GB2134597A

GB2134597A - Ignition system for an internal combustion engine

Info

Publication number: GB2134597A
Application number: GB08403344A
Authority: GB
Inventors: Adolf Fritz; Thomas Jakh
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1983-02-09
Filing date: 1984-02-08
Publication date: 1984-08-15
Also published as: JPS59147873A; JPH0472993B2; DE3304331C2; GB8403344D0; DE3304331A1; GB2134597B

Abstract

The ignition system comprises a speed limiting circuit 28 including a speed monitor 29. When a maximum admissible speed is reached, the monitor 29 disconnects a control terminal 30 from the earth lead 2. At the breakdown of a zener diode 40, a transistor 31 is switched on thereby switching off a transistor 37. In consequence, the voltage at the secondary winding 9 of an ignition coil 6 is insufficient to cause a spark, but the speed of the engine continued to be displayed on an indicating device 12. In a modification, when the maximum admissible speed is reached, a thyristor connected in parallel with the primary winding 5 is switched on to prevent the generation of sparks. <IMAGE>

Description

SPECIFICATION Ignition system for an internal combustion engine This invention relates to an ignition system for an internal combustion engine.

German Offenlegungsschrift No. 1 673 365 describes an ignition system in which a conventional rotational speed limiting means (as described in, for example, German Patent Specification No. 1 906 883) prevents constant generation of ignition voltage, or the generation of ignition voltage at all, during rotational speed limitation, and consequently a signal representative of the actual rotational speed cannot be tapped between the primary winding and the electronic contact breaker.

It is an object of the present invention to provide an ignition system which substantially overcomes the disadvantages of the prior art as described above.

In accordance with the present invention there is provided an ignition system for an internal combustion engine, comprising an ignition coil whose primary winding forms, together with an electronic contact breaker, a series combination connectible to a current source, and whose secondary winding is operatively connected to at least one spark plug, a device for deriving a signal representative of the engine speed from a point between the primary winding and the electronic contact breaker, and a speed-limiting circuit for allowing the induced voltage, appearing on the ignition coil upon the switching off of the electronic contact breaker, first to increase to a value required for an effective representation of the engine speed, and then to drop to an extent that an electrical spark over (ignition spark) cannot appear at the spark plug.

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 another embodiment of an ignition system in accordance with the present invention.

The ignition system, whose circuit diagram is shown in Figure 1, is intended for an internal combustion engine (not illustrated) of a motor vehicle (which is also not illustrated). This ignition system is fed from a direct current source 1 which may be the battery of the motor vehicle. A lead 2 is connected between the negative pole of the current source 1 and earth, and a supply lead 4 is connected to the positive pole and incorporates an operating switch (ignition switch) 3. The supply lead 4 is the starting point for a circuit branch which leads to the earth lead 2 by way of the primary winding 5 of an ignition coil 6 and then by way of an electronic contact breaker 7. The electronic contact breaker 7 is formed by the emitter-collector path of an n-p-n transistor 8.The secondary winding 9 forming part of the ignition coil 6 is connected to the earth lead 2 by way of a spark plug 10. A signal representative of the rotational speed for a device 11 to be acted upon by the rotational speed of the internal combustion engine, an indicating device 12 in the preferred case, is tapped from the lead between the primary winding 5 and the electronic contact breaker 7.

A protective circuit 1 3 for protection against excess voltages is associated with the transistor 8 and comprises a voltage divider 14, 1 5 connected in parallel with the emitter-collector path of the transistor 8, and a circuit element 1 7 which is connected between the tapping 1 6 of the voltage divider 14, 1 5 and the base of the transistor 8 and which has a predetermined breakdown voltage. In the preferred case, the circuit element 1 7 is a Zener diode 17'.

Furthermore, the supply lead 4 is the starting point for a circuit branch which first leads to an anode of a diode 18 for protection against connection with incorrect polarity, and then to a circuit point 20 by way of a current-limiting resistor 1 9 and then continues to the earth lead 2 by way of a buffer capacitor 21. A Zener diode 22 is connected in parallel with the buffer capacitor 21 and serves to stabilize the potential at the circuit point 20, the anode of the Zener diode being connected to the earth lead 2.

The circuit point 20 is connected to a control device 23 which is known per se and which ensures, over a relatively large range of rotational speed, a constant duration of the flow of current conducted through the primary winding 5 before each ignition operation, and which is to be constructed in the manner of, for example, the subject of German Patent Specification No.

2 244 781. Furthermore, the control device 23 is connected to the earth lead 2 and its input is connected to a signal generator 24 which is connected at one end to earth potentiai. The signal generator 24 is constructed in the manner of a small alternating current generator, it being ensured, in conjunction with the control device 23, that the electronic contact breaker 7 is conductive during the pulse duration t1 and non-conductive during the pulse duration t2. For this purpose, the output of the control device is connected to the base of an n-p-n transistor 25 whose emitter is connected to the earth lead 2 and whose collector is connected to the circuit point 20 by way of a resistor 26.Moreover, the collector of the transistor 25 is connected to the anode of a blocking diode 27 whose cathode is connected to the base of the transistor 8 forming the electronic contact breaker 7.

Finally, the ignition system is also provided with a speed-limiting circuit 28 which includes a monitor 29 for the speed of the internal combustion engine. The speed monitor 29 is connected to the circuit point 20 and also to the earth lead 2, and its output is connected to a control terminal 30. Furthermore, the speedlimiting circuit 28 includes an electronic switch 31 formed by the emitter-collector path of an n-p-n transistor 32. The emitter of the transistor 32 is connected to the earth lead 20, and its collector is connected to the circuit point 20 by way of a resistor 33. The base of the transistor 32 is connected to the control terminal 30, and also to the earth lead 2 by way of a resistor 34.The control terminal 30 is also connected to the collector of an n-p-n transistor 35 whose emitter is connected to the earth lead 2 and whose base is connected to the collector of the transistor 25 by way of a resistor 36. The emitter-collector path of the transistor 32 forming the electronic switch 31 is connected in parallel with the base-emitter path of a further n-p-n transistor 37 whose emitter is also connected to the earth lead 2. The emittercollector path of the further transistor 37 is connected between the earth lead 2 and that end of the voltage divider 14, 1 5 which, remote from the primary winding 5 and the collector of the transistor 37, is also connected to the base of the transistor 32 by way of a feedback resistor 38.

Moreover, the tapping 1 6 of the voltage divider 14, 1 5 is connected by way of a resistor 39 to the cathode of a Zener diode 40 whose anode is connected to the base of the transistor 32.

The ignition system which has just been described operates in the following manner: The system is ready for operation as soon as the operating switch 3 is closed. It will be assumed that the time interval t1 is just commencing in the control device 23, so that the emitter-collector path of the transistor 25 enters its non-conductive state, and the emitter-collector path, forming the electronic contact breaker 7 of the transistor 8, enters its conductive state.

Current is thereupon conducted through the primary winding 5 and hence ignition energy for the next ignition operation is stored in the ignition coil 6.

When the time interval t2 commences in the control device 23, the emitter-collector path of the transistor 25 becomes conductive, and the emitter-collector path, forming the electronic contact breaker 7, of the transistor 8 becomes non-conductive, whereupon an induced voltage appears on the ignition coil 6 and is fed as a highvoltage pulse to the spark plug 10 where it is used to produce an electronic spark-over (ignition spark). At the same time, the induced voltage pulses are evaluated as speed information in the device 11.

In order to suppress the generation of an ignition spark upon reaching a maximum admissible speed, and nevertheless to ensure that the actual speed information still remains available, the speed-limiting circuit 28 is brought into operation. This is effected in that the control terminal 30, which is normally connected to the earth lead 2 by the speed monitor 29, is now isolated from the earth lead 2 by the speed monitor 29, so that the electronic switch 31 formed by the emitter-collector path of the transistor 32 assumes a switching standby condition. When the induced voltage is produced on the ignition coil 6 by blocking the electronic contact breaker 7, the voltage on the primary winding 6 can in the first instance increase to a value adequate for satisfactory speed information.

The Zener diode 40 breaks down when this value is reached. whereupon control current flows through the transistor 32 and the electronic switch 31 assumes its conductive state, whereas the emitter-collector path of the transistor 37 assumes its non-conductive state. These switching states of the transistors 32, 37 are maintained by way of the feedback resistor 38 and cause the voltage divider resistor 1 5 to be isolated from the earth lead 2 and the limiting effect on the Zener diode 1 7' to be reduced to a value at which the voltage on the secondary winding 9, which is not in phase with the voltage on the primary winding 5, cannot cause an ignition spark at the sparkplug 10.

The speed-limiting circuit 28 is taken out of readiness again preferably when the primary winding 5 is next put into circuit, in that control current can then flow through the base-emitter path of the transistor 35 and results in the change-over of the associated emitter-collector path into its conductive state. The base of the transistor 32 is then connected to the earth lead 2 both by the transistor 35 and the switching path then closing again in the speed monitor 29, whereupon the electronic switch 31 ,formed by the emitter-collector path of the transistor 32, again assumes its non-conductive state, whereas the emitter-collector path of the further transistor 37 again assumes its conductive state, and the normal state is re-established.If the speed remains below the maximum admissible value, the normal state continues to exist, otherwise the speed-limiting operation just described sets in again.

The ignition system of Figure 2 differs from that of Figure 1 in that the switching element 1 7 having a specific breakdown voltage and forming part of the protective circuit 13 comprises two series-connected Zener diodes 41,42 whose cathodes face the tapping 16, and a circuit branch, commencing from the junction between the Zener diodes 41, 42, leads by way of a resistor 43 to the base of the transistor 32 forming the electronic switch 31. In the present instance, that end of the voltage divider 14, 1 5 which is remote from the primary winding 5 is connected directly to the earth lead 2. The collector of the further transistor 37 is connected to the circuit point 20 by way of a resistor 44 and to the base of an n-p-n transistor 46 by way of a capacitor 45. The emitter of the transistor 46 is connected to the earth lead 2, and its collector is connected to the circuit point 20 by way of a series combination comprising a first inductive winding 47 and a resistor 48. The first inductive winding 47 is coupled by an iron core 49, indicated by a broken line, to a second inductive winding 50, one end of which is connected to the control electrode of a thyristor 51 and its other end is connected to the cathode of the thyristor 51. The anode-cathode path of the thyristor 51 is connected in parallel with the primary winding 5. The resistor 52 connected in parallel with the base-emitter path of the transistor 46 serves to discharge the capacitor 45.

The action of the ignition system of Figure 2 differs from that of Figure 1 in the following manner: When the maximum admissible engine speed is attained, and hence the electronic switch 31 is brought into readiness for change-over, the induced voltage increasing on the ignition coil 6 gives rise to a value at which the Zener diode 41 breaks down, the speed information being adequate for operation of the device 11, and hence a pulse is applied to the base of the transistor 32 forming the electronic switch 31, whereupon the electronic switch 31 assumes its conductive state.The emitter-collector path of the further transistor 37 thereby becomes nonconductive, and a charging current for the capacitor 45 is conducted by way of the baseemitter path of the transistor 46, so that the emitter-collector of the transistor 46 becomes momentarily conductive and a short current pulse is fed by way of the resistor 48 and the inductive winding 47. This current pulse is transmitted to the inductive winding 50 which thereupon causes a firing current to flow through the thyristor 51. By virtue of the induced voltage on the ignition coil 6 and on the anode-cathode path of the thyristor 51, this anode-cathode path becomes conductive, so that the flow of induced current then setting in reduces the induced voltage to a value which can lead to an electrical spark-over at the spark plug 10. It is thereby also ensured that usable speed information is supplied to the device 11 if an ignition spark does not appear at the spark plug 10.

After the induced voltage has decayed, the anode-cathode path of the thyristor 51 becomes non-conductive again. The switching operations in the speed-limiting circuit 28 which have just been described are cancelled again as soon as the electronic contact breaker 7, and thus also the emitter-collector path of the transistor 35, assume their conductive states. The capacitor 45 can then discharge the way of the resistor 52 as a result of the emitter-collector path of the further transistor 37 assuming its conductive state.

Claims

1. An ignition system for an internal combustion engine, comprising an ignition coil whose primary winding forms, together with an electronic contact breaker, a series combination connectible to a current source, and whose secondary winding is operatively connected to at least one spark plug, a device for deriving a signal representative of the engine speed from a point between the primary winding and the electronic contact breaker, and a speed-limiting circuit for allowing the induced voltage, appearing on the ignition coil upon the switching off of the electronic contact breaker, first to increase to a value required for an effective representation of the engine speed, and then to drop to an extent that an electrical spark over (ignition spark) cannot appear at the spark plug.

2. An ignition system as claimed in claim 1, wherein the electronic contact breaker is a transistor which is protected against excess voltage by a protective circuit which comprises a voltage divider connected in parallel with the emitter-collector path, and a circuit member having a predetermined breakdown voltage which is located between the tapping of the voltage divider and the base, the protective circuit at the same time being used to limit the rotational speed.

3. An ignition system as claimed in claim 2, wherein said signal deriving device includes a speed monitor and an electronic switch which can be brought into change-over readiness in the speed-limiting circuit by the speed monitor when the maximum admissible speed is reached, and which can be changed over by way of a switching branch commencing from the protective circuit when the induced voltage appearing on the ignition coil reaches a predetermined value, whereupon means for reducing the induced voltage appearing on the ignition coil is brought into effect.

4. An ignition system as claimed in claim 3, wherein the change-over readiness of the electronic switch is switched off, in dependence upon the change-over of the electronic contact breaker into its conductive state.

5. An ignition system as claimed in claim 3, wherein the electronic switch is a second transistor whose emitter-collector path is connected in parallel with the base-emitter path of a third transistor, the emitter-collector path of the second transistor forming the electronic switch being normally non-conductive, and the emittercollector path of the third transistor being normally conductive.

6. An ignition system as claimed in claim 5, wherein that end of the voltage divider which is remote from the primary winding is connected by way of the emitter-collector path of the third transistor to the emitter of the first transistor which forms the electronic contact breaker, the tapping of the voltage divider is connected by way of a circuit member of predetermined breakdown voltage of the base of the second transistor forming the electronic switch, and a feedback resistor is interposed between the base of the second transistor forming the electronic switch, and the collector of the third transistor.

7. An ignition system as claimed in any of claims 2 to 4, wherein the circuit member of predetermined breakdown voltage which forms part of the protective circuit comprises a plurality of Zener diodes whose cathodes face the tapping of the voltage divider, the junction of two such Zener diodes is connected to the base of the second transistor forming the electronic switch, and a switching of this transistor into its conductive state is used to switch on a thyristor which is connected in parallel with the primary winding.

8. An ignition system for an internal combustion engine, constructed and adapted to operate substantially as herein described with reference to and as illustratedin the accompanying drawings.