This paper describes the development and experimental validation of a control-oriented, real-time-capable, Diesel engine instantaneous fuel consumption and brake torque model under warmed-up conditions. Such a model, with the capability of reliably and computationally-efficiently estimating the aforementioned variables at steady-state and transient engine operating conditions, can be utilized in the context of real-time control and optimization of hybrid powertrains. The only two inputs of the model are the torque request and the engine speed. While Diesel engine dynamics are highly nonlinear and very complex, by considering the Diesel engine and its control system (engine control unit (ECU)) together as an entity, it becomes possible to predict the engine instantaneous fuel consumption and torque based on only the two inputs. A synergy between different modeling methodologies including physically-based grey-box and data-driven black-box approaches were integrated in the Diesel engine model. The fueling and torque predictions have been validated by means of FTP72 test cycle experimental data from a medium-duty Diesel engine at steady-state and transient operations.