EP0529735B1 - Ignition device for an internal combustion engine - Google Patents

Description

The present invention relates to a device for igniting internal combustion engines, comprising a coil with a primary winding and a secondary winding, provided with a frame in which the passage of a magnet driven by the rotation of the engine produces a flux variable magnetic which induces in the primary winding a current pulse whose cut-off causes in the secondary winding a high voltage pulse intended for a spark plug, a primary current circuit in which the said current closes and which has an impedance in series with a switch transistor, and a circuit for controlling the conductive or non-conductive state of the switch transistor, causing the current to be cut off by cutting the said switch transistor when the value of a control signal applied to an input terminal of this circuit crosses a determined threshold value.

Such a device is used in small internal combustion engines, for example for lawn mowers, chain saws, hedge trimmers, brush cutters, etc.

Such a device is known from German patent DE 23 14559. The circuit described by this document comprises a switch transistor which is of the Darlington NPN type with, in series in its emitter connection an impedance, in this case a measurement resistance of current. The voltage generated by the current in this resistor is applied to the base-emitter space of an NPN transistor, which therefore becomes conductive when the current exceeds a determined value. This transistor is connected in such a way that its conduction causes the switching transistor to switch off.

Such a device works, but it is not protected against the operation of the engine turning upside down.

The invention proposes to provide a device which, when the engine turns upside down, does not trigger an ignition spark.

Another object of the invention is to provide a device which makes it possible to obtain, when the engine is running in the normal direction, a variable ignition advance with speed and well suited to the requirements of the engines.

The invention is based on the remark that the known magnetic flywheels provide, before the main current pulse which is used for ignition, another pulse of opposite polarity, and on the idea of introducing into the device a capacitance which is charged by the first pulse and whose charge thus acquired remains active in operation during the following main pulse, so that in the event of reverse engine operation, which reverses the order when pulses appear, operation is completely disrupted.

Thus a device according to the invention is remarkable in that, at least a first and a second immediately successive current pulses of opposite polarities being induced by the variable magnetic flux and the control circuit being designed to trigger the switching off of the switch transistor at during the second pulse, the variable voltage developed at the common point between the impedance and the switch transistor is transmitted to the input terminal of the control circuit by a capacitor whose armature is connected to said common point and, the transmitter and the collector of the switch transistor being shunted by a diode, there is a current path in series with the capacitance between the two terminals of the primary winding, path which allows the preloading of the capacitance, via the diode, during the first pulse.

The current path in question allows a certain charge of the capacity during the first pulse, a charge which will be added to the control signal thereafter, since it is by this same capacity that the useful control signal is transmitted .

It is true that a device comprising a resistor through which the primary current passes and also a capacitor is known from the German patent DE 15 39180. However in this device, the capacitor is directly connected to one end of the primary winding (point referenced 28 or "c"), and its charge cannot therefore be added or subtracted with the voltage created by the current in the resistor.

The device according to the invention operates with maximum security when said threshold value is determined in such a way that the breaking of the current is controlled by the variable voltage, developed as a common point between the impedance and the switch transistor during the second electrical pulse, only when the capacity is preloaded beyond a predetermined value by the current of the first pulse.

In a particular embodiment, the switch transistor being of a first polarity and said impedance being placed in its collector path, the control circuit comprises a transistor of a second polarity whose base is connected by the capacitance at said point common, whose emitter is connected to the end of the impedance which is not that connected to the common point, and whose collector is connected to the base of the switch transistor.

In another embodiment, the switch transistor being of a first polarity and said impedance being placed in its collector path, the control circuit comprises a transistor of the first polarity whose base is connected by the transmission capacity to said common point and whose collector is connected to the base of a first transistor of a second polarity whose emitter is connected to the end of the impedance which is not that connected to the common point and whose collector is connected to the base of the transistor of the first polarity, and a second transistor of a second polarity whose emitter is connected to the end of the impedance which is not that connected to the common point, whose collector is connected at the base of the switch transistor, and the base of which is connected to the emitter of the transistor of the first polarity.

In yet another embodiment, the switch transistor being of a first polarity and said impedance being placed in its collector path, the control circuit comprises a transistor of the first polarity, the base of which is connected by the transmission capacity to the said common point and whose collector is connected to the end of the impedance which is not that connected to the common point, and a transistor of a second polarity whose emitter is connected to the end of the impedance which is not that connected to the common point, the collector of which is connected to the base of the switch transistor, and the base of which is connected to the emitter of the transistor of the first polarity.

According to a variant, the switch control circuit further includes a transistor of a second polarity, the base of which is connected to the midpoint of a resistance bridge which connects the two ends. of the primary winding, whose emitter is connected to the end of the impedance which is not that connected to the common point, and whose collector is connected to the emitter of the above-mentioned transistor of the first polarity.

According to another variant, the control circuit further comprises a transistor of a second polarity, the base of which is connected to the midpoint of a resistance bridge which connects the two ends of the primary winding, the emitter of which is connected to the end of the impedance which is not that connected to the common point, and whose collector is connected to the end of the capacitance which is not that connected to the common point.

The description which follows, with reference to the appended drawings describing nonlimiting examples will make it clear how the invention can be implemented.

FIG. 1 schematically represents a device according to the invention.

FIGS. 2A and 2B are diagrams of the voltage developed by the winding over time.

Figures 3 to 6 show different variants or embodiments of electrical diagrams of a device.

The device shown in Figure 1 comprises a coil with a primary winding 1 and a secondary winding 2. The windings are placed on a frame 3 in which a magnet 4 driven by the rotation of the motor produces a variable magnetic flux which induces a current in the primary winding 1.

A cut in this current causes a high voltage pulse in the secondary winding for a spark plug.

The current of the primary winding 1 taken from terminals 5 and 6 closes in a primary current circuit made of a resistor R1 in series with a switch transistor Ti. The switch transistor Ti is of the NPN type and the resistor R1 is placed in its collector path.

Figure 2A shows the voltage pulses produced by the primary winding in the case of normal operation with a determined geometry of the magnet and the armatures chosen by way of example. This layout was noted for an idle operation, at 1500 rpm. Each passage of the magnet in front of the coil generates four pulses a, b, c, d, the fourth being very weak. The first and second immediately successive pulses of opposite polarities participate in the operation of the circuit. The other two play no role in normal operation. Figure 2B shows the voltage pulses produced in reverse. Due to the asymmetry of shape of the armature of the coil the shape of the pulses has changed, and the first pulse has become very weak, too weak to play a role, especially since it disappears completely as soon as the circuit is connected and draws current from the winding. Consequently, the first pulse which is not negligible is the pulse "b" of polarity opposite to the pulse "a" of FIG. 2A. The main pulse is now pulse "c", which is of opposite polarity to pulse "b" in Figure 2A.

In the circuit of FIG. 1, a control circuit 7 for the conductive or non-conductive state of the switch transistor Ti is provided with an input terminal 9 and is connected to the base of the transistor Ti. Its essential role is, during the second pulse b, to first make the transistor Ti conductive and then to cause the current of this transistor to be cut off. This interruption occurs when the increasing voltage on the input terminal 9 crosses a determined threshold value. The variable signal at the common point 8 between the resistor R1 and the transistor Ti is applied to the input terminal 9 via a capacitor C.

The transistor Ti is here a transistor of the so-called Darlington type which generally comprises by construction a diode D in shunt, diode D which is conductive in the direction of current opposite to that of the transistor Ti.

A current path in series with the capacitance between the two terminals of the primary winding, path which is conductive at least during the first pulse, is constituted by the diode D and a resistor 10 between the input terminal 9 of the control circuit 7 and terminal 5.

The threshold value on input 9 of the circuit of FIG. 1, for which the setting in the non-conducting state of the switch transistor Ti, is determined in such a way that the breaking of the current is controlled by the variable voltage developed at the common point 8 during the second electrical pulse only when the capacity is preloaded by the current of the first pulse. A person skilled in the art will easily obtain this by carrying out successive experiments during which he will vary elements of the circuit 7 at his choice, so as to vary the triggering threshold, until the above condition is satisfied. . This will be explained more precisely on the occasion of the description of the diagrams given below by way of examples.

In the device of FIG. 3, the control circuit 7 comprises a transistor T2 of PNP type, the base of which is connected to the capacitor C via a resistor 11. The other end of the capacitor is connected to the common point 8. The emitter of transistor T2 is connected to the end of resistor R1 which is linked to terminal 5, i.e. the end which is not connected to common point 8, and its collector is connected to base of the switch transistor Ti via a diode and a resistor in series. A diode D2 constitutes, in series with the resistor 11, a current path between the input of the control circuit, that is to say the point 9 connected to the capacitor C, and the end of the impedance R1 linked at terminal 5, i.e. the end which is not connected to common point 8.

The operation is as follows: the first pulse produces a signal between terminals 5 and 6 which is more negative on terminal 5; thus the capacitor C is charged via the elements D, 11, D2 with a negative polarity on its left armature in the figure; at the start of the second pulse, the negative voltage across the capacitor negatively polarizes the base of transistor T2 relative to its emitter, which makes it conductive; the pulse produces a more positive signal on terminal 5, and therefore it passes an emitter-collector current in the transistor T2 which makes the transistor Ti conductive and a current begins to pass in R1 and the transistor Ti; the capacitance C is discharged then possibly charges in the other direction due to the base current of the transistor T2, regulated by the value of the resistor 11, and in the end the transistor T2 passes to the cut state as well as the transistor Ti. In the absence of a first negative pulse, the capacitor is not charged and the transistor T2 as well as the transistor Ti are blocked from the start of the positive pulse; there is no spark.

This circuit which corresponds to the simplest variant of a circuit according to the invention has the disadvantage of not operating if the first pulse is small, that is to say at low speed, unless using a very powerful magnet.

In the circuits of FIGS. 4 to 6, the control circuit comprises a transistor T1 of NPN type, the base of which is connected by the capacitance C to the common point 8, via a resistor 21 in series with the capacitance C.

A second PNP type transistor T2 has its emitter connected to the resistor R1 on the side of terminal 5. In the circuit of FIG. 4, a diode is inserted in this link. In the circuits of Figures 5 and 6, a resistor 26 is inserted there. The collector of transistor T2 is connected to the base of the switch transistor. In the circuit of Figure 4, a resistor 28 is interposed in series in the link. The base of the PNP transistor T2 is connected to the emitter of the NPN transistor T1. In the circuit of Figure 4 a resistor 12 is inserted in the link. The base of transistor T2 is also connected to the emitter of the switch transistor via one or more resistors, namely resistance 13 in the circuits of FIGS. 5 and 6, and resistors 12 and 13 in series in the circuit of FIG. 4. A resistor 30, possibly integrated in the transistor Ti, connects the base of the transistor Ti to terminal 6.

We will assume later that the ground is connected to terminal 6. This is only a convention intended to remove any ambiguity in certain explanations.

In the circuit of FIG. 4, the base of the transistor T1 is connected to the collector of a transistor T4 of the PNP type, the emitter of which is connected to the terminal 5, and the base of which is connected to the collector of the NPN transistor T1. Thus the two transistors T1 and T4 are mounted as a thyristor in a known manner, which prevents the transistor T1, once it has become conductive, from relocking again. Two resistors 22, 23 in series are placed between terminal 5 and point 9. A resistor 24 is further connected between the base of transistor T2 and that of transistor T1 and a resistor 18 between terminal 5 and the base of transistor T2 .

In the circuits of FIGS. 5 and 6, the collector of the transistor T1 is connected to the end of the impedance R1 which is linked to the terminal 5, that is to say which is not connected to the common point 8. Its base is also connected to terminal 5 by a resistor 17.

A current path between the capacitance armature, on the left in the figures, and the end of the impedance R1, at the top in the figures, for charging the capacitance during the first negative pulse, is constituted by a or several resistors in series: the resistors 22, 23 or the resistors 18, 24, 21 in the circuit of FIG. 4, the resistors 17 and 21 in the circuits of FIGS. 5, 6.

The setting of the threshold value at input 9 for which the transistor Ti is cut can be done in the circuit of Figure 4 by the values of the resistors 12 or 13 or 18 and in that of Figures 5 and 6 by the ratio of the resistance bridge 17, 21. The value of the basic voltage which makes the transistor T2 conductive is also an important element for fixing the triggering threshold. Elements such as the diode D3 or the resistor 26 bring this voltage to the desired value.

These three circuits operate as follows: when a positive voltage appears on terminal 5 relative to terminal 6, a current flows through the emitter-base junction of transistor T2 and goes to terminal 6 via resistors 12 and / or 13; the transistor T2 is therefore conductive and supplies the base of the transistor Ti which becomes conductive; therefore the voltage at point 8 is close to that at point 6 and the voltage at point 9 is even lower given the prior charge of capacitance C on the first negative pulse (as explained with reference to the figure 3), therefore the transistor T1 is blocked; part of the current flowing in the transistor Ti passes through the resistor R1 of course, but also through the current path already mentioned which makes the voltage on the armature 9 of the capacitor C more and more positive; after a certain delay, this voltage at point 9 is high enough to make the transistor T1 conductive, which causes the voltage to rise on the base of the transistor T2; this one then blocks as well as the transistor Ti.

It is clear that when the motor turns upside down, there is no longer any precharge of the capacitance C by the first pulse and the operation is modified. There may still be a spark, but it is significantly shifted in time.

In the circuit of FIG. 4, the point common to the base of the transistor T4 and to the collector of the transistor T1 is connected by a resistor at the midpoint of the bridge of the two resistors 22, 23. This allows the direct triggering of the transistor T4 at large speed, the capacitance C then transmitting to the bridge 22, 23, before having had time to discharge, the voltage created at the terminals of the resistor R1 by the rise of the current. There is then more advance on ignition.

In the circuits of FIGS. 5 and 6, the emitter of transistor T2 is connected to the end 6 of the primary winding by a resistor 27.

In the circuit of FIG. 5, the control circuit further includes a PNP type transistor T5, the base of which is connected to the central point of a star with three resistors 19, 20, 14. Each of these resistors is connected, from the side opposite to the central point, respectively to terminal 5, i.e. the common point of the primary winding and the resistor R1, at the end 6 of the primary winding, and at the end 8 of the impedance R1. As a variant, the resistance 20 or else the resistance 14 may be infinite, that is to say not be present. The emitter of transistor T5 is connected to the end of resistor R1 which is not the one connected to common point 8, and its collector is connected to the emitter of transistor NPN T1, i.e. the base of transistor T2. In addition, a diode D1 is connected by its anode to the intermediate point of two resistors in series connecting terminal 5 to the base of the transistor T5, and by its cathode to the end of the capacitor C on the side of the transistor T1. This diode allows a faster rise than that which would be obtained via resistors 17 and 21 only, for the voltage on terminal 9 of the capacitor.

The transistor T5 is blocked at the start of a positive pulse because the voltage obtained at the terminals of the resistor 19 from that between the terminals 5 and 6 or between the terminals 5 and 8 by division in the bridges of resistors 20, 19 and 14, 19 is lower than the emitter-base voltage which would make the transistor T5 conductive. When this voltage has grown, the transistor T5 becomes conductive which blocks the transistor T2, as when the transistor T1 becomes conductive. Thus the transistors T5 and T1 act each on their own, and it is the first to act which matters: at high speed it is T5 which acts first, which then makes it possible to have more advance on ignition . The switching from one operating mode to the other takes place suddenly at a certain speed.

The circuit of FIG. 6 which is the preferred assembly, is derived from that of FIG. 5: the control circuit comprises a transistor T6 of PNP type whose base is connected to the central point of a star of three resistors 15, 16 , 31. Each of these resistors is connected, on the opposite side to the central point, respectively to terminal 5, that is to say the common point of the primary winding and of the resistor R1, at the end 6 of the primary winding , and at the end 8 of the impedance R1. As a variant, the resistor 16 or else the resistor 31 can be infinite, that is to say not be present. The emitter-collector path of transistor T6 is not connected in parallel with that of transistor T1 as is the case for transistor T5 in figure 5, but creates a connection between terminal 5 and the armature, on the left in the figure, of the capacitance C. Instead of acting directly to block the transistor T2 as does the transistor T5 of FIG. 5, the transistor T6 when it becomes conductive adds an additional current to that which crosses the resistor 17, which causes the tension of the armature 9 of the capacitor C to rise more quickly; thus is provided an ignition advance which, from a certain speed, varies gradually as a function of the speed.

When the transistor Ti is cut, the positive voltage increases very quickly on the terminal 5 of the primary winding, because of the self-inductance. The corresponding current in the chain of elements constituted by the resistor 17, the base-emitter space of the transistor T1 and the resistor 13 could develop at the terminals of the resistor 17 a voltage sufficient to make the transistor T2 again conductive. The bridge of resistors 26, 27 varies the voltage of the emitter of transistor T2 so as to follow the rise of the voltage applied to its base and to avoid this drawback.

Satisfactory operation of the circuit of FIG. 6 is obtained with a resistance R1 of the order of 1 to 2 Ω, a capacitance of the order of 27 nF, resistors 21, 17, 13, 15, 16, 26, 27 , 30 respectively of the order of 1.5 kΩ, 40 kΩ, 15 kΩ, 5 kΩ, 40 kΩ, 50 Ω, 7 kΩ, 3 kΩ. With these values, no resistor 31 is used. An element 29, made of a resistor of around 5 kΩ in series with a so-called CTN resistor of 15 kΩ, is furthermore connected between terminal 5 and the base of the transistor T6, so as to maintain the temperature characteristics. The Ti and T2 transistors are high voltage models.

The invention is not limited to the embodiments described above. It is possible for example to add to the circuit of FIGS. 5 and 6 the resistor 28 of the circuit of FIG. 4, or to introduce into the circuits of FIGS. 5 and 6 the chain of resistors 18, 12, 13 of the circuit of the figure 4. In addition, it is roughly equivalent to connect the lower terminal of the resistor 17 of figures 5 and 6 on the other side of the resistor 21, that is to say directly at point 9, the resistor 21 being of low ohmic value. In the diagram of FIG. 6, the high point of this same resistor 17, which is connected to the supply terminal 5, can also be connected to it through the resistor 15, that is to say that said resistor 17 is then connected between the base of transistor T6 and point 9.

More generally, by placing the resistor R1 in the emitter connection of the transistor Ti, the person skilled in the art can easily imagine a control circuit 7 which has the required voltage levels (it suffices for example to use transistors of opposite polarity and add or subtract a transistor to reverse the signal applied to the base of the switch transistor) so as to reproduce in another form the circuits described here. The transistor Ti can also be replaced by another controllable switch device, for example a MOS type transistor.

Claims (9)

1. An ignition device for internal combustion engines, comprising a coil which includes a primary winding (1) and a secondary winding (2) and is provided with an armature (3) in which the passage of a magnet, driven by the rotation of the engine, produces a variable magnetic flux which induces in the primary winding a current pulse whose interruption causes a high-voltage pulse for a spark plug in the secondary winding, a primary current circuit in which said current is restored and which comprises an impedance (R1) in series with an interrupt transistor (Ti), and a circuit (7) for controlling the turning on and off of said interrupt transistor, causing the interruption of the current by the turning off of said interrupt transistor when the value of a control signal applied to an input terminal (9) of this circuit exceeds a given threshold value, characterized in that at least a first (a) and a second (b) directly successive current pulse of opposite polarity are induced by the variable magnetic flux, that the control circuit is conceived to trigger the turning off of the interrupt transistor in the course of the second pulse (b), that the variable voltage developed at the junction (8) between the impedance (R1) and the interrupt transistor (Ti) is applied to the input terminal (9) of the control circuit via a capacitance (C), an armature of which is connected to said junction, that the emitter and the collector of the interrupt transistor are shunted by a diode, and that a current path (D, 10) exists, in series with the capacitance, between the two terminals (5, 6) of the primary winding, which path enables the precharging of the capacitance, via the diode, during the first pulse.
2. A device as claimed in Claim 1, characterized in that said threshold value is determined so that the interruption of the current is by the variable voltage developed at the junction (8) between the impedance and the interrupt transistor during the second electric pulse, be it only if the capacitance (C) has been precharged by the current of the first pulse (a).
3. A device as claimed in any one of the Claims 1 or 2, characterized in that the interrupt transistor (Ti) is of a first polarity and said impedance (R1) is arranged in its collector path, the control circuit comprises a transistor (T2) of a second polarity whose base is connected to said junction (8) via the capacitance (C), its emitter being connected to the end (5) of the impedance (R1) which is not the end connected to the junction (8), its collector being connected to the base of the interrupt transistor (Ti).
4. A device as claimed in any one of the Claims 1 or 2, characterized in that the interrupt transistor (Ti) is of a first polarity and said impedance (R1) is connected in its collector path, the control circuit comprises a transistor of the first polarity (T1) whose base is connected to said junction (8) via the transmission capacitance (C) and whose collector is connected to the base of a first transistor (T4) of a second polarity, the emitter of which is connected to the end (5) of the impedance which is not the end connected to the junction (8), its collector being connected to the base of the transistor (T1) of the first polarity, and also comprises a second transistor (T2) of a second polarity whose emitter is connected to the end (5) of the impedance which is not the end connected to the junction (8), whose collector is connected to the base of the interrupt transistor (Ti), and whose base is connected to the emitter of the transistor (T1) of the first polarity.
5. A device as claimed in any one of the Claims 1 or 2, characterized in that the interrupt transistor (Ti) is of a first polarity and said impedance (R1) is connected in its collector path, the control circuit comprises a transistor of the first polarity (T1) whose base is connected to said junction (8) via the transmission capacitance (C) and whose collector is connected to the end (5) of the impedance which is not the end connected to the junction, and also comprises a transistor (T2) of a second polarity whose emitter is connected to the end (5) of the impedance which is not the end connected to the junction (8), whose collector is connected to the base of the interrupt transistor (Ti), and whose base is connected to the emitter of the transistor (T1) of the first polarity.
6. A device as claimed in Claim 5, characterized in that the switch control circuit also comprises a transistor (T5) of a second polarity whose base is connected to the central point of a resistance bridge (19, 20) which interconnects the two ends (5, 6) of the primary winding, whose emitter is connected to the end (5) of the impedance which is not the end connected to the junction (8), and whose collector is connected to the base of said transistor (T2) of the second polarity.
7. A device as claimed in Claim 5, characterized in that the switch control circuit also comprises a transistor (T6) of a second polarity whose base is connected to the central point of a resistance bridge (15, 16) which interconnects the two ends (5, 6) of the primary winding, whose emitter is connected to the end (5) of the impedance which is not the end connected to the junction (8), and whose collector is connected to the end of the capacitance (C) which is not the end connected to the junction (8).
8. A device as claimed in Claim 5, characterized in that the switch control circuit also comprises a transistor of a second polarity (T5) whose base is connected to the central point of a resistance bridge (19, 14) which interconnects the two ends of the impedance (R1), whose emitter is connected to the end (5) of the impedance which is not the end connected to the junction (8), and whose collector is connected to the base of said transistor (T2) of the second polarity.
9. A device as claimed in Claim 5, characterized in that the switch control circuit also comprises a transistor of a second polarity (T6) whose base is connected to the central point of a resistance bridge (15, 31) which interconnects the two ends of the impedance (R1), whose emitter is connected to the end (5) of the impedance which is not the end connected to the junction (8), and whose collector is connected to the end of the capacitance (C) which is not the end connected to the junction (8).

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