汽油/乙醇双燃料复合喷射对发动机排放性能的影响

通过在一台汽油气道喷射(GPI)的单缸、四冲程发动机上增加一套乙醇缸内直喷(EDI)系统,将其改装成为双喷射发动机,研究了汽油/乙醇复合喷射对发动机排放性能的影响,并建立了发动机计算流体动力学(CFD)模型,详细分析了缸内各区域的温度和组分分布特点.结果表明:加入乙醇直喷可以有效降低CO、NO_x和soot排放,提高乙醇能量占比后,CO、HC和soot排放又有所增加;点火时刻提前,缸内整体温度和压力逐渐增加,NO_x排放显著上升,soot排放有所下降,由于局部混合气质量变差,燃料不完全燃烧导致CO和HC排放有所增加,同时受过冷壁面淬熄效应的影响,作为未燃碳氢(UHC)排出的燃料也会提高HC排放;乙醇喷射时刻提前,燃料有更充分的时间进行蒸发混合,缸内燃烧状况得到优化,CO和soot排放减少,NO_x排放呈上升趋势,间隙效应和壁面油膜吸附作用的增强导致HC排放呈上升趋势.

Effects of Gasoline-Ethanol Dual-Fuel Injection on Engine Emissions

An ethanol direct injection(EDI)system was installed to a single-cylinder four-stroke spark ingition(SI)engine which was originally equipped with gasoline port injection(GPI)system,making it possible for dualinjection operation.The influence of ethanol/gasoline on engine emission performance was first experimentally studied,and then a computational fluid dynamics(CFD)model was established to visually analyze the characteristics of in-cylinder temperature and composition distribution in order to better understand the experimental results.The experimental results show that ethanol direct injection can effectively reduce CO,NO_x and soot emissions,however,at high ethanol energy ratio,the CO,HC and soot emissions increase.With the gradual advance of spark timing,in-cylinder temperature and pressure increase,leading to substantial rising of NO_x emissions and moderate reduction of soot emissions.Further,with the advance of spark timing,poor fuel-air mixture is observed,leading to an incomplete fuel combustion and hence an increase in CO and HC emissions.Cold wall quenching is another major source of HC emissions in this dual-fuel injection system as the unburned hydrocarbon(UHC)is exhausted out.When the ethanol injection timing is advanced,fuel-air mixture preparation and combustion are improved,resulting in a reduction in CO and soot emissions and an increase in NO_x emission.The enhancement of gap effect and wall film adsorption leads to an upward trend in HC emissions.