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缸内直喷汽油机(gasoline direct injection,GDI)的颗粒物排放问题越来越引起人们的重视。为解析GDI汽油机缸内碳烟生成机理,文中提出了一种改进的两步法碳烟模型,将该模型应用于Kiva-Chemkin软件中,对一款典型GDI汽油机的喷雾碰壁、混合气形成、燃烧和碳烟生成过程进行了三维计算流体力学(CFD)数值模拟。结果表明,GDI汽油机的碳烟主要源于缸内油气混合不均、壁面油膜的燃烧即池火燃烧。在均质混合气工作模式下,增加喷油提前角可使燃油雾化更好、油气混合均匀,从而降低碳烟生成。但如果喷油提前角过大,会导致喷雾碰壁,引起碳烟生成量急剧增加。在分层混合气工作模式下,碳烟主要来源于局部浓区。过早喷油不利于产生喷雾诱导的滚流,不利于油气均匀混合,会产生局部燃油浓区,从而导致缸内碳烟生成量较高。结果表明,文中提出的碳烟模型可较好地预测缸内碳烟生产过程。
Particulate emissions from gasoline direct injection (GDI) have drawn more and more attention. In order to analyze the generation mechanism of soot in cylinders of GDI gasoline engine, an improved two-step soot model is proposed in this paper. The model is applied to Kiva-Chemkin software to simulate the spray wall of a typical GDI gasoline engine, Three-dimensional computational fluid dynamics (CFD) numerical simulations of combustion and soot generation were performed. The results show that the soot of GDI gasoline engine is mainly due to the uneven mixing of oil and gas in the cylinder and the burning of wall oil film, ie pool fire. In the homogeneous mixture mode of operation, increasing the fuel injection advance angle can make the fuel atomization better and mix the oil and gas evenly, so as to reduce the generation of soot. However, if the fuel injection advance angle is too large, will lead to spray hit the wall, causing a sharp increase in soot formation. In the stratified mixture mode of operation, soot mainly comes from local concentration. Premature fuel injection is not conducive to produce spray-induced tumble, is not conducive to the uniform mixing of oil and gas, will have a local fuel concentration, resulting in a higher amount of soot in the cylinder. The results show that the soot model proposed in this paper can predict the soot production process better.