Impact of diesel pilot distribution on the ignition process of a dual fuel medium speed marine engine

Recent emission legislation in the marine sector has emphasized the need to reduce nitrogen oxides (NO_x) emissions as well as sulphur emissions. The fulfilment of emission legislation limits with conventional marine diesel oil (MDO) requires complex and expensive aftertreatment systems and in this framework lean burn pilot ignited dual fuel (diesel and natural gas) is revealed as one of the most suitable engine platforms to decrease pollutant formations at its source and therefore to mitigate aftertreatment system requirements. For this reason, an experimental study has been carried out in an 8.8 l dual fuel single cylinder Wärtsilä 20DF engine in order to evaluate different diesel equivalence ratio distributions in the combustion chamber and to get a deeper insight into the interaction between the high reactivity (diesel) and the low reactivity (natural gas) fuels during the ignition process. Engine testing has been complemented with diesel spray pattern simulations for a better understanding of local combustion conditions. Results show the importance of local pilot fuel distribution as a way to control combustion phasing and consequently its impact on combustion instability, emissions and knock conditions. Stable combustion with engine-out NO_x levels below legislation have been achieved without the need of after-treatment system using appropriate high reactivity fuel (HRF) distribution control.