GB2038943A

GB2038943A - Spark Ignition Devices for Internal Combustion Engines

Info

Publication number: GB2038943A
Application number: GB7944099A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-12-22
Filing date: 1979-12-21
Publication date: 1980-07-30
Also published as: IT7928025A0; IT1127698B; GB2038943B; ES8101720A1; FR2444816B1; DE7935780U1; DE2951223A1; ES487221A0; FR2444816A1; CA1147797A; JPS5591770A

Abstract

A train of sparks is maintained over one or more prolonged periods of the engine cycle. The sparks are produced by a high frequency voltage, the application and the cut-off of which are controlled by a detector 4 which provides a signal representing the position of the cylinder in the engine. The high voltage is produced by a circuit 3 for cutting off a continuous current, which is controlled by a continuously-running low-power oscillator 1 via a switch 2 controlled by the detector 4. The device makes it possible to achieve total combustion of the mixture, and the sparks can be maintained possibly up to the end of the exhaust phase. <IMAGE>

Description

SPECIFICATION Spark Ignition Devices for Internal Combustion Engines The present invention relates to spark ignition devices for internal combustion engines.

For the purposes of reducing pollution, it is known for example from French patents Nos.

73 06 238 (UK Patent No. 1 450 478) and 2 298 012, to cause improved combustion of the combustible mixture in an internal combustion engine, by prolonging the ignition spark at the sparking plug over a period of greater or lesser length, in the cycle of each cylinder of the engine.

French patent No. 73 06 238 also seeks to improve combustion in this way and to provide for complementary post-combustion by injecting air into the cylinder after the actual ignition phase.

The system disclosed in above-mentioned French patent No. 2 298 012 uses a means for detecting the moments at which ignition begins in each cylinder, the detecting means controlling triggering of a high-power oscillator which in turn transfers its energy to the sparking plug or plugs by way of a coil. In principle, the spark produced at the sparking plug is then repeated at the speed of the frequency of the oscillator during part or the whole of the combustion phase in the cylinder in question.

The above-indicated patent specifies that the power oscillator must have a certain number of characteristics, the most important of which is rapidity in the rise and fall in the amplitude of the voltage which the oscillator is required to supply under steady-state operating conditions. Indeed, this is essential in order for the spark to begin to jump immediately at full power at a precise moment, in each cycle, that is to say, when, taking account of the ignition advance, the mixture must begin to burn.

It has been found that such an oscillator is extremely difficult to adjust and that the ignition device provided with an oscillator of this kind cannot in practice operate correctly, particularly when the engine is operating in the high regions of its output, precisely because the rise time in respect of the amplitude of the voltage of the oscillator is excessively long and moreover relaxation phenomena prevent the oscillator from rapidly returning to its non-oscillating state.

The device disclosed in the above-indicated patent also suffers from the disadvantage that it cannot supply a sufficiently high voltage to maintain the spark at the sparking plug throughout the period of time corresponding to the highest pressure level in the cylinder (the moment at which explosion of the combustible mixture occurs and the period of time immediately thereafter). Thus, it is not possible significantly to increase the degree of combustion of unburnt matter after burning of the combustible mixture, as in most cases the spark is extinguished by the high pressure and can be reignited only very late in the combustion phase of the operating cycle.

This means that the ignition device described in the above-indicated patent does not make it possible sufficiently to improve the pollution index of the exhaust gases of a motor provided with the ignition device.

Embodiments of the present invention seek to provide an ignition device which is capable of prolonging the spark at the sparking plug, selectively with or without interruption, beyond the combustion phase, that is to say, if necessary, virtually up to the end of the exhaust phase, to determine with a very high degree of accuracy the moment at which production of the sparks in the cylinder begins, taking due account of the desired ignition advance, and, finally, maintaining production of the sparks even during the period when the highest pressure obtains in the cylinder.

The present invention provides an ignition device for an additional combustion engine, which, with or without the additional introduction of air, during each combustion cycle of the engine, provides for the production of sparks at at least one sparking plug during one or more given prolonged periods of said cycle, which may be up to the end of the exhaust phase, said device comprising a detector for producing, in the course of each combustion cycle and taking account if appropriate of a desired ignition advance, on the one hand an ignition commencement signal and on the other hand an ignition maintenance signal which extends over the prolonged period or periods of said cycle, a high voltage generator for selectively applying a train of voltage waves to each sparking plug of the engine during said period or periods, and a pilot oscillator for producing a clock signal at the frequency of said voltage wave, said high voltage generator comprising a continuous current cut-off circuit controlled by the clock signal and feeding into a supply circuit for supplying the sparking plug or plugs during the given period or periods, under the control of said detector.

These characteristics provide, in preferred embodiments described below, a considerable advantage which is that the high voltage is produced by using the action of cutting off a continuous current whose transitory phenomena on each cut-off are quasi non-existent, the lowpower pilot oscillator operating continuously.

Thus, by using semiconductor components which are capable of effecting switching at a high frequency, it is possible for the frequency of the ignition voltage wave to be selected at a value which can be up to 20 kHz and more. Moreover, as the cut-off in the continuous current is quasi instantaneous each time, it is possible, for example by means of a logic control means, to trigger each voltage wave at the precise moment in the combustion cycle, at which the train of sparks in the engine is required to begin.

In accordance with one embodiment of the invention, the ignition device comprises, for each sparking plug of the internal combustion engine, a current cut-off circuit connected to a separate supply circuit for that sparking plug, the pilot oscillator and the detector being selectively connectable to each cut-off circuit by way of a logic control circuit.

In accordance with another embodiment of the invention, the ignition device comprises a single current cut-off circuit and a logic control circuit which, under the action of said oscillator and said detector, controls the cut-off circuit, and also a distribution circuit which is connected between said oscillator and each of a plurality of the supply circuits for supplying the sparking plugs of the engine.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a simplified diagrammatic view of an ignition device embodying the invention, showing in particular a current cut-off circuit; Figure 2 is a simplified diagrammatic view showing another device embodying the invention; Figure 3 is a simplified diagrammatic view of a further ignition device embodying the invention; Figure 4 is a view in axial section of an example of a detector which can be used in ignition devices embodying the invention; Figure 5 is a detail view of the detector showing a plan view of a disc for producing control signals; Figure 6 is a more detailed diagrammatic view of the embodiment of Figure 3; Figure 7 is a time graph showing the mode of operation of the ignition device of Figure 6;; Figure 8 is a graph showing the production of a spark and the pressure curve in a cylinder of an Internal combustion engine, this graph having been produced with the device of Figure 6; and Figure 9 shows a graph similar to that shown in Figure 8, for illustrating an alternative form of the invention in which each cycle comprises two successive periods in which sparks are produced.

Before beginning to describe the accompanying drawings, it should be noted that ignition devices embodying the invention may be used in internal combustion engines having any number of cylinders, the following description relating solely by way of example to a fourcylinder engine. It will also be noted that the ignition device can be used both in spark ignition petrol engines and in engines of the diesel type, in which case it provides for post-combustion in the cylinder or cylinders of the engine. Having said this, Figure 1 is a diagrammatic view illustrating the basic principle of an ignition device embodying the invention, which comprises a pilot oscillator 1 producing a high-frequency pulsed signal whose frequency is preferably higher than 10 kHz.This oscillator, which in comparative terms is only of very low power, passes positive and negative pulses by means of a logic circuit 2 to a voltage cut-off circuit 3. The logic circuit 2 is controlled by a detector 4 which is intended to produce signals representing the positions of the pistons in the cylinders of the engine. In addition, by way of a line 5, the logic control circuit 2 passes an ignition control signal to a circuit 6 for supplying the sparking plug 7 or plugs of the engine, the control signal being produced in the detector 4, taking account of the ignition advance required in dependence on the operating conditions of the engine.

The power required for producing the sparks at the sparking plug 7 is produced in the current cutoff circuit 3 which is supplied by a continuous current source 8 which for example forms part of the motor vehicle provided with the ignition device embodying the invention. The cut-off circuit 3 operates at the rhythm of the signals supplied by the pilot oscillator 1. In this embodiment, it comprises a set of two rapidswitching transistors 9 and 10 whose bases are controlled respectively by the direct and complementary pulses of the signal from the pilot oscillator 1. It will be appreciated that this circuit can also be asymmetric.

The collector-emitter circuits of the transistors 9 and 10 are connected in parallel between earth and a primary winding 11 of a step-up transformer 1 2 which is supplied symmetrically by the continuous current source 8; the centre tap 1 3 of the transformer is connected to the positive terminal of the source 8. Respective protection diodes 14 and 1 5 are connected in parallel in the collector-emitter circuits of the transistors 9 and 1 0. A respective polarisation or bias resistor 1 6 and 1 7 is connected between the base and the emitter of the transistors.

It will be appreciated that the cut-off circuit described above is only an example in which many variations and modifications may be made, the embodiment illustrated being advantageous by virtue of its simplicity and its reliability.

The above-described circuit makes it possible to produce a train of sparks between the electrodes of the sparking plug 7; the length, beginning and end of the train of sparks are determined by the detector 4. The train of sparks is produced by one or more trains of high voltage waves which appear at the terminals of the secondary winding 1 8 of the step-up transformer 12. The commencement of the train of voltage waves or the commencement of the first train of waves, that is to say, its first double alternation, is as the outset at the value required for producing a spark at the sparking plug 7, as the switching action produced by the transistors 9 and 10 does not suffer from any noticeable delay in each double alternation. Thus, it is possible to achieve a very high degree of accuracy in determining the moment at which ignition begins in the cylinder of the engine, and to prolong the spark throughout the period desired for achieving complete combustion of the mixture, with or without an interruption.

The circuit of the above-described composition also makes it possible substantially to increase the value of the voltage at the sparking plug by virtue of the provision of the step-up transformer and the supply circuit 6 which will be described in detail hereinafter. In a specific embodiment, the voltage may be up to 50 kV, which makes it possible to maintain the spark at the sparking plug even during the period of the cycle which corresponds to the pressure peak at the moment of maximum compression. In conventional ignition systems, this pressure peak prevented post-combustion of the mixture which remained unburnt after the ignition phase of the operating cycle of the engine, but the pressure peak is completely overcome by means of the circuit embodying the invention.Moreover, the ignition device whose principle has just been described above can be used with complementary air injection, in accordance with the arrangements disclosed in above-mentioned French patent No.

73 06 238. As regards the value of the voltage used, it may be noted that it may by up to 100 kV or more and that the air gap in the sparking plug used with this ignition device is optimised, in dependence on the level of maximum pressure produced in the cylinder. For example, the air gap may be 1.4 mm, with a voltage of 50 kV and pressures in the cylinder of the order of 50 bars at the moment of explosion of the combustible mixture.

Figure 2 is a diagrammatic view of another ignition device embodying the invention. In the embodiment illustrated, the ignition device is for an internal combustion engine which has four cylinders comprising sparking plugs 7a to 7d. The sparking plugs are respectively connected to four separate supply circuits 6a to 6d which are themselves connected to four separate cut-off circuits 3a to 3d. A logic circuit 2 provides for both transmission and distribution of the signals from the detector 4 and the pilot oscillator 1, to the four circuits 3a to 3d.

In the Figure 3 embodiment, there is only a single cut-off circuit 3 constructed as shown in Figure 1; in this embodiment, the output of the circuit is connected in common to four supply circuits 6a to 6d which are triggered in accordance with the desired ignition sequence, under the control of the logic circuit 2, by means of four triggering circuits 1 9a to 1 9d which are respectively associated with the four supply circuits 6a to 6d. The circuit in Figure 3 will be described in detail hereinafter with reference to Figures 6 and 7.

Reference will first be made to Figures 4 and 5 which show an embodiment of a detector which can be used in ignition devices embodying the invention. The device illustrated is preferably derived from a conventional contact breaker with platinum contact points, which is well known in the art and which has been modified for use in the present application. It comprises a fixed casing 20 in which a shaft 21 is rotatably mounted. The shaft 21 rotates synchronously with the internal combustion engine. The shaft 21 may drive a rotary member 22 by way of two means 23 and 24 which provide for an angular offset between the shaft 21 and the rotary member 22, respectively in dependence on the speed of rotation of the contact breaker (speed of the engine) and in dependence on the depression in the intake manifold of the engine.As is well known, the means 23 and 24 are intended to provide for ignition advance, in dependence on the operating mode of the engine.

The rotary member 2 is fixed with respect to a disc 25 (see Figure 5) which is associated with a fixed stirrup-like member 26 which straddles a small angular part of the periphery of the disc, extending radially over a distance which corresponds to three signal tracks 27, 28 and 29, in the example described. The disc is made for example of an opaque material, the signals being produced by transparent areas, as shown in Figure 5. The stirrup member 26 carries, facing each track, a light-emitting diode 30 on the one hand and, on the other hand, on the opposite side, a photoelectric detector 31, with each diode 30 and each detector 31 co-operating as a pair to produce signals in dependence on the configuration of the tracks 27, 28 and 29.In a first alternative form, the outside track 27 comprises four transparent areas which are displaced at angular spacings of 900 and which are respectively associated with the four cylinders of the engine, in order to determine therein the moment at which ignition begins with, if appropriate, the desired ignition advance.

The tracks 28 and 29 are each formed by a transparent area extending over virtually 900. By virtue of the binary nature of the signals which the tracks 28 and 29 produce in the photoelectric detectors 31, the tracks 28 and 29 supply four control signals which, for each cylinder, extend over virtually 1 800 from the moment of ignition (an angle of 900 on the disc 25). In this way, it is possible to determine the commencement of ignition, the length of the train of sparks produced in each cylinder, and equally the ignition sequence in the four cylinders of the engine.

It should be noted that the control signals required for this purpose can be produced in a different manner, whether optical-electronic, magnetic or otherwise; moreover, adapting the system to engines with a different number of cylinders can easily be effected in dependence on their per se known ignition diagram.

Reference will now be made to Figure 6 which shows a more detailed diagrammatic view of the ignition device of Figure 3. The essential components shown in Figure 6 are again the pilot oscillator 1, the logic circuit 2, the cut-off circuit 3, the detector 4 which in this case is the detector described above with reference to Figures 4 and 5, the supply circuits 6a and 6d and the triggering circuits 1 9a to 1 9d. In order to simplify the drawing, the circuits 6b to 6d and 1 9b to 1 9d are not shown in detail.

The detector 4 has three outputs 32a to 32c whose states respectively correspond to the configurations of the tracks 27 to 29 and which are formed by the photoelectric detectors 31. The three outputs 32a to 32c are connected by way of amplification circuits 33 to a dead or read-only memory 34 or a wired logic circuit which distributes the various signals which it receives to its outputs in dependence on its internal wiring which is set up once and for all when the device is built.

The pilot oscillator 1 comprises an oscillating portion proper as indicated at 35, comprising two amplifiers 36, a resistor 37 which is preferably variable, and a capacitor 38, these components being mounted in a feedback mode. The output of the oscillator portion 35 is applied to a shaping circuit 39 formed by a J-k type flip-flop whose output signal is a train of alternate pulses which are at the pilot frequency of 20 kHz for example (see curve D, Figure 7). The inverting output Q of the circuit 39 forms a clock signal for the memory 34. The direct output Q of the circuit 39 is connected to a divider-by-six counter 40.

The frequency of the pilot oscillator 1 may be constant or variable in dependence on a number of parameters such as for example the operating condition of the engine, by subjecting the value of the resistor 37 to the control of such a parameter by conventional means.

The memory 34 is intended to switch the signals which it receives to six conductors 41 to 46 of which the conductors 41 and 42 are provided for respectively controlling the bases of the transistors 9 and 10 of the circuit 3. The conductors 43 to 46 transmit the ignition sequence signals which depend on the configuration of the signals produced at the outputs 32b and 32c of the detector 4. All the conductors 41 to 46 are connected to respective amplifiers 47. The amplifiers 47 which are associated with the ignition sequence signals are connected to supplementary amplifiers 48 which adjust the signal level to that required for controlling the triggering circuits 1 9a to 19d.

The secondary winding 1 7 of the transformer 1 2 which forms the output of the cut-off circuit 3 is connected by way of a capacitor 49 to a distribution line 50 which is connected by way of a diode 51 to earth and also to the four triggering circuits 1 9a to 1 9d by way of diodes 52 which are mounted in opposition relative to the diode 51.

The assembly comprising the capacitor 49, diode 51 and each of the diodes 52 forms a voltage doubling means which raises the output voltage of the secondary winding 1 7 to double its value.

Each diode 52 is connected to the series assembly of a capacitor 53 and the primary winding of a coil 54, to the secondary winding of which is connected a respective sparking plug 7a to 7d. The diode 52 is also connected to the anode of a silicon controlled rectifier 55, for example a thyristor, whose cathode is connected to earth and whose trigger terminal is connected to the output of one of the amplifiers 48 respectively.

The ignition device of Figure 6 operates in the following manner (see Figure 7).

The pilot oscillator 1 permanently produces a control signal (waveform D in Figure 7) which is divided into equal periods of six double alternations (t,, t1, t2 ) by the divider 40. The frequency of the control signal is for example 20 kHz.

The process shown in Figure 7 corresponds to operation of one of the circuits 1 9a to 1 9d, the mode of operation of the other circuits being identical.

The assembly of the circuits is so designed that the voltage on the line 50 is constantly reestablished at its maximum value (56, waveform A, Figure 7) in spite of the repeated earthings to which it is subjected by virtue of one of the thyristors 55 opening.

It will be assumed that, at expiry of the period to, the sparking plug 7a must be set in operation, this order being produced by virtue of the signals from the outputs 32a to 32c of the detector 4.

At the moment at which the ignition order occurs at the output 32c by virtue of the corresponding opening of the track 27 passing between the diode 30 and the detector 31 which are associated with each other, the memory 34 switches that order on to the circuit corresponding to the sparking plug 7a, for example in dependence on the signals which it receives from the outputs 32a and 32b. In the case in question, the memory 34 transmits the order by way of the associated amplifiers 47 and 48 to the trigger terminal of the corresponding thyristor 55 (pulse 57, waveform B) which shortcircuits the series assembly of the capacitor 53 and the primary winding of the coil 54, in order to discharge therein the charge which has previously accumulated in the capacitor 53. This discharge gives rise to a first voltage wave 58 which produces a spark at the sparking plug 7a.By way of example, the voltage at the secondary winding 1 7 may be 400 V for example, which value is doubled to about 800 V by the assembly 49 to 51 and 52. The coil can finally increase the voltage at the sparking plug to a very high value (up to 50 kV for example).

At the moment at which the ignition order occurs, the divider 40 begins to count, with the cut-off circuit 3 still being in a de-activated condition, since the memory 34 prevents operation thereof, by waiting for it to receive the third pulse counted by the divider 40. When this time delay period has passed, the thyristor 55 is assumed to be closed again, the signal at its trigger terminal being suppressed and the voltage at its terminals having become zero. Moreover, the third pulse controls opening of the cut-off circuit 3 which begins to supply power for recharging the capacitor 50 which is then charged for example in three stages 59 over three cycles of the clock signal, up to double the voltage of the secondary winding 17. As the divider has then counted up to 5, there remains one pulse during which the level of the voltage on the line 50 is maintained (end of the period t,).

The memory 34 then again causes triggering of the thyristor 55 and a fresh spark is produced at the same sparking plug 7a as the configuration of signals at the outputs 32a and 32b has not whanged. It should be noted in this regard that the frequency at which the sparks occur at a sparking plug during the period in which it is operating is one sixth of the frequency of the pilot signal, namely 20/6kHz; for example, the period for which it is connected into circuit and operates corresponding to about 1 800 of the cycle of the cylinder in question (900 on the disc 25 of the detector).

It should also be noted that the charge at the capacitors 53 of the circuits 1 9b to 1 9d are maintained by their associated diodes 52, while these circuits are not activated by means of the thyristors 55 by virtue of the configuration of signals at the outputs 32a and 32b.

It will be seen therefore that a train of voltage waves of the form shown at 58 in Figure 7 is produced at each sparking plug 7a to 7d, in the ignition sequence required by the engine; triggering of the wave train is immediate at full voltage, and the duration thereof extends over virtually 1800 of the cycle of the cylinder.

Figure 8 shows a graph representing both a waveform in respect of the voltage at the sparking plug and a curve in respect of pressure in a cylinder in which an ignition device embodying the invention was used.

It will be seen that the voltage wave train is maintained virtually at maximum value, even when the pressure in the cylinder reaches its maximum level.

Tests carried out by the applicant with an ignition device embodying the invention on a current motor vehicle engine indicated that, by virtue of complete combustion of the mixture, with all other things being equal the savings in fuel may be up to 8% with respect to the fuel consumption of the same engine when provided with a conventional ignition device.

Figure 9 shows an alternative form of the invention wherein the production of sparks is interrupted for a certain period of time, being resumed thereafter and permitting ignition of the unburnt matter and providing for a postcombustion effect.

Indeed, as spark production was triggered in the first instance by the signals of the track 27 of the disc 25 (Figure 5), by providing transparent areas 27A it is very easy to interrupt the production of sparks in the cycle, as desired, and possibly several times in the cycle.

In use of devices embodying the invention, the formation of plasma can be obtained by the turbulence of gases inside the cylinder(s) on condition that the high voltage eiectric arc is maintained at the spark plug(s). The ignition arrangement must permit this arc to be maintained even with the very high pressure prevailing in the cylinder at the time of explosion and, until the end of the expansion phase, at a minimum. Tests have shown that the permanent electric arc gave rise to a plasma from 8 to 10 mm approximately by virtue of the turbulence of the gases inside the cylinder. This permanent plasma makes it possible to maintain combustion and to retard the low limit of extinction. It is clear that the more plasma there is, the better combustion will be. Furthermore, it is preferable to have a permanent rather than a sequenced plasma because it is necessary for striking the arc to have an excess voltage proportional to the pressure level prevailing in the cylinder.

As stated in the above-mentioned French Patent No. 7306238 (UK Patent No. 1 450 478), the plasma may be obtained by a source of air outside the engine. This air has other complementary functions: it oxidises the combustion gases, evacuates the residual gases in the cylinder (improved filling co-efficient) and activates turbulence in the cylinder.

Claims

1. An ignition device for an internal combustion engine, which, during each combustion cycle of the engine, provides for the production of sparks at at least one sparking plug during one or more given prolonged periods of said cycle, said device comprising a detector for producing, in the course of each combustion cycle, on the one hand an ignition commencement signal and on the other hand an ignition maintenance signal which extends over the prolonged period or periods of said cycle, a high voltage generator for selectively applying a train of voltage waves to the or each sparking plug of the engine during said period or periods, and a pilot oscillator for producing a clock signal at the frequency of said voltage wave, said high voltage generator comprising a continuous current cut-off circuit controlled by the clock signal and feeding into a supply circuit for supplying the sparking plug or plugs during the given period or periods, under the control of said detector, said device being so arranged that the voltage supplied for maintaining the spark at the sparking plug or plugs during the combustion phase is compatible with (or depends on) the distance between the electrode of the sparking plug, which is the most suitable, and the highest pressure level obtaining in the cylinder.

2. An ignition device according to claim 1, wherein the current cut-off circuit comprises a semiconductor switching component arranged to be triggered by said pilot oscillator under the control of said detector, said component being connected to a primary winding of a step-up transformer having a tap connected to a continuous current source.

3. A device according to claim 2, wherein the cut-off circuit is symmetrical and comprises two semiconductor components which are respectively connected to the end terminals of the primary winding of the transformer while said source is connected to the centre point of said primary winding.

4. An ignition device according to any one of claims 1 to 3, which comprises, for each sparking plug of the internal combustion engine, a current cut-off circuit connected to a separate supply circuit for each said sparking plug, the pilot oscillator and said detector being selectively connectable to each cut-off circuit by way of a logic control circuit.

5. An ignition device according to any one of claims 1 to 3, which comprises a single current cut-off circuit and a logic control circuit which, under the action of said pilot oscillator and said detector, controls said cut-off circuit, and also a distribution circuit which is connected between said oscillator and each of a plurality of the supply circuits for supplying the sparking plugs of the engine.

6. An ignition device according to claim 5, as appendant to claim 3. wherein said distribution circuit comprises, for each supply circuit, a semiconductor triggering component which is connected to a common distribution line which in turn is connected to a secondary winding of the step-up transformer by way of a blocking means which prevents current from returning towards said line, and control electrodes of said triggering components are connected to the logic control circuit for said components to be triggered during periods which are determined by said detector supplying signals representing the combustion cycle of said engine.

7. An ignition device according to claim 6, wherein said logic control circuit comprises means for providing for periodic triggering of each semiconductor triggering component during the periods determined by said detector.

8. An ignition device according to claim 7, arranged such that the periodic triggering of each semiconductor triggering component is effected at a frequency which is a sub-multiple of the frequency of said pilot oscillator.

9. An ignition device according to claim 8, wherein said distribution circuit comprises a voltage doubling means which is connected to the secondary winding of said step-up transformer.

10. A device according to any one of the preceding claims, wherein said detector is operative to supply a plurality of signals, each of which is characteristic of a given cylinder of the engine, each signal being of a duration which approximately corresponds to 1 800 of the cycle of said engine as reckoned from the required moment of ignition and taking account, if appropriate, of the ignition advance.

11. A device according to any one of the preceding claims, which is operative to supply additional air into the cylinder or cylinders of the engine.

12. A device according to any one of the preceding claims, wherein the or each said prolonged period extends to the end of the exhaust phase.

13. A device according to any one of the preceding claims, wherein the detector is operative, in producing said signal, to take account of a desired ignition advance.

14. An ignition circuit for an internal combustion engine, substantially as herein described with reference to Figures 1,4 and 5, Figures 2, 4 and 5, Figures 3 to 8 or Figure 9 of the accompanying drawings.

1 5. The use of an ignition device according to any one of the preceding claims in an engine of the diesel type as an ignition system for providing for post-combustion in the cylinder or cylinders of said engine.