Thermal management is a key issue to minimize fuel consumption while dealing with pollutant emissions. It paves the way for developing new methods and tools in order to assess the effects of warm up phase with different drivetrains architectures and to define the most suitable solution to manage oil and coolant temperatures.DEVICE (Downsized hybrid Diesel Engine for Very low fuel ConsumptIon and CO2 Emissions) project consists in designing hybrid powertrain to cut off significantly CO2 emissions. It combines a 2-cylinder engine with an electric motor and a 7-gear dual clutch transmission. Hybridization and downsizing offer a great improvement of fuel economy and it is valuable to study their effects on thermal management. Hence, a dedicated AMESim platform is developed to model the fluids temperatures as well as the energy balance changes due to the powertrain architecture.After using a 4-cylinder reference engine to validate the model, the warm up phase (comparing hot and cold start NEDC) leads to a 12% fuel consumption penalty with DEVICE powertrain. Hybridization and downsizing save up respectively 7% and 11% on cold start (20°C) NEDC. Since energy balance is changed compared to reference engine, new technologies, such as electric water pump, resizing oil/water exchanger and so on, should be used to enhance fuel economy and the modelling platform enables to study their effects.