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In recent years, catalyst systems such as a lean NOx trap (LNT) catalyst system and a urea selective catalytic reduction (SCR) system have been developed to obtain cleaner diesel emissions. At Nissan, we developed an emission control system for meeting Tier 2 Bin 5 requirements in 2003. On the basis of that technology, a new HC-NOx trap catalyst system has now been developed that complies with the SULEV standards without increasing the catalyst volume and precious metal loading. Compliance with the SULEV standards requires a further reduction of HC (NMHC) emissions by 84% and NOx by 60% compared with the emission performance Tier 2 Bin 5 compliant catalyst system. Consequently high conversion performance for both HCs and NOx is needed. An investigation of HC emission behavior under the FTP75 mode showed that a reduction of cold-phase HCs was critical for meeting the standard. Large quantities of HCs above C4 are emitted in the cold state.

An exhaust gas recirculation (EGR) system that recirculates a portion of the exhaust gas back to the intake system is effective in reducing nitrogen oxide (NOx) emissions from diesel engines. However, improved control accuracy over the EGR flow rate is required, because an excessively large flow rate causes emissions of particulate matter (PM) to increase. In recent years, direct injection (DI) diesel engines have also been used on ordinary passenger cars, because their fuel economy is superior to that of indirect injection (IDI) diesel engines. Since DI engines are more sensitive to the EGR flow rate than their IDI counterparts, improving the accuracy of EGR flow rate control has become even more significant. This study concerned an EGR control algorithm based on the results of calculations performed with an engine model capable of representing the dynamic states of the intake and exhaust systems.