## 3 Results and Discussion
### Combustion Modes Comparison
Figure 13: Operating conditions reached during the RDE cycle for CDC (**top**) and dual-mode combustion (**bottom**). The SOC variation during the cycle has a notable importance on the hybrid concept performance. The SOC value at the end of the cycle must reach the initial SOC to consistently deliver the same amount of energy for the driver upon each use of the car. For this study, an initial SOC value of 0.64 was used, and its behavior during the driving cycle is presented in Figure 15. It can be seen that the initial and the final values are the same. Whenever SOC reaches the lower limit, the engine starts and remains running during the time required to achieve the SOC upper limit, as illustrated by the blue line. The steady operation in a single condition defines an increasing step in the fuel consumption profile. On Figure 15b, the cumulative fuel consumption profiles during the WLTC cycle for the three modes are presented. It can be seen that the extra high phase is responsible for almost 50% of the total fuel consumption of the vehicle. In this sense, as the cycle progresses, the ICE starts to operate more frequently and at high load conditions to maintain the charge level. This can be visualized by looking at the engine state number in Figure 15a, as 1 stands for charge depleting and 4 for the higher power density condition. As a result, the extra high phase exerts a dominance on the final fuel consumption. Figure 16 shows the results of the RDE cycle. It is clear that the engine starts more frequently during this driving cycle than with the WLTC. This is directly related to the velocity profiles that were presented in Figure 7. The RDE cycle has higher velocities than the WLTC during the initial phases. Therefore, the energy requirements are also higher, making it necessary for the ICE to work. However, at the final phase of RDE, the state 4 is not achieved, i.e., the higher load condition is not required to charge the batteries. This is explained by the maximum velocity achieved during the driving cycle. While the WLTC cycle reaches maximum velocities of 130 km/h, the RDE remains under 120 km/h for the duration of the cycle. Therefore, low power density conditions are enough to maintain the charge or to drive the battery energy to its original condition. Figure 14: Cumulative fuel consumption and fuel savings for WLTC (**a**) and RDE (**b**) driving cycles. Figure 15: SOC, internal combustion engine (ICE) on-off state, and cumulative fuel consumption for the hybrid powertrain (**a**) and instantaneous fuel consumption comparison for the different concepts (**b**) during the WLTC cycle. Table 6 shows the engine-out emissions for the different combustion modes and the two driving cycles. It can be seen that the dual-mode concept is not able to reach engine-out levels that are below the Euro 6 regulation levels. Therefore, it will need the conventional after-treatment layout. This is because during the driving cycles some operating conditions fall inside the CDC zone, see Figure 14, leading to an increase in the total amount of NOx and soot produced. This results in differences of NOx versus CDC of only 0.1 g/km. In the case of the RCCI-SHV, the fixed operating conditions for the ICE allows for the best values in terms of fuel consumption and emissions to be obtained. The results confirm that the RCCI-SHV leads to the lowest engine-out emissions, with NOx and soot levels below the Euro 6 regulation. The NOx reduction versus CDC is 76.2% and 67.7% for the WLTC and RDE cycle, respectively. In terms of HC and CO, values with RCCI-SHV are higher than CDC but lower than dual-mode. However, it is expected that a conventional diesel oxidation catalyst (DOC) will reduce the RCCI-SHV tail-pipe levels below the regulation limits [47]. The CO\({}_{2}\) reduction is also clear from Table 7. Both concepts (dual-mode and RCCI-SHV) are able to reach consistent reductions of this pollutant, with levels below the upcoming 2021 target of 95 g/km [48].