## 1 Introduction  
Emissions from ships have negative impacts on human health and the Earth's environment. The International Maritime Organization (IMO) released many regulations on marine emissions to limit these impacts. Marine emission regulations are getting stricter and stricter nowadays. According to the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI, since 1 January 2020, all ships around the World must comply with the usage of fuels containing a maximum of 0.5% sulfur. The carbon (C) intensity per marine transport mean must be reduced by at least 40% by 2030 in comparison to 2008, with a reduction target of at least 70% by 2050. Greenhouse gases (GHGs) from ships must be reduced by at least 50% by 2050, compared to 2008. Since 1 January 2016, NOx emissions from ships have been limited to 3.4 g/kWh for engines with speeds less than or equal to 130 rpm (revolution per minute). This limitation gradually decreases with the increase of the engine speed and reaches only 2 g/kWh for engines with a rate higher than or equal to 2000 rpm [1]. Therefore, the emission reduction technologies must be applied to both new-building and existing ships to meet stricter emission regulations [2, 3, 4]. Technically, many solutions can be applied to reduce emissions from marine engines, however, each solution has its own advantages and disadvantages. Choosing an appropriate solution for an actual engine and application thus becomes a technical issue that needs to be considered carefully. This paper focuses on two effective solutions to reduce emissions in the marine sector: using alternative fuels and optimizing fuel injectors. Among the various alternative fuels, natural gas (NG) has been widely used for marine engines. It is expected to continue to be used for a long time in the future because it has many advantages. NG produces low exhaust gas emissions (EGEs), requires no processing, has low prices, abundant reserves, etc. Especially, if NG is used as a primary fuel in dual-fuel (DF) engines, pure diesel existing engines can be modified to NG-diesel DF engines very easily with only a low cost [5; 6; 7]. However, NG has a high auto-ignition temperature (low cetane number). Therefore, it always needs external energy for ignition, such as spark plugs in spark-ignition engines (SIEs) or pilot fuels (usually diesel oil) in DF engines. The detailed properties of NG and its effects on the combustion and emission formations of a NG-diesel DF marine engine have been presented and can be found in our previous studies [8; 9]. Methane (CH\({}_{4}\)) occupies approximately 96% of NG by volume [10; 11] and has been employed to represent NG in many previous studies by many researchers [5; 12; 13; 14; 15; 16]. These studies have demonstrated the ability of CH\({}_{4}\) to represent NG with a high accuracy in simulation results. Methane was thus used to represent NG in this study. Internal combustion engines (ICEs) convert thermal energy exported from the chemical energy of fuels through combs to the mechanical energy on engine shafts. Combustion is a process of fuel oxidation by oxygen (O\({}_{2}\)) in the air. Ideally, all injected fuels should be in contact with all of the O\({}_{2}\) molecules available in the engine's combustion chamber so that the fuel can be burned as completely as possible. In the direct injection engines (DIEs), fuel atomization and injection characteristics play very important roles in the combustion quality of engines. They are also the major factors in reducing EGEs, while keeping or even increasing the engine power [17]. The atomization of the fuel is not only influenced by fuel injector properties but also by the interaction between the injected fuel droplets and the air existing in the combustion chambers of engines [18]. Regarding the fuel injection characteristics, the injection method (port- or direct-injection), injection strategy (single- or multiple-injection), injection timing, and SA play important roles in the combustion and emission characteristics of DIEs [5; 6].