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The enhancement of vortex development, fuel-air mixing and heat release in diesel spray flame by inversed-delta injection rate shaping, having been predicted via LES simulation with detailed chemical kinetics, is experimentally confirmed for the first time. Newly developed 3-injector TAIZAC (TAndem Injector Zapping ACtivation) injector realizing aggressive inversed-delta injection rate shaping was used for single-shot combustion experiments in a constant volume combustion vessel. Simultaneous high-speed (120,000fps) and high-resolution (1,280 x 704 pixels) laser schlieren and UV OH* chemiluminescence imaging combined with subsequent Flame Imaging Velocimetry (FIV) analysis was employed to elucidate the correlation between vortex development and enhanced heat release.

Shadowgraph/schlieren imaging techniques have often been used for flow visualization of reacting and non-reacting systems. In this paper we show that high-speed shadowgraph visualization in a high-pressure chamber can also be used to identify cool-flame and high-temperature combustion regions of diesel sprays, thereby providing insight into the time sequence of diesel ignition and combustion. When coupled to simultaneous high-speed Mie-scatter imaging, chemiluminescence imaging, pressure measurement, and spatially-integrated jet luminosity measurements by photodiode, the shadowgraph visualization provides further information about spray penetration after vaporization, spatial location of ignition and high-temperature combustion, and inactive combustion regions where problematic unburned hydrocarbons exist. Examples of the joint application of high-speed diagnostics include transient non-reacting and reacting injections, as well as multiple injections.

For better understanding of soot formation and oxidation processes in a biodiesel spray flame, the morphology, microstructure and sizes of soot particles directly sampled in a spray flame fuelled with soy-methyl ester were investigated using transmission electron microscopy (TEM). The soot samples were taken at different axial locations in the spray flame, 40, 50 and 70 mm from injector nozzle, which correspond to soot formation, peak, and oxidation zones, respectively. The biodiesel spray flame was generated in a constant-volume combustion chamber under a diesel-like high pressure and temperature condition (6.7 MPa, 1000K). Density, diameter of primary particles and radius of gyration of soot aggregates reached a peak at 50 mm from the injector nozzle and was lower or smaller in the formation or oxidation zones of the spray.

Diesel low-temperature combustion strategies often rely on early injection timing to allow sufficient fuel-ambient mixing to avoid NOx and soot-forming combustion. However, these early injection timings permit the spray to penetrate into a low ambient temperature and density environment where vaporization is poor and liquid impingement upon the cylinder liner and piston bowl are more likely to occur. The objective of this study is to measure the transient liquid and vapor penetration at early-injection conditions. High-speed Mie-scatter and shadowgraph imaging are employed in an optically accessible chamber with a free path of 100 mm prior to wall impingement and using a single-spray injector. The ambient temperature and density within the chamber are well-controlled (uniform) and selected to simulate in-cylinder conditions when injection occurs at -40 crank-angle degrees (CAD) or fewer before top-dead center (TDC).

Future fuels will come from a variety of feed stocks and refinement processes. Understanding the fundamentals of combustion and pollutants formation of these fuels will help clear hurdles in developing flex-fuel combustors. To this end, we investigated the combustion, soot formation, and soot oxidation processes for various classes of fuels, each with distinct physical properties and molecular structures. The fuels considered include: conventional No. 2 diesel (D2), low-aromatics jet fuel (JC), world-average jet fuel (JW), Fischer-Tropsch synthetic fuel (JS), coal-derived fuel (JP), and a two-component surrogate fuel (SR). Fuel sprays were injected into high-temperature, high-pressure ambient conditions that were representative of a practical diesel engine. Simultaneous laser extinction measurement and planar laser-induced incandescence imaging were performed to derive the in-situ soot volume fraction.

In order to better understand in-flame diesel soot oxidation processes, soot particles at the oxidation-dominant periphery of diesel spray flame were sampled by a newly developed “suck” type soot sampler employing a high-speed solenoid valve and their morphology and nanostructure were observed via high-resolution transmission electron microscopy (HR-TEM). A single-shot diesel spray flame for the soot sampling experiment was achieved in a constant-volume vessel under a diesel-like condition. The sampler instantaneously sucks out a small portion of soot laden gases from the flame. A TEM grid holds inside the flow passage close to its entrance is immediately exposed to the gas flow induced by the suction at the upstream of the solenoid valve, so that the quick thermophoretic soot deposition onto the grid surface can effectively freeze morphology variation of soot particles during the sampling processes.

For better understanding of soot formation and oxidation processes in a diesel spray flame, two kinds of planar soot imaging techniques, Laser-Induced Incandescence (LII) and Laser Scattering (LS) techniques, were applied simultaneously to a diesel spray flame in a constant-volume combustion vessel under a diesel-like condition (2.5MPa, 940K). An analysis of LII and LS images yielded 2-dimensional distribution images of concentration, size and number density of soot particles in the spray flame, based on an assumption that LII and LS signals are proportional to the soot particle size to the power of 3 and 6, respectively. In order to obtain clearer variation trend in the soot concentration, size and number density distribution in significantly fluctuating single-shot diesel spray flames, spontaneous and time-integrated ensemble averaging of the laser-measured images were employed.

The reaction zone of a diesel fuel jet stabilizes at a location downstream of the fuel injector once the initial autoignition phase is over. This distance is referred to as flame lift-off length. Recent investigations have examined the effects of a wide range of parameters (injection pressure, orifice diameter, and ambient gas temperature, density and oxygen concentration) on lift-off length under quiescent diesel conditions. Many of the experimental trends in lift-off length were in agreement with scaling laws developed for turbulent, premixed flame propagation in gas-jet lifted flames at atmospheric conditions. However, several effects did not correlate with the gas-jet scaling laws, suggesting that other mechanisms could be important to lift-off stabilization at diesel conditions. This paper shows experimental evidence that ignition processes affect diesel lift-off stabilization.

This paper details the development and application of a Rayleigh imaging technique for quantitative mixing measurements in a vaporizing diesel spray under engine conditions. Experiments were performed in an optically accessible constant-volume combustion vessel that simulated the ambient conditions in a diesel engine. Two-dimensional imaging of Rayleigh scattering from a diesel spray of n-heptane and well-characterized ambient was accomplished by using a 532 nm Nd:YAG laser sheet and a low-noise back-illuminated CCD camera. Methods to minimize interference from unwanted elastic scattering sources (e.g. windows, particles) were investigated and are discussed in detail. The simultaneous measurement of Rayleigh scattering signal from the ambient and from the diesel spray provides important benefits towards making the technique quantitative and accurate.

In order to investigate early soot formation process in diesel combustion, spectral analysis and optical thermometry of early soot formation region in a transient spray flame under diesel-like conditions (Pg2.8 MPa, Tg620-820K) was attempted via laser-induced fluorescence (LIF) from pyrene (C16H10) doped in the fuel. Pyrene is known to exhibit a temperature\-dependent variation of LIF spectrum; the ratio of S2/S1 fluorescence yields, from the lowest excited singlet state S1 and the second excited singlet state S2, depends on temperature. In the present study, pyrene was doped (1%wt) in a model diesel fuel (0-solvent) and the variation of LIF spectra from the pyrene in the spray flame in a rapid compression machine were examined at different ambient temperatures, ambient oxygen concentrations, measurement positions and timings after start of fuel injection.

In our previous work, diesel late combustion heat release is suspected to originate from rich fuel mixture cloud stagnating at the spray tip. Injection rate shaping is gaining attention as an attractive strategy to control diesel spray combustion characteristics where it could be an effective approach in reducing the late combustion. Progressive ramp-down injection rate as in “inversed-delta” shape is achieved by using a novel rate shaping injector called TAIZAC (TAndem Injectors Zapping Activation); rate shaping can be realized by controlling the actuation timing of two directly-connected commercially available injectors. To investigate the potential of inversed-delta rate shaping for reduction of diesel late combustion, simultaneous high-speed UV laser diffuse back illumination (DBI), UV emissions and soot luminosity imaging of inversed-delta and conventional rectangle-injected spray flames conducted in a constant volume combustion chamber are compared.

For better understanding of soot formation and oxidation processes in diesel spray flame, the nanostructure of primary soot particles directly sampled in a diesel spray flame was investigated via High-Resolution Transmission Electron Microscopy (HRTEM). A single-shot diesel spray flame was achieved in a constant volume combustion vessel under diesel-like conditions (Ta=1000K, Pa=2.7 MPa) and a micro-grid for HRTEM observation was directly exposed to the spray flame to thermophoretically sample soot particles onto the grid surface. A preliminary nanostructure investigation was conducted for x500k magnification HRTEM images of soot particles directly sampled in diesel spray flames of Fischer-Tropsch Diesel (FTD) fuel seeded with naphthalene as a representative aromatic substance. A MATLAB code for HRTEM image processing and analysis of lattice fringes within primary soot particles was developed and used to characterize the length, tortuosity and separation of lattice fringes.

Measurements of Excitation-Emission Matrix (EEM) of shock-heated vapors of polycyclic aromatic hydrocarbons (PAHs) at high temperature (750-1500K) and high pressure (0.3-1.3MPa) conditions were conducted using a multi-wavelength excitation laser in order to demonstrate the potential of the single-measurement EEM fluorometry for investigation of soot precursors. Argon-diluted vapors of naphthalene and pyrene, as PAH model compounds, were heated in an optically accessible shock tube. The PAH vapors were excited by a coherent multi-wavelength “rainbow” laser light generated by converting the 4th harmonic (266nm) of a pulsed Nd:YAG laser using a Raman cell frequency converter filled with high-pressure (2MPa) methane-hydrogen mixture.

The liquid and vapor-phase spray penetrations of #2 diesel and neat (100%) soybean-derived biodiesel have been studied at late expansion-cycle conditions in a constant-volume optical chamber. In modern diesel engines, late-cycle staged injections may be used to assist in the operation of exhaust stream aftertreatment devices. These late-cycle injections occur well after top-dead-center (TDC), when post-combustion temperatures are relatively high and densities are low. The behavior of diesel sprays under these conditions has not been well-established in the literature. In the current work, high-speed Mie-scatter and schlieren imaging are employed in an optically accessible chamber to characterize the transient and quasi-steady liquid penetration behavior of diesel sprays under conditions relevant for late-cycle post injections, with very low densities (1.2 - 3 kg/m 3 ) and moderately high temperatures (800 - 1400 K).

In order to investigate the mechanism of heat transfer on the chamber wall of direct-injection diesel engines, 2-D temperature imaging and heat flux measurement in the flame impinging region on the chamber wall were conducted using laser-induced phosphorescence technique. The temperature of the chamber wall surface was measured by the calibrated intensity variation of the 355nm-excited laser-induced phosphorescence from an electrophoretically deposited thin layer of La2O2S:Eu phosphor on a quartz glass plate placed in a rapid compression and expansion machine (RCEM). Instantaneous 2-D images of wall temperature at different timings after start of injection and time-resolved (10kHz) heat flux near the flame impinging region were obtained for combusting and non-combusting diesel sprays with impinging distance of 23.4mm at different injection pressures (80 and 120MPa).

Laser plasma ignition has been pursued by engine researchers as an alternative to electric spark-ignition systems, potentially offering benefits by avoiding quenching surfaces and extending breakdown limits at higher boost pressure and lower equivalence ratio. For this study, we demonstrate another potential benefit: the ability to control the timing of ignition with short, nanosecond pulses, thereby optimizing the type of mixture that burns in rapidly changing, stratified fuel-air mixtures. We study laser ignition at various timings during single and double injections at simulated gasoline engine conditions within a controlled, high-temperature, high-pressure vessel. Laser ignition is accomplished with a single low-energy (10 mJ), short duration (8 ns) Nd:YAG laser beam that is tightly focused (0.015 mm average measured 1/e₂ diameter) at a typical GDI spark plug location.

As a new method to examine the extremely unsteady and spatially varying wall heat transfer phenomena on diesel engine combustion chamber wall, high-speed imaging of infrared thermal radiation from the chromium coated window surface impinged by a diesel spray flame has been conducted in a constant volume combustion chamber. The infrared radiation from a back surface of the chromium layer was successfully visualized at 10kHz frame rate and 128 × 128 pixel resolution through the window. The distributions of infrared radiation, temperature and heat flux exhibited coherent and streaky structure with radial stripes extending and waving from a stagnation point likely reflecting the near-wall turbulent structure in a wall impinging diesel flame. The experiments were conducted with various parameters such as fuel injection pressure, ambient gas oxygen concentration, wall impinging distance, wall surface roughness and wall materials.

Diesel injection parameters effect on liquid-phase diesel spray penetration after the end-of-injection (EOI) is investigated in a constant-volume chamber over a range of ambient and injector conditions typical of a diesel engine. Our past work showed that the maximum liquid penetration length of a diesel spray may recede towards the injector after EOI at some conditions. Analysis employing a transient jet entrainment model showed that increased fuel-ambient mixing occurs during the fuel-injection-rate ramp-down as increased ambient-entrainment rates progress downstream (i.e. the entrainment wave), permitting complete fuel vaporization at distances closer to the injector than the quasi-steady liquid length. To clarify the liquid-length recession process, in this study we report Mie-scatter imaging results near EOI over a range of injection pressure, nozzle size, fuel type, and rate-of-injection shape. We then use a transient jet entrainment model for detailed analysis.

For better understanding, model development and its validation of in-cylinder soot formation processes of Gasoline Direct Injection (GDI) engines, crank-angle-resolved mass and size distribution of in-cylinder soot during a GDI combustion cycle were investigated via optical measurements and total cylinder sampling technique in an optically accessible Rapid Compression and Expansion Machine (RCEM). A direct-injection, spark-ignited and single-shot combustion event was achieved in the RCEM operated with engine speed 600 rpm, compression ratio 9.0, equivalence ratio 0.9 and natural aspiration. A three-component (iso-octane 65%, n-heptane 10%, toluene 25%) gasoline surrogate fuel and a multi-hole injector shared within the Japanese SIP Innovative Combustion Technology research program were used.

Diesel spray mixture formation is investigated at target conditions using multiple diagnostics and laboratories. High-speed Particle Image Velocimetry (PIV) is used to measure the velocity field inside and outside the jet simultaneously with a new frame straddling synchronization scheme. The PIV measurements are carried out in the Engine Combustion Network Spray A target conditions, enabling direct comparisons with mixture fraction measurements previously performed in the same conditions, and forming a unique database at diesel conditions. A 1D spray model, based upon mass and momentum exchange between axial control volumes and near-Gaussian velocity and mixture fraction profiles is evaluated against the data.