【作者】 沈照杰；

【导师】 刘忠长；

【作者基本信息】 吉林大学 ， 动力机械及工程， 2013， 博士

【摘要】 柴油机问世至今，无论是系统附件还是燃烧模式都发生了很大变化，从粗笨、噪声大、冒黑烟的形象中彻底脱离，代之以高效、节油、可靠、清洁，并广泛应用于汽车领域。随之快速发展的高压共轨燃油喷射系统、EGR技术、后处理技术等已极大的解决了柴油机排放问题，然而在超低排放与能源危机的双重压力下，考虑到能源的全循环生命周期效率，排放控制问题不能只通过简单的物理吸附及化学催化反应来解决，需要从排放物生成的根源——燃烧过程进行控制。本研究为了解决柴油机燃烧过程中NO_x与soot排放的折衷关系，采用试验台架与数值模拟平台结合的方式对重型高压共轨涡轮增压柴油机稳态及瞬态工况下的工作过程、人为设定缸内EGR分层、进气过程实现缸内EGR分层等问题进行了研究，探索有效降低柴油机氮氧化物与微粒排放及实现显著的缸内组分分层的方法。本研究的研究对象为重型高压共轨涡轮增压柴油机。首先建立了重型柴油机瞬态及稳态试验平台，通过改变喷油参数研究柴油机稳态过程燃烧及排放特征的变化规律。稳态及瞬态过程研究一方面实现了对数值模型的多工况校验；一方面得到了柴油机缸内NO_x与soot生成特征及规律，为缸内EGR分层设定提供理论基础。瞬态过程的研究表明：随着瞬变循环数增加，缸内温度-当量比分布向双岛图的NO_x岛的递进减小，向soot岛的递进增加，对应NO_x排放降低而碳烟排放增加；缸内油气分布不均匀程度呈先增加后减小的趋势；瞬变过程第176循环与相应的50%稳态负荷点（相同转速，相同平均指示压力，IMEP）相比，缸内分布不均匀程度在碳烟生成持续期内偏高是导致瞬变过程缸内碳烟排放恶化的主要原因；瞬变过程中缸内高温区持续时间对NO_x生成的影响大于缸内高温区体积的影响，并且45个试验工况点的NO_x排放与缸内平均温度特征的相关性分析结果与模拟分析结果一致。课题组前期时序分区燃烧的研究表明，采用后喷结合缸内化学氛围迁移可以解决缸内碳烟与NO_x排放的折衷关系，但由于对后喷量及后喷时刻等参数十分敏感，时序分区燃烧降低排放的能力有限，因此本研究提出EGR分层的概念。基于缸内NO_x与碳烟生成区域与生成时间不同的机理，在NO_x生成区保持缸内较高的EGR，控制燃烧温度降低NO_x生成，而在碳烟生成区使其保持充足的氧氛围，并利用缸内流动对缸内组分的迁移作用向碳烟区补充氧气，促进碳烟后期氧化。旨在缸内局部燃烧区内形成EGR率浓区，其余位置为新鲜空气充量，这样既可以降低向气缸内引入EGR的量，使EGR冷却损失下降，涡轮增压能力提高，又可以使缸内平均氧浓度保持在较高的水平，燃烧效率得到提高。EGR分层研究首先采用人为设定缸内EGR分布的方式，探索径向与轴向分层特征对柴油机燃烧及排放的影响规律及同时降低NO_x与碳烟排放的潜力；其次，搭建重型柴油机全气道仿真模型，在切向进气道截面不同位置通入EGR，研究进气过程对缸内EGR分布的影响，结果表明：缸内设定EGR分层可同时降低NO_x与碳烟排放；轴向分层局部EGR率30%时，排放同时降低效果优于径向分层；EGR分层与时序分区燃烧耦合，主喷定时由-3°提前到-5°时，碳烟与NO_x同时降低，且此时对应的指示热效率也较高，能够打破NO_x与碳烟排放间的折衷关系；进气过程EGR与空气未经混合流入进气道，继而流入气缸，缸内EGR呈梯度分布，可实现EGR分层，最大EGR率梯度可达18.3%。人为设定EGR分层能够实现同时降低NO_x与碳烟排放，且与时序分区燃烧耦合通过调整喷油参数使柴油机保持较高的指示热效率。继而本研究设计了CO₂喷射试验，研究如何实现EGR分层，并结合数值仿真分析评价CO₂喷射试验的缸内组分分布。搭建CO₂进气喷射试验台架及数值模拟平台；改造重型柴油机第一缸进气系统，分别在第一缸的切向与螺旋气道安装CO₂喷管，并保证喷管出口靠近进气门位置，降低进气道内CO₂与空气的混合机会；在1缸排气道内安装废气采样管，对1缸废气采样并由Horiba废气分析仪测量废气排放；结合全气道数值模型研究CO₂喷射试验中缸内组分分布特征。研究结果表明：CO₂喷射试验存在CO₂窜缸的问题，CO₂质量流量为10kg/h时，误差约为4%，CO₂质量流量为50kg/h时，误差约为10%，因此CO₂喷射试验只分析切向、螺旋及双进气道三种不同CO₂喷射方式在随着CO₂喷射质量流量的增加，柴油机燃烧及排放的变化规律；作者提出两种缸内组分分布评价方法，一是缸内组分质量浓度场及其浓度梯度，一是结合经济学中的洛伦兹曲线与基尼系数，评价不同CO₂喷射方式缸内的氧气与CO₂分配不均匀程度；组分的浓度梯度可直观表征缸内分布不均匀性，但仅限于局部特征，洛伦兹曲线及基尼系数由缸内全局单元格数据统计计算而得，反映的是缸内整体分布不均匀性特征。两种方法互为印证，互为补充。均匀CO₂进气、切向气道、螺旋气道及双气道喷射CO₂的四种方式对比得出，控制氧浓度在0.21附近，可以使发动机排放满足欧四水平，继续降低氧浓度能够使NO_x与THC达到欧Ⅴ排放法规要求，但CO与碳烟排放较高，并会牺牲指示热效率，其中螺旋气道喷射CO₂方式在缸内平均氧浓度为0.21时，NO_x、碳烟、CO与THC排放同时满足欧Ⅳ排放水平，切向与双气道喷射也比较接近，但切向喷射NO_x排放略高，双气道喷射CO排放略高；数值模拟分析表明采用进气道加装CO₂喷管控制缸内组分分布虽然能实现缸内组分浓度梯度分布，但全局分层特征较低。为了解决CO₂喷射试验的缸内全局分层特征较低的问题，本研究提出时序进气概念，即将进气过程分段。时序进气可以控制进气组分在缸内的分布位置，先进入气缸的组分在缸内分布为下浓上稀，后进入气缸的组分为下稀上浓；两段时序进气，第一段通入空气，第二段通入CO₂，缸内氧气质量浓度梯度可达7.38%，缸内不均匀性特征高于0.2；与均匀进气相比，缸内平均CO₂率相同时，时序进气的NO_x与碳烟排放均较低，NO_x降低51.2%，碳烟降低13.4%，由于燃烧区域CO₂质量浓度高于均匀进气情况，燃油滞燃期更长，放热率峰值更高，但缸内压力偏低。更多还原

【Abstract】 Diesel has been changed not only on the appearance but also the combustionmode. It is never looked as heavy, noise and always with black smoke, instead,effective and reliable, save oil and clean are used to describe it, and widely used inautomobile since the first diesel came out. However, ultralow emissions and energycrisis challenge the diesel for combustion control to reduce emission on its generation,cycle life is a new topic for the requirement of energy efficiency, which meansafter-treatments and alternative fuel engines are limited compared with diesel. Tosolve the trade-off between NO_xand soot emissions, this work have analyzed steadyand transient operation of diesel combustion process, diesel combustion and emissionsof EGR stratification set artificially in cylinder, and how to realize in-cylinderstratification through intake process based on heavy and medium duty turbocharginghigh pressure common rail diesel engines by experiments and simulations.In this work steady and transient combustion process had been analyzed based onHD （heavy duty） diesel with common rail fuel injection system and turbocharger.Tested bed and simulation model were first established, and fuel injection parameterswere adjusted to obtain the relationship of CA50（crank angle when50%of fuelburned） and emissions with fuel injection parameters under steady condition.3Dcombustion and spray simulation on DI High pressure common rail HD diesel engineunder transient operations had also been conducted. Experimental results of constantspeeding increasing torque transient process rated at1650rpm, and torque increasedfrom10%to90%under10s, had been separated into discrete points based on cycle.NO_xemissions mechanism had been analyzed by volume and duration of in-cylinderhigh temperature area and φ-T diagram. As analysis shown, the simulation results ofseparated transient loads agreed with that of corresponding experimental data. NO_xemissions dropped under increasing torque process while soot emissions rosesubstantially as explained in φ-T diagram that in-cylinder equivalence ratio andtemperature march a lot to soot area and retreat to NO_xarea as loads increased.Uniformity of cylinder mixture was analyzed by Lorenz curve which is a graphicalrepresentation of the cumulative distribution function of the empirical probabilitydistribution of wealth. Under transient conditions, soot emission deterioration isattributable to more heterogeneous of in-cylinder fuel-air mixture compared to steadycondition. Simulation results and experimental analysises have reached an agreementthat NO_xemissions are dependent on duration of in-cylinder high temperature.Timing-sequential regionalized diesel combustion proposed in previous investigation is realized by post injection and proper in-cylinder chemical atmospheretransport to keep post injection fuel injecting to high oxygen concentration region.NO_xand soot emissions decrease simulanously, however, mass of post injection fueland post injection timimg are sensitive for threshold of emissions, consequently, EGRstratification is proposed based on the difference of NO_xand soot generation area andtiming, which means high EGR concentration at the region of spray while highoxygen at the other region. NO_xgeneration is depressed by high EGR during maininjection fuel combustion, and soot oxidation is enhanced by high oxygenconcentration realized by in-cylinder chemical atmosphere migration during postinjection fuel combustion. As a result, mass of EGR introduced in cylinder decreases,cooler lose of EGR decreases and exhaust energy flowing to turbocharger increasesmore importantly, in-cylinder mean oxygen concentration is higher than uniform EGRwhat will make combustion efficiency increasing.EGR stratification first discussed numerically using STAR-CD3.26by settingin-cylinder EGR distribution using subroutines. Combustion results of EGRstratification have similar heat release rate character compare to uniform EGR casewhile lower soot and NO_xemissions with the same mass of EGR introduced tocylinder. Axial and radical EGR stratification that defined artificially in cylinder underthe condition of1650r/min and50%load have been numerically analysed. It isindicated that both axial and radical distribution of EGR stratification make NO_xandsoot emissions decreasing simultaneously, however, radical EGR distribution worksbetter under low local EGR rate, axial EGR stratification degrades more in NO_xandsoot emissions. EGR stratification coupled with timing-sequential regionalized dieselcombustion has the potential to resovle the trade-off between NO_xand soot emissionswhile keep high indicated heat efficiency.EGR introduction strategies were discussed for diesel EGR stratification usingComputational Fluid Dynamics （CFD） and experimental test bed. And in-cylinderdistribution before the start of fuel injection for different stratification strategies wasanalyzed. In-cylinder gas heterogeneous distribution during intake and compressionstroke were realized by means of introducing EGR gas to tangential and spiral intakeport respectively and kept EGR away from fresh air using separated runner extendedto each intake valve. In this work, CO₂gas was selected to substitute for EGR.In-cylinder pressure of simulation results agreed with that of experiments. Twomethods were proposed to evaluate in-cylinder stratification, one is in-cylinderinhomogeneity and oxygen mass concentration field contour for local distribustioncharacter, the other is Lorenz curve and the Gini coefficient which is a graphicalrepresentation of the cumulative distribution function of the empirical probabilitydistribution of wealth in economics and the Gini coefficient is calculated based onLorenz curve and used to quantify in-cylinder inhomogeneity of different simulation cases for global distribustion character. Tested bed is reformed from CA6DL dieselengine for the intake system of first cylinder, two pipes were fitted separately totangential and spiral intake port of first cylinder, and exhaust sampling system wasinstalled to first cylinder for exhaust analysis. The problem of the CO₂injectionsystem is the leakage of CO₂to the other intake port, the maximum error is10%high,so the experimental results are just used to qualitative analysis. The results indicatethat CO₂injection strategies have the potential to meet the Euro4emisions stardardwhen in-cylinder mean oxygen mass concentration is0.21, as oxygen massconcentration lower further, NO_xand THC emissions reached the Euro5standardwith a little higher emission of CO and soot and lower indicated heat efficiency.In-cylinder distribustion simulation results suggest that in-cylinder stratification isobtained, however global inhomogeinty is very low by injecting CO₂to intake port.Timing intake has been proposed to obtain in-cylinder stratification for higherglobal inhomogeinty. Compare to CO₂injection strategy for different CO₂flow path,timing intake strategy obtain higher global inhomogienty. Further investigations onin-cylinder distribustin and combustion are conducted by splitting intake process fortwo phases, during first phase air flows into the cylinder and mainly distribute at thebottom of combustion charmber, and second phase for CO₂gas mainly distribute atthe top of cylinder, the in-cylinder inhomogeneity is higher and notable as oxygenmass concentration gradient is7.38%and the Gini coefficient is higher than0.2.Compared with uniform intake, NO_xand soot reduced substantially with the samemass of CO₂in-cylinder. NO_xdecreased51.2%and soot decreased13.4%. However,ignition delay lags as higher CO₂concentration compare to uniform intake atcombustion area, the peak of heat release rate is higher, while the cylinder pressure islower. Four characters have been used to evaluate in-cylinder distribution of intakecharge, mass ratio of O₂/CO₂distribution; it is the maximum value of CO₂and O₂distribution inhomogeneity. Axial distribution is gas stratification on piston movementdirection. And radical distribution is gas stratification on radical distance withcylinder axis, and spherical distribution is on fuel injection direction.更多还原