低负荷下天然气-二甲醚RCCI发动机燃烧和排放的仿真研究

Simulation study on combustion and emissions of a natural-gas/dimethyl-ether RCCI engine under low load

利用发动机仿真专用软件CONVERGE,研究低负荷工况下当量比、二甲醚喷射比例以及发动机转速对天然气-二甲醚双燃料在分隔式燃烧室中反应活性控制压燃(RCCI)燃烧和排放等性能的影响。以单缸Ricardo E6/MS配备MK.V预燃室的发动机为对象建立仿真模型,通过文献中的实验结果验证了模型的有效性。仿真结果表明,当量比为0.28~0.35时,放热率曲线表现为单个主峰,当量比为0.4~0.5时,放热率曲线表现为双峰;当量比为0.45时,发动机的动力性和经济性较好,NO_(x)、soot、HC和CO这4类污染物的排放量总体处于较低水平;发动机指示热效率和IMEP都随着二甲醚喷射比例的降低而增加,但二甲醚喷射比例过低时又会出现失火现象,二甲醚喷射比例为15%时,发动机的动力性和经济性最好,且NO_(x)排放量最低,但HC的排放量较高;在低负荷工况下,转速升高会导致燃烧相位推迟以及燃烧效率降低,甚至出现失火现象,转速为800 r/min左右时,双燃料发动机的燃烧和排放性能都表现得更好。

By using CONVERGE software for engine simulation,effects of equivalent ratio,dimethyl-ether(DME)injection ratio and engine speed on reactivity controlled compression ignition(RCCI)combustion and emissions of natural-gas/DME dual-fuel in separated chambers were studied under low engine load.A simulation model of single-cylinder Ricardo E6/MS engine with MK.V pre-chamber was built,and its validity was verified by the experimental results from a reference.The simulation results show that heat release rate curves have a single main peak at the equivalent ratio of 0.28~0.35,and double peaks at the equivalent ratio of 0.4~0.5.When equivalent ratio equals 0.45,the engine has better power performance and fuel economy,and the overall emissions of NO_(x),soot,HC and CO are relatively low.Indicated thermal efficiency and IMEP of the engine increase with the decrease of DME injection ratio,but the engine will misfire if the injection ratio is too low.When the injection ratio of DME is 15%,the engine has the best power performance and fuel economy,and NO_(x) emission is at the lowest level,but HC emission is higher.Under low load,rising engine speed will lead to delayed combustion phase,lower combustion efficiency and even misfiring.When engine speed is about 800 r/min,combustion and emission performances of the dual-fuel engine are better.