## 1 Introduction  
Economic development and industrialization have increased the use of fossil fuels worldwide, and fossil fuels are now central to modern society. Fossil fuels provide comfort in use, but the global warming caused by greenhouse gases, ozone layer destruction, and acid rain caused by sulfur oxides (SOx) and nitrogen oxides (NOx), are largely affected by emissions from the combustion of fossil fuels. This is occurring on a global scale [1; 2; 3; 4]. Due to increasing environmental pollution, emission regulations are being strengthened around the world, and the International Maritime Organization (IMO) is trying to tighten the regulation of air pollutants emitted from ships, which are becoming more significant. The Regulations for the Prevention of Air Pollution from MARPOL (The International Convention for Prevention of Marine Pollution for Ships) Annex [5; 6], which regulate the quality of sulfur oxides (SOX), volatile organic compounds (VOCs), off-gas incinerator emissions, and fuel oils, have been revised and adopted [7]. The main purpose of IMO emission regulations is to reduce NOx emissions levels, and Table 1 shows these NOx emission reduction levels in stages. In the case of ships, the emission standard is legally less stringent than that for automobiles, and, since marine fuels are high viscosity fuel oils, the concentration of pollutants is high, and the possibility of incomplete combustion is high [8; 9]. The low fuel consumption and high explosive power of diesel engines have been used in many applications, such as onshore and marine engines. However, air pollution caused by diesel engines is another challenge to humans, and research is being conducted in various ways, such as pre-treatment technology and post-treatment technology, to reduce such pollution. Specifically, as IMO regulations on ship diesel engines are tightened, corresponding pollution reduction technology is being developed. Emulsion fuel use, one of the technologies that attracts attention, characterizes the exhaust gas emitted from diesel engines. Because pollution reduction technology has an excellent effect on the reduction of NOx and PM (particle matter) [10; 11; 12; 13], that technology is put to practical use. Additionally, much fuel consumption reduction is expected for study continuation. Efforts have also been made to apply these technologies, which have a number of positive use effects, to heavy oil C, which constitutes a large portion of the fuel in ships [14], and is easier to emulsify than other fuels. However, emulsified fuel is difficult to heat to a high temperature and has a high viscosity, which makes it difficult for actual ship use. Emulsion fuels are also difficult to combust because of their unusual combustion characteristics compared to other fuels. There are two main ways to improve the exhaust gas. Before the fuel is used it can be treated to remove sulfur and metals that emit harmful gases, and to improve combustion. Ancillary technologies include delayed fuel injection, reshaped combustion chambers, improved fuel injection systems, and recirculated exhaust and water. Pre-treatment methods include the use of emulsified fuels, using diesel oil-water emulsification. Secondly, post-treatment methods include catalytic decomposition, catalytic reduction, absorption, and adsorption, to remove NOx contained in the exhaust gas. In the process of reducing NOx, by using after-treatment devices such as SCR (selective catalytic reduction), which is the most widely used post-treatment method in the case of ship oil, highly sulfurrous compounds cause soot, sulphate, and nitrogen oxides to be emitted in the exhaust gas. Due to the large amount of material discharged, the durability of SCR is significantly reduced, requiring pre-treatment of the fuel [15; 16; 17; 18]. However, alternative fuel technology is being actively researched to lessen disadvantages such as power degradation, corrosiveness, and fuel viscosity, for application to existing engines.