With the improvements in Diesel engine injection systems, the fueling-path, which is more accurate, flexible, and faster than the air-path, can be actively utilized in conventional and advanced combustion mode controls, especially for enhancing the combustion transient performance. In this paper, fuel injection split models are proposed to describe the relationship between fuel split ratio and two combustion outputs, i.e., the crank angle at 50% heat released (CA50) and the indicated mean effective pressure (IMEP). The model parameters are related to the engine in-cylinder thermal boundary conditions, referred to as the in-cylinder conditions (ICCs). The models were verified by engine experimental data with identical and different ICCs under different engine operating conditions. Such models can be potentially utilized in active fueling control for Diesel engine combustion control, and therefore benefit engine fuel efficiency and reduce engine-out emissions.