Publications

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Time-resolved visualization of fast processes using high-speed digital video-cameras has been widely used in most fields of scientific research for over a decade. In many applications, high-speed imaging is used not only to record the time history of a phenomenon but also to quantify it, hence requiring dependable equipment. Important aspects of two-dimensional imaging instrumentation used to qualitatively or quantitatively assess fast-moving scenes include sensitivity, linearity, as well as signal-to-noise ratio (SNR). Under certain circumstances, the weaknesses of commercially available high-speed cameras, i.e., sensitivity, linearity, image lag, etc., render the experiment complicated and uncertain. Our study evaluated two advanced CMOS-based, continuous-recording, high-speed cameras available at the moment of writing. Various parameters, potentially important toward accurate time-resolved measurements and photonic quantification, have been measured under controlled conditions on the bench, using scientific instrumentation. Testing procedures to measure sensitivity, linearity, SNR, shutter accuracy, and image lag are proposed and detailed. The results of the tests, comparing the two high-speed cameras under study, are also presented and discussed. Results show that, with careful implementation and understanding of their performance and limitations, these high-speed cameras are reasonable alternatives to scientific CCD cameras, while also delivering time-resolved imaging data.

All future high-efficiency engines will have fuel directly sprayed into the engine cylinder. Engine developers agree that a major barrier to the rapid development and design of these high-efficiency, clean engines is the lack of accurate fuel spray computational fluid dynamic (CFD) models. The spray injection process largely determines the fuel-air mixture processes in the engine, which subsequently drives combustion and emissions in both direct-injection gasoline and diesel systems. More predictive spray combustion models will enable rapid design and optimization of future high-efficiency engines, providing more affordable vehicles and also saving fuel.

The DOE project for Co-Optimization of Fuels and Engines seeks to define both fuel properties and engine hardware to create cleaner and more fuel-efficient engines. Fuel spray technologies are central to this goal as the spray injection determines the combustible mixtures formed within the engine. Sprays are known to affect bum rate and efficiency, particulate formation and emissions, as well as temperature and engine knock sites. Computational fluid dynamic models must predict complicated interaction between plumes and vaporization to be useful as a design tool for industry. Changes in fuel properties are expected to affect fuel delivery. While Co-Optima fuels may be selected for chemical criteria, such as high octane number rating, an understanding of how the physical properties affect spray performance is necessary to optimize fuel delivery. Many of the selected Co-Optima fuels have properties that are different than standard gasoline, requiring investigations for their performance. A new continuous-flow spray chamber facility has been completed, offering capability to control the pressure and temperature of the gases at engine-relevant conditions at the time of injection as well as a massive increase in data throughput. Direct-injection multi-hole gasoline sprays for different Co-Optima fuels are investigated in this chamber.

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The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel-air mixing. However, the mechanisms of spray collapse are not fully understood and are difficult to predict. The purpose of this work is to study the aerodynamics in the inter-spray region, which can potentially lead to plume collapse. High-speed (100 kHz) particle image velocimetry is applied along a plane between plumes to observe the full temporal evolution of plume interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using simultaneous diffused back illumination and Mie-scatter techniques. Experiments are performed under simulated engine conditions using a symmetric eight-hole injector in a high-temperature, high-pressure vessel at the “Spray G” operating conditions of the engine combustion network. Indicators of plume interaction and collapse include changes in counter-flow recirculation of ambient gas toward the injector along the axis of the injector or in the inter-plume region between plumes. The effect of ambient temperature and gas density on the inter-plume aerodynamics and the subsequent plume collapse are assessed. Increasing ambient temperature or density, with enhanced vaporization and momentum exchange, accelerates the plume interaction. Plume direction progressively shifts toward the injector axis with time, demonstrating that the plume interaction and collapse are inherently transient.

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The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel-air mixing. However, the mechanisms of spray collapse are not fully understood. The purpose of this work is to study the effects of injection duration and multiple injections on the interaction and/or collapse of multi-plume GDI sprays. High-speed (100 kHz) Particle Image Velocimetry (PIV) is applied along a plane between plumes to observe the full temporal evolution of plume-interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using Diffused Back Illumination (DBI). Experiments are performed under simulated engine conditions using a symmetric 8-hole injector in a high-temperature, high-pressure vessel at the "Spray G" operating conditions of the Engine Combustion Network (ECN). Longer injection duration is found to promote plume collapse, while staging fuel delivery with multiple, shorter injections is resistant to plume collapse.

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Designers of direct-injection compression-ignition engines use a variety of strategies to improve the fuel/charge-gas mixture within the combustion chamber for increased efficiency and reduced pollutant emissions. Strategies include the use of high fuel-injection pressures, multiple injections, small injector orifices, flow swirl, long-ignition-delay conditions, and oxygenated fuels. This is the first journal publication paper on a new mixing-enhancement strategy for emissions reduction: ducted fuel injection. The concept involves injecting fuel along the axis of a small cylindrical duct within the combustion chamber, to enhance the mixture in the autoignition zone relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). Finally, the results described herein, from initial proof-of-concept experiments conducted in a constant-volume combustion vessel, show dramatically lower soot incandescence from ducted fuel injection than from free sprays over a range of charge-gas conditions that are representative of those in modern direct-injection compression-ignition engines.

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A conceptual model for turbulent ignition in high-pressure spray flames is presented. The model is motivated by first-principles simulations and optical diagnostics applied to the Sandia n-dodecane experiment. The combined analysis established a conceptual model for turbulent ignition in high-pressure spray flames which is based on a set of identified characteristic time scales. The suddenly forming steep gradients from successful high-temperature ignition initiate the propagation of a turbulent flame. It rapidly ignites the entire spray head on time scales which are generally significantly smaller than the corresponding cool flame wave time scales.

Fuel and oxidizer mixing is a key parameter influencing combustion and emission performance in diesel engines. At the same time, quantitative mixing measurements in automotive sprays are very challenging such that only a few experimental results are available as targets for the development and tuning of numerical models. The caveat is that the experimental data mainly concern the quasi-steady part of the jet, while it can be argued that the injection process in current alternative thermal engines is mostly transient. This work applies planar laser Rayleigh scattering at high-frequency to resolve the development and mixing of vaporized diesel sprays injected in a highly-pressurized environment. The state-of-the-art equipment employed for these experiments include a purposely-built high-power, high-repetition rate pulsed burst laser, optimized optics and a state-of-the-art high-speed CMOS camera. Advanced image processing methods were developed and implemented to mitigate the negative effects of the extreme environments found in diesel engines at the time of injection. The experiments provided two-dimensional mean and variance of the mixture and temperature quantities. The optical system's high spatial and temporal resolution enables tracking of the mixing field with time and space, from which temporally and spatially correlated mixing quantities can be extracted. Further analysis of the detailed mixture and temperature fields offered information about the turbulent mixing process of high-pressure diesel sprays such as scalar dissipation rates or turbulent length scales. Substantial effort was made to assess the uncertainties and limitations of such experimental results due to the optically challenging environment.

Designers of direct-injection compression-ignition engines use a variety of strategies to improve the fuel/charge-gas mixture within the combustion chamber for increased efficiency and reduced pollutant emissions. Strategies include the use of high fuel-injection pressures, multiple injections, small injector orifices, flow swirl, long-ignition-delay conditions, and oxygenated fuels. This is the first journal publication on a new mixing-enhancement strategy for emissions reduction: ducted fuel injection. The concept involves injecting fuel along the axis of a small cylindrical duct within the combustion chamber, to enhance the mixture in the autoignition zone relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). The results described herein, from initial proof-of-concept experiments conducted in a constant-volume combustion vessel, show dramatically lower soot incandescence from ducted fuel injection than from free sprays over a range of charge-gas conditions that are representative of those in modern direct-injection compression-ignition engines.

Whilst the physics of both classical evaporation and supercritical fluid mixing are reasonably well characterized and understood in isolation, little is known about the transition from one to the other in the context of liquid fuel systems. The lack of experimental data for microscopic droplets at realistic operating conditions impedes the development of phenomenological and numerical models. To address this issue we performed systematic measurements using high-speed long-distance microscopy, for three single-component fuels (n-heptane, n-dodecane, n-hexadecane), into gas at elevated temperatures (700–1200 K) and pressures (2–11 MPa). We describe these high-speed visualizations and the time evolution of the transition from liquid droplet to fuel vapour at the microscopic level. The measurements show that the classical atomization and vaporisation processes do shift to one where surface tension forces diminish with increasing pressure and temperature, but the transition to diffusive mixing does not occur instantaneously when the fuel enters the chamber. Rather, subcritical liquid structures exhibit surface tension in the near-nozzle region and then, after time surrounded by the hot ambient gas and fuel vapour, undergo a transition to a dense miscible fluid. Although there was clear evidence of surface tension and primary atomization for n-dodecane and n-hexadecane for a period of time at all the above conditions, n-heptane appeared to produce a supercritical fluid from the nozzle outlet when injected at the most elevated conditions (1200 K, 10 MPa). This demonstrates that the time taken by a droplet to transition to diffusive mixing depends on the pressure and temperature of the gas surrounding the droplet as well as the fuel properties. We summarise our observations into a phenomenological model which describes the morphological evolution and transition of microscopic droplets from classical evaporation through a transitional mixing regime and towards diffusive mixing, as a function of operating conditions. We provide criteria for these regime transitions as reduced pressure–temperature correlations, revealing the conditions where transcritical mixing is important to diesel fuel spray mixing.

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This work explores the mechanisms by which a post injection can reduce unburned hydrocarbon (UHC) emissions in heavy-duty diesel engines operating at low-temperature combustion conditions. Post injections, small, close-coupled injections of fuel after the main injection, have been shown to reduce UHC in the authors’ previous work. In this work, we analyze optical data from laser-induced fluorescence of both CH₂O and OH and use chemical reactor modeling to better understand the mechanism by which post injections reduce UHC emissions. The results indicate that post-injection efficacy, or the extent to which a post injection reduces UHC emissions, is a strong function of the cylinder pressure variation during the post injection. However, the data and analysis indicate that the pressure and temperature rise from the post injection combustion cannot solely explain the UHC reduction measured by both engine-out and optical diagnostics. In conclusion, the fluid-mechanic, thermal, and chemical interaction of the post injection with the main-injection mixture is a key part of UHC reduction; the starting action of the post jet and the subsequent entrainment of surrounding gases are likely both important processes in reducing UHC with a post injection.

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An n-dodecane spray flame (Spray A from Engine Combustion Network) was simulated using a δ function combustion model along with a dynamic structure large eddy simulation (LES) model to evaluate its performance at engine-relevant conditions and to understand the transient behavior of this turbulent flame. The liquid spray was treated with a traditional Lagrangian method and the gas-phase reaction was modeled using a δ function combustion model. A 103-species skeletal mechanism was used for the n-dodecane chemical kinetic model. Significantly different flame structures and ignition processes are observed for the LES compared to those of Reynolds-averaged Navier-Stokes (RANS) predictions. The LES data suggests that the first ignition initiates in a lean mixture and propagates to a rich mixture, and the main ignition happens in the rich mixture, preferably less than 0.14 in mixture fraction space. LES was observed to have multiple ignition spots in the mixing layer simultaneously while the main ignition initiates in a clearly asymmetric fashion. The temporal flame development also indicates the flame stabilization mechanism is auto-ignition controlled. Soot predictions by LES present much better agreement with experiments compared to RANS, both qualitatively and quantitatively. Multiple realizations for LES were performed to understand the realization to realization variation and to establish best practices for ensemble-averaging diesel spray flames. The relevance index analysis suggests that an average of 5 and 6 realizations can reach 99% of similarity to the target average of 16 realizations on the mixture fraction and temperature fields, respectively. However, more realizations are necessary for the hydroxide (OH) and soot mass fractions due to their high fluctuations.

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In this LDRD project, we developed a capability for quantitative high - speed imaging measurements of high - pressure fuel injection dynamics to advance understanding of turbulent mixing in transcritical flows, ignition, and flame stabilization mechanisms, and to provide e ssential validation data for developing predictive tools for engine combustion simulations. Advanced, fuel - efficient engine technologies rely on fuel injection into a high - pressure, high - temperature environment for mixture preparation and com bustion. Howe ver, the dynamics of fuel injection are not well understood and pose significant experimental and modeling challenges. To address the need for quantitative high - speed measurements, we developed a Nd:YAG laser that provides a 5ms burst of pulses at 100 kHz o n a robust mobile platform . Using this laser, we demonstrated s patially and temporally resolved Rayleigh scattering imaging and particle image velocimetry measurements of turbulent mixing in high - pressure gas - phase flows and vaporizing sprays . Quantitativ e interpretation of high - pressure measurements was advanced by reducing and correcting interferences and imaging artifacts.

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We investigate the mixing, penetration, and ignition characteristics of high-pressure n-dodecane sprays having a split injection schedule (0.5/0.5 dwell/0.5 ms) in a pre-burn combustion vessel at ambient temperatures of 750 K, 800 K and 900 K. High-speed imaging techniques provide a time-resolved measure of vapor penetration and the timing and progression of the first- and second-stage ignition events. Simultaneous single-shot planar laser-induced fluorescence (PLIF) imaging identifies the timing and location where formaldehyde (CH2O) is produced from first-stage ignition and consumed following second-stage ignition. At the 900-K condition, the second injection penetrates into high-temperature combustion products remaining in the near-nozzle region from the first injection. Consequently, the ignition delay for the second injection is shorter than that of the first injection (by a factor of two) and the second injection ignites at a more upstream location near the liquid length. At the 750 K and 800 K conditions, high-temperature ignition does not occur in the near-nozzle region after the end of the first injection, though formaldehyde remains from first-stage reactions. Under these conditions, the second injection penetrates into cool-flame products that are slightly elevated in temperature (∼100 K) relative to the ambient. This modest temperature increase and the availability of reactive cool-flame products reduces the first- and second-stage ignition delay of the second injection by a factor of approximately two relative to the first injection. At the 750-K ambient condition, high-temperature ignition of the first injection does not occur until the second injection enriches the very fuel-lean downstream regions.

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The development of accurate predictive engine simulations requires experimental data to both inform and validate the models, but very limited information is presently available about the chemical structure of high pressure spray flames under engine- relevant conditions. Probing such flames for chemical information using non- intrusive optical methods or intrusive sampling techniques, however, is challenging because of the physical and optical harshness of the environment. This work details two new diagnostics that have been developed and deployed to obtain quantitative species concentrations and soot volume fractions from a high-pressure combusting spray. A high-speed, high-pressure sampling system was developed to extract gaseous species (including soot precursor species) from within the flame for offline analysis by time-of-flight mass spectrometry. A high-speed multi-wavelength optical extinction diagnostic was also developed to quantify transient and quasi-steady soot processes. High-pressure sampling and offline characterization of gas-phase species formed following the pre-burn event was accomplished as well as characterization of gas-phase species present in the lift-off region of a high-pressure n-dodecane spray flame. For the initial samples discussed in this work several species were identified, including polycyclic aromatic hydrocarbons (PAH); however, quantitative mole fractions were not determined. Nevertheless, the diagnostic developed here does have this capability. Quantitative, time-resolved measurements of soot extinction were also accomplished and the novel use of multiple incident wavelengths proved valuable toward characterizing changes in soot optical properties within different regions of the spray flame.

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Quantitative measurements of the total radiative heat transfer from high-pressure diesel spray flames under a range of conditions will enable engine modelers to more accurately understand and predict the effects of advanced combustion strategies on thermal loads and efficiencies. Moreover, the coupling of radiation heat transfer to soot formation processes and its impact on the temperature field and gaseous combustion pollutants is also of great interest. For example, it has been shown that reduced soot formation in diesel engines can result in higher flame temperatures (due to less radiative cooling) leading to greater NOx emissions. Whereas much of the previous work in research engines has evaluated radiation based on two- or three-color detection with limited spatial resolution, this work uses an imaging spectrometer in conjunction with a constant volume pre-burn vessel to quantify soot temperatures, optical thickness, and total radiation with spatial and spectral (360-700 nm) resolution along the flame axis. Sprays of n-dodecane were injected from a single hole, 90-m diameter orifice into a range of ambient temperature conditions while holding ambient density and oxygen concentration constant at 22.8 kg/m 3 and 15%, respectively. Soot surface temperatures derived by fitting a model to the spectral data were within 10 K of the stoichiometric computed adiabatic flame temperature for lower ambient temperature, lower sooting cases. As ambient temperature was increased, leading to greater soot formation, the spectrally derived peak soot temperature decreased relative to the calculated adiabatic flame temperature. For the highest ambient temperature case (1200 K), the spectrally derived soot surface temperature was more than 140 K lower than the calculated adiabatic flame temperature. Values of optical thickness, KL, were also derived by fitting the spectral data and these values were compared to extinction based KL measurements. The spectrally derived KL was within a factor of about 1.5 from the extinction based data for the higher sooting cases. Under lower sooting conditions the differences were larger. For the lowest sooting case, the radiant fraction-defined as the fraction of energy emitted by radiation relative to the chemical energy available from the fuel injection-was negligible at less than 0.01%. The highest temperature flame with the greatest optical thickness resulted in a radiant fraction of 0.46%. Copyright © 2014 SAE International.

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This paper provides an analysis of high-pressure phenomena and its potential effects on the fundamental physics of fuel injection in Diesel engines. We focus on conditions when cylinder pressures exceed the thermodynamic critical pressure of the injected fuel and describe the major differences that occur in the jet dynamics compared to that described by classical spray theory. To facilitate the analysis, we present a detailed model framework based on the Large Eddy Simulation (LES) technique that is designed to account for key high-pressure phenomena. Using this framework, we perform a detailed analysis using the experimental data posted as part of the Engine Combustion Network (see www.sandia.gov/ECN): namely the "Baseline n-heptane" and "Spray-A (n-dodecane)" cases, which are designed to emulate conditions typically observed in Diesel engines. Calculations are performed by rigorously treating the experimental geometry, operating conditions and relevant thermo-physical gas-liquid mixture properties. Results are further processed using linear gradient theory, which facilitates calculations of detailed vapor-liquid interfacial structures, and compared with the high-speed imaging data. Analysis of the data reveals that fuel enters the chamber as a compressed liquid and is heated at supercritical pressure. Further analysis suggests that, at certain conditions studied here, the classical view of spray atomization as an appropriate model is questionable. Instead, nonideal real-fluid behavior must be taken into account using a multicomponent formulation that applies to arbitrary hydrocarbon mixtures at high-pressure supercritical conditions.

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This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation model to accurately predict gas temperatures and thus soot formation rates. When including such a radiation model, the LES model predicts mean soot concentrations within 30% in the ethylene jet flame.

This paper presents experimental results for two fuel-related topics in a diesel engine: (1) how fuel volatility affects the premixed burn and heat release rate, and (2) how ignition quality influences the soot formation. Fast evaporation of fuel may lead to more intense heat release if a higher percentage of the fuel is mixed with air to form a combustible mixture. However, if the evaporation of fuel is driven by mixing with high-temperature gases from the ambient, a high-volatility fuel will require less oxygen entrainment and mixing for complete vaporization and, consequently, may not have potential for significant heat release simply because it has vaporized. Fuel cetane number changes also cause uncertainty regarding soot formation because variable ignition delay will change levels of fuel-air mixing prior to combustion. To address these questions, experiments are performed using a constant-volume combustion chamber simulating typical low-temperature-combustion (LTC) diesel conditions. We use fuels that have the same ignition delay (and therefore similar time for premixing with air), but different fuel volatility, to assess the heat-release rate and spatial location of combustion. Under this condition, where fuel volatility is decoupled from the ignition delay, results show almost the same heat release rate and spatial location of the premixed burn. The effect of ignition quality on soot formation has also been studied while maintaining similar levels of fuel-ambient mixing prior to combustion. To achieve the same ignition delay, the high-cetane-number fuel is injected into an ambient gas at a lower temperature and vice versa. The total soot mass within the spray is measured and compared for fuels with different cetane numbers but with the same premixing level (e.g. the same ignition delay and lift-off length). Experimental results show that the combination of high cetane number and low ambient gas temperature produces lower soot than the other combination, because the ambient temperature predominantly affects soot formation.

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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. Shadowgraph and schlieren image processing steps required to account for variations of refractive index within the high-temperature combustion vessel gases are also shown.

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. Results show that an increased injection pressure correlates well with increasing liquid length recession due to an increased entrainment wave speed. Likewise, an increased nozzle size, with higher jet momentum and faster entrainment wave, enhances the liquid length recession. A low-density, high-volatility fuel does not decrease the strength of the entrainment wave; however, it decreases the steady liquid length causing the entrainment wave to reach the liquid spray tip earlier, which ultimately results in faster liquid length recession. A slow ramp down in injection rate causes a weaker entrainment wave so that the liquid length recession occurs even prior to injector close.