【作者】 李楠；

【导师】 赵华； 谢辉；

【作者基本信息】 天津大学 ， 动力机械及工程， 2012， 博士

【摘要】 可控自燃着火（CAI）燃烧具有同时改善发动机的燃油经济性和NOX排放的优势。但由于CAI为混合气压缩自燃着火，缺少像传统汽油机的点火时刻或柴油机的喷油时刻那样控制着火的直接手段，给燃烧控制带来很大挑战。本文针对全可变气门机构汽油机，研究缸内成分和温度的不均匀分布及其对CAI自燃的影响规律，探索控制CAI着火及燃烧的一种有效手段，这对CAI燃烧应用具有重要意义。本文以一台配备全可变气门机构的单缸汽油机为对象，基于KIVA-3V软件，采用动网格技术建立了带有进、排气道的四气门汽油机计算模型，采用Shell模型和特征时间燃烧模型计算着火和燃烧过程。采用多循环模拟汽油机CAI燃烧过程，降低了仿真结果对初始条件和边界条件的敏感性。利用实验结果验证了着火延迟的有效性，在不同工况下的模型预测的CA10、CA50、CA90与实验结果误差在1-2度曲轴转角范围内。为了定量描述缸内废气和温度分布特征，采用统计学中的标准差概念定义了“不均匀度”来表征不均匀分布的程度。通过CFD仿真，研究了排气门关闭时刻、进气门开启时刻和进气门升程控制对缸内流动、废气分布和温度分布的影响。研究表明，在CAI燃烧可运行的范围内，可以通过进气门开启时刻和升程的调节，实现缸内不均匀度的控制。推迟进气门开启时刻和升高进气门升程都可以使缸内不均匀度增大，但是进气门开启时刻的控制作用在小废气率下无效，而进气门升程的控制作用在整个CAI运行范围内的废气率下均有效。随着废气率的增大，缸内废气不均匀度和温度不均匀度都有增大的趋势。通过CFD仿真，研究了废气和温度分布不均匀度对CAI着火和燃烧的影响规律。结果表明，缸内废气率和平均温度相同时，温度不均匀度可以有效控制着火时刻，但对燃烧持续期的影响较小。在本文研究范围内（缸内废气率为32%～85%），温度的不均匀度越大，着火时刻越早。因此，可以通过对温度不均匀度的控制来控制着火时刻。为了验证缸内分布不均匀性对着火燃烧的影响，本文针对单缸发动机，通过控制进气门相位，产生进气前回流，在缸内形成一定的废气和温度不均匀分布。通过CFD揭示了其分布特征；通过单缸机试验，研究了其着火特性。研究表明，进气前回流使缸内高废气率区和高温区分离，促进了小负荷的自燃着火，从而验证了缸内温度和废气分布不均匀性对自燃着火的控制作用。本研究也为实现CAI燃烧向小负荷的拓展提供了新思路。更多还原

【Abstract】 Controlled auto-ignition (CAI) combustion has the advantage in fuel economyand NOx emissions for engines. However, there is no direct ignition control in CAIengines, as the spark in gasoline engines or the fuel injection in diesel engines, whichis a great challenge for combustion control. The present paper explores the method tocontrol CAI auto-ignition through adjusting in-cylinder components and temperaturedistribution based on a fully variable valve technology. That is significant to thepractical application of CAI combustion.A gasoline engine model with four vavles and intake and exhaust ports is built inKIVA-3V. The Shell model is used to simulate the auto ignition, and thecharacteristic time model is used to simulate the combustion process. Multi-cyclesimulation is applied to reduce the sensitivity of the results in the initial conditionsand the boundary conditions. The validity of ignition delay is validated. The errors ofCA10, CA50and CA90of the simulation results and the experimental results arebetween1-2degrees crank angle. The "inhomogeneity" is defined as the standarddeviation to quantify the in-cylinder residual gas fraction (RGF) and temperaturedistributions.The effects of exhaust valve closing (EVC) timing, intake valve opening (IVO)timing and intake valve lift (IVL) on the in-cylinder flow, RGF and temperaturedistributions are studied by CFD simulation. The inhomogeneity of residual gas andtemperature has a wide range of variation under CAI mode, and the inhomogeneitycontrol can be achieved through the adjustment of both intake valve open timing andintake valve lift. The in-cylinder inhomogeneity increases with the intake valve opentiming delaying and intake valve lift increasing. However, the intake valve opentiming control strategy is only effective when applied to high in-cylinder RGF cases,whereas the intake valve lift control strategy is effective in controlling the in-cylinderinhomogeneity under the whole CAI range. The inhomogeneity of RGF andtemperature increases as the RGF increases.The abilities of the in-cylinder RGF and temperature distribution to control CAIignition and combustion processes are investigated by CFD simulations. The resultsshow that the temperature inhomogeneity can effectively control the CAI ignition timing with the same in-cylinder RGF and average temperature. However, thestratification of the RGF and temperature caused by the valve strategies is not strongeenough to influence the CAI combustion duration. The CAI auto-ignition timingadvances with the temperature inhomogeneity increasing in a wide range of the32%～85%RGF. Therefore, the auto-ignition timing can be controlled by thetemperature inhomogeneity.In order to validate the effects of the in-cylinder distribution on ignition andcombustion, the intake backflow is introduced by controlling the intake valve timing,which could lead to some RGF and temperature inhomogeneities. The distribution isrevealed through CFD simulation and the ignition characteristic is researched viasingle-cylinder gasoline engine experiments. The results show that the hightemperature zone and high RGF zone are separated by intake backflow, which is aadvantage for the ignition in low loads. A new concept for the realization of CAIcombustion in low load is provided.更多还原