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A method for automatic reduction of detailed kinetic to skeletal mechanisms for complex fuels is proposed. The method is based on the simultaneous use of sensitivity and reaction-flow analysis. The resulting skeletal mechanism is valid for the parameter range of initial and boundary values, the analysis have been performed for. The gas-phase chemistry is analyzed in the end gas of an SI-engine, using a two-zone model. Species, not relevant for the occurrence of autoignition in the end gas, are defined as redundant. They are identified and eliminated for different pre-set levels of minimum reaction flow and sensitivity. The error in the mechanism increases monotony with increasing pre-set level of minimum reaction flow.

Simultaneous measurements of the radial and the tangential components of velocity are obtained in a high-speed, direct-injection diesel engine typical of automotive applications. Results are presented for engine operation with fuel injection, but without combustion, for three different swirl ratios and four injection pressures. With the mean and fluctuating velocities, the r-θ plane shear stress and the mean flow gradients are obtained. Longitudinal and transverse length scales are also estimated via Taylor's hypothesis. The flow is shown to be sufficiently homogeneous and stationary to obtain meaningful length scale estimates. Concurrently, the flow and injection processes are simulated with KIVA-3V employing a RNG k-ε turbulence model. The measured turbulent kinetic energy k, r-θ plane mean strain rates ( 〈Srθ〉, 〈Srr〉, and 〈Sθθ〉 ), deviatoric turbulent stresses , and the r-θ plane turbulence production terms are compared directly to the simulated results.

The behavior of the engine-out UHC and CO emissions from a light-duty diesel optical engine operating at two PPCI conditions was investigated for fifteen different fuels, including diesel fuels, biofuel blends, n-heptane-iso-octane mixtures, and n-cetane-HMN mixtures. The two highly dilute (9-10% O₂) early direct injection PPCI conditions included a low speed (1500 RPM) and load (3.0 bar IMEP) case~where the UHC and CO have been found to stem from overly-lean fuel-air mixtures~and a condition with a relatively higher speed (2000 RPM) and load (6.0 bar IMEP)~where globally richer mixtures may lead to different sources of UHC and CO. The main objectives of this work were to explore the general behavior of the UHC and CO emissions from early-injection PPCI combustion and to gain an understanding of how fuel properties and engine load affect the engine-out emissions.

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.

This study investigates the potential of partial fuel stratification for reducing the knocking propensity of intake-boosted HCCI engines operating on conventional gasoline. Although intake boosting can substantially increase the high-load capability of HCCI, these engines would be more production-viable if the knock/stability load limit could be extended to allow higher loads at a given boost and/or to provide even higher thermal efficiencies. A technique termed partial fuel stratification (PFS) has recently been shown to greatly reduce the combustion-induced pressure-rise rate (PRR), and therefore the knocking propensity of naturally aspirated HCCI, when the engine is fueled with a φ-sensitive, two-stage-ignition fuel. The current work explores the potential of applying PFS to boosted HCCI operation using conventional gasoline, which does not typically show two-stage ignition. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) at 1200 rpm.

The potential of boosted HCCI for achieving high loads has been investigated for intake pressures (Piⁿ) from 100 kPa (naturally aspirated) to 325 kPa absolute. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) equipped with a compression-ratio 14 piston at 1200 rpm. The intake charge was fully premixed well upstream of the intake, and the fuel was a research-grade (R+M)/2 = 87-octane gasoline with a composition typical of commercial gasolines. Beginning with Piⁿ = 100 kPa, the intake pressure was systematically increased in steps of 20 - 40 kPa, and for each Piⁿ, the fueling was incrementally increased up to the knock/stability limit, beyond which slight changes in combustion conditions can lead to strong knocking or misfire. A combination of reduced intake temperature and cooled EGR was used to compensate for the pressure-induced enhancement of autoignition and to provide sufficient combustion-phasing retard to control knock.

End-gas temperature stratification has long been studied with respect to its effect on stoichiometric spark-ignition (SI) engine knock. The role of temperature stratification for homogeneous-charge compression ignition (HCCI) engine operation is also reasonably well understood. However, the role of temperature stratification in ultra-lean SI engines has had less coverage. Literature is lacking well-controlled studies of how knock is affected by changes in the full cylinder temperature fields, especially since cycle-to-cycle variability can impede a determination of cause and effect. In this work, the knocking propensity of specific cylinder conditions is investigated via 3D computational fluid dynamics (CFD) simulations utilizing a large eddy simulation (LES) framework.

Long chain alcohols possess major advantages over the currently used ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. The rapid developments in biofuel technology have made it possible to produce C 4 -C 5 alcohols cost effectively. These higher alcohols could significantly expand the biofuel content and potentially substitute ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C 5 alcohol, isopentanol, as a fuel for HCCI engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. Results are presented in comparison with gasoline or ethanol data previously reported. For a given combustion phasing, isopentanol requires lower intake temperatures than gasoline or ethanol at all tested speeds, indicating a higher HCCI reactivity.

Well-mixed lean or dilute SI engine operation can provide efficiency improvements relative to that of traditional well-mixed stoichiometric SI operation. However, the realized gains depend on the ability to ensure stable, complete and fast combustion. In this work, the influence of fuel type is examined for gasoline, E30 and E85. Several enabling techniques are compared. For enhanced ignition stability, a multi-pulse (MP) transient plasma ignition system is compared to a conventional high-energy inductive spark ignition system. Combined effects of fuel type and intake-gas preheating are examined. Also, the effects of dilution type (air or N2-simulated EGR) on lean efficiency gains and stability limits are clarified. The largest efficiency improvement is found for lean gasoline operation using intake preheating, showing the equivalent of a 20% fuel-economy gain relative to traditional non-dilute stoichiometric operation.

Combustion issued from an eight-hole, direct-injection spray was experimentally studied in a constant-volume pre-burn combustion vessel using simultaneous high-speed diffused back-illumination extinction imaging (DBIEI) and OH* chemiluminescence. DBIEI has been employed to observe the liquid-phase of the spray and to quantitatively investigate the soot formation and oxidation taking place during combustion. The fuel-air mixture was ignited with a plasma induced by a single-shot Nd:YAG laser, permitting precise control of the ignition location in space and time. OH* chemiluminescence was used to track the high-temperature ignition and flame. The study showed that increasing the delay between the end of injection and ignition drastically reduces soot formation without necessarily compromising combustion efficiency. For long delays between the end of injection and ignition (1.9 ms) soot formation was eliminated in the main downstream charge of the fuel spray.

Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies and extremely low NOx and particulate emissions, but controlling the combustion timing remains a challenge. This paper explores the potential of Partial Fuel Stratification (PFS) to provide fast control of CA50 in an LTGC engine. Two different compression ratios are used (CR=16:1 and 14:1) that provide high efficiencies and are compatible with mixed-mode SI-LTGC engines. The fuel used is a research grade E10 gasoline (RON 92, MON 85) representative of a regular-grade market gasoline found in the United States. The fuel was supplied with a gasoline-type direct injector (GDI) mounted centrally in the cylinder. To create the PFS, the GDI injector was pulsed twice each engine cycle. First, an injection early in the intake stroke delivered the majority of the fuel (70 - 80%), establishing the minimum equivalence ratio in the charge.

Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural thermal stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions. Enhanced natural thermal stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions.

This paper1 explores some of the important considerations in devising a practical and consistent framework and methodology for working with experiments and experimental data in connection with modeling and prediction. The paper outlines a pragmatic and versatile “real-space” approach within which experimental and modeling uncertainties (correlated and uncorrelated, systematic and random, aleatory and epistemic) are treated to mitigate risk in modeling and prediction. The elements of data conditioning, model conditioning, model validation, hierarchical modeling, and extrapolative prediction under uncertainty are examined. An appreciation can be gained for the constraints and difficulties at play in devising a viable end-to-end methodology. The considerations and options are many, and a large variety of viewpoints and precedents exist in the literature, as surveyed here. Rationale is given for the various choices taken in assembling the novel real-space end-to-end framework.

Methanol emerges as a compelling renewable fuel for decarbonizing engine applications due to a mature industry with high production capacity, existing distribution infrastructure, low carbon intensity and favorable cost. Methanol’s high flame speed and high autoignition resistance render it particularly well-suited for spark-ignition (SI) engines. Previous research showed a distinct phenomenon, known deflagration-based knock in methanol combustion, whereby knocking combustion was observed albeit without end-gas autoignition. This work studies the implications of deflagration-based knock on noise emissions by investigating the knock intensity and combustion noise at knock-limited operation of methanol in a single-cylinder direct-injection SI engine operated at both stoichiometric and lean (λ = 2.0) conditions. Results are compared against observations from a premium-grade gasoline.

The combustion behavior of conventional gasoline has been numerically investigated by means of detailed chemical-kinetic modeling simulations, with particular emphasis on analyzing the chemistry of the intermediate temperature heat release (ITHR). Previous experimental work on highly boosted (up to 325 kPa absolute) HCCI combustion of gasoline (SAE 2020-01-1086) showed a steady increase in the charge temperature up to the point of hot ignition, even for conditions where the ignition point was retarded well after top dead center (TDC). Thus, sufficient energy was being released by early pre-ignition reactions resulting in temperature rise during the early part of the expansion stroke This behavior is associated with a slow pre-ignition heat release (ITHR), which is critical to keep the engine from misfiring at the very late combustion phasings required to prevent knock at high-load boosted conditions.

Isopentanol is an advanced biofuel that can be produced by micro-organisms through genetically engineered metabolic pathways. Compared to the more frequently studied ethanol, isopentanol's molecular structure has a longer carbon chain and includes a methyl branch. Its volumetric energy density is over 30% higher than ethanol, and it is less hygroscopic. Some fundamental combustion properties of isopentanol in an HCCI engine have been characterized in a recent study by Yang and Dec (SAE 2010-01-2164). They found that for typical HCCI operating conditions, isopentanol lacks two-stage ignition properties, yet it has a higher HCCI reactivity than gasoline. The amount of intermediate temperature heat release (ITHR) is an important fuel property, and having sufficient ITHR is critical for HCCI operation without knock at high loads using intake-pressure boosting. Isopentanol shows considerable ITHR, and the amount of ITHR increases with boost, similar to gasoline.

The Federal Aviation Administration Airworthiness Assurance Nondestructive Inspection Validation Center (FAA-AANC) is currently conducting a detection reliability study pertaining to the detection of cracks in multi-layered aluminum sheets. This paper describes the design, production and characterization of test specimens that are currently being used to conduct third layer Probability of Detection (PoD) experiments. Pertinent aspects of the lap splice joints for Boeing 737 aircraft, Line Numbers 292 - 2565 are included in the test specimens. A preliminary analysis of the data indicates that for some inspectors, traditional measures of performance - in particular PoD curves based on maximum likelihood fit to two-parameter lognormal curve - may be misleading.

Ionization probes built into the head gasket and uniformly distributed around the cylinder bore of a knocking, spark-ignition engine have been used to locate the autoigniting end-gas region. As normal combustion evolves after spark ignition, the ionization probes individually respond to the arrival of the propagating flame. Then, when autoignition occurs, the probes located in the end-gas region respond in rapid succession. By utilizing pressure transducer measurements to determine when autoignition occurs, the ionization probe response becomes a means to locate the end-gas region. Knowledge of the location of the last ionization probe to detect the normal flame can then be used to infer where, within the end-gas region, autoignition first occurred.

This paper introduces the “Discrete Direct” (DD) model calibration and uncertainty propagation approach for computational models calibrated to data from sparse replicate tests of stochastically varying systems. The DD approach generates and propagates various discrete realizations of possible calibration parameter values corresponding to possible realizations of the uncertain inputs and outputs of the experiments. This is in contrast to model calibration methods that attempt to assign or infer continuous probability density functions for the calibration parameters-which incorporates unjustified information in the calibration and propagation problem. The DD approach straightforwardly accommodates aleatory variabilities and epistemic uncertainties in system properties and behaviors, in input initial and boundary conditions, and in measurement uncertainties in the experiments.

Reaching for higher loads and improving combustion-phasing control are important challenges for HCCI research. Although HCCI engines can operate with a variety of fuels, recent research has shown that fuels with two-stage autoignition have some significant advantages for overcoming these challenges. Because the amount of low-temperature heat release (LTHR) is proportional to the local equivalence ratio (ϕ), fuel stratification can be used to adjust the combustion phasing (CA50) and/or burn duration using various fuel-injection strategies. Two-stage ignition fuels also allow stable combustion even for extensive combustion-phasing retard, which reduces the knocking propensity. Finally, the LTHR reduces the required intake temperature, which increases the inducted charge mass for a given intake pressure, allowing higher fueling rates before knocking and NOx emissions become a problem. However, the amount of LTHR is normally highly dependent on the engine speed.