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Biodiesel is one of the most important alternative fuels due to limited conventional resources. Methyl octanoate with long alkyl chain and mono-alkyl ester is one of the surrogate fuels of actual biodiesel. In order to clarify distinctive combustion characteristics of methyl octanoate, comparison with alkane in traditional diesel is necessary. Therefore, this work chose n-nonane with same carbon number as methyl octanoate. The laminar flame speeds of two fuels were measured at different conditions (T = 423 K, P = 1 and 3 atm, and φ = 0.7–1.5) in a constant volume combustion bomb. Results show that laminar flame speed of n-nonane is higher than that of methyl octanoate in most conditions, but lower in fuel-rich conditions. The combined chemical kinetic model of methyl octanoate and n-nonane was developed through eliminating unnecessary reactions, which can well predict the experimental results of laminar flame speed and ignition delay time. The adiabatic flame temperature results from the updated kinetic model show that the temperatures of n-nonane are higher than methyl octanoate in whole conditions, which is unexpectedly opposite to their laminar flame speeds order in fuel-rich conditions. In overall conditions, the intermediates of methyl octanoate tend to produce CH3, which consumes H radicals, thereby reducing the laminar flame speed, which can explain the higher laminar flame speed of n-nonane in most conditions. Meanwhile, the oxidation reactions with oxygen molecules of intermediates of n-nonane are suppressed due to low combustion temperature at φ = 1.5 unlike at φ = 0.7 and 1.0. On the contrary, those of methyl octanoate have similar reactions between φ = 0.7, 1.0, and φ = 1.5. It means that the laminar flame speed of n-nonane decreases more significantly than methyl octanoate as the equivalence ratio increases from φ = 1.0 to φ = 1.5. [ABSTRACT FROM AUTHOR]

Organophosphorus compounds (OPCs) are known to be combustion inhibitors (CI), fire suppressants, or flame retardant molecules (FRMs) for polymers and as surrogates (simulants) for the disposal or thermal degradation of chemical war agents (CWAs). Despite a significant number of studies on the mechanism of their action, OPCs’ combustion chemistry is still insufficiently understood. There is a need for further understanding of their auto-ignition and oxidation characteristics at relevant conditions (high pressures and low temperatures). This study reports on new data on the autoignition delays of Trimethyl Phosphate (TMP)-in-air mixtures obtained from experiments performed on a high-pressure shock tube (HPST) at pressures of 5 and 10 bar in the initial temperature range from 1200 to 2200 K. An updated TMP kinetic model deduced from the Glaude et al. model for the thermal degradation of OPCs is also proposed for the estimation of the autoignition delays of the studied mixtures by incorporating new reaction pathways and corresponding rate constants estimation of some reactions involving TMP and some intermediate products of its degradation. The results indicate that the proposed model is in satisfactory agreement with all the investigated mixtures.

Unsaturated hydrocarbons are major components of transportation fuels, combustion intermediates, and unburnt exhaust emissions. Conversely, NOx species are minor species present in the residual and exhaust gases of gasoline-fueled engines and gas turbines. Their co-existence in transportation engines is quite common, particularly with exhaust gas recirculation, which can greatly influence engine combustion characteristics. Therefore, this paper presents a review on the combustion chemistry of unsaturated hydrocarbons and NOx mixtures, with a focus on their chemical kinetic interactions. First, a comprehensive overview of fundamental combustion experiments is provided, covering mixtures of C2–C5 unsaturated/oxygenated species (namely alkenes, alkynes, dienes, alcohols, ethers, ketones, and furans) and three major NOx species (namely NO, NO2, and N2O), as well as reactors including jet-stirred reactors, flow reactors, burners, shock tubes, and rapid compression machines. Then, two widely adopted nitrogen chemistry models are evaluated in conjunction with a core chemistry model (i.e., NUIGMech1.1) via detailed chemical kinetic modeling, and the model similarities and differences across broad temperature ranges are highlighted. Thereafter, the unique interconversions between the three major NOx species are presented. In particular, the controversy regarding the pathways governing NO and NO2 conversion is discussed. Following this, the key direct interaction reactions between unsaturated species and NOx species are overviewed. Finally, the distinguishing features of the combustion chemistry for unsaturated hydrocarbon and NOx mixtures are summarized, and recommendations for future research on this topic are highlighted. [ABSTRACT FROM AUTHOR]

Cyclopropane has the highest strain energy among all cycloalkanes. As a promising precursor for synthesizing high-energy-density (HED) hydrocarbon fuels, advanced exploration of the combustion properties of cyclopropane is a major priority. This study measures the ignition delay times of cyclopropane using a shock tube (ST) under the medium-low temperature range of 803 – 1101 K, 10 atm and equivalence ratio of 0.5 – 2.0. In the study of cyclopropane combustion, the reaction pathways diverge significantly between high and low temperatures, resulting in the ignition at low temperatures being much shorter than the extrapolation at high temperatures. Meanwhile, cyclopropane and propene exhibit comparable ignition characteristics at high temperatures, though cyclopropane demonstrates a notably accelerated ignition process at lower temperatures. Based on the NUIGMech1.1 C0-C3 mechanism, a detailed mechanism involving 174 species and 1044 reactions is developed to elucidate its wide temperature combustion characteristics. At a high temperature of 1300 K, the kinetic analysis reveals that cyclopropane primarily undergoes ring-opening isomerization to produce propene, the corresponding heat release significantly facilitates the ignition process. As the temperature decreases, the generation of cyclopropyl radical via the hydrogen abstraction channel progressively prevails, and the ring-opening isomerization changes to inhibit the ignition process. Moreover, sensitivity analyses reveals that the incorporation of a low-temperature reaction pathway substantially refined the predictive model for laminar flame speed. [ABSTRACT FROM AUTHOR]

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 studied 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.

Unsaturated hydrocarbons are major components of transportation fuels, combustion intermediates, and unburnt exhaust emissions. Conversely, NOx species are minor species present in the residual and exhaust gases of gasoline-fueled engines and gas turbines. Their co-existence in transportation engines is quite common, particularly with exhaust gas recirculation, which can greatly influence engine combustion characteristics. Therefore, this paper presents a review on the combustion chemistry of unsaturated hydrocarbons and NOx mixtures, with a focus on their chemical kinetic interactions. First, a comprehensive overview of fundamental combustion experiments is provided, covering mixtures of C2–C5 unsaturated/oxygenated species (namely alkenes, alkynes, dienes, alcohols, ethers, ketones, and furans) and three major NOx species (namely NO, NO2, and N2O), as well as reactors including jet-stirred reactors, flow reactors, burners, shock tubes, and rapid compression machines. Then, two widely adopted nitrogen chemistry models are evaluated in conjunction with a core chemistry model (i.e., NUIGMech1.1) via detailed chemical kinetic modeling, and the model similarities and differences across broad temperature ranges are highlighted. Thereafter, the unique interconversions between the three major NOx species are presented. In particular, the controversy regarding the pathways governing NO and NO2 conversion is discussed. Following this, the key direct interaction reactions between unsaturated species and NOx species are overviewed. Finally, the distinguishing features of the combustion chemistry for unsaturated hydrocarbon and NOx mixtures are summarized, and recommendations for future research on this topic are highlighted.

Ammonia (NH3) is considered a promising alternative fuel, capable of producing energy with zero CO2 emissions. Its combustion, however, poses a series of challenges due to the low reactivity of NH3 and the formation of very high quantities of NOx. This work numerically investigates the combustion and emission characteristics of ammonia in three modern stationary gas turbine concepts, namely (a) lean-burn dry-low emissions (DLE); (b) rich-burn, quick-quench and lean-burn (RQL); and (c) moderate or intense low oxygen dilution (MILD), under operating conditions typical of commercial gas turbines (inlet temperatures of 500 K and pressure of 20 bar). Numerical simulations employing detailed chemical kinetic mechanisms are carried out to study the propagation speed of ammonia, the combustor temperatures, and the emissions of NOx and NH3. The simulations are first validated against literature NOx data and then the most accurate mechanism is selected. The performance of the different gas turbine engine concepts is subsequently compared based on the results from the selected mechanism. The results show that the lowest emissions are achieved with the RQL and MILD combustion concepts, while the DLE combustion concept only presents acceptable emission values under conditions deemed unstable, where the laminar flame speeds are below 3 cm/s. [ABSTRACT FROM AUTHOR]

2,3,4-trimethyl pentane (234-TMP) is an isomer of octane with the same number of methyl branching groups as 2,2,4-trimethyl pentane (iso -octane). However, there are very few studies of this fuel available in the literature. In this work, a detailed chemical kinetic model is developed to describe the oxidation of 234-TMP using NUIGMech1.3 as the core mechanism. The rate constants for some important reaction classes are updated following a review of literature rate constants. Additionally, the impact of each rate constant on simulated ignition delay times for 234-TMP compared to 2,2,3-trimethyl pentane and 2,2,4-trimethyl pentane (iso -octane) is discussed. The thermodynamic data of the alkanes (RH), alkyl (Ṙ), alkyl peroxy (RȮ 2), hydroperoxy-alkyl Q ˙ OOH, and peroxy hydroperoxyalkyl (Ȯ 2 QOOH) radicals are newly estimated based on recently updated group values in the literature. Moreover, this study presents the first set of data available for the oxidation of 234-TMP at higher pressures (15 and 30 atm), in the temperature range 600–1600 K, and at fuel/'air' equivalence ratios (φ) of 0.5, 1.0 and 2.0. The chemical kinetic model shows general good agreement with the experimental measurements. In addition, flux and sensitivity analyses are conducted to identify the important pathways and reactions controlling fuel oxidation at different temperatures. Furthermore, the reactivity of 234-TMP is compared to that of iso -octane, indicating that 234-TMP is slower to react as it has more tertiary carbon sites compared to iso -octane. [ABSTRACT FROM AUTHOR]

p -Cymene is a renewable terpenoid bio-aromatic species that is a promising aromatic substitute for conventional transportation fuel. A comprehensive experimental and kinetic investigation of p -cymene pyrolysis and oxidation is undertaken in this study over a wide range of engine-relevant conditions. Ignition delay times are measured in a rapid compression machine at pressures of 20, 30, and 40 atm, at temperatures in the range of 776–1000 K, for equivalence ratios in the range of 0.5 ‒ 2.0. Higher-temperature ignition delay times are also measured in a high-pressure shock tube at three equivalence ratios of 0.5, 1.0, and 2.0, at pressures of 20 atm and 40 atm, and at temperatures in the range 900–1350 K. Furthermore, to better understand the high-temperature chemistry of p -cymene, CO time-history profiles, are measured behind reflected shock waves with a spectroscopic laser diagnostic. The measurements are performed near 1 atm and include equivalence ratios ranging from 0.5 to 2.0, at temperatures in the range 1363–1815 K. Laminar flame speeds are measured at T 0 = 403 K with p 0 = 1 atm, and at T 0 = 424 K with p 0 = 3 atm at equivalence ratios ranging from 0.7 to 1.4. A detailed kinetic model adding 32 low-to-high temperature reaction classes is developed to describe p -cymene combustion chemistry based on NUIGMech1.3 as the core mechanism. Sensitivity and flux analyses are performed to highlight important reaction classes at different temperatures and their rate constants are discussed in detail. The model is used to simulate the wide range of experimental data performed in this work and those already available in the literature and shows good agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]

Methanol is an attractive oxygenate increasingly used as primary fuel for dual-fuel combustion technology yielding beneficial thermal-efficiency and emissions in modern engines. As such, it is significant to fundamentally understand the autoignition behavior of methanol/diesel mixtures. This study measured the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios (30%, 50%, 70% methanol by mol.) on a heated shock tube and a heated rapid compression machine at temperatures of 650−1450 K, pressures of 6−20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. The typical two-stage ignition characteristics with the negative temperature coefficient response were observed for dual-fuel mixtures. In general, both the total and first-stage IDTs decrease with the increment of pressure and equivalence ratio as well as diesel proportion in mixtures. The simulation results performed with a published detailed mechanism in conjunction with a tri-component diesel surrogate demonstrate generally good agreement with the experimental data at all test conditions. Moreover, a crossover of IDTs occurs at a higher temperature (~1500 K) for varying equivalence ratios, and experiment and simulation both exhibit a non-linear mixing effect of methanol addition on diesel ignition. Interestingly, simulation results clearly suggest a crossover (~940 K) for mixtures with varying methanol content at an intermediate-temperature, where the IDT of the mixture with a lower methanol ratio becomes longer as the temperature is higher than that of the crossover. Furthermore, brute-force sensitivity analyses assisted with reaction path analysis were conducted to gain deeper insights into the autoignition chemistry of dual-fuel mixtures, especially for the chemical interaction between both fuels during the low-temperature oxidation process. It is found that methanol hardly generates •OH, whereas •OH is mainly produced by the low-temperature reaction pathways of diesel. Thus, •OH is the bridge for both fuels during the ignition process. At the high methanol ratio, the H-abstraction of diesel is mainly via •HO 2 while CH 3 OH consumes a large percentage of •OH. Consequently, the competition between methanol and diesel for •OH radicals inhibits the overall reactivity of reaction network. In addition, the original data reported here lay a foundation for the development and validation of more accurate and robust dual-fuel kinetic schemes. [ABSTRACT FROM AUTHOR]

Ethylene is an important intermediate during oxidation and pyrolysis process of higher hydrocarbons, and also serves as a fuel under practical engines. An experimental and modeling study is performed to investigate the combustion chemistry of ethylene–air mixtures under a wide range of conditions. The shock tube experiments are conducted at pressures of 1, 4, 10 and 19 atm with equivalence ratios of 0.5, 1.0 and 2.0 to determine the ignition delay times by measuring the emission signal of excited methylidyne (CH*). The ignition data under 10 and 19 atm both span a wide temperature range varying from a low temperature of 721 K to a high temperature of 1320 K. No typical negative-temperature-coefficient is observed, but the ignition at low temperatures is much shorter than the extrapolation at high temperatures. Simulations are performed and compared to the experiments using contemporary core models with consideration of a linear pressure rise of 3% /ms, but no simulation could predict the ignition trend under low-to-intermediate temperatures. Therefore, a detailed model of ethylene oxidation is proposed based on AramcoMech2.0 in the present work. The modeling behavior at high temperatures is improved by revision of addition reaction between ethylene and oxygen atom (C 2 H 4 +Ӧ). The modeling behavior at the region of low-to-intermediate temperatures is improved by detailed updating of β -hydroxyethyl oxidation (PĊ 2 H 4 OH+O 2). The effect of reactions at the potential energy surface of C 2 H 4 +HȮ 2 and Ċ 2 H 5 +O 2 is determined, and the second oxygen addition (Ċ 2 H 4 OOH+O 2) is added into current model. The present model is extensively validated by ignition delay times, concentration profiles of stable species in jet-stirred reactors and laminar flame speed under various test conditions. Sensitivity analyses are carried out to identify the dominant reactions during the auto-ignition process and laminar premixed flame propagation. Additional simulations are performed to investigate the effect of model modifications on predictions of ethane and ethanol oxidation. The present model slightly promotes the low-temperature ignition of both ethane and ethanol, and its prediction on the concentration profiles of some stable species is also improved. The proposed model of ethylene oxidation can be used as a cornerstone to develop models of larger hydrocarbon fuels. [ABSTRACT FROM AUTHOR]

A single-pulse shock tube study of the pyrolysis of 2% C 2 –C 6 1-alkenes is presented at 2 bar in the temperature range 900–1800 K in the current study. Reactant, intermediate and product species are obtained and quantified using gas chromatography-mass spectrometry (GC–MS) analysis. MS is used for species identification and a flame ionization detector is used for quantification. The experiments show the effect of carbon chain length on the production of smaller C 1 –C 3 fragments. A new detailed kinetic mechanism, NUIGMech1.0, is used to simulate the data and the predictions for the major species are satisfactory. The improvement in predictions of the current mechanism, which includes C 6 and C 7 species, is substantial compared to AramcoMech3.0. Kinetic analyses are conducted with the current mechanism to identify the formation and consumption pathways of the quantified species. The experimental data are expected to contribute to a database for the validation of mechanisms at pyrolytic conditions. Additionally, the mechanism aims to provide a fundamental understanding of the pyrolytic chemistry of olefins. [ABSTRACT FROM AUTHOR]

Ethanol is an attractive oxygenate increasingly used for blending with petroleum-derived gasoline yielding beneficial combustion and emissions behavior for a range of internal combustion engine schemes, including stoichiometric spark-ignition and low temperature combustion (LTC). As such, it is important to fundamentally understand the autoignition behavior of gasoline/ethanol blends. This work utilizes a rapid compression machine (RCM) and a homogeneous charge compression ignition (HCCI) engine to experimentally quantify changes in fuel reactivity, through ignition delay times and preliminary heat release, for blends of 0 to 30% vol./vol. into a full boiling range research gasoline (FACE-F). Diluted/stoichiometric and undiluted/fuel-lean conditions are explored covering a wide range of compressed temperatures and pressures relevant to conventional and advanced, gasoline combustion engines. Detailed chemical kinetic modeling is undertaken using a recently updated gasoline surrogate model in conjunction with a five-component surrogate to model the RCM experiments and provide chemical insight into the perturbative effects of ethanol on the autoignition process. The diluted/stoichiometric RCM measurements reveal that within the low-temperature regime ethanol retards first-stage and main ignition delay times, and suppresses both the rates and extents of low-temperature heat release (LTHR), while within the intermediate-temperature regime ethanol only causes slight changes. Good agreement of ignition delay time and preliminary heat release prediction is found between model and experimental results. Sensitivity and flux analyses further show that ethanol blending effects are dominated by the competition between the H-atom abstraction from ethanol and other fuel components by OH radical at low temperatures, and by HO 2 radical at intermediate temperatures. These findings are consistent across both fuel loading conditions explored in this study. In addition, when HCCI engine experiments are mapped onto undiluted/lean RCM measurements under a constant combustion phasing scenario, good correspondence between the two apparatuses is observed for LTHR and start of high-temperature heat release. The current study highlights the importance of characterizing LTHR in predicting fuel behaviors in high-boost/low-temperature engines, and demonstrates that RCM experiments can provide an alternative, and more-efficient avenue for such characterization. [ABSTRACT FROM AUTHOR]

• Chemical effect of H 2 on soot/PAH formation of jet fuel. • H 2 promotes the soot formation in co-flow diffusion flame. • PAH formation weakens as the PAH ring number increases. • Rapidly increased A1 and alkynes may lead to the promotion of soot formation. • Inhibiting effects on PAH-=>PAH affect the A4 formation. PAH and soot are harmful substances that can be produced in any type of combustion equipment including aircraft engines. The co-combustion of hydrogen and jet fuel has been applied in aero-engine combustors and large-scale hydrogen addition is a promising solution for reducing the consumption of fossil fuels in the aviation industry. However, it remains unclear how H 2 influences the soot and PAH formation characteristics of jet kerosene. In this study, to investigate the impact of H 2 on soot and PAH formation, planar laser-induced incandescence (PLII), planar laser-induced fluorescence of PAH (PAH-PLIF) and chemical kinetic simulation were conducted for the laminar diffusion flames of RP-3 jet kerosene and its surrogate S1 with different H 2 doping rates. It is found that the introduction of H 2 leads to the increased soot formation. However, the promotion effect of H 2 on the PAH formation weakens as the number of PAH rings increases, and the formation of A4 is significantly inhibited. But the rapidly increase of benzene and alkynes in the H 2 -doped kerosene flame may ultimately lead to the promotion of soot formation. Furthermore, the changes in direct synthesis reactions and PAH=>PAH- jointly affect the converse changes in A1 and A4 formation. These findings will contribute to the development of the soot model and soot/PAH-reduction strategy for the co-combustion of jet fuels and hydrogen. [ABSTRACT FROM AUTHOR]

To reach sustainable aviation, one approach is to use electro-fuels (e-fuels) within the gas turbine engines. E-fuels are CO2-neutral synthetic fuels which are produced employing electrical energy generated from renewable resources, where the carbon is taken out of the atmosphere or from biomass. Our approach is, to find e-fuels, which can be utilized in the lean premixed prevaporized (LPP) combustion, where most of the non-CO2 emissions are prevented. One of the suitable e-fuel classes is alcohols with a low number of carbons. In this work, the autoignition properties of propanol isomers and butanol isomers as e-fuels were investigated in a high-pressure shock tube (HPST) at temperatures from 1200 to 1500 K, the pressure of 10 bar, and lean fuel-air conditions. Additional investigations on the low-temperature oxidation and flame speed of C3 and C4 alcohols from the literature were employed to develop a comprehensive mechanism for the prediction of ignition delay time (IDT) and laminar burning velocity (LBV) of the above-mentioned fuels. A numerical model based on newly developed chemical kinetics was applied to further study the IDT and LBV of fuels in comparison to the Jet-A surrogate at the engine-related conditions along with the emissions prediction of the model at lean fuel-air conditions.

• Development of multi-component surrogate mechanism for diesel and gasoline. • Multi-component surrogate fuel better reproduces the combustion characteristics. • The overall reactivity of system is enhanced by using multi-component fuel. • Cyclohexane and toluene control the LTHR and HTHR during combustion in multi-component fuel. The role of chemical mechanisms in studying the combustion phenomenon is very important. These mechanisms, if formulated properly, can make a numerical investigation more accurate and time-saving. In this study, a multi-component surrogate has been presented after integrating two previous mechanisms to study the combustion chemical kinetics. The mechanism contains a total of six components, 168 species and 680 reactions and has been validated extensively. The validations have been carried out for individual and mixture of components over a wide range of temperatures and pressures. The mechanism showed a good overall agreement with the experimental data. In order to do an application-based kinetic study, this mechanism was later used to study the combustion characteristics of Reactivity Controlled Compression Ignition (RCCI), which is a low-temperature and dual fuel combustion concept. The combustion characteristics have then been investigated considering the impact of single and multi-component surrogates on chemical and physical properties. First, only chemical properties and then physical properties have been changed followed by a change in both properties by a multi-component mechanism. The in-cylinder pressure and heat release rates have been captured well by multi-component surrogate fuel representing both physical and chemical properties of multi-component surrogate. The NOx and soot emissions have been predicted well by using a combination of physical and chemical properties of multi-component and single component surrogates for diesel and gasoline, respectively. The chemical kinetics analysis was performed at points of five percent (CA5) and fifty percent (CA50) of total heat release using sensitivity and consumption reaction pathways analyses. These analyses showed an overall more production of OH, CH 2 O and CO 2 in multi-component surrogate fuel which enhanced the overall heat release rate. Toluene and cyclohexane were observed to inhibit the reactivity of multi-component surrogate fuel. They, on the other hand, also control the low-temperature heat release rates and enhance the high-temperature heat release rates. [ABSTRACT FROM AUTHOR]

Abstract Chemical structures of low-pressure premixed flames are routinely used as species-resolved validation targets for the development of chemical kinetic combustion mechanisms. Species identities, for example based on flame-sampling mass spectrometry, and species-specific mole fractions as function of distance to the burner are especially important targets. This work, which is based on a compilation of the chemical structures of 55 low-pressure premixed flames from various laboratories, unravels previously unexplored validation targets. It is shown here that such a compiled data set allows for the extraction of fuel-specific chemistry and can be used to identify inconsistent data, thus potentially improving the accuracy of the validation set. Most importantly, this large data set of flame structures was essential to discover that, despite the different fuels and flame conditions used in the individual flame studies, the maximum mole fractions of some intermediate species are correlated to each other in a systematic way across a large number of flames. Some examples, including the correlations between maximum mole fractions of benzene (C 6 H 6) vs. toluene (C 6 H 5 CH 3) and the correlation between the isomeric allene (CH 2 CCH 2) and propyne (CH 3 CCH), are discussed and it is shown that these obtained correlations can be used as targets for model refinement. [ABSTRACT FROM AUTHOR]

One of the major biofuels used today is biodiesel, composed of fatty acid methyl esters. The fats and oils feedstocks used to make biodiesel are in relatively short supply. To meet increasing global demand, engineered microorganisms have been developed to catalyze conversion of sugar to fatty acid esters that contain unique β‑hydroxy-esters. This study investigated the effect of a hydroxyl group on the combustion characteristics of methyl esters by comparing the chemical behavior of methyl hexanoate (MHx) and methyl 3-hydroxyhexanoate (M3OHHx)—used as surrogates for diesel-boiling-range esters with longer fatty acid chains. The oxidation of these esters was studied experimentally in a flow reactor at 0.84 and 10 bar, 600 to 1,100 K, and stoichiometric conditions; and in a constant volume combustion chamber at 5 and 10 bar, 600 to 900 K, and equivalence ratios of 0.3 and 0.6. MHx was more reactive in the constant volume chamber at temperatures below 800 K at 10 bar and equivalence ratio of 0.6. MHx also exhibited a higher indicated cetane number (16.4) than M3OHHx (8.1). We investigated this reactivity trend using an updated MHx kinetic model and M3OHHx model developed as part of this work. The kinetic models predicted that radicals formed from MHx were consumed by low-temperature chemistry reactions, whereas the M3OHHx reactivity was significantly governed by the β-radical (on the same carbon as the OH group) chemistry which mainly terminated via the less reactive chain propagation pathway to methyl-3-oxohexanoate + HO 2. This study extends our understanding of structural effects on ester reactivity, which will allow for accurate surrogate formulation and performance simulations. [ABSTRACT FROM AUTHOR]

This work develops a detailed chemical kinetic model for Polycyclic Aromatic Hydrocarbon (PAH) chemistry that builds on a chemical model for gasoline surrogates by Cai et al. [Proc. Combust. Inst. 37 (2019) 639–647]. Informed by ab-initio and experimental studies, the model development emphasizes the prediction of soot precursors starting from C 3 H 4 isomers up to the size of acepyrene. The proposed model was validated against experimental measurements from 79 publications, including ignition delay times, laminar burning velocities, and speciation data for several fuels in various conditions. A validation against measured peak mole fractions from 31 counterflow flames demonstrates an average prediction error of aromatic species up to the size of acenaphthalene below a factor of three. Additionally, the effects of experimental uncertainty are quantitatively discussed by computing confidence intervals for the true model prediction error. The formation of two-ring aromatic species is often still considered poorly understood. Accordingly, special attention is given to the formation of indene and naphthalene. Reaction flux analyses based on rigorously defined species selection criteria reveal that indene formation pathways are initiated by reactions of phenyl and benzyl and described by the reaction kinetics from theoretical calculations. In contrast, naphthalene formation is dominated by more uncertain pathways relying on estimated rates that are initiated by reactions of benzene and fulvenallenyl. Five-member rings on the periphery of PAH species and soot particles play a crucial role in recent experimental and theoretical studies. Therefore, this work investigates acenaphthene and other C 12 H 8 isomers. The most abundant C 12 H 8 isomers are acenaphthene, 1-ethynylnaphthalene, and 2-ethynylnaphthalene, and their respective concentrations are predicted to be similar in many flames. Furthermore, reaction flux analyses show that their formation is dominated by HACA routes starting from naphthalene. [ABSTRACT FROM AUTHOR]

• The PAH sub-mechanism is updated based on recent advances. • The updated model can well reproduce mole fractions of PAH at elevated pressures. • The influence of alkylbenzenes structure on PAH formation is illustrated. • The important reactions for PAH formation of each system are emphasized. Alkylbenzene is one of the significant components of practical transport fuel, which is considered as the main reason leading to serious emissions of polycyclic aromatic hydrocarbon (PAH) from combustion processes. This work reports a kinetic modeling study on the impact of alkylbenzenes structure, i.e. , the length and number of the alkyl side chain, on PAH formation at elevated pressures. A detailed kinetic model has been established and a large number of reactions concerning the PAH species formation have been updated according to the latest theoretical studies. The model is then verified by the previous experimental data containing the concentration profiles of PAH species. The fuel decomposition reactivity and the mole fraction curves of monocyclic aromatic hydrocarbon and polycyclic aromatic hydrocarbon can be well predicted by the present model. The kinetic analysis indicates that the dominant formation pathways of PAHs are barely influenced by the alkyl side chain length, especially at the high-temperature window. In the A1_C1-C4 linear alkylbenzenes pyrolysis system, the formation of benzene is mainly affected by the competition between the combination reaction of C 5 H 3 with CH 3 and the ipso-substitution reaction of A1CH 3 by H. Unlike the A1_C1-C4 linear alkylbenzenes pyrolysis chemistry, fuel-related reactions associated with the formation of PAHs become more usual as the number of alkyl side chain increases, such as the formation pathway of C 9 species in the pyrolysis chemistry of o-xylene and the formation pathway of phenanthrene in the oxidation chemistry of 1,3,5-trimethylbenzene. [ABSTRACT FROM AUTHOR]

Abstract Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in 'air' at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation. A detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ 2 radicals in the case of the resonantly stabilized radicals or O 2 for other radicals. (b) HȮ 2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3Ö atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH 2 O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ 2 H 3 and ĊH 2 CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ 4 H 5 -i radical and H 2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed. [ABSTRACT FROM AUTHOR]

Decahydronaphthalene (decalin), with both cis and trans isomers, is a bicyclic alkane that is found in aviation fuels, diesel fuels, and alternative fuels from tar sands and oil shales. Between the two decalin isomers, trans -decalin has a lower cetane number, is energetically more stable, and has a lower boiling point. Moreover, trans -decalin has often been chosen as a surrogate component to represent two-ring naphthenes in transportation fuels. Recognizing the importance of understanding the chemical kinetics of trans -decalin in the development of surrogate models, an experimental and modeling study has been conducted. Experimentally, the autoignition characteristics of trans -decalin were investigated using a rapid compression machine (RCM) by using trans -decalin/O 2 /N 2 mixtures at compressed pressures of P C   = 10–25 bar, low-to-intermediate compressed temperatures of T C   = 620–895 K, and varying equivalence ratios of ϕ = 0.5, 1.0, and 2.0. These new experimental data demonstrate the effects of pressure, fuel loading, and oxygen concentration on autoignition of trans -decalin. The current RCM data of trans -decalin at lower temperatures were also found to complement well with the literature shock tube data of decalin (mixture of cis  +  trans ) at higher temperatures. Furthermore, a chemical kinetic model for the oxidation of trans -decalin has been developed with new reaction rates and pathways, including, for the first time, a fully-detailed representation of low-temperature chemical kinetics for trans -decalin. This model shows good agreement with the overall ignition delay results of the current RCM experiments and the literature shock tube studies. Chemical kinetic analyses of the developed model were further conducted to help identify the fuel decomposition pathways and the reactions controlling the autoignition at varying conditions. [ABSTRACT FROM AUTHOR]

There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion. [ABSTRACT FROM AUTHOR]

In order to get insight into the decomposition and high-temperature oxidation kinetics of ethyl pentanoate in combustion processes, a stoichiometric premixed burner-stabilized flame of ethyl pentanoate/O 2 /Ar mixture at low pressure (20 Torr) was investigated by molecular-beam mass spectrometry combined with single-photon ionization by vacuum ultraviolet radiation from the Advanced Light Source (ALS) in Berkeley, CA, USA. Mole fraction profiles of 43 species were measured in the flame and compared with those calculated using a detailed chemical kinetic mechanism proposed by Dayma et al. (2012) for ethyl pentanoate oxidation. Although mole fraction profiles of major species and several intermediates were predicted quite accurately by the model, significant discrepancies between the measured and modeled peak mole fractions of many intermediates in the flame were observed. A kinetic analysis of the main reaction pathways of ethyl pentanoate oxidation was performed to trace the origins of these discrepancies. It was concluded that the reaction pathways responsible for consumption of primary radicals formed directly from the fuel molecule as well as of the products of successive β-scission reactions should be revised when developing a next-generation combustion model for ethyl pentanoate. [ABSTRACT FROM AUTHOR]

There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion.

This paper presents the results of experimental and numerical research on the process of biogas air combustion. The purpose of the study was to determine the effect of biogas CO 2 content on: (a) variations in SO 2 concentrations in flue gas, (b) variations in the rates of the key reactions of oxidation of H 2 S to SO 2 . The subject of investigation were the gas mixtures: CH 4 /CO 2 (of 25; 35 and 45 vol % CO 2 ) with a varying hydrogen sulfide content. The experiments were conducted in a three-zone pipe furnace equipped with a swirl burner (S g =1.26) with combustion substrate pre-mixing. It was noticed that the consumption of hydrogen sulfide was significantly reduced with the temperature decrease from 1223 K to 1023 K. The increase in the biogas carbon dioxide content inhibited the process of oxidation of H 2 S to SO 2 . [ABSTRACT FROM AUTHOR]

As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40 atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270 K. Results at temperatures above 900 K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900 K demonstrate a stronger negative temperature coefficient behavior for FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two- and three-component mixtures composed of n -heptane, iso -octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition. [ABSTRACT FROM AUTHOR]

Through a large interdisciplinary approach the “Tailor Made Fuels from Biomass” (TMFB) cluster of excellence has been pursuing the identification of next generation biofuels. By first using chemical synthesis to procure suitable chemical components from biomass followed by initial screening experiments a large information database is compiled and can be used to guide fuel design. As a result of this method butanone has been identified as a particularly interesting target owing to its potential usage as a spark-ignition fuel, thanks to its impressive knock resistant properties. This has motivated this study to consider the fundamental combustion chemistry controlling its reactivity under engine relevant conditions. A detailed chemical kinetic model which includes both high- and low-temperature oxidation reaction pathways for butanone is developed. This model (PCFCbutanone_v1) is validated using the available experimental data from the literature (included as supplemental material) and newly measured data collected during the course of this study. Ignition delay times are measured using both a shock tube and a rapid compression machine. They are measured for φ = 1.0 butanone/air mixtures covering a range of temperatures (850–1280 K) and pressures (20 and 40 bar) that incorporates engine relevant conditions. In addition, laminar burning velocities for butanone are measured for a range of equivalence ratios (0.7–1.3) and pressures (1 and 5 bar), an important parameter considering the potential use of butanone within spark ignition engines. The new model also incorporates new quantum chemical calculations of the thermodynamic parameters for butanone and the low-temperature species associated with its oxidation. The thermodynamic parameters used in the model are necessary to calculate the reverse rate constants in the model making them important parameters, in particular in predicting the low-temperature ignition delay times presented here. Butanone shows no evidence of negative temperature coefficient behavior. However, inclusion of the low-temperature oxidation pathways is shown to be important to accurately predict the low-temperature ignition delay times of butanone. Of particular importance are the radical β-scission and HȮ 2 elimination reactions which are essential in accurately predicting the ignition delay times. [ABSTRACT FROM AUTHOR]

Experimental and numerical atmospheric chemical structure of methane flames with and without hydrogen operating under sooting conditions has been investigated. Mole fraction profiles of reactants, final products and intermediate C 2 C 7 hydrocarbon species (from ethane to toluene) have been obtained using gas chromatography and gas chromatography -mass spectrometry. The most unsaturated species, which are believed to play a key-role in the formation of PAHs and soot, such as acetylene, propyne, allene, 1-butyne and small aromatic compounds, such as benzene and toluene, seemed to be particularly sensitive to the presence of hydrogen in the post-flame zone. A new kinetic scheme proposed predict well the present results (sooting premixed flames) and was also applied successfully to literature data on the oxidation of methane/hydrogen mixtures under shock-tube and jet-stirred reactor (JSR) conditions. Our results suggest that hydrogen can be able to affect the reaction steps that bring to the formation of PAH and soot particles, by inhibiting or promoting their formation in a function of the particular working conditions and inlet flow composition of the flames. We discuss here the main reactions which can explain the ambiguous effects of hydrogen. [ABSTRACT FROM AUTHOR]

A shock tube (ST) and a rapid compression machine (RCM) have been used to measure new ignition delay times for methanol oxidation over a wide range of pressures (2–50 atm) and equivalence ratios (0.5, 1.0, and 2.0). These measurements include dilute and fuel/‘air’ conditions (1.5–21.9% methanol), over a temperature range of 820–1650 K. The new data has been compared to previously published studies and provides insight into internal combustion engine relevant conditions which are previously un-studied at pressures of 10, 30, 40 and 50 atm. In addition to these ignition delay times, species concentrations have also been measured using a jet-stirred reactor (JSR). In these experiments methanol concentrations of 2000 and 4000 ppm were used at equivalence ratios of 0.2–2.0, at pressures of 1–20 atm, and in the temperature range of 800–1200 K with residence times varying from 0.05–2.00 s. The newly measured experimental data was used to develop a new detailed chemical kinetic model (Mech15.34). This model was also validated using available literature data. The new model is capable of predicting all of the validation data with reasonable accuracy, with some discrepancy in predicting formaldehyde in the JSR data. All of this, results in a robustly validated and accurate, new detailed chemical kinetic model. [ABSTRACT FROM AUTHOR]

[Display omitted] • Degradation of p -ASA and ROX in Fenton-type processes was found to be autocatalytic. • (Hydro)quinones produced from p -ASA and ROX oxidation accelerated their degradation. • (Hydro)quinones exhibited catalytic effect through promoting redox cycling of iron. • A chemical kinetic model was developed to study intermediates' autocatalytic effect. • Chemical kinetic modeling helped reveal important insight into Fenton-type processes. While the electron-shuttling properties of (hydro)quinones can facilitate the redox cycling of iron species (Fe2+ and Fe3+), the impact of in situ formed (hydro)quinones from degradation of non-phenolic aromatic pollutants in Fenton-type processes has not been reported. This study investigated the catalytic effect of (hydro)quinone intermediates produced from degradation of p -arsanilic acid (p -ASA) and roxarsone (ROX) in Fenton and photo-Fenton processes, and its chemical kinetic modeling. The bimolecular rate constants of the reactions of OH with p -ASA and ROX were determined to be (2.79 ± 0.29) × 109 and (2.03 ± 0.07) × 109 M−1·s−1, respectively. However, ROX degraded faster than p -ASA in Fenton process under the same conditions, which was attributed to the greater catalytic effect of the in situ formed (hydro)quinone intermediates. p -Hydroquinone, p -benzoquinone, and 1,2,4-benzenetriol were identified among the oxidation intermediates of p -ASA, while 2-nitrohydroquinone, 2-nitrobenzoquinone, 1,2,4-benzenetriol, and 2-hydroxy-benzoquinone were found for ROX oxidation. The autocatalytic degradation of p -ASA and ROX in Fenton and photo-Fenton processes could be well described by a chemical kinetic model after accounting for the reactions of the (hydro)quinone intermediates. Both experimental and modeling results consistently showed that the redox cycling of iron promoted by the in situ formed (hydro)quinone intermediates was critical for the oxidation of p -ASA and ROX in Fenton process. Chemical kinetic modeling revealed that (hydro)quinone-related reactions and photo-reduction of Fe3+-complexes were responsible for producing the vast majority of OH that sustained the continuous degradation of p -ASA and ROX in photo-Fenton process with the presence of limited amount of Fe2+. The autocatalytic effect observed for phenylarsonic acid compounds and the chemical kinetic model developed in this study could guide the development of Fenton and solar photo-Fenton treatment for other non-phenolic aromatic pollutants. [ABSTRACT FROM AUTHOR]

An experimental and modeling study is performed to develop a high accuracy combustion model of ethylcyclohexane (ECH) over a wide temperature range. The shock tube experiments are conducted to determine ignition delay times for ECH/air mixtures at 678–1287 K. Extensive quantum-chemical calculations concerning the low temperature kinetics of ECH including oxygen addition to hydroperoxyalkyl (·QOOH) and the following decomposition of hydroperoxyalkylperoxy radicals (·OOQOOH) have been investigated at the level of CBS-QB3//M06-2X/6-311++G(d,p). Due to the mutual influence of hydrogen bond and ring structure, the species configuration needs to be carefully determined in order to obtain thermodynamic data and then kinetic parameters. Different temperature- and pressure-dependent rate constants are computed for the side chain of ECH, the ring part, and the part where the ring and the side chain intersect. The rate constants of the hydrogen migration reaction are compared with the values estimated by analogizing with linear alkanes. Obvious deviations are observed especailly at low temperature. A detailed kinetic model is developed based on the existing low temperature model of ECH with the replacement of reaction calculated in present work and the core mechanism of AramcoMech 3.0. The model is validated comprehensively with experimental results of ignition delay time, laminar flame speed and species concentration profiles in jet-stirred reactor (JSR), which shows a good prediction over a wide temperature range. The results of sensitivity analyses show that the computed reactions of ·QOOH and ·OOQOOH have a great impact on the low temperature oxidation of ECH, which help to accurately reproduce the negative-temperature-coefficient behavior of ECH ignition. The kinetic parameters obtained in this work are conductive to improve the combustion models of substituted cycloalkanes with alkyl side chains, and the obtained kinetic model with high accuracy of ECH can be used as a basis to develop model of jet fuel. [ABSTRACT FROM AUTHOR]

• New ignition delay times of 1,2,4-trimethylbenzene are measured. • Comparisons of the ignition characteristics of three C9H12 fuels are performed. • Dominant reactions affecting ignition of three C9H12 fuels are different. • The reaction path of trimethylbenzene is different from n-propyl benzene. • Trimethylbenzene undergoes complicated reactions compared with n-propyl benzene. Alkyl aromatics comprise a significant portion of real fuels. Among various alkyl aromatics, the C 9 H 12 aromatic fuels including n-propylbenzene and trimethylbenzenes are representative alkyl aromatics, which are widely detected in real fuels, and they are widely employed as surrogate compounds in modeling real fuels. Thus, the combustion kinetic study of C 9 H 12 fuels is necessary and urgent for fuel combustion. In this work, comparative experimental and kinetic modeling study of the high-temperature ignition of three C 9 H 12 fuels is performed. New ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST) for 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene. The studied pressure is 2, 5 and 10 bar, the equivalence ratios are 0.5, 1.0 and 2.0, and the temperature range is from 1090 K to 1600 K for IDT in HPST. The experimental results are simulated using an updated detailed kinetic mechanism. Reaction path analysis and sensitivity analysis are performed to provide insight into the chemical kinetics controlling the ignition of the three C 9 H 12 fuels. The present experimental data set and kinetic model results should be valuable to improve our understanding of the combustion chemistry of alkyl aromatics and to offer practical guidance for the surrogate model development of real fuels. [ABSTRACT FROM AUTHOR]

Experimental data are reported on the structure of laminar premixed methane/oxygen/argon flames stabilized over a flat burner at 1, 3, and 5 atm with different equivalence ratios ϕ (0.8–1.2). Mole fraction profiles of the reactants (CH 4 , O 2 ), major stable products (CO 2 , H 2 O, H 2 , CO) and intermediates such as H, OH, CH 3 radicals, as well as ethylene and acetylene, were measured by molecular-beam mass spectrometry. The temperature profiles in the flames were measured by thermocouples in the presence of a sampling probe to take into account the flame cooling effect due to the probe. The structures of stoichiometric flames at 1, 3 and 5 atm were compared to elucidate the effect of pressure on the mole fractions of the flame species. Fuel-lean ( ϕ = 0.8) and fuel-rich ( ϕ = 1.2) flames at 5 atm were also investigated in this work. All the experimental data were compared with the numerical simulations using the Premix code and three detailed chemical kinetic mechanisms for methane combustion available in the literature: the GRI-Mech 3.0, AramcoMech 1.3 and USC Mech II. The absolute mole fractions of CH 4 , O 2 , H 2 O, CO, CO 2 , H 2 , H, OH, CH 3 in the flames and their dependences on pressure were captured by both mechanisms reasonably well. An analysis of the reaction mechanisms was performed to gain insights into the kinetics of methane combustion in stoichiometric conditions in the range of pressures from 1 to 5 atm and to explain the observed pressure effects on peak mole fractions of flame radicals. The decrease of peak mole fractions of acetylene and ethylene with pressure increase, which was observed in the experiments, was not reproduced by the mechanisms. Both mechanisms predicted the increase in their peak mole fractions with pressure (in the range from 1 to 3 atm). The kinetic analysis indicated the need to revise the pressure-dependent chemistry of acetylene and ethylene formation in the mechanisms. [ABSTRACT FROM AUTHOR]

Tri-propylene glycol monomethyl ether (TPGME) is an important oxygenated fuel additive that can be used to reduce soot in diesel engines. However, a validated chemical kinetic model that incorporates the low- to high-temperature chemistry, needed to simulate ignition in a diesel engine is not available for TPGME. In addition, no fundamental experimental data are available that can be used to validate a TPGME mechanism. In this study , a surrogate chemical kinetic model for TPGME that includes low- and high-temperature chemistry has been developed, and shock tube ignition delay time data has been acquired for its validation at 0.25% TPGME for temperatures in the range of 980–1545 K, at pressures of 10 and 20 atm, and at equivalence ratios of φ = 0.5, 1.0 and 2.0. The predictions from the model have been compared to the experimental measurements with good agreement. Under the experimental conditions investigated in the shock tube, TPGME was found to be consumed by molecular elimination reactions and also H-atom abstraction by H ̇ atoms and by O ̇ H and H O ̇ 2 radicals. In performing sensitivity analyses it was found that the ignition of TPGME is most sensitive to reactions involving propene. Considering how the sensitivity analyses change with pressure, the most sensitive reactions involved H ̇ atoms at 10 atm and H O ̇ 2 radicals at 20 atm. With respect to the effect of equivalence ratio, reactions involving H ̇ atoms are relatively more sensitive under fuel-rich conditions while those involving H O ̇ 2 radicals are relatively more sensitive under fuel-lean conditions. Further experimental work is needed to enable validation of the model under low-temperature conditions. TPGME was compared to n -heptane which has similar ignition properties based on Cetane Number. Predictions showed that TPGME has a higher overall reactivity compared to n -heptane. In addition, TPGME is shown to produce significantly less soot precursor species when TPGME predictions are compared to n -heptane. [ABSTRACT FROM AUTHOR]

The ignition behavior of 2,5-dimethyl furan (2,5-DMF), iso -octane, and their blends is investigated. To confirm that 2,5-DMF is the least reactive furan, its ignition behavior is compared with that of the isomer, 2-ethyl furan (2-EF), revealing the very high reactivity of 2-EF. For the 2,5-DMF/ iso -octane comparative study, ignition delay times are measured over a temperature range from 1009 to 1392 K and pressures up to 12 atm for lean, stoichiometric, and rich mixtures of fuel, oxygen, and argon. It is observed that 2,5-DMF generally has longer ignition delay times than iso -octane when the equivalence ratio ϕ , the argon-to-oxygen ratio D , and pressure p are kept constant over a range of temperatures, T . Further, ignition delay times of a 2,5-DMF/ iso -octane 50% blend (by liquid volume) are measured and compared to those of the pure fuels at stoichiometric and rich conditions and pressure of 12 atm. The blend shows intermediate reactivity between the pure fuels, albeit in closer alignment with iso -octane than 2,5-DMF. A combined model for 2,5-DMF and iso -octane combustion is developed, drawing from recent literature models for the pure components. Further modifications are carried out to improve agreement with the current and previous ignition data. The resulting model captures the ignition trends of the pure and blended fuels. Reaction pathway analysis and species sensitivity analysis are performed for more insight on the governing chemical kinetics. The reported experimental data set and the model advance combustion modeling of bio and conventional fuel blends for spark ignition engines. [ABSTRACT FROM AUTHOR]

2-Methylbutanol (2-methyl-1-butanol) is one of several next-generation biofuels that can be used as an alternative fuel or blending component for combustion engines. This paper presents new experimental data for 2-methylbutanol, including ignition delay times in a high-pressure shock tube and premixed laminar flame speeds in a constant volume combustion vessel. Shock tube ignition delay times were measured for 2-methylbutanol/air mixtures at three equivalence ratios, temperatures ranging from 750 to 1250 K, and at nominal pressures near 20 and 40 bar. Laminar flame speed data were obtained using the spherically propagating premixed flame configuration at pressures of 1, 2, and 5 bar. A detailed chemical kinetic model for 2-methylbutanol oxidation was developed including high- and low-temperature chemistry based on previous modeling studies on butanol and pentanol isomers. The proposed model was tested against new and existing experimental data at pressures of 1–40 atm, temperatures of 740–1636 K, equivalence ratios of 0.25–2.0. Reaction path and sensitivity analyses were conducted for identifying key reactions at various combustion conditions, and to obtain better understanding of the combustion characteristics of larger alcohols. [ABSTRACT FROM AUTHOR]

Diethyl ether (DEE) has favorable properties as a diesel fuel component, including its outstanding cetane number. To utilize this promising fuel, more and more knowledge on the chemical kinetics of DEE oxidation will be required. For the present article, the rate constants of unimolecular reactions of ethoxyethylperoxy radicals, which are main intermediates in the oxidation of DEE under the engine relevant conditions, have been computationally investigated and compared with those of alkanes. Geometries, vibrational frequencies, and energies of reactants, products, and transition states with pronounced barrier were calculated according to the procedure of the CBS-QB3 method. The high-pressure limiting rate constants were calculated in the temperature range of 500−2000 K by using a conventional transition state theory with hindered rotor approximation for low frequency torsional mode. The oxygen dissociation reactions have been investigated by using a variational transition state theory based on the CASPT2(7,5)/aug-cc-pvdz single point calculations at UB3LYP/CBSB7 geometries and vibrational frequencies. It was found that the oxidation pathways are equal to those of alkane oxidations, however, the rate constants are significantly different from those of alkanes due to the oxygen vicinity. The rate constants of intramolecular hydrogen shift reactions are from 3 to 8 times larger at 700 K than those of alkylic peroxy radical when the abstracted hydrogen is in the β-position of the ether. The rate constant of β-scission reactions for 1,5-intramolecular hydrogen shift products of 1-ethoxyethylperoxy radial is 163 times larger at 700 K than that of alkylic hydroperoxy radical, and this reaction becomes a main reaction pathway, whereas cyclic-ether is a main product in alkane oxidation. These characteristic rate constants are given in three-parameter modified Arrhenius form for the refinement of predictive chemical kinetic models being developed. [ABSTRACT FROM AUTHOR]

Isoprene dominates the carbon flux emitted by vegetation and constitutes 40% of non-methane biogenic emissions worldwide. Despite pyrolysis experiments at temperatures above 1000 K showing a link between isoprene combustion and aromatic species formation, comprehensive mechanistic research on isoprene is scarce in the literature. In this work, we carry out an experimental and theoretical study to build, for the first time, a chemical kinetic model describing isoprene pyrolysis. The formation of polycyclic aromatic hydrocarbon (PAH) precursor species, often observed in vegetation fire plumes, is partially explained by isoprene pyrolysis experiments and theoretical modeling. Molecular dynamics (MD) simulations unveil reaction pathways from allylic isoprenyl radicals to allene and cyclopentadiene (CPD) intermediates, two relevant species detected in the experiments. Rate constants for these identified pathways are calculated using variational transition state theory to update the kinetic model, which is validated against single-pulse shock tube (SPST), and jet-stirred reactor (JSR) experimental data in the temperature range of 850–1690 K. The kinetic model presents satisfactory agreement with the SPST experimental data, and a reaction pathway analysis shows that association of propargyl radicals results in benzene formation. The JSR pathway analysis also identifies the prominent reactions for CPD, benzene, styrene, and toluene formation. Our model does not reproduce the CPD experimental profiles, indicating that additional studies are necessary. Overall, our findings advance the understanding of isoprene pyrolysis and its related atmospheric pollutants in naturally occurring vegetation fires where smoldering and oxygen-deficient combustion processes are present. [ABSTRACT FROM AUTHOR]

Abstract: Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer–Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been simulated with a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The oxidation behavior of 2,5-dimethylhexane is compared with oxidation of other octane isomers to confirm the effects of branching on low and intermediate temperature fuel reactivity. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane. [Copyright &y& Elsevier]

Abstract: This study examines intermediate temperature heat release (ITHR) in homogeneous charge compression ignition (HCCI) engines using blends of ethanol and n-heptane. Experiments were performed over the range of 0–50% n-heptane liquid volume fractions, at equivalence ratios 0.4 and 0.5, and intake pressures from 1.4bar to 2.2bar. ITHR was induced in the mixtures containing predominantly ethanol through the addition of small amounts of n-heptane. After a critical threshold, additional n-heptane content yielded low temperature heat release (LTHR). A method for quantifying the amount of heat released during ITHR was developed by examining the second derivative of heat release, and this method was then used to identify trends in the engine data. The combustion process inside the engine was modeled using a single-zone HCCI model, and good qualitative agreement of pre-ignition pressure rise and heat release rate was found between experimental and modeling results using a detailed n-heptane/ethanol chemical kinetic model. The simulation results were used to identify the dominant reaction pathways contributing to ITHR, as well as to verify the chemical basis behind the quantification of the amount of ITHR in the experimental analysis. The dominant reaction pathways contributing to ITHR were found to be H-atom abstraction from n-heptane by OH and the addition of fuel radicals to O2. [Copyright &y& Elsevier]