## 4 Conclusions  
In the current eco-friendly trend of the maritime industry, the DC network presents itself as a new way of distributing electricity onboard. The variable speed engine generator set as well as the energy storage are the heart of the DC grid system. This concept allows the engine to run at an appropriate speed corresponding to each specific load. This results in achieving the best combustion efficiency over the entire operating range, thus improving SFOC with reduced emissions. This study evaluated the effects of varying speed on engine performance and vibration behavior. The ultimate purpose is to maximize the benefit of variable speed engine while maintaining safety throughout the system against vibration. Our results can be summarized as follows:  
1. Each engine has a fixed cylinder volume corresponding to a specific range of optimal fuel injected. In case the amount of fuel injected in each cycle is maintained in that range, the best combustion efficiency is always obtained. Outside this range, a higher SFOC is resulted. This fuel amount is closely related and can be controlled by adjusting the torque produced by the generator. 2. Variable speed operation requires a highly controllable fuel supply system, which can maintain the injection quality, including pressure, duration, and ignition time. The injection timing should be later, and an auxiliary air blower installation is essentially required to generate enough power at a lower speed. 3. At the same speed, the vibration increases with load. In addition, more torque generated at lower speed can produce larger vibration. This behavior is more serious once the engine is operating at a speed range close to resonance. Most generators have resonance at a lower speed, which is of little concern in fixed frequency generators that always operate at a constant speed. The vibration behavior should be confirmed to apply the variable speed genset. In case of operation under these conditions is necessary to improve SFOC, the vibration characteristic of the generator set should be changed. 4. Recalculating with a new design or changing the firing order is a possible solution to improve vibration behavior but has high cost and complexity. Altering the natural frequency by changing the rigidity of the frame is an economical and efficient solution for the diesel generator set. Replacing rubber vibration isolator with a spring type with many outstanding advantages is the best option for this experimental system. The above findings should be of concern when applying a variable speed diesel generator. In the next study, the vibratory behavior should be confirmed after the replacement of the vibration isolator. Figure 11: The rubber vibration isolator on this experimental genset. **Author Contributions:** Conceptualization, Q.D.V. and W.-J.L. ; methodology, Q.D.V. and W.-J.L. ; software, J.-u.L. ; validation, Q.D.V., W.-J.L. ; formal analysis, Q.D.V. ; investigation, H.J. ; resources, W.-J.L. ; data curation, J.-H.N. ; writing--original draft preparation, Q.D.V. and W.-J.L. ; writing--review and editing, J.-u.L., J.-w.L., and S.H.Y. ; visualization, H.J. ; supervision, J.-H.N. ; project administration, J.-H.C.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was supported by the Ministry of Education of Republic of Korea and the National Research Foundation of Korea (NRF-2019R1G1A1005342) and by the 'Autonomous Ship Technology Development Program (K_G012001614001)' funded by the Ministry of Trade, Industry &Energy (MOTIE, Korea). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Acknowledgments:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest. **Nomenclature**