Patent ID: 8713916

Claim:
A NOx purification system for reducing NOx in exhaust gas of an engine, comprising: an exhaust gas passage having an upstream side and a downstream side; in order from the upstream side to the downstream side, a first oxidation catalyst, a second oxidation catalyst, an ammonia solution supply configured to supply an ammonia solution to the exhaust gas passage, and a selective reduction NOx catalyst; an oxygen concentration sensor located on the upstream side of the first oxygen catalyst to detect an oxygen concentration in the exhaust gas; a controller configured to control an in-cylinder fuel injection so that hydrocarbon in the exhaust gas is oxidized by the first oxidation catalyst, and nitric oxide in the exhaust gas is oxidized by the second oxidation catalyst; and the controller is configured to calculate a target oxygen concentration by using a value detected by an exhaust gas temperature sensor disposed between the first oxidation catalyst and the second oxidation catalyst, wherein the controller controls an air-intake amount and a multi-injection quantity of the in-cylinder fuel injection such that the detected oxygen concentration is set to be a predetermined target oxygen concentration, which is the oxygen concentration of the exhaust gas flowing into the first oxidation catalyst that is necessary for most of the hydrocarbon of the exhaust gas to be oxidized by the first oxidation catalyst, and wherein a feedback control of the in-cylinder fuel injection is performed by the controller such that the exhaust temperature becomes equal to a predetermined control target temperature while monitoring the value detected by the exhaust gas temperature sensor.