## 1 Introduction  
To limit the impact of emissions on human health and the Earth's environment, the marine emission regulations released by the International Maritime Organization (IMO) are becoming increasingly strict. In order to meet these stricter emission regulations, emission reduction solutions are mandatorily applied on both new-build and existing ships nowadays [1; 2; 3]. According to the MARPOL (International Convention for the Prevention of Pollution from Ships) Annex VI, since 1 January 2020, all ships have to comply with the utilization of fuels with max. 0.5% sulfur globally. The minimum reduction in carbon intensity per maritime transport work must reach 40% by 2030 compared with 2008, with the target of reaching 70% by 2050. The reduction in greenhouse gas (GHG) emissions from marine shipping must reach at least 50% by 2050 compared with 2008. From 2016, NOx emissions from ships have been limited to 3.4 g/kWh for engines with speeds less than or equal to 130 rpm. This limit gradually decreases with increasing engine speed and reaches only 2 g/kWh for engines with a speed higher than or equal to 2000 rpm [4]. There are various solutions for reducing emissions from marine engines, but this paper focuses on two aspects: engine technologies and alternative fuels. Among combustion engines, internal combustion engines (ICEs) with their high thermal efficiency have been widely used in industry. Among ICEs, compression ignition (CI)diesel engines have higher thermal efficiency compared to spark ignition (SI) gasoline engines [5]. They are therefore currently contributing a very large portion in providing propulsion for propellers as well as generators on ships. However, EGEs such as NOx (nitrogen oxides), soot, and CO\({}_{2}\) (carbon dioxide--one of the strong greenhouse gases--GHGs) produced from the combustion process of such engines are major concerns nowadays. In ICEs, the combustion of fuel by the O\({}_{2}\) (oxygen) present in the supplied air inside the engine combustion chamber generates engine power, while also producing EGEs. Therefore, engine performance and EGE characteristics are greatly influenced by the fuel type, fuel-supplying method, fuel injection characteristics, fuel-air equivalent ratio, fuel-air mixing quality, combustion chamber geometry, etc. [6]. Among many kinds of fuels, natural gas (NG), which principally consists of methane (CH\({}_{4}\)), has been and will still be widely used in heavy-duty marine engines. It has many advantages, such as low EGEs, low price, no processing, abundant reserves, etc. However, since NG has a high self-ignition temperature (low cetane numbers), it needs an external energy source for ignition, such as a spark plug in SI engines or pilot diesel oil in dual-fuel (DF) engines. If NG is used in the DF engines, pure diesel engines can be modified to an NG-diesel DF engine very easily and with only a low cost [7; 8; 9]. Numerous studies have been performed to investigate the combustion and EGE characteristics of NG-diesel DF engines [10]. The detailed properties, as well as the effects of NG on the combustion and EGE characteristics of an NG-diesel DF engine, can also be referred to in our previous studies [11; 12]. Regarding engine technologies, ME-GI (M-type, Electronically Controlled Gas Injection) engines designed by MAN B&W are one of the state-of-the-art engines in the maritime industry today. In total, more than 200 engines have been ordered and in use since 2015, and the number will rapidly increase in the coming years [13]. Unlike the DF engines, which use intake port injection for primary gas introduction, ME-GI DF engines use the direct injection method for the primary gas introduction. The gaseous fuel is injected and burned directly, leading to high efficiency and combustion stability according to the diesel cycle principle. With direct gas injection, the injection timing, gas injection rate, and injection pressure can be controlled in a wide range of engine loads and speeds to keep the optimal spraying quality.