Internal combustion engine with exhaust-gas purification system, and method for purifying the exhaust gas from an internal combustion engine

Abstract

An exhaust-gas purification system and method are provided for an internal combustion engine having an NOx storage catalytic converter and a downstream SCR catalytic converter. The NOx storage catalytic converter can be supplied in a first operating mode with an oxidizing exhaust gas and in a second operating mode with a reducing exhaust gas. A third operating mode is provided between the first operating mode and the second operating mode, in which an exhaust gas which has a lower content of oxidizing constituents than the first operating mode and a lower content of reducing constituents than the second operating mode can be supplied to the NOx storage catalytic converter to improve NH 3 production at the start of converter regeneration.

What is claimed is: 
     
       1. A method for purifying the exhaust gas from an internal combustion engine having an exhaust-gas purification system including a nitrogen oxide storage catalytic converter and an SCR catalytic converter downstream of the nitrogen oxide storage catalytic converter, comprising the steps of:
 supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of oxidizing constituents;   supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of reducing constituents; and   supplying the nitrogen oxide storage catalytic converter, between the oxidizing constituents supplying step and the reducing constituents supplying step, with an exhaust gas which has a lower content of oxidizing constituents than in the oxidizing constituents supplying step and a lower content of reducing constituents than in the reducing constituents supplying step,   wherein the step between the oxidizing constituents supplying step and the reducing constituents supplying step is terminated at the earliest when the nitrogen oxide storage catalytic converter is predominantly filled by exhaust gas delivered in step between the oxidizing constituents supplying step and the reducing constituents supplying step.   
     
     
       2. The method as claimed in  claim 1 , wherein
 the nitrogen oxide storage catalytic converter is formed as a parallel arrangement of a first nitrogen oxide storage catalytic converter element and a second nitrogen oxide storage catalytic converter element, and   the first nitrogen oxide storage catalytic converter element and the second nitrogen oxide storage catalytic converter element are operated alternately by switching of a switching device arranged to selectively direct exhaust gas into said elements.   
     
     
       3. A method for purifying the exhaust gas from an internal combustion engine having an exhaust-gas purification system including a nitrogen oxide storage catalytic converter and an SCR catalytic converter downstream of the nitrogen oxide storage catalytic converter, comprising the steps of:
 supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of oxidizing constituents;   supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of reducing constituents; and   supplying the nitrogen oxide storage catalytic converter, between the oxidizing constituents supplying step and the reducing constituents supplying step, for a predetermined period with a constant exhaust gas composition which has a lower content of oxidizing constituents than in the oxidizing constituents supplying step and a lower content of reducing constituents than in the reducing constituents supplying step,   wherein in step of supplying the nitrogen oxide storage catalytic converter between the oxidizing constituents supplying step and the reducing constituents supplying step, an air/fuel ratio which set to control exhaust gas composition is set to be slightly greater than 1, such that the oxidizing constituents in the exhaust gas have an oxygen excess of 1% or less.   
     
     
       4. The method as claimed in  claim 3 , wherein the air/fuel ratio is approximately 1.05. 
     
     
       5. A method for purifying the exhaust gas from an internal combustion engine having an exhaust-gas purification system including a nitrogen oxide storage catalytic converter and an SCR catalytic converter downstream of the nitrogen oxide storage catalytic converter, comprising the steps of:
 supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of oxidizing constituents;   flushing the nitrogen oxide storage catalytic converter with lean exhaust gas after supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of oxidizing constituents, wherein the internal combustion engine is a diesel engine and the exhaust gas is diesel exhaust gas, and wherein the lean exhaust gas of the flushing step is generated by one of
 post-injecting fuel into one or more cylinders of the diesel engine; 
 throttling intake air of the diesel engine; and 
 increasing an exhaust gas recirculation rate of the diesel engine; and 
   supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of reducing constituents after the flushing step is completed,   wherein the lean exhaust gas supplied during the flushing step has a lower content of oxidizing constituents than the exhaust gas supplied in the oxidizing constituents supplying step and a lower content of reducing constituents than in the exhaust gas supplied in the reducing constituents supplying step,   wherein the flushing step is terminated at the earliest when the nitrogen oxide storage catalytic converter is predominantly filled by the lean exhaust gas of the flushing step,   wherein the fuel is post-injected in an expansion stroke of the one or more cylinders between an approximately 20° and 120° crank angle after top dead center.    
     
     
       6. A method for purifying the exhaust gas from an internal combustion engine having an exhaust-gas purification system including a nitrogen oxide storage catalytic converter and an SCR catalytic converter downstream of the nitrogen oxide storage catalytic converter, comprising the steps of:
 supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of oxidizing constituents;   flushing the nitrogen oxide storage catalytic converter with lean exhaust gas after supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of oxidizing constituents, wherein the internal combustion engine is a diesel engine and the exhaust gas is diesel exhaust gas, and wherein the lean exhaust gas of the flushing step is generated by one of
 post-injecting fuel into one or more cylinders of the diesel engine; 
 throttling intake air of the diesel engine; and 
 increasing an exhaust gas recirculation rate of the diesel engine; and 
   supplying the nitrogen oxide storage catalytic converter with exhaust gas containing an excess of reducing constituents after the flushing step is completed,   wherein the lean exhaust gas supplied during the flushing step has a lower content of oxidizing constituents than the exhaust gas supplied in the oxidizing constituents supplying step and a lower content of reducing constituents than in the exhaust gas supplied in the reducing constituents supplying step,   wherein the flushing step is terminated at the earliest when the nitrogen oxide storage catalytic converter is predominantly filled by the lean exhaust gas of the flushing step,   wherein the fuel is post-injected in an exhaust stroke of the one or more cylinders.