Reductant addition in exhaust system comprising NOx-absorbent

Abstract

An exhaust system for a vehicular lean-burn internal combustion engine comprises a NO x -absorbent, a reductant injector ( 78 ) disposed upstream of the NO x -absorbent and means ( 50 ), when in use, for controlling reductant addition, wherein the reductant addition control means supplies reductant to the NO x -absorbent at all vehicle speeds in a duty cycle at a rate which is predetermined to correlate with a desired NO x conversion at the average duty cycle speed of the vehicle.

1 . An exhaust system for a vehicular lean-burn internal combustion engine comprising: a NO x -absorbent; a reductant injector disposed upstream of the NO x  absorbent; and means, when in use, for controlling reductant addition, wherein the reductant addition control means supplies reductant to the NO x -absorbent at all vehicle speeds in a duty cycle at a rate which is predetermined to correlate with a desired NO conversion at the average duty cycle speed of the vehicle. 
     
     
         2 . An exhaust system according to  claim 1 , wherein the NO x -absorbent is selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds and mixtures of any two or more thereof. 
     
     
         3 . An exhaust system according to  claim 2 , wherein the alkaline earth metal is selected from the group consisting of barium, magnesium, strontium and calcium. 
     
     
         4 . An exhaust system according to  claim 2 , wherein the alkali metal is selected from the group consisting of potassium and caesium. 
     
     
         5 . An exhaust system according to  claim 2 , wherein the rare earth metal is selected from the group consisting of cerium, yttrium, lanthanum and praseodymium. 
     
     
         6 . An exhaust system according to  claim 2 , wherein the alkaline earth metal compound, the alkali metal compound or the rare earth metal compound is supported on a support material. 
     
     
         7 . An exhaust system according to  claim 6 , wherein the support is selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixtures or a composite oxide of any two or more thereof. 
     
     
         8 . An exhaust system according to  claim 6 , wherein the NO x -absorbent comprises the support. 
     
     
         9 . An exhaust system according to  claim 1 , wherein the NO x -absorbent is a component of a NO x -trap comprising a catalyst for oxidising NO. 
     
     
         10 . An exhaust system according to  claim 9 , wherein the NO oxidation catalyst comprises a platinum group metal. 
     
     
         11 . An exhaust system according to  claim 10 , wherein the NO x -trap comprises a NO reduction catalyst. 
     
     
         12 . An exhaust system according to  claim 1 , comprising an oxidation catalyst disposed between the reductant injector and the NO x -absorbent. 
     
     
         13 . An exhaust system according to  claim 1 , comprising a catalyst for oxidising NO to NO 2  disposed upstream of the reductant injector. 
     
     
         14 . An exhaust system according to  claim 13 , wherein the NO oxidation catalyst is platinum on an alumina support. 
     
     
         15 . An exhaust system according to  claim 13 , comprising an optionally catalysed particulate filter disposed between the oxidation catalyst and the reductant injector. 
     
     
         16 . An exhaust system according to  claim 9 , wherein the NO x -trap comprises a particulate filter. 
     
     
         17 . An exhaust system according to  claim 1 , comprising control means when in use, intermittently to enrich the exhaust gas composition for regenerating the NO x -absorbent. 
     
     
         18 . An exhaust system according to  claim 24 , wherein the control means, when in use, supplies reductant to the NO x -trap only when the catalyst is active for NO x  reduction. 
     
     
         19 . A diesel engine comprising an exhaust system according to  claim 1 . 
     
     
         20 . A light-duty diesel engine according to  claim 19 . 
     
     
         21 . A method of reducing NO in the exhaust gas of a vehicular lean-burn internal combustion engine, which method comprising absorbing NO x  from the exhaust gas in a NO x -absorbent, contacting the NO x -absorbent with a reductant to regenerate the NO x -absorbent, at all vehicle speeds in a duty cycle, and reducing NO x  to N 2 , wherein a rate of reductant injection correlates with a desired NO x  conversion at the average duty cycle speed. 
     
     
         22 . An exhaust system according to  claim 10 , wherein the platinum group metal is selected from the group consisting of platinum, palladium and a combination of platinum and palladium. 
     
     
         23 . An exhaust system according to  claim 11 , wherein the NO x  reduction catalyst comprises rhodium. 
     
     
         24 . An exhaust system according to  claim 9 , comprising control means when in use, to intermittently enrich the exhaust gas composition for regenerating the NO x -absorbent.