Partially premixed combustion (PPC) is a combustion concept with similarities to both Diesel and Homogeneous Charge Compression Ignition (HCCI) combustion. It provides a combustion mode with better controllability than HCCI without increasing the emissions of nitrogen oxides and soot to the level of traditional Diesel engines. The model described in this paper aims to describe the main features of Diesel PPC combustion within the closed part of an engine cycle. It is a single-zone model including heat losses to the cylinder walls as well as fuel evaporation losses. The simulation framework used allows optimization problems to be formulated based on the model equations. The overarching goal of this modeling is to be able to use the model explicitly for optimization.