Abstract:
The invention relates to a method for operating an internal combustion engine including a fuel-injection unit and a laser ignition-unit. According to the invention, during a compression cycle of the internal combustion engine, fuel is injected into the combustion chamber by means of the fuel-injection unit in such a way that an ignitable, round, flat mixed region of fuel and air forms on a plunger and a predetermined period elapses between the end of the fuel injection and the start of ignition in order to form the round flat mixed region. Ignition then takes place within the flat round mixed region by means of the laser-ignition unit.

Description:
CROSS-REFERNCE TO RELATED APPLICATION 
   This application is a 35 USC 371 application of PCT/EP2006/066747 filed on Sep. 26, 2006 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an internal combustion engine having a laser ignition device for direct gasoline injection, and to a method for operating such an engine. 
   2. Description of the Prior Art 
   Internal combustion engines for direct gasoline injection are known in various designs from the prior art. Such engines are lately used more and more, since they have lower fuel consumption and low emissions. A common feature of the known engines is that they typically have an injection valve in a central position and a spark plug disposed in such a way that they are located on a periphery of the spray of fuel injected into the combustion chamber. The ignition of the fuel, which propagates essentially conically from the injection valve, is effected at the periphery of the cone, since only there is an ignitable fuel-air mixture present. In practice, however, in positioning the spark site of the spark plug, problems arise in positioning this spark site precisely in the narrow peripheral zone in the region of the ignitable fuel-air mixture. This is due on the one hand to component tolerances of the injection valve, spark plug, and cylinder head, and on the other to the deviation in the spray geometry from one cycle to another, and thus the ignitable peripheral zone can vary to a certain extent. Cyclical deviations in the spark site within the electrode region can also occur in the spark plug, and the spray geometry can vary from aging because of deposits or as a function of the engine performance graph. These factors lead to reduced efficiency on the one hand and to problems with the exhaust gases on the other. 
   Moreover, it has lately been proposed that the conventional spark plug be replaced by a laser ignition device, with ignition again taking place at the periphery of an injected fuel vapor cloud. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   The method according to the invention for operating an internal combustion engine has the advantage over the prior art that reliable ignition can always be assured. Moreover, the efficiency is increased according to the invention, which results in fuel economy and improved exhaust emissions. This is attained according to the invention in that fuel is injected into a combustion chamber during a compression cycle of the engine. The fuel is injected into the combustion chamber in such a way that an ignitable patty-shaped mixture region comprising fuel and air forms on the piston. The patty-shaped mixture region rests on the bottom face of the piston. According to the invention, between an end of the fuel injection and a beginning of an ignition with a laser ignition device, a predetermined period of time elapses, to enable the formation of the patty-shaped mixture region on the piston. The period of time is selected such that an ignition site of a laser beam of the laser ignition device, aimed into the combustion chamber, is located in the interior of the ignitable patty-shaped mixture region. As a result, ignition occurs first inside the patty- shaped mixture region. According to the invention, ignition therefore no longer occurs at the periphery of a vapor cloud of injected fuel, but rather in the interior of a patty-shaped mixture region located on the piston. After the injection, there is a wait until the patty-shaped mixture region has formed on the piston which during the compression phase moves counter to the injection direction of the fuel. When the fuel injected into the combustion chamber is injected in the form of a stream that with increasing penetration depth into the combustion chamber increasingly vaporizes, the stream, particularly in the region of the point of the spray, produces an ignitable envelope of a gaseous fuel-air mixture, which wraps like a ruff around a center of the stream. This gaseous envelope is essentially in the form of a teardrop. Because the piston is moving counter to the thus-injected fuel, the piston deflects the fuel-air mixture horizontally to all sides, which leads to further mixing from turbulence. As a result, the ignitable patty-shaped mixture region according to the invention is formed on the piston bottom. Thus during the period of time after the injection and before the ignition, the mixture region of the invention is formed on the piston. The patty-shaped mixture region is surrounded by a non-ignitable gas mixture, especially air. In the process, the piston continues moving upward in the direction of the cylinder head, and ignition does not occur until the focal point of the laser is located in the interior of the mixture region. Since the ignition is thus generated in the interior of the mixture region, reliable ignition can be made possible. Moreover, beginning at the ignition site, the flame travels to the edge of the mixture region are markedly shorter than in comparison with an ignition point at a periphery, so that faster combustion is attained as well. The use of a laser ignition device makes it possible for ignition to take place at any arbitrary point in the mixture region. When conventional spark plugs are used, only spark locations that reach approximately 8 mm into the combustion chamber are possible, since otherwise the electrodes and ceramic of the spark plugs become too hot. In contrast to this, using the laser ignition device can be done at the periphery of a combustion chamber, without part of it protruding into the combustion chamber. Moreover, a laser ignition device has no ignition energy losses from quenching phenomena (heat dissipation) at metal spark plug electrodes. As a result, cyclical fluctuations in the course of combustion are reduced, since the laser ignition device makes high replicability of the sensitive flame core formation possible. Also by means of the laser ignition device, even dilute mixtures can be ignited. Moreover, ignition in the interior of the mixture region enables faster combustion of the entire mixture region, which can be utilized thermodynamically by enabling either a faster course of combustion (higher compression without knocking, resulting in better fuel economy) and/or mixture dilution (leaning down or exhaust gas recirculation, which thus reduces NOx and has advantages in terms of better combustion). 
   The injection of the fuel is effected preferably by means of a multiplicity of individual streams, which are generated by means of a multi-port valve having preferably between seven and fourteen ports, or by means of an outward-opening annular gap valve (A valve) with an opening angle α of between 70°≦α≦110°. 
   Preferably, the period of time between the end of the fuel injection and the beginning of the ignition is selected such that it corresponds to a crankshaft angle travel of between 5° and 15°, in particular between 5° and 10°, and especially preferably 7.5°. 
   To enable faster and more reliable formation of the patty-shaped mixture region, a hollow, in which the patty-shaped mixture region forms, is preferably provided in the piston bottom. The hollow is preferably circular and symmetrical. 
   Especially preferably, ignition occurs near or at a middle region of the patty-shaped mixture region, so as to have the shortest possible flame travels through the entire mixture region. 
   Also preferably, a protruding nub over which the patty-shaped mixture region lies is formed on the piston bottom, so as to form a region that protrudes essentially in the axial direction of the piston in the patty-shaped mixture region. The ignition of the patty-shaped mixture region can preferably be effected beginning at the protruding region. As a result, it can be attained that the onset of the ignition of the patty-shaped mixture region takes place at an even earlier motion segment in the direction of top dead center of the piston, so that precisely in the optimal crankshaft angle range for efficiency, the most complete possible combustion of the patty-shaped mixture region takes place, and the expansion cycle then follows. To achieve positioning of the injection valve as centrally as possible in the cylinder head, the protruding nub is preferably disposed on a periphery of the hollow on the piston bottom. To achieve faster formation of the patty-shaped mixture region, a fuel injection is preferably effected at the nub that protrudes from the piston bottom. 
   Preferably, the fuel injection takes place in a plurality of successive intervals. As a result, a higher proportion of air is present in the patty-shaped mixture region, since between the individual intervals of the fuel injection, there is a small air cushion each time. 
   To keep the flame transit times from an ignition site through the patty-shaped mixture region as short as possible, a multiplicity of ignition sites is preferably provided in the interior of the mixture region. The multiplicity of ignition sites can be generated by means of a plurality of laser ignition devices or by means of one laser ignition device which is actuated multiple times at different focal points. The multiplicity of ignition sites is preferably disposed symmetrically in the patty-shaped mixture region. The multiplicity of ignition sites is preferably located in one plane, to achieve the most homogeneous possible flame transit time through the mixture region. However, it should be noted that depending on the given geometrical conditions at the piston and/or of the combustion chamber, the multiplicity of ignition sites may also be disposed in different planes. 
   To enable load-dependent ignition, for instance, in a simple way, the focal length of the laser ignition device is preferably variable. As a result, the laser ignition device can be adapted to different ambient conditions. 
   In order in particular to attain reduced noise upon ignition of the patty-shaped mixture region at a plurality of ignition sites, the various ignition sites are preferably ignited at different instants. This makes further optimization of the course of combustion possible. 
   The method according to the invention is preferably employed in a stratified-charge mode of the engine. The term stratified-charge mode is understood here to mean a mode of operation in which only slight loads are placed on the engine. In the stratified-charge mode, combustion in the combustion chamber is defined essentially only by the fuel mass injected, and a throttle valve is typically opened wide. However, the method of the invention can also be employed in the normal operating mode of the engine. 
   In addition, according to the invention, an internal combustion engine for direct injection of fuel into a combustion chamber is proposed that includes a laser ignition device, a piston, and a fuel injection device. The engine moreover includes a control unit for determining an instant of ignition of the laser ignition device. The control unit does not activate the laser ignition device until the fuel injection operation has concluded, and an ignitable patty-shaped mixture region has formed on the piston. In other words, between the end of the fuel injection event and the onset of ignition, a predetermined period of time elapses, on the one hand to enable the formation of the patty-shaped mixture region and on the other so as not to perform an ignition until a focal point (ignition site) of the laser is located in the patty-shaped mixture region. It is thus assured that the mixture region is ignited in its interior, and hence the flame travels are very short. 
   The control unit determines the instant of ignition preferably as a function of a piston position. The position of the piston can preferably be determined from a crankshaft angle by means of a sensor. 
   To reinforce the formation of the ignitable patty-shaped mixture region, the piston preferably has a substantially circular hollow on a piston bottom face. The hollow is preferably symmetrical to a center axis of the piston. 
   Also preferably, the piston has a protruding nub on the piston bottom face. The nub may preferably be provided in a circular hollow, or on the periphery of the hollow. By means of the protruding nub in the region of the hollow, it can be assured that the patty-shaped mixture region forms above the nub as well, so that one region of the mixture region protrudes in the direction of motion of the piston. The laser ignition device is then preferably disposed in such a way that the focal point of the laser is located in the protruding region of the mixture region. As a result, ignition long before top dead center of the piston can for instance be attained, yet the ignition still reliably takes place in the interior of the mixture region. 
   In a further preferred feature of the invention, the hollow formed on the piston bottom face has a basic face inclined at an angle to the center axis of the piston. This makes it possible to dispose the laser ignition device centrally in the cylinder head and nevertheless to enable a fuel injection perpendicularly to the piston bottom. The central disposition of the laser ignition device has advantages in terms of installation space as well. 
   Preferably, the laser ignition device includes a focusable lens, for varying a position of the focal point of the laser beam. 
   To make a multiplicity of ignition sites in the patty-shaped mixture region possible, the engine preferably includes a multiplicity of laser ignition devices. The multiplicity of laser ignition devices are preferably disposed such that the ignition sites are disposed as symmetrically as possible in the patty-shaped mixture region. The control unit actuates the multiplicity of laser ignition devices preferably at different instants, to enable optimizing a desired course of combustion, in particular with regard to noise and to the flame transit times through the mixture region. 
   The fuel injection device is preferably a multi-port valve having a number of ports ranging between seven and fourteen, or an outward-opening annular gap valve (A-valve), preferably with an opening angle of between 70° and 110°. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the invention are described below in detail in conjunction with the drawings. In the drawings: 
       FIG. 1  is a schematic sectional view of an internal combustion engine, in a first exemplary embodiment of the invention; 
       FIGS. 2   a  and  2   b  are schematic illustrations of the injection of fuel by means of a multi-port valve; 
       FIG. 3  is a schematic illustration of a spray cloud of an outward-opening annular gap valve (A-valve); 
       FIGS. 4   a  and  4   b  are schematic views of an injection by means of a multi-port valve in a further exemplary embodiment of the invention; 
       FIGS. 5 and 6  are schematic sectional views of an internal combustion engine in a second exemplary embodiment of the invention; 
       FIGS. 7 and 8  are schematic sectional views of an internal combustion engine in a third exemplary embodiment of the present invention; 
       FIGS. 9 and 10  are schematic sectional views of an internal combustion engine in a fourth exemplary embodiment of the present invention; 
       FIG. 11  is a schematic sectional view of an internal combustion engine in a fifth exemplary embodiment of the present invention; 
       FIG. 12  is a schematic sectional view of an internal combustion engine in a sixth exemplary embodiment of the present invention; and 
       FIG. 13  is a plan view on the patty-shaped mixture region of  FIG. 12 ; and 
       FIGS. 14   a  and  14   b  are plan views of patty-shaped mixture regions with different ignition sites. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An internal combustion engine  20  in a first exemplary embodiment of the invention will now be described in conjunction with  FIGS. 1 and 2 . 
   As shown in  FIG. 1 , the internal combustion engine  20  includes a piston  21 , with a piston bottom  22  in which a circular hollow  23  is disposed centrally and symmetrically to a center axis X-X of the piston. The piston  21  moves in a cylinder in the known manner; an injection device  25  and a laser ignition device  26  are disposed in a cylinder head  24 . The injection device  25  is disposed centrally in the cylinder head on the center axis X-X of the piston and in this exemplary embodiment, it is a multi-port valve with ten ports. The disposition of the ports can be seen in  FIG. 2   a . The laser ignition device  26  is controlled via a control unit  34  and has an aspherical lens  26   a . The laser ignition device further includes a quality-switched optically pumped solid-state laser. The laser ignition device  26  generates a laser beam  27 , which is aimed into a combustion chamber  29 . The laser ignition device  26  is formed in planar fashion toward the inner wall of the cylinder head  24 , so that the laser ignition device  26  does not protrude into the combustion chamber  29 . 
   From  FIGS. 2   a  and  2   b , the injection of the fuel into the combustion chamber  29  becomes clear. The individual streams, identified by reference numerals  1  through  10 , of the injection device  25  are injected into the combustion chamber in the direction of the oncoming piston. With increasing penetration depth of each individual stream, increasing vaporization occurs, so that an envelope of a gaseous fuel-air mixture is generated around each injection stream  1  through  10 , especially in the region of the spray point of the injected fuel. This is indicated in  FIG. 2   b  by the large circles around each individual stream  1  through  10 . The gaseous fuel-air mixture envelope wraps around the stream in the manner of a ruff, and the point of the stream is also formed by a gaseous fuel-air mixture. As can also be seen from  FIG. 2   b , the fuel-air mixture envelopes of the individual streams are partially superimposed on one another because of turbulence and because the piston  21  is moving counter to the stream direction and deflects the fuel-air mixture envelope horizontally. As a result, an ignitable patty-shaped mixture region  28  forms on the piston bottom  22 . The formation of the patty-shaped mixture region  28  is still further reinforced by the hollow  23  provided in the piston bottom  22 . The patty-shaped mixture region  28  has a circular shape, with a thickness that decreases somewhat from the center toward the outer periphery. The mixture region  28  is a substantially homogeneous gaseous fuel-air mixture, which has a mean lambda value of between 0.8 and 1.5. 
   According to the invention, the injection of fuel is effected at a crankshaft angle of approximately 35° before top dead center OT. The ports in the multi-port valve should be designed such that each individual stream has a shape that is as bushy as possible. For that purpose, the diameter of an individual port is preferably between approximately 130 μm and 200 μm. Also preferably, tapering and in particular conical, outward-opening ports or graduated ports are favorable to a bushy spray shape with the desired fuel-air mixture gas envelopes. As shown in  FIG. 2   a , the beam axes of the ports in the multi-port valve are selected such that they have approximately the same dihedral angle spacing. 
   It should be noted that depending on a distance traveled by an individual stream to the piston bottom, the diameter of the individual ports may vary. Inner streams, which given the central disposition of the injection device shown in  FIG. 1  have the shortest distance to the piston, can have a smaller port diameter than the ports located on the circumference. Through the smaller ports, the fuel quantity passing through them, and thus the spray impetus, become less, so that the inner streams as well are reliably vaporized before they reach the piston bottom  22 . The choice of the port diameter and the opening angle of the injection ports and of the number of ports should be made such that the injected fuel has just vaporized when it reaches the piston bottom  22 . As a result, the oncoming piston can furnish especially good mixing and homogenizing of the ignitable mixture region  28 . 
   The formation of the mixture region  28  takes place in a range of between 35° before top dead center and approximately 20° before top dead center.  FIG. 1  shows the position approximately 20° before top dead center, in which the mixture region  28  has formed homogeneously in the hollow  23  on the piston bottom. However, ignition by means of the laser ignition device  26  does not take place until a focal point of the laser  27  is located in the interior of the mixture region  28 . This focal point defines the ignition site  27   a  in the interior of the mixture region. This exists at a piston position of approximately 20° before top dead center, as shown in  FIG. 1 , with the ignition site  27   a  located precisely on the center axis X-X. 
   The control unit  34  controls the instant of ignition of the laser ignition device  26  as a function of the position of the piston  21 . Preferably, the instant of ignition is located at a crankshaft angle of approximately 20° before top dead center. It can thus be assured that the mixture region  28  combusts completely in the crankshaft angle range that is optimal in terms of efficiency, and high efficiency of the engine is achieved. A period of time between an end of the fuel injection and the onset of ignition corresponds to a distance traveled by the piston over a crankshaft angle of between 5° and 10°, preferably 7.5°. 
   As a result of the ignition in the interior of the mixture region  28 , the flame travels through the mixture region  28  are reduced markedly, compared to ignition at the periphery. Hence on the one hand faster and also more-complete combustion can be attained. On the other, as a result an especially stable combustion process is also attained, especially in the stratified-charge mode of the engine. The patty-shaped mixture region  28  is due according to the invention to the interaction of the fuel being injected and of the piston, and a certain period of time elapses between the end of injection and the onset of ignition, so as to enable the formation of the mixture region and to perform ignition of the mixture region  28  in its interior. Also as a result of the ignition in the interior, the deviations that occur from component tolerances do not lead to uneven combustion. Deviations in the spray geometry from one cycle to the next, or performance-graph-dependent fluctuations in the spray geometry, have no influence on the method of the invention, either. The laser ignition device furthermore has the advantage that no ignition energy losses from quenching phenomena (heat dissipation) at metal spark plug electrodes occur, as they do in the prior art. Hence in an Otto engine, high replicability of the sensitive flame core formation is made possible for the first time. 
     FIG. 3  shows an alternative embodiment of an injection device  25 , in the form of an outward-opening annular gap valve (A-valve). The annular gap valve shown injects the fuel in conical shape, so that in the sectional view shown in  FIG. 3 , an annular fuel region  30  is brought about. On each side of the fuel region  30 , a respective envelope  31  and  32  of fuel-air mixture forms. An opening angle of the annular gap valve of  FIG. 3  is preferably between 70° and 110°. Because of the injection by means of the annular gap valve, a homogeneous, patty-shaped mixture region  28  on the piston is likewise generated as in  FIG. 1 , and the piston deflects the arriving fuel-air mixture horizontally both inward and outward. 
     FIGS. 4   a  and  4   b  show a further embodiment of an injection device  25 , which again is embodied as a multi-port valve. The multi-port valve shown in  FIGS. 4   a  and  4   b  has twelve injection ports  1  through  12 . The injection ports are distributed along two concentric circles and are offset from one another on these circles. The result is the spray distribution around the individual streams as shown in  FIG. 4   b . The large circles again represent the gaseous mixture envelopes. Otherwise, this exemplary embodiment is equivalent to the first exemplary embodiment so that the description thereof may be referred to. 
   In  FIGS. 5 and 6 , an internal combustion engine  20  is shown in a second exemplary embodiment of the invention, in which identical or functionally identical parts are identified by the same reference numerals. Unlike the first exemplary embodiment, in the engine  20  of the second exemplary embodiment, the laser ignition device  26  is disposed centrally on the center axis X-X. The injection device  25  is disposed laterally of the laser ignition device  26  and at an angle α to the center axis. Once again, the injection device  25  is a multi-port valve; in  FIG. 5 , three injection streams, with a still-liquid fuel stream  35  and a developing gaseous envelope  36  comprising a fuel-air mixture, are shown schematically. When the individual streams strike the piston bottom  22 , they are completely vaporized, so that only a gaseous fuel-air mixture strikes the piston bottom. As can also be seen from  FIG. 5 , a protruding nub  37  is also formed in the hollow  23  formed in the piston bottom  22 . The protruding nub  37  is disposed centrally in the hollow  23 , on the center axis X-X, and essentially has the shape of a segment of a sphere. The injection of fuel takes place precisely in the direction of the protruding nub  37 . As shown in  FIG. 5 , the piston position at the onset of injection is approximately 35° before top dead center. Once the fuel injection is concluded, the piston  21  moves onward in the direction toward the laser ignition device  26 , and then as a result of the deflection at the piston bottom, the homogeneous, ignitable mixture region  28  forms (see  FIG. 6 ). Here, the mixture region  28  is also formed above the protruding nub  37 , so that as shown in  FIG. 6 , an ignition site  27   a  is located in a protruding region  28   a  of the mixture region  28 , at a piston position of approximately 20° before top dead center. The position of the piston as shown in  FIG. 6  is the position in which the ignition of the mixture region  28  takes place. Since in this exemplary embodiment, the laser ignition device  26  is disposed centrally on the center axis X-X, and the center axis X-X is also an axis of symmetry for the mixture region  28 , the flame travels from the ignition site  27   a  to the peripheries of the mixture region  28  are especially short in this exemplary embodiment. As a result, especially fast, complete combustion can be achieved. 
     FIGS. 7 and 8  show an internal combustion engine  20  in a third exemplary embodiment of the invention, in which once again identical or functionally identical parts are identified by the same reference numerals as in the preceding exemplary embodiment. The engine  20  in the third exemplary embodiment is essentially equivalent to that of the second exemplary embodiment, except that the protruding nub  37  in the third exemplary embodiment is disposed at a periphery of the hollow  23 . As a result, the injection device  25  can be disposed centrally on the center axis X-X of the piston  21 , and the laser ignition device  26  is disposed in the cylinder head  24  in such a way that it is positioned above the protruding nub  37 . As a result the nub  37  and the laser ignition device  26  are located essentially on a common axis Y-Y that is parallel to the center axis X-X. The injection of fuel is effected directly into the hollow  23  formed in the piston bottom  22 . In cooperation with the hollow  23  and the motion of the piston  21  contrary to the incoming stream direction, the mixture region  28  is again formed in the hollow  23  after the end of injection and before the ignition, and the mixture region  28  has a protruding region  28   a  in the region of the protruding nub  27  (see  FIG. 8 ). In a position approximately 20° before top dead center, which is shown in  FIG. 8 , the focal point of the laser  27  is located in the protruding region  28   a  of the mixture region  28 , so that ignition can then take place in the interior of the mixture region  28 . As a result, it is also possible, in comparison to the first exemplary embodiment, to enable earlier ignition of the mixture region  28 , since the laser ignition device ignites the protruding region  28   a  of the mixture region  28 , and the flame travels extend from there through the entire mixture region  28 . Otherwise, this exemplary embodiment is equivalent to the preceding exemplary embodiments, so that the description of those may be referred to. 
   In  FIGS. 9 and 10 , an internal combustion engine  20  in a fourth exemplary embodiment of the invention is shown; once again, identical or functionally identical parts are identified by the same reference numerals as in the preceding exemplary embodiments. The fourth exemplary embodiment is essentially equivalent to the second exemplary embodiment; once again, a protruding nub  37  is formed centrally in the hollow  23  in the piston bottom  22 . The laser ignition device  26  is again disposed centrally on the center axis X-X of the piston  21 , and the injection device  25  is at an angle α. Unlike the second exemplary embodiment however, the piston bottom in the region of the hollow  23  is inclined relative to a plane E that is perpendicular to the center axis X-X. The inclination is indicated by the angle β in  FIGS. 9 and 10 . The inclination of the hollow  23  is preferably selected such that a central injection stream injects essentially perpendicular to the inclined surface of the hollow. The central injection stream strikes the protruding nub  37  in particular, which leads to faster formation of the patty-shaped mixture region  28 . As can be seen from  FIGS. 9 and 10 , the laser ignition device  26  is again disposed above the protruding nub  37 , so that early ignition of the mixture region is possible. Otherwise, this exemplary embodiment is equivalent to the preceding exemplary embodiments, so that the description of those may be referred to. 
     FIG. 11  shows a fifth exemplary embodiment of an internal combustion engine of the invention; again, identical or functionally identical parts are identified with the same reference numerals as in the preceding exemplary embodiments. The fifth exemplary embodiment is essentially equivalent to the second exemplary embodiment; unlike the second exemplary embodiment, both the injection device  25  and the laser ignition device  26  are disposed at an angle γ and δ, respectively, to a center axis X-X. The angles of inclination γ, δ of the injection device  25  and laser ignition device  26  are the same and differ only in their sign. A protruding nub  37  in the hollow  23  in the piston bottom  22  is again located below the laser ignition device  26 , resulting in an asymmetrical embodiment of the hollow. The patty-shaped mixture region  28  again forms with a protruding region  28   a  above the protruding nub  37 , so that an ignition site  27   a  of the mixture region  28  is located in this protruding region  28   a . Otherwise, this exemplary embodiment is equivalent to the preceding exemplary embodiment, and the description of that may be referred to. 
   Below, an internal combustion engine in a sixth exemplary embodiment of the invention will be described in conjunction with  FIGS. 12 and 13 ; once again, parts that are identical or functionally identical are identified by the same reference numerals as in the preceding exemplary embodiments. The engine in the sixth exemplary embodiment has two laser ignition devices  26 , as shown in  FIG. 12 . An injection device  25  is disposed centrally on a center axis X-X of the piston  21 . The two laser ignition devices  26  are disposed laterally at an angle γ and δ, respectively, to the center axis X-X, and the angles γ and δ are the same size. The disposition of two laser ignition devices  26  makes it possible to generate two ignition sites  27   a  in the patty-shaped mixture region  28 . As a result, in particular, the flame travels inside the mixture region  28  can be kept short. In  FIGS. 12 and 13 , the flame travels are schematically drawn as short arrows around the ignition sites  27   a . Ignition of the patty-shaped mixture region  28  is preferably effected simultaneously, since the patty-shaped mixture region  28  is embodied symmetrically to the center axis X-X. The two ignition sites  27   a  are located on a plane F that is perpendicular to the center axis X-X. Otherwise, this exemplary embodiment is equivalent to the preceding exemplary embodiments, and the description of those may be referred to. 
     FIGS. 14   a  and  14   b  also show two different examples for igniting the patty-shaped mixture region  28  by means of a plurality of ignition sites. In  FIG. 14   a , there are three ignition sites  27   a , disposed in a mixture region  28  of circular circumference at angles of approximately 120° each from one another, beginning at a center axis X-X. The spacings of each ignition site  27   a  to the periphery of the mixture region  28  are selected to be equal. In  FIG. 14   b , an embodiment with four ignition sites  27   a  is shown, which are disposed symmetrically to a center axis X-X. As can be seen from  FIG. 14   b , this disposition with four ignition sites  27   a  is especially advantageous, since the flame travels, beginning at each ignition site  27   a , through the entire mixture region  28  are essentially of equal length, until the entire mixture region  28  is ignited. 
   It is noted that it is understood that in all the exemplary embodiments shown, multiple injection can be performed. This has the advantage in particular that the mixture region  28  is constructed in stratified fashion, with a thin layer of air between each two fuel-air mixture layers. As a result, the proportion of air in the mixture region  28  can be increased. 
   A further advantage of the method of the invention, in all the exemplary embodiments described, is that for generating the mixture region  28 , injection devices can be used that generate symmetrical spray geometries (without a spray gap for the spark plug). As a result, it is also unnecessary to associate the injection device with a spark plug protruding into the combustion chamber. Moreover, moistening of the laser ignition device with liquid fuel, which in the prior art reduces the ignitability of a spark plug, does not occur. 
   An internal combustion engine according to the invention can be used both in vehicles and in stationary fashion. 
   The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.