## 3 Results and Discussion
### Comparison with HCCI Combustion  
For the comparison of the PC-HCCI results with the reference HCCI results, the operating point that results in the highest indicated efficiency while satisfying all the  
Figure 13: The calculated values of PC fuel energy at ST and MFB at AI for the optimal combustion phasing achieved with different control strategies at each of the investigated intake temperatures: (**a**) intake temperature = 80 \({}^{\circ}\)C; (**b**) intake temperature = 90 \({}^{\circ}\)C; (**c**) intake temperature = 100 \({}^{\circ}\)C.  
imposed constraints is selected for each intake temperature. At the intake temperatures of 90 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, the previously mentioned conditions are achieved by OP19 and OP30, respectively. Both operating points represent a combination of the most advanced spark timing and the lowest mixture reactivity. At the intake temperature of 80 \({}^{\circ}\)C, the operating point with the spark timing that is delayed by one step compared to the most advanced ST and with the slightly higher mixture reactivity (\(\alpha_{\text{PC}}\) @ ST = 1.28 instead of 1.63) is determined to be the one that achieves the previously mentioned conditions. The two reference operating points from the initial measurements are also included in the analysis to verify the performance benefits by changing the pre-chamber variant and optimizing the operating parameters. The comparison of performance metrics of selected operating points for each of the investigated intake temperatures with initial measurements and with reference HCCI operating points, at IMEP = 3.5 bar, is shown in Figure 14.  
When compared to the initial measurements, an approximately 2-percentage-point increase in indicated efficiency is achieved, at almost identical combustion phasing. Very stable combustion is maintained regardless of the significant decrease in the pre-chamber mixture reactivity when compared to initial measurements. A slight increase in THC emissions is evident as well as a slight decrease in CO emissions, resulting in an overall slightly lower combustion efficiency. On the other hand, the reduction in the pre-chamber mixture reactivity significantly reduced the NOx emissions, especially at the intake temperature of 80 \({}^{\circ}\)C. The only negative trend is the slight increase in knocking tendency, which can more likely be attributed to the change in pre-chamber geometry rather than the change in the operating parameters caused by the optimization. When the PC-HCCI results are compared to the reference HCCI results at the same compression ratio, an almost identical result is obtained in all the performance metrics available from the simulation. However, when compared to the experimentally obtained HCCI reference point, an efficiency more than one percentage point lower is noted. At the same time, the average MAPO value is two times higher for PC-HCCI, explained by the shift in the optimal combustion phasing of 2 \({}^{\circ}\)CA when compared to the HCCI result at the compression ratio of 16. The NOx emissions are higher but still insignificant, and the THC and CO emission levels are comparable. Overall, it can be concluded that a very similar performance is obtained with significant variations in intake temperature, proving the reliability and adaptability of the PC-HCCI combustion mode. The indicated efficiency is identical at the intake temperatures of 90 \({}^{\circ}\)C and 100 \({}^{\circ}\)C and slightly lower at the intake temperature of 80 \({}^{\circ}\)C, while the NOx emissions are almost identical for each intake temperature. The intake temperature of 100 \({}^{\circ}\)C leads to the most stable combustion and highest combustion efficiency, which also means the lowest THC and CO emissions. However, it was previously shown that the intake temperature of 90 \({}^{\circ}\)C offers the highest flexibility and most reliable control of combustion phasing.