WO2008017601A1

WO2008017601A1 - Spark plug for high-frequency plasma ignition

Info

Publication number: WO2008017601A1
Application number: PCT/EP2007/057809
Authority: WO
Inventors: Georg Bachmaier; Robert Baumgartner; Reinhard Freitag; Thomas Hammer; Oliver Hennig
Original assignee: Siemens Aktiengesellschaft
Priority date: 2006-08-08
Filing date: 2007-07-30
Publication date: 2008-02-14
Also published as: DE102006037037A1

Abstract

In a spark plug for a high-frequency plasma ignition having a primary electrode(1) and a secondary electrode (2), between which a plasma flashover can occur, the primary electrodes (1) and/or the secondary electrodes (2) are completely coated with an insulating dielectric layer (7,8,10,12).

Description

SPARK PLUG FOR HIGH FREQUENCY PLASMA IGNITION

description

The present invention relates to an ignition device for high-frequency plasma ignition according to the features of the preamble of claim 1.

For the ignition of inert fuel-air mixtures in internal combustion engines, especially lean or extremely rich mixtures and mixtures with a high proportion of exhaust gas or fuels that have a high flash point (eg compressed natural gas, CNG), a very high energy must be introduced into the gas mixture be ignited and / or a larger volume of mixture than this example is necessary for stoichiometric gasoline-air mixtures.

A known ignition possibility, which offers these features, is the high-frequency plasma ignition, as described for example in DE 10 2004 058 925. This is a

Resonant circuit or resonator, consisting of a coil inductively and a capacitance by a high-frequency source resonantly excited until the capacitance representing electrodes ignites a high-frequency plasma. The design used so far is based on the conventional spark plug, i. it consists of an outer ground electrode connected to the screwed connection and an insulated central rod electrode. The coupling of higher energy at the same boundary conditions leads to a greatly increased electrode wear. A resulting shorter service life compared to conventional ignition systems is disadvantageous for series production in motor vehicles.

In conventional, inductively or capacitively pulsed ignition systems could by combining materials with high melting point and materials of good thermal conductivity, the life of the spark plug electrodes on Values of more than 1000 operating hours can be improved. In the high-frequency plasma ignition in the form known in the prior art, however, the service life achieved with these materials are far below, so that an optimization of electrode materials does not promise a sufficient increase in service life.

It is therefore an object of the present invention to provide an ignition device for a high-frequency plasma ignition, in which the electrode load is significantly reduced.

This object is achieved by an ignition device having the features of claim 1. Advantageous developments are specified in the subclaims.

Advantageously, the insulating dielectric layer on at least one of the electrodes, by preventing a direct current flow, ensures that no narrowly defined bases of the plasma flashover can form, but that the electrical power in the plasma boundary layer is distributed over a larger area. While e.g. a positive half cycle of the exciting resonant frequency voltage drift electrons on the dielectric layer of the electrode of positive polarity. This loads the

Dielectric layer locally and the electrical potential at the charged surface decreases locally. More electrons are repelled and attach to other, non or less charged sites on the surface. This process continues until the surface is completely charged or the polarity of the electrode reverses. After reversing the polarity, the surface is discharged and possibly charged opposite. This can not lead to small foot areas of the plasma flashover, in which the entire current flow concentrates and in which the electrode material is strongly eroded. The electrical power is coupled into the plasma flashover exclusively capacitive high frequency. Are both electrodes, both the primary electrode and the secondary electrode covered with a dielectric layer, so occurs in neither of them erosion of the electrode surfaces and thus only reduced wear.

In a favorable embodiment, the thickness of the dielectric layer decreases to a combustion chamber-side electrode tip of the primary electrode and / or the secondary electrode. The distance of the plasma flashover between the primary electrode and the secondary electrode or the respective surface of the dielectric layer can increase to a combustion chamber-side electrode tip of the primary electrode and the secondary electrode.

At an increasing towards the electrode tips

Überschlagsweg ignites the high-frequency plasma in the range of a small flashover distance. The charge of

Dielectric layer in this area then ensures that the plasma flashover moves very quickly in areas larger rollover distance or spread and thereby increases in power. As a result, the propagation of the plasma flashover into a combustion chamber is promoted, and it is ensured that the plasma power is deposited to a large extent in the region of the electrode tips in the combustion chamber. This process is advantageously assisted as the thickness of the dielectric layer decreases toward the electrode tips, increasing the local capacitance.

The thickness of the dielectric layer may be between 1/10 mm and a few mm.

Advantageously, the surface of the dielectric-coated primary electrode and / or secondary electrode is up to max. about 10 mm ² .

By the thickness of the dielectric layer or the dielectric barrier and by the material properties Of the dielectric, such as the relative dielectric permittivity as well as the capacitive coupled to the plasma flash or the resonator effective surface of the electrodes, the capacity can be adjusted to the necessary values for the ignition of the high-frequency plasma and the required power so that a plasma flashover of the desired strength on the other hand, a sufficient dielectric strength of the barrier is given under all operating conditions and there are no breakdowns through the dielectric layer. Typical values are a few to a few 10 mm ² for the surface of the central high-voltage electrode and a few tenths of a millimeter to a few millimeters for the thickness of the dielectric layer.

The secondary electrode may be arranged surrounding the primary electrode.

In a favorable embodiment, a gap between the primary electrode and the secondary electrode is filled to one or more channels with the dielectric.

This makes it possible to control where and with what energy density plasma flashovers on the electrode tip come into contact with the combustion chamber.

The secondary electrode may be a cylinder arranged concentrically around the primary electrode.

This allows a simple, symmetrical design of the ignition device.

The secondary electrode may be arranged around the primary electrode at different distances. In particular, a non-concentric arrangement of a cylinder around the primary electrode is possible. By means of a corresponding electrode design, a gap between the outer secondary electrode and the inner primary electrode can be reduced locally to provide preferred conditions for a capacitively coupled radio-frequency plasma discharge.

In an advantageous embodiment, the dielectric is a glass or a ceramic, in particular aluminum oxide, aluminum nitride or boron nitride.

These materials are characterized by low electrical conductivity and good dielectric properties at high temperature.

The present invention will be explained in more detail by means of embodiments with reference to the accompanying drawings. It shows

1A shows in cross section the electrode region of an ignition device according to the invention,

FIG. 1B shows the electrode area of the ignition device of FIG. 1A in a plan view of the combustion chamber. FIG.

2A in cross section the electrode region of a further ignition device according to the invention,

2B in combustion chamber side view of the electrode region of the ignition device of Fig. 2A,

3A in cross section the electrode region of a further ignition device according to the invention,

3B in combustion chamber side view of the electrode region of the ignition device of Fig. 3A,

4A in cross section the electrode region of a further ignition device according to the invention, 4B in combustion chamber side view of the electrode region of the ignition device of FIG. 4A,

5A in cross section the electrode region of a further ignition device according to the invention,

5B, in the combustion chamber side view, the electrode area of the ignition device of FIG. 5A, FIG.

FIG. 1A shows an embodiment of an ignition device according to the invention in longitudinal section in the region of the electrodes with a primary electrode 1 and a secondary electrode 2, which concentrically surrounds the primary electrode 1 as a cylinder. Between the primary electrode 1 and the secondary electrode 2, a dielectric 3 is arranged. In the region of a primary electrode tip 4 and a secondary electrode tip 5, the dielectric does not fill the gap between primary electrode 1 and secondary electrode 2 and an annular gap 6 remains. In this annular gap 6 and on the primary electrode tip 4, the primary electrode 1 is completely covered with a dielectric layer 7.

FIG. 1B shows the electrode area of the ignition device of FIG. 1A with the dielectric layer 7, the annular gap 6 and the secondary electrode tip 5 in a plan view of the combustion chamber.

By preventing a direct current flow, the insulating dielectric layer 7 on the primary electrode 1 ensures that no narrowly defined base point of the plasma flashover on the primary electrode 1 can form. The electrical power is distributed over a larger area. Due to the local charging of the dielectric layer 7, a further charge build-up will take place at no or less charged points of the surface. If it comes to the rollover, it can not come to small foot areas of the plasma flashover, in which the entire Concentrated current flow and in which the electrode material is eroded greatly. The electrical power is coupled into the plasma flashover exclusively capacitive high frequency.

2A and 2B show a further embodiment of an ignition device according to the invention in a longitudinal section in the region of the electrodes. FIGS. 1A and 1B corresponding components are provided with the same reference numerals. The primary electrode 1 is from the cylindrical

Secondary electrode 2 concentrically surrounded. Between the primary electrode 1 and the secondary electrode 2, the dielectric 3 is arranged, which leaves free the annular gap 6 in the region of the primary electrode tip 4 and the secondary electrode tip 5. In the annular gap 6 and at the

Primary electrode tip 4, the primary electrode 1 with the dielectric layer 7 is completely covered. The secondary electrode 2 is also completely covered with a dielectric layer 8 in the region of the secondary electrode tip 5.

FIG. 2B shows in combustion chamber side view the electrode area of the ignition device of FIG. 2A with the dielectric layer 7, the annular gap 6 and the dielectric layer 8 above the secondary electrode tip.

When both electrodes, both the primary electrode 1 and the secondary electrode 2, are covered with the dielectric layers 7, 8, neither of them erosion of the electrode surfaces and thus only reduced wear of the igniter as a whole.

3A and 2B show a further embodiment of an ignition device according to the invention in longitudinal section in the region of the electrodes. The components corresponding to FIGS. 1A and 1B are in turn provided with the same reference numerals. The primary electrode 1 and the secondary electrode 2 are concentric with the dielectric 3 provided therebetween arranged. The secondary electrode tip 5 is correspondingly completely covered with the dielectric layer 8, as in the embodiment in FIGS. 2A and 2B. A primary electrode tip 9 has a conically tapering shape, so that a conically widening annular gap 11 remains between a dielectric layer 10 of the primary electrode tip 9 and the dielectric layer 8 of the secondary electrode 2.

Fig. 3B shows in combustion chamber side view of the

Electrode region of the ignition device of FIG. 3A with the dielectric layer 7, the annular gap 11 and the dielectric layer 10.

When a flashover path in the conical annular gap 11 increases towards the electrode tips 4, 9, the high-frequency plasma ignites in the region of a small flashover distance. As a result of the charging of the dielectric layer in this region, the plasma flashover spreads very rapidly into regions of greater flashover distance and increases in power.

4A and 4B show a further embodiment of an ignition device according to the invention in a longitudinal section in the region of the electrodes. The FIGS. 3A and 3B corresponding components are again provided with the same reference numerals. The primary electrode 1 and the secondary electrode 2 are insulated by the dielectric 3 interposed therebetween. The secondary electrode tip 5 is completely covered with the dielectric layer 8. The primary electrode 1 has a dielectric layer 12 decreasing in thickness to the primary electrode tip 9. The annular gap 11 widens conically.

Fig. 4B shows in combustion chamber side view the

Electrode region of the ignition device of FIG. 4A with the dielectric layer 12, the annular gap 11 and the dielectric layer 10. The reduction of the thickness of the dielectric layer 12 towards the primary electrode tip 4 enhances the rapid propagation of the plasma flashover into regions of greater flashover distance in the conical annular gap 11.

5A and 5B show a fifth embodiment of an ignition device according to the invention in longitudinal section in the region of the electrodes. The components corresponding to FIGS. 1A and 1B are in turn provided with the same reference numerals. The primary electrode 1 is surrounded concentrically by the cylindrical secondary electrode 2 and both are completely embedded in an insulating dielectric 13 which also covers the primary electrode tip 4 and the secondary electrode tip 5. Ignition channels 14 are excluded in the dielectric 13.

FIG. 5B shows in combustion chamber side view the electrode area of the ignition device of FIG. 5A with the dielectric 13, which contains the primary electrode tip and the

Secondary electrode tip covered and three firing channels 14, which are designed as holes.

In the ignition channels 14, the plasma flashover takes place with a greater energy density. As a result, the energy density and spatial arrangement of the plasma flashovers at the tip of the ignition device can be influenced in a simple manner.

The operation of the described capacitively coupled high frequency ignition devices is similar to that of conventional electrodes equipped with direct current flow, ie, a distinction can be made between the power supply to initiate the plasma flashover and the power supply to maintain a high-power plasma flashover with impedance matching to the resonator upon initiation of the plasma flashover. A further advantage of the dielectric layer ignition devices according to the invention is that they limit the power locally and overall. The plasma-facing surface of the dielectric layer is capacitively coupled to the area covered by its electrode, and the impedance of this capacitance C _SC hicht is at an angular frequency of the exciting voltage ω _f: Z _SC hicht = 1 / (ω _f C _S chicht) • For a given voltage U to the electrode and a low plasma impedance, the current I _P i _asma is limited by the condition

I piasma <WfCschichtU

LIST OF REFERENCE NUMBERS

1 primary electrode

2 secondary electrode

3 dielectric

4 Primary electrode tip

5 Secondary electrode tip

6 annular gap

7 dielectric layer

8 dielectric layer

9 Primary electrode tip

10 dielectric layer

11 conical annular gap

12 dielectric layer

13 dielectric

14 ignition channel

Claims

claims

1. Ignition device for high-frequency plasma ignition with a primary electrode (1) and a secondary electrode (2), between which a plasma flashover can take place, characterized in that the primary electrode (1) and / or the secondary electrode (2) completely with an insulating dielectric layer (7, 8,10,12) are coated.

2. Ignition device for high-frequency plasma ignition according to claim 1, characterized in that the thickness of the dielectric layer (12) to a combustion chamber-side electrode tip (4) of the primary electrode

(1) and / or the secondary electrode (2) decreases.

3. Ignition device for high frequency plasma ignition according to claim 1 or 2, characterized in that the thickness of the dielectric layer is between 1/10 mm and a few mm.

4. Ignition device for high-frequency plasma ignition according to one of the preceding claims, characterized in that the surface of the dielectric-coated primary electrode and / or secondary electrode to max. about 10 mm ² .

5. Ignition device for high frequency plasma ignition according to one of the preceding claims, characterized in that the distance of the plasma flashover between the primary electrode (1) and the secondary electrode (2) to a combustion chamber side electrode tip (4,5) of the primary electrode (1) and the secondary electrode ( 2) increases.

6. Ignition device for high frequency plasma ignition according to one of the preceding claims, characterized in that the secondary electrode (2) around the primary electrode (1) is arranged around.

7. Ignition device for high-frequency plasma ignition according to claim 6, characterized in that a gap between the primary electrode (1) and the secondary electrode (2) except for one or more channels (14) with the dielectric (13) is filled.

8. Ignition device for high-frequency plasma ignition according to claim 6 or 7, characterized in that the secondary electrode (2) concentrically around the

Primary electrode (1) is arranged cylinder.

9. Ignition device for high-frequency plasma ignition after

Claim 6 or 7, characterized in that the secondary electrode (2) around the primary electrode (1) is arranged at different distances.

10. Ignition device for high-frequency plasma ignition according to one of the preceding claims, characterized in that the dielectric is a glass or a ceramic, in particular aluminum oxide, aluminum nitride or boron nitride.