Combustion and Emission Performance of an HCCI Engine Fuelled by n-Heptane/Toluene Blends at a Low-Load Operating Condition

Hongsheng Guo, Mining Energy, W. Neill
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Abstract

Homogeneous charge compression ignition (HCCI) engine technology offers high fuel efficiency and extra low nitrogen oxide (NOx) and particulate matter (PM) emissions, which makes it a potential alternative combustion mode to conventional diesel engines. A diesel fuel is usually composed of many classes of hydrocarbons among which aromatic compounds have attracted special attention due to their specific combustion characteristics. Understanding the combustion and emission characteristics of different classes of hydrocarbons is crucial for identifying appropriate diesel fuels suitable for HCCI combustion. Toluene is a typical aromatic compound in a diesel fuel. A study of toluene content may provide implications of the effect of aromatic in a diesel fuel on HCCI combustion. In this paper, the combustion and emission performance of an HCCI engine fuelled by n-heptane/toluene blends at a low load operating condition was investigated by experiment and numerical simulation. A modified Cooperative Fuel Research (CFR) engine and a in-house-developed multi-zone model were employed. The engine was operated at the condition of engine speed of 900 rpm, relative air/fuel ratio of 3.5 and without external exhaust gas recirculation. The investigated fuel blends covered a range from pure n-heptane to 70% toluene by volume. Both experimental and numerical results showed that an increase in toluene fraction in the fuel blend retarded combustion phasing. As a result, the optimal compression ratio, at which thermal efficiency reached its maximum for a fuel blend, increased with increasing toluene fraction. The maximum thermal efficiency increased as the toluene fraction increased from 0 to 50%, but then decreased with further increasing toluene fraction to higher values. The peak pressure rise rate also increased with increasing toluene fraction at a constant combustion phasing. An increase in toluene fraction resulted in an increase in unburned hydrocarbon emissions but had little effect on NOx emissions.
低负荷工况下正庚烷/甲苯混合燃料HCCI发动机的燃烧与排放性能
均质装药压缩点火(HCCI)发动机技术提供了高燃油效率和超低氮氧化物(NOx)和颗粒物(PM)排放,使其成为传统柴油发动机的潜在替代燃烧模式。柴油通常由多种碳氢化合物组成,其中芳香族化合物因其特殊的燃烧特性而受到特别关注。了解不同类别碳氢化合物的燃烧和排放特性对于确定适合HCCI燃烧的柴油燃料至关重要。甲苯是柴油中典型的芳香族化合物。甲苯含量的研究可能会对柴油中芳香族对HCCI燃烧的影响提供启示。采用实验和数值模拟的方法,研究了正庚烷/甲苯混合燃料HCCI发动机在低负荷工况下的燃烧和排放性能。采用改进的协同燃料研究(CFR)发动机和自行开发的多区域模型。发动机在发动机转速900转/分、相对空燃比3.5、无外部废气再循环的条件下运行。所研究的燃料混合物涵盖了从纯正庚烷到70%甲苯的体积范围。实验和数值结果均表明,混合燃料中甲苯含量的增加延缓了燃烧的分相。结果表明,混合燃料的最佳压缩比随着甲苯分数的增加而增加,此时热效率达到最大值。最大热效率随着甲苯分数从0到50%的增加而增加,但随着甲苯分数的进一步增加而降低。在恒相燃烧条件下,峰值压力上升速率随甲苯分数的增加而增加。甲苯馏分的增加导致未燃烃排放增加,但对NOx排放影响不大。
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