Understanding the role of low reactivity fuel stratification in a dual fuel RCCI engine – A simulation study

Natural gas - diesel, Reactivity Controlled Compression Ignition (RCCI) is currently one of the most promising combustion strategies for the next generation heavy-duty engines. A major issue to be addressed for this dual fuel concept to become practically applicable is its low combustion efficiency and associated high methane emissions in partial engine loads. Introducing gaseous fuel stratification (by direct injection) is considered to be beneficial for combustion efficiency increase. Yet, the improvement potential of such natural gas stratification in RCCI engines is unknown. Modeling studies are needed to investigate this methane reduction measure because they can provide more insight to the combustion process than measurements alone. The objective of this study is to assess the potential of RCCI with direct injected low reactivity fuel in terms of thermal efficiency and methane emissions. A broad range of measured, single point injected gas, operating points served as a basis for the simulation study. TNO’s in-house, multi-zone, chemical kinetics based model was validated for those points and used to generate simulation results for different natural gas stratification profiles. It is shown that gaseous fuel stratification in dual fuel RCCI helps to increase combustion efficiency and decrease methane and carbon monoxide emissions. At the same time, nitrogen oxides increase and (at some operating points) the emissions of non-methane hydrocarbons slightly increase too. The effect is strongest for the low load cases showing maximum improvement of combustion efficiency by 11 percentage point for in-extensive natural gas stratifications. The improvement potential reduces with increasing engine load. Thanks to the insights to the combustion process (given by the simulation results), explanations for the observed trends are provided, and important phenomena are identified that are associated with increased low reactivity fuel stratification.