Characteristics of semi-premixed diesel combustion with a twin peak shaped heat release (twin combustion) were investigated under several in-cylinder gas conditions in a 0.55 L single cylinder diesel engine with common-rail fuel injection, super-charged, and with low pressure loop cooled EGR. The first-stage combustion fraction, the second injection timing, the intake oxygen concentration, and the intake gas pressure influence on thermal efficiency related parameters, the engine noise, and the exhaust gas emissions was systematically examined at a middle engine speed and load condition (2000 rpm, 0.7 MPa IMEP). The twin peak shaped heat release was realized with the first-stage premixed combustion with a sufficient premixing duration from the first fuel injection and with the second fuel injection taking place just after the end of the first-stage combustion. Optimizing the first-stage combustion fraction and the second injection timing to the position of the twin peak shape heat release, significant reductions in engine noise when compared with the premixed diesel combustion with a single injection were achieved, in addition to improved indicated thermal efficiency and decreased NOx emissions and without increases in other exhaust gas emissions, including the CO, THC, and smoke emissions. These excellent characteristics, with the exception of the smoke emissions, were confirmed for a wide range of intake oxygen concentrations. However, very large reductions in intake oxygen results in increases in the smoke emissions. Increasing the intake gas pressure effectively reduces the smoke emissions at lower intake oxygen concentrations and improves the indicated thermal efficiency, maintaining the low engine noise.