SCR EXHAUST EMISSION CONTROL SYSTEM AND METHOD THEREFORE, FOR FILLING THE UREA REDUCING AGENT AFTER RETURNING TO THE TANK

摘要

An exhaust emission control system for an internal combustion engine includes a oxidation catalyst and particulate filter combination, upstream of a reducing agent supply mechanism (200) in front of a SCR-catalyst (41) and ammonia oxidation catalyst (51) combination. The SCR-catalyst is configured to purify exhaust gas by being supplied with the reducing agent urea. The reducing agent supply mechanism has an addition injection valve (230) configured to inject the urea reducing agent to the exhaust passage. The reducing agent supply mechanism is configured to supply the reducing agent to the addition valve and to recover the reducing agent from the addition valve, i.e. return back the urea to the tank. The electronic control unit is configured to execute a filling process of filling the reducing agent into the addition valve (S230) when a temperature of the addition valve is higher than or equal to a boiling point of the reducing agent (S220).