| 摘要 |
PROBLEM TO BE SOLVED: To prevent overheating of an urea water addition valve by increasing an addition amount of an ammonia-based reducing agent such as urea water without generating an ammonia slip.SOLUTION: When an estimated exhaust temperature TH2est before SCR at the time of fuel addition exceeds an ammonia slip exhaust temperature threshold value Tc (YES in S116), an urea water addition amount is increased and the increase treatment is intermittently executed (S118-S126). Due to the intermittent increase, a period (tu1) when a normal amount of ammonia is supplied to an SCR catalyst and a period (tu2) when a large amount of ammonia is supplied are repeated. Even when an adsorption amount on the surface of the SCR catalyst comes close to saturation in the period tu2, ammonia moves to inside of the SCR catalyst or reacts with NOx in the next period tu1, so that ammonia adsorption allowance on the surface recovers. Therefore, compared to normal time, a larger amount of urea water can be added from the urea water addition valve without generating an ammonia slip so as to enable cooling of the urea water addition valve. |
