Patent ID: 7146799

Claim:
A system for an engine having at least a first group and a second group of cylinders, the system comprising: a first emission control device coupled to said first and second groups of cylinders; a second emission control device coupled downstream of said first device; and a computer storage medium having a computer program encoded therein for controlling fuel injected into the first and second group of cylinders, comprising: code for operating the engine in a first mode, said first mode including at least operating said first group of cylinders lean of stoichiometry and said second group of cylinders between a first rich air-fuel ratio and a second, less rich, air-fuel ratio based at least on a determination of conditions in or downstream of said first emission control device; and code for operating the engine in a second mode, said second mode including at least operating said first group of cylinders lean of stoichiometry and said second group of cylinders between a first rich air-fuel ratio and a second, less rich, air-fuel ratio based at least on a determination of conditions in or downstream of said second emission control device.