Multi-stage heat release is an important feature of hydrocarbon auto-ignition that influences engine operation. This work presents findings of previously unreported three-stage heat release in the auto-ignition of n-heptane/air mixtures at lean equivalence ratios and high pressures. Detailed homogenous gas-phase chemical kinetic simulations were utilized to identify conditions where two-stage and three-stage heat release exist. Temperature and heat release profiles of lean n-heptane/air auto-ignition display three distinct stages of heat release, which is notably different than two-stage heat release typically reported for stoichiometric fuel/air mixtures. Concentration profiles of key radicals (HO2 and OH) and intermediate/product species (CO and CO2) also display unique behavior in the lean auto-ignition case. Rapid compression machine measurements were performed at a lean equivalence ratio to confirm the existence of three-stage heat release in experiments. Laser diagnostic measurements of CO concentrations in the RCM indicate similar concentration-time profiles as those predicted by kinetic modeling. Computational singular perturbation was then used to identify key reactions and species contributing to explosive time scales at various points of the three-stage ignition process. Comparisons with two-stage ignition at stoichiometric conditions indicate that thermal runaway at the second stage of heat release is inhibited under lean conditions. H + O2 chain branching and CO oxidation reactions drive high-temperature heat release under stoichiometric conditions, but these reactions are suppressed by H, OH, and HO2 radical termination reactions at lean conditions, leading to a distinct third stage of heat release.

Liquefied Natural Gas (LNG) has become an increasingly important world energy resource and is a part of the European Union clean fuel strategy launched in 2013. Therefore, there are currently several ongoing measurement strategies considering quality specification of LNG. In this context, for application in gas engines, it is essential to understand the combustion behavior of these natural gas mixtures. The methane number (MN) which represents a scale for the knocking propensity, is one of the main indicators for this combustion behavior. In this study, we investigated the influence of the LNG composition on the ignition delay time and thus the knocking behavior of prototypical LNG Mixtures. Several LNG typical mixtures containing CH/CH/CH/n-CH/i-CH/n-CH/i-CH/N were studied in the temperature range 850–1450 K, with pressures of 20 and 40 bar and at equivalence ratios of 0.4 and 1.2. The use of a shock tube and a rapid compression machine facility allowed us to study the ignition behavior over a wide range of operating conditions relevant to gas engines. We report a detailed investigation of LNG autoignition with respect to temperature, pressure and equivalence ratio thereby providing crucial validation data for chemical kinetic models for real applications.

Extended wavelength tuning of an IH-QCL (integrated heater quantum cascade laser) is exploited for simultaneous detection of methane and acetylene using direct absorption spectroscopy. The integrated heater, placed within few microns of the laser active region, enables wider wavelength tuning than would be possible with a conventional DFB (distributed feedback) QCL. In this work, the laser current and heater resistor current are modulated simultaneously at 25 kHz to tune the laser over 1279.6-1280.1 cm, covering absorption transitions of methane and acetylene. The laser is characterized extensively to understand the dependence of wavelength tuning on modulation frequency, modulation amplitude and phase difference between laser/heater modulation. Thereafter, the designed sensor is validated in both room-temperature static cell experiments and non-reactive high-temperature-measurements in methane-acetylene-argon gas mixtures in the shock tube. Finally, the sensor is applied for simultaneous detection of methane and acetylene during the high-temperature pyrolysis of iso-octane behind reflected shock waves.

Methanol (CHOH) is the simplest alcohol and is considered to be a future fuel, produced by solar-driven reduction of carbon dioxide. The reaction of methanol and hydroxyl radicals is important in both combustion and atmospheric systems because this reaction is the dominant consumption pathway for methanol oxida- tion. Hydrogen abstraction at the CH or OH site of CH OH leads to different radical intermediates. The relative importance of these two channels is critical for combustion modeling as the subsequent chemistries of the product radicals (CHO and CHOH) are markedly different. In this work, we measured overall rate coefficients for the reaction of methanol (CHOH), methanol-d (CD OH) and methanol-d (CH DOH) with OH radicals over the temperature range of 900 -1300 K and pressures near 1.3 atm by employing shock tube/UV laser absorption technique. Combining our results with literature data, we recommend following three-parameter Arrhenius expressions (cm molecule s ): k1 (CH3OH + OH ) = 3.25 × 10 (T/300 K) exp(297.8K/T) 210 - 1344 K k2 (CDOH + OH ) = 4.69 × 10 (T/300 K) exp (-59.8K/T)293 - 1287 K Using our measured total rate coefficients, we determined site-specific H-abstraction rate coefficients and hence, branching ratios of the two abstraction channels. Our results show that abstraction at the CH site is the dominant channel, contributing more than 80% throughout our temperature range. Our calculated site-specific rate coefficients (per H atom) over 900-1300K are given by (cm molecule s ): k (CH2OH channel) = 2.55 × 10 exp (-2287.1 K/T ) k (CH3O channel) = 4.30 × 10 exp (-3463.2 K/T )

In this work, we investigated the self-reaction of allyl radicals and its cross-reaction with hydroxyl radical by employing shock tube and laser absorption techniques. We carried out the experiments behind reflected shock waves over the temperature range of 800-1200 K and pressures of 1.1-2.5 bar. We generated allyl (CH) and OH radicals by fast thermal decomposition of allyl iodide (CHI) and tert-butyl hydroperoxide (TBHP), respectively, and monitored reaction progress by detecting OH near 306.69 nm and CH near 220 nm using UV laser absorption. At the detection wavelength, we measured the temperature dependence of the absorption cross-sections of CH and CHI. Rate coefficient for the self-recombination reaction of allyl radicals showed a small negative temperature dependence and no noticeable fall-off behavior over 1.15-1.96 bar giving a mean value of kC3H5+C3H5=(1.0±0.2)×10-11cm3molecule-1s-1. Likewise, the cross-reaction of allyl and OH radicals did not exhibit discernible pressure and temperature dependence under our experimental conditions indicating a barrierless reaction, and an average value of kC3H5+OH=(9.3±0.7)×10-11cm3molecule-1s-1 best illustrates our measured rate coefficients. These measurements represent the first direct experimental determinations of the rate coefficients for these important radical-radical reactions at high temperatures and pressures.

A blend of low-octane (light and heavy naphtha) and high-octane (reformate) distillate fuels has been proposed for powering gasoline compression ignition (GCI) engines. The formulated 'GCI blend' has a research octane number (RON) of 77 and a motor octane number (MON) of 73.9. In addition to ~64 mole% paraffinic components, the blend contains ~20 mole% aromatics and ~15 mole% naphthenes. Experimental and modeling studies have been conducted in this work to assess autoignition characteristics of the GCI blend. Ignition delay times were measured in a shock tube and a rapid comparison machine over wide ranges of experimental conditions (20 and 40 bar, 640-1175 K, ϕ = 0.5, 1 and 2). Reactivity of the GCI blend was compared with experimental measurements of two surrogates: a multi-component surrogate (MCS) and a two-component primary reference fuel (PRF 77). Both surrogates capture the reactivity of the fuel quite well at high and intermediate temperatures. The MCS does a better job of emulating the fuel reactivity at low temperatures, where PRF 77 is more reactive than the GCI blend. Ignition delay times of the two surrogates are also simulated using detailed chemical kinetic models, and the simulations agree well with the experimental findings. The results of rate-of-production analyses show important role of cycloalkane chemistry in the overall autoignition behavior of the fuel at low temperatures.

This work reports thermal decomposition of cyclopentanone behind reflected shock waves over 1150 - 1590 K and 750 - 1800 Torr. Carbon monoxide is one of the main reaction products and its formation was monitored using a quantum cascade laser operating near 4.56 μm. Our results show that cyclopentanone undergoes decomposition, under the present experimental conditions, via reaction channels that produce CO almost exclusively. A recent ab initio study by Zaras et al. revealed that cyclopentanone decomposes to produce CO and ethylene by two channels: ring-opening to form a di-radical which subsequently decomposes, and concerted elimination to produce CO and C 2 H 4 directly; their predicted rate constants are much slower than literature experimental data. To resolve the rate constant discrepancy and to determine whether keto- enol tautomerism plays a significant role, we performed master equation simulations which produced results in good agreement both with the previous ab initio study and with the experimental data obtained in the present work.

Ethyl levulinate (Ethyl 4-oxopentanoate) is a liquid molecule at ambient temperature, comprising of ketone and ethyl ester functionalities and is one of the prominent liquid fuel candidates that may be easily obtained from lignocellulosic biomass. The combustion kinetics of ethyl levulinate have been investigated. Shock tube and rapid compression machine apparatuses are utilised to acquire gas phase ignition delay measurements of 0.5% ethyl levulinate/O2 mixtures at ϕ = 1.0 and ϕ = 0.5 at ∼ 10 atm over the temperature range 1000–1400 K. Ethyl levulinate is observed not to ignite at temperatures lower than ∼1040 K in the rapid compression machine. The shock tube and rapid compression machine data are closely consistent and show ethyl levulinate ignition delay to exhibit an Arrhenius dependence to temperature. These measurements are explained by the construction and analysis of a detailed chemical kinetic model. The kinetic model is completed by establishing thermochemical-kinetic analogies to 2-butanone, for the ethyl levulinate ketone functionality, and to ethyl propanoate for the ethyl ester functionality. The so constructed model is observed to describe the shock tube data very accurately, but computes the rapid compression machine data set to a lesser but still applicable fidelity. Analysis of the model suggests the autooxidation mechanism of ethyl levulinate to be entirely dominated by the propensity for the ethyl ester functionality to unimolecularly decompose to form levulinic acid and ethylene. The subsequent reaction kinetics of these species is shown to dictate the overall rate of the global combustion reaction. This model is then use to estimate the Research and Motored Octane Numbers of ethyl levulinate to be ≥97.7 and ≥ 93, respectively. With this analysis ethyl levulinate would be best suited as a gasoline fuel component, rather than as a diesel fuel as suggested in the literature. Indeed it may be considered to be useful as an octane booster. The ethyl levulinate kinetic model is constructed within a state-of-the-art gasoline surrogate combustion kinetic model and is thus available as a tool with which to investigate the use of ethyl levulinate as a gasoline additive.

Surrogate fuel formulation has drawn significant interest due to its relevance towards understanding combustion properties of complex fuel mixtures. In this work, we present a novel approach for surrogate fuel formulation by matching target fuel functional groups, while minimizing the number of surrogate species. Five key functional groups; paraffinic CH, paraffinic CH, paraffinic CH, naphthenic CH–CH and aromatic C–CH groups in addition to structural information provided by the Branching Index (BI) were chosen as matching targets. Surrogates were developed for six FACE (Fuels for Advanced Combustion Engines) gasoline target fuels, namely FACE A, C, F, G, I and J. The five functional groups present in the fuels were qualitatively and quantitatively identified using high resolution H Nuclear Magnetic Resonance (NMR) spectroscopy. A further constraint was imposed in limiting the number of surrogate components to a maximum of two. This simplifies the process of surrogate formulation, facilitates surrogate testing, and significantly reduces the size and time involved in developing chemical kinetic models by reducing the number of thermochemical and kinetic parameters requiring estimation. Fewer species also reduces the computational expenses involved in simulating combustion in practical devices. The proposed surrogate formulation methodology is denoted as the Minimalist Functional Group (MFG) approach. The MFG surrogates were experimentally tested against their target fuels using Ignition Delay Times (IDT) measured in an Ignition Quality Tester (IQT), as specified by the standard ASTM D6890 methodology, and in a Rapid Compression Machine (RCM). Threshold Sooting Index (TSI) and Smoke Point (SP) measurements were also performed to determine the sooting propensities of the surrogates and target fuels. The results showed that MFG surrogates were able to reproduce the aforementioned combustion properties of the target FACE gasolines across a wide range of conditions. The present MFG approach supports existing literature demonstrating that key functional groups are responsible for the occurrence of complex combustion properties. The functional group approach offers a method of understanding the combustion properties of complex mixtures in a manner which is independent, yet complementary, to detailed chemical kinetic models. The MFG approach may be readily extended to formulate surrogates for other complex fuels.

The ν1 fundamental band of N2O is examined by a novel spectrometer that relies on the frequency locking of an external-cavity quantum cascade laser around 7.8 μm to a near-infrared Tm:based frequency comb at 1.9 μm. Due to the large tunability, nearly 70 lines in the 1240 – 1310 cm−1 range of the ν1 band of N2O, from P(40) to R(31), are for the first time measured with an absolute frequency calibration and an uncertainty from 62 to 180 kHz, depending on the line. Accurate values of the spectroscopic constants of the upper state are derived from a fit of the line centers (rms ≈ 4.8 × 10−6 cm−1 or 144 kHz). The ν1 transitions presently measured in a Doppler regime validate high accuracy predictions based on sub-Doppler measurements of the ν3 and ν3-ν1 transitions.