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Technical Paper
2014-10-13
Magnus Sjöberg, Wei Zeng, Daniel Singleton, Jason M. Sanders, Martin A. Gundersen
It is well known that well-mixed lean or dilute SI engine operation can provide improvements of the fuel economy (FE) relative to that of traditional well-mixed stoichiometric SI operation. However, the potential is limited by the onset of unstable combustion for low fuel/air-equivalence (phi) ratios. This work examines the use of two methods for improving combustion stability for lean operation, namely multi-pulse transient plasma ignition and intake air preheating. These two methods are compared to standard SI operation using a normal inductive ignition system without intake air preheating. E85 is the fuel chosen for this study. The experimental results from single-cylinder testing show that the FE improvement for lean operation with the regular spark system without intake air preheating amounts to 12% for phi = 0.67. Using a combination of intake air preheating and multi-pulse ignition, the engine can be operated stably at lower phi, enabling a larger improvement of FE, amounting to 17% for phi = 0.59.
Technical Paper
2014-10-13
Julien Manin, Scott Skeen, Lyle Pickett, Eric Kurtz, James E. Anderson
The purpose of this study is to characterize extended lift-off combustion in a controlled engine combustion simulation facility using selected fuels of different cetane number, oxygen content, molecular structure, and blend ratio. Ignition delay time, lift-off length and soot KL extinction have been measured for ambient and injector operating conditions relevant to modern diesel engines. The experiments compared different fuel blends made of n-hexadecane, n-dodecane, methyldecanoate, TPGME, as well as m-xylene. Several optical diagnostics have been used simultaneously to monitor the ignition, combustion and soot formation/oxidation processes from spray flames from different fuels. The results confirmed the impact of oxygenated fuels on combustion, with a slight reduction in ignition delay times and a strong impact on soot formation. It has been observed that the changes in ignition timing and lift-off length, generally associated with changes in soot production levels were not following a direct relationship.
Technical Paper
2014-04-01
Lyle M. Pickett, Julien Manin, Alan Kastengren, Christopher Powell
A full understanding and characterization of the near-field of diesel sprays is daunting because the dense spray region inhibits most diagnostics. While x-ray diagnostics permit quantification of fuel mass along a line of sight, most laboratories necessarily use simple lighting to characterize the spray spreading angle, using it as an input for CFD modeling, for example. Questions arise as to what is meant by the “boundary” of the spray since liquid fuel concentration is not easily quantified in optical imaging. In this study we seek to establish a relationship between spray boundary obtained via optical diffused backlighting and the fuel concentration derived from tomographic reconstruction of x-ray radiography. Measurements are repeated in different facilities at the same specified operating conditions on the “Spray A” fuel injector of the Engine Combustion Network, which has a nozzle diameter of 90 μm. Long-distance microscopy at >100 kHz speeds is used to characterize the opening, steady, and closing phases of injection.
Technical Paper
2014-04-01
Shaoping Quan, Meizhong Dai, Eric Pomraning, P. K. Senecal, Keith Richards, Sibendu Som, Scott Skeen, Julien Manin, Lyle M. Pickett
Shock waves have been recently observed in high-pressure diesel sprays. In this paper, three-dimensional numerical simulations of supersonic diesel spray injection have been performed to investigate the underlying dynamics of the induced shock waves and their interactions with the spray. A Volume-of-Fluid based method in the CFD software (CONVERGE) is used to model this multiphase phenomena. An adaptive Mesh Refinement (AMR) scheme is employed to capture the front of the spray and the shock waves with high fidelity. Simulation results are compared to the available experimental observations to validate the numerical procedure. Parametric studies with different injection and ambient conditions are conducted to examine the effect of these factors on the generation of shock waves and their dynamics.
Technical Paper
2014-04-01
Joseph Oefelein, Guilhem Lacaze, Rainer Dahms, Anthony Ruiz, Antony Misdariis
Abstract This paper first summarizes a new theoretical description that quantifies the effects of real-fluid thermodynamics on liquid fuel injection processes as a function of pressure at typical engine operating conditions. It then focuses on the implications this has on modeling such flows with emphasis on application of the Large Eddy Simulation (LES) technique. The theory explains and quantifies the major differences that occur in the jet dynamics compared to that described by classical spray theory in a manner consistent with experimental observations. In particular, the classical view of spray atomization as an appropriate model at some engine operating conditions is questionable. Instead, non-ideal real-fluid behavior must be taken into account using a multicomponent formulation that applies to hydrocarbon mixtures at high-pressure supercritical conditions. To highlight the implications and needs related to modeling, we present a series of studies using LES that focus on experiments being conducted in the high-pressure combustion vessel at Sandia National Laboratories.
Technical Paper
2013-10-14
Paul C. Miles, Dipankar Sahoo, Stephen Busch, Johannes Trost, Alfred Leipertz
Measurements of ignition behavior, homogeneous reactor simulations employing detailed kinetics, and quantitative in-cylinder imaging of fuel-air distributions are used to delineate the impact of temperature, dilution, pilot injection mass, and injection pressure on the pilot ignition process. For dilute, low-temperature conditions characterized by a lengthy ignition delay, pilot ignition is impeded by the formation of excessively lean mixture. Under these conditions, smaller pilot mass or higher injection pressures further lengthen the pilot ignition delay. Similarly, excessively rich mixtures formed under relatively short ignition delay conditions typical of conventional diesel combustion will also prolong the ignition delay. In this latter case, smaller pilot mass or higher injection pressures will shorten the ignition delay. The minimum charge temperature required to effect a robust pilot ignition event is strongly dependent on charge O2 concentration. Measured fuel-air ratio distributions, coupled with the homogeneous reactor simulations, show this temperature dependency to be due to both kinetics and over-mixing caused by the long ignition delay associated with dilute mixtures.
Technical Paper
2013-10-14
Julien Manin, Lyle M. Pickett, Scott A. Skeen
Despite ongoing research efforts directed at reducing engine-out emissions, diesel engines are known to be one of the largest sources of atmospheric particulate matter (i.e., soot). Quantitative measurements are of primary importance to address soot production during the combustion process in the cylinder of diesel engines. This study presents the capabilities of an extinction-based diagnostic developed to quantitatively measure the soot volume fraction in n-dodecane sprays injected in a high-pressure, high-temperature vessel. Coupled with high-speed imaging, the technique yields time-resolved measurements of the soot field by relying on a diffused back-illumination scheme to improve extinction quantification in the midst of intense beam steering. The experiments performed in this work used two wavelengths, which, when combined with the Rayleigh-Debye-Gans theory, provide information about the optical and physical properties of soot. For validation of the extinction imaging technique and comparison with prior work, the path-averaged soot volume fraction has been simultaneously measured using laser extinction.
Technical Paper
2013-10-14
Yi Yang, John E. Dec
This paper studies autoignition characteristics and HCCI engine combustion of ketone fuels, which are important constituents of recently discovered fungi-derived biofuels. Two ketone compounds, 2,4-dimethyl-3-pentanone (DMPN) and cyclopentanone (CPN), are systematically investigated in the Sandia HCCI engine, and the results are compared with conventional gasoline and neat ethanol. It is found that CPN has the lowest autoignition reactivity of all the biofuels and gasoline blends tested in this HCCI engine. The combustion timing of CPN is also the most sensitive to intake-temperature (Tin) variations, and it is almost insensitive to intake-pressure (Pin) variations. These characteristics and the overall HCCI performance of CPN are similar to those of ethanol. In contrast, DMPN shows multi-faceted autoignition characteristics. On the one hand, DMPN has strong temperature-sensitivity, even at boosted Pin, which is similar to the low-reactivity ethanol and CPN. On the other hand, DMPN shows much stronger pressure-sensitivity than ethanol and CPN.
Technical Paper
2013-09-08
Dipankar Sahoo, Paul C. Miles, Johannes Trost, Alfred Leipertz
Fuel tracer-based planar laser-induced fluorescence is used to investigate the vaporization and mixing behavior of pilot injections for variations in pilot mass of 1-4 mg, and for two injection pressures, two near-TDC ambient temperatures, and two swirl ratios. The fluorescent tracer employed, 1-methylnaphthalene, permits a mixture of the diesel primary reference fuels, n-hexadecane and heptamethylnonane, to be used as the base fuel. With a near-TDC injection timing of −15°CA, pilot injection fuel is found to penetrate to the bowl rim wall for even the smallest injection quantity, where it rapidly forms fuel-lean mixture. With increased pilot mass, there is greater penetration and fuel-rich mixtures persist well beyond the expected pilot ignition delay period. Significant jet-to-jet variations in fuel distribution due to differences in the individual jet trajectories (included angle) are also observed. Increased injection pressure significantly increased the mixing rate, leading to leaner mixture distributions, and with lower ambient temperature modestly richer mixtures are found near the heads of the jets.
Technical Paper
2013-09-08
Lyle M. Pickett, Julien Manin, Raul Payri, Michele Bardi, Jaime Gimeno
Transients in the rate of injection (ROI) with respect to time are ever-present in direct-injection engines, even with common-rail fueling. The shape of the injection ramp-up and ramp-down affects spray penetration and mixing, particularly with multiple-injection schedules currently in practice. Ultimately, the accuracy of CFD model predictions used to optimize the combustion process depends upon the accuracy of the ROI utilized as fuel input boundary conditions. But experimental difficulties in the measurement of ROI, as well as real-world affects that change the ROI from the bench to the engine, add uncertainty that may be mistaken for weaknesses in spray modeling instead of errors in boundary conditions. In this work we use detailed, time-resolved measurements of penetration at the Spray A conditions of the Engine Combustion Network to rigorously guide the necessary ROI shape required to match penetration in jet models that allow variable rate of injection. The discrete control-volume jet model of Musculus and Kattke is utilized, and improved to account for variable spreading angle with axial distance, also based upon experimental measurements.
Technical Paper
2013-04-08
Garrett E. Barter, David Reichmuth, Todd H. West, Dawn K. Manley
We present a parametric analysis of electric vehicle (EV) adoption rates and the corresponding contribution to greenhouse gas (GHG) reduction in the US light-duty vehicle (LDV) fleet through 2050. The analysis is performed with a system dynamics based model of the supply-demand interactions among the fleet, its fuels, and the corresponding primary energy sources. The differentiating feature of the model is the ability to conduct global sensitivity and parametric trade-space analyses. We find that many factors impact the adoption rates of EVs. These include, in particular, policy initiatives that encourage consumers to consider lifetime ownership costs, the price of oil, battery performance, as well as the pace of technological development for all powertrains (conventional internal combustion engines included). Widespread EV adoption can have noticeable impact on petroleum consumption and GHG emissions by the LDV fleet. However, EVs alone cannot drive compliance with the most aggressive GHG emission reduction targets, even as the electricity source mix shifts away from coal and towards natural gas.
Technical Paper
2013-04-08
Vicente Romero, Joshua Mullins, Laura Swiler, Angel Urbina
This paper discusses the treatment of uncertainties corresponding to relatively few samples of random-variable quantities. The importance of this topic extends beyond experimental data uncertainty to situations involving uncertainty in model calibration, validation, and prediction. With very sparse samples it is not practical to have a goal of accurately estimating the underlying variability distribution (probability density function, PDF). Rather, a pragmatic goal is that the uncertainty representation should be conservative so as to bound a desired percentage of the actual PDF, say 95% included probability, with reasonable reliability. A second, opposing objective is that the representation not be overly conservative; that it minimally over-estimate the random-variable range corresponding to the desired percentage of the actual PDF. The presence of the two opposing objectives makes the sparse-data uncertainty representation problem an interesting and difficult one. The performance of a variety of uncertainty representation techniques is tested and characterized in this paper according to these two opposing objectives.
Technical Paper
2013-04-08
Emre Cenker, Gilles Bruneaux, Lyle Pickett, Christof Schulz
Within the Engine Combustion Network (ECN) spray combustion research frame, simultaneous line-of-sight laser extinction measurements and laser-induced incandescence (LII) imaging were performed to derive the soot volume fraction (fv). Experiments are conducted at engine-relevant high-temperature and high-pressure conditions in a constant-volume pre-combustion type vessel. The target condition, called "Spray A," uses well-defined ambient (900 K, 60 bar, 22.8 kg/m₃, 15% oxygen) and injector conditions (common rail, 1500 bar, KS1.5/86 nozzle, 0.090 mm orifice diameter, n-dodecane, 363 K). Extinction measurements are used to calibrate LII images for quantitative soot distribution measurements at cross sections intersecting the spray axis. LII images are taken after the start of injection where quasi-stationary combustion is already established. In addition, by changing the LII timing relative to the injection, the temporal variation of the soot cloud is ob-served from initial soot formation until soot oxidization.
Technical Paper
2013-04-08
Jacqueline O'Connor, Mark Musculus
This work is a technical review of past research and a synthesis of current understanding of post injections for soot reduction in diesel engines. A post injection, which is a short injection after a longer main injection, is an in-cylinder tool to reduce engine-out soot to meet pollutant emissions standards while maintaining efficiency, and potentially to reduce or eliminate exhaust aftertreatment. A sprawling literature on post injections documents the effects of post injections on engine-out soot with variations in many engine operational parameters. Explanations of how post injections lead to engine-out soot reduction vary and are sometimes inconsistent or contradictory, in part because supporting fundamental experimental or modeling data are often not available. In this paper, we review the available data describing the efficacy of post-injections and highlight several candidate in-cylinder mechanisms that may control their efficacy. We first discuss three in-cylinder mechanisms that have been frequently proposed to explain how post injections reduce engine-out soot.
Technical Paper
2013-04-08
Sanghoon Kook, Renlin Zhang, Kevin Szeto, Lyle M. Pickett, Tetsuya Aizawa
In-flame soot sampling based on the thermophoresis of particles and subsequent transmission electron microscope (TEM) imaging has been conducted in a diesel engine to study size, shape and structure of soot particles within the reacting diesel jet. A direct TEM sampling is pursued, as opposed to exhaust sampling, to gain fundamental insight about the structure of soot during key formation and oxidation stages. The size and shape of soot particles aggregate structure with stretched chains of spherical-like primary particles is currently an unknown for engine soot modelling approaches. However, the in-flame sampling of soot particles in the engine poses significant challenges in order to extract meaningful data. In this paper, the engine modification to address the challenges of high-pressure sealing and avoiding interference with moving valves and piston are discussed in detail. Of particular interest is the uncertainty caused by a selection of the on-grid locations for transmission electron microscope imaging and cycle-to-cycle fluctuations of the engine combustion.
Technical Paper
2013-04-08
Katsufumi Kondo, Tetsuya Aizawa, Sanghoon Kook, Lyle Pickett
For better understanding of soot formation and oxidation processes applicable to diesel engines, the size, morphology, and nanostructure of soot particles directly sampled in a diesel spray flame generated in a constant-volume combustion chamber have been investigated using Transmission Electron Microscopy (TEM). For this soot diagnostics, the effects of the sampling processes, TEM observation methodology and image processing methods on the uncertainty in the results have not been extensively discussed, mainly due to the complexity of the analysis. This study particularly aims to reveal the uncertainties due to sampling methods (mainly the effect of shot-by-shot fluctuation of “identical” single-shot spray flames on the sampled soot properties), TEM observation methods (mainly the fluctuation of soot morphology among different on-grid locations and the number of soot particle samples required to compensate for the fluctuations) and analysis methods (mainly operator-by-operator fluctuation of morphology measurements and the effects of averaged diameter and cutoff number for primary particles in aggregates on the determination of fractal dimension).
Technical Paper
2013-04-08
Jacqueline O'Connor, Mark Musculus
Partially premixed low-temperature combustion (LTC) using exhaust-gas recirculation (EGR) has the potential to reduce engine-out NO and soot emissions, but increased unburned hydrocarbon (UHC) emissions need to be addressed. In this study, we investigate close-coupled post injections for reducing UHC emissions. By injecting small amounts of fuel soon after the end of the main injection, fuel-lean mixtures near the injector that suffer incomplete combustion can be enriched with post-injection fuel and burned to completion. The goal of this work is to understand the in-cylinder mechanisms affecting the post-injection efficacy and to quantify its sensitivity to operational parameters including post-injection duration, injection dwell, load, and ignition delay time of the post-injection mixture. Three optical diagnostics - planar laser induced fluorescence of OH radicals, planar laser induced fluorescence of formaldehyde, and high-speed imaging of natural combustion luminescence - complement measurements of engine-out UHC with parametric variations of main- and post-injection timing and duration.
Technical Paper
2013-04-08
Charles J. Mueller
This study summarizes the peer-reviewed literature regarding the use of raw pyrolysis liquids (PLs) created from woody biomass as fuels for compression-ignition (CI) engines. First, a brief overview is presented of fast pyrolysis and the potential advantages of PLs as fuels for CI engines. Second, a discussion of the general composition and properties of PLs relative to conventional, petroleum-derived diesel fuels is provided, with emphasis on the differences that are most likely to affect PL performance in CI-engine applications. Next, a synopsis is given of the peer-reviewed literature describing experimental studies of CI engines operated using neat PLs and PLs combined in various ways with other fuels. This literature conclusively indicates that raw PLs and PL blends cannot be used as “drop-in replacements” for diesel fuel in CI engines, which is reflected in part by none of the cited studies reporting successful operation on PL fuels for more than twelve consecutive hours. Based on the reported failure modes, some recommendations are offered for improving performance, reliability, and safety when fueling CI engines with PLs.
Technical Paper
2013-04-08
Federico Perini, Adam Dempsey, Rolf D. Reitz, Dipankar Sahoo, Benjamin Petersen, Paul C. Miles
In a recent study, quantitative measurements were presented of in-cylinder spatial distributions of mixture equivalence ratio in a single-cylinder light-duty optical diesel engine, operated with a non-reactive mixture at conditions similar to an early injection low-temperature combustion mode. In the experiments a planar laser-induced fluorescence (PLIF) methodology was used to obtain local mixture equivalence ratio values based on a diesel fuel surrogate (75% n-heptane, 25% iso-octane), with a small fraction of toluene as fluorescing tracer (0.5% by mass). Significant changes in the mixture's structure and composition at the walls were observed due to increased charge motion at high swirl and injection pressure levels. This suggested a non-negligible impact on wall heat transfer and, ultimately, on efficiency and engine-out emissions. In this work, the extensive and quantitative local information provided by the PLIF experiments was used as the reference for assessing the accuracy of the CFD modeling of the engine.
Technical Paper
2012-09-10
Magnus Sjöberg, David Reuss
The lean-burn stratified-charge DISI engine has a strong potential for increased thermal efficiency compared to the traditional throttled SI engine. This experimental study of a spray-guided stratified-charge combustion system compares the engine response to injection-timing retard for gasoline and E85. Focus is on engine-out NO and soot, and combustion stability. The results show that for either fuel, injection-timing retard lowers the engine-out NO emissions. This is partly attributed to a combination of lower peak-combustion temperatures and shorter residence time at high temperatures, largely caused by a more retarded combustion phasing. However, for the current conditions using a single-injection strategy, the potential of NO reduction with gasoline is limited by both elevated soot emissions and the occurrence of misfire cycles. In strong contrast, when E85 fuel is used, the combustion system responds very well to injection-timing retard. By using near-TDC injection of E85 ultra-low emissions of NO can be achieved.
Technical Paper
2012-09-10
Brian Harries, Kenneth Leslie, Townsend Hyatt, Brandon Smith, Kenneth Meierjurgen, Jenna Beckwith, Marc Compere
With a growing need for a more efficient consumer based automotive platform, Embry-Riddle Aeronautical University (ERAU) chose to redesign the 2013 Chevrolet Malibu as a Plug-in Hybrid Electric Vehicle(PHEV). A Series architecture was chosen for its low energy consumption and high consumer acceptability when compared to the Series/Parallel-through-the-road and the Pre-Transmission designs. A fuel selection process was also completed and B20 Biodiesel was selected as the primary fuel due to lower GHG (Greenhouse Gases) emissions and Embry-Riddle's ability to produce biodiesel onsite using the cafeteria's discarded vegetable oil.
Technical Paper
2012-09-10
Srivatsava V. Puranam, Richard R. Steeper
Prior studies have shown that fuel addition during negative valve overlap (NVO) can both increase temperature and alter composition of the charge carried over to main HCCI combustion. Late NVO fuel injection, i.e., near top dead center, can cause piston wetting and subsequent localized rich flames. Since acetylene is a product of rich combustion and is known to advance ignition, it is hypothesized that the species could play a chemical role in enhancing main combustion. The objective of this work is to quantify the effects of acetylene on HCCI combustion. While the research topic is specifically relevant to NVO-fueled HCCI operation, the experiments are conducted without NVO fueling to avoid uncertainties of NVO reforming reactions. Instead, a single post-NVO injection of iso-octane fuels the cycle, and acetylene is seeded into the intake flow at varying concentrations to simulate a reformed product of NVO. Total chemical energy is held constant by adjusting injected mass of iso-octane to compensate for added acetylene.
Video
2012-06-18
High-load HCCI combustion has recently been demonstrated with conventional gasoline using intake pressure boosting. The key is to control the high combustion heat release rates (HRR) by using combustion timing retard and mixture stratification. However, at naturally aspirated and moderately boosted conditions, these techniques did not work well due to the low autoignition reactivity of conventional gasoline at these conditions. This work studies a low-octane distillate fuel with similar volatility to gasoline, termed Hydrobate, for its potential in HCCI engine combustion at naturally aspirated and low-range boosted conditions. The HCCI combustion with fully premixed and partially stratified charges was examined at intake pressures (Pin) from 100 to 180 kPa and constant intake temperature (60�C) and engine speed (1200 rpm). First, the key parameters for high-load HCCI operation were investigated, including 1) intermediate temperature heat release (ITHR), which determines the potential of using combustion retard to control the HRR, and 2) ?
Technical Paper
2012-04-16
Sage Kokjohn, Rolf D. Reitz, Derek Splitter, Mark Musculus
Premixed charge compression ignition (PCI) strategies offer the potential for simultaneously low NOx and soot emissions with diesel-like efficiency. However, these strategies are generally confined to low loads due to inadequate control of combustion phasing and heat-release rate. One PCI strategy, dual-fuel reactivity-controlled compression ignition (RCCI), has been developed to control combustion phasing and rate of heat release. The RCCI concept uses in-cylinder blending of two fuels with different auto-ignition characteristics to achieve controlled high-efficiency clean combustion. This study explores fuel reactivity stratification as a method to control the rate of heat release for PCI combustion. To introduce fuel reactivity stratification, the research engine is equipped with two fuel systems. A low-pressure (100 bar) gasoline direct injector (GDI) delivers iso-octane, and a higher-pressure (600 bar) common-rail diesel direct-injector delivers n-heptane. A sweep of the common-rail injection timing creates a range of fuel reactivity stratification.
Technical Paper
2012-04-16
Tetsuya Aizawa, Hiroki Nishigai, Katsufumi Kondo, Teruo Yamaguchi, Jean-Guillaume Nerva, Caroline Genzale, Sanghoon Kook, Lyle Pickett
For a better understanding of soot formation and oxidation processes in conventional diesel and biodiesel spray flames, the morphology, microstructure and sizes of soot particles directly sampled in spray flames fuelled with US#2 diesel and soy-methyl ester were investigated using transmission electron microscopy (TEM). The soot samples were taken at 50mm from the injector nozzle, which corresponds to the peak soot location in the spray flames. The spray flames were generated in a constant-volume combustion chamber under a diesel-like high pressure and high temperature condition (6.7MPa, 1000K). Direct sampling permits a more direct assessment of soot as it is formed and oxidized in the flame, as opposed to exhaust PM measurements. Density of sampled soot particles, diameter of primary particles, size (gyration radius) and compactness (fractal dimension) of soot aggregates were analyzed and compared. No analysis of the soot micro-structure was made. The overall morphology of the biodiesel soot bears similarity to that of #2 diesel, but the soot density, primary particle size, and fractal dimension are smaller for biodiesel.
Technical Paper
2012-04-16
Sanghoon Kook, Lyle M. Pickett
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. From experiments, it was found that fuels with long lift-off length generally produce less soot but fuel molecular structure also affects soot formation.
Technical Paper
2012-04-16
Adam B. Dempsey, Bao-Lin Wang, Rolf D. Reitz, Benjamin Petersen, Dipankar Sahoo, Paul C. Miles
In a recent experimental study the in-cylinder spatial distribution of mixture equivalence ratio was quantified under non-combusting conditions by planar laser-induced fluorescence (PLIF) of a fuel tracer (toluene). The measurements were made in a single-cylinder, direct-injection, light-duty diesel engine at conditions matched to an early-injection low-temperature combustion mode. A fuel amount corresponding to a low load (3.0 bar indicated mean effective pressure) operating condition was introduced with a single injection at -23.6° ATDC. The data were acquired during the mixture preparation period from near the start of injection (-22.5° ATDC) until the crank angle where the start of high-temperature heat release normally occurs (-5° ATDC). In the present study the measured in-cylinder images are compared with a fully resolved three-dimensional CFD model, namely KIVA3V-RANS simulations. The impacts of computational grid resolution and of the flow initialization method are discussed as they pertain to the mixture preparation process.
Technical Paper
2012-04-16
Bao-Lin Wang, Michael J. Bergin, Benjamin R. Petersen, Paul C. Miles, Rolf D. Reitz, Zhiyu Han
A generalized re-normalization group (RNG) turbulence model based on the local "dimensionality" of the flow field is proposed. In this modeling approach the model coefficients C₁, C₂, and C₃ are all constructed as functions of flow strain rate. In order to further validate the proposed turbulence model, the generalized RNG closure model was applied to model the backward facing step flow (a classic test case for turbulence models). The results indicated that the modeling of C₂ in the generalized RNG closure model is reasonable, and furthermore, the predictions of the generalized RNG model were in better agreement with experimental data than the standard RNG turbulence model. As a second step, the performance of the generalized RNG closure was investigated for a complex engine flow. The flow field generated by the generalized RNG closure model was compared to particle image velocimetry (PIV) velocity measurements from an optically accessible General Motors Company 1.9L HSDI engine equipped with helical and tangential intake ports.
Technical Paper
2012-04-16
Rainer N. Dahms, Michael C. Drake, Ronald O. Grover, Jr., Arun S. Solomon, Todd D. Fansler
Recently, high-speed optical imaging data for a single operating point of a spray-guided gasoline engine has, along with the flamelet model and the G-equation theory, enabled the development of the new spark-ignition model SparkCIMM. Within its framework, detailed chemistry flamelet models capture the experimental feature of multiple localized ignition events along the excessively stretched and restriking spark channel, as well as the observations of non-spherical highly corrugated early turbulent flame fronts. The developed flamelet models account for the substantial turbulent fluctuations in equivalence ratio and enthalpy present under spray-guided conditions. A non-unity Lewis number formulation captures the deficient species diffusion into the highly curved flame reaction zone. Previously, these diffusion processes were shown to have a significant effect on the early flame development in spray-guided engines due to, initially, locally rich and highly diluted low turbulent Damköhler number mixtures.
Technical Paper
2012-04-16
Joseph Oefelein, Rainer Dahms, Guilhem Lacaze
This paper provides an analysis of high-pressure phenomena and its potential effects on the fundamental physics of fuel injection in Diesel engines. In particular, 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. This framework is then used to perform a thermodynamic analysis of the flow. We focus on the experiments being conducted in the high-pressure combustion vessel at Sandia National Laboratories using n-heptane as a reference fuel. The calculations are performed by rigorously treating the experimental geometry and operating conditions, with detailed treatment of relevant thermophysical mixture properties. Results demonstrate that n-heptane enters the chamber as a compressed liquid, not a spray, and is heated at supercritical pressure.
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