## 3 Results and Discussion
### DoE and New Reference Results
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Targeted** & **Operating** & **Image [bar]** & **Intake Temperature** [\({}^{\circ}\)C] & **\(\lambda_{\text{global [-]}}\)** & **ST [\({}^{\circ}\)CA BTDC]** & **PC DOI [ms]** & **Head Temperature** [\({}^{\circ}\)C] \\ \hline \multirow{6}{*}{80} & OP39 & 3.02 & 80.8 & 2.98 & 18 & 1.01 & 118.4 \\ \cline{2-7}  & OP40 & 3.06 & 82.4 & 2.94 & 18 & 0.81 & 118.7 \\ \cline{2-7}  & OP41 & 2.99 & 82.5 & 2.97 & 16 & 1.11 & 114.0 \\ \cline{2-7}  & OP42 & 2.94 & 80.4 & 2.97 & 16 & 0.89 & 115.8 \\ \cline{2-7}  & OP43 & 3.00 & 79.7 & 2.96 & 14 & 1.21 & 119.4 \\ \cline{2-7}  & OP44 & 3.03 & 82.0 & 2.91 & 14 & 0.97 & 118.2 \\ \hline \multirow{6}{*}{90} & OP45 & 3.04 & 91.3 & 2.94 & 14 & 1.17 & 122.5 \\ \cline{2-7}  & OP46 & 2.98 & 92.1 & 2.95 & 14 & 0.94 & 121.1 \\ \cline{2-7}  & OP47 & 3.00 & 92.6 & 2.90 & 14 & 0.70 & 120.2 \\ \cline{2-7}  & OP48 & 2.99 & 90.4 & 2.93 & 12 & 1.27 & 121.9 \\ \cline{2-7}  & OP49 & 2.99 & 90.1 & 2.92 & 12 & 1.01 & 120.7 \\ \cline{2-7}  & OP50 & 3.05 & 90.7 & 2.86 & 12 & 0.76 & 119.9 \\ \cline{2-7}  & OP51 & 2.98 & 92.4 & 2.92 & 10 & 1.35 & 123.0 \\ \cline{2-7}  & OP52 & 2.99 & 92.8 & 2.89 & 10 & 1.08 & 122.0 \\ \cline{2-7}  & OP53 & 3.01 & 92.6 & 2.85 & 10 & 0.81 & 119.6 \\ \hline \multirow{6}{*}{100} & OP54 & 3.04 & 100.0 & 2.91 & 12 & 0.90 & 123.3 \\ \cline{2-7}  & OP55 & 3.00 & 100.0 & 2.92 & 12 & 0.72 & 122.6 \\ \cline{2-7}  & OP56 & 2.98 & 99.7 & 2.89 & 10 & 1.00 & 122.4 \\ \cline{2-7}  & OP57 & 2.96 & 100.2 & 2.89 & 10 & 0.80 & 122.2 \\ \cline{2-7}  & OP58 & 2.97 & 99.5 & 2.91 & 8 & 1.20 & 123.8 \\ \cline{2-7}  & OP59 & 3.00 & 100.3 & 2.86 & 8 & 0.96 & 122.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The list of all measured operating points at IMEP = 3.0 bar. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Compression** & **Reference** & **Image [bar]** & **Intake Temperature** [\({}^{\circ}\)C] & **\(\lambda_{\text{global [-]}}\)** & **ST [\({}^{\circ}\)CA BTDC]** & **PC DOI [ms]** & **Head Temperature** [\({}^{\circ}\)C] \\ \hline \multirow{6}{*}{17.5} & R8 & 2.94 & 34.1 & 2.97 & 16 & 1.17 & 110.9 \\ \cline{2-7}  & R9 & 3.56 & 32.6 & 2.69 & 16 & 1.13 & 119.7 \\ \cline{2-7}  & R10 & 4.03 & 32.1 & 2.52 & 16 & 1.04 & 127.7 \\ \cline{2-7}  & R11 & 5.63 & 31.9 & 1.89 & 4 & 0.40 & 132.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Reference PCSI operating points at CR = 17.5.  
approximately 5 \({}^{\circ}\)C higher in-cylinder temperature is obtained at top dead center (TDC) with \(\lambda\) = 2.88 due to the change in thermodynamic properties of the air-fuel mixture with the change in the dilution level. As a result, a similar combustion phasing is achieved at both loads (with both dilution levels). In reality, lower wall temperatures are expected at lower engine load, causing at least a slight increase in the required intake temperature to maintain the same combustion phase. Nevertheless, at both dilution levels, the operating points at the intake temperature of 110 \({}^{\circ}\)C could not maintain the desired IMEP. This indicates that the lower bound of the required intake temperature for HCCI combustion is exceeded, and those operating points are thus not further used in the analysis. Figure 7 shows the simulated pressure profiles of HCCI combustion at the compression ratio of 17.5, while the obtained CA50 values and the corresponding indicated efficiencies are given in Figure 8.  
Based on the presented results, it is expected that to achieve similar indicated efficiencies, the combustion phasing should be controlled such that the CA50 is somewhere between 6 \({}^{\circ}\)CA aTDC and 10 \({}^{\circ}\)CA aTDC.