## 3 Results and Discussion
### Analysis of Controllability and Resulting Performance
There are some exceptions worth mentioning, one of which is the sudden and very significant increase in CoV IMEP under the combinations of the most advanced spark timings and lowest mixture reactivity in the pre-chamber at the intake temperature of 80 \({}^{\circ}\)C. Another is the sudden combustion phasing advance at the spark timing of 2 \({}^{\circ}\)CA bTDC after reducing the mixture reactivity at the intake temperature of 100 \({}^{\circ}\)C. Both operating points are included in the following analysis to explain such behavior. Figure 12 shows the comparison of pressure profiles with the reference HCCI pressure profile at the optimal combustion phasing achieved with different control strategies at each of the investigated intake temperatures. Also included in the analysis are the two already mentioned operating points that stand out as not following the expected trend. It is already clear from the pressure plots that a large share of the main chamber charge is consumed by flame propagation prior to the autoignition, but the pressure plots by themselves do not indicate anything suspectable with OP2 when compared to the other two points at the same intake temperature. It is, however, clear for OP38 that the autoignition occurred in the pre-chamber, resulting in a much higher burn rate, which consequently reflects the combustion phasing in the main chamber, but it is not clear whether it occurred prior to or during the flame propagation. Figure 11: The analysis of controllability of PC-HCCI combustion for the entire set of investigated combinations of operating parameters: (**a**) intake temperature = 80 \({}^{\circ}\)C; (**b**) intake temperature = 90 \({}^{\circ}\)C; (**c**) intake temperature = 100 \({}^{\circ}\)C.  
To help further explain the suspectable behavior of OP2 and OP38, as well as further compare the entire set of operating points resulting in the optimal combustion phasing, the PC fuel energy at spark timing is calculated for each of the analyzed operating points and given in Figure 13. Additionally, the values of both the total MFB and the MFB in the main chamber only at the time of autoignition (AI) are extracted using a method previously described in Figure 6. As expected, there is a clear trend of increased ignition energy with the increase in intake temperature due to later spark timings and at the same time lower MFB values at AI due to conditions being closer to those required for HCCI combustion. Finally, with the help of the presented results, the suspectable behavior of both OP2 and OP38 can also be explained. Considering the unstable operating point at 80 \({}^{\circ}\)C (OP2, ST = 14 \({}^{\circ}\)CA bTDC, \(\lambda_{\text{PC}}\) @ ST = 1.63), it can be seen that in addition to the overall lowest reactivity of the pre-chamber mixture, the PC fuel energy is the lowest amongst all analyzed operating points, while the MFB at autoignition is the highest, where almost 30% of the main chamber charge is consumed by flame propagation prior to autoignition. Since flame propagation through such a highly diluted main chamber charge is susceptible to high cyclic variability, the results indicate that this case simply represents the exceeding of the lowest bound of the ignition energy required for reliable and stable flame propagation at the imposed intake temperature. Considering that the OP19 at the intake temperature of 90 \({}^{\circ}\)C, which achieves stable combustion, shows only a slightly higher value of PC fuel energy at ST and only slightly lower values of MFB at AI, it can be assumed that the OP2 could achieve the desired combustion stability with only a small modification of pre-chamber mixture reactivity. On the other hand, the plots of pressure profiles already indicated that the uncontrolled autoignition in the pre-chamber occurs before the mixture is consumed by flame propagation, and the assumption is backed up by the extracted values of MFB.