GB2163213A

GB2163213A - Ignition systems for internal combustion engines

Info

Publication number: GB2163213A
Application number: GB08516280A
Authority: GB
Inventors: Adolf Fritz
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1984-06-29
Filing date: 1985-06-27
Publication date: 1986-02-19
Also published as: JPS6119970A; JPH0579826B2; GB2163213B; DE3423949A1; GB8516280D0

Abstract

The primary winding (5) of the ignition coil (6) has a shunt circuit including a thyristor (21), and the primary winding (5) is, before each ignition point, supplied during transitory periods of time with current from a DC source (1) through an output transistor (7). At the end of each such time period, the current induced in the primary winding (5) can flow through the thyristor (21). Thus, while storing the ignition energy, the thyristor (21) is forward biased during the off state of the output transistor (7) and reverse biased during the on state of the output transistor (7). A measuring resistor (8) is in series with the primary winding (5) and the output transistor (7) and is used to switch the output transistor (7) on and off while the ignition energy is stored. At the ignition instant, a control unit 14 switches on a transistor (20) thereby grounding the gate of the thyristor (21) and preventing it from switching on so that when the transistor 7 is switched off, a spark is generated. <IMAGE>

Description

SPECIFICATION Ignition systems for internal combustion engines The ignition relates to ignition systems and more particularly to an ignition system in which, between ignition pulses, energy is stored in an ignition coil and such storage is maintained by switching on and off a power stage in series with the primary winding of the ignition coil, the current flowing in the primary winding when the output stage is switched off, being allowed to flow through a shunt circuit containing a thyristor which is fired when the output transistor is blocked.

An ignition system pointing in this direction is known (according to DE-PS 25 31 278 or the corresponding U.S.A. Patent No. 4 105 006), in which, however, triggering the thyristor in shunt with primary winding calls for relatively complicated circuitry.

The invention resides in an ignition system for an internal combustion engine comprising an ignition coil; a power transistor whose emitter-collector path is in series with the primary winding of the ignition coil; a shunt circuit connected to said primary winding, said shunt circuit containing a thyristor; a measuring resistor in series with the primary winding and the output transistor for switching the output transistor on and off during storing of the ignition energy, and a control circuit which contains at least a control resistor connected to the firing electrode of the thyristor, whose anode is connected to said primary winding, whereby the said primary winding, before each ignition point, is supplied through the output transistor with current from a DC source only during transitory time periods and whereby the current induced in the primary winding at the end of each such time period flows through the thyristor in the shunt circuit, and, while the ignition energy is stored, the thyristor in the off state of the output transistor is conductive and in the on state of the output transistor is non-conductive.

The invention is further described, by way of examples, with reference to the accompanying drawing, which is a circuit diagram of one embodiment of ignition system, constructed according to the invention.

The ignition system shown in the drawing is intended for the internal combustion engine (not shown) of a motor vehicle (also not shown). The system is fed from DC source 1, which may be the battery of the motor vehicle. A lead 2 is connected to the negative terminal of the DC source 1 and to earth and a supply lead 4 is connected to the positive terminal of the source 1 and contains an operating switch (ignition switch) 3. A branch circuit is connected to the supply lead 4 and comprises the primary winding 5 of an ignition coil 6, the collector-emitter path of a (npn) a final stage or output transistor 7, and a measuring resistor 8 connected to the earth lead 2. The secondary winding 9 of the ignition coil 6 is connected through the spark plug 10 to earth line 2.The base of the transistor 7 is connected to the emitter of a (npn) driver transistor 11, the collector of which is connected through resistor 1 2 to the supply lead 4. The base of the driver transistor 11 is connected to the output of an inverter stage 13, the input of which is connected to the output of a control unit 14.

The control unit 14, which may be a microcomputer, ensures that, for each ignition process, approximately the same amount of energy is available. The input to the control unit 14 is connected to the earth lead 2 through a signal transducer 15, which operates in the manner of a small AC generator and would be coupled to the crankshaft or camshaft of the internal combustion engine.

A lead is connected between the junction between output transistor 7 and the measuring resistor 8 and to the input of a comparator 1 6. The other input of the comparator 1 6 is connected through a regulated voltage source 1 7 to earth lead 2. The output of the comparator 1 6 is connected to the input of monostable flip-flop 18, the output of which is connected to the base of the driver transistor 11.

A lead containing a current-limiting resistor 1 9 is connected from the junction between control unit 1 4 and inverter stage 1 3 to the base of a further transistor 20 (npn-type), whose emitter is connected to earth lead 2 and whose collector is connected to the firing electrode of a thyristor 21. The thyristor 21 is arranged in a shunt circuit 22 of the primary winding 5 of the ignition coil 6 in such a way that it conducts the induction current of the primary winding 5. At the same time a control branch 23 containing at least a resistor 24 is connected to the firing electrode of thyristor 21 whose anode is connected by a lead 22 to the positive end of the primary winding 5.It is recommended to place a zener diode 25 in series with resistor 24 in such a way that this zener diode 25 is loaded by the induced current of the primary winding 5 in reverse biased direction. It is further recommended to add a parallel branch to resistor 24, which is at least partly capactive by virtue of a capacitor 27 and which, if necessary, contains a resistor 26.

The ignition system described above operates as follows: As soon as the operating switch 3 is closed, the system is ready for operation. It is assumed that the signal transducer 1 5 has influenced the control unit 14 in such a way that, at the output of the control unit 14 a signal, having at least the potential of the earth lead 2, is present. This causes the emitter-collector path of transistor 20 to be non-conductive and the emitter-collector paths of transistors 11, 7 to be conductive, whereupon the current in the switched on primary winding 5 rises and simultaneously the voltage drop across the measuring resistor 8 increases.

When the amount of current is sufficient to provide an effective ignition spark, the voltage drop at resistor 8 is such that the comparator 1 6 triggers the monostable flip-flop 18 to its unstable state. In the unstable state of flip-flop 18, the emitter-collector paths of transistors 11, 7 become blocked. By interrupting the current in the primary winding 5, an induced voltage acts at the anode of thyristor 21 and an induced current is conducted by branch circuit 23 to the firing electrode of thyristor 21, so that the anode-cathode path becomes forward biased, thus the energy stored in ignition coil 6 is slowly dissipated with the induced current flowing through the anodecathode path. This decay is determined in such a way that sufficient energy to provide an ignition spark remains in ignition coil 6, until the flip-flop 8 returns to its stable state.

When this is the case, the emitter-collector paths of transistors 11, 7 again become conductive, so that the current supply to the primary winding 5 is restarted by the output transistor 7 and the shunt 22 by the thyristor 21 becomes ineffective. This current supply is maintained again until the voltage drop at the measuring resistor 8 through comparator 1 6 again causes the flip-flop 18 to go unstable and thus the above operating sequence is repeated.

At the point (instant) of ignition, a signal is made available at the output of control unit 14, which at least has the potential of the supply lead 4. This causes the emitter-collector path of transistor 20 to become conductive and the firing electrode of the thyristor 21 to be connected to the earth lead 2, whereupon when transistors 11, 7 become nonconducting, thyristor 21 cannot be fired. The blocked state of the emitter-collector path of output transistor 7 rather induces a high tension peak in the secondary winding 9, which causes an electric discharge (ignition spark) in the spark plug 10.

It is recommended to use an easily fired" type of thyristor 21, that is, one which is highly sensitive to control.

Resistor 24 is required as a current limiter.

The use of the zener diode 25 is recommended because, without its use, interfeence could lead to undesired firing of thyristor 21.

The use of capacitor 27 makes it possible to fire thyristor 21 faster in relation to the induced current. It also delays the voltage rise at the thyristor anode.

Claims

1. An ignition system for an internal combustion engine comprising an ignition coil; a power transistor whose emitter-collector path is in series with the primary winding of the ignition coil; a shunt circuit connected to said primary winding, said shunt circuit containing a thyristor; a measuring resistor in series with the primary winding and the output transistor for switching the output transistor on and off during storing of the ignition energy, and a control circuit which contains at least a control resistor connected to the firing electrode of the thyristor, whose anode is connected to said primary winding, whereby the said primary winding, before each ignition point, is supplied through the output transistor with current from a DC source only during transitory time periods and whereby the current induced in the primary winding at the end of each such time period flows through the thyristor in the shunt circuit, and, while the ignition energy is stored, the thyristor in the off state of the output transistor is conductive and in the on state of the output transistor is non-conductive.

2. An ignition system according to claim 1, in which the control resistor within the control circuit is in series circuit with a zener diode, which is loaded by the induced current of the primary winding in a reverse-biased direction.

3. An ignition system according to claim 1 or 2, in which the control resistor is provided with a parallel branch which is at least partially capactive.

4. An ignition system for an internal combustion engine, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.