Abstract:
A method for the targeted initiation of a regeneration of a particle filter in an exhaust-gas duct of an internal combustion engine which has a catalytic converter downstream of the particle filter in the flow direction of the exhaust gas, the regeneration of the particle filter taking place by means of an oxidative burn-off of the particles during the regeneration phase.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for the targeted initiation of a regeneration of a particle filter in an exhaust-gas duct of an internal combustion engine which has a catalytic converter downstream of the particle filter in the flow direction of the exhaust gas, the regeneration of the particle filter taking place by means of an oxidative burn-off of the particles during the regeneration phase. 
         [0002]    The invention also relates to a device for carrying out the method according to the invention. 
         [0003]    To reduce the particle emissions of diesel engines, and in the future also increasingly of spark-ignition engines (EU6 limit values from 2014), particle filters are used in the exhaust-gas duct of the internal combustion engines. The exhaust gas is conducted through the particle filters, which separates the solid particles contained in the exhaust gas and retains said solid particles in a filter substrate. As a result of the mass of soot accumulated in the filter substrate, the particle filter becomes blocked over time, which manifests itself in an increase in the exhaust-gas counterpressure with an adverse effect on engine power and fuel consumption. For this reason, the accumulated mass of soot must be discharged from time to time. Said filter regeneration takes place during separate regeneration phases by means of an oxidative burn-off of the particles, which takes place independently as an exothermic reaction if an exhaust-gas temperature of at least 580° C. and an adequately high oxygen concentration in the exhaust gas are present. The course of the regeneration can be controlled by means of the composition of the exhaust gas and the exhaust-gas temperature. 
         [0004]    The exhaust-gas treatment of internal combustion engines requires further components aside from the particle filter. For example, in the case of spark-ignition engines operated on a homogeneous concept, the pollutants hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO x ) are converted by means of a three-way catalytic converter. In the case of lean-burn concepts, a storage catalytic converter for nitrogen oxides is usually connected downstream. The lowest possible discharge of pollutants is achieved by means of lambda regulation, wherein the fuel/air mixture supplied to the internal combustion engine is regulated on the basis of the oxygen concentration in the exhaust gas. The oxygen fraction present in the exhaust gas is described by a lambda value which has a value of 1 for stoichiometric combustion, a value of &gt;1 for an excess of oxygen and a value of &lt;1 for a lack of oxygen. The lambda value is measured by means of corresponding lambda probes arranged in the exhaust-gas duct. 
         [0005]    The regeneration of the particle filter takes place generally when a limit value for an exhaust-gas counterpressure is exceeded, as already described above. This may be detected by means of a suitable model and adapted by means of a differential pressure measurement. Here, the soot oxidation and therefore the regeneration of the filter are influenced significantly by the exhaust-gas temperature and the residual oxygen content. Since there must be an excess of oxygen in the exhaust gas for the burn-off of the particles, it is not possible in said phase for the mixture composition of the internal combustion engine to be freely selected according to the demands of the driving mode. It is therefore desirable to determine an end of the regeneration in order to be able to switch to normal driving operation. 
         [0006]    An as yet unpublished patent application from the applicant with the official application number DE 10 2009 028237.8 discloses a method for the monitoring and regulation of the regeneration of a particle filter in an exhaust-gas duct of an internal combustion engine, wherein the regeneration of the particle filter takes place by means of an oxidative burn-off of the particles during a regeneration phase. Here, it is provided that, during the regeneration phase of the particle filter, the internal combustion engine is operated at a lean operating point at least temporarily during lean-burn operating phases or during a mixture oscillation, and that the regeneration of the particle filter is monitored by means of the time profile of a second signal of a second lambda probe arranged downstream of the particle filter, or the time profile of a second characteristic variable derived therefrom, in comparison with the time profile of a first signal of a first lambda probe arranged upstream of the particle filter in the exhaust-gas direction, or the time profile of a first characteristic variable derived therefrom, during the lean-burn operating phases or during the mixture oscillation. It is disadvantageous here that the regeneration takes place within a lean-burn phase, in which other harmful exhaust-gas constituents cannot be optimally removed. 
         [0007]    Since it is also not possible to ensure that the engine is regularly in an operating state in which the regeneration of the particle filter can take place independently, facilities for a forced initiation of the regeneration must be provided. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the invention to provide a method for the forced initiation of the regeneration, which method permits reliable regulation and monitoring of the regeneration of the particle filter. 
         [0009]    It is also an object of the invention to provide a corresponding device for carrying out the method. 
         [0010]    The object of the invention relating to the method is achieved in that, when the internal combustion engine is in the warm operating state but the temperature is still insufficient for a regeneration of the particle filter, measures are temporarily taken to increase the exhaust-gas temperature upstream of and/or in the particle filter. 
         [0011]    The object relating to the device is achieved in that the device has a control unit by means of which the initiation, control and monitoring of the regeneration of the particle filter are carried out and by means of which signals of a first lambda probe arranged upstream of the particle filter in the exhaust-gas direction, signals of a second lambda probe arranged downstream of the particle filter and/or downstream of the catalytic converter in the exhaust-gas direction and/or signals of at least one temperature sensor can be evaluated, wherein by means of a program routine implemented in the control unit, measures can be carried out for a limited time to targetedly increase the temperature upstream of and/or in the particle filter. 
         [0012]    In the regeneration of a particle filter, use is made of the fact that, when the exhaust-gas temperature lies in a range which permits a regeneration, the regeneration takes place automatically while maintaining the conversion of the other pollutant components, since the untreated exhaust gas still has a certain residual oxygen fraction (0.5 to 0.7%). Furthermore, under suitable conditions, a regeneration may also take place by means of the so-called heterogeneous water-gas equilibrium reaction (C+H 2 O         CO+H 2 ). Here, use is made of the fact that, when there is a lack of oxygen, the reaction kinetics for the regeneration of the particle filter changes. Here, no additional oxygen is required in the exhaust gas. Water is always present in the exhaust gas from the reaction. Said reaction however takes place only at relatively high temperatures, that is to say &gt;800° C. The requirement for a regeneration of the particle filter may be detected from a suitable model or by means of a corresponding sensor arrangement, for example differential pressure measurement, wherein for the initiation of the regeneration, firstly the further ambient conditions, such as for example the exhaust-gas temperature and the present operating mode, are checked. 
         [0013]    With the method and the device, a regeneration of the particle filter may also be initiated within operating states of the internal combustion engine in which the ambient conditions for the regeneration are not optimal. This is the case for example if the internal combustion engine is operated for a very long time in part-load operation or at low loads, such as is often the case for example in city traffic. Furthermore, adequate conversion of all harmful exhaust-gas components must be ensured even during said phase, which can likewise be achieved by means of the method. 
         [0014]    One option for this is a targeted reduction in efficiency with the aim of increasing the exhaust-gas temperature. For this purpose, strategies may be used which conventionally serve for heating up a catalytic converter when the engine is still cold. Therefore, in one preferred method variant, it is provided that, for the targeted initiation of the regeneration of the particle filter, the ignition angle is shifted in the direction of a late ignition time, and therefore the exothermic reaction is displaced increasingly into the exhaust stroke, which leads to an increase in the exhaust-gas temperature, as a result of which the regeneration of the particle filter can, as described above, take place independently. 
         [0015]    To compensate the reduction in efficiency and the associated torque loss, it may be provided here that, during the shift of the ignition angle, a throttling action of the internal combustion engine is reduced. Since regulation is also carried out to a stoichiometric air ratio, this strategy engine is exhaust-gas-neutral, that is to say an adequate conversion of all harmful exhaust-gas components can be ensured. On account of the reduction in efficiency, however, a certain increase in fuel consumption must be accepted in this regeneration phase. 
         [0016]    Aside from the late adjustment itself, in another method variant, the so-called homogeneous-split (HSP) operating mode may be used in addition to the late adjustment of the ignition angle. Said operating mode is used for example to bring the catalytic converter up to operating temperature as quickly as possible after the start phase, that is to say when the engine is still cold. Here, a second injection, which has a stabilizing effect, in the compression stroke is utilized to subject the ignition to an extreme delay, such that a large proportion of the combustion energy can be utilized to increase the exhaust-gas enthalpy, which, in the application of the method according to the invention, heats the particle filter in a very short time (within a few seconds) to the operating temperature required for regeneration. High ignition reliability can be attained by means of said second injection or if appropriate by means of further injections. Furthermore, as a result of the placement of the rich mixture in the vicinity of the spark plug, increased burn-through stability can be obtained. 
         [0017]    A further option for actively initiating the regeneration is to increase the exothermic reaction in a three-way catalytic converter. It is provided here that, from a lambda regulating mode around a lambda value of 1, regulation to a lambda value of &gt;1 (lean-burn operation) is carried out for a limited time and an oxygen accumulator of the catalytic converter is thereby filled, and subsequently, after the filling of the oxygen accumulator, a lambda value of &lt;1 is set by pilot control, as a result of which the oxygen accumulator in the catalytic converter is emptied. Said process, which is highly exothermic overall, causes the exhaust system to be heated up, and on account of the overall lambda value of 1, is approximately neutral with regard to the conventional exhaust-gas components. 
         [0018]    Said change in the lambda regulating mode, which has hitherto been known for heating up the catalytic converter or for the desulfurization of a NO x  accumulator catalytic converter, can be used to particularly great effect if the particle filter is designed as a combined particle filter/catalytic converter and has a catalytic coating. Here, particularly effective heating of the particle filter can be obtained by means of said measure. 
         [0019]    The regeneration must be monitored for all active measures. It is therefore provided that the temperature in the particle filter is used as a regulating variable for the initiation of the regeneration and for the monitoring of the regeneration, wherein said temperature is determined directly by means of at least one temperature sensor arranged in or on the particle filter or being derived from signals of lambda probes which are arranged in the exhaust duct upstream and/or downstream of the particle filter and which serve for lambda regulation or being determined on a modeled basis from an exhaust-gas temperature model. The intensity and the duration of the heating-up process can thereby be influenced in a targeted manner. 
         [0020]    One device variant therefore provides that an exhaust-gas temperature model is implemented within the control unit and the regulating variable for the regeneration of the particle filter is a temperature of the particle filter derived from said exhaust-gas temperature model. Continuous monitoring can thereby be ensured. 
         [0021]    A preferred use of the method as described above in terms of its variants is the implementation for the regeneration of a close-coupled particle filter in the exhaust-gas duct of an internal combustion engine which is designed as a spark-ignition engine and which has intake pipe injection or direct injection. It is advantageous here that existing lambda probes or existing sensor concepts may be utilized, because lambda probes for lambda regulation are already provided in the exhaust-gas duct of spark-ignition engines, such that the signals thereof may be jointly used for the initiation, control and regulation of the regeneration of the particle filter, as a result of which the method can be implemented particularly cost-effectively in future spark-ignition engines with particle filters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be explained in more detail below on the basis of an exemplary embodiment illustrated in the figures, in which: 
           [0023]      FIG. 1  shows an internal combustion engine having a particle filter arranged in the exhaust-gas duct thereof and having a downstream pre-catalytic converter and a main catalytic converter or an LNT/SCR catalytic converter, and 
           [0024]      FIG. 2  shows the internal combustion engine with a combined particle filter/catalytic converter and a downstream main catalytic converter or an LNT/SCR catalytic converter. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  shows an internal combustion engine  10  having an air supply  11  and having a particle filter  15  arranged in an exhaust-gas duct  12  and having a downstream pre-catalytic converter  17  and also a main catalytic converter  18  which may be designed as a three-way catalytic converter. The exhaust gas of the internal combustion engine  10  which is purified in the particle filter  15  and the catalytic converters  17 ,  18  is discharged via an exhaust gas outlet  20 . The lambda value of the exhaust gas in the exhaust-gas duct  12  directly downstream of the internal combustion engine  10  is determined by means of a first lambda probe  13 . In said region, the temperature of the exhaust gas is additionally determined by means of a temperature sensor  14 . Particles are accumulated in the particle filter  15  during the operation of the internal combustion engine  10 . This increases the exhaust-gas counterpressure. The particle filter  15  must therefore be burned off, and thus regenerated, when required. A regeneration can take place only when the exhaust-gas temperature lies above approximately 580° C.; this can be detected by means of the temperature sensor  14 . Furthermore, an adequate amount of oxygen for a combustion must be present. This can be detected by means of the first lambda probe  13 . A second lambda probe  16  is arranged in the exhaust-gas duct  12  downstream of the particle filter  15  and the downstream pre-catalytic converter  17 . For post-cat lambda regulation, said second lambda probe may also be arranged downstream of the main catalytic converter  18 . 
         [0026]    From the difference between the output signals of the first lambda probe  13  and of the second lambda probe  16 , it is possible to determine the extent to which the burn-off of particles in the particle filter  15  consumes oxygen. If no difference can be detected between the signals, the burn-off has ended. The signals of the first lambda probe  13  and of the second lambda probe  16  and also the output signal of the temperature sensor  14  are supplied to a control unit  21 . 
         [0027]    A program sequence for comparing the signals and for initiating, controlling and monitoring the regeneration is implemented in the control unit  21 . It is provided here that the signals of the first lambda probe  13 , the signals of the second lambda probe  16  and/or signals of the temperature sensor  14  can be evaluated as significant regulating variables, wherein by means of the program routine implemented in the control unit  21 , measures can be carried out for a limited time to targetedly increase the temperature upstream of and/or in the particle filter  15 . In a further exemplary embodiment, an exhaust-gas temperature model is implemented within the control unit  21 . A temperature of the particle filter  15  modeled using said exhaust-gas temperature model is provided as a regulating variable for the regeneration of the particle filter  15 . Here, the control unit  21  may be integrated in the engine controller of the internal combustion engines  10 , in which the lambda regulation is conventionally implemented. 
         [0028]    Said basic design is shown for an operating variant in which regulation is carried out to a lambda value of 1. For a lean-burn operating mode with a lambda value λ&gt;1, it is alternatively possible for an LNT/SCR catalytic converter  19  to be provided instead of the main catalytic converter  18 . Here, LNT stands for “Lean NO x  Trap” and refers to a catalytic converter whose surface is impregnated with barium salts and platinum and other noble metals and which can therefore adsorb nitrogen oxides from the engine exhaust gas. SCR stands for “Selective Catalytic Reduction” and refers to a selective catalytic reduction of nitrogen oxides in exhaust gases. The chemical reaction in the SCR catalytic converter is selective, that is to say the nitrogen oxides are preferably reduced, while undesired secondary reactions (such as for example the oxidation of sulfur dioxide to form sulfur trioxide) are substantially suppressed. Said nitrogen oxides, typically NO and NO 2 , can be stored on the catalytic converter surface. If such a catalytic converter is periodically exposed to a rich fuel/air mixture, said nitrogen oxides can be converted into nitrogen, carbon dioxide and water. 
         [0029]      FIG. 2  schematically shows the arrangement from  FIG. 1  in a modified arrangement. Here, in contrast to  FIG. 1 , the particle filter  15  and the pre-catalytic converter  17  are combined to form a combined particle filter/catalytic converter, the particle filter having a catalytic coating. Here, too, a main catalytic converter  18  is provided for an operating variant in which regulation is carried out to a lambda value of 1. For a lean-burn operating mode with a lambda value λ&gt;1, it is alternatively possible for an LNT/SCR catalytic converter  19  to be provided instead of the main catalytic converter  18 , as shown in  FIG. 1 .