Development and validation of a comprehensive two-zone model for combustion and emissions formation in a DI diesel engine

A two-zone model for the calculation of the closed cycle of a direct injection (DI) diesel engine is presented. The cylinder contents are taken to comprise a non-burning zone of air and another homogeneous zone in which fuel is continuously supplied from the injector holes during injection and burned with entrained air from the air zone. The growth of the fuel spray zone, consisting of a number of fuel–air conical jets equal to the injector nozzle holes, is carefully modelled by incorporating jet mixing to determine the amount of oxygen available for combustion. Application of the mass, energy and state equations in each one of the two zones yields local temperatures and cylinder pressure histories. For calculating the concentration of constituents in the exhaust gases, a chemical equilibrium scheme is adopted for the C–H–O system of the 11 species considered, together with chemical rate equations for the calculation of nitric oxide (NO). A model for the evaluation of soot formation and oxidation rates is incorporated. A comparison is made between the theoretical results from the computer program implementing the analysis, with experimental results from a vast experimental investigation conducted on a fully automated test bed, direct injection, standard ‘Hydra’, diesel engine located at the authors' laboratory, with very good results, following a multi-parametric study of the constants incorporated in the various sub-models. Pressure indicator diagrams and plots of temperature, NO, soot density and of other interesting quantities are presented as a function of crank angle, for various loads and injection timings, elucidating the physical mechanisms governing combustion and pollutants formation. Copyright © 2003 John Wiley & Sons, Ltd.     

Validation and sensitivity analysis of a two zone Diesel engine model for combustion and emissions prediction

The present two zone model of a direct injection (DI) Diesel engine divides the cylinder contents into a non-burning zone of air and another homogeneous zone in which fuel is continuously supplied from the injector and burned with entrained air from the air zone. The growth of the fuel spray zone, which comprises a number of fuel-air conical jets equal to the injector nozzle holes, is carefully modelled by incorporating jet mixing, thus determining the amount of oxygen available for combustion. The mass, energy and state equations are applied in each of the two zones to yield local temperatures and cylinder pressure histories. The concentration of the various constituents in the exhaust gases are calculated by adopting a chemical equilibrium scheme for the C–H–O system of the 11 species considered, together with chemical rate equations for the calculation of nitric oxide (NO). A model for evaluation of the soot formation and oxidation rates is included. The theoretical results from the relevant computer program are compared very favourably with the measurements from an experimental investigation conducted on a fully automated test bed, standard “Hydra”, DI Diesel engine installed at the authors’ laboratory. In-cylinder pressure and temperature histories, nitric oxide concentration and soot density are among the interesting quantities tested for various loads and injection timings. As revealed, the model is sensitive to the selection of the constants of the fuel preparation and reaction sub-models, so that a relevant sensitivity analysis is undertaken. This leads to a better understanding of the physical mechanisms governed by these constants and also paves the way for construction of a reliable and relatively simple multi-zone model, which incorporates in each zone (packet) the philosophy of the present two zone model.     

喷雾锥角对柴油机燃烧过程影响的数值模拟

柴油机燃烧过程中喷雾内部的物理化学过程非常复杂,传热、蒸发、扩散、流动等物理过程控制着化学反应,影响着火和燃烧过程,进而决定着发动机的动力性、经济性以及排放性能。利用CFD分析软件FIRE对一台直列6缸增压柴油机的喷雾与燃烧过程进行模拟。研究了喷雾锥角对燃烧过程的影响规律以及喷雾锥角对碳烟和NOx生成的影响。     

柴油机加速工况时NOx排放模拟计算

在油滴蒸发准维燃烧模型基础上,结合生成NOx的详细化学动力学机理,建立了直喷式柴油机的NOx生成模型,与实测结果对比有较好的吻合性。利用此模型对柴油机加速过程NOx排放进行了模拟计算,计算结果表明：柴油机加速工况时的NOx浓度值变化较大,有的比标定工况时还大。     

Modeling study of soot formation and oxidation in DI diesel engine using an improved soot model

Particulate emission is one of the most deleterious pollutants generated by Diesel fuel combustion. The ability to predict soot formation is one of the key elements needed to optimize the engine performance and minimize soot emissions. This paper reports work on developing, a phenomenological soot model to better model the physical and chemical processes of soot formation in Diesel fuel combustion. This hybrid model features that the effect of turbulence on the chemical reaction rate was considered in soot oxidation. Soot formation and oxidation processes were modeled with the application of a hybrid method involving particle turbulent transport controlled rate and soot oxidation rate. Compared with the original soot model, the in-cylinder pressures, heat release rate and soot emissions predicted by this hybrid model agreed better with the experimental results. The verified hybrid model was used to investigate the effect of injection timing on engine performance. The results show that the new soot model predicted reasonable soot spatial profiles within the combustion chamber. The high temperature gas zone in cylinder for hybrid model case is distributed broadly soot and NOx emission dependence on the start-of-injection (SOI) timing. Retarded SOI timing increased the portion of diffusion combustion and the soot concentration increased significantly with retarding of the fuel injection timing. The predicted distributions of soot concentration and particle mass provide some new insights on the soot formation and oxidation processes in direct injection (DI) engines. The hybrid phenomenological soot model shows greater potential for enhancing understanding of combustion and soot formation processes in DI diesel engines.     

柴油机混合气形成条件对瞬态火焰温度和碳烟质量浓度影响的研究

进气涡流强度、喷嘴的喷孔直径与个数、喷油压力等直接影响直喷式柴油机燃烧室内混合气形成与燃烧过程的进行，从而影响柴油机的动力性、燃油经济性和碳烟排放。作者用同步光导纤维四色法测量系统，研究了进气涡流、喷孔直径和个数对燃烧过程中火焰温度和碳烟浓度的影响。本文介绍的是所进行的研究工作和得到的结果。     

基于CFD的增压柴油机不同喷油提前角的排放影响分析

在考虑进气涡流比的情况下,对某490增压直喷柴油机不同喷油提前角下的缸内喷雾及燃烧过程进行了三维CFD模拟,给出了4种不同喷油提前角时的缸内火焰、NOx和Soot的时空分布.计算和分析了不同的喷油提前角下,上止点前后喷入缸内的油量比例、着火时刻、上止点温度、最高燃烧温度等对燃烧与排放物的影响.结果表明上止点前后的油量比例对排放有较大影响,对于该机型在喷油提前角较小的情况下,应考虑适当减小喷油锥角以充分利用燃烧室底部的空气.仅靠喷油提前角的变化并不能同时降低NOx和Soot的排量,并通过性能与排放试验进行了验证.     

直喷柴油机喷油系统对燃烧和排放影响的三维模拟研究

直喷柴油机的喷油过程影响燃烧过程、影响发动机性能和污染物排放。为了研究喷油特性(如不同的喷油压力、喷油定时和喷孔直径等)对燃烧性能的影响,利用三维数值模拟软件K IVA-3代码,对喷油过程、燃烧过程和发动机性能进行了数值模拟。计算结果显示了不同喷油特性下的NO和碳烟排放的变化趋势。     

直喷式柴油机的碳烟预测模型

本文建立了一个直喷式柴油机的多区准维油滴蒸发燃烧模型,在此基础上采用Arrhenius型公式对实际过程中极为复杂的碳烟生成历程进行了模拟。在一台四冲程直喷式柴油机上对该模型进行了标定和试验验证,试验结果表明该模型对发动机变工况、变参数运行时的碳烟生成量有较好的预测能力。     

直喷式柴油机燃烧室形状对碳粒形成过程影响的试验研究

本文介绍了作者采用可视化技术和图象分析技术研究直喷式柴油机燃烧室形状对燃烧室内喷雾场和燃烧碳粒场影响的研究结果。分析了四角形燃烧室和盆形燃烧室喷雾发展历程和燃烧碳粒场发展历程，找出了四角形燃烧室碳粒排放量较低的原因。     

直喷柴油机碳烟生成和氧化历程的数值研究

提出了研究碳烟的生成和氧化历程及工作条件影响的方法。将Surovikin碳烟生成模型和Nagle碳烟氧化模型集成在ICFD-CN源程序中构建多维模型，采用数值循环模拟的方法，根据不同的工作条件计算碳烟的生成和氧化历程。结合试验数据分析和比较进气温度、空燃比、负荷、转速和喷油提前角对碳烟生成和氧化历程的影响。研究表明：用数值模拟的方法反映了1137柴油机碳烟的生成和氧化历程以及设计参数对碳烟的生成和氧化的影响规律，对柴油机的设计具有参考价值。     

正丁醇/柴油发动机燃烧排放特性及喷油、燃烧室形状影响

针对某型号直喷柴油机,建立了该柴油机中单缸完整燃烧室及气道三维模型,使用三维计算流体力学(computational fluid dynamics,CFD)分析软件CONVERGE对其进行模拟计算,研究了正丁醇掺混比例对柴油机燃烧排放的影响。结果表明:随着正丁醇掺混比例的提高,峰值缸压、滞燃期和燃烧速度均呈递增趋势,碳烟及CO排放量逐渐减少,NO_x排放量小幅增加。为了进一步改善缸内燃烧情况和降低污染物排放,对正丁醇掺混时喷油策略、燃烧室几何形状的综合影响进行了研究,结果表明:掺混时多次喷油及采用合适的燃烧室模型可以有效改善掺混后缸内油气混合情况,增加缸内湍动能强度,进一步降低碳烟排放量。与纯柴油工况对比,掺混并采用多次喷油策略后碳烟排放明显下降,且通过掺混能够有效简化喷油策略,但弱化了燃烧室形状对碳烟排放量的影响。     

高强化柴油机涡流比与喷油压力匹配的多维燃烧模拟研究

采用CFD模拟软件FIRE对高强化柴油机缸内工作过程进行了模拟,分析了涡流比和喷油压力对缸内燃烧过程的影响。在此基础上对涡流比和喷油压力进行了综合优化匹配模拟,为小缸径柴油机燃烧系统的设计提供了理论依据。     

Heat Transfer in Diesel and Partially Premixed Combustion Engines; A Computational Fluid Dynamics Study

This article presents findings from a Computational Fluid Dynamics (CFD) study performed on the heat transfer characteristics of diesel and partially-premixed combustion (PPC) engines. The study is confined to the combustion bowl, where numerical simulations have been performed on a part of the engine cycle, namely the compression, combustion, and expansion phases. Three engine geometries were simulated and after validating the results with experimental data, parameter variations were carried out, in order to estimate their effects on the heat transfer, engine performance, and emission levels. The work was performed using a commercial CFD tool, with which only a part of the engine cylinder was modeled, the enclosure of one spray. The results highlight some important characteristic differences between the conventional diesel combustion and the low-temperature combustion scheme PPC. The reduced in-cylinder temperatures for the PPC case lead to a reduced production of NO_x and soot emissions, without compromising the engine performance, only a small penalty in the increased intake air pressure is found. The importance of an appropriate injection strategy was also highlighted, as the presence of a pilot injection during the compression stroke enhanced the temperature stratification in a PPC engine. This leads to reduced heat losses and improved engine efficiency. Finally, the shape of the combustion bowl was shown to have significant effects on both heat losses as well as emission levels.     

Soot processes in compression ignition engines

While diesel engines are arguably superior to any other power-production device for the transportation sector in terms of efficiency, torque, and overall driveability, they suffer from inferior performance in terms of noise, NO_x and particulate emissions. The majority of particulate originates with soot particles which are formed in fuel-rich regions of burning diesel jets. Over the past two decades, our understanding of the formation process of soot in diesel combustion has transformed from inferences based on exhaust measurements and laboratory flames to direct in-cylinder observations that have led to a transformation in diesel engine combustion. In-cylinder measurements show the diesel spray to produce a jet which forms a lifted, partially premixed, turbulent diffusion flame. Soot formation has been found to be strongly dependent on air entrainment in the lifted portion of the jet as well as by oxygen in the fuel and to a lesser extent the composition and structure of hydrocarbons in the fuel. Soot surviving the combustion process and exiting in the exhaust is dominated by soot from fuel-rich pockets which do not have time to mix and burn prior to exhaust valve opening. Higher temperatures at the end of combustion enhance the burnout of soot, while high temperatures at the time of injection reduce air entrainment and increase soot formation. Using a conceptual model based on in-cylinder soot and combustion measurements, trends seen in exhaust particulate can be explained. The current trend in diesel engine emissions control involves multi-injection combustion strategies which are transforming the picture of diesel combustion rapidly into a series of low temperature, stratified charge, premixed combustion events where NO_x formation is avoided because of low temperature and soot formation is avoided by leaning the mixture or increasing air entrainment prior to ignition.     

船用直喷式天然气发动机喷射策略的优化

利用CONVERGE软件基于L23/30DF型船用天然气发动机建立了双天然气喷嘴、双引燃柴油喷嘴的直喷天然气发动机的缸内燃烧过程的CFD计算模型,计算了不同的柴油和天然气喷射时刻和间隔下发动机缸内燃烧和排放过程。结果表明:引燃柴油的喷射时刻及其与天然气喷射时刻的间隔,对直喷式天然气发动机燃烧和排放性能有重要影响。当喷射时刻为-25℃A时,发动机具有较高的缸内爆发压力和良好的排放性能。在引燃柴油和天然气喷射间隔为16℃A时,可获得最高的缸内爆发压力,此时soot排放降低了22%。     

直喷式柴油机缸内热辐射多区(多维)模型的研究

以准维现象学多区喷雾燃烧模型和碳粒生成预测了模型为基础,建立了缸内空间辐射多区（多维）模型,并以Ｇ４１３５直喷式柴油机为研究对象,用蒙特卡洛（Ｍｏｎｔｅ－Ｃａｒｌｏ）法计算和分析了燃烧室壁面辐射热量的分布,结果表明,热流量分布规律和数值与柴油机缸内燃烧过程,有关试验结果相符。     

现象学燃烧分析模型及其在柴油机燃烧研究中的应用

本文提出一个新的直喷式柴油机燃烧分析模型,该模型利用台架实验数据计算燃空混合率、放热率、NO_x生成率等燃烧过程参数,研究燃烧系统参数对燃烧过程的影响,分析充量间传热传质过程及其影响因素。本模型已用于天津动力机厂130柴油机燃烧系统改进的研究。结果表明,强化扩散燃烧阶段缸内空气与产物间紊流混合率可减少NO_x排放量,预计适当提高130柴油机空气涡流将会改善高、低转速工况的综合排放特性。     

二甲醚发动机燃烧模型的建立

根据直喷式四冲程柴油机的工作特点,结合传统柴油机的燃烧模型,建立适合于二甲醚发动机的燃烧模型,并进行模拟计算。模拟结果与试验结果比较吻合,表明该模型具有一定的适用性。     

小缸径直喷式柴油机喷雾,燃烧和碳粒生成过程试验研究

本介绍了作采用同步高速摄影技术和图象分析技术研究柴油机缸内碳粒生成和氧化过程的研究结果。研究结果表明：在柴油机燃烧过程中碳粒最早出现在燃烧室内有空气渗混的富油区；速燃期内高温富碳燃烧区的形状与着火前一时刻喷雾场的形状相吻合；富油区大直径油滴的燃烧裂解和活塞顶隙处外溢燃气的低温燃烧是柴油机产生碳烟微粒排放的主要原因。