Homogeneous charge compression ignition is a promising concept for achieving low emissions at part-load operations. This technique can be successfully applied to traditional direct injection diesel engines with low extra costs and no modification to the DI system by performing the mixture formation in the intake manifold. The present paper describes the development of a control-oriented model for the study of the combustion process in a HCCI diesel engine with external mixture formation. The model is based on a first-law thermodynamic analysis of in-cylinder processes in order to identify the influence of the main control parameters on HCCI auto-ignition. The combustion process is modeled through the definition of a gross heat release rate, avoiding a detailed description of the chemical reactions that could increase the complexity and the computation time. The model is then validated against experimental data obtained on a diesel engine equipped with an external fuel atomizer. The satisfactory agreement obtained and the low calibration effort make the model a useful tool for the development of applications related to HCCI engine control and diagnostics.