EP0000652A1

EP0000652A1 - Internal combustion engine spark ignition apparatus producing an oscillating discharge current of a capacitor

Info

Publication number: EP0000652A1
Application number: EP78300178A
Authority: EP
Inventors: Basil Earle Wainwright
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-07-21
Filing date: 1978-07-21
Publication date: 1979-02-07
Also published as: NO782501L; IT7825967A0; DK325278A; ES471943A1; IT1097200B; ZA784170B; AU3823978A; JPS5459530A

Abstract

An apparatus for producing spark ignition of an internal combustion engine in which a capacitor (C1) is connected in a resonant circuit with the primary (1a) of an ignition coil (1). An arrangement is provided to charge the capacitor (C1). and a thyristor (SCR) is connected in the circuit so that when fired an oscillatory current flows in the circuit so as to induce in the secondary (1b) of the coil (1) an oscillatory high voltage for producing spark ignition. The charging current to the capacitor (C1) is inhibited during the current discharge so as to maximise the period that the thyristor (SCR) can be held conductive without becoming locked in a permanently conductive condition.

Description

This inrsntion relates to improvements in and relating to apparatus for producing spark ignition of an internal soubmation engine.
It is well known that the electrical sparks fed to the spark pluse of an internal combustion engine are comventionally produced by means of an ignition eoil having its high voltage secondary wimding connected to the engine's spark plugs through a distributor, and having its lew veltage primary winding sonnected to a lew voltage seurse, typisally a is volt battery or an alternator drivem by the engine. An engine driven switohing devioe, typieally a meahamieal contact breaker, produces iinterruptions im the surrent flowing in the eoil's primmary winding and consequently high voltage pulses are produced in the seil's secondary winding, wwhich are applied te the spark plugs.
Heeantly, a proposal has been made to inerease the energy of the sparks applied to the spark plugs, by eounecting a capaeitor to the primary coil of the winding, charging the capacitor to a voltage much higher than the cenvemtional 12 volt supply veltage form the amgine's batiery and alternator, and diseharging the capaeitor through the eoil's primary winding sach time a spark is required. with such am arrangement, the tetal spark energy for each firimg of a eylinder of the engine, is increased substantially with respect to the conventional spark ignition apparatus, but the duration of the sparks produced by the arrangement is much less than those produced by the conventional apparatus. Such shorter sparks can prove disadvantageous with certain engines and certain engine operating conditions, particularly but not exclusively with large capacity racing engines, V-12 cylinder engines for example.
It is an object of the present invention to provide an apparatus for producing spark ignition of an internal combustion engine, which makes use of the capacitive discharge aforementioned but which provides electrical pulses for producing spark ignition of an increased duration with respect to that of the prior proposal.
In accordance with the present invention there is provided an apparatus for producing spark ignition of an internal combustion engine, comprising an ignition coil having primary and secondary windings, a capacitor connected in an electrical circuit with the primary winding, voltage generating means for receiving a supply voltage and charging said capacitor to a voltage greater than said supply voltage, switching means arranged to cause the capacitor to discharge a current through the primary winding of the coil, the apparatus being so arranged that the discharge current of the capacitor oscillates in the said circuit, and control circuit means adapted to operate the switching means to produce said discharge current in such a manner that more than one oscillation of the current passes through the primary winding whereby to induce in the secondary winding more than one cycle of an oscillatory voltage for producing spark ignition of the engine.
Preferably, the voltage generating means comprises a d.c. to d.c. converter for producing from low voltage input supply of typically 12 volts of a battery or alternator, a high charging voltage, of typically 200 volts, for the capacitor.
Preferably, the switching meane comprieee a thyristor and the control circuit means is responsive to operation of a contact breaker which may be of the conventional mechanical kind or of the more recently developed photocell or magnetically operated types. The control circuit is arranged to trigger the thyristor into conduction each time a apark is required, and to hold the thyristor in a conductive stats for more than one oscillation of the discharge current of the capacitor, thereby producing in the secandary winding an oscillatory voltage of more than one cycle. The oscillatory voltage can be arranged to continue for substantially the whole operative firing period of each cylinder of the engine thereby producing improved combustion of the fuel/air mixture as coirpared with the shorter duration sparks produced by the prior proposal.
Conveniently, the control circuit means is arranged to reduce the duration of conduction of the thyristor as the speed of the engine increases, so as to prevent the duration of sparking from extending from the firing period of one cylinder into the firing period of another cylinder.
A particularly preferred embodiment of the invention includes an ignition coil having primary and reeoadarprindinas, a capacitor connected in electrical circuit with the primary winding, voltage generating means for receiving a supply voltage and adapted to charge the capacitor to a voltage greater than said supply voltase, a thyristor connected in said circuit and arranged so that when fired to a conductive state the capacitor diseharges in an oscillatory manner through the thyriator and the secondary winding, and control circuit means adapted to apply to the gate of the thyristor a firing sisnal to render the thyristor conductive for a period sufficient for a plurality of oscillations of the capacitor diecharge current to pass through said primary winding, and wsans for inhibiting said voltage generating emans from charging the capacitor during the discharge thereof produced by firing of the thyristor.
When the capacitor is discharged by firing the thyristor, an oscillatory current is established in the circuit, the peak magnitude of the current decreasing with successive oscillations thereof. The particularly preferred form of the apparatus of the invention includes means for inhibiting operation of the voltage senerating means during discharge of the capacitor, which has the advantage of permitting substantially the entire stered energy established in the capacitor to be dissipated through the primary winding to generate in the iscondary winding a voltage for producing spark ignition. Without the inhibiting means aforesaid, the firing pulse applied to the thyristor would have to be terminated before the peak oscillatory capacitor discharge current besame less than the current snpplied to the capacitor by the voltage generating means during the capacitor dissharge, otherwise the thyristor would be held in a permanently conducting state thereby preventing the capacitor from being recharged for further operations of the apparatus. Thus, the inhibiting means permits the thyristor to be maintained conductive for a longer period than would otherwise be possible.
Further features and advantages of the invention will appear from the following description of embodiments thereof given by way of illustrative example with reference to the sccompanyign drawings wherein:

Figure 1 is a schematic circuit diagram of an apparatus in accordance with the invention;
Figure 2 illustrates a sparking waveform developed by the apparatus of the invention in comparison with sparking waveforms of the prior apparatus aforementioned, and
Figure 3 illustrates a modification of the circuit shown in Figure 1.

Referring now to Figure 1, the apparatus comprises a conventional ignition coil 1 having a primary winding 1a, and a second winding 1b connected through a conventional distributor 2 to spark pluas 3 of the engine. The prisary winding 1a is connected in series with a capacitor C1 and a seitching thyristor SCR, which together define a tuned oscillatory circuit when the thyristor SCR is conductive.
The capacitor C1 ia charged to a voltage of typically 200 volts by means of a d.c. to d.c. converter 4 shown in dotted outline. The converter 4 is fed with a d.c. voltage of typically 12 volts from an input terstinal 3. connected in use to the engine's battery and alternator system (not shown).
The thyristor SCR is triggered by means of a control circuit 6, responsive to a contact breaker (not shown) either of the conventional mechanical or the photocell type.
Considering now the d.c. to d.c. conyerter.4 in more detail, it comprises a step up transformer 7 having primary coils 8a, 8b and a secondary coil 9. The primary coils 8 are connected in a push-pull tranristor oscillator arrangement which includes two sets of transistors TR1, TR2; TR3, TR4 connected as complementary Darlington pairs, each pair supplying current to a respective one of the coils 8a, 8b. A pickup coil 8₀ is connected to supply positive feedback to the transistors, and a diode arrangement D1 to D4 is provided to bias the transistor pairs into conduction, the diodes being arranged so that when one of the Darlingtou pairs is switched on, the other is biased off and vice versa. A d.c. bias current is applied to the tranaistor pairs through a bias current path 4A which includes a voltage dropping resistor R. In operation of the oscillator, current is fed through the coils 8a, 8b sequentially and in opposite directione at a frequency which is a function of the inductance of chokes L1, L2, the values of a resistor R1 and a capacitor C2, and the inductance preserved by the primary coils 8a, 8b, thereby inducing a stepped up alternating voltage in the secondary coil 9. The output of the coil 9 is applied to a bridge rectifier 10 and thence to a smoothing arrangement comprieing resisters R2, R3 and a capacitor C3. Thus, in use, the d.c. to d.c. converter 4 applies a ateppsd up d.c. voltage of typically 200 volts to the capacitor C1.
The control circuit 6 for the switching thyristor SCR will now be described in more detail. The control circuit has a 7.5 volt stabilised voltage rail 11 established from the 12 volt supply terminal by a resistor R4 and a Zener diode ZD. The contact breaker of the engine (not shown) is connected between a terminal 12 connected to the supply rail 11, and a terminal 13 which is connected to the bass of a transistor TR5 connected as an emitter follower. The tranaistor TR5 is provided with a biasing resistor chain including resistors R5, R6, R7. A smoothing network including a capacitor °C4 and a diode D5 is provided to remove unwanted switching transients produced by the contact breaksr. The output of the transistor TR5 thus comprises a chain of pulses the frequency of which is a function of the engine speed, the phase of the pulses being indicative of the desired timing of the sparks to be produced by the spark plugs.
The output of the transistor TR5 is applied to two signal paths. The first path includes a capacitor C5. The electrical charge of each pulsa is dumped in turn into the capacitor C5 which then discharges betwesn, pulses through series resistors R8, R9, resistor R10 and the collector emitter path of a transistor TR6. A drive transistor TR? has its baas connected to the junction between the series resistors R8, R9, and is arranged, when switched on, to apply a switching drive signal to the gate of the thyristor SCR.
Thetramistor TR7 will switch off when the capacitor C5 has discharged to a voltage sufficient to allow an appropriate bias voltage to be developed across the resistor R9, and thus the thyristor SCR is switched on in response to each pulse in the pulse train from the contact breaker and for a tise depondent upon the discharge time of the capacitor C5. This discharge time is arranged to decrease with increased engine speed.
To this end, the pulse signal train from the transistor TR5 is applied to a second signal path including a differentiator comprising a capacitor C6 and a resistor R11, a diode pump circuit including a diode D6, a resistor R12 and a spacitor C?, and biasing resistors R13, R14. These components effectively differentiate the pulse chain from transistor TR5 and produce a d.c. voltage of snitude dependent upon the frequency of the puls i.e. the ensine speed. This voltage is applied to the baseof the transistor TR6 so as to deerease its collector; emitter impodamce with increasing engine speed, so as to increase the rate of discharge of capacitor C5 with increased engine speed. As a result, the time for which the thyristor SCR is switched on, is decreased with increased engine speed so as to prevent the high voltage signal produced in the secondary winding 1b of the coil for a particular cylinder firing, extending in time into the time appropriate for firing of another cylinder.
As previously mentioned, the capacitor C1 and the winding la of the ignition coil define a tuned circuit, so that when the thyristor SCR is fired, the capacitor C1 discharges current in an oscillatory manner. The control circuit is arranged to hold the thyristor SCR conductive for a period which corresponds to substantially all of the firing period of a cylinder, and during that period, the oscillatory current flowing in priwary winding 1a and the capacitor C1 induces in the secondary winding a plurality of cycles of high voltage signal having the frequency of the oscillation of the current flowing through the primary winding. I have found that this high voltage signal (typically of the order of 3,500 volts) produces an improved combustion in the cylinders because the spark produced by the spark plug extends for substantially the whole period that combustion can take place thereby maximising the likelihood of complete combustion of the fuel/air mixture applied to the engine.
In order to allow substantially all of the charge established on the capacitor C1 to be dissipated through the priimmary winding 1a of the coil, the apparatus includes inhibiting means to be described hereinafter fpr inhibiting operation of the d.c. to d.e. converter 4· during periods when the thyristor SCR is triggered into a conductive state, so as to stop a charging current being applied to the capacitor C1 whilst it is being dischargod. Now, it will be appreciated that the diseharge current of the capacitor will have an oscillatory waveform of which the peak magnitude decays with time in an exponential manner. When the firing pulse applied to the gate of the thyristor SCR terminates to terminate the conductive state of the thyristor, it is essential by virtue of the well known operating characteristics of the thyristor, that the curreat flowing in the thyrstor be decreased below a predetermined level in order for the thyristor to return to a non-conductive state. Without the inhibiting means, a limit would be set on the period for which the thyristor could be triggered conductive by a pulse from the control circuit 4, because after the thyristor has been fired and at a time when the capacitor Cl has become partially discharged, the sum of the charging current from the converter 4 and the decaying oscillatory discharge current would reach a level which would no longer pass below the threshold value required to switch off the thyristor SCR.
However, in the present embodiment, the inhibiting means is provided to effectively switch off the charging current supplied to the capacitor Cl by the converter 4 whilst the capacitor is being discharged, which permits the firing pulses applied by the circuit 4 to the thyristor SCR to be msade longer than would otherwise be possible and still ensure that the thyristor will reliably turn off at the end of the firing pulse. The inhibiting means thus permits the thyristor SCR to be turned on for a period which allows substantially all the oseillatory current produced by discharge of the capacitor C1 to be used to generate spark ignition.
The inhibiting means of the present embodiment is an inherent feature of the particular arrangement of the converter 4 shown in Figure 1. As previously mentioned, the frequency of oscillation of the transistors TR1-4 is determined in part by the inductance presented by the primary coils 8a, 8b. When the thyristor is in a non-conductive state, the inductance presented by the primary coils 8a, 8b is a function of the self-inductance of the primary coils and also the mutual inductance of the priimary and secondary coils 8 and 9. However, when the thyristor SCR is fired into a conductive state, the ends of the secondary coil 9 are short circuited. The transformer 7 is so arranged that upon short circuiting of the secondary coil 9, the mutal inductance of the coils 8, 9 is reduced substantially by allowing flux from the primary to leak out of the transformer. As a result, the inductance presented to the transistor oscillator by the coils 8a, 8b is reduced substantially when the thyristor SCR is fired, which causes the frequency of oscillation of the oscillator to increase substantially. The traasforaer 7 is however extremely inefficient at the higher frequency and as a result substantially no charging current is induced in the secondary coil 9 whilst the thyristor SCR is conductive. The operation of the transformer 7 and the transistor oscillator when the thyristor SCR is in its conductive state can also be understood in teerms of the leakage inductance of the transformer. Leakage inductance is effectively the pre- portion of flux generated by one coil that does not intersect the others, and can be considered as a smpall indue tor in series with the main winding. When the secondary coil 9 is short circuited, the converter-operates at a frequency and power level determined by the value, of this leakage inductance. The leakage inductance is preferably increased by winding the secondary coil on a bobbin first, followed by a relatively large inter-winding separation of about 0.025", which also provides primary/secondary insulation, followed by the primary coil which is wound Bifiler' to ensure good balance. This ensures that when the secondary is short circuited, the output current is typically less than 20 m.a. As soon as the thyristor SCR returns to its non-conducting state, the short circuit is removed and the d.c. to d.c. convertar 4 operates normally, charging up the capacitor Cl for the next spark.
Another example of the apparatus is shown in Figure 3, which has a different form of inhibiting means. Most of the circuit is the same as that shown in Figure 1 and like parts are marked with like reference numerals. Only those parts of the circuit which differ from those of Figure 2 will be described hereinafter.
The circuit of Figure 3 includes a further voltage dropping resistor R13 connected in the d.c. bias current supply path 4A, and a bias current by-pass path 14 connected to the path 4A between the resistors R R13. The path 14 is connected to the positive side of the thyristor SCR and includes a diode 07 arranged to prevent the charging current from the rectifier 10 from passing to the transistor oscillator. Thus. when the thyristor SCR is fired into its conductive state, the path 14 provides a low impedance path to earth through the thyristor SCR for the bias currept for the transistors TR1-4 of the oacillator, and thus the bias current preferentially flows to earth through the by-pass current path 14 rather than to the transistors, which results in the transistors being turned off. Hence, the oscillator is turned off whilst the thyriator SCR is in Its conductive state, so as to inhibit charging of the capacitor C1 whilat it is being discharged.
The elongation of the high frequency signal produced by the described examples of apparatus of the invention as compared with the prior arrangements can be seen clearly from Figure 2. All of the grapha of Figure 2 illustrate the output voltage developed acroae the aeeondary winding of the coil when a spark ie produaod. Figure 2A illustrating the output of a conventional contact breaker system, Figure 2B illustrating the prior proposed capacitive discharge system, and Figure 2C illustrating the output waveform of the described example of the present invention. The operative spark generating periodsare illustrated with shaded lines on the graphs. It will be seen that the spark period of the capacitive discharge period of Figure 2B is much shorter than the prior system of Figure 2A but has a much faster rise time. It will be furthermore noted that the deseribed apparatus of the present invention provides not only the shorter period rise time but also provides a substantialy elongated spark generation period.
I have found that the apparatus of the present invention makes a such more efficient use of the charge established on the charged capacitor, as compared with the prior proposal, by letting the charge dissipate during the whole of a cylinder firing period.
The apparatus of the invention has been described by way of example used with a mechanical contact brcaker, but it will be readily apparent to those skilled in the art that a photoelectric or like non-mechanical contact breaker system can be connected to the terminals 12, 13. Many other modifications and variations of the present invention will be readily apparent to those skilled in the art, which fall within the scope of the present invention as claimed hereinafter.

Claims

1. An apparatus for producing spark ignition of an internal combustion engine, comprising an ignition coil having primary and secondary windings, a capacitor connected in an electrical circuit with the primary winding, voltage generating means for receiving a supply voltage and charging said capacitor to a voltage greater than said supply voltage, switching means arranged to cause the capacitor to discharge a current through the primary winding of the coil, the apparatus being so arranged that the discharge current of the capacitor oscillates in the said circuit, and control circuit means adapted to operate the switching means to produce said discharge current in such a manner that more than one oscillation of the current passes through the primary winding whereby to induce in the secondary winding more than one cycle of an oscillatory voltage for producing spark iignition of the engine.

2. Am apparatus for producing spark ignition of an internal combustion engine, comprising an ignition coil having primary and secondary windings, a capacitor connected in electrical circuit with the primary winding, voltage generating means for receiving a supply voltage and adapted to charge the capacitor to a voltage greater than said supply voltage, a thyristor connected in said circuit and arranged so that when fired to a conductive state the capacitor discharges in an oscillatory manner through the thyristor and the secondary winding, and control circuit means adapted to apply to the gate of the thyristor a firing signal to render the thyristor conductive for a period sufficient for a plurality of oscillations of the capacitor discharge current to pass through said primary winding, and means for inhibiting said voltage generating means from charging the capacitor during the discharge thereof produced by firing of the thyristor.

3. An apparatus according to claim 2 wherein the voltage generating means comprises a transistor oscillator for producing an oscillating current from an input direct current, a step up transformer having a primary winding connected to receive said alternating current and having a secondary winding connected to a rectifier for producing a rectified direct current, said capacitor being connected to be charged by said rectified current.

4. An apparatus according to claim 3 wherein the frequency of oscillation of the transistor oscillator is an inverse function of the inductance presented by the primary coil of the transformer, and the secondary coil of the transformer is connected so that upon said firing of the thyristor the coil is shorted, the construction of the transformer being such that the mutual inductance of the primary and secondary coils reduces substantially upon said shorting of the secondary coil whereby to increase the frequency of oscillation of said transistor oscillator in such a manner as to decrease the electrical power transferred from said primary to said secondary coil, whereby to inhibit operation of the voltage generating means during said firing of the thyristor.

5. An apparatus according to claim 3 wherein said oscillator includes a bias current supply means adapted to supply to transistors of the oscillator a direct current to bias the transistors into operation, and means for preventing the supply of said bias current to the transistors in response to said firing of the thyristor whereby to inhibit operation of the oscillator during said firing of the thyristor.

6. An apparatus according to claim 4 or 5 wherein the transistor oscillator includes first and second complementary Darlington pairs of transistors each connected to a respective portion of said primary coil, a pick-up coil being connected to provide positive feedback from the primary coil to the translators.

7. An apparatus according to claim 6 including means defining a bias current supply path for said transistors, and a current by-pass path extending from said bias current supply path to the said circuit including said thyristor, the arrangement being such that upon firing of the thyristor, the bias current preferentially flows through the by-pass path rather than to the transistors whereby to inhibit operation of the oscillator during firing of the thyristor.

8. An apparatus according to any one of claims 3 to 5 including connected to the rectifier, a smoothing circuit for smoothing transients in said rectified current.

9. An apparatus according to claim 1 wherein said control circuit means includes an input for receiving electrical input pulses having a frequency indicative of engine speed and a phase indicative of the desired timing of spark ignition, and means responsive to said electrical pulses and arranged to generate firing pulses for the thyristor in such a manner that the duration of the firing pulses decreases with an increase in the frequency of the input pulses.

10. An apparatus as claimed in claim 1 installed on an internal combustion engine.