激光诱导火花点火和电火花点火着火过程的对比研究

应用高速纹影成像技术研究调Q Nd:YAG纳秒脉冲激光诱导火花点燃汽油/空气混合气的火核发展过程,并与传统电火花点火的火核发展过程相对比,详细阐述了激光诱导火花点火过程中有别于电火花点火过程的球形火核发展的三瓣结构火核的形成机理。进一步研究了0.1MPa、363K条件下,激光诱导火花、电火花点燃不同当量比下的汽油/空气混合气的着火概率。研究结果表明:激光诱导火花点火可以提高燃烧速度,并且能扩展稀燃极限,其在点燃当量比0.6(电火花点火稀燃极限)的稀混合气时,着火概率达65%~70%。     

利用火花点火式快速压缩装置进行天然气直喷分层燃烧可行性的研究

利用快速压缩装置进行了天然气直喷分层燃烧可行性的研究。结果表明 :天然气分层燃烧具有短的初期火焰发展期和主燃期以及高的燃烧压力。分层燃烧可使稀燃极限延伸到很小的当量比。由于过度分层 ,CO在当量比大于 0 .8时急剧增加 ,而 NOx 的峰值也因充量分层而出现在小当量比处。燃烧效率在当量比处于0 .1～ 0 .9范围时高于 0 .92 ,在极小当量比时由于未燃混合气淬熄 ,在当量比时由于过度分层而使燃烧效率降低。根据燃烧产物计算的燃烧效率与根据放热分析获得的燃烧效率一致。因此 ,天然气直喷分层燃烧在宽广的当量比范围内可望实现高效燃烧火花助燃发动机的宽广的高效燃烧。     

氢内燃机燃烧过程的数值模拟

基于三维CFD仿真软件FIRE,建立单缸进气歧管喷射的氢内燃机三维仿真模型,在验证仿真模型的有效性的基础上,重点研究了点火提前角和当量燃空比对氢内燃机燃烧及排放的影响。分析结果表明,推迟点火提前角,缸内最高压力不断增大。提高当量比会使燃烧放热率升高,燃烧持续期缩短,但过大的当量比易发生异常燃烧。延迟点火并配合稀燃技术可以降低NOx排放。     

直喷天然气发动机燃烧效率分析

利用快速压缩装置对直喷天然气发动机的效率进行了分析。在宽广的当量比范围内，分析了三种燃料喷射方式下和均匀混合气燃烧时的燃烧效率。结果表明，燃油喷射方式下的燃烧效率在0．2－0．9当量比范围内均具有较高的数值并与喷射方式无关；在当量比小于0．2和大于0．9时，由于CO的原因，使燃烧效率降低。均匀混合气燃烧时，燃烧效率在当量比大于0．7时较高，而当量比小于0．7时，由于很高的未燃甲烷的生成使燃烧效率损失较大。燃料喷射燃烧与均匀混合气燃烧相比，维持高燃烧的比范围宽。因未燃甲烷的生成造成的燃烧效率的损失与喷油时刻无关，因CO造成的燃烧效率的损失随喷油滞后而增加。     

直喷二甲醚对稀汽油混合气燃烧过程影响的研究

在一台四冲程单缸汽油机上,通过缸内直喷二甲醚(DME)实现了空气稀释汽油混合气的稳定燃烧。研究结果表明:在1 500r/min下,固定循环燃油热值时,直喷DME可以降低汽油机稀燃下的循环变动,加速初期火焰发展速度,缩短燃烧持续期,提高汽油稀燃稳定燃烧的过量空气系数上限。稀燃和直喷DME相结合可以改善发动机在稀燃下的燃油经济性。与理论空燃比混合气相比,稀燃能使指示燃油消耗率最多降低11.7%。改变点火时刻和直喷DME比例能实现不同过量空气系数下的最佳燃烧相位。随着过量空气系数的增加,最佳放热中心相位提前。     

预燃室射流点火对汽油发动机性能影响

基于一台单缸汽油发动机,设计了主动预燃室系统,试验了预燃室混合气状态对燃烧及排放的影响,通过对比不同点火能量的火花塞点火和预燃室点火,明确预燃室射流点火对燃烧过程影响机理．结果表明：随着预燃室内喷油量的增加,颗粒物数量(PN)排放增加;预燃室内浓混合气能改善燃烧相位、加快燃烧速度,提高点火性能,但预燃室内当量比附近的混合气有更大的节油潜力．当全局过量空气系数φglobal小于1.4时,预燃室点火燃油消耗率恶化;当φglobal大于1.4时,预燃室改善热效率的能力开始凸显．当预燃室中燃油量占总循环油量的分数为2%时,预燃室点火能将稀燃极限扩展至φglobal为2.1,在φglobal为1.8时总指示热效率达到48.5%的最大值．     

不同过量空气系数下射流点火发动机燃烧、排放和爆震特性的试验研究

基于单缸试验机研究了过量空气系数对射流点火发动机性能的影响.通过分析发动机性能曲线、缸内燃烧情况及爆震特性探究射流点火最佳运行区间,并与火花点火燃烧方式进行对比.结果表明,射流点火可以有效提升瞬时放热率并拓展发动机稀燃极限,缩短缸内混合气滞燃期与燃烧持续期,同时燃油经济性有一定提升.在稀燃条件下氮氧化物排放极低.爆震方...     

稀燃天然气掺氢发动机的热效率与排放特性

为了分析在天然气中掺入不同体积比的氢气对发动机经济性和排放性的影响,在一台6缸火花点火天然气发动机上开展了体积掺氢比在不同工况下对热效率和排放特性影响的试验研究.结果显示掺氢可以拓宽发动机的稀燃极限,提高燃烧速度,使得最佳转矩点火提前角(MBT)相对推迟;在点火提前角不变的情况下掺氢对热效率没有明显优势,而且会使NOx排放升高.而在MBT时,掺氢可以一定程度上提高发动机的指示热效率,降低未燃CH4和CO的排放,改善NOx与未燃碳氢(主要为CH4)的trade-off关系.掺氢的优势还体现在可以让发动机高效的工作在更稀的情况下,从而有利于降低NOx的排放和传热损失.     

用PLIF法研究缸内直喷发动机稀燃分层与当量比分层策略

采用平面激光诱导荧光法定量研究缸内直喷汽油机在稀燃分层和当量比分层的不同控制策略下火花塞附近的混合气分布,并根据图像结果讨论了多种策略下火花塞附近燃空比分布状态和循环变动状态,以及对混合气分布情况的评价方法.对于平均当量比为0.4的稀燃策略,恰当的二次喷射可以在火花寒附近适宜点火的浓区.当量比下大部分策略都能在火花塞附近达到可以点火的混合气浓度,但缸内浓区中心的位置不同.对当量比下100°CA BTDC第二次喷射的3种策略的分析表明,6:1喷射形成分层均质效果相对最好.对连续多个循环的循环变动的分析表明,大部分情况下火花塞附近混合气最高浓度的变动值小于±15%,部分策略产生的循环变动在±10%以内.     

主动预燃室式直喷汽油机稀薄燃烧性能研究

为明晰不同点火方式对汽油机稀薄燃烧特性的影响规律,在一款排量为0.5L的研究型单缸机上试验研究了传统火花塞和主动预燃室两种不同点火方式下发动机燃烧及排放特性,探索主动预燃室拓展稀薄燃烧极限的多种影响因素。研究结果表明,稀薄燃烧可有效降低油耗,提高发动机热效率。传统点火线圈的稀燃极限处于过量空气系数1.5附近,最高指示热效率为45.0%,而采用主动预燃室系统后,稀燃极限可进一步拓展,过量空气系数可达2.0,指示热效率提升至46.5%,氮氧化物排放比采用传统火花塞点火技术时降低约88%;主动预燃室匹配高压缩比14.80的燃烧系统,可进一步拓展稀燃极限至过量空气系数2.1,指示热效率可达48.0%,氮氧化物排放继续降低,在过量空气系数采用2.1时NOx排放最低可达58×10-6。     

用快速压缩装置研究直喷式天然气发动机排放特性

使用快速压缩装置进行了直喷式天然气发动机排放特性的研究。测量了三种不同方式下的排放，并与均相混合气燃烧情况进行了对比。实验结果表明，在宽广的当量比范围内，天然气直喷方式的燃烧效率高于0.95。由于混合气的分层燃烧，天然气喷射方式在宽广的当量比范围内保持较低的HC排放量，同等功率下的低CO2排放量，低NOx排放量，其NOx排放在理论当量比处的降低更为明显。直喷天然气发动机既具备柴油机发动机效率高的特点，又具备预混燃烧发动机排放低的特点。     

利用快速压缩装置研究天然气直喷燃烧循环变动

利用快速压缩装置研究了天然气直喷燃烧循环变动，研究结果表明：借助于分层燃烧和由燃料喷射的湍流引发的快速火焰传播，天然气直喷燃烧在小当量比条件下能实现良好的燃烧稳定性，低的压力峰值循环变动，低的压力升高率峰值循环变动和低的燃烧放热率峰值循环变动，研究发现燃烧期和燃烧产物的循环变动。CO和未燃碳氢的循环变动依赖于后续燃烧期的循环变动，NOx的循环变动依赖于快速燃烧期的循环变动。在燃烧最佳喷射条件下，天然气直喷燃烧的循环变动随当量比的变化并不敏感。     

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.     

Performance and emission characteristics of hydrogen fueled spark ignition engine

A study of alternate fuels leads to hydrogen as a candidate fuel for the future. Its remarkable properties provide the potential of high thermal efficiency at part load by operating the engine unthrottled with lean mixtures. The problems of pre-ignition and backfiring could be overcome at a wide range of operation by providing a cold spark plug with a narrow gap and by keeping combustion chamber walls clean. Hydrogen operation of spark-ignited engines has been found to be very profitable at low equivalence ratios both from the point of view of increased thermal efficiency and reduced nitrogen oxides emissions.     

Study of dimethyl ether homogeneous charge compression ignition combustion process using a multi-dimensional computational fluid dynamics model

A multi-dimensional Computational Fluid Dynamics (CFD) model is adopted to investigate the Dimethyl Ether (DME) Homogeneous Charge Compression Ignition (HCCI) combustion and emissions processes. A reduced chemical mechanism is coupled with a CFD code in the multi-dimensional CFD model. The pressure profiles predicted by the multi-dimensional CFD model are more accurate than the single-zone model, because the wall heat transfer and in-cylinder turbulence flow are considered. During the combustion process the in-cylinder temperature distribution undergoes a process from inhomogeneity to homogeneity. Both low and high temperature reactions don't occur simultaneously throughout the cylinder. The low temperature reactions are initiated near the piston surface and squish region, and the high temperature reactions are initiated in the combustion chamber core zone and squish region. Emission analysis indicates that unburned fuel and CH₂O account for the majority of unburned hydrocarbon (HC). The unburned fuel, CH₂O and CO emission mainly resides in the bottom, middle and upper part of the piston-ring crevice region, respectively. With the decrease of DME equivalence ratio, unburned fuel and CO increases. However, when the DME equivalence ratio is too small, CO emission decreases.     

The lean burn direct injection jet ignition gas engine

This paper presents a new in-cylinder mixture preparation and ignition system for various fuels including hydrogen, methane and propane. The system comprises a centrally located direct injection (DI) injector and a jet ignition (JI) device for combustion of the main chamber (MC) mixture. The fuel is injected in the MC with a new generation, fast actuating, high pressure, high flow rate DI injector capable of injection shaping and multiple events. This injector produces a bulk, lean stratified mixture. The JI system uses a second DI injector to inject a small amount of fuel in a small pre-chamber (PC). In the spark ignition (SI) version, a spark plug then ignites a slightly rich mixture. In the auto ignition version, a DI injector injects a small amount of higher pressure fuel in the small PC having a hot glow plug (GP) surface, and the fuel auto ignites in the hot air or when in contact with the hot surface. Either way the MC mixture is then bulk ignited through multiple jets of hot reacting gases. Bulk ignition of the lean, jet controlled, stratified MC mixture resulting from coupling DI with JI makes it possible to burn MC mixtures with fuel to air equivalence ratios reducing almost to zero for a throttle-less control of load diesel-like and high efficiencies over almost the full range of loads.     

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.     

Effect of water injection and spark timing on the nitric oxide emission and combustion parameters of a hydrogen fuelled spark ignition engine

One of the main problems with hydrogen fuelled internal combustion engines is the high NO level due to rapid combustion. Use of diluents with the charge and retardation of the spark ignition timing can reduce NO levels in Hydrogen fuelled engines. In this work a single cylinder hydrogen fuelled engine was run at different equivalence ratios at full throttle. NO levels were found to rise after an equivalence ratio of 0.55, maximum value was about 7500 ppm. High reductions in NO emission were not possible without a significant drop in thermal efficiency with retarded spark ignition timings. Drastic drop in NO levels to even as low as 2490 ppm were seen with water injection. In spite of the reduction in heat release rate (HRR) no loss in brake thermal efficiency (BTE) was observed. There was no significant influence on combustion stability or HC levels.     

Effect of hydrogen addition on the performance of methane-fueled vehicles. Part II: driving cycle simulations

The overall effect of hydrogen addition to methane fuel on the performance of an SI engine is determined for highway and urban driving cycles. Various engine operating schemes are investigated including fixed equivalence ratio and hydrogen fraction with varying load or throttle position (scheme 1), varying equivalence ratio and fixed hydrogen fraction with varying load (scheme 2), and varying equivalence ratio and hydrogen fraction with varying load (scheme 3). Scheme 2 is determined to be the best method of operating the engine with optimum hydrogen volume fractions in the range of approx. 11–31 vol% for the highway cycle and 11–38 vol% for the urban cycle. The optimum values are based on maximizing the total distance covered for a full tank of fuel and minimizing the total amount of CO₂ produced. It is also shown that except for unburned hydrocarbons, emissions of the other pollutants, CO and NO are substantially reduced below values for the base case of pure methane operating at a fixed equivalence ratio of unity.