NOx Conversion Improved with Model-Based Urea Injection Control: To maintain a high NOx conversion rate, the urea injection quantity is generally controlled using a map-based urea injection control system. Under transient operating conditions, the above control method may not be precise enough to keep non-reacted ammonia at a low level. A group of researchers from Waseda University, Tokyo Roki Ltd., and Nissan Diesel Motor Co., Ltd., investigated ways to reduce ammonia emission while increasing the NOx conversion rate or at least maintaining it at an acceptable level. They developed a one-dimensional chemical reaction model to calculate NOx and ammonia emissions. Using this model, they investigated the effectiveness of a model-based urea injection control system and reported the results compared with those of the map-based urea injection control system.