## 4 Conclusions  
Prior studies on exhaust gas emissions from ships have primarily focused on gaseous substances, such as NO\({}_{\rm X}\) and SO\({}_{\rm X}\), or particulate matter, such as PM10. In the case of CO\({}_{2}\) emissions, total emissions are calculated based on estimated fuel consumption. In this study, CO\({}_{2}\) emissions were measured using a PEMS and the results were compared with those of other studies. Using experimental ships (two passenger ships equipped with an SSD, two passenger ships equipped with an HSD, and one fishing boat equipped with an HSD), the CO\({}_{2}\) emissions from the main propulsion engine were measured in real time without setting any operational conditions for the experiment, and the following conclusions are drawn:  
Figure 14: Coefficient of experimental vessels for each RPM range variation. (**a**) SSD 1; (**b**) SSD 2; (**c**) HSD 1; (**d**) HSD 2; (**e**) HSD 3. * Depending on the type of engine size and displacement, there is a difference in CO2 emissions at the engine's maximum output (maximum rpm), whereby the higher the displacement, the higher the CO2 emissions. Therefore, a differentiated plan according to engine size should be prepared when establishing a policy for limiting CO2 emissions by ships. * Fuel-based emission factors are similar for both types of engine considered, and only a slight difference is observed in emission factors, even when both types of engines used bunker A and MGO. Therefore, when calculating the total CO2 emitted from ships, using a fuel-based emission factor is highly desirable. * According to the results of this study, it is found that there was little difference between the emission factors of SSD (3144.32 kg/tonne-fuel) using bunker A and HSD (3150.58 kg/tonne-fuel) using diesel. In addition, no difference in the fuel-based CO2 emission factor according to engine rpm is observed, and there is no significant difference arising from the comparison with the results of other studies and the emission factor presented by EEA, IMO, and other studies. However, to develop the CO2 emission factor for each fuel, continuous experiments are needed in the future. * Ships spend most of their time sailing in the ocean using high rpm and emitting immense amounts of CO2. Therefore, to minimize CO2 emissions, it is necessary to establish a method to reduce CO2 emissions in the ocean. To establish optimal operating conditions from economic and environmental perspectives, a method was proposed using the sailing duration and CO2 emission data. The optimal operating conditions for each ship are different, and the optimal operating conditions for the ship should be evaluated by approaching them from several standpoints. * This research method has the advantage of being able to measure ships for which it is impossible to acquire engine data from a data acquisition system, but it needs a lot of manpower and expertise such as fuel line modification. Additionally, it cannot be concluded that the vessels or engines used in this experiment are representative of all vessels. Therefore, we found that basic experiments on engine dynamo are absolutely necessary, like in the early stages of developing automobile emission factors. In future research, representative ships and engines will be selected through basic data on ship distribution, and basic experiments on various engines will be conducted on an engine dynamo. The raw data supporting the conclusions of this article will be made available by the authors on request. Methodology, J.O. ; Validation, J.P.; Formal analysis, S.L. ; Data curation, J.P. and D.L. ; Writing--original draft, S.L. ; Project administration, J.O. ; Funding acquisition, J.L. and D.L. All authors have read and agreed to the published version of the manuscript. This work was supported by a grant from the National Institute of Environmental Research (NIER), funded by the Ministry of Environment (ME) of the Republic of Korea (NIER-2022-04-02-099). Not applicable.