多喷油器分段喷射对大缸径低速柴油机燃烧及排放的影响

Influence of Split Injection on Combustion and Emissions of Large-Bore Low-Speed Diesel Engine with Multiple Injectors

针对大缸径低速二冲程船用柴油机,废气再循环(EGR)技术满足TierⅢ排放时造成的油耗恶化问题,采用三维数值模拟手段,研究了EGR氛围下多次喷射与燃油喷射角度对发动机油气混合、燃烧及排放的影响。结果表明,采用30%EGR率可满足TierⅢ排放标准;喷油角度直接决定缸内混合气形成过程,喷油角度过小会导致气缸中心油束重叠形成浓混合气区,缸内空气无法充分利用;喷油角过大会导致喷雾油束和火焰撞壁,缸壁散热损失增加;而适中的喷油角度(20°)有利于提升功率,降低油耗。在EGR氛围下通过协同优化多次喷射与喷油角度,采用30%EGR率结合10.0%预喷量,主预喷间隔为15°,喷油角20°,可使NO x排放降低65.5%,油耗仅增加1.24%;采用三次喷射可进一步改善油耗与NO x排放之间的折衷关系,但优化潜力有限。

To address the issue of a large two-stroke marine diesel engine having higher fuel consumption due to adopting the exhaust gas recirculation(EGR)to meet the TierⅢemissions regulations,the influence of split injection and injection angle on the fuel-air mixture,combustion and emissions were studied by using three-dimensional numerical simulation based on the single cylinder model of such engine with multiple injectors.The results show that 30%of EGR rate can meet TierⅢ,and the injection angle directly determines the formation of fuel-air mixture in the cylinder.If the injection angle is too small,the tips of fuel sprays of the injectors overlaps in the central cylinder and forms local rich region,leading to insufficient utilization of the in-cylinder air.If the injection angle is overlarge,fuel sprays and flames collids on the cylinder wall and hence results in increased heat loss through the wall.With a moderate fuel injection angle of 20°,in contrast,optimal mixing and heat release can be achieved with improved power output and reduced fuel consumption.In the presence of EGR,synergistically optimizing split injection and injection angle can reduce NO x by 65.5%with fuel consumption increasing only by 1.24%when using 30%of EGR rate,10.0%of fuel fraction in pilot injection,15°of interval between pilot and main injections and 20°of injection angle.Adopting three-time injection can effectively improve the compromise between fuel consumption and NO x emissions,but with a small room for a further optimization.