Effects of flash boiling injection on in-cylinder spray,mixing and combustion of a spark-ignition direct-injection engine

Higher engine efficiency and ever stringent pollutant emission regulations are considered as the most important challenges for today's automotive industry. Fast evaporation and combustion technique has caused unprecedented attention due to its potential to solve both of the above challenges. Flash boiling, which features a two-phase flow that constantly generates vapor bubbles inside the liquid spray is ideal to achieve fast evaporation and combustion inside direct-injection (DI) gasoline engines. In this study, three spray conditions, including liquid, transitional flash boiling and flare flash boiling spray were studied for comparison under cold start condition in a spark-ignition direct-injection (SIDI) optical gasoline engine. Optical access into the combustion chamber includes a quartz linear and a quartz insert on the piston. In separate experiments, we recorded the crank angle resolved spray morphology using laser scattering technique, and distribution of fuel before ignition employing laser induced fluorescence technology, as well as time-resolved color images of flame with high-speed camera. The spray morphology during the intake stroke shows stronger plume-plume and plume-air interaction under flash boiling condition, as well as smaller penetration. Then around the end of compression (before ignition), the fuel distribution is also shown to be more homogeneous with less cyclic variation under flash boiling. Finally, from the color images of the flame, it was found that with the increase of superheat degree, the diffusion rate of blue flame (generated by excited molecules) is higher, which is considered to be related with the larger fractal dimension of the flame front. Also, the combustion is more complete with less yellow flame under flash boiling.     

Ultra-lean limit extension for gasoline direct injection engine application via high energy ignition and flash boiling atomization

Extensive efforts have been made in achieving leaner combustion for gasoline direct injection (GDI) engines to further improve the thermal efficiency and reduce harmful emissions. Among these techniques, increasing ignition energy has been proven to be an effective method to achieve lean combustion. Few targets the atomization process of the fuel in generating a more homogenous fuel-air mixture, which is believed to be able to extend the lean flammability limit of the engine. This investigation explores the use of flash boiling atomization, a technique to improve spray atomization via elevating the fuel temperature, in combination with high energy ignition technique for better GDI engine performance under lean-burn conditions. For such purposes, a single-cylinder, optical GDI engine was used with high-speed imaging techniques, along with other measurement instruments. The fuel was preheated by a heating element and high energy ignitions were generated by a customized ignition system. ignitions with various initial currents (transistor coil ignition (TCI), 250 mA, and 500 mA) under both sub-cooled and flash boiling conditions were examined using different excess air ratios. It was found that using flash boiling atomization has extended the lean limit from 1.95 to 2.10 under the 500 mA initial current ignition. Other critical parameters such as indicative mean effective pressure (IMEP), emissions such as CO, NO_x, THC were also analyzed to demonstrate the impacts of high energy ignition and flash boiling atomization.     

Modelling biodiesel–diesel spray combustion using multicomponent vaporization coupled with detailed fuel chemistry and soot models

A multicomponent vaporization model is integrated with detailed fuel chemistry and soot models for simulating biodiesel–diesel spray combustion. Biodiesel, a fuel mixture comprised of fatty-acid methyl esters, is an attractive alternative to diesel fuel for use in compression-ignition engines. Accurately modelling of the spray, vaporization, and combustion of the fuel mixture is critical to predicting engine performance using biodiesel. In this study, a discrete-component vaporization model was developed to simulate the vaporization of biodiesel drops. The model can predict differences in the vaporization rates of different fuel components. The model was validated by use of experimental data of the measured biodiesel drop size history and spray penetration data obtained from a constant-volume chamber. Gas phase chemical reactions were simulated using a detailed reaction mechanism that also includes PAH reactions leading to the production of soot precursors. A phenomenological multi-step soot model was utilized to predict soot emissions from biodiesel–diesel combustion. The soot model considered various steps of soot formation and destruction, such as soot inception, surface growth, coagulation, and PAH condensation, as well as oxidation by oxygen and hydroxyl-containing molecules. The overall numerical model was validated with experimental data on flame structure and soot distributions obtained from a constant-volume chamber. The model was also applied to predict combustion, soot and NOx emissions from a diesel engine using different biodiesel–diesel blends. The engine simulation results were further analysed to determine the soot emissions characteristics by use of biodiesel–diesel fuels.     

Optical characterization of methanol compression-ignition combustion in a heavy-duty engine

In the search for renewable fuels, there are very few candidates as compelling as methanol. It can be derived from refuse material and industrial waste, while the infrastructure exists worldwide to support broad and fast adoption, potentially even as a “drop-in” fuel for existing vehicles with only minor modifications. The most efficient engines currently available are compression-ignition engines, however they often come with high emissions or compromises like the soot-NO_x trade-off. Methanol however, is a low sooting fuel that can potentially be used in such engines despite its high resistance to auto-ignition and reduce emissions while maintaining high engine efficiency. Due to the auto-ignition resistance, few studies of methanol compression-ignition exist and even fewer are conducted in an optically accessible engine. Here, two cases of premixed combustion and two of spray-driven combustion of methanol are studied in a Heavy-Duty optically accessible engine. Ignition and combustion propagation are characterized with a combination of time-resolved natural flame luminosity measurements and single-shot, acetone fuel-tracer, laser induced fluorescence. Additionally, Mie-scattering is used to identify the interaction between liquid spray and ignition sites in spray-driven methanol combustion. Results show that methanol combusts drastically different compared to conventional fuels, especially in spray-driven combustion. The evaporative cooling effect of methanol appears to play a major role in the auto-ignition characteristics of the delivered fuel. Ignition sites appear right at the end of injection when the evaporative cooling effect is withdrawn or at liquid length oscillations where, again the effect is momentarily retracted. To the authors’ knowledge, this has not been documented before.     

Impact of coolant temperature on piston wall-wetting and smoke generation in a stratified-charge DISI engine operated on E30 fuel

A late-injection strategy is typically adopted in stratified-charge direct injection spark ignition (DISI) engines to improve combustion stability for lean operation, but this may induce wall wetting on the piston surface and result in high soot emissions. E30 fuel, i.e., gasoline with 30% ethanol, is a potential alternative fuel that can offer a high Research Octane Number. However, the relatively high ethanol content increases the heat of vaporization, potentially exacerbating wall-wetting issues in DISI engines. In this study, the Refractive Index Matching (RIM) technique is used to measure fuel wall films in the piston bowl. The RIM implementation uses a novel LED illumination, integrated in the piston assembly and providing side illumination of the piston-bowl window. This RIM diagnostics in combination with high-speed imaging was used to investigate the impact of coolant temperature on the characteristics of wall wetting and combustion in an optical DISI engine fueled with E30. The experiments reveal that the smoke emissions increase drastically from 0.068 FSN to 1.14 FSN when the coolant temperature is reduced from 90 °C to 45 °C. Consistent with this finding, natural flame luminosity imaging reveals elevated soot incandescence with a reduction of the coolant temperature, indicative of pool fires. The RIM diagnostics show that a lower coolant temperature also leads to increased fuel film thickness, area, and volume, explaining the onset of pool fires and smoke.     

Diesel engine emissions and combustion predictions using advanced mixing models applicable to fuel sprays

An advanced mixing model was applied to study engine emissions and combustion with different injection strategies ranging from multiple injections, early injection and grouped-hole nozzle injection in light and heavy duty diesel engines. The model was implemented in the KIVA-CHEMKIN engine combustion code and simulations were conducted at different mesh resolutions. The model was compared with the standard KIVA spray model that uses the Lagrangian-Drop and Eulerian-Fluid (LDEF) approach, and a Gas Jet spray model that improves predictions of liquid sprays. A Vapor Particle Method (VPM) is introduced that accounts for sub-grid scale mixing of fuel vapor and more accurately and predicts the mixing of fuel-vapor over a range of mesh resolutions. The fuel vapor is transported as particles until a certain distance from nozzle is reached where the local jet half-width is adequately resolved by the local mesh scale. Within this distance the vapor particle is transported while releasing fuel vapor locally, as determined by a weighting factor. The VPM model more accurately predicts fuel-vapor penetrations for early cycle injections and flame lift-off lengths for late cycle injections. Engine combustion computations show that as compared to the standard KIVA and Gas Jet spray models, the VPM spray model improves predictions of in-cylinder pressure, heat released rate and engine emissions of NOx, CO and soot with coarse mesh resolutions. The VPM spray model is thus a good tool for efficiently investigating diesel engine combustion with practical mesh resolutions, thereby saving computer time.     

Interpreting the influence of fuel spray impact on mixture preparation for HCCI combustion with port-fuel injection

This paper addresses the influence of fuel spray impact on fuel/air mixture for combustion in port-fuel injection engines. The experiments include time resolved measurements of surface temperature synchronized with PDA measurements of droplet dynamics at impact and were conducted to quantify the effects of interactions between successive injections on the mixture preparation for combustion in homogeneous charge compression ignition (HCCI) engines. Analysis shows that, during engine warm up, the heat transfer over the entire valve surface occurs within the vaporization-nucleate-boiling regime and the local instantaneous surface temperature correlates with the dynamics of droplets impacting at the same point. A functional relation is found for the heat transfer coefficient, which also describes other experiments reported in the literature. Similarity does not hold after the engine warms up because heat transfer and droplet vaporization at the surface are dominated by multiple interactions between droplets arisen from diverse heat transfer regimes. However, results evidence the existence of a critical surface temperature which sets a transition between overall heat transfer regimes dominated by local nucleate boiling at lower temperatures and by local intermittent transition regimes at higher temperatures. The heat transfer within the overall nucleate boiling regime is shown to be due to a thin film boiling mechanism leading to breakdown of the liquid-film at a nearly constant surface temperature, regardless of injection frequency or any other spray conditions. While at low frequencies this regime is not limited neither by the delivery of liquid to the surface, nor by the removal of vapour from the surface, at higher frequencies it is triggered by enhanced vaporization induced by piercing and mixing the liquid film. The results further evidence the important role of spray impingement for mixture preparation as required for HCCI.     

Butanol–acetone mixture blended with cottonseed biodiesel: Spray characteristics evolution,combustion characteristics,engine performance and emission

Increasing energy demands and more stringent legislation relating to pollutants such as nitrogen oxide (NO_x) and carbon monoxide (CO) from fossil fuels have accelerated the use of biofuels such as biodiesel. However, current limitations of using biodiesel as an alternative fuel for CI engines include a higher viscosity and higher NO_x emissions. This is a major issue that could be improved by blending biodiesel with alcohols. This paper investigates the effect of a butanol–acetone mixture (BA) as an additive blended with biodiesel to improve the latter's properties. Macroscopic spray characteristics (spray penetration, spray cone angle and spray volume) were measured in constant volume vessel (CVV) at two injection pressures. A high-speed camera was used to record spray images. The spray's edge was determined using an automatic threshold calculation algorithm to locate the spray outline (edge) from the binary images. In addition, an engine test was carried out experimentally on a single-cylinder diesel engine. The engine's performance was measured using in-cylinder pressure, brake power (BP) and specific fuel consumption (SFC). Emission characteristics NO_x, CO and UHC were also measured. Neat biodiesel and three blends of biodiesel with up to 30% added BA were tested. The experimental data were analyzed via ANOVA to evaluate whether variations in parameters due to the different fuels were significant. The results showed that BA can enhance the spray characteristics of biodiesel by increasing both the spray penetration length and the contact surface area, thereby improving air–fuel mixing. The peak in-cylinder pressure for 30% BA was comparable to neat diesel and higher than that of neat biodiesel. Brake power (BP) was slightly improved for 10% BA at an engine speed of 2000 rpm while SFC was not significantly higher for any of the BA-biodiesel blends because of the smaller heating value of BA. Comparing the effect on emissions of adding BA to biodiesel, increasing the amount of BA reduced NO_x and CO (7%) and (40%) respectively compared to neat biodiesel, but increased UHC.     

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.     

蒸发性对燃油瞬态喷雾撞击壁面的动态传热影响

直喷发动机燃油喷雾撞击壁面形成油膜,导致燃烧效率降低,颗粒物排放增加。伴随撞壁的动态传热过程对油膜蒸发具有重要影响。本文针对正戊烷、甲醇、甲醇汽油混合燃料瞬态喷雾撞击壁面,研究了不同条件下蒸发性对燃油瞬态喷雾撞击壁面动态传热影响。结果表明,提高喷油温度可促进燃油雾化,增大喷油压力或降低喷油距离可提高液滴撞壁强度,缩短液膜存在时间。撞壁瞬态温度与热流密度动态变化特征受燃油蒸发性与喷雾条件联合影响。     

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

DMM燃料柴油机可控预混合燃烧的研究

本文介绍了在压燃式发动机上进行的预混合燃烧研究。在柴油机的进气道入口处安装了一个电控燃料喷射系统,喷入具有低十六烷值、低沸点的甲缩醛(DMM)燃料,在压缩冲程中形成均匀的混合气,并在上止点附近喷入少量柴油来点燃混合气。本文研究了预混合燃料比、发动机负荷、进气中CO2浓度和喷孔直径对发动机燃烧和排放的影响。试验结果表明,进气道喷射DMM的预混合燃烧能同时大幅降低NOx和碳烟排放,为降低柴油机有害排放提供了一种新途径。     

Vaporisation characteristics of methanol,ethanol and heptane droplets in opposed stagnation flow at low temperature and pressure

A computational model is developed and applied to study the vaporisation behaviour of three liquid fuels. This fundamental study is motivated by a need to understand how the performance of direct-injection-spark-ignition (DISI) engines may be affected by changes in fuel composition, especially alcohols. Currently, most DISI engines are designed for homogeneous-charge combustion, where the in-cylinder fuel injection, vaporisation and mixing is accomplished during the intake and early in the compression process. Thus the temperature and pressure are low, compared to post-compression conditions. The two-phase axisymmetric model is based upon an ideal opposed stagnation flow field. Liquid droplets are carried in one air stream that is met by an opposed air flow. Because of stagnation-flow similarity, the mathematical model can be represented as a one-dimensional boundary-value problem. Results show significant differences between methanol, ethanol and heptane fuels, which have potentially important impacts on the design and modification of fuel-injection systems for direct-injection engines with alternative fuels.     

A joint experimental and numerical study of ignition in a spray burner

Partly due to stringent restrictions on pollutant emissions, aeronautical engine manufacturers target lean operating conditions which raise new difficulties such as combustion stability as well as ignition and re-ignition at high altitude. The injection of liquid fuel introduces additional complexity due to the spray-flame interaction. It is then crucial to better understand the physics behind these phenomena and to develop the capacity to predict them in an industrial context. In this work, a comprehensive joint experimental and numerical investigation of the academic swirled-confined version of the KIAI-Spray burner is carried out. Experimental diagnostics, such as Phase Doppler Anemometry (PDA), Planar Laser Induced Fluorescence (OH-PLIF), high-speed visualization and high-speed particle image velocimetry (HS-PIV), together with Large Eddy Simulations coupled to Discrete Particle Simulations are used to study spray flame structure and spray ignition. The analysis of the swirled-stabilized spray flame highlights the main effects of the presence of droplets on the turbulent combustion, and the complementarity and validity of the joint experiment and simulation approach. Ignition sequences are then studied. Both experiment and simulation show the same behaviors, and relate the flame kernel evolution and the possible success of ignition to the local non reacting flow properties at the sparking location, in terms of turbulence intensity and presence of droplets.     

柴油混合闪蒸喷雾的探索研究

基于同时降低柴油机中的NOx和微粒的想法，作者提出混合闪蒸喷雾的设想，井建立了混合闪蒸试验台，以证实这种设想在喷雾阶段是否能实现．试验证实了混合闪蒸雾化改善柴油雾化的机理．文中分析了水油比、水的温度、压力对雾束形状、油滴平均直径的影响，对混合闪蒸用于实际柴油机的潜力做了估计．     

Flow, spray and combustion analysis by laser techniques in the combustion chamber of a direct-injection diesel engine

The purpose of this paper is to show how the analysis of in -cylinder flow, fuel injection, and combustion by means of state-of-the-art optical techniques, as laser light-sheet, laser doppler anemometry and laser shadowgraphy, can help to support the understanding of the interaction of swirl flow development, spray formation, auto-ignition and combustion in near production-line direct-injection diesel engines and thus advances the development of engines with lower fuel consumption and emissions.     

Endoscopic fuel film,chemiluminescence, and soot incandescence imaging in a direct-injection spark-ignition engine

In direct-injection spark-ignition engines, fuel films formed on the piston surface due to impinging sprays are a major source of soot. Previous studies investigating the fuel films and their correlation to soot production were mostly performed in model experiments or optical engines. These experiments have different operating conditions compared to commercial engines. In this work, fuel films and soot are visualized in an all-metal engine with endoscopic access via laser-induced fluorescence (LIF) and natural incandescence, respectively. Gasoline and a mixture of isooctane/toluene were used as fuel for the experiments. The fuel films were excited by 266 nm laser pulses and visualized by an intensified CCD camera through a modular UV endoscope. Gasoline yielded much higher signal-to-noise ratio, and this fuel typically took an order of magnitude longer to evaporate than isooctane/toluene. The effects of injection time, injection pressure, engine temperature, and combustion on the fuel-film evaporation time were investigated. This film survival time was reduced with higher engine temperature, higher injection pressure, and later injection time, with engine temperature being the most significant parameter, whereas skip-fired combustion had very little effect on the film survival time. In complementary experiments, LIF from fuel films and soot incandescence were simultaneously visualized by an intensified double-frame CCD camera. At lower engine temperatures the fuel films remained distinct, and soot formation was limited to regions above the films, whereas at higher temperatures, fuel films, and hence the soot, appeared to be spread over the whole piston surface. Finally, high-speed imaging showed the spray, chemiluminescence, and soot incandescence, with results broadly consistent with fuel-film LIF and soot incandescence imaging.     

Simultaneous rainbow schlieren deflectometry and OH* chemiluminescence imaging of a diesel spray flame in constant pressure flow rig

The physical and chemical phenomena that take place during fuel injection, entrainment and fuel-air mixing, cool-flame and ignition reaction, and combustion in diesel sprays still require extensive study. Global parameters such as liquid and vapor jet penetration lengths and spreading rates render useful yet still limited information. Understanding of the temporal evolution of the spray as it progresses through various steps is needed to develop advanced clean combustion modes and high-fidelity predictive models with sufficient accuracy. In this study, high-speed rainbow schlieren deflectometry (RSD) and OH* chemiluminescence are used to simultaneously image fuel-air mixing, cool-flame reactions, ignition, flame propagation and stabilization, and combustion in a transient diesel-like flame. A constant pressure flow rig (CPFR) is used to conduct multiple injections in quick succession to obtain a statistically relevant dataset. n-heptane was injected at nominal supply pressure of 1000 bar from a single-hole diesel injector into ambient at pressure of 30 bar and temperature of 800 K. About 500 injections were performed and analyzed to reveal structural features of non-reacting and reacting regions of the spray, quantify jet penetration and spreading rates, and study cool-flame behavior, ignition, flame propagation and stabilization at lift-off length, and combustion at upstream and downstream locations.     

Topological transitions of Jet A-1 lean azimuthal flames (LEAF)

Prior studies about liquid fuel combustion in a vitiated air environment have shown increased combustion efficiency with reduced NOx, CO, and soot emissions. The concept of lean azimuthal flame (LEAF), which can be associated to the latter combustion mode, is based on opposed injections of air and liquid fuel sprays in an axisymmetric chamber with a central outlet, which can result in a highly turbulent toroidal reaction zone. The mixture of fresh air and hot combustion products of each spray provides a vitiated cross-flow configuration to the next spray distributed along the chamber circumference, leading to ignition and sequential combustion of the sprays by the others. The present paper deals with a LEAF combustor with air-assisted spray atomization, which has not been investigated so far. The combustor is fueled with Jet A-1 and operated from 15 to 25 kW with variations in the atomization-air to liquid mass flow ratio (ALR). This study focuses on the flame topology transitions as a function of atomizer ALR. Experimental results based on flame chemiluminescence and OH planar laser-induced fluorescence show two flame topologies: tubular and LEAF topology for ratio of 2 and 4, respectively (denoted ALR2 and ALR4). The spray Mie scattering indicates a significant presence of unburnt droplets for ALR2, whereas quick evaporation is observed for ALR4 cases. In this paper, we propose and validate a basic model based on the spray droplet size distribution, and the evaporation and convection timescales, which are the prominent factors governing the flame topology. Indeed, for ALR2, the evaporation timescale is longer than the convective timescale, which causes incomplete spray evaporation and insufficient vitiated environment, leading to a tubular flame topology and preventing a LEAF to develop. In contrast, for ALR4, the spray evaporation timescales are smaller than the convective timescales, which aids the LEAF topology.