EGR对电喷LPG发动机快速稀薄燃烧的影响

文章介绍了废气再循环(EGR)对电喷LPG发动机快速稀薄燃烧的影响.研究结果表明,EGR的应用,对稀薄燃烧稀限及发动机燃烧特性都有较大的影响.本研究开发的基于高能双火花塞点火的快速燃烧系统可以加快LPG燃气的燃烧速率,有效地改善引入EGR的LPG发动机的快速稀燃过程.     

电控LPG发动机稀燃技术影响因素的试验研究

针对某小型单缸汽油机进行了LPG化改造,介绍了系统的组成并开发了发动机试验控制系统用于LPG发动机稀薄燃烧技术的研究.研究结果表明:不同的燃烧室形式对稀燃热效率和NOx排放水平具有重要影响,在稀燃状态下直口碗型燃烧室比浅碗型燃烧室的排放性和经济性好.发动机的转速、负荷、压缩比、点火提前角以及火花塞间隙等运转或结构参数是影响LPG发动机稀燃极限的主要因素,负荷和转速提高以及压缩比和火花塞间隙增加,均使稀燃界限拓宽;合理的点火提前角也有利于稀限拓宽;通过对各参数的优化可以拓展LPG发动机的稀燃极限,在保持LPG发动机低排放的情况下可将发动机的动力性基本恢复到原汽油机的水平.     

火花点火层状充气发动机的现状及展望

1.前言火花点火层状充气发动机是燃烧室内混合气的浓度分布发生层状变化,用装在可燃混合气层的火花塞点火,而后火焰沿着扩散形成的可燃层向混合气传播的方式工作的发动机。采用这种方式,总空燃比虽然很稀,已超过了可燃极限,但在局部却可形成可燃混合气层。因而不同于供给相同浓度混合气的普通汽油机,部分负荷时无需节流进气。     

天然气/氢气燃烧特性研究

在定容燃烧弹中研究了不同氢气掺混比例、燃空当量比和初始压力下的大然气／氢气混合气的燃烧特性,建立了适合用于容弹计算的准维双区模型。研究结果表明：在各种当量比和初始压力下,随着掺氢比例的增加,混合气的质量燃烧速率明显增加,燃烧持续期和火焰发展期娃著缩短。随着掺氢比例的增加,短的燃烧持续期所对应的当量比范围变宽,稀混合气和浓混合气条件下天然气掺氢对火焰发展期缩短的效果更明显。化学计量比附近（1．0—1．1）掺氢燃烧对燃烧最大压力值影响不大,浓混合气（燃空当量比大于1．1）和稀混合气燃烧时,随着掺氢比例的增加,最大燃烧压力值增加。     

喷雾-壁面复合引导LPG直喷发动机混合气形成过程的数值解析

利用液化石油气(LPG)优良的燃料特性,提出了一种新的喷雾-壁面复合引导燃烧系统.由激光诱导荧光法(PLIF)验证了计算模型准确性后,使用Fire V2011软件,对复合引导燃烧系统喷油器安装位置、燃料喷射方向和活塞凹坑形状等参数进行了优化,并确定了燃烧系统参数.最后数值解析了典型的部分负荷工况(2,000,r/min)喷雾-壁面复合引导LPG直喷发动机分层混合气的形成过程.结果表明:采用压缩冲程后期(40°,CA BTDC)喷射,在接近点火时刻(20°,CA BTDC)时从3孔喷油器的上部2孔喷出的LPG喷雾和下部1孔喷入燃烧室凹坑,在卷吸涡作用下引导至火花塞电极附近的LPG喷雾将交织在一起,在火花塞电极附近形成稳定的混合气分层构造;而整个燃烧室的空然比达60以上.     

缸内直喷CNG发动机喷射方式对火焰传播特性及性能的影响

火焰传播是点燃式发动机的主要燃烧方式,为了有效控制缸内直喷CNG发动机更稳定的稀薄燃烧过程,以实现高效率低排放,利用试验用单缸光学发动机,采用双喷射器缸内直喷CNG方式,在双火花塞点火的条件下,分析研究了不同喷射方式对火焰形成及传播特性以及发动机性能的影响.结果表明,双点点火时,缸内气流对首先形成的火焰具有"牵引"作用,而两个火焰由于形成时刻不同传播方式就有区别,且两个火焰之间具有"挤压或推动"作用,由此影响整体火焰传播速度;缸内火花塞附近混合气浓度越浓或不均匀性越大,循环波动越小,燃烧更稳定,但随混合气浓度分布不均匀性的增加,NOx排放也增加,表明基于混合气浓度不均匀性分布特性的燃烧过程中,NOx的生成机理不仅与高温富氧条件有关,而且还与浓度场分布特性有直接的关系.     

火花点火发动机稀混合气燃烧和失火界限

大家知道,使用稀混合气可以降低一氧化碳(CO)和氮氧化物(NO)等排放物,同时可以保持良好的燃料经济性。但是由于失火和燃烧不正常会引起运转不稳定,从而防碍了发动机使用很稀的混合气。过去为了扩大使用稀混合气的界限,把注意力集中在点火系统上,特别是改变火花塞的设计、火花能量及放电时间等。对于混合气的紊流也进行了研究。不过这方面的工作不太深入。     

压燃式天然气发动机着火和敲缸的试验研究

提出了一种压燃式天然气发动机燃烧系统。该燃烧系统采用了低散热的分隔式燃烧室和复合供气系统,即利用分别安装于进气管和气缸盖上的高、低压天然气喷射阀在一个工作循环中的分时供气,以在副燃烧室内形成较浓的混合气,在主燃烧室内形成稀混合气。在接近压缩终点处,副室内的混合气首先着火,其火焰喷入主燃烧室点燃其中的稀混合气。在单缸试验机上研究了这一燃烧系统的着火起动特性和敲缸现象。试验结果说明：仅采用进气道低压喷射天然气的供气方式在发动机气缸内形成天然气／空气的均质混合气,可很容易地实现压缩着火和起动发动机;电热塞温度、进气温度及副室与主室之间通道尺寸对发动机的着火和起动性有显著的影响,可以实现仅利用电热塞辅助加热即可在常温进气条件下起动发动机。在主、副燃烧室内实现混合气浓度的时间-空间控制,以实现混合气浓度分层,有助于避免敲缸现象。     

LPG单一燃料电控发动机冷起动控制的研究

根据LPG发动机冷起动的特点，设计了LPG发动机冷起动控制策略，在4缸直列单一LPG气态进气道多点顺序喷射发动机上进行了9℃、15℃和20℃的冷起动试验研究。试验结果表明：通过优化初始燃油喷射脉宽、起动喷射脉宽衰减梯度、初始节气门开度、节气门开度衰减梯度等控制参数，可以实现LPG发动机燃烧稳定、起动可靠，并尽可能缩短浓混合气供给时间；为保证可靠起动，LPG发动机和汽油发动机在起动阶段都需要浓混合气，但相对于汽油机，温度对LPG发动机能够成功起动的初始喷气脉宽的影响不是很明显，使用LPG燃料可以缩短起动阶段提供浓混合气的时间，起动过程中为产生相同的转矩，LPG发动机需要较大节气门开度。     

火花点火甲醇发动机工作过程多维数值模拟

研究对象为一台由柴油机改造的火花点火甲醇发动机,应用AVL-FIRE软件的FEP技术建立进气道-进气门-气缸部分三维动态网格,建立数学模型并验证了模型的可行性;详细分析了缸内流体速度场分布,甲醇与空气的混合气燃烧火焰扩散及传播过程;研究了火花塞位置对甲醇发动机燃烧过程的影响,结果表明火花塞设置在原柴油机喷油器处较为合理...     

Influence of laser ignition on characteristics of an engine for hydrogen enriched CNG and iso-octane usage

The study has focused on determining the laser plug effects on engine characteristics and the laser plug usage results have compared with spark plug usage. The laser ignition technique is a type of new ignition technique and an important solution that can make combustion systems more efficient. The testing of an engine with a laser plug is the novelty of the study and the tests were carried out with reference to equivalence ratio and plug power ranges. The behaviors of the engine at full load were examined so experimentally for both ignition techniques at hydrogen enriched CNG and iso-octane mixture usage. The tests were carried out for variations of 0.4–2.0 equivalence ratio and 20–120 W plug power. A mixture that 90% iso-octane and 10% HCNG in mass was used at two ignition modes in tests for 3300 rpm maximum engine torque speed. Also, the flame formation and propagation for both ignition techniques were detected via a high-speed camera. The tests have shown the laser ignition leads to more energy consumption in the rich mixture conditions and also, less energy is required in the lean conditions. The laser ignition discharge has extended the engine's lean combustion limits via a small energy input at the tests. The high-speed camera images have shown that the laser ignition reduces the Kernel flame formation and propagation time. The laser ignition technique was produced less NO_x than the conventional spark ignition method.     

Improvement on the combustion of a hydrogen fueled engine

More and more attention is being paid to hydrogen as a future fuel. Hydrogen has the intrinsic physical—chemical characteristics of wide ignition limits, high flame propagation rate and low ignition energy. By using hydrogen, the combustion process in engines can be improved and the rapid burning of the lean mixture in them can be realized, thus leading to improvements both in fuel economy and in emission characteristics of the original engine. The experimental results of burning hydrogen fuel, hydrogen-gasoline mixed fuel and hydrogen-diesel oil mixed fuel in the engine to improve the combustion processes are discussed.     

点燃式发动机掺烧甲醇裂解气的研究

摘要甲醇裂解气(D.M)是甲醇在一定温度下发生裂解反应的产物(2H_2+CO),而发动机排气余热提供甲醇蒸发和裂解所需热量.当发动机使用汽油和富氢的甲醇裂解气时,能在较稀混合气下运行;为了获得更稀的混合气,对发动机进行了补气实验.结果表明,燃用混合燃料时热效率有较大的改善,燃烧稳定性加强.通过对示功图和放热规律的分析,明确了发动机经济性提高的原因.     

On the ignition and flame development in a spray-guided direct-injection spark-ignition engine

High-speed fuel, flow, and flame imaging are combined with spark discharge measurements to investigate the causes of rare misfires and partial burns in a spray-guided spark-ignited direct-injection (SG-SIDI) engine over a range of nitrogen dilution levels (0–26% by volume). Planar laser induced fluorescence (PLIF) of biacetyl is combined with planar particle image velocimetry (PIV) to provide quantitative measurements of equivalence ratio and flow velocity within the tumble plane of an optical engine. Mie scattering images used for PIV are also used to identify the enflamed region to resolve the flame development. Engine parameters were selected to mimic low-load idle operating conditions with stratified fuel injection, which provided stable engine performance with the occurrence of rare misfire and partial burn cycles. Nitrogen dilution was introduced into the intake air, thereby displacing the oxygen, which destabilized combustion and increased the occurrence of poor burning cycles. Spark measurements revealed that all cycles exhibited sufficient spark energy and duration for successful ignition. High-speed PLIF, PIV, and Mie scattering images were utilized to analyze the spatial and temporal evolution of the fuel distribution and flow velocity on flame kernel development to better understand the nature of poor burning cycles at each dilution level. The images revealed that all cycles exhibited a flammable mixture near the spark plug at spark timing and a flame kernel was present for all cycles, but the flame failed to develop for misfire and partial burn cycles. Improper flame development was caused by slow flame propagation which prevented the flame from consuming the bulk of the fuel mixture within the piston bowl, which was a crucial step to achieve further combustion. The mechanisms identified in this work that caused slower flame development are: (1) lean mixtures, (2) external dilution, and (3) convection velocities that impede transport of the flame into the fuel mixture.     

火花点火式天然气发动机燃烧系统的研究

本介绍了天然气发动机预燃室燃烧系统的研制过程，新设计的燃烧系统工作可靠、性能优良。试验结果表明：预燃室内混合气着火稳定、火焰传播迅速；主燃室燃烧速度高、持续期短、无后燃现象，整机性能比原机有较大的提高。     

利用快速压缩装置进行直喷天然气发动机燃烧特性的研究

利用快速压缩装置开展了直喷天然气发动机燃烧特性的研究,分析了3种不同喷射方式下的燃烧特性并与均相混合气燃烧进行了对比。研究结果:上喷天然气燃烧比均相混合气燃烧的最大压力高,在宽广的当量比范围内具有短的火娄发展期和快速燃烧,克燃烧放热率和压力升高率基本上与喷射方式无关。喷射方式与均相混合气相比,燃烧放热率,压力升高率大。缩短喷油和点火间的时间间隔将缩短火焰发展期和快速燃烧期,其时间间隔的优化对直喷天然气发动机极为重要。直喷天然气发动机的燃烧方式为预混控制充量分层燃烧,此燃烧方式燃烧速率,排放低。     

Understanding ignition processes in spray-guided gasoline engines using high-speed imaging and the extended spark-ignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark re-strikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitz-number criteria).In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the G-equation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuel-rich regions along the spark channel. In this highly-turbulent highly-stratified environment, these ignition spots burn as quasi-laminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (cross-over temperature based) and turbulence (Karlovitz-number based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis.The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highly-turbulent highly-stratified engine conditions, (b) incorporate that understanding into a physically-based submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physically-based modeling of cycle-to-cycle variability in engines.     

Modelling of hydrogen-blended dual-fuel combustion using flamelet-generated manifold and preferential diffusion effects

In the present study, Reynolds-Averaged Navier-Stokes simulations together with a novel flamelet generated manifold (FGM) hybrid combustion model incorporating preferential diffusion effects is utilised for the investigation of a hydrogen-blended diesel-hydrogen dual-fuel engine combustion process with high hydrogen energy share. The FGM hybrid combustion model was developed by coupling laminar flamelet databases obtained from diffusion flamelets and premixed flamelets. The model employed three control variables, namely, mixture fraction, reaction progress variable and enthalpy. The preferential diffusion effects were included in the laminar flamelet calculations and in the diffusion terms in the transport equations of the control variables. The resulting model is then validated against an experimental diesel-hydrogen dual-fuel combustion engine. The results show that the FGM hybrid combustion model incorporating preferential diffusion effects in the flame chemistry and transport equations yields better predictions with good accuracy for the in-cylinder characteristics. The inclusion of preferential diffusion effects in the flame chemistry and transport equations was found to predict well several characteristics of the diesel-hydrogen dual-fuel combustion process: 1) ignition delay, 2) start and end of combustion, 3) faster flame propagation and quicker burning rate of hydrogen, 4) high temperature combustion due to highly reactive nature of hydrogen radicals, 5) peak values of the heat release rate due to high temperature combustion of the partially premixed pilot fuel spray with entrained hydrogen/air and then background hydrogen-air premixed mixture. The comparison between diesel-hydrogen dual-fuel combustion and diesel only combustion shows early start of combustion, longer ignition delay time, higher flame temperature and NOx emissions for dual-fuel combustion compared to diesel only combustion.     

涡流室式柴油机燃用纯甲醇的研究

本文介绍了涡流室式柴油机采用电热塞点火方式燃用纯甲醇的研究.通过设计以电热塞为热源的热面点火系统,采用部分稳定氧化锆(PSE)涂层的涡流室,以及改变涡流室通道的形状,使发动机燃用甲醇时的性能得到改善,在各工况下均降低了电热塞的功率和燃料消耗率(按热值计).发动机燃用甲醇时的NO_x排放量比燃用柴油时显著降低,但HC和CO排放量却有一定程度的上升.     

Improvement of thermal efficiency and reduction of NOx emissions by burning a controlled jet plume in high-pressure direct-injection hydrogen engines

A new combustion process called the Plume Ignition Combustion Concept (PCC), in which the plume tail of the hydrogen jet is spark-ignited immediately after the completion of fuel injection to accomplish combustion of a rich mixture has been proposed by the authors. This PCC combustion process markedly reduces nitrogen oxides (NOx) emissions in the high-output region while maintaining high levels of thermal efficiency and power. On the other hand, as burning lean mixture of fuel and air is the conventional way to improve thermal efficiency and reduce NOx, a high λ premixed mixture of hydrogen and air formed by injecting hydrogen in the early stage of the compression stroke has been used in direct-injection hydrogen engines. It was recently reported, however, that this mixture condition does not always offer expected improved thermal efficiency under even lean mixture conditions by increasing unburned hydrogen emissions caused by incomplete flame propagation in the non-uniform and extremely lean portion of the mixture. In this study, the effect of retarding the injection timing to late in the compression stroke but slightly advanced from original PCC was examined as a way of reducing unburned hydrogen emissions and improving thermal efficiency. These effects result from a centroidal axially stratified mixture that positions a fairly rich charge near the spark plug. This stratified mixture is presumably effective in reducing incomplete flame propagation thought to be the cause of unburned hydrogen emissions and also promoting increasing burning velocity of the mixture that improve thermal efficiency. Finally, this research is characterized by measuring the hydrogen fuel concentration at the point and the time of spark ignition quantitatively by spark-induced breakdown spectroscopy in order to identify the changes in mixture ratio mentioned above caused by the parameters involved.