[0001] This inrsntion relates to improvements in and relating to apparatus for producing
spark ignition of an internal soubmation engine.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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).
[0014] 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.
[0015] 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
8
0 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
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.