| 摘要 |
PROBLEM TO BE SOLVED: To improve activity of de-NOx catalyst by injecting a certain specified amount of hydrocarbon on the upstream of contact removal for nitrogen oxide at short time intervals periodically by a pulsating injection means, and performing contact conversion by use of platinum/alumina type catalyst. SOLUTION: When hydrocarbon is periodically pulsatively injected in an exhaust line on the upstream of a conversion catalyst provided in an exhaust system of an engine (in the drawing, A indicates a curve showing the change of hydrocarbon concentration), the change of NOx amount measured on the downstream of the conversion catalyst appears as a curve indicated by B in the drawing. Then, to reduce NOx in the exhaust gas, during the operation of the engine, preferably in the region of 1-10 seconds, hydrocarbon is intermittently injected. At this time, preferably gas oil is used for hydrocarbon to be injected, the injection temperature is 200 deg.C or more, and the injection is controlled according to the temperature of exhaust gas.
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