## 3 Results and Discussions
### Condition Migration Strategies  
Since the engine's operating state is decoupled from the vehicle's driving state under real driving conditions, the engine can operate under lower pollutant emission rate conditions to reduce pollutant emissions. As the emission limits progressively tighten, some engine operating points can migrate from the lowest fuel consumption to the lower pollutant emission rate conditions. As can be seen, these engine condition migrations tighten emission limits at the expense of fuel economy; also, the operating conditions for the lower emission rates of different pollutants do not overlap. These two factors make the engine condition migration more challenging to decide. To this end, the DP algorithm provides condition migration strategies that provide solutions to achieve a reasonable compromise between fuel consumption reduction and pollutant emission reduction. Figure 10 shows the DP solution for the stricter CO and NOx emission limits by engine condition migration. Given that the PN emissions have already achieved orders of magnitude lower reductions through the start-stop reduction strategy, the PN emissions are not discussed as being more restricted in the condition migration strategies. In Figure 10a, Strategy E given by the DP algorithm migrates the operating points located within the 1949 to 5366 s time period from the lowest fuel consumption rate condition and the engine stop condition to the lowest CO emission rate condition (1000 r/min \(\times\) 41.46 N\(\cdot\)m), so that the CO emissions less than those obtained by the non-start-stop strategy are obtained. To continue to tighten the CO emissions while liberalizing PN and NOx emissions, the DP algorithm gives Strategy F, which migrates more operating points located within the 1251 to 5431 s time period from the lowest fuel consumption rate condition to the lowest CO emission rate condition. Due to the low output power at the lowest CO emission rate condition, the operating points located within the 0 to 1251 s time period deviate from the lowest fuel consumption rate condition and relocate at the higher engine speed and torque condition (3550 r/min \(\times\) 121.22 N\(\cdot\)m) to compensate for the reduction of engine output power, as shown in Figure 10b. With this strategy, the total trip CO emissions are slightly reduced, but PN emissions, NOx emissions, and fuel consumption increase significantly, as shown in Figure 11.  
Figure 9: Accumulated real driving emissions of the start–stop reduction strategies. Figure 10c shows Strategy G given by the DP algorithm to further limit NOx emissions. Under this strategy, the operating points located within the 1787 to 5517 s time period are migrated from the lowest CO emission rate condition to the higher speed and lower torque condition (1200 r/min \(\times\) 30.38 N\(\cdot\)m); also, the operating points located within the 0 to 1786 s time period are migrated from the lowest fuel consumption rate condition to the higher speed and lower torque condition (3000 r/min \(\times\) 98.83 N\(\cdot\)m). Generally, higher engine speeds contribute to higher power output, while lower engine loads favor lower NOx emissions. Compared to the case in Figure 10a, the total trip NOx emissions and PN emissions under this strategy show a significant reduction, with little change in CO emissions and a slight increase in fuel consumption, as seen in Figure 11. In addition, Figure 10d shows Strategy H for further limiting NOx emissions while liberalizing CO and PN emissions. In the strategy, the engine almost operates in the region around the condition with the higher speed and lower torque (2850 r/min \(\times\) 41.14 N\(\cdot\)m). The operating duration of the high speed and high torque conditions (4200 r/min \(\times\) 113.02 N\(\cdot\)m) is significantly shorter, only within the 0 to 189 s time period. These actions contribute to the reduction of NOx emissions.