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A new combustion system targeting a drastic decrease in NOx emission and a brake specific energy consumption equivalent to that of a DI diesel engine has been developed. In this new combustion system, a lean burn system using early injection was employed to reduce NOx emission and an auto-ignition DI engine system was employed to achieve the low energy consumption. Methanol was used as the fuel for reducing NOx emission. The objective of this study is to clarify the possibility of the system for the auto-ignition of a premixed lean mixture of methanol fuel. This study shows that the gas temperature at ignition, Tig, is the predominant factor affecting auto-ignition. Auto-ignition occurs when Tig exceeds approximately 1000K. The methanol lean burn system in an auto-ignition DI engine drastically decreased NOx emission with almost the same brake specific energy consumption as a diesel engine in the middle load region.