System and Second Law Analysis of the Effects of Reformed Fuel Composition in “Single” Fuel RCCI Combustion

Dual-fuel reactivity controlled compression ignition (RCCI) combustion is a promising method to achieve high efficiency with near-zero NOx and soot emissions; however, the requirement to carry two fuels on board limits practical application. Advancements in catalytic reforming have demonstrated the ability to generate syngas (a mixture of CO and hydrogen) from a single hydrocarbon stream. This syngas mixture can then be used as the low reactivity fuel stream to enable single fuel RCCI combustion. The present effort uses a combination of engine experiments and system level modeling to investigate reformed fuel RCCI combustion. The impact of reformer composition is investigated by varying the syngas composition from 10% H₂ to approximately 80% H₂. The results of the investigation show that reformed fuel RCCI combustion is possible over a wide range of H₂/CO ratios. A system level and second law analysis are performed on the highest efficiency operating points, and comparisons are made between partial oxidation reforming, steam reforming, and conventional diesel. The results show that endothermic reforming (steam reforming) can achieve comparable system level efficiency to conventional diesel operation by recovering exhaust energy at similar system-out NOx emissions and near-zero soot emissions. An autoignition integral approach combined with reformer equilibrium modeling shows that in order to achieve higher system level efficiencies, lower H/C ratios of the parent fuel are necessary. Typical H/C ratio ranges for diesel fuels limit the range of operation of the engine to high hydrogen fractions, limiting the system level efficiency due to high heat transfer rates associated with these conditions.