Multi-pulse Capacitor Discharge Ignition System

Abstract

An electric ignition system for an automobile engine includes a capacitor and a SCR in series with the primary winding of a high voltage transfer. The SCR is triggered by means of an AND gate to which a recurrent control signal synchronized with the engine and a voltage derived from the capacitor voltage are applied. During each control signal, a plurality of high voltage pulses are generated by the repeated charge and discharge of the capacitor through the SCR and transformer.

Description

The present invention relates to an apparatus for producing high-voltage pulses, especially for producing sparks at a spark plug of an internal combustion engine. The invention particularly relates to an apparatus of the type known, for example, from U.S. Pat. No. 3,051,870, including a capacitor connected in series with the primary winding of a step-up high-voltage transformer across the secondary winding of which at least one spark plug may be connected. The apparatus also includes a voltage source by which said capacitor is charged, a controlled rectifier through which the capacitor is discharged through the primary winding of the high-voltage transformer, and a control circuit for causing the controlled rectifier to conduct.

In an internal combustion engine, especially a spark-ignition engine provided with an apparatus of the above defined type or of one of most of the other known types, starting difficulties frequently occur. These are usually due to a weak storage battery, for example, due to its age. When the engine is driven by an electric starter-motor energized by the battery, the battery voltage drops excessively and hence the driving torque developed by the starter-motor is too weak to bring the internal combustion engine up to a sufficient starting speed. These poor starting conditions are aggravated by the fact that, owing to the abnormally low battery voltage, the ignition spark also is weak and of short duration if the same battery is used for feeding the starter-motor and the ignition-apparatus.

With the known ignition-apparatuses, combustion frequently is far from complete so that the efficiency of the engine is reduced and the toxicity of its exhaust gases is increased. In the towns, the air is increasingly contaminated by exhaust gases, so that this toxicity must be combated with all available means. On the other hand, internal combustion engines of ever higher speeds and ever higher compression ratios continuously require better ignition apparatuses.

It is an object of the invention to provide an improved and yet simple apparatus of the above defined type.

The apparatus in accordance with the invention is characterized in that the control circuit includes an and-gate to which a recurrent control signal and a voltage derived from the voltage across the capacitor are applied, so that, during the application of the control signal, the controlled rectifier is repeatedly rendered conductive when the voltage across the capacitor reaches a predetermined value. Consequently, for example, when an internal-combustion engine is running slowly or is being started, and owing also to the comparatively long duration of the control signals, a train of high-voltage pulses and a corresponding train of ignition sparks are produced during each control signal.

It should be noted that the use of a train of high-voltage pulses for igniting the combustible gas mixture in a combustion chamber of a spark-ignition internal-combustion engine is known, inter alia from Belgian Pat. specification No. 509,801. The apparatus in accordance with the invention, however, is very simple and effective.

The invention will now be described more fully with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is the circuit diagram of a first embodiment of the apparatus in accordance with the invention,

FIG. 2 is the circuit diagram of a second embodiment,

FIG. 3 is the circuit diagram of a third embodiment,

FIG. 4 shows a detail of this third embodiment, and

FIG. 5 is the circuit diagram of a modification of the third embodiment.

The first embodiment shown diagrammatically in FIG. 1 includes a capacitor 1 connected in series with the primary winding 2 of a step-up high-voltage transformer 3. Across a secondary winding 4 of the transformer a spark plug 5 can be connected, for example, through a distributor, not shown. The system further includes a voltage source constituted by a source 6 of a low direct voltage in the form of a 12 volt battery and by a converter in the form of a blocking oscillator including a transistor 7 and a transformer which comprises a core 8 made of a square-loop ferromagnetic material, a primary winding 9 connected in the collector circuit of the transistor 7, a feedback winding 10 connected in the base-emitter circuit of this transistor and a high voltage secondary winding 11. The emitter of the transistor 7, which is of the p-n-p type, is connected to the positive terminal of the source 6, its base is connected to its emitter through the series combination of a feedback capacitor 13 shunted by a resistor 12 and the winding 10 and to the negative terminal of the source 6 through a biasing resistor 14. Its collector is connected to this negative terminal through the winding 9. One end of the secondary winding 11 is connected to ground and to the negative terminal of the source 6, and the other end is connected to a full-wave rectifier in voltage-doubling connection which comprises a coupling capacitor 15 and two rectifier elements 16 and 17. The rectifier elements 16 and 17 are connected in series and in the same pass direction across the series combination of the capacitor 1 and the winding 2, and the capacitor 15 is connected between the nongrounded terminal of the winding 11 and the junction of the rectifier elements 16 and 17. Both windings 2 and 4 of the transformer 3 are connected to ground at one end, and the electrode of the capacitor 1 which is not connected to the winding 2 is positively charged to a voltage of, say, 400 volts through the rectifier 15, 16, 17.

The apparatus further includes a controlled rectifier in the form of a semiconductor controlled rectifier or thyristor 18 through which the capacitor 1 is discharged through the primary winding 2, and a control circuit for rendering the controlled rectifier 18 conducting.

The control circuit includes a p-n-p transistor 19 which acts as an and-gate. The emitter of this transistor is directly connected to the positive terminal of the source 6, its collector is connected to ground through a load resistor 20 and its base is connected to the tapping of a voltage divider which comprises resistors 21 and 22 connected across the capacitor 1. A second voltage divider comprising resistors 23 and 24 is connected between the base of the transistor 19 and the positive terminal of the source 6, and its tapping is connected to ground through a switch 25, for example, the contact breaker of a spark-ignition internal-combustion engine having at least one combustion chamber which is provided with the spark plug 5. The control circuit further includes a p-n-p amplifying transistor 26. The base of this transistor is connected to the collector of the transistor 19, its emitter is directly connected to the positive terminal of the source 6 and its collector is connected to ground through a load resistor 27 and to the control electrode of the thyristor 18 through a resistor 28.

The resistors 21, 22, 23 and 24 have values such that the transistor 19 is highly conducting as long as the switch 25 is closed, even if the capacitor 1 is fully charged. Under these conditions the transistor 26 is cut off and supplies no forward current to the control electrode of the thyristor 18, so that this thyristor also remains cut off. When a control signal is produced by the circuit of the switch 25 being broken, the transistor 19 compares the voltage of the source 6 applied to its emitter with the voltage across the resistor 22 applied to its base and derived from the voltage across the capacitor 1. If the latter voltage exceeds a predetermined value, the transistor 19 is cut off. The transistor 26 becomes highly conducting and, through the resistor 28, part of its collector current flows to the control electrode of the thyristor 18, rendering the latter conducting. The capacitor 1 now discharges through the thyristor 18 and the winding 2, the current pulse through this winding produces a high-voltage pulse across the secondary winding 4 and causes a spark discharge to take place between the electrodes of the spark plug 5.

Owing to the capacitor 1 being discharged, the voltage at the base of the transistor 19 again becomes negative with respect to the voltage at its emitter, even if the contact of the switch 25 is still open. Hence, this transistor begins to conduct again and cuts off the transistor 26, enabling the thyristor 18 to be extinguished again at the next passage through zero of its main current.

Owing to the secondary winding 11 of the converter transformer being completely short-circuited by the thyristor 18 via the rectifier 15, 16, 17, the converter including the transistor 7 substantially stops oscillating until the thyristor 18 is again rendered non-conducting.

As soon as the thyristor has again become non-conducting, the converter including the transistor 7 starts oscillating strongly, and the capacitor 1 is thus charged again through the rectifier 15, 16, 17. At the instant at which the voltage across this capacitor again reaches the predetermined value, the transistor 19 is again cut off unless the contact of the switch 25 has been closed in the meantime. If this is not the case, the transistor 26 and hence the thyristor 18 become conductive again, the capacitor 1 again discharges through the thyristor and the winding 2 and the apparatus produces a second spark between the electrodes of the spark plug 5, and so on.

Thus, each time the contact of the switch 25 is broken the system produces a train of high-voltage pulses and a corresponding train of sparks, these trains being interrupted only by the contact of the switch 25 being closed again. The time interval between two successive pulses is determined by the capacitance of the capacitor 1, by the internal resistance of the source of charging voltage comprising the source 6, the converter 7 - 14 and the rectifier 15 - 17, by the predetermined voltage across the capacitor 1, which is determined by the resistors 21, 22, 23 and 24, and by the transformation ratio of the converter with the rectifier.

When the thyristor 18 is conducting, the capacitor 1 together with the inductance of the winding 2 forms a resonant circuit closed for both directions of current flow; for currents in the reverse direction of the thyristor 18 it is closed through the rectifier elements 16 and 17. This closed circuit is strongly excited by the discharge of the capacitor 1 through the thyristor 18 and oscillates with damped oscillations at its resonant frequency. If a half-cycle of this oscillation is sufficiently longer than the recovery time of the thyristor 18, say, 20 microseconds, the latter is cut off after a full oscillation cycle. If this half-cycle is shorter than the recovery time of the thyristor, for example, is equal to 10 microseconds (frequency of oscillation 50 kc/s), the thyristor remains conducting in its forward direction until the oscillation amplitude has decreased to such an extent that the forward current through the thyristor 18 can no longer maintain it in the conductive state.

When the full-wave rectifier 15-17 is replaced by a half-wave rectifier, for example, by omitting the capacitor 15 and the rectifier element 17 and connecting the winding 11 to the anode of the rectifier element 16, the capacitor 1 together with the winding 2 form, for one direction of current flow, a resonant circuit having a comparatively high natural frequency and closed through the thyristor 18 and, for the other direction of current flow, a resonant circuit having a lower natural frequency and closed through the rectifier 16 and the winding 11. Hence, the second and any further even-numbered half-cycles are of longer duration and lower amplitude than the first and any further odd-numbered half-cycles of the oscillation.

According to another modification, not shown in the drawings, the winding 2 may be shunted by a diode which would have its cathode connected to ground and would suppress the even-numbered half-cycles of the oscillation across the winding 2, so that each pulse produced is limited to only about the first quarter-cycle of the resonant circuit 1, 2.

The shape, the duration, the nature (of one polarity or of alternating polarities) and the mutual time intervals of the high-voltage pulses may thus be acted upon at will. It should be noted that if the converter 7 - 14 were to continue oscillating and supplying a charging current to the capacitor 1 during the pulses, the thyristor 18 could not be so readily extinguished and, under certain conditions, would not extinguish at all. These conditions obtain when the charging current supplied by the converter 7 - 14 through the rectifier 15 - 17 forms a sufficient holding current for the thyristor used. By ensuring that the converter 7 - 14 cannot oscillate when the thyristor 18 conducts, the thyristor 18 is prevented from being maintained in the conducting condition by the rectified alternating current supplied by the converter 7 - 14 through the rectifier 15 - 17. This also results in slightly shortening the high-voltage pulses produced, by suppressing a useless low-amplitude end portion or "tail," and also in reducing the dissipation in the element 7 of the converter 7 - 14 and the mean value of the current supplied by the source 6 of low direct voltage.

Especially for starting a cold engine, the described train of ignition sparks has proved more effective than a single continuous spark of the same duration. Combustion is accelerated and is more complete, the exhaust gases contain less carbon monoxide and less carbon is deposited in the exhaust and in the engine, especially on the electrodes of the spark plugs.

The second embodiment shown in FIG. 2 is equipped with a balanced converter including two transistors 7 and 7'. The transformer 8 - 11 has a primary winding 9, 10, 9', 10', which also acts as a feedback winding and is provided with a centre tapping connected to the positive terminal of the source 6. The ends of the winding are connected to the bases of the transistors 7 and 7' through biasing resistors 12 and 12', respectively, the emitters of said transistors being connected to intermediate taps on this winding. The transformer 8 - 11 has a high transformation ratio and its secondary winding 11 is connected to the input terminals of a rectifier bridge 15'. The control circuit includes a delay circuit comprising capacitors 32, 33, 34 and resistors 35, 36, 37 which determines, or at least limits, the duration of each pulse produced by a discharge of the capacitor 1. This network is connected between the emitter and the base of the transistor 19 so that the capacitors 32, 33, 34 are charged to the difference between the voltage across the resistor 22 of the voltage divider 21, 22 and the voltage of the supply source 6. The emitter of the transistor 19 is connected to the tapping on the voltage divider 21, 22 and its base is connected to the positive terminal of the supply source 6 through the resistors 35, 36, 37 of the delay network. Therefore, the transistor 19 becomes conductive when the said voltage difference is positive and exceeds its base emitter threshold. The collector current flows through the resistor 20 and the resulting voltage across this resistor biases the base of a second transistor 26' of the n-p-n type in the forward direction.

The collector of the transistor 26' is connected to the negative terminal of the supply source 6 through a switch, for example, a contact breaker 25, and its emitter is connected to the control electrode of the thyristor 18 through a resistor 28. Consequently, the transistor 26' can supply a forward current to this control electrode only if the transistor 19 is conducting and the contact of the switch 25 is broken. When these two conditions are simultaneously satisfied the thyristor 18 becomes conducting and the capacitor 1 discharges through this thyristor 18 and the winding 2. The resulting reduction of the potential of the emitter of the transistor 19 is delayed by the network 32 - 37, so that a reduction of the voltage across the capacitor 1 does not immediately cut off the transistors 19 and 26'. As a result the thyristor 18 is also maintained conducting, even after interruptions of its anode current of a duration longer than its recovery time. The duration of the forward current supplied to its control electrode is thus limited by the time constant of the network 32 - 37, provided the contact of the switch 25 remains broken during this time. During the time determined by this time constant, a train of high-voltage pulses may be produced across the secondary winding 4, for example, in the form of pulses of damped oscillations of a comparitively low frequency, for example, 20 kc/s, of the circuit 1,2 which is alternately closed through a thyristor 18 and through the rectifier 15'.

A practical embodiment of the apparatus of FIG. 1 included a balanced converter as shown in FIG. 2. The following components were used: Transistors 7 and 7' Philips type ASZ 18, Transistors 19 and 26 Philips type BC 186, Rectifier 15' Philips type BY 123, Thyristor 18 Philips type BTY 91, Capacitor 1 1 .mu.F, Resistors 12 and 12' 5 ohms each, Resistors 14 and 14' 270 ohms each, Resistor 20 22 kilo-ohms, Resistor 21 180 kilo-ohms, Resistor 22 5.6 kilo-ohms, Resistor 23 27 ohms, Resistor 24 4.7 kilo-ohms, Resistor 27 1 kilo-ohm, Resistor 28 220 ohms, Transformer 8-11 12 volts.fwdarw.400 volts, Transformer 2-4 normal ignition coil.

The control signal may be produced by various means other than a switch or contact breaker, for example, by a permanent magnet displacable relative to a coil or a Hall generator.

In the third embodiment, part of which is shown in FIG. 3, this signal is transmitted as a sudden large variation of a photosensitive resistor. The transistors 19 and 26' are connected in the manner shown in FIG. 2, with the exception that the base of the transistor 19 is directly connected to the positive terminal of the supply source 6, the delay network 32 - 37 being omitted, and that the collector resistor 20 of this transistor is also omitted and replaced by a photosensitive resistor or LDR 38 connected in series with a variable resistor 39 between the collector of the transistor 19 and the base of the transistor 26'. An electric incandescent lamp 40 is fed from the source 6 through a variable resistor 41. It is arranged so as to be capable of illuminating the LDR 38 through an aperture 42 in a screening member 43.

As shown in FIG. 4, the screening member 43 is a disc having two apertures 42. The disc may be driven by the crankshaft of a two-cylinder four-stroke spark-ignition internal-combustion engine together with the rotor of the distributor thereof.

As in the second embodiment shown in FIG. 2, the transistor 19 compares a proportional part of the charge voltage of the capacitor 1 with the voltage of the supply source 6 and conducts only if the capacitor voltage exceeds a predetermined value. The collector circuit of this transistor includes the control electrode cathode path of the thyristor 18, the resistor 28, the base emitter path of the transistor 26' and, in addition, the variable resistor 39 and the LDR-38. So long as the latter is not illuminated its resistance is so high that only a negligibly small collector current can flow to the base of the transistor 26' so that the emitter current of this transistor is not sufficient to render the thyristor 18 conducting. Consequently, the thyristor becomes conducting only when, the capacitor 1 being sufficiently charged, the LDR 38 is illuminated by the lamp 40.

The duration of the train of high-voltage pulses produced is equal to the time during which the LDR 38 is sufficiently illuminated to cause the transistor 26' to supply a sufficient forward current to the control electrode of the thyristor 18.

As shown in FIG. 4, the apertures or slots 42 of the screening member 43 have a gradually varying width, so that a gradually increasing and decreasing part of the light emitted by the lamp 40 illuminates the LDR 38. On the other hand, the sensitivity threshold of the control circuit can be varied by means of the variable resistor 39 and/or the brightness of the lamp 40 can be varied by means of the variable resistor 41. The duration of the train of high-voltage pulses and/or the starting instant of this train relative to the position of the screening member 43 can thus be controlled. In an ignition apparatus for a spark-ignition internal-combustion engine, this control can be automatically performed, for example, by varying the value of the resistor 41 under the control of data supplied by a gas-chromatograph and/or a tachometer-generator. A transistor can thereby obviously be used as the variable resistor. U.S. Pat. No. 3,361,123 (R. Kasama et al.) describes an ignition system in which the internal resistance of a transistor is varied as a function of engine speed to provide an automatic ignition advance capability. This automatic control is a very useful and desirable property because the composition and the pressure of the combustible gas mixture in a spark-ignition internal-combustion engine vary very strongly, whereas the most complete combustion possible is always desired or even required, incomplete combustion giving rise to strong corrosion of the spark plug electrodes, which again results in a still poorer combustion. An indispensable means for counteracting such incomplete combustion is a permanently operating automatic adjustment of the instant of ignition and of the duration of the ignition spark or train of ignition sparks. U.S. Pat. No. 3,314,407 (A. Schneider) provides background information for the invention and further illustrates the state of the art by describing a control system in which a parameter of an internal combustion engine is used to vary an electrical characteristic of an engine control system or the like. Bosch et al. discloses a fuel injection system for an internal combustion engine in which the airflow in the engine intake manifold influences the amount of fuel injection via a heated negative temperature coefficient resistor placed in the manifold and electrically connected in a transistor control circuit. Bosch et al also shows a centrifugal device coupled to the engine shaft and mechanically coupled to the arm of a variable resistor connected in the same transistor control circuit whereby the engine speed is made a factor in the control arrangement. The engine parameter control principles disclosed in the foregoing U.S. patents may be utilized in the embodiment of FIG. 3 of the invention.

In the modification shown in FIG. 5, the transistor 19 forms part of an integrated circuit 50 which also comprises base, emitter and collector resistors 29, 30 and 31 for this transistor, a protection diode 44, connected with the opposite polarity in parallel with its base emitter path, a phototransistor 38' which replaces the LDR 38 of FIG. 3, and three additional n-p-n amplifying transistors 46, 47 and 49 in grounded emitter connection with thin collector resistors 46 and 51 respectively, and coupling resistors 48 and 52 respectively. By using such an integrated circuit having only four connecting leads, namely two for the supply, one for the input and one for the output, the entire apparatus is reduced to very small dimensions, whilst its production is rendered very simple and hence cheap.

Claims

1. An apparatus for producing high-voltage pulses for a spark plug of an internal-combustion engine comprising, a capacitor connected in series with the primary winding of a step-up high-voltage transformer across the secondary winding of which at least one spark plug is connected, a voltage supply source including means by which said capacitor is charged to a predetermined voltage level sufficient to produce a spark discharge, a controlled rectifier through which the capacitor is discharged through the primary winding of the high-voltage transformer, and a control circuit for causing the controlled rectifier to become conductive including an and-gate to which a recurrent control signal synchronized with the engine rotation and a voltage derived from the voltage across the capacitor are applied so that the controlled rectifier is repeatedly rendered conductive when the voltage across the capacitor reaches said predetermined level and

2. An apparatus as claimed in claim 1 wherein the control circuit includes a transistor between the emitter and base electrodes of which a voltage is applied that is the difference between a proportional part of the voltage across the capacitor and a reference voltage proportional to the voltage of the supply source with the control signal present, so that the repetition frequency of the high-voltage pulses produced is substantially

3. An apparatus as claimed in claim 1 in which the series combination of the capacitor and the primary winding of the high-voltage transformer is shunted by the series combination of two rectifier elements so that when the controlled rectifier conducts the first mentioned series combination forms a resonant circuit closed for currents of both directions, characterized in that the capacitance of the capacitor and the inductance effective across the primary winding of the high-voltage transformer are chosen so that one half-cycle of the natural oscillation of the resonant circuit is shorter than the recovery time of the controlled rectifier, so that each high-voltage pulse produced is an oscillation pulse extending

4. An apparatus as claimed in claim 1 wherein the control signal is produced by the illumination of a photosensitive element through a slot formed in a screening member driven by the engine, characterized in that the width of this slot varies gradually, and means for varying the intensity of the source of light used for illuminating the photosensitive element and/or the sensitivity of said photosensitive element under control of the respective values of one or more operational parameters of

5. An apparatus as claimed in claim 4, further comprising means for varying the intensity of the light source and/or the sensitivity of the

6. An electric ignition system for an engine having one or more spark plugs comprising, a source of voltage, a transformer having a primary winding and a secondary winding coupled to a spark plug, a capacitor connected in series with said primary winding across the voltage source, means including said voltage source for charging the capacitor to a predetermined value sufficient to produce a spark discharge, a discharge circuit for said capacitor including a controlled rectifier connected in series with the capacitor and primary winding, and a control circuit coupled to the control electrode of the controlled rectifier so as to initiate current flow therein comprising, a gate circuit having an output coupled to said control electrode and an input, means controlled by the engine for generating a recurrent control signal that varies as a function of the engine speed, means for deriving a voltage proportional to the capacitor voltage, means coupling said signal generating means and said voltage deriving means to the input of said gate circuit whereby the gate circuit supplies a current initiating pulse to said control electrode when the control signal is present at the gate input and the capacitor voltage

7. A system as claimed in claim 6 wherein the electrical parameters of the transformer and capacitor are chosen so that a charge-discharge cycle of the capacitor is substantially shorter than the recurrence gating period of said control signal for low engine operating speeds whereby the capacitor is charged and discharged at least twice during each recurrence period of the control signal thereby to develop a plurality of spark discharge pulses across said spark plug during each recurrence gating

8. A system as claimed in claim 7 wherein said gate circuit includes an electronic switch having one input coupled to said signal generating means and said voltage deriving means and a second input coupled to a source of reference voltage of a value chosen so that said electronic switch passes a control pulse to said control electrode each time said predetermined capacitor voltage is developed and the control signal is present at the

9. A system as claimed in claim 6 wherein said voltage source comprises a DC-AC converter including electric oscillating means having an output circuit coupled to said capacitor and controlled rectifier via a rectifier circuit so that oscillations therein are inhibited during at least a part

10. A system as claimed in claim 6 wherein said control signal generating means comprises a contact breaker operated in synchronism with the engine.

11. A system as claimed in claim 6 wherein said control signal generating means comprises a photosensitive element connected in the input of said gate circuit, a source of light spaced from said photosensitive element, and an opaque member having a slot therein positioned between said light source and photosensitive element and rotatable in synchronism with the

12. A system as claimed in claim 11 further comprising means for varying the intensity of the light emitted by said light source as a function of

13. A system as claimed in claim 6 wherein said gate circuit comprises a transistor having a base electrode and an emitter electrode which together form the input thereof, and means for applying a reference voltage proportional to the voltage of the voltage source to one of said electrodes and said capacitor proportional voltage to the other of said transistor electrodes, the capacitor voltage being determined by the

14. A system as claimed in claim 6 wherein said voltage deriving means comprises a first voltage divider connected across said capacitor and to the input of the gate circuit and a second voltage divider connected across the voltage source and to the input of the gate circuit, and wherein said control signal generating means includes switching means synchronized to the engine shaft rotation and coupled to said first and second voltage dividers so as to alter the voltage distribution

15. A system as claimed in claim 6 wherein said control signal generating means comprises a photosensitive element conncted in the input of said gate circuit, a source of light spaced from said photosensitive element, an opaque screening member having a slot therein positioned between said light source and photosensitive element and rotatable in synchronism with the engine, and means for varying the intensity of the light emitted by said light source in a manner so as to control the instant of ignition of

16. A system as claimed in claim 6 wherein said control circuit further comprises a time delay network coupled to the input of the gate circuit and arranged to maintain the flow of said current pulse to said control electrode subsequent to the discharge of the capacitor and for a time

17. A system as claimed in claim 15 wherein the width of the slot in the

18. A system as claimed in claim 6 wherein said control signal generating means comprises a photosensitive element connected in the input of said gate circuit, a source of light spaced from said photosensitive element, an opaque screening member having a slot therein positioned between said light source and photosensitive element and rotatable in synchronism with the engine, the width of said slot varying gradually with its radial distance from the axis of rotation of the screening member, and means for varying the intensity of the light emitted by said light source as a function of the engine speed.