Ammonia borane (NH3BH3) is a promising high-energy solid fuel candidate for ramjet propulsion systems. However, the detailed chemical kinetic mechanisms remain insufficiently understood. In this study, the ignition delay times (IDTs) of NH3BH3/air mixtures were experimentally investigated using a high-pressure shock tube under conditions of 5.0 and 10.0 bar over 1000 to 2300 K. A comprehensive kinetic mechanism comprising 58 species and 292 reactions was developed to describe the combustion of ammonia borane. Ab initio calculations were conducted to study hydrogen abstraction, unimolecular, and chemically activated reactions on the potential energy surfaces of NBH6 and NBH5. Geometry optimizations, vibrational frequency calculations, and dihedral angle scans were performed at the M06-2X/6-311++G(d,p) level of theory. Single-point energies (SPEs) for all species were determined using the CCSD/CBS method. The kinetic and thermochemical parameters obtained from high-level calculations were incorporated into the C3MechV3.3 framework to construct a revised mechanism. The reliability of the developed mechanism was evaluated through IDT simulations, and sensitivity and flux analyses were performed to identify the key reactions controlling the reactivity. The results indicate that the phase transition reaction NH3BH3(s) -> NH3BH3 and the unimolecular decomposition reaction NH3BH3 -> NH3 + BH3 play dominant roles during ammonia borane combustion.

Understanding the pyrolysis mechanism of ammonium perchlorate (AP) is critical, as it is the major oxidizer of composite solid propellants. In this study, Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry (SVUV-PIMS) was employed to diagnose AP pyrolysis from 250°C to 550°C at photon energies of 11 and 14 eV. The gaseous products formed during the pyrolysis of AP were characterized, covering key intermediates involved in nitrogen, chlorine, and nitrogen-chlorine cooperative reactions, such as NH3, NO, HNO, N2O, NO2, HCl and Cl2. The identification of HN3 and NH2O2 expanded upon the products identified by previous studies. Based on the initial formation temperatures of the products and the dominant reactions, the pyrolysis of AP can be divided into three interrelated stages: the low- (260–330°C), the intermediate- (330–400°C) and the high-temperature decomposition stage (400–550°C). The newly identified species and their associated reaction pathways have been introduced to the existing mechanisms. The updated mechanism was validated, and the results at 550°C and 1.0 atm indicated critical roles of the aforementioned species in the AP pyrolysis. Based on experimental observations and kinetic analyses, the multi-stage pyrolysis reaction pathway of AP has been updated, laying a foundation for the development of a high-precision, experimentally constrained AP reaction kinetic model. Novelty and significance statement: In this study, SVUV-PIMS approach was employed to diagnose the AP pyrolysis process using different photon energies. The pyrolysis process can be divided into three interrelated stages according to temperature evolution of major species, including NH3, NO, HNO, N2O, NO2, HCl and Cl2, which have been previously identified. By introducing newly found species such as HN3 and NH2O2 into the existing AP pyrolysis mechanism, a more complete reaction pathway spanning from the initial proton transfer to the formation of final products was constructed. Building on previous experimental and theoretical studies, this study unravels the pyrolysis kinetics of pyrolysis.

Organophosphorus compounds (OPCs) are effective flame inhibitors due to their gas-phase radical-scavenging chemistry. However, the autoignition behavior of triethyl phosphite (TEPI, P(OC2H5)3), a trivalent organophosphorus compound, has not previously been characterized. This work presents the first experimental and kinetic modeling study of TEPI autoignition, addressing a key gap in the combustion chemistry of organophosphorus compounds. Ignition delay times (IDTs) of TEPI/air mixtures were measured behind reflected shock waves in a high-pressure shock tube at temperatures of 1000-1600 K, pressures of 5-10 bar, and equivalence ratios of phi = 0.1 and 0.5. A detailed TEPI oxidation mechanism was developed using high-level quantum chemistry, canonical transition-state theory with tunneling corrections, and RRKM/master-equation analysis, yielding over 40 TEPI-specific reactions that were integrated into a validated C0-C3 hydrocarbon/phosphorus base mechanism. The measured IDTs exhibit Arrhenius-type temperature dependence, strong pressure sensitivity, and longer ignition delays under leaner conditions. At 10 bar and phi = 0.5, TEPI ignites within 0.1 to 1 ms (1100-1500 K), approximately an order of magnitude faster than at 5 bar and phi = 0.1. The kinetic model reproduces the experimental IDTs within a factor of two across all conditions and identifies PO-containing intermediates (PO, HOPO, HOPO2) as key contributors to TEPI oxidation and inhibition of ignition. These results provide the first validated ignition dataset and chemical kinetic framework for TEPI, offering mechanistic insight into trialkyl phosphite combustion and establishing a foundation for extending trivalent phosphorus chemistry to future flame-inhibition and fire-suppression studies, where extinction and flame-speed measurements will be required.

Organophosphorus compounds are used in fire suppression, acting in the gas phase by scavenging flame-propagating radicals (H, OH) and in the condensed phase by promoting char formation. However, most studies emphasize pentavalent species such as trimethyl phosphate (TMP), leaving the gas-phase oxidation chemistry of trivalent phosphorus compounds underexplored. This work presents the first detailed kinetic mechanism for trimethyl phosphite (TMPI), with a focus on its gas-phase reactivity. Ignition delay times (IDTs) of TMPI/air mixtures were measured behind reflected shock waves at 5-10 bar under lean conditions (phi = 0.1, 0.5). IDTs decrease from similar to 1-3 ms at 1050 K to similar to 0.01-0.03 ms at 1500 K, with shorter delays at higher pressure and richer mixtures. A reaction mechanism was constructed using quantum-chemically derived thermochemical and kinetic parameters. Rate coefficients were obtained from transition state theory, Rice-Ramsperger-Kassel-Marcus (RRKM) theory, and master equation simulations using the Master Equation System Solver (MESS), which incorporates hindered rotor and anharmonic treatments. The mechanism reproduces experimental IDTs across all conditions. IDT sensitivity analysis reveals that ignition is promoted by chain-branching and TMPI radical-generating reactions, and inhibited by radical-scavenging and recombination reactions. IDT flux analysis indicates that TMPI oxidation proceeds primarily via H- and CH3-abstraction, forming PO/PO2 intermediates. At 5 bar, PO2 predominantly branches toward CH3PO(2), whereas at 10 bar, PO3 and HOPO pathways become more competitive. These results fill a critical gap in phosphorus combustion chemistry and provide a foundation for predicting the effectiveness of phosphite-based fire suppression.

2-Ethylhexyl nitrate (EHN) has attracted attention for its high reactivity, making it a promising candidate for use in propellants and as a combustion-enhancing fuel additive. To gain a fundamental understanding of its combustion behavior and support its practical application in advanced propulsion systems, it is essential to develop an accurate and reliable chemical kinetic model. In this study, ignition delay times (IDTs) of EHN/O2/N2 mixtures were systematically measured using a high-pressure shock tube. Experiments were conducted over a temperature range of 900-2000 K, at pressures of 5 and 10 bar, and under equivalence ratios of 0.5 and 1.0. The results clearly demonstrate the characteristic two-stage ignition behavior of EHN. Moreover, the IDTs were found to be highly sensitive to changes in both equivalence ratio and pressure. In the theoretical investigation, the initial decomposition pathways of EHN were systematically explored using high-level quantum chemical calculations at the QCISD(T)/CBS//M06-2X/6-311++G (d,p) level. The results indicate that cleavage of the O-N bond is the dominant reaction channel. A detailed kinetic model for EHN was developed based on the C3MechV3.3 reaction mechanism. The model predictions show good agreement with experimentally measured IDT. Furthermore, based on the current kinetic model, sensitivity, flux, and OH radical rate of production analyses were performed to identify key controlling steps and characterize radical-driven kinetics. The results show that in the first stage of ignition, over 90% of EHN is consumed via O-N bond cleavage, producing the 2-ethylhexoxy radical (EHO) and NO2, which spontaneously initiate the NO2-NO catalytic cycle and significantly enhance the system's initial reactivity. In contrast, during the second stage, the chain-branching reaction H + O2 -> O + OH becomes dominant and serves as the primary driving force behind the rapid acceleration of system reactivity.

Triethyl phosphite (TEPI) is an organophosphorus compound of interest as a flame inhibitor and additive. Yet, its high-temperature decomposition pathways remain poorly characterized. This study integrates ab initio quantum chemical calculations, ReaxFF molecular dynamics (MD) simulations, and numerical kinetic modeling to investigate TEPI's thermal degradation. Molecular geometries and thermochemical parameters were obtained using M06-2X/6-311++G(d,p) and composite methods (G3B3, CBS-QB3, G3), supported by MP2/CCSD-based complete basis set (CBS) extrapolations. The C-O bond was identified as the weakest link (similar to 60 kcal/mol), consistent across electronic structure methods, and machine learning-based bond dissociation energy predictions (ALFABET). Despite variations in absolute bond dissociation energies among ReaxFF parameterizations, all methods confirmed C-O cleavage as the dominant initiation step. Reactive MD showed that TEPI is stable below 1500 K, with decomposition initiated by homolytic C-O scission yielding ethyl and an oxygen-centered radical (adjacent to the phosphorus atom). At 5000 K, over 240 intermediates were identified, including C2H5, CH3, C2H4, CH2O, PO, PO2, and HOPO. A kinetic mechanism constructed from quantum chemistry and ReaxFF-MD-derived pathways was implemented in Chemkin to simulate ignition delay times (IDTs) for TEPI/oxidizer mixtures at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 1 and 10 bar, and temperatures between 800 and 2000 K. The results show that IDTs shorten with increasing pressure and equivalence ratio, while decomposition is slowed under fuel-lean conditions due to radical scavenging by phosphorus-containing fragments. Flux and sensitivity analyses highlight initial C-O bond cleavage and subsequent PO/HOPO radical recombination as the dominant kinetic pathways. This work provides new mechanistic insights into TEPI decomposition as a transferable modeling framework for trivalent phosphorus-based compounds.

Triethyl phosphite (TEPI), an organophosphorus compound, offers potential applications in flame-retardant materials, organic synthesis, homogeneous catalysis, agrochemicals, and pharmaceutical intermediate production. However, TEPI has received little attention compared to more extensively studied phosphates and phosphonates, with its thermal decomposition and chemical reactivity, particularly under combustion and high-temperature conditions, remaining largely unexplored. This study addresses that gap by analyzing the thermochemical properties and reaction kinetics of TEPI to clarify its combustion behavior and support the accurate modeling of its reaction pathways. In this study, the M06-2X/6-311++G-(d,p) level of theory was used for geometry optimization, vibrational frequency calculations, and dihedral scans. The single-point energies (SPEs) of TEPI and its five radicals were calculated at the MP2/cc-pVXZ (X = D, T, or Q) and CCSD-(T)/cc-pVXZ (X = D, T) levels of theory. We applied complete basis set (CBS) extrapolation to these energies to improve the accuracy and approximate the basis set limit. The bond dissociation energies (BDEs) of TEPI were calculated using single-point energies (SPEs) corrected with zero-point energies (ZPEs), as well as total energies at zero Kelvin (TEZK) obtained from an average of composite methods, including G3B3, G2, CBS-QB3, and G3B3. The thermochemical properties of TEPI and the rate constants for hydrogen atom abstraction (HAA) reactions with molecular oxygen, O2, and various radicals: •H (hydrogen), •OH (hydroxyl), •CH3 (methyl), CH3O• (methoxy), and HO2 • (hydroperoxyl) were calculated using the Master Equation System Solver (MESS). The computed rate constants were further correlated with the corresponding energy barrier heights to elucidate their relationship. The results show that HAA from the secondary hydrogen site is more favorable than from the primary site, with the highest reaction rates observed for •H and •OH abstractions. With a kinetic mechanism still developing, additional reaction pathways such as hydrogen atom transfer and scission of various single bonds were estimated using rate constants derived by analogy. Furthermore, sensitivity analysis of the ignition delay time (IDT) confirmed the significance of HAA reactions, which control the initial consumption of TEPI. However, further refinement of the kinetic mechanism and experimental validation are necessary to fully confirm these reaction pathways and establish a robust model for TEPI's behavior under practical conditions. These findings offer fundamental insights and quantitative kinetic parameters into TEPI's reactivity, which serve as inputs for constructing detailed chemical kinetic models, thereby offering a quantitative basis for predicting its combustion behavior and optimizing its performance in flame-retardant applications.

Iso-alkanes are found in large quantities in both novel and conventional fuels. Accurate kinetic models for these fuels are essential for numerical simulations of combustion engines. However, existing chemical kinetic mechanisms are insufficient to fully elucidate the combustion chemistry of alkane isomers. Additionally, the influence of molecular structure differences between iso-alkanes, specifically differences in the position and number of methyl branches on their low-temperature oxidation pathways has not been comprehensively studied. The relationship between fuel properties, such as auto-ignition behavior and flame characteristics, and molecular structure is still not fully understood. The present study proposes updated reaction rate rules to establish skeletal kinetic mechanisms for monomethyl and dimethyl iso-alkanes with different positions of methyl substitution. Firstly, the important reaction classes from the sub-mechanisms of the hexane isomers are identified using reaction-class-based global sensitivity analysis. Subsequently, skeletal chemical mechanisms for 2-methyl and 3-methyl pentane are constructed, following a comparison of their critical reaction pathways. It is observed that the location of the methyl group significantly influences the positions of the critical H-atom abstraction reactions. This work further extends the study to dimethyl hexane isomers, by considering 2,2- and 2,3-dimethyl butane, and 2,3- and 2,4-dimethyl pentane. By integrating the monomethyl and dimethyl alkanes, reaction rate rules are updated for the construction of skeletal chemical mechanisms for larger iso-alkanes with similar molecular structures. Using reaction rate rules, skeletal chemical mechanisms of monomethyl and dimethyl iso-alkanes up to C10 are constructed. Comparisons between experimental data and simulations show good agreement, demonstrating the robustness of the monomethyl and dimethyl iso-alkanes chemical mechanisms and the effectiveness of the proposed reaction rate rules.

Carbon-nitrogen interaction reactions play an important role in governing the reactivity of ammonia blended fuels. However, there remains uncertainties regarding their detailed reaction pathways and rate constants, hampering the development of high-fidelity chemical kinetic models. In this study, the kinetics of C(center dot)H3 + NH2, a key C-N interaction reaction in ammonia/methane blend combustion have been investigated. The potential energy surface has been explored using the high-level ANL0F method, yielding highly accurate stationary point energies that agree with ATcT values within 0.1 kcal mol-1. Variable reaction coordinate transition state theory is used to treat the barrierless association and decomposition reaction channels, based on directly sampled radical-radical interaction energies at the CASPT2-F12(2e,2o)/cc-pVTZ-F12 level of theory. The minimum transitional mode numbers of states obtained are then coupled with the RRKM/master equation to calculate temperature-and pressure-dependent rate constants. Our a priori calculations capture available experimental measurements from the literature very well. The calculated rate constants have been incorporated into an NH3/CH4 chemical kinetic model currently under development at the University of Galway. The effect of the updated kinetic data for C(center dot)H3 + NH2 on model predicted NH3/CH4 fuel reactivity is elucidated.

Ethyl tert-butyl ether (ETBE) has captured significant research attention due to its potential to reduce harmful emissions and consequently it is used as an oxygenate additive in gasoline. A comprehensive low- to hightemperature chemistry sub-model for ETBE has been developed for the first time and is validated against experimental data including ignition delay times (IDTs), species profiles, and laminar flame speeds. This paper focuses on the low- to intermediate-temperature kinetics of ETBE oxidation. IDTs of ETBE mixtures are measured in both a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) at pressures of 15 and 30 bar in the temperature range 615-1376 K at equivalence ratios of 0.5, 1.0, and 2.0 in 'air'. The observed negative temperature coefficient behavior in ETBE oxidation can be explained by the competition between the reactions involving the formation of cyclic ethers and tert-butyl vinyl ether (TBVE), and the reactions associated with the formation and consumption of carbonyl hydroperoxide species. Moreover, IDTs of 2,2-dimethylbutane (22DMB) and 2,2-dimethylpentane (22DMP) mixtures were also measured at 15 and 30 bar in the temperature range 666-1300 K at stoichiometric conditions in 'air' in order to compare the reactivities of these alkanes with their corresponding ethers, methyl tert-butyl ether (MTBE) and ETBE. The oxygen lone pair in both MTBE and ETBE reduces the adjacent alpha C-H bond dissociation energy, making hydrogen atom abstraction at that site more facile which results in higher ether fuel reactivity at temperatures above 1000 K. At temperatures below 1000 K, the substitution of the corresponding secondary carbon atom in alkanes with an oxygen atom in ethers results in a much lower flux of fuel forming Q(center dot)OOH radicals via a six-membered ring transition state which is the key species leading to low-temperature chain-branching reactions. This is why the reactivities of MTBE and ETBE are almost two orders of magnitude lower than their alkane counterparts 22DMB and 22DMP in the negative temperature coefficient region. Conversely, dimethyl ether displays nearly two orders of magnitude higher reactivity compared to propane at lower temperatures, because of the much higher fuel flux of (ROB)-B-center dot radicals proceeding to chain branching pathways through a six-membered ring transition state isomerization reaction compared to propane. This comparative analysis provides fundamental insights into structure-reactivity relationships in oxygenated fuel combustion chemistry.

Based on our latest detailed chemical reaction mechanism, C3MechV4.0, we have developed two reduced reaction mechanisms-C3MechLite and C3MechCore-targeting C0-C3 chemical species including NH3. C3MechLite (61 species), contains a number of species comparable to GRI-Mech (53 species), that can accurately predict the combustion characteristics of hydrogen, carbon monoxide, ammonia, methane, natural gas, nitrogen oxides, and their mixtures for a wide range of conditions. C3MechCore (118 species) targets a more comprehensive range of C0-C3 fuels, including ammonia, methanol, ethanol, and dimethyl ether. Both mechanisms demonstrate predictive accuracy comparable to C3MechV4.0 for the combustion characteristics of the target fuels. C3MechLite is designed with a component library structure, enabling further reduction in mechanism size depending on the fuel(s) of interest for 2D/3D numerical simulations. Various combinations of component libraries were validated, and the average prediction error remains within 1 % compared to C3MechLite. Furthermore, the mechanism was applied to 3D LES simulations of H2 lifted flames and was confirmed to reproduce flame characteristics with high accuracy. C3MechLite and its component library structure enable highfidelity and computationally efficient chemical kinetic mechanisms, paving the way for application in more complex combustion simulations. Novelty and significance statement: An integrated component library of compact kinetic mechanism is created based on C3MechV4.0, a comprehensive detailed chemical kinetic mechanism. The component library allows users to flexibly control the size of a mechanism to reduce computational costs without losing prediction accuracy. A new reduced chemical kinetic mechanism, C3MechLite, has a comparable number of chemical species (61 species) compared to GRI-Mech (53 species) and is applicable to a wider range of conditions (fuel blends, temperature and pressure) than GRI-Mech, with a comparable level of prediction accuracy as the detailed mechanism. The proposed component library and C3MechLite can be utilized in various simulation types and provide more accurate information of complex combustion phenomena.

It is important to investigate the first-stage ignition of alkane fuels as it is responsible for the cool flame heat release in combustors, particularly engines. In the present study, a new set of ignition delay time (IDT) data of npentane is measured in a rapid compression machine (RCM) at phi = 1.0, p = 30 atm, and T = 685-994 K. Moreover, the species concentration profiles of major intermediate species, including alkenes, cyclic ethers, and aldehydes are measured in an RCM at a two-stage ignition condition (T = 730 K) using an updated 2 x fast-acting-valves sampling system. A new kinetic model has been developed to simulate this data. Both the core chemistry and thermochemistry of the low-temperature species associated with n-pentane have been systematically updated. It is found that updating the HO2 + HO2 reaction, which leadstwo OH radicals and O2, has no obvious influence on the 1st-stage ignition but significantly affects the prediction of the total IDT. This is because OH radicals are mainly produced from the formation and consumption of carbonyl-hydroperoxide species before the 1st-stage ignition; HO2 radical recombination and the reaction H2O2 (+M) <-> OH + OH (+M) become the main source of OH radical production only at/after the 1st-stage ignition. The updated thermochemistry data inhibit both the 1st-stage and total IDTs due to the shift towards reactant in the equilibrium of the RO2 <-> QOOH reaction. The key reactions involved in the low-temperature chemistry are optimized using the Optima++ code within the uncertainty limits of reviewed rate constants in the literature. The present model can predict the experimentally measured data well and shows an improvement compared to previous models.

Phosphorus-based compounds are increasingly studied for energy applications due to their unique functional properties and practical uses. However, phosphites, particularly in the gas phase, remain relatively underexplored. As a result, significant gaps persist in our understanding of their reactivity and stability. This study investigates trimethyl phosphite (TMPI) by combining quantum-chemical calculations to determine its thermochemical properties, reactive molecular dynamics (ReaxFF-MD) simulations to identify decomposition pathways, and kinetic calculations of hydrogen-atom abstraction (HAA) reactions. Molecular geometries were optimized at the M06-2X/6-311++G(d,p) level. Single-point energies were obtained using composite methods (G3, G3B3, CBS-QB3) and wavefunction-based calculations (MP2 and CCSD(T)). Composite results were averaged, while MP2 and CCSD(T) energies were extrapolated from cc-pVDZ to cc-pVQZ to approach the complete basis set (CBS) limit. The resulting averaged and CBS-extrapolated values were used to derive consistent bond dissociation energies (BDEs) and reaction energetics across various pathways. BDEs computed using the CBS-extrapolated method for C-O, C-H, and O-P bonds were 98.3, 56.4, and 93.9 kcal/mol, respectively. ReaxFF-MD-postulated decomposition pathways and product evolution trends corroborated key HAA and initiation pathways identified by quantum calculations. Six HAA reactions with O2 and radicals (Ḣ, ȮH, HȮ2, ĊH3, and CH3Ȯ) were evaluated using the Master Equation System Solver (MESS). The trend in reactivity based on forward barrier heights follows the order ȮH < Ḣ < CH3Ȯ < ĊH3 < HȮ2 < O2. The ȮH radicals showed the lowest activation barrier (<1 kcal/mol) and the highest branching ratio at low temperatures. In contrast, the abstraction with Ḣ dominated the branching ratios at high temperatures. As experimental data on TMPI remain limited, these results provide insight into its gas-phase reactivity, which is relevant to combustion chemistry, flame inhibition, and the environmental degradation of organophosphorus compounds.

It is necessary for gasoline surrogate models to simulate the effect of NOx addition on fuel auto-ignition behavior, as NOx can affect engine combustion via exhaust gas recirculation (EGR). Toluene is often used as a representative aromatic component in gasoline surrogate models, and hence it is important to investigate the effect of NOx addition on its auto-ignition behavior and to fully understand the interaction chemistry between toluene and NOx. In this paper, high-pressure shock tubes and a rapid compression machine are used to measure the ignition delay times (IDTs) of toluene in 'air' mixtures with and without the addition of nitrogen dioxide (NO2), at a pressure of 20 atm and at temperatures in the range 600-1400 K. The IDTs of n-heptane, iso-octane and a mixture of toluene/n-heptane/iso-octane are measured at the same conditions for comparison. The experimental results show that the auto-ignition behavior of toluene exhibits significantly different sensitivity to NO2 addition compared to n-heptane and iso-octane. NO2 significantly promotes the reactivity of toluene at low temperatures (600-1000 K), in which the IDTs decreased by two orders of magnitude when 1000 ppm of NO2 is added, whereas there is an order of magnitude decrease with the addition of 200 ppm NO2. The promoting effect of NO2 on toluene oxidation reduces significantly at temperatures above 1000 K. The experimental results also show that NO2 addition exhibits a slight promoting effect on the reactivity of n-heptane and iso-octane at temperatures above 750 K at the conditions studied. A kinetic model is proposed based on C3MechV3.3 in which the interaction chemistry between these gasoline surrogates and NOx is updated. The proposed kinetic model can simulate well the effect of NO2 addition on the auto-ignition behavior of these surrogates. Flux and sensitivity analyses are performed to highlight the important interaction reaction pathways.

The effectiveness of two calculation methods—Cohen's pocket model and the cluster analysis method (specifically Density-Based Spatial Clustering of Applications with Noise, DBSCAN)—was compared in predicting aluminum agglomeration on the burning surfaces of solid propellants. The comparison focused on the effects of ammonium perchlorate (AP) particle size, AP size gradation, and the relative proportions of coarse and fine AP. Experimental investigations were also conducted on four hydroxyl-terminated polyether (HTPE) propellant samples and compared with the two methods. The results show that with larger AP particle sizes and higher coarse AP content, both methods predict an increase in average aluminum volume—whether in a pocket (according to Cohen's pocket model) or in localized aluminum-rich regions (as identified by DBSCAN)—which is consistent with experimental results on agglomerate diameter variations. Additionally, DBSCAN more accurately predicts the number mean (D1,0) and number-volume mean (D3,0) agglomerate diameters, while the pocket model provides results that are closer to the volume-moment mean diameters (D4,3). For monomodal AP size distributions, the average aluminum volume calculated by DBSCAN correlates with the cubic power of AP size, which aligns with the results from the pocket model. However, for bimodal distributions, DBSCAN results deviate from the pocket model due to particle gradation effects. Using the Voronoi diagram, it was found that the spatial volumes partitioned by AP particles approximate a log-normal distribution. This finding explains the log-normal-like agglomerate size distributions predicted by DBSCAN and suggests that the pocket volumes should also follow a log-normal distribution when AP particles are graded and randomly distributed.

With the growing demand for nitrogen-containing sustainable fuels and propellants, accurately predicting their thermochemical properties has become increasingly important. While quantum chemical calculation (QC) methods and calorimetric experiments offer high precision, they are often time-consuming and computationally intensive. In contrast, the group additivity (GA) method provides a faster alternative. However, its accuracy typically declines for complex nitrogen-containing compounds. In this study, we calculated the thermochemical properties of 283 nitrogen-containing species using ab initio composite methods (G3, G4, CBS-APNO, CBS-QB3). The QC results were used to optimize 43 existing GA groups and to develop 32 new groups for nitrogen-containing structures. Compared to Active Thermochemical Tables (ATcT), the QC methods achieved a 95% confidence interval (CI) of ±1.173 kcal/mol for ΔfH°0K. The optimized GA model (without the newly developed groups) achieved CIs of ±1.645 kcal/mol for ΔfH°298K and ±4.222 cal/(mol·K) for entropy, with specific heat capacity (Cp) uncertainties ranging from ±1.144 to ±1.441 cal/(mol·K) over 300-1000 K. After adding the newly developed groups, the GA model improved, yielding CIs of ±1.894 kcal/mol for ΔfH°298K and ±3.221 cal/(mol·K) for entropy. This work demonstrates an efficient framework for enhancing GA-based thermochemistry predictions using quantum data. This study's results could enable more accurate combustion modeling, better control of nitrogen oxide emissions, and the design of advanced nitrogen-containing materials.

The autoignition behaviour of E20 gasolines is investigated experimentally using both a rapid compression machine and a high-pressure shock tube. A detailed chemical kinetic model is employed to simulate the data. Four E20 gasolines are studied, two of which are formulated from refinery streams, thereby representing a “real” fuel, while the other two are surrogate blends, which can also be modelled, and which are designed to approximately represent the RON and MON of the fuels made from the refinery streams. The measured ignition delay times (IDTs) of the surrogate fuels are always measured to be shorter than those of the “real” fuels at all temperatures and equivalence ratios, although this is most apparent in the negative temperature coefficient (NTC) regime. This deviation can be, at least partly, explained by the fact that the RON of the surrogates is slightly lower than the RON of the corresponding “real” fuels. For the surrogate fuels, there is an under-prediction of IDTs using the Galway GS_MechV1 chemical kinetic mechanism compared to the experimental results. This under-prediction is found to be more prominent at an equivalence ratio of 0.5. A modification to the original surrogates was made which increased the RON of the surrogate fuels and brought the simulated IDTs closer to the experimental measurements.

As the world grapples with climate change, decarbonizing the transportation sector remains an immense challenge. Ethanol-containing gasolines offer a promising pathway that aligns with global initiatives to transition towards low-carbon transportation fuels. This study investigates ignition delay times (IDTs) of a research-grade oxygenated gasoline (Euro 6 E10) containing a substantial proportion (20-40 %, by vol.) of ethanol. Experiments were conducted across three domains: (a) a cooperative fuel research (CFR) engine, (b) two high-pressure shock tubes (HPSTs), and (c) two rapid compression machines (RCMs). IDTs were investigated over a broad range of temperatures (655-1470 K), pressures (20 and 40 bar), and equivalence ratios (phi = 0.5, 1, 1.5). The CFR engine results indicated that blending ethanol with Euro 6 E10 gasoline led to a synergistic increase in octane ratings across the two ethanol-blended gasoline mixtures. IDTs results showed a pronounced reactivity-inhibiting effect of ethanol at temperatures below approximate to 830 K across the entire range of conditions investigated. In contrast, intermediate- and high-temperature ignition delays of gasoline-ethanol blends exhibited close similarities regardless of the blend octane numbers, compositions, or ethanol content. A reactivity-promoting effect of ethanol was observed solely in fuel-rich scenarios (phi = 1.5) and at temperatures greater than approximate to 950 K. A recently published gasoline model by the authors was updated with the latest kinetic knowledge to evaluate the effects of ethanol blending and was subsequently used to validate the measured IDTs. The revised model demonstrated reasonable accuracy with both the 4-component and 8-multicomponent ethanol-containing surrogates developed in this study, with the 4-component surrogate demonstrating better performance. Finally, sensitivity analyses were performed to identify key reactions contributing to the reactivity perturbative effects of ethanol blending on Euro 6 reactivity characteristics.

Studying the oxidation of n-butane, a major component of LNG, is critical to improve the efficiency of transportation engines. Furthermore, its negative temperature coefficient (NTC) behavior provides insights into the oxidation of larger hydrocarbons. Several studies have investigated n-butane oxidation at engine-operating pressures using various methods, including ignition delay time (IDT) measurements in rapid compression machines (RCMs) and shock tubes, flame velocities, and species concentrations in flow reactors. While these species measurements provide deeper insights into oxidation networks than IDTs, they are limited to either low-pressure or highly diluted conditions. To address this gap, this study measures species concentrations during n-butane oxidation at 30 bar in the NTC region (742 K and 855 K, respectively), at stoichiometric and moderate dilution levels in an RCM. A novel two-valve setup allowed gas sample extraction for off-line gas chromatography-mass spectrometry analysis. Complementary IDT data were obtained in the temperature range of 680 - 910 K, at pressures of 15 and 30 bar, and equivalence ratios of 0.5, 1.0, and 2.0. The results suggest that while current n-butane models reasonably predict its autoignition characteristics, they fall short in predicting the formation of key oxidation intermediates at engine-relevant conditions. In this context, the n-butane submechanism within the NUIGMech1.3 framework was updated. Modifications involve recently computed thermochemical data for critical intermediates and adjustments to rate constants, using analogies with structurally similar molecules such as n-propane and n-pentane. The present model reproduces reasonably well both the measured IDT and species concentrations documented herein and data from the literature. Nevertheless, the model slightly underestimates the reactivity within the NTC domain and the formation of some intermediates at the NTC peak. This study highlights the importance of integrating species concentration and IDT measurements at application-relevant conditions to refine kinetic mechanisms and significantly advances the understanding of C4 hydrocarbon oxidation chemistry. Novelty and Significance Statement The novelty of this research lies in the measurement of species concentrations during the ignition delay of n-butane mixtures in an RCM at high pressures near the NTC minimum and maximum using a novel two-valve gas sampling setup. This, in combination with new thermochemical data and rate rules based on analogies with propane and n-pentane, allowed the refinement of the n-butane sub-mechanism within the NUIGMech1.3 framework. By combining species concentration measurements with ignition delay times in the RCM, this study examines the oxidation of n-butane, a major component of LPG, under conditions that closely mimic engine environments, overcoming the limitations of previous studies limited to highly dilute conditions. This research is part of a broader investigation of C4 oxidation chemistry, along with our companion work on 1-butene. The resulting kinetic model is capable of reproducing most of the available n-butane and 1-butene validation targets.