微量柴油引燃高压直喷天然气发动机排放低于同排量柴油机，热效率与同排量柴油机相当，具有良好的发展前景。对于高压直喷天然气发动机，循环变动特性直接影响了其运行的稳定性，但目前的研究大多针对发动机的燃烧和排放特性，对循环变动的研究十分有限。以一台微量柴油引燃高压直喷天然气发动机为研究对象，对其外特性工况下不同运行参数的循环变动进行了研究。结果表明：随天然气喷射提前角增加，最大燃烧压力的循环变动呈现先增加后减小的趋势，低速下最大燃烧压力的循环变动相对较高；滞燃期和50%燃料燃烧相位角的循环变动随天然气喷射的提前和转速的增加而增加；在24 MPa喷射压力下，平均有效压力的循环变动随天然气喷射提前角的增加而增加，而在30 MPa下，平均有效压力循环变动随天然气喷射提前角的变化规律与转速有关；在高转速下，平均有效压力的循环变动相对较高；燃料的喷射压力对各参数的循环变动影响很小。

Compared with diesel engine with the equivalent displacement, high pressure direct injection natural gas engine ignited by a small quantity of diesel has lower emission and comparable thermal efficiency, which has a promising prospect. For this kind of NG engine, the previous extensive studies mostly focused on the combustion and emission characteristics and the study on the cyclic variation which has significant effects on the stability of engine operation was very limited. The cyclic variations of different operating parameters were investigated at full load on a high pressure direct injection natural gas engine ignited by a small quantity of diesel. The results indicated that the cyclic variation of peak cylinder pressure first increases and then decreases with the increase of natural gas injection timing and tends to be higher at low speed. The cyclic variations of ignition delay and 50% mass burnt fraction (50%MBF) increase with the injection advancement and the increase of engine speed. The cyclic variation of indicated mean effective pressure (IMEP) increases with the injection timing at 24 MPa, but the changing law relates to engine speed at 30 MPa. In addition, the cyclic variation of IMEP is relatively high at high speed and that of each parameter is little affected by injection pressure.