Parametric investigation of pre-injection on the combustion, knocking and emissions behaviour of a large marine four-stroke dual-fuel engine

This study aims at the parametric investigation of a large marine four-stroke dual-fuel engine in order to identify the pre-injection effects on the engine combustion, knocking and emissions parameters. A model was employed that was developed by integrating a 1-D engine model in AVL-BOOST and a 3-D CFD model in CONVERGE. The MAN 51/60DF marine engine is modelled and the simulation results were validated against experimental data. Subsequently, parametric runs for various pre-injection timings and mass ratios are performed and the simulation results are analysed and discussed. The derived in-cylinder pressure oscillations at determined points are employed to calculate the knock index (KI), which was used as an evaluation indicator for the knocking intensity. A number of pre-injection strategies with varying timing and fuel mass ratios are studied. This study results reveal that a lower knock trend and NO_X emissions can be achieved by early pre-injection timing and increasing pre-injection fuel mass ratio. In addition, the medium pre-injection interval increases the engine IMEP while reducing the NO_X and total hydrocarbon emissions. Larger pre-injection mass ratio reduce the KI and NO_X emissions, but reduces IMEP and causes the wetted-wall phenomenon. Besides, the excessive pre-injection intervals and pre-injection mass ratio result in a change in combustion mode from the conventional diesel compression ignition mode to a two-stage auto-ignition mode. This study provides a better understanding of the underlying interactions of involved parameters and proposes pre-injection solutions to improve the engine performance, emissions and knocking behaviour.