柴油喷射压力对HCII燃烧过程影响的可视化

摘要

Homogenous charge induced ignition (HCII) is a promising combustion mode due to its potential advantages combined from both the compression ignition combustion and the spark ignition combustion,thus simultaneously improves the engine thermal efficiency and emission characteristics.An investigation of the impact of diesel injection pressure on the HCII combustion characteristics was carried out on an optical engine in comparison with the traditional diesel combustion mode.High speed in-cylinder ignition images were analyzed in conjunction with the combustion information derived from the in-cylinder pressure data.The results suggest that increasing diesel injection pressure results in shorter ignition delay and earlier ignition timing.An enlarged flame area and faster combustion rate were also observed.As a result,the peak cylinder pressure and the heat release rate are increased and the phase of the peak pressure is advanced.Under the same diesel injection pressure,the HCII combustion mode gives more ignition points scattered and more ignition timing advanced,but a slower flame development rate at the early stage than the diesel combustion mode.Furthermore,the spray based flame development of diesel combustion mode appears more obviously under medium and high injection pressures.While in the HCII combustion mode,it suggests the coexistence of premix combustion and diffusion combustion.At medium injection pressure,a large range of blue flame was observed,which suggests a typical premixed combustion.At high injection pressure,the combustion is mainly the premixed combustion of homogenous gasoline charge,while the diffusion combustion becomes predominant after the peak heat release rate.%均质混合气引燃(HCII)的燃烧方式融合了柴油机与汽油机的优点,具有提高发动机指示热效率、改善排放的潜力.通过光学发动机,采用高速摄影和燃烧分析系统,研究纯柴油(缸内直喷)与汽油均质混合气柴油引燃两种工作模式下柴油喷射压力对燃烧特性的影响.结果表明:随着柴油喷射压力的提高,两种燃烧模式的燃油雾化质量改善,滞燃期缩短,着火时刻提前,缸内压力和放热率峰值增大,峰值位置提前,同时着火面积增大,燃烧速率加快.在相同柴油喷射压力下,HCII燃烧模式的着火点较为分散,着火时刻相比纯柴油更早,但火焰发展初期速度较慢.纯柴油模式在各喷射压力下均有扩散燃烧特征,中、高喷射压力时扩散燃烧现象更加明显,HCII燃烧模式在低喷射压力下为预混合燃烧和扩散燃烧共存.中等喷射压力下,视窗内分布大片蓝色火焰,着火面积较大,为典型的预混燃烧.高喷射压力下,前期燃烧主要为汽油均质混合气的预混燃烧,放热率峰值点之后以柴油的扩散燃烧为主.