Patent ID: 8091350

Claim:
An exhaust purification apparatus for an engine comprising: a selective reduction-type NOx catalyst that is interposed in an exhaust passage of an engine and selectively reduces NOx contained in exhaust gas of the engine by using ammonia as a reducing agent; an EGR unit that recirculates exhaust gas of the engine to an intake side of the engine; and a control unit that is coupled to the EGR unit and controls the operation of the EGR unit; wherein the control unit includes: an exhaust-purification-efficiency estimation section that estimates exhaust purification efficiency of the NOx catalyst with respect to NOx; a target-exhaust-purification-efficiency calculation section that calculates target exhaust purification efficiency with respect to NOx on the basis of an operational state of the engine; an oxygen-concentration detection section that detects oxygen concentration in intake gas supplied into cylinders of the engine; a NOx-decrease-factor calculation section that calculates a decrease factor as an index numeral indicative of a decrease state of a NOx production rate in the cylinders of the engine at the oxygen concentration detected by the oxygen-concentration detection section on the basis of NOx production characteristics in the cylinders of the engine in relation to the oxygen concentration in the intake gas; a target-oxygen-concentration calculation section that corrects the NOx decrease factor according to a ratio between the exhaust purification efficiency estimated by the exhaust-purification-efficiency estimation section and the target exhaust purification efficiency, and calculating oxygen concentration corresponding to the corrected NOx decrease factor as target oxygen concentration on the basis of the NOx production characteristics; and an EGR control section that controls the EGR unit according to the target oxygen concentration calculated by the target-oxygen-concentration calculation section.