Threshold Sooting Index Research Articles

A threshold soot index-based fuel surrogate formulation methodology to mimic sooting tendency of real fuels in 3D-CFD simulations

Particulate Matter emission is an increasing concern for engine manufacturers due to the strong limits imposed by worldwide regulations. Fuel composition plays a key role in determining the extent to which soot is formed during the combustion process. The availability of advanced multidimensional computational fluid-dynamics soot models incorporating soot chemistry pushed researchers to formulate fuel surrogates able to represent sooting tendency of real fuels in the numerical framework. Such studies, which provide information to target research grade fuels, are scarcely present in literature. A methodology is proposed to estimate sooting tendency of commercial gasolines, based on Threshold Soot Index and basic composition information, as well as to formulate tailored ethanol-toluene reference fuel surrogates. The technique relies on a purely mathematical approach to estimate Threshold Soot Index of individual compounds and blends using a structural group contribution-based approach, available in literature, which allows to adapt fuel surrogate palette as needed. Firstly, this approach is demonstrated by means of constant pressure reactor simulations using the Method of Moments. Secondly, a methodology is proposed to formulate fuel surrogates simultaneously targeting the main chemical and physical auto-ignition characteristics and estimated Threshold Soot Index. Several oxygenated and non-oxygenated commercial gasolines available on the market are targeted to provide a wide number of validation cases. Finally, surrogates are used to generate a selection of constant pressure-based soot libraries tested in conjunction with the Sectional Method on a 3D-CFD model of a single-cylinder optically accessible gasoline engine. Surrogates exhibit the same qualitative ranking estimated based on Threshold Soot Index and retain a quantitative scaling in terms of particulate matter formation.

A new jet fuel surrogate formulated by emulating the distribution of pyrolysis products obtained from shock tube experiments

A new jet fuel surrogate is developed by emulating the distribution of pyrolysis products obtained from shock tube experiments. The distribution of pyrolysis products is described by the use of a series of pyrolysis curves, which refers to a graphical depiction of the concentrations of pyrolysis products plotted against temperatures. Besides, the matching on chemical properties of jet fuel, including molecular weight (MW), hydrogen/oxygen ratio (H/C ratio), derived cetane number (DCN), lower heating value (LHV) and threshold sooting index (TSI), is also taken into account during the formulation process. The new surrogate is a mixture of n-decane, iso-cetane and m-xylene, and these three components are selected through the simulation of pyrolytic process based on the consideration that their mixture can provide similar pyrolysis curves to the target fuel. The validity of the new surrogate has been evaluated extensively against available experimental results from homogeneous reaction systems and low dimensional premixed flames. Meanwhile, a definition of “Error function” is introduced to quantitatively analyze the agreement between calculated values by surrogate and measured data of Jet A. As a reference, the predictive capabilities of four surrogates published in the last ten years and one hybrid-chemistry model for jet fuel are also investigated under the same conditions. According to the simulated results, the new surrogate with a detailed chemical mechanism from CRECK Modeling group (Version 1412) is capable of presenting reasonable predictions on the major combustion behaviors of Jet A across a wide range of conditions, such as ignition delay time, laminar burning velocity and oxidative products distribution.

Variation of gas phase combustion properties of complex fuels during vaporization: Comparison for distillation and droplet scenarios

Surrogate mixtures for modeling real fuels are formulated based on gaseous combustion property targets and liquid physical properties. A batch distillation model was developed to evaluate the vaporization characteristics of some existing surrogates for Jet-A. Chinese aviation fuel RP-3 was then used as a target to experimentally obtain distillation curves and variation of chemical functional groups. A 24-component surrogate was formulated mainly to capture the distillation behavior of RP-3. This surrogate was then used in two droplet vaporization models (Finite Thermal Conductivity/Finite Diffusivity (FTC/FD), Infinite Thermal Conductivity/Infinite Diffusivity (ITC/ID)) to investigate the effects of preferential vaporization on gaseous combustion properties of a complex real fuel. The results obtained from FTC/FD and ITC/ID provide regional bounds for droplets in a vaporizing spray. Four combustion property targets (Molecular Weight (MW), Hydrogen to Carbon ratio (H/C), Derived Cetane Number (DCN) and Threshold Sooting Index (TSI)) were employed as indicators of gas combustion properties. It was found that due to a wide distribution of compounds’ volatility, gaseous combustion properties vary significantly during droplet vaporization. The results suggest development of vaporization models that well capture preferential vaporization of a target real fuel for further spray modeling.

Effect of 5-membered bicyclic hydrocarbon additives on nanostructural disorder and oxidative reactivity of diffusion flame-generated diesel soot

Soot emission from diesel engines is of serious concern, as it is carcinogenic and remains suspended in air for a long time to cause adverse effects on human health and the environment. A way to reduce soot emission is by altering soot nanostructures by introducing fringe curvatures to enhance reactivity and increase oxidation. Such disorder in soot nanostructure can be initiated by the introduction of 5-membered rings into 6-membered, graphite-like soot structures. This study investigates the effect of the addition of norbornane, a saturated, 5-membered bicyclic hydrocarbon additive to diesel on the physicochemical properties, sooting propensity, structural disorders, and the oxidative reactivity of soot. The physicochemical analysis revealed that the threshold sooting index (TSI) of the blend is reduced to 29.5 at an optimum blending percentage of 10% norbornane-90% Diesel (NBD) as compared to the TSI of 37 for diesel. The analyses using XRD, Raman, HRTEM, and EDX indicated that the addition of this additive resulted in an increased soot nanostructural disorder, smaller PAH size, increased fringe curvature, and significantly greater aliphatic content in soot as compared to an unsaturated 5-membered bicyclic additive, dicyclopentadiene (DCPD). The reactivity studies confirmed that NBD soot is easily oxidized in air, since it requires a lower initial activation energy (90 kJ/mol) as compared to DCPD (120 kJ/mol) and pure diesel soots (170 kJ/mol). Thus, norbornane, a saturated 5-membered bicyclic compound, which is available as a by-product of polymer industry and in crude oil, can serve as a potential fuel additive for designing advanced fuels.

Formulating of model-based surrogates of jet fuel and diesel fuel by an intelligent methodology with uncertainties analysis

In this study, an intelligent model to formulate fuel surrogates was proposed which includes intelligent surrogate blend optimizer and optimization module of chemistry kinetic mechanism. The components of surrogates were selected from the library of candidates with given compositions by the intelligent surrogate blend optimizer based on the targeted fuel properties. Hence, new jet fuel surrogate (JFS) and diesel fuel surrogate (DFS) models were formulated consisting of five components (n-dodecane/iso-octane/isocetane/decalin/toluene), with specified compositions of each component, respectively. The new surrogates contain major components of the target practical fuels and inherently emulate the key properties including liquid density, viscosity, surface tension, cetane number, hydrogen-carbon ratio, molecular weight, lower heating value and threshold sooting index. Moreover, a robust skeletal oxidation mechanism of fuel surrogate composed of five components was updated and multi-objective optimization algorithm of non-dominated sorting genetic algorithm II (NSGA-II) was introduced to achieve self-adaptive tuning of Arrhenius pre-exponential factor in fuel-related sub-mechanism. Thus, the optimal rate constants could satisfy the predictions of ignition delay times and species concentrations simultaneously. Then Generalized Polynomial Chaos (gPC) was employed to minimize the uncertainty of stochastic input coefficients and its propagation towards ignition behaviours. Consequently, the new surrogate models were well validated against available tested data from practical fuels, and better performance of current JFS model was highlighted when compared with the prediction results by previous model. It is feasible to employ this intelligent model to formulating surrogates for more practical fuels.

Surrogate fuels for RP-3 kerosene formulated by emulating molecular structures, functional groups, physical and chemical properties

The objective of the current study was to formulate RP-3 kerosene surrogate by emulating fuel properties affecting the physical and chemical processes of the target fuel under the engine relevant conditions. This study utilized two-dimensional gas chromatography with time-of-flight mass spectrometry (GC × GC-TOFMS) and 13C and 1H nuclear magnetic resonance (NMR) spectroscopy to characterize the compositional characteristics of RP-3 fuel, and various standard test methods were applied to measure the physical and chemical properties of the target fuel. Two surrogate fuels (K1, a mixture of five components and K2, a mixture of seven components) were optimally determined through a multi-property regression algorithm by matching carbon types (CTs), distillation curve, cetane number (CN), density, and threshold sooting index (TSI) of the target fuel. The measured and estimated values of both target properties and non-target properties of surrogates were validated against the experimental data of RP-3 kerosene. Ignition delay times (IDTs) of both surrogates were investigated in a heated shock tube and a heated rapid compression machine under engine relevant conditions and validated against the measured results of RP-3. Overall, K1 and K2 both exhibited good matching on the compositional characteristics, physical-chemical properties, and gas phase ignition behaviors with the target fuel. In contrast, the seven-component K2 was more competitive and more comprehensive.

Performance and emissions of a DISI engine fueled with gasoline/ethanol and gasoline/C-4 oxygenate blends – Development of a PM index correlation for particulate matter emission assessment

The increasing attention devoted to biomass-derived oxygenated molecules is among others related to their potential to reduce pollutant emissions at the exhaust of spark-ignition engines particularly when combined with the latest direct injection technologies. The impact of adding such compounds to gasoline on engine performances, however, strongly depends on the composition, the chemical structure and the physical properties of the considered additives. The formation of particulate matter (PM), whose emissions are subject to increasingly stringent standards, is especially influenced by such factors thus prompting the need for the formulation of predictive multi-parameter PM indexes helping to support the development of advanced and cleaner engine fuels. Within this context, the present work proposes to analyze the performance and emissions of a direct injection spark-ignition engine fueled with mixtures made of gasoline with five different oxygenated fuels under homogeneous injection condition. Ethanol and four C-4 oxygenates (1-butanol, butanone, butanal and methyl propionate) have been selected to be blended with gasoline in proportions of 10, 20 and 40 vol%. Engine performances and exhaust emissions have been characterized in terms of specific fuel consumption, flame development angle, fully developed combustion duration and concentrations of CO, HC, NOx and PM. While the addition of oxygenated fuels increases the specific fuel consumption proportionally to the mixture lower heating value, it tends to decrease the flame development angle while not significantly influencing the fully developed combustion phase. Besides, relatively unchanged CO and HC emissions have been observed contrary to NOx and PM releases which have been found to decline with increasing oxygenate contents. Measured particulate concentrations have been compared with the predictions of four PM and sooting indexes from the literature including the Particulate Matter Index (PMI), the Threshold Sooting Index (TSI), the Oxygen Extended Sooting Index (OESI) and the Fuel Equivalent Sooting Index (FESI). While the FESI has been found to be the most efficient to reproduce experimentally monitored trends, it has still been coupled with five different fuel properties and volatility metrics to propose, through a multi-parameter optimization procedure, a generic PM index correlation allowing reproducing well measured PM emissions. By this way, it has been shown that the heat of vaporization and the fuel energy content are key factors to be combined with a sooting propensity indicator so as to capture engine exhaust PM releases.

Experimental characterization of jet fuels under engine relevant conditions – Part 2: Insights on optimization approach for surrogate formulation

Computational combustion modeling is an essential complementary tool to engine experiments and the combination of computational fluid dynamics and detailed chemical kinetics provides the promise for optimizing engine performance. However, predicting the effects of chemical and physical properties of fuels on engine performance is a great challenge since transportation fuels are composed of several hundreds to thousands of chemical species. Surrogates are simpler representations of real fuels and are comprised of selected species of known concentrations that exhibit combustion characteristics similar to those of the real fuels. In this paper, drawing from our previous work on surrogate formulations, we investigate the effects of surrogate components on the autoignition characteristics of conventional and alternative jet fuels, using a combination of experimental and modeling approaches. As target fuels, we analyzed a conventional jet fuel (Jet-A) and two alternative jet fuels (coal-derived IPK, natural-gas-derived S8). Experimental data on the properties of surrogate mixtures, such as liquid density and threshold sooting index, their combustion behaviors, together with those from their corresponding target real fuels are compared and analyzed using predictions obtained from the surrogate model. Results from a modified CFR engine show that the ignition reactivity of Jet-A and Sasol IPK surrogates are stronger than their target fuels, while the S8 surrogate displayed an ignition behavior very similar to the target S8. Results from a constant volume spray combustion chamber provided reasonable agreements between the surrogates and their target jet fuels in terms of physical and chemical ignition delay times and apparent heat release trends, with the S8 surrogate showing the best agreement. In addition, surrogate mixtures that were modified from the original Jet-A and IPK surrogates to achieve a better agreement with CFR experiments performed poorly when tested in a spray chamber. These results imply that the agreement between the behaviors of surrogates and real fuels in one device or in one condition does not guarantee similarity in other devices or in other conditions. This study also highlights the need for improvements in the current surrogate formulation methodologies to provide a more universal emulation of the autoignition behaviors of target transportation fuels.

The blending effect on the sooting tendencies of alternative/conventional jet fuel blends in non-premixed flames

In order to study the blending effect on sooting tendencies for practical alternative/conventional jet fuel blends, the present study has investigated soot formation in non-premixed flames of Jet-A blended with three alternative jet fuels (AJFs), i.e., Hydroprocessed Esters and Fatty Acids from camelina (HEFA-Camelina), Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), and Alcohol-to-Jet (ATJ). The smoke points of the AJF/Jet-A blends were measured in a wick lamp per ASTM D1322 standard, and their Threshold Sooting Indices were derived. Soot volume fraction profiles of the AJF/Jet-A fuel blends were also measured using the laser-induced incandescence diagnostics in the counterflow non-premixed flames. The effects of AJF blending ratio, strain rate, and reactant concentration on the soot formation were examined. The experimental results show that the Threshold Sooting Indices of the AJF/Jet-A blends exhibit a linear relationship with AJF mole fraction. While the maximum soot volume fractions of fuel blends show a fairly linear relationship with AJF blending ratio, the nonlinearity becomes prominent at high strain rates and low reactant concentrations. Comparing the three AJF/Jet-A fuel blends, the sooting tendencies are ranked as ATJ/Jet-A > FT-SPK/Jet-A > HEFA-Camelina/Jet-A. Correlations of the maximum soot volume fraction with aromatics content, hydrogen content, smoke point, and Threshold Sooting Index were all also examined. It is found that neither aromatics content nor hydrogen content can correlate with the maximum soot volume fraction for all the fuel blends at a given condition; conversely, smoke point shows a nonlinear correlation and Threshold Sooting Index shows an approximately linear correlation with the maximum soot volume fraction for all the fuel blends. It is suggested that the different sooting tendencies of AJF/Jet-A blends are mainly caused by the different sooting propensities of individual AJFs, which are strongly dependent on their distinct paraffinic compositions.

The sooting tendency of aviation biofuels and jet range paraffins: effects of adding aromatics, carbon chain length of normal paraffins, and fraction of branched paraffins

ABSTRACTThe smoke point of several fuel mixtures was measured using a smoke meter defined by American Society for Testing Material D1322. The threshold soot index (TSI) was calculated to compare the sooting tendency of fuels. Two aviation biofuels produced by hydroprocessing of coconut oil, four jet range normal paraffins (n-paraffins), iso-octane, and propylbenzene were used to investigate the effects of chain length of n-paraffins, iso-paraffins, and aromatics on the sooting tendency. The results show that the effect of propylbenzene on the smoke point of the mixtures is dominant compared to that of the chain length of n-paraffins and the fraction of iso-paraffins. The differences in the sooting tendency among these n-paraffins are insignificant. In addition, blending aviation biofuels with Jet A1 leads to a decrease in the sooting tendency. Aromatics in Jet A1 play an important role on the sooting tendency of the mixtures of aviation biofuels and Jet A1.

Development of a new jet fuel surrogate and its associated reaction mechanism coupled with a multistep soot model for diesel engine combustion

A new jet fuel surrogate was developed in this work by emulating real jet fuel properties including physical, gas phase chemical properties and threshold sooting index (TSI). An intelligent optimization methodology was proposed to calculate the species composition that inherently satisfies both the physical and chemical characteristics as well as sooting tendency. Eight properties were selected as the target properties for the jet fuel surrogate development, including liquid density, viscosity, surface tension, cetane number (CN), hydrogen carbon (H/C) ratio, molecular weight (MW), lower heating value (LHV) and TSI. As a result, the CN, H/C ratio, LHV, TSI and density of the new jet fuel surrogate are reproduced excellently with very little deviations within 3%. The averaged deviation of viscosity is −6.318% and the deviations of MW is 9.776%. As the highest deviation among all properties, the averaged deviation of surface tension is 11.76%. Based on the newly developed jet fuel surrogate, a skeletal jet fuel surrogate mechanism with 5 components including decalin, n-dodecane, iso-cetane, iso-octane and toluene was developed. The skeletal jet fuel surrogate mechanism was significantly compacted into 74 species and 189 reactions by describing the chemistries for the oxidation of large molecules C4–Cn and small H2/CO/C1 molecules respectively, which makes it practical to be used for 3-D engine combustion simulations. The validations of ignition delay times present reasonable agreement between experiment and predictions over a wide range of equivalence ratios (0.5–2.0) and pressures (8–30 atm), except for a shift of negative temperature coefficient (NTC) region towards higher temperatures at Φ = 1.5, 20 atm: in the simulation the NTC region is from 830 K to 950 K while in the experiment the NTC region is from 740 K to 890 K. The predicted species concentrations can reproduce the trend of the experimental data, especially for O2, CO and CO2. The simulated laminar flame speed at 400 K and 470 K are with absolute averaged deviations of 3.5% and 4.06% respectively. The constant volume spray and combustion validations are reasonably good. In the engine combustion validations, a multistep soot model was embedded into the new jet fuel surrogate mechanism. The predicted in-cylinder pressure can reproduce the experimental data, expect for small deviations after the peak pressure (the averaged deviation is around 6.2% after the peak pressure). The embedded soot model can well reproduce the trend of the experimental data. It can be concluded that this new jet fuel surrogate mechanism is compact and robust for the utilization in diesel engine combustion simulation.

Development of an optimization methodology for formulating both jet fuel and diesel fuel surrogates and their associated skeletal oxidation mechanisms

In this study, an optimization methodology was developed for formulating both jet fuel and diesel fuel surrogates along with their associated skeletal oxidation mechanisms. Based on this methodology, new jet fuel surrogate (JFS) and diesel fuel surrogate (DFS) are formulated with same five components (n-dodecane/iso-octane/iso-cetane/decalin/toluene) by emulating practical fuel properties including liquid density, viscosity, surface tension, cetane number (CN), hydrogen-carbon (H/C) ratio, molecular weight (MW), lower heating value (LHV) and threshold sooting index (TSI). Based on the newly developed JFS and DFS, their associated chemical skeletal mechanisms were developed, which are described with same chemical reaction structure: five skeletal sub-mechanisms are employed into the skeletal fuel mechanisms, including decalin (C10H18), n-dodecane (C12H26), iso-cetane (C16H34), iso-octane (C8H18) and toluene (C7H8) sub-mechanisms. By describing the chemistries for the oxidation of large molecules C4-Cn and small H2/CO/C1 molecules respectively, the skeletal mechanisms are significantly compacted into 74 species and 189 reactions, which makes them practical to be used in 3-D engine combustion simulations. In addition to sufficient 0-D validations of ignition delay times, species concentrations and laminar flame speed in various environments, 3-D validations of constant volume spray and engine combustion were also performed for the new JFS and DFS skeletal mechanisms. It can be observed that the agreements between the computational results predicted by the present mechanisms and all the experimental data are reasonably good, which proves that the newly proposed JFS and DFS skeletal mechanisms are robust and accurate to be used in engine combustion computational fluid dynamics (CFD) studies.

An optimization method for formulating model-based jet fuel surrogate by emulating physical, gas phase chemical properties and threshold sooting index (TSI) of real jet fuel under engine relevant conditions

Two jet fuel surrogates (S1 and S2) were proposed in this work, aimed at improving the quantitative accuracy of fuel properties that affect both premixed and spray-guided combustion modes under engine relevant conditions by emulating real jet fuel properties including physical, gas phase chemical properties and threshold sooting index (TSI). An intelligent optimization approach was employed to calculate the composition that inherently satisfies both the physical and gas phase chemical characteristics as well as sooting tendency. The jet fuel surrogate S1 was composed of five components including decalin, n-dodecane, iso-cetane, iso-octane and toluene (0.005/0.4011/0.1249/0.098/0.371 by mole fraction), while surrogate S2 is a mixture of the same components but different in proportions (0.1449/0.3706/0.2059/0.0195/0.2591 by mole fraction). Based on the newly proposed surrogates, a skeletal jet fuel surrogate chemical reaction mechanism was developed by describing the chemistries for the oxidation of large molecules C4Cn and smaller H2/CO/C1 molecules respectively. The skeletal jet fuel surrogate mechanism was significantly compacted into 74 species and 189 reactions, making it practical to be used in 3-dimensional (3-D) engine combustion simulations. This newly developed mechanism was verified against the experimental results of ignition delay times, species concentrations and laminar flame speed under a wide range of conditions, while 3-D validations were conducted for spray liquid and vapor penetrations in a constant volume chamber. As a result, the proposed fuel surrogate is capable of predicting the spray and combustion characteristics and main species profiles under engine relevant circumstance. Due to the stringent target properties used in this work, the surrogate S1 performed best in assessing ignition characteristics attributed to its elaborate chemical properties, making it the most suitable candidate for chemical dominated combustion like premixed engine combustion. Alternatively, S2 displayed outstanding spray-guided combustion behavior because of the stringent chemical and physical target properties assigned. It is worthwhile to note that this new method of formulating surrogates for different applications was efficient and time-saving. Finally, we aim to perform experimental validation tests for CN, density, viscosity, surface tension, TSI, sooting tendency and particle size distribution to further support the validity of the current proposed jet fuel surrogates (S1 and S2) in the future.

A minimalist functional group (MFG) approach for surrogate fuel formulation

Surrogate fuel formulation has drawn significant interest due to its relevance towards understanding combustion properties of complex fuel mixtures. In this work, we present a novel approach for surrogate fuel formulation by matching target fuel functional groups, while minimizing the number of surrogate species. Five key functional groups; paraffinic CH3, paraffinic CH2, paraffinic CH, naphthenic CHCH2 and aromatic CCH groups in addition to structural information provided by the Branching Index (BI) were chosen as matching targets. Surrogates were developed for six FACE (Fuels for Advanced Combustion Engines) gasoline target fuels, namely FACE A, C, F, G, I and J. The five functional groups present in the fuels were qualitatively and quantitatively identified using high resolution 1H Nuclear Magnetic Resonance (NMR) spectroscopy. A further constraint was imposed in limiting the number of surrogate components to a maximum of two. This simplifies the process of surrogate formulation, facilitates surrogate testing, and significantly reduces the size and time involved in developing chemical kinetic models by reducing the number of thermochemical and kinetic parameters requiring estimation. Fewer species also reduces the computational expenses involved in simulating combustion in practical devices. The proposed surrogate formulation methodology is denoted as the Minimalist Functional Group (MFG) approach. The MFG surrogates were experimentally tested against their target fuels using Ignition Delay Times (IDT) measured in an Ignition Quality Tester (IQT), as specified by the standard ASTM D6890 methodology, and in a Rapid Compression Machine (RCM). Threshold Sooting Index (TSI) and Smoke Point (SP) measurements were also performed to determine the sooting propensities of the surrogates and target fuels. The results showed that MFG surrogates were able to reproduce the aforementioned combustion properties of the target FACE gasolines across a wide range of conditions. The present MFG approach supports existing literature demonstrating that key functional groups are responsible for the occurrence of complex combustion properties. The functional group approach offers a method of understanding the combustion properties of complex mixtures in a manner which is independent, yet complementary, to detailed chemical kinetic models. The MFG approach may be readily extended to formulate surrogates for other complex fuels.

Development of a Diesel Surrogate Fuel Library

Diesel fuel is composed of a complex mixture of hundreds of hydrocarbons that vary globally depending on crude oil sources, refining processes, legislative requirements and other factors. In order to simplify the study of this fuel, researchers create surrogate fuels to mimic the physical and chemical properties of Diesel fuels. This work employed the commercial software Reaction Workbench – Surrogate Blend Optimizer (SBO) to develop a Surrogate Fuel Library containing 18 fuels. Within the fuel library, the cetane number ranges from 35 to 60 (in increments of 5) at threshold soot index (TSI) levels representative of low, baseline and high sooting tendency fuels (TSI = 17, 31 and 48, respectively). The Surrogate Fuel Library provides the component blend ratios and predicted properties for cetane number, threshold soot index, lower heating value, density, kinematic viscosity, molar hydrogen-to-carbon ratio and distillation curve temperatures from T10 to T90. A market petroleum Diesel fuel with a cetane number of 50 and a threshold soot index of 31 was selected as the Baseline Diesel Fuel. The combustion, physical and chemical properties of the Baseline Diesel Fuel were precisely matched by the Baseline Surrogate Fuel. To validate the SBO predicted fuel properties, a set of five surrogate fuels, deviating in cetane number and threshold soot index, were blended and examined with ASTM tests. Good agreement was obtained between the SBO predicted and ASTM measured fuel properties. To further validate the Surrogate Fuel Library, key properties that were effected by altering the component blend ratios to control cetane number and TSI were compared to a set of five market Diesel fuels with good results. These properties included density, viscosity, energy density and the T10 and T90 distillation temperatures. The Surrogate Fuel Library provided by this work supplies Diesel engine researchers and designers the ability to analytically and experimentally vary fuel cetane number and threshold soot index with fully-representative surrogate fuels. This new capability to independently vary cetane number and threshold soot index provides a means to further enhance the understanding of Diesel combustion and design future combustion systems that improve efficiency and emissions.

Evaluation of sooting tendency of acetone–butanol–ethanol (ABE) fuels blended with diesel fuel

Acetone–Butanol–Ethanol (ABE), the intermediate product in bio-butanol production process using Biological Fermentation Purification technology, has been proposed to blend diesel directly for saving the high energy requirement of separating purity butanol. It is very important to study the sooting tendency of ABE-diesel blends, which determined whether it can be used as a clean alternative fuel. Therefore, in this paper, the smoke point and relationship between flame height and fuel uptake rate for a wide range ratio of ABE (0%, 5%, 10%, 20%, 30%, 50% in volume referred to as D100, ABE5, ABE10, ABE20,ABE30, and ABE50, respectively) were measured using wick-fed burner. Flame heights were captured by a Digital SLR camera and fuel uptake rate were gained from electronic analytical balance. It was observed that flame height versus fuel uptake rate started with a linear relationship, then disjointed points appeared, finally returned linear again. Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) calculated from fuel uptake rate and smoke point respectively were used to evaluate the blends’ sooting tendency. ABE-diesel has a lower sooting tendency than butanol-diesel because it owns higher oxygen content and lower carbon content for the same blend ratio. Besides, the contribution of acetone, butanol and ethanol to the intensity of ABE weaken blends’ sooting tendency were tested through increasing their relative concentrations in ABE. Since ethanol and butanol have higher H/C ratio and oxygen content from the view of molecular structure, their content in ABE play a positive role in weakening sooting tendency, while the acetone content in ABE has an opposite effect because of its unsaturation degree.

Reconstruction of chemical structure of real fuel by surrogate formulation based upon combustion property targets

The global chemical character of complex chemical fuel mixtures is explicitly determined by evaluating the abundances of chemical functional groups present within them rather than by applying a traditional interpretation based upon molecular species composition. Statistical analyses of the relationships among each chemical functional group and specific combustion property targets (CPTs) of the fuel are rigorously developed. The results demonstrate that the four CPTs currently used in aviation kerosene surrogate formulation - H/C molar ratio, derived cetane number (DCN), average molecular weight (MW), and threshold sooting index (TSI) - effectively constrain the chemical functional group distribution of the fuel, and, hence, the global combustion behaviors of pre-vaporized fuel/air mixtures. Successful emulation of the CPTs for a target real fuel involves developing a surrogate mixture that defines an “equivalent” chemical functional group distribution to that of the target fuel. Among the CPTs used for real fuel surrogate development, DCN does not abide by a linear blending rule, which generally frustrates development of surrogates. However, a quantitative structure–property relation (QSPR) regression for DCN is demonstrated here using the chemical functional group approach. Results of the regression reveal that the (CH2)n group plays the most significant role in determining the fuel autoignition propensity, followed by the influences of CH3 and benzyl-type groups. The QSPR functional group approach extends to provide a powerful tool to address potential preferential vaporization effects dictated by fuel distillation characteristics. Further analysis of fuel chemical property variation (DCN and H/C ratio) over the distillation curve (and other physical properties) provides a foundation for understanding the complex combustion behaviors of multi-phase and multi-component fuels relevant to real gas turbine engine applications.

Evaluation of sooting tendency of different oxygenated and paraffinic fuels blended with diesel fuel

In this study a comparative experimental work about the sooting tendency of different fuel blends has been carried out. Three groups of alternative fuels, blended with diesel fuel, were tested: (1) an animal fat biodiesel; (2) two paraffinic fuels: a hydro-treated vegetable oil (HVO) and a gas-to-liquid (GTL) from natural gas and (3) two alcohols: ethanol and butanol. The smoke point of different binary alternative-diesel fuel blends was determined by means of a standardized Smoke Point Lamp. A low sulphur diesel fuel without biodiesel content was used as reference fuel. As sooting tendency parameters, both Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) from blends with respect to diesel fuel were compared. Two main trends were obtained. While biodiesel-, HVO- and GTL-diesel fuel blends showed similar exponential decreasing trend in sooting tendency as a function of blending percentage (with an exponential factor between −0.005 and −0.01), alcohol-diesel fuel blends resulted to be more effective suppressors of the soot (around 20 times better).Comparing the sooting tendency of fuel blends to engine opacity/particle emissions achieved by other authors, when great differences were obtained, as in the case of alcohol blends compared to biodiesel, HVO and GTL, the translation are in good agreement.

Combustion characteristics of C4 iso-alkane oligomers: Experimental characterization of iso-dodecane as a jet fuel surrogate component

Global combustion characteristics of iso-dodecane (2,2,4,6,6-pentamethylheptane, iC12) are measured and compared to those of iso-octane (2,2,4-trimethylpentane, iC8), iso-cetane (2,2,4,4,6,8,8-heptamethylnonane, iC16) and a 50/50 molar blend of iC8/iC16. The fuels are experimentally characterized by measuring combustion property targets (derived cetane number and smoke point/threshold sooting index), reflected shock ignition delay times at 20 and 40 atm, and extinction limits of strained laminar diffusion flames at 1 atm. The derived cetane number (DCN) and threshold sooting index (TSI) of iC12 are measured to be 16.8 (±1) and 15.4 (±0.5), respectively. In addition to average molecular weight (MW) and overall hydrogen-to-carbon ratio (H/C ratio), the combustion property targets for iC12 are very nearly molar averages of those for iC8 and iC16. Values agree very well with the measured combustion property indicators for the 50/50 blend of iC8/iC16. Further analysis of fuel chemical functional group distributions also finds that the abundances of methylene and methyl groups in iC12 and the 50/50 blend of iC8/iC16 are identical, further supporting that the global combustion characteristics of iC12 can be emulated by the molar averaged mixture of iC8 and iC16. Measurements of reflected shock ignition delays show that the ignition delay times of iC12 are in close agreement with those of the 50/50 molar mixture of iC8/iC16 over a broad range of temperature conditions. It is also found that the ignition delays of the three neat iso-alkanes exhibit quantitatively identical behaviors for both high and low temperature regimes, an observation that can guide the construction of combustion kinetic models for iC12. Measurements of strained diffusion flame extinction and their interpretation by the transport-weighted enthalpy (TWE) metric also support that the high temperature chemical kinetic potentials of the three iso-alkanes are essentially identical. Moreover, it is shown that molecular diffusivity (inherent in molecular weight) is the major parameter that differentiates flame extinction behaviors amongst the three iso-alkanes. In summary, this experimental study further supports the utility and characteristics of combustion property target formulation concepts for producing mixtures that emulate the pre-vaporized global combustion properties of a specific target fuel. The work also points to a strong correlation of the combustion property target data for these iso-alkanes with methylene and methyl functional group distributions for each fuel.

Analysis of the sooting propensity of C-4 and C-5 oxygenates: Comparison of sooting indexes issued from laser-based experiments and group additivity approaches

Oxygenated biofuels have received a particular attention during the last few years due to their ability to reduce soot emissions. Numerous experimental and numerical studies focusing on the sooting propensity of such alternative fuels especially revealed that the ability of oxygenates to limit the soot production was strongly related to their molecular structure in addition to their oxygen content. The objective of the present work is thus to analyze and quantify the potential of soot reduction of several C-4 and C-5 oxygenated molecules used as additives in a Diesel surrogate (the IDEA fuel composed of 70vol% n-decane and 30vol% α-methylnaphthalene). To do so, soot concentration measurements have been carried out by Laser-Induced Incandescence (LII) in 33 turbulent spray flames burning mixtures of IDEA with 12.5, 25 and 50vol% of various oxygenated additives including alcohols, aldehydes, ketones and esters. The different oxygenates have been selected to assess the impact of the position of the oxygenated functional group, the number of oxygen atoms and the type of CO link (single or double) on the production of soot. Based on the peak soot volume fractions measured in standardized jet flames, a Fuel Equivalent Sooting Index (FESI) has been inferred for each considered oxygenated additive and compared with corresponding Yield Sooting Index (YSI), Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) values measured and/or estimated based on structural group contribution methods. Obtained results show that all the tested oxygenates reduce the sooting propensity of the base fuel in which they are added. For a given number of carbon atom in the molecule, the sooting tendency of oxygenated compounds increases in the following order: esters<aldehydes<ketones<alcohols. Reasons explaining such trends have been discussed considering the different effects that may impact the soot formation (i.e. the chemical effect induced by the presence of oxygenated functional groups in the fuel mixtures and the dilution of the base fuel by the additives which remains the most important factor for all the considered molecules). Finally, FESI values derived from a previous database obtained by analyzing flames burning gasoline-surrogate/ethanol blends [1] have been confronted with recently published OESI values proposed in [2] for gasoline/ethanol blends.