Exhaust gas purification device for an engine

摘要

In the present invention, the exhaust gas from the engine is divided into a first and a second branch exhaust passages after it passes through a three-way reducing and oxidizing catalyst, and the two branch exhaust passages merge into an exhaust gas outlet passage. In the first branch exhaust passage, an oxidizing catalyst is disposed, and in the exhaust gas outlet passage, a denitrating and oxidizing catalyst is disposed. NOx in the exhaust gas from the engine is all converted to N2 and NH3 by the three-way reducing and oxidizing catalyst and a part of the NH3 generated by the three-way catalyst flows into the first branch exhaust passage and is converted to NOx again by the oxidizing catalyst. The amount of NOx produced by the oxidizing catalyst and the amount of NOx flowing through the second branch exhaust passage is determined by the flow distribution ratio of the first and the second branch exhaust passages. In this invention, the flow ratio is determined in such a manner that the amount of the NOx produced by the oxidizing catalyst and the amount of the NH3 passing through the second branch exhaust passage are stoichiometric to produce N2 and H2O. Therefore, when these exhaust gases flow into the denitrating and oxidizing catalyst after they mix with each other in the exhaust gas outlet passage, all the NOx and the NH3 are converted to N2 and H2O by the denitrating and oxidizing catalyst without producing any surplus NOx or NH3.