Abstract In ethanol/diesel dual-fuel engines, premixed ethanol/air atmosphere could affect the ignition delay of diesel spray as well as the following soot formation. In this paper, a reduced mechanism describing the chemistry of ethanol and n-heptane was composed and validated with various experimental data. Based on this mechanism, simulations were conducted to investigate the combustion characteristics of n-heptane spray under premixed ethanol/air atmosphere in a combustion vessel. The effects of equivalence ratio of premixed ethanol/air atmosphere, ambient temperature and charge cooling effect of ethanol vaporization were investigated. Under premixed ethanol/air atmosphere, the results showed that high temperature ignition first occurred in the regions with mixture near stoichiometric ratio, rather than regions with largest n-heptane content (i.e. highest fuel reactivity). This is mainly because the vaporization of direct injected n-heptane decreased the gas temperature and consequently restricted the low temperature reaction. While the equivalence ratio of premixed ethanol/air atmosphere was further increased, the mixture near stoichiometric ratio exhibited lower fuel reactivity, which consequently resulted in prolonged ignition delay. It was also found that premixed ethanol could inhibit the soot formation of n-heptane spray under low ambient temperatures while contributed to significantly increased soot emission under high ambient temperatures. Moreover, the simulated results showed that the cooling effect of ethanol vaporization also played an important role in soot reduction of dual-fuel combustion.

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乙醇/空气预混气氛对燃烧容器内正庚烷喷雾燃烧特性影响的研究

摘要 在乙醇/柴油双燃料发动机中，乙醇/空气预混气氛会影响柴油喷雾的点火延迟以及随后的烟灰形成。在本文中，描述了乙醇和正庚烷化学的简化机制被组成并用各种实验数据进行了验证。基于此机理，通过模拟研究了正庚烷喷雾在燃烧容器中预混乙醇/空气气氛下的燃烧特性。研究了预混乙醇/空气当量比、环境温度和乙醇汽化进料冷却效果的影响。结果表明，在乙醇/空气预混气氛下，混合物接近化学计量比的区域首先发生高温点火，而不是具有最大正庚烷含量（即最高燃料反应性）的区域。这主要是因为直接注入正庚烷的汽化降低了气体温度，从而限制了低温反应。虽然进一步提高了预混乙醇/空气气氛的当量比，但接近化学计量比的混合物表现出较低的燃料反应性，从而导致点火延迟时间延长。还发现预混乙醇在低环境温度下可以抑制正庚烷喷雾的烟灰形成，同时在高环境温度下显着增加烟灰排放。此外，模拟结果表明，乙醇汽化的冷却效果在双燃料燃烧的烟尘减少中也发挥了重要作用。这主要是因为直接注入正庚烷的汽化降低了气体温度，从而限制了低温反应。虽然进一步提高了预混乙醇/空气气氛的当量比，但接近化学计量比的混合物表现出较低的燃料反应性，从而导致点火延迟时间延长。还发现预混乙醇在低环境温度下可以抑制正庚烷喷雾的烟灰形成，同时在高环境温度下显着增加烟灰排放。此外，模拟结果表明，乙醇汽化的冷却效果在双燃料燃烧的烟尘减少中也发挥了重要作用。这主要是因为直接注入正庚烷的汽化降低了气体温度，从而限制了低温反应。虽然进一步提高了预混乙醇/空气气氛的当量比，但接近化学计量比的混合物表现出较低的燃料反应性，从而导致点火延迟时间延长。还发现预混乙醇在低环境温度下可以抑制正庚烷喷雾的烟灰形成，同时在高环境温度下显着增加烟灰排放。此外，模拟结果表明，乙醇汽化的冷却效果在双燃料燃烧的烟尘减少中也发挥了重要作用。