催化燃烧中表面反应-气相反应间相互作用及其对均质压燃发动机着火特性的影响

通过对微元管中甲烷在铂表面的催化燃烧过程的数值计算,分析了当混合气入口压力很高时气相反应对整个催化燃烧过程的影响;通过敏感度分析,找出了对异相着火及气相着火起主要作用的基元反应步.结果表明,在异相着火过程中起主要作用的基元反应步为甲烷与氧气在催化剂表面的吸附反应及氧气的解吸反应,在气相着火过程中起主要作用的基元反应步为OH·及水的吸附与解吸反应.对活塞顶涂有铂催化剂的均质压燃（HCCI）发动机的燃烧过程进行了数值模拟,分析了催化效应及关键表面反应基元步对HCCI发动机着火时刻以及燃烧过程中中间组分的影响,结果表明,催化反应能促进混合气的着火,缩短着火延迟时间,对HCCI发动机着火时刻起主要影响的表面反应为OH·及水的吸附与解吸反应.     

燃油添加剂改善HCCI发动机着火性能的试验研究

针对均质预混合压燃着火(HCCI)发动机低负荷易失火的问题,通过在参考燃料(90%的正庚烷和10%的异辛烷)和93号汽油中分别加入适量的着火改进剂过氧化苯甲酸淑丁脂,在HCCI发动机台架上进行燃烧试验.试验结果表明,在参考燃料中添加质量分数为0.3%~1.2%的过氧化苯甲酸叔丁脂后,与参考燃料相比,低负荷界限拓宽了15%-28%,而高负荷界限却降低了0.5%-7%,表明添加剂在拓宽低负荷的同时使高负荷范围下降.93号汽油添加适量的过氧化苯甲酸叔丁酯能够实现HCCI稳定燃烧,而未加添加剂的汽油则不能燃烧.     

进气温度对甲醇均质压燃燃烧特性的影响

在一台快速压缩机上模拟甲醇HCCI的燃烧过程,通过缸压曲线和活塞位移曲线等数据进行计算分析,研究了进气温度对甲醇均质压燃燃烧特性的影响规律。结果表明,当其他边界条件一定时,随着甲醇混合气进气温度的增加,甲醇HCCI的燃烧始点提前,缸内压力升高率峰值增加,放热率峰值增加,燃烧持续期缩短。     

用汽油和甲醇燃油分层拓展HCCI燃烧高负荷的试验研究

在一台单缸HCCI发动机上研究了进气道喷射汽油缸内喷射甲醇形成汽油甲醇燃油分层的HCCI燃烧排放特性,探索了其拓展HCCI燃烧高负荷的潜力。试验结果表明:在汽油HCCI燃烧中喷射甲醇能够有效降低缸内混合气的温度,推迟着火时刻,延长燃烧持续期,从而降低压力升高率和缸内最高燃烧压力,有利于拓展HCCI燃烧高负荷。一定的HCCI负荷工况存在最佳的汽油甲醇比例,且汽油甲醇最佳比例随着负荷的增加不断减小。在最大压力升高率0.5MPa/°CA和较高的指示效率的限制下,自然吸气条件下采用汽油和甲醇燃油分层的HCCI燃烧最高负荷比汽油HCCI燃烧提高了近50%,达到0.62MPa。     

Intermediate temperature heat release in an HCCI engine fueled by ethanol/n-heptane mixtures: An experimental and modeling study

This study examines intermediate temperature heat release (ITHR) in homogeneous charge compression ignition (HCCI) engines using blends of ethanol and n-heptane. Experiments were performed over the range of 0–50% n-heptane liquid volume fractions, at equivalence ratios 0.4 and 0.5, and intake pressures from 1.4 bar to 2.2 bar. ITHR was induced in the mixtures containing predominantly ethanol through the addition of small amounts of n-heptane. After a critical threshold, additional n-heptane content yielded low temperature heat release (LTHR). A method for quantifying the amount of heat released during ITHR was developed by examining the second derivative of heat release, and this method was then used to identify trends in the engine data. The combustion process inside the engine was modeled using a single-zone HCCI model, and good qualitative agreement of pre-ignition pressure rise and heat release rate was found between experimental and modeling results using a detailed n-heptane/ethanol chemical kinetic model. The simulation results were used to identify the dominant reaction pathways contributing to ITHR, as well as to verify the chemical basis behind the quantification of the amount of ITHR in the experimental analysis. The dominant reaction pathways contributing to ITHR were found to be H-atom abstraction from n-heptane by OH and the addition of fuel radicals to O₂.     

Direct numerical simulations of HCCI/SACI with ethanol

Two and three dimensional direct numerical simulations (DNS) of an autoignitive premixture of air and ethanol in Homogeneous Charge Compression Ignition (HCCI) mode have been conducted. A special feature of these simulations is the use of compression heating through mass source/sink terms to emulate the compression and expansion due to piston motion. Furthermore, combustion phasing is adjusted such that peak heat release occurs after Top Dead Center (TDC) during the expansion stroke, as in a real engine. Zero dimensional simulations were first conducted to identify important parameters for the higher dimensional simulations. They showed that for ethanol, temperature and dilution are the parameters the problem is most sensitive to. One set of two dimensional simulations were conducted with a uniform mixture composition and different levels of temperature stratification, both with and without compression heating. Another set of simulations varied the mixture stratification with constant temperature stratification. Both sets showed considerable differences in ignition delay, heat release and peak temperature and peak pressure. Compression heating was also found to have a significant effect on the heat release profile. A three dimensional simulation was conducted for Spark-Assisted HCCI (SACI). It was initiated with a small spark kernel, which evolved into a premixed flame. The entire mixture eventually underwent autoignition. Distance function based analysis showed a strongly attenuating flame. Analysis of scalar mixing frequencies shows that differential diffusion and reaction induced mixing play an important role in predicting the mixing of reactive scalars. This has significant implications for mixing models for reactive flows. Chemical explosive mode analysis (CEMA) was applied to the 3D simulation and showed promise in identifying the transition from flame propagation to autoignition.     

Experimental investigation of performance and emissions of the SICAI-hybrid engine systems

The use of hybrid electrical engines can provide more efficiency by reducing fuel consumption and emissions. In the research, the experimental studies on the created hybrid electric engine were presented. The hybrid engine combines an electric motor with the internal combustion engine (ICE) which is operated under spark assisted controlled auto-ignition (SICAI) combustion mode with the alternative fuels consisting of different ratios of methane–hydrogen blends. In order to establish the hybrid engine, firstly, efficiency graphs of the electrical motor were obtained experimentally. The battery charge status was also checked. The operating range of the SICAI engine in the hybrid system was identified considering performance and efficiency parameters. Based on these parameters, a hybrid algorithm was established to control the operating of the created hybrid engine system. Thus, the experimental studies were carried out for 100% methane, 90% methane-10% hydrogen, 80% methane-20% hydrogen and, 70% methane-30% hydrogen blends (by volume) at wide opening throttle (WOT) and, 50% WOT positions. Consequently, the results were discussed in terms of efficiency and emissions.     

Experimental study of combustion and emission characteristics of ethanol fuelled port injected homogeneous charge compression ignition (HCCI) combustion engine

The homogeneous charge compression ignition (HCCI) is an alternative combustion concept for in reciprocating engines. The HCCI combustion engine offers significant benefits in terms of its high efficiency and ultra low emissions. In this investigation, port injection technique is used for preparing homogeneous charge. The combustion and emission characteristics of a HCCI engine fuelled with ethanol were investigated on a modified two-cylinder, four-stroke engine. The experiment is conducted with varying intake air temperature (120–150 °C) and at different air–fuel ratios, for which stable HCCI combustion is achieved. In-cylinder pressure, heat release analysis and exhaust emission measurements were employed for combustion diagnostics. In this study, effect of intake air temperature on combustion parameters, thermal efficiency, combustion efficiency and emissions in HCCI combustion engine is analyzed and discussed in detail. The experimental results indicate that the air–fuel ratio and intake air temperature have significant effect on the maximum in-cylinder pressure and its position, gas exchange efficiency, thermal efficiency, combustion efficiency, maximum rate of pressure rise and the heat release rate. Results show that for all stable operation points, NO_x emissions are lower than 10 ppm however HC and CO emissions are higher.     

添加剂对甲醇均质充量压燃燃烧的影响

采用零维详细化学动力学模型,模拟了分别加入H2O2、CH2O两种添加剂的甲醇HCCl燃烧,分析了这两种添加剂对甲醇燃烧的影响.结果表明:H2O2、CH2O在缸内分解产生OH活性基,提高了OH的浓度,甲醇着火时刻提前,并且提高了缸内的压力和温度峰值.同时,CH2O能提高平均指示压力,H2O2通过控制着火时刻来影响平均指示压力的大小.