柴油机PPCI燃烧模式下预喷策略对缸内高频压力振荡的影响机理研究

柴油机燃烧噪声产生的原因为气动载荷和高频压力振荡,其中高频压力振荡对燃烧噪声有重要影响。为研究柴油机PPCI燃烧模式下预喷对缸内压力高频振荡产生的影响机理,在一台直喷单缸柴油机进行预主喷燃烧试验。利用小波时频分析的方法,将试验测定的缸压曲线进行处理从而得到缸内高频压力振荡的时频图,并重点对其进行分析。分析结果表明,在预主喷试验条件下,压力高频振荡的影响无法通过放热率定性地反映出来。此外主喷燃烧对高频振荡的影响小于预喷燃烧,而且大预喷量结合较早的预喷时刻的喷油策略产生的高频压力振荡最小。预主喷产生高频压力振荡主要分布在-4～16°CA,压力振荡的频段主要集中于4000～5000 Hz和6000～8000 Hz,其中预喷燃烧产生的高频压力振荡的频率范围比主喷燃烧大。     

后喷时刻对柴油机燃烧影响的可视化研究

后喷可改善柴油机的颗粒物排放特性。在光学发动机上,保持总喷油量不变,变换不同主、后喷间隔时刻,对缸内燃烧过程进行高速摄影,研究后喷时刻变化对柴油机缸内燃烧特性的影响。将获取的缸内燃烧火焰图像,利用双色法计算得出缸内温度场和表征碳烟浓度的KL因子分布情况,并分析缸内燃烧温度和碳烟生成特性。研究表明,随主、后喷间隔角增加,温度分布及KL因子总量呈双峰分布,KL因子总量后期减少速度增加,即碳烟氧化速度加快。     

基于废气管理的汽油机缸内温度不均匀分布对自燃着火的影响

基于废气-负荷控制策略的汽油机HCCI燃烧,利用排气门管理内部残余废气率,控制发动机负荷,进气门升程和相位调节缸内状态,控制着火和优化燃烧相位。本文利用KIVA对全可变气门机构单缸机进行CFD仿真,研究缸内温度、残余废气分布特征,以及温度、废气不均匀分布对着火的影响。结果表明,采用废气-负荷控制策略,改变进气门相位和升程可以调节缸内温度和残余废气的不均匀分布。随着内部残余废气率增大,进气门对缸内温度和废气率分布不均匀度的调控能力增强。大废气率条件下,温度和废气率不均匀度对着火时刻调节作用较明显,可以实现10°曲轴转角的有效调节;小废气率条件下,由于缸内温度和废气不均匀分布差别较小,其对于着火时刻的调节作用表现得不明显。     

四气门汽油机缸内滚流运动对燃烧过程影响的研究

1引言近年来，四气门汽油机由于性能优越，已成为车用发动机的发展方向，而我国在这方面的研究和开发还没有开展。对其缸内空气运动的研究发现[1-4]：当两个进气门都打开时，缸内空气运动主要是绕与气缸轴线垂直方向旋转的滚流运动（Tumblemotion），与传统二气门发动机缸内形成的绕气缸轴线旋转的涡流运动（Swirlmotion）不同；它在压缩末期很容易破碎，大大增加了湍流强度，促进燃烧过程明显改善。但在低速时，由于进气速度下降，燃油在进气道蒸发和雾化变差，使燃烧过程恶化，发动机扭矩下降。为此，一些汽车公司采用在低速时将一个进气…     

进气中CO2浓度对预混合燃烧和排放影响的试验和模拟研究

本文研究了进气中CO2浓度对燃烧和排放特性的影响.研究表明在所有的预混合燃料比下,当CO2浓度增加时,NOx排放随之大幅减少,烟度排放有小的变化。利用KIVA3V和湍流与化学反应交互的燃烧模型对柴油机预混合燃烧进行了模拟研究,对缸内OH浓度的模拟计算表明,随着CO2浓度的增加,着火前期OH生成浓度明显向后推移,这表明燃料的氧化速率随CO2浓度的增加变慢,从而延长了着火滞燃期。进气中CO2浓度变大时,燃烧温度降低,有利于降低NOx的排放。     

6110柴油机进气过程流动的多维瞬态数值模拟研究

柴油机进气过程中气体流动是强瞬变的非定常三维湍流运动,直接影响到充气效率、缸内气流状态及进气道和缸内壁面换热,因而也影响到燃油雾化、燃烧及有害废气生成等,对柴油机的动力性、经济性起着重要作用。因此,本文利用大型通用CFD软件STAR-CD及ES-ICE对6110柴油机进气过程的气体流动进行多维瞬态数值模拟研究,通过计算可以得到不同时刻进气道和缸内流场分布,为柴油机进气系统的优化设计提供重要的理论依据。     

内燃机气道及缸内气体流动特性研究

本文对某型号内燃机气道稳态流动和缸内气流运动特性进行了研究.气道流动特性稳态计算结果与气道稳流试验结果相吻合,流动特征参数偏差在3%左右.瞬态计算结果表明,在进气行程初期,缸内涡流和湍流迅速增强,随后涡流变化缓慢,而湍流强度迅速下降.在气门叠开时,存在进气和排气回流现象.进气时,进气阀边缘处是缸内气体最大速度出现位置,气门最大升程时该处速度达到134.4m/s;切向气道缩口处最大速度高达134.7m/s,此处存在一定的进气节流损失.     

直接加热对空气喷气发动机燃烧室地面模拟试验燃烧效率的影响研究

本文提出了一个计算燃烧效率的理论模型,从本模型出发导出了由于直接加热使进气污染而导致对燃烧效率影响的关系式。本文综合了直流燃烧室模型、涡喷发动机主燃室和模型试验件的台架试验数据,对理论关系式进行了验证,并得到了可供工程使用的半经验关系式。该关系式可由进气污染空气试验所得的燃烧效率值和进气污染的程度直接算出该燃烧室在相同工况纯净空气下的燃烧效率。     

涡流管能量分离过程实验研究

利用涡流管部分轴线温度测量实验装置,考虑到进气压力和涡流室结构对涡流管能量分离过程的影响,针对不同进气压力和不同涡流室结构的涡流管部分轴线温度分布进行了实验研究。根据实测结果,得到了进气压力及涡流室几何结构对涡流管部分轴线温度分布的影响曲线。在此基础上,根据实验结果并结合热力学原理对涡流管制冷的物理行为作了分析。结果表明:涡流导致涡流室中心区域气流膨胀是涡流管产生制冷效应的一个重要原因。     

直喷柴油机火焰浮起长度的数值模拟研究

采用三维CFD模型模拟了直喷柴油机缸内喷雾燃烧过程,模拟缸压曲线得到了实验的验证.通过高温区与喷嘴之间的稳定距离来确定柴油机火焰浮起长度,研究在不同进气条件下火焰浮起长度的变化情况.该模型成功地预测了火焰浮起长度随着初始进气压力的增大而减少,随着进气温度的升高出现先增大后减少的趋势.同时模拟了在不同EGR率下柴油机缸内燃烧情况,发现火焰浮起长度和燃料着火延迟时间都随EGR率的增加而增大.     

Flashback-induced flame shape transition in a two-stage LPP aeronautical combustor

Large-Eddy Simulations were performed to study the flashback-induced flame shape transition of a lean premixed M flame in a staged liquid-fuelled aeronautical lean-burner, as observed experimentally. The BIMER combustor is a Lean Premixed Prevapourised (LPP) burner composed of two stages, each with its own injector and swirler: the main outer stage, called multipoint, uses jet-in-crossflow injection to achieve the LPP regime, while the central stage, called pilot, uses a pressure swirl injector to create a hollow cone spray to stabilise the flame. During LPP operation, this M flame presents a strong acoustic activity, promoting a periodic flashback of its leading edge. When, aiming to stabilise the flame, the pilot injection is increased and the multipoint injection decreased, the oscillating leading edge (due to the longitudinal acoustic perturbations) attaches to the pilot spray, changing the flame into a Tulip shape. Two phenomena were identified as being the most relevant causes of this flame shape transition. First, the leading edge position and the thermoacoustic instability amplitude are directly linked to the combustion chamber final temperature. The higher the temperature in the chamber, the more upstream the leading edge stabilises, and the higher the acoustic oscillation amplitude, both increasing the risk of a successful flashback. Second, the injection regime with high pilot injection allows the leading edge to attach to the pilot spray, as the flame only transitions when the pilot spray is sufficiently high. The higher the pilot fuel flow, the higher the amount of fuel sprayed in the critical region where the flame might attach for a transition to the Tulip shape. Therefore, as the change in injection regime is the main mechanism lean staged burners use to reduce emissions while increasing operability, this works shows that an M flame is unsuitable to such burners with similar aerodynamic topology and properties.     

Simulation of diesel spray combustion using LES and a multicomponent vapourisation model

Diesel spray and combustion in a constant-volume engine cylinder was simulated by a large eddy simulation (LES) approach coupling with a multicomponent vapourisation (MCV) modelling. The simulation focused on the inclusion of the interaction between fuel spray and gas-phase turbulence flow at the sub-grid scale. The LES was based on the dynamic structure sub-grid model, and an additional source term was added to the filtered momentum equation to account for the effect of drop motion on the gas-phase turbulence. The multicomponent drop vapourisation modelling was based on the continuous thermodynamics approach using a gamma distribution to describe the complex diesel fuel composition and was capable of predicting a more complex drop vapourisation process. The effect of gas-phase turbulence flow on the fuel drop vapourisation process was evaluated through the solution of the gas-phase moments of the distribution in the present LES framework. A non-evaporative spray in a constant-volume engine cylinder was first simulated to examine the behaviours of LES, in comparison with a Reynolds-averaged Navier–Stokes (RANS) simulation based on the RNG k–? model. More realistic diesel spray structures and improved agreement on liquid penetration length with the corresponding experimental data were predicted by the LES, using a grid resolution close to that of RANS. A more comprehensive simulation of diesel spray and combustion in cylindrical combustor was also performed. Predicted distributions of soot particles were compared to the experimental image, and improved agreement with the experimental data was also observed by using the present LES and MCV models. Consequently, results of the present models proved that improved overall performance of the fuel spray simulation can be achieved by the LES without a significant increase in the computational load compared to the RANS.     

溶有CO2燃油发动机NOx和碳烟排放的数值研究

本文在KIVA程序的基础上,建立了溶有CO2燃油低温着火模型、碳烟模型,修改了油滴的蒸发模型,并根据实测数据对溶有CO2燃油在缸内的喷射、蒸发、燃烧和燃烧排放物的生成过程进行了数值模拟,溶气燃油喷射时CO2析出过程产生的"微爆"作用促进了燃油的雾化,计算表明溶气燃油喷射形成的可燃混合气分布更均匀,从而改善了缸内局部高温和氧浓度分布不均的条件,减少了NOx和炭烟的生成.     

燃料敏感性对内燃机部分预混燃烧(PPC)光谱特性的影响研究

在一台光学发动机上,利用火焰高速成像技术和自发光光谱分析法,研究了燃料敏感性(S)为0和6时对发动机缸内火焰发展和燃烧发光光谱的影响。试验过程中,通过改变喷油时刻 (SOI=-25,-15和-5°CA ATDC) 使燃烧模式从部分预混燃烧过渡到传统柴油燃烧模式。通过使用正庚烷、异辛烷、乙醇混合燃料来改变燃料敏感性。结果表明,在PPC模式下(-25°CA ATDC),火焰发展过程是从近壁面区域开始着火,而后向燃烧室中心发展,即存在类似火焰传播过程,同时在燃烧室下部未燃区域也形成新的着火自燃点。敏感性对燃烧相位影响较大,对缸内燃烧火焰发展历程影响较小;高敏感性燃料OH和CH带状光谱出现的时刻推迟,表明高敏感性燃料高温反应过程推迟,且光谱强度更低,表明碳烟辐射强度减弱。在PPC到CDC之间的过渡区域(-15°CA ATDC),燃烧火焰发光更亮,燃烧反应速率比-25°CA ATDC时刻的反应速率更快。高、低敏感性燃料对缸压放热率的影响规律与-25°CA ATDC相近,此时的燃烧反应更剧烈,放热率更高,碳烟出现时刻更早。该喷油时刻下的光谱强度高于PPC模式下的光谱强度,说明此时的CO氧化反应与碳烟辐射更强。在CDC模式下(-5°CA ATDC),由于使用的燃料活性较低,燃烧放热时刻过于推迟,放热量很小,缸内燃烧压力低,因此燃料敏感性对缸压和放热率的影响不明显,但从燃烧着火图像中可以看到高敏感性燃料的火焰出现时刻较低敏感性燃料推迟。低敏感性燃料的燃烧初期蓝色火焰首先出现在燃烧室中心,着火火焰出现时刻更早,之后蓝色火焰从中心向周围扩散,呈现火焰传播为主导的燃烧过程;燃烧后期,局部混合气过浓区导致亮黄色火焰面积逐渐增大并向周围扩散。高敏感性燃料的火焰发展趋势与低敏感性燃料类似,黄色火焰的亮度与面积更小。尽管高、低敏感性燃料的OH和CH带状光谱的出现时间相近,但高敏感性燃料的光谱强度仍更低。综合分析,火焰发展结构与自发光光谱特征主要受喷油时刻的影响,燃料的敏感性主要影响着火时刻和火焰自发光光谱强度,且高敏感性燃料的光谱强度更低。     

Fuel spray visualization and its impingement analysis on in-cylinder surfaces in a direct-injection spark-ignition engine

Abstract  

Experiments are performed to investigate the effects of fuel spray on in-cylinder mixture preparation and its impingement on cylinder walls and piston top inside a direct-injection spark-ignition engine with optical access to the cylinder. Novel image processing algorithms are developed to analyze the fuel impingement quantitatively on in-cylinder surfaces. The technique is useful to optimize the fuel pressure, injection timing and the number of injections to minimize the fuel impingement on in-cylinder surfaces. E85, which represents a blend of 85% ethanol and 15% gasoline (by volume) is used in this study. Two types of fuel injectors are used; (i) low-pressure production-intent injector with fuel pressure of 3 MPa, and (ii) high-pressure production injector with fuel pressures of 5 and 10 MPa. In addition, the effects of split injection are also presented by maintaining the same amount of fuel used in single injection. It is found that the split injection is an effective way to reduce the overall fuel impingement on in-cylinder surfaces while maintaining a reasonably good air–fuel mixture in the cylinder.     

6110柴油机喷雾过程的多维瞬态数值模拟研究

将柴油机全体燃烧室部件(气缸盖-气缸套-活塞组)作为一个耦合体,在对耦合体进行传热数值模拟的基础上得到喷雾过程缸内计算的壁面边界条件.利用大型通用CFD软件STAR-CD及ES-ICE,在进气压缩过程多维瞬态数值模拟基础上,对6110柴油机喷雾过程进行多维瞬态数值模拟研究.通过计算,着重分析缸内两相流动,燃油喷射、雾化以及喷雾粒子的空间分布等.     

Large-eddy simulations of a stratified-charge direct-injection spark-ignition engine: Comparison with experiment and analysis of cycle-to-cycle variations

Multiple-cycle large-eddy simulations (LES) have been performed for an optically accessible, single-cylinder, four-stroke-cycle, spray-guided direct-injection spark-ignition (SG-DISI) engine operating in a stratified globally fuel-lean mode. The simulations combine a standard Smagorinsky turbulence model, a stochastic Lagrangian parcel method for liquid fuel injection and fuel spray modeling, a simple energy-deposition spark-ignition model, and a modified thickened flame model for turbulent flame propagation through highly stratified reactant mixtures. Comparisons between simulations and experiments include individual-cycle and ensemble-average pressure and apparent-heat-release-rate traces, individual-cycle and ensemble-average indicated mean effective pressures (IMEP), and instantaneous two-dimensional vapor-equivalence-ratio contours. Although the number of LES cycles is small (35), the results show that the simulations are able to capture the global combustion behavior that is observed in the experiments, including cycle-to-cycle variations. The simulation results are then analyzed further to provide insight into the conditions that lead to misfire versus robust combustion. As has been reported in earlier experimental and LES studies for homogeneous-charge SI engines, local conditions in the vicinity of the spark gap at the time of ignition largely determine the subsequent flame development. However, in contrast to homogeneous-charge engines, no single local or global quantity correlates as strongly with the eventual peak pressure or IMEP for each cycle. Rather, it is the interplay among the early flame kernel, the velocity field that it experiences, and the fuel distribution that it encounters that ultimately determines the fate of each combustion event. Deeper analysis and quantitative statistical comparisons between experiments and simulations will require the simulation of larger numbers of engine cycles.     

高温空气燃烧NOx排放特性的试验研究

通过两种结构烧嘴的热态燃烧试验对比,研究了烧嘴结构、燃气射流速度、过量空气系数对高温空气燃烧过程氮氧化物排放的影响特性。研究结果认为:在燃气喷口两侧布置两个矩形空气喷口的烧嘴,氮氧化物排放量低于圆形空气喷口烧嘴;随着燃气射流速度的提高,高温空气燃烧过程排放的氮氧化物逐渐减少。与普通燃烧过程不同的是,随着过量空气系数的提高,在一定范围内高温空气燃烧的氮氧化物排放量不断增加。分析认为,高温空气燃烧氮氧化物排放量与火焰体积、炉内氧气与燃气混合过程以及燃气射流和空气射流对炉内烟气的卷吸量有关。     

燃油分级多点喷射低污染燃烧室的化学反应网络模型分析

本文采用基于详细化学反应机理的化学反应网络模型分析了航空发动机燃油径向分级多点喷射低污染燃烧室的NO_x排放特性。该分级燃烧室不同于传统燃烧室,头部由值班区和主燃区两个不同的燃烧区域,根据CFD得到的流场特性和当量比的分布特性对燃烧室进行分区构建化学反应器网络模型,研究了值班级当量比以及值班级和主燃级两级供油比例对排放的影响。同时,还分析了空气进口温度对NO_x排放的影响。得到了较为合理的变化趋势,为低污染燃烧室的初步设计提供了有益的指导。