The invention pertains to a method for operating an exhaust gas aftertreatment with a diesel particulate filter and a device for controlling the exhaust gas aftertreatment. The invention also pertains to an exhaust gas aftertreatment and to an internal combustion engine.
It is known from the prior art that diesel particulate filters can be used to remove soot particles from an exhaust gas. Diesel particulate filters can comprise a fine-pored structure—e.g., a ceramic structure or, as described in US 2007-151,231 A, a fine-pored woven steel structure—on the walls of which the soot particles are deposited. It is known that diesel particulate filters can be regenerated. A distinction is made between passive regeneration and active regeneration; in the latter case, the soot particles are burned off at predetermined time intervals and/or after a predefinable trigger signal.
In an exhaust gas aftertreatment system with a passively regenerating diesel particulate filter, advantage is taken of the so-called CRT (Continuous Regeneration Trap) effect, and the diesel particulate filter is thus regenerated continuously, i.e., in particular without a fixed, predefined trigger signal. In the passive regeneration technique, the exhaust gas temperatures of the engine are sufficient under normal operating conditions to ensure the continuous removal of soot from the diesel particulate filter. Under certain conditions, e.g., special climate conditions or long, continuous periods of low-load operation, the regeneration process can be supported by more thorough measures. For example, the exhaust gas temperature of the engine and thus the soot burnoff can be increased significantly for a short time. US 2010-031,638 A, for example, describes implementing a passive regeneration by increasing the engine load. US 2011-265,456 A describes the possibility of increasing the soot burnoff by changing the combustion cycle and thus raising the temperature of the exhaust gas.
To determine a good time for regeneration, various mathematical and simulation or estimation methods for defining the soot load of a diesel particulate filter are described, some of which are quite complicated. It is known from WO 05/116413 that a neuronal network can be trained to determine the load state on the basis of the operating state of the engine, the differential pressure across the diesel particulate filter, and the exhaust gas values. The above-mentioned computational methods are comparatively complicated. In addition, there should be no need to carry out extra measurements to determine the state of the filter.
It is true that, to determine the load of the diesel particulate filter (DPF) and/or the best time for a regeneration process, the load state of the diesel particulate filter can be usefully determined by measuring the differential pressure (ΔP) across the diesel particulate filter. In the simplest case, regeneration by means of a thermomanagement measure as cited above could be initiated whenever a predetermined or calculated reference value for the differential pressure (ΔP) is exceeded.
It has been found, however, that the cause of the differential pressure is comparatively complex. The differential pressure increases over the service life of the diesel particulate filter not only as a result of the loading with soot but also as a result of a loading with ash; the accumulation of ash is a fundamental factor in determining the service life (T_L) of the diesel particulate filter. Whereas soot comprises essentially combustible carbon components, ash is defined as the incombustible component of a filter load, which is almost impossible to remove during operation or which at best requires considerable effort to do so. Thus, DE 1 002 951 describes measures for reducing the amount of ash.
During operation, however, the increase in the differential pressure caused by the ash load leads in particular to the situation that, either a reference value pertaining to the differential pressure for initiating regeneration is reached at an increasingly earlier point, or a reference value calculated on the basis of the combustion ends up being too low. As a result, this leads in the case of a passive regeneration system to the initiation of thermomanagement more frequently than would in fact be necessary, i.e., it would lead regularly to the initiation of thermo-management measure even though the soot load of the diesel particulate filter is not yet critical. It is known from DE 12 034 340 A1, for example, that a combustion calculation for a known fuel specification can be carried out, which also takes into account the ash residues in the diesel particulate filter. It is also desirable, however, to have available an improved method and a device for operating an exhaust gas aftertreatment system by means of which the influence of ash on the differential pressure across a diesel particulate filter of the exhaust gas aftertreatment system can be taken into account, i.e., in particular so that the influence can be determined, in the case of passively regenerating diesel particulate filters.