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2010 and future EPA regulations introduce stringent Oxides of Nitrogen (NOx) reduction targets for diesel engines. Selective Catalytic Reduction (SCR) of NOx by Urea over catalyst has become one of the main solutions to achieve these aggressive reductions. As such, urea solution is injected into the exhaust gas, evaporated and decomposed to ammonia via mixing with the hot exhaust gas before passing through an SCR catalyst. Urea mixers, in this regard, are crucial to ensure successful evaporation and mixing since its liquid state poses significant barriers, especially at low temperature conditions that incur undesired deposits. Intensive efforts have been taken toward developing such urea mixers, and multiple criteria have been derived for them, mainly including NOx reduction efficiency and uniformity. In addition, mixers must also satisfy other requirements such as low pressure drop penalty, mechanical strength, material integrity, low cost, and manufacturability.

EPA 2015 Tier IV emission requirements pose significant challenges to large diesel engine after treatment system development with respect to reducing exhaust emissions including HC, CO, NOx and Particulate Matter (PM). For a typical locomotive, marine or stationary generator engine with 8 to 20 cylinders and 2500 to 4500 BHP, the PM reduction target could be over 90% and NOx reduction target over 75% for a wide range of running conditions. Generally, HC, CO and PM reductions can be achieved by combining DOC, cDPF and active regeneration systems. NOx reduction can be achieved by injecting urea as an active reagent into the exhaust stream to allow NOx to react with ammonia per SCR catalysts, as the mainstream approach for on-highway truck applications.

The introduction of stringent EPA 2015 regulations for locomotive / marine engines and IMO 2016 Tier III marine engines initiates the need to develop large diesel engine aftertreatment systems to drastically reduce emissions such as SOx, PM, NOx, unburned HC and CO. In essence, the aftertreatment systems must satisfy a comprehensive set of performance criteria with respect to back pressure, emission reduction efficiency, mixing, urea deposits, packaging, durability, cost and others. Given multiple development objectives, a systematic approach must be adopted with top-down structure that addresses top-level technical directions, mid-level subsystem layouts, and bottom-level component designs and implementations. This paper sets the objective to provide an overview of system development philosophy, and at the same time touch specific development scenarios as illustrations.