Selective Catalytic Reduction (SCR) systems have been shown to be the most promising exhaust aftertreatment system for near term in-use applications to meet the stringent US 2010 oxides of nitrogen (NOx) emissions regulations of 0.2 g/bhp-hr for on-highway heavy duty diesel engines. SCR systems use the ammonia-containing compound urea, as a reducing agent. In order to control the urea dosage during transient operation of the engine, sophisticated control strategies are needed. This study discusses the development of an open loop, non-sensor based fuzzy logic urea dosage controller. The goal of the fuzzy logic based control was to achieve maximum NOx emission reduction, while limiting the amount of ammonia slip. The open loop controller was implemented on a heavy duty diesel engine equipped with a catalyzed diesel particulate filter (DPF) and a SCR system. The control system was quantified by operating the engine over different test cycles on an engine dynamometer. This study shows that the fuzzy logic approach is a simple and effective way to control NOx, as well as ammonia slip.

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