## 3 Results and Discussion
### DoE and New Reference Results  
As mentioned, the main target of the investigation is to achieve stable, efficient, and low NO\({}_{\mathrm{X}}\) operation at mid to low engine loads. The lean limit in PCSI combustion mode at the compression ratio of 12.8 was determined to be \(\lambda\) = 2.2, which resulted in an IMEP of 3.9 bar and a significant drop in indicated efficiency when compared to the operation at \(\lambda\) = 1.6-1.8. One of the targets of this study is thus not only to extend the lean limit but to maintain a high indicated efficiency at the same time. On the other hand, the upper limit of the engine load that could be achieved with HCCI combustion on the same experimental engine without intake boosting was approximately 3.55 bars of IMEP at \(\lambda\) = 2.44 [19]. Considering the available reference results, the investigation is performed at engine loads of 3.5 and 3.0 bars of IMEP. In [23], the numerical simulations indicated, as later confirmed by the initial measurement, that the required intake temperature for achieving PC-HCCI combustion with the compression ratio of 17.5 is between 80 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, so the DoE includes three different intake temperatures, 80 \({}^{\circ}\)C, 90 \({}^{\circ}\)C, and 100 \({}^{\circ}\)C. On each combination of intake temperature and engine load, a spark sweep and PC DOI sweep were performed to obtain the highest indicated efficiency while satisfying the restrictions in terms of combustion stability and knock intensity. The PC DOI sweep is imposed such that the injected fuel amount is reduced in steps of 20% of the fuel amount, determined by simulations to achieve a stoichiometric mixture in the pre-chamber at the spark timing. Such a strategy should result in a very similar range of \(\lambda_{\mathrm{PC}}\) values at each of the imposed spark timings. The list of all measured operating points at IMEP = 3.5 bar is given in Table 5, and for IMEP = 3.0 bar in Table 6.  
Additionally, the measurements are performed at the intake temperature of 33 \({}^{\circ}\)C but without the injected PC fuel mass sweep. The stoichiometric mixture in the pre-chamber is rather targeted at each spark timing, which is the usual approach for PCSI combustion, to provide the reference results of PCSI combustion at the same compression ratio of 17.5. Along with the IMEP of 3.0 and 3.5 bar, the upper and lower bounds of the reference set taken from measurements of PCSI combustion at the compression ratio of 12.8 (5.5 bar and 4.0 bar) are measured. The operating points with the highest indicated efficiencies at the imposed engine loads are given in Table 7 and represent another set of reference results. Finally, since there are no available experimental results of HCCI combustion at the compression ratio of 17.5, the MultiZone HCCI combustion model that was validated in [23] was used to obtain some reference results for the evaluation of PC-HCCI combustion, whose operating parameters are given in Table 8. The operating parameters of the operating point used for validation are also given in Table 8, as it will be used as the only experimentally obtained reference HCCI result, regardless of the different compression ratio. The air excess ratios were kept constant for each of the imposed intake temperatures. Since all other boundary conditions, such as intake and exhaust pressures, wall temperatures, etc., are taken from the validated operating point (R12) and are imposed on both targeted engine loads, resulting in the same conditions at the intake valve closed position (IVC),