Article

Recent Progress in the Development of Diesel Surrogate Fuels

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Abstract

There has been much recent progress in the area of surrogate fuels for diesel. In the last few years, experiments and modeling have been performed on higher molecular weight components of relevance to diesel fuel such as n-hexadecane (n-cetane) and 2,2,4,4,6,8,8-heptamethylnonane (iso-cetane). Chemical kinetic models have been developed for all the n-alkanes up to 16 carbon atoms. Also, there has been experimental and modeling work on lower molecular weight surrogate components such as n-decane and n-dodecane that are most relevant to jet fuel surrogates, but are also relevant to diesel surrogates where simulation of the full boiling point range is desired. For two-ring compounds, experimental work on decalin and tetralin recently has been published. For esters, kinetic mechanisms for compounds of lower molecular weights but similar to those found in typical biodiesel blendstocks also have been published. For multi-component surrogate fuel mixtures, recent work on modeling of these mixtures and comparisons to real diesel fuel is reviewed. Detailed chemical kinetic models for surrogate fuels are very large in size, so it is noteworthy that significant progress also has been made in improving the mechanism reduction tools that are needed to make these large models practicable in multi-dimensional reacting flow simulations of diesel combustion. Nevertheless, major research gaps remain. In the case of iso-alkanes, there are experiments and modeling work on only one of relevance to diesel: iso-cetane. Also, the iso-alkanes in diesel are lightly branched and no detailed chemical kinetic models or experimental investigations are available for such compounds. More components are needed to fill out the iso-alkane boiling point range. For the aromatic class of compounds, there has been little work for compounds in the boiling point range of diesel. Most of the new work has been on alkyl aromatics that are of the range C7–C9, below the C10–C20 range that is needed. For the chemical classes of cycloalkanes and esters, experiments and modeling on higher molecular weight components are warranted. Finally for multi-component surrogates needed to treat real diesel, the inclusion of higher molecular weight components is needed in models and experimental investigations.

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... Diesel is a mixture of up to hundreds of pure components, and considering all of these species in a simulation is difficult and time-consuming, if it is not impossible. Three approaches were proposed in the literature to model diesel composition, namely surrogates [9], pseudo-components [10], and continuous description [11]. Surrogates are defined as a mixture of known components so that the mixture's desired properties match those of the real fuel [9], and components and their percentage in the surrogate are determined by minimizing the deviation between the estimated and measured values of chemical and/or physical properties. ...
... Three approaches were proposed in the literature to model diesel composition, namely surrogates [9], pseudo-components [10], and continuous description [11]. Surrogates are defined as a mixture of known components so that the mixture's desired properties match those of the real fuel [9], and components and their percentage in the surrogate are determined by minimizing the deviation between the estimated and measured values of chemical and/or physical properties. Using surrogate fuels is the most common method to capture diesel properties without the exact description of its chemical composition. ...
Article
This paper presents the prediction capability of the density of near- and super-critical hydrocarbon mixtures, especially diesel fuel, by three cubic equations of state (EoS), which include PPR78, PSRK and RK-PR, two virial EoS that are BWR and SBWR, and PC-SAFT EoS. Comparison with published measured density of three different hydrocarbon mixtures, which are methane-propane-pentane, heptane-octane, and RP3 aviation kerosene, revealed that all the equations of state predict well the density of the gas phase. On the other hand, a comparison with RP3 experimental data showed that all EoS over-predict the density of gas-like supercritical phase. However, to ascertain the findings of this comparison, experimental data for other hydrocarbon mixtures at such a state are required. The results showed also that PC-SAFT is a suitable choice for liquid and liquid-like density calculations with an AAD of 2.9%, closely followed by SBWR and BWR with AAD values of 3.7% and 4.0%, respectively, while RK-PR and PPR78 produce reliable prediction of the density of liquid and liquid-like supercritical phases of hydrocarbons with an AAD of 5.5% and 4.4%, respectively. RK-PR and SBWR are found as the most accurate EoSs for calculating the density of hydrocarbons mixtures at the transition from liquid-like to gas-like supercritical phases with an AAD of 11.2% and 12.8%, respectively. Using zero and non-zero binary interaction parameters with PPR78, PSRK, and PC-SAFT showed negligible effect on the density calculation. Comparison of the predictions with the experimental diesel density showed that BWR has the best results, followed by PC-SAFT. RK-PR is found to deviate by less than 5% in predicting the diesel density for all examined surrogates. The predictions of diesel density by different EoS at near- and super-critical conditions are compared with each other because of the lack of published experimental data. This comparison showed that RK-PR can be used, with a reasonable accuracy and computational cost, for predicting the density of both the liquid-like phase and transition interval, and PPR78 for the gas-like supercritical phase. On the other hand, more accurate predictions of the liquid-like phase density can be achieved using BWR, and that of the transition interval and gas-like phase can be realized by SBWR but with a higher computational cost.
... The surrogate fuel methodology using several representative components to match the practical fuel's chemical kinetic characteristics is widely used. 21 In our previous works, 22,23 the formulation of RP-3 surrogate fuel which contains 54.3% ndodecane/32.1% 2,5-dimethylhexane/13.6% toluene was proposed by the functional group-based surrogate methodology. ...
Article
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... This creates difficulties in the CFD modeling of combustion processes. The works [37][38][39][40][41] present some options for modeling the combustion of diesel fuel. An analysis of the composition of commercial diesel fuel shows that the mass fraction of carbon is approximately 86% [18]. ...
Article
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... The n-decane (n-C10H22) is an alkane with a large carbon number, and it is suggested as a major component in the surrogates for jet and diesel fuels. 63,64 The fuel droplets with uniform sizes are filled in the tube at a temperature of 300 K and pressure of 0.1 MPa. The computational domain is schematically depicted in Fig. 1. ...
Article
In this work, to promote deflagration to detonation transition (DDT), a designed hot jet in a pre-detonator is produced to initiate the detonations in the main detonation tube. We perform two-dimensional simulations of the DDT process for low-volatile fuel (n-decane) mixed with nitrogen and oxygen based on the Eulerian−Lagrangian approach. The effects of fuel atomization, vaporization, and shock focusing on the flame acceleration and DDT are discussed under different nitrogen dilution ratio and droplet size conditions. The results show that the flame acceleration process can be divided into slow and fast deflagration stages. Additionally, initiation times are mainly determined by the fuel atomization and evaporation in the slow deflagration stage, which dominates the entire DDT time. Furthermore, there are different intensities of hot jets rather than stable detonation waves formed at the pre-detonator exit. Moreover, local decoupling and re-initiation events are detected near the internal wall of the U-bend, inducing the overdriven detonation decaying into stable detonation waves in the smooth tube. Results also demonstrate that the detonation pressure and velocity decrease by 13.56% and 12.55% as the nitrogen dilution ratio increases from 0.5 to 2. In particular, as the nitrogen dilution ratio continued to increase to 2.25, the development in DDT is similar, but the jet intensity is significantly weakened. While as the droplet size increases from 10 to 40 um, the detonation pressure and velocity decrease only by 2.69% and 1.49%, respectively.
... The four-component diesel surrogate fuel model from the work of Chang and coworkers [22] was adopted as the base mechanism, and this model was constructed by applying the decoupling methodology [22,23]. As known, commercial diesel fuel is a mixture of hundreds to thousands of hydrocarbon components, and its main components can be classified as n-alkanes, iso-alkanes, cycloalkanes and aromatics [24]. Those four components in actual diesel fuel were represented by n-decane, iso-octane, MCH and toluene, respectively, in the diesel surrogate of Chang et al. [22]. ...
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... Butylbenzene is not only an aromatic representative component in jet-fuels and diesel fuels [103], [104] but also a model fuel for large alkylbenzenes, such as n-pentylbenzene, etc… [105]. Much research on the fundamental combustion parameters of n-butylbenzene was performed, including ignition delay [105]- [109], flame speeds [75], [110], [111], species concentrations [112]- [114]. ...
Thesis
Polycyclic aromatic hydrocarbons (PAHs) are considered important soot precursors. Exploring and understanding the PAH formation pathways are essential building-blocks toward developing reliable kinetic models that can accurately simulate soot formation. This work aims at developing a comprehensive kinetic model emphasizing on PAH formation chemistry based on the detailed species profiles obtained in a high­purity single-pulse shock tube coupled to gas chromatography / mass spectrometry techniques. ln particular, the pyrolysis of different aromatic fuels and mixtures is studied under combustion-like conditions, over a wide temperature range (900-1800 K), pressures of around 20 bar, and reaction times of 4 ms. The fuels include propylene, propyne, benzene and toluene with and without addition of C2/C3 fuels, phenylacetylene, phenylacetylene + acetylene/ethylene, Ca-C10 alkylbenzenes. The outputs of the present work include: i) an extensive experimental database of species profiles, including PAHs up to four-rings; ii) development of a detailed and comprehensive chemical kinetic mode!; iii) advanced understanding on the kinetic schemes and mechanisms involved in the fuels thermal decomposition and formation of soot precursor molecules. These results can serve for future mode! developments concerning more complex fuels and surrogates as well as the base for the construction of soot codes for simulation of particle formation in combustion applications.
... The list of fuel surrogates presented is especially relevant for high-reactivity fuel. Diesel for instance can contain as many as 200 different components [35], each impacting its properties. The fuel property most relevant for the present application is ignition behaviour. ...
Conference Paper
With low-temperature combustion engine research reaching an applicable level, physics-based control-oriented models regain attention. For reactivity controlled combustion concepts, chemical kinetics-based multizone models have been proven to reproduce the governing physics for performance-oriented simulations. They offer accuracy levels similar to high-fidelity computational fluid dynamics (CFD) models but with a fraction of their computational effort. Nevertheless, state-of-the-art reactivity controlled compression ignition (RCCI) simulations with multizone model toolchains still face challenges related to predictivity and calculation speed. This study introduces a new multizone modelling framework that addresses these challenges. It includes a C++ code, deeply integrated with open-source, thermo-kinetic libraries, and coupled to an industry standard 1-D modelling framework. Incorporating a predictive turbulence mixing model, it aims to eliminate dependence on CFD-based initialisation, while applying a novel zonal configuration to achieve sensitivity to the combustion chamber´s geometrical features. Basic sensitivity simulations performed for zonal resolution and chemical kinetic mechanisms prove the approach is fit for purpose. Aiming for optimal trade-off between accuracy and simulation speed, the 12-zone model has a simulation time below three minutes per closed cycle. These achievements are validated against a medium-speed, large-bore, single-cylinder research engine, running in a dual-fuel mode with natural gas and light fuel oil. Using basic submodels, the framework reproduces measured in-cylinder pressure trace within an RMS error of 0.85 bar, and combustion performance indicators within a 5% error margin target. Ultimately, this is the first time the multi-zone kinetic framework has been proven suitable to reproduce RCCI combustion on a state-of-the-art marine engine geometry.
... Compared to the cryogenic liquid injection, diesel injection is more complicated, since the fuel is composed of hundreds of species [14]. From a thermodynamic point of view, the mixture's critical properties are significantly different from those of a single species, exhibiting a complicated nonlinear relationship with temperature and pressure [15]. ...
... There are a variety of surrogates for diesel fuels; in this work it is formed from 70% n-decane and 30% 1-methylnaphthalene [166,167]. The thermodynamic simulation routine is similar to that shown in Fig. S8. ...
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The present review article aims to study different aspects involving ethanol combustion and utilization. The review deals with the production and sustainability of corn-based and sugarcane-based ethanol as a starting point to present ethanol as a renewable and sustainable biofuel. The second step is to present the current understanding of ethanol combustion, regarding the application of thermodynamic theory and detailed chemical kinetic mechanisms to model the combustion processes. The utilization of ethanol implies consideration of safety parameters related to its combustion. Therefore, a section was dedicated to discussing the flammability limits and detonability limits of ethanol and their experimental determination. The application of gasoline-ethanol blends in spark ignition engines and diesel-ethanol blends in compression ignition engines is also addressed. New techniques (as dual injection) are being considered to improve the combustion efficiency. Also, the use of biodiesel in diesel-ethanol-biodiesel blends has shown to be a promising possibility. The greenhouse gas emissions obtained in different experimental works with internal combustion engines running on different fuel blends involving ethanol were also reviewed. Ethanol potentially increases the thermal efficiency of internal combustion engines and reduces the NOx emissions. Finally, the possibility of integrating internal combustion engines and ethanol fuel cells is also considered.
... Xylenes are found as important components in crude oils and also widely used to formulate surrogates for liquid transportation fuels [1][2][3] . Predictive combustion kinetic models for xylenes consumption and the subsequent formation of pollutants such as polycyclic aromatic hydrocarbons (PAHs) are highly necessary towards practical applications. ...
Article
Full-text available
This work reports a kinetic study on the pyrolysis of three xylene isomers based on single-pulse shock tube experiments and detailed kinetic modeling. Speciation measurements for the post-shock gas mixtures are obtained via sampling and gas-chromatography-mass spectrometry over a temperature range of 1150−1650 K at a nominal pressure of 20 bar. A sub-mechanism incorporating consumption reactions of xylene isomers as well as subsequent pathways leading to polycyclic aromatic hydrocarbons (PAHs) is developed and integrated into our on-going PAH formation kinetic model. The model can satisfactorily predict the qualitative measurements and accurately characterize both the similarities and fuel-specific features in the pyrolysis of three xylenes. The three isomers decompose at similar rates, though m-xylene exhibits a slightly weaker reactivity. This arises from the fact that m-xylyl, different from its o- and p- counterparts, cannot undergo hydrogen loss to form m-xylylene diradical. Distinct consumption schemes of the three xylyl radicals also result in the different species pools observed at temperatures below 1400 K. A large amount of styrene results from a stepwise isomerization of o-xylylene following dehydrogenation of o-xylyl. An early formation of anthracene is noted as a unique phenomenon in o-xylene pyrolysis, which is attributed to specific reactions of o-xylyl. Due to a relatively high abundance of m-xylyl, the self-recombination product 3,3′-dimethylbibenzyl is among the major species formed at the initial stage of m-xylene pyrolysis. p-Xylyl predominantly converts to p-xylylene which is largely consumed via polymerization processes. Besides, mutual conversions among the three xylyl radicals are found to play an essential role in the pyrolysis of three xylenes. Toluene is measured of significant concentrations in the pyrolysis of all three isomers, which rationalizes that PAH speciation in the pyrolysis of three xylenes and toluene is highly similar at elevated temperatures. Modeling analyses show that apart from the benzyl/toluene chemistry, reactions involving C8 species also have important contributions to PAH formation in xylenes pyrolysis.
... Due to the complexity of this fuels, surrogates ones were used for simulations instead. For diesel, n-heptane was used as it is a widely accepted as a surrogate for conventional Diesel [29,30]. For OME , it must be taken into account that in experiments the fuel was a mixture of different OME chains where varied from 1 to 6. ...
Article
The reduction of the carbon footprint of internal combustion engines and the pollutant emissions is mandatory for the survival of this technology. In this sense, e-fuels are considered as a potential pathway to achieve this reduction and even remarkable carbon footprint mitigation in compression ignition engines. Among numerous e-fuels, oxymethylene ethers stand out because of their low soot formation characteristics. However, the complexity of their physical and chemical properties makes it a challenge to be used in conventional engines. The aim of the current study is to investigate the effects of the stoichiometry of oxymethylene ether on the in-cylinder combustion behaviour and the pollutant formation when blended with fossil diesel. For this purpose, numerical simulations of a medium duty optical engine fuelled with these blends were carried out using CONVERGE CFD, which were validated with experimental data. Different reaction mechanisms that can be found in the literature were evaluated, using n-heptane as to the fossil diesel surrogate and OME3 as the oxymethylene ether surrogate. Results highlight the differences in terms of equivalence ratio fields achieved when varying the e-fuel content in the blend. As a consequence, the combustion process is faster and the soot formation is drastically reduced when the oxymethylene ethers content is above 30%. This makes these blends interesting to reduce the well-known soot-NO trade off of compression ignition engines.
... Hydrocarbon catalytic cracking over zeolites is considered to be a chain reaction of the carbenium ion. [13][14][15][16][17] Experimental and theoretical investigations into the representative components have been carried out to reveal the details of hydrocarbon catalytic cracking. Due to their specific molecular structures, n-hexane, 1-hexene, cyclohexane and cyclohexene are usually chosen as model compounds of normal alkanes, linear alkenes, cyclic alkanes and cyclic alkenes, respectively. ...
Article
In order to reveal the effects of the molecular structure, the catalytic cracking of n-hexane, 1-hexene, cyclohexane and cyclohexene over HZSM-5 zeolites was carried out at 260-550 °C under atmosphere....
... Hexadecane and 1-decane molecules contain 16 carbons and 10 carbons, respectively. Both fuels are knows as surrogate fuels for diesel [52]. 1-Dodecanol was used as the surfactant to prepare microemulsions. ...
Article
This study is motivated by the need to present a robust methodology for preparing stable methanol-in-diesel emulsions for use in compression ignition engines with the specific objective of maximizing the methanol content. Specifically, it involved exploring the feasibility of methanol-in-diesel emulsions with conventional surfactants such as Tween-80 and Span-80 and non-conventional surfactants such as 1-dodecanol, pentanol, and butanol. The hydrophilic-lipophilic balance (HLB) values of the surfactant were varied from 7 to 15 to investigate the role of the surfactant HLB on stability of the macroemulsion. It is observed that the macroemulsion with an HLB value of 10 provides the best stability results. Using surfactant HLB value of 10, three macroemulsions with 10 wt.%, 15 wt.%, 20 wt.% of methanol were prepared using ultrasonication. However, only the macroemulsion with 10 wt.% of methanol was observed to be stable for at least 20 days after preparation. Next, the microemulsions of diesel-methanol were produced by using non-conventional surfactants such as 1-dodecanol, pentanol, and butanol. Among these, 1-dodecanol was found out as the most suitable surfactant owing to its ability to form microemulsions with any mixing ratio of diesel-methanol, and its high cetane number (63.6). This study has clearly brought out the strategies for preparing both macro and microemulsions. Overall, the results presented in the current work are expected to aid efforts in adapting compression ignition engines for diesel-methanol fuel blends.
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The dynamics of low temperature combustion (LTC) and intermediate temperature combustion (ITC) dramatically affect the combustion limits and properties of high temperature combustion (HTC) and engine performance. To reveal the transition from LTC to ITC and HTC and the characteristics of heat release in spray combustion under low ambient temperatures, optical experiments were conducted in a constant volume combustion chamber (CVCC). The high-speed natural luminosity method and the high-speed schlieren method were used to obtain images of the evolution of spray ignition and vapor-phase spray combustion, respectively. The results reveal a significant increase in CVCC pressure when the flame luminosity is not captured by the high-speed natural luminosity method at a low ambient temperature (740 K), which is attributed to heat release by the LTC process. The cumulative heat release obtained from the first law of thermodynamics based on pressure attains 29.72 % of the total lower heating value of the fuel. The transition from LTC to ITC and HTC is diagnosed based on the luminosity variation of the vapor-phase spray tip region in the schlieren images. Furthermore, the transition from LTC to ITC and HTC can be monitored based on the average luminosity of the spray tip. Finally, with the decrease in ambient temperature, the end of LTC is gradually delayed by a smaller magnitude and rate of change. These results demonstrate that the transition from LTC to ITC and HTC is sensitive to ambient temperature.
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In this work, we present a methodology on automatic generation of predictive lumped sub-mechanisms for normal and branched alkanes. This methodology aims at obtaining lumped reaction mechanisms that preserve the chemical behavior of each reaction class in the detailed model. To achieve this goal, detailed sub-mechanisms for combustion of alkanes are generated by employing an updated version of the MAMOX++ software developed in this work; recent progress in the low-temperature reaction classes and rate rules are incorporated into the updated software. Instead of computing the selectivities of several primary products with MAMOX++ and fitting the selectivities between the detailed and lumped models, this work proposes a new methodology to generate the lumped sub-mechanisms for fuel molecules. The stoichiometric parameters and the reaction rates for each reaction class in the lumped sub-mechanism are fitted to match those in the detailed model. Based on the present methodology, both the detailed and lumped sub-mechanisms for normal C5C10 alkanes and branched C5C8 alkanes, that is for 15 different fuels, are automatically generated and merged into a base chemistry model (i.e. AramcoMech 2.0), respectively. The detailed and lumped models are validated against the experimental data in the literature. The automatically generated detailed models for alkanes are able to capture the experimental targets across a wide range of conditions, demonstrating the robustness of the reaction classes and rate rules adopted. The lumped models for normal alkanes have similar performance to their respective detailed models, and are able to predict the oxidation behavior of normal alkanes. However, prediction deviations between the detailed and lumped models for branched alkanes are shown to be slightly greater.
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There is an important influence of alcohols in shale oil on its thermochemical properties. In this work, the fractions and species of the alcohols in shale oil from retorting oil shale were studied experimentally. The alcohols exist mainly in the form of linear carbon chains with different lengths, as well as different sites of hydroxyls. Then, 7-tetradecanol and pentadecanol were selected for further exploring the pyrolysis mechanisms through theoretical calculation and kinetic modeling, due to their abundance and representativeness in alcohols of shale oil. Density functional theory (DFT) method was utilized to reveal the difficulty of the thermal cracking of these two alcohols. The results indicated that the dissociation energies of C-C bonds are lower than those of C-H bonds, the reaction of eliminating water has a low activation energy. The Mulliken charges imply that there is a large electrostatic attraction between the O atom and its attached C atom in 7-tetradecanol, but an electrostatic repulsion between them in pentadecanol, which is thought to have an effect on the elimination of water. Finally, the detailed pyrolysis kinetic mechanisms of these two alcohols were written in a systematic manner by the GRI-Mech3.0 mechanism. A large number of the kinetic parameters were computed by the group-additivity method, which are consistent with the thermochemical data calculated by DFT. In summary, this research provides a theoretical basis for development of the kinetic mechanisms of large molecular alcohols and the accurate surrogate shale oil model.
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Benzaldehyde is a vital intermediate during the oxidation of toluene and oxygenated aromatics, but benzaldehyde's combustion chemistry currently gains little attention. The H-atom abstraction of benzaldehyde is one critical reaction class, yet its rate coefficients rely on the analogy of molecular similarity. For this reason, we have employed the ab initio calculations at the CCSD(T)/cc-pVTZ//M06-2X/6-311+g(d,p) level of theory, along with the RRKM master equation to predict the rate coefficients of H-abstraction reactions of benzaldehyde by H, O3(P), 3O2, OH, HO2, and CH3 radicals. The calculated rate coefficients of benzaldehyde + OH generally agree with literature measurements. From the branching ratio analysis, all the H-abstractions from ring sites can be neglected in kinetic model construction except by OH radical at temperature above 1700 K, where the ring sites contribute a relatively large branching ratio to consume benzaldehyde. A random sampling method has been used to estimate the global uncertainty in the calculated rate coefficients. The logarithm of uncertainty is proportional to the reciprocal temperature, and the global uncertainty is primarily derived from the energy errors and is almost independent of the attacking species. Comparison between benzaldehyde and acetaldehyde reveals that their rate coefficients are consistent only in H-abstractions by H and HO2 but are conflictive in other reactions, especially in the case of OH. By incorporating the new calculations, existing models show a faster prediction in autoignition delay times of benzaldehyde. This study reports the first high-level ab initio calculations for the H-atom abstraction reaction class of benzaldehyde. It is necessary for the future comprehensive kinetic modeling of aromatic aldehyde.
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α-Methylnaphthalene (AMN) is the primary reference bicyclic aromatic compound of diesel, and it is commonly used as a component of diesel, kerosene and jet-fuel surrogates formulated to describe real fuel combustion kinetics. However, few experimental data on neat AMN combustion are available in the literature. This work provides the first measurements of laminar flame speed profiles of AMN/air mixtures at 1 bar varying the initial temperature from 425 to 484 K, and equivalence ratio (φ) between 0.8 and 1.35 paving the way for the kinetic study of AMN combustion chemistry at high temperatures (>1800 K). The experimental data obtained in a spherical reactor are compared with kinetic model simulations. Specifically, the AMN kinetics is implemented from its analogous monocyclic aromatic compound, i.e., toluene, through the analogy and rate rule approach. This method allows to develop kinetic mechanisms of large species from the kinetics of smaller ones characterized by analogous chemical features, namely the aromaticity and the methyl functionality in the case of toluene and AMN. In doing so, it is possible to overcome the need of high-level electronic structure calculations for the evaluation of rate constants, as their computational cost increases exponentially with the number of heavy atoms of the selected species. To assess the validity of this approach, ab initio calculations are performed to derive the rate constants of the H-atom abstraction reactions by H, OH and CH3 radicals from both toluene and AMN. The kinetic model obtained satisfactorily agrees with the measured laminar flame speed profiles. Sensitivity and flux analyses are performed to investigate similarities and differences between the main reaction channels of toluene and AMN combustion, with the former leading to ∼6 cm/s faster flame speed at almost identical conditions (P=1 bar, T∼425 K), as evidenced by both kinetic model simulations and experimental findings.
Article
This work seeks to characterize the fidelity needed in a gasoline surrogate with the intent to replicate the complex autoignition behavior exhibited within advanced combustion engines, and specifically Homogeneous Charge Compression Ignition (HCCI). A low-temperature gasoline combustion (LGTC) engine operating in HCCI mode and a rapid compression machine (RCM) are utilized to experimentally quantify fuel reactivity, through autoignition and preliminary heat release characteristics. Fuels considered include a research grade E10 U.S. gasoline (RD5-87), three multi-component surrogates (PACE-1, PACE-8, PACE-20), and a binary surrogate (PRF88.4). Each fuel was studied at lean/HCCI-like conditions covering a wide range of temperatures and pressures that are representative of naturally aspirated to high boost engine operation. Detailed chemical kinetic modeling is also undertaken using a recently updated gasoline surrogate kinetic model to simulate the RCM experiments and to provide chemical insight into surrogate-to-surrogate differences. The LGTC engine experiments demonstrate nearly identical reactivity between PACE-20 and RD5-87 across conditions, while faster phasing is seen for both PACE-1 and PACE-8 due to their stronger intermediate- and low-temperature heat release (ITHR/LTHR) at naturally aspirated and boosted conditions, respectively. The RCM experiments reveal typical low-temperature, negative temperature coefficient (NTC) and intermediate-temperature autoignition behaviors at all pressure conditions for RD5-87, which are qualitatively reproduced by all surrogates. Quantitative discrepancies in both autoignition and preliminary heat release are observed for all surrogates, while their ability to replicate RD5-87 autoignition behavior follows the order of PACE-20 > PACE-1 > PACE-8 > PRF88.4. Excellent mapping is obtained between the LGTC engine and the RCM, where the engine pressure-time trajectories can be characterized by the regimes represented by the RCM autoignition isopleths. The kinetic model performs commendably when simulating both autoignition and preliminary heat release of PACE-20, while typically overpredicting ignition delay times for PACE-1, PACE-8 and PRF88.4 at high-pressure and low-temperature/NTC conditions. Sensitivity and rate of production (ROP) analyses highlight surrogate-to-surrogate differences in the governing chemical kinetics where n-pentane initiates rapid OH branching at a faster rate and an earlier timing for PACE-20 than iso-pentane does for PACE-1 and PACE-8, making it computationally more reactive than the other surrogates. The current study highlights the need to include non-standardized properties, such as the lean/HCCI-like autoignition characteristics, in addition to ASTM properties (e.g., RON, MON) as metrics of fuel reactivity and targets to be matched when formulating high-fidelity surrogates that fully capture gasoline advanced combustion behavior such as HCCI-like autoignition.
Article
C10 naphthenic ring-containing species are important components in transportation fuels and have been widely used for surrogate formulation of diesel and jet fuels to represent cycloalkanes or aromatic fractions. In this study, experimental and modeling studies were carried out to compare the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with n-butylcyclohexane, decalin and tetralin. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were applied to measure 2D maps of soot volume fractions and relative PAH concentrations, respectively. Yield sooting index (YSI) was obtained experimentally to indicate the fuel sooting tendency. It can be observed that the soot volume fractions, sooting tendencies and pyrene concentrations of C10 naphthenic ring-containing species follow the order of tetralin > decalin > n-butylcyclohexane. The chemical kinetic simulation reveals that decalin decomposes through two main pathways, including the H-atom abstraction route and ring opening route. Especially, the pathway of ring opening reactions coupled with dehydrogenation results in the strongest benzene formation process among the test components. In the reacting system of tetralin flames, ring opening reactions are suppressed, while the dominant decomposition way of tetralin is via H-atom abstraction reactions, which offer a more direct pathway to form naphthalene and indene without benzene formation, leading to the highest naphthalene concentration of tetralin flame and the strongest soot formation process among all the test components. This fundamental investigation aims to provide valuable data on sooting behaviors and PAH formation characteristics of typical C10 naphthenic ring-containing species and get insight into the kinetic pathways from fuels to soot precursors. Useful information has been obtained for soot reduction from the perspective of the fuel design and surrogate formulation in terms of the sooting tendency.
Article
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.
Article
Conformational effect is an important feature of the low-temperature (low-T) oxidation chemistry of cycloalkanes. In this work, both theoretical calculation and conformational analysis were performed for the first oxygen addition in methylcyclohexane (MCH) oxidation to better understand the influence of the spatial orientation and the steric hindrance of substituted functional groups. Based on synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) measurements and adiabatic ionization energy calculations, reactive hydroperoxides, highly oxygenated molecules and stable products were identified in the jet-stirred reactor (JSR) oxidation of 1.0% and 0.5% MCH at 1.04 bar, 500–900 K and the equivalence ratio of 0.25. The abundant formation of 1-methylcyclohexene and the observation of alkenylhydroperoxides and their decomposition products verify the existence of conformation-dependent pathways in first and second oxygen addition in low-T MCH oxidation, respectively. A MCH oxidation model incorporated with conformation-dependent pathways was constructed to consider the conformational effects of cyclohexane ring and sidechain, and was validated against experimental data over a wide range of conditions in this work and literature. The consideration of conformational effects of cyclohexane ring and sidechain can generally better predict the measured species profiles and ignition delay time under low-T conditions. Based on modeling analysis, conformational effects of cyclohexane ring were found to play important roles in first oxygen addition and chain-branching in low-T MCH oxidation. Compared with the situation in low-T cyclohexane oxidation, the methyl substitution on cyclohexane ring reduces its conformational effects on chain-branching process.
Article
Cyclohexane oxidation chemistry was investigated using a near-atmospheric pressure jet-stirred reactor at T = 570 K and equivalence ratio ϕ = 0.8. Numerous intermediates including hydroperoxides and highly oxygenated molecules were detected using synchrotron vacuum ultraviolet photoelectron photoion coincidence spectroscopy. Supported by high-level quantum calculations, the analysis of photoelectron spectra allowed the firm identification of molecular species formed during the oxidation of cyclohexane. Besides, this work validates recently published gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry data. Unambiguous detection of characteristic hydroperoxides (e.g., γ-ketohydroperoxides) and their respective decomposition products provides support for the conventional O2 addition channels up to the third addition and their relative contribution to the cyclohexane oxidation. The results were also compared with the predictions of a recently proposed new detailed kinetic model of cyclohexane oxidation. Most of the predictions are in line with the current experimental findings, highlighting the robustness of the kinetic model. However, the analysis of the recorded slow photoelectron spectra indicating the possible presence of C5 species in the kinetic model provides hints that the substituted cyclopentyl radicals from cyclohexyl ring opening might play a minor role in cyclohexane oxidation. Potentially important missing reactions are also discussed.
Article
The chemical reactions of each component of hydrocarbon fuels at low temperatures become more complex and the interactions become stronger, which is considered to potentially affect the soot formation characteristics. A 0-dimensional simulation was applied to analysis the component coupling effect on polycyclic aromatic hydrocarbon(PAH) forming process for diesel surrogate fuel, n-heptane/cyclohexane/toluene. The results show that, for the single component, the low temperature reaction of cyclohexane could contribute to the PAH formation while n-heptane inhibit it, and toluene is hardly involved in the soot generation at low temperature. Due to the component coupling effect of multi-component diesel surrogate fuels, the soot generation is significantly reduced at low temperatures, and the pyrene(A4) generation is reduced by 3 orders of magnitude compared to individual components weighted fuels. The generation of O radicals from n-heptane rapidly consumes C6H5O, which replaces its original reaction road to naphthalene(A2). As a result, the reaction time of A2 is shortened by 2 orders of magnitude, then, the generation of A4 and soot are reduced due to the reduction of A2. Therefore, the competition and consumption between the low temperature reaction products and the core radicals of soot generation are the key to reducing soot accumulation.
Article
As a major chemical class of fossil fuels, mono-alkylated cyclohexanes are frequently employed to construct the surrogate fuels of fossil fuels. Much effort has been made to study the oxidation behavior of mono-alkylated cyclohexanes, but reduced/skeletal mechanisms suitable for multi-dimensional combustion simulations are still scarce. In this work, a set of skeletal models for mono-alkylated cyclohexanes from methyl-cyclohexane (MCH) to octyl-cyclohexane (n-OTCH) were built and deduced by integrating the decoupling methodology and reaction rate rules. The development process contains two parts, i.e., the skeletal model establishment for the base fuel and the skeletal model deduction for other fuels utilizing the reaction rate rules. For mono-alkylated cyclohexanes, the reactions in the fuel-relevant sub-model also dominate the laminar flame speed, apart from the C0–C3 sub-model, which differs from that of n-alkanes. To well capture the flame propagation behavior, the local sensitivity analysis on the comprehensive mechanism was introduced. For each mono-alkylated cyclohexane, the skeletal model includes 52 species and 216 reactions. The final skeletal models were validated against extensive experimental measurements in jet-stirred reactors, shock tubes, rapid compression machines, and premixed flames over wide operating conditions. Satisfactory agreements between the observed data and simulated results are achieved, which indicates the practicability of the proposed method.
Article
The presence of a liquid wall film caused by spray impingement is inevitable in modern direct injection engines and can have a substantial effect on flame dynamics and pollutant emissions. The present study investigated the effects of a wall film on turbulent flame propagation characteristics under different initial conditions. In these trials, methane-air premixed turbulent jet flames were established in a constant volume vessel with a small pre-chamber connecting to the main chamber through a narrow orifice. Various fuel films were prepared on the surface of a side plate installed in the main chamber. The flame morphologies were captured using a high-speed schlieren photographic system and the intensity of pool fire was examined by an image intensifier with UV lens. Meanwhile, temporal pressures in both chambers were recorded by two pressure sensors respectively. Changes in turbulent combustion phases in conjunction with single and multi-component wall films were analyzed based on flame propagation features, pressure data and film pyrolysis characteristics over ranges of equivalence ratios (0.8–1.2) and initial pressures (0.2–0.4 MPa). Gas phase emissions were also evaluated to further explore the effects of wall films on turbulent combustion. The high reactivity of the fuel films was found to increase the turbulent flame speed and provide higher peak pressures, although the heat absorbed by evaporation delayed the pressure rise time. The volatility of the film material affected the propagation speed together with the jet flame area growth and these effects were more obvious under lean combustion conditions. Film pyrolysis was triggered as multiple turbulent flows converged near the film region and produced a pool fire with soot deposition. The peaks of pressure and pressure rise rate observed when using a diesel film were intermediate between those obtained with n-dodecane and n-tetradecane films. A lubricating oil film pool fire also appeared after increasing the initial pressure but its intensity was weaker than those of the fires generated by the fuel films.
Article
Accurate determination of fuel properties of complex mixtures over a wide range of pressure and temperature conditions is essential to utilizing alternative fuels. The present work aims to construct cheap-to-compute machine learning (ML) models to act as closure equations for predicting the physical properties of alternative fuels. Those models can be trained using the database from MD simulations and/or experimental measurements in a data-fusion-fidelity approach. Here, Gaussian Process (GP) and probabilistic generative models are adopted. GP is a popular non-parametric Bayesian approach to build surrogate models mainly due to its capacity to handle the aleatory and epistemic uncertainties. Generative models have shown the ability of deep neural networks employed with the same intent. In this work, ML analysis is focused on two particular properties, the fuel density and diffusion, but it can also be extended to other physicochemical properties. This study explores the versatility of the ML models to handle multi-fidelity data. The results show that ML models can predict accurately the fuel properties of a wide range of pressure and temperature conditions.
Article
Cyclic Ethers (CEs) belong to a class of compounds of importance to understand the chemistry of both the engine auto-ignition of hydrocarbon fuels and the combustion of oxygenated biofuels. This article, divided in six parts, aims at systematically analyzing how up-to-date experimental and theoretical methods were applied to unveil the gas-phase oxidation chemistry of these compounds. The first part gives a brief overview on the significance of CEs as intermediates formed during alkane low-temperature oxidation summarizing its generally accepted chemical mechanism. This part also addresses the role of CEs as potential biofuels derived from lignocellulosic biomass and discusses the production methods of these molecules and their combustion performances in engine. The second part presents the different theoretical methods dedicated to calculate the electronic structure, thermochemical and kinetic data of CEs. The third part introduces the experimental methods used in studies related to CEs with a special focus on mass spectrometry and gas chromatography. The fourth part reviews the experimental and modeling studies related to CE formation during the low-temperature oxidation of linear, branched, cyclic alkanes, alkylbenzenes, olefins, and oxygenated fuels. The fifth part analyses the published work concerning the CE degradation chemistry and highlights the dominant involved reactions. To finish, the sixth part concludes and proposes future research directions.
Article
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.
Article
Mitigation of the carbon footprint of internal combustion engines is mandatory to ensure a future for this technology. Within this scope, e-fuels are considered a potential solution to replace conventional fossil fuels. However, in some cases, their physical and chemical properties are so different that its application in conventional engines is complex. For this reason, this work focuses on the study of oxymethylene ethers (OME X ) as a potential low-carbon fuel alternative. The aim is to improve the understanding of the combustion process of these e-fuels when they replace fossil Diesel in internal combustion engines under equivalent operating conditions. To achieve this objective, a computational fluid dynamics model of an optical compression ignition engine has been developed. The operating conditions chosen are representative of a medium load point of the engine, which coincide with experimental work previously done on this platform. n-Heptane was used as surrogate of fossil Diesel while OME X was simulated as a simpler mixture of oxymethylene ether molecules. Results show remarkable differences between Diesel and OME X . This fuel provides lower equivalence ratio fields. Thus, oxidation reactions are promoted in wider areas within the combustion chamber, leading to a faster combustion process. Besides, the soot formation is also drastically decreased in comparison to the other fuel. These results have been corroborated with experimental information.
Article
Methylcyclohexane is one of the important components of transportation alternative fuels, and its reaction kinetics of low temperature combustion are crucial for the performance of advanced combustion engines, especially for autoignition. In the present work, we study 3-methylcyclohexyl radical (3MCHyl) as a model compound to clarify the kinetics of methylcyclohexyl radical with O2 and the subsequent isomerization and dissociation of cis 3-methylcyclohexylperoxyl (cis-3MCHylOO). Combined the electronic structure calculations with Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) method, we determine previously missing kinetics data in a wide temperature and pressure range. The similarities and difference of kinetics between various methylcyclohexyl conformation with O2 are clarified, which is helpful to better understand the formation mechanism of highly oxygenated molecules (HOM) and combustion pollutants. The accurate calculated kinetics data in the present work are crucial and valuable for the prediction of ignition properties of alternative fuels.
Article
Methylcyclohexane (MCH) and n-heptane (nC7) were used to perform experimental studies in high-temperature oxidation pretreated STS304 reactor (ϕ2 × 0.5 × 1000 mm) at 873–1073 K, 1.0 MPa, and 1.0 mL/min feed rate. The heat sinks of the two fuels were calculated based on the conversion and product distribution. MCH was found to have difficulty in achieving high heat sink at low and high temperatures. The formation pathways of ethylene and benzene were then compared. Results showed their main precursors were similar, but the contribution proportion of each path was different affected by initial cracking reactions of MCH and nC7. The coking rate of MCH was lower than that of nC7 at low temperatures, while the trend was the opposite at high temperatures. The variation in C/H ratio of coke for MCH and nC7 with temperature was the same, whereas the C/H ratio of the former was significantly lower. The unique unimolecular demethylation and intramolecular stepwise dehydrogenation reactions of MCH caused its coking rate to be higher than that of nC7. An experimental method was proposed to determine the coking active site density based on composition and structure of coke. Finally, a coking mechanism based on the growth of spherical coke particles was proposed.
Article
Accurate and efficient determination of thermodynamics and kinetics is long pursued but hindered by electronic energy calculations due to very high computational cost. In this work, we propose a Cascaded Group Additivity (CGA)-ONIOM-DFT framework targeting fast thermodynamic and kinetic predictions. We studied the reaction of cyclopentane with OH radical as a proof-of-principle demonstration. The CGA-ONIOM method accurately constructed the potential energy surface with a mean unsigned error of 0.20 kcal∙mol-1 compared to the full-level CCSD(T)/CBS results. The calculated rate constants agree well with the experimental data, suggesting the feasibility of using the CGA-ONIOM-DFT framework for combustion chemical kinetics.
Article
As an important component of transportation fuels, toluene has little reactivity in the low temperature regime. However, the low temperature reactivity of toluene may be enhanced by the reaction of other reactive components (e.g., n-heptane) in fuel mixtures. This work examines low temperature oxidation of toluene in jet stirred reactor oxidation of an n-heptane/toluene mixture (1:1 in mole, 500–800 K, ϕ=0.5, τ=2.0 s, p = 1 bar). Two measurement techniques, time of flight molecular beam mass spectrometry using synchrotron vacuum ultraviolet radiation as the photon ionization source and gas chromatography mass spectrometry, were applied to identify and measure 32 species, including four polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH (OPAH). Numerical simulations using the latest kinetic model from Lawrence Livermore National Laboratory predicted the mole fractions of fuel molecules and small intermediates well, but under-predicted the mole fractions of oxygenated aromatics (phenol, benzyl alcohol, and cresol). The identification of benzyl peroxide–an important intermediate–supported the proposed formation pathways for the identified aromatics. Model analysis highlighted the influence of H-atom abstraction, OH/H radical ipso substitution, and OH addition reactions of toluene on the formation of phenol, benzyl alcohol, and cresol, which may further grow to OPAH molecules by the addition of benzyl radical from H-atom abstraction of toluene.
Article
The understanding of autoignition characteristics of fuels is essential for the design of engines and the optimization of combustion organization. However, the autoignition and chemical kinetics of 0# diesel, a typical diesel fuel designed aimed at Chinese national stage VI emission standard, are seldom investigated. In this paper, ignition delay times (IDTs) for 0# diesel/air were measured using a shock tube over the temperature range of 1041−1307 K, pressures of 2−10 atm, and equivalence ratios of 0.5−1.5. The effects of pressure and equivalence ratio on ignition delay times were systematically analyzed. It was found that the ignition delay times were sensitive to the factor of pressure and less sensitive to the factor of equivalence ratio. In order to clarify the chemical mechanism that govern the ignition process, a kinetic model was further developed to describe the reaction network of 0# diesel. Firstly, a ternary surrogate fuel (30.73% n-hexadecane, 28.31% iso-cetane, and 40.96% propyl-benzene by mole) was proposed for 0# diesel based on the combustion property target matching strategy using an automatically iterated method. The results showed that the proposed surrogate model reproduced the crucial property targets of the real diesel with a deviation of less than 3%, including cetane number (CN), molecular weight (MW), low heat value (LHV) and hydrogen/carbon ratio(H/C). On this basis, a skeletal mechanism was proposed for 0# diesel and validated against the experimental results obtained in the current study. Comparisons between results from simulation and present experimental results showed that the mechanism can accurately describe the autoignition characteristics of 0# diesel. Experimental data and mechanism presented in this study will provide insight into the understanding the combustion progress of 0# diesel and can be used for numerical simulations.
Article
The combustion of diesel fuel in a pre-chamber vortex burner with a steam blast atomizer has been studied numerically. The well-proven k-w SST (URANS) model was chosen for calculations. Combustion was simulated using the Eddy Dissipation Concept (EDC) model, taking into account detailed chemical mechanisms in turbulent reacting flows. The results of testing the selected mathematical model showed good agreement of calculations with the known experimental data, a high agreement of the results was achieved: temperature, CO, NOx, O2, and CO2. The influence of steam and fuel flow rates on the flow structure and physicochemical processes in the burner and at the nozzle outlet has been studied. The structure of the flow inside the burner was shown. A high-velocity jet of steam ejects fuel, oxidizer flow and hot combustion products, forming a large zone of circulation and intense mixing. The influence of burner operating regimes on fuel underburning and environmental performance has been investigated. It was found that in the selected range an increase in the relative steam flow rate leads to almost complete combustion of fuel inside the burner. In the combustion chamber with a natural air supply, the excess air ratio was (0.96–1.23). For the considered fuel flow rate of (0.8–1.2 kg/h), the concentration of toxic emissions meets the environmental standard EN:267 – CO < 40 ppm, NOx < 50 ppm. The minimum emissions of CO = 5 ppm and NOx = 27 ppm are achieved at mass steam concentration of 44–50%.
Article
The necessity to reveal the formation mechanism of not only polycyclic aromatic hydrocarbons (PAHs) but also oxygenated PAHs (OPAHs) during combustion is increasing. Although many studies on PAHs have been conducted, fundamental studies investigating OPAH formation are still limited. Phenol, benzofuran, and dibenzofuran were selected as OPAHs in this study. We experimentally investigated fuel-rich oxidation in a flow reactor at atmospheric pressure, mean gas temperatures from 1050 to 1350 K, residence times from 0.2 to 1.5 s, and equivalence ratios from 3.0 to 12.0. Ethylene, toluene, and n-decane were used as fuels. Three kinds of OPAHs as well as 23 kinds of PAHs including monocyclic structures were quantitatively measured through a direct sampling using gas chromatography mass spectrometry. The results showed that concentrations of PAHs and OPAHs were strongly affected by the fuel type, with increasing concentrations in the following order: ethylene < n-decane < toluene. A chemical kinetic model for OPAH formation was developed based on a recent PAH growth model. The model showed that the predicted concentrations of OPAHs and PAHs were in reasonable agreement with the measured data in this study. Some modifications were made to the previous model based on recent literature studies and on the comparison of the simulated and measured results. The effect of the fuel type on the formation of PAHs and OPAHs was investigated through kinetic analysis using the model to discuss their reaction pathways.
Article
Diesel from indirect coal liquefaction (DICL) is a kind of extremely promising alternative fuel to alleviate the oil security problem caused by excessive dependence on imported oil and reduce the pollutant emission. However, the mechanism of diesel from indirect coal liquefaction is very few, and the numerical simulation of combustion and emission characteristics of diesel from indirect coal liquefaction is even less. Therefore, a reduced mechanism of DICL, entailing 178 components and 650 reactions, was put forward to research the combustion and emission characteristics of engines fueled with DICL under different loads in this work. n-Hexadecane (HXN) mechanism and 2,2,4,4,6,8,8-heptamethylnonane mechanism (HMN) were respectively considered as representative species of straight-chain paraffin and branched alkane in surrogate model. Firstly, direct relation graph and direct relation graph with error propagation were used in turn. Then, sensitivity analysis coupled with rate of production analysis has been used to further reduce the detail 2,2,4,4,6,8,8-heptamethylnonane mechanism. After that, the skeleton mechanism of n-hexadecane and the reduced polycyclic aromatic hydrocarbon (PAH) mechanism were combined with the simplified HMN mechanism to develop a novel multi-component mechanism of HXN-HMN-PAH. Brute-force sensitivity analyses were respectively conducted at different temperatures to find out these key reactions that have great effects on the ignition delay times (IDTs). Then, the optimizations of the reduced mechanism were made based on the ignition delay times of the experimental datum and the detail mechanism. A proportion of 71.5% HXN and 28.5% HMN by mole fraction was determined according to the properties of practical DICL. Furthermore, the optimized mechanism was employed to verify experimental values including the ignition delay times, the species concentrations on jet-stirred reactors (JSR) and the laminar flame speeds. Eventually, the mechanism was coupled into computational fluid dynamic (CFD) to perform multi-dimensional numerical simulation validation in a directed injection compression ignition (DICI) engine under different loads.
Article
Time-resolved soot and PAH formation from gasoline and diesel spray pyrolysis are visualized and quantified using diffuse back illumination (DBI) and laser induced fluorescence (LIF) at 355 nm, respectively, in a constant-volume vessel at 60 bar from 1400 to 1700 K for up to 30 ms. The delay, maximum formation rate, and yield of soot and PAHs are compared across fuels and temperatures and correlated with the yield sooting indices on either the mass or mole basis. The delays generally decrease with increasing temperature, and the formation rates of both PAHs and soot generally increase with temperature. The apparent PAH-LIF yield may decrease with temperature due to PAH growth and conversion into larger species, signal trapping, and thermal quneching. Soot yield generally increases with temperature. The mass-based YSI correlates reasonably well with soot delay, but YSI does not correlate well with soot yield. The mass-based YSI is a more appropriate predictor of sooting propensity than the mole-based YSI.
Article
As the smallest cycloalkane, cyclopropane is a highly strained three-membered ring hydrocarbon that exhibits high reactivity. In this work, the autoignition characteristics of cyclopropane have been investigated behind reflected shock waves. Experiments were conducted at pressures of 2, 5, and 10 atm, equivalence ratios of 0.5, 1.0, and 2.0, and temperatures ranging from approximately 1100 to 1500 K. The effects of temperature, pressure and equivalence ratio on ignition delay time have been investigated. Quantitative relationships have been yielded by the regression analysis of the experimental data. A high-temperature combustion mechanism of cyclopropane based on NUIGMech1.1 has been developed, and the predicted results are in good agreement with the experimental results. Reaction pathway and sensitivity analyses have been carried to determine the significant reaction pathways in the ignition process and key reactions that affect the ignition delay time. Finally, comparisons between the ignition delay time of cyclopropane and that of propene, cyclopentane, and cyclohexane have been conducted, and kinetic analyses have been performed to interpret the ignition difference between cyclopropane and the above-mentioned fuels.
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The ignition of n-decane in air has been studied experimentally at temperatures between 800 and 1300 K and at pressures up to 80 atm. The experiments have been carried out in a preheated shock tube. Ignition delay times of stoichiometric and lean mixtures of n-decane and air have been measured behind reflected shock waves. The experimental data have been compared with four kinetic mechanisms. Differences between the experimental data and the modeling results are discussed.
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A numerical study was performed to compare the formation of nitric oxide (NO) and nitrogen dioxide (NO 2), collectively termed NOx, resulting from biodiesel and diesel combustion in an internal combustion engine. It has been shown that biodiesel tends to increase NOx compared to diesel, and to-date, there is no widely accepted explanation. Many factors can lead to increased NOx formation and it was of interest to determine if fuel chemistry plays a significant role. Therefore, in order to isolate the fuel chemistry from mixing processes typical in a compression ignition engine, sprays were not considered in the present investigation. The current study compares the NOx formation of surrogates for biodiesel (as represented by methyl butanoate and n-heptane) and diesel (n-heptane) under completely homogeneous conditions. Combustion of each fuel was simulated using the Senkin code for both an adiabatic, constant volume reactor, and an adiabatic, single-zone HCCI engine model. The fuel chemistry is represented using an updated version of a mechanism that combines reduced mechanisms for methyl butanoate and n-heptane. NOx chemistry is predicted using a 19-step model that includes species and reactions for both thermal and prompt NOx. It was found that the biodiesel surrogate can cause a NOx increase when compared to diesel surrogate, but the relative increase was small (<3%) for most equivalence ratios. The differences in initial temperatures required to match ignition time make it difficult to definitively link the NOx increase to the oxygen in the fuel under these conditions. The largest NOx increase (26%) was seen at near-stoichiometric conditions. However, it was found that the fuel-bound oxygen in biodiesel did not increase NOx to the extent that the same amount of oxygen would create if it were available in the surrounding air. While the presence of O 2 in the biodiesel surrogate does slightly impact NOx formation, it does not appear to be a dominant factor for HCCI engines, where mixture conditions are well below stoichiometric. In conventional diesel combustion, where equivalence ratios are often above stoichiometric, these results suggest that the fuel chemistry can play a role in the observed NOx increase.
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A phenomenological description, or "conceptual model," of how direct-injection (DI) diesel combustion occurs has been derived from laser-sheet imaging and other recent optical data. To provide background, the most relevant of the recent imaging data of the author and co-workers are presented and discussed, as are the relationships between the various imaging measurements. Where appropriate, other supporting data from the literature is also discussed. Then, this combined information is summarized in a series of idealized schematics that depict the combustion process for a typical, modern-diesel-engine condition. The schematics incorporate virtually all of the information provided by our recent imaging data including: liquid- and vapor-fuel zones, fuel/air mixing, autoignition, reaction zones, and soot distributions. By combining all these elements, the schematics show the evolution of a reacting diesel fuel jet from the start of fuel injection up through the first part of the mixing-controlled burn (i.e. until the end of fuel injection). In addition, for a "developed" reacting diesel fuel jet during the mixing-controlled burn, the schematics explain the sequence of events that occurs as fuel moves from the injector downstream through the mixing, combustion, and emissions-formation processes. The conceptual model depicted in these schematics also gives insight into the most likely mechanisms for soot formation and destruction and NO formation during the portion of the DI diesel combustion event discussed.
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Conference Paper
The ignition delay times of toluene-oxygen-argon mixtures with fuel equivalence ratios from 0.5 to 1.5 and concentrations of toluene from 0.1 to 2.0% were measured behind reflected shock waves for temperatures 1270 to 1755 K and at a pressure of 2.4 ± 0.7 atm. A detailed chemical kinetic model has been developed on the basis of a kinetic mechanism proposed by Pitz et al. [1] to reproduce our experimental results as well as some literature data obtained in other shock tubes at pressures from 1.1 to 50 atm. It is found that the present chemical kinetic model could give better agreement on the pressure dependence of the ignition delay times than the previously proposed kinetic models.
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Low-temperature combustion (LTC) strategies for diesel engines are of increasing interest because of their potential to significantly reduce particulate matter (PM) and nitrogen oxide (NOx) emissions. LTC with late fuel injection further offers the benefit of combustion phasing control because ignition is closely coupled to the fuel injection event. But with a short ignition-delay, fuel jet mixing processes must be rapid to achieve adequate premixing before ignition. In the current study, mixing and pollutant formation of late-injection LTC are studied in a single-cylinder, direct-injection, optically accessible heavy-duty diesel engine using three laser-based imaging diagnostics. Simultaneous planar laser-induced fluorescence of the hydroxyl radical (OH) and combined formaldehyde (H2CO) and polycyclic aromatic hydrocarbons (PAH) are compared with vapor-fuel concentration measurements from a non-combusting condition. Through comparative analysis of OH, H2CO, and PAH fluorescence, mixtures are identified as either fuel-lean, fuel-rich, or of intermediate stoichiometries. The impacts of combustion chamber design on in- cylinder mixing processes are explored by comparing three piston bowl diameters of 60%, 70% and 80% of the cylinder bore. The data show that piston-bowl diameter influences in-cylinder mixing and pollutant formation processes by altering jet-jet and jet-wall interactions. When the fuel jets impinge on the bowl wall prior to ignition, adjacent jets merge, forming fuel-rich regions where soot formation occurs. By using a larger diameter bowl, wall impingement prior to ignition is reduced and delayed, and mixtures are leaner throughout the jet. However, a greater fraction of the jet becomes too lean for complete combustion. By using a smaller diameter bowl, a strong jet-wall interaction pushes the fuel-rich jet- jet interaction regions into the center of the chamber, where mixtures are predominantly lean. This reduces net soot formation and displaces fuel-lean regions of otherwise incomplete combustion into the combusting regions near the bowl wall.
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It is generally accepted that emissions of nitrogen oxides (NOx) increase as the volume fraction of biodiesel increases in blends with conventional diesel fuel. While many mechanisms based on biodiesel effects on in-cylinder processes have been proposed to explain this observation, a clear understanding of the relative importance of each has remained elusive. To gain further insight into the cause(s) of the biodiesel NOx increase, experiments were conducted in a single-cylinder version of a heavy-duty diesel engine with extensive optical access to the combustion chamber. The engine was operated using two biodiesel fuels and two hydrocarbon reference fuels, over a wide range of loads, and using undiluted air as well as air diluted with simulated exhaust gas recirculation. Measurements were made of cylinder pressure, spatially integrated natural luminosity (a measure of radiative heat transfer), engine-out emissions of NOx and smoke, flame lift-off length, actual start of injection, ignition delay, and efficiency. Adiabatic flame temperatures for the test fuels and a surrogate #2 diesel fuel also were computed at representative diesel-engine conditions. Results suggest that the biodiesel NOx increase is not quantitatively determined by a change in a single fuel property, but rather is the result of a number of coupled mechanisms whose effects may tend to reinforce or cancel one another under different conditions, depending on specific combustion and fuel characteristics. Nevertheless, charge-gas mixtures that are closer to stoichiometric at ignition and in the standing premixed autoignition zone near the flame lift-off length appear to be key factors in helping to explain he biodiesel NOx increase under all conditions. These differences are expected to lead to higher local and average in-cylinder temperatures, lower radiative heat losses, and a shorter, more-advanced combustion event, all of which would be expected to increase thermal NOx emissions. Differences in prompt NO formation and species concentrations resulting from fuel and jet-structure changes also may play important roles.
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The oxidation of methyl hexanoate has been studied experimentally in a jet-stirred reactor at 10 atm and a constant residence time of 1 s, over the temperature range 500-1000 K and for fuel-Jean to fuel-rich conditions. Concentration profiles of reactants, stable intermediates, and final products have been obtained by sonic probe sampling followed by online and offline GC analyses. The oxidation of methyl hexanoate under these conditions showed the three regimes of oxidation well-known for large hydrocarbons, namely cool flame, negative temperature coefficient, and high temperature oxidation. It was modeled using a detailed chemical kinetic reaction mechanism (435 species and 1875 reversible reactions). This mechanism was validated by comparing the present experimental results to the simulations. The main reaction pathways involved in methyl hexanoate oxidation were delineated computing rate of formation and consumption. The present results show a strong similitude with the oxidation of n-alkanes.
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Detailed intermediate species profiles for the oxidation of 1-methylnaphthalene have been obtained by gas-phase sampling from an atmospheric-pressure flow reactor operating at about 1170 K. These are the first detailed oxidation results reported for any polycyclic aromatic hydrocarbon, despite the presence of these compounds in many practical fuels. No evidence was found to indicate that fuel consumption through ring reactions was competitive with consumption through attack of the methyl side chain. Based on the observed intermediate species profiles, a mechanism is proposed for the oxidation of 1-methylnaphthalene which is strongly analogous to a previously developed mechanism for the oxidation of toluene under flow reactor conditions. In this mechanism, the resonantly stabilized 1-naphthylmethyl radical undergoes radical-radical reactions to form 1-naphthaldehyde, which decomposes to 1-naphthyl radical. Naphthalene and 1-naphthyl radical are oxidized to 1-naphthoxy radical, which decomposes to indenyl radical. Further reaction and decomposition results in the formation of phenylacetylene, styrene, phenyl radical, and acetylene. This mechanism is consistent with the observed sequential formation of the major aromatic intermediates 1-naphthaldehyde, naphthalene, indene, phenylacetylene, and benzene. Other pathways to the formation of minor intermediates such as 2-methylnaphthalene, acenaphthylene, 2-naphthol, and 1-methylindene are considered.
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We present an adaptive algorithm for low Mach number reacting flows with complex chemistry. Our approach uses a form of the low Mach number equations that discretely conserves both mass and energy. The discretization methodology is based on a robust projection formulation that accommodates large density contrasts. The algorithm uses an operator-split treatment of stiff reaction terms and includes effects of differential diffusion. The basic computational approach is embedded in an adaptive projection framework that uses structured hierarchical grids with subcycling in time that preserves the discrete conservation properties of the underlying single-grid algorithm. We present numerical examples illustrating the performance of the method on both premixed and non-premixed flames.
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Laminar flame speeds and extinction strain rates of premixed C{sub 5}-C{sub 12}n-alkane flames were determined at atmospheric pressure and elevated unburned mixture temperatures, over a wide range of equivalence ratios. Experiments were performed in the counterflow configuration and flow velocities were measured using Laser Doppler Velocimetry. The laminar flame speeds were obtained using a non-linear extrapolation technique utilizing numerical simulations of the counterflow experiments with detailed descriptions of chemical kinetics and molecular transport. Compared to linearly extrapolated values, the laminar flame speeds obtained using non-linear extrapolations were found to be 1-4 cm/s lower depending on the equivalence ratio. It was determined that the laminar flame speeds of all n-alkane/air mixtures considered in this investigation are similar to each other and sensitive largely to the H{sub 2}/CO and C{sub 1}-C{sub 4} hydrocarbon kinetics. Additionally, the resistance to extinction decreases as the fuel molecular weight increases. Simulations of the experiments were performed using the recently developed JetSurF 0.2 reaction model consisting of 194 species and 1459 reactions. The laminar flame speeds were predicted with good accuracy for all the n-alkane-air mixtures considered. The experimental extinction strain rates are well predicted by the model for fuel-lean mixtures. For stoichiometric and fuel-rich mixtures, the predicted extinction strain rates are approximately 10% lower than the experimental values. Insights into the physical and chemical processes that control the response of n-alkane flames are provided through detailed sensitivity analyses on both reaction rates and binary diffusion coefficients. (author)
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Laminar flame speeds of n-decane/air and n-dodecane/air mixtures are measured using the counterflow twin-flame configuration at preheat temperatures ranging from 360 to 470 K and equivalence ratios ranging from 0.7 to 1.4. Extinction stretch rate measurement as a function of equivalence ratio is also carried out for fuel/O{sub 2}/N{sub 2} mixtures with [N{sub 2}/(O{sub 2} + N{sub 2})] = 0.84 by mole and preheat temperature of 400 K. All experiments are conducted under atmospheric pressure conditions. In addition, the overall activation energies of n-decane/air mixtures at varying equivalence ratios are deduced. The experimental data for laminar flame speeds and extinction stretch rates are also simulated using chemical kinetic mechanisms available in the literature. Comparison of the experimental and computed results demonstrates the deficiencies of the existing mechanisms. Although sensitivity analysis is performed to identify the most sensitive reactions pertinent to laminar flame speed and extinction limit, the results are unable to assess the adequacy of the chemistry involving large hydrocarbons. (author)
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A method of updating and reusing legacy FORTRAN codes for combustion simulations is presented using the DAEPACK software package. The procedure is demonstrated on two codes that come with the CHEMKIN-II package, CONP and SENKIN, for the constant-pressure batch reactor simulation. Using DAEPACK generated code, analytical derivative calculations, sparsity pattern information, and hidden discontinuity information can be obtained for the models of interest. This information can be easily integrated with different solvers giving the modeler great flexibility in selecting the best solution procedure. Using the generated code, the CONP code was connected to three different solvers, and the SENKIN code was connected to two different solvers. The effect of model formulation, analytical derivatives, sparsity, and sensitivity equation solution method were analyzed for three large kinetic mechanisms for methane, acetylene, and n-heptane. For the n-heptane model, with 544 species and 2446 reactions, a factor of 10-speed improvement over the original solution procedure was found using analytical derivatives and sparse linear algebra. For sensitivity calculations, for a small number of parameters, a factor of 55 improvement over the original solution procedure was found for the n-heptane problem. Upon closer examination of results, no one method is found to always be superior to other methods, and selection of the appropriate solution procedure requires an examination of the specific kinetic mechanism, which is easily conducted using DAEPACK generated code.
Article
A detailed chemical kinetic model for the mixtures of Primary Reference Fuel (PRF: n-heptane and iso-octane) and toluene has been proposed. This model is divided into three parts; a PRF mechanism [T. Ogura et al., Energy & Fuels 21 (2007) 3233-3239], toluene sub-mechanism and cross reactions between PRF and toluene. Toluene sub-mechanism includes the low temperature kinetics relevant to engine conditions. A chemical kinetic mechanism proposed by Pitz et al. [Proc. the 2nd Joint Meeting of the U.S. Combust. Institute (2001)] was used as a starting model and modified by updating rate coefficients. Theoretical estimations of rate coefficients were performed for toluene and benzyl radical reactions important at low temperatures. Cross-reactions between alkane, alkene, and aromatics were also included in order to account for the acceleration by the addition of toluene into iso-octane recently found in the shock tube study of the ignition delay [Y. Sakai et al, SAE 2007-01-4014 (2007)]. Validations of the model were performed with existing shock tube and flow tube data. The model well predicts the ignition characteristics of toluene and PRF/Toluene mixtures under the wide range of temperatures (500-1700 K) and pressures (2-50 atm). It is found that reactions of benzyl radical with oxygen molecule determine the reactivity of toluene at low temperature. Although the effect of toluene addition to iso-octane is not fully resolved, the reactions of alkene with benzyl radical have the possibility to account for the kinetic interactions between PRF and toluene.
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The CRC Fuels for Advanced Combustion Engines working group has worked to identify a matrix of research diesel fuels for use in advanced combustion research applications. Nine fuels were specified and formulated to investigate the effects of cetane number aromatic content and 90% distillation fraction. Standard ASTM analyses were performed on the fuels as well as GC/MS and /u1H//u1/u3C NMR analyses and thermodynamic characterizations. Details of the actual results of the fuel formulations compared with the design values are presented, as well as results from standard analyses, such as heating value, viscosity and density. Cetane number characterizations were accomplished by using both the engine method and the Ignition Quality Tester (IQT/sT) apparatus.
Article
This work, which parallels a recent study of cyclohexane and methylcyclohexane by the authors, concerns the oxidation chemistry of methylcyclopentane (MCP), 1,2,3,4-tetrahydronaphthalene (tetralin), and decahydronaphthalene (decalin) in a motored engine at low to intermediate temperatures. The experiment is conducted with variable compression ratio from 4 to 15 at equivalence ratio of 0.25 and fixed intake temperature. Results show dramatically different reactivity in low temperature oxidation for the three compounds. MCP and tetralin show little low temperature reaction prior to autoignition, while decalin shows significant low temperature reactivity. Detailed product analysis showed that conjugate olefins, the olefin having the identical structure with the reactant except the only C=C bond, account for over 70% of the products from MCP and an even higher percentage of the products from tetralin. Tetralin oxidation under the present conditions is essentially oxidative dehydrogenation with little oxygenated cyclic compound being formed. Hydronaphthalenes with various degrees of unsaturation are detected in the products from decalin, but are not as prevalent as in the case of MCP and tetralin, because of the high selectivity toward low temperature chain branching. The ring-opening paths in decalin oxidation are discussed, suggesting that breaking the common C-C bond of the two rings is more likely than opening the two rings one after the other. Methyl substitution on the ring was found to significantly promote the formation of propene relative to ethene. Reaction mechanisms are proposed to explain the major products formed from each compound. (author)
Article
The experimental study of the oxidation of two blend surrogates for diesel and biodiesel fuels, n-decane/n-hexadecane and n-decane/methyl palmitate (74/26 mol/mol), has been performed in a jet-stirred reactor over a wide range of temperatures covering both low, and high-temperature regions (550-1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (hydrocarbon inlet mole fraction of 0.002) and at stoichiometric conditions. Numerous reaction products have been identified and quantified. At low and intermediate temperatures (less than 1000 K), the formation of oxygenated species such as cyclic ethers, aldehydes and ketones has been observed for n-decane, n-hexadecane, and methyl palmitate. At higher temperature, the formation of these species was not observed any more, and small amounts of unsaturated species (olefins and unsaturated methyl esters) have been detected. Results obtained with methyl palmitate and n-hexadecane have been compared in order to highlight similarities and differences between large n-alkanes and methyl esters. (author)
Article
The autoignition of CH aromatic/air mixtures (ortho-xylene, meta-xylene, para-xylene, and ethylbenzene in air) has been studied in a shock tube at temperatures of 941-1408 K, pressures of 9-45 atm, and equivalence ratios of =1.0 and 0.5. Ignition times were determined using electronically excited OH emission and pressure measurements. The measurements illustrate the differences in reactivity for the CH aromatics under the studied conditions. Ethylbenzene was by far the most reactive CH aromatic with ignition times a factor of two to three shorter than the xylenes. The xylene isomers exhibited ignition times that were similar, with o-xylene the most reactive, p-xylene the least reactive, and m-xylene just slightly more reactive than p-xylene. The p-xylene ignition times are almost identical to previous measurements for toluene at the same conditions. The differences in reactivity can be attributed to the C-H and C-C bond strengths in the alkyl side chains and the proximity of the methyl groups in the xylenes. These results represent the first ignition measurements for CH aromatics at the elevated-pressure moderate-temperature conditions studied, providing needed targets for kinetic modeling at engine-relevant conditions. Kinetic modeling illustrates the importance of the methylbenzyl + HO reaction and indicates further study of this reaction is warranted. (author)
Article
The influence of oxygenated hydrocarbons as additives to diesel fuels on ignition, NOx emissions and soot production has been examined using a detailed chemical kinetic reaction mechanism. N-heptane was used as a representative diesel fuel, and methanol, ethanol, dimethyl ether and dimethoxymethane were used as oxygenated fuel additives. It was found that addition of oxygenated hydrocarbons reduced NOx levels and reduced the production of soot precursors. When the overall oxygen content in the fuel reached approximately 25% by mass, production of soot precursors fell effectively to zero, in agreement with experimental studies. The kinetic factors responsible for these observations are discussed.
Article
Exhaust emissions and combustion characteristics in single-cylinder engines, with well-characterized test fuels having carefully controlled molecular composition and conventional distillation characteristics and cetanme number (CN) were investigated. It was observed that under low and medium loads and at equivalent CNs, cycloparaffins have a higher PM formation tendency than isoparaffins or n-paraffins. It was also observed that under high-load conditions, the paraffin molecular structure had a very small effect on PM formation. The results show that the combustion was initiated under a state of insufficient fuel-air mixing, and also dense soot clouds were formed in the fuel spray jet near the injector.
Article
A cell agglomeration algorithm is proposed to mitigate the computational cost of incorporating detailed chemical kinetics in multi-dimensional Computational Fluid Dynamics (CFD) simulations. Cells that are close in species and energy composition space are agglomerated before calling the reaction integrator, substantially reducing the number of chemistry integrations. The algorithm is generalized and applicable to any reacting flow configuration, and the accuracy is fully controllable. A dynamic hash table is used to efficiently bin cells into high dimensional hyper-cubes in composition space. The method is applied to four different CFD simulations and the speed-up and incurred error are assessed for a range of agglomeration tolerances and table dimensions. The proposed approach exhibits up to an order of magnitude speed-up with a relatively moderate decrease in accuracy.