New 12V/48V power net architectures are potential solutions to close the gap between customer needs and legislative requirements. In order to exploit their potential, an increased effort is needed for functional implementation and hardware integration. Shifting of development tasks to earlier phases (frontloading) is a promising solution to streamline the development process and to increase the maturity level at early stages.This study shows the potential of the frontloading of development tasks by implementing a virtual 48V mild hybridization in an Engine-in-the-Loop (EiL) setup. Advanced simulation technics like Functional Mock-up Interface (FMI) based co-simulation are utilized for the seamless integration of the real time simulation models and allow a modular simulation framework as well as a decrease in development time. As base line, an existing and validated co-simulation consisting of a GT-POWER engine model, a SimulationX transmission model, and a dSPACE ASM vehicle dynamics model is used. A Simulink-based dual 12V/48V power net model is developed to extend the base model. The 48V side is mainly composed of a belt driven starter generator (BSG) that is directly connected to the combustion engine (P0-layout) and a 48V Li-Ion battery. The 48V side is coupled via a bidirectional DC/DC to the 12V Absorbent Glass Mat (AGM)-battery and the 12V loads.In the next step, an engine test bench is coupled with the real time simulation by replacing the simulated combustion engine. Extensive tests are carried out on the Engine-in-the-Loop test bench, considering new legislative test requirements like WLTC (Worldwide harmonized Light vehicles Test Cycle) and RDE (Real Drive Emission). The results show the great emission reduction potential of 48V mild hybrids and proof that the frontloading-based EiL methodology is a promising solution to validate the system behavior with a heterogeneous cyber-physical test setup.