Abstract:
In an internal combustion engine, fuel is directly injected into at least one combustion chamber at least during a compression stroke in such a way that a stratified mixture is present in the combustion chamber. This mixture is then externally ignited. The fuel is introduced during the compression stroke by at least one main injection and an ignition injection, the ignition injection taking place immediately before an ignition and producing at least essentially no torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method and a control device for operating an internal combustion engine. 
         [0003]    2. Description of Related Art 
         [0004]    In the article “Die neue Emissionsstrategie der Benzin-Direkteinspritzung” (“The new emissions strategy of direct gasoline injection”), MTZ 11/2003, pp. 916-923, the possibility of a “stratified start” is described. In contrast to a homogenous low-pressure start, in which injection takes place during an intake stroke, in a stratified start the injection does not take place until late during the compression stroke. This results in a stratified fuel-air mixture having a rich mixture cloud in the area of the spark plug. 
         [0005]    The advantage of such a stratified start in comparison with a conventional low-pressure start is that, above all at low ambient and engine temperatures, a lower enrichment is required, which improves the emission characteristic of the internal combustion engine. Such an enrichment is required because during startup a portion of the injected fuel adheres to the cold combustion chamber walls as a film, and thus does not fully take part in the actual combustion during the start phase. The fuel that remains uncombusted in this way is ejected without being combusted and results in undesirable hydrocarbon emissions. In order to compensate the fuel mass that does not participate in the combustion, the injected quantity must be correspondingly increased. Given a homogenous low-pressure start, enrichment factors of 2 to 3 over the stoichiometric quantity are not unusual. 
         [0006]    After the stratified start phase, which usually lasts approximately 1 to 2 seconds, i.e. after the very first injections and ignitions, a catalytic converter heating phase takes place during which a homogenous split injection is carried out. In the above-named article, concerning this it is proposed that a first injection take place during the intake stroke, producing a lean homogenous base mixture in the combustion chamber. There then follows a second injection during the subsequent compression stroke, which provides a rich mixture cloud in the area of the spark plug. The ignition takes place relatively late, shortly after top dead center, between the compression stroke and the following expansion stroke. In this way, a late center of the combustion is realized, with the result that a large part of the released energy is not converted into mechanical energy, but rather is released in the exhaust gas as heat. This enables a very rapid heating of a catalytic converter. The production of the charge stratification is in both cases preferably produced by a spray-guided method. 
         [0007]    From published German patent document DE 10 2004 017 989, it is known, in operation with a very lean charge mixture, to make a pre-injection during an intake stroke and to make a main injection during a compression stroke immediately before ignition. However, here the mixture preparation is not optimal. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to provide a method for operating an externally ignited internal combustion engine having direct fuel injection, with which the emissions and the fuel consumption can be reduced. 
         [0009]    In the method according to the present invention, a reliable ignition can be ensured even given the most various and lean charge stratifications in the combustion chamber. In this way, the operating range of the internal combustion engine is significantly expanded without fear of ignition misses. The actual torque-producing main injection can however in addition be set to that injection angle during the compression stroke at which a desired charge stratification and mixture preparation is best ensured. This simultaneously reduces emissions and consumption. The quantity of the main injection (which can also be introduced by a multiple injection) is significantly greater than the quantity of the ignition injection. The ignition injection itself is so small that it does not contribute, or at least does not contribute significantly, to the torque of the internal combustion engine, and also does not significantly increase fuel consumption. With this injection, a mixture is produced only in a very small area directly at the spark plug that is so rich that an “ignition torch” results in the combustion chamber that can reliably ignite the rest of the mixture, which in the normal case is lean. 
         [0010]    The ignition injection takes place immediately before the ignition, before top dead center between the compression and expansion stroke has been reached. “Immediately” means that the distance, expressed in degrees of crank angle (° KW), with a maximum of approximately 5° KW, is preferably only approximately 1° KW. If necessary, the ignition and the ignition injection can also take place simultaneously. The ignition angle is standardly in the area of a crank angle of approximately 15° KW before top dead center, i.e. still in the compression stroke. 
         [0011]    Through the method according to the present invention, in particular at low ambient temperatures the fuel enrichment required for a reliable stratified start can be further reduced, which has a favorable effect in particular on the hydrocarbon emissions of the start phase. It is even possible to realize a stratified start with a lean start lambda. For example, at an engine temperature of approximately 20° C. the internal combustion engine can be lean-started with a lambda value of 1 to 1.5; given an optimized combustion method a lambda value &gt;2 is even possible. The start lambda to be realized is advantageously a function of various parameters in addition to engine temperature, e.g. fuel quality, ambient temperature, etc. Through the method according to the present invention, during the start phase the combustion is also improved, i.e. stabilized, which also has a favorable effect on the level of emissions during the start phase. In addition, an internal combustion engine operated in this way has increased robustness relative to different fuel qualities. The start behavior when “poor fuel” is supplied is thus more reliable. 
         [0012]    This is realized in that during the start phase, i.e. during the very first injections or rotations of the crankshaft, during the compression stroke at least one main injection is made, preferably at a crank angle of approximately 80-30° KW before top dead center between the compression stroke and the expansion stroke. This injection injects fuel into the pre-compressed and thus pre-heated air in the combustion chamber. The ignition injection takes place subsequently. 
         [0013]    In addition, it is advantageous if, after the end of the start phase, a catalytic converter heating phase is carried out with a homogenous split injection and with an ignition angle that is situated in the expansion stroke, i.e. after top dead center between the compression stroke and the expansion stroke. This is because the transition between the start phase and the catalytic converter heating phase can be realized essentially only by a preferably continuous late setting of the ignition angle, which can be easily applied and can be carried out in such a way that the torque produced by the internal combustion engine is not influenced thereby, or at least not noticeably. 
         [0014]    In order to trigger the ignition injection, only a small amount of computing capacity is required if this injection, preferably the end thereof, is coupled at least indirectly to the crank angle of the ignition. 
         [0015]    Here, the interval between the ignition injection and the ignition can be at least at times rigid, which further conserves resources. However, a better emission and start behavior is achieved if the interval between the ignition injection and the ignition is a function, at least at times, of at least one state quantity (e.g. number of injections) of the internal combustion engine. 
         [0016]    It is preferable if in addition at least one injection takes place during an intake stroke, e.g., at a crank angle in the area of approximately 280° KW before the top dead center between the compression stroke and the expansion stroke. In this way, preferably during the start phase a method is realized that corresponds to a combination of a high-pressure stratified start method and a homogenous split injection method. In this way, the advantages of a spray-guided homogenous split injection can be exploited already during the start phase, i.e. without the otherwise standard late injection angle required for the heating of a catalytic converter. Such a method is distinguished by a particularly robust combustion with simultaneous low emissions. Such a method can also help avoid engine knock. 
         [0017]    The emissions level can be further reduced if the parameters that characterize the start phase are not rigid, but rather are a function of environmental state quantities and/or operating quantities of the internal combustion engine. In particular, the partitioning of the injected quantity and/or the crank angle of the injections can be a function of at least one environmental state quantity, in particular of an ambient temperature and/or of an intake air temperature, and/or of at least one operating quantity of the internal combustion engine, in particular of a relative charge and/or of a temperature of a component of the internal combustion engine, and/or of an ignition angle. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0018]      FIG. 1  shows a schematic representation of an internal combustion engine having a plurality of cylinders, each having a combustion chamber. 
           [0019]      FIG. 2  shows a partial section through an area of a cylinder of the internal combustion engine of  FIG. 1 . 
           [0020]      FIG. 3  shows a diagram in which fuel injections and ignitions of the individual cylinders of  FIG. 1  are plotted over a crank angle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    In  FIG. 1 , an internal combustion engine is designated  10  as a whole. It is used to drive a motor vehicle (not shown), and comprises four essentially identical cylinders  12   a  to  12   d  having corresponding combustion chambers  14   a  to  14   d.  In  FIG. 2 , one cylinder  12  is shown in more detail as an example (if a reference character does not include the index a-d, this means, here and in the following, that the statements hold correspondingly for all similar components a-d). 
         [0022]    Combustion air enters combustion chambers  14   a  to  14   d  via an intake pipe  16  and intake valves  18   a  to  18   d.  Fuel is injected into combustion chambers  14   a  to  14   d  by a respective injector  20   a  to  20   d.  Injectors  20   a  to  20   d  are connected to a rail (not shown) in which fuel is stored under high pressure. The fuel is predominantly gasoline, and the internal combustion engine shown in  FIG. 1  is therefore an engine that operates with gasoline direct injection (GDi). However, it is also possible to use a gaseous fuel, biofuel, or a synthetic fuel. 
         [0023]    The fuel/air mixture in combustion chambers  14   a  to  14   d  is ignited in each case by a spark plug  22   a  to  22   d.  The hot combustion exhaust gases are conducted away from combustion chambers  14   a  to  14   d  via outlet valves  24   a  to  24   d,  into an exhaust pipe  26 . This pipe leads to a catalytic converter  28  that converts pollutants in the exhaust gas, thus cleaning the exhaust gas. 
         [0024]    The operation of internal combustion engine  10  is regulated by a control and regulating device  30  that obtains signals from various sensors and actuators (not shown in  FIG. 1 ) via which certain state quantities ZS of internal combustion engine  10  are acquired. These include for example a gas pedal sensor with which a user of internal combustion engine  10  can express a desired torque. 
         [0025]    These sensors also include temperature sensors that acquire for example the temperature of a cylinder head and/or of a coolant of internal combustion engine  10  or of intake air flowing through intake pipe  16 , an HFM (air mass) sensor that acquires the air mass flowing into combustion chambers  14   a  to  14   d  via intake pipe  16 , and lambda sensors that are situated in the area of catalytic converter  28  and that acquire the ratio of the fuel/air mixture in combustion chambers  14   a  to  14   d.  Such a sensor is shown as an example in  FIG. 1 , designated  31 . Control and regulator device  30  controls for example injectors  20 , spark plugs  22 , and a throttle valve (not shown in  FIG. 1 ) in intake pipe  16 . 
         [0026]    As can be seen in particular in  FIG. 2 , internal combustion engine  10  realizes what is known as a “spray-guided” combustion method. In such a method, injector  20  is preferably centrally situated. Electrodes  32  of spark plug  22  are standardly situated relatively close to injector  20 . A piston base  34  of a piston  36  has a design that supports the charge stratification. 
         [0027]    A stratified start method is used to start internal combustion engine  10  in the present case. This method is now explained with reference in particular to  FIG. 3 . In  FIG. 3 , the individual strokes of a work cycle for each cylinder  12   a  to  12   d  are shown over the crank angle (° KW) of a crankshaft (not shown in  FIGS. 1 and 2 ) of internal combustion engine  10 . The top dead centers, situated in each case between a compression stroke and an expansion stroke, of a cylinder  12  are designated ZOT 12a  to ZOT 12d . As an example, cylinder  12   b  will now be used to explain the injection strategy realized during the start phase of internal combustion engine  10 , i.e. during the very first injections and combustions or revolutions of the crankshaft of internal combustion engine  10 . However, the same holds in principle for the other cylinders  12   a,    12   c,  and  12   d  as well. 
         [0028]    During an intake stroke  38  of cylinder  12   b,  injector  20  emits a pre-injection  40   b  into combustion chamber  14   b  in the area of a crank angle of approximately 300-260° KW, preferably approximately 280° KW before top dead center ZOT 12b . This pre-injection  40   b  produces a base mixture in combustion chamber  14   b  that is homogenous as a whole, i.e. is distributed uniformly in combustion chamber  12   b,  and that is very lean, designated by reference character  42  in  FIG. 2 . In a subsequent compression stroke  44   b,  in the area of a crank angle of approximately 80-30° KW, preferably approximately 50° KW, before top dead center ZOT 12b  a first injection  46   b , called the main injection, is emitted into combustion chamber  14   b.  In this way, a mixture cloud is produced in the center of combustion chamber  14   b  that is richer than the homogenous lean base mixture  42 ; the cloud is designated  48  in  FIG. 2 . Towards the end of compression stroke  44   b,  injector  20  makes a second injection  50   b,  called the ignition injection, into combustion chamber  14   b.  In this way, in a limited local area around electrodes  32  of spark plug  22 , a comparatively rich and small local mixture cloud is produced, designated  52  in  FIG. 2 . 
         [0029]    This ignition injection  50  takes place during the compression stroke and immediately (maximum approximately 5° KW, preferably only approximately 1° KW) before a subsequent ignition (reference character  54   b  in  FIG. 3 ), carried out in the area of a crank angle of approximately 20-10° KW, preferably approximately 15° KW, before top dead center ZOT 12b . If warranted, the ignition injection and the ignition can also occur simultaneously. The crank angle of the end of ignition injection  50  is temporally coupled to the crank angle of ignition  54  (“ignition angle”). This is represented in  FIG. 3  by a double arrow designated  56 . This coupling can be rigid or variable, the latter being a function of current state quantities ZS of internal combustion engine  10 . The exact interval between ignition injection  50   b  and ignition  54   b  can be prespecified for example by a characteristic field. 
         [0030]    The partition of the injected quantity between pre-injection  40   b,  main injection  46   b,  and ignition injection  50   b,  as well as the crank angle of pre-injection  40   b  and main injection  46   b,  are determined by control and regulating device  30  as a function of an engine temperature, an ambient temperature, an engine rotational speed, and an intake air temperature, as well as of a relative charge, a temperature of the cylinder head of internal combustion engine  10 , and the crank angle of ignition  54   b  (ignition angle). 
         [0031]    In the example embodiment according to  FIG. 3 , during the start phase a pre-injection  40  is made during intake stroke  38 . In an exemplary embodiment that is not shown, this pre-injection does not occur. In a corresponding method, therefore, only main injection  46   b  and ignition injection  50   b  take place during compression stroke  44   b.    
         [0032]    The injection strategy shown in  FIG. 3  is applied during the start phase, i.e. during the very first rotations of the crankshaft of internal combustion engine  10 . Subsequently, a transition is made to a homogenous split injection in which, for the example of cylinder  12   b  during expansion stroke  58   b , the ignition angle is situated after top dead center ZOT 12b . This late adjustment of the ignition angle moves the combustion center to a later time, resulting in an increase in the exhaust gas temperature and therefore an improved heating of catalytic converter  28 . Detailed explanations of homogenous split injection can be found in the article “Die neue Emissionsstrategie der Benzin-Direkteinspritzung” (“The new emissions strategy of direct gasoline injection”), MTZ 11/2003, pp. 916-923, whose disclosure is hereby expressly incorporated into the subject matter of the present specification.