GB2060771A

GB2060771A - Ignition system for internal combustion engines

Info

Publication number: GB2060771A
Application number: GB8031732A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-10-03
Filing date: 1980-10-02
Publication date: 1981-05-07
Also published as: JPS6345506B2; DE2940036C2; DE2940036A1; GB2060771B; JPS5656966A

Abstract

The ignition system comprises a series arrangement constituted by the primary winding (5) of an ignition coil (6) and a switching transistor (7), and an overvoltage protection circuit (12) which includes a resistor (13) and a switching element (14) which has a predetermined breakdown voltage. In the event of a fault such as a decrease in load in the secondary current circuit, the switching element (14) breaks down to prevent excessive voltage in the secondary circuit. <IMAGE>

Description

SPECIFICATION Ignition system for internal combustion engines The invention relates to ignition systems for internal combustion engines.

Ignition system for internal combustion engines are known which comprise a series circuit which is constituted by the primary winding of an ignition coil and the switching path of a switching transistor, and to which current is supplied via the primary winding, an overvoltage protection circuit, which starts at the junction between the primary winding and the switching path and leads via a first voltage-dividing resistor, and then via a switching element, having a predetermined breakdown voltage, to the control electrode of the switching transistor, and a resistance circuit leading from the junction between the first voltage-dividing resistor and the switching element via a second voltage-dividing resistor, the resistance circuit being permanently connected to the end of the switching path remote from the primary winding.

An ignition system of the latter type is already known from German Auslegeschrift No. 2339896, in which the breakdown value of the switching element provided in the overvoltage protection circuit is so predetermined that a maximum permissible peak voltage across the primary winding forms the response criterion of this protection circuit. It has now been shown, however, that such limitation of the primary voltage is not in every case sufficient, for example, if there is a fault on the secondary side of the ignition coil, which may, in the simplest instance, be a break in one of the connecting leads forming part of the secondary current circuit of the ignition coil. In that event, owing to the small capacitive load in the secondary current circuit, an increased secondary voltage is produced, which may cause damage to the ignition coil, the distributor and the associated wiring harness.

It is an object of the present invention to provide an ignition system in which this problem is obviated.

In accordance with the present invention there is provided an ignition system for an internal combustion engine, comprising a series circuit which is constituted by the primary winding of an ignition coil and the switching path of a switching transistor, and to which current is supplied via the primary winding, an overvoltage protection circuit, which starts at the junction between the primary winding and the switching path and leads via a first voltage-dividing resistor, and then via a switching element, having a predetermined breakdown voltage, to the control electrode of the switching transistor, and a resistance circuit leading from the junction between the first voltage-dividing resistor and the switching element via a second voltage-dividing resistor, the resistance circuit being permanently connected to the end of the switching path remote from the primary winding, wherein if the load in the secondary current circuit of the ignition coil decreases, the voltage drop across the resistance circuit is arranged to increase, and at the same time, evaluation of the current leaving the switching path of the switching transistor is effected.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of one embodiment of an ignition system in accordance with the present invention; and Fig. 2 is a circuit diagram of a modification of the system of Fig. 1.

The ignition system shown in Figure 1 is intended for an internal combustion engine (not shown) of a motor vehicle (also not shown). This ignition system is powered from a direct-current source 1, which may be the battery of the motor vehicle. To the negative terminal of the current source 1 there is connected an earth-connected lead 2, and to the positive terminal there is connected a supply line 4 which includes an operating switch 3. The supply line 4 is the point of origin of a circuit leading first of all via the primary winding 5 of an ignition coil 6, then via the switching path 8 formed by the emittercollector path of an npn transistor 7, and lastly via a measuring resistor 9, to the earth lead 2.One end of the secondary winding 10 of the ignition coil 6 is connected to the junction between the primary winding 5 and the switching path 8, and its other end is connected via a sparking plug 11 to the earth lead 2. The junction between the primary winding 5 and the switching path 8 is also the staring point of an overvoltage protection circuit 12, which leads first of all via a first voltage-dividing resistor 13 and then via a switching element 14, which has a predetermined breakdown voltage, to the gate electrode 1 5 formed by the base of the transistor 7.In the preferred embodiment, the switching element 14 is formed by a Zener diode 1 6. The junction between the resistor 1 3 and the switching element 14 is the starting point of a resistance circuit 1 7, which leads first via a second voltagedividing resistor 1 8 and then via a third voltagedividing resistor 1 9 to the earth lead 2. The junction between the switching path 8 and the measuring resistor 9 is connected to the base of the transistor 7 by way of a circuit which includes a metering resistor 20. Furthermore, from the junction between the switching path 8 and the measuring resistor 9 an evaluation circuit 21, which includes a metering resistor 22, leads to the first input 23 of an operational amplifier 24.The second input 25 of the operational amplifier 24 is connected via a resistor 26 and a constant-voltage source 27 to the earth lead 2. The output 28 of the operational amplifier 24 is connected to a point 29 of the control circuit located between the second voltage-dividing resistor 1 8 and the third voltage-dividing resistor 19, and also via a feedback circuit including a resistor 30 to the second input 25. In the present case, the first input 23 of the operational amplifier 24 is formed by the non inverting input, and the second input 25 is formed by the inverting input The operational amplifier 24 receives its supply voltage via leads 31, 32.

In the present case, for the sake of simplicity the ignition process is initiated by means of a mechanical contact breaker 33, which is controllable by means of a cam 34 which can be rotated by the internal combustion engine and which is connected between the earth lead 2 and the base of an npn transistor 35. The base of the transistor 35 is also connected via a metering resistor 36 to the supply line 4. The transistor 35 is furthermore connected via its emitter to the earth lead 2, and via its collector both via a blocking diode 37 to the gate electrode 1 5 of the transistor 7 and via a resistor 38 to the supply line 4.

The method of operation of the abovedescribed ignition system is as follows: When the operating switch 3 is closed, the system is operational. It is assumed that the contact breaker 33 is closed, that is, that it is in the current-conducting state. In this state the emitterbase path of the transistor 35 is bridged, so that the emitter-collector path of this transistor 35 is turned off. Consequently, control current is able to flow via the diode 37 to the control electrode of the transistor 7, so that the switching path 8 is turned on. Therefore, current can now flow through the primary winding 5, with the result that magnetic energy for the next ignition process is stored in the ignition coil 6.

When the mechanical contact breaker 33 is then opened at the ignition instant, the emittercollector path of the transmitter 35 is turned on, and consequently the switching path 8 is turned off. The current flow in the primary winding 5 is thereby interrupted, and in the secondary winding 10 a high-voltage surge is induced which results in an electrical spark (an ignition spark) across the sparking plug 11.

When the ignition system is operating under normal conditions, the included voltage across the ignition coil 6 on the initiation of the ignition process is limited by the overvoltage protection circuit 12 to a value at which no damage can occur either in the primary or the secondary ignition current circuit.

If, however, a connection taking the ignitionvoltage surge to the sparking plugs is interrupted, only a relatively smali load is active in the secondary current circuit of the ignition coil 6. If no precautions were taken, electrical spark discharges, generality resulting in damage due to burning of insulating components, could occur in the secondary circuit of the ignition coil. In the present case, however, before damage occurs, the increased current at the switching element 1 4 during the ignition process will produce such a voltage drop across the measuring resistor that, amplifed by the operational amplifier 24, this voltage drop causes such a potential rise at the point 29 of the control circuit that the resultant induced voltage produces a breakdown at the switching element 14 at lower voltage values than is normally the case.Consequently, the voltages then produced in the secondary current circuit of the ignition coil 6 can also be prevented from causing damage.

The ignition system shown in Figure 2 differs from that of Figure 1 in that the output 28 of the operational amplifier 24 is connected to the base of an npn transistor 39, whose emitter-collector path lies in a shunt circuit of the third voltage dividing resistor 1 9. Moreover, in this case the first input 23 of the operational amplifier 24 is formed by the inverting input, and the second input 25 is formed by the non-inverting input. The other circuit elements bearing the same reference numerals as in Figure 1 have the same method of operation.

The method of operation of the ignition system shown in Figure 2 differs as follows from that of Figure 1.

In this case, on the initiation of the ignition process when a fault exists in the secondary current circuit, a current will flow via the switching element 14, causing increased conductivity of the switching path 8, and consequently, such a substantial voltage drop across the measuring resistor 9 that the emitter-collector path of the transistor 39 is turned off by the operational amplifier 24. Both voltage-dividing resistors 1 8 and 19 then come into operation in the resistance circuit 17, with e resultant increase in the voltage drop across the resistance circuit 1 7.

Consequently, the induced voltage in the primary winding 5 is now limited to a lower value, and therefore, in this case also, no unduly high voltages can occur there in the event of a fault in the secondary current circuit.

In the embodiments described the ignition process is initiated by means of a mechanical switch. The invention of course applies also to ignition systems in which a contactless method of initiation of the ignition process is used, for example, by means of an inductive or optical transducer.

The measuring resistor 9 may, for example, also be used at the same time for limitation of the current flowing in the primary winding 5, by connection of the evaluation circuit 21 to the base of a limiting transistor (not shown) whose emittercollector path is connected in shunt with the series arrangement comprising the base-emitter path of the transistor 7 and the measuring resistor 9.

Claims

1. An ignition system for an internal combustion engine, comprising a series circuit which is constituted by the primary winding of an ignition coil and the switching path of a switching transistor, and to which current is supplied via the primary winding, an overvoltage protection circuit, which starts at the junction between the primary winding and the switching path and leads via a first voltage-dividing resistor, and then via a switching element, having a predetermined breakdown voltage, to the control electrode of the switching transistor, and a resistance circuit leading from the junction between the first voltage-dividing resistor and the switching element via a second voltage-dividing resistor, the resistance circuit being permanently connected to the end of the switching path remote from the primary winding, wherein if the load in the secondary current circuit of the ignition coil decreases, the voltage drop across the resistance circuit is arranged to increase, and at the same time, evaluation of the current leaving the switching path of the switching transistor is effected.

2. An ignition system according to claim 1, in which in current-flow sequence, the switching path is followed by a measuring resistor, and in which an evaluation circuit is connected to the junction between the switching path and the measuring resistor.

3. An ignition system according to claim 1, in which in the resistance circuit, in current-flow sequence, a third voltage-dividing resistor follows the second voltage-dividing resistor, the junction of the iatter two resistors constituting a point in the control circuit.

4. An ignition system according to claim 1, 2 or 3, in which evaluation of the current leaving the switching path is effected by an operational amplifier, a first input of which is connected to the junction between the switching path and the measuring resistor, and a second input of which is connected to a stabilised voltage.

5. An ignition system according to claim 4, when appqndant to claim 3, in which the first input of the operational amplifier is its noninverting input and the second input is its inverting input, the output of the operational amplifier being connected to said point in the control circuit.

6. An ignition system according to claim 5, in which a feedback connection is provided between the second input and the output of the operational amplifier.

7. An ignition system according to claims 1 to 4, in which the first input of the operational amplifier is its inverting input and the second input is its non-inverting input, and in which the output of the operational amplifier is connected to the control electrode of a transistor whose switching path is included in a shunt circuit of the thirdvoltage-dividing resistor.

8. An ignition system substantially as hereinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.

9. An ignition system substantially as hereinbefore described with reference to and as illustrated in Fig. 2 of the accompanying drawings.