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Journal Article

Understanding the Chemical Effects of Increased Boost Pressure under HCCI Conditions

2008-04-14
2008-01-0019
One way to increase the load range in an HCCI engine is to increase boost pressure. In this modeling study, we investigate the effect of increased boost pressure on the fuel chemistry in an HCCI engine. Computed results of HCCI combustion are compared to experimental results in a HCCI engine. We examine the influence of boost pressure using a number of different detailed chemical kinetic models - representing both pure compounds (methylcyclohexane, cyclohexane, iso-octane and n-heptane) and multi-component models (primary reference fuel model and gasoline surrogate fuel model). We examine how the model predictions are altered by increased fueling, as well as reaction rate variation, and the inclusion of residuals in our calculations. In this study, we probe the low temperature chemistry (LTC) region and examine the chemistry responsible for the low-temperature heat release (LTHR) for wide ranges of intake boost pressure.
Technical Paper

Thermodynamic and Chemical Effects of EGR and Its Constituents on HCCI Autoignition

2007-04-16
2007-01-0207
EGR can be used beneficially to control combustion phasing in HCCI engines. To better understand the function of EGR, this study experimentally investigates the thermodynamic and chemical effects of real EGR, simulated EGR, dry EGR, and individual EGR constituents (N2, CO2, and H2O) on the autoignition processes. This was done for gasoline and various PRF blends. The data show that addition of real EGR retards the autoignition timing for all fuels. However, the amount of retard is dependent on the specific fuel type. This can be explained by identifying and quantifying the various underlying mechanisms, which are: 1) Thermodynamic cooling effect due to increased specific-heat capacity, 2) [O2] reduction effect, 3) Enhancement of autoignition due to the presence of H2O, 4) Enhancement or suppression of autoignition due to the presence of trace species such as unburned or partially-oxidized hydrocarbons.
Technical Paper

The Potential of HCCI Combustion for High Efficiency and Low Emissions

2002-06-03
2002-01-1923
Homogeneous Charge Compression Ignition (HCCI) engines can have efficiencies as high as compression-ignition, direct-injection (CIDI) engines (an advanced version of the commonly known diesel engine), while producing ultra-low emissions of oxides of nitrogen (NOx) and particulate matter (PM). HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels. While HCCI has been demonstrated and known for quite some time, only the recent advent of electronic sensors and controls has made HCCI engines a potential practical reality. This paper provides a comprehensive overview of the current state-of-the-art in HCCI technology, estimates the potential benefits HCCI engines could bring to U.S. transportation vehicles, and lists the R&D barriers that need to be overcome before HCCI engines might be considered for commercial application.
Technical Paper

The Influence of Fuel Volatility on the Liquid-Phase Fuel Penetration in a Heavy-Duty D.I. Diesel Engine

1998-02-23
980510
The objective of this investigation is to verify and characterize the influence of fuel volatility on maximum liquid-phase fuel penetration for a variety of actual Diesel fuels under realistic Diesel engine operating conditions. To do so, liquid-phase fuel penetration was measured for a total of eight Diesel fuels using laser elastic-scatter imaging. The experiments were carried out in an optically accessible Diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. In addition to liquid-phase fuel penetration, ignition delay was assessed for each fuel based on pressure-derived apparent heat release rate and needle lift data. For all fuels examined, it was observed that initially the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum characteristic length. Beyond this characteristic length, the fuel is entirely vapor phase and not just smaller fuel droplets.
Technical Paper

The Effects of Injection Timing and Diluent Addition on Late-Combustion Soot Burnout in a DI Diesel Engine Based on Simultaneous 2-D Imaging of OH and Soot

2000-03-06
2000-01-0238
The effects of injection timing and diluent addition on the late-combustion soot burnout in a direct-injection (DI) diesel engine have been investigated using simultaneous planar imaging of the OH-radical and soot distributions. Measurements were made in an optically accessible DI diesel engine of the heavy-duty size class at a 1680 rpm, high-load operating condition. A dual-laser, dual-camera system was used to obtain the simultaneous “single-shot” images using planar laser-induced fluorescence (PLIF) and planar laser-induced incandescence (PLII) for the OH and soot, respectively. The two laser beams were combined into overlapping laser sheets before being directed into the combustion chamber, and the optical signal was separated into the two cameras by means of an edge filter.
Technical Paper

The Effect of TDC Temperature and Density on the Liquid-Phase Fuel Penetration in a D. I. Diesel Engine*

1995-10-01
952456
A parametric study of the liquid-phase fuel penetration of evaporating Diesel fuel jets has been conducted in a direct-injection Diesel engine using laser elastic-scatter imaging. The experiments were conducted in an optically accessible Diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. The density and temperature at TDC were varied systematically by adjusting the intake temperature and pressure. At all operating conditions the measurements show that initially the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum length. Then, the liquid-fuel penetration length remains fairly constant although fuel injection continues. At a TDC density of 16.6 kg/m3 and a temperature of about 1000 K the maximum penetration length is approximately 23 mm. However, it varies significantly as TDC conditions are changed, with the liquid-length being less at higher temperatures and at higher densities.
Technical Paper

Spatial Analysis of Emissions Sources for HCCI Combustion at Low Loads Using a Multi-Zone Model

2004-06-08
2004-01-1910
We have conducted a detailed numerical analysis of HCCI engine operation at low loads to investigate the sources of HC and CO emissions and the associated combustion inefficiencies. Engine performance and emissions are evaluated as fueling is reduced from typical HCCI conditions, with an equivalence ratio ϕ = 0.26 to very low loads (ϕ = 0.04). Calculations are conducted using a segregated multi-zone methodology and a detailed chemical kinetic mechanism for iso-octane with 859 chemical species. The computational results agree very well with recent experimental results. Pressure traces, heat release rates, burn duration, combustion efficiency and emissions of hydrocarbon, oxygenated hydrocarbon, and carbon monoxide are generally well predicted for the whole range of equivalence ratios. The computational model also shows where the pollutants originate within the combustion chamber, thereby explaining the changes in the HC and CO emissions as a function of equivalence ratio.
Technical Paper

Soot and Fuel Distributions in a D.I. Diesel Engine via 2-D Imaging

1992-10-01
922307
Soot and fuel distributions have been studied in an optically accessible direct-injection diesel engine of the “heavy-duty” size class. Laser-induced incandescence (LII) was used to study the effects of changes in the engine speed on the in-cylinder soot distribution, and elastic (Mie) scattering and laser-induced fluorescence (LIF) were used to examine the fuel distribution. The investigation showed that, in this engine, soot is distributed throughout the cross section of the combusting region of the fuel jet for engine speeds ranging from 600 to 1800 rpm. No indication was found that soot occurs preferentially around the periphery of the plume. The LII images showed that the soot concentration decreases with increasing engine speed and injection pressure, and that the soot distribution extends much further upstream (toward the injector) at the lower engine speeds than at higher speeds.
Technical Paper

Soot Distribution in a D.I. Diesel Engine Using 2-D Laser-Induced Incandescence Imaging

1991-02-01
910224
Laser-induced incandescence (LII) has been explored as a diagnostic for qualitative two-dimensional imaging of the in-cylinder soot distribution in a diesel engine. Advantages of LII over elastic-scatter soot imaging techniques include no interfering signals from liquid fuel droplets, easy rejection of laser light scattered by in-cylinder surfaces, and the signal intensity being proportional to the soot volume fraction. LII images were obtained in a 2.3-liter, single cylinder, direct-injection diesel engine, modified for optical access. To minimize laser sheet and signal attenuation (which can affect almost any planar imaging technique applied to diesel engine combustion), a low-sooting fuel was used whose vaporization and combustion characteristics are typical of standard diesel fuels. Temporal and spatial sequences of LII images were made which show the extent of the soot distribution within the optically accessible portion the combusting spray plume.
Technical Paper

Soot Distribution in a D.I. Diesel Engine Using 2-D Imaging of Laser-induced Incandescence, Elastic Scattering, and Flame Luminosity

1992-02-01
920115
A combusting plume in an optically accessible direct-injection diesel engine was studied using simultaneous 2-D imaging of laser-induced incandescence (LII) and natural flame luminosity, as well as simultaneous 2-D imaging of LII and elastic scattering. Obtaining images simultaneously via two different techniques makes the effects of cycle-to-cycle variation identical for both images, permitting the details of the simultaneous images to be compared. Since each technique provides unique information about the combusting diesel plume, more can be learned from comparison of the simultaneous images than by any of the techniques alone. Among the insights gained from these measurements are that the combusting plume in this engine has a general pattern of high soot concentration towards the leading edge with a lower soot concentration extending upstream towards the injector. Also, the soot particles are found to be larger towards the leading edge of the plume than in the upstream region.
Technical Paper

Smoothing HCCI Heat-Release Rates Using Partial Fuel Stratification with Two-Stage Ignition Fuels

2006-04-03
2006-01-0629
This work explores the potential of partial fuel stratification to smooth HCCI heat-release rates at high load. A combination of engine experiments and multi-zone chemical-kinetics modeling was used for this. The term “partial” is introduced to emphasize that care is taken to supply fuel to all parts of the in-cylinder charge, which is essential for reaching high power output. It was found that partial fuel stratification offers good potential to achieve a staged combustion event with reduced pressure-rise rates. Therefore, partial fuel stratification has the potential to increase the high-load limits for HCCI/SCCI operation. However, for the technique to be effective the crank-angle phasing of the “hot” ignition has to be sensitive to the local ϕ. Sufficient sensitivity was observed only for fuel blends that exhibit low-temperature heat release (like diesel fuel).
Journal Article

Smoothing HCCI Heat Release with Vaporization-Cooling-Induced Thermal Stratification using Ethanol

2011-08-30
2011-01-1760
Ethanol and ethanol/gasoline blends are being widely considered as alternative fuels for light-duty automotive applications. At the same time, HCCI combustion has the potential to provide high efficiency and ultra-low exhaust emissions. However, the application of HCCI is typically limited to low and moderate loads because of unacceptably high heat-release rates (HRR) at higher fueling rates. This work investigates the potential of lowering the HCCI HRR at high loads by using partial fuel stratification to increase the in-cylinder thermal stratification. This strategy is based on ethanol's high heat of vaporization combined with its true single-stage ignition characteristics. Using partial fuel stratification, the strong fuel-vaporization cooling produces thermal stratification due to variations in the amount of fuel vaporization in different parts of the combustion chamber.
Technical Paper

Simulating HCCI Blending Octane Number of Primary Reference Fuel with Ethanol

2017-03-28
2017-01-0734
The blending of ethanol with primary reference fuel (PRF) mixtures comprising n-heptane and iso-octane is known to exhibit a non-linear octane response; however, the underlying chemistry and intermolecular interactions are poorly understood. Well-designed experiments and numerical simulations are required to understand these blending effects and the chemical kinetic phenomenon responsible for them. To this end, HCCI engine experiments were previously performed at four different conditions of intake temperature and engine speed for various PRF/ethanol mixtures. Transfer functions were developed in the HCCI engine to relate PRF mixture composition to autoignition tendency at various compression ratios. The HCCI blending octane number (BON) was determined for mixtures of 2-20 vol % ethanol with PRF70. In the present work, the experimental conditions were considered to perform zero-dimensional HCCI engine simulations with detailed chemical kinetics for ethanol/PRF blends.
Technical Paper

Refinement and Validation of the Thermal Stratification Analysis: A post-processing methodology for determining temperature distributions in an experimental HCCI engine

2014-04-01
2014-01-1276
Refinements were made to a post-processing technique, termed the Thermal Stratification Analysis (TSA), that couples the mass fraction burned data to ignition timing predictions from the autoignition integral to calculate an apparent temperature distribution from an experimental HCCI data point. Specifically, the analysis is expanded to include all of the mass in the cylinder by fitting the unburned mass with an exponential function, characteristic of the wall-affected region. The analysis-derived temperature distributions are then validated in two ways. First, the output data from CFD simulations are processed with the Thermal Stratification Analysis and the calculated temperature distributions are compared to the known CFD distributions.
Technical Paper

Reduced Gasoline Surrogate (Toluene/n-Heptane/iso-Octane) Chemical Kinetic Model for Compression Ignition Simulations

2018-04-03
2018-01-0191
Toluene primary reference fuel (TPRF) (mixture of toluene, iso-octane and heptane) is a suitable surrogate to represent a wide spectrum of real fuels with varying octane sensitivity. Investigating different surrogates in engine simulations is a prerequisite to identify the best matching mixture. However, running 3D engine simulations using detailed models is currently impossible and reduction of detailed models is essential. This work presents an AramcoMech reduced kinetic model developed at King Abdullah University of Science and Technology (KAUST) for simulating complex TPRF surrogate blends. A semi-decoupling approach was used together with species and reaction lumping to obtain a reduced kinetic model. The model was widely validated against experimental data including shock tube ignition delay times and premixed laminar flame speeds. Finally, the model was utilized to simulate the combustion of a low reactivity gasoline fuel under partially premixed combustion conditions.
Technical Paper

Quantitative 2-D Fuel Vapor Concentration Imaging in a Firing D.I. Diesel Engine Using Planar Laser-Induced Rayleigh Scattering*

1994-03-01
940682
The application of planar laser-induced Rayleigh scattering for quantitative 2-D measurements of vapor-phase fuel concentration in the main combustion zone of a direct-injection Diesel engine has been explored, developed and demonstrated. All studies were conducted in an optically accessible direct-injection Diesel engine of the “heavy-duty” size class at 1200 rpm and motored TDC conditions which were typical of the production version of this engine. First, this study verifies that beyond 27 mm from the injector all the fuel is vapor phase. This was done by investigating the Diesel jet under high magnification using 2-D elastic scatter imaging and subsequently evaluating the signal intensities from the droplets and other interfering particles (Mie scattering) and the vapor (Rayleigh scattering).
Technical Paper

Primary Reference Fuels (PRFs) as Surrogates for Low Sensitivity Gasoline Fuels

2016-04-05
2016-01-0748
Primary Reference Fuels (PRFs) - binary mixtures of n-heptane and iso-octane based on Research Octane Number (RON) - are popular gasoline surrogates for modeling combustion in spark ignition engines. The use of these two component surrogates to represent real gasoline fuels for simulations of HCCI/PCCI engines needs further consideration, as the mode of combustion is very different in these engines (i.e. the combustion process is mainly controlled by the reactivity of the fuel). This study presents an experimental evaluation of PRF surrogates for four real gasoline fuels termed FACE (Fuels for Advanced Combustion Engines) A, C, I, and J in a motored CFR (Cooperative Fuels Research) engine. This approach enables the surrogate mixtures to be evaluated purely from a chemical kinetic perspective. The gasoline fuels considered in this study have very low sensitivities, S (RON-MON), and also exhibit two-stage ignition behavior.
Technical Paper

Potential of Thermal Stratification and Combustion Retard for Reducing Pressure-Rise Rates in HCCI Engines, Based on Multi-Zone Modeling and Experiments

2005-04-11
2005-01-0113
This work investigates the potential of in-cylinder thermal stratification for reducing the pressure-rise rate in HCCI engines, and the coupling between thermal stratification and combustion-phasing retard. A combination of computational and experimental results is employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKIN III kinetics-rate code, and kinetic mechanisms for iso-octane. This study shows that the potential for extending the high-load operating limit by adjusting the thermal stratification is very large. With appropriate stratification, even a stoichiometric charge can be combusted with low pressure-rise rates, giving an output of 16 bar IMEPg for naturally aspirated operation. For more typical HCCI fueling rates (ϕ = 0.38 - 0.45), the optimal charge-temperature distribution is found to depend on both the amount of fuel and the combustion phasing.
Journal Article

Partial Fuel Stratification to Control HCCI Heat Release Rates: Fuel Composition and Other Factors Affecting Pre-Ignition Reactions of Two-Stage Ignition Fuels

2011-04-12
2011-01-1359
Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock. This study focuses on three factors, engine speed, intake temperature, and fuel composition, that can affect the pre-ignition processes of two-stage fuels and consequently affect their performance with partial fuel stratification. A model fuel consisting of 73 vol.% isooctane and 27 vol.% of n-heptane (PRF73), which was previously compared against neat isooctane to demonstrate the superior performance of two-stage fuels over single-stage fuels with partial fuel stratification, was first used to study the effects of engine speed and intake temperature.
Journal Article

PLIF Measurements of Thermal Stratification in an HCCI Engine under Fired Operation

2011-04-12
2011-01-1291
Tracer-based PLIF temperature diagnostics have been used to study the distribution and evolution of naturally occurring thermal stratification (TS) in an HCCI engine under fired and motored operation. PLIF measurements, performed with two excitation wavelengths (277, 308 nm) and 3-pentanone as a tracer, allowed investigation of TS development under relevant fired conditions. Two-line PLIF measurements of temperature and composition were first performed to track the mixing of the fresh charge and hot residuals during intake and early compression strokes. Results showed that mixing occurs rapidly with no measureable mixture stratification remaining by early compression (220°CA aTDC), confirming that the residual mixing is not a leading cause of thermal stratification for low-residual (4-6%) engines with conventional valve timing.
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