Optimizing intake flow for enhanced lean burn in a methanol-fueled SI engine: An experimental and numerical study

Methanol stands as a prime alternative fuel for spark-ignition engines, offering a viable path towards carbon neutrality due to its potential for renewable production and favorable combustion properties. Lean burn is key to maximizing its efficiency, yet is fundamentally limited by mixture preparation and ignition stability. This work presents a combined experimental and numerical investigation into optimizing the intake system of a direct-injection methanol engine to extend its lean-burn capability. Experimentally, a variable intake deflector was evaluated at a representative operating point (1500 rpm). An optimal flap opening angle (FOA) of 45° was found to generate a coherent tumble flow, extending the stable lean-burn limit (coefficient of variation on indicated mean effective pressure <5 %) to λ = 1.67. In contrast, a 30° FOA disrupted tumble formation, degrading stability. Complementing this, three-dimensional computational fluid dynamics simulations were employed to redesign the fixed intake geometry. Results demonstrated that the intake manifold flap (IMF) design is paramount for achieving mixture homogeneity, while the port divider (PD) primarily modulates tumble intensity. The synergistic coupling of optimal components (IMF 4 & PD 4) yielded a further improvement, increasing indicated mean effective pressure and indicated thermal efficiency by 0.11 bar and 0.79 % over the (IMF O & PD O) under the optimal 45° FOA setting. This study concludes that a coordinated strategy—combining active flap control for on-demand tumble generation with synergistic passive geometry optimization—is highly effective for stabilizing ultra-lean combustion and unlocking the high-efficiency potential of methanol fuel.