Patent ID: 8534053

Claim:
An exhaust purification apparatus for an internal combustion engine comprising: an NOx catalyst provided in an exhaust system of the internal combustion engine to selectively reduce NOx; a reducing agent inlet provided upstream of the NOx catalyst, through which a urea aqueous solution or ammonia is supplied to the NOx catalyst as a reducing agent; a first NOx sensor provided upstream of the reducing agent inlet and a second NOx sensor provided at an outlet of the NOx catalyst, each of the NOx sensors reacting both to NOx and to ammonia; and a controller configured to: determine whether a bed temperature of the NOx catalyst has exceeded a threshold set in a predetermined temperature region; calculate an actual amount of reducing agent added through the reducing agent inlet using outputs from the first and second NOx sensors when the bed temperature of the NOx catalyst has exceeded the threshold; and correct the actual amount of reducing agent so that the actual amount of reducing agent follows a target addition amount if a deviation between the actual amount of reducing agent and the target addition amount is equal to or larger than a predetermined amount, wherein the predetermined temperature region is a temperature region that is equal to or higher than such a temperature that half of the reducing agent added is converted into NOx in a process where a rate at which the reducing agent is converted into NOx increases as the bed temperature of the NOx catalyst increases.