Lean combustion effects on a high-compression methanol SI engine derived from a turbocharged miller cycle gasoline engine

Methanol, a versatile carbon-neutral fuel, offers significant potential for reducing greenhouse gas emissions in spark-ignition (SI) engines. This study systematically compares the combustion and emission characteristics of methanol and gasoline under varying compression ratios (CRs) and lean-burn conditions, combining experimental measurements with three-dimensional computational fluid dynamics (CFD) simulations. Experimental results demonstrate that Methanol exhibits superior knock resistance and a higher octane rating than gasoline, enabling stable combustion at CRs up to 15.2. However, its early combustion phase results in a 15% shorter burn duration and 18% higher peak in-cylinder pressure, posing challenges to mechanical durability. Under lean combustion (excess air coefficient (λ) = 1.0–1.8), methanol achieves optimal thermal efficiency at brake mean effective pressure (BMEP) = 1.1 MPa with λ = 1.6, where total hydrocarbon (THC) emissions increase marginally by 8% compared to stoichiometric conditions. CFD simulations reveal localized fuel-rich zones (equivalence ratio > 1.2) and thermal gradients (>250 K reduction in peak temperature at λ = 1.6), directly linking spatial inhomogeneity to THC and nitrogen oxides (NO x ) emission trends. Notably, NO x peaks at λ = 1.2 under high loads (BMEP = 1.6 MPa) due to elevated combustion temperatures, despite excess air dilution. This work highlights methanol’s viability in high-compression engines and provides critical insights for optimizing lean-burn strategies, advancing the development of efficient, low-emission methanol engines for carbon-neutral transportation.