【作者】 肖合林；

【导师】 张煜盛； 魏明锐；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2007， 博士

【摘要】 液化石油气(LPG)以其良好的排放及混合气形成特性,已成为汽油和柴油重要的替代燃料,被视为“绿色能源”,并在发动机上得到了广泛的应用。然而,国内外在现有发动机上使用LPG,通常采用LPG缸外气化与空气混合后进入气缸燃烧的工作方式。这种燃料供给方式存在许多不足之处。为了充分利用LPG的蒸发潜热高和快速气化特性,降低燃烧温度,减少NOx排放并提高其动力性和燃油经济性,充分发挥压燃式内燃机压缩比大、热效率高的特点,克服进气道供气方式存在的诸多缺点,本文以液态LPG与柴油混溶后缸内直喷压燃改善发动机性能的作用机理为研究对象,通过开展理论分析与试验研究,深入探讨了LPG/柴油混合燃料的喷雾特性、变化规律、影响因素,以及LPG碰壁对缸内混合气形成及燃烧过程的影响等科学问题;借助于自行开发的液态LPG/柴油比例混合电控系统,进行了直喷式LPG/柴油双燃料压燃发动机的性能试验;并就LPG进气道喷射与二甲醚(DME)缸内直喷实现准均质压燃的技术问题开展了基础试验研究,对LPG、DME组合燃料准均质压燃所产生的常规及非常规污染物排放进行了定量测试。本文的研究工作为LPG等液化气体燃料在柴油机上的应用提供了一条新的技术途径,对我国多元化汽车能源战略的实施具有较高的学术和应用价值。本文的研究内容和创新点主要有以下几个方面:在广泛阅读国内外文献的基础上,对LPG应用研究的现状进行了综述,分析了现有发动机使用LPG存在的诸如由进、排气气门重叠角导致的燃料的短路损失、发动机容积效率和输出功率较低、NOx排放没有明显改善,以及难以得到良好的发动机综合性能等弊端。针对目前关于LPG/柴油混合燃料缸内直喷压燃的研究仍未见有深入报道的现状,提出了一种新的全面改善LPG发动机性能的技术方案,即采用LPG/柴油混合燃料缸内直喷压燃模式来优化LPG发动机的动力与排放性能。通过向定容室内喷射液态LPG、柴油及不同比例的LPG和柴油的混合燃料,采用高速摄影的方法,研究了喷射参数、LPG和柴油的混合比对喷雾特性的影响;对不同燃料之间的喷雾特性进行了详细的比较和分析。试验研究结果表明,在相同的喷射参数下,各种燃料中,LPG的喷雾贯穿距最短,蒸发气化的速度最快;混合燃料中LPG的含量越高,其雾化特性越好。在此基础上,采用KIVA-3V计算程序,对LPG及柴油在定容室内的喷雾过程进行了数值模拟,对燃料的喷雾场进行了详细的计算分析,并与试验结果进行了对比,计算分析结果与试验结果基本吻合。在分析、归纳前人关于喷雾碰壁研究成果的基础上,基于试验研究的结果,建立了喷雾碰壁的数学模型。该模型给出了离散液滴油膜铺展和撞壁飞溅等物理过程的数学描述,考虑了液滴、壁面、油膜上方气体层之间的相互作用,即动量和热量的交换过程。利用该模型对LPG碰壁过程进行了模拟计算,并与柴油进行了对比分析。研究表明,LPG液体蒸发较快,与柴油相比,油膜厚度较薄,LPG碰壁后在湿壁面效应会大为减弱,这在缸内混合气形成及燃烧过程中是非常有利的因素。燃料碰壁分析计算结果与试验结果对比表明,两者之间吻合较好。为了开展LPG和柴油按任意比例混合后直喷压燃的试验,设计了一套混合燃料电控供油系统。该系统使液态LPG和柴油预先在混合器中按比例混合,再通过喷油泵及喷油器向缸内喷射。液态LPG和柴油的混合比例可任意调节。为此,电路采用MCS-51系列单片机AT89C52,控制电磁阀导通时间来控制柴油和LPG流量,以实现LPG和柴油的任意混合比例。电磁阀导通时间的控制则是通过控制驱动电磁阀导通的脉宽来实现。借助于混合燃料电控供油系统,在ZS195发动机上实现了液态LPG和柴油混合、缸内直喷压燃的燃烧方式,研究了LPG与柴油混合燃料的燃烧情况。通过试验发现,在适当的LPG/柴油混合比例下,与纯柴油燃烧相比,在功率不下降的前提条件下,混合燃料燃烧时缸内的最高燃烧爆发压力变化不大,CO排放量有所上升,但HC、NOX及碳烟的排放则有较大幅度的下降。试验中还发现,适当比例混合燃料燃烧时,发动机的耗油率及排气温度较燃烧柴油时低,但燃烧产生的噪声则增大;当混合燃料中LPG的量超过一定比例时,燃烧恶化,甚至不能燃烧。在经改装的ZS195柴油机上,通过采用LPG进气道进气、DME缸内直喷引燃的方式,开展了不同比例的高辛烷值燃料(LPG)和高十六值燃料(DME)相组配的可变燃料特性准均质压燃(QHCCI)基础试验研究。试验发现,采用这种燃烧方式,可在一定程度上实现着火点的控制,并使发动机在较大的负荷范围内工作。同时,当混合气中LPG较浓时,发动机燃烧不太稳定,排放、噪声都会升高。试验中还测试了燃烧所产生的HC、CO、NOX及非常规污染物甲醛和甲酸甲酯的排放。测试结果表明,随着混合气中LPG浓度的升高,HC、NOX、甲醛及甲酸甲脂的排放都会升高,CO的排放则呈现中间高,两头低的特点。且随着负荷的增大,发动机的污染物排放量均有所下降。更多还原

【Abstract】 The Liquefied-Petroleum Gas(LPG), which is of the better property of low exhaust emissions and rapid evaporation, is used extensively as an important substitute for the diesel and gasoline in the engines. However, the LPG is combusted in the chamber by premixing with the air in outside of the engine cylinder today, which leads to the low charge- efficiency and high exhaust emissions and can’t better use of the LPG’s capability of low exhaust emissions. In order to avoid these shortcomings, fueling the engine with the LPG injected directly into the cylinder will be the future aim of the LPG engine development. After studying most of the articles on the use of the LPG in the engine this paper researches the property on the spray and the combustion of the LPG injected directly into the cylinder in theory and experiments.Through spraying into the constant volume chamber with LPG or diesel or some kinds of mixtures of LPG and diesel, the effects of spray parameters and mixture ratio for LPG and diesel on the property of the spray were investigated and the characteristics of the sprays of all kinds of fuels were compared each other. The experiment shows that the penetration distance of the LPG is the shortest among the fuels and the velocity of the evaporation is the fast at the same spray parameters. It also finds that the higher the ratio of LPG in the mixture fuel, the better the spray property of the fuel. In addition of the experiment, this paper also performs the numeric simulation of the spray process for the LPG and diesel in the constant volume chamber with KIVA-3V program and analyzes the spray field in detail. Through comparison with the experiment, it finds that they are consistent with each other.Based on the detailed comparisons and analysis to many kinds of spray impingement models, two representative spray impingement models, wall spread and splashing, are introduced in this thesis. In the model, the exchange of heat and momentum between the wall, droplets and the gas above liquid film are modeled. The spray impingement of LPG is simulated and compared with that of diesel. Due to LPG quickly evaporating, its film thickness is much smaller than the one of diesel, which shows that the accumulation of LPG on the wall is not serious to be of advantage to combustion process.In order to perform the combustion experiment with the various mixture ratio of LPG and diesel injected directly into the cylinder, this paper designs a fuel supplement system, which can premix the LPG and diesel in the mixture corrector randomly. By use of the AT89C52 SCM, the system can control the open times of the electromagnetic valve through controlling the width of the electrical current pulse to determine the flux of the LPG and diesel and mix the LPG and diesel with predetermined ratio, which can be changed randomly.In this thesis, we study the combustion of the mixture of LPG and diesel in the ZS195 diesel engine by injecting it directly into the cylinder. It shows that with the suitable ratio of LPG, the maximum combustion pressure of the mixture and the exhaust emission of CO are higher but the HC and NOX are lower than that of the diesel. Through the experiment, it can be found that the mixture fuel consumption and the exhaust temperature are lower but the combustion noise is higher than that of the diesel and also can find that when the LPG ratio is too high, the combustion can’t occur.As a new manner of combustion, Homogeneous Charge-Compression Ignition(HCCI), is regarded as an important technology advancement of engine because of its great potentials of reducing the fuel consumption and the exhaust emissions. In this thesis, the homogeneous charge compression ignition of LPG in ZS195 by inducing the LPG into the cylinder through the intake pipes and injecting the DME into the cylinder as an ignition additive was conducted. Through the experiment, it can be found that the engine can run over a wide range of load and when the LPG is too high in the cylinder, the combustion can’t occur steadily and the exhaust emissions and noise would increase. In the experiment, it can also be found that the emissions of the HC, CO, NOx, HCHO and CH3OCHO would increase when the LPG concentration became high in the cylinder and all the emissions would decrease as the load became heavy.更多还原