Abstract: This paper investigates the effect of the excess air ratio λ and ignition advance angle θ_i on idle characteristics of a turbocharged hydrogen fueled SI engine.The experimental data are conducted under various operating conditions including different λ and θ_i.It is found that the ignition advance angle at MBT point gradully increases with increasing excess air ratio from 1.1 to 2.5.The indicated thermal efficiency increases as λ increases.NO_x emission decreases as λ increases.When λ is kept constant,NO_x emission increases as θ_i increases.During idle conditions of a hydrogen fueled engine,a lean mixture with a λ more than 2.5 is suitable,and θ_i should increase appropriately.The maximum cylinder pressure increases with an increase of λ and θ_i.The trend of the maximum rate of pressure rise is similar at different λ.Only under the conditions of λ=2.5 and θ_i<10°CA,the maximum pressure rise rate remains almost unchanged.