## 1 Introduction  
On board all operative ships, internal combustion engines (ICEs) are installed with the function of both propulsion engines and auxiliaries in electric energy generators. Specifically in large ships, ICEs are mainly 2-stroke and 4-stroke turbocharged diesel engines. Two-stroke engines are used exclusively for the propulsion of merchant ships such as very large container ships, tankers, and ro-ro due to the required great power. Moreover, the layout of the general arrangements of these types of ships does not create particular constraints on the installation of these mammoth-sized engines. On the other hand, 4-stroke engines, coupled with suitable generators, are used for the production of electric energy practically on board all ships. Moreover, thanks to the high specific power, they are mainly employed in the propulsion of ferries, naval ships (up to frigates), pleasure crafts, fishing vessels, and support vessels for offshore operations. It is still worth underlining that 4-stroke engines are also used in passenger ship gensets, which, as known, adopt an electric propulsion system. For both types of engines, the power they can deliver ranges from 10 to 100 MW; therefore, they must be fed with low-cost fuels that make the shipping of goods and passengers profitable for shipowners. In fact, as reported in [1], global shipping is responsible for moving about 90% of the world trade, while cruise ships in 2017 allowed nearly 25 million of passengers to sail the oceans. These significant results were obtained thanks to the great flexibility offered by shipping and the low cost per ton transported of this mode of transportation. Despite maritime transport still being the most fuel-efficient means of good transit, it should be noted that ships have, as mentioned before, big ICEs installed on board; as a consequence, shipping greatly contributes to the emission of air pollutants (PM, SO\({}_{X}\), NOx), sea pollutants (noise, thermal, discharges), and greenhouse gases (GHG). To be honest, in order to reduce all forms of pollution from ships, since 1973, the International Maritime Organization (IMO) has already issued a whole series of regulations. Moreover, the IMO has drawn up a roadmap for reducing emissions from ships in order to provide shipowners and shipbuilders with a clear long-term strategy to guide their research and development activities. With particular reference to air emissions, in 2006, MARPOL Annex 6 [2] was issued introducing the definition of Emission Control Areas, which are the areas of the planet with precise limits of air pollutant from ships allowed. Given the size of the world fleet and the partially retroactive effect of these regulations, several steps and implementation dates have been identified. On the same occasion, the compulsory adoption of the Ship Energy-Efficiency Management Plan (SEEMP) was introduced in order to limit waste as much as possible. Subsequently, IMO also ruled in favor of reducing GHG emissions [3], setting the 50% reduction of CO\({}_{2}\) produced by ships as a target in 2050 compared to that produced in 2011 (year of entry into force of [2]). On this premise, from 2006 onwards, countless studies have been carried out to evaluate reliable, economic, and efficient technological solutions capable of achieving these objectives. In particular, three solutions have been identified [4]: reducing the hull resistance; increasing the overall efficiency of the propulsion and power generation systems; and using alternative fuels with low or zero pollutant contents. It is easy to understand that the first strategy is really effective if applied on the construction of new ships, while the other two ones have found multiple applications also in the retrofitting of existing ships. In particular, the use of Liquefied Natural Gas (LNG) as a fuel is now a consolidated reality; additionally, many studies are being carried out in order to evaluate the use of other fuels such as hydrogen, methanol, and ammonia [5; 6; 7].