Abstract
Exhaust gas regulations are becoming more stringent as a result of growing global concern towards environmental issues. Three-way catalysts are widely used in order to reduce exhaust emissions from gasoline engines, but it is difficult to apply the same method to diesel engines, which produce excess O2. For this reason, research on NOx reduction performance in diesel engines represents a considerable challenge.
In order to respond to ever-stricter exhaust gas regulations, the authors developed an exhaust gas after-treatment system for diesel engines. The system employs a lean NOx catalyst (LNC) which utilizes NH3-based selective catalytic reaction mechanism for NOx reduction. Some of the NOx adsorbed by the catalyst under lean mixture conditions is converted into NH3 under rich mixture conditions. When the mixture becomes lean again, this NH3 reduces NOx in the exhaust gas. By repeating this process, the LNC system realizes effective NOx reduction.
However, the reduction of NOx emissions during engine start-up is a difficult challenge, even with the use of the LNC system. A variety of approaches toward resolution of this issue are being researched. These include the application of cooperative engine control for rapid achievement of catalyst activation temperature following engine start and the development of catalyst materials capable of lower activation temperatures. However, to date these approaches have proved unsatisfactory.
This paper presents the results of a feasibility study on a new exhaust gas after-treatment system, focused in particular on NOx reduction, as an alternative approach to those mentioned above. The system combines a compact fuel reformer and a De-NOx converter utilizing a catalyst able to adsorb NOx at low temperatures, and includes a downstream LNC converter. Tests demonstrated that this modified LNC system achieves significant NOx reductions during the initial stage of engine start-up.
Keywords: Diesel Engine, NOx Reduction, Fuel Reformer, Exhaust Control, H2-SCR