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An external injection system was introduced to control the lean Nox trap(LNT). LNT absorbs Nox in lean condition of exhaust gas and discharges N2 by reducing Nox in rich condition. To make exhaust gas lean or rich, fuel injection into the exhaust gas is used with the engine control. There are two ways to add a reducing agent into exhaust gas. One is post injection using common rail system and the other is external injection. The external injection has prior benefit; can be controlled independently without disturbing engine control, can be adapted to various layout of exhaust system, has no oil dilution problem, and etc. In this study the effect of an external fuel injection on the control of LNT system was investigated. At first, the injection characteristic of the external injection was analyzed: flow rate and atomization characteristic was controlled by the PWM signal and the fuel pressure. Then the external injection system was introduced into NPRS(Nox PM Reduction System).