Abstract:
A laser spark plug, e.g., for an internal combustion engine of a motor vehicle or a heavy-duty gas engine, has an antechamber for receiving an ignitable medium and a combustion chamber window which separates the antechamber from a part of the laser spark plug facing away from the combustion chamber. At least one flow-guiding element is provided in the antechamber and is situated in such a way that it deflects a fluid flow, which occurs in the antechamber and moves toward the combustion chamber window, into a radially inner direction.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a laser spark plug, in particular for an internal combustion engine of a motor vehicle or a heavy-duty gas engine, having an antechamber for receiving an ignitable medium and a combustion chamber window which separates the antechamber from the part of the laser spark plug facing away from the combustion chamber. 
         [0003]    2. Description of Related Art 
         [0004]    A laser spark plug of this type is already known from published German patent application document DE 10 2006 018 973 A1. The disadvantage of the known laser spark plug is the fact that a residual gas blanket made of a combusted air/fuel mixture cannot form or be preserved on a surface area of the combustion chamber window facing the antechamber, the residual gas blanket being capable of contributing to the protection of the combustion chamber window against the continuous input of additional combustion products. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    It is thus an object of the present invention to improve a laser spark plug of the type mentioned above in such a way that the formation or preservation of a residual gas blanket in the area of the combustion chamber window is facilitated. 
         [0006]    This object is achieved according to the present invention in a laser spark plug of the type mentioned at the outset in that at least one flow-guiding element is provided in the antechamber and is situated in such a way that it deflects a fluid flow, which occurs in the antechamber and moves toward the combustion chamber window, into a radially inner direction. In the sense of the present invention, the indication of the radially inner direction refers to a longitudinal axis or optical axis of the laser spark plug or the antechamber, i.e., the radially inner direction stands essentially perpendicularly on the longitudinal axis of the laser spark plug. 
         [0007]    According to studies by the applicant, an effective deflection of the fluid flows moving unimpededly toward the combustion chamber window surface, facing the antechamber, in conventional laser spark plugs is advantageously achieved by providing the flow-guiding element according to the present invention; such fluid flows are created, for example, during a compression stroke, during which an ignitable air/fuel mixture flows into the antechamber of an internal combustion engine containing the laser spark plug. 
         [0008]    The deflection of the fluid flow according to the present invention advantageously takes place in such a way that the fluid flow continues into a radially inner area of the antechamber and does not act on a volume region lying directly in the surface area of the combustion chamber window. In this way, during the operation of the laser spark plug, a residual gas blanket may form in this surface area which is not continuously disturbed by the new fluid flows originating in the inside of the antechamber. 
         [0009]    A particularly efficient deflection of the fluid flows interfering with the residual gas blanket formation is possible according to the present invention in that the flow-guiding element has a flow-guiding surface which has an essentially concave design. Other variants of the present invention may also provide flow-guiding surfaces which are concave only sectionally, for example, as long as the fluid flow is prevented from directly acting on the combustion chamber window surface or an adjoining volume region. 
         [0010]    In another variant of the present invention, a particularly advantageous fluid flow deflection is achieved in that the flow-guiding surface faces an ignition point which is situated in the antechamber and onto which the laser spark plug focuses laser radiation, and/or at least one overflow passage which enables a fluid connection between the antechamber and a combustion chamber. In this way, the fact that fluid flows in the antechamber usually propagate starting from the ignition point or starting from the overflow passage is taken into account. 
         [0011]    In another very advantageous specific embodiment of the laser spark plug according to the present invention it is provided that the flow-guiding element is situated directly in the area of the combustion chamber window, thus resulting in additional mechanical protection of the volume region in the area of the combustion chamber window in which the formation of the desired residual gas blanket is intended. 
         [0012]    The flow-guiding element is particularly advantageously designed in such a way that it surrounds, together with a surface of the combustion chamber window facing the antechamber, an essentially frustoconical volume region whose base area lies in the area of the combustion chamber window or is formed by a surface of the combustion chamber window. A corresponding cover area of the frustoconical volume region represents here an opening through which the laser radiation passes into the interior of the antechamber. 
         [0013]    In another variant of the present invention, a particularly uniform flow deflection according to the present invention is provided in that the flow-guiding element surrounds, in particular concentrically, an optical axis of the laser spark plug. 
         [0014]    In another variant of the present invention, a particularly stable configuration is provided if the flow-guiding element is designed as a single piece together with a housing of the laser spark plug. 
         [0015]    Another very advantageous specific embodiment of the laser spark plug according to the present invention is characterized in that at least one overflow passage is provided, which enables a fluid connection between the antechamber and a combustion chamber, and that the overflow passage is curved. Due to the nature according to the present invention of the curvature of the overflow passage, in contrast to the overflow passages which are conventionally designed as bore holes, an additional degree of freedom is achieved for the guidance of a fluid flow in the interior of the antechamber, thus advantageously supporting the flow-guiding elements according to the present invention with regard to a desired fluid deflection. 
         [0016]    A particularly advantageous fluid guidance in the area of the overflow passages is achieved in that a longitudinal axis of a first section of the overflow passage, which ends in an interior of the antechamber, is at a greater angle to the optical axis of the laser spark plug than a longitudinal axis of a second section of the overflow passage, which ends in an outside area surrounding the antechamber. The first longitudinal axis of the first section of the overflow passage may by all means be at a right angle to the optical axis of the laser spark plug, while the second longitudinal axis of the second section of the overflow passage may be preferably at an acute angle, in particular an angle &lt;20°, to the optical axis of the laser spark plug. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a first specific embodiment of a laser spark plug according to the present invention in a partial cross section. 
           [0018]      FIG. 2  shows a second specific embodiment of a laser spark plug according to the present invention. 
           [0019]      FIG. 3  shows a third specific embodiment of a laser spark plug according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 1  shows a partial cross section of a first specific embodiment of a laser spark plug  100  according to the present invention. Laser spark plug  100  has an integrated laser device  105  which may generate laser radiation  20  and focus it onto ignition point ZP lying in an antechamber  110  of laser spark plug  100 . 
         [0021]    Alternatively, laser spark plug  100  may also be configured in such a way that it is supplied with laser radiation by a remotely situated source (not shown) which it focuses onto the ignition point. In this case, local laser device  105  may be dispensed with. 
         [0022]    The interior of antechamber  110  is separated from part  100   a  of laser spark plug  100 , facing away from the combustion chamber, by a combustion chamber window  140 . 
         [0023]    Laser spark plug  100  or its antechamber  110  further has overflow passages  120  which enable a fluid connection between antechamber  110  and an outside area  200 , which may involve a combustion chamber of the internal combustion engine, for example, if laser spark plug  100  is installed in an internal combustion engine. 
         [0024]    According to the present invention, laser spark plug  100  further has at least one flow-guiding element  130  which is designed essentially rotation-symmetrically in the present case and is situated in the area of a surface of combustion chamber window  140 , facing antechamber  110 . 
         [0025]    Flow-guiding element  130  advantageously causes a fluid flow F, which is directed toward combustion chamber window  140 , to be deflected into a radially inner area or a radially inner direction with regard to the optical axis OA of laser spark plug  100 . 
         [0026]    Arrow F′, which is drawn as a dashed line in  FIG. 1 , shows the fluid flow deflected according to the present invention. 
         [0027]    By deflecting the flow according to the present invention from F to F′, a residual gas blanket  150  may advantageously form in volume region  111 , the residual gas blanket advantageously protecting the surface of combustion chamber window  140  against contaminants and other undesirable elements, such as those developing during the combustion due to the laser ignition in antechamber  110 . 
         [0028]    Flow-guiding element  130  preferably has a flow-guiding surface  130   a  which has an essentially concave design and which advantageously faces ignition point ZP or an overflow passage  120 , so that an efficient flow deflection is possible into a radially inner area. 
         [0029]    Although flow-guiding element  130  according to the present invention does not necessarily have to adjoin combustion chamber window  140 , this advantageously results in the formation of a volume region  111  which is particularly well protected against the flow and which highly reliably allows for the formation and lasting preservation of a residual gas blanket  150 . 
         [0030]      FIG. 2  shows another specific embodiment of laser spark plug  100  according to the present invention. 
         [0031]    In contrast to the variant according to the present invention according to  FIG. 1 , flow-guiding element  130  is here designed in such a way that it surrounds, together with a surface of combustion chamber window  140  facing antechamber  110 , an essentially frustoconical volume region  111  whose base area lies in the area of combustion chamber window  140  or is directly formed by a surface of combustion chamber window  140  in the present case. A cover area of frustoconical volume region  111  still allows for an, albeit limited, fluid communication between volume region  111  and the residual volume of antechamber  110  and the transmission of laser radiation  20  onto ignition point ZP. 
         [0032]    According to the present invention, a particularly uniform flow guidance or deflection of fluid flow F ( FIG. 1 ) away from combustion chamber window  140  may take place in that flow-guiding element  130  surrounds, in particular concentrically, an optical axis OA of laser spark plug  100 . 
         [0033]    Flow-guiding element  130  may preferably also be designed as a single piece together with housing  101  of the laser spark plug, whereby a configuration which is particularly mechanically stable is achieved. 
         [0034]      FIG. 3  shows another advantageous specific embodiment of laser spark plug  100  according to the present invention in which lateral overflow passages  120 ′ are curved. 
         [0035]    In the present case, a longitudinal axis L 1  of a first section of overflow passage  120 ′, which ends in the interior of antechamber  110 , is at a greater angle α1 with optical axis OA of laser spark plug  100  than a second longitudinal axis L 2  of a second section of overflow passage  120 ′, which ends in an outside area  200  surrounding antechamber  110 . Angle α2 between second longitudinal axis L 2  and optical axis OA accounts for less than approximately 50°, particularly preferably less than approximately 20°. 
         [0036]    This ensures, on the one hand, that an ignitable air/fuel mixture flows directly toward ignition point ZP or at least not directly toward combustion chamber window  140 , when the mixture is delivered to antechamber  110  from outside area  200 , i.e., the combustion chamber. On the other hand, it is advantageously ensured due to the configuration according to the present invention of second longitudinal axis L 2  that ignition torches emanating from antechamber  110 , which are used to ignite the mixture present in combustion chamber  200 , do not extend purely radially away from antechamber  110  but rather also extend at least to a certain extent in an axial direction with regard to the longitudinal axis or optical axis OA of laser spark plug  100 , i.e., propagate to the right in  FIG. 3 . 
         [0037]    According to the present invention, the shaping of curved overflow passages  120 ′ and flow-guiding element  130  takes place in such a way that an optimized flow deflection away from volume region  111  to be protected ( FIG. 2 ) is ensured, while at the same time ignition torches (not shown) may emanate, having a particularly advantageous orientation, from antechamber  110  into combustion chamber  200 . 
         [0038]    To ensure that the laser ignition itself does not interfere with the formation of residual gas blanket  150  ( FIG. 1 ), ignition point ZP is preferably selected in such a way that it lies outside of volume region  111  to be protected, in particular in a right-hand chamber half of antechamber  110  in  FIG. 1 .