An experimental and numerical investigation on the influence of external gas recirculation on the HCCI autoignition process in an engine: Thermal, diluting, and chemical effects
An experimental and numerical investigation on the influence of external gas recirculation on the HCCI autoignition process in an engine: Thermal, diluting, and chemical effects
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DOI：
10.1016/j.combustflame.2008.05.001
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发表时间：
2008-11
影响因子：
4.4
通讯作者：
H. Machrafi;S. Cavadias;P. Guibert
中科院分区：
文献类型：
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作者：
H. Machrafi;S. Cavadias;P. Guibert
为了解决均匀装药压缩点火（HCCI）发动机的自燃控制问题，与外部气体再循环（EGR）相关的参数似乎特别令人感兴趣。用EGR进行的实验在解释仅使用EGR的稀释和热方面的结果时存在一些困难。最近，EGR的化学方面得到了更多的考虑，因为这方面会与EGR的稀释和热方面产生复杂的相互作用。本文研究了EGR对自燃过程的影响，特别是EGR的化学特性。EGR中存在的稀释剂由n2和CO2模拟，稀释系数从0到46 vol%。对于EGR中可能存在的化学活性物质，使用CO、NO和CH2O等物质。CO和NO的初始浓度在0 ~ 170 ppm之间变化，CH2O的初始浓度在0 ~ 1400 ppm之间变化。为了研究化学物质对自燃的影响，将稀释系数固定为23 vol%， EGR温度固定为70℃。进口温度保持在70°C，等效比在0.29和0.41之间，压缩比为10.2。用于自燃的燃料是正庚烷和PRF40。结果表明，在所研究的区域内，CO对点火延迟没有影响，而NO有两种不同的影响。在浓度达到45 ppm之前，NO提前了PRF40的点火延迟，在更高的浓度下，点火延迟。NO对正庚烷自燃的影响不大，可能是由于正庚烷的燃烧速率较高。CH2O似乎延缓了点火。结果表明，OH自由基的形成或OH自由基被化学添加剂消耗决定了自燃反应性的变化。
In order to contribute to the solution of controlling the autoignition in a homogeneous charge compression ignition (HCCI) engine, parameters linked to external gas recirculation (EGR) seem to be of particular interest. Experiments performed with EGR present some difficulties in interpreting results using only the diluting and thermal aspect of EGR. Lately, the chemical aspect of EGR is taken more into consideration, because this aspect causes a complex interaction with the dilution and thermal aspects of EGR. This paper studies the influence of EGR on the autoignition process and particularly the chemical aspect of EGR. The diluents present in EGR are simulated by N2and CO2, with dilution factors going from 0 to 46 vol%. For the chemically active species that could be present in EGR, the species CO, NO, and CH2O are used. The initial concentration in the inlet mixture of CO and NO is varied between 0 and 170 ppm, while that of CH2O alters between 0 and 1400 ppm. For the investigation of the effect of the chemical species on the autoignition, a fixed dilution factor of 23 vol% and a fixed EGR temperature of 70 °C are maintained. The inlet temperature is held at 70 °C, the equivalence ratios between 0.29 and 0.41, and the compression ratio at 10.2. The fuels used for the autoignition are n-heptane and PRF40. It appeared that CO, in the investigated domain, did not influence the ignition delays, while NO had two different effects. At concentrations up until 45 ppm, NO advanced the ignition delays for the PRF40 and at higher concentrations, the ignition delayed. The influence of NO on the autoignition of n-heptane seemed to be insignificant, probably due to the higher burn rate of n-heptane. CH2O seemed to delay the ignition. The results suggested that especially the formation of OH radicals or their consumption by the chemical additives determines how the reactivity of the autoignition changed.