EP2002117B1 - Method for measuring an ionization current of a spark plug of the type with resonant structure, and corresponding device - Google Patents

Abstract

The method involves charging a resonant structure spark plug (BR) by a voltage generated with respect to a charged ballast capacitor (Cb) during ignition phase. Ionization current (Ii) of the spark plug is periodically measured between two ignition phases by a measurement unit (MMES), where the ionization current is measured between the ballast capacitor and a weight after polarizing the spark plug. An independent claim is also included for a device for measuring ionization current of a resonant structure type spark plug.

Description

The present invention relates generally to the measurement of an ionization current of a spark plug, in particular resonant-structure type spark plugs fitted to ignition systems for a motor vehicle.

The invention is particularly suitable for so-called "radiofrequency" ignition systems comprising candles with resonant structure of multi-spark or BME type.

These ignition systems using alternative currents are described, for example, in French patent applications. FR 2,859,830 , FR 2,589,869 , FR 2,859,831 , in the name of the Applicant.

At the end of the compression cycle, the spark plug is responsible for the formation of an electric arc whose energy is sufficient to trigger the ignition process of the gas mixture contained in the combustion chamber of the engine.

This electric arc corresponds to the ionization of the gaseous mixture located between the electrodes of the spark plug, respectively a positive central electrode and a ground electrode.

However, during the combustion of the mixture, after the generation of the spark by the candle, the flame front can propagate. His breath can then push some of the mixture against the walls of the cylinder and the top of the piston.

The elevation of the pressure and temperature is so great that the fuel can remain stuck against the walls, reach its self-ignition point and then ignite in several places.

This results in microexplosions producing vibrations in the acoustic dominance (between 5 and 10 KHz approximately). These vibrations are very vivid and can quickly create hot spots that further accentuate the problem. The accumulation of microexplosions will tear or melt a small amount of metal on the top of the piston and / or on the walls of the cylinder, which may lead after some time to the destruction of the piston and the walls of the cylinder.

It is possible to detect the occurrence of these pinging phenomena, by measuring the ionization current, that is to say the current flowing through the candle, see for example DE 19524539 . Indeed, an ionization current appears through the candle as if a resistance was temporarily placed across the electrodes (according to a first approximation).

For this, the measuring means or sensors must be able to operate in a very narrow bandwidth, for example of the order of 7 kHz.

An object of the invention is to provide means for measuring the polarization current in the case of candles of resonant structure type.

Another object of the invention is to propose measurement means that are sufficiently precise to be able to work in the desired narrow frequency bandwidth.

For this purpose, the invention proposes a method for measuring an ionization current of a resonant structure-type candle equipping an ignition system for a motor vehicle, in which, during a phase of ignition, said candle is supplied by a voltage generated using a previously loaded control capacitor.

According to a general characteristic of this aspect of the invention, said ionization current is periodically measured, between two ignition phases, between said regulating capacitor and the ground, after having polarized the spark plug.

In other words, instead of measuring the ionization current at the level of the candle, which one would have to do to solve the problem, this ionization current is measured directly at a capacitor regulator that powers the candle by unloading.

As a result, the inaccuracy of the measurement is minimized.

According to one embodiment, said ionization current is measured by means of measuring means connected between said regulating capacitor and the ground, which is short-circuited during the ignition phases.

In other words, the measurement means are connected only between two ignition phases.

According to another embodiment, the ionization current is measured at the end of a damping phase during which the current flowing through the candle decreases progressively.

According to another aspect of the invention, there is provided a device for measuring an ionization current of a resonant structure-type plug fitted to an ignition system for a motor vehicle, said spark plug being coupled to a generator comprising a regulating capacitor.

According to a general characteristic of this other aspect of the invention, said generator further comprises biasing means capable of biasing the spark plug, connected between the generator and said spark plug and means for measuring the ionization current of the said spark plug, connected between the control capacitor and the ground.

Thus, the measuring means being connected between the control capacitor and the ground and not directly to the terminals of the spark plug, it is possible to choose a polarization resistance of the spark plug of low value, adapted to the current intensity of the spark plug. ionization, which is generally less than 1 mA, and a particular frequency band, for example the frequency band of observation of pinging phenomena.

Preferably, the device may further comprise controllable short-circuit means capable of short-circuiting the measuring means.

For example, the measuring means may comprise a measurement resistor.

According to one embodiment, the short-circuit means may comprise a short-circuit transistor connected between the regulation capacitor and the ground, and controlled by a short-circuit voltage generator, and a bias supply connected between the measuring resistance and the mass and adapted to bias said short-circuit transistor.

According to one embodiment, the bias supply may comprise on the one hand a supply resistor and a local power supply connected in series, and on the other hand a supply capacitor connected in parallel with the supply resistor and the power supply. local power supply, between the measuring resistor and the mass.

• la figure 1 illustre un mode de réalisation de l'invention ;
• la figure 2 illustre plus précisément un mode de réalisation de l'invention ;
• la figure 3 représente plus en détail un module d'un mode de réalisation de l'invention ;
• la figure 4 représente un chronogramme de différentes étapes d'un mode de mise en oeuvre de l'invention ;
• les figures 5 et 6 représentent des modes de réalisation d'un autre bloc de l'invention.

Other advantages and features of the invention will appear on examining the detailed description of an embodiment of the invention, in no way limiting, and the appended drawings, in which:

• the figure 1 illustrates an embodiment of the invention;
• the figure 2 illustrates more precisely one embodiment of the invention;
• Figure 3 shows in more detail a module of an embodiment of the invention;
• the figure 4 represents a chronogram of different steps of an embodiment of the invention;
• Figures 5 and 6 show embodiments of another block of the invention.

On the figure 1 , the reference SYS represents an ignition system for a motor vehicle comprising a BR candle of resonant structure type, well known to those skilled in the art, and described for example in French patent applications FR 2,859,830 , FR 2,589,869 , FR 2,859,831 , in the name of the Applicant.

An ionization current Ii flows through the BR candle.

More specifically, as illustrated schematically in the figure 1 , the spark plug BR comprises a resonant assembly RS1 (called coil-plug), comprising an inductive coil L1 and a capacitor C1 which comprises in this example a base 1-ceramic 2-central electrode 3.

The BR candle is connected to a GEN generator capable of generating a voltage called "intermediate voltage" high value. This high voltage is supplied by the central electrode 3 of the capacitor C1. An electric arc occurs when the current passes between the central electrode 3 and a ground electrode 4, generating a spark 5.

The BR candle is connected to the GEN generator via a DHT stage called "high voltage driver" connected in series with decoupling means MDEC. MPOL spark plug bias means are connected parallel to the high voltage driver DHT and decoupling means MDEC.

The GEN generator comprises measuring means MMES able to measure the ionization current Ii flowing through the candle BR.

We now refer to the figure 2 which illustrates in greater detail an embodiment of the blocks of the SYS system according to the invention.

GEN generator can be achieved using a booster voltage boost type, according to the expression of the skilled person.

The generator GEN comprises a supply Vbat here of 12 volts, able to charge a coil called "tank" BRES connected by a first terminal b1 to the supply Vbat. The loading of the BRES coil is controlled by a transistor M1 connected between the other terminal b2 of the BRES coil and the ground. The transistor M1 is controlled by a voltage generator GM1.

The reservoir coil BRES discharges into the portion of the circuit connected to its terminal b2, via a rectifying diode DR, at a voltage greater than the voltage of 12 volts delivered by the supply Vbat. This relatively high voltage is called "intermediate voltage" Vint. It is of the order of a hundred volts. In order to keep this intermediate voltage Vint substantially constant, the generator GEN comprises a so-called "ballast" capacitor Cb connected to the output of the rectifying diode DR.

The generator GEN is connected to the high voltage driver DHT fed by the intermediate voltage Vint, and controlled by a control signal Scom by control means MCOM.

The Scom control signal is directly at the origin of the creation of the spark generation by the BR candle.

Figure 3 illustrates an exemplary embodiment of the high voltage driver DHT.

This comprises an assembly formed of a coil L2 and a capacitor C2 connected in parallel, receiving as input the intermediate voltage Vint.

The assembly L2-C2 is connected at the output to a control transistor M5 receiving on its control electrode the control signal Scom.

The control signal Scom corresponds to a pulse train, generated periodically.

Thus, at each pulse train, the transistor M5 charges the coil L2, which resonates with the capacitor C2 and the resonant assembly RS1, so as to produce high voltage pulses at the natural frequency of the spark plug BR.

When the resonant assembly RS1 is excited at its natural frequency, and its quality factor is high (for example greater than 40), this results in a very high voltage across the capacitor C1. The central electrode of the spark plug BR which is one of the terminals of the capacitor C1, is then brought to a very high voltage capable of triggering sparks.

We refer again to the figure 2 .

The excitation generated by the high voltage driver DHT is transmitted to the resonant structure RS1 of the spark plug BR via the decoupling means MDEC, here a decoupling capacitor Cd.

The decoupling capacitor Cd prevents the continuous connection between the intermediate voltage Vint and the central electrode of the candle 3. This bond breakage can prevent electric shocks or electrocutions for humans.

Moreover, if an "electric arc" type discharge were to be started, this would lead to a rapid destruction of the electrodes, in particular of the central electrode 3. In fact, if a spark with a sufficiently high conductivity is created between central electrode and ground, the generated voltage drop can fall below the intermediate voltage Vint. All the charges accumulated in the capacitor Cd are then transferred into the bond created by the spark. This charge transfer takes place with strong currents that can damage the central electrode 3.

The function of the decoupling capacitor Cd is to prevent this type of charge transfer.

Alternatively, the generator may be a lift-type transformer that prevents DC transfer. In this case, the use of a decoupling capacitor is no longer necessary.

In order to be able to measure the ionization current, MPOL biasing means are used to maintain a preferentially positive polarization after the generation of the spark, on the central electrode 3 of the BR candle.

Conventionally, the polarization means MPOL may be formed by a resistor Rpol connected between the output of the rectifying diode DR delivering the intermediate voltage Vint and the output of the decoupling means MDEC, here the capacitor Cd.

A simple solution to measure then the ionization current would be to connect across the polarization resistor Rpol an assembly capable of dividing the value of the voltage, to convert the value of the voltage thus divided into current, and then to measure it.

These conventional assemblies and well known to those skilled in the art, can be achieved using a discrete transistor differential amplifier, or an operational amplifier, or using a mounting using mirrors of current. However, these assemblies, comprising a voltage divider, reduce the accuracy required for a measurement of a very low ionization current.

In contrast to these solutions, the invention consists in using a polarization resistor with a small value so as to maintain maximum precision when measuring the ionization current, and to couple the measurement means not to the terminals of the polarization resistor Rpol but between the capacitor Cb and the mass, within the GEN generator.

These measurement means MMES comprise a measurement resistor Rm and a measurement terminal Bm where the ionization current is measured.

In addition, these measuring means MMES are associated with MCC short-circuit means comprising an INT switch connected in parallel to the measurement resistor Rm, this INT switch being controlled by a GCC short-circuit generator.

The switch is preferably fast and very low impedance.

The figure 4 illustrates the different steps of an operating mode of the invention, during a period T.

At time t0, the transistor M1 becomes on and allows the loading of the capacitor Cb.

At a time t1, the control signal Scom controls the transistor M5, using a pulsed control signal (the pulses being for example at the frequency of 5 MHz), triggering the phase ignition itself, and the generation of sparks by the BR candle. At time t2, the control signal becomes inactive again.

During a damping phase (between t2 and t3), the ignition current (having a high amplitude) attenuates naturally and gradually within the BR candle, due to the existence of parasitic resistances.

Between times t0 and t3, the short-circuit means are active and short-circuit the measurement resistance. Therefore, the capacitor Cb is connected between the rectifying diode DR and the ground.

At time t3, the transistor M2 makes the short-circuit means inactive, and the capacitor Cb then discharges through the measuring resistor Rm. The discharge current of the capacitor Cb corresponds to the ionization current flowing through the resistance Rpol, in the candle BR then in the mixture in combustion.

The value of the ionization current is then measured at the measuring terminal Bm.

The measurement phase ends at a time t4, and at a time t5 another cycle of loading, ignition and measurement is repeated.

Figure 5 shows an embodiment of the switch INT. In this example, the controllable switch is made by a transistor, here MOS type M2, whose control electrode is connected to the GCC generator. In order to counteract the effect of the structural diode of the MOS transistor M2, a polarization is introduced using an Apol bias supply connected between the measurement resistor Rm and the ground.

In Figure 6, there is shown an embodiment of this Apol bias supply.

In this example, the Apol bias power supply comprises a capacitor Cal connected to a local supply Aloc via a power supply resistor Ral. The local supply Aloc can be for example a battery voltage or a power supply at 5 volts.

Those skilled in the art will know how to size the components used, so as to know the voltage Val across the capacitor Cal. From this voltage value Val, the ionization current Ii is deduced by the relation: $$\mathrm{li}=\left(\mathrm{Tension\_Apol}-\mathrm{Tension\_Bm}\right)/\mathrm{rm}$$

The invention therefore makes it possible to measure the ionization current very precisely and in a well-defined frequency range, for example adapted to the detection of pinging phenomena.

Claims (8)

1. Method of measuring an ionization current of a resonant structure spark plug, used in an ignition system for a motor vehicle, in which, during an ignition phase, said spark plug (BR) is powered by a voltage generated using a previously charged control capacitor (Cb), characterized in that said ionization current (Ii) is measured periodically, between two ignition phases, between said control capacitor (Cb) and the ground, after having polarized the spark plug (BR).
2. Method according to Claim 1, in which said ionization current is measured using measurement means connected between said control capacitor (Cb) and ground, short circuited during the ignition phases.
3. Method according to Claim 1 or 2, in which the ionization current is measured on completion of a damping phase during which the current passing through the spark plug progressively decreases.
4. Device for measuring an ionization current of a resonant structure spark plug, used in an ignition system for a motor vehicle, said spark plug (BR) being coupled to a generator (GEN) comprising a control capacitor, characterized in that said generator also comprises polarization means (MPOL) able to polarize the spark plug (BR), connected between the generator (GEN) and said spark plug (BR) and means (MMES) of measuring the ionization current of said spark plug (BR), connected between the control capacitor (Cb) and ground.
5. Device according to Claim 4, also comprising controllable short-circuit means (MCC), able to short circuit the measurement means (MMES).
6. Device according to Claim 5, in which said measurement means (MMES) comprise a measurement resistor (Rm).
7. Device according to Claim 5 or 6, in which the short-circuit means (MCC) comprise a short-circuit transistor (M2) connected between the control capacitor (Cb) and ground, and controlled by a short-circuit voltage generator (GCC), and a polarization power supply (Apol) connected between the measurement resistor (Rm) and ground, and able to polarize said short-circuit transistor.
8. Device according to Claim 7, in which the polarization power supply comprises, on the one hand, a power supply resistor (Ral) and a local power supply (Aloc) connected in series, and on the other hand a power supply capacitor (Cal) connected in parallel to the power supply resistor (Ral) and the local power supply (Aloc), between the measurement resistor (Rm) and ground.

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