Purging of an NOx trap

A lean burn engine having a lean NOx trap in the form of a matrix (16) of narrow flow passages and means (20,22,24,26) for injecting reducing gases upstream of the NOx trap matrix (16) to purge the trap periodically, characterized in that a flow straightening matrix (12) of narrow flow passages is arranged upstream of the NOx trap matrix (16) and is separated from the trap matrix by a narrow chamber (14), and in that the means for injecting reducing gases upstream of the NOx trap matrix are operative to inject the reducing gases in bursts into the narrow chamber, each burst having sufficient mass and flow rate to fill the narrow chamber (14) with the reducing gases and to displace the exhaust gases previously present in the narrow chamber into the narrow flow passages of the flow straightening matrix (12) and of the trap (16) without significantly mixing with the exhaust gases previously present in the narrow chamber.

A system for treating a flow of exhaust gases generated with an internal combustion engine, the system comprising:

• an emission control device adapted to receive the flow of exhaust gases, the emission control device including an upstream, flow-straightening first matrix including a plurality of narrow flow passages, a downstream catalyst including a plurality of narrow flow passages, and a narrow chamber separating the flow-straightening matrix from the catalyst;
• and
• an injector for periodically injecting a reducing gas in bursts into the narrow chamber, each burst having a mass and a flow rate sufficient to fill the narrow chamber with the reducing gas and to displace the exhaust gases previously present in the narrow chamber into the narrow flow passages of the flow straightening matrix and of the catalyst without significantly mixing with the exhaust gases previously present in the narrow chamber.