Ignition system for an internal combustion engine

This invention relates to an ignition system for an internal combustion engine. The ignition system comprises an ignition circuit to generate a spark at an ignition plug in synchronism with a rotation of the internal combustion engine. The ignition circuit may be of a capacitor discharging type or of an ignition primary current interrupting type. The ignition system further comprises an ignition preventing circuit to prevent the ignition circuit from being operated when the internal combustion engine is rotated at a speed of revolution higher than a first set value of r.p.m. and when a throttle valve operating member is quickly returned to a position at which the r.p.m. of the engine is lower than the second set value of r.p.m. which is lower than the first set value of r.p.m. The ignition preventing circuit may be adapted to fully prevent the operation of the ignition circuit. Alternatively, the ignition preventing circuit may be adapted to thin the operation of the ignition circuit.

BACKGROUND OF THE INVENTION 
In the prior arts, there have been proposed various ignition systems for an 
internal combustion engine in which an ignition preventing circuit is 
provided to prevent an ignition circuit from being operated when a 
revolution per minute of the engine is higher than a critical one. 
However, such ignition systems cannot reduce the r.p.m. of the engine when 
an operator returns a throttle valve operating member in order to avert 
danger because it takes substantial time to reduce the r.p.m. of the 
engine. Furthermore, in a snowmobile having such ignition systems 
provided, even though the throttle valve operating member is returned, a 
throttle valve cannot be sometimes returned because a throttle wire 
freezes. This prevent the r.p.m. of the engine from being reduced. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the invention to provide an 
ignition system for an internal combustion engine wherein the speed of 
revolution per minute of the engine can be quickly reduced when a throttle 
valve operating member is quickly reduced in order to avert danger. 
It is another object of the invention to provide an ignition system for an 
internal combustion engine wherein the engine speed can be slowly reduced 
when a throttle valve operating member is moderately returned. 
It is further object of the invention to provide an ignition system for an 
internal combustion engine wherein an explosion noise is never generated 
in a muffler, which tends to occur due to remaining gases in an exhaust 
system. 
In accordance with the invention, there is provided an ignition system for 
an internal combustion engine comprising an ignition circuit to generate a 
spark at an ignition plug in synchronism with a rotation of the internal 
combustion engine, said ignition system further comprising an ignition 
preventing circuit to prevent said ignition circuit from being operated 
when the internal combustion engine is being rotated at a speed of 
revolution higher than a first set value of revolution per minute and when 
a throttle valve operating member is returned to a position at which the 
internal combustion engine is to be rotated at a revolution per minute 
lower than a second lower set value of revolutions per minute.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring now to FIG. 1, there is shown an ignition system for an internal 
combustion engine constructed in accordance with one embodiment of the 
invention. The ignition system comprises a capacitor discharging type 
ignition circuit 1 which includes an ignition coil 2, a capacitor 3 and an 
ignition electric source 4. The ignition electric source 4 which may be in 
the form of magneto has a first generating coil 5 and a second generating 
coil 6 provided therein. The first generating coil 5 at one end is 
connected to one end of a primary coil 2a of the ignition coil 2 through a 
forwarded diode 7 and through the capacitor 3 and at the other end is 
grounded. An ignition plug 8 at one end is connected to one end of a 
secondary coil 2b of the ignition coil 2. The other ends of the primary 
and secondary coils 2a and 2b of the ignition coil 2 and the ignition plug 
8 are grounded. 
An igniting thyristor 9 has an anode connected to the point of junction 
between the diode 7 and the capacitor 3 and a cathode connected to the 
other end of the primary coil 2a of the ignition coil 2. A capacitor 10 
has one end connected to the gate of the igniting thyristor 9 and the 
other end connected through a forwarded diode 11 to one end of the second 
generating coil 6 of the ignition electric source 4, the other end of 
which is connected to the point of junction between the first generating 
coil 5 of the ignition electric source 4 and the diode 7. An electric 
resistor 12 has one end connected to the point of junction between the 
capacitor 10 and the gate of the igniting thyristor 9 and the other end 
connected to the cathode of the igniting thyristor 9. Another thyristor 13 
has a cathode connected to the point of junction between the capacitor 10 
and the diode 11 and an anode connected to the other end of the electric 
resistor 12. A variable electric resistor 14 is connected in parallel to 
the thyristor 13 and a Zener diode 15 has anode connected to the gate of 
the thyristor 13 and a cathode connected to the point of junction between 
the anode of the thyristor 13 and the variable electric resistor 14. 
When an output signal indicated by the solid arrow is generated from the 
first generating coil 5 of the ignition electric source 4, the electric 
capacitor 3 is charged through the diode 7 and the primary coil 2a of the 
ignition coil 2. When an output signal indicated by the broken arrow is 
generated from the second generating coil 6, the capacitor 10 is charged 
through the diode 7, the anode and gate of the igniting thyristor 9 and 
the diode 11. When the voltage across the capacitor 10 exceeds the Zener 
voltage of the Zener diode 15, the Zener diode 15 is turned on to 
discharge the capacitor 10 through the gate and cathode 9 of the igniting 
thyristor 9, the electric resistor 12, the Zener diode 15 and the gate and 
cathode of the thyristor 13. This causes the thyristors 9 and 13 to be 
turned on. Thus, the capacitor 3 is discharged through the first thyristor 
9 and the primary coil 2a of the ignition coil 2 whereby a high voltage is 
established across the secondary coil 2b of the ignition coil to generate 
a spark at the ignition plug 8. Since the ignition electric source 
generates the outputs in synchronism with the rotation of the internal 
combustion engine, the sparks can be generated at the ignition positions 
of the engine. 
The ignition system of the invention further comprises an ignition 
preventing circuit 16 to prevent the ignition circuit 1 from being 
operated when the internal combustion engine is being rotated at a speed 
of revolution higher than a first set value of revolution per minute and 
when a throttle valve operating member is returned to a position at which 
the internal combustion engine is to be rotated at a revolution per minute 
lower than a second set value of revolution per minute (r.p.m.). The 
ignition preventing circuit 16 includes an ignition preventing thyristor 
17 with its anode connected to the point of junction between the first and 
second generating coils 5 and 6 of the ignition electric source 4 and the 
diode 7, and with its cathode grounded through a throttle position 
detecting switch 18 in a throttle valve operating member 19 which is in 
the form of lever as shown in FIG. 2. A diode 20 and electric resistors 21 
and 22 are connected in series to each other with an anode of the diode 20 
connected to the point of junction between the one end of the second 
generating coil 6 and the cathode of the diode 11 and with an outer end of 
the electric resistor 22 connected to the cathode of the ignition 
preventing thyristor 17. A capacitor 23 is connected in parallel to the 
electric resistor 22. There is provided a Zener diode 24 an anode of which 
is connected to the gate of the ignition preventing thyristor 17 and a 
cathode of which is connected to the point of junction between the 
electric resistors 21 and 22. 
As shown in FIG. 2, the throttle valve operating member 19 comprises a grip 
25 for a car and a throttle lever 26 which is connected to a throttle wire 
27 for controlling an opening degree of a throttle valve not shown. The 
throttle position detecting switch 18 is provided on the grip 25 so that 
it is closed when the throttle lever 26 is at a position at which the 
internal combustion engine is to be rotated at an r.p.m. lower than a 
given r.p.m. 
Since the electric resistors 21 and 22 serve as a voltage divider, the 
voltage across the electric resistor 22 which appears when the switch 18 
is closed is substantially in proportion to the output voltages of the 
first and second generating coils 5 and 6 of the ignition electric source 
4 and therefore the r.p.m. of the engine. When the voltage across the 
electric resistor 22 exceeds the Zener voltage of the Zener diode 24, the 
latter is turned on to apply a firing signal to the gate of the ignition 
preventing thyristor 17. There is a maximum r.p.m. at which the engine may 
be operated, which, merely as an example, might be 8000 r.p.m. There is a 
lower speed of the engine which is the minimum r.p.m. at which the Zener 
diode will conduct. This will be referred to later as a high or first set 
value of r.p.m. That first set value is so determined to be higher than a 
low or second set value of r.p.m. which is the r.p.m. below which the 
throttle position detecting switch 18 is closed. For example, the high set 
value of r.p.m. of the engine is 2,000 r.p.m. while the low set value of 
r.p.m. of the engine is 500 r.p.m. Thus, it will be noted that the 
ignition preventing circuit 16 is operated when the actual r.p.m. of the 
engine is higher than the high set value of r.p.m. and when the throttle 
lever 26 is at the position at which the engine is to be rotated at the 
r.p.m. lower than the low set value of r.p.m. 
In operation, after the internal combustion engine starts to be operated, a 
movement of the throttle lever 26 in the direction indicated by the arrow 
of solid line in FIG. 2 causes the opening degree of the throttle valve to 
be increased so that the r.p.m. of the engine becomes higher gradually. 
When the r.p.m. of the engine is relatively low, the throttle position 
detecting switch 18 is closed, but the ignition preventing thyristor 17 is 
never fired because the voltage across the electric resistor 22 is lower 
than the Zener voltage of the Zener diode 24. Therefore, the charge and 
the discharge of the capacitor 3 are repeated in synchronism with the 
rotation of the engine with the result that the igniting operation is 
normally made. 
The throttle lever 26 is further moved until it exceeds the position 
corresponding to the low set value of r.p.m. of the engine. At that time, 
the throttle position detecting switch 18 is open, and the r.p.m. of the 
engine becomes higher because the ignition preventing circuit 16 has no 
effect on the ignition circuit 1. 
The throttle lever 26 is further moved until the r.p.m. of the engine 
exceeds the high set value of r.p.m. As described hereinabove, since the 
throttle position detecting switch 18 is already open, the ignition 
preventing circuit 16 is never operated and the igniting operation is 
maintained. 
In case the throttle lever 26 is slowly returned toward the position at 
which the engine is to be rotated at the lower r.p.m., the r.p.m. of the 
engine is gradually decreased and the throttle position detecting switch 
18 is closed at the r.p.m. of the engine corresponding to the low set 
value of r.p.m. 
Meantime, in case the throttle lever 26 is quickly returned toward the 
idling position at which the engine is to be rotated at a lower r.p.m. 
when the engine is being actually rotated at the r.p.m. higher than the 
high or first set value of r.p.m., the throttle position detecting switch 
18 is closed, but the engine is being rotated at the r.p.m. higher than 
the high or first set value of r.p.m. because there is a delay of 
variation r.p.m. of the engine relative to the movement of the throttle 
lever 26. At that time, the voltage across the electric resistor 22 is 
higher than the Zener voltage of the Zener diode 24. Thus, it will be 
noted that the ignition preventing thyristor 17 is turned on with the 
result that the charging current which is to flow through the electric 
capacitor 3 is shorted therefrom. This prevents the ignition circuit from 
being operated and causes the r.p.m. of the engine to be quickly 
decreased. When the r.p.m. of the engine is lower than the high or first 
set value of r.p.m., the ignition preventing thyristor 17 is turned off 
because the voltage across the electric resistor 22 is lower than the 
Zener voltage of the Zener diode 24, which permits the engine to be idled. 
FIG. 3 shows another embodiment of the ignition system of the invention in 
which the ignition circuit 1 is substantially identical to that of the 
embodiment of FIG. 1 except that the second generating coil 6 is connected 
through the diode 11 to the gate of the igniting thyristor 9. In the 
embodiment of FIG. 3, the second generating coil 6 of the ignition 
electric source 4 generates an output signal therefrom everytime the 
engine reaches the ignition position. In FIG. 3, the same numerals 
designate the same components. 
In the embodiment of FIG. 3, the ignition preventing circuit 16 comprises 
an ignition preventing electric source 30, a pluse signal generator 31 
connected through the throttle position detecting switch 18 in the 
throttle valve operating member. The pulse signal generator 31 comprises 
an oscillating circuit 32 to oscillate when the throttle position 
detecting switch 18 is closed, a duty control circuit 33 to receive an 
output pulse from the oscillating circuit 32 to determine the duty of 
turning on or off the ignition preventing thyristor 17, and an amplifying 
circuit 34 to amplify a pulse signal from the duty control circuit 33. The 
output of the amplifying circuit 34 is connected across the gate and 
cathode of the ignition preventing thyristor 17. A third generating coil 
6A may be disposed in the ignition electric source 4 to drive loads such 
as a headlamp and so on. The throttle position detecting switch 18 may be 
mounted on the throttle valve operating member in the same manner as shown 
in FIG. 2 and is closed when the throttle valve operating member is at the 
position at which the engine is to be rotated at the r.p.m. lower than a 
low set value of r.p.m. which corresponds to an r.p.m. slightly higher 
than an idling r.p.m. At that time, a normal load is supposed to be 
applied to the engine. 
As shown in FIG. 4, the ignition preventing electric source 30 comprises a 
diode D.sub.1 having an anode connected to the non-grounded end of the 
third generating coil 6A of the electric source 4, an electric resistor 
R.sub.1 having one end connected to the cathode of the diode D.sub.1, a 
capacitor C.sub.1 having one end connected to the other end of the 
electric resistor R.sub.1 and the other end grounded, and an electic 
resistor R.sub.2 and a Zener diode ZD.sub.1 connected in parallel to the 
capacitor C.sub.1. The ignition preventing electric source 30 generates an 
output substantially equal to the peak value of the output voltage of the 
third generating coil 6A when the latter is lower than the Zener voltage 
of the Zener diode ZD.sub.1, and generates a constant voltage equaling to 
the Zenor voltage of the Zener diode ZD.sub.1 when the peak value of the 
output voltage from the third generating coil 6A exceeds the Zener voltage 
of the Zener diode ZD.sub.1. 
As also shown in FIG. 4, the oscillating circuit 32 which may be in the 
form of relaxation oscillator comprises a programmable unijunction 
transistor PUT. The cathode of the PUT is grounded through an electric 
resistor R.sub.3, and an electric capacitor C.sub.2 is connected between 
the anode of the PUT and earth. The anode of the PUT is connected to one 
end of an electric resistor R.sub.4, the other end of which is connected 
through the throttle position detecting switch 18 to the non-grounded end 
of the electric source 30. An electric resistor R.sub.5 is connected 
between the point of junction between the position detecting switch 18 and 
the electric resistor R.sub.4 and the gate of the PUT, while an electric 
resistor R.sub.6 is connected between the gate of the PUT and earth. An 
output terminal of the oscillating circuit 32 is led from the non-grounded 
end of the electric resistor R.sub.3 or the cathode of the PUT. The 
capacitor C.sub.2 is charged from the electric source 30 through the 
electric resistor R.sub.4 with the polarities shown in FIG. 2. FIG. 5(A) 
shows the voltage across the capacitor C.sub.2 or the potential at the 
point A of FIG. 2 which varies with time. When the voltage across the 
capacitor C.sub.2 increasing with time exceeds the gate biasing voltage of 
the PUT (which corresponds to the voltage across the electric resistor 
R.sub.6), the PUT becomes conductive because of its negative resistance 
characteristics. This causes the capacitor C.sub.2 to be discharged 
through the anode and cathode of the PUT and through the electric resistor 
R.sub.3. Thus, a narrow pulse voltage P is established across the electric 
resistor R.sub.3 every time the capacitor C.sub.2 is discharged. When the 
discharge of the capacitor C.sub.2 is finished, the PUT is turned off. 
Then, the capacitor C.sub.2 is charged to prepare for the above operation 
of the oscillating circuit 32. FIG. 5(B) shows the pulse P periodically 
established across the electric resistor R.sub.3 or at the point B of FIG. 
4. 
As shown in FIG. 4, the duty control circuit 33 comprises an npn transistor 
TR.sub.1 to the base and emitter of which the pulses from the oscillating 
circuit 32 are applied. The collector of the transistor TR.sub.1 is 
connected through an electric resistor R.sub.7 and the throttle position 
detecting switch 18 to the non-grounded end of the electric source 30. A 
capacitor C.sub.3 is connected between the collector and emitter of the 
transistor TR.sub.1 while the point of junction between the capacitor 
C.sub.3 and the collector of the transistor TR.sub.1 is connected through 
a Zener diode ZD.sub.2 to a base of an npn transistor TR.sub.2. The 
transistor TR.sub.2 has a collector connected through an electric resistor 
R.sub.8 and the position detecting switch 18 to the non-grounded end of 
the electric source and has an emitter grounded. The narrow pulse P from 
the oscillating circuit 32 permits the transistor TR.sub.1 to be 
momentarily turned on to thereby discharge the capacitor C.sub.3. After 
that, the transistor TR.sub.1 is turned off. The capacitor C.sub.3 is 
charged from the electric source 30 through the electric resistor R.sub.7 
with the polarities shown in FIG. 4 while the transistor TR.sub.1 is 
nonconductive. FIG. 5(C) shows the voltage V.sub.c3 across the capacitor 
C.sub.3 (at the point C of FIG. 4) which varies with time so as to form a 
saw tooth wave. When the voltage V.sub.c3 exceeds the Zener voltage of the 
Zener diode ZD.sub.2, a base current flows through the base and emitter of 
the transistor TR.sub.2 to turn it on. Thus, every time the transistor 
TR.sub.2 is turned on, a rectangular wave pulse is generated at the 
collector of the transistor TR.sub.2 or at the point D of the duty control 
circuit 34. 
As shown in FIG. 4, the amplifying circuit 34 comprises an npn transistor 
TR.sub.3 and an electric resistor R.sub.9. The base of the transistor 
TR.sub.3 is connected to the collector of the transistor TR.sub.2 in the 
duty control circuit 33 while the emitter of the transistor TR.sub.3 is 
grounded. The collector of the transistor TR.sub.3 is connected through 
the electric resistor R.sub.9 and the throttle position detecting switch 
18 to the non-grounded end of the electric source 30 and also directly to 
the gate of the ignition preventing thyristor 17. The amplifying circuit 
34 serves to reverse the output from the duty control circuit 33 to apply 
a rectangular control signal V.sub.g shown in FIG. 5(E) to the thyristor 
17. 
The output voltage from the electric source 30 is too low to generate the 
control signal V.sub.g from the amplifying circuit 34 until the r.p.m. of 
the engine reaches a high set value of r.p.m. slightly higher than the low 
set value of r.p.m. This is accomplished by the fact that the output 
voltage from the electric source 30 at the r.p.m. of the engine lower than 
the high set value of r.p.m. never breaks down the Zener diode ZD.sub.2 
which is accomplished by the voltage across the capacitor C.sub.3 of the 
duty control circuit 33. 
Let it be supposed that the normal load is applied to the engine and that 
the engine is normally accelerated from the idling condition. While the 
r.p.m. of the engine is lower than the low set value of r.p.m., the 
throttle position detecting switch 18 is closed, but the ignition 
preventing thyristor 17 is not conductive because the voltage of the 
electric source 30 is too low to generate the control signal V.sub.g from 
the amplifying circuit 34. Therefore, the ignition circuit 1 is normally 
operated to ignite the engine. 
When the revolution of the engine reaches the high set value of r.p.m. 
higher than the low set value of r.p.m., the output voltage from the 
electric source 30 reaches the one enough to generate the control signal 
V.sub.g from the amplifying circuit 34, but since at that time the 
throttle position detecting switch 18 is open, the control signal V.sub.g 
is never generated. Therefore, the ignition preventing thyristor 17 is 
never also turned on and the engine is normally operated to be 
accelerated. 
In case that the throttle valve operating member is slowly returned, the 
r.p.m. of the engine follows the movement of the throttle valve operating 
member to be decelerated. When the r.p.m. of the engine becomes lower than 
the high set value of r.p.m., the output voltage of the electric source 30 
is too low to generate the control signal V.sub.g from the amplifying 
circuit 34. Thus, the ignition preventing thyristor 17 is never turned on 
and, as a result, the ignition preventing circuit 16 is never operated. 
This causes the ignition circuit to be normally operated. 
In case that the throttle valve operating member is quickly returned to the 
position corresponding to an r.p.m. lower than the low set value of r.p.m. 
while the engine is being rotated at high r.p.m., higher than the high set 
value of r.p.m., the throttle position detecting switch 18 is closed to 
generate the control signal V.sub.g from the amplifying circuit 34 because 
the voltage of the electric source 30 is enough high to generate it. While 
the control signal V.sub.g is generated, the ignition preventing thyristor 
17 is turned on every time the voltage which causes the anode of the 
thyristor 17 to be positive is generated at the third generating coil 6A. 
Thus, it will be noted that the capacitor 3 is not charged every time the 
voltage is generated at the third generating coil 6A. Therefore, while the 
control signal V.sub.g is generated from the amplifying circuit 34, the 
sparks which are to be generated at the ignition plug 8 are thinned, which 
causes the engine to be intermittently ignited. The r.p.m. of the engine 
is quickly decelerated by this. When the r.p.m. of the engine becomes 
lower than the high set value, the control signal V.sub.g is no longer 
generated from the amplifying circuit 34, and as a result the ignition 
circuit 1 is normally operated. FIG. 5(F) shows the voltage V.sub.3 of the 
capacitor 3 of the ignition circuit 1 when the control voltage V.sub.g is 
generated. In this figure, the waveforms indicated at broken lines are the 
voltage waves which are to be charged at the capacitor 3 if the control 
signal V.sub.g is not generated. As noted from FIG. 5(F), the capacitor 3 
is prevented from being charged continuously four times and therefore 
continuous four sparks are prevented from being generated. 
It should be noted that the period of the control signal V.sub.g is 
constant in spite of variation in the revolution of the engine. Therefore, 
the number of the sparks which are thinned by the ignition preventing 
circuit 16 varies on the r.p.m. of the engine. As the r.p.m. of the engine 
is high, the number of the sparks to be thinned is increased. This quickly 
decreases the r.p.m. of the engine. It should be noted that the number of 
the sparks which are thinned is decreased as the r.p.m. of the engine is 
lowered. This causes the ignition system to be transferred to the normal 
ignition in a smooth manner. Thus, the explosion noise in the muffler or 
the mechanical shock is avoided. 
It is when the engine has the normal load applied thereto that the engine 
is rotated at the low set value at the position of the throttle valve 
operating member corresponding to the low set value. Practically, the 
r.p.m. at which the engine is rotated at various positions of the throttle 
valve operating member varies on the loading conditions of the engine such 
as the condition of the road surface and so on. The r.p.m. at which the 
engine is rotated sometimes tends to be higher than the low set value of 
r.p.m. even if the throttle valve operating member is positioned 
corresponding to the low set value. This occurs if the road surface 
freezes so that the load applied to the engine is relatively lighter. In 
this case, if the engine is rotated at the r.p.m. higher than the high set 
value, the thinned ignition is made and, therefore, the r.p.m. of the 
engine is caused to be lowered by the thinned ignition. However, if the 
load applied to the engine is relatively larger, the r.p.m. at which the 
engine is rotated sometimes tends to be lower than the low set value even 
if the throttle valve operating member is positioned corresponding to the 
low set value. In this case, the thinned ignition is not necessary and is 
not made by the ignition system of FIG. 3. 
It will be understood that the low set value of r.p.m. may be set at the 
idling r.p.m. In this case, the throttle position detecting switch 18 is 
arranged to be closed when the throttle valve operating member is returned 
to the free position at which the engine is to be rotated at the lowest 
r.p.m. 
Although two preferred embodiments of the invention have been described and 
illustrated with reference to the accompanying drawings, it will be 
understood to those skilled in the art that they are by way of example, 
and that various changes and modifications may be made without departing 
from the spirit and scope of the invention. For example, in the 
embodiments of FIGS. 1 and 3, the throttle valve operating member may be 
mounted on an accelerating pedal or on a throttle pedal. In the embodiment 
of FIG. 4, the oscillating circuit 32 may be formed of a unijunction 
transistor instead of the PUT and alternatively the oscillating circuit 32 
and the duty control circuit 33 may be composed of an asymmetrical 
unstable multivibrator. Also, in the embodiment of FIG. 4, the throttle 
position detecting switch 18 may be provided between the output of the 
pulse signal generator 31 and the gate of the ignition preventing 
thyristor 17. Furthermore, in the embodiments of FIGS. 1 and 3, the 
igniting thyristor 9 may serve as the ignition preventing thyristor 17. 
The ignition preventing thyristor 17 may be provided so as to short the 
second generating coil 6. It will be noted that an ignition preventing 
semiconductor switch may be provided instead of the thyristor 17 and 
arranged to interrupt the ignition circuit 1 to prevent it from being 
operated. Such an ignition preventing switch may be of multi-stage 
transistors. It will be also noted that the invention may be applied to a 
primary current interrupting type ignition system in place of a capacitor 
discharging type ignition system. Thus, it will be understood that the 
invention is intended to be defined only by the appended claims.