Oven-rotation prevention method and circuit in the non-contact type ignition circuit for the internal combustion engine

In a non contact ignition circuit for an internal combustion engine, a current induced in the primary winding of an ignition coil is controlled and cut-off by operation, on and off, of a thyristor so as to produce a discharge in a spark plug. A capacitor connected to the gate of the thyristor is charged with an inverse voltage. When the rotational speed of the internal combustion engine exceeds a predetermined level, i.e. a state of overrotation, the inverse voltage stored in the capacitor is discharged whereby the gate potential of the thyristor is negatively biased relative to the cathode. Accordingly, during the discharging period of the capacitor, the triggering time of the thyristor and sparking are retarded as compared to the normal running condition of the internal combustion engine. Thereby overrotation of the internal combustion engine is prevented.

This invention relates to an overrotation prevention method in a 
non-contact ignition circuit for an internal combustion engine and an 
overrotation prevention circuit embodying the method. 
The term "overrotation" herein refers to a state where engine rotational 
speed abnormally increases, and particularly, this tends to occur when 
load is rapidly changed from full load to no-load. Prior art approaches 
adapted to prevent overrotation of an internal combustion engine often 
include a governor mechanism of mechanical structure, or when rotational 
speed of the internal combustion engine exceeds a predetermined level, 
spark discharge at the spark plugs is stopped. 
In accordance with the above-mentioned second approach in which spark 
discharge at the plug is stopped, gas is forced into the engine cylinder, 
which poses a difficulty in providing re-ignition and various other 
inconveniences. For this reason, generally, the governor mechanism has 
been utilized. 
The governor mechanism, comprises a flyweight and a spring coupled to the 
flyweight. An increase of the centrifugal force acting on the flyweight, 
which increases in proportion to the rotational speed of the crank shaft, 
causes a displacement against the spring force of the flyweight so as to 
control excessive rotational speed of the internal combustion engine. 
As described above, the conventional mechanical governor mechanism requires 
the flyweight and the spring. In addition, there is required a space large 
enough to allow the flyweight to rotate since the latter rotates integral 
with the crank shaft. Space enough to allow the crank shaft to be 
displaced as the rotational speed thereof varies is also required. Hence, 
the mechanism becomes extremely bulky. As a consequence, it is difficult 
for the governor mechanism, which is often mounted in a very narrow space, 
to be mounted on the internal combustion engine, and above all, the 
mechanical service life thereof is decreased due to deterioration of the 
spring and the like. 
Various overrotation prevention circuits have recently been proposed for 
electrically retarding ignition timing in an ignition circuit to prevent 
overrotation of the internal combustion engine in an effort to avoid 
various disadvantages noted above with respect to the mechanical governor 
mechanism. In accordance with most of the aforementioned circuits, 
however, there is a limitation in the angle of delay; the amount of delay 
is maintained at a given value by operation of the overrotation prevention 
circuit. Thus, in the case where the rotational speed of the internal 
combustion engine tends to increase, for some reason despite operation of 
the overrotation prevention circuit, it is impossible in the prior 
circuits to inhibit such an increase of the rotational speed. Another 
problem with the prior art circuits is that the overrotation prevention 
circuit at a time of normal rotational speed of the internal combustion 
engine, causes normal ignition timing to be retarded slightly. 
The present invention eliminates the disadvantages and inconvenience noted 
above with respect to the aforementioned prior art examples by providing a 
method and apparatus for preventing overrotation of an internal combustion 
engine. The apparatus comprises a non-contact ignition circuit for an 
internal combustion engine in which a current induced in a primary winding 
or an ignition coil, with a plug connected to a secondary winding thereof, 
is controlled in its conduction and cut-off by the operation, on and off, 
of a thyristor so as to produce a spark discharge in the plug. When 
rotational speed of the internal combustion engine exceeds a predetermined 
level, i.e., overrotation, the inverse voltage stored in a capacitor 
connected to the gate of the thyristor is discharged to drop the gate 
potential of the thyristor, thereby retarding the triggering time of the 
thyristor and the time of sparking. During the discharging of the 
capacitor, overrotation of the internal combustion engine is prevented. 
Accordingly, it is an object of this invention to electrically prevent 
overrotation of an internal combustion engine. 
Another object of this invention is to increase the overrotation prevention 
response as the overrotation of the internal combustion engine increases. 
Another object of this invention is to initiate the overrotation prevention 
operation in accordance with an induced voltage which increases in 
proportion to rotational speed of the internal combustion engine. 
Still another object of this invention is to prevent the ignition circuit 
from being electrically influenced by the overrotation prevention circuit 
when the internal combustion engine is in normal running condition. 
Still another object of this invention is to enable free selection of the 
rotational speed for commencing an overrotation prevention operation of 
the internal combustion engine without influencing the normal ignition 
circuit.

Various embodiments of the present invention will now be described with 
reference to the accompanying drawings. 
As previously mentioned, the present invention is applied to a non-contact 
ignition circuit for an internal combustion engine in which a current, 
induced in the primary winding T.sub.1 of an ignition coil T having a plur 
P connected to the secondary winding T.sub.2, is controlled and cut-off by 
the on and off action of a thyristor SCR. The ignition circuits, to which 
this invention is applied, are roughly divided into two types, namely, an 
induction discharge type ignition circuit (TCI) and a capacity discharge 
type ignition circuit (CDI). 
The induction discharge type ignition circuit (FIG. 2) TCI comprises a 
resistor R.sub.1 inserted as a base resistor between the collector and 
base of a transistor Tr. The transistor Tr is connected in parallel with 
the primary winding T.sub.1 of the ignition coil T. A thyristor SCR is 
inserted between the base and emitter of the transistor Tr with the 
thyristor anode connected to the base. A resistance circuit comprising a 
resistor R.sub.2 (in the form of a variable resistor for setting the 
trigger time of the thyristor SCR) and a series resistor R.sub.3 are 
inserted between the gate of the thyristor SCR and collector of the 
transistor Tr. A series circuit comprising a diode D.sub.1 (for 
temperature compensation) in series with a resistor R.sub.4 is inserted 
between the gate and cathode of the thyristor SCR. 
In the ignition circuit (TCI), as is apparent from its construction (see 
FIGS. 2,5), when a forward induced voltage is produced in the primary 
winding T.sub.1, such that a base current flows into the base of the 
transistor Tr through the resistor R.sub.1, the transistor Tr is placed in 
conduction. Thus, current flows in the primary winding T.sub.1 through the 
transistor Tr. 
When the primary current increases in value, as the induced voltage in the 
primary winding T.sub.1 increases, a shunt current flowing into the gate 
circuit of the thyristor SCR through resistors R.sub.2, R.sub.3, R.sub.4, 
R.sub.5 and D.sub.1 also increases. Finally, the voltage drop in the gate 
circuit reaches the trigger voltage of the thyristor SCR at a time in the 
induced voltage cycle set by the value of the resistor R.sub.2. As a 
consequence the thyristor SCR turns on. 
When the thyristor SCR turns on, the potential difference between the base 
and emitter of the transistor Tr is almost zero, because the thyristor SCR 
shunts across the base and emitter so that the transistor Tr is cut off at 
the moment when the thyristor SCR turns on. When the transistor Tr current 
is cut off, the current flowing into the primary winding T.sub.1 is 
rapidly cut off. 
This rapid cut off of the current flowing into the primary winding T.sub.1 
causes a high voltage to be induced in the secondary winding T.sub.2 of 
the transformer T and produces a spark discharge in plug P. 
Thus, according to the present invention, the capacitor C connected to the 
gate of thyristor SCR used in the ignition circuits TCI and CDI is charged 
with an inverse voltage, and when rotational speed of the internal 
combustion engine exceeds a predetermined value, i.e., a state of 
overrotation, the inverse voltage stored in the capacitor C is discharged 
through a discharge circuit having a suitable time constant. This 
discharge of the capacitor C causes the gate potential of the thyristor 
SCR to be biased to a lower potential than that of the cathode of the 
thyristor SCR over a period of time, in accordance with the time constant 
of the discharge circuit of the capacitor C. Thus discharge disables 
triggering of the thyristor SCR so that the trigger time of the thyristor 
SCR is retarded for a period of time in accordance with the time constant 
of the discharge circuit of the capacitor C to thereby retard ignition 
timing of plug P. This retarded firing of the plug decreases overrotation 
and the rotational speed of the internal combustion engine. 
Thus, in the present invention, the inverse voltage stored in the capacitor 
C, which is connected to the gate of the thyristor SCR used in the 
ignition circuits (TCI and CDI), is discharged when the internal 
combustion engine is in a state of overrotation to bias the gate of the 
thyristor SCR negatively with respect to the cathode so that the trigger 
time of the thyristor SCR is retarded over a period of time in accordance 
with the discharging time of the capacitor C to prevent overrotation of 
the internal combustion engine. Accordingly, an overrotation prevention 
circuit ESG (see FIGS. 1,2) embodying the present invention would require 
at least; a capacitor C of which one terminal is connected to the gate of 
the thyristor SCR used in the ignition TCI or CDI; a charging circuit 
(Jcl) for charging the capacitor C with an inverse voltage; a discharging 
circuit (Hcl) for discharging the inverse voltage stored in the capacitor 
C; and a discharge switch circuit (Scl) for closing the discharging 
circuit (Hcl) to discharge the capacitor C when rotational speed of the 
internal combustion engine is in overrotation. 
A basic embodiment of the circuit ESG in accordance with the present 
invention, which is presumably a simplest form, will be discussed with 
reference to FIG. 2. 
In the embodiment shown in FIG. 2, an overrotation prevention circuit (ESG) 
is connected to the ignition circuit (TCI) described above. Inserted 
between the gate and cathode of the thyristor SCR is a circuit loop 
comprising in series a resistor R.sub.9 a capacitor C, and a second 
thyristor SCR.sub.1 having its anode connected to the capacitor C and its 
cathode connected to the cathode of the thyristor SCR. 
The gate circuit of the thyristor SCR.sub.1 is a loop circuit comprising in 
series a trigger coil TC.sub.1, a rectifying diode D.sub.10 and a resistor 
VR. The thyristor SCR.sub.1 has its gate connected to a movable contact of 
the resistor VR. 
A rectifying diode D.sub.7 is inserted between the negative terminal, i.e., 
lower end in FIG. 2, of the primary winding T.sub.1 also connected to the 
cathode of the thyristor SCR.sub.1 ! and anode of the capacitor C. The 
anode is also connected to the cathode of the thyristor SCR.sub.1. It 
should be noted that as defined herein and in FIG. 2 the anode of the 
capacitor C is the left terminal of the capacitor and the right terminal 
of the capacitor is defined as the cathode. A rectifying diode D.sub.8 is 
inserted between the cathode of the capacitor C and positive terminal, 
i.e., upper end in FIG. 2, of the primary winding T.sub.1 with the diode 
cathode connected to the positive terminal of the primary winding T.sub.1. 
Both the diode and D.sub.7 and D.sub.8 form a charging circuit (JCL) for 
the capacitor C when an inverse voltage, that is negative at the 
transistor collector and positive at the emitter is induced in winding 
T.sub.1. 
Accordingly the inverse voltage induced in the primary winding T.sub.1 is 
charged into the capacitors C. The capacitor is charged positive at its 
anode and negative at its cathode. 
Further, a rectifying diode D.sub.9 is inserted between the gate of the 
thyristor SCR with resistor R.sub.9 connected thereto! and the cathode of 
the thyristor SCR.sub.1 with the anode of the diode D.sub.9 connected to 
the cathode of the thyristor SCR.sub.1. The combination of the diode 
D.sub.9 and the resistor R.sub.9 forms a portion of the discharging 
circuit HCL! for the capacitor C. 
This discharging circuit HCL! forms a time constant circuit so that when 
the circuit comprising the capacitor C, thyristor SCR.sub.1, diode D.sub.9 
and resistor R.sub.9 is closed, the electric charge stored in the 
capacitor C is discharged in a period of time predetermined by the values 
of the capacitor C and resistor R.sub.9. 
With the embodiment shown in FIG. 2 constructed as above described, when 
the rotational speed of the internal combustion engine reaches a 
preselected level (set by the resistor VR), the thyristor SCR.sub.1 is 
placed in conduction to discharge the electrical charge stored in the 
capacitors C. Discharge current flows from the anode (left terminal, FIG. 
2) through the thyristor SCR.sub.1, the diode D.sub.9 and the resistor 
R.sub.9, whereby gate potential of the thyristor SCR is decreased to the 
value representing the voltage drop across conducting diode D.sub.9. 
Conduction of thyristor SCR is delayed until discharge of capacitor C is 
completed, that is, by the time set by the capacitor C and the resistor 
R.sub.9. Thereby conduction of thyristor SCR is delayed (retarded) as 
compared to the normal firing time set by the resistors in the gate 
circuitry of the thyristor SCR and more particularly by the variable 
resistor R.sub.2. Thus, above a preselected speed, spark retardation 
occurs to prevent overrotation of the internal combustion engine. 
It should be noted that it is the setting of the variable resistor VR 
connected to the gate of the second thyristor SCR.sub.1 which determines 
the speed at which retardation begins. 
The operation of the aformentioned circuit will further be described in 
detail. 
When the internal combustion engine is driven at a normal rotational speed, 
the gate voltage V.sub.2 (see FIG. 3 (b)) of the thyristor SCR.sub.1 due 
to the voltage induced, e.g., from a magnetic field associated with the 
engine flywheel, in the trigger coil TC.sub.1 does not reach the trigger 
voltage of the thyristor SCR.sub.1. Hence, the thyristor SCR.sub.1 is not 
placed in conduction; as a consequence the retard circuit ESG which 
discharges the capacitor C, is not operated but only the ignition circuit 
(TC.sub.1) of the transistor Tr and the thyristor SCR is operated. The 
engine operates normally. 
When the rotational speed of the internal combustion engine increases for 
some reason from the normal state as described above to a speed level 
predetermined by the setting of the resistor VR, the gate voltage 
(V.sub.2) of the thyristor SCR.sub.1, said gate voltage being due to the 
voltage induced in the trigger coil TC.sub.1, reaches the trigger voltage 
(see FIG. 3) (b)) of the thyristor SCR.sub.1 to place the thyristor 
SCR.sub.1 in conduction. 
Conduction of SCR.sub.1 imposes the voltage of capacitor C on the resistor 
R.sub.9 because of the low voltage drop across the thyristor SCR, and 
diode D.sub.9 during conduction. Accordingly, the gate of the ignition 
thyristor SCR which connects to the cathode of the diode D.sub.9 is at a 
low potential relative to its cathode and will not fire. This inhibited 
condition of the thyristor SCR continues while capacitor C discharges. 
After the capacitor C is discharged, the gate of the ignition thyristor 
SCR goes positive relative to its cathode and the thyristor SCR fires. 
Firing of the thyristor SCR as stated above, shunts the transistor (Tr) 
base and emitter together and interrupts emitter collector current flow 
causing a spark at the plug P. 
In other words, the thyristor SCR.sub.1 is triggered, and as a consequence, 
the trigger time of the thyristor SCR is delayed by the time set by the 
time constant circuit formed by the capacitor C and the resistor R.sub.9. 
When the thyristor SCR conducts, the emitter-collector current is 
interrupted, as a consequence of which the voltage between the collector 
and the emitter of the transistor Tr is rapdly increased as shown in FIG. 
3 (a) due to the well-known inductive kick in the primary winding T.sub.1 
to rapidly create a high voltage in the secondary winding T.sub.2. 
If the rotational speed of the internal combustion engine is greater than 
the speed set for firing the thyristor SCR.sub.1 by the resistor VR, the 
protection circuit (ESG) is continuously operated to fire thyristor 
SCR.sub.1 on every cycle. The delay time in firing thyristor SCR is fixed, 
set by the time constant circuit formed by the capacitor C and the 
resistor R.sub.9 irrespective of the rotational speed of the internal 
combustion engine. Hence, a fixed time period represents a larger portion 
of the engines rotation cycle when the speed of rotation is higher; the 
higher the rotational speed of the internal combustion engine, the greater 
is the magnitude of angle of lag in firing the plug P thereby increasing 
the overrotation prevention effect accordingly. 
FIG. 4 is a graphic representation showing the experimentally determined 
relation between the rotational speed of an internal combustion engine and 
the angle of lag produced by the protection circuit (ESG). Curve I 
illustrates the case where the lowest rotational speed for initiating 
firing of the protective circuit (ESG) is set to 3,000 rpm by the resistor 
VR. Curve II is the case where the starting rotational speed is set to 
5,000 rpm; curve III is the case where the starting rotational speed is 
set to 7,000 rpm; and curve IV is the case where the starting rotational 
speed is set to 8,000 rpm. 
It will be noted that the time constant in the time constant circuit is the 
same in all the cases and a flywheel, mounted on the internal combustion 
engine, is driven by the motor. This experiment was carried out merely to 
see the relation between the increase in the engines rotational speed and 
the effect on angle of lag as produced by the circuit ESG of the 
invention. 
As may be seen clearly in comparison of the various curves, the magnitude 
of lag angle is conspicuously higher when the set speed for initiating 
firing delay is higher in spite of the same time constant. Thus, higher 
setting speeds increase the rotation-reducing effect produced by angle of 
lag. 
As is also obvious from the curves in FIG. 4, even in the same setting 
condition, the degrees of lag angle increase as the rotational speed 
increases, and the firing delay which directly acts to prevent 
overrotation becomes greater in proportion to the increase in rotational 
speed. 
It will be appreciated that if the thyristor SCR is triggered prior to the 
operation of the switching circuit (ESG), then the firing is not delayed 
even when overrotation exists. Accordingly, it is necessary to set the 
trigger time t.sub.1 of the thyristor SCR.sub.1 at a time slightly earlier 
than the trigger time of the thyristor SCR. 
Accordingly, the width of angle of lag of the trigger time of the thyristor 
SCR by the circuit ESG is a value slightly smaller than the time constant 
set by the capacitor C and the resistor R.sub.9. 
In the embodiment shown in FIG. 2, the trigger time of the thyristor 
SCR.sub.1 is set by the trigger coil TC.sub.1 irrespective of the value of 
the inverse voltage charged in the capacitor C. Separately from the 
embodiment shown in FIG. 2, FIG. 5 illustrates another embodiment of the 
invention in which the thyristor SCR.sub.1 is triggered in accordance with 
the value of the inverse voltage charged in the capacitor C. 
The embodiment shown in FIG. 5 which is similar to the circuit of FIG. 2 
except for the switching circuitry (ESG) which comprises a thyristor 
SCR.sub.1 having its cathode connected directly to the cathode of the 
thyristor SCR to form a discharge circuit. The anode of the thyristor 
SCR.sub.1 is connected to one terminal of the capacitor C while the other 
terminal of capacitor C connects to the gate of thyristor SCR via the 
resistor R.sub.9. A rectifying diode D.sub.7 shunts the thyristor 
SCR.sub.1 with the diode D.sub.7 anode connected to the cathode of the 
thyristor SCR.sub.1, and the diode D.sub.7 cathode connected to the anode 
of SCR.sub.1. The diode D.sub.8 connects its anode at the junction between 
the capacitor C and resistor R.sub.9 ; the cathode of diode D.sub.8 
connects to the upper positive (FIG. 5) terminal of the primary winding 
T.sub.1. Resistor R.sub.13 is connected between the gate and cathode of 
the thyristor SCR.sub.1. The cathode of thyristor SCR.sub.1 connects to 
the cathode of the thyristor SCR. The Zener diode ZD.sub.1 connects 
between the anode and gate of the thyristor SCR.sub.1 with the Zener 
cathode connected to the thyristor anode. Diode D.sub.9 has its cathode 
connected to the gate of the thyristor SCR and its anode connected to the 
cathode of thyristor SCR. The ignition circuit, identified as TCl in FIG. 
5 and connected across the primary winding T.sub.1 of the ignition 
transformer T, is substantially identical to those circuits identified as 
TCl in FIG. 2 and operates identically. 
That is, the circuit shown in FIG. 5 is virtually identical in construction 
to that shown in FIG. 2 with the exception of the gate circuit of the 
thyristor SCR.sub.1. 
Thus, when an inverse voltage is induced in the primary winding T.sub.1, 
that is, when the lower end (FIG. 5) of primary winding T.sub.1 is 
positive, an inverse voltage is charged into the capacitor C, the charging 
current passing through the circuit (Jcl) which comprises the lower end of 
primary winding T.sub.1, diode D.sub.7, capacitor C, diode D.sub.8 and 
back to the upper end of primary winding T.sub.1. 
On the other hand, the inverse voltage stored in the capacitor C is 
discharged by a current passing through the discharge circuit (Hcl) from 
the capacitor C, through thyristor SCR.sub.1, diode D.sub.9, resistor 
R.sub.9, and back to the capacitor C. 
Incidentally, since the inverse voltage to be stored in the capacitor C is 
the voltage induced in the primary winding T.sub.1, it increases in 
proportion to the rotational speed of the internal combustion engine. 
Further, a series circuit comprising the Zener diode ZD.sub.1 and the 
resistor R.sub.13 is connected in parallel with a series circuit 
comprising the capacitor C, the resistor R.sub.9 and the diode D.sub.9. 
When the voltage between electrodes, positive on the left electrode, of 
the capacitor C, impressed across the Zener, exceeds the Zener breakdown 
voltage of the Zener diode ZD.sub.1, the Zener diode ZD.sub.1 conducts. As 
a result, an electric current is passed through the resistor R.sub.13 and 
applies a gate voltage which triggers the thyristor SCR.sub.1. 
The value of the inverse voltage charged into the capacitor C in an 
overrotation state of the internal combustion engine can be determined 
beforehand whereby only when the rotational speed of the internal 
combustion engine is in the overrotation state, will the Zener diode 
ZD.sub.1 breakdown and as a result, an electric current flows into the 
resistor R.sub.13 to trigger the thyristor SCR.sub.1. 
This will be explained in accordance with the operation of the entire 
circuit (ESG) (FIG. 5). 
When the rotational speed of the internal combustion engine is within the 
range of normal speeds, the inverse voltage charged into the capacitor C 
will not reach the Zener breakdown voltage of the Zener diode ZD.sub.1 so 
that the circuit ESG is not operated and the thyristor SCR comes in 
conduction at the time t.sub.1 set by the resistor R.sub.2 to produce a 
spark discharge in plug P exactly as in the circuit of FIG. 2 described 
above. 
Then when the rotational speed of the internal combustion engine increases 
for some cause or other, the inverse voltage induced in the primary 
winding T.sub.1 also increases and the value of the voltage stored in the 
capacitor C also increases. 
When the rotational speed of the internal combustion engine increases up to 
the overrotation value set previously to accommodate the Zener diode 
ZD.sub.1, the inverse voltage stored in the capacitor C and impressed 
across the thyristor SCR.sub.1 reaches the Zener breakdown voltage of the 
Zener diode ZD.sub.1 and thyristor SCR.sub.1 is fired to delay the spark 
at the plug P. 
Incidentally, the breakdown of the Zener diode ZD.sub.1 is not achieved at 
the same time when a potential difference between electrodes of the 
capacitor C first reaches the Zener voltage but occurs at time t.sub.3 
when the voltage between collector and emitter of the transistor Tr, i.e., 
the potential difference between terminals of the primary winding T.sub.1, 
gradually changes from the maximum value in the inverse direction to the 
forward voltage, as shown in FIG. 6 (a). 
As shown in FIG. 6 the change in voltage between anode and cathode of the 
thyristor SCR.sub.1 for causing the Zener diode ZD.sub.1 to breakdown 
assumes a minimum value, by forward conduction of diode D.sub.7, when the 
inverse voltage of the primary winding T.sub.1 is at maximum but an 
electric charge corresponding to the maximum value of the inverse voltage 
is charged into the capacitor C. As a consequence, the aforesaid anode to 
cathode voltage of SCR.sub.1 increases as the inverse voltage of the 
primary winding T.sub.1 decreases, and finally reaches the Zener voltage 
of the Zener diode ZD.sub.1 at time t.sub.3. 
When the Zener diode ZD.sub.1 breaks down, the trigger voltage of the 
thyristor SCR.sub.1 is produced in the resistor R.sub.13 to place the 
thyristor SCR.sub.1 in conduction. 
When the thyristor SCR.sub.1 comes in conduction, the electric charge 
stored in the capacitor C passes through the thyristor SCR.sub.1, the 
diode D.sub.9 and the resistor R.sub.9, and capacitor C is discharged in 
accordance with the time constant set by the capacitor C and the resistor 
R.sub.9. 
The discharge of the inverse voltage stored in the capacitor C, i.e., the 
conduction of the thyristor SCR.sub.1, causes the positive electrode (left 
side, FIG. 5) of the capacitor C to be virtually short circuited to the 
gate of thyristor SCR via the diode D.sub.9. As a consequence, the 
potential of the gate of the thyristor SCR drops to the negative side to 
render conduction of the thyristor SCR impossible. 
This state is retained for a period of time .DELTA.t.sub.1 set by the time 
constant to completely discharge the capacitor C, that is, for a period of 
time from t.sub.3 to t.sub.2. 
The timing in overrotation for conduction of the thyristor SCR.sub.1, i.e., 
breakdown time t.sub.3 of the Zener diode ZD.sub.1, is set at a time 
earlier than normal conduction of the thyristor SCR would begin without 
overrotation, i.e., normal ignition time t.sub.1 is within the range of 
time .DELTA.t.sub.1. Accordingly, the normal ignition time t.sub.1 is 
within the period for discharging the capacitor C. However, the thyristor 
SCR gate is shunted by diode D.sub.9 to its cathode during the discharge 
of the capacitor C as previously mentioned, hence, it is impossible to 
place the thyristor SCR in conduction. 
However, near the time t.sub.2 at which discharge of the capacitor C is 
completed, the forward voltage (FIG. 6 (a) ) induced in the primary 
winding T.sub.1 also increases. As a consequence, the gate potential of 
the thyristor SCR also rises gradually as shown in FIG. 6 (c) and has 
reached the trigger potential at time t.sub.2, when discharge of the 
capacitor C is completed, to trigger the thyristor SCR, thus producing a 
spark discharge in plug P. 
That is, ignition timing of the ignition circuit, indicated in FIG. 5 as 
TCI will delay by the time .DELTA.t.sub.2 from time t.sub.1 set by the 
resistor R.sub.2 to time t.sub.2 when discharge of the capacitor C 
completes. 
This delay of ignition timing causes output of the internal combustion 
engine to decrease abruptly, thereby decreasing the rotational speed 
thereof. 
In the ESG circuit, therefore, the Zener breakdown voltage of the Zener 
diode ZD.sub.1 may suitably be set (a Zener diode ZD.sub.1 having a 
suitable value of Zener voltage may be selected), whereby the rotational 
speed of the internal combustion engine for operating the switching 
circuit (ESG) may suitably be set. The value of angle of lag of ignition 
timing may freely be set by adjusting the time constant of the RC time 
constant circuit. 
When for some cause of other, the rotational speed of the internal 
combustion engine tends to remain excessive in spite of the fact that the 
switch circuit (ESG) is operating, the amount of electric charge stored in 
the capacitor C on the inverse voltage cycle increases and the time 
.DELTA.t.sub.1 increases. The amount of rotation of the flywheel per unit 
time increases for a given time delay. Thereby angle of lag which acts 
directly to reduce rotational speed of the internal combustion engine is 
greater as rotational speed is greater. 
It will be appreciated that the switching circuit (ESG) affords the added 
advantage of prespark prevention since an inverse current flows in the 
primary winding T.sub.1 when an inverse voltage is induced in the primary 
winding T.sub.1. The same is true for the embodiment shown in FIG. 2.