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In-cylinder EGR stratification for NOx and PM reduction

Engineers from the SNU Automotive Laboratory, School of Mechanical and Aerospace Engineering, Seoul National University (S. Korea) report that in-cylinder stratified exhaust gas recirculation (EGR) in a direct-injection diesel engine potentially reduces NOx by up to 45%, without penalties of performance and other emissions.

A proper in-cylinder swirl with stratified EGR maximizes the effects and achieves simultaneous reduction of NOx by 7% and PM by 23%, they report in a paper in the journal Energy.

To achieve in-cylinder EGR stratification, external EGR rates of two intake ports are varied by supplying EGR asymmetrically using a separated intake runner. The EGR stratification pattern is improved using a 2-step bowl piston and an offset chamfer at the tangential intake port. When high EGR gas is supplied to the left (tangential) port, a high EGR region is formed at the central upper region of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and PM is reduced significantly.

When high EGR gas is supplied to the right (helical) port, a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NOx is reduced without PM penalty.

—Choi et al.

They evaluated the robustness of stratified EGR under various operating conditions and injection strategies.


  • Seungmok Choi, Wonah Park, Sangyul Lee, Kyoungdoug Min, Hoimyung Choi (2011) Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine, Energy DOI: 10.1016/j.energy.2011.09.016


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