Autoignition Chemistry Research Articles

Development of an ammonia/diesel combustion mechanism for high ammonia energy ratio: Validation of the mechanisms in kinetic simulation and RCCI optical engine simulation

Ammonia blending in diesel is an efficient combustion strategy that overcomes the combustion resistance of ammonia while maintaining high engine adaptability. A smaller and accurate chemical kinetic mechanism is crucial for exploring the application of ammonia/diesel in engine. This study, based on the stepwise validation and optimization mechanisms, developed a novel ammonia/diesel mechanism comprising 130 species and 772 reactions. The mechanism was validated through kinetic verification for ammonia, n-dodecane, diesel and ammonia/diesel fuel. For profound understanding of the low-temperature autoignition chemistry of ammonia/diesel, applying this mechanism construct precise CFD models in RCCI optical engine. The results indicate that this mechanism can accurately capture ignition under high AER, wide working conditions, and low-temperature conditions. In the kinetic analysis, the C–N interaction reaction, particularly C2H4+NH2=C2H3+NH3, plays a crucial role in predicting the IDT of ammonia/diesel. In addition, NH2 undergoes deoxygenation reaction with HO2, converting inactive radicals HO2 into OH active radicals, enhances the reactivity of ammonia/diesel under low-temperature conditions. Applying the mechanism to CFD models, the model accurately predicts the pressure and heat release rate in RCCI optical engine, capturing the phenomenon of the high-temperature flame rapidly spreading towards the low-temperature regions in the cylinder. The research on this mechanism can construct accurate CFD model for optimizing efficient and clean combustion simulations of ammonia/diesel.

The Structure Characteristics of Laminar Premixed Flames of Gasoline-like Fuel Under CI Engine-Relevant Conditions.

Gasoline compression ignition characterized by partially premixed and long ignition delays typically features complex flame structures such as deflagration or spontaneous ignition fronts. In this study, the flame structure and propagation characteristics of PRF90/air mixtures under compression ignition engine-relevant conditions are investigated numerically. Similar to other types of fuels, under such conditions, the propagation speed of PRF90 laminar premixed flames depends not only on the unburnt mixture properties but also on the residence time, and the transition of the flame regime depends only on the residence time. Nevertheless, due to the temperature-dependent autoignition chemistry of PRF90, flames with excessively high unburnt temperatures show different combustion behaviors after the transition from deflagration to autoignition-assisted flames. Sensitivity analysis showed that, the dominant chain branching reactions in the deflagration mode are H + O2 = OH + O and CO + OH = CO2 + H, and that in the autoignition-assisted flames with lower unburnt temperature are H2O2(+M) = 2OH(+M) and IC8H18 + HO2 = AC8H17 + H2O2, while for higher unburnt temperatures, the reactions C3H5 + HO2 = C2H3 + CH2O + OH and IC8H18 = IC4H9 + TC4H9 are more important than the fuel low-temperature oxidation reactions. In addition, a criterion based on chemical explosive mode analysis is used to analyze the local combustion mode. The results show that the difference in diffusion/chemical structure at the crossover progress variables C 0 and crossover temperature allows both C 0 and to be used as a flame location for distinguishing propagation modes in premixed flame. However, the effects of the equivalence ratio on C 0 are different from that on , which means that the selection of C 0 and may lead to different discriminant results for stratified mixtures. Comparing the applicability of C 0-based and -based locations in three-dimensional gasoline compression ignition flame, it is found that the flame location based on the value of C 0 at ϕ = 1.0 can more completely reflect the flame development characteristics in stratified premixed combustion.

Numerical Study of Ignition and Combustion of Hydrogen-Enriched Methane in a Sequential Combustor

Ignition and combustion behavior in the second stage of a sequential combustor are investigated numerically at atmospheric pressure for pure CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document} fueling and for two CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document}-H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} fuel blends in 24:1 and 49:1 mass ratios , respectively, using Large Eddy Simulation (LES). Pure CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document} fueling results in a turbulent propagating flame anchored by the hot gas recirculation zones developed near the inlet of the sequential combustion chamber. As the H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} content increases, the combustion process changes drastically, with multiple auto-ignition kernels produced upstream of the main flame brush. Analysis of the explosive modes indicates that, for the highest H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} amount investigated, flame stabilization in the combustion chamber is strongly supported by auto-ignition chemistry. The analysis of fuel decomposition pathways highlights that radicals advected from the first stage flame, in particular OH, induce a rapid fuel decomposition and cause the reactivity enhancement that leads to auto-ignition upstream of the sequential flame. This behavior is promoted by the relatively large mass fraction of OH radicals found in the flow reaching the second stage, which is approximately one order of magnitude greater than it would be at chemical equilibrium. The importance of the out-of-equilibrium vitiated air on the ignition behavior is proven via an additional LES that features weak auto-ignition kernel formation when equilibrium is artificially imposed. It is therefore concluded that parameters affecting the relaxation towards chemical equilibrium of the vitiated flow can have an important influence on the operability of sequential combustors fueled with varying fractions of H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} blending.

Temperature -and pressure-dependent branching ratios for 2,6-dimethylheptyl radicals (C9H19) + O2 reaction: An ab initio and RRKM/ME approach on a key component of bisabolane biofuel

Chemical kinetics mechanisms contain elementary reactions and associated Arrhenius rate parameters are necessary to understand the modeling of hydrocarbon autoignition chemistry. The complexity of such mechanisms is increased due to interest in operating next-generation combustion strategies in the low-temperature region (≤1000 K), which is governed by O2-addition to alkyl radicals (R), subsequent radical isomerization and decomposition steps thereafter. In this work, we report theoretically the reaction of molecular oxygen to the four isomers of 2,6-dimethylheptyl radicals (C9H19). The stationary points on potential energy surfaces (PES), rate constants, and branching ratios from C9H19 isomers initiated by O2 has been investigated by a combination of ab initio/density functional theory (CBS-QB3) and advanced statistical rate theory i.e., microcanonical variational transition state theory (μCVTST) and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulations. The temperature- and pressure-dependent rate constants and branching ratios over temperature range 400 K–1000 K (with an interval of 50 K) and with different pressures 0.001, 0.01, 0.1, 1, 10 and 100 bar were computed. The RRKM/ME calculations reveal for addition of O2 to the primary radical, 2,6-dimethylhept-1-yl + O2 reaction, the formation of 2-iso-butyl-4-methyl-tetrahydrofuran (a five membered cyclic ether) + OH via concerted C–C and O–O bond scission of primary-secondary QOOH and formation of ROO-1 is the kinetically favorable pathway below 600 K. For 2,6-dimethylhept-2-yl + O2, three competitive favorable channels lead to the formation of 5-iso-propyl-2,2-dimethyl-tetrahydrofuran + OH, a five membered cyclic ether formed coincident with OH in a chain-propagating step from decomposition of tertiary-secondary hydroperoxyalkyl (QOOH), formation of a six membered cyclic ether i.e., 2,2,6,6-tetramethylpyran and ROO-2, however at higher temperature (>900 K) and 2–6-dimethyl-2-heptene is contributed equally over other products. For 2,6-dimethylhept-3-yl + O2 reaction, leading to 5-iso-propyl-2,2-dimethyltetrahydrofuran + OH (same products as in ROO-2) from decomposition of secondary–tertiary hydroperoxyalkyl (QOOH). For 2,6-dimethylhept-4-yl + O2 reaction, it reveals the formation of chain-propagation channels leading to iso-butene + methyl-isobutanal + OH. The results of this study reveals the importance of temperature- and pressure-dependent branching ratios, which is necessary to understand the low temperature internal combustion mechanism. The unimolecular rate constants were also compared with the previously reported values for 2,6-dimethylheptyl radical + O2 and similar reaction system i.e., 2,5-dimethylhexyl radical + O2. The updated rate constants and branching ratios may also serve as general prototypes for low-temperature oxidation of branched alkanes of next-generation biofuels with similar structural motifs, such as bisabolane and farnesane.

An experimental and modeling study on autoignition of 2-phenylethanol and its blends with n-heptane

2-Phenylethanol (2-PE) is an aromatic alcohol with high research octane number, high octane sensitivity, and a potential to be produced using biomass. Considering that 2-PE can be used as a fuel additive for boosting the anti-knocking quality of gasoline in spark-ignition engines and as the low reactivity fuel or fuel component in dual-fuel reactivity controlled compression ignition (RCCI) engines, it is of fundamental and practical interest to understand the autoignition chemistry of 2-PE, especially at low-to-intermediate temperatures (<1000 K). Based upon the experimental ignition delay time (IDT) results of neat 2-PE obtained from our previous rapid compression machine (RCM) investigation and the literature shock tube study, a detailed chemical kinetic model of 2-PE is developed herein, covering low-to-high temperature regimes. Besides, RCM experiments using binary fuel blends of 2-PE and n-heptane (nC7) are conducted in this work to investigate the nC7/2-PE blending effects, as they represent a dual-fuel system for RCCI operations. Furthermore, the newly developed 2-PE model is merged with a well-validated nC7 kinetic model to generate the current nC7/2-PE binary blend model. Overall, the consolidated model reasonably predicts the experimental IDT data of neat 2-PE and nC7/2-PE blends, as well as captures the experimental effects of pressure, equivalence ratio, and blending ratio on autoignition. Finally, model-based chemical kinetic analyses are carried out to understand and identify the controlling chemistry accounting for the observed blending effects in RCM experiments. The analyses reveal that nC7 enhances 2-PE autoignition via providing extra ȮH radicals to the shared radical pool, while the diminished nC7 promoting effect on 2-PE autoignition with increasing temperature is due to the negative temperature coefficient characteristics of nC7.

Exploring the chemistry behind low temperature auto-ignition of isopropyl nitrate in an RCM: An experimental and kinetic modeling study

The low-temperature auto-ignition chemistry of isopropyl nitrate (iPN) was experimentally and numerically investigated in the present study. The ignition delay times (IDTs) of iPN were measured stoichiometrically over a temperature range of 560–600 K at effective pressures of 5 and 10 bar in a rapid compression machine. A two-stage ignition phenomenon of iPN was observed. Both the first-stage IDTs and total IDTs vary rapidly within the narrow temperature range investigated (∼40 K). A recent iPN kinetic mechanism proposed by Fuller and Goldsmith for pyrolysis studies was extended. The reaction kinetics of CH3CHO + NO2 has been theoretically calculated at 500–1500 K and 0.01–100 atm. The rate information of CH3 + NO2 was updated based on previous theoretical results. The O2-addition channel of acetyl radical (CH3CO), which accounts for the first-stage IDT, was also considered in the present work. The extended iPN kinetic model predicts the two-stage IDTs well. Simulation results suggest that the IDTs are most sensitive to the following two reactions: (1) CH3 + NO2 = CH3O + NO; (2) CH3 + NO2 = CH3NO2. The former promotes the overall reactivity by yielding the reactive methoxy radical, while the latter forms a relatively stable product (i.e., CH3NO2). The reaction of CH3CHO + NO2 = CH3CO + HONO supplements the formation of CH3CO. The different consumption channels of CH3CO radicals (the O2-addition reaction and the decomposition reaction) lead to different chain reactions yielding OH radicals with increasing temperature in the ignition process. The “NONO2 loop” is the main route for OH formation in the studied conditions, which is mainly responsible for the iPN ignition.

Elucidating NO coupling effects on ignition of toluene reference fuels by chemical functional group analysis

Derived cetane number (DCN), Research and Motor Octane Numbers (RON and MON) have been fundamentally analyzed using Quantitative Structure-Property Relationship (QSPR) regression models with key chemical functional groups. Both RON and MON exhibit strong sensitivities to the abundances of (CH2)n and benzyl-type groups but lack sensitivity to the CH3 group, most dominant in real gasolines. Residual and EGR gases contain NOx known to synergize with fuel autoignition chemistry. Two TRF mixtures having high and low aromatic content but sharing the same RON and MON values were used to evaluate NOx coupling effects. DCN measurements with NO addition were found to be strongly correlated with the abundance of the (CH2)n group. Similar experiments of 200 ppm NO in a Rapid Compression Machine show promotion (inhibition) of ignition for the high (low) aromatic TRF fuel. Kinetic modeling attributes the promotion to the NONO2 interconversion reactions, NO + HO2 = NO2 + OH, CH3 + NO2 = CH3O + NO and NO2 + H = NO + OH. The inhibitive effect relates specifically to low temperature kinetics and high NO loading conditions, leading to the formation of meta-stable species (e.g. CH3 + NO2 (+M) = CH3NO2 (+M)) that decelerate the rate of conversion of HO2 to more reactive OH radicals. The coupling of NO with real gasolines depends on chemical composition and temperature conditions not only encompassed by RON and MON criteria, but by the chemical functional group characteristics. The relevance of this finding to the significance of preferential vaporization of multi-component gasolines on low-speed pre-ignition (LSPI) is discussed. Within the context of chemical functional group distributions of five distillation cuts of a marketed ethanol-free gasoline determined by NMR spectroscopy, the analyses identify considerable variations of key functionalities with fuel distillation properties, indicating chemical kinetic autoignition behaviors that are dependent on preferential vaporization.

Development of a phenomenological model for the description of RCCI combustion in a dual-fuel marine internal combustion engine

Increasingly stringent pollutant and CO2 emission standards require engine manufacturers to investigate innovative solutions. Among these techniques, low-temperature combustion (LTC) concepts have a large potential to simultaneously reduce NOx emissions and fuel consumption. A promising manner to realize LTC consists of adopting ultra-lean mixtures, where the combustion evolution is controlled by a proper spatial distribution of fuels with different chemical reactivities. In this context, depending on the proportion and stratification of the fuels, the heat release can primarily depend on chemistry progression, leading to a Reactivity Controlled Compression Ignition (RCCI) mode, or on flame propagation, locally initiated by a high reactivity fuel.In this work, the combustion characteristics of a large-bore research engine are experimentally investigated. Natural gas is supplied into the intake port, while light fuel oil (LFO) is directly injected in the cylinder. An experimental campaign is carried out including sweeps of engine load, air/fuel proportion, LFO amount, valve timing, and intake air temperature. Global engine operating parameters as well as cylinder pressure traces are recorded and analyzed.Based on the available experimental data, a phenomenological model handling both chemistries of fuels with different reactivities and flame propagation is developed and validated. The model is based on a multi-zone approach, where auto-ignition chemistry is solved by a tabulated method to preserve the computational effort. The proposed numerical approach shows the ability to simulate the experimental data with good accuracy, using a fixed tuning constant set. A dedicated correlation is built to reproduce the expected in-cylinder distribution of the directly injected liquid fuel. Global performance and combustion parameters are predicted with an average error below 5%. The model demonstrates to correctly describe the behavior of the tested engine under different operating conditions and to capture the physics behind such advanced combustion concepts.

Parametric study on dual-fuel ignition characteristics under marine engine-relevant conditions

Dual-fuel (DF) combustion strategies with low-carbon fuels are attractive for the shipping industry because of their better economy and lesser emissions. However, DF ignition process is highly complex and difficult to control due to its strong sensitivities to mixture-states. As such, a comprehensive numerical study was conducted to investigate the influence of thermodynamic states, ambient-fuel reactivity and micro-mixing process on the autoignition characteristics of DF mixtures (methane/n-heptane and methanol/n-heptane) under marine engine-relevant conditions. A novel parameter-sensitivity evaluation method by log–log definition was firstly proposed to quantify the ignition-delay-time (IDT) dependence on thermodynamic trajectory and charge reactivity, based on a newly detailed kinetic model (NUIGMech1.1). It was found that the methanol-addition has a larger impact on IDTs than methane, and also exhibits ignition-promoting behaviors at high temperatures. Besides, the IDT displays a higher sensitivity to temperature under methanol/air atmosphere but a higher sensitivity to pressure under methane/air atmosphere, suggesting that methane/diesel DF engines are prone to occur cycle-to-cycle variations under diesel pilot-ignition (DPI) mode while methanol/diesel DF engines exist the larger cycle variations under reactivity-controlled compression-ignition (RCCI) mode. There are small IDT-changes for methane/n-heptane mixtures with varying ϕCH4 while large variations for methanol/n-heptane as varying ϕCH3OH. Moreover, methanol shows the more significant chemical-effect on ignition. The mixture fraction of methanol-based blends with the shortest IDT, defined as the most-reactive mixture fraction (ZMR), is larger than that of methane at a given temperature. This demonstrates that the mixing degree (the high-reactivity fuel to low-reactivity fuel ratio) plays an important role in controlling the ignition. The mixing-time is also critical for ignition behavior of fuels, and autoignition of methanol/n-heptane blends exhibits higher sensitivity to mixing rates than methane/n-heptane. Therefore, adjusting injection parameters could tailor mixing-trajectories to offset changes in fuel autoignition chemistry. Additional reaction sensitivity analyses demonstrate that the OH radicals play a key role in the whole system reactivity at low temperatures, while HO2 becomes more important at intermediate-temperatures and CH3OH + OH <=> CH2OH + H2O displays the highest ignition-inhibition. Significantly, CH3OH + HO2 <=> CH2OH + H2O2 shows a high promoting effect on IDTs at high temperatures, resulting in the IDT-reduction.

A Rapid Compression Machine Study of 2-Phenylethanol Autoignition at Low-To-Intermediate Temperatures

To meet the increasing anti-knocking quality demand of boosted spark-ignition engines, fuel additives are considered an effective approach to tailor fuel properties for satisfying the performance requirements. Thus, screening/developing bio-derived fuel additives that are best-suited for advanced spark-ignition engines has become a significant task. 2-Phenylethanol (2-PE) is an attractive candidate that features high research octane number, high octane sensitivity, low vapor pressure, and high energy density. Recognizing that the low temperature autoignition chemistry of 2-PE is not well understood and the need for fundamental experimental data at engine-relevant conditions, rapid compression machine (RCM) experiments are therefore conducted herein to measure ignition delay times (IDTs) of 2-PE in air over a wide range of conditions to fill this fundamental void. These newly acquired IDT data at low-to-intermediated temperatures, equivalence ratios of 0.35–1.5, and compressed pressures of 10–40 bar are then used to validate the 2-PE model developed by Shankar et al. (2017). It is found that this literature model greatly overpredicts the current RCM data. The comparison of experimental and simulated results also provides insights into 2-PE autoignition behaviors at varying conditions. Further chemical kinetic analyses demonstrate that the absence of the O2-addition pathway of β-R. radical in the 2-PE model of Shankar et al. (2017) could account for the model discrepancies observed at low-to-intermediated temperatures.

Investigation on the autoignition characteristics of propanol and butanol isomers under diluted lean conditions for stratified low temperature combustion

Low-temperature combustion (LTC), which can be achieved through either fuel-lean mixtures or high dilution, is an effective way to reduce NOx and soot emission for an internal combustion engine. Alcohols are considered suitable fuels for LTC as they have the advantage of better fuel-air mixing, such as high autoignition resistance and fast vaporization. Propanol and butanol can be produced as bio- or electro-fuels, which are carbon-neutral and become candidates as transportation fuels. This work focuses on investigating the autoignition and emission behavior of these alcohols regarding the application in stratified LTC. The autoignition properties of propanol and butanol isomers have been investigated in a rapid compression machine (RCM) under 90% dilution, at temperatures from 800 K to 1100 K, pressures of 20 and 40 bar and equivalence ratios of 0.25, 0.5 and 0.9. Under all investigated conditions, the order of reactivity among butanol isomers is consistent, namely n-butanol > isobutanol ≅ 2-butanol > tert‑butanol, while n-propanol is always more reactive than isopropanol. The reactivity of propanol isomers is similar to that of iso and 2-butanol. It was observed in the measurement that the difference between the reactivity of different fuels was not constant in the investigated temperature range, which was explained by the autoignition chemistry transition between 800 K and 950 K. According to the modeling, emissions of all butanol and propanol isomers are similar. The non-diluted mixtures can only achieve low-temperature combustion (LTC) in extreme lean conditions, while the diluted mixtures can achieve LTC in a wider range of equivalence ratios and have both low NOx and hydrocarbon emissions.

Analysis of Auto-Ignition Chemistry in Aeroderivative Premixers at Engine Conditions

Abstract The minimization of auto-ignition risk is critical to the design of premixers of high power aeroderivative gas turbines as an increased use of highly reactive future fuels (for example, hydrogen or higher hydrocarbons) is anticipated. Safety factors based on ignition delays of homogeneous mixtures are generally used to guide the choice of a residence time for a given premixer. However, auto-ignition chemistry under aeroderivative conditions is fast (0.5–2 ms) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period necessarily involves the study of low-temperature auto-ignition precursor chemistry, but precursors can change with fuel and local reactivity. Chemical explosive modes (CEMs) are a natural alternative to study this as they can provide a measure for auto-ignition risk by considering the whole thermochemical state in the framework of an eigenvalue problem. When transport effects are included by coupling the evolution of the chemical explosive modes to turbulence, it is possible to obtain a measure of spatial auto-ignition risk where both chemical (e.g., ignition delay) and aerodynamic (e.g., local residence time) influences are unified. In this article, we describe a method that couples large eddy simulation (LES) to newly developed, reduced auto-ignition chemical kinetics to study auto-ignition precursors in an example premixer representative of real life geometric complexity. A blend of pure methane and di-methyl ether (DME), a common fuel used for experimental auto-ignition studies, was transported using the reduced mechanism (38 species/238 reactions) under engine conditions at increasing levels of DME concentrations until exothermic auto-ignition kernels were formed. The resolution of species profiles was ensured by using a thickened flame model where dynamic thickening was carried out with a flame sensor modified to work with multistage heat release. This paper is outlined as follows: First, a reduced mechanism is constructed and validated for modeling methane as well as DME auto-ignition. Second, sensitivity analysis is used to show the need for chemical explosive modes. Third, the thickened flame model modifications are described and then applied to an example premixer at 25 bar/890 K preheat. The chemical explosive mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentrations and identifies where the premixer is sensitive and flame anchoring is likely to occur.

Quantitative Detection of Products and Radical Intermediates in Low-Temperature Oxidation of Cyclopentane.

We present a combined experimental and theoretical investigation of the autoignition chemistry of a prototypical cyclic hydrocarbon, cyclopentane. Experiments using a high-pressure photolysis reactor coupled to time-resolved synchrotron VUV photoionization mass spectrometry directly probe the short-lived radical intermediates and products in cyclopentane oxidation reactions. We detect key peroxy radical intermediates ROO and OOQOOH, as well as several hydroperoxides, formed by second O2 addition. Automated quantum chemical calculations map out the R + O2 + O2 reaction channels and demonstrate that the detected intermediates belong to the dominant radical chain-branching pathway: ROO (+ O2) → γ-QOOH + O2 → γ-OOQOOH → products. ROO, OOQOOH, and hydroperoxide products of second-O2 addition undergo extensive dissociative ionization, making their experimental assignment challenging. We use photoionization dynamics calculations to aid in their characterization and report the absolute photoionization spectra of isomerically pure ROO and γ-OOQOOH. A global statistical fit of the observed kinetics enables reliable quantification of the time-resolved concentrations of these elusive, yet critical species, paving the way for detailed comparisons with theoretical predictions from master-equation-based models.

An experimental and modeling study of autoignition characteristics of two real low-octane gasoline fuels in a heated rapid compression machine at elevated pressures

Gasoline compression ignition (GCI) has received much attention due to its high efficiency and low emissions. The low-octane gasoline can be applied to extend the GCI operating range. However, there is limited knowledge of the autoignition chemistry for the low-octane gasoline. In this study, the ignition delay times (IDTs) of two real distillate gasoline fuels with research octane numbers (RON) of 72 and 83, denoted as G72 and G83 respectively, were measured in a heated rapid compression machine (RCM) over wide ranges of pressures (10, 15 and 20 bar), temperatures (670–940 K), equivalence ratios (0.5, 1.0 and 2.0), and diluted conditions. Both G72 and G83 exhibit apparent two-stage ignition characteristics with negative temperature coefficient (NTC) behavior in the low-to-intermediate temperature region. Considering the effects of exhaust gas recirculation (EGR) technology on GCI combustion, ignition characteristics of low-octane gasolines under simulated-EGR conditions were also studied by varying the N2/O2 ratio while fixing the fuel mole fraction. It is found that the NTC region moves towards the higher temperature side as the oxygen concentration decreases. Moreover, two six-component surrogates were formulated for G72 (28.4% n-pentane, 3.5% n-heptane, 24.5% 2-methylhexane, 26.3% 2,2,4-trimethylpentane, 13.8% cyclopentane, and 3.3% toluene, by mol.), and G83 (16.5% n-pentane, 10.6% n-heptane, 5.6% 2-methylhexane, 37.4% 2,2,4-trimethylpentane, 10.2% cyclopentane and 19.7% toluene, by mol.). Kinetic modeling was then conducted using a published kinetic model coupled with proposed surrogates. Sensitivity analysis results further revealed that compared with isooctane, cycloalkane chemistry is more important in the overall autoignition behavior at low temperatures.

Ignition delay time measurements and kinetic modeling of methane/diesel mixtures at elevated pressures

Natural gas/diesel dual-fuel (DF) combustion technology has attracted substantial attention in terms of thermal-efficiency improvement and emission reduction in advanced compression ignition engines. As known, autoignition behavior of DF mixtures plays a significant role in controlling the ignition timing and combustion performance in engines. In the present study, ignition delay time (IDT) measurements of methane/diesel mixtures with three varying diesel substitution ratios (denoted as DSR30, DSR50, and DSR70) were conducted on both a heated rapid compression machine and a heated shock tube under wide-range conditions (T = 640–1450 K, p = 6–20 bar, ϕ = 0.7, 1.0, and 2.0 in ‘air’ mixtures). Experimental results show that DF blends exhibit typical two-stage autoignition characteristics along with the negative temperature coefficient (NTC) response at the temperature region currently investigated, owing to the addition of high-reactivity diesel fuel. Moreover, both the total and first-stage IDTs decrease with the rise of pressure and equivalence ratio as well as diesel substitution ratio. Additionally, a crossover of IDTs occurs at a high temperature (~1400 K) for varying equivalence ratios, and there exists a non-linear promoting effect of diesel contents on IDTs. The simulation results performed using a detailed chemical kinetic mechanism (POLIMI_1412) in conjunction with a well-validated tri-component diesel surrogate show generally good agreement with the experimental results at the current test conditions. Furthermore, species evolution assisted with brute-force sensitivity analysis was carried out to further understand the autoignition chemistry of the DF mixture, particularly the chemical interplay between methane and diesel during the low-temperature ignition processes. It is found that methane hardly generates OH radicals, which are mainly produced via the low-temperature oxidation pathways of diesel fuel. The competition between methane and diesel for OH radicals inhibits the consumption of diesel while promotes the depletion of methane, resulting in an inhibiting impact on the overall reactivity of the reaction system. What's more, the experimental data reported herein provide a foundation for the development of DF kinetic models with high accuracy and robustness.

Autoignition behavior of methanol/diesel mixtures: Experiments and kinetic modeling

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 •HO2 while CH3OH 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.

Thermal-pyrolysis induced over-driven flame and its potential role in the negative-temperature dependence of iso-octane flame speed at elevated temperatures

Recently, high-temperature flame speed measurement up to 900 K has been successfully realized at Stanford University using a conventional shock tube, assisted by laser ignition and chemiluminescence image systems. For a near stoichiometric iso-octane/O2/N2/He mixture at atmospheric pressure, their results show that while flame speed increases with increasing unburnt temperature in the low and high-temperature range, it decreases with increasing unburnt temperature in the intermediate temperature regime (750–800 K). The measurement time for the flame initiation and propagation is orders of magnitude shorter than the first-stage ignition delay time, implying that the unburnt mixture upstream of the flame is largely chemically frozen. Therefore, such non-monotonic behavior in the flame speed is unlikely to be induced by autoignition chemistry. To understand this interesting phenomenon, 1-D transient simulation of spherical flame initiation and propagation with different ignition energy has been performed and compared with the experiment data. It is shown that for the relatively low ignition energy 7.5 mJ, an over-driven flame regime characterized by flame speeds and stretch rates well above the steady-state values occurs in the intermediate temperature range. The chemical structure of an over-driven flame involves substantial fuel thermal decomposition in the preheat zone, followed by C1-C4 oxidation in the reaction zone. It is found that the non-monotonicity in the observed flame speed is strongly correlated with the occurrence of over-driven flames in simulations. These results demonstrate a new flame structure which can potentially explain the observed non-monotonic flame speed for iso-octane at high temperatures. Higher ignition energy or smaller ignition kernel is suggested in future experiments to mitigate such over-driven effects.

Understanding the synergistic blending octane behavior of 2-methylfuran

The autoignition kinetics of hydrocarbons is an important criterion for selecting fuels for piston reciprocating engines, and it can be determined by relative performance to mixtures of alkanes, n-heptane and iso-octane, under certain standardized operating conditions. 2-methylfuran is a potential biofuel candidate, whose autoignition chemistry is markedly different from alkanes. Its octane behavior when blended with paraffins also shows a marked difference. The blending octane behavior of a fuel is characterized by its Blending Octane Number (BON). The BON of 2-methylfuran was extensively characterized in this work. 2-methylfuran's BON was mapped from experimental ignition delay times measured in a constant volume combustion chamber using established correlations. The effect on BON was studied depending on the RON of the base fuel into which 2-methylfuran was blended, as well as the quantity of 2-methylfuran blended. BON of 2-methyfuran was greater than its RON by a factor of four or more for some blends studied. BON reduced with increasing RON of the base fuel, as well as with increasing quantity of 2-methylfuran blended. A chemical kinetic model was created by integration of well validated sub-models for the blend components, and then used to explain the chemical kinetics leading to the extremely high BON values of 2-methylfuran. The synergetic anti-knock blending effect of 2-methylfuran is partially due to its physical properties leading to a greater molar fraction per volume fraction in the blend compared to iso-octane. Analysis using chemical kinetic model revealed that the chemical action behind 2-methylfuran's blending octane behavior was due to its ability to quench OH radicals which are important to the low-temperature oxidation chemistry of alkanes. This quenching effect is achieved due to the more rapid reaction rate of 2-methylfuran with OH radical compared to iso-octane, followed by the immediate conversion of the adduct shifting the equilibrium towards the product.

Low-temperature auto-ignition characteristics of NH3/diesel binary fuel: Ignition delay time measurement and kinetic analysis

Ammonia (NH3) is gaining increasing interest as a carbon-free alternative fuel in engine systems. Co-firing NH3 with diesel can overcome the high auto-ignition temperature and narrow flammability limits of pure NH3, while exploit its advantage. This study presents the auto-ignition properties of NH3/diesel binary fuel, at various NH3 blending ratios (10%, 30% and 50%) in a rapid compression machine. Ignition delay times (IDTs) were measured spanning a temperature range of 670–910 K, pressures of 10–20 bar, and equivalence ratios of 0.5–1.5. Typical two-stage ignition and negative temperature coefficient (NTC) response were identified for the blends. Both the first-stage and the total IDTs increase with the increasing NH3 blending ratio, and there exists a non-linear inhibiting effect of NH3 fraction on IDT. A blending mechanism was then constructed based on the existing diesel mechanism and NH3 mechanism. The mechanism can predict the inhibiting effect of NH3 addition, but fails to well capture the IDTs over the whole temperature range, especially for the first-stage IDT and the NTC response. Further kinetic analysis, including species mole fraction history, brute force sensitivity analysis and reaction pathway analysis, were conducted to gain deeper insight into the auto-ignition chemistry of the blending fuel. These analyses suggest the rate parameters of reactions NH3 + OH ⇒ NH2 + H2O and NH2 + NO ⇒ N2 + H2O are critical to accurately predict IDTs, and the competition role of NH3 for OH radical inhibits the diesel low-T chain-branching sequence, which eventually leads to restraining the reactivity of the whole reaction system.

A kinetic investigation on the synergistic low-temperature reactivity, antagonistic RON blending of high-octane fuels: Diisobutylene and cyclopentane

A synergistic effect on low-temperature autoignition reactivity of blending two high-octane compounds, i.e. cyclopentane and diisobutylene, was observed in engine experiments and ignition delay time measurements at 700–880 K and up to 25 bar with a rapid compression machine. Results show that the low-temperature ignition delay and RON of a cyclopentane-diisobutylene blend are inferior to those of its isolated blendstocks, therefore exhibiting an increased reactivity towards ignition at the tested conditions. Sampling and speciation of the reacting mixture during the ignition delay of pure compounds and the most reactive blend was conducted to support discussions focused on the kinetic perspective to this phenomena, which suggest that the separate chain initiation pathways of these potential high-octane blendstocks can interact with each other to help the radical pool growth at the beginning stage of the autoignition chemistry, and finally accelerate the propagation and branching pathways to promote the global reactivity. This result in significant two-stage ignition behavior for the blend, which is not observed at this temperature for both isolated blendstocks. These findings are especially meaningful for designing and blending suitable high-octane fuels for advanced combustion mode engines.