Patent ID: 8359841

Claim:
An exhaust purifying apparatus for an internal combustion engine comprising: a SCR catalyst, which is disposed in an exhaust passage of said engine and purifies nitrogen oxides contained in an exhaust gas discharged from said engine by reducing said nitrogen oxides by ammonia; a downstream air-fuel ratio sensor, disposed at a position downstream of said SCR catalyst in said exhaust passage and including a solid electrolyte layer, an exhaust gas side electrode layer, an atmospheric air side electrode layer exposed in a space to which atmosphere is introduced, and a diffusion resistance layer, in which said exhaust gas side electrode layer and said atmospheric air side electrode layer are formed on both surfaces of said solid electrolyte layer so as to oppose to each other to sandwich said solid electrolyte layer, and in which said exhaust gas side electrode layer is covered by said diffusion resistance layer, said downstream air-fuel ratio sensor outputting an output value varying in accordance with a concentration of oxygen at said exhaust gas side electrode layer of a gas which reaches said exhaust gas side electrode layer after passing through said diffusion resistance layer; additive agent supplying means for supplying an additive agent including urea-water or ammonia at a position upstream of said SCR catalyst in said exhaust passage in response to an instruction; and a controller that includes control logic, which when executed: obtains a first output value which varies in accordance with a concentration of oxygen of said exhaust gas which is in a state where nitrogen oxides contained in said exhaust gas have not been purified by said SCR catalyst; obtains a second output value which is a value based on said output value of said downstream air-fuel ratio sensor when said additive agent supplying means supplies said additive agent; obtains an ammonia-amount-relating-value which relates to an amount of ammonia which flows out from said SCR catalyst based on a difference between said first output value and said second output value; determines an amount of said additive agent to be supplied based on said ammonia-amount-relating-value, and sends to said additive agent supplying means an instruction to supply said determined amount of said additive agent, wherein the controller includes control logic to: obtain, as said first output value, said output value of said downstream air-fuel ratio sensor when said additive agent supplying means does not supply said additive agent; obtain, as said second output value, said output value of said downstream air-fuel ratio sensor when said additive agent supplying means supplies said additive agent; and send to said additive agent supplying means an instruction to stop supplying said additive agent in order to obtain said first output value, when a first predetermined condition is satisfied.