## 3 Results and Discussion
### Comparison with PCSI Combustion  
Figure 17 shows the comparison of the PC-HCCI combustion with the PCSI combustion at compression ratios of CR = 12.8 and CR = 17.5, for which the NOx optimized operating points were achieved by imposing the current Euro VI limit for heavy-duty vehicles in steady-state testing (0.4 g/kWh).  
Figure 17: The comparison of performance metrics of overall optimal PC-HCCI operating point at each of the investigated engine loads with reference PCSI operating points.  
First, it is worth noticing that even the PCSI combustion at the increased compression ratio significantly extends the lean limit, from \(\lambda\)-2.2 to \(\lambda\)-3.0, resulting in the lowest engine load of IMEP = 3.0 bar. At the same engine load of IMEP = 4 bar, the increase in compression ratio results in an increase in indicated efficiency of around 4 percentage points, which can be mainly attributed to the increased combustion efficiency. However, optimal combustion phasing for the PCSI combustion at such high dilution levels is still shifted closer to the TDC. This is because the increase in combustion efficiency while advancing the combustion phasing overcomes the increase in heat losses. However, the spark advance becomes restricted due to the inevitable effect of lowering the ignition energy, which then increases the cyclic variability and the risk of misfires. The highest efficiency is achieved at IMEP \(\approx\) 5.5 bar (CR = 17.5), which is 1 percentage point higher than with the compression ratio of 12.8. This is because the same engine load is obtained at the higher dilution level (combustion efficiency is maintained) and the optimal combustion phasing is not limited by the imposed NOx limit. However, further reduction in NOx emissions just under the expected Euro VII limit of 0.2 g/kWh [26] would once again require further mixture dilution, which comes at the expense of indicated efficiency, reducing it by 3 percentage points.  
The PC-HCCI combustion has an indicated efficiency 4 percentage points higher when compared to the PCSI combustion (CR = 17.5) at the same load, due to the higher combustion efficiency, which is maintained at the combustion phasing closer to the expected optimal range. The THC and CO emissions are consequently lower. A more stable combustion is achieved, with CoV(IMEP) further away from the imposed limit of 5%. On the other hand, the knocking tendency is higher, which is evident from the higher average MAPO values. The PC-HCCI combustion also has two times lower NOx emissions than the PCSI combustion at the same load. When the PC-HCCI is compared to the most efficient PCSI operating point, an indicated efficiency 1 percentage point lower is noted, but the specific NOx emissions are reduced by approximately 70%. This means that the PC-HCCI combustion can achieve ultra-low NOx emissions of <0.1 g/kWh, while at the same time maintaining the indicated efficiency at a much higher level, which was not possible with the ultra-lean PCSI combustion.  
Based on the presented results, it can be derived that by combining the two existing and widely investigated lean burn approaches (PCSI and HCCI) into a single combustion mode (PC-HCCI), some major drawbacks of each individual combustion mode can be resolved. Without some significant limitations or challenges identified, this makes PC-HCCI a feasible technology for achieving reliable, stable, and efficient operation with ultra-low NOx emissions in both new and retrofit engines.