均质压燃发动机燃烧与排放的多区模型模拟

应用一个有质量交换的6区模型模拟正庚烷在HCCI发动机中的燃烧和排放特性．通过把缸内划分为缝隙区、边界层区、外核心区和内核心区,加入Woschni传热模型计入了缸内的温度和浓度的不均匀分布．全部计算基于正庚烷燃烧的包含57种组分290个反应的详细机理,结果表明,该多区模型合理地模拟了HCCI发动机的燃烧过程,并可满意地预测出HC、CO和NO的排放．最后采用此多区模型分别讨论了缝隙容积、边界层厚度和壁面温度对HCCI发动机的燃烧和排放的影响．     

均质压燃(HCCI)单区和多区燃烧模型的比较

利用耦合化学动力学软件包CHEMKIN建立了汽油HCCI发动机的单区和多区燃烧模型。与试验数据的对比表明：虽然单区模型在参数化研究中能较准确地预测混合气的自燃定时,但其准确性在很大程度上取决于缸内的均匀性;包含缝隙区、淬熄区、质量交换区和绝热核心区的九区燃烧模型通过设置特定的功能分区能更好地模拟HCCI的燃烧过程,它在对自燃定时的预测和对热量释放过程和排放物的形成机理的模拟等方面都有很好的性能。     

催化燃烧对均质压燃发动机燃烧特性影响的数值模拟

通过运用DETCHEM软件包,对甲烷在催化剂Rh表面的详细反应机理进行了分析,结果表明数值模拟结果与实验数据相当吻合;通过耦合DETCHEM软件包及CHEMKIN软件包中的SENKIN模块,对活塞顶涂有催化剂铑的均质压燃(HCCI)发动机的燃烧过程进行了数值计算,建立了单区和多区模型．利用单区模型分析了催化燃烧对HCCI发动机着火时刻的影响,同时讨论了催化燃烧对燃烧过程中主要化学组分浓度变化的影响,结果表明催化燃烧会使HCCI发动机着火时刻提前;利用多区模型分析了催化燃烧对HCCI发动机的未燃碳氢化合物(UHC)、氮氧化合物(NOx)排放的影响,结果表明催化燃烧能降低UHC的排放,但会提高NOx的排放．     

HCCI发动机燃烧多维数值模拟(Ⅰ)——模型的建立和比较

使用多维骨架动力学模型模拟了以异辛烷为燃料的均质压燃(HCCI)发动机的燃烧过程。通过结合多维CFD程序KIVA和反应动力学程序CHEMKIN实现了化学反应与流动的耦合运算。结果表明网格密度和时间步长对HCCI的燃烧过程影响不大,而初始温度对着火点影响显著。通过修改多维模型比较了RNG k-ε湍流模型和标准k-ε湍流模型、Han和Reitz传热模型和传统传热模型、Kong的混合模型和无混合模型,以及不同的缝隙区模型,发现使用RNG k-ε湍流模型、Han和Reitz传热模型和缝隙流动模型计算结果与实验值更为接近。修改后的多维模型在不同当量比下计算得到的压力、放热率和排放值与实验完全一致。     

均质压燃发动机燃烧特性的详细反应动力学模拟

应用CHEMKIN化学动力学软件包中的SENKIN模块模拟了正庚烷在HCCI发动机中的燃烧过程。通过修改SENKIN程序，加入了Woschni传热模型，并在正庚烷详细氧化机理中加入氮氧化物的生成机理，将此程序纳入发动机燃烧的零维单区模型。对多种工况参数下的HCCI燃烧和NOx排放进行了系统的计算，并分别讨论了进气温度、进气压力、压缩比、过量空气系数和转速等参数变化对HCCI发动机燃烧过程的影响。     

催化燃烧对HCCI发动机着火点、燃烧性能及排放的影响

对甲烷在催化剂铑(Rh)表面的反应机理进行了分析。通过修改CHEMKIN软件包中的SENKIN模块,对活塞顶涂有催化剂的HCCI发动机的燃烧过程进行了数值计算,建立了单区、多区模型。利用单区模型分析了催化燃烧对HCCI发动机着火时刻的影响,结果表明在控制HCCI发动机着火时刻方面催化燃烧有其他方式所没有的优势;利用多区模型分析了催化燃烧对HCCI发动机的燃烧性能及HC、CO、NOx排放的影响,结果表明催化燃烧对燃烧效率、着火持续期有较大的影响,同时能降低HC、CO的排放,但会提高NOx的排放。     

采用多维数值模拟研究汽油HCCI发动机的排放物生成及演变规律

将发动机多维CFD程序KIVA-3V与化学动力学程序CHEMKINⅢ相耦合,分析了汽油HCCI发动机燃烧特性及排放物的生成机理及演变规律.发动机以异辛烷为燃料,其化学反应采用了详细的动力学机理.结果表明,此计算模型所得到的缸内压力及放热率的变化趋势与实验基本吻合;由于缝隙区温度较低,因而其成了汽油HCCI发动机HC排放的主要来源,而CO排放的主要来源则是边界层区;由于气缸中心区域温度较高,而NO的生成与温度呈指数关系,因此该区域为汽油HCCI发动机NO排放的主要来源.     

应用多区模型预测HCCI发动机的运行范围

应用多区模型预测了均质压燃(HCCI)发动机的运行范围.运行范围的边界由敲缸(由声响强度计算)、部分燃烧(由燃烧效率计算)和循环变动(由平均有效压力对进气温度的敏感度计算)确定.通过模拟一台以异辛烷为燃料的HCCI发动机发现,在不同当量比和进气温度下,计算得到的敲缸极限和循环变动极限均与试验相当吻合,从而很好地再现了HCCI发动机的运行范围.进一步研究运行范围随转速的变化,讨论了通过调节进气温度和当量比,在不同转速下HCCI发动机的最大可运行范围.最后考察了通过使用变压缩比和增压两种策略扩展HCCI发动机运行范围的潜力.结果表明:通过调节压缩比可以有效地控制着火点,在向低负荷扩展方面,使用高的进气温度和较低的压缩比显得更为有利;在向高负荷扩展方面,使用高压缩比更好;而增压有利于HCCI发动机的控制,同时增加了向高负荷扩展的范围.     

多孔介质发动机的双区燃烧模型

多孔介质(PM)发动机是基于多孔介质燃烧技术的新型发动机,能够实现均质和稳定燃烧。在考虑了区间质量分布、壁面传热、区间质量交换等因素的基础上,结合多孔介质换热模型,建立了多孔介质发动机的一种双区模型,对其燃烧过程进行模拟。着重讨论了进气温度和压强、压缩比、过量空气系数等参数对多孔介质发动机性能的影响。计算结果表明多孔介质对混合气的预热作用,促进了液体燃料汽化和燃烧反应发生,多孔介质初始温度对发动机的压燃着火起决定性作用。     

适用于HCCI发动机的异辛烷氧化的化学动力学模型(Ⅱ)——一个新的骨架机理

构造了一个包括40种组分和69个反应的异辛烷氧化的骨架机理，该机理可以较准确计算着火点、燃烧速率以及HC、CO和NOx的排放，适用于HCCI发动机的多维模型的计算。通过与激波管、速压机、喷射搅拌反应器和HCCI发动机实验数据的比较表明，该反应机理在不同温度、压力和当量比下，尤其在高压、稀薄的反应条件下具有很好的性能。在HCCI发动机的单区模型计算中骨架机理计算结果与Curran等人的详细机理计算结果基本相同。     

Modeling HCCI combustion: Modification of a multi-zone model and comparison to experimental results at varying boost pressure

The present study presents a comparison of the results obtained from a modified HCCI multi-zone model to experimental measurements, at different load and boost pressure conditions. The multi-zone model includes a modified sub-model for the wall heat transfer and accounts for the heat transfer between zones. Gas mixing between cold and hot regions of the combustion chamber, which is of major importance for the emissions formation, is also accounted for throughout compression, combustion and expansion. Combustion is modeled using a reduced set of chemical reactions coupled with a chemical kinetics solver. A refined zone configuration near the combustion chamber wall was used, in order to obtain a high resolution at the emissions formation regions. The pressure traces and emissions of nine experimental cases were compared to the multi-zone model results. In these cases the equivalence ratio and the boost pressure were varied, while maintaining constant engine speed. The results show adequate agreement with the pressure traces. The emissions trends are also adequately captured, with the absolute values presenting some deviation from the experimental cases especially for the HC and CO emissions at the relatively low air-fuel equivalence ratios.     

Numerical analysis of two-stroke free piston engine operating on HCCI combustion

An opposed-piston hydraulic free piston engine operating with homogenous charge compression ignition (HCCI) combustion, has been proposed by State Key Laboratory of Engines as a means of significantly improving the IC engine’s cycle thermal efficiency and lowering exhaust emissions. Single and multi-zone Chemkin model with detailed chemical kinetics, and unique piston dynamics extracted from one dimensional gas dynamic model, have been used to analyze the combustion characteristics and engine performance. Intake heating, variable compression ratio and internal EGR are utilized to control the combustion phasing and duration in the cycle simulations, revealing the critical factors and possible limits of performance improvement relative to conventional crank engines. Furthermore, real engine effects such as heat transfer with air swirl, residual mass fraction, thermal stratification, and heat loss fraction between zones are considered in the sequential CFD/multi-zone method to approach the realistic engine performance at an acceptable knock level.     

Heat transfer in HCCI multi-zone modeling: Validation of a new wall heat flux correlation under motoring conditions

The present study focuses on the development and a preliminary validation of a heat transfer model for the estimation of wall heat flux in HCCI engines via multi-zone modeling. The multi-zone model describes heat flow between zones and to the combustion chamber wall. Mass, species and enthalpy transfer, which affect the temperature field within the combustion chamber, are also considered between zones, accounting for the convective heat transfer terms. The multi-zone heat transfer model presented herein has been developed for HCCI combustion simulation and although it has been used in the past, its validation was based on cylinder pressure data under firing conditions. In the present study a more accurate validation of the model is conducted. This is achieved by comparing the multi-zone model heat loss rate predictions to the corresponding predictions of a validated CFD code. The cases examined correspond to actual motoring cases, against which the CFD code has been validated in a previous work. Moreover, a sensitivity analysis is presented, to assess the effect of the zone configuration, i.e. zone thickness and number, on the predicted heat loss rate and temperature profiles. In addition, a comparison is made between the results obtained from the proposed heat flux correlation and one in which the temperature gradient at the wall is approximated via finite differences.     

代理汽油在均质压燃内燃机中燃烧特性的模拟研究

在均质压燃（HCCI）内燃机中,燃烧主要由化学动力学控制。研究燃料的化学动力学反应机理对了解和控制HCCI具有重要意义。本文利用CHEMKIN多区模型,研究了由正庚烷、异辛烷和甲苯混合而成的代理汽油的燃烧特性。计算结果显示,多区模型弥补了单区模型中出现的温度压强陡升缺点,能更好地反映缸内真实燃烧过程。多区温度分布区间越广,则燃烧提前,燃烧持续期长。各区NOx排放和温度分布趋势类似。HC和CO排放主要集中在燃烧不完全的第1区。     

催化燃烧对均质压燃发动机排放影响的数值模拟

通过耦合DETCHEM软件包及CHEMKIN软件包中的SENKIN模块,对活塞顶涂有催化剂的均质压燃(HCCI)发动机的燃烧过程进行了数值计算,建立了多区模型.利用此模型分析了催化燃烧对HCCI发动机缸内温度、热释放速率以及未燃碳氢化合物(UHC)、氮氧化合物(NOx)、一氧化碳(CO)排放的影响,结果表明催化燃烧能降低UHC、CO的排放,但NOx的排放会有所升高.对不同催化剂及混合催化剂对HCCI发动机缸内温度、热释放速率以及UHC、NOx、CO排放的影响进行了探索,结果表明,和金属铂相比,以铑作催化剂时UHC的排放降低,但NOx、CO排放会有所升高;采用500/0Pt-500/0Rh的混合催化剂时,UHC、NOx的排放介于1000/0Pt与1000/0Rh之间,但CO的排放却比采用1000/0Pt与1000/0Rh时都要低.     

The effect of hydrogen addition on combustion and emission characteristics of an n-heptane fuelled HCCI engine

The mechanisms of the influence of hydrogen enrichment on the combustion and emission characteristics of an n-heptane fuelled homogeneous charge compression ignition (HCCI) engine was numerically investigated using a multi-zone model. The model calculation successfully captured the most available experimental data. The results show that hydrogen addition retards combustion phasing of an n-heptane fuelled HCCI engine due to the dilution and chemical effects, with the dilution effect being more significant. It is because of the chemical effect that combustion duration is reduced at a constant compression ratio if an appropriate amount of hydrogen is added. As a result of retarded combustion phasing and reduced combustion duration, hydrogen addition increases indicated thermal efficiency at a constant combustion phasing. Hydrogen addition reduces indicated specific unburned hydrocarbon emissions, but slightly increases normalized unburned hydrocarbon emissions that are defined as the emissions per unit burned n-heptane mass. The increase in normalized unburned hydrocarbon emissions is caused by the presence of more remaining hydrocarbons that compete with hydrogen for some key radicals during high temperature combustion stage. At a given hydrogen addition level, N₂O emissions increases with overly retarding combustion phasing, but hydrogen addition moderates this increase in N₂O emissions.     

Simulation of HCCI combustion in air-cooled off-road engines fuelled with diesel and biodiesel

The present work describes the elaboration of a predictive tool consisting on a phenomenological multi-zone model, applicable to the simulation of HCCI combustion of both diesel and biodiesel fuels. The mentioned predictive tool is created with the aim to be applied in the future to perform engine characterization during both pre-design and post-design stages. The methodology applied to obtain the proposed predictive model is based on the generation of an analytical mechanism that, given a set of regression variables representing the engine operative conditions, provides the user with the optimal figures for the scaling coefficients needed to particularize both the ignition delay and the heat release rate functional laws, which rule the combustion development in the proposed multi-zone model for HCCI engines. The validation of the proposed predictive multi-zone model consists on the comparison between chamber pressure curve derived from the simulations and experimental data based on a DEUTZ FL1 906 unit modified in order to allow HCCI combustion operation mode using diesel EN590 and rapeseed biodiesel. Finally, evidences of the capabilities of the proposed model to be used as a predictive tool applicable to the analysis of off-road engines under HCCI conditions are provided, consisting in the characterization and optimization of the operational maps related to both Brake Specific Fuel Consumption and NOx emissions.