Exhaust emission purification apparatus for engine

1. An exhaust emission purification apparatus for an engine, comprising:

a reduction catalyst disposed in an exhaust passage of the engine;

an addition device for adding a NOx reducing agent into the exhaust gas of the engine, the addition device including an injection nozzle disposed in the exhaust passage at a position upstream of the reduction catalyst, the addition device for injecting the NOx reducing agent via the injection nozzle; and

a passage member interposed in the exhaust passage, the passage member forming an internal circumferential surface of at least a part of the exhaust passage between the injection nozzle and the reduction catalyst,

wherein the passage member is joined by welding in an axial direction of the passage member into a hollow shape with the internal circumferential surface, and is disposed in the exhaust passage in a situation where the welding portion is located on the internal circumferential surface at a position where a normal line of the internal circumferential surface that is drawn at said position inclines vertically downward with respect to a horizontal direction extending from said position,

wherein the passage member comprises, in the welding portion, a weld bead swelling on the internal circumferential surface.

2. The exhaust emission purification apparatus for an engine according to claim 1 , wherein the passage member is rectangular in cross section.

3. The exhaust emission purification apparatus for an engine according to claim 1 ,

wherein the welding portion is disposed vertically higher than a central axis of the passage member.

4. The exhaust emission purification apparatus for an engine according to claim 1 , further comprising:

a first oxidation catalyst disposed in the exhaust passage at a position upstream of the injection nozzle; and

a second oxidation catalyst for treating the reducing agent, the second oxidation catalyst being disposed at a position downstream of the reduction catalyst.

5. The exhaust emission purification apparatus for an engine according to claim 1 , further comprising a urea water tank for storing a urea water as the reducing agent, the urea water tank being connected to the addition device.

6. An engine comprising: an engine body having a combustion chamber; and the exhaust emission purification apparatus according to claim 1 , the apparatus being disposed in an exhaust system of the engine.

7. The exhaust emission purification apparatus for an engine according to claim 1 , wherein the passage member is cylindrical.

8. The exhaust emission purification apparatus for an engine according to claim 7 , wherein the passage member is oval in cross section.

9. The exhaust emission purification apparatus for an engine according to claim 1 , further comprising:

an oxidation catalyst disposed in the exhaust passage at a position upstream of the injection nozzle; and

a NOx reduction catalyst as the reduction catalyst, disposed at a position downstream of the injection nozzle,

wherein the passage member has a plurality of exhaust distribution holes arranged in a circumferential direction of the passage member and is disposed between the injection nozzle and the reduction catalyst substantially concentric with the oxidation catalyst.

10. The exhaust emission purification apparatus for an engine according to claim 9 , further comprising:

a muffler housing for accommodating therein the oxidation catalyst and the reduction catalyst, the inside of the muffler housing being divided into a first muffling chamber, a second muffling chamber, and an intermediate chamber located between the first and second muffling chambers, by a plurality of partitions perpendicular to the axial direction,

wherein an exhaust passage arranged upstream of the muffler housing is inserted in the muffler housing in the axial direction, thereby communicating with the first muffling chamber,

the oxidation catalyst is accommodated in a first catalyst storage cylinder disposed in the intermediate chamber on one side with respect to a central axis of the muffler housing so as to allow a communication between the first and second muffling chambers,

the reduction catalyst is accommodated in a second catalyst storage cylinder disposed in the intermediate chamber on the other side with respect to the central axis so as to be connected with the second muffling chamber,

the passage member is disposed concentric with the oxidation catalyst in the second muffling chamber, and

an exhaust passage downstream of the muffler housing is inserted into the muffler housing to pass through the first muffling chamber in the axial direction, and thereby communicating with the second muffling chamber via the second catalyst storage cylinder.

11. An exhaust emission purification apparatus for an engine, comprising:

reduction means for reducing NOx in an exhaust gas of the engine;

addition means for adding a NOx reducing agent to the exhaust gas at a position upstream of the reduction means; and

passage formation means for forming at least a part of an exhaust passage between the addition means and the reduction means,

wherein the passage formation means is joined by a welding portion in an axial direction of this means into a hollow shape with an internal circumferential surface along which the exhaust gas and the added reducing agent are guided toward the reduction means, and the passage formation means is disposed in the exhaust passage in a situation where the welding portion is located on the internal circumferential surface in a position where the internal circumferential surface of this means extends to overhang,

wherein the passage formation means comprises, in the welding portion, a weld bead swelling on the internal circumferential surface.

12. A method for supplying a NOx reducing agent to a reduction catalyst disposed in a passage for an exhaust gas containing NOx, the method comprising the steps of:

adding the reducing agent into the exhaust gas at a position upstream of the reduction catalyst; and

introducing the added reducing agent into the reduction catalyst via a passage member which forms an internal circumferential surface of at least a part of an exhaust passage extending upstream of the reduction catalyst,

wherein the passage member is joined by a welding portion in an axial direction of the passage member into a hollow shape with the internal circumferential surface, and is disposed in the exhaust passage in a situation where the welding portion is located on the internal circumferential surface in a position where the internal circumferential surface of the passage member overhang,

wherein the passage member comprises, in the welding portion, a weld bead swelling on the internal circumferential surface.