**7. Conclusions and Recommendations**

In this paper, the influences of the ship propulsion control modes, electric power generation modes, ship operational speeds, propulsion modes as well as sailing on different fuels on the fuel consumption and emissions of an ocean-going benchmark chemical tanker have been investigated taking the ship's operational profiles into account. The current IMO's EEDI considers only one operating point when estimating ship energy efficiency, however, a lower EEDI does not necessarily mean less ship fuel consumption and emissions when sailing with a certain mission profile over the whole voyage. So, the mean value indicators weighted over the ship mission profile should be used when estimating the fuel consumption and emissions performance of the ship over the voyage.

When transiting in open sea, reducing the ship average operational speed will effectively reduce both the fuel consumption and emissions of the ship over the voyage. To reduce the ship operational speed, reducing the propeller revolution rather than the propeller pitch is more preferable, as pitch reduction will reduce the propeller efficiency and consequently increase the fuel consumption especially for voyage where the ship speed is reduced. Generating the electric power by the shaft generator (PTO mode) rather than the auxiliary generator (Aux mode) will further reduce the fuel consumption while the NOx and HC emissions could increase. However, compared to the propulsion control modes, the electric generation modes have relatively minor influence on fuel consumption and emissions of the ship.

When the ship is sailing and manoeuvring in the coastal and port areas, changing the fuel for the main engine from heavy fuel oil (HFO) to marine diesel fuel (MDF) will reduce the NOx and HC emissions significantly while slightly reducing the fuel consumption and CO2 emissions. Providing the power for ship propulsion (PTI mode) and onboard electric loads by the auxiliary engines and shutting down the main engine will further reduce the local NOx and HC emissions significantly while the fuel consumption and CO2 emission will increase notably mainly due to the lower engine efficiency of the auxiliary engines.

Using LNG (liquefied natural gas) as the fuel for both the main and auxiliary engines will reduce the NOx emission significantly compared to using HFO (heavy fuel oil) or MDF (marine diesel fuel). So, sailing the ship on LNG in close-to-port areas will produce much less local environmental impact due to the much less local pollutant emissions. In particular, sailing the ship in PTI mode on LNG will further reduce the local pollutant emissions in coastal and port areas. The fuel consumption and CO2 emission of the ship will also decrease notably over the whole voyage when sailing on LNG instead of HFO and MDF. However, the hydrocarbon (HC) emission is much higher when using LNG as a marine fuel than traditional diesel fuel due to the methane (CH4) slip and unburnt methane during engine operations and although it has no direct effects on human health, it may have a worse impact on climate change (global warming) when taking the life-cycle emissions of natural gas into consideration (although the lifetime of the emitted substance should then also be taken into account, which is outside

the scope of this paper). It is clear either way that methane emissions from LNG engines should be minimised as much as possible.

Last but not least, there are still some limitations and uncertainties existing in this paper and these will be further studied in future work. One of the limitations is that only CO2, NOx and HC emissions are investigated and the other emissions, such as sulphur oxides (SOx), particulate matter (PM) and soot (C), etc., generated by the ship are not considered. Another limitation is that only fuel consumption and emissions are investigated in this paper while the ship capital expenditure (CAPEX), operating expenditure (OPEX) and the operational safety of both the engine and ship have been left outside the scope. The uncertainty is in the simplistic assumption on the fuel consumption and emission performance of the engines when using LNG as the fuel. In future work, in addition to improving the fuel consumption and emissions models, the potential influence of the application of the hybrid propulsion and alternative fuels on the CAPEX and OPEX of the ship, and the operational safety of both the engine and ship especially in adverse sea conditions will be investigated. The trade-o ff relationships between the ship CAPEX and OPEX, and between the energy e ffectiveness of the ship and the operational safety, need to be investigated.

**Author Contributions:** Conceptualisation, C.S. and D.S.; Data curation, C.S.; Formal analysis, C.S. and P.d.V.; Investigation, C.S. and P.d.V.; Methodology, C.S. and D.S.; Project administration, K.V.; Resources, Y.D.; Software, C.S.; Supervision, P.d.V. and K.V.; Validation, P.d.V., D.S. and Y.D.; Visualisation, C.S.; Writing–original draft, C.S.; Writing–review and editing, P.d.V., D.S., K.V. and Y.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project partly is financially supported by the International Science and Technology Cooperation Program of China, 2014DFA71700; Marine Low-Speed Engine Project-Phase I; China High-tech Ship Research Program.

**Acknowledgments:** The authors would like to thank CSSC Marine Power Co., Ltd. (CMP) for providing the ample data of the benchmark ship and the engines, and the support for the measurements of the two-stroke marine diesel engines.

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