【作者】 闫妍；

【导师】 张煜盛；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2011， 博士

【摘要】 在能源和环境问题的双重压力下,开发、应用各种代用燃料和新型燃烧方式已经成为当下内燃机研究的热点问题。考虑到甲醇和二甲醚作为石油的替代燃料,除了可由煤或天然气制取外,也可由可再生生物质制取,从而有利于减少全球CO2总排放量。本项研究工作针对当前国内外在醇醚燃料均质充量压燃(HCCI)发动机燃烧与污染物排放研究方面存在的薄弱环节,以二甲醚/甲醇双燃料HCCI发动机的非常规排放物为主要研究对象,进行替代燃料燃烧和排放特性的研究,通过数值模拟计算与试验研究相结合,开展醇醚燃料非常规排放物排放特性、生成和控制机理的基础研究,发展了适合醇醚燃料HCCI发动机运行条件下非常规排放物的分析模型和检测技术,以探索全面改善醇醚燃料HCCI发动机污染物排放特性的新途径,为丰富新一代内燃机燃烧理论,为国家石油替代能源的决策和未来超低排放标准的制定提供科学依据。本文的研究内容和创新点主要有以下几个方面：对二甲醚(DME)和甲醇发动机试验与数值模拟研究的现状进行了综述,全面分析了两种燃料的理化性质,阐明了醇醚燃料在内燃机上应用的各种技术问题及其在新型燃烧模式开发中的发展潜力。开展了醇醚燃料发动机非常规排放物的检测技术与分析方法研究,以先进的气相色谱(GC)和傅立叶红外(FTIR)光谱检测仪为基本测试手段,构建了醇醚燃料发动机非常规排放物的科学检测方法与系统。在原有工作的基础上,建立了更加有效的非常规排放物检测技术,在气相色谱中采用毛细管FFAP柱取代原有填充柱,增大柱容量,提高排放物分离度,优化了色谱工作条件。针对GC对酸类物质反应度不灵敏的问题,采用FTIR对尾气中的甲酸进行了准确的定量测量。通过醇醚双燃料燃烧及其排放物的测试分析,获得了排气中甲醛、甲酸、甲酸甲酯,以及未燃二甲醚、甲醇的排放浓度等一系列较为精确的基础数据,为污染物排放控制策略的制定以及后续综合治理技术的开发提供了科学依据。以ZS195型直喷式柴油机为原型机,构建了醇醚燃料发动机专用试验台架,开展了进气道喷射醇醚混合燃料HCCI燃烧与排放特性试验研究。通过改变醇醚双燃料混合比、发动机转速和负荷以及进气加热等多种技术措施,探讨了实现HCCI发动机着火正时控制和发动机稳定工作范围拓宽,以及改善HCCI燃烧和污染物排放的有效技术途径。试验研究结果表明,甲醇的添加能够抑制二甲醚低温反应,降低缸内最大爆发压力和燃烧温度,推后主燃烧反应时刻,从而解决二甲醚的过快和过早燃烧。通过调节甲醇喷油量,可实现HCCI燃烧优化控制,还能明显扩展HCCI发动机运行的负荷范围。双燃料HCCI发动机的指示热效率最高可以达到49%左右,且具有较低的NO、排放,但其HC和CO的排放量明显高于原机柴油排放水平。进气温度提升到90度时,HC排放会有5%～20%左右的降幅,而CO的降低则非常明显。对HC排放影响最大的还是二甲醚/甲醇的喷射比例,精确的电子控制能够有效地降低HC排放。HC排放中二甲醚大约占据了60%,甲醇30%,甲醛,乙醛和甲酸甲酯等物质约为10%左右。在醇醚燃料发动机专用试验台架上就缸内直喷甲醇、进气道电控喷射二甲醚的复合燃烧模式分层充量压燃(PCCI)燃烧模式进行了试验研究。通过改变发动机转速和负荷、供油提前角、二甲醚/甲醇喷射比、以及在进气中添加不同比例CO2等变参数试验,探讨了PCCI燃烧模式下不同燃烧边界条件和燃料组配对PCCI发动机燃烧与排放特性的影响规律。研究表明,PCCI燃烧主要包括二甲醚低温放热反应、二甲醚高温放热反应和甲醇扩散燃烧等三个阶段。二甲醚作为辅助点燃剂,应维持相对较低的浓度,并应随着混合气中甲醇浓度的降低而降低。在进气中添加一定量的CO2气体,可将受试发动机的输出扭矩提高至原机最大输出扭矩的80%以上。通过优选供油提前角,可明显改善发动机的指示热效率,使NOx排放量在全工况范围内不大于400ppm,该值仅为燃用柴油的原机NOx排放量的30%。通过精确控制二甲醚/甲醇喷油比例,PCCI燃烧能实现全工况下NOx排放量均低于燃用柴油的原机,且在较低负荷工况下优势更加明显。醇醚燃料PCCI燃烧模式所排放的HC明显低于均质压燃方式,其中甲醇和甲醛是未燃HC的主要组成部分,其体积分数约为70%左右；甲酸的排放量很低,不超过20ppm；甲酸甲酯在低负荷时排放浓度较高,功率上升和进气道二甲醚喷油量的降低都能显著减少甲酸甲酯的排放。基于二甲醚和甲醇的详细化学反应动力学机理,开展了二甲醚/甲醇混合燃料详细反应动力学机理模型的研究。该机理模型包括97种组分和508个基元反应步骤。在此基础上,发展了零维HCCI发动机燃烧数学模型及其计算方法。借助于CHEMKIN软件,对二甲醚/甲醇混合燃料发动机性能与排放特性进行了计算研究。研究表明,混合燃料反应特性的改变主要发生在低温氧化阶段。由于甲醇汽化潜热大、十六烷值低,添加甲醇后,缸内温度降低,低温反应速率减缓,高温氧化反应延迟。为了节省计算时间,实现化学反应动力学与CFD多维模型的耦合计算,还开展了醇醚燃料简化化学反应动力学模型的研究。通过CHEMKIN软件的敏感度分析程序和基于计算机程序的自动机理简化方法,得到了二甲醚/甲醇混合燃料HCCI燃烧的简化机理,简化机理包括38种组分和99个基元反应。通过算例对比分析,发展的简化模型能正确揭示HCCI发动机燃烧过程和主要生成物组分的变化规律,能准确计算低温和高温阶段的放热特征时刻,且其计算结果与详细机理计算结果较为吻合。建立了二甲醚/甲醇HCCI发动机燃烧与排放的多维数学模型：1)带有进排气道的3D模型；2)忽略气道及内部不对称结构的2D旋转模型。利用CFD软件FLUENT耦合简化化学反应动力学模型,对二甲醚/甲醇发动机的HCCI燃烧过程和排放物生成机理进行了模拟计算,并研究了进气温度和顶岸间隙容积对发动机排放特性的影响。计算结果表明,2D和3D模型计算得到的示功图要比零维模型更接近于实测示功图,吻合度更高。3D模型能够较好地预测缸内压力,温度,物质浓度随曲轴转角的变化历程和着火时刻,但是由于忽略了缝隙效应,3D模型对于未燃HC的模拟精度较差。在2D模型的计算中,由于添加了顶岸间隙,使得HC排放的模拟精度有所提升,与试验结果吻合较好。同时在2D模型中研究了进气温度和顶岸间隙容积对HC排放的影响,计算结果表明提升进气温度和增大间隙容积都能促进HC排放的降低。更多还原

【Abstract】 With stronger the voice of environmental protection and oil crisis, the application of alternative fuels and new combustion mode have became hot research points. Taking into account China’s abundant coal resources, methanol and DME can obtained from coal are good alternative fuels. The research project is to utilize the fuel of DME and methanol in diesel engines for new combustion models. Combined with the National Natural Science Fund, Doctoral Fund and the content of the research works from home and abroad, make the following results of simulation and experimental validation in the combustion:Reviewed the utilization of DME and methanol in the engine experiments and numerical simulations. got a comprehensive analysis of physical and chemical properties and potential applications in the internal combustion engine of two fuels.On the basis of original works, a more efficient exhaust HC emissions testing technology was established. A capillary FFAP column was used to replace the original packed column in gas chromatography, the increasing column capacity improved emissions separation and chromatographic working conditions. Though optimizing the separation column temperature, carrier gas flow rate, vaporization temperature and detector temperature, GC was used for emissions of DME, methanol, formaldehyde and methyl formate. Fourier Transform Infrared spectroscopy method was also used to quantitatively investigate the characteristics of formic acid emissions. The emissions test obtained a series of precise emissions data of formaldehyde, formic acid, methyl formate, unburned DME and methanol, provided a scientific basis of pollutant emission control strategy and comprehensive management technology.The performances and emissions of the DME/methanol HCCI mode were studied by experiments. Experimental results showed that with the addition of combustion inhibitor methanol, the dual-fuel HCCI mode process has changed. The addition of methanol can inhibit reactions at low temperature, reduce the maximum cylinder pressure and combustion temperature, postpone the moment of main combustion reaction, solve the excessive and premature DME combustion. By adjusting the amount of methanol can effectively control the dual-fuel combustion process to achieve optimal control of HCCI mode, while adding methanol can significantly extend HCCI mode working range. Dual-fuel HCCI mode can reach the highest indicated thermal efficiency of about 49%, but at the burning border the indicating thermal efficiency reduced because of the detonation phenomenon. Dual-fuel HCCI engine had very low NOx emissions, but HC was significantly higher than the original disel engine. Inlet temperature raised to 90 degrees, HC emissions would decline 5%～20%, but in the case of low methanol concentration, the improvement became not obvious. DME accounted for approximately 60% of HC emissions, methanol was about 30%, formaldehyde, methyl formate and other substances was about 10%. Emission of CO had similar trend with HC, CO emission reduced with the increase of load and inlet air temperature.By changing the engine speed and load, fuel supply advance angle, dimethyl ether/ methanol injection ratio, and adding different proportions of CO2 into air, the tests of the PCCI mode with different boundary conditions were studied on PCCI test bench.PCCI mode consisted of three stages:low temperature reaction of DME, high-temperature reaction of DME and diffusion combustion reaction of methanol. DME as auxiliary lighting agent should be kept relatively low concentration, and with the decrease of methanol, its concentration need to be reduced. Utilized the EGR mode, engine output torque can exceed 80% of the maximum output torque of the original machine. Through the optimization of fuel supply advance angle, the indicated thermal efficiency can be significantly improved, and in the whole range of working conditions, the NOx emissions were not more than 400ppm, the value is only 30% amount of the original machine fueled with diesel. By precise control of DME/methanol injection ratio, the NOx emissions of PCCI mode were lower than the original machine.The HC emissions of PCCI mode were significantly less than the HCCI mode, methanol and formaldehyde were important parts of HC emission, their volume fraction was about 70%. Emissions of formic acid was very low, less than 20ppm. Methyl formate had higher concentration at the low load condition, power rise and DME concentration reduce can significantly reduce emissions of methyl formate.According to the single detailed mechanism of DME and methanol, through the CHEMKIN software can obtained DME/methanol detailed chemical kinetic mechanism, including 97 species and 508 elementary reactions. Through zero-dimensional simulation of HCCI combustion, the change of the mixed fuel reaction characteristics occurred mainly in low-temperature oxidation stage. The addition of methanol made the cylinder temperature decreased, low temperature reaction rate became slower and high temperature oxidation reaction delayed. In order to save computing time, a new simplified chemical kinetic model of DME/methanol HCCI mode was obtained, includes 38 species and 99 elementary reactions.Established two models of DME/methanol HCCI mode:1) a three-dimensional model with intake and exhaust channel; 2) two-dimensional rotation model ignored the airway and asymmetry structure of combustion chamber. The results showed that the indicator diagrams of multi-dimensional models were closer to the experimental results.3D model can predict cylinder pressure, temperature, fuel concentration in combustion process, but because of ignoring the gap effect,3D model had poor simulation of unburned HC. In the 2D model calculation, the addition of crevice zone made the simulation of HC emissions more accuracy, had well agreement with the experimental results. The 2D model simulation results showed that increasing the inlet temperature and the volume of crevice zone can reduce HC emissions, but when the temperature rise to a certain range, HC emissions decrease started slowing down.更多还原