Laser ignition device

An ignition laser includes a laser-active solid body, a housing, and a combustion chamber window. The heat value of the ignition laser is settable in accordance with the requirements of the internal combustion engine.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ignition laser for a laser ignition device of an internal combustion engine.

2. Description of Related Art

A so-called laser igniter is known from published international patent application document WO 2005/066488 A1. This laser igniter includes an ignition laser, which protrudes into the combustion chamber of an internal combustion engine. This ignition laser is connected via an optical fiber to a pump light source.

A so-called combustion chamber window is provided on one end of the ignition laser facing toward the combustion chamber, which is transmissive for the laser pulses generated in the ignition laser. The laser wavelength is between 500 nm and 1500 nm, preferably between 900 nm and 1100 nm. This combustion chamber window must be sealingly accommodated in a housing of the ignition laser. High requirements are placed on the seal between the combustion chamber window and the housing, because surface temperatures of greater than 600° C. may occur on the combustion chamber window during the operation in an internal combustion engine, for example. In addition, intermittent pressure loads of greater than 250 bar occur. If an ignition laser is used to ignite a gas turbine, lower pressures do prevail in the combustion chamber of the gas turbine; however, the surface of the combustion chamber window may reach temperatures of up to 1000° C.

In order for the laser igniter to operate without interference, the combustion chamber window must be transparent to the laser beams over the entire lifetime of the internal combustion engine. However, deposits may accumulate on the combustion chamber window during the operation, so that the required optical transparency of the combustion chamber window is lost. In order to prevent these deposits, the ignition laser is to be designed in such a way that the operating temperature on the surface of the combustion chamber window facing toward the combustion chamber is sufficiently high to burn off or oxidize deposits, above all of an organic nature.

However, the surface temperature of the combustion chamber window may not be so high that self-ignition of the fuel-air mixture may occur on the surface of the combustion chamber window or deposits, in particular oil ashes, may be formed on the combustion chamber window.

An ignition laser is known from published German patent application document DE 198 10 750 A1, in which the temperature of the combustion chamber window may be reduced by increasing the heat dissipation into the base of the ignition laser.

As a result, the surface temperature of the combustion chamber window in the entire operating range of the internal combustion engine must be kept above the burnoff temperature for organic coatings, which is approximately 400° C., and below a temperature of 600° C., from which a growth of crystalline phosphates from the oil ashes is known. This means that a temperature range between 420° C. and 550° C. is to be permanently achieved on the surface of the combustion chamber window.

Since this mentioned temperature range may not be maintained at least during the cold start of the internal combustion engine, rapid heating of the ignition laser or the combustion chamber window is desirable. Simultaneously, during steady operation of the internal combustion engine, the heat from the ignition laser must be sufficiently dissipated to avoid excess temperatures.

BRIEF SUMMARY OF THE INVENTION

The present invention is based on the object of providing an ignition laser which allows in particular rapid heating of the ignition laser or the associated combustion chamber window to the required temperature of 400° C., so that the burnoff temperature for organic coatings is achieved as rapidly as possible after the cold start of the internal combustion engine, and these coatings therefore may not accumulate permanently on the combustion chamber window.

This object is achieved according to the present invention for an ignition laser according to the definition of the species in that means are provided for influencing the heat conduction, which allow an adjustment of the heat transport from the combustion chamber via the housing of the ignition laser to the combustion chamber window, so that rapid heating of the combustion chamber window is achieved for all applications.

The means for influencing the heat conduction on one end of the housing facing toward the combustion chamber may include a heat conduction element, in particular a heat conduction ring made of a material having good thermal conductivity, in particular nickel and/or nickel-plated copper. This heat conduction element may either be integrally designed with the housing of the ignition laser or it may be made of a different material than the housing of the ignition laser and may be connected thereto in a proper heat conducting way, for example, by soldering or welding. Nickel or copper have proven themselves as suitable materials for such a heat conduction element, since they have very good thermal conductivity. The heat conduction element according to the present invention functions similarly to a cooling rib, since the heat absorption from the hot exhaust gases and the introduction of this heat into the housing of the ignition laser are accelerated and improved by the enlarged surface area of the heat conduction element.

The means for influencing the heat conduction, in the case of an ignition laser on whose housing an external thread is formed, may be that the external thread ends at a distance from one end of the ignition laser facing toward the combustion chamber. Specifically, if the external thread, which is typically used for screwing the ignition laser into the cylinder head of an internal combustion engine, is not led up to the end of the ignition laser, the heat dissipation from the end of the ignition laser facing toward the combustion chamber is reduced and the heating of the ignition laser is therefore accelerated.

In a further advantageous embodiment of the present invention, it is provided that at least one intermediate element, in particular a ring-shaped intermediate element, made of a material having poor thermal conductivity, preferably made of mica, is provided between the housing and the combustion chamber window. This intermediate element is used for the purpose of reducing the heat dissipation from the combustion chamber window and thus accelerating the heating of the combustion chamber window or increasing the stationary surface temperature of the combustion chamber window.

The intermediate elements may be designed as discs in the form of circular rings and may be provided in the axial direction in front of or behind one or both sides of the combustion chamber window between the housing and the combustion chamber window. For example, if only one intermediate element is provided and this intermediate element is situated on the contact surface facing away from the combustion chamber between the combustion chamber window and the housing, the dissipation of heat from the combustion chamber window into the base of the housing is thus reduced and as a result the heating is accelerated.

Furthermore, however, it is also possible to provide such an intermediate element on the contact surface facing toward the combustion chamber between the combustion chamber window and the housing. The heat introduction from the front side of the housing into the combustion chamber window is thus reduced, so that in this way the heating speed or the stationary surface temperature of the combustion chamber window may be influenced and adapted in accordance with the requirements of the specific case.

The object mentioned at the outset is also achieved in that the combustion chamber window is made of two different materials. Of course, the combustion chamber window must be transparent or transmissive for light having a wavelength of the ignition laser, so that the laser pulse from the interior of the ignition laser may reach the combustion chamber. A particularly suitable material for focusing the laser pulse is crystalline sapphire, which has a relatively high thermal conductivity of approximately 40 W/mK at room temperature, however.

It is provided according to the present invention to use crystalline sapphire in the area of the combustion chamber window directly adjoining the optical axis of the ignition laser as the material for the combustion chamber window and to manufacture the radial outside areas of the combustion chamber window from a different material, which has a lower thermal conductivity. The combustion chamber window is thus formed according to the present invention from a central area having optimum optical properties and a radial outer ring-shaped area which is optimized with respect to the thermal conductivity. Suitable materials for the second, outer area of the combustion chamber window are, for example, quartz glass, which has a thermal conductivity of only approximately 4 W/mK at room temperature, YAG, having a thermal conductivity of approximately 14 W/mK at room temperature, or zirconium oxide, having a thermal conductivity of approximately 10.5 W/mK at room temperature. It is already clear from the comparison of the thermal conductivities of the listed materials that the heat dissipation from the combustion chamber window may be controlled in very wide limits through a combustion chamber window which is made of two different materials, so that the surface temperature of the combustion chamber window may be set to the desired values and simultaneously very rapid heating of the combustion chamber window to temperatures above 400° C. may be achieved through suitable dimensioning and combining of the mentioned materials.

It is obvious that the features of the other independent claims may be used individually in an ignition laser according to the present invention. However, it is also readily possible to combine several of these features with one another. Thus, for example, it is readily possible, as shown inFIG. 2, to provide a heat conduction ring and simultaneously not to lead the external thread of housing38of the ignition laser up to the front side of the ignition laser.

This also applies accordingly to the intermediate elements between combustion chamber window58and the housing or the one combustion chamber window, which is made of two different materials.

DETAILED DESCRIPTION OF THE INVENTION

The ignition device according to the present invention is explained on the basis of the figures for exemplary purposes in an internal combustion engine. However, the ignition laser may also be implemented in a gas turbine.

An internal combustion engine bears reference numeral10as a whole inFIG. 1a. It may be used to drive a motor vehicle (not shown) or it may be used as a gas engine for power generation. Internal combustion engine10typically includes multiple cylinders, only one of which is identified by reference numeral12inFIG. 1a. A combustion chamber14of cylinder12is delimited by a piston16. Fuel reaches combustion chamber14through an injector18, which is connected to a fuel pressure storage20, also referred to as a rail. Alternatively, the fuel-air mixture may also be produced outside combustion chamber14, for example, in the intake manifold.

Fuel-air mixture22present in combustion chamber14is ignited using a laser pulse24, which is emitted into combustion chamber14by an ignition device27including an ignition laser26. For this purpose, ignition laser26is supplied with a pump light, which is provided by a pump light source30, via an optical fiber device28. Pump light source30is controlled by a control unit32, which also activates injector18.

As is apparent fromFIG. 1b, pump light source30supplies multiple optical fiber devices28for various ignition lasers26, which are each assigned to one cylinder12of internal combustion engine10. For this purpose, pump light source30has multiple individual laser light sources34, which are connected to a pulse current supply36. A “steady” distribution of pump light to various laser devices26is also implemented by the presence of multiple individual laser light sources34, so that no optical distributors or the like are required between pump light source30and ignition lasers26.

Ignition laser26has a laser-active solid body44having a passive Q-switch46, which forms an optical resonator together with a coupling mirror42and a decoupling mirror48. Upon application of the pump light generated by pump light source30, ignition laser26generates a laser pulse24in a way known per se, which is focused by a focusing lens52on an ignition point ZP located in combustion chamber14(FIG. 1a). The components present in housing38of ignition laser26are separated from combustion chamber14by a disk-shaped combustion chamber window58. Combustion chamber window58may be designed to be square or preferably round.

Different specific embodiments according to the present invention of laser devices26are shown enlarged inFIGS. 2through4. The end of ignition laser26facing toward combustion chamber15is primarily shown.

The surface of combustion chamber window58facing toward combustion chamber14(seeFIG. 1a) is provided with reference numeral60inFIG. 2.

In the exemplary embodiment shown inFIG. 2, housing38has a two-part design. In the exemplary embodiment shown, an inner part62is screwed together with an outer part64of housing38. Other connections are also possible, in particular integral bonds by soldering or welding. Combustion chamber window58is clamped sealingly between a front side (without reference numeral) of the inner part and a shoulder66of outer part64. Possibly required additional sealing elements are not shown inFIG. 2. Since outer part64and inner part62may be connected to one another by a thread68, the contact force, using which combustion chamber window58is clamped between inner part62and shoulder66of outer part64, may be set by a corresponding clamping torque. In the case of welded or soldered bonds, this contact force is to be implemented by suitable process control, for example, by mechanical pressing during the joining procedure.

Outer part64or housing38has an external thread70, which is used to screw ignition laser26into a cylinder head (not shown) of an internal combustion engine, for example.

It is provided according to the present invention that external thread70is not led up to a front side72of housing38, but rather is ended at a distance L before front side72. This means that the diameter of housing38in the area between the end of external thread70and front side72of the housing is smaller than the core hole diameter of external thread70. As a result, the cross section of housing38, in particular of outer part64, is reduced in this area and therefore the heat flow is reduced from front side72into the areas of housing38facing away from the combustion chamber. This has the result that during a cold start of the internal combustion engine, combustion chamber window58and in particular surface60of combustion chamber window58facing toward the combustion chamber heat up more rapidly and the burnoff temperature of 400° C. is therefore reached more rapidly.

A further design feature according to the present invention for controlling or increasing the surface temperature of combustion chamber window58is shown in the exemplary embodiment according toFIG. 2. Specifically, a heat conduction ring74is formed on front side72, which may either be designed integrally with outer part64of housing38or may be manufactured as a separate component made of a material having good thermal conductivity, for example, nickel or copper. In this case, connecting heat conduction ring74to housing38by integral bonding, for example, by soldering or welding, and therefore in a way having good thermal conductivity, is recommended.

Heat conduction ring74more or less has the task of a cooling rib, so that according to the present invention it is to introduce heat from the combustion chamber into housing38and therefore also into combustion chamber window58. It is obvious that further parameters for influencing the surface temperature or the operating temperature of the combustion chamber window are provided by the geometry and the dimensions of the heat conduction ring. Fundamentally, a large heat-transferring surface between heat conduction ring74and the surrounding combustion air and the exhaust gases in the combustion chamber results in an increased heat transfer. A large mass or a large volume of the heat conduction ring results in increased thermal inertia. These parameters may be used in order to allow rapid heating, on the one hand, and to achieve a certain thermal inertia, on the other hand, so that the maximal permissible surface temperature of combustion chamber window58is not reached or exceeded under all operating states.

It is obvious that the two features according to the present invention, namely heat conduction ring74and external thread70which is not formed up to front side72, may also be designed individually on an ignition laser26. It is not necessary to implement these two features together on one ignition laser26.

A further exemplary embodiment of an ignition laser26according to the present invention is shown inFIG. 3.

In this exemplary embodiment, intermediate rings76and78are inserted between housing38and combustion chamber window58. These intermediate rings76and78are made of a material having poor thermal conductivity, for example, mica. However, all other materials which have a poor thermal conductivity and sufficient temperature resistance are also suitable. First intermediate ring76and second intermediate ring78may also be used individually, depending on the extent to which the heat conduction or the heat dissipation is to be controlled within ignition laser26.

For example, if only first intermediate ring76is inserted between inner part62and combustion chamber window58, the heat conduction from combustion chamber window58into inner part62is reduced, so that the temperature of laser-active solid body44is reduced. The operating temperature of combustion chamber window58increases simultaneously.

The heat dissipation from the combustion chamber window into inner part62may be controlled within wide limits by the thickness of first intermediate ring76. If second intermediate ring78is inserted between combustion chamber window58and shoulder66of outer part64, the heat transfer from housing38into combustion chamber window58is reduced.

Therefore, the operating temperature of combustion chamber window58and the speed at which combustion chamber window58heats up during a cold start may be controlled within wide limits by the attachment of first intermediate ring76and/or second intermediate ring78.

A further exemplary embodiment of an ignition laser26according to the present invention is shown inFIG. 4. Combustion chamber window58is constructed from two different materials. Combustion chamber window58is manufactured from a first material having optimized optical properties concentrically or coaxially to an optical axis80of ignition laser26. This first area is provided with reference numeral82inFIG. 4. For example, combustion chamber window58may be manufactured from a crystalline sapphire (Al2O3) in this first area82. Crystalline sapphire has a relatively high thermal conductivity of approximately 40 W/mK at room temperature.

In a radial outer second area84, combustion chamber window58is manufactured from another transparent material, for example, quartz, YAG, or zirconium oxide. The mentioned materials are mentioned as examples and are not to be understood as a conclusive enumeration. All of these materials share the feature that they have a significantly lower thermal conductivity than crystalline sapphire. Therefore, it is possible through the construction according to the present invention of combustion chamber window58from two different materials to control the heat dissipation from or the heat introduction into combustion chamber window58within wide limits.

In order to manufacture such a combustion chamber window, for example, it is possible to fuse a quartz ring having lower thermal conductivity onto a sapphire window. Of course, it is also possible to combine other materials with one another. In the context of the present invention, the general term “quartz” certainly includes quartz glass, fused silica, and SiO2in amorphous form.

All specific embodiments which have been shown and explained for exemplary purposes on the basis ofFIGS. 2 through 4share the feature that they may be implemented individually or in any arbitrary combination in an ignition laser26. A mass-produced ignition laser26may therefore be adapted to the usage conditions within wide ranges with respect to the operating temperature and the heating speed of combustion chamber window58by combination of individual or multiple features according to the present invention.