ABSTRACT The potential improvement in fuel economy expected from Hybrid Electric Vehicles is between 5% and 25%, depending on the level of hybridization selected [1]. But in the same time, consumers want cars that are fun to drive as well as safe, comfy and ever cheaper. Translated to vehicle engineering targets, this means that seemingly conflicting engineering challenges need to be reconciled to manufacture the cars that will determine the world's future mobility. Numerical simulators at system level are now mandatory to keep the development time and cost of vehicles into acceptable limits, allowing to simulate not only the impact of new powertrains and vehicle technologies on the fuel consumption but also on other attributes like drivability and thermal comfort.