## 1 Introduction
This is considered to be one of the most important and necessary factors to achieve better combustion efficiency [14]. Furthermore, the engines designed according to the diesel cycle principle allow the operators to switch smoothly between different fuel modes, i.e., the fuel oil and DF mode [13]. Through the above analysis, the combination of using cleaner NG fuel and fuel direct injection technology in ME-GI DF engines provides an extremely effective solution for reducing EGEs from ships. To maximize the advantages of ME-GI engines, operators need to have insight into the influence of operating parameters on the engine. However, because the ME-GI engine family is so new, not much research has been done on these engines. In addition, the parameters related to the structure and layout of the fuel injectors in the combustion chamber are confidential documents of companies, so they should always be carefully preserved to avoid technology theft. As a result, it is impossible for engine operators to access these documents. This study aims to analyze the effectiveness of using NG as the primary fuel in ME-GI DF engines and the impacts of some of the main parameters of the fuel injector on the combustion and emission formation of the engine. This research will help engine designers and operators to be aware of the key factors that greatly impact engine power and emissions so that they can design and operate engines more efficiently. In fuel direct-injection engines, the diffusion combustion phase accounts for most of the engine's combustion process. In this phase of combustion, the mixing quality between the injected fuel and supplied air inside the engine combustion chamber strongly affects the rate of heat release (RoHR) and the formation of harmful emissions. Ideally, all the injected fuel will be in contact with all the oxygen available in the combustion chamber so that the combustion of fuel can take place as completely as possible. Both the fuel injection characteristics and atomization are the major factors in improving engine power while reducing EGEs [6]. Regarding the fuel injection characteristics, the injection method (port- or direct-injection), injection timing, injection strategy (single- or multiple-injection), SA, and IP play important roles in the combustion and EGE characteristics of engines. In fuel direct-injection engines, SA and IP have great influences on the combustion and EGE formation of the engine as they determine the fuel injection targeting point in the combustion chamber [5]. Therefore, studies for SA and IP are very important and will have implications for both engine design and operating engineers. Regarding the multiple-injection strategy, a series of studies have been carried out and demonstrated the effectiveness of this strategy in reducing soot or even soot and NOx (nitrogen oxides) emissions simultaneously in heavy-duty direct-injection diesel engines [15; 16; 17; 18; 19]. A systematic analysis of the mechanisms of droplet breakup and mixing with the surrounding fluid of fluid injection, and its relation to vorticity generation and transport, has been reported and can be referred to [20]. The effectiveness of the double-injection strategy in reducing EGEs applied on a heavy-duty direct-injection diesel marine engine has also been demonstrated and can be referenced in our previous study [21]. [PERSON] et al. reported in [22] that the nozzle axial location, which determines IP, considerably affected both the in-cylinder pressure and EGEs of fuel direct-injection engines. Here, a change in the axial location of the fuel nozzle by only a few millimeters led to a considerable change in the engine performance and emission behavior. [PERSON] and [PERSON] reported in [23] that the narrow-spray-angle injection helped to reduce the NOx and soot emissions without affecting the fuel oil consumption due to improved fuel-air mixture quality. [PERSON] et al.