## 10 Conclusions  
This study aimed to advance the design of an environmentally sustainable combustion chamber by enabling the controlled combustion of highly diffusive fuel, such as hydrogen, within an internal combustion engine. Based on validated PIV-CFD comparisons of the hydrogen air-fuel mixture, optimization of the injector hole configuration embedded in the CAP design was performed, with a focus on enhancing both performance and emissions characteristics.  
The findings highlight that  
* Enhancing in-cylinder tumbling motion significantly improves cycle-to-cycle variations and reduces the risk of misfire. Varying injection angles promote more homogeneous air-fuel mixing through enhanced tumble flow, resulting in more stable and consistent combustion.
* Although a target lambda value of 2.15 was adopted to ensure stable combustion, this parameter alone proved insufficient due to the inherent diffusivity of hydrogen. Baseline analyses demonstrated that irregular air-fuel distribution can lead to misfire or knocking tendencies across different cycles.  
* Cycle-to-cycle variations further revealed that unburned fuel trapped within the cylinder, resulting from improper valve timing, can lead to incomplete expulsion during the exhaust phase. This underscores the critical importance of optimizing valve timings during the preliminary design phase to avoid combustion instabilities, such as backfire.  
Finally, the optimization strategy implemented in Design 21, featuring three injector holes and a 15.24\({}^{\circ}\) injection angle, successfully achieved a more homogeneous in-cylinder mixture, maintaining the equivalence ratio below 1.2.  
As a result, NO\({}_{\text{x}}\) emissions were significantly reduced, with a decrease of approximately 39.4% compared to the baseline configuration. Furthermore, the design led to notable improvements in combustion stability and engine performance by mitigating cycle-to-cycle variations and significantly reducing the risk of misfire. These findings underscore the effectiveness of targeted injector and combustion chamber design modifications in advancing hydrogen-fueled engine technology.  
The outcomes of this study offer practical implications for engine designers working with hydrogen-fueled heavy-duty engines. By focusing on manufacturable parameters such as the number of injector holes and their inclination angles, the presented optimization framework enables the fine-tuning of injector designs without altering the base injector architecture or requiring higher injection pressures. This ensures compatibility with current LPDI systems while achieving measurable improvements in both combustion performance and NO\({}_{\text{x}}\) emissions. As such, the findings support data-driven decision-making in injector calibration and design workflows under lean hydrogen combustion conditions.  
However, certain limitations of the current study must be acknowledged to ensure balanced interpretation of the results. The simulations were conducted under full-load and steady-state conditions, without accounting for transient effects or cycle-to-cycle variability. The EGR level was kept constant at 2.6% throughout the optimization process, and no aftertreatment systems or combustion control strategies (e.g., spark timing, injection phasing) were included. Furthermore, the optimization was intentionally restricted to geometric parameters; namely, the number of injector holes and their inclination angles in order to focus on variables that are both manufacturable and directly applicable within current LPDI architectures. This design constraint ensures that the findings can be realistically implemented without requiring significant modifications to existing engine hardware or control strategies.