Effects of research octane number and octane sensitivity on Knock-Limited and Propagation-Limited combustion in a Lean-Burn DISI engine

The pursuit of higher thermal efficiency in next-generation spark-ignition (SI) engines is driving the development of ultra-lean combustion, where intensified flame quenching and knock create new challenges for fuel formulation. This study investigated the combined effects of research octane number (RON) and octane sensitivity (OS = RON – MON, with MON being the motor octane number) on performance, stability, and emissions in a 2.5-L GDI engine at 2000 rpm and 9 bar IMEP over λ = 1.0–1.9. Nine PRF/TPRF-based surrogates covering RON 95/100/105 and nominal OS ≈ 0/6/12 (OS ≈ 5–12 at RON 105), plus a Euro-6 E10 reference, were tested. Ignition timing was varied up to either maximum brake torque (MBT) or knock-limited spark advance (KLSA). At RON 95, the higher-OS formulations operated with approximately 1–5 CAD more spark advance than the OS = 0 fuel, bringing combustion phasing closer to the reference target. Higher OS also allowed CA50 to reach MBT at λ ≈ 1.5–1.6, shortened the burn duration by ∼ 1–2 CAD, and increased both peak pressure and efficiency. At RON ≥ 100, the engine operated near MBT over most of the λ range; the higher-OS formulations reached the reference CA50 with spark timing retarded by only ∼ 1–2 CAD relative to the lowest-OS fuel in each RON group, with negligible differences in CA50, ignition delay and burn duration. With leaner mixture, ignition delay and burn duration increased for all fuels; by λ ≈ 1.7, HRR peaks dropped by ∼ 40 % relative to stoichiometric operation and pressure/HRR traces converged within each RON class, indicating heat release governed primarily by mixture leanness. Combustion stability (COV IMEP ) stayed below 5 % to λ ≈ 1.75–1.80 for all fuels, extending to ≈1.85 for high-OS RON-95 blends. NOx peaked near λ ≈ 1.1 and fell by ∼ 93–97 % at λ ≈ 1.8; CO was minimized just lean of stoichiometric with Euro-6 E10, while HC rose sharply beyond λ ≳ 1.6. CO 2 emissions decreased nearly linearly with λ, and indicated thermal efficiency peaked at ∼ 44–46 % for λ ≈ 1.6–1.8 before falling as stability declined. The higher-OS formulations were most beneficial when combustion phasing was knock-limited (notably at RON 95), whereas at high RON and under ultra-lean operation, performance appeared to be limited by flame-kernel development and propagation rather than by octane quality.