生物柴油-柴油-乙醇混合燃料的燃烧与排放特性

为了改善发动机燃用高比例生物质混合燃料的性能，在中等比例的生物柴油-柴油混合燃料中分别添加5%、10%和20%体积比的乙醇（分别用BD50E5，BD50E10和BD50E20表示），在一台6缸增压共轨柴油机上，将发动机的转速稳定在1 600 r·min^-1，选择7个不同的负荷点测定不同掺混比生物柴油-柴油-乙醇混合燃料的燃烧与排放性能，并将其与柴油进行对比。结果表明：在平均有效压力为0.322 MPa的低负荷条件下，发动机为预喷加主喷喷油策略，在预喷的低温反应阶段生物柴油-柴油-乙醇混合燃料产生了大量羟基自由基，因此混合燃料的缸内最大压力和最大瞬时放热率均高于柴油；随着负荷的增大，当平均有效压力为0.805 MPa时，发动机的喷油策略转变为单段喷射，乙醇的热值较低导致生物柴油-柴油-乙醇混合燃料的缸内最大压力和最大瞬时放热率低于柴油；随着乙醇掺混比的增大，受乙醇低十六烷值和高汽化潜热的影响，生物柴油-柴油-乙醇混合燃料的滞燃期明显延长；强烈的预混燃烧和乙醇的高含氧量使混合燃料的燃烧速度明显加快，乙醇的添加有利于燃料集中放热从而缩短燃烧持续期；与纯柴油相比，BD50E5，BD50E10和BD50E20的NO_x排放量分别升高了10.46%、12.59%和17.52%，碳烟排放量分别降低了37.91%、45.85%和49.25%，CO排放量分别降低了20.24%、36.43%和46.43%，HC排放量分别降低了12.53%、4.40%和0.76%。

Combustion and Emission Characteristics of Biodiesel-diesel-ethanol Fuel Blends

To improve the performance of engines fueled with a high proportion of biomass fuel blends, ethanol was added to medium proportion biodiesel-diesel fuel blends by a volume percentage of 5%, 10%, and 20%, respectively denoted as BD50E5, BD50E10, and BD50E20. The combustion and emission performances of biodiesel-diesel-ethanol fuel blends were investigated on a turbocharged, 6-cylinder, common rail diesel engine at a constant speed of 1 600 rpm and seven engine loads, and the engine performances of the fuel blends were compared to that of diesel fuel. The results show that the fuel injection strategy of the test engine is pilot injection plus main injection at low loads when the brake mean effective pressure (BMEP) is equal to 0.322 MPa. The maximum cylinder pressure and the maximum heat release rates of the biodiesel-diesel-ethanol blends are higher than that of diesel, because a large number of hydroxyl radicals are generated in the low-temperature reaction of the pilot injection stage. With increased engine loads, the fuel injection strategy becomes single-stage injection at 0.805 MPa BEMP. The maximum cylinder pressure and the maximum heat release rate of the biodiesel-diesel-ethanol blends are lower than diesel fuel owing to the lower caloric value of ethanol. Moreover, with the increasing proportion of ethanol, the ignition delay of biodiesel-diesel-ethanol blends is obviously prolonged due to the lower cetane number and higher latent heat of ethanol, and the combustion speed of the fuel blends are distinctly accelerated due to the stronger premixed combustion and the higher oxygen content of ethanol. Therefore, the addition of ethanol is helpful to concentrate the heat release and shorten the combustion duration. In terms of exhaust emissions, compared to diesel fuel, NO_x emissions from biodiesel-diesel-ethanol blends of BD50E5, BD50E10, and BD50E20 increased by 10.46%, 12.59%, and 17.52%, respectively; whereas, soot emissions decreased by 37.91%, 45.85%, and 49.25%. CO emissions decreased by 20.24%, 36.43%, and 46.43%, and HC emissions decreased by 12.53%, 4.40%, and 0.76%, respectively.