FR2946190A1 - METHOD FOR DETECTING THE TYPE OF SPARK GENERATED BY A RADIOFREQUENCY IGNITION CANDLE COIL AND CORRESPONDING DEVICE - Google Patents

Abstract

The invention relates to a method for detecting the type of spark generated by a radiofrequency ignition coil-coil, for a motor vehicle engine, comprising the steps of: - generating a spark, and - measuring the ionization current. According to the invention, it essentially further comprises the steps of: - detecting the first ionization peak of the ionization current after the end of the spark, - determining the time (T mes) between the end of the spark and the first ionization current ionization peak, - compare the time value (T_mes) with a reference value (T_th), and - consider that the spark is of the mono filamentary type if the value of time ( T mes) is smaller than the reference value (T_th), and of branched type if the time value (T_mes) is greater than the reference value (T_th).

Description

METHOD FOR DETECTING THE TYPE OF SPARK GENERATED BY A RADIOFREQUENCY IGNITION CANDLE COIL AND CORRESPONDING DEVICE

The present invention relates to the field of radiofrequency ignition of a motor vehicle spool-plug, and more particularly the measurement of the ionization current. The measurement of the ionization current of the gases in the cylinders of the engine is carried out typically after the end of the ignition and finds particularly advantageous applications, for example for the detection of the crankshaft angle corresponding to the pressure peak of the chamber of combustion, knocking or for the identification of misfires. The operation of ionization current measuring circuits for a conventional ignition system is to bias the mixture of the combustion chamber after the generation of the spark between the electrodes of the spark plug, in order to measure the current resulting from the spread of combustion. Such circuits are conventionally arranged at the foot of the secondary of an ignition coil connected to the spark plug.

These circuits, however, need to be dedicated to the characteristics of the conventional ignition and are therefore not adaptable as such to plasma-generated ignition systems, using ignition plugs of the radiofrequency coils-candles type, as described in detail in the following patent applications filed in the name of the applicant FR 03-10766, FR 03-10767 and FR 03-10768. The specificities of radiofrequency ignition generate several constraints for measuring the ionization current.

The measurement of the ionization current is carried out after the end of the ignition. Its amplitude depends on the DC voltage or bias voltage applied between the high-voltage electrode of the spark plug and the motor mass. The bias voltage is typically between the battery voltage and several hundred volts. Experience shows that the signal representative of the ionization current has an amplitude of between 0.1 pA and 1 mA depending on the conditions of the combustion chamber (temperature, pressure, composition of the mixture, etc.). However, the ignition control signal induces large currents that have an amplitude difference of nearly 120 dB with the ionization current that is to be measured. The measuring circuit therefore undergoes a glare time during which it can not acquire a weak current. In addition, this type of ignition can develop two types of discharge: a branched-or multi-filament spark, and a single-filament arc spark. However, the type of spark directly affects the quality of the combustion. It is desirable to be able to detect the type of spark generated by a radiofrequency ignition coil-coil. Also, more specifically, according to a first of its objects, the invention relates to a method for detecting the type of spark generated by a radiofrequency ignition coil-coil, for a motor vehicle engine, comprising the steps of: generate a spark, and measure the ionization current. The measurement of an ionization current by a device for measuring an ionization current is known to those skilled in the art, in particular by the example given in the document of the prior art. 07-58795 filed by the Applicant. However, such a device does not detect the type of spark generated.

The present invention aims to overcome these disadvantages by providing a method for detecting in real time the type of spark generated by a radiofrequency ignition coil-plug. With this objective in view, the device according to the invention, furthermore in accordance with the preamble cited above, is essentially characterized in that it further comprises the steps of: detecting the first ionization peak of the current ionization after the end of the spark, - determine the time between the end of the spark and the first ionisation current peak, - compare the value of time with a reference value, and - consider that the spark is of the monofilament type if the value of the time is less than the reference value, and of the branched type if the value of the time is greater than the reference value. In one embodiment, it is expected to modify at least one of the ignition parameters as a function of the difference between the time and the reference value. In one embodiment, the method further comprises a step of slaving the reference value to the operating point of the engine. Alternatively, the reference value is constant over time, regardless of the operating point of the motor. According to another of its objects, the invention relates to a device for detecting the type of spark generated by a radiofrequency ignition coil-coil, for a motor vehicle engine, capable of implementing the method according to the invention and comprising means for generating a spark, and means for measuring the ionization current.

The device according to the invention is essentially characterized in that it further comprises: - means for detecting the first ionization current ionization peak after the end of the spark, - means for determining the time between the end of the spark and the first ionisation current peak, - means for comparing the value of time with a reference value, and - means for considering that the spark is of mono type filamentary if the value of the time is less than the reference value, and branched if the value of the time is greater than the reference value. In one embodiment, the device further comprises means for modifying at least one of the ignition parameters as a function of the difference between the time and the reference value. In one embodiment, the device further comprises means for controlling the reference value at the operating point of the motor. Preferably, the distance between the coil-spark plug electrode and the ground electrode is greater than or equal to 4mm. According to another of its objects, the invention relates to a computer program, comprising program code instructions for executing the steps of the method according to the invention, when said program is executed on a computer.

Finally, the invention also relates to a motor vehicle equipped with the device according to the invention or a programmable digital core such as a processor comprising the computer program or onboard computer according to the invention. Other features and advantages of the present invention will emerge more clearly on reading the following description given by way of illustrative and nonlimiting example and with reference to the appended figures in which: FIG. 1 illustrates an embodiment of the device according to the invention, FIG. 2 illustrates an embodiment of the device for measuring the ionization current according to the invention, FIG. 3 illustrates the equivalent circuit of a monofilament spark, FIG. a branched spark; FIG. 5 illustrates the relative ranges of the values of TO and T1 after the end of the spark, and FIG. 6 is a map illustrating the different types of spark as a function of the inter-electrode distance and of the supply voltage.

The detection device according to the invention is configured to be connected to a motor vehicle radiofrequency ignition coil / spark plug. Also in one embodiment, the device may include a spark plug-coil.

The coil-spark plug implemented in the context of the controlled radiofrequency ignition is electrically equivalent to a resonator 1, whose resonance frequency F, is greater than 1 MHz, and typically close to 5 MHz. The resonator comprises in series a resistor Rs, an inductance coil Ls and a capacitance Cs. Ignition electrodes 111 and 112 of the spark plug are connected across the capacitor Cs of the resonator, making it possible to generate electrical discharges, preferably branched, to initiate the combustion of the mixture in the combustion chambers of the engine, when the resonator is powered at its resonance frequency.

Indeed, when the resonator is powered by a high voltage at its resonant frequency F, (1 / (2nVLs * Cs)), the amplitude across the capacitor Cs is amplified so that multi-filament discharges occur. develop between electrodes, over distances of the order of a centimeter, at high pressure and for peak voltages of less than 40 kV. These are referred to as branched sparks, insofar as they involve the simultaneous generation of at least several lines or ionization paths in a given volume, their branches being moreover omnidirectional. This radiofrequency ignition application requires the use of a power supply circuit, capable of generating voltage pulses, typically of the order of 100 ns, which can reach amplitudes of the order of 1 kV, at a voltage of frequency very close to the resonance frequency of the plasma generation resonator of the radiofrequency coil-plug. The detection device according to the invention therefore comprises a power supply circuit 2, which conventionally implements a so-called pseudo-class E power amplifier arrangement. This assembly makes it possible to create the voltage pulses with the aforementioned characteristics.

This arrangement comprises a Vinter intermediate feed that can vary from 0 to 250V, a MOSFET power transistor M and a parallel resonant circuit 4, comprising a coil Lm (primary of a transformer T) in parallel with a capacitor Cp, with a frequency of resonance close to 5 MHz also.

The primary winding Lm of the transformer is connected on one side to the supply voltage Vinter and on the other side to the drain of the switch transistor M, controlling the application of the supply voltage Vinter across the winding primary at the frequency defined by the control signal V1.

The secondary winding Ln of the transformer, one side of which is connected to ground by a grounding wire 6, is in turn intended to be connected to the spark-plug. In this way, the resonator 1 of the coil-plug, connected to the terminals of the secondary winding by connecting son 5 and 6, whose ground return wire 6, is thus fed by the secondary of the transformer.

The transistor M is used as a switch to control the switching at the terminals of the parallel resonant circuit and the plasma generation resonator 1 to be connected to an output interface OUT of the supply circuit. The transistor M is driven on its gate by a logic control signal V1, supplied by a control stage 3, at a frequency which must be substantially set to the resonance frequency of the resonator 1. The intermediate continuous supply voltage Vinter, which can vary between 0 and 250 V, can advantageously be provided by a high voltage power supply, typically a DC / DC converter. Thus, close to its resonant frequency, the parallel resonator 4 transforms the continuous supply voltage Vinter into an amplified periodic voltage, corresponding to the supply voltage multiplied by the overvoltage coefficient of the parallel resonator and applied to an interface of output of the power supply circuit at the drain of the switch transistor M.

The switch transistor M then applies the amplified supply voltage to the output of the power supply, at the frequency defined by the control signal V1, which is sought to make as close as possible to the resonant frequency of the coil -bougie, so as to generate the high-voltage across the electrodes of the coil-candle necessary for the development and maintenance of the multi-filament discharge. The transistor thus switches high currents (Istart 20A) at a frequency of about 5 MHz, and with a drain-source voltage of up to 1 kV. The choice of the transistor is critical and requires a compromise between voltage and current. Also, according to the embodiment illustrated in FIG. 1, the transformer T has a variable transformation ratio, for example between 1 and 5. Preferably, the transformation ratio is adapted so as to reduce the drain-source voltage of the transformer. transistor switch M. The adaptation of the transformation ratio reduces the drain-source voltage of the transistor. The decrease in the primary voltage, however, induces an increase in the current flowing through the transistor. It is then possible to compensate for this constraint by, for example, placing two transistors in parallel controlled by the same control stage 3. The measurement of the ionization current consists in polarizing, at the level of the spark plug, the air / fuel mixture. present in the combustion chamber. An electrical image of the development of combustion is thus obtained as: I (t) _ U POLAR Z (t) With: 1 (t) UPOLAR Z (t) the ionization current measured over time , the applied bias voltage, and the equivalent impedance of the burned fuel / gas mixture. In order to measure the ionization current, the solution adopted consists in connecting a CMES measurement capacitor in series between the secondary winding of the transformer T and the resonator 1, on the ground return wire 6. The measurement capacitor is thus advantageously placed in the circuit at a place where the potential differences with respect to the mass are the lowest possible. Preferably, the measurement capacitor CME S is of reduced capacitance, typically about ten nanofarad, which makes it possible not to disturb the ignition system while having the possibility of making low frequency measurements of the ionization current. The detection device according to the invention thus comprises, in addition to the measuring capacitor CMES, a measuring circuit 10. The measuring circuit 10, connected to the terminals of the measuring capacitor CMES, advantageously comprises a low-impedance voltage generator. input, typically of the order of ten ohms, so as to reduce the glare time of the measuring circuit, and capable of providing a DC bias voltage VPOLAR for charging the measuring capacitor CMES. The voltage VPOLAR, may for example be between 12 and 250V. The low input impedance of the generator makes it possible to maintain the constant voltage across the capacitor and / or to quickly return its voltage to VPOLAR after the spark. This impedance is sufficiently low that the current 1ION representative of the evolution of the combustion of the gases in the combustion chamber is provided by the transistor TE, whose operation will be described in more detail below, and not by the capacitor. WCHE. It is this 1ION discharge current that the measuring circuit 10 measures.

The bias voltage VPOLAR is applied to the circuit via a bias stage 12, comprising a bipolar transistor TE mounted in common base with an output on the emitter of the transistor, connected to a terminal of the measurement capacitor CMES. The mounting of the transistor TE common base is characterized in particular by its low input impedance, advantageously to provide the desired reactivity to the measuring circuit. For example, by connecting this assembly to the measurement capacitor, an input impedance ZE equal to: ZE ù RN / Where: RIN is the resistance placed at the input of the measuring circuit, Rbe indicates the intrinsic resistance of the transistor TB, and 5 corresponds to the gain of the transistor TB.

Typically, by choosing: RIN = 8k52, Rbe = 1kS2 and 5 = 100, ZE 1052 is obtained. The output current IS of the assembly 12, representative of the ionization current TION, is measured via the output resistor. RS of the measuring circuit, which, as will be seen in more detail later, is in fact traversed by a current conferring a voltage Vs thereto, the measurement of which will then provide a voltage image of the ionization current.

This current IS is substantially equal to the difference in current between the current entering it in the transistor TB and the current IP flowing in the input resistance of the circuit RIN Gold, the amplitude of the ionization current that one seeks to Since the measurement circuit is generally small and predominantly less than 1 mA, it advantageously comprises current amplification means. For this purpose, the measuring circuit comprises a first current mirror MI, connected between the bias voltage source VPOIAR and the input of the transistor TB, and having an amplification gain Gm = RA / RE, defined by the values resistors RA and RB respectively present in each branch of the current mirror M. The current mirror M1 therefore makes it possible to amplify the current entering the transistor TB, to copy this amplified signal to the resistor R5 , connected at the output of the current mirror M. The current is the sum of the ionization current 1ION and the current IR flowing in the input resistor RIN.

Also, in order to measure a voltage VS across RS that is representative of the single ionization current, it is necessary to subtract the useless component corresponding to the current flowing in the input resistor RIN, the amplified signal obtained at the output of the mirror current M.

To do this, the measurement circuit 10 comprises a second current mirror M2, connected between the input resistance RIN of the circuit and the ground, and having an amplification gain Gm identical to the first current mirror MI, defined by the values of the resistances R'A and R'B respectively present in each branch of the current mirror M2. The output resistor RS, connected at the output of the second amplification means M2, is therefore traversed by the current difference the-IR, substantially equal to the ionization current IION, amplified by the ratio Gm = RA / RB = R'A / R'B. In other words, the output resistor RS is traversed by an amplified image of the ionization current, so as to obtain the output voltage Vs at its terminals according to the relationship: VS = Gm × RS × IION Where: Gm is the gain of the current mirror, RS is the output resistance, and LION is the ionization current. In order to obtain a large ionization current, the circuit must be polarized with the highest possible DC voltage, limited however by the maximum voltages and currents supported by the transistors of the circuit. Also, the maximum voltage accepted by the transistors of the measurement circuit determines the bias voltage of the assembly. Similarly, the input current must remain low enough to ensure linear operation. This constraint conditions the gain applied to the current mirrors. Thus, in the event of a short-circuit on the input (on the terminals of the measuring capacitor), the current increases in the resistor RA of the current mirror MI.

By amplification the current in the resistance RB increases. To protect the circuit, a diode D2 of the collector can be added to the base of the second transistor of the current mirror MI.

A spark, in particular a single-filament spark, between the spark plug electrode and a ground plane causes a sudden increase in the current flowing in the measuring capacitor and, consequently, a large variation in the voltage at its terminals, which is potentially harmful. for the measuring circuit. The measurement circuit can therefore provide a protection diode D1r for transferring the excess energy into a buffer capacitor CT and to ensure that the voltage across the measuring capacitor does not exceed the bias voltage VPOLAR. This sudden increase in the ionization current is called the ionization peak. In this case, it is a first ionization peak; there is a second subsequent peak corresponding to a thermal peak of ionization which does not take into account in the present invention. The circuit 10 and / or a computer connected to the ground return wire 6 can therefore be used as means for detecting the first ionization current ionization peak after the end of the spark.

The ionization peak appears at the moment when the flame front of the combustion touches the wall (mass) of the combustion chamber. There is therefore a delay (propagation of the flame) between the end of the spark and the ionization peak.

However, the moment at which the spark comes to an end is known data at the input of the control stage 3. The present invention therefore aims to determine, in this case by measurement, the time T mes between the end of the spark and the appearance of the first ionization peak of the ionization current. Comparison means are then used to compare the value of the time T mes to a reference value T th.

And, depending on the difference between the value of the time T mes and the reference value T th, we can deduce, via a computer for example, the type of spark. For this purpose, it is considered that the spark is of the monofilament type if the value of the time T mes is less than the reference value T th, and of the branched type if the value of the time T mes is greater than the reference value. T th. Indeed, the generation of a spark from a radiofrequency ignition creates a thermal footprint and localized radicals at the filament or filaments of the discharge. The equivalent resistance of the filaments is between 1kS2 and 10kQ. On the other hand, the inter-electrode resistance of the un-ionized gas is greater than 40MS2. Knowing that the thermal effect of the discharge is maintained in the gas for several milliseconds, it can be estimated that the equivalent resistance of the ionized channels remains identical at least 1 millisecond after the end of the spark. Thus, at the beginning of the combustion, two equivalent circuits of the ionization current measuring circuit are conceivable depending on the type of discharge generated: • A monofilament discharge regime (single channel between the spark plug electrode and a wall at the potential of the mass) (see FIG. 3): the impedance Z (t) of the mixture is equal to RMONO E [1kS2; 10kS2]. An ionization peak is then measured at a first time To, in l To <80ps from the end of the spark. • In branched regime (see Figure 4): the equivalent impedance Z (t) is equal for each channel to RMONO + RcAz 40MS2 30 The ionization current is zero. However, during the development of the combustion, RcAz decreases and then vanishes during the contact between the flame front and the wall of the combustion chamber: a resistive ionization peak is measured at a second time Tlr with T1> To, compared to the end of the spark. The respective value of TO and that of T1 is substantially dependent on the operating point of the motor, and varies in a respective set. However, whatever the respective values of TO and T1, we always have T1> T0. The time at which the ionization peak appears after the end of the spark thus makes it possible to identify the type of discharge generated. In one embodiment, it is possible to determine the reference value T th as a function of the operating point.

It is then possible to provide means, in this case in the form of a correspondence table, for controlling the reference value T th at the operating point of the motor. In another embodiment, FIG. 5, it is possible to determine the reference value T th independently of the operating point, for example by choosing a value T th greater than the maximum value of the set of values T0, and less than the minimum value of the set of T1 values, after the moment Ti of the end of the spark.

As illustrated in FIG. 6, an inter-electrode distance, i.e., the distance between the coil-spark plug electrode and the ground electrode, greater than 4mm is required to generate a branched spark . In this case, the time between the end of the spark and the first ionization peak verifies the relation T1> 2T0, ie T1> 160ps. Thus, the time T mes between the end of the spark and the appearance of the first ionization current ionization peak compared to the T th reference value makes it possible to determine the type of spark generated. It is therefore possible to provide means for modifying at least one of the ignition parameters, for example the current or the supply voltage Vinter, as a function of the comparison. With this characteristic, as illustrated in FIG. 1, it is possible to loop the measuring circuit 10 on the supply circuit 2. As illustrated on the map FIG. 6, if the spark generated is a mono-filamentary spark (with bridging), zone Z1, it is then necessary to reduce the power supply Vinter to return to branched area (without bridging) zone Z2. It will be noted that within a distance of 4 mm, zone Z3, there is no branched spark; and that beyond a value substantially equal to 14 or 15mm, zone Z4, the spark is branched, even if there is a bridging to ground, which is undesirable. In order to optimize the efficiency of the combustion, many devices are used to increase the turbulence of the gases (swirl, tumble) in the combustion chamber. The speed of a highly turbulent flame can reach 25m / s. The propagation time of the flame has a direct influence on the appearance of the peak of ionization following a branched discharge regime. Whatever the artifices used, we always have T1> T0. Moreover, with regard to the acquisition of the signal, an acquisition frequency of 30 kHz is necessary to extract the rattling information of the ionization signal. The first ionization peak has a width of 50us at -3dB amplitude. Such a frequency is therefore sufficient to detect it.

Claims (10)

REVENDICATIONS1. A method of detecting the type of spark generated by a radiofrequency ignition coil-coil, for a motor vehicle engine, comprising the steps of: - generating a spark, and - measuring the ionization current, characterized in that it further comprises the steps of: - detecting the first ionization current ionization peak after the end of the spark, - determining the time (T mes) between the end of the spark and the first peak of the spark ionization current, - compare the value of time (T mes) with a reference value (T th), and - consider that the spark is of mono filamentary type if the value of time (T mes) is less than the reference value ('th), and of branched type if the value of the time (T mes) is greater than the reference value (T th). The method of claim 1, further comprising a step of: modifying at least one of the ignition parameters as a function of the difference between the time (T mes) and the reference value (T th) . The method of any of the preceding claims, further comprising a step of slaving the reference value (T th) to the operating point of the engine. 30 4. Method according to any one of claims 1 or 2, wherein the reference value (T th) is constant in time, regardless of the operating point of the engine. 5. Device for detecting the type of spark generated by a radiofrequency ignition coil-ignition, for motor vehicle engine, capable of implementing the method according to any one of the preceding claims, and comprising: - means for generating a spark, and - means for measuring the ionization current, characterized in that it further comprises: - means for detecting the first ionisation current peak 10 of the ionization current after the end of the spark means for determining the time ('mes) between the end of the spark and the first ionization current ionization peak; - means for comparing the time value (' mes') with a value of reference ('th), and - means for considering that the spark is of mono-filamentary type if the value of time (' mes) is lower than the reference value ('th), and of branched type if the value of the time ('mes') is greater than the value of 20 ref erence ('th). The apparatus of claim 5, further comprising means for modifying at least one of the ignition parameters as a function of the difference between the time ('mes) and the reference value (' th). 25 7. Device according to claim 5 or 6, further comprising means for slaving the reference value ('th) to the operating point of the engine. 8. Device according to claim 5 to 7, wherein the distance between the electrode of the coil-spark plug 30 and the ground electrode is greater than or equal to 4mm. An on-board computer or computer program, comprising program code instructions for executing the steps of the method according to any one of claims 1 to 4, when said program is executed on a computer or on an onboard computer. 10. Motor vehicle equipped with the device according to any one of claims 5 to 8.