**4. Conclusions**

The main goal of the present paper is to characterize the most appropriate pre-injection pattern in a marine diesel engine, the Wärtsilä 6L 46, in order to optimize the pilot injection process. A CFD model previously validated with experimental results was employed to obtain data corresponding to a set of 125 injection patterns using pilot injection. The pre-injection rate, duration, and starting instant were varied in the ranges of 5% to 25%, 1º to 5º CA, and −22º to −18º CA ATDC, respectively. Since the manipulation of these parameters has conflicting results on consumption and emissions of NOx, CO, and HC, a MCDM approach was employed to select the most appropriate injection pattern. Due to the importance of criteria weights on the overall result, several criteria weighting methods were compared. In particular, a subjective weighting method was compared with four objective weighting methods: entropy, CRITIC, variance, and standard deviation. The CRITIC, variance, and standard deviation methods led to the same injection pattern: −19º CA pre-injection starting angle, 20% preinjection rate, and 5º CA pre-injection duration. Nevertheless, the entropy method provided a −22º CA pre-injection starting angle, 25% pre-injection rate, and 5º CA pre-injection duration as the most appropriate injection pattern, and the subjective method determined this as a −19º CA pre-injection starting angle, 20% pre-injection rate, and 1º CA pre-injection duration. The main contribution of the present work consists in emphasizing the differences between the results obtained using various methods for the determination of the criteria weights, showing the advantage of subjectivism over objectivism. Based on the overall results, the subjective method is recommended since the criteria weights are defined by experts in the field. In fact, in practical applications, subjective methods are more frequently employed than objective ones. Objective methods are only recommended when the objectivity of the research is too important or when there is no agreemen<sup>t</sup> between the weights proposed by the experts.

**Author Contributions:** Conceptualization, M.I.L. and C.G.R.; methodology, M.I.L. and C.G.R.; software, M.I.L. and C.G.R.; validation, M.I.L. and C.G.R.; formal analysis, M.I.L. and L.C.-S.; investigation, M.I.L., C.G.R., and L.C.-S.; resources, M.I.L. and C.G.R.; writing—original draft preparation, M.I.L. and L.C.-S.; writing—review and editing, M.I.L. 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 Norplan Engineering S.L. and recommend the courses "CFD with OpenFOAM" and "C ++ applied to OpenFOAM" available at www.technicalcourses.net.

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