## 3 Materials and Methods  
### Development Environment  
For the results of the coupled test benches example presented in this paper, a vehicle simulation of a parallel P2 hybrid was set up to generate a suitable environment for the coupled test benches. Gas and brake pedal positions are calculated from a driver model, developed and validated for a driving robot on a full vehicle test bench [51]. As a result, control problems due to inertias in the powertrain or latencies caused by test bench communication can be avoided. The resulting load requirements are acquired by a hybrid controller and distributed to the two test benches depending on the operating  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Name** & **Literature** & **Research Focus** \\ \hline ACoRTA & [18; 19; 20; 21; 22; 23] & RT co-simulation \\  & & Satisfying HRT conditions \\ \hline XiL-BW-e & [24; 25; 26; 27; 28] & Regional network for XiL approaches \\  & & Distributed delevopment and connected validation \\ \hline SmartLoad & [29] & Validation concept for highly automated vehicle \\  & & Implementation of DCP standard called ’DCPLite’ \\ \hline TechReaL & [30; 31; 32; 33] & Technology readiness level raise of BEVs \\  & & Early realistic testing through connected test benches \\ \hline Virtual shaft & [34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45] & Network setup between different test benches \\  & & RT performance on driving cycle reproducibility \\ \hline Hy-Nets & [46; 47] & Connected and automated HEVs \\  & & Traffic simulation in ECU testing \\ \hline HIFI-ELEMENTS & [48; 49] & Validation environment standards for BEV \\  & & Thermal HiL tests \\ \hline DUETT & [50] & Combined physical–virtual environments \\  & & Closed-loop approach for virtual HV battery systems \\ \hline \hline \end{tabular}
\end{table}
Table 1: Categorization of a literature selection on test bench coupling projects and approaches.  
modes. The rule-based hybrid controller has common operating modes, such as load point shifting, electric driving, engine driving, and boost. For the ICE, values for the accelerator pedal position, and for the EM, values for the torque requirements are calculated. The torques provided by the test benches are combined by a detailed transmission model and transmitted to the road via the differential and the wheels to calculate subsequent vehicle conditions. A detailed gearbox model is needed to perform temperature calculations, because the real EM and the virtual gearbox had a cooling circuit connected in parallel. The measured EM temperature and the gearbox temperature calculated from the gear losses are then used to calculate the cooling fluid's inlet temperature and transmitted to the EM test bench's conditioning system as a reference value. For the EM, a battery model consisting of an equivalent circuit with two RC elements was also built to calculate the state-of-charge (SoC) dependent DC link voltage. The battery model, as well as the transmission model and some hybrid controller conditions were derived from the reference vehicle's measurement data. Regarding the developed vehicle simulation models, a transition from simulation-only to test bench experiments needs to be guaranteed, including adequate interface identification and configuration. For this case, the models were prepared step-by-step in a simulation toolchain for application on the test bench. MiL and SiL approaches were used, and the HiL method was then applied to validate the models with the separate test benches ensuring the usage for coupled test bench operation.