This paper discusses a streamlined approach for characterizing the heat flows from the combustion chamber to the engine coolant, engine oil circuit and the ambient. The approach in this paper uses a built-in flow and heat transfer solver in the CAD model of the engine to derive heat transfer coefficients for the coolant-block interface, oil-block interface and the block-ambient interface. These coefficients take into account the changing boundary conditions of flow rate, temperatures, and combustion heat to help characterize the complex thermal interactions between each of these sub-systems during the warm-up process. This information is fed into a larger system model of the engine to get a more accurate prediction of the engine warm-up and the effect of various fuel economy improvement strategies being evaluated. One of the key benefits shared in this paper is the practicality of the process that can be replicated on every production vehicle simulation model.