Abstract:
A method for the formation of a combustible fuel/air mixture in a combustion chamber of a direct-injection internal combustion engine with an injection nozzle, which has a closure body includes the steps of injecting at least two partial quantities of fuel into the combustion chamber and moving the closure body of the injection nozzle into its closed position after the injection of each of the partial quantities of fuel.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for the formation of a combustible fuel/air mixture in the combustion chamber of a direct-injection internal combustion engine with an injection nozzle, which has a closure body and via which the fuel is introduced in at least two partial quantities into the combustion chamber. 
     BACKGROUND INFORMATION 
     A method for the formation of a combustible fuel/air mixture is described, for example, in German Published Patent Application No. 196 42 653. According to this method, a combustible fuel/air mixture can be formed in the cylinders of direct-injection internal combustion engines by a process in which, after the exposure of a nozzle opening by raising a valve member from a valve seat that includes the nozzle opening, fuel is injected by an injector into each combustion chamber delimited by a piston. In order to allow internal mixture formation that is optimized for consumption and emissions at every operating point of the entire engine map under all operating conditions of the internal combustion engine, especially in stratified-charge mode, provision is made for the opening stroke of the valve member and the injection time to be variable. In this case, the geometry of the jet may be altered by coking of the injection valve, and increased emissions of soot due to poor mixture formation in stratified lean-mixture operation and a reduction in the reliability of ignition due to the changing quality of the mixture at the spark plug and due to coking in stratified lean-mixture operation are possible. In addition, there are increased proportions of unburnt fuel due to dilution of zones of the mixture in stratified lean-mixture operation. In addition, there is wetting of the spark plug and hence failure thereof due to soot deposition, increased emissions of pollutants owing to incomplete combustion of the mixture at the spark plug due to random scatter of the injection jet and collapse of the injection jet under the back pressure in the combustion chamber in stratified lean-mixture operation, i.e., an increased likelihood of misfires. 
     It is therefore an object of the present invention to provide a method that ensures reliability of ignition and avoids coking of the spark plug at all operating points. 
     SUMMARY 
     The above and other beneficial objects of the present invention are achieved by providing a method in which the closure body of the injection nozzle can be moved into its closed position after the injection of each partial quantity. This ensures that the fuel input or the two fuel pulses are injected in a defined manner at the respective instant and thus make a significant contribution to optimum mixture formation. Closing the nozzle opening without reducing the fuel pressure applied significantly improves the respective fuel pulse. 
     It may be advantageous for this purpose that the first partial quantity is greater than the second partial quantity, 70% to 99% or 80% to 99% of the total quantity of fuel being introduced first, and the remainder being introduced after 0.05 ms to 0.4 ms or 1° of crank angle to 5° of crank angle and the injection cycle being ended between 50° of crank angle and 5° of crank angle before TDC (Top Dead Center). The main quantity of fuel initially introduced is prepared in an optimum manner by the extended mixture formation time before ignition and by the second pulse including the remaining quantity of fuel, and an undiluted combustible fuel/air mixture is formed. 
     According to one example embodiment of the present invention, the fuel may be introduced as a fuel cone and may produce a toroidal vortex at the end of the cone envelope in the region of a piston. Thus, an undiluted combustible fuel/air mixture that ensures initiation of ignition may be formed in the region of the spark plug. Inside and outside the fuel cone, the toroidal vortex carries the fuel introduced into the other regions of the combustion chamber and particularly into the region of the spark plug. 
     The nozzle opening of the injection nozzle may be disposed at a distance (A) of 1 mm to 8 mm from a combustion-chamber roof and at a distance (B) of 10 mm to 15 mm from a spark plug, the injection pressure of the injection nozzle varying between 100 bar and 300 bar or between 150 bar and 250 bar. The fuel jet emerging from the injection nozzle may be formed approximately conically and may include a constant jet angle α that is independent of the position or location of the closure element. The form of fuel jet required for optimum mixture formation, i.e., a toroidal vortex, is thereby achieved. The position of the spark plug and the position of the fuel jet may define the formation of the optimum mixture. 
     According to an example embodiment of the present invention, the jet angle α may be 10% to 50% or 20% to 40% smaller than the angle β of the combustion-chamber roof. It is thus possible to prevent wetting of the combustion-chamber roof and to prevent the toroidal vortex from striking the combustion-chamber roof. 
     The fuel jet may include at least one or one inner and one outer toroidal vortex at the end of its cone envelope in the region of the piston. Optimum mixture formation is thus achieved throughout the combustion chamber. 
     According to the present invention, the closure element may be mounted in a coaxially rotatable manner and may be moved axially by between 0 μm and 80 μm or 10 μm and 50 μm into the combustion chamber at any time via the piezoelectric element. The rotatable closure body thus contributes a circumferential velocity component to the fuel jet or fuel cone, thus improving mixture formation and fuel input. 
     The closure body may include a conical sealing surface with an angle δ of between 70° and 90° or between 70° and 85°, and a housing of the injection nozzle may include a curved, parabolic or conical outlet cross section, thus forming the sealing seat or the sealing surface of the injection nozzle. Thus, the gap or nozzle opening tapers continuously towards the outlet with a curved or parabolic outlet cross section, and the fuel jet is thus accelerated continuously up to its emergence. In this arrangement, the fuel jet has a jet angle α that is independent of the position of the closure element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional schematic view of a cylinder with a piston, an injection nozzle and a spark plug. 
     FIG. 2 is a cross-sectional schematic view of a cylinder with a piston, an injection nozzle, a spark plug and a toroidal vortex. 
     FIG. 3 is a cross-sectional schematic view of the injection nozzle of the injection valve. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 each illustrate a cylinder  12  of a direct-injection internal combustion engine, in which a piston  9  delimits a combustion chamber  2  together with a cylinder head  13  that closes off the cylinder  12 . A fuel injection nozzle  1  is arranged coaxially at a distance of, for example, 0 mm to 10 mm from a cylinder axis  15  in the cylinder head  13 . In this area, the cylinder head  13  or a combustion-chamber roof  8  has a conical or a roof-shaped design, the injection nozzle  1  being arranged at the highest point, i.e., in the region of the actual tip of the cone or roof. 
     A control unit (not shown) determines the instant of exposure of a nozzle opening  4  of the injection nozzle  1  specifically for each operating point of the internal combustion engine, the instant being associated with the position of a crankshaft or of the respective piston  9 . Through this opening, the fuel enters the combustion chamber  2  as a fuel cone  7  in various phases of an injection cycle. 
     In the combustion chamber  2 , a combustible fuel/air mixture is formed with the fuel injected and the charge air fed into the cylinder  12  through the inlet duct (not shown). 
     In stratified-charge mode, fuel injection occurs during the compression stroke. Starting with the fuel cone  7  injected, the injection process results in the formation of a cloud of mixture in the combustion chamber  2 . In this process, the fuel cone  7  forms an angle α of between, for example, 70° and 90°, this angle always being somewhat smaller than the angle β of the combustion-chamber roof  8 . A spark plug  3  is positioned in the combustion chamber  2  so that its center line is approximately perpendicular to the fuel cone envelope  7 , i.e., the deviation is from, for example, 0° to 30°, an earth electrode  3 ′ of the spark plug  3  being essentially unwetted by the fuel cone envelope  7 . At an injection pressure of between, for example, 100 bar and 300 bar, so-called toroidal vortices  11 ,  11 ′ may be formed in the region of the piston  9 , starting from the generatrix of the fuel jet (see FIG.  2 ). The toroidal vortex  11  may be formed by the fuel cone  7  rolling up from the generatrix of the fuel cone  7  before the fuel cone  7  strikes the piston  9 . A toroidal vortex  11  is formed on the outer side of the cone, extending beyond the circumference of the cone towards the combustion-chamber roof  8 . With the toroidal vortex  11  formed or in the region of the toroidal vortex  11 , the fuel is mixed with the air in the combustion chamber. Since the outer toroidal vortex  11  forms above the fuel cone  7 , a combustible undiluted fuel/air mixture forms in the region of the spark plug  3  and at its electrode  3 ′. A second toroidal vortex  11 ′ is formed within the fuel cone  7 . Thus, a combustible undiluted fuel/air mixture is produced in the region of the injection nozzle  1 . 
     FIG. 3 illustrates the injection nozzle  1  having a closure element  6  and a closure body  10 . The injection nozzle  1  further includes a cylindrical housing  17 , formed around a is longitudinal axis, and a fuel chamber  18  disposed between the housing wall  17  and the closure element  6 . 
     At its upper end, the closure element  6  is coupled mechanically to an actuator (not shown) and to a return spring. The actuator may be, for example, a piezoelectric element that expands when supplied with an electric voltage and hence provides the stroke of the closure element  6 . The pressure prevailing in the fuel chamber  18  exerts a restoring force in addition to that of the spring force on an upper end face (not shown) of the closure element  6 . This arrangement ensures the leak-tightness of the injection nozzle  1  at all times. 
     The injection nozzle  1  includes the nozzle opening  4  and the closure body  10 . The nozzle opening  4  is formed, at least in part, by a curved part  25  on the lower end of the housing wall  17 . The curved part  25  of the housing wall  17  has a curved or parabolic cross section on the inside, i.e., at the end of the fuel chamber  18 . 
     The closure body  10  is designed as a double cone, i.e., the closure body  10  includes one cone  26  or conical outer surface facing downwardly toward the combustion chamber and one cone or conical outer surface facing inwardly toward the fuel chamber. This inner part forms a conical sealing surface  24  and, with the inner curved or parabolic or conical part  25  of the housing  17 , forms the sealing seat  14  or nozzle opening  4 . In this arrangement, the generatrix of the cone  24  forms a tangent to the inner, curved part  25  of the nozzle opening  4 . Both sealing surfaces  24 ,  25  may extend in parallel toward an outer side or end  27  of the injection nozzle  1  and may form a right angle with respect to the outer generatrix  25  of the closure body  10 . The end  27  of the housing wall  17  in this region is formed in a corresponding manner as a frustoconical surface and, when the injection nozzle  1  is closed, forms a smooth transition or common conical surface with the generatrix  26 . In the closed state, the cone envelope  26  is thus extended by the lower part of the housing  17  or end  27 . The cross section of the fuel chamber  18  thus tapers continuously towards the sealing seat  14  and, at that point, is substantially zero in the closed state. 
     When the closure element  6  is displaced axially, the sealing surface  24  of the closure body  10  rises from the parabolic part  25  of the housing  17  into the combustion chamber  2  and thus opens the nozzle opening  4  for the fuel. The opening stroke of the closure element  6  and the duration of opening of the nozzle opening  4  determine the fuel flow rate through the nozzle opening  4  and hence the overall quantity or partial quantity of fuel supplied.