Simultaneous high-speed visualization of soot luminosity and OH1 chemiluminescence of alternative-fuel combustion in a HSDI diesel engine under realistic operating conditions

Crude-oil independent liquid fuels are currently being developed for future HSDI diesel engines. Thus, it is the primary objective of the present study to characterize the combustion of selected reference fuels under realistic conditions, in particular with regard to flame lift-off and soot formation. The experiments are conducted in an optically-accessible and a comparable all-metal HSDI engine at part load, using n-decane, n-heptane, 1-decanol, and conventional diesel, respectively, as the fuel. Two image-intensified, high-speed CMOS cameras are employed simultaneously, in order to visualize the highly unsteady combustion process in terms of OH¹ radicals and soot, respectively, with relatively high temporal resolution and data throughput.The results demonstrate the influence of the fuel properties, in particular cetane number and volatility, on mixture formation, ignition, combustion, soot formation, and emissions. Relatively high soot emissions for n-decane can basically be explained by its short ignition delay, small lift-off length, and lack of fuel-bound oxygen. The soot formation process seems to be more important for the relative engine-out emissions than soot oxidation under the investigated conditions. Furthermore, a very strong correlation between the ignition delay and the flame lift-off length (during injection) is found. This indicates that lift-off stabilization is essentially determined by autoignition.     

不同海拔条件下柴油机撞壁喷雾燃烧特性的可视化研究

使用定容燃烧弹模拟一款重型柴油机在海拔0m、3 000m和4 500m条件下运行时缸内的热力学状态,利用多种成像技术实现了撞壁燃烧过程的可视化,研究了不同海拔对撞壁喷雾燃烧的高温着火、火焰传播和碳烟生成特性的影响。结果表明,海拔从0m升高到4 500m,撞壁着火延迟从0.55ms延长到0.88ms,撞壁着火距离从20.38mm增大到26.87mm,高海拔对撞壁着火过程有明显的抑制效果;撞壁火焰呈现圆盘状火焰形态,火焰发展进入稳定阶段后,由于前锋区域的涡流强度增大,撞壁火焰铺展高度、撞壁火焰铺展宽度和撞壁火焰面积都随海拔升高而增大,但在火焰发展初期,由于撞壁平板的冷却作用,撞壁火焰尺寸在海拔升高到4 500m时减小;空间积分火焰亮度和时间积分火焰亮度均随海拔升高而增大,海拔升高到4 500m时碳烟生成速率和氧化速率均增大。     

Study of the characteristic of diesel spray combustion and soot formation using laser-induced incandescence (LII)

In this paper, the planar images of diesel spray combustion flame and soot formation were measured and analyzed by using LII, in a constant volume combustion vessel. The effects of combustion flame and fuel–air mixing characteristics on soot formation and distribution of soot concentration were studied at different conditions. The result indicates that, with increase in ambient temperature and pressure, the ignition delay of diesel fuel is shorter. The increase of ambient temperature and pressure and the reduction of injection pressure shorten the diesel flame lift-off length. The lower the ambient temperature and pressure, the weaker LII signal intensity. At the same ambient temperature and pressure condition, the higher the diesel injection pressure, the smaller the soot production in diesel jet spray, and soot particles are primarily produced in the relative fuel-rich region, which is encompassed by the flame surface front at the downstream of the diesel jet.     

稀释气体组分和浓度对柴油喷雾燃烧特征影响

通过一台高温、高压可视化定容燃烧弹,采用高速摄像技术研究了不同稀释气体(Ar、N2和C02)在10％～40％的氧体积分数范围内对柴油喷雾着火和火焰发展过程的影响;采用羟基自发光成像和双色法研究了低氧体积分数下稀释气体组分(Ar和N2)对火焰发展特征和碳烟分布的影响.结果表明:随氧体积分数增加,柴油机喷雾火焰自发光亮度增...     

Fuel effects on flame lift-off under diesel conditions

An apparent relation between the lift-off length under diesel conditions and the ignition quality of a fuel has previously been reported. To cast light on the underlying mechanism, the current study aims to separate flame lift-off effects of the chemical ignition delay from those of other fuel properties under diesel conditions. Flame lift-off was measured in an optical diesel engine by high-speed video imaging of OH-chemiluminescence. Fuel and ambient-gas properties were varied during the experiment. Only a weak correlation was found between ignition delay and lift-off length. The data indicate that this correlation is due to a common, stronger correlation with the ambient oxygen concentration. The chemical ignition delay and the fuel type had similar, weak effects on the lift-off length. A recently proposed mechanism for lift-off stabilization was used to interpret the results. It assumes that reactants approaching the lift-off position of the jet are mixed with high-temperature products found along the edges of the flame, which trigger autoignition. In this picture, the fuel effect is most likely due to differences in the amount of mixing with high-temperature products that is required for autoignition. In the current experiment, all lift-off effects seem to arise from variations in the reactant and product temperatures, induced by fuel and ambient properties.     

Effects of variable O2 concentrations and injection pressures on the combustion and emissions characteristics of the petro-diesel and hydrotreated vegetable oil-based fuels under the simulated diesel engine condition

This experimental research investigates the effects of variable O₂ concentrations and injection pressures on the combustion and emissions characteristics of the diesel (B7) and the hydrotreated vegetable oil (HVO)-based fuels. The O₂ concentrations included 21%, 15% and 10% O₂, while the injection pressures were 80 and 120 MPa. The experimental fuels were the diesel fuel (B7), the neat HVO, the 20%, 50% and 80% HVO (by mass fraction) blended with the diesel. The experiments were carried out in a rapid compression-expansion machine (RCEM) under the direct injection (DI) diesel combustion condition. The analysis was undertaken using the two-color method. The experimental results indicated that the ignition delay, the heat release rate, the flame temperature, the soot density-KL factor, the NO_x and soot-out emissions were inversely correlated to the HVO fraction in the blend. In addition, the findings revealed the similar flame profiles in which the higher flame temperature region and the darker KL density were concentrated around the spray flame upstream, regardless of the HVO mixing ratio. Besides, the decrease in the O₂ concentration resulted in the lower heat release rate, integral heat release, flame temperature, KL factor and NO_x emissions but the longer ignition delay and higher soot concentration, with the highest soot concentration observed under the 15% O₂ environment. Nevertheless, the higher pressure differential (i.e. between the injection pressure and the ambient pressure) contributed to the shorter ignition delay, higher heat release rate, early peak of the flame temperature, wider combustion area, faster soot oxidation rate and higher NO_x production.     

柴油含水乙醇乳化燃料物性及喷雾燃烧特性研究

试验使用不同配比的柴油含水乙醇乳化燃料,对其理化、喷雾和燃烧特性进行了研究。随着柴油含水乙醇乳化燃料中含水乙醇含量的增加,乳化燃料的密度和运动黏度上升,表面张力略微下降,初始蒸馏温度下降,含氧量升高,十六烷值和低热值降低。试验使用定容燃烧弹,在常温高压和高温高压环境下,对乳化燃料非蒸发喷雾、蒸发喷雾及喷雾燃烧的特性进行了测试。研究结果表明:随着乳化燃料中含水乙醇比例升高,非蒸发喷雾贯穿距和喷雾锥角变化不大;蒸发喷雾贯穿距和喷雾锥角略微减小,但无明显规律,而蒸发喷雾中液相贯穿距离明显增加;燃烧火焰自发光亮度逐渐降低,表征碳烟生成量逐渐减少;在900K环境温度、21%氧体积分数条件下着火滞燃期变化不大。     

Simulations of transient n-heptane and n-dodecane spray flames under engine-relevant conditions using a transported PDF method

Time-dependent Reynolds-averaged CFD is performed for transient turbulent spray flames in a high-pressure, constant-volume chamber for two single-component fuels using skeletal chemical mechanisms. The simulations span a range of initial pressures, temperatures and compositions that correspond to conventional and advanced (e.g., low-temperature) compression-ignition engine combustion. The objectives are to establish the extent to which turbulent fluctuations in composition and temperature influence ignition delays and lift-off lengths and turbulent flame structure under engine-relevant conditions, and to provide insight into turbulence-chemistry interactions. This is done by comparing results from a model that accounts for turbulent fluctuations using a transported composition probability density function (PDF) method with those from a model that ignores the influence of turbulent fluctuations on local mean reaction rates (a locally well-stirred reactor – WSR – model). For robust diesel combustion conditions, the WSR and PDF computed ignition delays and lift-off lengths are close to each other, and both are in good agreement with experiment. For lower initial temperatures, ignition delays and lift-off lengths from the two models are significantly different, and the results from the PDF model are in better agreement with experiment. The differences are especially striking for n-dodecane. There the PDF-model computed ignition delays and lift-off lengths are within 10% of measured values for initial temperatures of 900 K and higher (for 22.8 kg/m³ density, 15% oxygen), while the WSR model predicts an ignition delay that is three times the measured value at 900 K. At an initial temperature of 800 K, the WSR model fails to ignite, whereas the PDF model computed ignition delay and lift-off length are within 30% of the measured values. In all cases, the WSR and PDF models produce significantly different turbulent flame structures, and the differences increase with decreasing initial temperature and oxygen level. The WSR model produces a thin laminar-like flame, while the PDF model gives a broadened turbulent flame brush that is qualitatively more consistent with what is expected for these highly turbulent flames and what is observed experimentally. Thus, while it may be possible to reproduce some global ignition characteristics using a WSR model (depending on the choice of chemical mechanism), turbulent fluctuations play an increasingly important role at lower initial temperatures and oxygen levels.     

不同喷油压力RP-3航空煤油、柴油碰壁喷雾着火和燃烧特性的对比研究

利用直径0.22mm的单孔喷嘴高压共轨喷油器,以喷油器油量标定数据及控制参数为基础,采用高速相机成像技术在定容燃烧室内在等喷油量变喷油压力的前提下测量了着火点、着火滞燃期、燃烧持续期、火焰面积(AF)和火焰自然发光强度(SINL)的变化规律,对比研究了RP-3航空煤油、柴油碰壁喷雾的着火和燃烧特性。结果表明:在低喷油压力下着火点分布在离壁面较远的区域,在较高喷油压力下着火点位于壁面上,距喷油器中心线的距离随喷油压力的增加而增加,且RP-3航空煤油着火点距喷油器的距离比柴油更远。随着喷油压力的增加,RP-3航空煤油碰壁喷雾火焰的着火滞燃期先降低后增加,柴油碰壁喷雾火焰的着火滞燃期不断降低,且RP-3航空煤油具有更短的着火滞燃期。燃烧持续期随喷油压力的增加而降低,RP-3航空煤油的燃烧持续期比柴油短。喷油压力越高,火焰面积(AF)和自然发光强度(SINL)的变化速率越高,而AF和SINL的最大值及达到最大值所需的时间越小。与柴油相比,RP-3航空煤油的AF、SINL具有更高的变化速率,且AF、SINL的峰值更高,达到峰值的时间更短。     

The influence of molecular structure of fatty acid monoalkyl esters on diesel combustion

The subject of this paper is a series of experiments conducted on a single-cylinder research engine investigating the influence of molecular structure on the combustion behaviour of fatty acid alcohol ester (biodiesel) molecules under diesel engine conditions. The fuels employed in these experiments comprised various samples of pure individual fatty acid alcohol ester molecules of different structure, as well as several mixtures of such molecules. The latter consisted in biodiesel fuels produced by the transesterification of naturally occurring plant oils or animal fat with a monohydric alcohol. It was observed that the molecular structure of the fuel significantly influenced the formation of NO_x and particulate matter and their respective concentration in the exhaust gas. The influence on the formation of NO_x in particular, appeared to be exerted first through the effect which the molecular structure had on the auto-ignition delay occurring after the fuel was injected into the combustion chamber, and second through the flame temperature at which the various molecules burned. The emission of particulates on the other hand showed correlation with the number of double bonds in the fuel molecules for the case of larger accumulation mode particles, and with the boiling point of the fuel samples for the case of the smaller, nucleation mode particles. The effect of ignition delay on the exhaust emissions of these pollutants was isolated by adding the ignition promoting molecule 2-ethylhexyl nitrate to some of the fuel samples in closely specified concentrations, so as to equalise the ignition delay for the relevant fuel samples. The removal of the ignition delay as a main influence on the combustion process enabled the observation of the lesser effects of adiabatic flame temperature.     

柴油两段喷射中预喷对主喷燃烧特性影响试验研究

采用可视化定容弹搭配高速数码相机研究柴油两段喷射过程中预喷对主喷喷雾在低温下的燃烧特性的影响。通过对单段喷雾燃烧过程的试验研究发现,随喷射压力增大单段喷雾燃烧的着火延迟期与燃烧持续期减小,而火焰浮起长度与液相长度增大。同时,在相同喷射压力与喷射脉宽下对比单段喷雾燃烧过程与两段喷射主喷燃烧过程得到:两段喷雾的主喷着火位置更靠近喷嘴;主喷的着火延迟期大大缩短,但预喷量对其影响较小;两段喷雾的主喷燃烧持续期增大,且随着预喷量的增大而增大;两段喷雾的主喷燃烧火焰浮起长度小于单段喷雾燃烧火焰浮起长度,且随着预喷量的增大略有增大;两段喷雾燃烧的液相长度小于单段喷雾燃烧的液相长度,且随着预喷量的增大而减小。     

Influence of operating parameters on performance and emissions for a compression-ignition engine fueled by hydrogen/diesel mixtures

Hydrocarbon exhaust emissions are mainly recognized as a consequent of carbon-based fuel combustion in compression ignition (CI) engines. Alternative fuels can be coupled with hydrocarbon fuels to control the pollutant emissions and improve the engine performance. In this study, different parameters that influence the engine performance and emissions are illustrated with more details. This numerical work was carried out on a dual-fuel CI engine to study its performance and emission characteristics at different hydrogen energy ratios. The simulation model was run with diesel as injected fuel and hydrogen, along with air, as inducted fuel. Three-dimensional CFD software for numerical simulations was implemented to simulate the direct-injection CI engine. A reduced-reaction mechanism for n-heptane was considered in this work instead of diesel. The Hiroyasu-Nagel model was presented to examine the rate of soot formation inside the cylinder. This work investigates the effect of hydrogen variation on output efficiency, ignition delay, and emissions. More hydrogen present inside the engine cylinder led to lower soot emissions, higher thermal efficiency, and higher NO_x emissions. Ignition timing delayed as the hydrogen rate increased, due to a delay in OH radical formation. Strategies such as an exhaust gas recirculation (EGR) method and diesel injection timing were considered as well, due to their potential effects on the engine outputs. The relationship among the engine outputs and the operation conditions were also considered.     

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.     

Analysis of ignition delay period of a dual fuel diesel engine with hydrogen and LPG as secondary fuels

In this study, experiments were performed on 4 cylinder turbocharged, intercooled with 62.5 kW gen-set diesel engine by using hydrogen, liquefied petroleum gas (LPG) and mixture of LPG and hydrogen as secondary fuels. The experiments were performed to measure ignition delay period at different load conditions and various diesel substitutions. The experimental results have been compared with ignition delay correlation laid down by other researchers for diesel and dual fuel diesel engine. It is found that ignition delay equation based on pressure, temperature and oxygen concentration for a dual fuel diesel engine run on diesel-biogas gives variation up to 6.56% and 14.6% from the present experimental results, while ignition delay equation for a pure diesel engine gives 7.55% and 33.3% variation at lower and higher gaseous fuel concentrations, respectively. It is observed that the ignition delay of dual fuel engine depends not only on the type of gaseous fuels and their concentrations but also on charge temperature, pressure and oxygen concentration.     

Spatial combustion analysis of biodiesel fueled engine using combustion chamber endoscopy and modeling

Main objective of this study was to demonstrate endoscopic visualization of combustion events in the combustion chamber of a production grade compression ignition (CI) engine. High speed endoscopic imaging was used to provide qualitative information about the in–cylinder combustion for mineral diesel and biodiesel fueled engine operating at different engine loads. These images were analyzed using image processing program developed in MATLAB, in order to determine the ‘start of combustion’ (SoC), ‘spatial soot distribution’ and ‘spatial flame temperature distribution’. In–cylinder pressure and rate of heat release (RoHR) were validated using simulation results obtained by using a KIVA-3V code. The luminosity of flames in the combustion images was relatively lower for biodiesel compared to baseline diesel. Area of soot distribution decreased in later stages of combustion for both test fuels, which indicated superior oxidation of soot particles formed, during the post combustion events. Biodiesel showed relatively lower dissipation of heat, which caused lower soot radiations in the flames. Simulated soot distribution and flame temperature distribution obtained from KIVA-3V code also showed similar behavior and verified the trends observed by combustion chamber endoscopy.     

Experimental study of the fuel jet combustion in high temperature and low oxygen content exhaust gases

The performance of high temperature air combustion (HiTAC) depends on the heat regenerator efficiency and on the way fuel is mixed with furnace gases. In this work, combustion of a single fuel jet of gasol (>95% of propane) was investigated experimentally. Experiments were carried out in steady-state conditions using a single jet flame furnace. The jet of fuel was co-axially injected into high temperature exhaust gases generated by means of a gas burner also fired with gasol. Thus, instead of highly preheated and oxygen depleted air, which was normally used by other researches for such studies, this work has used high temperature and low oxygen content exhaust gases as the oxidiser. A water-cooled fuel nozzle was used to control fuel inlet temperature. Influence of the oxygen content in the oxidiser, at temperatures of 860–890 °C, on the flame visibility and the reactants composition was investigated. The combustion of gasol in hot flue gases appeared to be very stable and complete even at very low oxygen concentration. The oxygen concentration in the oxidiser was found to have a substantial effect on flame size, luminosity, colour, visibility and lift-off distance. Reduced oxygen concentration increases the flame size and lift-off distance, and decreases luminosity and visibility. The HiTAC flame first became bluish and then non-visible at sufficiently low concentration of oxygen in the oxidiser. In this work, results are presented for the constant ratio between fuel jet velocity and velocity of co-flowing flue gases. This ratio was equal to 26.     

Experimental study of the diesel spray combustion and soot characteristics for different double-injection strategies in a constant volume combustion chamber

In this work, broadband luminosity was used to detect ignition delay and diffused back-illumination extinction imaging was employed to measure soot property with different double-injection strategies in a constant volume combustion chamber under different ambient conditions. The interaction between the two-stage injections have been studied experimentally with two key parameters, dwell time and first injection duration. Results show that in two-stage injection cases, a faster flame propagation speed and a more homogeneous soot concentration distribution were observed in the second injection. Besides, averaged KL value in soot regions of the second injection is slightly less than that of the first one. The first injection has a better effect on the ignition delay of the second injection under low ambient pressure. Meanwhile, the soot onset time of the second injection is advanced more obviously. However, compared with the first injection, the advance amount of the ignition delay and the soot onset time of the second injection shows no particular sensitivity with ambient temperature. With longer pilot injection duration, the initial sooting location of the main injection shows a slight reduction but the total soot mass seems to be constant with the variation of the pilot injection duration.     

Use of hydrogen in dual-fuel diesel engines

Hydrogen is a promising future energy carrier due to its potential for production from renewable resources. It can be used in existing compression ignition diesel engines in a dual-fuel mode with little modification. Hydrogen's unique physiochemical properties, such as higher calorific value, flame speed, and diffusivity in air, can effectively improve the performance and combustion characteristics of diesel engines. As a carbon-free fuel, hydrogen can also mitigate harmful emissions from diesel engines, including carbon monoxide, unburned hydrocarbons, particulate matter, soot, and smoke. However, hydrogen-fueled diesel engines suffer from knocking combustion and higher nitrogen oxide emissions. This paper comprehensively reviews the effects of hydrogen or hydrogen-containing gaseous fuels (i.e., syngas and hydroxy gas) on the behavior of dual-fuel diesel engines. The opportunities and limitations of using hydrogen in diesel engines are discussed thoroughly. It is not possible for hydrogen to improve all the performance indicators and exhaust emissions of diesel engines simultaneously. However, reformulating pilot fuel by additives, blending hydrogen with other gaseous fuels, adjusting engine parameters, optimizing operating conditions, modifying engine structure, using hydroxy gas, and employing exhaust gas catalysts could pave the way for realizing safe, efficient, and economical hydrogen-fueled diesel engines. Future work should focus on preventing knocking combustion and nitrogen oxide emissions in hydrogen-fueled diesel engines by adjusting the hydrogen inclusion rate in real time.     

柴油在甲醇/空气高温热氛围中的着火和燃烧特性

在定容燃烧弹上进行了柴油分别在空气和在甲醇/空气预混均质混合气中着火燃烧的实验研究.结果表明,与空气热氛围相比,甲醇混合气热氛围延长了柴油的滞燃期和加长了火焰浮起高度.采用正庚烷-甲醇的详细化学反应机理,利用数值模拟的方法计算了零维模型中正庚烷及正庚烷加甲醇的燃烧反应过程和中间产物历程.其结果表明,甲醇的加入使得正庚烷的高低温放热反应开始时刻后移,滞燃期延长,低温放热反应峰值明显下降,且无明显的负温度系数区,高温反应放热峰值高于其在空气氛围中,归其原因在于甲醇大量消耗着火的OH自由基,并将其转化为低温氧化中不活跃的H2O2,使得系统着火前反应活性减弱.实验和计算结果均表现出甲醇具有抑制柴油及其参比燃料着火的作用.