In order to make better air utilization in combustion chamber to improve the mixture quality, enhance combustion efficiency and reduce emissions, a new swirl chamber combustion system of DI (direct injection) diesel engines is proposed. The in-cylinder spray, mixture formation and combustion progresses are simulated and investigated at different nozzle angles with AVL-FIRE code. Results show that in the current combustion system, spray angle of 146 degrees is the best on the aspect of mixture homogeneity among all the cases. NO mass fraction is the lowest at spray angle of 154 degrees, while Soot mass fraction is the lowest at spray angle of 146 degrees. At the initial stage of spray, the fuel/air equivalence ratio distribution was primarily controlled by the squish and swirl, while after ignition in expansion stroke, the combustion swirl and reverse squish had great impact on the in-cylinder temperature distribution. To sum up, the spray angle and air motion matching with the combustion chamber is a key factor on combustion process and emissions performance. Considering the emission performance, spray angle of 150 degrees is a relatively better compromise in the new combustion system. (C) 2014 Elsevier Ltd. All rights reserved.