【作者】 刘勇强；

【导师】 左承基；

【作者基本信息】 合肥工业大学 ， 车辆工程， 2013， 博士

【摘要】 随着汽车保有量的不断增加和排放法规的日益严格，进一步降低内燃机排放成为当前能源与环境领域的一个重要课题。柴油机作为动力装置以动力性强、热效率高、燃油消耗率低而得到广泛应用，但其NO_x和微粒（碳烟）排放量较高，且排放控制受“NO_X－微粒”折中曲线的束缚。为了改善柴油机的排放特性，本文以ZS195柴油机为研究对象，通过燃烧可视化实验和数值模拟相结合的方法，研究柴油机掺混燃烧过程的燃烧机理，探讨从机内净化入手达到同时降低NO_x和微粒的目的，破解柴油机的“NO_x－微粒”排放难题。本文综合各种可视化系统结构型式的优缺点和研究的实际情况，研制了一套视窗上置式柴油机可视化实验装置。在应用三基色测温法进行温度测量前，对光学测试系统进行温度标定，标定结果表明，用三基色法算出的温度场数值与实测值是非常相近的，其结果在工程上是可信的。利用三基色测温法在视窗上置式柴油机可视化实验装置上，对柴油机的着火和燃烧过程进行了可视化分析，结果表明，用三基色法分析缸内燃烧温度场与用其它方法得到的结果相一致，说明该方法是一种测试分析内燃机缸内着火、燃烧及火焰传播过程的有效方法；结合温度场和示功图曲线对柴油机的着火及燃烧过程进行了分析，结果符合实际柴油机燃烧过程的发展规律。利用柴油机可视化实验装置对柴油机掺氢、EGR（用CO₂模拟）和掺氢EGR的燃烧特性进行了试验研究，结果表明，柴油机采用EGR技术后，随着EGR率的增加，着火时刻呈现推迟的趋势，燃烧持续时间因不完全燃烧而缩短；缸内最大爆发压力减小，达到峰值压力的时间推迟，放热率峰值下降，放热率曲线整体推后，对柴油机的动力性和经济性产生不利影响；另外，EGR的加入使缸内燃烧的最高温度和平均温度降低，缸内氧体积数下降，有利于降低NO_X的排放。柴油机进气掺氢后，随着掺氢比的增加，着火时刻呈现提前的趋势，燃烧持续时间因燃烧速率加快而缩短；缸内最大爆发压力增大，达到峰值压力的时间提前，放热率峰值上升，放热率曲线整体提前，有利于提高柴油机的动力性和经济性；另外，掺氢使得缸内燃烧的最高温度和平均温度相应升高。对于柴油机的掺氢EGR燃烧，当EGR率一定时，随着掺氢比的增加，着火时刻呈现提前的趋势，小EGR率时，燃烧持续时间随着掺氢比的增加而缩短，大EGR率时，燃烧持续时间随着掺氢比的增加而延长；缸内最大爆发压力增加，达到峰值压力的时间提前，放热率峰值增加，放热率曲线整体提前；缸内最高温度和平均温度相应增高；另外，掺氢EGR可以在对发动机的动力性和经济性不产生较大影响和氧体积分数减少的前提下，改善柴油机的燃烧，有利于降低柴油机有害排放物的生成。柴油机掺烧模拟废气柴油重整气时，当EGR率一定时，随着掺氢比的增加，着火时刻呈现提前的趋势，其中H2对着火发生的影响作用大于CO；从火焰发展照片来看，H2对混合气燃烧的影响也要大于CO气体；由于模拟废气柴油重整气的掺混气体量较大，而总进气量和喷油量保持不变，导致过量空气系数变小，混合气变浓，氧体积数下降，所以造成燃料不能完全燃烧，燃烧结束较早导致燃烧持续时间较短，随着掺氢比的增大，燃烧逐步改善，燃烧持续时间开始延长；当EGR率一定时，随着掺氢比的增加，缸内最大爆发压力增加，达到峰值压力的时间提前；放热率峰值增加，放热率曲线整体提前；缸内最高温度和平均温度相应增高。利用FLUENT软件建立了三维燃烧模型，分别模拟计算了标定工况下ZS195柴油机不同方案的缸内压力变化趋势，与ZS195柴油机试验值进行比较，模拟结果和测试结果较为吻合，变化趋势一致，说明论文建立的燃烧模型真实可行。通过三维燃烧模拟，对缸内流场、温度、氧体积数和有害排放进行了分析，对比了掺烧模拟废气柴油重整气不同方案对柴油机工作过程的影响。在喷油初期，缸内速度矢量分布受柴油喷射速率的影响较大；随着掺混气体量的增加，缸内氧浓度下降；当EGR率一定时，随着掺氢比的增大，缸内最高温度升高；当掺氢比一定时，随着EGR率的增大，缸内最高温度降低，而CO的加入对燃烧温度的影响不太明显，这与试验结果基本一致。利用建立的三维燃烧模型，对掺烧模拟废气柴油重整气不同方案下柴油机的有害排放进行了预测分析。原机方案A的NO_X排放和碳烟排放值都比较大；5%EGR对应的B、C、D三种方案中，NO_X排放的降低效果差不多，碳烟排放降低效果从D至C再到B越来越好；15%EGR对应的E、F、G三种方案中，NO_X排放的降低效果从E至F再到G越来越好，碳烟排放大幅降低，降低最明显的是F方案。因此，模拟废气柴油重整气可以实现同时降低NO_x和碳烟排放的目的，为破解柴油机“NO_X－微粒”排放难题提供一条重要的技术途径。更多还原

【Abstract】 With the increase of vehicle population and the stringency of emission regulation, one keyproblem in energy and environmental fields is to decrease emission from internal combustionengines (ICE). Diesel engine, as power device with strong dynamic, high thermal efficiency, lowfuel consumption, has been widely used. However, it also has higher NO_x and particles (carbonsmoke) emissions and which restricted by “NO_X－particles” tradeoff curve. In order to improve theemissions characteristic of diesel engine, this paper studies the burning mechanism of mixingcombustion by the way of the burning visualization experiment and the value simulation in theZS195diesel engine, explores the purpose of reducing NO_x and particulate by the internalpurification to crack “NO_x–particles” emission problems of diesel engine.An optical engine modified from production engine with observation window provided on thecylinder-head has been developed beyond the actual situation and the advantages and disadvantagesof structure type of visualization system. Before applying the way of three-primary-color fortemperature measure, calibrating the optical measurement system for temperature. The results showthat the temperature values of calculation and measurement are very similar, and which is credible inthe project. The ignition and combustion process of diesel engine are analyzed by the use ofthree-primary-color in the optical engine. The results show that the temperature distribution analyzedby three-primary-color consistent with which obtained by other methods, so the methods ofthree-primary-color is an effective way to measure and analyze the ignition, combustion andflame-propagation process in cylinder of ICE. In the meantime, the research result of the fuelignition and combustion process in diesel engine taken by the methods of combining the temperaturefields with indicator diagram is corresponded with that of real engine.The combustion characteristics of intake air blended with hydrogen, EGR, hydrogen and EGRhas been studied in the optical diesel engine. The experimental results show that the ignition timeshowing delay of the trend and the combustion duration reduction because of incomplete combustionas EGR rate increasing when EGR technique is adopted. EGR system with increasing of EGR ratewill result in peak value of the cylinder pressure and heat release rate decreasing, the curve ofcylinder pressure and heat release rate postponing which will lead to negative effects on powerperformance and fuel economy of the diesel engine. In addition, EGR system with increasing ofEGR rate will also result in maximum temperature and average temperature of cylinder reducing andoxygen concentrations decreasing which will lead to beneficial effects on NO_Xemission reducing.When the intake air mixes with hydrogen, the ignition time appears ahead of trend as the hydrogenratio increasing; the combustion duration shorten because of the rapidity of combustion speeds upand the peak of both the cylinder pressure and heat release rate increase and becomes ahead of timewhich will lead to beneficial effects on power performance and fuel economy of the diesel engine. Inaddition, cylinder maximum temperature and average temperature increase accordingly as thehydrogen ratio increasing. When the intake air blends with hydrogen and EGR and the EGR rate isconstant, the ignition time appears ahead of trend as the hydrogen ratio increasing; the combustionduration shortens with small EGR rate and extends with big EGR rate, the peak of both the cylinder pressure and heat release rate increase and becomes ahead of time and cylinder maximumtemperature and average temperature increase accordingly as the hydrogen ratio increasing. Inaddition, under not producing a significant impact on engine power and economy and decreasing inoxygen concentrations, the intake air mixing with hydrogen and EGR can improve combustionwhich is advantageous in reducing the harmful emission of diesel engine.In simulated exhaust diesel reforming gas (SEDRG) conditions, when the EGR rate is constant,there is fire timing advance with increasing of hydrogen ratio and more effect on ignition in H2thanCO. Hydrogen has more effect on combustion of mixed gas from the flame development picture.Due to mixed gas of SEDRG larger and total intake gas and spray oil keep not variable, there is theexcessive air coefficient small, mixed gas concentrated and oxygen volume number declined whichwill led to fuel not full burning and combustion duration shorten with burning end early. Asincreasing of hydrogen ratio, combustion gradually improved and combustion duration time beganextended. When the EGR rate is constant, the peak of both the cylinder pressure and heat release rateincrease and becomes ahead of time as the hydrogen ratio increasing; while cylinder maximumtemperature and average temperature increase accordingly.Three-dimensional-combustion model has been developed using FLUENT software. Thecylinder pressure of the different programs is simulated in the ZS195diesel engine under calibrationconditions. The modeling results are good agreement with the measured data obtained from aparallel experimental investigation, so the combustion model of establishment is feasible. The flowfield, temperature, oxygen concentrations and harmful emission are analyzed and the effects ofdifferent programs in SEDGR condition on work process of diesel are contrasted. The velocityvector was influenced largely by the fuel injection quantities at the begging of fuel injection. Thecylinder oxygen concentration drops with increasing of mixed gases volume. When the EGR rate isconstant, cylinder maximum temperature increases with increasing of hydrogen ratio. When thehydrogen ratio is constant, cylinder maximum temperature decreases with increasing of EGR rate.The effect of CO admission is little on combustion temperature which is agreed with experimentalresults.The forecast analysis is studied on the diesel harmful emission of different programs withthree-dimensional-combustion model in SEDGR condition. The value of both NO_x and sootemission of program A are large. The results of NO_x emission decreasing are the same and sootemission are better and better from program D to program C and then program B among program B,program C and program D with5percent EGR. In the program E, program F and program G with15percent EGR, the soot emission of three programs especially program F are all decreased largely andNO_x emission decreasing are better and better from program E to program F and then program G.Therefore, the SDEGR technique can reduce NO_x and soot emission and provide an importanttechnical approach for resolving “NO_x–particles” emission problems of diesel engine.更多还原