EP2321524B1 - Device for measuring the ionization current in a radiofrequency ignition system for an internal combustion engine - Google Patents

Description

The present invention relates to the field of resonant radiofrequency ignition of an internal combustion engine. It relates more particularly to a device adapted to perform the measurement of the ionization current of the gases in the cylinders of the engine.

The measurement of the ionization current of the gases in the cylinders of the engine is typically carried out after the end of the ignition and is then used to perform diagnostics on the course of the combustion, for example for the detection of the corresponding angle. at maximum pressure of the combustion chamber, knocking or for the identification of misfires.

It is known ionization current measuring circuits for a conventional ignition system, the operation of which consists in polarizing the air / fuel mixture present in the combustion chamber after the generation of the spark between the electrodes of the candle. in order to measure the current resulting from the propagation of the flame.

These circuits, however, need to be dedicated to the characteristics of conventional ignition and are not adaptable as such to plasma-generated ignition systems using radiofrequency coil-type spark plugs (BME ), 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 Another example is described in the application published under the number FR 2 899 394 .

Indeed, the specificities of the radiofrequency ignition generate several constraints to measure the current resulting from the combustion.

First, the ignition control signal induces large currents that have an amplitude difference of more than 120 dB with the ionization current due to combustion of the fuel mixture. Since the measurement of this current takes place after the end of the ignition, a glare time is then experienced during which the measuring circuit can not acquire a weak current.

In addition, since the measuring circuit fits into the ignition system, it is important not to significantly reduce the efficiency of the ignition system.

Finally, this type of radiofrequency ignition makes it possible to develop two types of discharges, among a multi-filament spark and a mono-filament arc, which influence the ignition system differently. There is therefore a difficulty in guaranteeing an independence of the measurement of the ionization current with respect to the type of discharge generated.

The present invention therefore aims at providing a device for measuring the ionization current in a radiofrequency ignition system, adapted to meet the aforementioned constraints, in particular by making it possible to minimize the masking period of the measurement and guaranteeing independence of the measurement with respect to the type of discharge generated.

• un condensateur de mesure connecté en série entre l'enroulement secondaire du transformateur et le résonateur,
• un circuit de mesure d'un courant aux bornes dudit condensateur de mesure, ledit courant fournissant une image électrique de l'évolution de la combustion,
• un circuit de protection, connecté entre le condensateur de mesure et le circuit de mesure, adapté à affranchir le temps d'acquisition de la mesure dudit courant des effets électriques induits par la commande d'allumage.

With this object in mind, the invention thus relates to a radiofrequency ignition device of an internal combustion engine composed of a power supply circuit comprising a transformer including a winding secondary is connected to at least one resonator having a resonance frequency greater than 1 MHz and comprising two electrodes capable of generating a spark to initiate the combustion of a fuel mixture in a cylinder of the engine in response to an ignition control, characterized in that it comprises:

• a measuring capacitor connected in series between the secondary winding of the transformer and the resonator,
• a circuit for measuring a current at the terminals of said measuring capacitor, said current providing an electrical image of the evolution of the combustion,
• a protection circuit, connected between the measurement capacitor and the measurement circuit, adapted to free the acquisition time from the measurement of said current of electrical effects induced by the ignition control.

According to one embodiment, the measurement capacitor is connected in series between the secondary winding of the transformer and the resonator, at a ground return wire of the transformer and the resonator.

The device according to the invention comprises polarization means of the fuel mixture, adapted to apply a bias voltage between an electrode of the resonator and a motor mass.

In addition, the protection circuit comprises a diode bridge biased by resistances to a supply voltage proportional to the bias voltage.

Preferably, the measurement circuit comprises a current-voltage converter produced using an operational amplifier.

According to one embodiment, the operational amplifier has a non-inverting input connected to the bias voltage and an inverting input connected to a terminal of the measurement capacitor via the protection circuit.

Advantageously, the current-voltage converter comprises a feedback resistance and a feedback capacitance connected in parallel with the feedback resistance.

Preferably, the input impedance of the current-voltage converter is at least one hundred times lower than the impedance of the measurement capacitor.

According to one embodiment, a primary winding of the transformer is connected on one side to an intermediate supply voltage and on the other side to the drain of at least one switching transistor controlled by a control signal, the switching transistor applying the supply voltage across the primary winding at a frequency defined by the control signal.

Preferably, the transformer comprises a variable transformation ratio.

• la figure 1 est un schéma d'un résonateur modélisant une bobine-bougie radiofréquence de génération de plasma;
• la figure 2 est un schéma illustrant un circuit d'alimentation selon l'état de la technique, permettant d'appliquer une tension alternative dans la gamme des radiofréquences aux bornes de la bobine bougie modélisée à la figure 1,
• la figure 3 est un schéma illustrant une variante du circuit de la figure 2,
• la figure 4 est un schéma illustrant un circuit d'alimentation adapté selon l'invention à la mesure du courant d'ionisation et de la tension aux bornes des électrodes de la bougie lors d'une commande d'allumage, et
• la figure 5 illustre un mode de réalisation du circuit de mesure du courant d'ionisation.
• la figure 5bis illustre une première variante du mode de réalisation de la figure 5, et
• la figure 5ter illustre une seconde variante du mode de réalisation de la figure 5.

Other characteristics 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:

• the figure 1 is a diagram of a resonator modeling a plasma generating radiofrequency coil-plug;
• the figure 2 is a diagram illustrating a power supply circuit according to the state of the art, making it possible to apply an alternating voltage in the range of radio frequencies at the terminals of the candle coil modeled at the figure 1 ,
• the figure 3 is a diagram illustrating a variant of the circuit of the figure 2 ,
• the figure 4 is a diagram illustrating a power supply circuit adapted according to the invention for measuring the ionization current and the voltage across the spark plug electrodes during ignition control, and
• the figure 5 illustrates an embodiment of the ionization current measurement circuit.
• the figure 5bis illustrates a first variant of the embodiment of the figure 5 , and
• the figure 5ter illustrates a second variant of the embodiment of the figure 5 .

The coil-spark plug implemented in the context of the controlled radiofrequency ignition is electrically equivalent to a resonator 1 (see FIG. figure 1 ), whose resonant frequency F _c 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 11 and 12 of the coil-plug are connected across the capacitor Cs of the resonator, making it possible to generate multi-filament discharges to initiate combustion mixing in the combustion chambers of the engine, when the resonator is powered.

l'amplitude aux bornes de la capacité Cs est amplifiée de telle sorte que des décharges multi-filamentaires se développent entre les électrodes, sur des distances de l'ordre du centimètre, à forte pression et pour des tensions crêtes inférieures à 20 kV.Indeed, when the resonator is powered by a high voltage at its resonant frequency F _c $\left(1/\left(2\mathrm{\pi }\sqrt{\mathit{ls}*\mathit{cs}}\right)\right),$

the amplitude across the capacitance Cs is amplified so that multi-filament discharges develop between the electrodes, over distances of the order of one centimeter, at high pressure and for peak voltages below 20 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 application to radio frequency ignition then 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 frequency very close to the resonance frequency of the plasma generation resonator of the radiofrequency coil-plug.

The figure 2 schematically illustrates such a power supply circuit 2. The supply circuit of the radiofrequency coil-plug conventionally implements a so-called "Class E pseudo power amplifier" assembly. This assembly makes it possible to create the voltage pulses with the aforementioned characteristics.

This assembly consists of a Vinter intermediate supply that can vary from 0 to 250V, a MOSFET transistor M power and a parallel resonant circuit 4 comprising a coil Lp in parallel with a capacitor Cp. 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 DC supply voltage V _inter may advantageously be provided by a high voltage power supply, typically a DC / DC converter.

Thus, near its resonant frequency, the parallel resonator 4 converts the intermediate DC supply voltage V _inter into an amplified periodic voltage, corresponding to the supply voltage multiplied by the overvoltage coefficient of the parallel resonator and applied to a power circuit output interface 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 multifilament discharge.

The transistor thus switches high currents at a frequency of about 5 MHz and with a drain-source voltage of up to 1 kV.

According to a variant illustrated in figure 3 , the parallel coil Lp is then replaced by a transformer T, having a transformation ratio of between 1 and 5. The primary winding L _M of the transformer is connected, on one side to the intermediate supply voltage V _inter and the other hand, the drain of the switch transistor M, controlling the application of the intermediate supply voltage V _inter across the primary winding at the frequency defined by the control signal V1.

The secondary winding L _N 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 adaptation of the transformation ratio then makes it possible to reduce 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 placing for example two transistors in parallel controlled by the same control stage 3.

When lighting, it is essential that the branched spark develops in volume to ensure optimal combustion and engine operation. For this application, the presence of the The combustion is symbolized by a variable resistance R _ION between the terminals of the capacitor CS.

The ionization signal, representative of the evolution of the combustion, has an amplitude of between 0.1μA and 1mA depending on the conditions of the combustion chamber (temperature, pressure, composition of the mixture, etc.). It is therefore sought to measure a signal having an amplitude ratio of up to 120 dB with respect to the ignition signal.

The ionization signal is a low frequency signal and sampling at 100 kHz extracts all the useful information. In the case of radiofrequency ignition, the plasma generation resonator R _S L _S C _S is controlled at a frequency greater than 1 MHZ and typically between 4 MHz and 6 MHz. We therefore benefit from a frequency difference of almost two decades, which can then be used to compensate for differences in amplitude levels.

The realization of the measurement of the ionization current requires the use of a component that does not degrade the energy efficiency of the ignition.

The solution adopted for this purpose consists, with reference to the figure 4 , to connect a measurement capacitor C _MES 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 location where the Potential differences with respect to mass are as small as possible.

A capacity capacitor of about ten nanofarad, allows not to disturb the system ignition while having the ability to perform low frequency measurements of the ionization current.

Thus, the main advantage of the choice of this measurement component compared to other passive components lies in its behavior in radiofrequency. Indeed, at high frequencies, those skilled in the art know that the equivalent high frequency circuit of a capacitor is constituted by a series resonator. Now, a resonator has an impedance that changes according to the frequency of the signal applied to its input, and is minimal at the resonance frequency of the resonator. This characteristic of the evolution of the impedance of a resonator as a function of the frequency then allows the capacitor to have a very low impedance in the vicinity of the resonance frequency of the ignition and a high impedance in the frequency band used. for the ionization signal (F _ION <15kHz). The measurement capacitor is therefore judiciously chosen so as to have its lowest impedance in the frequency range used for the ignition control signal. This makes it possible to minimize the voltage across the measuring capacitor to protect the measuring circuit, which will now be described with reference to the figure 5 .

A DC power supply, not shown, providing a voltage V _polar , is provided for biasing the high voltage electrode of the spark plug coil connected at the output of the power supply circuit with respect to the motor yoke, so as to make it possible to polarize the combustible mixture after the end of ignition.

The ionization current _ION , representative of the combustion, is indeed a measured signal after the end of the ignition, that is to say after the formation of the spark. Its amplitude therefore depends, among other things, on the bias voltage applied between the coil-spark electrode and the motor ground.

The polarization voltage is unipolar and typically between 1V and 100V. We will speak of positive polarization when the high voltage electrode of the spark plug is polarized at a potential higher than that of the motor mass.

However, it is possible to polarize the fuel mixture negatively. The potential of the central electrode of the candle is then lower than that of the motor mass. The bias voltage is in this case typically between -100V and -1V.

A measuring circuit 40 of the ionization current I _ION across the capacitor C _MES , providing an electrical image of the evolution of the combustion is described in FIG. figure 5 . With reference to this figure, the measurement circuit 40 is made in the form of a current-voltage converter adapted to provide a voltage V _s output proportional to the input current.

The converter comprises an operational amplifier MN1 and a feedback resistance R _R.

The operational amplifier MN1 has a non-inverting input (+) connected to the polarization voltage V _polar and an inverting input (-) connected to a terminal of the capacitor C _MES via a protection circuit 30, adapted to freeing the acquisition time from the measurement of the effects of spark formation and to which we will return in more detail later.

The resistor R _R is connected between the inverting input (-) and the output of the operational amplifier MN1.

Alternatively, as illustrated in figure 5bis in the case where the fuel mixture is negatively polarized, the non-inverting input (+) is connected to the negative bias voltage V _polar and the inverting input (-) connected to the measurement capacitor terminal via of the protection circuit 30, while the resistor R _R is connected between the inverting input (-) and the output of the operational amplifier MN1.

According to another variant illustrated in figure 5ter it is also possible to choose any polarization of the fuel mixture with a polarization voltage V _polar respecting: $${\mathrm{V}}_{\mathrm{EE}}<{\mathrm{V}}_{\mathrm{thriller}}<{\mathrm{V}}_{\mathrm{CC}}{\mathrm{with\; V}}_{\mathrm{EE}}<\mathrm{0}{\mathrm{and\; V}}_{\mathrm{CC}}>\mathrm{0}$$

Such a current / voltage arrangement is able to accurately measure very small currents.

The input of the operational amplifier is equivalent to an inductance of value L _e . This leads to the appearance of pseudoperiodic oscillations of frequency F _osc greater than 100 kHz after the end of ignition, due to the circuit formed by the input impedance | Z _E | the current-voltage converter and the measuring capacitor C _MES , which reduce the desaturation time of the measuring circuit. It is therefore necessary to add a feedback capacitance C _R in parallel with the feedback resistor R _{R in} order to damp these oscillations. We therefore choose a capacity that checks: $$\mathit{Fosc}>f=\frac{2\pi }{R_R{\mathrm{VS}}_R}>100\mathit{kHz}$$

The feedback capacity is therefore negligible for the useful frequency band of the measured signal representative of the evolution of the combustion (typically less than 100kHz), while optimizing the desaturation time of the measuring circuit.

In addition, it is important that the feedback impedance be carefully chosen to ensure that the voltage V _S at the output of the measuring circuit is well proportional to the current _ION resulting from the combustion.

Typically, the measuring capacitor C _MES is charged during the spark generation phase. It is important that the input impedance Z _E of the current-voltage converter is low (at least 100 times smaller) in front of the impedance of the measurement capacitor Z _MES . This condition ensures that the current-to-voltage converter, not the measurement capacitor, provides the image current of the development of the combustion. In other words, the impedance of the capacitor C _{MES must} be high compared to the input impedance of the amplifier so that the totality of the ionization current _ION is found in the amplifier MN1.

It is known that this converter has an input impedance which follows the following relation: $$\left|Z_E\right|=\frac{\left|Z_R\right|}{\mathrm{BOY\; WUT}}$$

G being the gain of the operational amplifier.

With: $$\left|Z_R\right|=\frac{R_R}{1+{\mathit{j\omega R}}_R{\mathrm{VS}}_R}\approx R_R$$

The following relation must therefore be verified for all frequencies below 100MHz: $$\frac{\left|Z_{\mathit{MY}}\right|}{\left|Z_R\right|}\mathrm{.}\mathrm{BOY\; WUT}>\alpha ,\mathrm{or}\alpha \ge 100$$

So, if the previous conditions are true, we have: $${\mathrm{V}}_{\mathrm{S}}={\mathrm{R}}_{\mathrm{R}}\mathrm{.}{\mathrm{I}}_{\mathrm{ION}}+{\mathrm{V}}_{\mathrm{POLAR}}$$

We will now return in more detail on the protection circuit 30, thus eliminating the effects of ignition by filling an anti-glare function of the measuring circuit 40 described above. In this way, the acquisition of the measurement of the current I _ION representative of the evolution of the combustion can be advantageously carried out independently of the effects of the formation of the spark.

Indeed, useful information on the combustion is extractable from the ionic signal soon after the end of the ignition.

However, it has been seen that the strong currents induced by the ignition control signal, which have an amplitude difference of nearly 120 dB with the current representative of the combustion, cause a glare time, or masking period, during which the acquisition of a weak current can not be carried out.

Also, in order to overcome the effects of the ignition control, it is planned to connect the protection circuit 30 between the measurement capacitor and the current-voltage converter forming the measurement circuit 40. current-voltage converter must maintain the best possible dynamics and have a desaturation time of preferably less than 300 μs to allow a reliable measurement of combustion at maximum speed.

The protection circuit 30 comprises a diode bridge 31, biased by resistors R _H and R _B at a supply voltage V _ALIM , preferably close to the bias voltage V _POLAR .

This architecture is stable and does not disturb the measurement if the bias current I _D flowing in the diodes of the protection circuit is large compared to the current supplied by the converter.

We can check that: $$I_D=\frac{V_{\mathit{ALIM}}}{2\left(r_{\mathit{dyn}}+R_B+R_H\right)}{\mathrm{and}r}_{\mathit{dyn}}\approx \frac{1}{40\times I_D}$$

R _dyn being the dynamic resistance of a diode. Therefore : $$I_D\approx \frac{V_{\mathit{ALIM}}-1/20}{R_B+R_H}$$

Let V _ALIM = 12V and R _B = R _H = 1kΩ, we obtain: $$\mathrm{ID}=\mathrm{3}\mathrm{my}>{\mathrm{I}}_{\mathrm{IONmax}}=\mathrm{500}\mathrm{uA}\mathrm{.}$$

This equation makes it possible to find the right compromise between the stability of the assembly and the average consumption of the protection circuit. Resistors R _B and R _H may typically have a value between 100Ω and 50kΩ and may be of different values.

The optimal polarization voltage V _POLAR is thus defined by: $$V_{\mathit{POLAR}}=\frac{R_H}{R_H+R_B}\mathrm{.}{V}_{\mathit{ALIM}}$$

The voltage V _POLAR can for example be obtained from the voltage V _ALIM via a resistive divider circuit, well known per se.

The protection circuit 30 thus has a dual role. It makes it possible to maintain a low desaturation time of the measuring circuit whatever the conditions of spark generation. In addition, it promotes robustness of the measuring circuit to each type of spark that a resonant ignition system can generate.

Claims (8)

1. Device for the radiofrequency ignition of an internal combustion engine consisting of a power supply circuit (2) comprising a transformer (T), a secondary winding (L_N) of which is connected to at least one resonator (1) that has a resonant frequency in excess of 1 MHz and comprising two electrodes (11, 12) that are able to generate a spark to initiate the combustion of a combustible mixture in a cylinder of the engine in response to an ignition command, characterized in that it comprises:

   - a measuring capacitor (C_MES) connected in series between the secondary winding of the transformer and the resonator,

   - a circuit (40) for measuring a current (I_ION) at the terminals of said measuring capacitor, said current providing an electrical image of the trend of the combustion,

   - means of polarizing the combustible mixture, designed to apply a polarization voltage (V_polar) between an electrode of the resonator and an engine ground,

   - a protection circuit (30), connected between the measuring capacitor and the measurement circuit, designed to free the acquisition time for the measurement of said current (I_ION) from the electrical effects induced by the ignition command, comprising a diode bridge polarized by resistances (R_H, R_B) at a power supply voltage (V_ALIM) that is proportional to the polarization voltage (V_polar).
2. Device according to Claim 1, characterized in that the measuring capacitor (C_MES) is connected in series between the secondary winding of the transformer and the resonator, at the level of a ground return wire (6) of the transformer and of the resonator.
3. Device according to either one of the preceding claims, characterized in that the measurement circuit (40) comprises a current-voltage converter produced using an operational amplifier (MN1).
4. Device according to Claim 3, characterized in that the operational amplifier has a non-inverting input linked to the polarization voltage and an inverting input linked to a terminal of the measuring capacitor via the protection circuit.
5. Device according to either one of Claims 3 and 4, characterized in that the current-voltage converter comprises a feedback resistor and a feedback capacitor connected in parallel to the feedback resistor.
6. Device according to any one of Claims 3 to 5, characterized in that the input impedance of the current-voltage converter is at least a hundred times lower than the impedance of the measuring capacitor.
7. Device according to any one of the preceding claims, characterized in that a primary winding of the transformer is connected on one side to an intermediate power supply voltage (V_inter) and on the other side to the drain of at least one switch transistor (M) controlled by a control signal (V1), the switch transistor applying the power supply voltage to the terminals of the primary winding at a frequency defined by the control signal.
8. Device according to any one of the preceding claims, characterized in that the transformer has a variable turns ratio.

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