Numerical study on the effect of a two-stage pilot injection strategy on the performance of medium-speed diesel/natural gas dual-fuel marine engine

The need to reduce marine engine emissions continues to grow with the implementation of stricter emission standards. Numerical simulations were conducted to study the effect of a two-stage pilot diesel injection strategy on combustion and emissions in a medium-speed diesel/natural gas dual-fuel marine engine. A comprehensive analysis was performed on the start of the first pilot diesel injection (SOI1) and the proportion of the first pilot diesel injection (POI1), identified as two coupling factors of the injection strategy. The results indicated a significant improvement in indicated thermal efficiency (ITE) of over 48% and a reduction in nitrogen oxide (NO_x) emissions of over 50% compared to the baseline single-stage injection strategy can be achieved through the use of high POI1 and delayed SOI1. In addition, the first pilot diesel spray covered the squish and piston bowl regions, enhancing the reactivity of the fuel-air mixture in these regions. The mass fraction of CH₂O and H₂O₂ radicals determined the distribution of reactivity in different regions and characterized the transition process from low-temperature reaction to high-temperature reaction. The strategy of high POI1 and delayed SOI1 resulted in a better low-temperature reaction distribution, reducing combustion duration, improving ITE, and decreasing NO_x, HC, CO emissions at medium load condition.