EP2438289B1 - Laser spark plug for an internal combustion engine - Google Patents

Description

State of the art

The invention relates to a laser spark plug for an internal combustion engine.

Laser spark plugs are arranged similar to those used in high-voltage ignition conventional spark plugs in the region of a cylinder head of an internal combustion engine and couple high-energy laser pulses in their associated combustion chamber to ignite an air / fuel mixture therein. In order to ensure a reliable operation of a laser spark plug with optical components integrated therein under the ambient conditions prevailing in the area of the cylinder head (inter alia large temperature fluctuations, vibrations), considerable design effort is required.

From the JP 2006-242038 A A laser-based ignition system for an internal combustion engine is already known, in which a converging lens is arranged in a laser spark plug. In addition, the known laser spark plug on deformation means which are adapted to deform the converging lens. In this way, a firing location of the known laser ignition system can be varied.

DE 10 350 101 discloses a laser based braiding system with a boleto-optical element for focusing.

A disadvantage of the known laser-based ignition system is the design effort that goes hand in hand with the provision of the deformation means for the targeted deformation of the convergent lens. Substantial drive energy is required for the deforming means to deform the relatively high-mass converging lens in the desired manner for adjusting the focus position.

Disclosure of the invention

Accordingly, it is an object of the present invention to improve a laser spark plug of the type mentioned in that with a relatively small design effort, the possibility for a spatial multiple ignition is given in a combustion chamber of the internal combustion engine.

This object is achieved in the laser spark plug of the aforementioned type according to the invention that at least one volume Bragg grating element is arranged in a beam path of the laser spark plug.

The volume Bragg grating element, also referred to as volume Bragg grating, in short VBG, consists of a spatially arranged optical grating whose transmission or reflection behavior for impinging electromagnetic radiation can be adjusted by specifying inter alia a lattice constant in a manner known per se. The optical grating is formed by a periodic variation of the refractive index of a carrier medium containing the VBG.

In order to realize a spatial multiple ignition in which laser ignition pulses can be radiated simultaneously to at least two different ignition points, it is provided in the laser spark plug according to the invention that the volume Bragg grating element is designed as a beam splitter. For this purpose, the volume Bragg grating element can preferably have at least two different volume Bragg gratings, which can be arranged in a manner known per se in the same volume range of a suitable carrier medium and have different properties or orientations.

In contrast to the conventional systems with deformable converging lens, advantageously no movable component is provided in this variant of the invention, and the integration of a plurality of volume Bragg gratings in a corresponding carrier element allows a very small-sized arrangement.

According to a further variant, the volume Bragg grating element according to the invention can advantageously also be integrated directly into a focusing optics and / or a combustion chamber window of the laser spark plug. In these cases, one or more volume Bragg gratings are written directly into the respective components, which also results in a very compact construction.

In a further very advantageous variant of the present invention, it is provided that the volume Bragg grating element has a diffraction efficiency which is less than about 95%.

By virtue of the diffraction efficiency of the volume Bragg grating element deliberately chosen according to the invention at less than 95%, several partial beams which have mutually divergent beam axes result when laser radiation passes through the volume Bragg grating element, so that focusing is again achieved by downstream focusing optics the multiple partial beams to different ignition points is possible.

Particularly preferably, the volume Bragg grating element has a diffraction efficiency of about 50%, so that in addition to a first laser partial beam, a second, also referred to as off-axis beam, laser partial beam is generated when the volume Bragg grating element is properly aligned the optical axis of a laser device arranged in the laser spark plug. The diffraction efficiency is u.a. in a conventional manner. influenced by an angle of incidence of the incident laser radiation and its wavelength. In order to achieve the aforementioned effects of the beam splitting, the properties of the volume Bragg grating element and its orientation in the laser spark plug must be selected accordingly.

In a further very advantageous embodiment of the laser spark plug according to the invention it is provided that the volume Bragg grating element is arranged to be movable relative to the optical axis of the laser spark plug, resulting in further degrees of freedom in the spatial multiple ignition. For example, by controlling a tilt angle between a surface normal of the volume Bragg grating element and the optical axis of the laser spark plug, the propagation direction of the volume Bragg grating element can be determined exiting laser radiation can be influenced, whereby advantageously a temporally successive multiple ignition at different ignition points is possible.

The drive means for moving the volume Bragg grating element according to the invention are arranged in a further advantageous variant of the invention directly in the laser spark plug. In contrast to conventional systems, which are based on a deformation of converging lenses, a comparatively low drive energy is required for the movement according to the invention of the volume Bragg grating element.

In a further preferred embodiment of the laser spark plug according to the invention, the volume Bragg grating element is arranged such that it directly faces an end face, provided for coupling out the laser radiation generated, of a laser device arranged in the laser spark plug. The required for proper operation of the volume Bragg grating element according to the invention precise angular alignment of the volume Bragg grating element to the decoupling surface of the laser device or to the optical axis of the laser device according to the invention advantageously achieved in that a spacer between the volume Bragg grating element and the laser device is provided. The spacer according to the invention preferably has a parallelism of its surfaces of less than about 2 microns and preferably has a substantially annular geometry, so that laser radiation from the laser device can pass through the volume Bragg grating element according to the invention. Particularly preferably, the volume Bragg grating element according to the invention is spring-loaded so that it is pressed in its rest position by the spring forces on the spacer, whereby the volume Bragg grating element occupies the required precise parallel position to the outcoupling surface of the laser device.

An inventively provided in the laser spark plug drive for the volume Bragg grating element may for example be designed so that it displaces the volume Bragg grating element in its radial outer region axially against the spring forces, so that a tilting of the volume Bragg grating element opposite the optical axis of the laser device results.

In a further particularly preferred embodiment of the laser spark plug according to the invention, it is provided that the volume Bragg grating element has a location-dependent lattice constant, thus being designed as a chirped volume Bragg grating element. By using a chirped VBG, which advantageously has an increased spectral acceptance over VBGs with a location-independent, constant lattice constant, the relatively large temperature fluctuations occurring in the area of the spark plug can be taken into account, which usually have a negative effect on the wavelength stability of the laser device contained in the laser spark plug ,

To generate high-energy laser ignition pulses, the laser spark plug according to the invention preferably has a laser-active solid with a, preferably passive, Q-switching. The VBG according to the invention can be easily integrated into the laser-active solid and is especially suitable for operation with high-energy laser pulses, which greatly reduce the service life of conventional, dielectric reflection layers due to the high power densities.

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 in the drawing.

Figur 1

eine Brennkraftmaschine mit einer erfindungsgemäßen Laserzündkerze,

Figur 2

schematisch eine erste Ausführungsform der erfindungsgemäßen Laserzündkerze aus Figur 1 im Detail,

Figur 3a, 3b, 3c

Ausführungsformen der erfindungsgemäßen Laserzündkerze mit einem mehrere Volumen-Bragg-Gitter aufweisenden Volumen-Bragg-Gitterelement,

Figur 4a, 4b

Ausführungsformen der erfindungsgemäßen Laserzündkerze mit einem Volumen-Bragg-Gitterelement verminderter Beugungseffizienz zur Erzeugung von off-axis-Laserstrahlen,

Figur 5

eine weitere Ausführungsform einer erfindungsgemäßen Laserzündkerze, und

Figur 6

noch eine weitere Ausführungsform einer erfindungsgemäßen Laserzündkerze.

In the drawing shows:

FIG. 1

an internal combustion engine with a laser spark plug according to the invention,

FIG. 2

schematically a first embodiment of the laser spark plug according to the invention from FIG. 1 in detail,

Figure 3a, 3b, 3c

Embodiments of the laser spark plug according to the invention with a volume Bragg grating element having a plurality of volume Bragg gratings,

Figure 4a, 4b

Embodiments of the laser spark plug according to the invention with a volume Bragg grating element of reduced diffraction efficiency for the generation of off-axis laser beams,

FIG. 5

a further embodiment of a laser spark plug according to the invention, and

FIG. 6

Yet another embodiment of a laser spark plug according to the invention.

An internal combustion engine carries in FIG. 1 Overall, the reference numeral 10. It is used to drive a motor vehicle, not shown. The internal combustion engine 10 comprises a plurality of cylinders, of which in FIG. 1 only one is designated by the reference numeral 12. A combustion chamber 14 of the cylinder 12 is limited by a piston 16. Fuel enters the combustion chamber 14 directly through an injector 18, which is connected to a designated also as a rail fuel pressure accumulator 20.

In the combustion chamber 14 injected fuel 22 is ignited by means of a laser beam 24, which is preferably emitted in the form of a laser pulse 24 from a laser device 26 having a laser spark plug 100 to the ignition point ZP in the combustion chamber 14. For this purpose, the laser device 26 is fed via a light guide device 28 with pumping light, which is provided by a pumping light source 30. The pumping light source 30 is controlled by a control unit 32, which also controls the injector 18.

The pumping light source 30, together with the optical waveguide device 28 and the laser spark plug 100 having the laser device 26, forms a laser-based ignition system 27 of the internal combustion engine 10.

How out FIG. 2 is apparent, the laser device 26 in addition to a laser-active solid 44 according to the invention also has a passive Q-switching circuit 46, so that the components 44, 46 together with a coupling mirror 42 and a Auskoppelspiegel 48 form a laser oscillator.

The basic mode of operation of the laser device 26 is as follows: Pumplicht 60, which is supplied to the laser device 26 via the optical fiber device 28, passes through the transparent for a wavelength of the pumping light 60 Einkoppelspiegel 42 in the laser-active solid 44 a. There, the pumping light 60 is absorbed, resulting in a population inversion. The initially high transmission losses of the passive Q-switching circuit 46 prevent a laser oscillation in the laser device 26. However, as the pumping time increases, so does the radiation density in the interior of the resonator formed by the laser-active solid 44 and the passive Q-switching 46 and the mirrors 42, 48. From a certain radiation density, the passive Q-switching 46 or a saturable absorber of the passive Q-switch 46 bleaches, so that a laser oscillation occurs in the resonator.

By this mechanism, a laser beam 24 is generated in the form of a so-called. Giant impulse, which passes through the Auskoppelspiegel 48 and is hereinafter referred to as Laserzündimpuls.

Instead of the passive Q-switching 46 described above, the use of an active Q-switching is also conceivable.

According to the invention, at least one volume Bragg grating element is arranged in the beam path of the laser spark plug 100. In the present case, the coupling-out mirror 48 is formed by such a volume Bragg grating element, so that a conventional dielectric reflection coating can advantageously be dispensed with at this point.

The volume Bragg grating element 48 forming the output mirror can advantageously be integrated directly into the laser device 26, for example by writing a corresponding grating pattern into the laser-active solid 44 or the Q-switching 46.

Alternatively, the volume Bragg grating element 48 can also be embodied as a separate component, which is arranged externally to the components 44, 46 or can be connected to the laser device 26, for example by wringing or adhesion-free bonding.

In the present case, the volume Bragg grating element 48, in addition to its function as a coupling-out mirror, is designed such that it acts as a beam splitter. As a result, the laser pulses 24 generated by the laser device 26 are advantageously divided into a plurality of partial beams. These partial beams can be arranged downstream of the volume Bragg grating element 110, not in FIG. 2 illustrated focusing optics advantageous to several different ignition points in the combustion chamber 14 (FIG. FIG. 1 ) of the internal combustion engine 10 are bundled.

FIG. 3a shows a further embodiment of the laser spark plug 100 according to the invention, in which a arranged outside the laser device 26 volume Bragg grating element 110 is provided. The volume Bragg grating element 110 according to FIG FIG. 3a has at least two different volume Bragg gratings, so that the laser radiation 24 generated by the laser device 26 is divided into two sub-beams 24a, 24b as already described. The integration of the at least two different volume Bragg gratings into the volume Bragg grating element 110 according to the invention is shown in FIG FIG. 3a symbolized by extending in two different spatial directions line crowds.

How out FIG. 3a 1, the laser radiation 24a, 24b emerging from the volume Bragg grating element 110 is first widened by a biconcave diverging lens 49a, so that it then passes through a biconvex converging lens 49b downstream of the diverging lens 49a to the ignition points ZP1, ZP2 in the combustion chamber 14 Internal combustion engine 10 ( FIG. 1 ) is focusable. In the present case, the converging lens 49b simultaneously forms a combustion chamber window which terminates the interior of the laser spark plug 100 with respect to the combustion chamber 14 of the internal combustion engine.

FIG. 3b shows a further variant of the laser spark plug 100 according to the invention, in which the at least two different volume Bragg gratings comprising volume Bragg grating element 110 is monolithically integrated into the laser device 26th

Figure 3c shows a further embodiment of the laser spark plug 100 according to the invention, in which the at least two different volume Bragg gratings for the realization of a spatial multiple ignition are advantageously integrated directly into the combustion chamber window 49c.

In this variant of the invention, the diverging lens 49a and the converging lens 49b are disposed in the inner space of the laser spark plug 100.

FIG. 4a shows a further very advantageous embodiment of the laser spark plug 100 according to the invention, in which a spatial multiple ignition is advantageously realized in that the volume Bragg grating element 110 has a lower diffraction efficiency than about 95%. In addition to a primary partial beam 24a coaxial with the optical axis OA of the laser device 26, this also produces a so-called off-axis continuous beam 24b, which is focused by the focusing optics, which are not further described here, onto a second ignition point ZP2 outside the optical axis OA.

By selecting the diffraction efficiency of the volume Bragg grating element 110 according to the invention, advantageously, the distribution of the laser power to the different laser ignition points ZP1, ZP2 can be controlled.

FIG. 4b shows another very advantageous embodiment of the laser spark plug 100 according to the invention, in which an off-axis laser partial beam 24b is in turn generated by the provision of a volume Bragg grating element 110 with a low diffraction efficiency of about 50%. Unlike the in FIG. 4a illustrated variant of the invention, the laser spark plug 100 according to FIG. 4b a movably arranged volume Bragg grating element 110, which in the present case is rotatably mounted on a housing inner wall of the laser spark plug 100 by means of a rotary joint 111. In this way, the volume Bragg grating element 110, under the drive of the drive device 112, can advantageously be tilted relative to an end face of the laser device 26 that decouples the laser radiation 24 such that the off-axis laser partial beam 24b is generated when the volume Bragg grating grid element 110 is arranged at a corresponding angle with respect to the laser device 26 or the optical axis OA of the laser spark plug.

That is, at the in FIG. 4b illustrated variant variant can be selected by appropriate alignment of the volume Bragg grating element 110, which is controllable by the drive unit 112, between a laser ignition only in the ignition point ZP1 or in the ignition point ZP2.

In contrast to conventional deformable converging lenses for realizing a spatial multiple ignition, the configuration according to the invention requires a relatively small drive energy and a very small axial displacement of the drive unit 112 parallel to the optical axis OA of the laser spark plug 100, due to the relatively low angular acceptance of the volume Bragg grating element 110 is conditional.

FIG. 5 shows a further embodiment of the laser spark plug 100 according to the invention, in which the volume Bragg grating element 110 preferably has the highest possible diffraction efficiency, in particular a diffraction efficiency of 99% or more.

In this way, by appropriate alignment of the volume Bragg grating element 110 relative to the laser device 26, an off-axis ignition point ZP 'can be realized whose position in the combustion chamber 14 is dependent on the tilt angle between the volume Bragg grating element 110 and the associated end face of the laser device 26 and the optical axis OA.

FIG. 6 shows a further very advantageous embodiment of the laser spark plug 100, in which a volume Bragg grating element 110 is movably arranged relative to the laser device 26 and its optical axis OA. In this case, the volume Bragg grating element 110 is bordered by a mounting ring 115, which is as shown in FIG FIG. 6 apparent Federkraftbeaufschlagt and is arranged relative to the housing of the laser spark plug 100, that held in the mounting ring 115 volume Bragg grating element 110 is located in plan position with respect to a decoupling surface 48, as long as it is not deflected by the drive unit 112 as shown.

Between the laser device 26 or its decoupling surface 48 'and the corresponding end face of the volume Bragg grating element 110, a spacer 116 is presently provided according to the invention. The spacer 116 may, as well as the mounting ring 115, preferably have an annular geometry, so that the laser radiation generated by the laser device 26 can pass through the mounting ring 115 on the volume Bragg grating element 110.

At the same time, the spacer 116 ensures a precise flatness of the volume Bragg grating element 110 relative to the laser device 26. For this purpose, the corresponding end faces of the spacer 116 preferably have a parallelism of approximately 2 μm or less.

The provision of the spacer 116 according to the invention also advantageously ensures that no contact takes place between the volume Bragg grating element 110 and the laser device 26, whereby a mechanical impairment of the decoupling surface 48 'is prevented. About the drive means 112, which may be, for example, a piezoelectric actuator, the volume Bragg grating element 110 as shown FIG. 6 can be tilted from a rest position in which its longitudinal axis has a right angle to the optical axis OA of the laser device 26, so that the spatial position of the ignition point in the combustion chamber 14 in the manner already described above is precisely adjustable.

In further preferred variants of the invention, the volume Bragg grating element 110 may have a location-dependent lattice constant in order to increase the spectral acceptance of the volume Bragg grating element 110 in a manner known per se.

The volume Bragg grating element 110 according to the invention advantageously makes it possible to provide a cost-effective and compact laser spark plug 100, which simultaneously provides spatial and temporal multiple ignition. In addition, the volume Bragg grating element 110 according to the invention is advantageously suitable for the occurring during the laser ignition high optical pulse power and up to at least about 400 ° C temperature stable.

Claims (10)

1. Laser ignition plug (100) for an internal combustion engine, characterized in that at least one volume Bragg grating element (48, 110), which is configured as a beam splitter for splitting laser radiation (24) guided in the laser ignition plug (100) into a plurality of partial beams, is arranged in a beam path of the laser ignition plug (100).
2. Laser ignition plug (100) according to Claim 1, characterized in that the volume Bragg grating element (110) has at least two different volume Bragg gratings.
3. Laser ignition plug (100) according to one of Claims 1 to 2, characterized in that the volume Bragg grating element (110) is integrated in focusing optics (49b) and/or a combustion space window (49c).
4. Laser ignition plug (100) according to one of the preceding claims, characterized in that the volume Bragg grating element (110) has a diffraction efficiency that is less than approximately 95 percent.
5. Laser ignition plug (100) according to one of the preceding claims, characterized in that the volume Bragg grating element (110) is arranged such that it is moveable relative to an optical axis (OA) of the laser ignition plug (100).
6. Laser ignition plug (100) according to Claim 5, characterized by drive means (112) for moving the volume Bragg grating element (110).
7. Laser ignition plug (100) according to one of Claims 5 to 6, characterized in that a spacer (116), which preferably has a substantially annular geometry and whose end faces, which are provided for making contact with a coupling-out surface (48') of a laser device (26) arranged in the laser ignition plug (100) and the volume Bragg grating element (110), are plane-parallel with respect to one another, is arranged between the coupling-out surface (48') and the volume Bragg grating element (110).
8. Laser ignition plug (100) according to one of the preceding claims, characterized in that the volume Bragg grating element (110) has a location-dependent grating constant.
9. Laser ignition plug (100) according to one of the preceding claims, characterized in that the volume Bragg grating element (110) is monolithically integrated in a laser-active solid body (44) and/or a further optical component arranged in the beam path.
10. Laser ignition plug (100) according to one of the preceding claims, characterized in that a laser-active solid body (44) with a passive Q switch (46) is integrated in the laser ignition plug (100).

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