Revealing CO2-H2O synergies behind soot and CO surges under high EGR rates for cleaner RCCI engines

• Dilution, heat capacity and chemical effects of EGR are decoupled and quantified. • Combustion characteristics are primarily governed by heat capacity and dilution effects. • Dilution effect dominates NO x while the chemical effect controls CO and soot at high EGR rates. • CO 2 and H 2 O show chemical synergies that increase CO and soot emissions at high EGR rates. • Selective suppression of specific chemical effects (particularly CO 2 ) may enable cleaner combustion. Gasoline-diesel reactivity controlled compression ignition (RCCI) is a promising technology to achieve high thermal efficiency and ultra-low NO x emissions, but faces the problems of increased CO and soot emissions and combustion instability under high exhaust gas recirculation (EGR) rates. To address this challenge, this study systematically decouples the EGR effects (including heat capacity, dilution and chemical effects) by innovatively combining three-dimensional computational fluid dynamics with detailed chemical kinetics and a variable-separation method. The results show that heat capacity is the predominant factor inhibiting combustion, and dilution effect intensifies significantly as EGR rate increases. At low EGR rates (11–22%), heat capacity effect primarily drives NO x reduction; at 33% EGR, dilution and heat capacity effects contribute equally; and at 46% EGR, dilution effect becomes the dominant factor for NO x abatement (76% reduction). Heat capacity effect promotes soot formation (37% increase), while chemical effect dominates CO (93% increase) and soot (51% increase) production. The CO 2 and H 2 O in EGR exhibit distinct nonlinear synergistic interactions. When acting individually, their effects on the key intermediate C 2 H 2 and free radicals (OH, O) are counteracting. However, their combined action produces a synergistic effect that significantly inhibits C 2 H 2 oxidation and substantially promotes soot and CO formation. This synergistic interaction is the core mechanism for the sharp rises of soot and CO emissions at high EGR rates (46%). Consequently, targeted suppression of the chemical effects of specific EGR components (either CO 2 or H 2 O) could achieve ultra-low NO x emissions while effectively inhibiting soot and CO formation. The findings of this study provided a theoretical basis for designing clean and efficient RCCI combustion strategies based on precise regulation of EGR components.