【作者】 钱叶剑；

【导师】 左承基；

【作者基本信息】 合肥工业大学 ， 车辆工程， 2009， 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。气体燃料发动机以其优良的排放、良好的经济性以及能部分替代石油资源而受到了空前重视。本文从改善燃烧降低排放的目标出发,重点研究了煤层气和氢气对内燃机燃烧过程和排放性能的影响。根据煤层气的理化性质,将S195柴油机改装成火花点火式煤层气发动机,并详细试验研究了煤层气组分变化对发动机燃烧和性能的影响。结果表明,当压缩比和点火定时一定时,煤层气发动机的怠速稳定性随甲烷浓度的增加而增加,当点火定时一定时,增加压缩比对提高怠速稳定性有利;在小负荷工况下,煤层气浓度的变化对缸内压力的影响不大,但在高负荷时,最大爆发压力受煤层气浓度的影响较大。当负荷一定时,压力升高率随甲烷浓度的增加而增加,主燃期变短。煤层气中甲烷的浓度达到一定程度后,煤层气中惰性气体对燃烧过程的影响较小。NOX排放浓度随着甲烷浓度的增加而增加,HC和CO排放随负荷和甲烷浓度的增加而降低。利用MATLAB软件,模拟了燃烧室内涡流强度,并利用随机点法计算火焰相关参数,最后建立了涡流室式煤层气发动机准维双区燃烧模型;在燃烧模型的基础上,建立了煤层气发动机主要排放(NOX、CO和HC)的预测模型。模拟结果与试验值的比较表明:论文建立的燃烧和排放预测模型基本合理,能较好反映煤层气发动机缸内工作过程,是研究涡流室式煤层气发动机燃烧机理的有效工具。试验研究了不同EGR率、掺氢率和掺氢EGR(HEGR)对ZS195柴油机性能的影响。结果表明,ZS195柴油机采用EGR技术后,缸内最大爆发压力和压力升高率峰值减小,工作粗暴性有所改善,发动机的动力性和经济性有一定退化;EGR技术能显著降低NOX排放,但CO、HC和烟度排放会增加。ZS195柴油机掺氢燃烧后,随着掺氢率的增加,缸内最大爆发压力和压力升高率峰值都会增加。掺氢燃烧会提高ZS195柴油机的热效率,经济性得到改善;掺氢燃烧可以减少HC、CO和烟度排放,但NOX排放增加;ZS195柴油机使用HEGR技术后,当EGR率一定时,随着掺氢率的增加,最大爆发压力、压力升高率峰值和放热率峰值都有所增加;ZS195柴油机使用HEGR技术后,能同时降低NOX和烟度排放,发动机的有效热效率略有改善。利用CHEMKIN软件对柴油废气重整反应进行了化学动力学模拟。结果表明,无论有无催化剂,重整器入口温度是影响柴油重整反应的主要因素。没有催化剂时入口温度在900K以上重整反应才能开始,反应速率慢,重整产物中H2和CO的体积分数少。有催化剂时入口温度在600K以上重整反应即可进行,反应速率快,重整产物H2和CO的产量大。没有催化剂时,柴油废气重整反应是吸热反应,水蒸汽重整反应和水煤气反应是主要反应,氧化反应是次要反应。有催化剂时,柴油废气重整反应是放热过程,放热量主要由氧碳比决定。水碳比(H2O/C)和氧碳比(O2/C)是影响柴油重整反应的外部因素。增加水碳比(H2O/C)和氧碳比(O2/C),可以增加H2和CO的产量。最优水碳比(H2O/C)在1.5-2之间,氧碳比(O2/C)在0.3-0.5之间。不考虑催化作用,重整产物H2和CO体积分数随着流速得增加而下降;考虑催化作用时,空速对重整产物H2和CO体积分数几乎没有影响。重整反应器的结构(长度和直径)对柴油废气重整反应的影响很小。利用FLUENT软件建立了ZS195柴油机三维燃烧模型,对ZS195柴油机在标定工况下采用EGR、掺氢燃烧和HEGR技术进行了三维燃烧模拟。模拟结果和测试结果较为吻合,变化趋势一致,说明建立的燃烧模型真实可行。缸内速度矢量分布受柴油喷射量的影响较大,但在喷油前和燃烧后期,EGR率对缸内最大速度和流场分布影响不大。随着EGR率的增加,缸内局部最高温度下降,O2浓度下降,NO的局部质量浓度也下降。随着掺氢量的增加,缸内流场的最大速度有所下降,但压缩过程前期和膨胀冲程后期缸内速度矢量分布没有明显变化。缸内局部最高温度和NO质量浓度也随掺氢量的增加而增加。模拟结果显示ZS195柴油机采用HEGR技术可以同时降低NO和微粒排放。更多还原

【Abstract】 With increasing concerns on environment protection, the worldwide communities are bringing forward increasingly rigorous regulations for reducing toxic emissions from Internal Combustion Engines (ICE). As a result, improving fuel economy and reducing exhaust emissions are among the highest priorities for automotive industry and also remain as the most demanding subject for academia engaged in the research of combustion engines. Gas fuel can offer a promising perspective to enable the internal combustion engine to meet these challenges. In order to improve combustion process and decrease emissions, this paper studies the effect of gas fuel on performance and emission of ICE.Based on the physical and chemical characters of coal-bed gas, a model S195 diesel engine with swirl chamber has been modified to fuel with coal-bed gas. Detailed experiments have been carried out to investigate the combustion and emission characteristics of the engine operating with different grades of coal-bed gas. The experimental results show that the idling stability of coal-bed gas engine increases with the methane percentage increasing. Increasing compression ratio is helpful to enhance idling stability when ignition timing is constant. The combustion pressure difference resulting from the variation of methane content in coal-bed gas is not evident at low load condition, but it is evident at high load condition. The pressure rise rate increases with the methane percentage increasing when engine load condition is constant, and the combustion duration also reduces. The effect of inertia gas on combustion process is small when methane percentage reaches a certain level. The HC and CO emission decreases with methane percentage and engine load increasing, but it is reverse for NO emission.A quasi-dimensional 2-zone combustion model has been developed by using the MATLAB platform. The combustion model includes two sub-models, with the first calculating the turbulence intensity history and the second calculating flame parameters by random point method. Emissions of NO, HC and CO have been predicted using the numerical model based on an extended Zeldovich mechanism, wall quenching and incomplete oxidation theory. The effects of various components in coal-bed gas on the engine combustion process and emissions have been investigated, with a satisfactory agreement between the simulation and experimental data achieved.The influence of EGR ratio and hydrogen addition on performance and emissions of diesel engine has been studied. The experimental results show that the peak pressure and maximum rate of pressure rise reduces when EGR technique is adopted. EGR system can improve the operation stability. EGR system will result in engine output decreasing, fuel consumption increasing and smoke increasing. The peak pressure and maximum rate of pressure rise increases with the quantities of hydrogen increasing at all kind of load condition. It is concluded that induction of small quantities of hydrogen can significantly enhance the thermal efficiency and economics of ZS195 diesel engine. With hydrogen induction, NOX level increases, but smoke, HC and CO emissions reduce. In addition, hydrogen-enriched air is also used as intake charge in a diesel engine adopting exhaust gas recirculation (HEGR) technique. When HEGR technique is adopted, peak pressure reduces and the maximum rate of pressure rise and heat release increase at low load condition. But they are increase at high load condition. The HEGR technique can reduce NOX and smoke emissions simultaneously while the engine efficiency is improved.The chemical kinetics model of diesel fuel exhaust gas reforming has been developed using the CHEMKIN platform. The results show that the reaction profile of diesel fuel exhaust gas reforming was significantly influenced by the reformer inlet temperature with or without catalyst. The reforming reaction can start without catalyst until the reformer inlet temperature reach 900K, H2 and CO content in the reformer product is small. When the catalyst is adopted, the reforming reaction can start at the reformer inlet temperature of 600K, the reforming reaction rate is high, H2 and CO content increase. With the catalyst, the diesel fuel exhaust gas reforming reaction is endothermic, steam reforming reaction and water gas shift reaction are main reaction; partial oxidation reaction is secondary. Without catalyst, the reforming reaction is exothermic which the quantities of heat release are decided by the O2/C ratio. H2O/C ratio and O2/C ratio also influence the reforming reaction. The H2 and CO content of reformer product increases with the H2O/C ratio and O2/C ratio increasing. The optimum H2O/C ratio and H2O/C ratio is 1.5-2 and 0.3-0.5 respectively. Without catalyst, the H2 and CO percent in reformer product decreases with flow rate while the volume of reformer is constant. With the catalyst, the effect of Gas Hourly Space Velocity (GHSV) on H2 and CO percent is small while volume of reformer is constant. The structural parameter of reformer (length and volume) slightly influences the reaction profiles of diesel fuel exhaust gas reforming.Three-dimensional combustion model has been developed using FLUENT software. The effect of EGR, hydrogen addition and HEGR on performance of ZS195 diesel engine is studied. The modeling results are compared with the measured data obtained from a parallel experimental investigation, with a good agreement. The velocity vector was influenced by the fuel injection quantities, although the influence is small during the prophase of fuel injection and anaphase of combustion. The partial temperature, O2 and NO mass fraction decreases with the EGR ratio increasing. The local temperature, O and NO mass fraction increases with the quantities hydrogen addition increasing. The HEGR technique can reduce NO and smoke emission simultaneously.更多还原