**4. Conclusions**

This work presents a CFD analysis to study NOx reduction in a commercial marine engine, the Wärtsila 6 L 46. The NOx reduction is based on ammonia injection directly into the cylinder, and this measure was compared with water injection. The so-called artificial inert species method was employed. Inert species with the same properties of water, ammonia, and air were used to characterize the chemical, thermal, and dilution effects of water and ammonia injection.

It was found that the chemical effect using ammonia injection is extremely dependent on the injection timing. The optimum NOx reduction using ammonia is obtained when this is injected during the expansion stroke, leading to a significant chemical effect, and negligible thermal and dilution effects. The optimum NOx reduction using water is obtained when this is injected near TDC. Injected near TDC, water promotes NOx reduction by the dilution and, more significantly, the thermal effect. If ammonia is injected near TDC, the thermal and dilution effects become more significant but the global NOx reduction is noticeably lower than the values obtained when using water.

**Author Contributions:** Conceptualization, M.I.L.G. and C.G.R.V.; methodology, M.I.L.G. and C.G.R.V.; software, M.I.L.G. and C.G.R.V.; validation, M.I.L.G. and C.G.R.V.; formal analysis, M.I.L.G. and L.C.-S.; investigation, M.I.L.G. and C.G.R.V.; resources, M.I.L.G. and C.G.R.V.; writing—original draft preparation, M.I.L.G. and L.C.-S.; writing—review and editing, M.I.L.G. and L.C.-S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to express their gratitude to Talleres Pineiro S.L., sale and repair of marine engines, as well as to Norplan Engineering S.L.

**Conflicts of Interest:** The authors declare no conflict of interest.
