## 3 Results and Discussion
### Analysis of Controllability and Resulting Performance  
Figure 9 shows the comparison of experimentally obtained PC-HCCI pressure profiles for different strategies of combustion phasing control with simulated HCCI pressure profiles, at the intake temperature of 90 \({}^{\circ}\)C. It can be seen that by varying either the spark timing or by varying the PC DOI at the fixed spark timing, the combustion phasing can be controlled with an effect that equals that of changing the intake temperature by 5 \({}^{\circ}\)C in an HCCI combustion mode. To give some more insight into how the operating parameters are affecting the combustion phasing, the calculated values of PC fuel energy at ST and CA50 are shown against different combinations of the imposed spark timing and the calculated value of \(\lambda_{\text{PC}}\) at ST (Figure 10). It can be seen that the adjustment of spark timing and PC DOI which targets constant mixture reactivity in the pre-chamber (fixed \(\lambda_{\text{PC}}\) at ST) has an almost identical effect as adjusting the mixture reactivity at fixed spark timing by reducing the injected  
Figure 8: CA50 values and the corresponding indicated efficiencies: (**a**) \(\lambda\) = 2.6; (**b**) \(\lambda\) = 2.88. Figure 9: The comparison of obtained PC-HCCI pressure profiles for different strategies of combustion phasing control with reference HCCI pressure profiles: (**a**) spark sweep with fixed \(\lambda_{\text{PC}}\) @ ST; (**b**) PC DOI or \(\lambda_{\text{PC}}\) @ ST sweep at fixed spark timing. fuel amount by 20%. It should be noted that, while lowering the value of \(\lambda_{\rm PC}\) at fixed ST advances the combustion due to the combined effect of higher PC fuel energy and higher laminar flame speeds, keeping the same values of \(\lambda_{\rm PC}\) at more advanced spark timings leads to lower PC fuel energy. This is a known restriction of PCSI combustion of highly diluted mixtures where the significant spark advance, which is necessary due to low laminar flame speeds, consequently reduces the ignition energy, which can cause unstable combustion or even misfires. The values of \(\lambda_{\rm PC}\) @ ST are calculated for all the remaining operating points from Table 5, and at each step of the PC DOI sweep, they vary only by \(\pm 4\%\), meaning that the imposed strategy achieved the desired effect. Figure 11 shows the analysis of the controllability of PC-HCCI combustion for the entire set of investigated operating points from Table 5. The obtained results of indicated efficiencies, CoV MMEP, average MAPO, and NOX emissions are plotted against the CA50 value to show how much the combustion phasing can be controlled, what the limitations are, and how different control strategies affect the efficiency, combustion stability, knocking tendency, and NOX emissions, as some of the most important performance metrics. Some general trends can be observed as follows:  
* The achievable combustion phasing range is limited on one side by the high knocking tendency and on the opposite side by combustion instability. * For the same CA50 value, higher efficiency, more stable combustion, and lower NOX emissions are obtained with a higher intake temperature. * At each of the imposed intake temperatures, for a given CA50, the combination of earlier spark timing and lower mixture reactivity in the pre-chamber leads to both higher efficiency and lower NOx emissions, at the expense of slightly higher cyclic variability. * The optimal combustion phasing corresponds to that of the simulated HCCI combustion at the same compression ratio at each of the imposed intake temperatures. Figure 10: The calculated values of PC fuel energy at ST and CA50 for different strategies of combustion phasing control: (**a**) spark sweep with fixed \(\lambda_{\rm PC}\) @ ST; (**b**) PC DOI or \(\lambda_{\rm PC}\) @ ST sweep at fixed spark timing.