Patent Publication Number: US-2012024251-A1

Title: Gas engine having a laser ignition device

Description:
FIELD OF THE INVENTION 
     The present invention relates to an internal combustion engine equipped with a laser ignition device, in particular a gas engine. 
     BACKGROUND INFORMATION 
     German patent document DE 10 2004 001 554 A1 discusses a laser ignition device for igniting an air-fuel mixture in an internal combustion engine, an ignition laser of the laser ignition device protruding into a combustion chamber of the internal combustion engine. The ignition laser is supplied optically by a pumped light source via an optical fiber. 
     Large-scale gas engines are usually operated near the lean limit of a fuel-air mixture in order to achieve a good efficiency. A stable flame core must be formed during ignition, so that the fuel-air mixture in the combustion chamber may be burned afterward as rapidly as possible. Specifically in the case of gas engines operated in the extremely lean range, it is of utmost importance to reduce the combustion time and thereby increase engine efficiency. The combustion time is defined as the period of time within which between 10% and 90% of the energy conversion takes place. 
     In the case of conventional high voltage ignitions using spark plugs, the ignition spot is necessarily close to the roof of the combustion chamber, so that the flame propagates into the combustion chamber in the direction of the piston bottom approximately hemispherically. The combustion time is therefore comparatively long. To counteract this long combustion time, internal combustion engines today are often designed as short-stroke engines, i.e., having a bore diameter larger than the piston stroke length. The flame pathways in the direction of the piston are shortened in this way. To nevertheless achieve rapid and thorough combustion of the mixture, a high velocity of flow and consequently a high turbulence must prevail in the combustion chamber. In traditional internal combustion engines, this is achieved by swirl flows and quench flows. 
     To generate these flows and the resulting turbulence, substantial charge cycle losses occur and/or unfavorable chamber geometries in which the ratio between the surface area and the volume is great are required, so that high wall heat losses occur. The charge cycle losses as well as the wall heat losses reduce the efficiency of the internal combustion engine. 
     SUMMARY OF THE INVENTION 
     An object of the exemplary embodiments and/or exemplary methods of the present invention is to improve upon an internal combustion engine having a laser ignition device, so that a reliable and low-emission combustion is ensured at various operating points, in particular even with a lean fuel-air mixture, while ensuring a very good efficiency at the same time. 
     This object may be achieved by an internal combustion engine having an ignition laser with the features described herein. Features important for the exemplary embodiments and/or exemplary methods of the present invention are also to be found in the following description and in the drawings, where the features either alone or in various combinations may be important for the present invention without any explicit reference thereto. Advantageous refinements are found in the subclaims. 
     According to the exemplary embodiments and/or exemplary methods of the present invention, it is provided that the combustion chamber is designed to be spheroidal. 
     This makes it possible for the flame front to propagate in the form of a sphere in all directions of the chamber, significantly shortening the flame paths and the compression time. Consequently the efficiency of the internal combustion engine also increases. 
     Since it is provided, according to the exemplary embodiments and/or exemplary methods of the present invention that the combustion chamber is to be designed to be spheroidal, in particular spherical, and in particular point-symmetrical with the ignition spot of the ignition laser at the point in time of ignition of the fuel-air mixture in the combustion chamber, the travel times of the flame front until reaching the combustion chamber wall are almost of the same short length everywhere. The result is a very compact combustion chamber shape and a favorable surface/volume ratio of the combustion chamber. The wall heat losses are therefore reduced and the efficiency of the internal combustion engine is further increased. The risk of so-called knocking combustion is also reduced and the internal combustion engine according to the present invention often manages without swirl flows or quench flows. 
     Due to the oscillating movement of the piston, it is self-evident that the shape of the combustion chamber naturally depends on the position of the crankshaft or the position of the piston in the cylinder. Therefore, in the internal combustion engine according to the present invention, emphasis is placed on the shape of the combustion chamber at the start of ignition. In other words, this ensures that even during the subsequent very short combustion time, the combustion chamber will have a favorable geometry for the combustion process. 
     The combustion chamber has a spheroidal geometry in particular when the combustion chamber encloses a spherical volume, which constitutes no less than 50%, in particular no less than 67%, in particular no less than 80% of the total volume of the combustion chamber. 
     A spheroidal geometry of the combustion chamber is also obtained in particular when the combustion chamber has a total surface area and a total volume, the total surface area being no greater than 1.5 times, in particular no greater than 1.25 times, in particular no greater than 1.15 times of the surface area of a sphere whose volume is equal to the total volume of the combustion chamber. 
     An advantageous embodiment of the present invention provides that a combustion chamber roof formed in the cylinder head and a piston bottom of the piston have the same shape. 
     Advantageous embodiments of the present invention provide that the combustion chamber roof and the piston bottom are planar, frustoconical and/or dome-shaped. The advantages according to the exemplary embodiments and/or exemplary methods of the present invention are easily implemented by using these geometries. 
     It has additionally proven advantageous if, at the ignition point, the maximum distance between the combustion chamber roof and the piston bottom corresponds approximately to the diameter of the piston. This ensures a very compact combustion chamber shape. 
     In an additional advantageous embodiment of the present invention, it is provided that the ignition laser has an antechamber having at least one opening, which may be at least one borehole, and that one ignition spot of the ignition laser is located in the antechamber. 
     It is additionally provided that the antechamber has openings, which may be boreholes, which are directed into the combustion chamber. This achieves the result that the fuel-air mixture ignited in the antechamber passes through the opening in the form of an ignition flare into the combustion chamber according to the exemplary embodiments and/or exemplary methods of the present invention, where it ensures a rapid burn-up of the fuel mixture in the combustion chamber. Here again, the geometries according to the exemplary embodiments and/or exemplary methods of the present invention of the combustion chamber are advantageous. 
     Additional advantages and advantageous embodiments of the present invention are shown in the following drawings and described in the description and the patent claims. All the features disclosed in the drawings, the description and the patent claims may be essential to the present invention, either individually or in any combination with one another. 
     Exemplary embodiments of the present invention are explained as an example below on the basis of the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a schematic diagram of an internal combustion engine having a laser-based ignition device. 
         FIG. 1   b  shows a schematic representation of the ignition device from  FIG. 1   a.    
         FIG. 2  shows a detailed representation of an internal combustion engine according to the present invention. 
         FIG. 3  shows another detailed representation of an internal combustion engine according to the present invention. 
         FIG. 4  shows another detailed representation of an internal combustion engine according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An internal combustion engine is labeled with reference numeral  10  on the whole in  FIG. 1   a . It may be used to drive a motor vehicle (not shown). Internal combustion engine  10  usually has several cylinders, only one of which is labeled with reference numeral  12  in  FIG. 1   a . A combustion chamber  14  of cylinder  12  is delimited by a piston  16 . Fuel enters combustion chamber  14  directly through an injector  18 , which is connected to a fuel pressure accumulator  20 , also known as a rail. Alternatively, the fuel-air mixture may also be formed outside of combustion chamber  14 , for example, in an intake manifold. 
     Fuel-air mixture  22  in combustion chamber  14  is ignited by a laser pulse  24  emitted into combustion chamber  14  by a laser ignition device  27 , which includes an ignition laser  26 . The ignition in combustion chamber  14  may also be prepared in an antechamber (not shown in  FIG. 1 ) situated upstream from the combustion chamber. Ignition laser  26  is supplied via a fiber optic light guide  28  with a pumped light, which is supplied by a pumped light source  30 . Pumped light source  30  is controlled by a control unit  32 , which is also able to trigger injector  18 . 
     As shown in  FIG. 1   b , pumped light source  30  has multiple fiber optic light guides  28  for various ignition lasers  26 , each of which is assigned to one cylinder  12  of internal combustion engine  10 . For this purpose, pumped light source  30  has multiple individual pumped laser light sources  34 , which are connected to a pulsed current supply  36 . Due to the presence of multiple individual pumped laser light sources  34 , the pumped light is distributed, so to speak, “latently” to the various laser devices  26 , so that no optical distributors or the like between pumped light source  30  and ignition lasers  26  are necessary. 
     Ignition laser  26  has, for example, a laser-active solid-state body  44  having a passive Q-switch  46 , which together with an input mirror  42  and an output mirror  48  forms an optical resonator. Under the action of the pumped light generated by pumped light source  30 , ignition laser  26  generates in a known manner a laser pulse  24 , which is focused by a focusing lens  52  on an ignition spot ZP situated in combustion chamber  14  (or in an antechamber, not shown here). The components in housing  38  of ignition laser  26  are separated from combustion chamber  14  by an exit window  58  for laser beams  24 . 
     A cylinder head  17  having indicated charge cycle valves  19  and a screwed-in ignition laser  27  is apparent in  FIG. 2 . Ignition spot ZP of ignition laser  27  lies in the center of combustion chamber  14  at the ignition point. 
     The possible spherical propagation of the flame front in the internal combustion engine according to the present invention is indicated in  FIG. 2  by dash-dot rings. Since the flame front in  FIG. 2  has already reached one-third of combustion chamber  14 , it thus illustrates a situation in which piston bottom  15  has moved further in the direction of top dead center TDC, i.e., in the direction of combustion chamber roof  69  in comparison with the ignition point. The position of piston bottom  15  at the ignition point is indicated by a dashed line  15 ′. In this position, combustion chamber  14  is spheroidal and also point-symmetrical with respect to ignition spot ZP. Thus a very short combustion time is implemented, and at the same time a very favorable ratio between the surface area and volume of the combustion chamber is achieved. 
     The height of the combustion chamber at the ignition point is indicated by reference letter H in  FIG. 2 . In the exemplary embodiment shown in  FIG. 2 , a borehole diameter B of cylinder  12  and of piston  16  is greater than height H of the combustion chamber at the point in ignition time. It is now also possible according to the present invention to reduce diameter B of piston  16  while at the same time increasing the stroke of the piston. The internal combustion engine would thus have a longer stroke and the combustion chamber geometry would be even more compact. 
     In the exemplary embodiment according to  FIG. 2 , combustion chamber roof  69  and piston bottom  15  are both planar. This nevertheless yields a very short combustion time because the distance between ignition spot ZP and the walls (in particular the cylinder borehole of cylinder  12 , combustion chamber roof  69  and piston bottom  15 ) delimiting combustion chamber  14  is very short. 
       FIG. 3  shows a further optimized exemplary embodiment of an internal combustion engine according to the present invention. In this exemplary embodiment, combustion chamber roof  69  and piston bottom  15  both have a frustoconical design, resulting in a diamond-shaped combustion chamber  14  in the longitudinal section, which comes even closer to the ideal shape of a spherical combustion chamber. With this combustion chamber geometry, charge cycle valves  19  and thus the inlet and outlet channels (without reference numerals) may be integrated very favorably and easily into the combustion chamber geometry. 
     At the point in time illustrated in  FIG. 3 , piston  16  is at the top dead center and combustion chamber  14  is delimited almost exclusively by combustion chamber roof  69  and piston bottom  15 . Only a very narrow ring of cylinder  12  also delimits combustion chamber  14  between cylinder head  17  and piston bottom  15 . The heat dissipation over the cylinder wall is therefore very minor. Here again, it is clear that the flame front propagating from ignition spot ZP travels very short distances and hits combustion chamber roof  69  as well as piston bottom  15  almost simultaneously. The advantages according to the present invention may also be implemented in this way. 
     It is, of course, also possible to design a combustion chamber roof  69  and a piston bottom  15  in the form of a hemisphere or a spherical calotte, so that an even closer approach to the ideal shape of a complete spheroidal combustion chamber is achieved. This exemplary embodiment is not shown. 
     In the exemplary embodiment according to  FIG. 4 , ignition spot ZP is placed in antechamber  63  of ignition laser  26 . Laser ignition device  27  shown in  FIG. 4  includes ignition laser  26  having housing  38 , exit window  58  for laser beams  24  and focusing lens  52 . Housing  38  of ignition laser  26  is screwed by a thread  60  into an opening provided for this purpose in a cylinder head of internal combustion engine  10 . Alternative fastening options using a bayonet closure or a clamping claw, for example, are also possible. Ignition spot ZP of ignition laser  26  is in a cylindrical insert  62 , which is integrated into and/or installed in housing  38  of ignition laser  26  in  FIG. 2 . Insert  62  is thus an integral component of ignition laser  26 . 
     Insert  62  delimits an antechamber  63  of combustion chamber  14 . Insert  62  includes a cylindrical lateral area  64 , which is closed at the bottom by a bottom plate  66  in  FIG. 4 , bottom plate  66  having boreholes  68  running obliquely downward in an edge area. Bottom plate  66  forms a planar connection to the area of the cylinder head facing combustion chamber  14 . Combustion chamber  14  is formed partially by a frustoconical combustion chamber roof  69  in the cylinder head and a piston bottom  15  of piston  16  of internal combustion engine  10 . Piston  16  is guided in cylinder  12  and has piston rings  74  on its circumference. 
     When the fuel-air mixture in antechamber  63  is ignited, combustion begins there and ignition flares  76  of the fuel-air mixture, which is already burning, are blasted into the combustion chamber through boreholes  68 . These ignition flares ensure rapid and thorough combustion of the fuel-air mixture in combustion chamber  14  according to the present invention. Since it is possible to provide multiple boreholes  68 , including those running obliquely, the ignition flares may be aligned in such a way that the combustion paths, starting from ignition flares  76  up to the outermost corners of the combustion chamber, are as short as possible. For this reason, boreholes  68  are pivoted outward at an angle with respect to the longitudinal axis of cylinder  12 . 
     It is of course also possible to further optimize the combustion performance of the internal combustion engine via this angle and the number of boreholes  68 . Here again, it is important that at the point in time when ignition flares  76  enter combustion chamber  14  from antechamber  63 , combustion chamber  14  has a spheroidal shape. 
     Laser ignition device  27  functions as follows: Ignition laser  26  sends a laser pulse  24 , which is focused in insert  62  at ignition spot ZP close to bottom plate  66 . An ignitable air-fuel mixture, which is ignited at ignition spot ZP, is present in antechamber  63  as well as in combustion chamber  14 . Therefore, a stable flame core, capable of igniting the air-fuel mixture in combustion chamber  14 , is formed in antechamber  63 . 
     The flame core therefore escapes in the form of flares  76  through boreholes  68 . The direction of outflow and the shape of the flares  76  are determined by the design of boreholes  68  and are adapted to the shape of combustion chamber  14 . Flares  76  may be aligned in such a way that flares  76  are able to propagate in the largest possible area of combustion chamber  14 . In the determination of the desired direction and size of flares  76 , the flow conditions in combustion chamber  14  are also taken into account. The larger the end face  72  of piston  16 , the greater are also the so-called quench flows and swirl flows  80  in combustion chamber  14  when the piston is near top dead center TDC. In order to take this into account, suitably aligned boreholes  68  are required, depending on the design of combustion chamber  14  and trough  15  in bottom plate  66  and/or lateral area  64  of ignition laser  26  or of insert  62 . This then permits rapid combustion of the charge in combustion chamber  14 . 
     Since insert  62  may also be designed as a separate component, insert  62  may be selected or exchanged, depending on the design of combustion chamber  14  in internal combustion engine  10 . Insert  62  may thus be exchangeable, which makes it possible to adapt ignition laser  26  to different use conditions by replacing insert  62 .