| abstract |
In performing of a reduction purification of NOx contained in exhaust gas (G) comprising spouting urea water (U) through addition nozzle (7) to the upstream side of NOx reducing catalyst (6) incorporated in exhaust passage (5) of diesel engine (1), a target adsorption rate curve as obtained by shifting, to a low temperature side, an ammonia saturated adsorption rate curve of NOx reducing catalyst (6) is set up. Then, an ammonia target adsorption rate corresponding to catalyst temperature is computed, and an actual rate of ammonia adsorbed onto the NOx reducing catalyst (6) is determined. Further, the rate of urea water (U) to be added to the upstream side of NOx reducing catalyst (6) is decreased when the actual adsorption rate reaches a target adsorption rate and is increased when the actual adsorption rate becomes lower than the target adsorption rate. Thus, while preventing desorption of ammonia, the adsorption rate of ammonia is ensured to thereby attain saving of urea water (U). |