EP2496827A1

EP2496827A1 - Laser ignition plug

Info

Publication number: EP2496827A1
Application number: EP10770526A
Authority: EP
Inventors: Werner Herden; Martin Weinrotter; Juergen Raimann; Andreas Letsch
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2009-11-06
Filing date: 2010-10-04
Publication date: 2012-09-12
Also published as: WO2011054610A1; JP2013510258A; DE102009046472A1; US20120304959A1

Abstract

The invention relates to a laser ignition plug (100), comprising an antechamber (110), wherein the laser ignition plug (100) is designed to radiate, in particular to focus, laser radiation (24) guided and/or generated in the laser ignition plug (100) onto at least two ignition points (ZP1, ZP2) located in the antechamber (110), which are different from one another. According to the invention, the laser ignition plug (100) is designed to radiate the laser radiation (24) into the antechamber (110) such that a distance (d12) between at least one first ignition point (ZP1) and a second ignition point (ZP2) adjacent thereto is greater than a minimum distance (d2min) of the first and/or second ignition points (ZP1, ZP2) to an inside surface (110a) of the antechamber (110).

Description

description

title

Laser spark plug prior art

The invention relates to a laser spark plug with an antechamber, wherein the laser spark plug is adapted to irradiate in the laser spark plug guided and / or generated laser radiation to at least two mutually different in the antechamber lying ignition points, in particular to focus.

A laser spark plug of the aforementioned type is described in FR 2 873 763 A1. Disclosure of the invention

It is an object of the present invention to develop a laser spark plug of the type mentioned in that an improved, in particular more uniform and faster, combustion in the pre-chamber is possible.

This object is achieved in the laser spark plug of the aforementioned type according to the invention that the laser spark plug is adapted to irradiate the laser radiation in the antechamber, that a distance between at least a first ignition point and a second adjacent thereto ignition point is greater than a minimum distance of first and / or second ignition point to an inner surface of the antechamber.

The inventive design of the laser spark plug advantageously allows a more uniform and efficient combustion of a pre-chamber located in the air / fuel mixture, because in compliance with the

Abstandskriteriums invention in the individual ignition points Emerging flame cores or flame fronts developing therefrom can propagate for a maximum of unhindered until they hit the inner surface of the pre-chamber or an adjacent flame front.

The inventive principle advantageously allows a maximum

Pressure increase during combustion in the antechamber, whereby particularly high-energy ignition torches from the antechamber by a

Fluid connection to a main combustion chamber realizing overflow can escape.

In a particularly preferred embodiment of the laser spark plug according to the invention it is provided that the distance between the adjacent ignition points is at least about 120 percent, preferably at least about 160 percent, of the minimum distance of the first and / or second ignition point to the inner surface of the prechamber, whereby investigations of the According to the applicant, results in a particularly efficient combustion.

According to a further variant of the invention, it can be provided quite particularly advantageously that a distance between all adjacent ignition points is at least about 120 percent, preferably at least about 160 percent, of the minimum distance of a point of ignition to the inner surface of the pre-chamber, ie the principle according to the invention is used in the case of transmitted more than two ignition points on the totality of all ignition points. Furthermore, it is particularly advantageous in this case to have as uniform a distribution as possible

Ignition points in the antechamber, in each case in consideration of the boundary conditions according to the invention for the distance between adjacent ignition points and their distance from the inner surface of the pre-chamber.

In a further very advantageous embodiment of the laser spark plug according to the invention, it is provided that a minimum distance of an ignition point to the inner surface of the pre-chamber is about 10 percent to about 40 percent of a maximum extent of an interior of the pre-chamber, in the case of a substantially at least partially spherical or ellipsoidal In particular, antechamber about 10 percent to about 40 percent of a radius of the antechamber. This configuration is particularly useful for systems with two to about eight points of ignition. In a further very advantageous variant of the invention, in which the antechamber has at least partially substantially spherical or ellipsoidal shape, it is provided that the distance between the first ignition point and the second ignition point is about twice as large as a minimum distance of the first and / or second Ignition point to the inner surface of the antechamber.

Generally it can be provided according to the invention that a mean distance between adjacent ignition points is greater than a mean distance of the ignition points to the inner surface of the antechamber. The mean distance between adjacent ignition points to each other can be determined, for example, as an average over the respective distances to each other adjacent ignition points. The same applies to the determination of the average distance of the ignition points to the inner surface of the pre-chamber, wherein preferably only a minimum distance of the respective ignition point is considered to the inner surface.

The setting according to the invention of the plurality of ignition points in the antechamber of the laser spark plug can, according to a preferred embodiment, be carried out by means of an optical element which focuses the laser radiation onto the different ignition points.

The optical element may according to a further variant of the invention

For example, be formed as a multiple focal lengths focusing optics, the focusing optics having at least two substantially concentric with each other or juxtaposed focusing areas each having a different focal length. Such a focusing optics can be realized, for example, by a lenticular optical body, the surfaces of which correspond in sections, in particular

have different curvature properties. Alternatively or in addition to lens arrangements, it is also possible to use (hollow) mirrors in order to realize the position of the ignition points provided according to the invention.

Another very advantageous embodiment of the invention

Laser spark plug provides that the laser spark plug is adapted to irradiate the laser radiation in such a way in the antechamber that a Rayleigh length the irradiated laser radiation is at least about 10 percent, preferably at least about 30 percent, of a maximum extent of an interior of the prechamber.

Investigations by the Applicant may be in such

Configuration results in a string of spark points called multiple ignition points along the optical axis of the laser spark plug in the antechamber, which also allows fast and efficient combustion. To achieve such a spark chain in particular long focal length focusing optics are used, which combine the laser radiation in a corresponding manner in the antechamber.

Other features, applications and advantages of the invention will become apparent from the following description of embodiments of the invention, which are illustrated in the figures of the drawing. All described or illustrated features alone or in any combination form the subject matter of the invention, regardless of their

Summary in the claims or their dependency and regardless of their formulation or representation in the description or drawing.

In the drawing shows:

1 shows a combustion chamber-facing end portion of a first embodiment of the laser spark plug according to the invention in a partial cross-section,

FIGS. 2, 3, 4 each show different pre-chamber configurations of further embodiments of the laser spark plug according to the invention,

FIGS. 5, 6, 7 each show a combustion chamber-facing end section of further embodiments of the laser spark plug according to the invention, a course of operating variables of

Laser spark plug according to the invention applied over a beam axis of the laser spark plug, schematically a partial cross section of another embodiment of the laser spark plug according to the invention, and

Figure 10a, 10b, 1 1 a, 1 1 b different embodiments of optical elements for use in the invention

Laser spark plug.

FIG. 1 shows a combustion chamber-facing end section of a first

Embodiment of a laser spark plug 100 according to the invention

In the installation position of the laser spark plug 100 shown in FIG. 1 in a cylinder head 200 of an internal combustion engine, the prechamber 110 of the laser spark plug 100 projects into the combustion chamber 300 of a cylinder of the internal combustion engine. The internal combustion engine may, for example, be a stationary one

Large gas engine or even act on an internal combustion engine of a motor vehicle.

The laser spark plug 100 has an integrated laser device 120, which may be, for example, a laser-active solid with a passive Q-switching, which generates high-energy laser ignition pulses 24 in a conventional manner with appropriate exposure to pump radiation.

The laser radiation 24 is radiated by the laser device 120 onto an optical element 130, which i.a. for focusing the laser radiation 24 in a manner to be described later.

In the present case, the optical element 130 is formed so that it

Laser radiation 24 to a total of two in the prechamber 1 10 arranged and different ignition points ZP1, ZP2 bundles. In the ignition points ZP1, ZP2 arises in the laser ignition in a conventional manner a flame kernel, which spreads as a function of the present air / fuel mixture - neglecting non-vanishing flow velocities in the pre-chamber 1 10 - approximately spherical and thereby forms a corresponding flame front ,

In this case, through the fluid connection between the interior of the pre-chamber 1 10 and the combustion chamber 300 producing overflow 150 so-called ignition flares from the pre-chamber 1 10 out pressed into the combustion chamber 300, ignite a present in the combustion chamber 300 air / fuel mixture.

According to the invention, the laser spark plug 100 is designed to radiate the laser radiation 24 into the prechamber 110 such that a distance d12 between the first ignition point ZP1 and the second ignition point ZP2 adjacent thereto is greater than a minimum distance d2min of the second ignition point ZP2 to an inner surface 1 10a of the prechamber 1 10.

Particularly preferred is a corresponding minimum distance dl min of the first ignition point ZP1 to the inner surface 1 10a also smaller than the distance d12 of the ignition points ZP1, ZP2 with each other.

The inventive arrangement of the ignition ZP1, ZP2 in the prechamber 1 10 is advantageously ensured that an efficient and rapid combustion of the pre-chamber 1 10 located air / fuel mixture takes place, whereby a maximum pressure increase in the prechamber 1 10 is realized. Accordingly, in the laser ignition according to the invention in the pre-chamber 1 10 high-energy ignition torches that allow a safe ignition of the present in the combustion chamber 300 mixture.

The interior of the antechamber 1 10 is of an interior of the

Laser spark plug 100 separated by a combustion chamber window 140, the focusing optics 130 with respect to the primary propagation direction of the

Laser radiation 24 is optically downstream. Although a laser device 120 for generating laser radiation is preferably provided directly in the laser spark plug 100, the inventive principle of the configuration of the ignition points ZP1, ZP2 in the prechamber 110 can also be used with laser spark plugs which are not designed for the local generation of laser ignition pulses 24. but rather, from a source located remotely, generate laser firing pulses 24 into the prechamber 10.

In the variant of the invention depicted in FIG. 1, the optical element 130 has the function of a focusing lens, wherein, as shown in FIG. 1, two focusing areas of respectively different focal lengths arranged essentially concentrically with one another are provided. In a radially inner region, for example, the optical element 130 has a first one

Radius of curvature or generally a curved surface with first

Refractive properties, while a radially outer region of a different radius of curvature or generally a curved surface with different from the first refractive properties second

Has refractive properties.

As a result, the core beam 24a of the laser ignition pulse 24 is focused onto the first ignition point ZP1, while the sheath beam 24b of the

Laserzündimpulses 24 is focused on the different thereof, the second ignition point ZP2.

Although it is not necessary for the function of the principle according to the invention, the laser spark plug 100 is formed so that it focuses the laser radiation 24 to ignition ZP1, ZP2, which at or about in the region of an optical axis of the laser spark plug, especially in pre-chambers 1 10 with rotationally symmetrical geometry 100 lie.

Due to the inventive arrangement of the ignition ZP1, ZP2 in the interior of the prechamber 1 10 is advantageously ensured that the emanating from the ignition ZP1, ZP2 flame fronts for as long as possible from the laser ignition unhindered in the interior of the prechamber 1 10, resulting in the already described above

desired pressure increase in the prechamber 1 10 leads.

In a further preferred embodiment, both ignition points ZP1, ZP2 are exposed to laser radiation substantially simultaneously. It may - depending on the design of the laser device 26 - but also a temporal

Delay between the irradiation of the laser ignition pulses to the different ignition points ZP1, ZP2 be provided. A particular advantage of the invention is that none of the

Ignition points ZP1, ZP2 outgoing flame fronts unnecessarily early contact with the inner surface 1 10a of the prechamber 1 10 or one

adjacent flame front undergoes, which prevents efficient burning of the pre-chamber volume in conventional systems. The

In contrast, laser ignition according to the invention allows an efficient

Combustion in the pre-chamber 1 10 and thus the production of particularly high-energy ignition torches, which emerge in the combustion chamber 300 and reliably ignite the mixture present there. In a particularly preferred embodiment of the invention

Laser spark plug 100 is provided that the distance d12 between the adjacent ignition points ZP1, ZP2 at least about 120 percent,

preferably at least about 160 percent, of the minimum distance d2min of the second ignition point ZP2 to the inner surface 110a of the prechamber 1 is 10.

If the first ignition point ZP1 closer to the inner surface 1 10a of

Prechamber 1 10 is located as the second ignition point ZP2, the inventive distance criterion d12> = d2min or preferably d12> = 1.6 ^* d2min in a corresponding manner to the relation between the ignition point distance d12 and the minimum distance dl min of the ignition point ZP1 to the inner surface 1 10a the antechamber 1 10 apply.

FIG. 2 shows a further variant of the invention, which deviates from the system described with reference to FIG

Having a prechamber configuration. The at least partially approximately spherical or ellipsoidal-shaped pre-chamber 1 10 according to Figure 2 is in turn of the remaining interior, not shown in Figure 2 of the laser spark plug 100 by a

Combustion chamber window 140 separated.

According to the invention, the laser spark plug 100 or its focusing optics (not shown in FIG. 2) is designed so that the laser radiation 24 (FIG. 1) generated in the laser spark plug 100 is focused onto the ignition points ZP1, ZP2 apparent from FIG.

In addition to the primary distance criterion according to the invention d12> = dl min and / or d12> = d2min, in the present case the minimum distance dl min of the first ignition point ZP1 to the inner surface 110a of the pre-chamber 110 is selected to be approximately 25 percent of a maximum longitudinal extent of the interior corresponds to the antechamber 1 10, which extends in Figure 2 in the vertical direction.

Investigations by the Applicant have shown that this configuration according to the invention, particularly in the case of at least partially spherically or ellipsoidally shaped prechambers 1 10, leads to a particularly efficient combustion in the prechamber 110 and thus promotes the formation of high-energy ignition flares 155.

FIG. 3 shows a further embodiment of the invention, in which the prechamber 110 has a substantially cylindrical shape.

Even with such an antechamber geometry, the realization of the distance condition d12> = dl min, d2 min according to the invention is advantageous with regard to a rapid and efficient combustion of the air / fuel mixture located in the prechamber 110.

FIG. 4 shows a further variant of the invention, in which the prechamber 110 has a substantially ellipsoidal basic shape. Unlike the described above with reference to Figures 1 to 3

Embodiments of the laser spark plug according to the invention, the configuration shown in Figure 4 has a total of three ignition ZP1, ZP2, ZP3, of which two ignition points ZP1, ZP3 outside the optical axis of

Laser spark plug 100, in particular relatively close to a region

Inner surface 1 10a of the prechamber 1 10, lie.

This is due to the fact that investigations by the Applicant According to a feed of the pre-chamber 1 10 with from the combustion chamber 300 through the overflow channels 150a, 150b einströmendem air / fuel mixture

Turbulences result that lead to different flow velocities of the present in the pre-chamber 1 10 mixture. In particular, the overflow ducts 150a, 150b can advantageously be arranged relative to one another in such a way that the partial flows 151a, 151b flowing through them overlap with respect to their flow direction and speed in such a way that in the region of the inner wall 110a of the prechamber 110 they are particularly large Flow rates result. For this purpose, the overflow channels 150a, 150b are arranged with their longitudinal axis, in particular tangentially to the optical axis or the longitudinal axis of the prechamber 110, and not approximately radially.

For the configuration of FIG. 4 described above, it is expedient for at least one, but preferably two, ignition points ZP1, ZP3 to be arranged in the region of high flow velocity in order to enable reliable ignition of the air / fuel mixture present in the prechamber 110. In addition, the further ignition point ZP2 arranged centrally on the optical axis of the laser spark plug 100 effects a safe ignition

Ignition of the in the pre-chamber 1 10 located air / fuel mixture even in those areas where the flow velocity

has relatively low values.

The ignition points ZP1, ZP2, ZP3 described above with reference to FIG. 4 advantageously satisfy the criteria according to the invention d12> = dl min, d23> = d3min, wherein the relevant spark gap distances d12, d23 are advantageously even greater than approximately 160 percent greater than that concerned

Minimum distances d1 min, d3min of the considered ignition points ZP1, ZP3. The combination of the distance criteria according to the invention for the arrangement of the ignition points ZP1, ZP2, ZP3 in the interior of the pre-chamber 1 10 with the

Taking into account expected flow velocities in the prechamber 1 10 leads to particularly good results in terms of efficient and rapid combustion in the prechamber 1 10.

FIG. 5 shows a further embodiment of a device according to the invention

Laser spark plug 100, in which a beam splitter by means of two mirrors 132a, 132b, one of which is partially reflecting, realized, to irradiate the laser ignition pulses 24 to two different ignition points ZP1, ZP2.

Optically downstream of the beam splitter 132a, 132b is a focusing optics 133 which has different refractive properties in sections such that a partial beam 24_1 passing through the mirror 132a is focused on the first ignition point ZP1 and a partial beam 24_2 reflected by the mirror 132b is focused on the second ignition point ZP2.

Although in the above-described embodiment according to FIG. 5 the ignition points ZP1, ZP2 lie outside the optical axis of the laser spark plug 100, the distance criterion according to the invention is again fulfilled such that the distance between the ignition points ZP1, ZP2 is significantly greater than a minimum distance between the two present Ignition points ZP1, ZP2 and a relevant portion of an inner surface 1 10a (Figure 1) of

Prechamber 1 10.

FIG. 6 shows a further embodiment of a device according to the invention

Laser spark plug 100, in which instead of a focusing optics diffractive optics 135 is used to disassemble the laser ignition pulse 24 into a plurality of partial beams. At the same time, the diffractive optic 135 can advantageously be designed so that it focuses the two partial beams onto corresponding ignition points ZP1, ZP2. As an alternative or in addition, focusing of the partial beams can also be carried out by the combustion chamber windows 140 which are optically arranged downstream of the diffractive optics 135.

FIG. 7 shows a further embodiment of a device according to the invention

Laser spark plug, in which the focusing optics not shown above so is formed, in particular as a long focal length focusing lens, that a Rayleigh length of irradiated into the pre-chamber 1 10 laser radiation 24 is at least about 10 percent, preferably at least about 30 percent, a maximum extent L of the interior of the pre-chamber 1 10.

For example, the Rayleigh length may be selected to be about 0.4 cm to about 1.2 cm.

As a result, the formation of a so-called spark chain is advantageous

ensures that the present arises from the fact that the irradiated into the pre-chamber 1 10 laser radiation 24 over a corresponding length range within the pre-chamber 1 10 has a sufficiently high electric field strength or optical power density. This means that the air / fuel mixture in the pre-chamber 1 10 can be ignited substantially simultaneously over the entire Rayleigh length.

Although the above described with reference to Figure 7

Embodiment of the invention does not necessarily meet the primary criterion of the underlying distance criterion, also results from the further inventively relatively large selected Rayleigh length a particularly uniform and rapid combustion in the pre-chamber 1 10th

A further possibility according to the invention of realizing two laterally offset ignition points consists of a simply astigmatic laser beam 24 and / or an astigmatic optical element, i. H. an element with

different refractive power in different spatial directions, to use.

8 shows a profile of a beam diameter of the laser radiation 24 in mutually perpendicular planes x, y along a spatial coordinate z, which corresponds to the beam direction of the laser radiation 24.

In the present case, the reference numeral SDx is the beam diameter of the

Laser radiation 24 in a first plane x denotes, and with the

Reference character SDy is the beam diameter of the laser radiation 24 in the second plane x orthogonal to the first plane x. As can be seen from FIG. 8, the locations z1, z2 of minimum beam diameter fall apart along the beam coordinate z, so that the power density S logarithmically plotted in the graph of FIG. 8 accordingly has two local maxima at the locations z1, z2.

The locations z1, z2 from the diagram according to FIG. 8 correspond to a first and a second ignition point ZP1, ZP2 in the pre-chamber 110.

FIG. 9 shows a further embodiment of a device according to the invention

Laser spark plug 100, in which a focusing optical element 136, which is embodied for example as an aspheric, is provided.

A first possibility according to the invention of generating the beam diameter SDx, SDy of the laser radiation generated by the laser device 120 according to FIG. 8 consists in the provision of a cylindrical lens 138 which is arranged in the

Beam path of the laser spark plug 100 is introduced, in particular between the laser device 120 and the focusing optics 136th

Another possibility for the modification of the invention

Beam diameter SDx, SDy, cf. Figure 8, is to provide a combustion chamber window 140, the first optical surface is designed, for example, as a cylindrical surface. That is, the combustion chamber window 140 has the shape of a cylindrical lens.

Also possible is the use of an optical element 139, cf. 10a, which is embodied on a first optically active side 139a as a sphere or as an asphere, and which is embodied on the opposite optical side 139b as a cylinder.

FIGS. 10a, 10b show corresponding sections through such an optical element 139 in the x-z and y-z planes, respectively.

With sufficient stability, the optical element 139 can also take over the function of the combustion chamber window 140, so that the element 139 simultaneously realizes the focusing, the generation of two or more ignition points, and the function of the combustion chamber window. Furthermore, it is possible to use an optical element 139 ', cf. 1 1 a, which is formed on a first optically active surface 139 'a as a cylinder or sphere or asphere, and which has a second cylindrical surface on an opposite second optical surface 139' b, which realizes a different focal length than the geometry of the first optical

Surface 139'a. This can be achieved, for example, by the respective surfaces 139'a, 139'b, cf. Figure 1 1 a, Figure 1 1 b, have different curvature parameters and / or that the axes of symmetry of the respective surfaces are perpendicular to each other.

Another way to generate multiple, different ignition points is to use a simple astigmatic laser beam that has different waist positions. Such a laser beam generates two ignition points by itself in combination with a rotationally symmetrical optical system. An astigmatic laser beam can be emitted, for example, by a solid-state laser having a

has corresponding asymmetry. The asymmetry can be achieved either by a correspondingly asymmetric pumping profile, e.g. be generated in the case of a Q-switched laser or by impressing a temperature and / or mechanical stress profile. This can be impressed on the solid-state laser, for example, by an asymmetric coupling to heat source sinks.

Alternatively or additionally, one or more cylindrical surfaces may be used as resonator mirrors for the laser resonator to produce an astigmatic laser beam. An astigmatic laser beam generated in the manner described above can preferably also be combined with the further measures according to the invention described above.

In general, it is advantageous to use the principle according to the invention, taking into account the given in the pre-chamber 1 10 flow conditions, the ignition points are particularly advantageous chosen so that all emanating from them flame fronts fill the pre-chamber volume as quickly as possible. This effect is achieved according to the invention in particular when the ignition points are arranged in the antechamber so that the flame fronts propagating away from them as late as possible on the

Prechamber inner wall or adjacent flame fronts meet.

Claims

claims

1 . Laser spark plug (100) having an antechamber (1 10), wherein the laser spark plug (100) is adapted to, in the laser spark plug (100) guided and / or generated laser radiation (24) on at least two of each other

various sources of ignition (ZP1, ZP2) lying in the antechamber (1 10), in particular to focus, characterized in that the laser spark plug (100) is designed to irradiate the laser radiation (24) into the prechamber (1 10) a distance (d12) between at least one first ignition point (ZP1) and a second ignition point (ZP2) adjacent thereto is greater than a minimum distance (d2min) of the first and / or second ignition point (ZP1, ZP2) to an inner surface (110a) the antechamber (1 10).

2. laser spark plug (100) according to claim 1, characterized in that the distance (d12) between the adjacent ignition points (ZP1, ZP2) at least about 120 percent, preferably at least about 160 percent, of the minimum distance (d2min) of the first and / or second ignition point (ZP1, ZP2) to the inner surface (1 10a) of the prechamber (1 10).

3. laser spark plug (100) according to any one of the preceding claims, characterized in that a distance between all adjacent

Ignition points (ZP1, ZP2, ..) is at least about 120 percent, preferably at least about 160 percent, of the minimum distance of an ignition point (ZP1, ZP2, ..) to the inner surface (1 10a) of the prechamber (1 10).

4. laser spark plug (100) according to any one of the preceding claims, characterized in that a minimum distance (dl min, d2min) of a

Ignition point (ZP1, ZP2) to the inner surface (1 10a) of the prechamber (1 10) is about 10 percent to about 40 percent of a maximum extent of an interior of the pre-chamber (1 10), in the case of a substantially at least partially spherical or ellipsoidal In particular, antechamber about 10 percent to about 40 percent of a radius of the antechamber (1 10).

5. laser spark plug (100) according to any one of the preceding claims, characterized in that the pre-chamber (1 10) at least partially substantially spherical or ellipsoidal shape, and that the distance (d12) between the first ignition point (ZP1) and the second ignition point (ZP2) is about twice as large as a minimum distance (dl rmin, d2min) of the first and / or second ignition point (ZP1, ZP2) to the inner surface (1 10a) of the antechamber (1 10).

6. laser spark plug (100) according to any one of the preceding claims, characterized in that a mean distance between adjacent ignition points to each other is greater than an average distance of the ignition points to the inner surface (1 10a) of the pre-chamber (1 10).

7. laser spark plug (100) according to any one of the preceding claims, characterized in that an optical element (130, 133) is provided, which bundles the laser radiation (24) to the different ignition points (ZP1, ZP2).

8. laser spark plug (100) according to claim 7, characterized in that the optical element (130, 133) is designed as a focal having multiple focal optics.

9. laser spark plug (100) according to claim 8, characterized in that the

Focusing optics (130, 133) has at least two substantially concentric with each other or juxtaposed focusing areas each having a different focal length.

10. laser spark plug (100) according to any one of the preceding claims, characterized in that the laser spark plug (100) is adapted to irradiate the laser radiation (24) into the pre-chamber (1 10) such that a Rayleigh length of the irradiated laser radiation (24 ) is at least about 10 percent, preferably at least about 30 percent, of a maximum extent (L) of an interior of the prechamber (1 10).