A control strategy of low-pressure loop exhaust gas recirculation and NO x storage catalyst for diesel engines

Abstract

Low-pressure loop exhaust gas recirculation systems are effective means of simultaneously reducing the NO_x emissions and fuel consumption of diesel engines. Further lower emission levels can be achieved by adopting a system that combines low-pressure loop exhaust gas recirculation with a NO_x storage and reduction catalyst. However, this combined system has to overcome the issue of combustion fluctuations resulting from changes in the air–fuel ratio due to exhaust gas recirculation from rich operating conditions. The aim of this research was to reduce combustion fluctuations by developing low-pressure loop exhaust gas recirculation control logic. In order to control the combustion fluctuations caused by low-pressure loop exhaust gas recirculation, it is necessary to estimate the recirculation time. First, recirculation delay was investigated, and a model was developed. A good correlation was found between actual measurements and the recirculation delay estimated by this model. Next, the control logic for low-pressure loop exhaust gas recirculation was studied. The recirculation gas under rich operating conditions was detected by an air–fuel ratio sensor to examine a method of controlling the exhaust gas recirculation valve in accordance with the timing for the rich gas to reach the exhaust gas recirculation valve actually. Thus, fluctuations in torque and combustion noise were improved.