Patent ID: 7765800

Claim:
An exhaust emission control device comprising a selective reduction catalyst incorporated in an exhaust pipe of an engine and capable of selectively reacting NO x with ammonia even in the presence of oxygen, a pair of differently oxidative oxidation catalysts arranged in parallel with each other and upstream of the selective reduction catalyst, a divided passage for parting of the exhaust gas into the respective oxidation catalysts and for rejoining of the same after passage through the respective oxidation catalysts, exhaust distribution means for adjusting amounts of the exhaust gas distributed to the respective oxidation catalysts with respect to the divided passage such that the NO/NO 2 ratio in the exhaust gas is about 1-1.5, urea water addition means for selectively adding the urea water to either upstream or downstream side of the selective reduction catalyst, a first connection pipe branched from the exhaust pipe upstream of the divided passage and connected to the exhaust pipe downstream of the selective reduction catalyst, and a second connection pipe branched from the exhaust pipe at a position between the divided passage and a portion of the exhaust pipe from which a leading end of said first connection pipe is branched and connected to the exhaust pipe downstream of a portion of the exhaust pipe to which a trailing end of the first connection pipe is connected, proper changeover being conducted by passage changeover means between a first passage pattern for causing the exhaust gas from the engine to flow normally through the exhaust pipe and a second passage pattern for causing the exhaust gas from the engine to flow through the first connection pipe and in backflow through the selective reduction catalyst and oxidation catalysts to the second connection pipe.