【作者】 张炜；

【导师】 宋崇林；

【作者基本信息】 天津大学 ， 动力机械及工程， 2010， 博士

【摘要】 柴油机微粒是由基本碳粒子组成的聚集体。在基本碳粒子中,碳层内部原子的排列次序及碳层之间的相互关系称为微粒的微观结构。初始燃料、温度、时间等反应条件的不同,微粒的微观结构呈现出无定形或石墨化结构;而微粒的微观结构又可以影响微粒在氧化过程中的反应活性。此外,在碳层边缘位置形成的、容易与空气发生反应的微粒表面官能团也会影响微粒的反应活性。鉴于此,本文利用全气缸取样系统,应用图像处理、拉曼光谱、红外光谱、光电子能谱、同步热分析等测试技术和分析方法,开展了柴油机燃烧过程中缸内微粒的微观结构、表面官能团、氧化特性的演化历程和形成机理的研究,研究工作对今后柴油机燃烧系统设计和后处理技术具有重要的指导意义。论文研究工作和取得主要研究成果如下:1.根据柴油机微粒微观形貌图像的特点,定义了微粒微观结构特征参数;采用包括图像规格化、方向图计算、Gabor滤波法、局部阀值法、改进的OPTA法等一系列图像处理算法,对源微粒形貌图像进行处理,得到微粒微观结构特征参数信息。2.在燃烧过程中,层面间距、曲率分别在0.36~0.40nm、1.22~1.33之间变化,且呈在急燃期内减小,缓燃期内增加,此后逐渐减小的趋势;而微晶尺寸在1~2.2nm之间变化,且变化规律与层面间距和曲率的正相反。随着共轨压力的升高、燃空当量比的增加,层面间距、曲率减小而微晶尺寸增加;而随着发动机转速的增加,层面间距、曲率增加而微晶尺寸减小。拉曼光谱中G峰与D峰峰高比的变化规律与微晶尺寸的变化规律完全一致,验证了微粒微观结构特征参数提取结果的准确性。3.在燃烧过程中,缸内微粒碳氢官能团当量浓度在急燃期快速下降,随后降低趋缓。总含氧官能团(羟基官能团和羰基官能团)摩尔百分含量在急燃期后期和后燃期出现峰值,峰值为11.45 % ~ 22.37%,这种变化规律主要受羟基官能团的影响。羰基官能团和羟基官能团的比值呈单峰状分布,且在缓燃期初期达到最大值。此外,氧、碳元素比的变化规律与总含氧官能团的变化规律相同。4.除在缓燃期内略微减小外,缸内微粒起始氧化温度、最大氧化速率温度和燃尽温度在燃烧过程中逐渐增加。在燃烧过程中,微粒的指前因子和活化能分别在7.37×105～1.55×108/s、123.05～154.53kJ/mol之间变化,且微粒活化能呈在急燃期内急剧增加,缓燃期内略微减小,此后逐渐增加的趋势。微粒活化能随着微粒石墨化程度的增加而增加,而随着微粒表面含氧官能团的增加而减小。更多还原

【Abstract】 Diesel particulates are known to be the aggregates of individual primary particles. Within a primary particle, the degree of atomic order as manifested by graphitic layer plane segments and their physical relation to each other are defined as soot microstructure. Soot microstructure can range from amorphous to graphitic depending upon initial fuel identity and synthesis conditions, such as temperature and time. Meanwhile, variations in the microstructure can in turn affect the particulate reactivity toward oxidation. In addition, surface functional groups formed on edge sites which are directly involved in the attack by air are likely to be another factor affecting particulate reactivity. Therefore, based on a novel total cylinder dumpling system, the evolution process and formation mechanisms of microstructure, surface functional groups and oxidation reactivity of diesel in-cylinder soot were investigated using the measurement techniques and analysis method, for example, digital image processing, Raman spectrum, Fourier Transform Infrared spectroscopy, X-ray photoelectron spectroscopy, and simultaneous thermal analysis. The database obtained from this study will be of instructive significance for combustion cycle design and post-treatment processes of a diesel engine. The major research work and achievements of this dissertation are listed as follows:1. The definition of soot microstructure characteristics (i.e. fringe length, fringe separation distance and tortuosity) were introduced according to the nature of diesel particulate TEM images, and the digital image processing algorithm consisted of image normalization, orientation image estimation, Gabor filter, local threshold method, improved OPTA algorithm, et. al., were applied to quantify the microstructure characterization of diesel particulates.2. During combustion process, fringe separation distance and tortuosity decreased sharply in the rapid combustion phase, increased slightly in the mixing-controlled combustion phase, and decreased again afterwards, with a size range of 0.34~0.42nm and 1.22~1.33 respectively. In contrast, fringe length showed a contrary tendency to fringe separation distance and tortuosity, with the size range of 1~2.2nm. An increase in common rail pressure and fuel/ air ratio resulted in larger fringe length and shorter fringe separation distance and tortuosity; on the contrary, an increase in engine speed resulted in larger fringe separation distance and tortuosity and shorter fringe length. The peak intensity ratio of G/D bands in Raman experiment showed the same tendency as fringe length as the combustion proceeded, verifying the accuracy of image processing algorithm used.3. During combustion process, the molar concentration of the carbon-hydrogen groups on the in-cylinder particulate surface, which was indicated by the normalized peak height ratio (IC-H/IC=C), decreased strongly in the rapid combustion phase and then decreased slowly afterwards. The molar percentage of surface oxygen groups (i.e. hydroxyl and carbonyl) of in-cylinder soots presented two peaks through the combustion phases, with the heights of peaks ranging from 11.45 % to 22.37 %. The first peak appeared in the end of the rapid combustion phase, and the second in the late combustion phase. The total surface oxygen groups were significantly influenced by the hydroxyl. The variation tendency of the hydroxyl/ carbonyl ratio was unimodal during combustion process with a maximum value appeared at the beginning of the mixing-controlled combustion phase. In addition, the oxygen/carbon molar ratio had the same tendency as the total surface oxygen groups as the combustion proceeded.4. The onset temperature, maximum rate temperature and burnout temperature of the diesel in-cylinder particulates increased gradually during combustion process, except for a slight decrease in the mixing-controlled combustion phase. The pre-exponential factor and the activation energy of diesel in-cylinder soots were in the range of 7.37×105～1.55×108/s and 123.05～154.53kJ/mol, respectively. The activation energy intensively increased in the rapid combustion phase, declined to some extent in the mixing-controlled combustion phase, and gradually increased again afterward. More graphitic microstructure could lead to higher activation energy, whereas more surface oxygen groups would lessen the activation energy.更多还原