Patent ID: 6901749

Claim:
An exhaust emission control system for an internal combustion engine, having an exhaust system comprising: a nitrogen oxide removing means provided in the exhaust system of said engine for absorbing nitrogen oxide contained in exhaust gases in an exhaust lean condition; a sulfur oxide amount estimating means for estimating the amount of sulfur oxide absorbed in said nitrogen oxide removing means; and a sulfur oxide removing means for removing the sulfur oxide when the sulfur oxide amount estimated by said sulfur oxide amount estimating means has reached a set value; wherein said sulfur oxide amount estimating means estimates an amount of change per unit time in the sulfur oxide amount according to an air-fuel ratio of an air-fuel mixture supplied to said engine and an operating condition of said engine, and accumulates the estimated amount of change to thereby estimate the sulfur oxide amount, wherein said sulfur oxide amount estimating means includes a first estimating means for estimating the amount of change in the sulfur oxide amount according to the operating condition of said engine when the air-fuel ratio is set in the vicinity of a stoichiometric ratio, a second estimating means for estimating the amount of change in the sulfur oxide amount according to the operating condition of said engine when the air-fuel ratio is set in a lean region with respect to the stoichiometric ratio, and a third estimating means for estimating the amount of change in the sulfur oxide amount according to the operating condition of said engine when the air-fuel ratio is set in a rich region with respect to the stoichiometric ratio.