【作者】 李珏；

【导师】 洪伟；

【作者基本信息】 吉林大学 ， 工程热物理， 2012， 硕士

【摘要】 随着汽车工业的发展，汽车产量越来越高。人们对汽车排放造成的污染也越来越重视，因此各个国家的汽车排放法规日益严格。我国的排放法规也在日趋严格，其重点领域便是柴油机的氮氧化物和微粒的排放。柴油机瞬态工况下的燃烧和排放与稳态相比有着很大的不同，这种差异在涡轮增压的柴油机上体现得更为明显。因此欧美国家相继推出了自己的瞬态排放测试标准EPA（美国）和ETC（欧洲）。对柴油机瞬态工况下的燃烧和排放的研究也越来越受到人们的关注。本文应用AVL-BOOST和AVL-Fire diesel建立一维/三维仿真平台，对车用增压中冷、六缸高压共轨柴油机进行建模仿真。在仿真平台的基础上，通过对稳态工况，不同负荷瞬变率下的瞬态工况的模拟计算，对比这些不同工况的燃烧过程及排放水平，寻找瞬态运行规律，最终得到了以下结论：1.应用AVL-BOOST和AVL-Fire diesel建立一维/三维仿真平台。应用实验数据校准模型。同时在搭建好的联合仿真平台的基础上，设定了两种不同的仿真模式：全循环结合模式和部分循环标定点模式。通过与传统一维计算方式的结果对比，验证了联合仿真平台在进行燃烧过程的复杂计算时有着较好的准确性；同时和CFD软件整机计算相比，其节省了70%以上的时间。2.对1800r/min转速下的稳态工况与瞬态工况的燃烧和排放进行了模拟计算，并通过结果对比分析瞬态工况参数变化规律：（1）和稳态工况相比，恒定转速增扭矩的瞬态工况存在着进气和转矩响应滞后的现象。这种现象是由喷油量—进气量滞后度和增压器响应延迟造成的；（2）和稳态工况相比，瞬态工况燃烧过程中的参数有着较大的差异。瞬态工况的空燃比较低，缸内燃烧最高温度较高，最高压力较低，燃烧持续期增加，放热率重心后移；（3）和稳态工况相比，瞬态工况下烟度排放量增加。这是由于瞬态工况相同负荷下，喷入缸内的柴油增多，后续喷入的柴油多数位于空气流动弱，氧气含量低的区域。瞬态工况下的氮氧排放较低。这是因为低负荷时缸壁温度较低，影响燃烧温度；高负荷时，涡轮增压器滞后度引起的进气量下降，从而导致相同输出扭矩下，瞬态工况NOx的排放量更低。3.通过分别对1800r/min转速，负荷在2s、4s、8s内负荷从88N.m(10%)到800N.m（90%）的三种瞬变过程进行数值模拟计算，并对实验结果进行对比，从而得出以下结论：（1）恒定转速增扭矩工况下，瞬变率越大，输出扭矩及涡轮增压器响应的延迟性越明显。增压器响应延迟性的原因包括两点：每一个阶段下供油量先增加而进气量还达不到这一循环水平，需要一定的时间才能达到理论水平；排气管中气体存在可压缩性，单一循环无法完全压缩气体，提升涡轮能量。（2）恒定转速增扭矩工况，相同扭矩条件下，当瞬变率增大时，进气量减小，2S和8S进气量差值达到82.24kg/h；燃油消耗量增加，输出扭矩660N.m（80%负荷）下，2S和8S的燃油消耗量差值达到了3.5kg/h；缸内空燃比下降；柴油机的最高燃烧温度上升；柴油机的最高缸内压力下降；燃烧持续期增加。其原因在于，瞬变率增大时，进气量-喷油量的滞后的增大；涡轮增压器响应延迟性增大。（3）不同瞬变率下的排放水平也不相同。瞬变率增加，烟度排放量增加。这是燃油循环喷射量随瞬变率增大而上升，较多的燃油处于贫氧状态，从而生成更多Soot；同时因为缺氧，后续生成的Soot无法被氧化，最终导致Soot排放量明显上升。氮氧化合物排放量随瞬变率增大而下降。这是因为在中低负荷时，瞬变率越大，燃烧室内的温度相对于燃油喷射滞后度大，导致相同转矩下，瞬变率大的NOx的排放量更低；而在高负荷时，由于缸内平均温度和最高温度均在1600K以上，因此增压器响应滞后导致的瞬时空燃比降低，氧的浓度下降成为主要影响因素。更多还原

【Abstract】 With the developing of economy and technology, more and more vechiles areproduced to meet people daily needs. Emissions regulations become s increasingly strict,especially in PM and NOx emissions of diesel. Additionally, there are obviously differencesbetween transient conditions and steady conditions and many new transient emissionregulations are created, such as EPA (USA) and ETS (Europe). Therefore, more researcherstransfer their study on transient conditions.Establishment of1D/3D simulation platform for diesel engineIn this study, a new simulation platform combining1-D and3-D models wasestablished for diesel combustion process and emissions formation research. In the newsimulation platform,3-D model is used in combustion simulation and1-D model is used inother parts. Compared with traditional1D simulation, the improved model can predict heatrelease rate, A/F ratio, combustion pressure and emissions more accurately. The changingtrends of simulation results are similar with the experiment results. Compared withtraditional CFD simulation, the improved simulation saved70%times in the simulation oftransient conditions.Investigation of transient conditions on simulation platformIn the sutdy, based on the improved simulation platform, combustion and emissions ofsteady conditions and transient conditions are calculted and finally some conclusions aredrawn.(1) Compared with steady conditions, there are delays in output torque andturbocharger of transient conditions, and delays become larger as transient rate increasing.The reason is compressibility of gas and interance of turbocharge.(2) Compared withsteady conidtions, air inflow decreases; fuel consumption increases; temperature peak valueincreases; pressure peak value decreases. Additionally, these phenomenons become moreobviously as transient rate increasing.(3) There are some differences of emissions betweentrasient and steady conditions. More Soot are produced in transient conditions andincreaseing as transient rate being higher.The reason is: in the same output torqueconditions, there are more fuels in transient conditions which means more fules would beinjected into the area of lack oxygen and air flow. Based on this, more Soot are produced;Fewer NOx are produced in transient conditions. In low load conditions, low temperatureof chamber will absorb heat which causes combustion temperature decreases; In hig loadconditions, the average temperature are higher than1600K which means temperatr has littleeffect on NOx production. Then, there is less oxygen in chamber in transient conditions.Based on these, fewer NOx are produce in transient conditions.更多还原