Simulation and performance comparison between diesel and natural gas engines for marine applications

The article shows the performance comparison between two marine engines, fuelled by natural gas and diesel oil, respectively, both belonging to the Bergen' engine series of Rolls-Royce Marine, suitable as prime movers for ship propulsion. Two different simulation codes, one for each engine, validated by means of geometrical and performance data provided by the manufacturer, have been developed to extend the comparison to the whole working area of the examined engines. Although the maximum continuous power is very similar (about 2MW at the same rotational speed), some differences exist in size, efficiency and pollutant emissions of the two types of engines. The reasons are investigated through a specific thermodynamic analysis, aimed to explain such differences, in terms of efficiency and emissions (particularly carbon dioxide), when varying the working conditions. The analysis is carried out by comparing the respective real cycles, at the same working condition, and repeating the comparison for different engine delivered powers and rotational speeds. In addition, a study of the different modes of combustion is developed to explain the major differences found in the emissions of nitrogen oxides.