Optimised selection of marine dual-fuel low-speed diesel engines: introducing relative specific fuel consumptions

Low-speed diesel engines are the most used ones in large ships as main propulsion, and due to severe environmental rules, there has been an intensive search for using alternative fuels which generate less emissions in these engines. In this context, dual-fuel diesel engines have been developed and their employment keeps growing. Therefore, the present study aimed to elaborate a simple and fast approach to assist the selection of the optimal marine dual-fuel low-speed diesel engine concerning fuel consumption to be fitted as primary mover. Relative specific fuel consumptions were introduced to develop unified expressions, and a polynomial approach was applied in order to fit curves. The accuracy of the method was verified through comparisons with a web-based application provided by an engine manufacturer and the highest deviation peaked at about 2.7 %. A practical liquefied natural gas carrier design of matching propeller/ship and engine was performed so that the convenience of the method could be explored. Thus, through the proposed optimisation methodology, annual savings of over 8.1 and 9.6 % were reached regarding heavy fuel oil and boil-off gas, respectively. At last, it was found that the optimal matching might not be due to the most efficient either propeller or engine.