2.2 L 직접분사 디젤 엔진에서 LNT 흡장률이 NOx 정화 효율에 미치는 영향
Effect of LNT adsorption fraction on NOx conversion efficiency in 2.2 L direct-injection diesel engine
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초록
디젤엔진은 가솔린엔진 대비 우수한 열효율 및 적은 CO2 배출과 더불어 우수한 토크 성능을 지닌다. 그러나 디젤엔진은 국부적인 고온 반응에 의해 발생하는 질소산화물(NOx) 및 확산연소에 의한 입자상물질의 배출이 높다. 이러한 NOx배출 저감을 위하여 흡장형 NOx 촉매(lean NOx trap, LNT)는 실차 장착의 용이성으로 배기가스재순환 기술 및선택적 환원 촉매와 같이 사용할 경우 디젤엔진의 NOx 저감 성능을 극대화할 수 있다. 본 연구에서는 2.2 L 직접분사커먼레일 디젤 엔진에서 NOx 흡장률에 따라 재생을 실시하여 LNT 촉매의 NOx 정화 효율을 분석하였다. LNT 재생 시의 촉매 전단 공기과잉률은 0.92 ~ 0.97의 범위를 갖으며, CO 농도는 약 1.6 ~ 2.6 %, O2 농도는 1.1 ~ 1.4 % 수준이었다. 엔진 운전 조건에 관계없이 NOx 흡장률이 높을수록 NOx 정화 효율이 감소하였다. 또한 NOx 흡장률 50 % 이하로 흡장 후 재생 하였을 때 운전조건에 관계없이 NOx 전환 효율은 60 % 이상이었다. 흡장률이 낮은 영역에서의 촉매재생은 연료소모율이 증가하므로, 연료소모율의 최소화 및 NOx 정화 효율의 극대화를 동시에 고려하여 LNT 재생제어를 실시하여야한다.
Abstract
Diesel engines have higher thermal efficiency and lower CO2 emissions than gasoline engines, as well excellent low-to-medium speed torque characteristics. However, diesel engines produce higher levels of nitrogen oxides owing to the local high temperature reactions and the higher amount of particulate matter produced by diffusive combustion. To reduce NOx emissions, a lean NOx trap (LNT) maximizes its effectiveness to minimize NOx emissions from the diesel engine when used with exhaust gas recirculation and selective catalytic reaction technologies owing to its ease to mount on the actual vehicle. In this study, NOx conversion efficiency of an LNT catalyst was analyzed via the LNT regeneration according to the NOx adsorption fraction in a 2.2 L direct-injection common-rail diesel engine. The lambda values of the exhaust gas during the regeneration of the LNT were within the range of 0.92 ~ 0.97, the CO concentration was approximately 1.6 ~ 2.6%, and the O2 concentration was approximately 1.1 ~ 1.4%. Regardless of the engine operating conditions, the NOx conversion efficiency decreased as the NOx adsorption fraction increased. When the LNT was regenerated at less than 50% of the NOx adsorption fraction, the NOx conversion efficiency was observed to be more than 60%, regardless of the operating conditions. Because the LNT regeneration at the lower NOx adsorption fraction leads to an increase in the fuel consumption, the LNT regeneration strategy should consider the simultaneous minimization of the fuel consumption rate and the maximization of the NOx conversion efficiency.
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