The effects of in-cylinder EGR stratification on combustion and emission characteristics are investigated in a single cylinder direct injection diesel engine. 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. Stratified EGR potentially reduces NOx by maximum 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%. Moreover, the robustness of stratified EGR is evaluated under various operating conditions and injection strategies. (C) 2011 Elsevier Ltd. All rights reserved.