| 摘要 |
A NOx purification system having a selective reduction catalyst (33), adapted so that even when no ammonia or urea is supplied, NOx is converted to ammonia by a first catalyst (32) consisting of a lean NOx catalyst or a ternary catalyst at ammonia formation control and the ammonia is adsorbed by the selective reduction catalyst (33) disposed downstream and so that by the adsorbed ammonia, there is carried out reduction purification of the NOx contained in the exhaust gas when no ammonia formation control is performed by the selective reduction catalyst (33). In the system, at the ammonia formation control, an ammonia adsorption target amount (At) being a target value of ammonia adsorbed by the selective reduction catalyst (33) is computed, and the ammonia formation control is carried out only when a cumulative value (A3) of an ammonia formation amount (A4) formed by the first catalyst (32) at the ammonia formation control is under the ammonia adsorption target amount (At). Consequently, ammonia is stably supplied in just proportion to the selective reduction catalyst (33). |
