EP1515594A2 - Arrangement for plasma generation - Google Patents

Abstract

The plug has a series resonator with a resonance frequency higher than 1MHz and having a capacitor (111) whose terminals are connected to two electrodes (103, 106) for plasma generation. One electrode forms a spark plug shell and another electrode behaves as an anode. A radiofrequency excitation is applied to the plug and an insulating sleeve (100) separates free ends of the electrodes. Independent claims are also included for the following: (a) a plasma generation system including a spark plug; and (b) a plasma generation method.

Description

The present invention relates generally to the plasma generation in a gas, and more particularly plasma generating systems between two electrodes. Plasma generation is especially used for the controlled ignition of engines internal combustion by the electrodes of a candle.

Ignition of internal combustion engines essence, consisting in initiating the combustion of a air-fuel mixture in a combustion chamber said engine, is relatively well controlled in the current engines. In spark-ignition engines with indirect injection, conventionally, a candle and a upstream electronic device can generate a spark capable of transmitting to the mixture a sufficient energy for its combustion. The formation of this discharge requires breakdown voltages high (of the order of 30 kV per mm), so that one limits the inter-electrode space of the candles to approximately 1 mm, relatively unfavorable distance to the initiation of combustion.

To meet the standards of depollution, the car manufacturers have developed engines to controlled ignition capable of operating with mixtures poor carburets, that is to say, presenting an excess of air relative to the amount of fuel injected. These developments have been applied in particular to direct injection engines, in which the injection of fuel is done directly in the room of combustion.

Conventional ignition devices apply quite badly on lean-burn engines and direct injection. Indeed, the ignition devices are very difficult to develop. A front of flame propagates properly in a very mixture poor (wealth less than 0.3) but the initiation of burning usually requires wealth greater than 0.7, and preferably for wealth close to stoichiometry. It is therefore essential to maintain a sufficiently high level of wealth at the level of the inter-electrode space.

The generation of stratified mixtures has therefore been developed. As opposed to a homogeneous mixture where the wealth is globally the same in every respect, a stratified blend presents a wealth that decreases at as you move away from the candle. The stratification of the mixture in the combustion chamber is for example obtained by guiding the fuel jet so that the jet meets the candle at the moment of the production of the spark. The guidance of the jet is especially obtained by aerodynamic phenomena, generated for example by a suitable form of the piston.

Stratified mixtures pose several problems. It is difficult to match the moment of spark and the presence in the vicinity of the inter-electrode space of a mixing cloud presenting a wealth close to 1, in a mix environment overall poor. In addition, the mixture around the candle at the moment of the spark significant inhomogeneities of wealth, variable in time, which do not guarantee the initiation of combustion at the time of development of the spark. The size and spark duration of the classical candles then imply a failure rate Ignition incompatible with performance requirements and current pollution. Moreover, the jet of fuel often hits the candle directly, which leads to fouling of the insulation of the candle. This fouling promotes leakage currents between the central electrode and the mass. The generation of sparks is affected because the spark is short-circuited by a carbon path of low impedance which reduces the potential difference between the electrodes of the candle.

New spark plugs on the surface produce larger sparks to treat the problem of the space-time appointment. We turn on thus a higher mixing volume. The probability initiation of combustion is then very widely increased in an ignition direct injection engine ordered and laminated mix. Such candles are especially described in patent applications FR97-14799, FR99-09473 and FROO-13821. Such candles generate significant sparks from reduced potential differences. The candles to surface sparks exhibit a dielectric separating the electrodes in the area where the distance the separating is the weakest; we guide sparks formed between the electrodes on the surface dielectric. These candles amplify the inter-electrode field on the surface of the dielectric. We charge for this gradually the basic abilities formed by the dielectric and an underlying electrode. The candles generate a spark propagating along the surface of the insulation in areas where the field electric in the air is the strongest. A device conventional engine ignition, coupled with such candles typically generates sparks presenting a length of 4 mm with breakdown voltages between 5 and 25 kV. When the candle presents globally a symmetry of revolution around his main axis, the discharge has a probability of substantially identical appearance anywhere around insulation. On the contrary, classical candles generate an electric arc occurring systematically in the same volume extremely reduced. This ignition method by plasma generation still has disadvantages. It happens in particular a passage to the arc following a single line. The initiation of combustion is not optimal.

There is therefore a need, which the invention aims to satisfy, for a plasma generating system between electrodes solving one or more of these disadvantages.

• deux électrodes;
• un matériau diélectrique séparant les électrodes;
• un générateur de tension, connecté aux électrodes et appliquant sélectivement une tension alternative présentant une fréquence supérieure à 1 MHz et une amplitude entre crêtes supérieure à 5kV entre les électrodes.

The invention thus relates to a plasma generation system, comprising:

• two electrodes;
• a dielectric material separating the electrodes;
• a voltage generator, connected to the electrodes and selectively applying an AC voltage having a frequency greater than 1 MHz and an amplitude between peaks greater than 5kV between the electrodes.

The invention also relates to a method of plasma generation between two electrodes separated by a dielectric material, the method comprising a step of applying an alternating voltage with a frequency greater than 1 MHz and a amplitude between peaks greater than 5 KV between electrodes.

The term branched plasma used later refers to the simultaneous generation of at least several lines or ionization paths in a given volume, their branches are also omnidirectional.

While a plasma of volume implies the warming of the whole volume in which he has to generated, the branched plasma only requires the heating in the path of formed sparks. So, for a given volume, the energy required for a plasma branched is significantly lower than that required by a volume plasma.

The invention makes it possible to easily generate a plasma of volume or a branched plasma. Such plasmas can especially be used to initiate a combustion of fuel in a combustion engine. The plasma as well generated can also be used in other applications, such as gas depollution exhausts, the disinfection of air conditioning, air filter cleaning or surface treatment.

In a general manner, the following will be understood by high density, any molar density greater than 2.5 * 10 ^-3 mol / L. Combustion density will be called any molar density of gas greater than 5 * 10 ^-2 mol / L. A stream of positive ionization propagating from the anode will be referred to as a streamer.

• Figure 1, un schéma de fonctionnement d'une bougie d'allumage à étincelle de surface;
• Figure 2, la représentation de champs appliqués et de l'étincelle générée entre les électrodes de la bougie durant l'initiation de l'allumage;
• Figure 3, un diagramme du champ électrostatique entre les deux électrodes de la bougie durant l'initiation de l'allumage;
• Figure 4, une représentation schématique du développement d'un streamer pour une unique montée en tension (champ local et champ global);
• Figure 5, une représentation schématique d'un mode de réalisation du système de génération de plasma selon l'invention ;
• Figure 6, un modèle électrique utilisé pour le dimensionnement du résonateur série;
• Figure 7, une variante dans laquelle l'amplificateur comprend un transformateur à point milieu;
• Figure 8, une autre variante du système dans laquelle l'amplificateur comprend une commande de transistor de puissance par un transistor bipolaire;
• Figure 9, des chronogrammes de signaux durant l'excitation du résonateur de la figure 7;
• Figure 10, les différents éléments de l'alimentation de la figure 7 intégrés sur un même circuit;
• Figure 11, une représentation schématique d'une boucle d'asservissement incluse dans l'amplificateur;
• Figure 12, une variante du système comprenant une boucle d'asservissement et des circuits de génération des premières oscillations de tension;
• Figure 13, une autre variante de système comprenant une boucle d'asservissement et des circuits de génération des premières oscillations de tension;
• Figure 14, un exemple de transformateur formant une sonde de courant de l'amplificateur, réalisé sur un circuit imprimé;
• Figure 15, un mode de réalisation d'une inductance parallèle sur un circuit imprimé;
• Figure 16, un autre mode de réalisation d'une inductance parallèle sur un circuit imprimé;
• Figure 17, une variante d'un système présentant une alimentation et un amplificateur communs pour deux résonateurs;
• Figures 18 et 19, des représentations schématiques en coupe d'un exemple de bougie utilisable dans le système de génération de plasma;
• Figures 20 à 27, différentes configurations de têtes de bougie adaptées pour une excitation radiofréquence.

Other features and advantages of the invention will become clear on reading the following description which is given by way of non-limiting example and with reference to the figures. These figures show:

• Figure 1, a diagram of operation of a spark plug surface spark;
• Figure 2, the representation of applied fields and the spark generated between the electrodes of the candle during ignition initiation;
• Figure 3, a diagram of the electrostatic field between the two electrodes of the candle during ignition initiation;
• Figure 4, a schematic representation of the development of a streamer for a single voltage rise (local field and global field);
• Figure 5 is a schematic representation of an embodiment of the plasma generating system according to the invention;
• Figure 6, an electric model used for the design of the series resonator;
• Figure 7, a variant in which the amplifier comprises a mid-point transformer;
• Figure 8, another variant of the system wherein the amplifier comprises a power transistor control by a bipolar transistor;
• Figure 9, timing of signals during the excitation of the resonator of Figure 7;
• Figure 10, the various elements of the power supply of Figure 7 integrated on the same circuit;
• Figure 11 is a schematic representation of a control loop included in the amplifier;
• 12, a variant of the system comprising a control loop and generating circuits of the first voltage oscillations;
• Figure 13, another system variant comprising a control loop and generating circuits of the first voltage oscillations;
• FIG. 14, an example of a transformer forming a current probe of the amplifier, produced on a printed circuit;
• Figure 15, an embodiment of a parallel inductance on a printed circuit;
• Figure 16, another embodiment of a parallel inductance on a printed circuit;
• Figure 17, a variant of a system having a common power supply and amplifier for two resonators;
• Figures 18 and 19, schematic sectional representations of an example of candle usable in the plasma generation system;
• Figures 20 to 27, various configurations of candle heads adapted for radio frequency excitation.

The invention proposes to apply a voltage alternative of frequency higher than 1MHz and amplitude between peaks greater than 5 kV between two plasma generation electrodes, separated by a element made of dielectric material. This excitement will be referred to as the radiofrequency excitation suite. This generates a plasma of volume or a plasma branched.

Figure 1 illustrates details of the structure of a surface spark spark plug for which the application of a radiofrequency excitation is particularly advantageous. We go previously describe the operation of such candle.

The surface effect candle 110 includes a head of candle destined to debouch in the room of combustion in the lower wall of the cylinder head of an engine. The candle comprises an electrode cylindrical low voltage that serves as a metal base 103 for screwing into a recess made in the engine cylinder head and opening inside the combustion chamber. The pellet 103 is intended to be electrically connected to the ground.

The cap 103 surrounds a high voltage electrode cylindrical 106 disposed in a central position. The electrode 106 is intended to be connected to a generator of a high ignition voltage. The electrode 106 is isolated from the cap 103 via a insulating sleeve 100. The insulating sleeve is constituted of a material whose relative permittivity is greater than 3, for example a ceramic. The candle has a space 105 separating the dielectric 100 and one end of the electrode 103.

The electrode 106 and the insulating sleeve 100 protrude by a length 1 outside the base 103. This length 1 corresponds substantially to the length of the spark generated when a high voltage is applied between the electrodes 106 and 103.

The base or low voltage electrode 103 includes monobloc a body and a connecting piece supporting a collapsed flange 101. The flange 101 has a beveled edge extending nearby immediate of the surface of the insulation 100.

The dielectric 100 creates a field amplification electrostatic in the air in its vicinity. The spark generated between the beveled edge of the flange 101 of the base 103 and a free end 104 of the electrode 106 is propagated on the surface of the insulation 100, where the electric field in the air is the strongest.

The formation of a spark is initiated by tearing in the middle of a few electrons subject to an important electric field. When applying significant voltage between the electrodes, electrons of the collar are accelerated by the electrostatic forces generated and strike molecules of the air. The end of the collar is the area that undergoes the most electrostatic field important, and thus constitutes the starting point of the first avalanche. The molecules of the air release a electron and an ionizing photon in turn other molecules of air. A chain reaction ionizes the air in the space 105 between the electrode 103 and the dielectric 100. The gas space 105 makes it possible to perform a prior ionization with a difference potential between the electrodes 103 and 106 relatively limited.

A conductive channel is thus created, as illustrated in Figure 2. The broken lines represent equipotentials of the electrostatic field when a high voltage is applied between the electrodes 103 and 106.

FIG. 3 represents an example of amplitude of electrostatic field between the end of the flange 101 and the end of the electrode 106, A designating the end of the flange, B designating the end 104 of the electrode 106. Once the air is ionized at the end of the collar, the ionization of the air creates a space charge with a potential close to that of the collar and that behaves so like a extension of it. When spreading avalanche front, the electric field is amplified in upstream of the front and promotes the creation of new avalanches. The phenomenon is self-sustaining along the sleeve 100, to create a conductive ionized channel to the end 104 of the central electrode.

In the candle of Figure 1, the insulation is separated of the electrode 103 by an air space. This space is not essential for the operation of the candle but facilitates the making of the candle with a collar with a very sharp angle near the surface of the insulation. It also reduces the influence of fouling phenomena.

The physical phenomenon implemented thanks to radiofrequency excitation has similarities with the propagation described above but allows significantly improve the effects. Figure 4 schematically represents the electrostatic field when leaving an avalanche. We can notice that Avalanche propagation is limited by the field locality due to the separation of atoms and their electrons. This local field limits in particular the spread of the discharge over lengths important. The present invention proposes, other, an electrical excitation capable of reversing the polarity of the global field imposed before the electrons could not recombine with the atoms present in the middle. With each alternation of polarity, the electrons are becoming more and more accelerated in sense reverse. A polarization wave propagates thus oscillatory way at the frequency of the excitation, recovering at each period the charges deposited at the previous period. Each alternation then produces a wave propagation larger than the previous; it is thus possible to obtain sparks of very long lengths with voltage amplitudes between the electrodes relatively limited. Radio frequency excitation also removes voltage variations from breakdown between successive cycles.

For an application to automotive ignition, the skilled person will use electrodes and a insulation showing materials and geometry suitable for initiating combustion in a mixture with a density of combustion and to resist the plasma thus formed.

Plasma thus formed has many advantages in the context of automotive ignition: significant reduction in the rate of misfires in a stratified lean mixture system, reduction of wear of the electrodes and adaptation of the ignition initiation volume to density function. It is found that the excitation described is adapted to achieve ignition of a mixture having a density greater than 5 * 10 ^-2 mol / L. For this ignition application, the generator applies the excitation between 1.5 and 200 times per second, with an application duty ratio of between 10 and 1000, and preferably between 72 and 720.

The radiofrequency excitation described is also adapted to a plasma deposition application in a gas having a density of between 10 ^-2 mol / L and 5 * 10 ^-2 mol / L. The gas used in this application may typically be nitrogen.

The radiofrequency excitation is further adapted to an application for the depollution of a gas having a density of between 10 ^-2 mol / L and 5 * 10 ^-2 mol / L.

The radiofrequency excitation is further adapted to a lighting application using a gas having a molar density of between 0.2 mol / L and 1 mol / L.

• un générateur capable de faire résonner une structure L-C à une fréquence supérieure à 1MHz avec une tension aux bornes du condensateur supérieure à 5kV, de préférence supérieure à 6kV.
• un résonateur connecté en sortie du générateur et présentant un facteur de surtension compris entre 40 et 200 et présentant une fréquence de résonance supérieure à 1 MHz.
• une tête de bougie comprenant deux électrodes séparées par un isolant, permettant de générer un plasma lors de l'application de l'excitation radiofréquence.

An envisaged plasma generation system mainly comprises three functional subsystems:

• a generator capable of resonating an LC structure at a frequency greater than 1 MHz with a voltage across the capacitor greater than 5 kV, preferably greater than 6 kV.
• a resonator connected at the output of the generator and having an overvoltage factor between 40 and 200 and having a resonance frequency greater than 1 MHz.
• a candle head comprising two electrodes separated by an insulator, for generating a plasma during the application of the radiofrequency excitation.

• une alimentation basse tension 3 (générant une tension continue inférieure à 1000 V);
• un amplificateur radiofréquence 5, amplifiant la tension continue et générant une tension alternative à la fréquence commandée par la commande de commutation 4.

FIG. 5 represents an embodiment of a plasma generation system 1 and its voltage generator 2. The voltage generator advantageously comprises:

• a low voltage supply 3 (generating a DC voltage of less than 1000 V);
• a radio frequency amplifier 5, amplifying the DC voltage and generating an AC voltage at the frequency controlled by the switching control 4.

The AC voltage of the amplifier 5 is applied on the resonator LC 6. The resonator LC 6 applies the alternating voltage according to the invention between the electrodes 103 and 106 of the candle head.

The voltage supplied by the power supply 3 is less than 1000V and the power supply presents preferably a limited power. We can thus foresee that the energy applied between the electrodes is limited to 300mJ per ignition, for reasons of security. We also restrict the intensity in the voltage generator 2 and its power consumption. To generate DC voltages greater than 12 V in an automotive application, the power supply 3 can include a 12 Volt to Y Volt converter, Y being the voltage supplied by the power supply to the amplifier. We can thus generate the level of desired DC voltage from a voltage of drums. The stability of the DC voltage generated being a priori not a decisive criterion, we can plan to use a switching power supply for power the amplifier, for its qualities of robustness and simplicity.

One can also envisage, according to a variant, to apply to the terminals of the amplifier a voltage 42 V taken from the vehicle's electrical circuit. This is indeed the level of tension that will be in future future standards motor vehicles. This variant, avoiding the voltage conversion through the power supply 3, reduced substantially the cost and complexity of the generator of voltage 2.

This voltage generator helps to focus the highest voltages on the resonator 6. The amplifier 5 thus deals with the tensions a lot smaller than the tensions applied between electrodes: we can therefore use an amplifier 5 reasonable cost and with characteristics of common components for the mass automobile production, whose reliability is furthermore proven. In addition, such a generator of voltage has a relatively large number of components reduced. There is thus a system for generating voltage with reliability, volume, weight and an attractive production facility, in especially for large series in one application automobile.

The amplifier 5 can accumulate energy in the resonator 6 at each alternation of its voltage. A class 5 amplifier will preferably be used E, as detailed in US Pat. No. 5,187,580. amplifier can maximize the factor of surge. Such an amplifier achieves switching out of phase with the amplifier described in US Pat. No. 3,919,656 which aims to achieve switching at zero voltage and / or intensity and does not optimize the surge factor of the amplifier. The skilled person will associate well heard a switching device adapted to the chosen amplifier, to support the requirements of mounted in tension and have a speed of adequate switching.

The preferred class E amplifier features a parallel resonator 62. This parallel resonator 62 is preferably made on the same map as the amplifier 5 and its switching control 4. The parallel resonator 62 temporarily stores the energy provided by the amplifier 5, and provides periodically this energy to the series 61 resonator. With specified supply voltage values in addition, an amplifier 5 will be used having an overvoltage coefficient of the order of 3. This overvoltage coefficient corresponds to the ratio between the voltage supplied by the low power supply voltage 3 and amplitude between peaks of voltage applied on the series resonator. The coefficient of overvoltage of the resonator series 61 associated is then of preferably between 40 and 200. The coefficient of overvoltage of the series resonator is notably limited by its angle of loss.

A preferred dimensioning of the inductive and capacitive elements of the series resonator 61 will be explained. FIG. 6 illustrates an electric model of this resonator. Thus, the inductance series 65 has in series an inductance L and a resistor Rs taking into account the skin effect in the radiofrequency domain. The capacitor 119 has in parallel a capacitance C and a resistor Rp. The resistor Rp corresponds, if appropriate, to the dissipation in the ceramic of the spark plug. When the series resonator 61 is supplied with a voltage at its resonance frequency f ₀ (1 / (2π√ (L * C))), the amplitude across the capacitor C is amplified by the overvoltage coefficient Q defined by the following formula: Q = 1 The VS Rs + rp The VS

From the equation ω ₀ ² = (2π * f ₀ ) ² = 1 / (L * C), we deduce that the following equation must be checked to obtain the maximum value of Q: Rs = Rp * L / C

• f₀ est de l'ordre de 5MHz;
• les valeurs Rs et Rp sont constantes;
• Rp est principalement induite par l'encrassement de la tête de bougie et vaut en moyenne 50 kQ;
• Rs vaut approximativement 10 Ω en prenant en compte l'effet de peau.

We will take into account the following conditions:

• f ₀ is of the order of 5 MHz;
• the values Rs and Rp are constant;
• Rp is mainly induced by the fouling of the candle head and is worth on average 50 kQ;
• Rs is approximately 10 Ω taking into account the skin effect.

We then deduct VS ≈ 1 ω Rs * rp ≈45 pF and The ≈ Rs * rp ω ≈ 22 uH

Another modeling also allows determine these characteristics. The resistance of the capacity is modeled by the dissipation factor dielectric (tan (δ) = 1 / (Rp * C)) in the material insulation of the candle head, which is considered constant and solely dependent on the chosen material.

The overvoltage coefficient is then defined as follows:

The maximization of the overvoltage coefficient Q is then equivalent to the minimization of The VS . A high capacitance C and a reduced inductance L are then preferably selected.

These determination rules apply regardless of the type of series resonator used and therefore also apply to the described coil-candle later.

A compromise in the choice of values is however necessary for the variant using a MOS transistor power as a switch, as described below. Indeed, the current through the MOS switch then increases with the capacity C. value of the capacitance C must therefore be fixed in function of the nominal current of the MOS switch.

We will now describe several variants amplifiers 5. In general, we will use preferably an amplifier having a transistor MOSFET power as 51 commander switch the commutations at the terminals of the resonator 6. The Figures 7 and 8 illustrate two embodiments amplifiers 5 including M4 MOSFETs, such as Switches 51. Amplitude and frequency concerning the voltage to be generated between electrodes can be solved with a transistor Power MOSFET with characteristics following: insulation greater than 500 V, one drain current capacity greater than 30 A, a switching time less than 20 ns (and preferably of the order of 10ns in case of use of a servo loop) and a capacity in grid current up to 10A.

This MOSFET transistor will also present preferably an inductance of less than 7 nH on its connections between its active silicon surface and the circuit board on which it is implanted. We avoid thus transients during high voltage peaks which would be detrimental to the rapid switching of the transistor.

Figure 7 shows a first mode of realization of an amplifier 5 having such a switching control transistor M4. A midpoint transformer 56 is interposed between the command 4 and the M4 power MOSFET. The MOSFET M4 power can be controlled very quickly with a symmetrical voltage able to block it effectively. Indeed, the application of a tension negative on the gate of the MOSFET M4 allows to compensate for overvoltages caused by inductance M4 link with the rest of the circuit. The blocking of the transistor is thus facilitated, especially since negative voltage can discharge the capacity grid-drain particularly quickly.

The amplifier 5 shown comprises two intermediate transistors M1 and M2 arranged for alternately feed the coils L1 and L2 of the primary of the midpoint transformer. A circuit 57 applies control signals respective on transistors M1 and M2. The signals of order do not overlap temporally for avoid a short circuit in the primary. Signals order also advantageously have substantially equal activation times to limit the magnetizing current in the transformer 56. It can be also compensate for unequal times activation by a high value of the inductor magnetising transformer 56.

The timing diagram in Figure 9 illustrates different signals during the excitation of the series 61 resonator. curve 91 represents the current flowing through the resonator series 61. Curve 92 illustrates the voltage of the MOSFET M4 grid. Curve 93 illustrates the voltage at the input of the series 61 resonator.

The amplifier 5 is advantageously integrated on a same circuit board 8. It is thus possible to integrate the transformer 56, the transistors M1 to M4 and the control circuit 57 on the same printed circuit, according to the diagram shown in Figure 10. We get so for a reduced cost an amplifier 5 very compact. The leakage inductance of the transformer and surges at the terminals of intermediate transistors M1 and M2.

The left part of Figure 10 represents several elements of the amplifier 5 and their connections. The central part of Figure 10 represents the transistors M1 and M2 and their winding respective L11 and L12. The right part of Figure 10 schematically represents the different elements integrated on the printed circuit board 8. The assembly formed by transistors M1 to M4, coils L11, L12 and L2, is preferably disposed on an edge of the circuit 8. The windings can thus be arranged in the air gap of a split torus 81.

FIG. 8 represents a second embodiment of an amplifier 5 having a MOSFET switching control transistor M4. The gates of the transistors M1 and M2 are linked. Transistors M1 and M2 thus switch simultaneously. The bipolar transistor M3 is therefore mounted as a follower. When M1 and M2 conduct, the bipolar transistor M3 is off, and therefore the MOSFET transistor M4 is also blocked. Intermediate transistors M1 and M2 having the following characteristics are preferably used: a control voltage of 5V, a nominal intensity of 8A at this voltage, a resistance R _on less than 150 milliOhm and a response time of less than 20ns.

As shown in FIGS. 11 to 13, advantageously a servo amplifier 5 the charge current applied to the resonator. practice, we try to enslave a switch 51 controlling the commutations at the terminals of the resonator 6. The amplifier 5 thus has a device for measure 54 of the current applied to the input of the resonator 6. The instruction is applied to an input 58 of a comparator. The output signal of the comparator is applied on an amplification device 53 schematically represented. We optimize the overvoltage factor of amplifier 5 while driving the resonator 6 at its own frequency despite the behavior drifts of the components. This avoids to use components whose cost and complexity are inappropriate for mass production. The enslavement is for example achieved by reinjecting in the amplifier 5 a voltage proportional to the current flowing in the load. We can also apply a phase correction to the measured signal through the phase shift module 55.

In such a transformer 54, combined with a loop enslavement, the parallel resistance R2 of the transformer secondary fills preferentially two functions of servitude: the feedback of a signal proportional to the current in the load, and the phase shift of the intensity crossing the load in depending on its resistance value.

It is advantageous to use a transformer 54 having a very low inductance value (for example between 10 and 20 nH) and whose windings support a current of the order of 10A. Figure 14 thus presents an example of transformer made on a printed circuit, facilitating the obtaining of such characteristics. The left part of Figure 14 represents independently the useful layers of the printed circuit. The right part of the figure represents these superimposed layers and assemblies. The conductive element 151 forms the primary of a transformer, and is arranged on a first face of the substrate 152. This conductive element 151 is in the example realized in substantially wire form. The conductive elements 153 and 154 form the secondary of the transformer. These conductive elements 153 and 154 are arranged on a second face of substrate 152, opposite the conductive element 152. The elements 153 and 154 are electrically connected on the one hand following the dotted line, and on the other hand by resistance 155. Resistance 155 can be used to measure the through current the conductive element 151 and to form the module of phase shift 55 described above.

Advantageously adapts the servocontrol of the switch 51 controlling the switches to 6 resonator terminals previously described to the modes of embodiments having a control transistor of Power MOSFET switching as a switch. We can thus switch the MOSFET M4 transistor to optimal moments.

In order for the servo structure to produce quickly oscillations, despite a current of zero initial charge, there are several variants advantages of the system.

LC 6 resonator includes a series 61 resonator and a parallel resonator 62. The series 61 resonator has a 119 series capacitance and a series inductor 65. According to a first variant, the structure servo control includes an astable oscillator 52 (eg a slot generator) to generate the first alternations in the 119 series capacity and stabilize the oscillations in steady state. We predicts that the frequency of the oscillator is close of the frequency of excitation generated between electrodes. The servo structure adds the current measurement signal and the signal of the astable oscillator 52 and thus allows the amplifier in class E to achieve the commutations at the most favorable moments.

In addition, the first niche generated by the oscillator 52 is approximately twice as much shorter than the following: thus, we can initialize the current in the series 65 inductance to the value of this current in steady state. The parallel resonator 62 includes an inductor 621 and a capacitor 622 arranged in parallel. All impulses to inductance 621 and capacitance 622 terminals are then equal. We can avoid over-dimensioning the switch 51 and exploit it optimally.

Figure 12 shows a second variant. The control signal applied to the switch 51 generates a low voltage time slot, that is to say of the order of 5μs, initiating the first alternation in the resonator 6. The servo signal then controls the switch 51. The loop of feedback of the present servo structure a high gain. Thus, the initial impulse operational servoing is sufficiently short, and the current flowing through the switch 51 remains reasonable. It is not necessary to oversize the switch 51 to perform the starting the servo, especially when the switch is formed of a MOSFET transistor of power.

An advantageous combination of the parallel resonator 62 and Series 61 resonator optimizes operation of the system when the natural frequency of the resonator parallel 62 is slightly greater than that of resonator series 61. Thus, the voltage pulse generated by closing the switch transistor M4 has a duration less than the half-period of the resonator series 61. Thus, the impulse when closing the transistor switch M4 is anticipated by the diode internal inverse of the M4 transistor when the voltage of its drain passes by a null value. We are planning while the ratio between the impedances respective characteristics of the parallel resonator 62 and the series 61 resonator is less than 100 greater than 40. The lower value guarantees a good overvoltage coefficient. The upper limit value the currents in the transistor M4. We use typically a capacity of 1 nF and an inductance of 1 μH for the parallel resonator 62. The impedance characteristic of the parallel resonator 62 then approximately 32 ohms.

Moreover, in the parallel resonator 62, can consider that the abilities between the turns of inductance 621 will be negligible compared to the capacity of the capacitor 622. It can therefore be realized the inductor 621 in the form of a superposition of substantially circular conductive tracks 623, carried out on the superimposed layers of a circuit printed. Examples of inductance structures 621 printed circuit boards are shown in FIGS. 16. The embodiments of these figures allow thus to realize a 621 inductor without core of ferrite. This reduces the cost and improves the performance of the inductor 621.

In Figures 15 and 16, the thick dots represent connection pads of different tracks. The vertical lines joining the studs connection represent conductive links between the studs. The connected tracks thus form a coil. Advantageously, each track 623 is surrounded of a closed loop 625, in order to reduce the radiation inductance 621 formed by the tracks.

The diagram of FIG. 15 represents a variant having a top layer and a layer lower that does not have a coil track. The upper layer and the lower layer present each a connection terminal 624 of the inductor 621.

The diagram of FIG. 16 represents a variant, in which the lower layer and the layer each have a coil track and a connection terminal. Curved lines 626 joining a connection pad at a connection terminal 624 represent an electrical connection reported on these printed circuit layers.

At the working frequencies of the resonator 6, the losses are important. In order to limit these losses, limit of preference to the maximum the presence of material in the series 61 resonator.

It should be noted in the foregoing that the reference of a series resonator does not necessarily imply that the resonator also includes a parallel resonator.

As shown in Figure 17, we can consider a variant, in which a power supply is used and a common amplifier for two resonators 6 arranged in parallel. This variant allows reduce the weight, cost and overall complexity of voltage generation system 1 for a motor controlled ignition. Each resonator 6 corresponds to a respective combustion chamber 141 and 142, both combustion chambers being in phase opposition. The amplifier 5 is controlled so that the voltage ignition is generated at a time during the compression and during relaxation for each room of combustion. Indeed, compression in a room 141 is synchronized with the trigger 142 in the other. When generating the voltage, the snapping in the relaxation room 142 is a lot faster than in the compression chamber 141. In indeed, the gas density in the relaxation chamber is much lower than the density in the chamber in compression. The equivalent discharge resistance of the relaxation chamber 142 is thus much more higher than that of the chamber in compression. The candle present in the chamber in compression continues then its rise in tension until breakdown. The gas density in the room in relaxation is weak enough not to change so annoying the overvoltage coefficient in the chamber in compression; the spark generation in the chamber in compression is thus undisturbed by the generating the voltage in the other chamber.

Figure 18 shows a sectional view of a candle advantageously integrating a series resonator 61. The spark plug 110 has a connection terminal 131, connected to a first end of a winding inductive 112. The second end of winding inductive 112 is connected to an inner end of the high voltage electrode 106. This end is also in contact with an insulating element 111 forming the capacitor.

The electrodes 103 and 106 are in this example separated by the dielectric material 100 for guiding the sparks between these electrodes. The 61 series resonator built into the 110 candle includes the inductive winding 112 and the insulating element 100 also forming the capacitor between the electrodes 103 and 106. The capacitor and the inductive winding 112 are arranged in series. The resonator series capacitance series 61 is formed of capacitor and capacitors internal parasites of the candle. This capacity 119 is arranged in series with an inductor 65 to form the 61 series resonator. The length of the connection between the inductance and the capacitor being thus reduced, reduces parasitic capacitances in the candle. It is thus easy to obtain a coefficient of overvoltage of the series resonator within the preferred range from 40 to 200 described above. The candle 110 is thus used to maintain the AC voltage between the electrodes 103 and 106, in the field of desired frequency.

The series resonator built into the present candle preferably a single winding 112, facilitating the making such a candle.

A large number of turns in the winding unique 112 is necessary to obtain an inductance of the order of 50 μH (order of magnitude detailed by the after). However, a large number of turns generates stray capacitances. The only inductive coil 112 preferably has an axis (identified by the line dotted line) and consists of a plurality of superimposed turns along its axis. We thus hear that the projection of a turn is identical to the projection of all the turns along this axis. We then limit parasitic capacitances by not superimposing turns radially.

The candle furthermore advantageously comprises a shielding 132 connected to a mass and surrounding the inductive winding 112. The field lines are thus closed within shielding 132. Shielding 132 thus reduces electromagnetic emissions parasite of the candle 110. The winding 112 can in effect generate intense electromagnetic fields with the radiofrequency excitation that is envisaged to apply between the electrodes. These fields can notably disrupt embedded systems of a vehicle or exceed thresholds defined in standards resignation. The shield 132 is preferably constituted a non-ferrous material with high conductivity, such than copper. One can use a loop conductor as shielding 132.

For a 132 shield and a single coil 112 each having a generally cylindrical shape, the optimal ratio between their diameter is worth the number from Euler, approximately 2.72, if you want minimize the maximum electric field generated at the surface of the turns. This avoids phenomena of breakdown causing energy dissipation in the candle. We will then preferably choose a report between their diameter between 2.45 and 3.

The use of two coils 112 wound on one another and connected in parallel makes it possible to reduce the resistance of the winding formed. The skin effect, significantly increasing the resistance of the winding in the radio frequency range, is minimized by the winding one over the other of these two windings. If it is desired to minimize the length of the winding 112 for a predetermined inductance, the optimum ratio between the diameter of the shield 132 and the coil 112 is 2 by winding on one another two windings 112 connected in parallel by their ends. The two coils wound on one another have slightly different winding diameters and therefore slightly different inductances, which can disturb the operation of the candle in the radio frequency range. It has been determined that for the value 2 mentioned above, the difference of the inductances did not disturb the operation of the candle in the radiofrequency domain. In this case, a ratio of diameters between 1.35 and 1.5 will preferably be chosen.

The coil 112 and the shield 132 are of preferably separated by an insulating sleeve 133 into one suitable dielectric material, in order to further reduce the risk of breakdown or effluvia, the cause of energy dissipation. Of course, the more energy dissipations are low, the greater the amplitude the voltage applied between the electrodes is high and the longer the life of the candle is high. The dielectric material may for example be one of the silicone resins marketed under the references Elastosil M4601, Elastosil RTV-2 or Elastosil RT622 (the latter having a breakdown voltage of 25 kV / mm and a dielectric constant of 2.8). We can provide that the outer surface of the sleeve 133 is metallized to form the shielding 132 above.

In general, we will favor a winding winding 112 around a solid element 134 made made of insulating and non-magnetic material. It reduces still the risks of breakdown and the capacities parasites.

The set of dielectric materials is preference strongly debulled, in order to further reduce the risks of breakdown. All materials dielectric of the candle preferably has melting temperatures above 150 ° C.

Generally, when the coil-candle includes several insulating elements contiguous, it exists a significant risk of creating air inclusions at the interface between these elements, especially are made of ceramic. However, for reasons constructive, it is envisaged that the coil-candle in most cases understands several elements contiguous insulators. In particular, the link between the insulation 134 of the coil and the insulator 111 of the head candle is also for the same reasons corona, a very important source of dissipation. The technique mentioned above can, according to a new embodiment, be put to use at the level of the ceramic to create equipotentials preventing the formation of electric discharges.

Figure 19 shows a section of an element insulator 111 of candle head, also solving this problem. This insulating element 111 is intended to be associated with an insulating element 133 in the form of resin of silicone. This insulating element 111 has a non-circular section and is included in a room circular 136 belonging to the cathode 103. Thus, this element forms passages intended to let the silicone resin during its injection. Resin silicone can thus eliminate most of the air inclusions of the surface of the insulating elements.

The dielectric material used for the insulation 100 can for example be a ceramic based on alumina, of aluminum nitride, aluminum oxide or silicon carbide.

At the working frequencies of the series resonator, the losses are important. In order to limit these losses, the skilled person will limit to the maximum the presence of magnetic material in the series resonator.

According to a particularly advantageous variant illustrated in Figure 18, the candle 110 presents in in addition to a current measuring winding 139 fulfilling the function of module 54. This winding 139 includes several turns surrounding the winding 112. The winding 139 is preferably arranged to proximity of the connector 131 and at a distance from the head of candle, in an area where the voltages are relatively bass.

The candle of the invention can integrate a certain number of other features, such as the seal of seat 130 of Figure 18 disposed against a shoulder of the cathode 103, and intended to ensure the sealing of the breech at the level of the candle light.

The candle head is the part of the candle that is placed in the gas in which the plasma has to be form. This candle head preferably comprises three elements: a central electrode 106, a ground electrode 103 and an insulator 100. Geometry of these elements is decisive for ensuring formation of plasma volume or plasma branched to the desired location of the room, with the optimum properties, especially for ignition (volume important, optimal energy transfer to the gas, etc ...).

Figures 20 to 27 illustrate different configurations of candle heads, advantageously included in candles adapted to generate a plasma between their electrodes and adapted to be powered by radiofrequency excitation.

Figure 20 shows a first group of variants of candle heads, which we will call candles with capacitive propagation. These geometries of candle have a cathode 103 partially covered by the insulation 100 in the axis of the candle. This geometry generates a capacitive propagation of the spark on the surface of the insulation 100.

Figure 20.I shows a head geometry of candle known in itself. In this figure, we see that the cathode 103 protrudes axially beyond insulation 100. It is also noted that there is a direct path in the air between the anode 106 and the cathode 103. An electric arc can be formed according to this direct route.

The geometries of Figures 20.II and 20.III generate a better distribution of the plasma on the surface of insulation 100. By lengthening the connecting air path both electrodes, we reduce the probability of forming an arc. This creates landfills multidirectional between the electrodes. The plasma is distributes more evenly around the candle and the volume of gas affected is increased. We also decrease the capacitive propagation effect between the electrodes; the plasma can thus be generated at a greater distance large of the surface of the insulation.

In the variant of Figure 20.II, the cathode 103 no longer projecting axially with respect to the insulation 100. Insulator 100, cathode 103 and anode 106 substantially form a flat surface, avoiding the forming an electric arc between the anode 106 and the cathode 103.

In the variant of Figure 20.III, the insulation 100 is protruding axially from the ends of the electrodes 103 and 106. This still allows extend the air path between the electrodes 103 and 106. The protrusion of the insulator 100 forms a boss round.

The variant of Figure 21 proposes to reduce the capacitive effect. So, in the candle head, the cathode 103 does not extend radially under the insulation 100.

To lengthen the air path between the cathode and the anode, the cathode 103 of this variant is arranged axially recessed with respect to the insulation. The central electrode or anode 106 is arranged flush with the insulation.

Figure 22 proposes to make a cavity or a recess 120 in the insulator in order to amplify the depolarization phenomenon. The anode 106 presents also a growing section at its end, at 120. Thus, in the recess, the final section of the anode 106 is greater than its intermediate section. This creates axially a vacuum 121 between the end of the anode and the insulator 100, which locally amplifies the electric field.

In general, the variants intended to avoid the formation of a direct arc between the electrodes function optimally in combination with radiofrequency excitation. excitation radiofrequency makes it possible to lengthen and bend the trajectory of the sparks.

Figures 23 to 25 show examples of peak effect candles characterized by a part pointed anode protruding axially from at one axial end of the insulation and with respect to the cathode.

Fig. 23 shows an embodiment preferential of a spark plug head effect. The anode 106 consists of a core 1061 and a sheath 1062. The core 1061 is for example made of copper to promote the evacuation of heat on along the anode 106. This reduces erosion electrochemical end of the anode. Sheath 1062 may be made of any suitable material, such only nickel.

Figure 24 shows several examples of heads of high-tech candles. These candles present thus a ground electrode 103 recessed axially by compared to insulation 100, to reduce the effect capacitive. The protruding end of the anode 106 also has a pointed shape.

Examples 24.II to 24.IV each present a cathode 103 forming an axial recess 122 near insulation 100. This withdrawal 122 furthermore presents a round shape. This increases the capacity of the candle to generate a branched spark. We reduce effect the probability that a plasma will spread only on the surface of the insulation. Plasma thus tends to be distributed in a volume distant from the surface of the insulation 100.

Examples 24.III and 24.IV show an insulator 100 whose end has a rounded shape 123, to reduce its internal constraints. These constraints are related to the high levels of the fields electrical and temperature gradients nearby from the end of the insulation 100.

The example of Figure 24.IV includes an anode 106 whose axial end 1063 has several tips. We thus generate a larger number of sparks during the excitement and we split the erosion of the anode 106 on all the points used.

Peak effect candles can accidentally generate arcing between the anode 106 and the piston, when the distance between the piston and the candle head is weak. These arches prematurely erode the tip of the anode 106 and prevent the formation of plasma volume or plasma branched. The candle head of Figure 25 presents thus a solution to this problem. The tip of the anode 106 is thus disposed in a counterbore 124 formed in the insulation 100. In order to reduce the electric field to the interface between the anode 106 and the insulator 100, preferably provides a counterbore and anode forms cylindrical and having diameters whose ratio is equal to the number of Euler. This is expected to preferably the ratio of their diameter is included between 2.45 and 3. In Figure 25, we also distinguish that the insulator 100 protrudes axially relative to the tip of the anode 106. The insulation 100 presents also an edge protruding 125 axially relative to the cathode 103.

Figures 26 and 27 illustrate heads of candles with dielectric barriers that will be designated by the following by one-eyed candles. At the level of the head of candle, the anode 106 is completely covered by insulation 100. Such candles allow in particular to eliminate the formation of an electric arc between the anode and a piston, and eliminate the erosion of the anode. The life of the candle is so very greatly increased, and can equal the lifespan a heat engine without requiring maintenance. Of such candles only work because of the capacitive character of the insulation 100.

The operation of a blind candle is rendered possible by the use of excitement radio frequency. Applying an excitement radiofrequency between the electrodes of a blind candle is also particularly advantageous. excitation electrodes form loads of space on the outer surface of the insulation. Insulator 100 is then comprises as an anode and a plasma of volume or a branched plasma is generated on its surface. Although the insulation has a relatively low load, the radiofrequency excitation makes it possible to generate a very large number of sparks on the surface of the insulation in a very short time. We can predict in this variant that the insulator 100 forms the capacitor of the resonator. This reduces the energy dissipated in the candle.

According to a variant illustrated in FIG. 27, can also consider using a breech of thermal engine as mass electrode. The cost and the complexity of the candle can thus be strongly reduced. In the blind candle of Figure 27, the cathode is constituted by the breech.

We can also integrate the candle into the engine cylinder head, again using the cylinder head as the cathode of the candle. The skilled person will then take all appropriate measures to ensure that the duration life of the candle is at least equal to the duration of life of the engine.

By the way, although the heads of candles represented have a symmetry of revolution around their axis, we can also provide heads candle with other geometries, in the frame of the invention.

Claims (20)

Candle (110) comprising: two plasma generating electrodes (103, 106), a series resonator (61) having a resonance frequency greater than 1 MHz and comprising: a capacitor (111) provided with two terminals, the electrodes being connected to the respective terminals of the capacitor; and an inductive winding (112), the capacitor and the winding being arranged in series; the candle being characterized in that : among said electrodes, a first electrode forms a base of the spark plug and is connected to ground, a second electrode being an anode; the candle further comprises a piece of dielectric material separating the free ends of the electrodes. Spark plug according to claim 1, characterized in that the series resonator comprises a single inductive winding (112). Spark plug according to Claim 2, characterized in that the inductive winding has an axis and consists of a plurality of superimposed turns along this axis. Spark plug according to any one of the preceding claims, characterized in that it further comprises a probe (139) for measuring the current flowing through the winding comprising a winding radially surrounding the winding. Spark plug according to any one of the preceding claims, characterized in that it further comprises a shield (132) connected to a ground and surrounding the inductive winding (112). Spark plug according to Claim 5, characterized in that the shielding and the inductive coil are generally cylindrical in shape and in that the ratio of their respective diameters is between 2.45 and 3. Spark plug according to Claim 5 or 6, characterized in that the shielding and the inductive coil are separated by an insulating sleeve (133) made of a material having a dielectric coefficient greater than 1. Spark plug according to any one of claims 5 to 7, characterized in that the outer surface (132) of the insulation sleeve is metallized and constitutes the shielding. Candle according to any one of claims 5 to 7, characterized in that the shield comprises a conductive loop. Spark plug according to any one of the preceding claims, characterized in that the inductive winding (112) is wound around a solid element (134) made of a material having a dielectric coefficient greater than 3. Spark plug according to claim 7 or claim 10, characterized in that one of said insulation materials has a breakdown voltage greater than 20 kV / mm. System according to claims 9 and 11, characterized in that said series capacitance is formed by parasitic capacitances and a capacitor arranged inside the spark plug. System according to Claim 12, characterized in that the series inductance is arranged inside the spark plug. System according to one of Claims 11 to 13, characterized in that the amplifier (5) is a phase-shifted class E amplifier. System according to claim 14, characterized in that the amplifier has a switch (51) and a servo control loop of the switch on the intensity passing through the resonator. System according to any one of claims 11 to 15, characterized in that the power supply (3) is a DC voltage supply at 42V. Method of generating plasma between two electrodes (106, 103) separated by a material dielectric (100), the method comprising a step of applying an alternating voltage with a frequency greater than 1 MHz and amplitude between peaks greater than 5 KV between the electrodes. A method of generating plasma according to claim 17, characterized in that said voltage is applied between electrodes immersed in a fuel mixture having a density greater than 5 * 10 ^-2 mol / L, between 1.5 and 200 times per second , with an application duty cycle of between 10 and 1000. Method according to claim 18, characterized in that the voltage is applied between the electrodes of a surface spark plug. Method according to claim 19, characterized in that the voltage is applied between the electrodes of a peak effect candle.

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