To maximize the contribution of hybrid electric vehicles (HEV) to the decarbonization of the transport sector, their energy performance is a key factor that needs to be considered from the design phase. For an HEV, optimizing the design in order to reduce consumption involves not only optimizing a sizing aspect but also the EMS. This co-optimization of sizing and energy management is generally tackled either by nesting optimal control algorithms within an optimization algorithm dedicated to sizing, or by using convex optimization to simultaneously optimize the design levels. However, the former approach is known to induce computational constraints (for example relating to calculation time), while the latter can affect modeling fidelity due to the constraints inherent in convex optimization...