【作者】 肖广飞；

【导师】 黄震； 乔信起；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2007， 博士

【摘要】 随着汽车保有量的不断增加和排放法规的日益严格,进一步降低内燃机排放是当前能源与环境领域的一个重要课题。本文从改善燃烧降低排放的目标出发,开展了内燃机氧化剂设计的研究。在对不同氧化剂环境中内燃机气体燃料LPG和液体燃料轻柴油稳态燃烧火焰特性研究基础上,本文重点研究了LPG发动机和柴油机冷起动以及柴油机稳态运行时使用组合进气的燃烧和性能。首先构建了杯式燃烧装置。基于该燃烧装置,研究了气体燃料LPG和液体燃料轻柴油在不同氧化剂环境下的稳态燃烧火焰特性。对LPG,研究了氧化剂组分对其部分预混合火焰和扩散火焰的影响,并对LPG扩散火焰进行了数值模拟研究。对LPG部分预混合火焰,当O2/N2氧化剂总流量不变时,扩散火焰长度和亮度随O2浓度下降先增加后减少,火焰根部与燃烧杯口的距离增大,直至火焰熄灭。对LPG扩散火焰,O2流量保持不变时,随N2流量的增加,火焰高度增加,而火焰可见长度先增加后降低,亮度减弱;火焰最高温度下降,而且火焰高温区减小。当N2流量过大时,火焰熄灭。LPG扩散火焰的数值模拟表明,随O2/N2氧化剂氧浓度提高,燃烧区域缩小并向燃烧杯杯口集中,同时火焰高温层变厚且高温区域集中。对轻柴油,首先考察了液面燃烧时扩散火焰和黑烟随氧化剂氧浓度增加的变化,然后重点研究了氧化剂组分和流量对以灯芯方式燃烧时的扩散火焰的影响。研究表明,柴油的液面燃烧火焰和灯芯火焰都包括两部分:刷子形的上部火焰和圆锥形的下部火焰。对柴油液面燃烧,O2/N2氧化剂总流量不变时,增加氧浓度后扩散火焰高度增加。氧浓度较低时,黑烟较多;氧浓度增大到某范围内,黑烟消失;氧浓度继续增加,则又产生黑烟。而对柴油灯芯火焰,高度的变化趋势是先增加后降低。无论是LPG燃烧还是柴油燃烧,比较N2,CO2对火焰的影响更显著。分别构建了基于膜法富氧技术的富氧进气LPG发动机和柴油机冷起动研究系统。对LPG发动机,基于循环控制方法研究了富氧进气冷起动首循环的燃烧排放,并比较了不同氧浓度进气时冷起动/暖机和热怠速阶段的运行行为。对柴油机,重点研究了最低自行起动温度时进气氧浓度对燃烧和性能的影响。研究表明,无论是LPG发动机还是柴油机,富氧进气的使用均能够提高发动机的起动速度。同时,随着进气氧浓度的增加,排气烟度、HC和CO排放显著降低,但NOx排放增加。LPG发动机冷起动首循环缸内燃烧分析表明,各种循环燃料量下,使用富氧进气均使缸内峰值压力增加,对应出现时刻提前,同时使缸内燃烧放热更为集中,最大放热率出现时刻提前。柴油机冷起动缸内燃烧分析表明,与常规进气相比,富氧进气后着火时刻明显提前,缸内压力升高。构建了柴油机组合进气供给系统。在供油时刻不变的条件下,研究了柴油机富氧进气和常规进气添加惰性气体时的燃烧排放特性。结果表明,富氧进气后排气烟度、HC和CO排放显著降低而NOx排放增加明显。常规进气添加Ar、N2和CO₂时排气烟度均上升而NOx排放均下降。相同Ar、N2和CO₂添加量,使用CO₂后烟度增加幅度最小而NOx排放下降幅度最大。针对富氧进气和常规进气添加惰性气体后的燃烧排放特点,研究了O2/CO₂/N2组合进气时柴油机燃烧排放特性,得到了实现柴油机排气烟度和NOx排放同时降低的优化组分进气,并对柴油机使用优化组分进气进行了研究。研究表明,柴油机使用O2/CO₂/N2优化组分进气,NOx排放可降至原机排放的40%,而排气烟度的下降可达50%以上,而保持指示热效率下降很小。柴油机使用优化组分进气时,随进气中O2浓度和CO₂浓度同时增加,绝热指数和多变指数逐渐变小,滞燃期延长,平均指示压力降低。排气烟度和NOx排放随优化组分进气中O2浓度和CO₂浓度同时增加均降低,但HC和CO排放变化不大。对柴油机在两种富氧燃烧方式—进气富氧和燃料含氧时的燃烧排放特性进行了比较。进气氧浓度选择22.1%;使用DMC添加比例10%的DMC/柴油混合燃料作为含氧混合燃料。结果表明,与使用含氧混合燃料时相比,在供油时刻不变情况下,富氧进气时燃烧持续期长,指示热效率低,排气烟度低而NOx排放高。HC和CO排放均较使用含氧混合燃料时的低。柴油机采用富氧进气方式时,排气再循环具有更高的容忍度。利用化学反应动力学与KIVA3V耦合对柴油机采用优化组分进气时的燃烧和排放进行了数值模拟,并对进气中CO₂与O2影响柴油机燃烧和排放的作用机理进行分析。结果表明,随优化组分进气中O2浓度和CO₂浓度的同时增加,缸内温度、NO和O原子浓度逐渐降低,且高温、高浓度区域缩小。就降低NO效果而言,进气CO₂浓度较低时,热作用效果较小,而CO₂浓度增大到15%后,热作用的贡献起主导作用。与使用CO₂置换N2比较,CO₂直接稀释常规进气时缸内温度、NO、O2和OH浓度的下降趋势更为明显。O2/N2进气氧浓度分别为21%、25%的缸内燃烧过程数值模拟显示,进气氧浓度提高后,缸内O2量增加,OH浓度上升,同时O浓度上升,OH和O原子的高浓度区域增加。更多还原

【Abstract】 With the increase of vehicle population and the stringency of emission regulation, one key problem in energy and environmental fields is to decrease emissions from internal combustion （IC） engines. In order to improve combustion and decrease emissions, this paper studies oxidizer design of IC engines. Based on the researches on the characteristics of steady-state combustion flame of LPG and lightweight diesel fuel which are mainly used for IC engines as gas and liquid fuel in different oxidizer environment, the paper emphatically studys the oxygen-enriched combustion and performance of LPG engine during cold-phase and diesel engine during both cold-phase and steady-state with composite intake air.A cup burner equipment was designed and built. With the burner, the characteristics of steady-state combustion flame of LPG and lightweight diesel fuel in different oxidizer environment were studied. For LPG, the effects of oxidizer composition on LPG partial premixing and diffusion flame were observed. When the flowrate of O2/N2 oxidizer is fixed, with the decrease of O2 concentration, the length and brightness of the diffusion flame first increase and then decrease, the distance between the bottom of the visible flame and the rim of the combustion cup increases till the flame disappears. When the flowrate of O2/N2 oxidizer is fixed, with the increase of N2 flowrate, the height of LPG diffusion flame increases, the visible length firstly increases and then decreases, the brightness gradually weakens, peak flame temperature drops and the high temperature area reduces. Through the numerical simulation of LPG diffusion flame, it is shown that with the increase of oxygen concentration of O2/N2 oxidizer, the combustion area reduces and its center moves to the rim of the cup. At the same time, the high temperature layer becomes thicker and the area of high temperature centralizes. For lightweight diesel, the effects of oxygen concentration of oxidizer on the diffusion flame and smoke of fluid level combustion were investigated and the effects of composition and flowrate of oxidizer on the flame of wick combustion were emphatically studied. The results show that the diesel diffusion flame of both fluid level combustion and wick combustion can be divided into two main parts: the lower taper flame and the upper brush-shaped flame. For diesel fluid level combustion, with constant O2/N2 oxidizer flowrate, the increase of oxygen concentration in oxidizer makes the height of diffusion flame increases. when the oxygen concentration is low, the smoke is much; when the oxygen concentration rises, the smoke disappears and the further increase of oxygen concentration makes smoke re-appear. For wick combustion, with the increase of oxygen concentration, the height of flame first increases to the peak value and then drops. Compared with N2, CO₂ has more evident effect on the combustion of both LPG and diesel.The intake air oxygen-enrichment systems for cold-start study of LPG engine and diesel engine were built based on gas membrane seperation technology. For LPG engine, the characteristics of combustion and emission in first cycle were studied based on cycle-controlled method and the running behaviors during start/warm-up and hot idle stages with different oxygen concentration intake air were compared. For the diesel engine, the effects of oxygen concentration in intake air at borderline temperature on combustion and emissions were emphatically studied. The results show that for both LPG engine and diesel engine, OEA makes start speed rise. At the same time, with the increase of oxygen concentration in intake air, soot, HC and CO emissions significantly decrease while NOx emissions increase. Through the analyses of in-cylinder combustion in first cycle of LPG engine during cold start, it shows that with OEA, the peak of in-cylinder pressure increases and the corresponding phase advances. At the same time, the heat release becomes quicker and the phase corresponding to the maximum heat release rate advances too. The analyses of in-cylinder combustion of diesel engine during cold start show that with OEA, the ignition timing obviously advances and the in-cylinder pressure rises compared with ambient air.A system which can supply composite intake air was built. With the same fuel supply timing, the characteristics of combustion and emission were studied when the diesel engine was supplied with oxygen-enriched intake air （OEA） and the intake air diluted with inert gas Ar、N2 and CO₂. The results show that with OEA, the soot, HC and CO emissions all decrease significantly while NOx emissions increase obviously. When the ambient air is diluted with Ar, N2 or CO₂, the soot emissions increase while NOx emissions decrease. With the same addition amount into intake air, the use of CO₂ leads to the smallest increase of soot and largest decrease of NOx emissions. Based on the obvious differences of OEA and inert gas on combustion, the characteristics of combustion and emission of diesel engine with composite intake air including O2, CO₂ and N2 were studied and obtained the composition-optimized intake air with which the soot and NOx emissions can be reduced simultaneously. Then composition-optimized intake air was applied to diesel engine for further study. The results show that with the above composition-optimized intake air, the NOx and soot emissions were reduced by 40% and 50%, respectively, compared with ambient air. At the same time, the indicated thermal efficiency just dropped a little bit.When the composition-optimized intake air is supplied to the diesel engine, with the simultaneous increase of O2 and CO₂ concentration in intake air, the adiabatic and polytropic exponent gradually decrease, ignition delay lengthens and mean indicated pressure drops. Soot and NOx emissions both reduce and HC and CO emissions scarcely vary.The characteristics of combustion and emission of the diesel engine using OEA was compared to that using oxygenated blend fuel. For OEA, the oxygen concentration was 22.1%. The oxygenated blend fuel included 10%DMC and 90%diesel. The fuel supply timing was fixed at 28℃A. The research results show that compared to oxygenated blend fuel, OEA makes longer combustion duration and lower indicated thermal efficiency and soot emissions as well as higher NOx emissions. With OEA, the HC and CO emissions are lower than those with oxygenated blend fuel and the diesel engine can tolerate more amount of EGR.The in-cylinder combustion and emissions with composite-optimized intake air were simulated using KIVA3V code coupled with the turbulence chemistry interaction model. At the same time, the effects of CO₂ and O2 in intake air on combustion and emission were analyzed. The results show that, with the simultaneous increase of O2 and CO₂ concentrations in composite-optimized intake air, in-cylinder temperature, NO and O atom concentrations all decrease and the areas of high temperature and concentration reduce. When the CO₂ concentration in intake air is relatively low, thermal effect on reducing NO is weak. But after the CO₂ concentration is more than 15%, thermal effect becomes the mail factor for NO. Compared with the replacement of N2 with CO₂, the decrease of in-cylinder temperature, NO, O2 and OH concentration is more obvious than CO₂ is directly added into intake air. The in-cylinder combustions of diesel engine were respectively simulated with 21% and 25% oxygen concentration intake air. With the increase of oxygen concentration, the in-cylinder O2 amount increase, OH and O concentration rises. The area of the high OH and O concentration expand too.更多还原