March 19, 1975 and entitled !lFerroresonant Capacitor Discharge Iynition System".
Our copending patent application identified above relates to a capacitor discharye ignition system which has an ignition coil with a primary winding and a secondary winding wound about a ferromagnetic core. The system includes a spark gap which is connected in series with a first capacitor. The series-connected first capacitor and spark gap are connected across the ignition coil secondary winding and a second capacitor i9 coupled to the ignition coil primary winding and to a DC source of electrical energy. Circuit means are provided for charging the second capacitor and for discharging it through the primary winding in timed relation to engine operation. This produces breakdown of the spark gap in the secondary circuit and subsequent oscillations in this circuit.
The secondary circuit oscillations are at a fre~uency f defined by the expression f = Vm/4NS~s where Vm is the instantaneous maximum voltage acxoss the first capacitor, Ns is the number of turns in the secondary winding of the ignition coil and ~s is the magnetic flux enclosed by the secondary winding of the ignition coil at saturation of its . ~ .

j - 2 - ~
,~

~ o3~0Z7 ferromagnetic core.
The present invention is an improvement over the capacitor discharge ignition system described a~ove in that it provides an ignition system which operates.in a ferro-resonant mode as defined by the above expression, but which also provides ~ustained and controllable spar~ duration characterized by ferroresonant oscillations in the secondary circuit of the ignition coil.
A capacitor discharge ignition system in accordance with the invention comprises an ignition coil having first and second primary windings, a secondary winding and a ferro-magnetic core about which the windings are wound~ ~ spark plug has electrode~ ~paced to form a spark gap. One o~ the electrodes is coupled to one terminal of the secondary winding and the spark gap is connected in series with a first capacitor. One terminal o the capacitor is coupled to the other terminal of the secondary winding. A second capacitor is coupled to the first primary winding and a DC source of electrical energy is provided.
First circuit means are coupled to the second capacitor and to the first primary winding. The first circuit means controls the charging of the second capacitor from the DC source of electrical energy and controls the discharge of the second capacitor through the first primary winding in timed relation to operation of the engine. Second circuit means, coupled to the second primary winding, are provided or producing an oscillatory current in the second primary winding for a predetermined time interval subsequent to each discharge of the second capacitor through the first primary winding. The discharge of the second capacitor through the first primary winding and the subsequent production of the 1~)38~7 oscillatory current in the second primary winding produces, for at least a portion of the predetermined time interval, a voltage in the secondary circuit of the ignition ~oil which oscillates at a frequency f defined by the expression f = Vm/4NS~9 where Vm is the instantaneous maximum voltage across the first capacitor, Ns is the number of turns in the secondary winding and ~s is the magnetic flux within the secondary winding when the ferromagnetic core of the ignition coil is magnetically saturated.
The capacitor discharge ignition system of this invention, therefore, has a controllable spark duration and ferroresonant oscillations occur i~ the secondary circuit of the ignition coil for a predetermined time interval.
The invention may be better understood by reference to the detailed description which follows and to the drawings, wherein:
Figures la and lb together form a complete schematic diagram of a capacitor discharge ignition system in accordance with the invention; and Figures 2 through 13 are reproductions of actual voltage and current waveforms observed on an oscilloscope, the waveforms having the same time base and illustrate the phase relationships of signals which occur at various points in the circuit shown schematically in Figures la and lb.
With reference now to the drawings, wherein like numerals refer to like parts in the several views, there is shown in Figures la and lb a complete schematic diagram of a capacitor discharge ignition system capable of operation in a ferroresonant mode in accordance with the invention. Various portions of the electrical circuit are enclosed by broken lines ~'~

~03~
and given designations with respect to their function in the circuit. The complete ignition circuit of Figures la and lb is designated by the numeral 10.
In Figure la, it may be seen that the ignition system 10 includes an ignition coil 12 which has a first primary winding Pl, a second primary winding P2 and a secondary wind-ing 5. The ignition coil 12 has a ferromagnetic core 14 which in the circuit 10 is capable of being saturated repetitively after the initial breakdown of a spark gap 26. More speci-fically, the secondary winding S of the ignition coil hasone of its leads connected to one terminal of a capacitor Cl.
The other terminal of the capacitor Cl is connected to g.round at 16. A lead 18 extends rom the other terminal o~ the secondary winding S t~ the rotor 20 of a conventional distri-butor 22 for a spark-ignition internal combustion engine.
The distributor 22 has eight contacts 24 which are repeti-tively and serially contacted by the rotor 20 such that repetitive electrical contact is made with the eight spark gaps 26 contained in the spark plugs of the internal combus-tion engine. Thus, each of the spark plugs has one of its electrodes, represented by a lead 25, connected to the secondary winding S of the ignition coil and has its other electrode 27 connected to ground at 28. It should be noted that the ground connections 16 and 28.are common and, there-fore) each of the spark gaps 26 is connected, sequentially as the rotor 20 rotates, in series with the capacitor Cl.
The capacitor Cl need not be located as shown in Figure 1, but rather may be connected in series with the spar]c gap 26, for example, by its insertion in the lead 18, the lead 25 or the lead 27. If the capacitor Cl is inserted in the leads . ~

~;)3~
25 or 27, a separate capacitor is reguired for each spark gap. Similarly, a separate secondary winding S may be provided for each of the spark gaps 26 if desired. Separate secondary windings S and capacitors Cl for each of the spark' gaps 26 may be housed within the spark plug, for example, as depicted in the spark plug design of U.S. Patent 3,267,325 issued August 16, 196~ to J.F. Why.
The first primary winding Pl of the ignition coil 12 has one of its terminals connected to ground at 30 and has its other terminal 32 coupled, through a saturable, ferromagnetic core induckor L2 and a lead 34, to a capacitor C2. The capaaitor C2 i5 connected to a junction 36 ~ormed between a resistor Rl and the anode of a semiconductor controlled rectifier (SCR) Q7. The cathode o~ the SCR is connected to ground. The SCR has a gate or control electrode 38. The current limiting resistor Rl is connected through another saturable, ferromagnetic core inductor Ll to a -~350 volt DC
source of electrical energy. This voltage, as well as the other DC voltages shown in Figure 1, may be obtained from a 12 volt DC source of electrical energy, such as the storage battery 44 conventional'in motor vehicles, through use of a DC to DC converter well known to those skilled in the art.
An input matching circuit, a duration gate generator, a restrike oscillator, an SCR driver and an SCR switch comprise circuit means for charging the capacitor C2 from the DC
source o~ electrical energy and ~or discharging'this capacitor through the first primary winding Pl in timed relation to operation of the engine. The charging and discharging o~
the capacitor C2 in timed relation to engine operation may be obtained in the conventional manner by a cam 40 mechanically ~3~027 coupled to the distributor rotor 20, driven by the engine, and used to intermittently open and close a set of breaker points 42, one of which is connected to ground and the other of which is connected at a junction 46. Because the DC
source of electrical energy 44 has its negative terminal connected to ground and has its positive terminal connected through a resistor RZ to the junction 46, the junction 46 is at ground potential when the breaker points 42 are closed and is at the ~12 volt potential of the storage battery 44 when the breaker points are open. The voltage rise at khe junction 46 which occurs each tlme the hreaker points open is supplied to an input matching cîrcuit to cause the produckion o~ a spark in one o~ the spark gaps 26.
As indicated above, the circuikry 10 includes an input matching circuit. The function oE this circuit is to couple the pulses occurring at the junction 46 to a duration gate generator. The duration gate generator produces a pulse output signal which has a controllable duration and which is t supplied to the restrike oscillator. The func~ion of the restrike oscillator is to produce one or more pulse signals during the duration of the signal from the duration gate generator. Each pulse produced at the outp~t of the restrike oscillator is used to initiate the discharge of the capacitor C2 through the ignition coil first primary winding Pl. The output pulses from the restrike oscillator circuit are supplied to an SCR
driver circuit which utilizes the restrike oscillator pulses to produce pulse spikes which are applied to the gate 38 of the SCR Q7. An interlock circuit is provided to prevent, when the ignition circuit 10 is first put into operation, the supply of a pulse to the gate electrode 38 until the capacitor i~ 7 ? ~i ~380~7 C2 has had su~ficient time to charge. In the paragraphs which follow, the above circuit portions are described in detail.
The input matching circuit includes a choke inductor L3 which has one of its terminals connected to the junction 46 and which has its other terminal connected to the cathode of a zener diode D1. The anodé of this zener diode is coupled to ground through a resistor R3 connected in parallel with a noise suppression capacitor C3. The anode of the zener diode also is connected through the series combination of a DC blocking capacitor C4 and a current limiting resistor R5 to the base of an NPN transistor Ql. The junct~on farmed between the capacitor C4 and the resistor RS is connected to the cathode o~ a zener diode D2 whose anode :Ls connected to ground. A resistor R4 is connected in parallel with the zener diode D2. The emitter of the transistor Ql also is connected to ground and its collector is connected through resistors R6 and R7 to a +18 volt DC supply lead 48.
The function o~ the resistor R3 and capacitor C3 is to suppress high $re~uency noise signals that may appear at the anode of the zener diode Dl. The capacitor C4 permits the positive step voltage, which occurs at the junction 46 when the breaker points 42 open, to momentarily pass through the resistor R5 to the base o~ the transistor Ql to render it momentarily conductive in its collector-emitter output circuit. This permits current to 10w through the resistors R7 and R6 to ground.
The duration gate generator has a blocking capacitor C5 connected to the junction formed between the resistors R6 and R7. The opposite terminal o~ the capacitor CS is connected through a current limiting resistor R9 to the base ~4 .~ , .

~.~3~3027 of a~PNP transistor Q2. The junction formed ~etween the capacitor C5 and the resistor R9 is connected through a resistor R8 to the voltage supply lead 48. The emitter o~
the transistor Q2 also is connected to the supply lead 48 and its collector is connected through series-connected resistors R10, Rll and R12 to a -18 volt DC supply lead 50. The resistor R12 is variable and controls the duration (total length of time) of multiple spark discharges produced in a given spar]c gap 26 during one combustion cycle in the engine.
More speciically the resistor R12 controls the duration of the output signal pulse ~rom the duration gate generator. In a reciprocating spark-ignition internal combustion engin~, the length or duxation o~ thls output pulse is th~ length o~
time available for the production o~ one or more sparks in the spark gap 26 in a yiven cylinder to cause ignition of a combustible mixture of fuel and air and a resultant power stroke of the piston in that cylinder.
The capacitor C6 has one of its terminals connected to the voltage supply lead 48 and has its other terminal connected to the junction formed between the resistors R10 and Rll. Also connected to this junction is the cathode of a clamping diode D9 which has its anode connected to ground.
The diode D9 limits the negative voltage at this junction to one diode voltage drop below ground potential. The junction formed between the resistors R10 and Rll also i9 connected through a coupling capacitor C7 and a current limiting resistor R15 to the base of a PNP transistor Q3.
The junction formed between the capacitor C7 and resiStor R15 is connected through a resistor R13 to the negative voltage supply lead 50. The collector of the transistor Q3 also is _ g _ ~3138027 connected through a resistor R15 to the supply lead 50, and the emitter of this transistor is connected to the positive voltage supply lead 48. The collector of the transistor Q3 is connected through a resistor R16 to the base of an NPN
transistor Q4 whose emitter is connected to ground. A clamping diode D3 has its cathode connected to the base of the trans-istor Q4 and has its anode connected to ground to limit the base voltage to one dlode voltage drop below ground potential.
The output signal of the duration gate generator is taken at the collector of the transistor Q4 which is connected to pin 7 of a dual monostable multivibrator Ul, which as shown is a Teledyne type 342. A Texas Instr~ents type 153~2 or the e~uivalent also may be used for Ul.
The duration gate generator is a sawtooth generator which is triggered when the transistor Ql is rendered conduc-tive, which occurs, as previously stated, when the breaker points 42 open. When the transistor Ql is rendered conductive, the resistor R8 and capacitor C5 differentiate the resulting negative voltage step at the collector of Ql. The negative voltage spike which results is applied to the base of the transistor Q2. This renders.the transistor Q2 conductive in its emitter-collector outpu-t circuit ~or a time sufficient to permit the discharge of the capacitor C6 through the resistor R10 and the emitter-collector circuit o~ the trans-istor Q~. The capacitor C6 will have previously been charged to a voltage slightly in excess o~ 18 volts DC. The transistor Q3 is normally conductive in its emitter-collector output circuit due to the flow o~ current from the voltage supply lead 48, through its emitter-base junction, through the resistor R15, and primarily through the resistor R13 to the 9~0380Z7 negative voltage supply lead 50. However, when the capacitor C6 discharges, a positive voltage approximately equal to the voltage on the supply lead 48 appears at the junction formed between resistors R10 and Rll. This voltage is applied through the capacitor C7 and the resistor R15 to the base o~
the transistor Q3 to render it nonconductive. The transistor Q3 remains nonconductive for the length of time required for the capacitor C6, after ~he transistor Q2 again becomes nonconductive, to recharge through the series resistors Rll and R12. Typically, the transistor Q3 is nonconductive for a time period of from 1 to S ms. When the transistor Q3 is rendered nonconductive and for so long as it i5 nonconductive, the transistor Q4 has no base drive and also is nonconductive which results in the application o~ a positive voltage at the pin 7 of the dual monostable multivibrator Ul.
The dual monostable multivibrator Ul has one mono-stable multivibrator with an input Al and an output Ql The other monostable multivibrator in the integrated circuit Ul has an input A2 and an output ~2. By the connection of the Ql output to the A2 input and the connection of the Q2 output to the Al input, as is accomplished by the connection of the lead 52 between the pins 5 and 10 and the connection of the pins 6 and 11 at a junction 54, the dual monostable multi-vibrator Ul becomes a pulse generatox, the output of which is taken at its pin 2. The Ql output at pin 2 alternates between a high voltage level o~ about 10 volts and a low voltage level near ground potential. With the circuit values indicated in the drawing, the high voltage portion o~ the signal at pin 2 is approximately 68% of the signal period. Dual variable resistors R18 and Rl9 are connected, respectively, through a 103l~0;~
resistor R20 and a capacitor C9 to the pins 3 and 4 and through a resistor R21 and a capacitor C10 to the pins 12 and 13. These components determine the duty cycle or pulse width at output pin 2 of the multivibrator and permit the period of the signal at pin 2 to be varied from about 0.30 ms to 1.5 ms. The period o the signal at pin 2 represents the restrike delay, that is, the delay between multiple ignition sparks produced in each of the spark gaps 26 by repetitive triggering of the SCR Q7.
The dual monostable multivibrator Ul is triggered or gated when the output circuit of the transistor Q4is rendered nonconductive. When the transi~tor Q4 is conductive, the signal at pin 2 of the dual monostable multivibrator Ul remains constant at a low voltage level, but when the transistor Q4 becomes nonconductive, gating multivibrator Ul, the signal at pin 2 becomes a series of pulses which continually gate the SCR Q7 to produce a spark in a spark gap 26 each time a pulse occurs at pin 2. These repetitive and restriking sparks continue to occur until the transistor Q4 is once again rendered conductive.
The dua~ monostable multivibrator Ul receives its positive voltage supply from a voltage regulator comprising a resistor R17 connected in series with the parallel combination of a zener diode D4 and a capacitor C8. The junction formed between these components is connected to the volta~e supply pin 16 of Ul and also is connected to the variable resistors R18 and Rl9. Pin 8 of the multivibrator Ul is connected to ground. Pin 2 of the multivibrator is connected through a current limiting resistor R22 and a zener diode D5 to the base of an NPN transistor Q5.

. .

~380Z7 The transistor Q5 is located in the SCR driver por-tion of the circuit 10 and has its emitter connected to ground.
Its collector is connected through a resistor R27 to the voltage supp~y lead 48 and also is connected through a current limiting resistor R28 to the base of a PNP transistor Q6. The emitter of the transistor Q6 is connected to the voltage supply lead ~8 and its collector is connected through a resistor R29 and a lead 60 to a -18 volt DC voltage supply. The collector of the transistor Q6 also is connected, through a ~eries circuit including diferentiating capacitor C12, resistor R30 and zener diode D8, to the gate electrode 38 of the SCR Q7.
The wave~orms shown in Flgures 2 through 13 ar~
representations of signals which occur at various points in the circuit schematically illustrated in Figure 1, with the exception that the waveorms 11, 12 and 13 pertain to a 35 mil spark gap located in air at atmospheric pressure rather than to a spark gap located in the cylindèr of an operating internal combustion engine.
Figure 2 shows the voltage waveform that occurs at pin 2 of the dual monostable multivibrator Ul. This voltage is the oscillatory output voltage of the multivibrator which occurs so long as the input transistor Q4 connected to its pin 7 is in a nonconductive state. Of course, Q~ is rendered nonconductive each time, and for a predetermined time established by the duration gate generator, that the cam 40 opens the breaker points 42. On each positive going edge of the pulses in Figure 2, the transistor Q5 is rendered conductive. This reduces its collector voltage to substan-tially ground potential to cause the conduction of the PNP
transistor Q6. When nonconductive, the collector of the ~0381~)27 transistor Q6 is at approximately -18 volts DC, but when rendered conductive, its collector achieves a voltage of al~ost ~18 volts DC. This step voltage on the collector of the transis-tor Q6 is differentiated by the capacitor C12 to produce a voltage spike which gates the SCR Q7. The voltage spikes are represented in Figure 6, which illustrates the voltage spikes occuring on the resistor R30 at points corresponding to the positive going edges of the pulses of Figure 2, which pulses occur at pin 2 of the multivibrator.
Thus, it is apparent that the SCR Q7 is gate.d or triggered on each positive going edge o~ the oscillatory signal occuring at pin 2 o~ the multivibrator Ul and th~t this continues so long as the txansistor Q4 is nonconductive.
If the duration gate generator is adjusted such that the transistor Q4 is nonconductive for 5 milliseconds and if the restrike delay resistors R18 and Rl9 are adjusted such that the signal of Figure 2 has a period of 0.33 ms, then the gate 38 of the SCR Q7 will receive 16 trigger pulses during the course of the 5 ms that the transistor Q4 is nonconductive.
This produces a corresponding 16 spark discharge in a single one of the spark gaps 26. It should be noted that 5 ms is approximately the time required for the piston in an eight-cylinder, four-cycle reciprocating internal combustion engine to travel from its top-dead-center position to its bottom-dead-center position when the engine is operating at 6,000 rpm.
With respect to the interlock portion of the circuitry 10, it may be seen that this circuit portion comprises NPN
transistors Q8 and Q9. The emitters of these transistors are connected to ground potential. The collector of the transistor Q9 is connected, through a diode D6, to the junction formed ~ 80;~7 between.the resistor R22 and the zener diode D5. The collector of this transistor also is connected through a resistor 23 to a lead 57 connected to a +18 volt DC source of electrical energy. A current limiting resistor R25 is connected between the lead 57 and the collector of the trans-istor Q8. The collector of the transistor Q8 also is con-nected through a current limiting resistor R25 to the base of the transistor Q9. A series-connected resistor R26 and capacitor Cll are connected between the lead 57 and ground potential. The junction formed between the resistor R26 and the capacitor Cll is connected through a zener diode D7 to the base o~ the transistor Q8. Upon the initial application of the DC supply potential to the lead 57, th~ transi~tor Q9 immediately is conductive in its collector-emitter output circuit. This has the ef~ect of connecting the pin 2 output of the multivibrator Ul to ground potential to prevent the conduction of the transistor Q5 and, consequently, to prevent the supply of a triggering pulse to the gate electrode 38 of the SCR Q7. At this time, the transistor Q8 is nonconductive in its output circuit because the capacitor Cll forms an effective short circuit of its base-emitter circuit. However, the continued application of the DC voltage on the lead 57 causes the capacitor Cll to be charged through the.resistor R26.
When the voltage on the upper terminal of the capaci-tor Cll exceeds the sum o~ the breakdown voltage of the zener diode D7 and the base-emitter voltage drop required to render the transistor Q8 conductive, then the collector-emitter circuit of trans.istor Q8 becomes conductive and shunts the base-emitter circuit of the transistor Q9. The transi~stor .~, 111)3~ 7 Q9 then becomes nonconductive and the positive going edges of the oscillatory signal at pin 2 of the multivibrator Ul are permitted to cause the repetitive triggering of the gate electrode 3~ of the SCR Q7. The time required to charge the capacitor Cll exceeds considerably the time re~uired to charge the capacitor C2 connected to the first primary winding Pl of the ignition coil 12. The capacitor C2 must be fully charged before the SCR Q7 is triggered ~ecause the latter is self-commutated as a result of the discharge of the capacitor C2 through it and the first primary winding Pl.
Of course, the interlock circuitry shown in Figure la may be replaced by gate circuitry which prevents the application of a trigger signal on the gate electrode 38 of the SCR
prior to the required charge level on the capacitor C2 being attained.
When the SCR Q7 is nonco~ductive between its anode and cathode, the capacitor C2 is charged from the -~350 volt DC power supply through the current path including the inductor Ll, the resistor Rl, the inductor L2, the first primary winding Pl of the ignition coil 12 and the ground circuitO When the.SCR Q7 is triggered by a positive pulse applied to its gate electrode 38, a current spike is produced.
Two such current spikes, caused by two successive trigger pulses applied to the gate electrode 38 are shown in the waveform of Figure 9. It may be seen that these current spikes have an alternating current waveform. ~t the end of the spike, the SCR Q7 is self-commutated. This sel~-commuta-tion is aided by the saturable inductor L2 which o~fers little impedance to current flow due to its saturable character.
Figure 10 shows the voltage across the first primary ~ ~i ....

winding Pl upon the occurrence of the current spikes shown in Figure 9. It may be seen that this voltage is oscillatory, that it has a voltage spike which corresponds to the breakdown of one of the spark gaps 26, and that the amplitude is sub-stantially constant for the time interval during which current flows through the spark gap (this current is shown in Figure 11 hereinafter described).
The sustain oscillator, the sustain gate, the sustain driver and the sustain power amplifier generally comprise circuit means for producing a fixed ~requency oscillatory current in the second primary winding P2 for a predetermined time interval subse~uent to each discharge of the capacitor C2 through the ~irst primar~ wind:lng Pl. The sustain gate is triggered by a ~ignal which triggers the SCR Q7 and produces oscillations of a square-wave character and of fixed frequency. These oscillations receive current and power amplification through the sustain driver and sustain power amplifier circuits, and the amplified oscillatory currents flow through the second primary winding P2 of the ignition coil 12.
The sustain oscillator includes a dual monostable multivibrator integrated circuit U2. The dual monostable multivibrator U2 as shown has the pin connections of a Moto-rola Semiconductor Corporation type MC 667, but equivalent devices ma~ be substituted. Dual monostable multivibrator U2 has its Q2 output connected to its Tl input and has its Ql output connected to its T2 input. Thus, lead 64 inter-connects pins 1 and 8 o~ U2 and pins 6 and 13 are intercon-nected at a junction 66 which forms the trigger input to the multivibrator U2. The trigger input is supplied via a lead ~31B~7 68 connected to the collector of a transistor Qll. The emitter of the transistor Qll is connected to ground.
A lead 62 is connected to the junction 54 connected to pins 6 and ll of the dual monostable multivibrator Ul in the restrike oscillator. The signal on these pins is the same as the pin 2 signal shown in Figure 2. Lead 62 is connected through a resistor R31 to the cathode of a zener diode Dl0, the anode of which is connected to the base of an NPN transistor Ql0. The emitter of the transistor Ql0 is connected to ground and its collector is connected through a c~rrent limiting resistor R33 to a -~18 volt DC source of electrical energy. A resistor R32 is connected to this source and to the junction formed between the resistor R31 and the cathode o the zener diode Dl0. The collector of the trans-istor Q10 also is connected through a curren~ limiting.
resistor R34 to the base of an NPN transistor Qll~ ~hen the voltage on the lead 62 is at its high voltage level, the transistor Ql0 is conductive in its output circuit and its collector voltage is substantially at ground potential. This renders the transistor Qll nonconductive in its output circuit and its collector is isolated from ground potential. On the other hand, when the signal on the lead 62 is a low voltage, the transistor Ql0 is nonconductive, which causes the trans-istor Qll to be conductive in its collector-emitter output circuit and results in the connection of the pins 6 and 13 of the dual monostable multivibrator U2 to substantially ground potential.
The dual monostable multivibrator U2 is connected as a squaxe-wave oscillator which has a duty cycle and period determined by the parallel-connected resistors R35 and R36 ~0~8~27 connected across pins 10 and 11 and the capacitor C13 connected between pins 9 and 11 and by the parallel-connected resistors R37 and R38 connected across pins 3 and 4 and the capacitor C14 cQnnected between the pins 3 and 5. Resistors R36 and R37 are variable to provide an oscillator output signal on the Ql output at pin 2 of the multivibrator U2 which has a fre~
quency variable between 17 KHz and 35.7 KHz. The output on the pin 2 of the dual monostable multivibrator U2 is a low level voltage whenever the voltage on pin 2 of the dual mono-stable multivibrator Ul is a low voltage, and the voltage onpin 2 of the dual monostable multivibrator U2 is oscillatory between 12 volts and ground potential whenever the voltage on pin2 of the dual monostable multivibratox Ul is at a high .
voltage level. The oscillatory voltage at pin 2 o~ the multivibrator U2 is applied through a current limiting resis-tor R40 to the base of an NPN transistor Q12. The emitter of the transistor Q12 is connected to ground and its collector is connected through a current limiting resistor R41 to a lead 58 connected to a +18 volt DC source of electrical energy.
The voltage supply to the multivibrator U2 is obtained from a resistor R39 connected to the lead 58 and to the parallel combination of a filter capacitor C15 and a zener diode Dll which are connected between the pin 14 of U2 and ground potential. This provides a regulated supply voltage for multivibrator U2. Pin 7 of the multivibrator U2 is connected to a ground lead 70.
The output signal of the sustain oscillator is obtained on a lead 72 connected to the collector of the transistor Q12. This signal is shown in Figure 4 where it may be seen that the voltage oscillates between about ~18 volts ~L038027 DC and 0 volts DC. Because each of the high voltage-level pulses at pin 2 of the multivibrator Ul results in a trigger signal being applied to the gate 38 of the SCR Q7, and from the waveform of Figure 4, it is clear that an oscillatory signal is produced on the lead 72 of the sustain oscillator each time the 5CR Q7 is triggered. This oscillatory signal has a duration corresponding to the duration of the high-voltage-level pulses shown in Figure 2. These sustained oscillations on the lead 72 cause, in a manner hereinafter described, current oscillations in the second primary winding P2 of the ignition coil 12.
With particular reference now to Figure lb, there is shown the sustain gate, the su8kain driver and the su~tain power ampli~ier, the functions of which are to provide current and power amplification of the oscillatory signals occurring on the lead 72 which is connected through a current limiting resistor R48 to the base of a PNP transistor Q15 in the sustain gate. The emitter of the transistor Q15 is connected to a ~18 volt DC supply lead 74 and its collector is connected through a current limiting resistor R49 to a -18 ~olt DC supply - lead 76. The voltage on the collector of the transistor Q5 in the SCR driver portion of the circuitry is shown in Figure 3 as the complement of the signal on pin 2 of the dual mono-stable multivibrator Ul and is supplied via a lead 59 and through a current limiting resistor R~2 to the base of a P~P
transistor Q13. The emitter o~ this trans.istor is connected to the voltage supply lead 74 and its collector is connected through a resistor R43 to the negative voltage supply lead 76. Its collector also is connected through a current limiting resistor R45 to the base of a PNP transi.stor Q14.

. - 20 -~038~27 The collector of Q14 is connected through a current limiting resistor R46 to the negative voltage supply lead 76 and its emitter is connected to the voltage supply lead 74.
A diode gate is formed by diodes D12, D13, D14 and D15. The anodes of the diodes D12 and D13 are connec~ed together and, through a resistor R44, are connected to the collector of the transistor Q13. The cathode and anode junction formed between diodes D12 and D14 is connected by a lead 78 to the collector of the transistor Q15 and the cathodes of the diodes D14 and D15 are connec-ted, through a resistor R47, to the collector o~ the transistor Q14. The junction formed between the c~thode o the diode D13 and the anode o~
the diode D15 is connected by a lead 80~ whlah is thc output of the sustain gate, to one terminal of a resistor R50 the other terminal of which is connected to ground. The lead 80 also is connected through a resistor R51 to the base of an NPN transistor Q16 and through a resistor R52 to the base of a PNP transistor Q17. Transistors Q16 and Q17 form a push-pull amplifier and thus have their emitters connected together and to ground potential. The collector of the transistor Q16 is connected through a current limiting resistor R53 to the voltage supply lead 74, and tha collector of the transistor Q17 is connected through a resistor R54 to the negative voltage supply lead 76. Also, the collector of the transistor Q16 is connected to the base of a PNP transistor Q18 whose emitter is connected to the voltage supply.lead 74 and whose collector is connected via a lead 82 and a resistor R55 to ground. Similarly, the collector of the transistor Q17 is connected to the base of an NPN transistor Ql9 whose emitter is connected to the negative voltage swpply lead 76 and whose collector is connected to the lead 82 and, through the resistor R55 to ground potential. It may be appreciated that when the transistor Q16 is conductive in its collector-emitter output circuit, the transistor Q18 also is conductive to permit current flow from the voltage supply lead 74 to the lead 82, and, through the resistor R55, to ground. Like-wise, when the transistor Q17 is conductive in its emitter-collector output circuit, the output circuit of the transistor Ql9 is conductive to permit current to flow from ground, through the resistor R55 and through the collector-emitter output circuit of the transistor Ql9 to the negative volkage supply lead 76.
As may be seen from F~gures 3 and 4, prior to the occurrence o~ oscillations on the lead 72, the voltage on this lead is at about +18 volts, as is the voltage on the gate signal lead 59. Thus, the emitter-base junctions of the tran-sistors Q15 and Q13 are reverse-biased and these transistors are non-conductive. In such case, the voltage on the sustain-gate output lead 80 is at ground potential. When the voltage at pin 2 of the dual monostable multivibrator Ul rises to about 10 volts to cause the application of a trigger signal on the gate lead 38 of the SCR Q7, the gate signal on lead 59 falls to a few volts as shown in Figure 3. At the same time, the voltage on the lead 72, connected to the collector of the transistor Q12 in the sustain oscillator, oscillates between about ~18 volts DC and substantially ground potential as shown in Figure 4. The low voltage on the lead 59 renders the transistor Q13 conductive. This results in the applica-tion of about ~18 volts to the base of the transistor Q14 and it is rendered nonconductive in its output circuit. The , , ~38~7 oscillations on the lead 72 are applied through -the resistor R48 to the base of the transistor Q15 to render its emitter-collector output circuit conductive and nonconductive in a corresponding oscillatory manner. Thus, the lead 78 alter-nates between ~18 volts and -18 volts. When the lead 78 is at ~18 volts, current flows from the collector o~ the trans-istor Q13 through the resistor R44, through the diode D13 and into the lead 8~. At the junction formed between lead 80 and the resistor R50 the current divides, part of it-flowing to ground throuyh the resistor R50 and the remainderflowing through the resistor R51 and base-emitter junction of the transistor Q16 to ground. When the lead 80 i~ at -18 volts~ currents ~low from ground through the resistor R50 and ~rom ground through the emitter-base junction of the transistor Q17 and the resistor R52 to the lead 80 where these currents are combined. The combined current flows from the lead 80, through the diode D15, the resistor R47 and the resistor R46 to ^the negative voltage supply lead 76~ Under such circumstances, the voltage waveform on the lead 80 is as shown in Figure 5.
The transistors Q16 and Q17 are alternately conductive during the oscillatory voltage which occurs on the lead 72. These transistors amplify the alternating voltage signal on the lead 80.
When the transistor Q16 is conductive on alterna-tive half cycles, the transistor Q18 also is conductive to provide current and power amplification. Similarly, when the transistor Q17 is conductive, the transistor Q19 also is conductive to provide amplification. ~he voltage on the collectors of the transistors Q18 and Ql9, during the .,f ~
~' ~31~027 oscillations on the lead 72, also oscillates between about ~18 and - 18 volts. This alternating voltage, when positive, is applied through a current limiting resistor R56 to the base of a transistor Q20 to render it conductive, and, when negative, is applied through a current limiting resistor R56 to thé base of a transistor Q21 to render it conductive. The emitters of the transistors Q20 and Q21 are connected together and to ground, the collector of the transistor Q20 is connected through a resistor R58 to the voltage supply lead 74, and the collector of the transistor Q21 is connected through a resistor R59 to the voltage supply lead 76. The transistors Q20 and Q21 form a push-pull amplifier.
The collec~or o~ khe transistor Q20 i~ connected through a curr~nt limiting resistor R60 to the base o~ a tran-sistor Q22, the emitter of which is connected through a resi~tor R62 to the voltage supply lead 74. The collectox o~
the transistor Q21 is connected through a current limiting resistor R61 to the base of a transistor Q23 whose emitter is connected -through a resistor R63 to the ~oltage supply lead 76. The collectors of the transistors Q22 and Q23 are con-nected together. A diode D16 has its cathode connected to the emitter of the transistor Q22 and has its anode connected to the collector of this transistor. Similarly, a diode D17 has its cathode connected to the collector o~ the transistor Q23 and has its anode connected to the emitter oE this tran sistor. Transistor Q22 is conductive when transistor Q20 is conductive, and transistor Q23 is conductive when transistor Q21 is conductive.
The junction formed between the collectors of the transistors Q22 and Q23 is connected by a lead 84 to the ~38~:)27 junction formed between a resistor R64 and a saturable inductor L4. The opposite ter~inal of the resistor R64 is connected to ground. Lead 19 connects the opposite terminal of the saturable inductor L4 to the second primary winding P2 of the ignition coil 12 and the lead 21, connected to the opposite terminal.of this second primary winding, is connected to ground. Thus, the resistor R64 is connected in parzllel with the series-connected saturable inductor L4 and second primary winding P2. The alternating conduction of the tran-sistors Q22 and Q23 in respons~ to the oscillations on thelead 7Z causes an alternating current to flow through the saturable inductor L4 and the second primary winding P2 of the ignition coil to sustain a spark in khe gap 26 o~ a spa.rk plug for a time period determi~ed by the length of time the oscillation continues on the lead 72. The alternating voltage across and current flo~7 through the second primary winding P2 are shown, respectively, in Figures 7 and 8.
As was previously ~entioned, Figure 9 shows the current flow through the primary winding Pl for two spark discharges through a spark gap 26. It may be seen that two alternating current spikes occur, one ~or each of the SCR Q7 gate signal pulses which occur as shown in Figure 6. These gate signal pulses result in conduction of the SCR Q7 and the discharge of the capacitor C2 through the first primary winding Pl. This breaks down ~ spark gap 26, causes fexro-resonant oscillations to occur in the secondary c~rcuit o~
the ignition coil 12, and caus~s the sustain gate, sustain oscillator, and sustain ampliier circuitry to produce alternating current in the second primA.ry winding P2. The frequency of this alternating current is selected to sustain .

~38~27 a ferroresonant mode of oscillation in the ignition coil secondary circuit.
Figure ll depicts the current through a 35 mil spark gap, located in air at atmospheric pressure, for two spark discharges, each of which is initiated by the discharge of the capacitor C2 through the first primary winding Pl and each of which is sustained for a predetermined time interval as a result of the alternating current flow through the second primary winding P2. It may be seen that this current flow through the spark gap is alternating in direction, that the initial amplitude and frequency, that is, ~or about the ~irst 75 microseconds of the spark di.scharge, is h.igher than the ~ixed ~requency and amplitud~ o current ~low wh:lch occurs therea~ter, and that the alternating current 10w through the spark gap is nonsinusoidal, which is the result of erro-resonant oscillation in the secondary circuit o the ignition coil 12, this ferroresonant oscillation resulting ~rom repetitive variation o the ignition coil ferromagnetic core between saturated and unsaturated conditions.
Figure 12 shows the voltage across the 35 mil spark gap, located in air at atmospheric pressure, during the current discharge through this spark gap as depicted in Figure 11. The waveform of Figure 12 has notch-like portions 86 which cor-respond to the current spikes shown in Figure 11, leading to strong arcs within the spark gap 26.. The spark is extinguished at the point 88. Following this, a sinusoidal and decreasing amplitude oscillation 90 take place.
Figure 13 depicts the voltage across the capacitor Cl or two spark discharges corresponding to -the current and voltage wave:Eorms shown, respectively, in Figures 11 and 12.

~. ~

~La38~)~7 It may be seen that the frequency of this voltage across the capacitor Cl for about the first 75 microseconds oscil-lates at a voltage and frequency which is in excess of that which follows. The oscillations of voltage across the capa-citor Cl during this initial 75 microseconds is a ~errore~-onant oscillation defined by the equation f = Vm/4NS~s. The oscillations which ollow also behave in accordance with this equation, but the frequency o oscillation is that produced by the alternating current flowing through the second primary winding P2. In other words, the ferroresonant oscillations lock-in at the fixed fre~uency of the sustaining alternating current oscillations in the second primary winding P2. The voltage V~ across the capacitor Cl assumes a value defin~d by the ~oregoing e~uation ~or operation at such fl~ed fre-quency.
The voltage and current waveforms shown in Figure 2 through 13 were obtained with an ignition coil 12 having a first and second primary windings Pl and P2 each of one turn and a secondary winding of 160 turns. The primary windings Pl and P2 and the secondary winding S were wound on a ferrite (manganese zinc~ core having the shape of a closedt hollow cylinder with a central core running along its axis. The cylinder had an outside diameter of 42 millimeters and a height of 29 millimeters. The primary and secondary windings were wound about the central core. The capacitor Cl had a value of 500 picofarads. The remaining components in the circuit of Figures la and lb were of the values indicated therein. The capacitance values are given in microfarads, unless otherwise specified, and the resistance values are in ohms or, as indicated, in kilohms.

Z~
The design oE the saturable ferromagnetic ignition coil 12 is not critical and may take various forms other than that described in the p.receding paragraph. Also, the value of the capacitor Cl is of importance in producing ferro-resonance in the secondary circuit during the discharge o~
the capacitor C2 through the ignition coil primary winding Pl, but the capacitance Cl may be wlthin a broad range.
Values in excess of 1,000 picoEarads for the capacitor Cl have been used.
The DC voltage supply for charging the capacitor C2 and the value o~ this capacitor must ba su~Eiciently large to permit the discharge o~ this capacitor through th~ ~irst primary winding Pl of the ignition coil 12 to p.roduce a ~erroresonant condition, as depicted in Figures 7 through 13, in the ignition system.
The circuitry of Figures la and lb is designed to provide multiple sustained sparks during a given combustion cycle in a given combustion chamber oE an engine. If it is desired to produce only one sustained spark per combustion cycle, then the circuitry may be simplified considerably. Of course, a transistorized ignition system using a pulse genera-tor driven by a distributor or the like may be used in place of the cam 40 and breaker points 42. Such breakerless ignition systems are well known.
The inven tors have Eound that the ~Eirst and second primary windings Pl and P2 may, if desired, be replaced by a single primary winding connected to the SCR Q7 in the manner shown in Figure la, but also having its terminal leads con-nected, Eor example, by the leads 19 and 21 in Figure lb, to the output o:E the sustain oscillator.

- ~8 -

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A capacitor discharge ignition system for an internal combustion engine, which comprises:
an ignition coil having first and second primary windings, a secondary winding and a ferromagnetic core about which said windings are wound;
a spark plug having electrodes spaced to form a spark gap, one of said electrodes being coupled to one terminal of said secondary winding;
a first capacitor connected in series with said spark gap, one terminal of said first capacitor being coupled to the other terminal of said secondary winding;
a second capacitor coupled to said first primary winding;
a DC source of electrical energy;
first circuit means, coupled to said second capacitor and to said first primary winding, for charging said second capacitor from said DC source of electrical energy and for discharging said second capacitor through said first primary winding in timed relation to operation of said engine; and second circuit means, coupled to said second primary winding, for producing an oscillatory current in said second primary winding for a predetermined time interval subsequent to each discharge of said second capacitor through said first primary winding;
the discharge of said second capacitor through said first primary winding and the subsequent production of said oscillatory current in said second primary winding producing, for at least a portion of said predetermined time interval, a voltage in the secondary circuit of ignition coil which oscillates at a frequency defined by the expression f =
Vm/4Ns.PHI.S where Vm is the instantaneous maximum voltage across said first capacitor, Ns is the number of turns in said secondary winding, and .PHI.s is the magnetic flux within said secondary winding when said ferromagnetic core of said ignition coil is magnetically saturated.

2. The ignition system of claim 1, wherein said first circuit means includes means for generating a gating signal for causing the discharge of said second capacitor through said first primary winding and wherein said second circuit means includes an oscillator for generating an oscillatory signal and an amplifier means for amplifying said oscillatory signal, said amplifier means being coupled to said second primary winding to produce said oscillatory current in said second primary winding, said oscillator being controlled by said gating signal generated by said first circuit means.

3. The ignition system of claim 2, wherein said means for generating said gating signal includes a second oscillator, said second oscillator being triggered in timed relation to operation of said engine, said second oscillator having an output signal from which said gating signal is derived and which determines said predetermined time interval.

4. A capacitor discharge ignition system for an internal combustion engine which comprises:
an ignition coil having first and second primary windings, a secondary winding and a ferromagnetic core about which said windings are wound;
a spark plug having electrodes spaced to form a spark gap, one of said electrodes being coupled to one terminal of said secondary winding;
a first capacitor connected in series with said spark gap, one terminal of said capacitor being coupled to the other terminal of said secondary winding;

a second capacitor coupled to said first primary winding;
a DC source of electrical energy;
first circuit means, coupled to said second capacitor and to said first primary winding, for charging said second capacitor from said DC source of electrical energy and for discharging said second capacitor through said first primary winding in timed relation to operation of said engine; and second circuit means for producing an alternating current through said spark gap subsequent to each discharge of said second capacitor through said first primary winding, said alternating current having a frequency f defined by the expression f = Vm/4Ns.PHI.s where Vm is the instananeous maximum voltage across said first capacitor, Ns is the number of turns in said secondary winding and .PHI.s is the magnetic flux within said secondary winding when said ferromagnetic core of said ignition coil is magnetically saturated.

5. The ignition system of claim 4, wherein said second circuit means includes an oscillator controlled by said first circuit means, said oscillator being coupled to said second primary winding to cause an oscillatory current to flow through said second primary winding subsequent to each discharge of said second capacitor through said first primary winding.

6. The ignition system of claim 5, wherein said alter-nating current through said spark gap, during at least a portion of the time it exists, has a frequency equal to the frequency of said oscillatory current in said second primary winding.

7. The ignition system of claim 6, wherein said alter-nating current through said spark gap has a frequency greater than 17 KHz.

8. The ignition system of claim 6, wherein said oscillator has an output frequency in the range from 17 to 35.7 KHz.

9. A capacitor discharge ignition system for an internal combustion engine, which comprises:
an ignition coil having a primary winding, a secon-dary winding and a ferromagnetic core about which said windings are wound;
a spark plug having electrodes spaced to form a spark gap, one of said electrodes being coupled to one term-inal of said secondary winding;
a first capacitor connected in series with said spark gap, one terminal of said first capacitor bring coupled to the other terminal of said secondary winding;
a second capacitor coupled to said primary winding;
a DC source of electrical energy;
first circuit means, coupled to said second capaci-tor and to said primary winding, for charging said second capacitor from said DC source of electrical energy and for discharging said second capacitor through said primary winding in timed relation to operation of said engine; and second circuit means, coupled to said primary winding, for producing an oscillatory current in said primary winding for a predetermined time interval subsequent to each discharge of said second capacitor through said primary winding;
the discharge of said second capacitor through said primary winding and the subsequent production of said oscil-latory current in said primary winding producing, for at least a portion of said predetermined time interval, a voltage in the secondary circuit of ignition coil which oscillates at a frequency defined by the expression f = Vm/4Ns.PHI.s where Vm is the instananeous maximum voltage across said first capacitor, Ns is the number of turns in said secondary winding, and .PHI.s is the magnetic flux within said secondary winding when said ferromagnetic core of said ignition coil is magnetically saturated.