## 4 Conclusions  
This study numerically investigated the effects of the SA and IP of the fuel injector on the combustion, performance, and emission characteristics of an ME-GI DF marine engine aiming to find out a set of optimal injection parameters for the injector to reduce engine EGEs. The major results of the study are as follows:  
* The IP of 0.02 m with an SA of 40 degrees produced the highest IMEP for the engine in both diesel and DF modes. This set of injection parameters increased the IMEP of the engine by 11.15% and 6.08% compared to when using the original set of injection parameters (IP = 0.01/SA = 45) in the diesel and DF mode, respectively. It is thus considered to be the optimal parameter that should be set for the fuel injector to enhance engine performance in both diesel and DF modes. * The IP of 0.01 m with an SA of 50 degrees produced the lowest specific NO emission in both diesel and DF modes. This set of injection parameters decreased the specific NO emissions of the engine by 18.92% and 18.97% compared to when using the original set of injection parameters (IP = 0.01/SA = 45) in the diesel and DF mode, respectively. It is thus considered to be the optimal parameter that should be set for the fuel injector to reduce NO emissions in both diesel and DF modes. Figure 17: Wall heat fluxes in all simulation cases: (**a**) IP = 0.01 m; (**b**) IP = 0.015 m; (**c**) IP = 0.02 m.  
The IP of 0.01 m with an SA of 40 degrees produced the lowest specific soot emission in both diesel and DF modes. This set of injection parameters decreased the specific soot emissions of the engine by 23.35% and 16.81% compared to when using the original set of injection parameters (IP = 0.01/SA = 45) in the diesel and DF mode, respectively. It is thus considered to be the optimal parameter that should be set for the fuel injector to reduce soot emissions in both diesel and DF modes. - The IP of 0.01 m with an SA of 45 degrees produced the lowest specific CO2 emission in both diesel and DF modes. This set of injection parameters is thus considered to be the optimal parameters that should be set for the fuel injector to reduce CO2 emissions in both diesel and DF modes. - The IP of 0.01 m helps to reduce heat losses through heat transfer from the combustion gas inside the engine cylinder to the engine combustion chamber walls. Based on the simulation results presented above, it can be concluded that, even though the IP of 0.02 m with an SA of 40 degrees helps to enhance the engine performance, if the main target is reducing engine exhaust gas emissions, an IP of 0.01 m is highly recommended to be used. At this IP, the specific SA of 40, 45, or 50 degrees that should be used will depend on which emissions need to be reduced. Conceptualization, W.-J.L., V.C.P. and J.-H.C.; methodology, V.C.P. and J.-S.K. ; software, V.C.P. and J.-S.K. ; validation, [PERSON]-J.L. and [PERSON]-[PERSON]; formal analysis, V.C.P. ; data curation, [PERSON]-H.C. and [PERSON]-J.L. ; writing--original draft preparation, J.-S.K. ; writing--review and editing, V.C.P., [PERSON]-[PERSON] and [PERSON]-[PERSON]; project administration, [PERSON]-H.C.; funding acquisition, W.-J.L. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Not applicable. Not applicable. Not applicable. Not applicable. The authors declare no conflict of interest. The chemical reaction mechanism calculates the NO formation using the classical Extended Zeldovich scheme as follows [33]:  
\[\text{N}_{2}+\text{O}\quad \begin{matrix}\overset{\text{k}_{\text{lf}}}{ \underset{\text{k}_{\text{lb}}}{\leftarrow}}\\ \overset{\text{k}_{\text{lb}}}{\leftarrow}\\ \overset{\text{k}_{\text{lb}}}{\leftarrow}\end{matrix} \quad\text{NO}+\text{N} \tag{1}\]  
\[\text{N}+\text{O}_{2}\quad \begin{matrix}\overset{\text{k}_{\text{2f}}}{ \underset{\text{k}_{\text{lb}}}{\leftarrow}}\\ \overset{\text{k}_{\text{lb}}}{\leftarrow}\end{matrix} \quad\text{NO}+\text{O} \tag{2}\]  
\[\text{N}+\text{OH}\quad