【作者】 张登攀；

【导师】 袁银男；

【作者基本信息】 江苏大学 ， 动力机械及工程， 2011， 博士

【摘要】 我国缺油、少气、富煤。随着煤制甲醇工艺的成熟,甲醇被视为最具潜力的内燃机替代燃料。甲醇十六烷值很低,柴油机直接燃用甲醇比较困难,但通过合适的掺烧手段,完全可以燃用甲醇。在柴油机掺烧甲醇的常见方式中,进气预混甲醇的掺烧方式显示出良好的推广应用前景。然而涡轮增压柴油机应用该方式掺烧甲醇时,还需开展柴油机进气状态对甲醇掺烧量的影响、甲醇在进气道的雾化混合过程以及预混甲醇对柴油机进气温度影响方面的大量深入研究。本文针对柴油机进气预混甲醇的特点,开展了涡轮增压柴油机进气预混甲醇的台架试验和进气预混甲醇混合过程的数值模拟。为研究涡轮增压柴油机在宽广工况范围内掺烧甲醇的工作过程,依据进气预混甲醇雾化混合的特点,设计一套甲醇低压喷射系统,用进气压力和柴油机转速信号作为运行工况的辨识信号和甲醇喷射的控制信号,完成4B26涡轮增压柴油机进气喷射甲醇的改造。台架试验表明,设计的甲醇喷射系统可满足双燃料发动机的运行要求,能适应柴油机整个运行工况范围内甲醇喷射的灵活控制。柴油机在双燃料模式运行时,要达到较好的燃油经济性,不同负荷点的甲醇掺烧比需要优化。当柴油机进气压力高于130kPa时,进气状态是影响双燃料工作过程的主要因素,甲醇的掺烧量受爆燃的限制,推迟柴油的供油提前角,可降低高负荷掺烧甲醇的爆燃倾向。柴油机掺烧甲醇后,燃油经济性获得改善,排气温度下降,NOx和碳烟排放能得到同时降低,但HC和CO排放增加。用数值模拟的方法研究柴油机进气预混甲醇的缸内温度和甲醇蒸气的分布特点,以指导甲醇／柴油双燃料发动机的改造。柴油机进气预混甲醇涉及进气系统和甲醇喷射两大系统,深入理解柴油机进气流动和醇气混合机理是掌握电控喷射甲醇的重要内容。尝试以KIVA-3V程序为计算仿真平台,详细分析柴油机进气预混甲醇的气液两相混合的模拟过程,发展和完善了液滴碰撞和附壁液膜在内的醇气混合数学模型。对柴油机精确造型的螺旋进气道—气门—气缸结构化六面体网格的划分方法进行研究,借助ICEM-CFD软件,生成初始计算网格。添加程序代码,完善气门运动模型的算法,使4B26柴油机的模型网格能在KIVA-3V程序上顺利通过检查。完成计算程序所需的输入文件,以试验数据为初始和边界条件,建立基于KIVA-3V的涡轮增压柴油机精确造型的整机进气压缩瞬态仿真模型。对柴油机进气压缩的气体流程进行详细的数值模拟研究。结果表明,缸内速度流场结构的变化对柴油机的转速和负荷不敏感,不同转速下的缸内流场变化过程相似,压缩过程中缸内始终存在涡流运动。这一规律对于柴油机进气喷射甲醇而言,意味着通过合适的喷射方式,克服甲醇的雾化混合过程和缸内浓度分布因柴油机转速和负荷的剧烈变化而出现较大差异。对柴油机进气预混甲醇的瞬态混合过程进行了模拟。计算表明,附壁甲醇的蒸发对甲醇的混合过程非常重要。对于特定的发动机工况,存在一个较佳甲醇喷射时刻,能较好折衷附壁甲醇量和液膜蒸发过程,甲醇喷射方向的选择要以减少甲醇附壁为原则。当甲醇不直接喷到气门表面时,气道甲醇在气门开启前主要以液态形式存在,甲醇的雾化混合主要在进气过程中进行,进入气缸的甲醇基本为气态形式。当甲醇直接喷到气门表面时,甲醇液滴与高温气门表面发生碰撞,导致甲醇液滴快速蒸发,气门开启时,大部分甲醇液滴已变为气态形式。壁面温度主要对附壁液膜的蒸发产生影响,进气温度对甲醇液滴和附壁液膜的蒸发都会产生影响,进气温度对甲醇蒸发的作用强于壁面温度的作用。在引燃柴油喷射时,不同甲醇喷射方向和喷射时刻的缸内甲醇蒸气存在一定的浓度分层,气缸轴向浓度分层比较明显,而径向趋于均匀。进气预混甲醇能降低进气的温度,在甲醇掺烧量较大的1800r/min和2200r/min两个转速,在压缩上止点前10°CA,缸内温度比不预混甲醇的缸内温度分别下降52K和43K。更多还原

【Abstract】 China is abundant in coal resource, less storage in natural gas and poor in oil. Methanol, made from coal, is the most promising alternative fuel for internal combustion engines. Methanol applied directly to diesel engines is very difficult because of the lower cetane number of methanol, while using an appropriate method of methanol blended, diesel engines can well operate on diesel and methanol, and intake premixed methanol appears to be an attractive means in several usual ways of diesel engines fueled with methanol. However, when methanol is used in turbocharged diesel engines, it is very necessary to do many researches on methanol premixed ratio and methanol atomization at different operation conditions, as well as intake temperature descent with methanol premixed. In the paper, experimental study on a turbocharged diesel engine with methanol premixed is conducted, and methanol/air mixture formation is simulated.To investigate the turbocharged diesel engine operating with methanol premixed within the range of full conditions, a methanol injection system was designed according to the characteristics of methanol premixed in induction system. Used the intake pressure and engine speed signals as identification of engine operating conditions and methanol injection control, 4B26 diesel engine was reformed to the dual fuel engine with methanol premixed. The test results show that the methanol injection system meets the demand of the dual fuel engine, controlling the methanol injection flexibly at different operation conditions. In dual fuel operation mode, methanol premixed ratio should be optimized so as to achieve better fuel economy than diesel used only. When the intake pressure is higher than 130kPa, the inlet flow state plays an important role in combustion process of dual fuel mode, and the mass of methanol blended is limited by knock, and knock tendency can be reduced by means of diesel fuel injection timing delayed. Compared to single diesel direct injection, the dual fuel mode can get better fuel economy and lower exhaust temperature. Although diesel compound combustion produces high CO and HC emissions than diesel combustion only, methanol premixed in port can reduce NOx and soot emissions simultaneously.Utilized numerical simulation tool, cylinder air temperature and methanol distribution characteristics of diesel engine intake methanol premixed were studied to guide the methanol/diesel dual fuel engine modification. Methanol sprayed into induction system covers diesel engine induction process and methanol injection system. It is important to investigate the mechanism of intake flow and air-methanol mixture formation inside internal combustion engine, which is one of the essential theories of methanol electronic control injection. Based on KIVA-3V code, the numerical model of air and methanol mixing process in diesel engine, including droplet collision and wall film dynamic model, was analyzed and developed. The structured hexahedral mesh for exact helical intake port-valve-cylinder shape was studied and the initial grid of 4B26 model was generated with ICEM-CFD software. Added and modified valve motion model algorithm, the grid can succeed in checking under KIVA-3V code.Completed input files of KIVA-3V code and used tested data as the initial and boundary conditions, the transient simulation model for 4B26 turbocharged engine with induction and compression stroke was created. The process of the diesel engine induction and compression at different conditions were simulated. The calculations show that the structure of cylinder velocity fields is insensitive to engine speed and load, and has similar change process at different operation conditions. During the compression stroke, there is always vortex motion in cylinder. The flow characteristics in cylinder mean that the diesel engine operating condition has little effect on the process of methanol atomization and contribution in cylinder by methanol injection control.The transient mixing process of methanol injected into the diesel engine induction system was simulated. The calculations show that methanol film evaporation is important to the methanol mixing formation. There is a better methanol injection timing at a given engine operation condition, which can obtain a good compromise between methanol film mass and film evaporation. Methanol injection orientation is determined by the principle of decreasing methanol film. When methanol is not directly sprayed onto inlet valve surface, methanol is almost in liquid state before the valve is opened. The methanol vaporization mainly carries out in engine induction stroke, nearly gaseous methanol into cylinder. When sprayed directly onto the valve surface, methanol droplets collide with the hot surface of the valve, resulting in rapid evaporation of droplet. Most of the methanol droplets have become gaseous form before the valve opened. High air temperature, making both droplets and film vaporize fast, has more influence on methanol mixture process than hot port wall which mostly improves film evaporation. At the time of diesel injected, methanol concentration in cylinder is stratified more obviously at cylinder axis direction than that at cylinder radial direction under different methanol injection orientation and timing. Methanol premixed can reduce the intake temperature. At 1800r/min and 2200r/min with high methanol premixed ratio, the cylinder temperature with methanol premixed is decreased by 52K and 43K respectively before top death center 10°CA compared with that without methanol premixed更多还原