Patent ID: 8336298

Claim:
An addition-amount controller for an exhaust gas purifying agent, the controller being applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine, the exhaust emission control system including a catalyst for promoting a specific exhaust gas purification reaction in a temperature range having a critical reaction temperature as a lower limit, and an addition valve for adding an additive of NH 3 (ammonia) or an additive serving as a generating source of the NH 3 to the catalyst itself or the exhaust gas on an upstream side with respect to the catalyst, the additive being adapted to purify NO x (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst, the addition-amount controller being adapted to control an amount of addition by the addition valve, the catalyst having properties of storing NH 3 and further decreasing the critical reaction temperature as an amount of NH 3 storage is increased, the addition-amount controller comprising: a mode selection unit configured to select one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; and an execution condition determining unit configured to determine whether the execution condition of the storage control mode is satisfied, wherein the execution condition determining unit determines that the execution condition of the storage control mode is satisfied when a temperature of the catalyst becomes lower than a predetermined temperature, and when a load on an output shalt of the internal combustion engine becomes higher than an allowable load; wherein a required NH 3 storage amount is variably set according to a target value of a NO x purification ratio on the catalyst.