Low pressure loop (LPL) EGR systems are effective means of simultaneously reducing the NOx emissions and fuel consumption of diesel engines. Further lower emission levels can be achieved by adopting a system that combines LPL EGR with a NOx storage and reduction (NSR) catalyst. However, this combined system has to overcome the issue of combustion fluctuations resulting from changes in the air-fuel ratio due to EGR gas recirculation from either NOx reduction control or diesel particulate filter (DPF) regeneration. The aim of this research was to reduce combustion fluctuations by developing LPL EGR control logic.
In order to control the combustion fluctuations caused by LPL EGR, it is necessary to estimate the recirculation time. First, recirculation delay was investigated. It was found that recirculation delay becomes longer when the LPL EGR flow rate or engine speed is low. In contrast, even if the engine speed changes, the delay was found to be proportional to the number of engine cycles. A model was developed to express this phenomenon. A good correlation was found between actual measurements and the recirculation delay estimated by the model.
Next, the control logic for LPL EGR was studied. The recirculation gas under rich operating conditions was detected by an air-fuel ratio sensor to examine a method of controlling the EGR valve in accordance with the timing for the rich gas to actually reach the EGR valve. Thus, fluctuations in torque and combustion noise were improved. On the other hand, an air-fuel ratio sensor may not be equipped in the system which does not use a NSR catalyst. Therefore, a method of predicting air-fuel ratio and controlling the intake oxygen concentration was developed separately for DPF regeneration. As a result, combustion variations due to DPF regenerations have also been improved.