EP0559540A1 - Electronic ignition device with coil for engine with controlled ignition - Google Patents

Abstract

The device comprises an electronic cut-off switch (18) for an internal combustion engine with controlled ignition using a spark plug placed in series with the secondary of a coil, comprising a cut-off switch (18) placed in the charging circuit of the primary (16) from a source (Vbat), means for measuring the charging current which are intended to supply, on an output, a signal in response to exceeding a given value which is less than the current necessary for creating the ignition voltage by cutting off the primary. The device additionally comprises means for transposing the primary voltage to form the image of the secondary voltage and to supply, on the said output, a signal when the secondary voltage becomes lower than a given threshold indicating the end of the spark at the spark plug. <IMAGE>

Description

The subject of the present invention is an ignition device controlled by spark plugs, for an internal combustion engine, of the so-called "electronic" type, comprising a calculation and control unit, such as a microprocessor, controlling a cut-off switch placed in the primary charge circuit of a coil from a source, such as the battery of a vehicle equipped with the engine.

We have already proposed such a device comprising detection means intended to supply, on an output to the control unit, a signal when the charging current of the primary exceeds a determined value I₁ (less than the current I₂ to be cut to create the spark ignition voltage).

Because the instant T₀ of the start of the charging of the coil is fixed by the control unit, knowledge of the instant T₁ at which the current exceeds the determined value I₁ makes it possible to determine the duration t₂ just sufficient to obtain the primary current necessary for its breaking to give the required ignition voltage.

The start time of the charge can thus be optimally adjusted by the control unit.

The invention aims in particular to provide a device of the above type allowing the control unit to have information on the duration of the spark between the spark plug electrodes, without this requiring additional output to the control unit.

For this, the invention proposes in particular a device which further comprises means for transposing the primary voltage to form the image of the secondary voltage and supply, on said output, a signal when the secondary voltage becomes less than a determined threshold indicating the end of the spark at the candle.

The presence of these transposition means allows the control unit to detect an abnormal state (short circuit or open circuit), revealed by the maintenance of the arc voltage for a duration greater than or less than a range that the experience revealed as normal.

In practice, maintaining the arc voltage for a period of between approximately 0.4 ms and 2 ms will indicate correct operation; an arc voltage for less than 0.4 ms will indicate a high probability of open circuit while an arc voltage of a duration greater than 2 ms will indicate a high probability of short circuit.

The transposition means can in particular use the fact that, when the current at the primary is canceled, the difference between the voltage V _p at the terminals of the primary and the voltage of the source Vbat is equal to the voltage at the secondary V _s , divided by the transformation ratio N. A transposition circuit using this observation can in particular include, in series between the terminal of the primary not connected to the source and the ground, the emitter-collector junction of a transistor whose collector resistances and d emitters are proportioned so that the voltage across one of the resistors is representative of the difference between the voltage across the primary and the source voltage.

This arrangement makes it possible in particular, by comparing the voltage representative of the secondary voltage to several thresholds, to determine the characteristics of the arc, for example by taking into account that the ionization voltage (generally of the order of 10 KVolts) is significantly higher than the arc holding voltage, generally between 500 and 1000 Volts. The signals indicating the successive passage to the various thresholds can still be transmitted on the same output.

In another alternative embodiment, precise information on the spark voltage can be provided to the calculation and control unit by digitizing the voltage representative of the secondary voltage.

When the device comprises several coils, the diagnostic means (means for detecting the passage of a load current above a threshold and means for transposition) may be common to all of the coils.

The measurement of the spark duration can be taken into account by the calculation and control unit to adjust the primary current at which the cut is made, this in order to guarantee for example a sufficient spark duration to avoid unburned.

• la figure 1 est un schéma fonctionnel de principe d'un dispositif selon un mode particulier de réalisation de l'invention;
• la figure 2 montre les courbes représentatives de la variation, en fonction du temps t, du courant I_p dans le primaire de la bobine, de la tension V_c image de la tension d'arc et de la sortie V_d vers l'unité de calcul, au cours d'une séquence d'allumage;
• la figure 3 montre une constitution possible du dispositif de la figure 1.

The invention will be better understood on reading the following description of a particular embodiment, given by way of non-limiting example. The description refers to the accompanying drawings, in which:

• Figure 1 is a block diagram of a device according to a particular embodiment of the invention;
• FIG. 2 shows the curves representative of the variation, as a function of time t, of the current I _p in the primary of the coil, of the voltage V _c image of the arc voltage and of the output V _d towards the unit calculation, during an ignition sequence;
• FIG. 3 shows a possible construction of the device of FIG. 1.

The device, the principle of which is shown in FIG. 1, is intended for the ignition of a combustion chamber of an internal combustion engine, equipped with at least one spark plug 10 placed in series with the secondary 12 of a coil. 14. The primary 16 of the coil is connected to a power source (vehicle battery in general), of voltage Vbat. The current I _p which flows through the primary 16 is controlled by a power switch 18. A resistor 20 is connected in series between the switch 18 and the ground so as to provide a voltage at its terminals V representative of the current I _p .

The switch 18 is controlled on closing and opening by a calculation and control unit 22, generally comprising a microprocessor, fixing the instant T₀ of closing of the switch and the instant T₂ of opening. This switch can be controlled by the unit 22 by means making it possible to avoid the appearance of a pre-spark, of the kind described in French patent application No. 92 02300.

The device of FIG. 1 comprises detection means providing a signal when the current I _p of charge in the primary exceeds a determined value I₁, equal for example to 4.5 Amperes when the current I₂ to be cut to obtain a quality spark satisfactory is 6 Amps. These means include a comparator 24, one input of which receives the voltage V across the resistor 20 and the other of a reference voltage V _r1 . Thus the voltage V _d applied to the diagnostic input of the calculation and control unit 22 goes from the logic voltage V _cc (+ 5 volts for example) to zero when I _p exceeds the value I₁.

From the duration t₁ between the start T₀ of the primary charge, fixed by the unit 22, and the instant T₁ of exceeding I₁, the unit 22 can determine the time t₂ necessary to arrive at the current I₂ and, for the next ignition, determine T₀ appropriately.

The means for transposing the device in FIG. 1 include, for example, from the junction between the primary 16 and the switch 18, the emitter-collector junction of a bipolar transistor 26, of PNP type, and the resistors of transmitter and collector 28 and 30, with respective values R _e and R _c . The voltage V _c across the resistor R _c is, when the base of the transistor 26 is connected to the source voltage Vbat: $${\text{V}}_{\text{vs}}{\text{= (V}}_{\text{p}}{\text{- Vbat) .R}}_{\text{vs}}{\text{/ R}}_{\text{e}}$$

We thus have a voltage representative of the secondary spark voltage, when the current I _p is zero. A capacitor 30 of low value (100 nF for example) is advantageously placed in parallel with the resistor 30, to ensure filtering.

The voltage V _c is an image of the spark voltage at the secondary, when the current I _p is zero because the switch 18 is cut. This voltage V _c can be compared to a reference voltage V _r2 , in a comparator 34. The reference voltage V _r2 is chosen so that the passage of V _c below V _r2 indicates the end of the spark at the candle 10.

In the illustrated embodiment, the output of comparator 34 attacks the diagnostic input of the calculation and control unit 22. All of the comparators 24 and 34 then constitute the equivalent of an OR circuit, which passes in the low state at time T₃ (Figure 2). Indeed comparator 34 goes low at time T₂, when the primary circuit is cut, and remains there until time T₃.

Several comparators 34 can be connected in parallel, to supply successive signals, which this time can be impulse, on the diagnostic input of the unit 22, for increasingly low values of the voltage at the secondary, for example when the secondary voltage changes from the value necessary for ionization to the holding value.

In the case where several coils are equipped with the same diagnostic assembly (elements in the dashed line frame of FIG. 1) an element 36 with unidirectional conduction is interposed between each coil and this assembly.

It should also be noted that the duration t₁ gives an indication of the state of the ignition system, a short duration t₁ (for example less than 1.5 ms for a source voltage Vbat of 13.5 volts), indicating a probability short circuit in primary).

Various hardware constitutions of the principle circuit of FIG. 1 are possible. Figure 3, where the elements corresponding to those of Figure 1 have the same reference number assigned to the index a, shows a constitution in which each switch 18a associated with a coil 14a is constituted by an IGBT transistor. In addition, in the case illustrated in Figure 3, each coil 14a has two secondary 12a, each in series with a spark plug 10a. This solution allows, for a four-cylinder engine, to have only two switch assemblies 38 associated with a single diagnostic assembly of the kind shown by a dashed line frame in FIG. 1. Only one of the assemblies 38 is shown in detail. in figure 3.

The comparators 24a and 34a receive respective reference voltages supplied by resistance bridges placed between the logic voltage V _cc and the ground. The set of two comparators also constitutes an OR circuit, which attacks the diagnostic input of the calculation and control unit 22 via a resistance-capacitance filter and an inverter 40.

Each of the IGBT transistors 18a can be controlled from a particular output pin of the calculation and control unit 22, for example by means of an assembly of the type described in the French patent application already mentioned.

Claims (5)

Electronic ignition device for internal combustion engine with spark plug ignition placed in series with the secondary of a coil, comprising a cut-off switch (18,18a) placed in the charging circuit of the primary (16,16a) at from a source (Vbat), means for measuring the load current intended to supply, on an output, a signal in response to the exceeding of a determined value, less than the current necessary to create the ignition voltage by breaking from primary,
characterized in that it further comprises means (28, 26, 30 or 28a, 26a, 32a) for transposing the primary voltage to form the image of the secondary voltage and supply, on said output, a signal when the secondary voltage becomes below a determined threshold indicating the end of the spark at the spark plug. Device according to claim 1, characterized in that the transposition means comprise, in series between the terminal of the primary (16,16a) not connected to the source and the ground, the emitter-collector junction of a transistor (26,26a ) and the collector and emitter resistors, proportioned so that the voltage across one of the resistors is representative of the difference between the voltage across the primary and the source voltage. Device according to claim 1 or 2 for an engine with several cylinders and at least one spark plug per cylinder, characterized in that the measuring means and the transposition means are common to all the cylinders and in that a respective diode (36, 36a) is interposed between the primary of each coil and the transposition means. Device according to any one of the preceding claims, characterized in that the measuring means and the transposition means also comprise a comparator (34,34a), one input of which receives a voltage reference and the other input receives the voltage across that of the emitter and collector resistors which is connected to ground. Device according to any one of the preceding claims, characterized in that the transposition means are connected to an analog-digital converter providing an indication of the variation of the arc voltage of the respective spark plug.

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