Abstract: The rule of ignition advance angle ( θi ) and excess air ratio ( λ) influencing on the combustion and emissions characteristics is investigated on a CNG engine at idle condition． The throttle valve is fully closed． When the parameters are changed，the stepper motor is adjusted automatically to control the engine speed at 800 r /min． When the ignition advance angle is changed from 40 to 10o crank angle ( CA) before top dead center ( BTDC) ，the maximum indicated thermal efficiency ( ITE) and the maximum burning speed are observed at λ = 1． 1． As the excess air ratio λ increases，the maximum ITE is corresponding to a greater θi ． The CoVimep will be increased if the ignition is too early or too late． When the excess air ratio λ is more than 1． 3，the CoVimep increases obviously． When λ = 1． 1，NOx emissions get the highest level．

Key words: CNG engine, idle, ignition advance angle, excess air ratio, emissions

摘要： 在一台CNG 发动机上研究怠速工况条件下，点火提前角( θi ) 和过量空气系数( λ) 对燃烧排放性能的影响规 律。节气门全关，参数调节时，步进电机自动调节，将发动机转速控制在800 r /min。θi 在上止点前40 ～ 10°CA BTDC 变化，λ = 1． 1 时指示热效率和燃烧速度最高; 随着λ 增加，最高指示热效率所对应的θi 越大; θi 过大或过小，都会加 剧循环变动; λ ＞ 1． 3，平均指示压力循环变动( CoVimep ) 明显增加; λ = 1． 1 时，NOx 排放最高。

关键词: CNG 发动机, 怠速, 点火提前角, 过量空气系数, 排放