In order to select optimum diesel engine systems and develop new control logic, a zero-dimensional diesel combustion model for Total Engine Simulation System (TESS) has been developed. It is possible to predict fuel efficiency and exhaust emission with relatively high accuracy considering its high computational speed. This paper presents model improvements in heat loss, in-cylinder flow, turbulence and soot of the combustion model. Furthermore validation results of prediction accuracy at a wide range of operating points and engine displacement are shown.