【作者】 佘金平；

【导师】 张煜盛；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2012， 博士

【摘要】 为了满足日益严格的排放法规和燃油经济性要求,并减轻排气后处理器的负担,近年来,开发具有“均质压燃(HCCI),低温燃烧(LTC)”特点的发动机燃烧新技术的研究十分活跃。HCCI被认为是传统压燃和点燃式内燃机最具潜力的替代燃烧方式,并已成为国际内燃机界竞相研究的前沿科学问题。然而,常规柴油饱和蒸汽压较低、液体粘度和表面张力较大、喷雾雾化性能较差,致使其直接应用于HCCI发动机尚存在均质混合气制备十分困难的问题。因此,预混合气制备是柴油机实现同时降低NOx和碳烟排放的准均质预混合燃烧方式的主攻关键技术。在国家重点基础研究发展计划项目(2007CB210001)的支持下,本文提出了燃油在高压油泵后进行高温预热以在缸内快速形成可燃混合气的技术方案。本文利用先进的光学可视化诊断技术、发动机台架实验和数值模拟计算研究,开展了燃油热强化对直喷式柴油机喷雾雾化、混合气形成和燃烧过程影响的基础研究,深刻揭示了高温燃油促进喷雾雾化,高效、快速制备准均质可燃混合气的作用机理；借助于燃油热强化与不同喷油策略以及燃烧边界条件的优化匹配,实现了直喷式柴油机的高效清洁燃烧。本文的研究成果为实现柴油喷雾强化混合过程以适应现代内燃机均质压燃燃烧模式的需要提供了一条新的技术途径。本文的研究内容和所取得的成果主要有以下几个方面：就国内外研究机构对HCCI发动机试验与数值模拟研究的现状进行了综述,全面分析了目前推广应用HCCI燃烧方式存在的一些技术瓶颈问题。通过重点分析了解“瞬态”条件下燃烧边界条件与燃料设计协同控制关键技术问题的最新研究进展,认识到在控制HCCI燃烧过程的诸多方法中,燃料理化特性的差异对HCCI预制混合气的制备及其燃烧过程的影响值得加以深入探究。针对常规柴油饱和蒸汽压较低、液体粘度和表面张力较大、喷雾雾化性能较差、以及因滞燃期短所导致的柴油均质预混合气制备时间受限等难题,首次开展了柴油热强化以在缸内快速形成可燃混合气的机理探索性研究。根据CFD多维模型与详细化学反应机理耦合计算的需要,建立了燃料热物性参数与燃烧反应机理的数据库。以多维CFD程序KIVA-3V为平台,在其中加入了过热油滴两区蒸发模型、PaSR湍流燃烧模型以及Hyroyasu-Nagle碳烟排放模型,建立了探索柴油热强化改善喷雾雾化混合和燃烧过程机理的数值模拟平台,完成了相关应用程序的调试及模型参数的调整,改进和完善了三维数值计算程序。藉此对早喷条件下不同喷油始点、预喷射策略及喷射燃油温度对缸内混合气充量浓度和温度不均匀性的影响进行了计算研究。研究表明：燃油温度对喷雾的蒸发混合过程有重要的影响,温度越高缸内的浓度及温度分布就越均匀,缸内居于预喷射平均当量比附近的混合气体积就越大,且高温燃油能有效地解决早喷燃油“湿壁”问题。在高温燃油喷射条件下,预喷射次数的变化对缸内混合气形成的影响趋于减弱。结合台架实验受试发动机的结构与运转参数的实际,利用激光粒度仪,率先对定容室内柴油热强化温度对其喷雾场微观特性的影响进行了实验观测研究。研究结果表明：随着燃油喷射温度的升高,液滴沿轴向和径向的SMD均减小,且不同位置SMD的差异减小。在实验研究的基础上,就燃油热强化对定容室喷雾的宏观及微观特性的影响进行了数值模拟研究。结果表明：燃油温度对喷雾宏观及微观特性具有重要影响,随着喷射燃油温度的升高,喷雾液相和气相贯穿距缩短,燃油温度愈高则其对液相贯穿距的影响更为显著；喷雾的SMD随燃油温度的升高而减小：喷雾的气相体积随燃油温度的升高而迅速增大。与此同时,燃油温度升高对喷雾体内当量比的分布影响明显,浓混合气所占的比重随着燃油温度的升高而增加,当燃油温度达530k时,气相混合气中燃空当量比大于2的油气质量约占所蒸发油气总质量的70%。实验与计算结果均表明,柴油热强化有利于改善唢雾的雾化、蒸发和混合气形成过程。以受试柴油机ZS195为研究对象,构建了柴油热强化改善喷雾雾化混合和燃烧过程的专用实验台架,开发了高压油管燃油加热控制系统。针对燃油热强化后其热物性参数的变化,进行了预热柴油经高压燃油泵柱塞套间隙的泄漏撮测量试验,分析了预热柴油热物性对燃油喷射特性和高压燃油泵泄漏量的影响。实验表明，采用直接诶加热高压油管的方式,温度越高燃油泵的泄漏率越大,转速越低,燃油泵的泄漏量亦越大；但即使在燃油强化温度达513k、发动机转速为1000转/分条件下,燃油泄漏率也仅为1.2%。结合燃油热强化技术措施,研究了原机及换装多孔喷嘴配合模拟EGR技术(进气添加二氧化碳)对发动机燃烧和排放特性的影响。研究表明,燃油预热温度对发动机燃烧与排放的影响程度与发动机负荷、喷油特性参数、EGR等燃烧边界条件密切相关。借助于超多孔喷油嘴以及合适的燃油喷射温度、喷油时间、进气添加少量C02等技术措施,能够改善直喷柴油机喷雾混合与燃烧过程。对于实际发动机存在着一个改善燃油经济性的最佳燃油温度范围,在此温度范围内,可兼顾发动机的燃油经济性与低排放性能。应用改进的KIVA-3V程序,就ZS195柴油机燃油高温热强化及不同喷射策略实现DI,PCCI-DI和PCCI三种燃烧模式进行了数值模拟研究,探讨了主、预喷射定时,预喷比例及EGR对混合气形成,燃烧和排放的影响。结果表明：对于DI燃烧模式的中等负荷工况,仅通过燃油早喷即可达到同时降低NOx和碳烟排放的目的；而在高负荷工况,NOx排放显著增加,且单纯利用早喷策略,发动机的IMEP降低明显。对于PCCI-DI燃烧模式,预喷射始点越靠后,主喷射始点时刻缸内预喷射燃油的分别越不均匀,且这种不均匀性导致缸内过早出现局部高温反应区,从而导致着火始点提前；预喷射始点为65BTDC,主喷射终点为5～10BTDC时,能在较宽的预喷射范围内同时获得较低的排放和较好的燃油经济性。对于进气添加二氧化碳的PCCI燃烧模式,在预喷射始点为65BTDC时,主喷时刻在较大的范围内变化,发动机都能获得较低的排放和较好的燃油经济性。三种燃烧模式对比可知,单纯采用早喷策略对发动机经济性的负面影响明显；采用PCCI-DI燃烧模式,经济性得到改善,部分排放物大幅减少,但中等负荷时CO排放增加明显,而高负荷时NOx排放明显增加；高负荷下采用PCCI燃烧模式,发动机在碳烟,NOx和CO均大幅下降的同时,其燃油经济性也能得到一定程度的改善。更多还原

【Abstract】 With severe worldwide energy crisis and stricter exhaust emission law and regulations, as one of the main fuel consumption and environment pollution sources, internal combustion engine is being paid more and more attention. As a new type engine combustion, homogeneous charge compression ignition (HCCI) and low temperature combustion(LTC) technology is being highly regarded as a research focus.There are some problems for conventional diesel fuel to achieve a uniform mixture distribution in the cylinder because of its high viscosity and poor fuel vaporization. So the homogenous mixture preparation is the key issue for diesel fuel to achieve HCCI combustion. With the support of National Basic Research Priorities Program administrated by the State Ministry of Science & Technology of China(Project 2007CB210001), this paper develops a technique to enhance diesel spray atomization, evaporation and mixture formation in the combustion chamber by fuel preheated after High-Pressure Fuel Pump. As we know when the fuel is heated to a very high temperature, the fuel boils when it is injected into the combustion chamber. This can significantly speed up burning due to better dispersing of the fuel drops.The mechanism of improved mixture formation and combustion process by preheated fuel injection is investigated by advanced optical visualization diagnostic technology, test-bed experiments and multi-dimensional numerical simulations. The new combustion technology can achieve the high efficiency and low exhaust emissions by optimum injection strategy with preheated fuel. This paper gives a novel way to speed up air-fuel mixture formation for HCCI engine. The main contents and achievements of this research are as follows:This paper reviews the research of the experiment and numerical simulation about HCCI, and then analyzes the bottleneck problems of application of HCCI. Through deep-going analyses of the latest research progress about the key technological issues of combustion boundary conditions combined with control of fuel design, it is realized that the difference of the fuel’s physicochemical properties play an important part in mixture preparation and combustion process.To solve the diesel mixture formation problems, the mechanism relevant to strengthen and speed up mixture formation of the conventional diesel fuel by preheated fuel injection is studied. A multi-dimensional numerical model is developed based on KIVA-3 V, which includes two zone evaporation model, PaSR turbulent combustion model, Hiroyasu-Nagle soot emission model. Based on this model, the effects of fuel injection timing, injection strategy and fuel temperature on the fuel-air equivalence ratio and temperature distribution in cylinder are studied. The results show that the pre-injection fuel temperature plays a very important role in fuel spray atomization, evaporation and mixture formation of diesel fuelled direct injection engine. As fuel temperature increases, the more uniform distributions of fuel-air mixture and temperature in the cylinder are achieved, the volume of prepared combustible mixture increases, and the wall impingement of fuel spray is almost disappeared. The influence of pilot injection timing on mixture formation weakens with high temperature injection.The effect of temperature on spray microscopic features of injection in constant volume chamber was measured by a Laser Particle Size Analyzer. The results show that the SMD of droplet in the axial and radial direction decrease, and the difference is small with fuel temperature increased. Based on the structural and operating parameters of the tested engine and the experimental conditions, a numerical simulation is developed and applied to the diesel engine. Results of calculation show that fuel temperature plays a very important role in macroscopic and microscopic features of the fuel spray. AS the fuel temperature increases, the penetrations of liquid/vapor phase and the SMD decrease, the volumetric quantity of vapor phase increases quickly. The higher the fuel temperature is, the more significant influence on the equivalence ratio of vapor phase, and the fraction of rich mixture increases. At fuel temperature of 530K, the mass fraction of fuel vapor with fuel-air equivalence ratio larger than 2.0 increases to about 70% of the whole evaporated fuel.A Model ZS195 single cylinder direct injection diesel engine is used to test the HCCI combustion. The electric heating and temperature controlling system of high pressure fuel supply pipe is installed to the rear of the high-pressure fuel injection pump which has been retrofitted for HCCI fuel supply system. The fuel leakage of pump with high fuel temperature injection was measured. The results show that leakage rate increases as fuel temperature increases, and the leakage rate decreases as engine speed increases. However, even though the fuel temperature is preheated up to 513K and the engine speed is at 1000 r/min., the fuel leakage rate is only 1.2%.Using the preheated fuel injection technology, the EGR influence on engine combustion and emission characteristics is studied with special multi-hole injector. The results show that the combustion and emission characteristics have closely related to the combustion boundary conditions (such as engine load, the fuel injection parameters, EGR, etc.). Through optimizing the special multi-hole injector, the fuel temperature, the injection timing and EGR, the processes of spraying, mixing and combustion of d. i. diesel engine can be improved obviously. There exists an optimum temperature range for preheated fuel temperature and around the optimum fuel temperature the trade-off between fuel economy and lower emission performance can be obtained.By making use of the improved KIVA-3V code and considering the preheated fuel with the different inject strategy, the numerical simulation of DI, PCCI-DI, PCCI combustion modes have been carried out. The effects of the main injection timing, the pilot injection timing, the pilot injection proportion and EGR on the mixture formation, combustion and exhaust emissions are studied quantitatively. The results show that at the middle load of DI model, the emissions of NOx and soot can be decreased only by using fuel early injection. But at the high load conditions, the emission of NOx increases obviously. And if only using the early injection, the IMEP of the engine decreases obviously. For the PCCI-DI combustion mode, if the pilot injection timing is retarded, the distribution of fuel manifests heterogeneity and more serious rich mixing zone at the beginning of the main fuel injection. Because of this, some parts of cylinder have the area of overheating. So the ignition timing occurs earlier. When the pilot injection timing is 65BTDC and the main injection timing is 5～10BTDC, the engine can get the best balance of fuel economy and exhaust emissions. For the PCCI mode with the addition of CO2 in the intake air, the engine can work well when the pilot injection timing is 65BTDC and the main injection timing can be changed in a wide range of adjustment. Comparing the three combustion modes, it can be concluded that it is not good for economy only by using the early injection strategy. Regarding to the PCCI-DI combustion mode, the fuel economy can be improved and the part of the emission can significantly reduced. But in the middle load the CO emission increases obviously and in the high load the NOx emission increases obviously. While in the high load and using PCCI combustion mode, the soot, NOx and CO emissions can be significantly reduced and fuel economy improved simultaneously.更多还原