Autoignition of gasoline and its surrogates in a rapid compression machine

The analysis and interpretation of the combustion chemistry is greatly simplified by using simple mixtures of pure components, referred to as surrogates, in lieu of fully-blended transportation fuels, such as gasoline. Recognizing that the ability to model autoignition chemistry is critical to understanding the operation of Homogeneous Charged Compression Ignition engines, this work is an attempt to experimentally and computationally assess the autoignition responses of research grade gasoline and two of its proposed surrogates reported in the literature using a rapid compression machine (RCM), for the low-to-intermediate temperature range and at high pressures. The first surrogate studied is a three-component mixture of iso-octane, n-heptane, and toluene. The second is a four-component mixture that includes an olefin (2-pentene), in addition to the ones noted above. Ignition delay times of stoichiometric mixtures, for gasoline and the two surrogates in air, are measured using an RCM for pressures of 20 and 40 bar, and in the temperature range of 650–900 K. The four-component surrogate is found to emulate the ignition delay times of gasoline more closely when compared to the three-component surrogate. Additionally, the experimental data are compared against the computed results from a recently developed surrogate model for gasoline combustion. A good agreement between the experimental and computed results is observed, while discrepancies are also identified and discussed.     

High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels

Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p₅ = 10, 20, and 40 bar) for equivalence ratios ? = 0.5 and 1.0 and over a wide temperature range 690 K < T₅ < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, ? = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated.     

Experimental and modeling study of the autoignition characteristics of commercial diesel under engine-relevant conditions

Knowledge of the autoignition characteristics of diesel fuels is of great importance for understanding the combustion performance in engines and developing surrogate fuels. Here ignition delays of China's stage 6 diesel, a commercial fuel, were measured in a heated rapid compression machine (RCM) under engine-relevant conditions. Gas-phase autoignition experiments were carried out at equivalence ratios ranging from 0.37 to 1.0, under compressed pressures of 10, 15, and 20?bar, and within a temperature range of 685–865?K. In all investigated conditions, negative temperature coefficient (NTC) behavior of the total ignition delays is observed. The autoignition of the diesel fuel exhibits pronounced two-stage characteristics with strong low-temperature reactivity. Experimental results indicate that the total ignition delays shorten with increasing compressed pressure, oxygen mole fraction and fuel mole fraction. The first-stage ignition delays are mainly controlled by compressed temperature and also affected by oxygen mole fraction and compressed pressure but show a very weak dependence on fuel mole fraction. Correlations describing the first-stage ignition delay and the total ignition delay were proposed to further clarify the ignition delay dependence on the multiple factors. Additionally, it is found that the newly measured ignition delays well coincide with and complement the diesel ignition data in the literature. A recently developed diesel mechanism was used to simulate the diesel autoignition on the RCM. The simulation results are found to agree well the experimental measurements over the whole temperature ranges. Species concentration analysis and brute force sensitivity analysis were also conducted to identify the crucial species and reactions controlling the autoignition of the diesel fuel.     

An experimental and modeling study of the shock tube ignition of a mixture of n-heptane and n-propylbenzene as a surrogate for a large alkyl benzene

Alkyl aromatics are an important chemical class in gasoline, jet and diesel fuels. In the present work, an n-propylbenzene and n-heptane mixture is studied as a possible surrogate for large alkyl benzenes contained in diesel fuels. To evaluate it as a surrogate, ignition delay times have been measured in a heated high pressure shock tube (HPST) for a mixture of 57% n-propylbenzene/43% n-heptane in air (≈21% O₂, ≈79% N₂) at equivalence ratios of 0.29, 0.49, 0.98 and 1.95 and compressed pressures of 1, 10 and 30 atm over a temperature range of 1000–1600 K. The effects of reflected-shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. A combined n-propylbenzene and n-heptane reaction mechanism was assembled and simulations of the shock tube experiments were carried out. The simulation results showed very good agreement with the experimental data for ignition delay times. Sensitivity and reaction pathway analyses have been performed to reveal the important reactions responsible for fuel oxidation under the shock tube conditions studied. It was found that at 1000 K, the main consumption pathways for n-propylbenzene are abstraction reactions on the alkyl chain, with particular selectivity to the allylic site. In comparison at 1500 K, the unimolecular decomposition of the fuel is the main consumption pathway.     

Ignition behavior of pure and blended methyl octanoate,n-nonane,and methylcyclohexane

Excited-state species profiles and ignition delay times were obtained under dilute conditions (99% Ar) using a heated shock tube for methyl octanoate (C₉H₁₈O₂), n-nonane (n-C₉H₂₀), and methylcyclohexane (MCH) over a broad range of temperature and equivalence ratio (? = 0.5, 1.0, 2.0) at pressures near 1 and 10 atm. Measurements were then extended to include two ternary blends of the fuels using 5% and 20% (vol.) of the methyl ester under stoichiometric conditions. Using three independently validated chemical kinetics mechanisms, a model was compiled to assess the influence of methyl ester concentration on ignition delay times of the ternary blends. Under near-atmospheric pressure, ignition delay times were of the following order for the pure fuels: methyl octanoate < n-nonane < methylcyclohexane. Experimental results indicate that the ignition behavior of the higher-order methyl ester approaches that of the higher-order linear alkane with increased pressure regardless of equivalence ratio. Methyl octanoate also displayed significantly lower pressure dependence relative to the linear alkane and the cycloalkane species. Both of these results are supported by model calculations. Blending of methyl octanoate with n-nonane and methylcyclohexane impacted ignition delay time results more strongly at 1.5 atm, yet had only a small effect near 10 atm for temperatures above 1400 K. The study provides the first shock-tube data for a ternary blend of a linear alkane, a cycloalkane, and a methyl ester. Ignition delay time measurements for the C₉:0 methyl ester were also measured for the first time.     

Autoignition characteristics of bio-based fuels,farnesane and TPGME,in comparison with fuels of similar cetane rating

Bio-based alternative fuels have received increasing attention with growing concerns about depletion of fossil reserves and environmental deterioration. The development of new combustion concepts in internal combustion engines requires a better understanding of autoignition characteristics of the bio-based alternative fuels. This study investigates two cases of alternative fuels, namely, a kerosene-type fuel farnesane and an oxygenated fuel, TPGME, and compares those fuels with full-boiling range of fuels with similar cetane number. The homogeneous autoignition and spray ignition characteristics of the selected fuels are studied using a modified CFR octane rating engine and a cetane rating instrument, respectively. When comparing farnesane with a full-boiling range counterpart (HRJ8), their similar cetane ratings result in comparable combustion heat release, but the overall ignition reactivity of farnesane is stronger than HRJ8 during the pre-ignition process. Results from a constant volume spray combustion chamber indicate that the spray process of farnesane and HRJ8 strongly influences the overall ignition delay of each fuel. Despite the similar cetane ratings of TPGME and n-heptane, TPGME shows greater apparent low-temperature oxidation reactivity at low compression ratios in the range from CR 4.0-5.5 than n-heptane. A simplified model focused on the key reaction pathways of low-temperature oxidation of TPGME has been applied to account for the stronger low-temperature reactivity of TPGME, supported by density functional theory (DFT) calculations. Regardless of the similar cetane ratings of the fuels, n-heptane and JP-8/SPK lead to similar total ignition delay times, while TPGME shows the shortest overall ignition delay times in the constant volume combustion chamber.     

A comparative reactivity study of 1-alkene fuels from ethylene to 1-heptene

A comparative reactivity study of 1-alkene fuels from ethylene to 1-heptene has been performed using ignition delay time (IDT) measurements from both a high-pressure shock tube and a rapid compression machine, at an equivalence ratio of 1.0 in ‘air’, at a pressure of 30 atm in the temperature range of 600–1300 K. At low temperatures (< 950 K), the results show that 1-alkenes with longer carbon chains show higher fuel reactivity, with 1-pentene being the first fuel to show negative temperature coefficient (NTC) behavior followed by 1-hexene and 1-heptene. At high temperatures (> 950 K), the experimental results show that all of the fuels except propene show very similar fuel reactivity, with the IDTs of propene being approximately four times longer than for all of the other 1-alkenes. To analyze the experimental results, a chemistry mechanism has been developed using consistent rate constants for these alkenes. At 650 K, flux analyses show that hydroxyl radicals add to the double bond, followed by addition to molecular oxygen producing hydroxy?alkylperoxy radicals, which can proceed via the Waddington mechanism or alternate internal H-atom isomerizations in chain branching similar to those for alkanes. We have found that the major chain propagation reaction pathways that compete with chain branching pathyways mainly produce hydroxyl rather than hydroperoxyl radicals, which explains the less pronounced NTC behavior for larger 1-alkenes compared to their corresponding alkanes. At 1200 K, flux analyses show that the accumulation of hydroperoxyl radicals is important for the auto-ignition of 1-alkenes from propene to 1-heptene. The rate of production of hydroperoxyl radicals for 1-alkenes from 1-butene to 1-heptene is higher than that for propene, which is due to the longer carbon chain facilitating hydroperoxyl radical formation via more efficient reaction pathways. This is the major reason that propene presents lower fuel reactivity than the other 1-alkenes at high temperatures.     

Mass spectrometric investigation of the low-temperature dimethyl ether oxidation in an atmospheric pressure laminar flow reactor

Low-temperature combustion is a major strategy today to reduce both soot and NO_x emission. Kinetic reaction models for low-temperature combustion which are validated against a wide range of experimental data are necessary e.g. for control purposes or as a basis for subsequent model reduction.In this study, the low-temperature oxidation of dimethyl ether in a highly diluted gas mixture was investigated experimentally in an atmospheric laminar flow reactor. The respective gas composition was analyzed by a time-of-flight mass spectrometer. This technique allows detection of all species simultaneously within the investigated temperature regime. Stoichiometries of ? = 0.8, 1.0, and 1.2 were studied with high temperature resolution in the range of 400–1200 K, and quantitative species mole fraction profiles have been determined.This wide temperature range comprises the different kinetic regimes occurring during the DME oxidation, which have been clearly resolved. The distinct negative temperature coefficient (NTC) region of the system was observed and extensive speciation is available. Special attention is given to species only occurring in the low-temperature region including formic acid and methyl formate.     

Ignition of non-premixed cyclohexane and mono-alkylated cyclohexane flames

Ignition temperatures of non-premixed cyclohexane, methylcyclohexane, ethylcyclohexane, n-propylcyclohexane, and n-butylcyclohexane flames were measured in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 373 K, a local strain rate of 120 s^?1, and fuel mole fractions ranging from 1% to 10%. Using the recently developed JetSurf 2.0 kinetic model, satisfactory predictions were found for cyclohexane, methyl-, ethyl-, and n-propyl-cyclohexane flames, but the n-butylcyclohexane data were overpredicted by 20 K. The results showed that cyclohexane flames exhibit the highest ignition propensity among all mono-alkylated cyclohexanes and n-hexane due to its higher reactivity and larger diffusivity. The size of mono-alkyl group chain was determined to have no measurable effect on ignition, which is a result of competition between fuel reactivity and diffusivity. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to fuel diffusion and also to H₂/CO and C₁–C₃ hydrocarbon kinetics.     

Low temperature oxidation of benzene and toluene in mixture with n-decane

The oxidation of two blends, benzene/n-decane and toluene/n-decane, was studied in a jet-stirred reactor with gas chromatography analysis (temperatures from 500 to 1100 K, atmospheric pressure, stoichiometric mixtures). The studied hydrocarbon mixtures contained 75% of aromatics in order to highlight the chemistry of the low-temperature oxidation of these two aromatic compounds which have a very low reactivity compared to large alkanes. The difference of behavior between the two aromatic reactants is highly pronounced concerning the formation of derived aromatic products below 800 K. In the case of benzene, only phenol could be quantified. In the case of toluene, significant amounts of benzaldehyde, benzene, and cresols were also formed, as well as several heavy aromatic products such as bibenzyl, phenylbenzylether, methylphenylbenzylether, and ethylphenylphenol. A comparison with results obtained with neat n-decane showed that the reactivity of the alkane is inhibited by the presence of benzene and, to a larger extent, toluene. An improved model for the oxidation of toluene was developed based on recent theoretical studies of the elementary steps involved in the low-temperature chemistry of this molecule. Simulations using this model were successfully compared with the obtained experimental results.     

Ignition of non-premixed counterflow flames of octane and decane isomers

Ignition temperatures of non-premixed flames of octane and decane isomers were determined in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 401 K, a local strain rate of 130 s^?1, and fuel mole fractions ranging from 1% to 6%. The experiments were modeled using detailed chemical kinetic mechanisms for all isomers that were combined with established H₂, CO, and n-alkane models, and close agreements were found for all flames considered. The results confirmed that increasing the degree of branching lowers the ignition propensity. On the other hand, increasing the straight chain length by two carbons was found to have no measurable effect on flame ignition for symmetric branched fuel structures. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to the H₂/CO and C₁–C₃ hydrocarbon kinetics for low degrees of branching, and to fuel-related reactions for the more branched molecules.     

Ignition delay times of methyl oleate and methyl linoleate behind reflected shock waves

Ignition delay times for methyl oleate (C₁₉H₃₆O₂, CAS: 112-62-9) and methyl linoleate (C₁₉H₃₄O₂, CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. (2011) [10], which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements.     

Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550–700 K) oxidation chemistry of isobutanol and tert-butanol

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4–5.1 Torr) experiments at 550 and 700 K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O₂ by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the α-hydroxyalkyl radical with O₂ is predominantly linked to chain-terminating formation of HO₂. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O₂ with β-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO₂ elimination is not possible in the β-hydroxyalkyl + O₂ reaction because of the absence of a beta C–H bond; this channel is available in the β-hydroxyalkyl + O₂ reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K.     

A rapid compression machine study of the low temperature combustion of cyclohexane at elevated pressures

Ignition delay times have been measured in a rapid compression machine for cyclohexane/O₂/N₂/Ar mixtures with equivalence ratios of 0.5, 1.0 and 2.0 at elevated pressures of up to 40 bar and temperatures between 680 and 910 K. These data clearly show the negative-temperature-coefficient behavior for cyclohexane in the temperature range investigated. The predictions of several detailed kinetic models are compared to these new experimental validation data and these mechanisms have been analyzed to explain the obtained differences with a focus on the crucial peroxy-chemistry of the primary radicals. The presented data are the first set of ignition delay times at elevated pressures for the low-temperature range.     

Effects of NOx addition on autoignition and detonation development in DME/air under engine-relevant conditions

Exhaust gas recirculation (EGR) technology can be used in internal combustion engines to reduce NOx emission and improve fuel economy. However, it also affects the end-gas autoignition and engine knock since NOx in EGR can promote ignition. In this study, effects of NOx addition on autoignition and detonation development in dimethyl ether (DME)/air mixture under engine-relevant conditions are investigated. Numerical simulation considering both low-temperature and high-temperature chemistry is conducted. First the kinetic effects of NOx addition on the negative temperature coefficient (NTC) regime are assessed and interpreted. It is found that NOx addition greatly promotes both low-temperature and high-temperature ignition stages mainly through increasing OH production. Then the autoignitive reaction front propagation induced by either local NO accumulation or a cold spot within NTC regime with different amounts of NO addition is investigated. For the first time, supersonic autoignition modes including detonation induced by local NO accumulations are identified. This indicates that local accumulation of NOx in end gas might induce super-knock in engines with EGR. A new parameter quantifying the ratio of sound speed to average reaction front propagation speed is introduced to identify the regimes for different autoignition modes. Compared to the traditional counterpart parameter used in previous studies, this new parameter is more suitable since it yields a detonation development regime in a C-shaped curve which is almost unaffected by the initial conditions. The results in this study may provide fundamental insights into knocking mechanism in engines using EGR technology.     

Effects of fuel properties on self-ignition and flame-holding performances in SCRAM jet combustor for n-alkane fuels

Combustion characteristics of liquid hydrocarbon fuels are studied in a model combustor of SCRAM jet engines. The Mach number and total pressure of main flow in the combustor are 2.0 and 0.38 MPa, respectively, and the total temperature is varied from 1800 to 2400 K. Five kinds of n-alkane fuels such as n-heptane, n-octane, n-decane, n-tridecane and n-hexadecane are employed in experiments. Fuels are injected with a carrier nitrogen gas perpendicular to the mail flow in the combustor and the self-ignition behavior is investigated. The results show that the liquid fuels with lower carbon number have better self-ignition performance. This suggests that physical properties of liquid fuels such as volatility have a dominant effect on the self-ignition. The flame-holding behavior is investigated with the addition of pilot hydrogen to carrier nitrogen gas. The critical equivalence ratio at which the stable combustion keeps after cut-off of the pilot hydrogen is obtained. The relationship between the critical equivalence ratio and carbon number of fuel shows that fuels with the carbon numbers from 8 to 10 have the best flame-holding performance among the tested fuels. These experimental results can be expressed qualitatively by the simplified analysis with the concept of physical and chemical induction times.     

High-spatial-resolution monitoring of strong magnetic field using Rb vapor nanometric-thin cell

We have implemented the so-called λ-Zeeman technique (LZT) to investigate individual hyperfine transitions between Zeeman sublevels of the Rb atoms in a strong external magnetic field B in the range of 2500 ? 5000 G (recently it was established that LZT is very convenient for the range of 10 ? 2500 G). Atoms are confined in a nanometric thin cell (NTC) with the thickness L = λ, where λ is the resonant wavelength 794 nm for Rb D₁ line. Narrow velocity selective optical pumping (VSOP) resonances in the transmission spectrum of the NTC are split into several components in a magnetic field with the frequency positions and transition probabilities depending on the B-field. Possible applications are described, such as magnetometers with nanometric local spatial resolution and tunable atomic frequency references.     

Piloted ignition of solid fuels at low ambient pressure and varying igniter location

In order to investigate the effects of ambient pressure and igniter location on piloted ignition of solid fuels, the ignition mass flux of PMMA was experimentally determined for locations of the igniter between 6 and 70 mm above the solid surface, under two external heat fluxes of 21.2 and 25.4 kW/m². The experimental results show that the ignition mass flux decreases as the igniter approached the solid surface until it reached a minimum, and then the ignition mass flux remains nearly constant followed by a slight increase with a further decrease of the igniter location. In addition, in another series of experiments the ignition mass flux for elm wood decreases by a factor 0.6 at reduced pressure 0.67 (Tibet 0.67 atm) compared to the ignition mass flux at normal pressure (Hefei, 1.0 atm). The results of this work are explained well by a numerical piloted ignition model which also explains recent observations on the ignition mass flux at reduced pressures in a forced-flow ignition and flame spread apparatus.     

Experimental and semi-detailed kinetic modeling study of decalin oxidation and pyrolysis over a wide range of conditions

Decalin is the simplest polycyclic alkane (polynaphtenic hydrocarbon) found in liquid fuels (jet fuels, Diesel). In order to better understand the combustion characteristics of decalin, this study provides new experimental data for its oxidation in a jet-stirred reactor. For the first time, stable species concentration profiles were measured in a jet-stirred reactor at a constant mean residence time of 0.1 s and 0.5 s at respectively 1 and 10 atm, over a range of equivalence ratios (? = 0.5–1.5) and temperatures (750–1350 K). The oxidation of decalin under these experimental conditions was modeled using a semi-detailed chemical kinetic reaction mechanism (11,000 reactions involving 360 species) derived from a previously proposed scheme for the ignition of the same fuel in a shock-tube. The proposed mechanism that includes both low- and high-temperature chemistry shows reasonably good agreement with the present experimental data set. It can also represent well decalin pyrolysis and oxidation data available in the literature. Reaction path analyses and sensitivity analyses were conducted to interpret the results.