Sooting Tendency Research Articles

Design and evaluation of surrogate mixtures for diesel based on the isolated droplet configuration

The development of simplified surrogate mixtures able to replicate combustion-related behaviors of chemically complex fuels is essential for their simulation with computational tools, a key step towards the design of high-efficiency and low-emission combustion applications. This work proposes to use the isolated droplet configuration as a benchmark to formulate and validate surrogates that capture the vaporization and soot production characteristics of a first-fill diesel and a diesel-biodiesel mixture. To that end, droplet vaporization experiments and a multicomponent model were coupled to produce blends matching the evaporation behavior, whereas the soot tendency was incorporated through tests at the ASTM D1322 smoke point lamp and the Oxygen Extended Sooting Index (OESI). The so-obtained surrogate blends were subsequently validated for both characteristics. Their evaporation curves proved to match remarkably well those obtained for the target fuels, with noticeable improvements when increasing the number of compounds in the mixture. As for the sooting behavior, the proposed blends achieved a good emulation in terms of the design parameter (OESI), confirming the validity of the proposed methodology. On the other hand, an additional and independent validation of the sooting propensity through the quantification of the mass of soot produced by isolated droplets under a high-temperature and reducing atmosphere revealed significantly higher soot yields for the surrogates when compared to the target fuels. These results highlight the relevance of the configuration used when designing and validating surrogates, since the same blends can provide substantial differences when evaluated through different sooting indices.

Droplet Combustion and Thermogravimetric Analysis of Pure Coconut Oil, Clove Oil, and Their Mixture

The droplet combustion and thermal behavior of pure coconut oil (PCO), clove oil, and their mixture were experimentally investigated. The mixture fuels were PCO and clove oil at the percentage of 10% based on volume (PCO-CO10). The experimental method uses droplet combustion and thermogravimetric analysis. The fuel droplet was suspended in the junction of k-type thermocouple and ignited by a coil heater. The ignition and combustion processes of droplets were recorded using a digital single-lens reflex camera at 25 fps. Thermogravimetric analysis with alumina crucible was prepared to investigate the thermal behavior of fuel. The result showed that the sequence of ovoid flame for PCO and PCO-CO10 take place until 0.4 second and 0.44 second, respectively. Complete combustion was explained in that sequence. The ovoid flame was formed when eugenol, terpene, and lauric acid were evaporated first in both PCO and PCO-CO10. Minimum ovoid flame takes place in clove oil due to soot tendency in the burning process that marked flame as the open tip. PCO-CO10 has the highest peak temperature due to the presence both of double carbon chains in fatty acid and aromatic ring structures, which were easy to decompose in high energy input. Clove oil was the lowest onset temperature, which indicates more volatile matter in this fuel and PCO has the highest thermal stability due to the fatty acid component in their fuel.

The effects of 1-methylnaphthalene addition to n-dodecane on the formation of soot and polycyclic aromatic hydrocarbons in laminar coflow diffusion flames

1-Methylnaphthalene (1-MN) is a typical diaromatic component in petroleum-based fuels and has been widely chosen to represent aromatic fractions in surrogate fuels of diesel and jet fuels. In this study, experimental and modeling studies were performed to investigate the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with 1-methylnaphthalene/n-dodecane mixtures, and explore chemical effects of 1-methylnaphthalen on key soot formation steps. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to obtained the soot volume fraction and relative concentration of typical PAHs along the flame centerline, respectively. A novel apparatus-independent indicator, yield aromatic index (YAI), was first proposed to quantify the formation tendency of a specific aromatic product in a flame. It can be found that the addition of 1-methylnaphthalene in the fuel mixtures has a significantly promoting effect on soot formation processes. The correlation between the mass fraction of 1-methylnaphthalene and sooting tendency shows a strong linear relationship. As to the PAH formation tendency, YAIA2R5 have the smaller deviations from corresponding YSIs than YAIA2&A3 and YAIA4, which indicating that A2R5 can be a type of important species leading to soot inception and have strong contribution to soot formation. Through the chemical kinetic analysis, A2R5 is proved to be a key coupling point of two A4 formation pathways, which start from n-dodecane and 1-methylnaphthalene, respectively. As a small amount of 1-methylnaphthalene is introduced into the fuel, the dominant approach to A2R5 rapidly shifts to a more efficient pathway, and is continually enhanced with increasing 1-methylnaphthalene at a moderate trend. The results reveal that 1-methylnaphthalene in the fuel can stride over the rate limiting step of the soot formation process of non-aromatic fuels, eventually leading to an obviously increasing sooting tendency. This study aims to provide useful information for the development of soot nucleation mechanisms and the control of soot formation processes.

Incipient sooting tendency of oxygenated fuels doped in ethylene counterflow diffusion flames

The incipient sooting tendencies of oxygenated fuels in counterflow diffusion flames were systematically assessed by doping selected oxygenate fuels into the baseline fuel of ethylene with various mixing ratios. Laser light scattering and planar laser-induced fluorescence (PLIF) techniques were adopted. The critical oxygen mole fractions at different mixing ratios were identified with the use of laser scattering. It is found that the critical oxygen mole fraction varies with oxygenates, suggesting that the effect of mixing ethylene with oxygenates is highly sensitive to the types of oxygenate. While the doping of acetone moderately promoted the incipient sooting tendency, the doping of methanol, acetic acid, formic acid, and diethyl carbonate suppressed the formation of incipient soot. On the other hand, the doping of ethanol, dimethyl carbonate, diethyl ether, and dimethoxymethane had weak influences on the incipient sooting tendency. The production of polycyclic aromatic hydrocarbons (PAHs) of different sizes were measured by PLIF. The results show there was a strong correlation between the incipient sooting tendency and the production of large PAHs. One important finding is that, at their sooting limits, the total oxygen-to-carbon ratio of a stoichiometric fuel/oxygen mixture is linearly correlated with the mixing ratio of the ethylene-oxygenate mixture. This slope of these two quantities can be defined as an incipient sooting index (ISI) of oxygenated fuels for ranking their tendency of incipient sooting formation. Having the slopes correlated well with the oxygenates’ hydrogen/oxygen ratio, a reasonably accurate prediction of critical oxygen mole fractions from the molecule information of oxygenated fuels can be achieved.

Relative particle emission tendencies of 2-methyl-3-buten-2-ol–gasoline, isobutanol–gasoline, and ethanol–gasoline blends from premixed combustion in a spark-ignition engine

Critical sooting equivalence ratios and subsequent particulate formation during spark-ignition combustion were studied for blends of ethanol, isobutanol (2-methyl-1-propanol), and methylbutenol (2-methyl-3-buten-2-ol) with gasoline using premixed prevaporized (PMPV) fueling. The air–fuel mixture was supplied to the engine in a well-mixed gaseous state allowing the chemical sooting tendency of the fuels, as measured by the critical sooting equivalence ratio, to be quantified. The engine was operated at a fixed load and phasing with varying equivalence ratio to determine the critical sooting equivalence ratio and evolution of the particle size distribution (PSD) for each fuel. Each alcohol was splash blended with a gasoline blendstock for oxygenate blending (BOB) at three different blend levels. Ethanol was blended at 10, 20, and 30% by volume (E10, E20, and E30). Isobutanol and methylbutenol blends were matched to ethanol on an oxygen weight percent basis resulting in blends of 16, 32, and 49% and 18, 37, and 56% by volume for isobutanol and methylbutenol, respectively. The results of this work show that increases in alcohol content consistently decreased the chemical sooting tendency and particulate emissions for all alcohol–gasoline blends tested. The level of decrease in particulate on a volume basis can be ordered isobutanol < methylbutenol < ethanol. On an oxygen weight percent basis, the order is ethanol < isobutanol < methylbutenol. The results demonstrate that methylbutenol has a lower chemical sooting tendency than isobutanol, and, given its other properties, is a potentially promising biofuel for blending with gasoline.

Fuel oxygenation as a novel method to reduce sooting propensity of fuels: An investigation with gasoline surrogate fuels

Blending transportation fuels like gasoline with biofuels reduce our reliance on fossil fuels and pollutant emission due to the presence of fuel-bound oxygen. However, biofuel availability is limited to regions with an abundance of biomass and land for energy crops. To gain the advantage of fuel-bound oxygen on emission reduction, this work explores the effect of replacing a fraction of soot-producing aromatics present in gasoline with oxygenated aromatics to check if a fuel formulation strategy, where a fraction of aromatics-rich stream in a refinery is catalytically oxygenated to form oxygenated aromatics before blending with other hydrocarbon streams to produce gasoline, could be advantageous in producing clean fuels. A gasoline surrogate containing 17% n-heptane, 47% isooctane, and 36% toluene, which closely predicted gasoline properties such as density, molecular weight, threshold sooting index, and octane number, is used for the experiments. To mimic the above fuel formulation strategy, a fraction of toluene in gasoline surrogate is replaced with oxygenated aromatics, benzyl alcohol and anisole that are structurally similar to toluene. The oxygenated blends showcased lower sooting tendency than the surrogate fuel. Soot particles, collected from the three fuel blends, are analyzed using TGA, XRD, HRTEM, and elemental analyzer, where soots from oxygenated blends are found to have higher reactivity with lower particle diameter and crystallite size. The results indicate that such fuel formulation can not only reduce soot emission, but also improve soot physicochemical properties to reduce its lifetime in the environment.

Probing sooting limits in counterflow diffusion flames via multiple optical diagnostic techniques

Sooting limit representing the critical flame condition where soot starts to appear is a useful quantitative index to assess the sooting tendency of a particular fuel. Experimental determination of sooting limit is closely related to the establishment of a well-defined “nucleation” flame which facilitates further scrutinization of the soot formation process. The present study was devoted to probing the earliest soot appearance in typical counterflow diffusion flames. The sooting limit was defined as the critical fuel (or oxygen) mole fraction in the nitrogen-balanced fuel (or oxidizer) stream. Multiple optical diagnostic techniques including flame luminosity, light extinction and scattering, spectral soot emission (SSE) as well as laser-induced incandescence (LII) were used to determine the sooting limits. The results showed that the value of sooting limit can be quantitatively different among the used detection techniques. Flame luminosity and SSE were found to be most sensitive to the presence of nascent soot, followed by LII and laser light scattering, while light extinction is the least sensitive technique. The effects of the wavelength of probing light on sooting limits were also assessed for the laser extinction and scattering technique. The contribution of the present work can be summarized into the following two aspects: (1) a quantitative database of sooting limits as determined from different soot detection measurements is provided to facilitate the choice of nascent soot-probing technique; (2) the first appearance of soot particles in nucleation flames are fully characterized and interpreted through a cross-comparison among multiple diagnostics techniques. It is also the authors’ expectation that the present results may provide new insights into the understanding of soot nucleation process and the interaction between light and nascent soot.

Effects of the addition of a high energy density fuel, adamantane to diesel on its cetane number, sooting propensity, and soot nanostructural properties

Soot particles, released during diesel combustion, adversely affect human health and the environment. Some studies suggest blending diesel with oxygenated fuels to reduce soot emissions, though it also reduces fuel economy and may produce new pollutants such as aldehydes. Another method to reduce soot could be to increase its reactivity by altering its physicochemical properties (e.g., introducing curvatures in its nanostructures) so that it oxidizes inside the engine or flame. In this direction, a cyclic high-density hydrocarbon fuel, adamantane is studied as an additive for diesel to determine its effect on cetane number, threshold sooting index, soot reactivity, and soot nanostructural properties. The addition of up to 6.75 wt% of adamantane in diesel significantly increased the smoke point of diesel from 18 to 29 mm (and threshold sooting index decreased by 44%), indicating a drastic drop in fuel sooting tendency. Adamantane addition in a small amount to diesel minimally affected the cetane number. To understand the impact of adamantane addition to diesel on the characteristics of soot particles, different techniques including TGA, HRTEM, XRD, EDX, and EELS were used. It is observed that adamantane enhances curvatures in soot nanostructures and reduces the size of fringes in soot and its aromatic content, and all of these property variations improve soot reactivity to improve its oxidation in flames or engines and reduce soot emission.

Modeling soot particles as stable radicals: a chemical kinetic study on formation and oxidation. Part I. Soot formation in ethylene laminar premixed and counterflow diffusion flames

Based on recent theoretical and experimental evidences, this work proposes a detailed discrete sectional soot model considering particles and aggregates behaving as stable radicals, paving the way for the implementation of a game-changing assumption in soot kinetic models. The concentration of large closed-shell and open-shell polycyclic aromatic hydrocarbons (PAHs) levels out with the increase of their aromatic structure and, contrary to conventional assumptions in existing soot models, their reactivity approaches that of persistent radical species. These findings were implemented in the model here proposed to describe soot formation (Part I) and oxidation (Part II). While the number of lumped pseudo-species involved is significantly reduced compared to previous discrete sectional soot models since no distinction between the kinetics of closed-shell and open-shell particles is taken into account, the physical meaning and consistency with recent evidences improves together with model performances. Rate parameters of gas-phase reactions involving resonantly stabilized radical PAHs were used as a reference for soot particle kinetics. The model was validated against 65 target ethylene laminar premixed flames and counterflow diffusion flames, with a general good agreement between experimental data and numerical simulations at different pressures (P = 1–8 atm), maximum flame temperatures (Tmax∼1500–2400 K) and soot yields (fv∼0.01 to ∼170 ppm). Parity diagrams allow to identify systematic deviations of the present model from the experiments, while the comparison between all the flames included in the database highlights the relative contribution to soot formation and growth of the main reaction classes constituting the soot model, spotting analogies and peculiarities of the two flame configurations considered. Similar kinetic patterns characterize most of the flames within the database, independently of flame configuration and sooting tendency of the specific case. Large PAH condensation dominates the formation of the first soot nuclei, while the condensation of smaller PAHs up to four aromatic rings is primarily responsible for the growth of larger soot particles and aggregates. Finally, the contribution of the different fluid dynamics in premixed and counterflow flames to particle evolution, resulting in a higher hydrogenation level of soot particles in the latter, is discussed.

Hydrogenated orange oil: A waste derived drop-in biojet fuel

Orange oil was extracted by steam distillation from the peels of oranges produced as waste in the orange juice factories. This raw orange oil, a potential source for biojet fuel, was analysed by FT-IR and GC-MS, and compared with distilled orange oil and pure d-limonene, which is its main chemical constituent. Both distilled orange oil and d-limonene were hydrogenated under reaction conditions (from 3 to 18 bar) which are mild enough to be industrially feasible, to improve its properties, especially to reduce their sooting tendency. Some important properties such as density, viscosity, heating values, lubricity, flash point, crystallization onset temperature, and smoke point were measured for hydrogenated orange oil and d-limonene (as a reference for comparison) at different conversions. These hydro-biofuels were blended with Jet A1 to check their suitability as biobased blending components for aviation. Based on the results obtained for the main aviation fuel properties, it is concluded that up to 15 vol% of partially hydrogenated orange oil could be blended with Jet A1 without any significant drawback for the performance of the actual airplanes. Flammability reduction systems would be needed to further increase the blend proportion of this drop-in biofuel in Jet A1.

Statistical study on soot volume fractions of buoyant turbulent diffusion fires

SummaryThe statistical soot properties of buoyant turbulent diffusion propane fire were experimentally and theoretically studied with different fuel sooting tendencies by nitrogen addition. Results show that nitrogen addition decreases the instantaneous soot volume fraction and increases soot intermittency. The combination of these two factors together leads to a decrease in mean soot volume fraction when nitrogen is added. Both radially integrated and the total soot volume fraction decrease with nitrogen addition. The radially integrated soot volume fraction is found to be a function of fuel concentration. A dimensionless correlation is proposed to predict the total soot volume fraction based on theoretical analysis and in good agreement with the experimental data. The results could provide guidance for predicting soot yield for buoyant turbulent fires.

Synergistic effect analysis on sooting tendency based on soot-specialized artificial neural network algorithm with experimental and numerical validation

Interaction between components is an important characteristic in a fuel mixture, which has significant influence on sooting tendency and is strongly related to chemical structures of components. It is time-consuming to study mixing effects between multiple fuels on soot formation through experimental methods, so it is essential to develop a simple and reliable statistical method to supplement experimental data. All sooting tendency data was transformed into a dimensionless index called Yield Sooting Index (YSI). A machine learning algorithm, named soot-specialized artificial neural network (S-ANN), was developed in this paper to predict sooting tendency of fuel surrogates. The mean relative error of S-ANN prediction results can be reduced to about 6%, which is better than that of linear method (13.72%). As a widely used index, YSI can characterize the relative scale of fuel sooting tendency but cannot clearly represent the synergistic or neutralization effects of mixture components. Therefore, this work proposes Soot Synergistic/Neutralization Index (SSNI) using the deviation between predicted value and linear fitting value. SSNIs are transformed from the prediction results of S-ANN and can indicate the interactions between fuels in terms of sooting tendency, which can also provide important information for component selection and sooting tendency adjustment during formulation process of surrogates. Two groups of binary mixtures were selected to validate the algorithm and index. From validation experiment, it is indicated that negative synergistic effect exists between n-cetane and iso-cetane; positive synergistic effect exists between iso-cetane and butylcyclohexane. These two synergistic effects are simulated and analyzed through previously developed chemical mechanism. The results reveal that soot-specialized artificial neural network algorithm have a strong ability to predict sooting tendency of fuel mixtures, which can be used as an accurate and low-cost approach to supplement experimental data. SSNI can help to understand the interactions of fuel mixtures and provide valuable guidance for the construction of surrogates.

Variation in sooting characteristics and cetane number of diesel with the addition of a monoterpene biofuel, α-pinene

Diesel fuel combustion generates soot particles, which are harmful for human health and the environment. To reduce soot emission, various solutions are proposed in the literature such as the use of metal-additives in fuels, fuel blending with biofuels, and the use of diesel particulate filters. This study analyses the effect of the addition of a bicyclic monoterpene hydrocarbon biofuel, α-pinene to diesel in different proportions on the fuel cetane number, sooting propensity, and the physicochemical properties of soot nanoparticles. The addition of 10% α-pinene to diesel exhibited a synergistic effect on sooting tendency and reduced the threshold sooting index of diesel by 21%, even though α-pinene is an unsaturated hydrocarbon, and had a minimal impact on cetane number, which reduced from 63.5 for diesel to 61.5 for the blended fuel. The influence of α-pinene addition to diesel on soot nanostructural characteristics and reactivity is determined through different characterization approaches including HRTEM, TGA, XRD, SEM-EDX, and EELS. The results indicate that α-pinene addition to diesel imposes curvatures in soot nanostructure, creates relatively smaller fringes (PAHs) in soot, and reduces soot aromatic content to improve soot oxidation rate.

Chemical and Sooting Structures of Counterflow Diffusion Flames of Butanol Isomers: An Experimental and Modeling Study

ABSTRACT This work reports an experimental and numerical analysis on the sooting characteristics of butanol isomers. Light extinction and gas chromatography were used to measure soot and gas-phase species, respectively. Kinetic analysis of the tested flames was performed with detailed gas-phase mechanism and a sectional soot model. The present work aims to provide an understanding on the effects of butanol isomeric structures on sooting tendencies. For a comprehensive analysis, flames of both neat butanol fuels and butanol/hydrocarbon mixtures were studied. The results showed that the relative ranking of sooting tendencies among the butanol isomers were similar in neat butanol flames and in butanol-doped ethylene flames. In addition, we show that a small amount of butanol addition in ethylene flame enhanced soot formation. It was found that different isomeric structures mainly affected the formation of C3H3, which led to the different concentrations of important aromatic soot precursors. In the butanol-doped ethylene flames, the blending of the four butanol isomers increased the number of C3H3 formation pathways, which in turn significantly increased the production of benzene. Structural effects explaining for the differences in the sooting tendencies of the four butanol isomers in neat butanol flames and ethylene/butanol flames were found to be the same.

Machine learning and deep learning enabled fuel sooting tendency prediction from molecular structure

Soot formation models become increasingly important in advanced renewable fuels formulation for soot reduction benefit. This work evaluates performance of machine learning (ML) and deep learning (DL) to predict yield sooting index (YSI) from chemical structure and proposes a tailor-made convolution neural network (CNN)-SDSeries38 for regression problem. In ML, a novel quantitative structure-property relationship (QSPR) is developed for feature extraction and the relationship between molecular structure and YSI is built by ML algorithm. In DL, SDSeries38 contains 9 feature learning modules, 1 regression module for automated feature learning and regression. It adopts standard series network architecture and modular structure, each feature learning module is a stack of convolution, batch normalization, activation, pooling layers. ML-QSPR model outperforms SDSeries38 in accuracy (RMSE = 7.563 vs 19.58), computational speed and the former applies to fuel mixtures. In DL, SDSeries38 network exceeds 10 classical CNN and provides a generic architecture enabling transfer application to other regression problem. DL application to regression is still in its infancy and there is no complete guide on how to develop specific CNN architectures for regression. Some gaps need to be filled: (1) Specially developed CNN architectures for regression are required; (2) The performances of direct transfer learning the classical CNN architectures from classification to regression are modest. A modular structure with typical function modules may provide an ideal solution; (3) Going deeper into the sequence of convolution layers improves predictive accuracy, but bears in mind to keep the number of layers below the threshold to avoid vanishing gradient.

Effect of alkyl substituent for cyclohexane on pyrolysis towards sooting tendency from theoretical principle

The fuel structure is an important controlling factor during the process of soot formation. Alkylated cyclohexane is a typical surrogate fuel with more complex structure than alkanes or aromatic fuels. Five alkylated cyclohexanes (i.e., cyclohexane, methylcyclohexane, ethylcyclohexane, 1,3-dimethylcyclohexane, 1,3,5-trimethylcyclohexane) and the n-alkane were selected to investigate the effect of the alkyl substituent on sooting tendency during the pyrolysis process from the theoretical principle. The reaction energy barriers profiles of alkylated cyclohexanes obtained by the density functional theory (DFT) illustrate the length and number of the alkyl substituent will affect the production of alkenes fragments which finally decompose to C2H2 and C3H3 species. The molecular dynamics simulations show the abundance of C2H2 and C3H3 fragments from the initial pyrolysis of fuels will accelerate the sooting tendency, which follow the trend of cyclohexane < methylcyclohexane < ethylcyclohexane ≈ 1,3-dimethylcyclohexane < 1,3,5-trimethylcyclohexane, in agreement with experimental findings. We hope this study can provide a more comprehensive understanding of the effect of fuel structure on sooting tendency and ideas for soot inhibition.

How do the oxygenated functional groups in ether, carbonate and alcohol affect soot formation in Jet A2 diffusion flames?

Four oxygenated fuels: ethanol (EtOH), dimethyl carbonate (DMC), dimethoxymethane (DMM) and polyoxymethylene dimethyl ether (PODE3) were blended with Jet A2 to investigate the sooting behaviour of the fuel mixtures. The smoke point was measured using wick-fed laminar diffusion flames as per the ASTM D1322 standard. The oxygen extended sooting index (OESI) was calculated to determine the sooting tendency of each mixture. Colour-ratio pyrometry and differential mobility spectrometry were used to measure the soot volume fraction (fv) and particle size distribution (PSD). The addition of oxygenated fuels caused a strong reduction in sooting tendency (i.e. OESI) at low blend strengths (5%) and a weaker linear reduction at higher blend strengths (10% and 20%). Each mixture showed a similar reduction at a given mole fraction of oxygenated fuel. The OESI broadly correlated with the soot volume fraction and particle size measurements. Increasing blend strengths resulted in smaller particles at the tip of the flame. The average particle size at the tip was influenced by the oxygen content but not the molecular structure of the oxygenated fuels, whereas the soot volume fraction in the wings was influenced by both the molecular structure of the oxygenated fuels and the oxygen/carbon ratio of the mixture. For the first time, fv and PSD have been reported for flames produced using Jet A2 blends in an ASTM D1322 lamp. The ability to relate data gathered using the ASTM D1322 standard for the sooting behaviour of different mixtures is going to be increasingly important as the aviation industry seeks to switch to sustainable fuels.

Production, fuel properties and combustion testing of an iso-olefins blendstock for modern vehicles

With the increasing pressure to decarbonize the transportation sector, exploring strategies that can reduce emissions from light-duty vehicles (LDV) has become critical. Bioblendstocks that allow for higher engine efficiency and fuel economy could complement vehicle electrification and help reach carbon neutrality by 2050. In this context, the potential of a mixture of iso-olefins as a bioblendstock was investigated for multimode boosted spark-ignition (SI)/advanced compression ignition (ACI) engine operation designed to achieve higher overall vehicle fuel economy. By establishing the relationship between the molecular structure of iso-olefins and research octane number (RON), octane sensitivity (S) (i.e., the difference between RON and motor octane number [MON]), and phi-sensitivity (i.e., the sensitivity of autoignition to the fuel-air equivalence ratio) a dimethyl-hexenes rich olefins mixture (DMHROM) was identified as a preferred blendstock for SI/ACI combustion engines. A pathway for DMHROM production from biomass-derived ethanol was developed and scaled up. More than 1 gallon of DMHROM blendstock was produced for fuel properties assessment including engine testing. Measurements in a Cooperative Fuel Research Engine showed that the DMHROM blendstock possesses a RON of 94 and S of 13.5, and blends synergistically. Rapid compression machine tests coupled with single-cylinder gasoline direct injection engine measurements demonstrated the 20 vol% DMHROM blend has higher phi-sensitivity than an olefin-free gasoline base fuel and a typical California Reformulated Gasoline Blendstock for Oxygenate Blending (CARBOB) gasoline fuel. These results demonstrate the potential of DMHROM for improving gasoline fuel performance and quality for operation under ACI conditions. The effectiveness of the aftertreatment system in mitigating emissions was verified and showed that the pure DMHROM blendstock and 20 vol% blend would not increase non-methane organic gases, NOx, or carbon monoxide (CO) emissions. The DMHROM blendstock was found to slightly decrease sooting tendency when added to a gasoline-base fuel (i.e., ∼6% reduction at 20 vol% blending level). Oxidation stability and lubricant compatibility were both confirmed for the 20 vol% blend. Overall, these results demonstrate that dimethyl-hexenes have potential for improving engine efficiency and fuel economy while meeting emissions regulations and ASTM specifications for gasoline fuel.

Experimental investigation of the lift-off height and soot formation of a spray flame for different co-flow conditions and fuels

Since the combustion process in spray flames is a highly complex multi-phase phenomenon, which involves several simultaneous processes such as atomization, vaporization, and chemical kinetics, it is still not fully understood. In the present work, an experimental investigation has been performed for three different hydrocarbon fuels (n-Heptane, n-Decane, n-Dodecane), in order to understand the effect of varying co-flow conditions, fuel mass flow rate, and fuel type on both the flame lift-off height and soot formation. The fuels were injected through a hollow-cone spray injector, with a nominal spray cone angle of 80∘ and orifice diameter of 120μm in an annular non-swirled preheated air co-flow. The flame lift-off height was determined by recording the OH* chemiluminescence, whereas soot formation has been determined through the color diffused back-illumination extinction technique. From the results, it has been observed for a certain fuel that the flame lift-off height is mainly controlled by the co-flow velocity and air co-flow temperature. The results also show that the fuel that yields largest droplet size and that possesses the lowest volatility exhibits the highest flame lift-off height. Furthermore, the results evidence a strong influence of the co-flow velocity on the soot formation. With an increase in co-flow velocity, the flame lift-off height is increased and so the amount of air entrainment, leading to a less rich reaction zone just downstream of the lift-off height, which in turn results in less soot formation. Finally, the comparison among different fuels shows that their differences in soot formation are likely related to fuel sooting tendency. This property, in turn, depends on the fuel molecular structure playing an important role on its determination.

Pyrolysis of bio-derived dioxolane fuels: A ReaxFF molecular dynamics study

Alkyl-substituted 1,3-dioxolanes, including 4,5-dimethyl-2-pentan-3-yl-1,3-dioxolane (Fuel 1), 4,5-dimethyl-2-pentyl-1,3-dioxolane (Fuel 2), and 2-(heptan-3-yl)-4,5-dimethyl-1,3-dioxolane (Fuel 3), have been recently suggested as potential biodiesels. In this paper, we investigate the initial pyrolysis of the alkyl-substituted 1,3-dioxolanes at high temperatures using ReaxFF molecular dynamics (MD) simulations. We analyze the decomposition rate, reaction mechanism, and product distribution in the pyrolysis of the three alkyl 1,3-dioxlanes. The three fuels primarily decompose to 4,5-dimethyl-1,3-dioxolane radical and hydrocarbons derived from the alkyl side-chains. The further decomposition of 4,5-dimethyl-1,3-dioxolane radical primarily leads to 2-C4H8 and CO2 within a few decomposition steps. The hydrocarbon product distribution is significantly affected by the molecular structure of the alkyl side-chain, which would have a strong influence on the sooting tendency of these fuels. The ReaxFF simulations predict that the order of sooting tendency would be Fuel 3 > Fuel 1 > Fuel 2, which agrees with the measured sooting tendency trend. Based on the pyrolysis mechanism identified by ReaxFF, we propose a new alkyl dioxolane, 4-hexyl-5-methyl-1,3-dioxolane (Fuel 4), which might produce even less soot, by modifying the molecular structure of Fuel 2. Our ReaxFF simulation shows that Fuel 4 produce much less C4H8, an effective non-aromatic soot precursor, than Fuel 2. Moreover, more carbon atoms are bonded to each oxygen atom in Fuel 4 than Fuel 2, which would help reduce soot yield by removing more carbon atoms from the soot-producing pool of species. The major decomposition pathways identified in this work can be used to develop chemical kinetic models for 1,3-dioxolane based compounds, as biodiesel components, applicable to combustion engine simulations. We also demonstrate that the chemical kinetic insight offered by ReaxFF simulations can be used to design new fuel molecules with more desired properties.