Abstract:
A method for operating an internal combustion engine having a motor and an exhaust gas aftertreatment system having an exhaust gas aftertreatment component, wherein exhaust gas formed in the motor during combustion of fuel is guided via the exhaust gas aftertreatment system for cleaning, includes: determining an exhaust gas actual value that depends upon an actual value of a nitrogen dioxide fraction in the exhaust gas upstream of the exhaust gas aftertreatment component; and changing at least one operating parameter for the motor such that the actual value of the nitrogen dioxide fraction is brought closer to a corresponding reference value for the nitrogen dioxide fraction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of application No. PCT/EP2014/066196, filed on 28 Jul. 2014, which claims priority to the German Application No. 10 2013 012 566.9, filed 29 Jul. 2013, the content of both incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to a method for the operation of an internal combustion engine. 
     2. Description of the Related Art 
     In addition to a motor, known internal combustion engines also have an exhaust gas aftertreatment system so that exhaust gas formed in the motor of the internal combustion engine during the combustion of fuel can be cleaned in the exhaust gas aftertreatment system. An efficient exhaust gas aftertreatment is required in order to meet increasingly strict exhaust gas limits. 
     In addition to solid particles, nitrogen oxides (NOx) in the exhaust gas must comply with increasingly strict limits. The nitrogen oxides in the exhaust gas are reduced using catalysts, for example, using selective catalytic reduction (SCR) catalysts. These SCR catalysts utilize ammonia as reductant for converting nitrogen oxides. The ammonia can be generated in an ammonia generator and introduced into the exhaust gas stream. In contrast, it is also possible to introduce an ammonia precursor substance, e.g., an aqueous urea solution, into the exhaust gas stream, which is then converted in the exhaust gas stream to ammonia, carbon dioxide and water vapor. The conversion of the ammonia precursor substance to ammonia in the exhaust gas is typically carried out using a hydrolysis catalyst. 
     The conversion of nitrogen oxides in the SCR catalytic converter is carried out when nitrogen monoxide exclusively is present in the exhaust gas as expressed by the following equation:
 
4NO+4NH 3 +O 2 →4N 2 +6H 2 O.
 
     The conversion of nitrogen monoxide in a SCR catalytic converter according to the equation given above takes place relatively gradually. Therefore, in order to accelerate the conversion of nitrogen oxides in the exhaust gas it is already known from practice to position an NO oxidation catalytic converter, particularly a platinum-containing NO oxidation catalytic converter, upstream of the SCR catalytic converter to convert nitrogen monoxide into nitrogen dioxide upstream of the SCR catalytic converter. In this case, when nitrogen dioxide is also present in the exhaust gas in addition to nitrogen monoxide, the conversion of the nitrogen oxides in the catalytic converter takes place according to the following equation:
 
NO+2NH 3 +NO 2 →2N 2 +3H 2 O.
 
     The conversion of nitrogen oxides according to the formula cited above in the presence of nitrogen monoxide and nitrogen dioxide in the SCR catalytic converter takes place faster than the pure conversion of nitrogen monoxide in the SCR catalytic converter. 
     Accordingly, the rapidity of the conversion of nitrogen oxides contained in the exhaust gas in the SCR catalytic converter depends on the proportion of nitrogen dioxide in the exhaust gas. However, the use of a separate NO oxidation catalytic converter for converting the nitrogen monoxide into nitrogen dioxide upstream of a SCR catalytic converter is disadvantageous because this increases expenditure on apparatus and, therefore, the costs for an internal combustion engine. 
     SUMMARY OF THE INVENTION 
     Based on the foregoing disadvantages of the prior art, an object to of the present invention is to provide novel method for the operation of an internal combustion engine. 
     According to one aspect of the invention, an exhaust gas actual value is determined depending on the actual value of a nitrogen dioxide fraction in the exhaust gas upstream of an exhaust gas aftertreatment component of the exhaust gas aftertreatment system, wherein at least one operating parameter for the motor is changed such that the actual value of the nitrogen dioxide fraction is brought closer to a corresponding reference value of the nitrogen dioxide fraction so that the respective exhaust gas aftertreatment component is operated in an optimized manner. 
     The present invention proposes for the first time to adjust the nitrogen dioxide fraction in the exhaust gas upstream of an exhaust gas aftertreatment component of an exhaust gas aftertreatment system in a defined manner by changing at least one operating parameter for the motor of an internal combustion engine so that the exhaust gas aftertreatment component can be operated in an optimized manner. This makes it possible either to use smaller NO oxidation catalytic converters or to dispense with the use of NO oxidation catalytic converters completely. 
     The reference value for the nitrogen dioxide fraction is preferably selected as a function of load point. In particular, the reference value for the nitrogen dioxide fraction is determined depending on at least one operating parameter of the motor and/or depending on at least one operating parameter of the exhaust gas aftertreatment system. The use of a load point-dependent or operating point-dependent reference value for the nitrogen dioxide fraction in the exhaust gas is particularly preferred because in this way an optimal operation of the motor on the one hand and an optimal operation of the exhaust gas aftertreatment system of the internal combustion engine on the other hand can be ensured for all load points or operating points of the internal combustion engine. 
     According to an advantageous further development, a lambda value and/or an ignition time and/or valve control times and/or a motor compression and/or an exhaust gas proportion in the motor combustion chamber are/is changed in this way as operating parameter(s) for the motor. The nitrogen dioxide fraction in the exhaust gas can be adjusted in a simple and reliable manner via at least one of the above-mentioned operating parameters for the motor. 
     According to an advantageous further development, a NOx actual value is measured as an exhaust gas actual value downstream of the exhaust gas aftertreatment component of the exhaust gas aftertreatment system by a NOx sensor, the actual value of the nitrogen dioxide fraction in the exhaust gas is determined upstream of the exhaust gas aftertreatment component depending on this exhaust gas actual value, this actual value of the nitrogen dioxide fraction is compared with the reference value of the nitrogen dioxide fraction, and at least one operating parameter for the motor is changed depending on this comparison such that the actual value of the nitrogen dioxide fraction is brought closer to the reference value of the nitrogen dioxide fraction. This arrangement is particularly preferred because the NOx actual value downstream of the exhaust gas aftertreatment component can be measured in a simple manner by a NOx sensor. Based on this NOx actual value that can be detected by measuring techniques, the actual value of the nitrogen dioxide fraction in the exhaust gas can be deduced so that at least one operating parameter of the motor can then be changed depending on a comparison between the actual value of the nitrogen dioxide fraction and the reference value of the nitrogen dioxide fraction such that the actual value of the nitrogen dioxide fraction approaches the reference value. 
     Operating parameters for the motor are preferably changed such that raw NOx emissions of the motor are reduced by a maximum 15%. In this way, it is possible to operate the motor with good efficiency and to prevent an increase in fuel consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred further developments of the invention are indicated in the following description. Embodiment examples of the invention are described more fully with reference to the drawings without the invention being limited to these embodiment examples. In the drawings: 
         FIG. 1  is a schematic view of an internal combustion engine; and 
         FIG. 2  is a diagram illustrating the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is directed to a method for operating an internal combustion engine. 
     In a highly schematic manner,  FIG. 1  shows an internal combustion engine  10  comprising a motor  11  with a plurality of cylinders  12  and an exhaust gas aftertreatment system  13  with at least one exhaust gas aftertreatment component  14 . Exhaust gas formed during the combustion of fuel in the cylinders  12  of the motor  11  of the internal combustion engine  10  can be guided via the exhaust gas aftertreatment system  13  to clean the exhaust gas in the exhaust gas aftertreatment system  13 . Positioned downstream of the exhaust gas aftertreatment system  13  according to  FIG. 1  there is a sensor  15 , which can be a NOx sensor in order to measure NOx emissions in the exhaust gas downstream of the exhaust gas aftertreatment system  13 . The exhaust gas aftertreatment component  14  of the exhaust gas aftertreatment system  13  can be a SCR catalytic converter, a particle filter or a NOx storage catalytic converter. 
     For operating an internal combustion engine  10  of this type, an exhaust gas actual value is determined within the meaning of the invention, which exhaust gas actual value depends on the actual value of a nitrogen dioxide fraction in the exhaust gas upstream of the exhaust gas aftertreatment component  14  of the exhaust gas aftertreatment system  13 . Depending on this exhaust gas actual value, at least one operating parameter for the motor  11  is changed such that the actual value of the nitrogen dioxide fraction is brought closer to a corresponding reference value for the nitrogen dioxide fraction so that the respective exhaust gas aftertreatment component  14  of the exhaust gas aftertreatment system  13  can be operated in an optimized manner. 
     Accordingly, it lies within the scope of the present invention to selectively influence the nitrogen dioxide fraction in the exhaust gas by changing at least one operating parameter of the motor  11  so that an exhaust gas aftertreatment component  14  of an exhaust gas aftertreatment system  13  located downstream of the motor  11  can be optimally operated. 
     The invention is used particularly in internal combustion engines  10  whose motor  11  is constructed as an Otto gas motor in which gaseous fuel is burned. Natural gas, which contains methane as constituent, is typically burned as gaseous fuel in Otto gas motors of this type. 
     The reference value for the nitrogen dioxide fraction in the exhaust gas is selected depending on the load point. Accordingly, it is possible to determine the reference value for the nitrogen dioxide fraction in the exhaust gas as a function of at least one operating parameter of the motor  11  and/or as a function of at least one operating parameter of the exhaust gas aftertreatment system  13 . Thus it is possible to determine the reference value for the nitrogen dioxide fraction in the exhaust gas depending on one or more exhaust gas temperatures and depending on the efficiency of the exhaust gas aftertreatment system  13  and depending on the efficiency of the motor  11 . 
     Preferably, a lambda value and/or an ignition time and/or valve control times and/or a motor compression and/or an exhaust gas proportion in the motor combustion chamber are/is changed in this way as operating parameter(s) for the motor. 
     When the lambda value is reduced, the nitrogen dioxide fraction in the exhaust gas tends to increase. 
     Further, by shifting the ignition time in direction of earlier ignition times and/or by increasing the proportion of exhaust gas in the motor combustion chamber, the nitrogen dioxide fraction in the exhaust gas tends to increase. 
     Further, it is possible to increase the nitrogen dioxide fraction in the exhaust gas by delayed opening of inlet valves of the cylinders  12  and by delayed closing of outlet valves of the cylinders  12 . 
     By increasing the motor compression, the nitrogen dioxide fraction in the exhaust gas tends to decrease. 
     The relationships mentioned above for influencing the nitrogen dioxide fraction in the exhaust gas are described by way of example for some operating parameters referring to  FIG. 2 . In  FIG. 2 , the percentage of nitrogen dioxide NO 2  in the nitrogen oxides NOx of the exhaust gas is plotted over the lambda value for a gas Otto motor, namely, depending on the load point of the motor  11  and depending on ignition times of the motor  11 . 
     Characteristic lines  16  and  17  relate to characteristic lines for full load operation of the motor  11 . In characteristic line  16 , ignition times are late-shifted, and in characteristic line  17  ignition times are early-shifted. 
     Characteristic lines  18  and  19  relate to characteristic lines for partial load operation of the motor  11 . In characteristic line  18 , ignition times are late-shifted, and in characteristic line  19  ignition times are early-shifted. 
     In a particularly preferred variant of the invention, an NOx actual value is measured as an exhaust gas actual value by the NOx sensor  15  shown in  FIG. 1  downstream of the exhaust gas aftertreatment component  14  of the exhaust gas aftertreatment system  13  that is to be operated in an optimized manner. Depending on this exhaust gas actual value, the actual value of the nitrogen dioxide fraction in the exhaust gas upstream of the exhaust gas aftertreatment component  14  is determined. This actual value of the nitrogen dioxide fraction is compared with a reference value for the nitrogen dioxide fraction. Depending on this comparison, at least one operating parameter for the motor  11  is changed such that the actual value of the nitrogen dioxide fraction in the exhaust gas upstream of the exhaust gas aftertreatment component  14  is brought closer to the reference value of the nitrogen dioxide fraction. 
     As stated earlier, the exhaust gas aftertreatment component  14  that is to be operated in an optimized manner through influencing the nitrogen dioxide fraction in the exhaust gas according to the invention can be a SCR catalytic converter. Alternatively, this exhaust gas aftertreatment component  14  can also be a particle filter or a NOx storage catalytic converter. 
     As stated earlier, the reference value for the nitrogen dioxide fraction in the exhaust gas is selected depending on the operating point. If the exhaust gas aftertreatment component  14  of the exhaust gas aftertreatment system  13  that is to be operated in an optimized manner as a result of the adjustment of the actual value of the nitrogen dioxide fraction is a SCR catalytic converter, then 50% is preferably selected as the reference value for the nitrogen dioxide fraction in the exhaust gas. However, it is also possible to select a reference value for the nitrogen dioxide fraction in the exhaust gas of less than 50%, particularly at high exhaust gas temperatures. 
     In particular, the reference value for the nitrogen dioxide fraction in the exhaust gas is selected such that the raw NOx emissions of the motor  11  are not reduced by more than 15% due to the operating parameter for the motor  11  that has been changed depending on this reference value. In this way, increased consumption can be prevented in the motor  11 . 
     Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  internal combustion engine 
           11  motor 
           12  cylinder 
           13  exhaust gas aftertreatment system 
           14  exhaust gas aftertreatment component 
           15  sensor 
           16  characteristic line 
           17  characteristic line 
           18  characteristic line 
           19  characteristic line