【作者】 伊克拉沃；

【导师】 张煜盛；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2009， 博士

【摘要】 上世纪九十年代后期以来,国内外广泛开展了均质压燃（HCCI ）、低温燃烧（LTC）研究。这是一种结合了传统柴油机和汽油机优点的全新的内燃机燃烧方式。人们发现,根据这一理论可以组织最清洁、热效率最高的燃烧过程,有望显著减少传统汽油机和柴油机的有害污染物排放,并降低其燃油消耗。HCCI燃烧主要受化学反应动力学因素的影响,但燃油与空气的混合过程对HCCI燃烧过程和燃烧控制也有着较为重要的作用。因此,燃料与空气的混合仍然是HCCI发动机需要解决的一个重要课题,提高燃油与空气的混合速率成为最近几年HCCI研究的一个重点。为了探讨混合气准备策略对HCCI发动机燃烧和排放性能的影响,本文从HCCI发动机工作过程的数值模拟入手,通过燃油/空气混合过程与化学动力学的耦合联算与实机试验研究,探讨了提高压缩天然气（CNG） HCCI发动机燃烧效率,降低其未燃碳化氢（UHC）和NO_x等燃烧污染物排放的技术途径,揭示出纯CNG、CNG掺氢或掺DME,以及CNG/DME双燃料掺氢发动机HCCI发动机的燃烧特性及其污染物排放的变化规律。本文的研究内容和取得的成果主要体现在以下几个方面:对CNG发动机实验与数值模拟研究的现状进行了综述,较全面地分析了实现CNG燃料HCCI燃烧的技术措施及其有关问题。以ZS195型直喷式柴油机为原型机,构建了CNG燃料HCCI发动机专用实验台架,在其上开展了纯CNG、CNG掺氢或掺DME,以及CNG/DME双燃料掺氢发动机的性能及污染物排放的测试研究。针对CNG特殊的理化性能,探讨了发动机不同燃烧边界条件（进气温度、空燃当量比、压缩比、掺氢或掺DME等）对CNG燃料HCCI发动机的性能与排放的影响。台架实验结果表明,CNG燃料HCCI发动机的进气加热温度随负荷和压缩比的增加而降低,掺烧氢可显著降低CNG燃料HCCI发动机对进气加热的要求,并可使燃烧相位提前,加快燃烧速度,减少燃烧持续期,增加发动机的指示热效率。同时,掺氢还可拓宽CNG的稀燃极限范围,从而有利于CNG发动机实现更稀薄的燃烧,降低发动机的有害排放。应用零维热力学模型与详细化学反应动力学模型的耦合联算,在计及缸壁传热的情况下,研究了纯天然气以及天然气掺氢的均质压燃燃烧反应机理,探讨了发动机不同运行工况和燃烧边界对燃烧过程的影响规律。CNG燃烧的化学反应动力学模型涉及53种组分、325步基元反应。此外,还应用含有99种组分、477步基元反应的化学动力学模型,分析研究了掺烧10%和20%体积分数的氢气对CNG/DME混合燃料发动机着火和燃烧过程的影响。结果表明,控制天然气HCCI着火过程的反应步骤包括HO₂与RH反应生成H₂O₂,以及后续H₂O₂与H₂反应生成OH活性基。天然气掺氢可使其在低温、低压条件下产生高浓度的H₂O₂和OH活性基。通过掺氢量与燃料空燃比控制相结合,可以有效控制着火定时和拓展HCCI发动机的负荷范围。鉴于零维单区模型关于混合气在缸内处于完全均匀混合状态的假设与实际情况并不完全符合,因此,有必要开展多组分均质混合气湍流混合过程与燃烧化学动力学耦合作用的理论研究。本文应用三维CFD和化学反应动力学的耦合模型,对由缸内复杂流动和燃烧室形状所引起的混合气不均匀性及其对燃烧过程的影响进行了数值模拟计算。通过KIVA-3Vr2程序和CHEMKIN的耦合运算,分析研究了进气道喷射CNG与缸内流场的相互作用及其对燃烧过程的影响,阐明了天然气发动机在不同氢气体积分数下的燃烧特性和在最高热效率下稳定运行的条件,对天然气掺氢的最佳氢气体积分数与发动机运行工况之间的关系进行了优化计算,为确定CNG发动机台架试验的最佳氢气体积分数提供了理论依据。天然气掺氢发动机的缸内压力、温度的预测值与实验值的对比表明,两者具有较好的一致性。在保证计算精度符合要求的条件下,为了尽可能地提高计算效率,并为化学反应动力学模型与CFD多维模型耦合的燃烧计算提供一个行之有效的途径,在DME详细化学反应动力学模型（包含了81种组分、362步基元反应）的基础上,构建了包含27种组分、145步基元反应的DME简化机理模型,建立了一整套从零维到三维耦合化学反应动力学的数值模型,初步形成了开展控制参数优化和燃烧控制策略研究的计算平台。通过简化模型与KIVA-3Vr2相耦合,进行了DME进气道喷射和不同起喷相位下的DME缸内多脉冲直喷对混合气均匀性和燃烧过程影响的对比研究。模拟计算给出了缸内压力、全历程混合气浓度分布、温度分布和燃烧相位随预混合气形成模式的变化情况,计算结果为理解均质混合气的制备策略提供了参考。对采用高十六烷值（DME）和高辛烷值（CNG）双燃料,以及DME/CNG掺氢拓宽均质压燃（HCCI）发动机运行工况范围的技术途径开展了探索性研究。应用详细化学反应动力学模型研究了DME/ CNG双燃料HCCI燃烧模式的燃烧反应机理,以HCCI非正常燃烧（即部分燃烧和爆燃）所涉及的着火极限和最高压力极限为界限值,通过调整DME/CNG双燃料的不同比例,计算得到了HCCI发动机运行范围的MAP图。研究结果表明,通过调整DME/CNG双燃料的不同比例,可使HCCI发动机的运行范围分别向小负荷或大负荷工况扩展,并显著提高其最高平均指示压力。通过数值模拟计算,揭示了DME/CNG掺氢燃烧可实现着火定时控制和工况范围拓宽的作用机理。详细化学反应动力学数值模拟研究表明,氢预氧化产生的大量活性基团能抑制和延缓DME的预氧化,从而达到有效控制其着火延迟期、改善预混合气制备并扩大发动机正常运转负荷范围的目的。在不掺氢的情况下,DME/CNG混合燃料HCCI发动机缸内压力示功图和UHC、NO_x排放的计算值与实验结果的对比表明,二者在数量上虽然存在一定的差别,但其变化趋势仍较符合。但在掺氢的情况下,由于氢燃烧可改善发动机的循环变动,致使发动机不同负荷工况的缸内压力示功图和UHC、NO_x排放的计算值与实验结果吻合较好。更多还原

【Abstract】 During the last decade an alternative to conventional internal combustion engines, i.e. spark-ignited and Diesel engines, has been under investigation by an ever increasing number of research groups. This alternative process is called Homogeneous Charge Compression Ignition (HCCI). That engine tests have elucidated that it has a great potential to reduce significantly the harmful emissions, and it is expected to reduce fuel consumption as a comparison with conventional engines operation. HCCI is defined as the process in which a homogeneous mixture of air and fuel, diluted with excess air as well as combustion products, is compressed under such conditions in which the auto-ignition occurs near the end of the compression stroke, followed by a combustion process that is significantly faster than conventional Diesel or Otto combustion.This study concerns the numerical and experimental investigation of HCCI engine combustion process. The principal aim of the work underlying the thesis has been using and developing the simulation tools that can contribute for understanding and further development of CNG HCCI engine process. Since HCCI combustion is strongly dependent on chemical kinetics, most of the work presented achieved with the capabilities of the detailed oxidation mechanisms which can describe the oxidation of CNG fuels with and without hydrogen or/and DME additives.Two models, with varying levels of complexity and corresponding calculation times, have been developed, validated, and used in this analysis. These simulation tools discussed are the so-called single-zone (CHEMKIN) and multi-zone (KIVA-3Vr2) models. The detailed reaction mechanisms are implemented in these models and a detailed-chemistry approach is used to predict the auto-ignition timing and the rate of combustion. Whereas the zero-dimensional (single-zone) model is based on the assumption of a perfectly homogeneous mixture, while the 3D-CFD/chemistry coupling model is more sophisticated since multiple zones are used to represent the mixture inhomogeneity in the combustion chamber of complex engines geometry. Modeling of HCCI engine requires balanced approach that captures both fluid motion as well as low and high temperature fuel oxidation. A fully coupled 3D-CFD and chemistry scheme would be the ideal HCCI modeling approach, but is computationally very expensive. As a result, modeling assumptions are required in order to develop tools that are computationally efficient, yet maintain an acceptable degree of accuracy.The major accomplishments and findings from this research can be recognized and traced by summarizing the sequence of collecting data from the results as follows:1- A zero-dimensional, thermodynamic model with detailed chemical kinetics and cylinder wall heat transfer correlations have been used to study the detailed oxidation mechanism of pure CNG and CNG/hydrogen in HCCI engine. This mechanism made up of 325 reversible elementary reactions among 53 species. The mechanism was numerically investigated at different operating and geometry conditions of HCCI engine during the time period in which both intake and exhaust valves are closed. In addition, an extended hydrocarbon oxidation reaction mechanism including 81 species and 362 elementary reactions has been used to simulate the combustion and emission behaviors of HCCI engine with DME fuel. Also, a detailed chemical kinetic mechanism considers 99 species and 477 reversible elementary reactions has been used to simulate the autoignition and combustion of CNG/DME fuel mixture with the effect of 10 and 20 % of hydrogen mole fraction.2- The gas fuel injection and its interaction with in-cylinder flow field and combustion behaviors of an HCCI engine with port fuel injection were numerically investigated by using numerous capabilities of multi-dimensional computational fluid dynamic (KIVA-3VR2) code coupled with a detailed chemical kinetics mechanism consisted from 314 elementary reactions among 52 species. The combustion process have been simulated successfully with the optimal hydrogen dose related to the specified operating conditions which give the highest thermal efficiency under stable working conditions of pure CNG and CNG with 20% of hydrogen blend. In addition, KIVA-3V is used to investigate the mixing process in HCCI engines prior to combustion, particularly for operation with liquid DME fuel. The calculations of complex chemical-kinetics/turbulent interaction, was carried out using a reduced oxidation mechanism of DME that mainly contains 145 elementary reactions among 27 species.3- Engine operating maps based on a series of simulation runs in the case of pure CNG fuel with and without hydrogen blends in addition with the effect of DME fuel additives into the engine missfire and knocking limits at different engine loads have been concluded. These tasks were achieved to find the optimum operating conditions of the HCCI engine fueled mainly by CNG with the minimum number of the laboratory engine tests.4- Experimental studies have been conducted in a commercial single cylinder diesel engine to investigate the basic performance characteristics of HCCI engines combustion and emissions. The CHEMKIN and improved KTVA-3Vr2/chemistry model results were validated using the HCCI engine experimental data. The predicted ignition timing by different model agree well with experimental data at different engine load since homogeneous charge was obtained with varying hydrogen or/and DME additives into the CNG/air mixture.更多还原