**1. Introduction**

Even though marine propulsion systems have been in constant development since the 18th century, nowadays the most common system used on board large carriers, i.e., container ships and tankers, is a system considering a diesel engine as the prime mover. Most of these engines are of the crosshead type, operating on the two-stroke cycle at low speed having long strokes, turbocharged, and directly coupled to a single fixed-pitch propeller. The power installed for these configurations vary from 10 MW and up to 80 MW [1]. These type of propulsion systems include the use of a Waste Heat Recovery System (WHRS), which relates taking the remaining heat of the exhaust gases generated from the combustion process of the diesel engine. This system was previously known as an economizer and was used to generate steam for heating processes only. Recently, in some cases, the WHRS uses heat to generate steam to be used in turbo-generators [2–4].

Diesel engines for marine applications have many advantages when compared to other prime movers such as turbines. They have a higher thermal efficiency and a low fuel oil consumption of

low-cost residual fuels. The disadvantage of consuming residual fuels is the high amount of CO2, SOx, and NOx emissions, which are related to the ship's operational condition [5,6]. Alternative fuels with low carbon and sulfur content have been considered to be used to replace these residuals fuels; however, its use in large carriers is not cost efficient and still presents a poor environmental performance. In this regard, alternative fuels such as Liquefied Natural Gas (LNG), biofuels and hydrogen are some of the most promising alternatives in study as replacement, but still some concerns about their storage, technological maturity and safety mitigating measures [7,8].

The ship's power design requirement represents the main constrain when the operational condition of the ship is assessed and compared to the power demands at normal operating conditions. The prime mover is forced, most of the time, to operate under underrated conditions increasing its fuel oil consumption therefore the amount of emissions. Diesel engines, as the one described, found their optimum fuel oil consumption point at ~75–80% of the MCR [9].

High fuel oil consumptions lead to an increase of emissions, e.g., CO2 emissions, which have been monitored and since 2011 have been measured using an index called EEDI. This index is part of a mandatory regulation coming from the IMO and applicable to every new ship since 2013 [10,11]. An operational indicator called EEOI has also been considered but this is not mandatory yet, although is integral part of the Ship Energy Efficiency Monitoring Plant (SEEMP), which is mandatory to be implemented on board ships but that is not auditable by any means yet. The EEOI is used as a measuring tool to voluntary assess the efficiency of an existing ship.

The EEDI encourage the use of technologies such as shaft generator to reduce the use of the power installed on board through auxiliary generator sets [12]. This reduction of the use reduces the fuel oil consumption, therefore, as was mentioned before, reduces the amount of CO2 emissions. The shaft generator force to use the diesel engine in a loading range, quite close to the optimum fuel oil consumption point [13,14]. The use of shaft generators above this point has been considering unjustified because of the possibility to overload the diesel engine leading to increase the fuel oil consumption.

The present work presents a marine diesel engine propulsion system with a direct driving shaft generator and a back to back converter based on the use of a selective control scheme. This scheme enables for the diesel engine to operate at its optimum fuel oil consumption point, which has been renamed as its Minimum Emissions Operating Point (MEOP). The scheme considers the use of the shaft generator as a Power Take Off (PTO) drive when the diesel engine operates below the MEOP and as a Power Take In (PTI) when the diesel engine operates above the MEOP. The shaft generator, at PTO, generates enough power to turn-off the generator set of the ship. These operational conditions have a repercussion on the EEOI, which is to be estimated and analyzed to prove the positive influence to lower the amount of emissions based on the reduction of the specific fuel oil consumption of the diesel engine. After EEOI results, the selective control scheme is going to be used to evaluate its influence over the EEDI of a new design looking for the development of an efficient propulsion system that ensures the compliance with the IMO regulations.
