Abstract:
A method and a device for use in the aftertreatment of exhaust gas of an internal combustion engine, in particular of a diesel engine, are described, having a device for providing oxidizing agents and an exhaust-gas aftertreatment unit, a temperature sensor ( 40 ) being provided to measure the temperature of the exhaust gas before it enters the exhaust-gas aftertreatment unit ( 10 ) and the device ( 20 ) for providing oxidizing agents being set up to vary the chemical composition of the oxidizing agents as a function of the temperature. This method of exhaust-gas aftertreatment ensures energy-efficient preparation of the exhaust gas.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed to a method and a device for use in the aftertreatment of exhaust gas of an internal combustion engine.  
         BACKGROUND INFORMATION  
         [0002]    German Published Patent Application No.198 26 831 describes a device, in which a plasma reactor is positioned before a particle filter in the exhaust gas system, but the plasma reactor is always operated in the same way, independently of the operating state of the engine.  
         SUMMARY OF THE INVENTION  
         [0003]    The method according to the present invention and the device according to the present invention have the advantage over the related art of ensuring energy-efficient aftertreatment of exhaust gas.  
           [0004]    In particular, it is advantageous to perform soot combustion using different oxidizing agents depending on the exhaust gas temperature and to use ozone, which is energetically costly to provide but is also effective at low temperatures, only at these low temperatures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 shows a device for exhaust-gas aftertreatment.  
         [0006]    [0006]FIG. 2 shows a diagram according to the present invention.  
         [0007]    [0007]FIG. 3 shows a method according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0008]    [0008]FIG. 1 shows an exhaust gas line  30 , in which an exhaust-gas aftertreatment unit  10 , implemented as a particle filter, is positioned. Before exhaust gas  11 , which comes from the diesel engine, may reach the particle filter, it must flow through an oxidation reactor  20 , implemented as a plasma reactor. Downstream from the particle filter, purified exhaust gas  100  leaves the exhaust gas system (for example, after passing through a muffler (not shown)) and reaches the open air. Pressure sensors  42  and/or  44 , which are connected electrically, for example, to a control unit  50  to transmit signals and which project into the exhaust gas line, are mounted directly upstream and downstream from the particle filter. A temperature sensor  40 , which is also connected to the control unit, projects into exhaust gas line  30  upstream from the oxidation reactor. Finally, the plasma reactor is also electrically connected to the control unit. The power supply for the measurement sensors and/or the oxidation reactor is not illustrated in greater detail. The oxidation reactor produces a non-thermal plasma in exhaust gas  11  flowing into it, using a dielectrically hindered electrical discharge, for example, between two electrodes whose power supply may be controlled via control unit  50 .  
         [0009]    The load state of the particle filter may be determined by the control unit, for example, by analyzing the pressure signals of the two pressure sensors, i.e., by determining a differential pressure. When the load state of the particle filter has exceeded a specific value, which is stored in the control unit, the control unit causes the application of a suitable voltage, for example, a high-frequency AC electrical voltage, to the electrodes of the plasma reactor in order to obtain oxidizing agents from exhaust gas components, which results in burn-off of the soot in the particle filter and therefore in the regeneration of the particle filter. If the load state falls below a second value, which is less than the first value, the plasma generator is switched off again. The electrical energy supplied in the switched-on state of the plasma reactor results in the production of high-energy electrons and UV light, which favors the development of radicals. Depending on the electrical power supplied, nitrogen monoxide molecules are primarily oxidized to nitrogen dioxide or (at higher electrical power) the residual oxygen remaining in the exhaust gas is also oxidized to ozone. The control unit is implemented in a way such that, if regeneration is required, the electrical power is set as a function of the exhaust gas temperature. At exhaust gas temperatures below a temperature threshold value of 250 degrees Celsius, preferably below 200 degrees Celsius, a higher electrical power is selected in order to produce additional ozone in greater quantities, while at exhaust gas temperatures above 200 degrees Celsius, preferably above 250 degrees Celsius, the regeneration runs sufficiently rapidly using nitrogen dioxide as an oxidizing agent, so that a lower power level, at which mainly nitrogen dioxide and hardly any ozone is produced, is sufficient for the plasma reactor. Interpolation between the two power levels may be performed in the exhaust gas temperature range of between 150 and 250 degrees Celsius. For ozone generation, an amount of energy of 10 to 15 watt-hours per gram of ozone is typically necessary, so that for soot oxidation using ozone, energy consumption of between 1 and 300 joules per liter of exhaust gas, in particular 10 to 50 joules per liter of exhaust gas, is typically needed. The wide range is explained by the strong dependence on the type and layout of the internal combustion engine. In addition, there is the conversion of nitrogen monoxide to nitrogen dioxide, which runs simultaneously in the low-temperature range of the exhaust gas, having an energy requirement of 2 to 200 joules, in particular 5 to 50 joules, per liter of exhaust gas. The oxidation of soot using nitrogen dioxide begins above 200 degrees Celsius, in particular above 250 degrees Celsius, as already explained, so that only the latter amount of energy is required. Therefore, the consumption may be significantly reduced, typically by at least 30 percent, through the changeover.  
         [0010]    In an alternative embodiment, temperature sensor  40  may also be positioned between the oxidation reactor and the particle filter. Alternatively, other measurement methods or a model-supported temperature prediction via an engine characteristics map stored in the controller are also possible. In a further alternative embodiment, the plasma generator may also be operated continuously. The exhaust gas counterpressure then does not have to be determined, and the pressure sensors may be dispensed with. Alternatively, for a continuous mode of operation of the plasma reactor, the pressure sensors may be used for detecting anomalous operating states such as a filter obstruction. As an alternative to a smooth transition between the two power levels of the plasma reactor, a changeover point may also be selected, at 200 degrees Celsius exhaust gas temperature, for example, at which a changeover between the two power levels is performed suddenly. In a further embodiment, the system may switch over to nitrogen monoxide conversion above 200 degrees Celsius; however, when the temperature of 250 degrees Celsius, in particular 300 degrees Celsius, is exceeded, the plasma may be completely dispensed with if an oxidation catalytic converter is also provided, because nitrogen monoxide conversion using a catalytic converter is energetically more favorable than that using a plasma method. In further alternatives, the degree of filling of the particle filter may also be determined via a single pressure sensor using counterpressure measurement instead of via differential pressure measurement. Furthermore, it is possible to determine the degree of filling using a soot sensor positioned upstream from the particle filter and time integration of its soot signal. It is also possible to analyze engine characteristics map data, which is stored in the control unit, in regard to the soot production and to integrate over time.  
         [0011]    [0011]FIG. 2 shows a diagram which illustrates the effectiveness of the two oxidizing agents used, ozone and nitrogen dioxide, in a plot of soot oxidation rate R (in arbitrary units) over exhaust gas temperature T (in degrees Celsius). Curve  110  represents the rate for ozone; curve  120  represents the rate for nitrogen dioxide.  
         [0012]    Both curves rise strictly monotonically, the soot oxidation using nitrogen dioxide starting noticeably only at higher exhaust gas temperatures than ozone. As curve  110  shows, filter regeneration is possible using ozone as the oxidizing agent at exhaust gas temperatures below 150 degrees Celsius. At temperatures above 250 degrees Celsius, the lower power level for the plasma reactor, at which only nitrogen dioxide is still mainly generated as an oxidizing agent, may be selected.  
         [0013]    [0013]FIG. 3 illustrates through a flowchart the method of exhaust-gas aftertreatment, having a first step  150  and a further step  170 .  
         [0014]    In method step  150 , oxidizing agents are first generated by oxidizing nitrogen monoxide and/or oxygen molecules contained in the exhaust gas. In this case, as a function of the temperature of the exhaust gas, either nitrogen monoxide is primarily produced or ozone is also produced in larger quantities in the way described above. In further method step  170 , the exhaust gas is treated using the oxidizing agent, for example, regenerating the particle filter by combusting the soot which has collected therein.  
         [0015]    A further embodiment of the present invention includes exhaust-gas aftertreatment in the form of oxidation of hydrocarbon residues within the exhaust at exhaust gas temperatures below 200 degrees Celsius, in particular below 150 degrees Celsius, for example.