Rapid Compression Machine Experiments Research Articles

Autoignition enhancement of methane by admixture of low fraction of acetaldehyde: Simulations and RCM experiments in stoichiometric and rich mixtures

AbstractThe effect of small fractions of acetaldehyde (CH3CHO) on the ignition delay time of methane (CH4) was examined at high pressure. Measurements are reported for the ignition delay time obtained in a rapid compression machine (RCM) at a compression pressure (Pc) of ∼60 bar and temperatures after compression (Tc) in the range 750–900 K for fuel‐air equivalence ratios ϕ in the range 1–4. The results show that mixtures of 2%–5% CH3CHO in CH4 ignite under conditions at which pure methane does not ignite experimentally. The efficiency of acetaldehyde as a promoter seems to be comparable to that of other oxygenated fuels like alcohols and ethers. For comparison with the experimental results, ignition delay times are computed using an updated reaction mechanism and two mechanisms from the literature for CH3CHO oxidation. For most conditions, the simulations using the current mechanism agree with the measurements to within a factor of two. The ignition profile shows a pre‐ignition temperature rise and two‐stage ignition similar to that previously observed in low fractions of dimethyl ether in ammonia; both phenomena are captured by the simulations. Analysis of simulations at constant volume indicates that CH3CHO is oxidized much more rapidly than CH4, producing reactive species that initiate the oxidation of CH4 and generates heat that accelerates oxidation toward ignition. The low‐temperature chain‐branching reactions of CH3CHO are important in the early oxidation of the fuel mixture. Additional simulations were performed for equivalence ratios of ϕ = 1 and 4, at a compression pressure (Pc) of 100 bar and Tc = 750–1000 K. The simulations indicate that CH3CHO has a strong ignition‐enhancing effect on CH4, with small fractions reducing the ignition delay time by up to a factor of 100, depending on the temperature, as compared to pure CH4.

Low-temperature ignition and oxidation mechanisms of tetrahydropyran

Cyclic ethers are relevant as next-generation biofuels and are also combustion intermediates that follow directly from unimolecular decomposition of hydroperoxyalkyl radicals. Accordingly, cyclic ether reactions are crucial to understanding low-temperature oxidation for advanced compression-ignition applications in combustion where peroxy radials are central to degenerate chain-branching pathways. Reaction mechanisms relevant to low-temperature ignition of cyclic ethers contain intrinsic complexities due to competing reactions of carbon-centered radicals formed in the initiation step undergoing either ring-opening or reactions with O2. To gain insight into mechanisms describing tetrahydropyran combustion, ignition delay time and speciation measurements were conducted. The present work integrates measurements below 1000 K of ignition delay times and species profiles in rapid compression machine experiments from 5 – 20 bar, spanning several equivalence ratios, with jet-stirred reactor experiments at 1 bar and stoichiometric conditions. The experiments are complemented with the development of the first chemical kinetics mechanism for tetrahydropyran that includes peroxy radical reactions, including O2-addition to tetrahydropyranyl (Ṙ), HOȮ-elimination from tetrahydropyranylperoxy (ROȮ) and hydroperoxytetrahydropyranyl (Q˙OOH), bi-cyclic ether formation, β-scission of Q˙OOH, and ketohydroperoxide formation.Negative-temperature coefficient (NTC) behavior is exhibited in the experiments and is reflected in the model predictions, which were within experimental uncertainty for several conditions. Disparities between the model predictions and experiments were analyzed via sensitivity analysis to identify contributing factors from elementary reactions. The analyses examine the role of ring-opening products of tetrahydropyranyl and hydroperoxytetrahydropyranyl isomers to identify reaction mechanisms that may contribute to model uncertainties. The detection of 66 species in the JSR experiments indicates that tetrahydropyran undergoes complex reaction networks, which includes interconnected reactions of aldehyde radicals and alkyl radicals. Primary radicals pentanal-5-yl and butanal-4-yl are derived from ring-opening reactions of tetrahydropyran-1-yl and undergo subsequent decarbonylation to form alkyl radicals (1-butyl and 1-propyl) that undergo reaction with O2 and may contribute to chain-branching, in addition to pathways involving tetrahydropyran-derived ketohydroperoxides.

Autoignition of cyclohexane at low-to-intermediate temperatures: Rapid compression machine experiments and improved comprehensive chemical kinetic model

Cycloalkanes are an important class of hydrocarbons found in conventional and alternative transportation fuels. Being one of the simplest practical cycloalkanes, cyclohexane is often used as a surrogate in studies to understand the oxidation, ignition, and sooting characteristics of cycloalkanes more broadly. Although cyclohexane has been extensively studied, very limited datasets for cyclohexane oxidation at low-to-intermediate temperatures are available in the literature. Moreover, the literature autoignition data from rapid compression machines (RCMs) did not provide information on volume histories or experimental heat transfer characteristics which can significantly impact model predictions, thereby hindering the detailed validation of chemical kinetic models. It has also been observed that the cyclohexane kinetic models developed so far are not able to predict the literature data across a wide range of operating and mixture conditions. Hence, further studies are recommended to understand and improve model predictions for cyclohexane oxidation. In this study, RCM experiments were conducted using cyclohexane/air mixtures at varying compressed pressures (PC =15, 20, and 30 bar), equivalence ratios (ϕ=0.5, 1.0, and 2.0), and compressed temperatures (TC=670–960 K). The current RCM results of cyclohexane were found to complement well with the literature data, with the first-stage IDTs at high pressures and volume histories at all test conditions being first reported for better development and validation of chemical kinetic models. In addition, a detailed chemical kinetic model for the cyclohexane oxidation has been developed and validated using both newly acquired experimental IDTs in this study and literature experimental studies (IDTs in RCMs and STs, species profiles in jet-stirred reactors, and laminar burning velocities). Unlike literature models, the proposed model was observed to provide an overall better prediction for experimental results over a wide range of conditions. Chemical kinetic analysis of the developed model was further conducted to identify the reaction pathways controlling the autoignition of cyclohexane.

Experimental and kinetic modeling study of the autoignition and oxidation of ammonia/ethane mixtures in a rapid compression machine and a jet-stirred reactor

Ignition delay times and speciation data of ammonia/ethane (NH3/C2H6) fuel blends were measured in a rapid compression machine (RCM) and in a jet-stirred reactor coupled with molecular beam mass spectrometry (JSR-MBMS), respectively. Specifically, the RCM experiments were conducted at elevated pressures of 20 and 40 bar, intermediate temperatures between 890 and 1110 K, three equivalence ratios of 0.5, 1.0 and 2.0 and for three C2H6 mole fractions from 1% to 10%. Besides, the JSR measurements were conducted at atmospheric pressure, intermediate temperatures between 700 and 1180 K, three equivalence ratios of 0.5, 1.0 and 2.0 and three C2H6 mole fractions from 10% to 50%. The experimental results demonstrate that adding C2H6 into NH3 leads to a more reactive mixture. A kinetic model from our recent work on ammonia/ethanol fuel blends (M. Li et al., Proc. Combust. Inst., 2022) has been updated and validated with the new data obtained in this work. The updated model (PTB-NH3/C2 mech) can reproduce the effects of ethane fraction and equivalence ratio on IDT/speciation and show an overall satisfactory agreement between experimental results and predictions. Based on the kinetic analyses, for all conditions investigated in this study, the generation of OH radicals from HO2 and H radicals dominates the characteristic of the auto-ignition process in the RCM and the NH3 consumption process in the JSR, respectively. Apart from that, the addition of ethane provides additional OH from a relatively low temperature (e.g., around 850 K in a JSR condition) and triggers the interactions between hydrocarbon and ammonia systems, e.g., the reactions of C2H6/C2H4/CH3+NH2, which play an important role in the combustion of ammonia.

Experimental and kinetic modeling study of tetramethylethylenediamine: A promising green propellant fuel

Tetramethylethylenediamine (TMEDA) is a promising green propellant fuel and reactivity promoter. However, the reactions between TMEDA and O2 are usually overlooked under ignition conditions. In this study, significant low-temperature reactivity was observed for 2%TMEDA/O2 mixtures, and autoignition could occur even at 470 K. To probe the chemical kinetics of TMEDA/O2, ignition delay times were measured in a rapid compression machine and a high-pressure shock tube. TMEDA pyrolysis products were also obtained in a single pulse shock tube. The autoignition of TMEDA/O2 in rapid compression machine experiments showed multi-stage heat release, a characteristic which became more obvious at oxygen-lean conditions. During the oxidation experiments, a non-Arrhenius temperature dependence of ignition delay time was observed. A chemical kinetic model of TMEDA was developed hierarchically, based on current reaction kinetics knowledge about hydrocarbons, small amines and NOx. Ignition delay times and pyrolysis products measured in this study, along with experimental data on small amines from the literature, were used to validate the kinetic model, which generally produced good predictions across different temperature, pressure and equivalence ratio conditions.

Exploring low-temperature oxidation chemistry of 2- and 3-pentanone

The low-temperature oxidation chemistry of 2- and 3-pentanone was investigated over a wide range of conditions using a jet-stirred reactor (JSR) and a rapid compression machine (RCM). The JSR oxidation experiment was performed at the pressure of 93.3 kPa over a temperature range of 600-1000 K. Detailed speciation information was obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. Ignition delay times (IDTs) of 2- and 3-pentanone were measured in an RCM from 640 to 820 K at pressures of 15 and 25 bar and an equivalence ratio of 1.0. The two C5 ketones showed NTC behavior and two-stage ignition phenomena. Mole fraction time histories of intermediate species during the two-stage ignition process of both ketones were obtained using a fast-sampling system coupled with gas chromatography. There are distinct differences between 2- and 3-pentanone in species concentration profiles and IDTs. A kinetic mechanism for the low-temperature oxidation of 2- and 3-pentanone was developed, which can satisfactorily predict all available measurements. The reaction path analyses indicate that the intramolecular hydrogen migration reaction of ROO radicals tends to produce resonance-stabilized QOOH radical structures. The secondary oxygen addition reaction of resonance-stabilized QOOH radicals thus is the most important source of OH radicals in the low-temperature oxidation of ketone fuels. The intramolecular hydrogen migration reactions are slowed down in the presence of the carbonyl functional group, which makes the low-temperature reactivity of the two C5 ketones lower than that of n-pentane. The position of the carbonyl functional group affects the species pools during the oxidation of the two ketones to a great extent. Larger production of CH4, C3H6, CH3COCH3, and C2H5CHO were observed in 2-pentanone oxidation, while the production of CH3CHO was favored during 3-pentanone oxidation both in the JSR and RCM experiments. The different lengths of the carbon chain on both sides of the carbonyl group in 2- and 3-pentanone resulted in the difference in the species distribution.

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.

Experimental and modeling study of the low to high-temperature oxidation of the methyl isopropyl ketone in O2/N2/Ar and O2/CO2/Ar atmospheres

Few studies on the low-temperature combustion behavior of MIPK, being a promising fuel additive, have been conducted. In this work, ignition delay times (IDTs) of MIPK were measured in the temperatures ranging between 780–910 K and pressures of 20 and 25 bar using a rapid compression machine (RCM). Oxy-fuel combustion combined with biofuel could remove CO2 from the atmosphere. The IDTs of MIPK were measured in the temperatures ranging between 1125–1600 K under the O2/CO2 atmosphere at the pressures of 1 and 10 bar using a shock tube. A low to high-temperature MIPK kinetic model (HUST-MIPK model) was proposed, in which the low-temperature sub-model consists of 19 low-temperature reaction classes and was constructed by analogy-based method, the high-temperature sub-model was adapted from the works of Cheng et al.. The predictions of HUST-MIPK model are in good agreement with the present low-temperature IDTs, high-temperature O2/CO2 atmosphere IDTs, and the literature experimental data. The negative temperature coefficient (NTC) behavior was not observed in the temperature range from 790 to 910 K in the present RCM experiments, but was observed for methyl propyl ketone (MPK) and diethyl ketone (DEK) under similar conditions. The low-temperature chemistry of three pentanone isomers (MIPK, MPK, and DEK) was compared using the flux and sensitivity analysis. The comparison of the experimental high-temperature IDTs between O2/CO2 and O2/Ar atmospheres indicates the IDTs of MIPK under O2/CO2 atmosphere are longer than those under O2/Ar atmosphere at 1 bar, and the effects of CO2 are almost independent of the pressure. The physical and chemical effects of CO2 on the ignition were studied in detail.

Replicating HCCI-like autoignition behavior: What gasoline surrogate fidelity is needed?

This work seeks to characterize the fidelity needed in a gasoline surrogate with the intent to replicate the complex autoignition behavior exhibited within advanced combustion engines, and specifically Homogeneous Charge Compression Ignition (HCCI). A low-temperature gasoline combustion (LGTC) engine operating in HCCI mode and a rapid compression machine (RCM) are utilized to experimentally quantify fuel reactivity, through autoignition and preliminary heat release characteristics. Fuels considered include a research grade E10 U.S. gasoline (RD5-87), three multi-component surrogates (PACE-1, PACE-8, PACE-20), and a binary surrogate (PRF88.4). Each fuel was studied at lean/HCCI-like conditions covering a wide range of temperatures and pressures that are representative of naturally aspirated to high boost engine operation. Detailed chemical kinetic modeling is also undertaken using a recently updated gasoline surrogate kinetic model to simulate the RCM experiments and to provide chemical insight into surrogate-to-surrogate differences.The LGTC engine experiments demonstrate nearly identical reactivity between PACE-20 and RD5-87 across studied conditions, while faster phasing is seen for both PACE-1 and PACE-8 due to their stronger intermediate- and low-temperature heat release (ITHR/LTHR) at naturally aspirated and boosted conditions, respectively. The RCM experiments reveal typical low-temperature, negative temperature coefficient (NTC) and intermediate-temperature autoignition behaviors at all pressure conditions for RD5-87, which are qualitatively reproduced by all surrogates. Quantitative discrepancies in both autoignition and preliminary heat release are observed for all surrogates, while their ability to replicate RD5-87 autoignition behavior follows the order of PACE-20 > PACE-1 > PACE-8 > PRF88.4. Excellent mapping is obtained between the LGTC engine and the RCM, where the engine pressure-time trajectories can be characterized by the regimes represented by the RCM autoignition isopleths. The kinetic model performs commendably when simulating both autoignition and preliminary heat release of PACE-20, while typically overpredicting ignition delay times for PACE-1, PACE-8 and PRF88.4 at high-pressure and low-temperature/NTC conditions. Sensitivity and rate of production (ROP) analyses highlight surrogate-to-surrogate differences in the governing chemical kinetics where n-pentane initiates rapid OH branching at a faster rate and an earlier timing for PACE-20 than iso-pentane does for PACE-1 and PACE-8, making it computationally more reactive than the other surrogates. The current study highlights the need to include non-standardized properties, such as the lean/HCCI-like autoignition characteristics, in addition to ASTM properties (e.g., RON, MON) as metrics of fuel reactivity and targets to be matched when formulating high-fidelity surrogates that fully capture gasoline advanced combustion behavior such as HCCI-like autoignition.

Modeling the non-Arrhenius ignition behavior of 2‐butanol: A detailed theoretical and experimental study

2-butanol is a promising alternative to fossil fuels for which production pathways are already established and which was proven to be suitable for usage within modern internal combustion engines. It is a potential octane booster for gasoline fuels, can be used in diesel engines to reduce soot production, and is applicable as a stand-alone fuel. In the present study, the previously not discussed non-Arrhenius ignition behavior of 2-butanol was revealed by recalculating and adjusting the thermodynamic properties of the fuel and its most important radicals. All relevant fuel consumption reaction rate coefficients were replaced by a consistent set of analogies without any further modifications of the rate parameters. The existing spectrum of validation targets for 2-butanol was extended by new high-pressure Rapid Compression Machine (RCM) experiments for end-of-compression pressures of peoc = 20, 30, 40, and 50 bar with high resolution of temperature to highlight the non-Arrhenius ignition behavior and to study the model performance in more detail. The kinetic model proposed in the present study can reproduce the observed non-Arrhenius behavior within the RCM regime. Kinetic analysis demonstrated that simulated Ignition Delay Times (IDTs) are very sensitive to the equilibria of the R. + O2 reactions for all 2-hydroxybutyl radicals. With the newly determined thermodynamic properties, the equilibria of the R. + O2 reactions are moved towards lower temperatures, enabling low-temperature chain branching. The shift of the equilibrium from the RO.2 side to R. + O2 within the temperature regime covered by the RCM experiments could be identified as the main reason for the observed non-Arrhenius behavior.

On multi-stage autoignition of n-propylcyclohexane at low temperatures

n-Propylcyclohexane (nPCH) has been widely used as a surrogate component representing alkylated cycloalkanes in conventional transportation fuels. Since the autoignition characteristics of nPCH at low temperatures have not been extensively studied in the literature, a series of rapid compression machine (RCM) experiments were conducted using nPCH/air mixtures over a wide range of engine-relevant conditions. Specifically, ignition delay times were measured over the compressed temperature range of TC = 615‒750 K, compressed pressures of pc = 10‒30 bar, and equivalence ratios of φ = 0.25‒2.0. For the first time, a three-stage ignition phenomenon of nPCH was observed at ultra-fuel-lean conditions (φ = 0.25). Using the chemical kinetic model of nPCH developed in our recent work (Combustion and Flame 238 (2022) 111944), experimental results were simulated to better understand the changes in temperature and heat release rate at each stage of autoignition. In addition, the simulated concentrations of key species (ȮH, HȮ2, CO, and CO2) and values of the heat production rate per reaction were analyzed to identify the controlling chemistry at each stage. The current model can well capture the three-stage, two-stage, and one-stage ignition for different mixtures.Based on experimental pressure traces, heat release rate (HRR) profiles of representative one-, two-, and three-stage ignition cases were derived, showing that the trends of the deduced HRR characteristics agree well with those predicted by RCM simulations using the current model. Moreover, the main heat production chemistry of different stages in the multi-stage ignition is summarized in this work. For the three-stage ignition, the low-temperature chain-branching reactions of nPCH oxidation dominate at the first stage, while the intermediate-to-high temperature chain termination reaction of Ḣ + O2 (+M) 〈=〉 HȮ2 (+M), the chain branching reaction of H2O2 (+M) 〈=〉 ȮH + ȮH (+M), and the reaction sequence of ĊH2CHO → CH2O → HĊO → CO play the major role at the second and third stages. When increasing the equivalence ratio from 0.25 to 0.5, a two-stage ignition is observed and its heat production chemistry is different from those of the three-stage ignition. The reactions related to the Ḣ-atom abstraction reaction from small species by O2 and ȮH radical and the formation reaction of C2 species (CHX1*O2J =〉 Ċ2H3 + CH2CO + C2H4) are dominant at the first stage, while the high-temperature reactions involving the competitive reactions of Ḣ + O2 (+M) 〈=〉 HȮ2 (+M) and Ḣ + O2 〈=〉 Ö + ȮH and the exothermic reaction of CO + ȮH 〈=〉 CO2 + Ḣ play an essential role at the second stage. When pressure increases from 10 bar to 20 bar, only one-stage ignition behavior is observed. The heat production analysis also shows that the high-temperature chemistry is dominated by the Ḣ-atom associated reactions, namely Ḣ + O2 (+M) 〈=〉 HȮ2 (+M) and Ḣ + O2 〈=〉 Ö + ȮH, as well as the CO conversion reactions such as CO + ȮH 〈=〉 CO2 + Ḣ.

Chemical kinetic interactions and sensitivity analyses for 2-ethylhexyl nitrate-doped PRF91 using a reduced mechanism

A numerical and experimental investigation about the chemical kinetic interactions between 2-ethylhexylnitrate (EHN) and PRF91 was performed in this study. Rapid compression machine experiments were conducted to investigate the effect of EHN on the autoignition reactivity of the fuel, and a reduced chemical kinetic mechanism was developed including an EHN sub-model. Experiments showed that the ignition delay decreases as the fuel is doped with EHN, but the effect of the doping level of EHN on the ignition is highly non-linear. Moreover, experiments showed that the EHN effectiveness is lowest during the transition between the low-temperature regime and the negative temperature coefficient (NTC) regime, and it rapidly increases as the temperature increases. Both detailed and (developed) reduced mechanisms were validated against the experimental results, allowing a more in-depth EHN-fuel chemistry study. Additionally, ignition delay sensitivity and EHN effectiveness sensitivity analyses were performed with the reduced mechanism to identify the reactions that control the interaction between EHN and the fuel. As the result, EHN thermal decomposition is only relevant for very low temperatures. The chemistry of EHN-doped fuel is more sensitive to intermediate temperature reactions than that of straight fuel, especially at lower temperatures, due to the effect of EHN on the NTC behavior of the fuel. Finally, the chemistry of EHN-doped fuel is very sensitive to NO2-to-NO reactions, especially at high temperatures, because these reactions transform the NO2 generated by EHN into NO, which is a very effective fuel reactivity enhancer.

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.

Oxidation and pyrolysis of methyl propyl ether

AbstractThe ignition, oxidation, and pyrolysis chemistry of methyl propyl ether (MPE) was probed experimentally at several different conditions, and a comprehensive chemical kinetic model was constructed to help understand the observations, with many of the key parameters computed using quantum chemistry and transition state theory. Experiments were carried out in a shock tube measuring time variation of CO concentrations, in a flow tube measuring product concentrations, and in a rapid compression machine (RCM) measuring ignition delay times. The detailed reaction mechanism was constructed using the Reaction Mechanism Generator software. Sensitivity and flux analyses were used to identify key rate and thermochemical parameters, which were then computed using quantum chemistry to improve the mechanism. Validation of the final model against the 1–20 bar 600–1500 K experimental data is presented with a discussion of the kinetics. The model is in excellent agreement with most of the shock tube and RCM data. Strong non‐monotonic variation in conversion and product distribution is observed in the flow‐tube experiments as the temperature is increased, and unusually strong pressure dependence and significant heat release during the compression stroke is observed in the RCM experiments. These observations are largely explained by a close competition between radical decomposition and addition to at different sites in MPE; this causes small shifts in conditions to lead to big shifts in the dominant reaction pathways. The validated mechanism was used to study the chemistry occurring during ignition in a diesel engine, simulated using Ignition Quality Test (IQT) conditions. At the IQT conditions, where the MPE concentration is higher, bimolecular reactions of peroxy radicals are much more important than in the RCM.

Autoignition study of iso-cetane/tetralin blends at low temperature

iso-Cetane and tetralin are the two fuel components commonly considered in literature diesel surrogates, and are hydrocarbon classes representative of iso-alkanes and naphthoaromatics, respectively. Since both surrogate components are involved in the key reactions controlling the autoignition reactivity of diesel surrogates, autoignition studies of iso-cetane, tetralin, and their blending behavior at low temperatures can be helpful in developing/refining chemical kinetic models of diesel surrogates, in order to better predict diesel ignition response under low temperature combustion conditions. In this investigation, autoignition experiments of iso-cetane, tetralin, and their binary mixtures at varying blending ratios have been conducted in a rapid compression machine (RCM) in the temperature/pressure range of 630–930 K and 10–20 bar. For the neat fuel components, the newly-acquired RCM data have been compared with the literature experimental results at some overlapping conditions. In addition, a chemical kinetic model describing the oxidation of iso-cetane and tetralin compiled/updated from the diesel surrogate model developed by Lawrence Livermore National Laboratory is used to simulate the experimental datasets. It is shown that this model exhibits good agreement with the experimental results of tetralin and also predicts the total ignition delay times of iso-cetane at lower temperatures reasonably well. However, the current model fails to fully capture the increase of total ignition delay time with increasing amount of tetralin in the binary blends observed in the present RCM experiments. Therefore, chemical kinetic analyses of the current model are conducted to help identify possible reasons leading to the discrepancies between simulations and experiments. Furthermore, the chemical kinetic interactions between iso-cetane and tetralin are explored by including additional cross-reactions among iso-cetane, tetralin, and their fuel fragments in the current model to assess their effects on model predictions. Based on the chemical kinetic analysis results, future direction for model refinements is discussed.

Experimental investigation of auto-ignition of ethylene-nitrous oxide propellants in rapid compression machine

Considering the available experimental investigations of auto-ignition behaviors of N2O-C2 hydrocarbons propellants are limited in high-temperature regime, in the work the ignition delay times for N2O-C2H4 propellants have been measured under the conditions of TC = 885–940 K, PC = 2.5–4.3 MPa, φ = 1.05 and 1.35 using the rapid compression machine experiments. Then the relational expression between ignition delay time and condition factors is derived as lgτig=11.781000/Tc-0.26Pc-0.83φ-9.41 by linear fitting. The expression can be used to predict ignition delay times of N2O-C2H4 propellants in practice. Moreover, the calculated ignition delay times by three kinetic models are compared with experimental data. The results show that these models can well predict ignition behaviors at high temperature regime, but fail at low-temperature regime. Through temperature sensitivity analysis, at high-temperature regime the N2O decomposition reaction has the highest sensitivity coefficient and dominated the overall reaction. While at low-temperature regime, the oxidation reactions of N2O and hydrocarbons become more important steps. For better prediction of low-temperature ignition behaviors, the low-temperature oxidation mechanism of N2O and hydrocarbons needs further development, and the kinetic parameters of the highly sensitive reactions should be improved.

Chemical effects of ferrocene and 2-ethylhexyl nitrate on a low-octane gasoline: An experimental and numerical RCM study

An in-depth understanding of fuel additives chemical effects is crucial for optimal use or additive design dedicated to more efficient and cleaner combustion. This study aims at investigating the effect of an organometallic octane booster additive named ferrocene on the combustion of a low-octane gasoline at engine-relevant conditions. Rapid compression machine experiments were carried out at 10 bar, from 675 to near 1000 K for stoichiometric (Φ = 1) and lean (Φ = 0.5) mixtures. The neat surrogate fuel was a blend of toluene and n-heptane whose research octane number was 84. The doping level of additive was set at 0.1% molar basis. Ferrocene does not show a remarkable effect on the 1st- stage ignition but presents a strong inhibiting effect on the main ignition of the surrogate fuel at both equivalence ratios. The inhibiting effect increases with temperature within the investigated range. The negative temperature coefficient (NTC) behavior of the surrogate fuel is enhanced by ferrocene. A kinetic model developed by literature data assembly as well as a novel sub-mechanism involving the formation of alcohols from the reactions of iron species is proposed. The kinetic model developed simulates the inhibiting effect of ferrocene reasonably well at both equivalence ratios. Thanks to the validated kinetic model, the chemical effect of ferrocene on the fuel combustion is explored and compared with 2-ethylhexyl nitrate (EHN), which is a conventional reactivity enhancer. Three major differences between the two additives were identified: the high-temperature stability of the fuel additive, the influence of additive on the toluene reactivity and the effect of the additive on the NTC behavior. The results presented in this study contribute to the in-depth comprehension of chemical effect of two fuel additives (ferrocene and EHN) having opposite effects on fuel reactivity.

Oxidation of di-n-butyl ether: Experimental characterization of low-temperature products in JSR and RCM

The oxidation of di-n-butyl-ether (DBE) was performed in a jet-stirred reactor (JSR) at 1 atm, 520 and 530 K, and from 480 to 670 K at 10 atm, at a residence time of 1 s, an equivalence ratio of 0.5, and an initial fuel concentration of 5000 ppm. Ignition experiments on DBE/air mixtures were also performed in a rapid compression machine (RCM) under stoichiometric conditions, 5 bar, and from 550 to 630 K. Low-temperature products formed in JSR and RCM experiments were characterized. To this end, high-resolution mass spectrometry analyses (HRMS) with flow injection analyses or ultra-high pressure liquid chromatography coupling were used to characterize hydroperoxides and diols (C8H18O3), ketohydroperoxides (C8H16O4), carboxylic acids (C2H4O2, C3H6O2, C4H8O2), di-keto ethers (C8H14O3), and highly oxygenated molecules (C8H14O5, C8H16O6, C8H14O6, C8H14O7, C8H16O8, and C8H14O9) resulting from up to five O2 additions on fuel's radicals. Whereas HOMs are of minor importance in combustion, they are considered key species for the formation of secondary organic aerosols. In addition, cyclic ethers and esters (C8H16O2) were observed. Heated electrospray and atmospheric pressure chemical ionizations (HESI and APCI) were used in positive and negative modes for MS analyses. H/D exchange with D2O was used to confirm the presence of –OH or –OOH groups in the products. The present results show that DBE oxidation proceeds similarly under JSR and RCM conditions. Whereas the CH2 groups neighboring the ether group are the most favorable sites for H-atom abstraction reactions, speciation indicated that other sites can react by metathesis forming a large pool of intermediates and products. Our kinetic reaction mechanism needs to be extended for simulating the formation of newly detected species.

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 impact of the third O2 addition reaction network on ignition delay times of neo-pentane

We studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O2 addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O2 addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O2 addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O2 addition reactions.