## 3 Results and Discussion
### Combustion Modes Comparison  
The objective of this section is to assess the potential of the RCCI-SHV architecture compared to the dual-mode concept described in Section 2.2 and conventional diesel combustion. For this purpose, performance and emissions maps of the original diesel engine and dual-mode RCCI were used as input data for the model. The driving cycles were simulated using the same gear shifting strategy for CDC and dual-mode combustion, while the hybrid concept relied on direct transmission with a fixed gear ratio. The operating conditions reached during the RDE cycle with CDC and dual-mode concepts are shown in Figure 13. It is interesting to note that the charging strategy for the hybrid vehicle was based on RBC (rule-based control), where the engine operating conditions are defined on SOC ranges. This allows for tailoring of the operating conditions to obtain the desired values of emissions. This means that the ICE will operate in a defined operating condition based on the previous state and the actual SOC value from the battery. The basic control strategy is summarized in Table 6 where CD stands for charge depleting operation and CS for charge sustaining mode. Other control strategies such as dynamic programming and a model predictive controller are able to obtain higher energy savings, but RBC has easier implementation from the control standpoint. Therefore, the energy savings achieved with the hybrid powertrain can potentially be even higher than the values presented here. Future works will be focused on the optimization of the charging strategy. Figure 12: Instantaneous SOC (**a**) and cumulative fuel consumption (**b**) for the three-charging strategy during the real driving cycle (RDE) driving cycle. Figure 13: _Cont._Figure 14 presents the results obtained for the cumulative fuel consumption and the respective fuel saving for each combustion mode. In the dual-mode case, most of the operating conditions fall inside the RCCI zone, which leads to a fuel consumption improvement of almost 5% compared to CDC. This is possible because the RCCI zone has lower values of brake-specific fuel consumption (BSFC) than CDC due to the highly diluted and fast combustion process. The RCCI-SHV strategy results in higher fuel savings since all the charging operating conditions come from the RCCI zone. In addition, the three points selected allow the ICE to run the engine in optimized conditions, overcoming the drawbacks associated with regular powertrains. The comparison with the CDC mode shows an energy saving of almost 19%. The reduction in the energy consumption is also higher compared to the dual-mode case, reaching absolute values of around 13%. Thus, despite the fact that the charging strategy is not optimal, the hybrid vehicle was able to reach considerable fuel savings compared to both RCCI and CDC operations. The values obtained for the RDE driving cycle are presented in Figure 14b. Since the RDE cycle is longer than the WLTC, the effect of the RCCI zone is observed. This is directly translated in a higher fuel saving compared to the CDC case, reaching values around 6%. Nonetheless, the same cannot be observed for the hybrid powertrain. The increased number of starts of the engine during the cycle leads to an increase in the total fuel consumption since a fuel penalty constraint was used to assess the higher fuel consumption upon starting the engine. Therefore, the final fuel saving reached 13.4%, an absolute decrease of almost 5% compared to the WLTC cycle. \begin{table}
\begin{tabular}{c c} \hline \hline
**Transitions** & **Conditional Statement** \\ \hline CD to CS Level 1 & State = 1 \& SOC \(<\) 0.62 \\ CS Level 1 to CD & State = 2 \& SOC \(>\) 0.64 \\ CS Level 1 to CS Level 2 & State = 2 \& SOC \(<\) 0.60 \\ CS Level 2 to CS Level 1 & State = 3 \& SOC \(>\) 0.62 \\ CS Level 2 to CS Level 3 & State = 3 \& SOC \(<\) 0.58 \\ CS Level 3 to CS Level 2 & State = 4 \& SOC \(>\) 0.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Rule-based control (RBC) control strategy.