GB2298003A

GB2298003A - Circuit for stopping an engine

Info

Publication number: GB2298003A
Application number: GB9606885A
Authority: GB
Inventors: Yosuke Kubota
Original assignee: Fuji Jukogyo KK; Fuji Heavy Industries Ltd
Current assignee: Subaru Corp
Priority date: 1992-09-21
Filing date: 1993-09-20
Publication date: 1996-08-21
Anticipated expiration: 2013-09-20
Also published as: GB9606885D0; GB2298003B

Abstract

A circuit for stopping an engine having a magneto 22 for generating a high voltage current to an ignition plug 24 via an ignition coil 23 and an ignition circuit 21 connected to the coil 23 for inducing the high voltage current by a transistor TR1 comprises a trigger means 25b for generating a trigger stop signal, a thyristor SCR1 responsive to the trigger signal, a holding capacitor C1 for automatically retriggering the thyristor after a predetermined time delay, a resistor R4 directly connected to the thyristor for short circuiting the ignition circuit 21 when the thyristor is triggered to stop the engine and a zener diode ZD connected to the thyristor in parallel with the resistor for preventing an occurrence of a peak voltage across the resistor which would inhibit the stopping of the engine. The engine may be stopped in response to a low oil level.

Description

DESCRIPTION STOP SWITCH CIRCUIT FOR AN ENGINE This invention relates to a stop switch circuit for an engine. More particularly, the invention concerns a stop switch circuit for an engine in which a source circuit and magneto are earthed to forcibly stop the engine.

This application is divided out of application 9319384.5, in which an engine stop switch is integrated with an engine stop switch circuit.

Heretofore, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 18668/1991 or Japanese Utility Model Laid-Open Publication No.

30381/1982, in most stop switch apparatuses, when a stop switch is closed a thyristor for stopping the engine of an engine stop switch circuit is turned on, charging a holding capacitor. Then, a turn-on voltage is applied to a gate of the thyristor for stopping the engine by a discharge voltage from the holding capacitor with a certain time constant. Even if the engine stop switch is returned to an off state, cutoff of a primary side of an ignition coil is prevented by turning on the thyristor for stopping the engine, thereby causing the engine to misfire and stop.

A normally open switch having a metal contact is frequently used as the engine stop switch.

In the above-described prior art, the structures of the engine stop circuit and the engine stop switch are not clearly disclosed, but the engine stop circuit and the engine stop switch are generally separate.

Accordingly, the engine stop switch apparatus is bulky, and assembly is complicated due to the necessity of wiring the engine stop circuit to the engine stop switch. In addition, the number of components is increased, and hence there is a problem with the reliability of the product.

Since a metal contact is used in the switch, contact defects due to rust occur readily, and there is a problem with durability where the switch is used out of a housing for a long period of time.

Heretofore, in stopping general-purpose engine having a magneto as a power supply, a source circuit voltage applied to a primary side of an ignition coil has been short-circuited by a thyristor circuit to eliminate induction of an insulating breakdown voltage at a secondary side of the ignition coil, thereby forcibly stopping the engine.

In general, a thyristor in the thyristor circuit is turned on by a signal from an engine stop switch or a control circuit for outputting an engine stop signal if an oil level becomes lower than a reference level, and a short-circuit current from a magneto power supply is supplied to the thyristor through a shortcircuiting resistor connected in series with the thyristor. For example, prior art is disclosed in Japanese Patent Laid-Open Publication No. 18668/1991 filed by the same assignee as this patent application.

However, when the thyristor is turned on to short-circuit the magneto power supply, the relatively large peak voltage generated across the shortcircuiting resistor is amplified by the ignition coil due to irregularity in the voltage across the shortcircuiting resistor caused by irregularity in the magneto and further superposition of high frequency components in the case of the short-circuit on the voltage across the short-circuiting resistor, and applied to the ignition plug. Then, there is a risk of inductive discharge due to the irregularity in the ignition system as well.

Therefore, even when the thyristor is turned on to short-circuit the magneto power supply, the engine is not easily stopped, and afterburn occurs, thereby reducing the durability of the engine.

An object of this invention is to provide a stop switch apparatus for an engine which eliminates the influence of irregularity in output voltages of individual magnetos when a source circuit voltage supplied by a magneto to an ignition system is short circuited by a thyristor, and a high frequency vibration when a source circuit is short-circuited, and obtains excellent engine stopping performance.

In accordance with the present invention, there is provided an engine stop switch circuit for an engine having a magneto for generating a high voltage current to an ignition plug via an ignition coil and an ignition circuit connected to said ignition coil for inducing said high voltage current by a power transistor, comprising a trigger circuit responsive to an engine stop detection signal for generating a trigger signal for stopping said engine; a first thyristor responsive to said trigger signal for stopping said engine and for producing a stop signal; a capacitor connected to said trigger circuit for delaying the stopping of said engine; a holding capacitor connected to said trigger circuit for automatically retriggering said first thyristor after a predetermined time delay;; a resistor directly connected to said first thyristor for short-circuiting said ignition circuit when said first thyristor is triggered to stop said engine; and a zener diode connected to said first thyristor in parallel with said resistor and provided to supply a reverse current in order to eliminate an occurrence of peak voltage across said resistor so as to avoid malfunction of stopping said engine and generation of afterburn.

In this apparatus, according to the invention, the thyristor is turned on to stop the engine, shortcircuiting the source circuit voltage, supplied by the magneto to the ignition system, and causing a shortcircuit current to pass through the short-circuiting resistor. Then, if the voltage across the shortcircuiting resistor becomes a predetermined voltage or higher due to the irregularity in the output voltage of the magneto and a high frequency vibration in the case of short-circuiting, a portion of the shortcircuiting current is fed as a reversal current through the zener diode.

A specific embodiment of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a sectional view taken along the line I-I of Fig. 3 illustrating an example of a stop switch apparatus for an engine, but not in accordance with the present invention; Fig. 2 is an exploded perspective view of the apparatus of Fig. 1 in partial section; Fig. 3 is a plan view of the apparatus; Fig. 4 is a left side view of the apparatus shown in Fig. 3; Fig. 5 is a circuit diagram of an example of engine stop switch circuit and an ignition circuit but not in accordance with the present invention; Fig. 6 is a circuit diagram of an embodiment of engine stop switch circuit and an ignition circuit in accordance with the invention; ; Fig. 7 is a waveform diagram showing the variation over time of a source circuit voltage when a thyristor for stopping the engine is turned off; and Fig. 8 is a waveform diagram showing the variation over time of a source circuit voltage when the thyristor for stopping the engine is turned on.

Figs. 1 to 5 show an example of a stop switch apparatus for an engine, but not in accordance with the present invention.

In Figs. 1 to 5, character A indicates a stop switch apparatus for an engine, numeral 1 designates an outside case of the stop switch apparatus A with an open base la, a switch window lb formed at an upper surface, and a wiring window Ic formed at one side.

An inside case 2 is mounted in the outside case 1. The inside case 2 is formed of rubber in a bag shape. A switch presser 2a, to be exposed through the switch window lb of the outside case 1, is formed at its upper surface, and a cable holder 2b to be mounted in the wiring window lc is integrally formed with one side.

A conductive rubber member container 2c of a square shape is formed substantially at the centre of the inner surface of the switch presser 2a of the inside case 2. Further, a reversal spring container 2d of a circular shape in contact with the conductive rubber member container 2c is formed at the centre of the conductive rubber member container 2c.

Positioning bosses 2e project at both sides of the conductive rubber member container 2c. In addition, there is a guide hole 2f through the cable holder 2b.

A reversal spring 3 serving as a reversal member is contained in the reversal spring container 2d formed on the inner surface of the inside case 2.

Further, a pressure sensitive conductive rubber member 4 serving as a pressure sensitive member is contained in the conductive rubber member container 2c. The pressure sensitive conductive rubber member 4 is in a so-called insulating state having a large resistance in a normal state in which a predetermined force is not applied and being in a conductive state when a predetermined force is applied at the centre thereof.

The outer periphery of the reversal spring 3 is mounted on the pressure sensitive conductive rubber member 4, and the centre thereof is formed in a shape to be deformed into a convex shape facing the top of the reversal spring container 2d. When a force more than a set value is exerted at the centre of the reversal spring 4, it is elastically deformed into a reversal state.

A substrate 6 is mounted on the inner surface of the inside case 2 through an insulating sheet 5.

Positioning holes 5a and 6a are respectively provided in the insulting sheet 5 and the substrate 6. In use, the positioning holes 5a and 6a respectively receive the positioning bosses 2e to be positioned. The outer periphery of the substrate 6 is airtightly mounted on the inner periphery of the inside case 2.

The engine stop switch 7 comprises a switch electrode 7a printed in a pectinated state of the surface of the substrate 6 opposed to the pressure sensitive conductive rubber member 4. Further, a window Sb for forming an insulating space is opened in the insulating sheet 5, between the switch electrode 7a and the pressure sensitive conductive rubber member 4. The window 5b is smaller than the pressure sensitive conductive rubber member 4 and hence the depth of the insulating space between the switch 7a and the pressure sensitive conductive rubber members is the same as the thickness of the insulating sheet 5, ie., 200 ym in the example shown in the drawings.

Electronic components such as capacitors and diodes for constituting an engine stop switch circuit 10 are mounted on the opposite surface of the substrate 6 to the switch electrode 7a, and base ends of two connecting cables 8a and 8b extending from the guide holes 2f through the cable holder 2b of the inside case 2 are connected thereto.

Further, a curable insulating base plate 9 made of epoxy resin or the like is filled between the substrate 6 and the bottom la of the outside case 1.

The substrate 6, the insulating sheet 5 and the inside case 2 are fixed in a close contact state in the outside case by filling the curable insulating base plate 9 therein. The outer periphery of the inside case 2 is in close contact with the inner surface of the outside case 1, and further the pressure sensitive conductive rubber member 4 and the reversal spring 3 are interposed and held between the switch presser 2a of the inside case and the insulating sheet 5.

As shown in Fig. 5, a cathode side of a thyristor SCR for stopping the engine of the engine stop switch circuit 10 is connected to an anode side of a diode D1, connected to an ignition source circuit VC supplied by a magneto (not shown) through a resistor R4, and an anode side of the thyristor SCR for stopping the engine is further connected to an earth or ground G.

The other end of a holding capacitor C1 connected at its one end to the ignition source circuit VC is connected to the cathode side of the diode D1.

Similarly, the other end of a capacitor C2 connected at one end to the ignition source circuit VC is connected to the cathode side of the diode D1 through a resistor R5, and connected to the gate of the thyristor SCR for stopping the engine.

The other end of the switch electrode 7a of the engine stop switch unit 7 connected at one end to the ground G is connected to a connecting point P of the resistor R4, the anode side of the diode D1 and the cathode side of the thyristor SCR for stopping the engine.

An ignition circuit 16 is connected to the engine stop switch circuit 10. The emitter sides of transistors TR1 and TR2 of the ignition circuit 16 are connected to the ignition source circuit VC. The collector side of the power transistor TR1 is connected to the ground G, and further the collector side of the transistor TR2 is connected to the ground G through the resistor R1.

The base of the power transistor TR1 is connected to the collector side of the transistor TR2, and further the base of the transistor TR2 branches from a point between resistors R2 and R3 which are connected in series between the ignition source circuit VC and the ground G.

Numeral 17 denotes an ignition coil. One end of a primary side of the ignition coil 17 is connected to the ignition source circuit VC, and the other end thereof is connected to the ground G. An ignition plug 18 is connected to a secondary side of the ignition coil 17.

The operation of the arrangement described above will now be described.

While the engine is operating, an ac output synchronized with the revolution of the engine is normally applied by the ignition source circuit VC to the ignition circuit 16, the engine stop switch circuit 10 and the primary side of the ignition coil 17.

The thyristor SCR for stopping the engine is maintained in an off state in the engine stop switch circuit 10 as long as the engine stop switch unit 7 is not once turned on.

On the other hand, when the voltage of the source circuit VC of the ignition circuit 16 becomes negative, a bias voltage is applied to the base of the transistor TR2 via the resistors R1 and R2, the base voltage of the transistor TR2 is hence raised and the transistor TR2 is turned on. Then, the power transistor TR1 is turned on, the primary side of the ignition coil 17 is short-circuited, and hence a high voltage is generated at the primary side of the ignition coil 17.

As a result, a high voltage of an insulating breakdown voltage or higher between the electrodes of the ignition plug 18 is induced at the secondary side of the ignition coil 17 to be sparked, and further energy necessary for discharging is provided for a predetermined period of time by means of an attenuating vibration upon cutting-off of the primary side of the ignition coil 17.

To stop the engine, the centre of the switch presser 2a of the inside case 1 exposed through the switch window lb of the outside case of the stop switch apparatus A for the engine is pressed. The reversal spring 3 contained in the reversal spring container 2c formed on the inner surface of the switch presser 2a is thereby reversed, pressing the pressure sensitive conductive rubber member 4.

As a result, the pressure sensitive conductive rubber member 4 is pressure-contacted with the switch electrode 7a formed on the substrate 6, shortcircuiting the switch electrode 7a, thereby turning on the engine stop switch unit 7.

Since the stroke of the pressure sensitive conductive rubber member 4 corresponds merely to a thickness of the insulating sheet 5, there is little fatigue of the pressure sensitive conductive rubber member 4 even if the engine stop switch is turned on and off repeatedly. Further, even if the space between the switch electrode 7a and the pressure sensitive conductive rubber member 4 expands and contracts due to temperature change, its volumetric change is small and hence it does not adversely affect the pressure sensitive conductive rubber member 4.

Excellent durability is thus obtained.

Further, since the pressure sensitive conductive rubber member 4 becomes conductive due to a reduction in its resistance value when a predetermined force is applied thereto, the engine stop switch circuit 10 is not, even if the pressure sensitive conductive rubber member 4 is, for example, erroneously contacted with the switch electrode 7a due to aging change, erroneously operated in a normal state in which a predetermined force is not applied thereto. Further, a metal contact is eliminated by the use of the pressure sensitive conductive rubber member 4, and hence contact defects due to rust rarely occur, thereby further improving durability and reliability.

In addition, since the surface of the side to be pressed of the pressure sensitive conductive rubber member 4 is protected by the reversal spring 3, the engine stop switch unit 7 is not, even if a slight external force is applied thereto, erroneously operated, resulting in excellent durability and high reliability. Further, since the pressure sensitive conductive rubber member 4 is merely interposed to be held between the reversal spring 3 and the insulating sheet 5, there is no need for an adhesive bond and assembly is simplified.

Since the engine stop switch circuit 10 and the engine stop switch 7 are integrally assembled in the stop switch apparatus A for the engine, assembly and handling thereof are facilitated. Further, since the switch electrode 7a is formed by printing on the substrate 6, wiring is eliminated. Hence, not only can the number of components be reduced, but also the entire apparatus can be produced compactly.

Further, since the outer surface of the inside case 2 is in close contact with the inner surface of the outside case 1, water droplets, for example, are scarcely introduced from the switch window lb of the outside case 1 into the stop switch apparatus, thereby obtaining excellent waterproofness and dustproofness.

Since the outer periphery of the substrate 6 is mounted airtightly on the inside case 2 and yet the engine stop switch unit 7 is mounted at the centre of the substrate 6, ingress of water droplets can be completely prevented and hence the stop switch apparatus can be used out of a housing for a long period of time.

When the engine stop switch unit 7 is tuned on at the time of stopping the engine, a charging current IC flows from the ground G side to the holding capacitor C1 through the diode D1 thereby charging the holding capacitor C1.

When the ignition source circuit VC then becomes negative, a turn-on current is supplied to the gate of the thyristor SCR for stopping the engine with a time constant determined by C1 and R5. Then, a charging current flows from the thyristor SCR for stopping the engine to the holding capacitor C1.

Therefore, if the engine stop switch unit is once turned on, the thyristor SCR for stopping the engine is turned on and off repeatedly irrespective of the on/off state of the engine stop switch unit 7.

When the thyristor SCR for stopping the engine is turned on, a forward bias current flows when the ignition source circuit voltage is negative, preventing cut-off of the primary side of the ignition coil 17, thereby causing the engine to misfire and stop.

As described above, according to this invention, the switch electrode of the engine stop switch unit and the engine stop switch circuit are integrally provided on the substrate. Therefore, wiring is eliminated, the engine apparatus can be simplified and decreased in size and hence the number of components and the number of assembly steps can be reduced.

Further, since the switch contact opposed to the switch electrode of the engine stop switch unit is formed as a pressure sensitive member, rust is not generated, thereby avoiding contact defects and durability and reliability are improved.

Moreover, since the reversal member for setting a contact force to the switch electrode of the pressure sensitive member is interposed between the switch contact and the switch presser, erroneous operation is eliminated, and reliability of the product can be further improved.

Figs. 6 to 8 show an embodiment of this invention.

In Fig. 6, numeral 21 designates an ignition circuit of known transistor type. A source circuit voltage VC is supplied from a magneto 22 to the ignition circuit 21.

A primary side of an ignition coil 23 is connected to the ignition circuit 21 between an output end of the magneto 22 and a ground G, and an ignition plug 24 is connected to a secondary side of the ignition coil 23.

The ignition circuit 21 comprises a power transistor TR1 and a transistor TR2 as a front stage for the power transistor TR1 as main components.

Emitters of the power transistor TR1 and the transistor TR2 are connected to the source circuit VC.

A collector of the power transistor TR1 is connected to the ground G, and a collector of the transistor TR2 is connected to a base of the power transistor TR1, and further connected to the ground G through a resistor R1. A base of the transistor TR2 is connected between resistors R2 and R3 which are connected in series between the source circuit VC and the ground G.

An engine stop switch circuit 25 is connected to the ignition circuit 21. The engine stop switch circuit 25 has a source short circuit 25a and a trigger unit 25b for operating the source short circuit 25a.

The source short circuit 25a comprises a thyristor SCR1 for stopping the engine to short circuit the source circuit VC from the magneto 22, and a holding capacitor C1 for automatically retriggering the thyristor SCR1 for stopping the engine. The trigger unit 25b is connected, for example, to a controller (not shown) for outputting an engine stop detection signal if an oil level of the engine becomes lower than a reference level, and comprises a trigger thyristor SCR2 for firing the thyristor SCR1 for stopping the engine.

The thyristor SCR1 for stopping the engine is connected at an anode therefore to the ground G and at a cathode thereof to one end of a short-circuiting resistor R4 and a cathode of a zener diode ZD. The short-circuiting resistor R4 and the zener diode ZD are connected in parallel with each other, and the other end of the short-circuiting resistor R4 and a anode of the zener diode ZD are connected to the source circuit VC.

The gate of the thyristor SCR1 for stopping the engine is connected to one end of a capacitor C2, the other end thereof being connected to the source circuit VC, and further connected to the cathode of the diode D1 through a resistor R5. The other end of the holding capacitor C1 connected at one end to the source circuit VC is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the cathode of the thyristor SCR1 for stopping the engine.

Further, the gate of the thyristor SCR1 for stopping the engine is connected to the anode of the triggering thyristor SCR2 through a resistor R6, the gate of the thyristor SCR2 is connected to a signal input terminal SIN through a resistor R7, and the cathode of the thyristor SCR2 is connected to the ground G.

When an engine stop signal of a high level is input from a control circuit (not shown) to the signal input terminal SIN, the thyristor SCR2 is turned on, turning on the thyristor SCR1 for stopping the engine, and the source circuit VC is short-circuited form the magneto 22, thereby forcibly stopping the engine.

The operation of the embodiment described above will now be described.

An AC output synchronized with the revolution of the engine is normally supplied from the magneto 22 to the source circuit VC during operation of the engine, and applied to the ignition circuit 21, the primary side of the ignition coil 23 and the engine stop switch circuit 25.

When an engine stop signal of a high level is not input from the signal input terminal SIN, the thyristor SCR1 for stopping the engine of the engine stop switch circuit 25 is not turned off by the thyristor SCR2. When the AC output from the magneto 22 becomes negative in this state, a bias voltage is applied to the base of the transistor TR2 by the resistors R1 and R2 of the ignition circuit 21.

Then, when the base voltage of the transistor TR2 reaches a level for turning on the transistor TR2, for example, at a predetermined timing such as BTDC 200, the transistor TR2 is turned on. Thus, the power transistor TR1 is turned on to short-circuit a primary current of the ignition coil 23. The variations of the voltage across the primary side of the ignition coil 23 is shown in Fig. 7. A voltage of about 400 V is, for example, generated at the negative side of the source circuit VC by self-induction at the primary side of the ignition coil 23.

As a result, a high voltage of an insulating breakdown voltage or higher between electrodes of the ignition plug 24 is induced at the secondary side of the ignition coil 23 to spark the ignition plug 24.

Further, an attenuating vibration causing energy necessary for discharging to be applied for a predetermined period of time (e.g., about 2msec.) is generated at the primary side of the ignition coil 23.

In this case, when the engine stop signal of a high level is input to the signal input terminal SIN of the engine stop switch circuit 25, the thyristor SCR2 is forwardly biased between the gate thereof and the cathode thereof, and the thyristor SCR2 is turned on.

If the polarity of the source circuit VC is positive, a forward current flows to the thyristor SCR2 through the short-circuiting resistor R4, the diode D1 and the resistors R5 and R6. Then, when the polarity of the source circuit VC is inverted to negative, the thyristor SCR2 is reverse biased between the anode thereof and the cathode thereof, and shifted to be turned off.

In this case, a recovery current flows to the thyristor SCR2, charging the capacitor C2 and the holding capacitor Cl, and to the thyristor SCR1 for stopping the engine. As a result, when the gate voltage of the thyristor SCR1 for stopping the engine is raised to reach the turn-on voltage, the thyristor SCR1 for stopping the engine is turned on. The recovery current is a current for recovering the thyristor SCR2 to a reverse blocking state. Thus, the thyristor SCR2 is chosen to have a relatively long recovery time to effectively utilize the recovery current.

When the thyristor SCR1 for stopping the engine is turned on, short-circuiting the negative side of the source circuit VC, a short circuit current flown through the short-circuiting resistor R4 and charge flows to and is stored in the holding capacitor C1 through the diode D1 due to voltage generated across the short-circuiting resistor R4. Then, the holding capacitor C1 is soon discharged to the gate of the thyristor SCR1 for stopping the engine at a rate determined by the time constant R5.C1. The time constant R5.C1 is so set that the gate voltage of the thyristor SCR1 for stopping the engine rapidly reaches the turn-on voltage before the turn-on of the transistor TR2 (and so at power transistor TR1) of the ignition circuit 21.Thus, when the thyristor SCR1 is once turned on, charge stored in the holding capacitor C1 is circulated to the gate of the thyristor SCR1 for stopping the engine through the resistor R5, and the thyristor SCR1 for stopping the engine is turned on and off repeatedly irrespective of the on/off state of the thyristor SCR2 (i.e., irrespective of any engine stop signal), thereby short-circuiting the source circuit VC.

In the case where the source circuit VC is shortcircuited, when the voltage VR across the shortcircuiting resistor R4 reaches a predetermined voltage or higher due to the irregularity in the output voltages of the magnetos 22 and the high frequency vibration when the source circuit VC is shortcircuited, a portion of the short-circuiting current flows as a reversal current through the zener diode ZD connected in parallel with the short-circuiting resistor R4. As shown in Fig. 8, a voltage VR across the short-circuiting resistor R4, i.e., a voltage of the negative side of the source circuit VC is maintained substantially constant.

More specifically, the irregularity in the output of the magnetos 22 and the high frequency vibration are removed by the zener diode ZD connected in parallel with the short-circuiting resistor R4, a peak voltage to generate an inductive discharge at the ignition plug 24 through the ignition coil 23 is removed from the voltage VR across the shortcircuiting resistor R4, thereby preventing malfunction in stopping the engine and generation of afterburn.

When ignition energy necessary to discharge the ignition plug 24 is removed from the primary side of the ignition coil 23 by the short-circuit of the source circuit VC, the ignition plug 24 is misfired and the rotation of the engine is gradually decelerated to stop the engine. The thyristor SCR1 for stopping the engine is completely turned off when the charge of the holding capacitor C1 is dissipated and the final discharge is finished, and the engine is once more enabled to start.

The gate voltage of the thyristor SCR1 for stopping the engine is, due to the delay time determined by the time constant R5.C2 caused by the capacitor C2, suitably set at the resistor R5 and the capacitor C1 by considering the delay time of the turn-on. The resistor R6 is suitably set by considering the charging times of the holding capacitor C1 and the capacitor C2 by the recovery current of the thyristor SCR2.

In the embodiment described above, the engine stop switch circuit 25 has source circuit 25a and trigger unit 25b. However, this invention is not limited to the particular embodiments. For example, the trigger unit 25b may be omitted, a normally open contact switch being connected between a cathode and a ground of the thyristor SCR1 for stopping the engine, and the switch may be closed only for a period of time in which the thyristor SCR1 for stopping the engine is automatically turned on by the holding capacitor C1, thereby stopping the engine.

According to the embodiment described above, when the thyristor is turned on, to stop the engine, shortcircuiting the source circuit to be supplied from the magneto to the ignition system, a short-circuiting current flows through the short-circuiting resistor.

Then, since a portion of the short-circuiting current flows as a reversal current through the zener diode if the voltage across the short-circuiting resistor becomes a predetermined voltage or higher due to the irregularity in the output voltages of the magneto and the high frequency vibration in the case of shortcircuiting, a peak voltage capable of generating discharge through the ignition system does not occur across the short-circuiting resistor. Therefore, malfunction in stopping the engine and generation of afterburn are prevented, providing excellent engine stopping performance.

While the preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention.

Claims

1. An engine stop switch circuit for an engine having a magneto for generating a high voltage current to an ignition plug via an ignition coil and an ignition circuit connected to said ignition coil for inducing said high voltage current by a power transistor, comprising a trigger circuit responsive to an engine stop detection signal for generating a trigger signal for stopping said engine; a first thyristor responsive to said trigger signal for stopping said engine and for producing a stop signal; a capacitor connected to said trigger circuit for delaying the stopping of said engine; a holding capacitor connected to said trigger circuit for automatically retriggering said first thyristor after a predetermined time delay; a resistor directly connected to said first thyristor for short-circuiting said ignition circuit when said first thyristor is triggered to stop said engine; and a zener diode connected to said first thyristor in parallel with said resistor and provided to supply a reverse current in order to eliminate an occurrence of peak voltage across said resistor so as to avoid malfunction of stopping said engine and generation of afterburn.

2. An engine stop switch circuit as claimed in claim 1, further comprising: a second thyristor in said trigger circuit and provided to trigger said first thyristor when said stop signal is received.

3. An engine stop switch circuit for an engine, substantially as herein described, with reference to, and as illustrated in, Figs. 6 to 8 of the accompanying drawings.

4. An engine comprising a circuit as claimed in any of the preceding claims.

5. A vehicle comprising an engine as claimed in claim 4.