## 5 Discussion  
In this paper, an EMS combining CED guidance and an A-ECMS was evaluated using the IDHIL simulator, which integrates a VANET simulator, V2X communication devices, and a vehicle simulation environment. CED guidance provides guide speeds and guide signals to the driver by considering road information (curvature, slope, distance, limit speed) and V2X communication information (traffic light, surrounding vehicle information). The A-ECMS performs optimal power distribution control by applying the target SOC derived based on road information to the equivalence factor. The connected PHEV with the proposed EMS was compared to a normal PHEV without eco-driving and with a conventional ECMS. The connected PHEV showed a fuel economy improvement of 60.28% compared to the normal PHEV. This equates to an average reduction in CO\({}_{2}\) emissions of 0.54 kg (36.57%). We came to the following conclusions from our simulation results. * By driving with CED guidance using V2X communication and road information, brake and acceleration pedal usage is reduced, which can be seen through regenerative braking energies and engine operating points. This in turn reduces energy loss and the usage of vehicle systems. * We can see the change in engine behavior by applying an A-ECMS using road information. Based on the road information, the target SOC was determined and reflected in the equivalent factor. As a result, the engine operated more often in areas with lower BSFC (higher efficiency). * CED guidance encourages drivers to drive more efficiently, and the A-ECMS enables engines to operate more efficiently. We have found that the integration of CED guidance and the A-ECMS can be synergistic in terms of improving fuel efficiency. Although we confirmed the improvement in fuel economy from the test results, further studies are needed to further improve fuel economy by utilizing the IDHIL simulator test bench with a connected vehicle environment. The first is to apply detailed road information in real time. In order to apply road information when driving in a real environment, real-time information must be guaranteed. In addition, applying a trajectory-based SOC reference using detailed road information will show a higher fuel efficiency improvement effect [26]. The second is to evaluate various EMS methods. EMSs for connected (P)HEVs have been well researched, but validation in a real V2X communication environment is lacking. Therefore, the IDHIL simulator can be used to test existing methods to validate the applicability and performance of existing EMSs in real-world V2X environments. Finally, application of an autonomous driving system should be verified. One of the problems that reduces the performance of eco-driving is human driver error. Since driver behavior is unpredictable and inconsistent, the performance of eco-driving systems cannot be guaranteed. Therefore, if an autonomous driving system is applied, its performance can be guaranteed because its ability to follow the reference speed is increased. **Author Contributions:** Formal analysis, S.H. ; project administration, S.H. and H.L. ; software, S.H. ; validation, S.H. ; investigation, S.H. ; writing--original draft preparation, S.H. ; writing--review and editing, S.H. and H.L. ; supervision, H.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Case** & **Connected PHEV** & **Normal PHEV** \\ \hline \#1 & 24,183.69 [] & 145,252.51 [] \\ \hline \#2 & 52,978.61 [] & 144,172.89 [] \\ \hline \#3 & 48,528.40 [] & 130,616.57 [] \\ \hline \#4 & 34,439.04 [] & 141,808.66 [] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation results: regenerative braking energy. Data Availability Statement: Not applicable.