Comparison of a natural gas engine with a diesel engine for marine propulsion

The paper reports a comparison, obtained by numerical simulation, between a natural gas sparkignition engine and a liquid fuel compression-ignition engine, both belonging to the "Bergen" engine series of Rolls-Royce Marine, suitable as prime movers for ship propulsion. The delivered power of both supercharged engines is very similar (about 2 MW) at the same rotational speed. Two thermodynamic simulation codes, one for each engine, have been developed and validated by means of geometrical and performance data provided by the manufacturer. 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. The comparison, extended to the whole working range of the engines, concerns mainly the following aspects: dimension and weight, overall efficiency, pollutant emissions. A specific thermodynamic analysis is developed to explain the differences between the two types of engines, in terms of efficiency and emissions.