Exhaust Volume Flow based Time Alignment in NO _x Measurements in NOx Reduction Systems 2012-01-1959

With the emission norms becoming more and more stringent along with the focus on reducing ownership and operating costs, the need to optimize the aftertreatment system becomes much more evident. Thus, the well monitored, optimized usage of urea or ammonia (NH₃) for the NO_x reduction in an SCR system is critical to reduce the operating cost of the vehicles and to comply with emission regulations. In Ammonia Storage and Delivery System (ASDS), pure gaseous NH₃ from the NH₃ cartridges is being used for the reduction of the engine-out NO_x in the exhaust stream over the NPF (NO_x Particulate Filter). In almost all NO_x reduction systems, NO_x sensors play an important role in determining the amount of urea or NH₃ to be dosed for efficient NO_x reduction with minimal NH₃ consumption and slippage for best possible fluid economy. In the NH₃ dosing strategy the NO_x conversion efficiency plays a vital role as a feedback variable for calculating NH₃ dosing quantity and is based on the upstream and downstream NO_x sensor measurements. Since the NO_x sensors are separated by certain volume of aftertreatment, there is a time delay involved in the measurements. This paper discusses about the importance of the time delay correction needed in NO_x measurements between engine-out (pre-aftertreatment system) and tail pipe-out (post-aftertreatment system) locations for the effective implementation of the NH₃ dosing strategy. This is particularly important when the dosing strategy is in feed-forward mode purely based on NO_x sensors measurement with no ammonia storage and during transient engine operation.