Multiple spark discharge ignition system

In an ignition system for an internal combustion engine, current from a d.c. power supply is applied through an input inductor alternately through an ignition coil and through a circuit path in shunt of the ignition coil. The current through the ignition coil is made of substantially constant predetermined magnitude while flowing through the shunt circuit, preferably by means of a fixed ballast resistor in the shunt circuit. Switching of the current into the ignition coil is enabled during a predetermined fraction of the engine cycle, with no current applied to the ignition coil between predetermined periods. The current is switched back and forth between the two paths a plurality of times during each predetermined period.

This invention relates generally to automotive ignition systems and more 
specifically to multiple spark ignition systems. Still more specifically 
it relates to such systems wherein the energy for ignition is supplied 
from a d.c. power supply through an inductor. 
In multiple spark discharge ignition systems ignition timing pulses 
indicate when sparks are to be initiated in the spark gaps of spark plugs 
in the respective cylinders of an internal combustion engine to cause 
combustion in the respective cylinders. Following each timing pulse, 
sparks are generated repetitively in a respective cylinder for a 
predetermined fraction of each engine cycle. In the systems illustrated in 
U.S. Pat. No. 4,131,100, the sparking in each cylinder each cycle was 
caused by repetitively interrupting current passing through an ignition 
coil, thus causing repetitive sparks to be produced by spark plugs in the 
respective cylinders during the predetermined interval. That is, current 
was built up in the primary of the ignition coil and interrupted 
periodically to produce sparks at the spark plugs, the current being 
reestablished between sparks. Systems as illustrated in the aforementioned 
U.S. Pat. No. 4,131,100 have proven effective, but they have certain 
limitations under certain conditions. At high engine speeds, there may be 
insufficient time for the current to build up in the ignition coil before 
its interruption, hence limiting the voltage and energy available for 
producing sparks at high engine speeds. Further, as the spark energy is 
derived entirely from the energy stored in the ignition coil at the moment 
of interruption, the spark energy is limited to the amount of energy 
storage capacity in the coil. 
In multiple spark discharge systems illustrated in U.S. Pat. No. 4,149,508, 
sparks are struck in both halves of an interruption cycle. That is, 
current through the ignition coil is repetitively interrupted and sparks 
are struck both upon current interruption and upon reapplication of the 
current through the coil. In the system illustrated in FIG. 6 of U.S. Pat. 
No. 4,149,508, current is supplied to the ignition coil through a storage 
inductor. During the ignition period the current through the ignition coil 
is periodically interrupted, the current through storage inductor being 
shunted to ground during the interruption interval. 
The system as illustrated in FIG. 6 of U.S. Pat. No. 4,149,508 has a number 
of shortcomings. For one thing, the currents through the storage inductor 
and the ignition coil are undefined. The patent refers to the battery and 
storage inductor as comprising a constant current source, but in the 
illustrated circuit the storage inductor can never be so large that the 
current therethrough cannot change at all, it can only remain 
substantially constant over the short interruption intervals when the 
current is shunted to ground. Actually, the current must rise slightly 
during each such interval and would rise to a very large magnitude if the 
storage inductor were shunted to ground for a long period. Further, there 
is nothing shown to limit the current even when it is directed through the 
ignition coil, and current will rise through the storage inductor and the 
ignition coil during such intervals, including the quiescent time between 
ignition times, except as current may be limited by the internal 
resistance of the battery and the inherent resistances of the storage 
inductor and the ignition coil. As a practical matter, with ignition coils 
as commonly used, the steady state current therethrough must be limited to 
3-5 a. to avoid overheating. 
In accordance with the present invention, energy is also applied from a 
battery to the ignition coil through an inductor. However, current of 
predetermined magnitude is developed in the inductor prior to energizing 
the ignition coil. Hence, when this inductor current is switched into the 
ignition coil, the current in the ignition coil immediately jumps to a 
relatively high level, initiating a spark in the respective spark gap 
almost instantaneously. Energy is thereafter supplied by the battery 
during the spark. When the ignition coil current is thereafter 
interrupted, the collapse of the magnetic field in the ignition coil 
results in a spark in the spark gap of polarity opposite to that of the 
spark created when the current was switched into the ignition coil. The 
current is thereafter switched back and forth to produce discharges 
following so closely as to be practically a continuous discharge. As the 
systems of the aforementioned U.S. Pat. No. 4,131,100 produce but one 
spark each cycle, much of the time the ignition coil is merely being 
reenergized. In the present invention, the energy is not limited by the 
energy storage capacity of the ignition coil, but is added from the 
battery during the initial spark. There is no lack of time for the primary 
current to build up in the ignition coil as it starts with the current 
built up in the inductor prior to the initiation of the spark. 
In accordance with the present invention, a defined current is developed in 
the input inductor during the quiescent period before the ignition period, 
and no current flows through the ignition coil. In consequence of having 
no current flowing through the ignition coil during the quiescent period, 
its average current is relatively low, permitting relatively high currents 
during the ignition period, substantially higher than the average currents 
permissible without overheating. Further, the input inductor need not be 
so large as to maintain a constant current; it need only be large enough 
to provide the energy for prompt initial spark breakdown without 
substantial drop in current. In the preferred embodiment of the present 
invention, current in the input inductor is maintained substantially 
constant by making the discharge voltage across the spark gaps, as viewed 
at the primary of the ignition coil, substantially equal to the battery 
voltage. 
Thus, it is a primary aspect of the invention to provide a multiple spark 
ignition system in which the energy for ignition is supplied from a power 
supply through an inductor, and in which current of predetermined 
magnitude is developed through the inductor prior to an ignition period, 
the current through the inductor being thereafter switched to flow 
alternately through two paths, one through the inductor and the other in 
shunt thereof.

The ignition system illustrated in FIG. 1 is intended for use in a cyclic 
internal combustion engine, not shown. Power to the ignition system is 
supplied from a direct current supply which may comprise a 12 v. battery 
with its positive terminal connected to a terminal 10 and with its 
negative terminal grounded. An ignition switch 12 connects the control 
circuit of FIG. 1 to the battery. The battery is also connected through an 
inductor 14 to an ignition coil 16. Energy from the ignition coil is 
supplied to a plurality of spark plugs 18 through a distributor 20, as is 
well known. The ignition coil 16 has a primary winding that receives 
current from the battery through the inductor 14 and a secondary winding 
connected to the respective spark plugs by the distributor 20 at 
appropriate times in the engine cycle. The ignition coil 16 produces high 
voltage pulses at the respective spark gaps of the spark plugs 18 to 
produce spark discharges in respective combustion chambers at appropriate 
times in their respective cycles as initiated by timing signals. In the 
circuit illustrated, the distributor 20 is coupled to a cam 22 which 
operates breaker points 24 to produce the timing signals. The breaker 
points 24 may be identical to the breaker points conventionally used to 
interrupt the current flow through an ignition coil. 
In the embodiment illustrated, the breaker points 24 are connected between 
ground and an input terminal 26 of an input circuit 28. The input circuit 
28 is principally a debounce circuit to assure the proper matching of the 
timing signal from the points 24 to a firing duration control circuit 30 
at its input terminal 32. With the points 24 in their normally closed 
position, the input terminal 26 is at ground potential, resulting in the 
terminal 32 being held at ground potential through a resistor R5. When the 
points 24 are opened by action of the cam 22, the potential of the 
terminal 26 rises toward the battery potential of +12 v. The rising signal 
is applied through a coupling capacitor C2, a diode D1 and a resistor R4 
to the terminal 32, sharply raising the potential at the terminal 32 and 
momentarily applying a positive timing control signal to a transistor Q1 
of the firing duration control circuit 30. When the points 24 close, the 
capacitor C2 is discharged through a resistor R3. Because it takes time 
for the capacitor to discharge, should the points bounce and momentarily 
open again, the subsequent pulse through the capacitor C2 will be 
insufficient to retrigger the transistor Q1. 
Power is supplied to the firing duration control circuit 30 through a 
resistor R2 to develop a fixed voltage of about 5 v. on a bus 34 as 
determined by a zener diode D2. The firing duration control circuit 30 is 
essentially a univibrator triggered to change state by the signal applied 
at the terminal 32. In the quiescent state of the firing duration control 
circuit 30, a transistor Q3 is rendered conducting by the connection of 
its base to the bus 34 through a resistor R7. This draws current through a 
resistor R10 and holds an output terminal 36 at a ground potential. This 
acts through a resistor R9 to hold a transistor Q2 off. When the trigger 
pulse is applied at the terminal 32, the transistor Q1 is turned on, 
drawing current through a resistor R6. This applies a signal through a 
capacitor C5 to turn off the transistor Q3, thus raising the potential at 
the terminal 36 and the base of the transistor Q2 and turning on the 
transistor Q2. Current continues to be drawn through the resistor R6 upon 
removal of the triggering pulse on the terminal 32, with the transistor Q2 
conducting and the transistor Q3 nonconducting. Current flows through the 
resistor R7 to charge the capacitor C5 until the charge on the capacitor 
C5 reaches sufficient voltage to cause the transistor Q3 to conduct again. 
This turns off the transistor Q2. 
The firing duration control circuit 30 thereafter remains in its quiescent 
condition until a subsequent trigger pulse appears at the terminal 32. The 
relative magnitudes of the resistances of resistors R6 and R7 and their 
relationship to the capacitance of the capacitor C5 determine the 
effective duty cycle of the firing duration control circuit 30. The 
charging and discharging rates of the capacitor C5 are made such that the 
capacitor is not fully discharged by the time the next subsequent timing 
pulse appears at the terminal 32. Preferably the magnitudes of the 
resistances and the capacitance are made such that the duty cycle of the 
firing duration control circuit 30 is about 20.degree. of rotation of the 
engine, producing an output signal on the output terminal 36 as shown in 
FIG. 2A. The signal is applied through a diode D3 to an oscillator 38. 
The oscillator 38 is shown as a gated multivibrator, enabled by the signal 
from the firing duration control circuit 30. When the oscillator 38 is so 
enabled, the output appearing at its output terminal 40 is a square wave 
signal as illustrated in FIG. 2B. The signal is created by alternately 
turning a transistor Q4 on and off. The transistor Q4 is alternately off 
and on for periods T2 and T3, respectively, as determined by the relative 
times for charging capacitors C6 and C7, respectively, to the points of 
conduction of transistors Q5 and Q4, respectively. The time intervals T2 
and T3 are preferably made substantially equal, for reasons that will 
appear below. 
In the absence of a firing duration control signal from the firing duration 
control circuit 30, the base of the transistor Q4 is held low through the 
diode D3. This keeps the transistor Q4 off, hence keeping the transistor 
Q5 on. This causes the capacitor C6 to be substantially fully discharged. 
Upon the appearance of the positive firing duration control signal 
illustrated in FIG. 2A, the base of the transistor Q4 is permitted to 
rise. The capacitor C7 then charges through a resistor R13. Because the 
resistor R13 had previously been conducting through the diode D3, the 
capacitor C7 was charged to the extent of the drop through the diode D3. 
Hence, the capacitor is charged in a relatively short time T0 sufficiently 
to turn on the transistor Q4 and hence turn off the transistor Q5. Because 
the capacitor C6 was initially fully discharged, the time thereafter for 
it to be charged sufficiently to permit the transistor Q5 to conduct is a 
time T1 which is longer than the time T3, hence making the first half 
cycle of the wave form illustrated in FIG. 2B substantially longer than 
the other half cycles. Thus, the gated oscillator 38 is enabled by the 
enabling signal (FIG. 2A) from the univibrator 30 to begin oscillating a 
short time T0 after application of the enabling signal. The oscillator 
thereafter oscillates in half cycles T1, T2, T3, T2, T3 . . . until the 
termination of the enabling signal. At the end of the enabling signal, the 
20.degree. firing duration signal from the firing duration control circuit 
30, the terminal 36 goes low, hence operating through the diode D3 to turn 
off the transistor Q4 and assure that the signal at the terminal 40 goes 
low at that time. The output signal on the terminal 40 is limited to 0.6 
v., which is the junction drop in a transistor Q6 to which the terminal 40 
is connected. 
The output signal on the terminal 40 is applied to the base of the 
transistor Q6 in a switching circuit 42. The signal from the gated 
oscillator 38 thus causes the transistor Q6 to turn on and off 
alternately, thus driving low and permitting to go high a terminal 44. The 
terminal 44 is connected to the 12 v. battery through a resistor R15; 
however, when the terminal 44 is permitted to go high, there is a 
conductive path from the terminal 44 to ground through the base to emitter 
circuits of a transistor Q7 and a Darlington circuit Q9. Hence, the 
terminal 44, when at its high potential, is at a voltage of about 2 v. The 
signal at the terminal 44 is hence that shown in FIG. 2C. This signal is 
applied to the bases of the transistor Q7 and a transistor Q8, turning 
them on and off oppositely and alternately. In the quiescent state, that 
is, the state before the 20.degree. firing duration control signal is 
applied, the signal at terminal 44 is high, hence turning on the 
transistor Q7 and turning off the transistor Q8. 
The turning on and off of the transistor Q7 controls the signal developed 
at a terminal 46. When the transistor Q7 is off, the terminal 46 is held 
low through a resistor R18. When the transistor Q7 is on, the terminal 46 
attains the voltage of the potential drop across the Darlington circuit 
Q9. This voltage is about 1.5 v. In the quiescent state, that is, between 
enabling pulses from the firing duration control circuit 30, the signal at 
the terminal 46 is high, keeping the Darlington Q9 on. With the Darlington 
Q9 on, the current flowing from the battery through the inductor 14 is 
directed through a ballast resistor R22 which limits the current through 
the induction coil, providing a constant predetermined current through the 
inductor 14 in the quiescent state and storing energy in the core of the 
inductor 14. In the quiescent condition, the high signal at the terminal 
44 holds the transistor Q8 off which in turn holds a Darlington circuit 
Q10 off. This precludes current from the battery through the inductor 14 
from flowing through the ignition coil 16. Hence, in the quiescent state, 
no current flows through the ignition coil, and no energy is stored 
therein. 
The turning off and on of the transistor Q7 by the signal developed at the 
terminal 44 causes the signal at the terminal 46 to follow the signal at 
the terminal 44. Hence, this signal, shown by the wave form 2D, is similar 
to the signal shown by wave form 2C but is limited to 1.5 v., the voltage 
drop across the Darlington circuit Q9. Similarly, the transistor Q8 is 
turned on and off oppositely by the signal applied at the terminal 44 and 
hence develops a signal at a terminal 48 which is opposite in phase to the 
signal at the terminal 46 and is of the wave form illustrated in FIG. 2E, 
limited in amplitude by the voltage drop across the Darlington circuit 
Q10. 
When the signal at the terminal 46 goes low, it turns off the Darlington 
circuit Q9 and hence stops the flow of current through the ballast 
resistor R22. Simultaneously, the transistor Q8 turns on the Darlington 
circuit Q10. This permits current to flow from the battery through the 
inductor 14 and thence through the ignition coil. The flow continues thus 
over the period T1. During the interval T2 the Darlington circuit Q9 and 
Q10 are reversed in their states of conduction. The states continue 
alternating over periods T2 and T3 until the end of the 20.degree. firing 
duration control signal. The current from the battery through the inductor 
14 hence is switched alternately into the two conduction paths, one 
through the ballast resistor R22 and the other through the ignition coil 
16. 
The ignition coil 16 may be a typical ignition coil as currently used in 
automobiles and, in particular, may be a current General Motors standard 
ignition coil in the form of a transformer having a turns ratio of 1:100, 
thus stepping up the voltage by a factor of 100. In a transformer, the 
application of a unidirectional current to the primary winding causes a 
magnetic field to be built up in the core of the transformer, adding 
energy to the core, where the energy is stored. At the same time, the 
change of current induces a current spike in the secondary winding of the 
transformer. For the sake of simplicity of explanation, the ignition coil 
16 is illustrated in FIG. 1 in one simplified form of its equivalent 
circuit. The equivalent circuit is shown as an input iron core inductor LP 
of 8 mh inductance in parallel with the primary winding WP of a 1:100 
turns ratio ideal transformer having a secondary winding WS. In the 
equivalent circuit, the components performing the two primary functions of 
the transformer are thus illustrated separately. That is, the power is 
stored in the input inductor LP and the current transformation takes place 
between the primary transformer winding WP and the secondary transformer 
winding WS, stepping up the voltage by a factor of 100 and stepping down 
the current by a factor of 100. The simplified equivalent circuit 
illustrated ignores the effects of such things as the resistance and 
capacitance of the windings and the hysteresis losses in the core, but the 
circuit is practically accurate for the purposes of the present 
description under the conditions of use at the intended currents, energy 
and frequencies. 
In the quiescent state between the 20.degree. enabling pulses and through 
the time T0, current from the battery through the inductor 14 is directed 
through the current path through the ballast resistor R22, developing the 
aforementioned constant predetermined current. When the enabled oscillator 
38 begins to oscillate at the beginning of the time interval T1, the 
current flowing through the inductor 14 is directed to the other path, 
through the primary winding of the ignition coil 16 (the primary winding 
being shown in two parts LP and WP). As the nature of an inductor is to 
resist changes in the flow of the current, the current continues to flow 
through the inductor 14 at substantially the same predetermined rate. As 
it can no longer flow through the Darlington circuit Q9, it must flow 
through the primary winding of the ignition coil 16 and the Darlington 
circuit Q10. However, the current through the equivalent input inductor LP 
cannot change promptly either, being zero at the time of switching. Hence, 
the current initially flows through the equivalent primary winding WP at 
the predetermined rate and is transformed to a corresponding current in 
the secondary winding WS. The secondary winding is, however, connected to 
a respective spark plug 18, as determined by the distributor 20, the spark 
plug presenting an open circuit having a certain effective capacitance. 
Because current must continue to flow through the inductor 14 and cannot 
all flow immediately through the equivalent inductor LP, the current in 
the ideal transformer builds up a charge on the spark plug terminals until 
the voltage therebetween rises to the breakdown potential of the 
respective spark gap. With the secondary winding WS connected to drive the 
center of the spark plug negative with respect to its ground connection, 
the spark plug will break down at about 30,000 v. With a 100:1 turns 
ratio, this transforms to 300 v. at the equivalent primary winding WP, 
developing about the same 300 v. at a terminal 50 of the Darlington 
circuit Q9, as shown by the wave form illustrated in FIG. 2F. The voltage 
at the spark gap is illustrated by the wave form shown in FIG. 2K. Once 
the spark gap is broken down, it maintains a voltage thereacross of about 
1,200 v., so long as current is applied thereto. This transforms to about 
12 v. at the terminal 50, as illustrated in the wave form of FIG. 2F, 
being, not coincidentally, the battery voltage. 
With the resistance of ballast resistor R22 about 1.3 ohms, the current 
through the inductor 14 is about 10 a. during the quiescent condition and 
through time T0. When the Darlington circuits Q9 and Q10 change state at 
the beginning of the time T1, the 10 a. current through the inductor 14 
that formerly flowed through the Darlington circuit Q9 is switched to flow 
through the ignition coil 16 and the Darlington circuit Q10. As stated 
before, the current through the equivalent inductor LP cannot change 
instantaneously. It therefore starts at zero and increases gradually 
during the interval T1. The remainder of the 10 a. current flows through 
the equivalent primary winding WP of the induction coil 16, starting at 10 
a. and decreasing thereafter as the current builds up in the inductor LP. 
The function of the inductor 14 is to provide the voltage for substantially 
instantaneously breaking down the respective spark gaps. To this end the 
inductance of the inductor 14 must be large enough that with the desired 
current flowing therethrough, in this example 10 a., there is sufficient 
energy stored in the core of the inductor to break down the spark gap, 
preferably without suffering any substantial diminution in current. This 
requires a large enough inductance as to maintain the flow of current near 
10 a. until the capacitance of the respective spark plugs and other 
circuitry has charged up to the breakdown voltage of the respective spark 
gap. In the particular embodiment illustrated, an inductance of about 8 mh 
has proven satisfactory, this being about the same as the inductance of 
the equivalent inductor LP. 
During the interval T1, while the Darlington circuit Q10 is conducting, the 
battery supplies power through the inductor 14 to the ignition coil, hence 
supplying power to maintain the discharge in the spark gap. It also 
provides the energy for storage in the equivalent inductor LP. Because the 
spark gap discharge potential as viewed across the equivalent primary 
winding WP is about the same as the battery potential, namely 12 v., there 
is substantially no voltage drop across the inductor 14. With no voltage 
drop to change the current I.sub.L1 through the inductor 14, it remains 
the same, namely 10 a. At the same time the current increases in the 
equivalent inductor LP at a constant rate of 1500 a/sec, the constant 12 
v. divided by the 8 mh inductance. As the sum of the current I.sub.WP 
through the equivalent primary winding WP plus the current I.sub.LP 
through the equivalent inductor LP remains constant at about 10 a., the 
current I.sub.WP decreases linearly, and energy is dissipated in the spark 
gap, heating the air and evaporated fuel in the respective combustion 
chamber. The current through a spark discharge is not determined by the 
voltage across the gap. It is whatever current is available, as determined 
by the other circuit elements, in this case the steady 10 a. through the 
inductor 14 less the current developing in the equivalent inductor LP. In 
the particular embodiment illustrated, the current I.sub.LP rises to about 
5 a. at the end of the interval T1. Hence the current I.sub.WP declines to 
about 5 a. in the same interval. 
At the beginning of the time interval T2, the Darlington circuits Q9 and 
Q10 switch states, and the current through the inductor 14 again passes 
through the Darlington circuit Q9. Current no longer flows through the 
Darlington circuit Q10 and the magnetic field in the ignition coil 16 
starts to collapse. As viewed in respect to the equivalent circuit shown, 
with the Darlington circuit Q10 open, the current flowing through the 
equivalent inductor LP of the ignition coil 16 flows through the 
equivalent primary winding WP in the direction opposite to current flow 
during the interval T1, raising the voltage at the spark gap until the 
spark gap again breaks down, this time in the reverse direction. The 
breakdown voltage is at a lower level because the gas has previously been 
ionized and heated. After breakdown, the voltage is again maintained 
across the spark gap at about 1,200 v., hence, making the voltage at the 
primary about 12 v. The voltage at an input terminal 52 of the Darlington 
circuit D10 hence rises promptly to the breakdown voltage plus the battery 
voltage as applied to the resistor R22. After breakdown, the voltage at 
the terminal 52 is reduced to the voltage necessary to sustain the 
discharge. The voltage at the terminal 52 is illustrated by the wave form 
2G. 
During the secondary discharge occurring in the interval T2, the energy 
stored in the equivalent inductor LP is dissipated in the discharge, this 
being the energy supplied to the core of the ignition coil 16 from the 
battery during the interval T1. However, the timing is such that the 
energy is not entirely dissipated by the time the period T3 begins; that 
is, the field has not entirely collapsed to zero. During the interval T2, 
the constant spark gap voltage as transformed to the equivalent primary 
winding is -12 v. The rate of change in current I.sub.LP through the 
equivalent inductor LP is therefore equal but opposite to the rate of 
change of the current I.sub.LP during the interval T2. The current 
I.sub.WP in the equivalent primary winding is equal and opposite to the 
current I.sub.LP during the interval T2 in which the Darlington circuit 
Q10 is non-conducting. It is important to the efficient operation of this 
invention that the spark discharge not be extinguished during the 
20.degree. firing duration signal, except at the changes in direction. 
Therefore, the current must be maintained at all times and not be 
permitted to drop to zero, again excepting the change in direction. In 
fact, it is desirable that the spark current be maintained substantially 
above zero at all times, as a weak spark can be extinguished by gas 
turbulence in the gap. As the rate of change of the current I.sub.LP is 
the same but of opposite sense in the intervals T1 and T2, the second half 
cycle T2 is made shorter than the first half cycle T1 so that there is 
insufficient time for the current I.sub.LP to fall to zero during the 
interval T2. 
At the end of time interval T2, the oscillator 38 again switches the 
Darlington circuits Q9 and Q10, this time back to the conditions of the 
interval T1. The action of the interval T1 then is repeated during the 
interval T3, except that breakdown of the spark gap is at a lower 
potential, the current I.sub.LP starts above zero at whatever current was 
flowing at the end of interval T2, and the interval T3 is shorter than the 
interval T1. In fact, it is preferred that the half cycles T2 and T3 be 
substantially equal in order that the current I.sub.LP not drift in one 
direction or the other. As the rates of change are substantially equal but 
opposite in the two half cycles, if one half cycle were longer, the 
current would change more during that half cycle, accumulating a net 
change after several cycles. The half cycles T2 and T3 are made equal by 
the timing of the half cycles in the oscillator 38. Small differences in 
timing are not significant, as the 20.degree. firing duration terminates 
before the current change accumulates to the point of extinguishing the 
discharge. The half cycles T2 and T3 thereafter alternate until the end of 
the 20.degree. firing duration control signal. At that time the circuit 
reverts to its quiescent condition, and the energy left in the equivalent 
inductor LP at that time is dissipated in the gap. The wave forms for the 
currents I.sub.WP and I.sub.LP are shown in FIGS. 2H and 2I. The spark 
current I.sub.WS is shown by the wave form of FIG. 2J. 
It may be noted that current flows into the ignition coil 16 at a magnitude 
of 10 a. during intervals T1 and T3 and that no current flows into the 
ignition coil 16 during intervals T0 and T2 or during the time between 
enabling pulses. In eight cylinder engines, a cylinder fires each 
90.degree. of engine rotation. Hence, there is 70.degree. between 
20.degree. enabling pulses, during which no current is applied to the 
ignition coil 16. As this substantially lowers the average current through 
the ignition coil, it permits relatively large currents to be applied 
during the intervals T1 and T3, much larger than permissible in the system 
of U.S. Pat. No. 4,149,508. 
Zener diodes D6 and D7 provide overload protection. The sum of the 
breakdown potentials of these diodes is about 360 v. In normal 
circumstances, these diodes do not conduct any current. However, should 
there be a failure in the breakdown of any spark gaps so that the spark 
gaps do not break down below 36,000 v., the diodes D7 and D6 conduct 
through a diode D9 or a diode D8 in order to prevent overload between the 
collectors and bases of the respective Darlington circuits Q9 and Q10. 
More particularly, in the event of overload, the current passes through a 
respective diode D4 or D5, a respective resistor R19 or R20 and a 
respective resistor R18 or R21 to turn on the non-conducting Darlington Q9 
or Q10 to dissipate the excess energy without overloading the respective 
collector to base circuit. 
As noted above, the current through the inductor 14 in the quiescent state 
of the circuit is determined primarily by the resistance of the resistor 
R22. It is this resistor that develops the current I.sub.L1 at a suitable 
constant predetermined magnitude for operating the ignition coil 16. The 
magnitude of the current I.sub.L1 is made whatever is suitable for the 
operation of the system in order to provide power of appropriate magnitude 
through the spark gap to ignite the fuel reliably at the proper time. The 
periods T1, T2, and T3 as determined by the gated oscillator 38 are such 
that with the particular ignition coil utilized, the currents I.sub.WP and 
I.sub.LP remain in appropriate ranges. 
Although a preferred embodiment of the invention has been shown and 
described with particularity, various changes in the circuit may be made 
within the scope of the present invention. For example, it may be noted 
that a substantial amount of power is lost in the resistor R22. This 
reduces the efficiency of the system, particularly at low engine speeds, 
as most of the power lost in the resistor R22 is lost during the interval 
between the 20.degree. firing duration control signals. It is within the 
scope of the invention to provide other means for developing a 
substantially constant current of suitable predetermined magnitude through 
the inductor 14 at least during the intervals T2 when the current 
therethrough is directed through the path shunting the ignition coil 16. 
This may be achieved, for example, by measuring the current through the 
inductor 14 and switching the battery in and out of the circuit as 
required to keep the current within predetermined limits, as between 9 and 
10 a. 
It should also be understood that other timing means may be utilized 
instead of the breaker points 24. For example, the timing signals may be 
electronically developed and controlled, as shown in U.S. application Ser. 
No. 899,355 filed by James Walter Merrick on Apr. 24, 1978 for "Electronic 
Engine Control", now U.S. Pat. No. 4,284,053, issued Aug. 18, 1981.