Development of Next-Generation NOx Reduction System for Diesel Exhaust Emission

Diesel particulate and NOx reduction system (DPNR) is an effective technology for the diesel after-treatment system, which can reduce particulate matter (PM) and nitrogen oxides (NOx) simultaneously. The DPNR has been developed under the Toyota D-CAT (Diesel Clean Advanced Technology) concept. Further improvement of the DPNR is hoped for cleaner air in the future.

This paper reviews the results of our study to improve the NOx purification performance on the DPNR. The NOx reduction performance of the catalysts deteriorates due to thermal deterioration and sulfur poisoning. In order to improve the thermal resistance of the catalysts, the suppression of precious metal sintering in the catalyst has been studied. As a result, higher catalytic activity after aging especially under lower temperature conditions was obtained. On the other hands, improvement of desulfurization performance is one of the key technologies in order to keep the high NOx reduction capability of the catalyst. Keeping high and uniform catalyst bed temperature has accelerated the sulfur desorption from the storage material. For the development of the DPNR with high efficiency, the effect of rich condition for NOx reduction has also been investigated. The NOx reduction by the combination of oxygen reduction in cylinder, the air fuel ratio is leaner than stoichiometry, and the exhaust port injection (EPI) was as effective as having a rich combustion for the higher conversion efficiency at low bed temperature.

As a result of these improvements, it is shown that the improved DPNR has higher NOx conversion efficiency of over 70% on new European driving cycle (NEDC) after aging.