## 9 Full Cycle Comparison Results  
Comprehensive full-cycle CFD simulations (2160 [CA]) were performed for both the baseline and Design 21 configurations to assess the impact of valve timing events on engine performance. Despite targeting a lambda value of 2.15 at the end of the injection phase, the baseline case (absent an injector hole configuration) exhibited misfire during the first and third cycles. This was attributed to excessively lean conditions near the spark plug region, where local lambda values exceeded 2.5, as depicted in Figure 17. In contrast, Design 21 achieved consistent ignition across all three cycles. The hydrogen injection strategy aimed to deliver approximately 70.144 mg of fuel per cycle. Nevertheless, in the baseline case, approximately 7.23 mg of  
Figure 16: Baseline (without injector holes configuration at left) and Design 21 Equivalence Ratio Distribution at 6 [CA] before top dead center. Figure 17: Baseline (without injector holes configuration) first (**left**) and third cycle (**right**) lambda Distribution at 6 [CA] before top dead center. trapped within the cylinder following the first cycle due to incomplete expulsion during the exhaust event. Consequently, the second cycle experienced an unintended increase in the effective fuel quantity by approximately 10%, leading to abnormal combustion characteristics and a propensity for knocking, as previously illustrated in Figure 15.  
Despite the additional fuel contribution from the first cycle, combustion during the second cycle proceeded effectively, resulting in comparable specific fuel consumption values between the baseline and Design 21 cases, as summarized in Table 4 above. Additionally, in the optimization study, NOx emissions were evaluated based on the cumulative in-cylinder NOx mass at +50 \({}^{\circ}\)CA after TDC. For the full-case comparison between the baseline and Design 21, total NOx was calculated as the time-dependent cumulative NOx mass released through the exhaust port over the full 720 \({}^{\circ}\)CA cycle. Thermal NOx formation in hydrogen-fueled combustion is highly sensitive to localized high-temperature regions, particularly where post-flame gases remain above 1800 [K] for extended durations. At +50 \({}^{\circ}\)CA ATDC, the combustion process is nearly complete, and the spatial temperature field reflects the thermal environment that governs NOx accumulation. Figure 18 shows that in the baseline injector configuration, large volumes of gas remain at temperatures above 1800 K, especially in peripheral regions of the combustion chamber. These areas indicate that hot gases are not only generated but also persist near the chamber walls and corners, where mixing and convective heat loss are less effective. The extended exposure of these gas volumes to high temperatures facilitates the time-integrated chemical pathways for NOx formation, especially in the presence of excess oxygen under lean conditions. In contrast, the optimized Design 21 confines the highest temperature zones near the chamber center, minimizing both the spatial extent and the residence time of gases in NOx-prone temperature ranges. The thermal field in this case is more symmetric and centralized, reducing heat transfer to walls and suppressing localized hot spots. This \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Baseline** & **Design 21** \\ \hline IMEP (720) [Bar] & 12.3 & 11.1 \\ PFP [bar] & 184.7 & 144.23 \\ Indicated Power [kW] & 59.07 & 53.43 \\ Indicated Specific Fuel Consumption [g/kWh] & 70.73 & 70.89 \\ Indicated Specific Emission [g/kWh] & 12.93 & 7.83 \\ Knock Index \(\left(dP/d\theta\right)_{max}\) [bar/\({}^{\circ}\)CA] & 64.7 & 17.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Baseline and Design 21 Performance and Emission Result Comparison. Figure 18: Baseline (without injector holes configuration) first **(left)** and third cycle (right). Temperature Distribution at 50 [CA] after top dead center.