Patent Application
20090056319
The invention relates to aftertreatment systems for internal combustion engine exhaust, and more particularly to chemical species injection, and catalysis.
To address engine emission concerns, new standards continue to be proposed for substantial reduction of various emissions, including NOx and particulate emissions. Increasingly stringent standards will require installation of aftertreatment devices in engine exhaust systems. Some of the aftertreatment technologies require certain chemical species to be injected into the exhaust system. For example, HC or fuel is injected in some active lean NOx systems, and additives such as cerium and iron are injected for diesel particulate filter regeneration, and urea solution is injected in selective catalytic reduction (SCR) systems for NOx reduction. These injected chemical species mix with exhaust gas before reaching downstream catalysts or filters.
Sole
Exhaust tube 12 has a catalyzed inner wall 30, e.g. provided by a catalytic coating, catalytically reacting with the exhaust prior to reaching aftertreatment element 24. Such catalyzed inner wall is upstream of aftertreatment element 24 and downstream of injector 26. Aftertreatment element 24 is supported in housing section 25. It is preferred that inner wall 30 extend to and along housing section 25 at outward taper 27 to aftertreatment element 24. Exhaust flows axially along an axial flow direction 32 to aftertreatment element 24. Inner wall 30 extends axially and parallel to exhaust flow along axial direction 32 and then is tapered outwardly at taper 27 along housing section 25 to aftertreatment element 24. Aftertreatment element 24 extends across and spans housing section 25 transversely to axial direction 32. In one embodiment, mixer 28 is downstream of injector 26. In another embodiment, as shown in dashed line, mixer 28a is upstream of injector 26. In a further embodiment, the mixer is catalyzed, e.g. with a catalytic coating, and catalytically reacts with the exhaust, for example as in commonly owned co-pending U.S. patent application Ser. No. 11/360,941, filed Feb. 23, 2006.
In the preferred embodiment, exhaust aftertreatment system 10 is an SCR, selective catalytic reduction, system for engine exhaust, including an exhaust tube or housing 12 having the noted upstream inlet 14 for receiving engine exhaust at 18, and the noted downstream outlet at 20 for discharging the exhaust at 22, an injector 26 for injecting urea reductant into the housing to hydrolyze to ammonia to react with and reduce NOx in the exhaust, an SCR catalyst 24 in the housing catalytically reacting with the exhaust, and wherein housing 12 has a catalyzed inner wall 30 catalytically reacting with the exhaust prior to reaching SCR catalyst 24 and accelerating the rate of urea hydrolysis to ammonia and reducing urea crystalline formation on inner wall 30.
The system further enables and provides a desirable method for reducing housing length in an SCR system, namely by providing housing 12 with a catalyzed inner wall 30 catalytically reacting with engine exhaust prior to reaching SCR catalyst 24 and accelerating the rate of urea hydrolysis to ammonia and reducing the length of inner wall 30 otherwise required between injector 26 and SCR catalyst 24. The urea reductant is injected in the exhaust flow upstream of the SCR catalyst, and the urea decomposes and hydrolyzes to ammonia on or upstream of SCR catalyst 24. The decomposition and hydrolysis typically takes place over a sufficiently long exhaust tube upstream of SCR catalyst 24. The length of such exhaust tube upstream of the SCR catalyst to enable sufficient decomposition and hydrolysis may be objectionable in vehicles with limited space and packaging constraints. The present system provides a simple and effective way of reducing such length.
Current production SCR systems used on low temperature duty cycle applications may experience issues with crystallization of the urea, which builds up on the inner surface of the exhaust tube and can cause blockage or restriction of exhaust gas flow. The present system reduces the occurrence of urea crystalline formation on inner wall 30 due to the catalytic reaction thereat. The present system thus both: a) reduces the length required of the decomposition or hydrolysis exhaust tube; and b) reduces the occurrence of crystalline formation on the inner tube wall.
In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. The different configurations, systems, and method steps described herein may be used alone or in combination with other configurations, systems and method steps. It is to be expected that various equivalents, alternatives and modifications are possible within the scope of the appended claims.
