## 3 Results and Discussion
### Comparison with HCCI Combustion
The results prove that, unlike in conventional HCCI combustion, the variations in boundary conditions during engine operation can be easily compensated for, ensuring adequate combustion stability and eliminating potential misfires. Finally, a similar indicated efficiency can be obtained as with conventional HCCI combustion at the same compression ratio, with up to 30 \({}^{\circ}\)C lower intake temperatures. Figure 15 shows the comparison of the PC-HCCI pressure profiles with the reference HCCI pressure profile at the optimal combustion phasing for each of the investigated intake temperatures at IMEP = 3.0 bar. The pressure profiles at 80 \({}^{\circ}\)C and 90 \({}^{\circ}\)C indicate that, compared to the previously analyzed operating points at IMEP = 3.5 bar, the combustion is much more dominantly governed by the flame propagation. At 100 \({}^{\circ}\)C, however, the pressure profile characteristic for PC-HCCI combustion starts to be evident. This indicates that in order to achieve the full performance benefits of HCCI combustion for lower engine load, an adjustment of the intake temperature range could be beneficial, especially considering that the difference in recorded cylinder head temperatures is approximately 20 \({}^{\circ}\)C between the two investigated engine loads. However, in this study, the optimization of operating parameters is performed at IMEP = 3.5 bar, and the intention of the performed investigation  
at lower engine load without adjusting the intake temperatures is to show the benefits in terms of reliability and adaptability of PC-HCCI when compared to HCCI combustion. To prove those assumptions, the comparison of performance metrics of optimal operating points at each of the investigated intake temperatures with the reference HCCI operating point, at IMEP = 3.0 bar, is shown in Figure 16.  
Figure 14: The comparison of performance metrics of optimal operating points at each of the investigated intake temperatures with initial measurements and with reference HCCI operating points, at IMEP = 3.5 bar. Figure 16: The comparison of performance metrics of optimal operating points at each of the investigated intake temperatures with reference HCCI operating point, at IMEP = 3.0 bar. Figure 15: The comparison of pressure profiles of optimal operating points at each of the investigated intake temperatures with reference HCCI pressure profile, at IMEP = 3.0 bar. It can be seen that for the intake temperature of 80 \({}^{\circ}\)C, there is the highest penalty in terms of indicated efficiency (2.4 percentage points when compared to HCCI), and the CoV IMEP is at the limit, even with a stoichiometric PC mixture. At the intake temperatures of 90 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, a stable operation is achieved, and the obtained efficiency is increased by approximately 1 percentage point. However, it is still lower than the indicated efficiency of the reference HCCI combustion. The results of the CA50 show that PC-HCCI has some features that resemble the PCSI combustion at high dilution levels, mainly the shift of the optimal combustion phasing towards the TDC to maintain the combustion efficiency at the expense of higher heat losses. Only at 100 \({}^{\circ}\)C is the combustion phasing close to that of the reference HCCI point, and it can be safely assumed that a further increase in intake temperature would further improve the performance. Nevertheless, the presented results prove that even when the operation is not in the optimal intake temperature range, the engine continues to operate reliably with only a slight penalty in indicated efficiency and increased cyclic variability, but without misfires or total loss of the combustion phasing control.