Abstract: Based on a mass-produced gasoline engine with adapted methanol fuel， the mechanisms by which stoichiometric combustion and ultra-lean burn technologies enhance the thermal efficiency of spark-ignition methanol engines were studied systematically. The core parameters and system configurations of the engine were optimized. For the stoichiometric combustion system， the compression ratio was increased to 15， and the corrosion resistance of the fuel injectors was enhanced. For the ultra-lean burn system， a 230 mJ high-energy ignition coil， a high-efficiency exhaust gas turbocharger and an optimized valve train were integrated to expand the lean burn limit. Bench tests were conducted to analyze the engine performance， emission characteristics and energy loss properties， and the challenge of identifying pre-ignition in methanol engines was addressed simultaneously. The results show that the maximum effective thermal efficiency of the methanol engine under stoichiometric combustion reached 45.96%， representing a 5.86 percentage point increase compared with that of the original gasoline engine， with total hydrogen carbon（THC） and particle number（PN） emissions reduced significantly. The maximal thermal efficiency of the methanul engine under ultra-lean burn was further increased to 48.67%， and the area with thermal efficiency above 46.00% accounted for half of the entire map. The NOₓ emissions decreased sharply compared with those under stoichiometric combustion.