European Published Patent Application No. 0 758 713 describes an exhaust gas cleaning unit in which an oxidation catalytic converter is connected upstream of the particle filter, as seen in the direction of flow of the exhaust gas, and the nitrogen oxide store is connected downstream of the particle filter. As an alternative to the nitrogen oxide store, a nitrogen oxide reduction catalytic converter may be used. The oxidation catalytic converter is used to convert nitrogen monoxide which is contained in the exhaust gas in soot regeneration phases of the particle filter into nitrogen dioxide, which promotes the soot burn-off in the particle filter. The nitrogen oxide store or the nitrogen oxide reduction catalytic converter is used to collect or convert nitrogen monoxide which is formed by the reaction of the nitrogen dioxide with the soot particles. The soot regeneration operations occur, for example, every 60 minutes for about three minutes with a lean exhaust gas composition and temperatures of between approximately 400° C. and 500° C. Conventional nitrogen oxide regeneration phases for the nitrogen oxide store are performed approximately every 10 seconds to every few minutes, in each case for approximately 0.5 seconds, with a rich exhaust gas composition. When a nitrogen oxide regeneration phase of this type falls in the period of a soot regeneration phase, it is performed, by an additional increase in temperature, as a correspondingly short sulfur regeneration phase, in order to prevent gradual sulfur poisoning of the nitrogen oxide store.
Various methods for nitrogen oxide (NOx) regeneration and sulfur regeneration of a nitrogen oxide store are conventional. For example, German Published Patent Application No. 197 50 226 describes a method for NOx regeneration in which a desired, rich exhaust gas composition is generated substantially only by engine measures, including suitable control of exhaust gas recycling.
It is an object of the present invention to provide a new type of exhaust gas cleaning unit and a corresponding operating method for this unit.