The possible NOx and soot limits that a conventional diesel engine could meet without the assistance of aftertreatment system were investigated on an engineering level. A methodology combining both experiment and numerical simulation was used to evaluate favorable and unfavorable effects of various in-cylinder strategies quantitatively. These strategies or factors include combustion chamber geometry, fuel injection strategy, exhaust gas recirculation (EGR), intake valve close (IVC) timing, and turbocharger. Interactions among these strategies were paid special attention. Two steps to achieve as low as possible emissions were proposed based on analysis of these strategies. The first step would shift the NOx-soot trade-off curve closer to low emission regions via optimization of injection strategy, combustion chamber geometry, IVC, and turbocharger. As a result, NOx and soot could be simultaneously reduced by approximately 14% and 62% respectively. In the second step, NOx and soot emissions were adjusted along the trade-off curve of the first step by changing EGR rate. At 20% EGR, NOx could be reduced by 65% at the cost of soot 20% increase. Thus the original engine’s emission of NOx and soot could be reduced approximately from 7.8 g/(kW·h) and 0.011 g/(kW·h) to 2.3 g/(kW·h) and 0.005 g/(kW·h), respectively. Therefore, with the assistance of aftertreatment system, Euro VI NOx and soot emission limits could possibly be achieved without much effort, leaving some margin for stricter restrictions in the future.