## 4 Conclusions  
Direct injection in the cylinder can inhibit backfire and increase the power of H2 ICEs. However, optimizing the injection strategy is quite complex and requires an understanding of the formation process of the in-cylinder mixture. Three-dimensional CFD is a convenient tool for simulating mixture formation. By studying the effects of the number and direction of hydrogen injector nozzle holes, injection direction, injection timing, and combustion  
Figure 19: NSD and mass fraction of mixtures with different equivalent ratios for different combustion chambers at TDC.  
Figure 18: Distribution of turbulent kinetic energy with different combustion chamber geometries at TDC.  
chamber geometry on the formation of the hydrogen-air mixture inside the cylinder, the formation quality of the mixture was evaluated from several perspectives, such as the spatial distribution of equivalence ratios, non-uniformity of the mixture, and suppression of abnormal combustion. The main results can be summarized as follows:  
1. For engines with side-mounted injectors and spark plugs near the injectors, using a multi-hole nozzle design with small spacing distances can cause multiple jets to coalesce due to the Coanda effect. This is not conducive to hydrogen diffusion, making it impossible to form a more uniform mixture in a short time.
2. The six-hole nozzle design exhibits an NSD of 0.3495, greater than the NSD associated with any single-hole nozzle designs. Therefore, it is preferable to use a nozzle arrangement with fewer holes and the largest possible spacing between jets.
3. A single-hole injector oriented towards the center of the combustion chamber or the spark plug can help achieve more uniform mixture formation during spark plug ignition.
4. The formation of vortex structures, as a result of hydrogen jet impingement on the walls, can significantly influence fuel diffusion. Particularly when these vortex structures are small in size, a stronger constraint effect on the fuel can be achieved.
5. There is little effect on the non-uniformity coefficient of the mixture in the cylinder with the delay of hydrogen injection timing from \(-144\)\({}^{\circ}\)CA to \(-136\)\({}^{\circ}\)CA, while the distribution of the mixture is apparently affected.
6. By guiding the jet direction of the hydrogen, the sloping combustion chamber accelerates the fuel-air mixing after the jet impinges on the wall, significantly improving the uniformity of the mixture in the cylinder.  
In the future, research is essential for H\({}_{2}\)ICEs operating with more economical hydrogen-air ratios using multiple hydrogen injection strategies. These research findings could help enhance the thermal efficiency of H\({}_{2}\)ICEs, thus saving hydrogen consumption. Such advancements would be beneficial for promoting the usage of H\({}_{2}\)ICEs by reducing costly consumption, thereby further mitigating vehicle emissions.  
Conceptualization, L.L.; methodology, H.C., Z.G., L.L., X.L., Z.H. and PQ.; validation, X.L., P.Q. and K.Z.; investigation, Z.G.; data curation, H.C.; writing--original draft preparation, H.C.; writing--review and editing, Z.M.; supervision, Z.M., Y.P. and Y.A.; project administration, Z.M. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Frontier Exploration Project of Longmen Laboratory (LMQYTSKT038).  
Not applicable.  
Data Availability Statement: Data will be made available on request.  
[PERSON] and [PERSON] were employed by the company CATARC Automotive Test Center (Changzhou) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.