【作者】 姚远；

【导师】 赵华； 谢辉；

【作者基本信息】 天津大学 ， 动力机械及工程， 2018， 硕士

【摘要】 火花点火-可控自燃(SI-CAI)混合燃烧是一种双阶段放热的稀释燃烧方式,可以提升点燃式发动机的热效率。但是其前期的稀释火焰易产生明显的放热量波动,进而导致后期自燃过程失稳。分层火焰引燃策略(SFI)利用浓混合气火焰传播速度快的特点,抑制混合燃烧前期火焰传播的不稳定性。但是,混合燃烧具有高温高稀释的特点,其缸内条件濒临自燃,火焰特征与传统存在区别。为设计合理的分层火焰引燃策略,本文通过光学发动机试验平台,采用外部EGR和进气加热模拟了混合燃烧的缸内边界条件,拍摄火焰发展的图像,研究了不同缸内条件下的汽油机分层火焰特征及发展规律。首先,考虑到混合燃烧前期火焰传播对整个燃烧放热过程的调控作用,本文研究了燃油加浓、废气-空气稀释以及初始温度对前期火焰传播速度的影响规律。结果表明,分层策略使高稀释条件下的火焰传播速度提升明显,浓混合气相对于稀混合气火焰传播速度提升10%以上,分层火焰引燃策略具有可实现性。其次,统计了火焰面上的曲率半径的PDF。火焰褶皱结构表现出对分层和稀释的敏感性。稀释的火焰锋面前会出现少量自燃点,在自燃方向上的火焰传播最大达到17m/s,相对平均火焰传播速度提升一倍左右;而废气的不均匀分布则会阻碍火焰的发展,导致火焰发展的各向异性,减慢火焰传播速度。最后,研究了稀释条件下直喷参数的调控规律以及分层火焰的传播特征。结果显示:分层燃烧主要受直喷时刻的控制,燃烧相位随直喷时刻的推迟而提前。当直喷时刻早于60°CA BTDC时,火焰表现出均质燃烧特征,单循环火焰发展趋于非分层火焰。而直喷时刻晚于60°CA时,燃烧形成分层火焰,相比于λ=1的均质燃烧,分层火焰在浓区的火焰传播速度提升9.4%,相比λ=0.9提升3.1%。与均质燃烧相比,分层火焰策略加浓了中心混合气,显著提升前期火焰速度与着火稳定性。更多还原

【Abstract】 Spark Ignition-Controlled Auto-Ignition(SI-CAI)Hybrid Combustion is a twostage dilution combustion that enhances the thermal efficiency of spark-ignition engine.However,the early dilutied flame is prone to produce obvious fluctuations,which leads to instability of the post-ignition process.The stratified flame ignition strategy(SFI)utilizes the characteristics of fast flame propagation in enriched mixture to suppress the instability of flame propagation in the early stage of hybrid combustion.However,the hybrid combustion is under high temperature and high dilution in-cylinder condition which is on the verge of spontaneous combustion.The flame characteristics are different from the conventional ones.In order to design a reasonable stratified flame ignition strategy,this paper uses an optical engine test platform with external EGR and intake air heating to simulate the in-cylinder boundary conditions of hybrid combustion,captures the images of flame development,and studies the characteristics and development of stratified flame in gasoline engine under different in-cylinder conditions.Firstly,considering that the early flame propagation has important influence on the whole hybrid combustion process,the influence of the fuel enrichment,exhaust-air dilution and initial temperature on flame propagation speed are studied.The results show that the stratification can obviously increase the flame propagation speed under the condition of high dilution.The flame propagation speed in dense mixture can be increased by more than 10% compared with that of dilute mixture.The stratified flame ignition strategy can be realized.Secondly,the PDF of the radius of curvature on the flame front is calculated.The flame fold structure shows sensitivity to stratification and dilution.A small amount of spontaneous ignition points will appear in front of diluted flame front.The flame propagates in the direction of spontaneous combustion up to 17 m/s,and the relative average flame propagation speed increases by about 50%.The uneven distribution of exhaust gas will hinder the development of flame,lead to the anisotropy of flame development and slow down the flame propagation speed.Finally,the regulation of direct injection parameters and the propagation characteristics of stratified flame under dilution conditions are studied.The results show that the stratified combustion is mainly controlled by the direct injection time,and the combustion phase is advanced with the delay of the direct injection time.When the direct injection time is earlier than 60 °CA BTDC,the flame exhibits homogeneous combustion characteristics.Meanwhile,flame tends to develop as non-stratified state.When the direct injection time is later than 60 °CA,stratified flame is formed.Compared with the homogeneous combustion of lambda=1,the flame propagation speed of stratified flame in the dense zone increases by 9.4% and 3.1% compared with lambda=0.9.Compared with homogeneous combustion,the stratified flame strategy enriches the central area,thus significantly improves the flame speed and ignition stability in the early stage.更多还原