GB2112061A - Electronic ignition systems for internal combustion engines - Google Patents

Abstract

Electrical control signals for determining the generation of ignition pulses are derived by way of a fibre optic system in which a light beam is led from a light emitting device (20) to an actuating device such as a disc (10) or a vane (16) which rotates at a speed related to the prevailing engine speed. The light beam is arranged to be modulated by the actuator (10) or (16) in dependence upon engine speed and the number of engine cylinders before being led to a light receiver (24) which provides a trigger signal for controlling the generation of the ignition pulses. Preferably, the trigger signals are initiated at a fixed angle of advance in relation to the engine operating cycle, and a control voltage (42) is generated for delaying the initiation of ignition pulses for a duration determined in dependence upon the prevailing engine speed and a predetermined engine speed/advance characteristic. <IMAGE>

Description

SPECIFICATION Improvements in electronic ignition systems for internal combustion engines The present invention relates to electronic ignition systems for internal combustion engines.

Conventional ignition systems have involved the use of a mechanically operating contact breaker ("points") to derive a low-voltage signal controlling the generation of high-voltage pulses which are fed to the various sparking plugs of the engine by way of a mechanical distributor. The contact breaker has had associated with it an advance/retard mechanism which operates in response to engine rpm and vacuum level and which has comprised a plurality of rotating centrifugal weights mounted on the distributor shaft and a diaphragm connected by a linkage to a contact-breaker plate on which the movable contact of the contact breaker mechanism is mounted.

A problem in practice with such a mechanical arrangement is that it is virtually impossibla to modify the advance/retard characteristic of a given mechanism so that entirely different mechanical configurations have to be devised to suit each different engine.

One object of the present invention is to provide an advance/retard mechanism which enables relatively simple adjustment and/or modification to enable it to be adapted to operate with engines having a wide range of required advance/retard characteristics.

It is conventional practice in ignition systems for the low-voltage signal, which controls the generation of the high-voltage pulses, to be generated in the vicinity of the distributor, for example by the aforementioned contact breaker.

In other systems, particularly in electronic ignition systems, the signal is generated in the region of the distributor and is then led by way of electrically conductive wires to a control box which may be several feet away in another part of the engine compartment. For example, such lowvoltage signals might be derived by breaking an infra-red beam by means of a rotating vane, or by moving a magnet past a fixed coil. In all such known cases, the electrical signal is developed in the relatively hostile environment which exists close to the engine where the distributor is usually located, signal then being led through this hostile environment and fed to the control unit at the remote location.

A problem in practice with such arrangements is that spurious signals can be picked by the generating device or by the lead wires and be fed to the electronic control unit which may then operate on such signals to produce false ignition pulses resulting in some cases in misfiring and/or mis-timing of the engine ignition.

Another object of the present invention is to provide an ignition system where the possibility of such spurious signals being picked up in this manner is eliminated.

In accordance with a first aspect of the present invention there is provided an electronic ignition system for an internal combustion engine wherein electrical control signals for determining the generation of ignition pulses are derived by way of a fibre optic system in which a light beam is led from a light emitting device to an actuating device which rotates at a speed related to the prevailing engine speed, the light beam being modulated by the actuator in dependence upon engine speed and the number of engine cylinders before being led to a light receiver which reacts to the resulting modulated light beam to provide a trigger signal for the generation of said ignition pulses.

Preferably, the fibre optic system includes a first fibre optic lead which carries the light beam from the emitter to the actuating device and a second fibre optic lead which carries the modulated light beam from the actuating device to the light receiver. Both emitter and receiver can be positioned away from the engine.

The actuating device may, for example, comprise a disc having a plurality of reflective segments cn its periphery, one for each engine cylinder, which serve to periodically reflect the light beam back to the receiver, or a plurality of vanes or filters, one for each engine cylinder which serve to periodically break the light beam which otherwise passes back continuously to the receiver. However, the invention is not restricted to the latter arrangements and other arrangements could equally well be used.

In accordance with a second aspect of the present invention, there is provided an electronic ignition system for an internal combustion engine comprising an actuating device, such as that described above, which is arranged to initiate electrical trigger signals at a fixed angle of advance in relation to the engine operating cycle, and means for delaying the initiation of ignition pulses from said triggering pulses for a duration determined in dependence upon the prevailing engine speed and a predetermined engine speed/advance characteristic.

Advantageously, the trigger signals are used to initiate pulses whose width is arranged to be varied in accordance with prevailing engine speed and said predetermined engine speed/advance characteristic, the trailing edges of said variable width pulses being used to initiate the ignition pulses.

Preferahly, the particular engine speed/advance characteristic can be altered merely by changing a circuit board whereby to facilitate adjustment of the system to suit different engines.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which is a circuit diagram of one embodiment of an electronic ignition system constructed in accordance with the present invention.

As in the case of many of the known systems, the present system uses an actuator device which rotates in synchronism with the engine and which may comprise for example (I) a disc 10 which is mounted on a shaft 12 rotating at a speed related to engine speed and which carries angularly spaced reflective segments 14 on its periphery or (2) a plurality of vanes 16 mounted on a similar shaft 1 21 rotating at a speed related to engine speed. The distinction in the case of the present invention lies in the fact that electrical signals are not generated at or in the region of the actuator device itself. Instead, a first fibre optic lead 1 8 carries a light beam from an emitter device 20 and projects this continuously onto the periphery of the disc 10 in the embodiment (I) or towards the path of the vanes 1 6 in the case of embodiment (2).In embodiment (I), a second fibre optic lead 22 is positioned to pick up light reflected from the reflective segments 14 and transmit the resulting reflected beam to a receiver 24. In the case of embodiment (2), the second fibre optic lead 22 is positioned to normally pick up the projected beam from the fibre optic lead 1 8 and transmit same to the receiver 24. In this case the received beam is broken each time that a vane 1 6 passes between the ends of the fibre optic leads 18, 22. Thus, in case (I) the receiver receives a positive light signal when the segments on the actuator pass the emitter fibre optic lead 18 whereas in case (2) the light signal to the receiver is cut-off when the vanes 1 6 pass the emitter fibre optic lead.

In both cases, it will be appreciated that no electrical signal as such is present upstream of the receiver 24 so that there is no opportunity for spurious electrical signals to be carried to the receiver in the previously possible manner. Thus, whether it be an input signal corresponding to the presence of a light beam (case I) or an input signal corresponding to the absence of a light beam (case 2), the receiver is provided with an essentially "clean" input signal to which it can react to control the generation of the required ignition pulses as described hereinafter.

The following description assumes that the actuator is constructed as embodiment 1, i.e. the actuator comprises a disc having aiternate reflective and non-reflective segments, one segment for each cylinder of the engine, (or has alternate clear and opaque segments). The receipt of the light signals at the receiver 24 results in a stream of electronic pulses 26 which are amplified and shaped by a pulse generator 28 so as to be of constant amplitude and duration. The resulting signal is indicated at 30 and is passed to the trigger inputs of a first monostable A and a second monostable B.

As explained in detail below, the arrangement is such that initiation of an ignition pulse is determined by the occurrence of the trailing edge of the output pulse of monostable B, the length of the latter pulse being variable in dependence upon a control signal which is dependent upon (a) engine speed and (b) a predetermined engine speed/advance characteristic for the engine in question and which is obtained by way of the monostable A.

To this end, the monostable A is triggered by the pulse train 30 and provides pulses 32 of fixed duration which are inverted relative to the pulses 30. Since the pulses 32 are derived from the original signals 26 from the receiver 24, they are generated at a frequency dependent upon the prevailing engine speed. The pulses 32 are fed to an integrator D whose feedback circuit 34 is designed to shape the output waveform of the integrator such as to achieve a predetermined input frequency/DC output voltage characteristic whose particular shape will be determined to suit a particular engine. A typical characteristic is indicated by the curve 36 in the drawing.

In order to enable the characteristic curve 36 to be change to suit different engines, the feedback circuit acting as the shaper is preferably mounted on a separate circuit board 38 so that it can be changed for a different board without affecting the remainder of the circuitry. Alternatively, more than one circuit board can be incorporated so that different shapes can be selected to suit changes in fuel or operating conditions.

In order to use the curve 36 to control the monostable B, it is necessary to convert the curve 36 into a more suitable form and this is achieved by means of a first inverter E, which simply inverts the curve 36, to achieve the curve 38, and a second inverter F which inverts the siope of the curve 38 to achieve the curve 40. Thus, the curve 40 is one in which the voltage level reduces with increasing frequency in accordance with a predetermined characteristic. The latter characteristic shape is provided by the engine manufacturer to comply with the required operating characteristics of the particular engine.

The signal on the line 42 from the element F is thus a variable voltage level whose magnitude depends on the engine speed in accordance with the characteristic curve 40. This signal is uses as a control voltage input to the monostable B to vary the pulse width of the pulses 44 from monostable B in dependence upon engine speed. The trailing edges of the pulses 44 are used to trigger a further monostable C whose output pulses 46 are inverted by an inerter G to provide control pulses 48. The latter pulses are used in a power module (not shown) to trigger the initiation of ignition pulses on their leading (negative going) edges.

The arrangement is such that the actuator operates at a fixed "angle of advance" which is slightly larger than the greatest angle of advance required under any running conditions. The control voltage developed on the line 42 is arranged to delay the ignition pulses from this maximum angle of advance to achieve the appropriate degree of advance for a given engine speed and in accordance with the manufacturers engine speed/advance curve corresponding to the curve 40.

Claims (11)

1. An electronic ignition system for an internal combustion engine wherein electrical control signals for determining the generation of ignition pulses are derived by way of a fibre optic system in which a light beam is led from a light emitting device to an actuating device which rotates at a speed related to the prevailing engine speed, the light beam being modulated by the actuator in dependence upon engine speed and the number of engine cylinders before being led to a light receiver which reacts to the resulting modulated light beam to provide a trigger signal for the generation of said ignition pulses.

2. An ignition system as claimed in claim 1, wherein the fibre optic system includes a first fibre optic lead which carries the light beam from the emitter to the actuating device and a second fibre optic lead which carries the modulated light beam from the actuating device to the light receiver.

3. An ignition system as claimed in claim 1 or 2, wherein the actuating device comprises a disc having a plurality of reflective segments on its periphery, one for each engine cylinder, which serve to periodically reflect the light beam back to the receiver.

4. An ignition system as claimed in claim 1 or 2, wherein the actuating device comprises a plurality of vanes or filters, one for each engine cylinder, which serve periodically to break the light beam which otherwise passes back continuously to the receiver.

5. An electronic ignition system as claimed in any of claims 1 to 4, wherein the actuating device is arranged to initiate electrical trigger signals at a fixed angle of advance in relation to the engine operating cycle, and a means is provided for delaying the initiation of ignition pulses from said triggering pulses for a duration determined in dependence upon the prevailing engine speed and a predetermined engine speed/advance characteristic.

6. An ignition system as claimed in claim 5, wherein the trigger signals are used to initiate pulses whose width is arranged to be varied in accordance with prevailing engine speed and said predetermined engine speed/advance characteristic, the trailing edges of said variable width pulses being used to initiate the ignition pulses.

7. An ignition system as claimed in claim 6, wherein said delaying means comprises a first monostable triggered by the trigger signals from the actuating device, the output signals of the first monostable being integrated to provide a d.c. level whose magnitude is dependent upon engine speed in accordance with a predetermined characteristic relationship, this d.c. level being used to control the width of said pulses.

8. An ignition system as claimed in claim 7, wherein the integration is performed in an integrator whose operating characteristic is determined by a feedback circuit which can be adjusted to vary said characteristic and hence to vary the relationship between said d.c. level and the engine operating speed.

9. An ignition system as claimed in claim 8, wherein said feedback circuit is contained on a separate circuit board from the remainder of the circuit whereby it can be changed for a board having a different characteristic without affecting the remainder of the circuitry.

10. An ignition system as claimed in any of claims 7 to 9, wherein the variable width pulses are generated by a second multivibrator which is also triggered by the trigger signals of the actuating device.

11. An electronic ignition device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.