Negative Temperature Coefficient Region Research Articles

Development and verification of RP-3 aviation kerosene surrogate fuel models using a genetic algorithm

For emulation of the physicochemical characteristics and hydrogen-carbon group composition of RP-3 aviation kerosene, two models were composed of three components including n-dodecane, n-butylcyclohexane and n-butylbenzene by genetic algorithm optimization methodology with physical properties measurements (S1, 27.9/63.0/9.1 mol%; S2, 58.0/30.0/12.0 mol%). The numerical results of properties using the surrogate fuels were compared with experimental data. Both models had good performance in predicting the physical properties by analyzing the trends of density and kinematic viscosity under different temperature and pressure conditions. Based on the proposed surrogates, a detailed RP-3 surrogate chemical reaction mechanism was developed by decoupling methodology which including 1408 species and 8965 reactions. Autoignition properties, oxidation properties and laminar combustion properties of surrogates were compared with RP-3 experimental data. Two models exhibited good matching on ignition behaviors with RP-3 at high and low temperatures. Moreover, the components proportion difference had a great influence on the ignition properties in the negative temperature coefficient region. It also made the predicted value of the laminar combustion velocity of S1 higher than that of S2.

Biphasic sensitization effect of NO2 on n-C4H10 auto-ignition

NOx in the exhaust gas can influence the auto-ignition properties, thus are important for the design and operation of HCCI engines and predicting engine knock in spark ignition engines. More work is clearly required to improve the understanding on fuel/NOx chemistry for both reaction mechanism construction and engine application. In this study, n-C4H10, as a representative hydrocarbon fuel with the negative temperature coefficient (NTC) behavior, was selected for test. Ignition delay times of n-C4H10/air mixtures with varying NO2 addition (0, 500 ppm, 1%, and 5%) have been measured behind reflected shock waves. The sensitization effect of NO2 on n-C4H10 auto-ignition was investigated at engine relevant conditions covering temperatures from 700 to 1200 K, pressure of 20 atm, and equivalence ratios from 1.0 to 2.0. Besides, the mixtures with excessive amounts of NO2 (5% and 10%) were also studied at relatively high temperatures and 10 atm, aiming at highlighting the interactions between NO2 and n-C4H10. Results indicated the effect of NO2 on n-C4H10 oxidation exhibits prominent temperature and NO2 concentration dependences. Trace NO2 addition (500 ppm) promotes low-temperature reactivity of n-C4H10 and reduces the ignition delay times. While higher levels of NO2 no longer promote ignition, instead, it turns gradually to be an inhibiting effect. At high temperatures, NO2 dramatically accelerates the n-C4H10 oxidation, and the promoting effect peaks at around 5% NO2 then saturates with further NO2 addition. In the negative temperature coefficient region, the NTC behavior of n-C4H10 is weakened and even disappears by the introduction of NO2. A preliminary detailed kinetic model was proposed to describe the n-C4H10/NO2 chemistry, and in general, is capable of capturing the NO2 sensitization behaviors under different conditions. Kinetic analyses were then performed to clarify the sensitization mechanisms of NO2 and the chemical interactions between NO2 and n-C4H10.

Is ortho-Cresol a Viable Lignocellulosic Blendstock? A Kinetic Study of Its Co-Oxidation within a Surrogate Fuel

The viability of the use of ortho-cresol as a bio-blendstock or antiknock additive from lignocellulosic biomass is assessed; Ignition delays of ortho-cresol within blends with iso-octane are measured with the ULille rapid compression machine, and compared with results from the literature; It is shown that ortho-cresol has a strong inhibiting effect on the reactivity towards ignition, most notably in the Negative Temperature Coefficient region; This effect is found to originate from competition with iso-octane on the OH radicals, where the reactivity of ortho-cresol with these radicals does not lead to radical chain-branching.

A Theoretical Study on Cool Flame Oxidation as an Effective Way for Fuel Reforming: Emphasis on Ignition Characteristics and Chemical Analysis

ABSTRACT Cool flame oxidation of hydrocarbon fuels produces substantial intermediates with various levels of chemical reactivity, which can be used for combustion control of advanced engines with low-temperature combustion. Previous investigation has evaluated the performance of low-temperature reforming under high fuel-rich conditions, whereas the potential of a broader scope of reforming conditions (especially fuel-lean scenarios) is not clear. In this work, cool flame oxidation as a reforming way was proposed to investigate the effect of reforming products from various reforming conditions on ignition characteristics of typical operating conditions, and the role of the reforming products from typical reforming conditions on ignition characteristics of different operating conditions was clarified under engine-relevant conditions. The results show that the reforming products via cool flame oxidation are more sensitive to reforming temperature than pressure and equivalence ratio. Residence time also plays an important role in reforming products, and depending on the oxidation degree, two kinds of reforming products can be identified, i.e. mild reforming and severe reforming, manifesting distinct chemical reactivity. Meanwhile, it is found that different from the previous findings of very fuel-rich reforming conditions, the ignition delay time of different blends is significantly shortened under most operating conditions, except for some scenarios (e.g. the negative temperature coefficient region) manifesting a “cross-point” behavior. Moreover, the promotion on ignition processes becomes enhanced under higher temperature, higher pressure, leaner mixture, and severe reforming conditions. Globally, cool flame oxidation as a reforming way can reduce the ignition delay time by 30 ~ 50% under engine-relevant conditions. The current work can provide new perspectives of cool flame oxidation as an effective way to control advanced engine combustion processes.

Diastereomers and Low-Temperature Oxidation

Diastereomers have historically been ignored when building kinetic mechanisms for combustion. Low-temperature oxidation kinetics, which continues to gain interest in both combustion and atmospheric communities, may be affected by the inclusion of diastereomers in radical chain-branching pathways. In this work, key intermediates and transition states lacking stereochemical specification in an existing diethyl ether low-temperature oxidation mechanism were replaced with their diastereomeric counterparts. Rate coefficients for reactions involving diastereomers were computed with ab initio transition state theory master equation calculations. The presence of diastereomers increased rate coefficients by factors of 1.2-1.6 across various temperatures and pressures. Ignition delay simulations incorporating these revised rate coefficients indicate that the diastereomers enhanced the overall reactivity of the mechanism by almost 15% and increased the peak ketohydroperoxide concentration by 30% in the negative temperature coefficient region at combustion-relevant pressures. These results provide an illustrative indication of the important role of stereomeric effects in oxidation kinetics.

Flame propagation and auto-ignition behavior of iso-octane across the negative temperature coefficient (NTC) region on a rapid compression machine

Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε-ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε-ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture.

CFD modeling of pre-spark heat release in a boosted direct-injection spark-ignition engine

Accurate predictions of low-temperature heat release (LTHR) are critical for modeling auto-ignition processes in internal combustion engines. While LTHR is typically obscured by deflagration, extremely late ignition phasing can lead to LTHR prior to the spark, a behavior known as pre-spark heat release (PSHR). In this research, PSHR in a boosted direct-injection spark-ignition engine was studied using 3-D computational fluid dynamics (CFD) and detailed chemical kinetics. The turbulent combustion was modeled via a hybrid approach that incorporates the G-equation model for tracking the turbulent flame front, and the well-stirred reactor model with detailed chemistry for assessing the low-temperature reactions in unburnt gas. Simulations were conducted using Co-Optima alkylate and E30 fuels at operating conditions characterized by different PSHR intensities. The predicted in-cylinder pressure and heat release rate were found to agree well with experiments. It was found the estimate of previous-cycle trapped residuals is of utmost importance for capturing PSHR correctly. A simulation best practice was developed which keeps the detailed chemistry solver active throughout the entire simulation, allowing to track the evolution of intermediate species from one cycle to the next. Following the validation, the dynamics of PSHR were analyzed in detail employing the pressure-temperature (P-T) trajectory framework. It was shown that PSHR correlated with the first-stage ignition delay of the fuel, hence showing close relation to the in-cylinder P-T trajectory and the chemical kinetics. Besides, it was indicated that LTHR is a self-limiting process that has the effect of attenuating the thermal stratification in the combustion chamber. Furthermore, it was observed the occurrence of PSHR caused the P-T trajectory of end-gas to overlap with the negative temperature coefficient region of the fuel’s ignition-delay maps. This effect was more significant in the fuel-rich regions where engine knock tendency would be generally higher, with potential implications on knock control and mitigation.

Exploring the interaction kinetics of butene isomers and NOx at low temperatures and diluted conditions

The oxidation of 1-butene and i-butene with and without addition of 1000 ppm NO was experimentally and numerically studied primarily at fuel-rich (ϕ = 2.0) conditions under high dilution (96% Ar) in a flow reactor operated at atmospheric pressure in the low temperature range of approximately 600-1200 K. Numerous intermediate species were detected and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). An elementary-step reaction mechanism consisting of 3996 reactions among 682 species, based on literature and this work, was established to describe the reactions and interaction kinetics of the butene isomers with oxygen and nitrogenous components. Model predictions were compared with the experimental results to gain insight into the low- and high-temperature fuel consumption without and with NO addition and thus the respective interaction chemistry. This investigation firstly contributes a consistent set of temperature-dependent concentration profiles for these two butene isomers under conditions relevant for engine exhaust gases. Secondly, the observed oxidation kinetics is significantly altered with the addition of NO. Specifically, NO promotes fuel consumption and introduces for i-butene a low-temperature behavior featuring a negative temperature coefficient (NTC) region. The present model shows reasonable agreement with the experimental results for major products and intermediate species, and it is capable to explain the promoting effect of NO that is initiated by its contribution to the radical pool. Further, it can describe the observed NTC region for the i-butene/NO mixture as a result of the competition of chain propagation and chain terminating reactions that were identified by reaction flow and sensitivity analyses.

Roles of alkanes, alcohols, and aldehydes for crude oil oxidation during the air injection process

Although the air injection process (AIP) has been applied for enhanced oil recovery purposes for many decades now, the oxidation mechanism behind the exotherm has not yet been widely discussed. Previous methane oxidation studies have stated that hydrocarbon oxidation is a chain reaction. Four types of hydrocarbons have been proposed to significantly affect the hydrocarbon oxidation, namely, alkanes, alkyl peroxides, alcohols, and aldehydes. Therefore, the functional groups of hydrocarbons might be the key to understanding the oxidation behaviors of crude oil during the AIP.This study used an accelerated-rate calorimeter to investigate the self-heating behaviors of three types of hydrocarbons and their corresponding mixtures with crude oil under high pressure. n-Decane, 1-decanol, and 1-decanal were selected for use herein because of their availability to represent the self-heating behaviors of the hydrocarbon types. The oxidation behaviors of hydrocarbons with a longer hydrocarbon chain can be explained by analogy with the results of previous methane oxidation studies; thus, the enthalpy from the low-temperature oxidation mainly comes from the oxidation process from alkanes to alcohols. The negative temperature coefficient region is mainly caused by the reactions that turn alcohols into aldehydes. Aldehydes are the direct fuel for high-temperature oxidation and have the lowest activation energy among the three tested types of hydrocarbons. More aldehydes in the crude oil should be favorable for the AIPs in low-temperature reservoirs.

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.

An experimental and kinetic modeling study of the ignition delay and heat release characteristics of a five component gasoline surrogate and its blends with iso‐butanol within a rapid compression machine

AbstractThis study investigates the performance of a five component gasoline surrogate (iso‐octane, toluene, n‐heptane, 1‐hexene, and ethanol) in representing the ignition delay time (IDT) behavior of gasoline (reference gasoline PR5801—research octane number 95.4, motor octane number 86.6), at conditions of 675–870 K, 20 bar, and Ф = 1 (stoichiometric) within a rapid compression machine (RCM). Experimentally, the surrogate produces a good representation of the ignition behavior of the gasoline at these conditions, displaying a similar IDT profile. The influence of blending with iso‐butanol on the surrogate's ignition delay behavior is also investigated, at blends from 5% to 70% of iso‐butanol by volume. The surrogate continues to produce a reasonable representation of the experimental IDTs of gasoline and iso‐butanol blends, except under a high degree of iso‐butanol blending (50% iso‐butanol), where the surrogate produced longer IDTs, particularly at temperatures below 740 K. Blends of 5% and 10% iso‐butanol produce IDTs shorter than that of any other blend, including the “neat” surrogate, at temperatures of 740–770 and 830 K, respectively. Kinetic modeling of RCM IDTs is performed using CHEMKIN‐PRO (Reaction Design: San Diego, CA, 2011) and a combined mechanism of the Sarathy et al. butanol isomers mechanism (Progress in Energy and Combustion Science 2014; 44: 40–102) and Lawrence Livermore National Laboratories “Gasoline Surrogate” mechanism (Proceedings of the Combustion Institute 2011; 33(1): 193–200). The model produces good IDT predictions below 740 K but overpredicts reactivity in the negative temperature coefficient region. Heat release rate analysis is conducted for experimental and modeling results to investigate low‐temperature heat release (LTHR) behavior. Simulations largely fail to accurately reproduce this behavior. This analysis, combined with local OH and brute force Δhf sensitivity analyses, indicates the significance of LTHR in the determination of IDTs and provides RCM heat release rates for future model validation.

Analysis of the Ignition Behavior Based on Similarity Factor Method

The chemical kinetics mechanism is an important factor to accurately predict the combustion characteristics of constant-volume bomb (CVB). In this study, an n-heptane oxidation mechanism constructed by Wang et al. is introduced to study the correlation of the ignition behaviors with the mechanism constructed by Chang et al. The effects of the similarity factor method in the analysis of ignition behaviors of fuel in CVB were repeatedly verified by changing the important spraying parameters: injection pressure and hole diameter. Through further verification, it was found that the combustion process was controlled at approximately 850 K and stoichiometric ratio mixture of fuel/air in CVB, which corresponds to the negative temperature coefficient region at stoichiometric ratio mixture in shock tube (ST). The mechanism verified by the experiment under the condition in ST can reflect the chemical ignition in CVB. In addition, the similarity factor method was less dependent on the chemical reaction mechanism and boundary conditions.

Measurements and simulations of ignition delay times and laminar flame speeds of nonane isomers

Ignition delay times (IDTs) and laminar flame speeds ( S L ) of C 9 H 20 (nonane) isomers are systematically investigated. IDTs of normal nonane (n-C 9 ), 2-methyloctane (2mC 8 ), 2,4-dimethylheptane (24mC 7 ), and 2,2,4,4-tetramethylpentane (2244mC 5 ) are experimetally obtained by a shock tube facility and numerically simulated by a chemkin 0-D reactor model. Further, laminar flame speeds ( S L ) of n-C 9 and 2244mC 5 are measured by spherical expanding flames in a constant-temperature, constant-pressure dual-chamber cruciform burner over a wide range of the equivalence ratio (Φ = 0.7–1.4), which are used to compare with numerically simulated results obtained by chemkin 1-D flame speed model. Detailed reaction mechanisms of KUCRS, LLNL and JetSurF ver.02 are used for numerical simulations. It is found that experimental IDTs increase with the number of methyl branches, especially in low-temperature and negative temperature coefficient (NTC) regions, where the increase of IDT with the number of methyl branches are well predicted by KUCRS. We also find that the measured values of S L of highly branched 2244mC 5 are smaller than those of n-C 9 at all values of Φ studied, of which measured S L data are successfully reproduced by the 1-D flame speed model with KUCRS. These results are important to our understanding of reaction characteristics for highly branched nonane isomers and for the designing of optimal alternative fuels in internal combustion engines.

Speciation and Heat Release Studies during n-Heptane Oxidation in a Motored Engine

ABSTRACT The low-to-intermediate temperature oxidation of n-heptane was investigated by means of speciation and heat release analysis using a motored single-cylinder engine. The experiments were conducted under a fixed engine speed and variable compression ratios in the range of 5.0 – 7.75. A premixed fuel-air mixture corresponding to an equivalence ratio of ϕ = 0.25 was examined for stable intermediate product evolution with varying compression ratios. The experiments focus on pre-ignition species evolution and avoid explosive autoignition. The progress of reactions was followed from the onset of the low-temperature heat release (LTHR) through the negative temperature coefficient (NTC) region. A Fourier transform infrared spectrometer (FTIR) was used to detect and quantitate carbon monoxide, carbon dioxide, and n-heptane, while a gas chromatograph-mass spectrometer (GC-MS) was used to identify species that evolve between the first and second stages of ignition. A total of fifty-six stable intermediates were identified. In-cylinder pressure measurements were used to carry out a heat release analysis and correlate the global markers such as the first and second stage pressure rise to species evolution. Quantitative concentration measurements were carried out for selected alkenes, aldehydes, ketones, ethers, and alcohols. The relative experimental trend for the production of compounds within each of these chemical classes was obtained. The experimental results for speciation and heat release were compared to numerical results obtained using a detailed chemical kinetic mechanism in the literature.

Effects of initial temperature on ignition and flame propagation of dual-fuel mixture in mixing layer

The study aims to identify the characteristics of ignition and flame propagation in an n-heptane/methane–air dual-fuel mixture based on a specified mixing layer scheme using a group of numerical simulations by incorporating a skeletal chemistry mechanism. The effects of different temperature stratifications and uniform temperature distribution on autoignition kernel development, flame propagation, and flame structure in the dual-fuel combustion have been comprehensively investigated. The results indicate that with the increase in the oxidizer stream temperature, the high-temperature ignition (HTI) location in the mixing layer changes non-monotonously, as determined by the most reactive mixture fraction in the mixture fraction–temperature (ξ−T) space obtained from zero-dimensional simulation. The HTI points located in the negative temperature coefficient (NTC) region shift to a richer mixture with temperature increase, whereas the points located in the high temperature region shift to the smaller mixture fraction region. Additionally, the results demonstrate that for the ignition points in the NTC region, the shift to the richer mixture weakens the combustion rate, resulting in a prolonged time interval for steady flame propagation although the oxidizer stream temperature is increased. Furthermore, the low-temperature ignition (LTI) front propagation is driven by the gradient of the ignition delay time, and the HTI propagates as a spontaneous ignition front at first and then transforms to a diffusion-driven flame. The results also demonstrate that the maximum temperature of the LTI propagates to the richer mixture with temperature stratification, in contrast to that with a uniform temperature. Lastly, the comparative analyses of chemical explosion mode (CEM) and flame structure are conducted for two representative cases. A wide non-CEM region induced by low-temperature chemistry (LTC) occurs in the oxidizer stream when its temperature is relatively lower. And double peaks of heat release rate (HRR), that represent the HTI of n-heptane and post-combustion of methane respectively, are observed within the ignition front at the oxidizer side.

Data-driven chemical kinetic reaction mechanism for F-24 jet fuel ignition

A data-driven chemical kinetic mechanism for the military version of Jet A, F-24, is developed for numerical simulations of the ignition process. The main purpose of this study is to obtain a practical F-24 mechanism across wide temperature and equivalence ratio ranges, with a particular focus on the negative temperature coefficient and low temperature regions. The new mechanism (ARLMech-HC-F24) is based on the HyChem model of a similar fuel and optimized using a micro-genetic algorithm against an experimental ignition delay data set of the target fuel. The development and optimization processes include reaction selection, population creation, shuffled tournament implementation based on a merit function, and child-individual creation for the next generation. Several techniques and parameters are proposed to generate an accurate mechanism through an efficient process. The newly introduced data-driven mechanism based on these techniques shows better merit value convergence and represents the ignition behavior more accurately than that without the techniques. This practical mechanism is suitable for the numerical simulations of the F-24 or Jet A ignition problem, and the suggested strategies can be employed in similar problems of rate coefficients estimation.

Mechanism of Methane Addition Affects the Ignition Process of n-heptane under Dual Fuel Engine-Like Conditions

for saving energy and protecting the environment, natural gas has been widely used in internal combustion engines, which makes the study on the ignition characteristics of natural gas/diesel mixtures important. In this work, the effects of trace methane addition on the ignition delay of n-heptane/air mixtures are numerically studied using a detailed n-heptane mechanism under marine engine-like conditions. The simulations are carried out based on the software CHEMKIN-PRO 18.0 with a closed homogeneous reactor. Results show that the prolonged ignition delay times (IDs) of n-heptane/air mixtures are observed over the whole initial temperature range after methane is added, and the increment of IDs in the negative temperature coefficient (NTC) region is significantly higher than that in high temperature region. The sensitivity analysis indicates that both inhibition and promotion effects of important elementary reactions on n-heptane oxidation are weakened because of methane addition. However, the weakening influence on the promoting effect is more prominent. In addition, the inhibition effect of some elementary reactions that are related to the methane oxidation is enhanced. Thus, the IDs of n-heptane/air mixture are prolonged. The analyses of reaction rate of production (ROP) show that the both the production and consumption rates of key radicals decrease significantly in NTC region after methane is added, but it is negligible in the high temperature region. The study can extend the theoretical basis of ignition characteristics of methane/n-heptane blends under elevated temperatures and pressures.

Experimental and numerical investigation of the promoting effect of a cetane booster in a low-octane gasoline fuel in a rapid compression machine: A study of 2-ethylhexyl nitrate

Modern societies require cleaner and more efficient internal combustion engines. Low-temperature combustion (LTC) has been proved to be a good step toward this goal. This study aims at investigating the promoting effect of a cetane booster additive named 2-ethylhexyl nitrate (EHN) on the reactivity of a low-octane gasoline at LTC-relevant conditions. Rapid compression machine experiments were conducted at 10 bar, from 675 to 960 K for stoichiometric mixtures. The neat fuel was a mixture of toluene and n-heptane whose research octane number is 84. The doping levels of the additive were set at 0.1 and 1% molar basis. At the experimental conditions, it is found that EHN provides a promoting effect on the surrogate reactivity over all the whole temperature range. This effect increases with EHN doping levels. The negative temperature coefficient (NTC) behavior of the surrogate fuel is mitigated by the presence of the additive. The EHN reactivity promoting effect is lowest around 710 K and then increases with temperature. Under some conditions, heat releases are observed during the compression process. The chemical reactivity of the fuel gas mixture during the piston movement has to be considered to get reliable simulations. Kinetic modeling works show a good agreement with experiments. The model of this study reproduces properly the EHN promoting effect over the whole range of investigated temperatures and doping levels. Numerical analyses were conducted. EHN can totally decompose during the compression process resulting in heat releases. EHN is less effective at low Tc (< 800 K) at lean condition than at stoichiometric condition. It is found that the EHN effect links to the OH radical formation and the NO2NO loop. The reactions between NO and n-heptyl peroxy radicals are found to be the main reason for the EHN effect in NTC region of the surrogate fuel oxidation.

The influence of pressure and equivalence ratio on the NTC behavior of methane

Methane based polygeneration processes in piston engines offer the possibility of a controllable and flexible conversion of energy, to up-convert low value chemicals and to store energy. These processes preferably take place under fuel-rich conditions and at high pressures. Under fuel-rich conditions, there was one experimental report that a distinctive negative temperature coefficient (NTC) behavior occurs in methane oxidation (Petersen et al., 1999). To design a polygeneration process, reliable kinetic models are required to capture the impact of pressure and equivalence ratio variations on reactivity of the gas mixtures. Here, the experimental basis for methane oxidation is expanded to high pressures and very fuel-rich conditions and compared to literature models, both with special emphasis on the NTC behavior. The oxidation of methane/oxygen mixtures at 2 ≤ Φ≤ 20 and pressures ranging from 1 to 20 bar is investigated. The literature reaction mechanisms are assessed with respect to their ability to predict this phenomenon and used to identify reaction pathways. It is found that NTC behavior occurs in a temperature range between 700 and 1000 K and at pressures higher than 5 bar. The lower temperature limit is slightly shifted towards higher temperatures with decreasing equivalence ratio. In addition, the higher the equivalence ratio, the broader the pressure range, in which the NTC behavior is observed. In general, predictions of some models are in good agreement with the experimental data. Reaction path analyses reveal that the competition between oxidation and recombination pathways are responsible for the NTC region in methane oxidation.

Exploring the low-temperature oxidation chemistry of 1-butene and i-butene triggered by dimethyl ether

In order to better understand the low-temperature oxidation chemistry of alkenes, 1-butene and i-butene oxidation experiments triggered by dimethyl ether (DME) were conducted in a jet-stirred reactor at 790 Torr, 500–725 K and the equivalence ratio of 0.35. Low-temperature oxidation intermediates involved in alcoholic radical chemistry and allylic radical chemistry were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). To better interpret the experimental data, a kinetic model was proposed based on our low-temperature oxidation model of DME and comprehensive oxidation models of 1-butene and i-butene in literature. Based on present experimental results and modeling analysis, alcoholic radical chemistry initiated by OH addition is mainly responsible for the low-temperature chain propagation of butenes, since the Waddington mechanism plays a dominant role compared with the chain-branching pathways through the second O2 addition. Allylic radical+HO2 reactions producing alkenyl hydroperoxides and fuel+O2 serve as the major chain-branching and chain-termination pathways, respectively, and they are competitive in the negative temperature coefficient (NTC) region. In contrast, chain-branching pathways originating from allylic radical+O2 and alkyl-like radical+O2 reactions have little contribution to the OH formation. Comparison with the simulation results of butane/DME mixtures demonstrates that butenes can largely inhibit the reactivity of DME at low temperatures due to its reduced low-temperature chain-branching process. However, in the NTC region, butenes may not be good OH absorbents since the allylic radicals can convert HO2 to OH and consequently enhance the oxidation reactivity.