Thermal Efficiency Improvement and Emission Reduction of Methanol Spark Ignition Engine Using Lean-Burn Strategy

Methanol is a promising fuel for achieving carbon neutrality in the transportation sector, particularly for internal combustion engine vehicles. With its high-Octane number, methanol enables higher thermal efficiency compared to gasoline engines. Additionally, its wide flammability range allows stable engine operation under lean burn conditions at low to mid-load levels. These characteristics make methanol well-suited for lean-burn strategies, which reduce pumping losses and enhance thermal efficiency. However, there remains a lack of studies on the influence of injection timing under different lean conditions, particularly in a wall-guided spark ignition engine. Wall-guided systems use the chamber wall or piston surface to redirect and stratify the fuel-air mixture near the spark plug at the time of ignition. The combustion performance of lean-burn engines in highly sensitive to variations in injection and excess air ratio. In this study, experiments were conducted on a single-cylinder engine to examine the combustion and emission characteristics under varying excess air ratios and the injection timings. At an SOI of -180 CAD aTDC, a thermal efficiency of 47.5% was achieved when the excess air ratio was increased. This corresponds to a 5.62% improvement in efficiency compared to the condition with excess air ratio (λ) 1.2 condition, representing the largest increase among all tested conditions. Due to high thermal efficiency, high vaporization heat of methanol, and low combustion temperature of lean conditions, nitrogen oxides emission decreased from 10.24 g/kWh to 2.23 g/kWh. However, corrected hydrocarbon emission increased from 3.07 g/kWh to 6.98 g/kWh under SOI -120 CAD aTDC condition, leading to the decline in combustion efficiency.