Multiphysics simulation of reactivity-controlled compression ignition: synergistic integration of hydrogen supplementation and piston crown recess design for simultaneous knock suppression and emission abatement

This study explores the effects of diesel–hydrogen dual-fuel combustion in a Reactivity Controlled Compression Ignition (RCCI) engine, focusing on performance enhancement, knock suppression, and emission reduction. Numerical simulations were performed across a range of hydrogen ratios (7–33%) combined with piston bowl geometry optimization. Results indicated that up to 10% hydrogen substitution, paired with an optimized piston design featuring a central radius of 10.00 mm and height of 21.44 mm, improved combustion uniformity and thermal efficiency. The optimized geometrical configuration, with 10% hydrogen and 17 mg diesel per cycle, yielded a 24.3% increase in brake mean effective pressure, a 1.5 kW rise in engine power, and a 27.4% reduction in brake specific fuel consumption compared to the baseline. While higher hydrogen concentrations further enhanced performance in simulations—improving fuel economy and output by up to 51% and 69%, respectively—these conditions exceeded the knock threshold (ringing intensity > 5), limiting their real-world applicability. The improvements are attributed to hydrogen’s faster flame speed, wide flammability range, and enhanced combustion kinetics. Overall, the findings highlight that a carefully tailored fuel blend and piston geometry can significantly improve efficiency and sustainability in Hydrogen-Diesel RCCI engines without compromising durability.