Study of diesel spray combustion and ignition using high-pressure fuel injection and a micro-hole nozzle with a rapid compression machine: improvement of combustion using low cetane number fuel

This study aimed to reduce NO_x and soot by creating a more homogeneous lean fuel distribution in a diesel spray using high-pressure fuel injection and a micro-hole nozzle. This injection system shortened the ignition delay, but a homogeneous lean fuel distribution in the diesel spray was not achieved. Using a lower cetane number fuel, the resulting longer ignition delay made a uniform, lean fuel distribution in the diesel spray possible with this injection system. Ignition and combustion were analyzed by the combustion chamber pressure history, and flame temperatures and KL values were analyzed by the two-color method.     

缸内喷雾燃烧CFD仿真模型校验

摘要：采用AVLFIRE软件,对某重型柴油发动机进行缸内喷雾燃烧过程仿真。利用内窥镜系统,获得缸内喷雾燃烧图像。通过CFD仿真结果与喷雾过程、燃烧火焰、碳烟浓度分布进行定性对比分析,校验了缸内喷雾燃烧CFD仿真模型的准确性。     

柴油、乳化油和甲醇喷雾燃烧光强波的研究

探讨了柴油机燃烧光强波的性质。在不同压力、氧浓度的条件下，对柴油、乳化油、甲酵喷雾的燃烧光强波进行了测量分析。结果表明，柴油机的燃烧光强波所反应的是火焰内碳粒的氧化历程。对于不同的燃料，燃烧光强波的强弱并不仅仅取决于火焰中碳粒的数量，同时还取决于火焰中的氧浓度。     

Soot and temperature distribution in a diesel diffusion flame: 3-D CFD simulation and measurement with laser diagnostics

In this study, a 3-D CFD simulation and laser diagnostics were developed to understand the characteristics of soot generation in a diesel diffusion flame. The recently developed RANS (Reynolds-averaged Navier-Stokes equations) hybrid combustion model (Extended Coherent Flame Model — 3 Zones, ECFM-3Z model) was used. This industrial, state-ofthe-art model of the diffusion flame is commonly used in diesel combustion models as well as for propagating (premixed) flame combustion. The simulation results were validated with measurements from a constant volume combustion chamber. The experiment revealed that soot accumulated in the chamber where the temperature decreased. Where the temperature increased rapidly, only a little soot accumulated. The temperature and soot distribution were independently examined using both the two-color method and a 3-D CFD simulation for a turbulent diesel diffusion flame.     

Effects of diesel fuel characteristics on spray and combustion in a diesel engine

Fuel properties play a dominant role in the spray, mixture formation and combustion process, and are a key to emission control and efficiency optimization. This paper deals with the influence of the fuel properties on the spray and combustion characteristics in a high-pressure and temperature chamber. Light diesel fuel spray and combustion images were taken by using a high-speed video camera and analyzed by their penetration and evaporation characteristics in comparison with current diesel fuel. Then, a single-cylinder DI engine was used to investigate combustion and exhaust characteristics. The mixture formation of the light diesel fuel is faster than that of the current fuel depending on physical properties like boiling point, density, viscosity and surface tension. Engine test results show that smoke is reduced without an increase in other emissions.     

发动机燃用水乳化柴油的研究进展

综述了柴油机燃用水乳化柴油的燃烧与喷雾特性、动力性与经济性及排放特性,对比分析了发动机燃用水乳化柴油与普通柴油在性能上的差异及其原因,总结了水乳化柴油在柴油机上的应用优化方法。结果表明:与柴油相比,乳化柴油着火滞燃期延迟,燃烧持续期缩短,喷雾贯穿距变长或相差不大,火焰升起高度增加;燃用乳化柴油时动力性下降,但有效热效率较柴油升高;乳化柴油可以明显降低NOx和炭烟排放,但多数工况下HC和CO排放有所升高,低转速和中低负荷工况下尤为明显;燃用乳化柴油时颗粒物数量浓度增加,体积浓度减小,且对于醛类和噪声排放并没有改善作用;添加合适添加剂或结合发动机技术协同作用,可以针对性地改善乳化柴油的燃烧过程,进一步起到节能减排的效果。基于燃料稳定性与燃料理化特性综合优化目标的燃料设计,以及适用于乳化柴油的高压共轨柴油机燃烧组织参数优化是未来的研究方向。     

Knock in dual-fuel diesel combustion with an E85 ethanol/gasoline blend by multi-dimensional simulation

In this paper, knocking combustion in dual-fuel diesel engine is modeled and investigated using the CFD code coupled with detailed chemical kinetics. The ethanol/gasoline blend E85 is used as the primary fuel in a dual-fuel combustion concept based on a light-duty diesel engine equipped with a common-rail injection system. The E85 blend is injected and well mixed with intake air in the intake manifold and is ignited by the direct injection diesel fuel. A 46-species, 187-reaction Multicomponent mechanism is adopted to model the auto-ignition process of the E85/air/diesel mixture ahead of the flame front. Based on the model validation, knocking combustion under boost and full load operating condition for 0 %, 20 %, 50 %, as well as 70 % E85 substitute energy is simulated. The effects of E85 substitute rate and two stage injection strategies on knock intensity, power output, as well as location of the auto-ignition initiation is clearly reproduced by the model. The calculation result shows that, for a high E85 rate of 50 % and 70 % with single injection strategies, the most serious knock and the origin of auto-ignition always occurs far away from where the flame of diesel spray is first generated, at the center of combustion chamber, due to higher pressure wave, relatively richer E85 mixture and longer distances of flame propagation. The two stage injection strategies with a small amount of diesel pilot injection ahead of the main injection primarily influence the ignition behavior of the directly injected fuel, leads to a lower pressure rise rate and a reduced propagation distance, both of which contribute to the attenuation of knock intensity for a higher E85 rate.     

基于可视化装置的柴油机喷雾燃烧过程试验研究

利用喷雾燃烧多功能动态可视化试验装置,获得了清晰直观的燃烧室内喷雾燃烧过程图片,分析研究了两种燃烧室对喷雾和燃烧过程的影响。结果表明:燃烧室内喷雾贯穿力比涡流和挤流强,喷雾以近似直线形式扩展到凹坑壁后,大部分随涡流方向扩展;火焰首先在靠缩口型燃烧室中心处出现,而圆柱型燃烧室则在靠凹坑上边缘处出现;缩口型燃烧室比圆柱型燃烧室燃烧结束得稍早,燃烧性能更好。     

用光学可视化方法研究乙醇柴油混合燃料的燃烧特征

应用直接图像法对乙醇柴油的燃烧过程进行研究。在一台单缸直接喷射式柴油机上,建立了直接图像法拍摄燃烧火焰图像的光学系统,对15%乙醇柴油、15%乙醇柴油加十六烷值改进剂、纯柴油在同一转速下的燃烧过程进行可视化研究。对火焰照片分析表明:柴油中加入乙醇后,无论是否恢复其十六烷值,其着火滞燃期都延长了,燃烧持续期缩短,火焰辉度减弱。在乙醇柴油中加入十六烷值改进剂后,着火滞燃期相对提前,燃烧持续期和火焰辉度增加,但仍然没有达到柴油机水平,这说明十六烷值改进剂有利于改善乙醇柴油的燃烧性能。通过温度场分析发现:乙醇柴油的缸内平均温度峰值要比纯柴油低很多,而且乙醇柴油燃烧时平均温度上升相当平缓。     

Phenomenological two-phase multi-zone combustion model for direct-injection diesel engines

This paper proposed a quasi-dimensional combustion model from a new observed two-phase penetration and combustion phenomenon in diesel spray. In the model, fuel spray was divided into two of liquid and gas phase areas. Considering the phenomenon that separation of gas and liquid phase in diesel spray occurs during spray penetration, gas and liquid area of spray are discretized respectively. Liquid phase areas play important role in fuel mass transport, however gas phase areas are the main region for fuel combustion in the model. Fuel and air mixing rate of gas phase zone is the key for the calculation of combustion rate. Validation experiments are designed by using optimal Latin hypercube design method. Comparison of calculations and experiments show that the model is able to predict diesel engine performance at different engine speeds, loads, and injection pressure and timing, and provides guidance for the design of engines.     

Instantaneous 2-D visualization of spray combustion and flame luminosity of GTL and GTL-biodiesel fuel blend under quiescent ambient conditions

An experimental study has been performed on spray combustion and two-dimensional soot concentration in diesel (ULSD), GTL and GTL-biodiesel fuel jets under high-pressure, high-temperature quiescent conditions. Instantaneous images of the fuel jets were obtained with a high-speed camera. It was confirmed that by blending GTL with 20% rapeseed biodiesel, certain fuel properties such as kinematic viscosity, density, surface tension, volatility, lower heating value and others may be designed and improved to be more like those of conventional diesel fuel but with considerable decrease in the amount of sulfur, PAH, cold filter plugging point, etc. The results showed that the spray tip penetration increased and the spray cone angle decreased when 20% biodiesel fuel was added to GTL fuel. Autoignition of the GTL-biodiesel blend occurred slightly earlier than that of diesel fuel. Experiments under high-pressure, high-temperature conditions showed that higher injection pressure induced a lower soot formation rate. The integrated flame luminosity, which serves as an indicator of soot concentration in the fuel jet, was slightly higher for the GTL-biodiesel blend than for pure GTL fuel due to the slightly higher sulfur content of pure biodiesel fuel.     

喷油器参数对柴油机燃烧特性影响的数值模拟

应用三维CFD模拟软件FIRE,对1台135柴油机的喷雾与燃烧过程进行了数值模拟。通过缸内流场分析和浓度分析,研究了喷孔数、喷孔锥角对燃烧过程的影响及对NOx和炭烟生成的影响。结果表明,喷孔数和喷孔锥角影响燃油的喷雾质量和油气混合质量,进而影响燃烧过程和排放物的生成;CFD模拟计算对优化喷油器参数,从而改进燃烧系统具有指导意义。     

柴油机高压及微孔径下燃油雾化特性的数值研究(Ⅱ)——湍流模型对燃烧的影响

为了有效模拟燃油燃烧过程,向计算流体力学平台KIVA-3V中加入了大涡模拟程序,同时耦合了化学反应动力学软件Chemkin,建立了喷雾燃烧过程的大涡模拟燃烧平台。研究了在高压和微孔径条件下两种湍流模型对燃油雾化燃烧过程的影响,同时对1台Caterpillar 3401重型柴油发动机缸内燃油喷雾燃烧过程进行了数值研究。结果显示,大涡模拟得到的燃油的燃烧过程与试验结果较为接近,燃烧温度分布及缸内压力变化趋势都要好于雷诺平均模型。     

Low temperature premixed combustion within a small bore high speed direct injection (HSDI) optically accessible diesel engine using a retarded single injection

An optically accessible single-cylinder high speed direct-injection (HSDI) Diesel engine equipped with a Bosch common rail injection system was used to study low temperature Modulated Kinetics (MK) combustion with a retarded single main injection. High-speed liquid fuel Mie-scattering was employed to investigate the liquid distribution and evolution. By carefully setting up the optics, three-dimensional images of fuel spray were obtained from both the bottom of the piston and the side window. The NOx emissions were measured in the exhaust pipe. The influence of injection pressure and injection timing on liquid fuel evolution and combustion characteristics was studied under similar fuel quantities. Interesting spray development was seen from the side window images. Liquid impingement was found for all of the cases due to the small diameter of the piston bowl. The liquid fuel tip hits the bowl wall obliquely and spreads as a wall jet in the radial direction of the spray. Due to the bowl geometry, the fuel film moves back into the central part of the bowl, which enhances the air-fuel mixing process and prepares a more homogeneous air-fuel mixture. Stronger impingement was seen for high injection pressures. Injection timing had little effect on fuel impingement. No liquid fuel was seen before ignition, indicating premixed combustion for all the cases. High-speed combustion video was taken using the same frame rate. Ignition was seen to occur on or near the bowl wall in the vicinity of the spray tip, with the ignition delay being noticeably longer for lower injection pressure and later injection timing. The majority of the flame was confined to the bowl region throughout the combustion event. A more homogeneous and weaker flame was observed for higher injection pressures and later injection timing. The combustion structure also proves the mixing enhancement effect of the liquid fuel impingement. The results show that ultra-low sooting combustion is feasible in an HSDI diesel engine with a higher injection pressure, a higher EGR rate, or later injection timing, with little penalty on power output. It was also found that injection timing has more influence on HCCI-like combustion using a single main injection than the other two factors studied. Compared with the base cases, simultaneous reductions of soot and NOx were obtained by increasing EGR rate and retarding injection timing. By increasing injection pressure, NOx emissions were increased due to leaner and faster combustion with better air-fuel mixing. However, smoke emissions were significantly reduced with increased injection pressure.     

高强化柴油机双卷流燃烧系统喷油参数匹配试验研究

针对采用双卷流燃烧系统的单缸柴油机进行了强化燃烧试验研究,研究内容包括不同喷雾锥角燃烧特性试验和相同负荷下不同转速的燃烧特性试验。研究发现,柴油机喷雾锥角的合理设计对燃烧特性具有重要影响,提高转速是提高柴油机强化程度的有效途径,但是转速提高会带来排温升高及燃油经济性恶化等问题。通过调节喷油定时可以在提高发动机转速时使排温及燃油经济性的问题在一定程度上得以改善。采用双卷流燃烧系统并配以合适的喷油参数(喷雾锥角、喷油定时)可以达到提高柴油机强化程度的目的。     

进气预热对柴油机起动过程动力性能影响的试验分析

为了改善某柴油机的起动特性,自行设计了柴油机进气火焰预热系统。利用基于时间的瞬时数据采集系统,测量了瞬时转速和缸内燃烧压力的变化规律,对不进气预热和进气预热条件下柴油机的起动特性进行了试验研究。试验结果表明:采用进气预热系统后,进气温度升高了18.5℃,柴油机起动过程中转速升高率增大,起动时间明显缩短;输出扭矩增大且波动明显减小;各个循环的最大燃烧压力升高且波动减小,燃油的着火滞燃期明显缩短,其着火过程中发生后燃、循环失火的概率明显减小,燃烧过程进行得更加充分,极大地改善了柴油机的起动性能。     

Evaluation of natural and tracer fluorescent emission methods for droplet size measurements in a diesel spray

Spray sizing that records fluorescent emission and scattered light has been widely applied to spray diagnostics over the last two decades. Different experimental strategies have been developed, but comparing the different solutions offered has remained of interest to experimentalists. In this work, a comparison of two fluorescence strategies for measuring droplet size in the liquid phase of a last-generation DI diesel spray is conducted. The natural fluorescent emission of a commercial diesel fuel and the fluorescence emitted by a tracer (Rhodamine B) are compared using theoretical and experimental approaches. The LIF/Mie ratio commonly called Planar Droplet Sizing (PDS) technique is applied in two different ways to elucidate the possible advantages of using a fluorescent dopant. The sprays were injected under non-evaporative conditions into a constant pressure vessel that simulates densities present at the moment of injection in currently used passenger car diesel engines. Characterization of the signal properties was performed by measuring the absorption coefficient, fluorescence emission spectrum, quantum yield and lifetime of both configurations. The scattered light and fluorescence intensities were calculated to verify the dependencies of the droplet surface and volume. When applying the two techniques to quantify droplet size in dense diesel sprays, the results show that signal weakness and lack of control over the properties of natural fluorescence produce distortion in the shape of the spray and cause measurements to be unreliable.     

缸内直喷式汽油机工作过程三维数值模拟

将一台柴油机改装为缸内直喷式汽油机,采用KIVA-Ⅱ软件时缸内直喷式汽油机的两个典型工况(分层燃烧和均质预混合燃烧)的燃烧过程进行了三维数值模拟。计算结果表明,采用分层燃烧和均质预混合燃烧具有不同的火焰传播方式和特点。     

Two-dimensional laser induced fluorescence measurement of spray and OH radicals of LPG in constant volume chamber

It is recognized that alternative fuels such as liquid petroleum gas (LPG) have less polluting combustion characteristics than diesel fuel. In this study, LPG was injected into a high pressure and temperature chamber to reproduce the stratification processes in an engine. The spray images were taken by the use of a PLIF method with Nd:YAG laser to analyze their penetration and evaporation characteristics. Also the characteristics of combustion were investigated by simultaneous visualization of OH radical and flames.The results show that the mixture moves along the impingement wall that reproduced the piston bowl and reaches the ignition spark plug. Also, OH fluorescence rises sharply and then decreases gradually when the combustion is carried out actively.     

Effect of a 2-stage injection strategy on the combustion and flame characteristics in a PCCI engine

Recently, to reduce environmental pollution and the waste of limited energy resources, there is an increasing requirement for higher engine efficiency and lower levels of harmful emissions. A premixed charge compression ignition (PCCI) engine, which uses a 2-stage type injection, has drawn attention because this combustion system can simultaneously reduce the amount of NOx and PM exhausted from diesel engines. It is well known that the fuel injection timing and the spray angle in a PCCI engine affect the mixture formation and the combustion. To acquire two optimal injection timings, the combustion and emission characteristics of the PCCI engine were analyzed with various injection conditions. The flame visualization was performed to validate the result obtained from the engine test. This study reveals that the optimum injection timings are BTDC 60° for the first injection and ATDC 5° for the second injection. In addition, the injection ratio of 3 to 7 showed the best NOx and PM emission results.