The recent updates of the European Union regulations concerning emissions from heavy-duty vehicles greatly tightened CO2 emission limits and widened the groups of vehicles that must withstand these thresholds to most of the subgroups defined within the VECTO methodology. To meet these regulations, manufacturers are introducing fully electrified powertrains for trucks or alternative-fueled internal combustion engines, while heavy-duty hybrid systems are not common in constructors’ catalogs. The aim of this analysis is to investigate and assess the effectiveness of a diesel-electric P2P4 series architecture for emission reduction purposes, in both full and plug-in hybrid configuration, while also considering techno-economic aspects. The first step of this work was to select a suitable test case; a rigid truck belonging to VECTO group 3 was identified and the design space for the most relevant hybrid powertrain components was defined.