The spray and combustion characteristics of gasoline and diesel were investigated in a direct injection compression ignition engine equipped with a common rail injection system. The spray evolution was observed under a non-evaporating condition in a constant volume chamber and under an evaporating condition in an optical engine. Under the non-evaporating condition, the liquid penetration length was similar between the gasoline and diesel. The gasoline spray exhibited a relatively larger spray cone angle than that of diesel spray. However, the gasoline spray exhibited a significantly shorter liquid penetration length and narrower spray angle than that of the diesel spray under the evaporating condition. The maximum liquid penetration length was maintained constant regardless of the injection pressure for each fuel at the evaporating condition. The diesel spray formed wall wetting through the fuel impingement on the combustion chamber due to the long liquid penetration length at an early injection timing of -32 crank angle degree after top dead center (CAD ATDC). A series of combustion experiments was performed in order to investigate the performance and emissions in a metal engine and the flame characteristics in an optical engine. A low load condition (indicated mean effective pressure (IMEP) of approximately 0.45 MPa) was tested under an injection timing range from -40 to 0 CAD ATDC. The maximum thermal efficiency was similar between the two fuels with injection in close vicinity of the TDC. The gasoline combustion created a larger amount of hydrocarbon, carbon monoxide, and comparable nitric oxides (NOx) but had a lower soot emission compared with diesel combustion. However, the NOx emission of the gasoline combustion was significantly reduced with the pre-mixed charge compression ignition (PCCI) combustion via early injection. The direct combustion visualization demonstrated that the natural luminosity (NL) of the gasoline combustion was dominated by the chemiluminescence from the premixed burn while the NL of the diesel combustion was primarily attributed to the soot incandescence from the diffusion burn. However, the PCCI combustion via the early injection was dominated by the chemiluminescence from the premixed burn for both fuels. (C) 2013 Elsevier Ltd. All rights reserved.