De-NOx catalytic systems generally perform better in rich or stoichiometric engine operating conditions due to high hydrocarbon (HC) concentrations in the exhaust gas. However, the hydrocarbon concentration in a diesel engine is typically low, so post or supplemental fuel injections into the exhaust gas have been employed in order to increase the HC concentration. A hydrocarbon-type lean NOx trap (HC-LNT) catalyst system that is a De-NOx system based on NOx-absorbing catalyst has also been developed to optimize control of the external HC injection into the diesel exhaust pipe. The system has a secondary injector that injects diesel fuel (HC) into an exhaust manifold. The typically high temperature (250-350 degrees C) in the exhaust manifold affects the spray behavior of the secondary injector, and the high temperature also makes it particularly difficult to achieve uniform distribution of the reducing agent in the manifold. Thus, it is necessary to use a mixer to improve the fuel distribution in the exhaust manifold. In this study, the effects of various mixer types on the spatial distribution of the LNT reducing agent were investigated to improve the performance of the LNT catalyst system. While the LNT reducing agent was injected directly toward the mixers in a transparent manifold, spray images were collected using a high-speed camera, and the spatial distribution of the spray was analyzed using image processing techniques. The analysis on the spatial distribution of the spray represented that mixers were beneficial to achieve uniform distribution of the reducing agent in exhaust pipe and improve the performance of LNT catalyst system. (C) 2011 Elsevier Ltd. All rights reserved.