Circuit for controlling the primary dwell time of ignition transformer

A circuit arrangement for controlling the duration of a current generated in the primary winding of an ignition transformer comprises a ramp generator responsive to the ignition timing or an internal combustion engine for generating a ramp voltage having a constant peak value and a variable rate of increase in voltage as a function of the speed of the engine. A pulse generating means is provided for successively generating a pulse of which the pulse height is substantially equal to the peak value of the ramp voltage and of which the pulse duration is variable inversely as a function of the speed of the engine. To the pulse generating means is coupled an integrator for integrating the pulses supplied thereto to generate an integrated output. The output of the integrator is supplied to a comparator for making a comparison with the instantaneous value of the ramp voltage to generate an ignition current pulse having a duration corresponding to the difference between the compared input variables.

BACKGROUND OF THE INVENTION 
The present invention relates generally to ignition systems for automotive 
internal combustion engines, and specifically to a circuit arrangement for 
controlling the primary dwell time of an ignition transformer 
substantially at a constant value. 
It is generally known in the art that in conventional ignition current 
control systems the primary dwell time of ignition transformer is detected 
and compared with a reference interval to detect the difference between 
them. The prior art systems include a negative feedback circuit which 
controls the duration of the ignition primary current so that the 
difference is reduced to a minimum. With the feedback operation the 
duration of the ignition primary current is substantially controlled to a 
desired constant value that ensures ignition at all engine speeds. 
However, the amount of the output variable of the closed-loop system needs 
to be controlled to within a relatively narrow range to prevent 
undesirable effects of system's hunting and this in turn requires the 
system's component parts to be manufactured to strict tolerances and 
adjusted constantly during operation to keep them in specified operating 
conditions. 
SUMMARY OF THE INVENTION 
The primary object of the invention is to provide an improved circuit 
arrangement for controlling the duration of an ignition primary current 
which overcomes the problems associated with the feedback-controlled 
ignition current control system. 
According to the present invention, the circuit arrangement comprises a 
ramp generator responsive to the ignition timing of the internal 
combustion engine for generating a ramp voltage having a constant peak 
value and a variable rate of increase in voltage as a function of the 
speed of the engine. A pulse generating means is provided for successively 
generating a pulse of which the pulse height is substantially equal to the 
peak value of the ramp voltage and of which the pulse duration is variable 
inversely as a function of the speed of the engine. To the pulse 
generating means is coupled an integrator for integrating the pulses 
supplied thereto to generate an integrated output. The output of the 
integrator is supplied to a comparator for making a comparison with the 
instantaneous value of the ramp voltage to detect a difference 
therebetween. An ignition current pulse having a duration corresponding to 
the detected difference is generated in the primary winding of an ignition 
transformer. Since the rate of increase in ramp voltage is proportional to 
the engine speed while the output of the integrator is inversely 
proportional to the engine speed, the ignition current pulse is controlled 
to a substantially constant value at all engine speeds. 
The present invention requires that the ramp voltage have a constant peak 
value and that the input to the integrator be derived from a pulse having 
a pulse height which is controlled to a value substantially equal to the 
peak value of the ramp voltage. The output of the integrator is rendered 
variable with the peak value of the ramp voltage and hence with its rate 
of voltage increase so that when the system is subjected to variations in 
power supply voltage and in circuit operating parameters such variations 
tend to automatically readjut the relative values of the inputs to the 
comparator to keep the duration of the ignition current pulse at a 
constant value. 
In a preferred embodiment, the input to the integrator is derived from a 
circuit comprising a pulse generator for generating a constant-duration 
pulse in response to the ignition timing and means for inverting the 
polarity of the constant-duration pulse and clamping the pulse height of 
the polarity inverted pulse to the peak value of the ramp voltage.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown a preferred embodiment of the spark 
ignition pulse generator of the invention for internal combustion engines. 
The ignition pulse generator generally comprises a ramp generator 100 
coupled to a conventional electronic ignition timing control unit (not 
shown), a constant duration pulse generator 200, a clamping circuit 380 
which clamps the amplitude of the constant duration pulse to the peak 
value of the ramp voltage, an integrator 390 for integrating the clamped, 
constant duration pulse and applying it to a comparator 350 which compares 
the instantaneous value of the ramp voltagen with the integrated signal 
for providing a spark ignition pulse to the primary winding of an ignition 
coil 600. 
In response to the application of an ignition timing pulse A (FIG. 2) a 
transistor 1 is rendered conductive to turn off a transistor 101 of the 
ramp generator 100. A capacitor 170 is charged by a constant current i1 
supplied from a multi-collector transistor 124 to develop an increasing 
voltage B as seen in FIG. 2 in response to the leading edge transistion 
"a" of the ignition timing pulse A. When the voltage developed in the 
capacitor 170 reaches a level C determined by resistors 142 and 144, a 
transistor 102 is turned off, causing a transistor 103 to turn on and a 
transistor 104 to turn on and then transistor 105 to turn off. If the 
voltage B developed in the capacitor 170 reaches a higher level D equal to 
the voltage developed between resistors 148 and 149 plus the base-emitter 
voltage of a transistor 107, the latter is rendered conductive to turn on 
a transistor 108 which forms with a transistor 109 a NOR gate to generate 
a pulse E having a constant duration T1. The constant duration pulse E is 
inverted in polarity by a transistor 110 to turn off a transistor 111 
during the interval T1 so that a capacitor 171 is charged through a diode 
131 by a constant current i2 supplied from the transistor 124 with a 
current value equal to one-half the current i1. The transistor 105 forms a 
NOR gate with a transistor 106 to generate a pulse F of a constant 
duration T2 in response to the voltage B reaching the reference voltage C. 
The capacitor 171 thus develops a voltage G which is proportional to the 
engine revolution speed. The voltage G is applied through a resistor 157 
to a current mirror circuit formed by a pair of transistors 112 and 113 so 
that an engine speed proportional current i3 is generated in the 
transistor 113. The current i3 is converted into a current i4 twice the 
value of current i3 by means of transistors 114 and 115. The current i4, 
which is proportional to engine speed, is used to charge a capacitor 172 
having the same capacitance value as capacitor 170, so that the capacitor 
172 is charged at a rate proportional to engine speed and thus develops a 
ramp voltage H having a constant peak value regardless of the engine 
speed. The peak value of the ramp voltage is determined by the reference 
voltage D developed at the junction between resistors 148 and 149 plus the 
base-emitter voltage of transistor 107. The capacitor 172 is discharged 
rapidly through a short circuit provided by a resetting transistor 116 
when the latter is biased into conduction in response to the leading edge 
transition of each pulse F which occurs at the collector of transistor 105 
in response to each ignition instant "a". 
The capacitor 172 is coupled to the inverting input of a comparator 180, 
the noninverting input of which is coupled to the junction between 
resistors 148 and 149 for making a comparison between the instantaneous 
value of the ramp voltage H and the voltage D corresponding to the peak 
value of the ramp voltage. The output of comparator 180 is coupled to the 
base of a transistor 190 and to the collectors of transistors 120 and 121. 
The transistor 120 having its base coupled to the collector of the 
transistor 110 remains conductive to short the output of the comparator 
180 until transistor 120 is biased off by transistor 110 for the interval 
T1. When the engine is accelerated, the ignition instant "a" would advance 
in time and therefore the ramp voltage H would be still lower than the 
constant voltage D at the instant "a". As a result, a high voltage at the 
output of the comparator 180 biases the transistor 119 into conduction for 
an interval T3. The conduction of transistor 119 turns off transistors 117 
and 118 to allow the capacitors 171 and 172 to be charged through diodes 
132 and 133, respectively, with a constant current supplied from 
multi-collector transistor 123 during the period T3. Therefore, the 
voltages G and H are increased by an amount that compensates for the 
increase in engine speed. 
The constant duration pulse generator 200 includes a capacitor 220 which is 
normally charged through a resistor 212 and a transistor 201 having its 
base coupled through a line 16 to the collectors of transistors 105, 106 
of the ramp generator 100 through a line 16. Transistor 201 is turned on 
in response to a pulse F to provide a discharge path to rapidly discharge 
the capacitor 220. At the trailing edge of the pulse F, the capacitor 220 
is again charged at a time constant rate determined by resistors 212, 213 
and capacitor 220 and the source voltage so that it builds up a voltage J 
which is applied to the noninverting input of a comparator 230 for making 
a comparison with a reference voltage K developed at a junction between 
resistors 214 and 215 which are coupled in series to a constant voltage 
source 400. The comparator 230 thus generates an output pulse having a 
duration determined by the voltage supplied from the source 400 in 
response to the leading edge transition "a" of each input timing pulse A. 
The output of the comparator 230 is coupled to a clamping circuit 380 
formed by transistors 301 and 302 having their conductive paths connected 
in series between the voltage source 400 and ground. The circuit junction 
between transistors 301 and 302 is coupled, on the one hand, to the output 
of comparator 230 and, on the other hand, to the base of an 
emitter-follower transistor 303 having its emitter coupled to ground by a 
resistor 311 which is in shunt with an integrator circuit 390 formed by a 
resistor 312 and a capacitor 320. The base of transistor 302 is coupled by 
a line 17 to the circuit junction between resistors 148 and 149 of ramp 
generator 100 so that the output voltage of the comparator 230 is clamped 
to the voltage D which is the peak value of the ramp voltage as previously 
described. 
The transistor 303 serves to invert the polarity of the constant duration 
pulse from comparator 230. At the emitter of transistor 303 is thus 
developed a train of pulses L having the same pulse height as voltage D 
with a constant pulse spacing T4 corresponding to the pulse duration of 
the output of comparator 230. The pulses L are integrated by the 
integrator circuit 390 into a DC voltage M which appears at the circuit 
junction between resistor 312 and capacitor 320. Therefore, the integrated 
voltage M is inversely proportional to engine speed. Voltage M is coupled 
to a transistor 351 of the comparator 350 for comparison with the ramp 
voltage H applied through a line 11 from the capacitor 172 to the base of 
transistor 357. Comparator 350 provides output pulses 0 with a spacing T5 
having the same duration as T4 but occur during the time prior to the 
ignition instant "a" as seen in FIG. 2 and supplies the pulses 0 to the 
base of a transistor 3. Since the rate of increase in ramp voltage H is 
proportional to engine speed while the voltage M is inversely proportional 
thereto, the pulse duration T5 has a constant value regardless of 
variations in engine speed. If, during engine high speed operations, the 
voltage M decreases below a reference level N established by resistors 313 
and 314 which are coupled in series between a source voltage Vb through 
resistor 32 and ground, the decrease in voltage M is compensated for by 
the action of transistors 304 and 305, whereby the voltage M is clamped to 
the reference level N and therefore the duty ratio of the output of 
comparator 350 is maintained constant when the engine is operated at 
excessively high speeds. Since the reference voltage N is variable with 
the source voltage Vb, the duty ratio of the comparator 350 output is 
rendered inversely variable as a function of the source voltage during 
such high engine speed operations. 
The detail of the operation of the comparator 350 is given as follows. When 
the base potential of transistor 357 reaches the base potential of 
transistor 351, the transistor 357 is turned off to turn off transistor 
354. As a result, transistors 351 and 353 are turned on, so that the 
collector voltage of transistor 356 is switched to a low voltage level. 
The comparator 350 includes transistors 358 and 359 which form constant 
current sources for the transistors 353, 354. Further included are 
transistors 360, 361 and 363 which provide an offset current that cancels 
the bias current of transistors 354 and 357 which might charge the 
capacitor 172. Transistors 307 and 352 are coupled so that they provide a 
feedback path 18 to the collectors of transistor 3 to the base of which 
the output of comparator 350 is supplied. With this circuit arrangement 
the comparator 350 has a hysteresis characteristic that prevents it from 
erratically switching its output state in response to an input which 
fluctuates about the threshold. The transistor 3 forms a NOR gate with a 
transistor 4 having its base coupled through a line 19 to the collectors 
of transistors 108, 109 of the ramp generator 100 to generate a train of 
pulses P with a duration T6 to correct the deviation of ignition timing. 
Specifically, the pulse duration T6 is theoretically equal to T5-(T1-T2). 
However, for practical purposes T6 can be considered substantially equal 
to T5 since T1-T2 is of a negligibly small constant value (approximately 
100 microseconds). The pulses P at the collectors of transistors 3 and 4 
are inverted in polarity by a transistor 5 into pulses Q which are applied 
to the base of an emitter-follower transistor 7. The emitter of transistor 
7 is coupled to ground by a resistor 23 to drive a Darlington power 
transistor amplifier 10 so that the latter is rendered conductive during 
the period T6 to generate an ignition current in the primary winding of an 
ignition transformer 600. 
In a preferred embodiment, a resistor network formed by resistors 29, 30 
and 31 is coupled in series to the emitter of the power amplifier 10 to 
develop a voltage corresponding to the primary current. This voltage is 
coupled through a resistor 28 to a current peak control circuit 500 to 
permit it to detect when the primary current exceeds a predetermined 
current value and turn on a transistor 8 so that the latter provides a 
conductive path of a relatively low conductivity to resistor 24 for the 
purpose of decreasing the base current of power transistor 10, whereby the 
primary current is precisely controlled to the predetermined value. This 
feedback operation could be omitted, if desired, since the primary current 
tends to have a uniform value due to the fact that the duration of this 
current generally corresponds to the pulse duration determined by the 
circuit 200. 
The embodiment of FIG. 1 is modified to additionally include a feedback 
circuit shown in FIG. 2. This feedback circuit comprises a compensation 
circuit 395 formed by a transistor 308 having its base coupled to the base 
of transistor 302 of the clamp circuit 380 and a transistor 309 having its 
base coupled through the emitter-collector path of transistor 308 to 
ground, the circuit junction between the base of transistor 309 and the 
emitter of transistor 308 is coupled to a collector of a multi-collector 
transistor 401 which replaces the transistor 301 of the previous 
embodiment. The X terminal of the constant voltage source 400 is connected 
through the collector-emitter path of transistor 309 and resistor 319 to 
the junction between the resistor 312 and capacitor 320 of integrator 390. 
As will be described the current peak control circuit 500 provides pulses 
U with a duration T7 to the base of transistor 309. With the application 
of a pulse U the transistor 309 is turned on charging the integrating 
capacitor 302 via resistor 319, so that the voltage M increases as a 
function of the pulse duration T7. As a result, pulse spacing T5 becomes 
smaller than pulse duration T4 and this in turn causes a reduction in 
pulse duration T7. Hence the primary dwell time of the ignition 
transformer 600 can be reduced to a value just needed to effect ignition. 
It is noted that the power consumed during the peak period T7 of current 
pulse S does not contribute to the generation of useful power for 
ignition. Therefore, the current peak control circuit 500 minimizes the 
amount of ignition power. 
Alternatively, instead of modifying the integrated voltage M this can be 
done by modifying the rate of increase in ramp voltage H with the 
compensation circuit 395. 
FIG. 4 is an illustration of the detail of current peak control circuit 
500. The source voltage Vb is supplied via terminal Y to a voltage divider 
formed by resistors 533 and 531, between which the junction is coupled by 
diodes 521, 522 to the base of a transistor 501 and further coupled by a 
resistor 532 to ground through the collector-emitter path of transistor 
501. A resistor 534 and a diode 520 are connected in series to the 
collector of transistor 501 to bias the base of a transistor 502 to 
provide a constant voltage through a resistor 551 to a circuit junction 
between resistors 541 and 542. The voltage detected by the ignition 
current detecting resistor network is applied to a terminal W and thence 
to the base of a transistor 503 having its emitter coupled by resistor 535 
to the base of a transistor 504. The circuit junction between resistors 
541 and 542 is coupled by resistor 540 to the base of a transistor 508 of 
which the emitter-collector path is connected in a circuit including 
resistors 538 and 539, the junction between resistors 538 and 539 being 
coupled to the base of a transistor 507. The emitters of transistors 504 
and 507 are coupled by a common resistor 537 to ground. The base of 
transistor 508 is impressed with a reference voltage with which the 
voltage detected in response to the ignition primary current is compared. 
This reference voltage is generated by a circuit including resistors 544, 
545, diode 523, a transistor 510 having its base coupled to a junction 
between resistor 544 and diode 523, the emitter of transistor 510 being 
coupled to the base of a transistor 509. The collector-emitter path of 
transistor 509 is connected in series with the resistors 542 and 541. 
The voltage applied to terminal W increases in proportion to the primary 
current value. When this voltage reaches said reference voltage, 
transistor 503 is turned off causing transistor 507 to turn off, so that 
the voltage at the collector of transistor 507 increases to thereby turn 
on transistors 8 and 9 of FIG. 3. This results in a decrease in current to 
the base of power transistor 10 so that the ignition primary current is 
controlled to the predetermined value. The rise in voltage at the 
collector of transistor 507 turns on transistors 511 and 512 and turns off 
transistor 513. The collector of the transistor 513 is connected to 
terminal 549 at which pulses U appear. As illustrated in FIG. 2, the 
primary current S that flows into the emitter of transistor 10 is 
generated in response to the falling edge of a pulse Q developed at the 
collectors of transistors 5, 6 with a linearly increasing rate until it 
reaches a predetermined peak level whereupon the current value is 
maintained constant. Ignition primary voltage signal R, developed at the 
collector of transistor 10, decreases rapidly in response to the falling 
edge of the pulse Q and then increases rapidly in response to the 
termination of the primary current S. 
The embodiments shown and described above show only preferred forms of the 
invention. Various modifications and additions are readily apparent to 
those skilled in the art without departing from the scope of the invention 
which is only limited by the appended claims. For example, the ignition 
trigger timing could also be obtained from the instant "b" in FIG. 2, or 
from a cylinder position sensor as employed in electronic ignition control 
systems.