A comparative study on the effects of air-fuel mixing quality on combustion characteristics was carried out in both conventional and low temperature diesel combustion (LTC) regimes. The injection pressure and intake pressure were considered as variables as they are important factors which influence the air-fuel mixing process. The intake O-2 concentration was varied to realize different combustion regimes. Improved air-fuel mixing with a higher injection pressure enhanced the combustion process in both conventional combustion and LTC regimes, resulting in higher peaks of in-cylinder pressure and heat release rate. The combustion phase in the LTC regime was more influenced by injection pressure due to longer premixing time than that of conventional combustion. A higher injection pressure reduced CO and HC emissions over a wide range of intake O-2 concentrations. The reduction of CO and HC emissions in the conventional combustion regime was due to higher combustion temperature, while that in the LTC regime was due to decreased under-mixed fuel by improved air-fuel mixing. Soot emissions at a higher injection pressure were reduced, particularly, in the conventional combustion regime where the soot formation rate is high. The increase of intake pressure was also advantageous in reducing CO, HC and soot emissions due to improved air-fuel mixing as well as enrichment of absolute amount of oxygen, which lead to enhanced combustion process. A direct flame image was taken to observe the flame structure of two different combustion regimes to correlate with the exhaust emission results and combustion characteristics. High flame luminosity was observed around the periphery of the spray jet in the conventional combustion regime, which was a direct indication of soot formation and high temperature combustion; while low luminosity was observed around the piston bowl in the swirl direction in the LTC regime, which indicated a longer air-fuel mixing period and low temperature combustion. (C) 2014 Elsevier Ltd. All rights reserved.