【作者】 郑朝蕾；

【导师】 张惠明；

【作者基本信息】 天津大学 ， 动力机械及工程， 2008， 博士

【摘要】 围绕φ? T燃烧原理图探索能避开NOx和soot排放区域的柴油机高效清洁新型燃烧方式是目前国际内燃机界研究的热点。本文以CFD与化学动力学耦合数值模拟研究为主要手段,结合实验研究,对均质充量压缩燃烧(HCCI)、混合气分层充量压燃(SCCI)和EGR稀释的低温燃烧(LTC)三种燃烧方式燃烧排放机理及高效清洁燃烧基础理论进行了系统研究。本文首选以正庚烷为对象,研究了HCCI燃烧反应动力学机理,结果表明:正庚烷燃烧呈现低温反应和高温反应阶段双阶段燃烧。低温反应阶段发生的关键是庚烷基的两次加氧过程,高温反应阶段是CH2O氧化成CO2的过程。在此基础上发展了HCCI简化动力学模型,并建立与CFD耦合的数学模型。结果表明:低温反应先在气缸压缩余隙和燃烧室内壁发生;高温反应开始于燃烧室内核区及气缸压缩余隙。未燃碳氢(UHC)排放主要来自于侧隙,大部分CO排放在气缸壁面附近。HCCI燃烧和排放特性实验研究表明,混合气变浓,缸内最大爆发压力和放热率峰值都升高,主燃烧持续期缩短,燃烧效率升高,指示热效率先上升后下降,UHC排放先降低再升高,CO排放降低。对正庚烷HCCI燃烧简化机理进行扩展和修正,发展了正庚烷分层充量压燃简化动力学机理。应用简化机理与CFD耦合,对缸内七种理论分层模式的燃烧及排放生成机理进行研究,结果表明:中心浓的前四种分层使着火时刻提前,最大压力升高率降低。七种理论分层燃烧UHC排放都比HCCI燃烧低,中心浓的分层使CO排放和NOx排放恶化,周边浓的后两种分层能使NOx排放基本不变而UHC排放和CO排放降低。对于预混/直喷分层燃烧来说,低温反应先发生在压缩余隙和凹坑内活塞表面附近的燃料均质分布区,而高温反应先发生在喷雾导致的浓混合气区域。在预混合气低温燃烧开始时刻附近喷油与中心浓的分层类似,小比例喷油条件下,-65 deg. ATDC前早喷与周边浓的分层类似。最后,本文应用实验和CFD数值模拟对在低氧浓度条件下LTC机理进行了研究,结果表明:随着氧浓度的降低,缸内压力峰值降低,滞燃期延长且预混燃烧放热比例增加;碳烟排放先升高后降低,低氧浓度soot排放降低的本质是低缸内温度抑制了soot的生成。缸内温度降低导致NOx和CO排放快速降低;HC排放先降低,在低氧浓度条件下又快速升高。随着喷油压力增加,缸内流场速度和湍动能都增加。促进燃油与空气的混合,碳烟排放的最高值和最终值都很显著降低。NOx排放其最终生成量随喷油压力增加而增大。进气压力增大,增大进气压力能改善低氧浓度局部缺氧的情况。soot生成的最大值和最终值都随进气压力的增大而降低。进气压力增大,最大局部温度的降低使得NOx排放降低。更多还原

【Abstract】 Many researchers in the world focus on the advanced combustion modes of diesel engines which can avoid NOx and soot forming regions. In this study, combustion mechanisms of three advanced combustion modes were investigated: Homogenous charge compression ignition (HCCI), Charge stratification compression ignition (SCCI) and Low temperature combustion (LTC). The potential of clean and high-efficiency of the three combustion modes was investigated using fully coupled multi-dimensional CFD and reduced chemical kinetics model combined with experiments.Based on the analysis of detailed mechanism, a reduced mechanism of n-heptane HCCI combustion is developed. The simulation results with CFD coupled the reduced mechanism model indicate that low temperature reaction begins from the vicinity of the cylinder wall and the bottom of the piston bowl, while the high temperature reaction takes place around the center of the bowl region and is widely distributed in space. UHC emissions mainly reside in the piston-ring crevice region. The majority of CO emissions are located in the region near the top surface of the piston. The Experimental study on n-heptane HCCI combustion shows that the increase of fuel/air equivalence ratio increases the maximum cylinder pressure and the maximum rate of heat release of combustion. With the increase of fuel/air equivalence ratio, the combustion efficiency increases; the indicated thermal efficiency increases first and then decreases; the UHC emissions decreases first and then increases; and the CO emissions decreases.A reduced chemical kinetics model of n-heptane SCCI combustion which consists of 42 species and 58 elementary reactions is developed. Applying CFD coupled the reduced mechanism model, seven different kinds of imposed stratification have been introduced according to the position of the maximal local fuel/air equivalence ratio in the cylinder at intake valve close. The results show that: The former four kinds of stratification, whose maximal local equivalence ratios locate between the cylinder center and half of the cylinder radius, advance ignition timing, reduce the pressure-rise rate, and retard combustion-phasing. All kinds of stratification can reduce UHC emissions. The last two kinds of stratification can reduce UHC and CO emissions while maintain low NOx emissions simultaneously. For premixed/direct-injected fuel combustion, the combustion process by fuel injection at the timing near the start of low temperature reaction is similar to the imposed stratification cases whose maximal local equivalence ratios locate between the cylinder center and half of the cylinder radius; the combustion process by early fuel injection before -65 deg. ATDC is similar to the imposed stratification cases whose maximal local equivalence ratios appear between half of the cylinder radius.Effects of oxygen concentration on combustion process and emissions of diesel engine are investigated by engine experiments and numerical simulation. The results indicate: Soot emissions increase first then decrease. It is the essential reason of low soot emissions at low oxygen concentration that the low in-cylinder combustion temperature leads to the inhibition of soot formation. The decrease of in-cylinder temperature leads to rapidly decrease of NOx emissions and the increase of CO emissions. HC emissions increase decrease first then rapidly increase at very low oxygen concentration. With the increase of injection pressure, the flow velocity and turbulent kinetic energy in the cylinder increases which are benefit to improving the mixing of fuel and air. The enhancement of intake pressure improves the mixing of fuel and air in the condition of oxygen lack in local regions at lower oxygen concentration. They are significant measures to reduce soot emissions in LTC combustion.更多还原