Abstract

Nowadays marine propulsion systems based on thermal machines that operate under the diesel cycle have positioned themselves as one of the main options for this type of applications. The main comparative advantages of diesel engines, compared to other propulsion systems based on thermal machines, are the low specific fuel consumption, residuals and their higher thermal efficiency. However, its main disadvantage lies in the emissions pro- duced by combustion, such as carbon dioxide (CO2), oxide sulphur (SOx) and oxide nitrogen (NOx). These emissions are directly related to the operating conditions of the propulsion system. Over the last decade, the International Maritime Organization (IMO), has adopted a series of regulations to reduce these emissions, based on the introduction of an energy efficiency design index (EEDI) and an energy effi- ciency operational indicator (EEOI). EEDI is mandatory for any new ship and the EEOI is optional to be applied. In this context, adding a shaft generator allows to reduce the nominal design power of the auxiliary generation system and, under nominal operating conditions, the propulsion plant, favouring lower EEDI and EEOI values, which means lower CO2, SOx and NOx emissions. However, the use of shaft generators can only be justified if the propulsion system operates, most of the time, under 75%–80% of the maximum continuous rating (MCR) design. In addition, the incorrect operation of the shaft generator can result in overloading the main engine, which means an increase of CO2, NOx and SOx emissions. The present work proposes a selective control system with a Maximum Power Point Tracking (MPPT) that allows operating the shaft generator in Power Take Off (PTO) or Power Take In (PTI) mode ensuring that the main engine operates, always, at the optimum point to generate minimum CO2, SOx and NOx emissions.