초록

An external reductant for an SCR vehicular NO.sub.x emission control system is selectively injected to SCR catalysts which are only at catalytically active temperatures thereby avoiding reductant absorption in catalytically unactive catalysts and improving conversion response time.

대표청구항 ▼

[ What is claimed is:] [1.] A method for converting NOx emissions in the exhaust gases of a vehicle powered by a diesel engine comprising the steps of:a) providing a first SCR catalyst catalytically active when its catalyst bed temperature is within a temperature range of a first low temperature to

[ What is claimed is:] [1.] A method for converting NOx emissions in the exhaust gases of a vehicle powered by a diesel engine comprising the steps of:a) providing a first SCR catalyst catalytically active when its catalyst bed temperature is within a temperature range of a first low temperature to a first high temperature;b) providing a second SCR catalyst chemically different from the first SCR catalyst and catalytically active when its catalyst bed temperature is within a temperature range of a second low temperature to a second high temperature, the second low temperature being less than the first low temperature;c) sequentially passing the exhaust gas in any order from one of the SCR catalysts to the other SCR catalysts;d) metering a reductant selected from the group consisting of ammonia and an ammonia precursor(s) during warm-up of the engine to the second SCR catalyst until the first SCR catalyst bed is heated from ambient temperature to a temperature whereat it ceases to function as a heat sink; and,e) thereafter and during warm-up, metering the reductant to any one of the SCR catalysts having a catalytically active bed whereby oxidation of reductant and substantial retention of reductant on the catalyst bed is avoided.