Iso-octane Ignition Research Articles

Effect of nitric oxide and exhaust gases on gasoline surrogate autoignition: iso-octane experiments and modeling

Exhaust gas recirculation (EGR) is widely used in advanced internal combustion engines to reduce engine emissions as well as control combustion phasing. Among various species present in EGR gases, CO2 and H2O are two major components that can thermally and chemically affect fuel autoignition. It is of fundamental interest to isolate the thermal and chemical effects of CO2 and H2O on fuel autoignition, especially as such an effort has not been reported in the literature. Moreover, nitric oxide (NO) is known to exhibit strong chemical effects on fuel autoignition, which in turn affects engine combustion phasing. The effects of ultra-low NO addition (< 100 ppm) on fuel autoignition at low temperatures are also not well understood. Recognizing these problems, autoignition experiments of iso-octane (a major gasoline surrogate component) in air are performed in this study using a rapid compression machine at varying compressed pressures, equivalence ratios, dilution levels with an EGR gas analogue (consisting of CO2, H2O, O2, and N2) and N2 only, and varying amounts of NO addition. The thermal and chemical effects of the EGR gas analogue are isolated and evaluated by comparing the ignition delay time datasets of EGR and N2-only diluted cases. It is found that the dilution/thermal effect on autoignition dominates at all test conditions, and the combined chemical effect of CO2 and H2O is insignificant. The NO doping experiments show that the promoting and inhibiting effects of NO addition on iso-octane ignition delay time are temperature dependent. Furthermore, a chemical kinetic model for gasoline surrogate components and NOx is developed by validating it against the experimental data obtained in this study and those available in the literature for various gasoline surrogate components. The validation of the model across different experimental datasets and for several fuels demonstrates its reliability and wide applicability in several operating conditions. Comparisons with previously developed models shed light on the gaps still present in understanding the effect of nitrogen species on the reactivity of gasoline surrogate molecules. Chemical kinetic analyses have also been conducted to identify the important reaction pathways controlling iso-octane autoignition at various conditions, and to elucidate the underlying mechanism leading to different reactivity trends at different NO doping levels and temperatures.

Plasma-assisted pyrolysis and ignition of pre-vaporized n-heptane, iso-octane and n-decane

We intensively investigated the kinetic enhancement of dielectric-barrier-discharge (DBD) plasma on the pyrolysis and ignition of n-heptane, iso-octane, and n-decane fuels in a counterflow diffusion burner. The DBD plasma generated upstream from the fuel nozzle can contribute to low-temperature pyrolysis and high-temperature oxidation of pre-vaporized liquid fuels. Stable and short-lived intermediate species generated by plasma were measured by ex situ gas chromatography (GC) and in situ planar laser induced fluorescence (PLIF), respectively. GC results indicate that with plasma assistance approximately 2–3% of large hydrocarbons decompose at temperatures as low as 370 K and within 1 s. High-temperature oxidation experiments demonstrate that DBD can lower the ignition temperature of all three fuels with various global flow stretch rates. Experiments were compared with numerical analysis performed using the OPPDIF code that is formulated for the counterflow geometry. Finally, the discrete chemical explosive mode analysis (CEMA) elucidates the promotion of plasma-induced species, such as H2, C2H4, and OH radicals.

Effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer

Hydrogen addition is widely used to improve the combustion performance of single-component fuel. In this study, the effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer were examined and interpreted numerically. Detailed chemistry and transport were considered in simulation. The non-premixed ignition delay times at different hydrogen blending levels were obtained and analyzed. It was found that hydrogen addition greatly reduces the ignition delay. This is mainly due to the fact that the preferential mass diffusion of hydrogen over iso-octane significantly increases the local hydrogen blending level at the ignition kernel. Besides, for the non-premixed ignition process, two modes of reaction front propagation were identified through the analysis based on Damköhler number and consumption speeds. One is the reaction-driven mode characterized by local or sequential homogeneous autoignition; and the other is the diffusion-driven mode, which depends on the balance of mass diffusion, heat transfer and chemical reaction. These two modes lead to different ignition behaviors. For pure iso-octane with low mass diffusivity, ignition is mainly caused by local homogeneous reaction occurring at the most reactive position. With the increase of diffusion layer thickness, the local temperature at the most reactive position increases and therefore the non-premixed ignition delay time of pure iso-octane decreases. However, when hydrogen with high mass diffusivity is added into iso-octane, the non-premixed ignition is controlled by fuel diffusion. With the increase of diffusion layer thickness, the concentration gradient becomes smaller and thereby less hydrogen diffuses into the ignition kernel. Consequently, unlike pure iso-octane, the non-premixed ignition delay time of hydrogen/iso-octane blends increases with the diffusion layer thickness.

Combustion chemistry of iso-octane/ethanol blends: Effects on ignition and reaction pathways

This work presents new experimental data on the ignition of iso-octane and ethanol fuel blends, including measurements of pollutant species and precursors, using the University of Michigan Rapid Compression Facility (UM RCF). Ignition delay times were determined from pressure–time histories of ignition experiments for stoichiometric mixtures of iso-octane, ethanol, and 5, 11, 26, 50 and 67% by volume iso-octane and ethanol blends with air. A range of temperatures (900–1080 K) were studied at a pressure of 10 atm. Speciation experiments were performed for pure iso-octane (E0), pure ethanol (E100) and a 50% by volume blend of the two fuels (E50) at 10 atm and ∼930 K. Fast-gas sampling, gas chromatography and mass spectrometry were used to identify and quantify 14 stable intermediate species formed during the ignition delay periods for the three fuels. The measurements of eight stable intermediates: iso-butene (i-C4H8, 2-methyl-1-propene), propene (C3H6), ethanal (CH3CHO), ethene (C2H4), ethane (C2H6), methane (CH4), carbon monoxide (CO), carbon dioxide (CO2) were considered in detail and were used to describe reaction pathways important during iso-octane and ethanol ignition and how they were altered for iso-octane/ethanol blends. Simulations were carried out using a detailed reaction mechanism for gasoline surrogates available in the literature and the agreement with the ignition and speciation experiments was generally excellent (within the experimental uncertainty and expected computational uncertainty) for ignition delay times and for the species time-histories, with a few small discrepancies. The results indicate the reaction pathways of iso-octane and ethanol in the blend develop in parallel with a shared radical pool with no significant fuel-to-fuel interactions.

A skeletal iso-octane reaction mechanism for low temperature plasma ignition with ozone surrogate

In this work, we present a skeletal reaction mechanism for modeling ignition of iso-octane with low temperature plasma. The low temperature plasma was modeled by its main product ozone as a surrogate. A key chain branching reaction of oxygen radical attack on iso-octane was identified from reaction pathway analysis. A new skeletal reaction mechanism was developed by incorporating the key reaction along with ozone chemistry to the skeletal iso-octane reaction mechanism from Ra and Reitz [19]. Reaction rate constants of the key reactions and five other related reactions were calibrated to match predictions from the comprehensive reaction mechanism from Curran et al. [Combust. Flame 129 (2002) 253–280]. The optimized reaction mechanism shows good agreement with the comprehensive mechanism. It provides sufficient accuracy and computational simplicity to enable practical CFD simulation of low temperature plasma ignition in internal combustion engines.

Investigation of Iso-octane Ignition and Validation of a Multizone Modeling Method in an Ignition Quality Tester

An ignition quality tester was used to characterize the autoignition delay times of iso-octane. The experimental data were characterized between temperatures of 653 and 996 K, pressures of 1.0 and 1.5 MPa, and global equivalence ratios of 0.7 and 1.05. A clear negative temperature coefficient behavior was seen at both pressures in the experimental data. These data were used to characterize the effectiveness of three modeling methods: a single-zone homogeneous batch reactor, a multizone engine model, and a three-dimensional computational fluid dynamics (CFD) model. A detailed 874 species iso-octane ignition mechanism (Mehl, M.; Curran, H. J.; Pitz, W. J.; Westbrook, C. K. Chemical kinetic modeling of component mixtures relevant to gasoline. Proceedings of the European Combustion Meeting; Vienna, Austria, April 14–17, 2009) was reduced to 89 species for use in these models, and the predictions of the reduced mechanism were consistent with ignition delay times predicted by the detailed chemical mechanism acr...

A Comparative Study of Conditional Moment Closure Modelling for Ignition of iso-octane and n-heptane in Thermally Stratified Mixtures

This paper presents a comparative study of the premixed conditional moment closure (CMC) model for modelling ignition of thermally stratified mixtures under homogeneous charge compression ignition (HCCI) conditions. For this purpose, the CMC model is applied to two sets of direct numerical simulations (DNSs) modelling ignition of lean n-heptane/air and iso-octane/air mixtures with various levels of thermal stratification. The results show excellent agreement for all n-heptane cases with thermal stratification of 15-60 K. However, an advanced ignition is predicted by the CMC model for the iso-octane case with thermal stratification of 60 K in comparison with the DNS data. Inspection of homogeneous ignition delay demonstrates that the ignition delay time fluctuations are much higher in the iso-octane cases compared with the n-heptane cases having same level of temperature inhomogeneities. This is because of the differing ignition responses to temperature between these two fuels. The observed discrepancies in the iso-octane case with ${T}^{\prime }=60$ K are due to the dominance of deflagration mode of combustion resulting in large conditional fluctuations, which occurs in the iso-octane case and not the n-heptane case because the temperature dependence of ignition delay is stronger for iso-octane. To further investigate the reasons for the observed discrepancies, a transport equation for the conditional variance is derived for premixed combustion. Assessment of the conditional variance equation using the DNS data shows that correlations between dissipation and conditional fluctuation and correlations between reaction and conditional fluctuations are the dominant sources of conditional fluctuations.

Investigation of 2,5-dimethyl furan and iso-octane ignition

The ignition behavior of 2,5-dimethyl furan (2,5-DMF), iso-octane, and their blends is investigated. To confirm that 2,5-DMF is the least reactive furan, its ignition behavior is compared with that of the isomer, 2-ethyl furan (2-EF), revealing the very high reactivity of 2-EF. For the 2,5-DMF/iso-octane comparative study, ignition delay times are measured over a temperature range from 1009 to 1392K and pressures up to 12atm for lean, stoichiometric, and rich mixtures of fuel, oxygen, and argon. It is observed that 2,5-DMF generally has longer ignition delay times than iso-octane when the equivalence ratio ϕ, the argon-to-oxygen ratio D, and pressure p are kept constant over a range of temperatures, T. Further, ignition delay times of a 2,5-DMF/iso-octane 50% blend (by liquid volume) are measured and compared to those of the pure fuels at stoichiometric and rich conditions and pressure of 12atm. The blend shows intermediate reactivity between the pure fuels, albeit in closer alignment with iso-octane than 2,5-DMF. A combined model for 2,5-DMF and iso-octane combustion is developed, drawing from recent literature models for the pure components. Further modifications are carried out to improve agreement with the current and previous ignition data. The resulting model captures the ignition trends of the pure and blended fuels. Reaction pathway analysis and species sensitivity analysis are performed for more insight on the governing chemical kinetics. The reported experimental data set and the model advance combustion modeling of bio and conventional fuel blends for spark ignition engines.

Effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane

Experimental and numerical studies have been performed on the thermal and chemical effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane. Experiments were conducted using a recently developed rapid compression machine in the temperature range of 600–850K. Three buffer gases were studied including nitrogen (N2), argon (Ar), and a mixture of Ar and carbon dioxide (CO2) at a mole ratio of 65.1%/34.9%. Iso-octane was studied at 20bar, ϕ=1, and a dilution level of buffer gas to O2 of 3.76:1 (mole ratio). n-Heptane was studied at 9bar, ϕ=1, and a dilution level of buffer gas to O2 of 5.63:1 (mole ratio). For experiments where two-stage ignition was observed, the buffer gas composition had no impact on the first-stage ignition time but, as expected, it caused differences in the total heat release, pressure and temperature rise after the first-stage ignition. As a consequence, significant differences were observed for the total ignition delay time as a function of the buffer gas composition, with up to 40% and 42.5% faster total ignition time for iso-octane and n-heptane, respectively, by using Ar instead of N2. The chemical effects of the buffer gas composition were studied experimentally by comparing the results of the N2 and Ar/CO2 (65.1%/34.9%) mixtures, recognizing that while the Ar/CO2 mixture has the same heat capacity as N2, its predicted combined third-body collision efficiency is about 76% higher than N2. The experimental results showed negligible chemical effects on the first-stage and total ignition delay times. Numerical simulations were carried out over a wider range of temperatures for pure N2, Ar, and CO2 as buffer gases. Results showed that thermal effects are very pronounced and dominated at the negative temperature coefficient and two-stage ignition conditions, which is consistent with the experimental results and previous studies in the literature. However, the simulation results also showed at temperatures higher than 850K, the chemical effects of CO2 became more important than the thermal effects.

Experimental and numerical analysis of nitric oxide effect on the ignition of iso-octane in a single cylinder HCCI engine

Small concentrations of very reactive chemical species such as nitric oxide (NO) have a very important impact on the onset of combustion. In engine applications, this may lead to irregular combustion phenomena in spark ignition engines or contribute to the control of the homogeneous charge compression ignition (HCCI) engines. To numerically analyze and predict the effect of these species, detailed chemical mechanisms are required. However, including these mechanisms in computational fluid dynamics (CFD) simulations often results in prohibitive computational cost. Using a single-cylinder HCCI engine test bench, we have analysed the effect of initial NO concentrations ranging from 0 to 500ppm on the ignition of iso-octane. We have also investigated whether the tabulation of dynamic adaptive chemistry (TDAC) method, that significantly reduces the CPU time associated with using detailed chemistry in CFD simulations, could capture this effect. This paper first presents the experimental setup and the validation data. Then it compares these experimental data with the CFD simulation results using a detailed mechanism involving more than 1000 species and 4500 reactions. The significant effect of NO on iso-octane ignition is efficiently captured over the whole range of NO concentration with a speed-up factor of up to 1500 (compared to simulations without TDAC). This work further demonstrates that TDAC represents a very efficient tool to include detailed kinetic mechanisms able to describe complex phenomena, such as the kinetic effect of small concentrations of reactive species, on ignition in a HCCI engine.

Combustion characteristics and influential factors of isooctane active-thermal atmosphere combustion assisted by two-stage reaction of n-heptane

This paper presents an experimental study on the isooctane active-thermal atmosphere combustion (ATAC) which is assisted by two-stage reaction of n-heptane. The active-thermal atmosphere is created by low- and high-temperature reactions of n-heptane which is injected at intake port, and isooctane is directly injected into combustion chamber near the top dead center. The effects of isooctane injection timing, active-thermal atmosphere intensity, overall equivalence ratio, and premixed ratio on combustion characteristics and emissions are investigated. The experimental results reveal that, the isooctane ignition and combustion can be classified to thermal atmosphere combustion, active atmosphere combustion, and active-thermal atmosphere combustion respectively according to the extent of n-heptane oxidation as well as effects of isooctane quenching and charge cooling. n-Heptane equivalence ratio, isooctane equivalence ratio and isooctane delivery advance angle are major control parameters. In one combustion cycle, the isooctane ignited and burned after those of n-heptane, and then this combustion phenomenon can also be named as dual-fuel sequential combustion (DFSC). The ignition timing of the overall combustion event is mainly determined by n-heptane equivalence ratio and can be controlled in flexibility by simultaneously adjusting isooctane equivalence ratio. The isooctane ignition regime, overall thermal efficiency, and NO x emissions show strong sensitivity to the fuel delivery advance angle between 20 °CA BTDC and 25 °CA BTDC.

Experimental Study on Influencing Factors ofiso-Octane Thermo-atmosphere Combustion in a Dual-Fuel Stratified Charge Compression Ignition (SCCI) Engine

This paper focuses on a novel combustion mode that is developed from the original concept of homogeneous charge compression ignition (HCCI) combustion. Experiments on iso-octane thermo-atmosphere combustion were carried out on a modified single-cylinder engine. n-Heptane was induced from the intake manifold to promote the ignition of iso-octane spray that was directly injected into the cylinder, through HCCI combustion of n-heptane. The combustion and emission characteristics of this novel combustion strategy were analyzed, and the effects of the iso-octane supply advance angle and n-heptane premixed ratio were investigated. The results show that the influence of the thermal effect and active radical produced by n-heptane HCCI combustion on the iso-octane ignition and emission is varied, depending upon the iso-octane supply advance angle. With the advancing of this fuel supply angle, the ignition of iso-octane is enhanced at first but then suppressed. At an iso-octane supply advance angle of 25 °CA BTDC, iso-octane is best ignited and the highest combustion efficiency and indicated thermal efficiency are obtained. Meanwhile, the level of CO in the exhaust gas is the lowest, with the penalty of the worst NOx emission.

A shock tube study of iso-octane ignition at elevated pressures: The influence of diluent gases

The ignition of iso-octane/air and iso-octane/O 2/Ar (∼20% O 2) mixtures was studied in a shock tube at temperatures of 868–1300 K, pressures of 7–58 atm, and equivalence ratios Φ = 1.0 , 0.5, and 0.25. Ignition times were determined using endwall OH ∗ emission and sidewall piezoelectric pressure measurements. Measured iso-octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the influence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.

Experimental investigation of the intermediates of isooctane during ignition

AbstractDirect measurements of intermediates of ignition are challenging experimental objectives, yet such measurements are critical for understanding fuel decomposition and oxidation pathways. In the current work, a new gas‐sampling system is used to provide quantitative discrete measurements of 30 hydrocarbon and oxygenate species during rapid compression facility studies of isooctane ignition. Two target conditions and equivalence ratios (based on molar fuel to oxygen ratio) were studied (P = 5.2 atm, T = 1000 K, ϕ = 0.4 and P = 4.8 atm, T = 975 K, ϕ = 1.2). The results are compared with model predictions that use the detailed reaction mechanism developed by Curran et al. (Combust Flame 2002, 129, 253–280). In general, the model predictions are in excellent agreement with the experimental data, including several trace species. Isobutene (i‐C4H8) and propene (C3H6) were the major olefin species identified in the experiments. The results are consistent with an intermediate temperature reaction path sequence, where isooctane is consumed by H‐atom abstraction to yield isooctyl radicals that undergo β‐scission to form olefin and alkyl radical species. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 498–517, 2007

An experimental investigation of iso-octane ignition phenomena

High-speed digital imaging has been used in rapid compression facility (RCF) studies to investigate ignition phenomena of iso-octane/air mixtures. Sequential images were captured for each experiment. The results indicate the existence of two ignition regimes. In one domain, ignition is rapid, typically less than 76 μs, and ignition occurs simultaneously throughout the test volume. In the other domain, reaction fronts form and propagate within the test volume prior to volumetric ignition. The data span equivalence ratios from ϕ = 0.20 to 1.98, with inert/O 2 gas ratios from 1.38 to 5.89, pressures from 8.7 to 16.6 atm, and temperatures from 903 to 1020 K. The transition between the two regimes is discussed in the context of the mixture composition and experimental conditions. The analysis shows that the fuel mole fraction is a key parameter dictating the boundary between the modes of ignition. Below a critical mole fraction limit, volumetric ignition is observed; above the critical limit, reaction fronts are consistently present prior to volumetric ignition. The ignition delay times for both ignition regimes are well reproduced using a homogeneous simulation with detailed reaction chemistry, when the state conditions are modified to account for the presence of the reaction fronts. The results are discussed in terms of proposed reaction chemistry, ignition theory, and previous studies of iso-octane ignition.

A rapid compression facility study of OH time histories during iso-octane ignition

Iso-octane ignition delay times ( τ ign ) and hydroxyl (OH) radical mole fraction ( χ OH ) time histories were measured under conditions relevant to homogeneous charge compression ignition engine operating regimes using the University of Michigan rapid compression facility. Absolute quantitative OH mole fraction time histories were obtained using differential narrow-line laser absorption of the R 1(5) line of the A 2 Σ + ← X 2 Π i ( 0 , 0 ) band of the OH spectrum ( ν 0 = 32606.56 cm −1 ). Ignition delay times were determined using pressure and OH data. Diluted iso-octane/argon/nitrogen/oxygen mixtures were used with fuel/oxygen equivalence ratios from ϕ = 0.25 to 0.6 for τ ign measurements and from ϕ = 0.25 to 0.35 for χ OH measurements. The pressures and temperatures after compression ranged from 8.5 to 15 atm and from 945 to 1020 K, respectively, for the combined τ ign and χ OH data. The maximum mole fraction of OH during ignition and the plateau value of OH after ignition are compared with model predictions using different iso-octane oxidation mechanisms. Sensitivity and rate of production analyses for OH identify reactions important in iso-octane ignition under these lean, intermediate-temperature conditions. The OH time histories show significant sensitivity to the OH + OH + M = H 2O 2 + M, CH 3 + HO 2 = CH 3O + OH, and CH 3 + HO 2 = CH 4 + O 2 reactions, which have rate coefficients with relatively high uncertainties. Improved predictions of the OH time histories can be achieved by modifying the rate coefficient for these reactions. The enthalpy of formation used for OH also has a significant effect on the predicted ignition delay times.

Experimental study of the kinetic interactions in the low-temperature autoignition of hydrocarbon binary mixtures and a surrogate fuel

The oxidation and autoignition of five undiluted stoichiometric mixtures, n-heptane/toluene, isooctane/toluene, isooctane/1-hexene, 1-hexene/toluene, and isooctane/1-hexene/toluene, has been studied in a rapid compression machine below 900 K. Ignition delay times of two- and one-stage autoignition have been measured and compared to those for pure hydrocarbons. The largest influence of mixing is in the region of the negative temperature coefficient. Intermediate products have been analyzed. The main reaction paths of low-temperature co-oxidation are discussed according to current knowledge of the oxidation paths of pure hydrocarbons. The influence of toluene on the temperature coefficient of the first stage of ignition of isooctane cannot be accounted for by the current theories of low-temperature autoignition. Each hydrocarbon generates a pool of radicals whose reactivity and selectivity toward further attack changes with temperature and with the family of hydrocarbons. The overall behavior of mixtures may result from changing competition for HO 2 and OH as temperature increases during the delay time. Termination reactions between stable radicals seem to have a minor impact at low temperature.

An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions

Autoignition of iso-octane was examined using a rapid compression facility (RCF) with iso-octane, oxygen, nitrogen, and argon mixtures. The effects of typical homogeneous charge compression ignition (HCCI) conditions on the iso-octane ignition characteristics were studied. Experimental results for ignition delay times, τ ign, were obtained from pressure time-histories. The experiments were conducted over a range of equivalence ratios ( ϕ = 0.25 – 1.0 ), pressures ( P = 5.12 – 23 atm ) , temperatures ( T = 943 – 1027 K ), and oxygen mole fractions ( χ O 2 = 9 – 21 % ), and with the addition of trace amounts of combustion product gases (CO 2 and H 2O). It was found that the ignition delay times were well represented by the expression τ ign = 1.3 × 10 −4 P −1.05 ϕ −0.77 χ O 2 −1.41 exp ( 33 , 700 / R ( cal / mol / K ) T ) , where P is pressure (atm), T is temperature (K), ϕ is the equivalence ratio (based on iso-octane to O 2 molar ratios), χ O 2 is the oxygen mole percent (%), and τ ign is the ignition delay time (ms). Carbon dioxide was found to have no chemical effect on τ ign. Water was found to systematically decrease τ ign by a small amount (less than 14% for the range of conditions studied). The maximum uncertainty in the measured τ ign is ±12% with an average uncertainty of ±6%. The performance of several proposed chemical reaction mechanisms (including detailed, reduced, and skeletal mechanisms) was evaluated in the context of the current experimental results.

Shock tube ignition measurements of iso-octane/air and toluene/air at high pressures

Ignition delay times were measured in a shock tube for iso-octane/air and toluene/air at conditions similar to those found in homogeneous charge compression ignition (HCCI) engines. Initial reflected shock conditions spanned the following ranges: temperature 855–1269 K, pressure 14–59 atm, and equivalence ratios ϕ of 0.5 and 1 in synthetic air. Ignition delay times were measured using sidewall pressure via piezo-electric transducers and confirmed with OH* and CH* emission measurements. The iso-octane ignition delay times are in excellent agreement with existing measurements by Fieweger et al. [Proc. Combust. Inst. 25 (1994) 1579; Combust. Flame 109 (1997) 599] and modeling by Ogink and Golovitchev [SAE Technical Paper Series, 2001, Paper No. 2001-01-3614]. No comparable high-pressure data exist for toluene/air, and modeling by Pitz et al. [U.S. Sections of the Combustion Institute 2nd Joint Spring Meeting, 2001, Paper 253] and Dagaut et al. [Fuel 81 (2002) 173] significantly over-predicts the toluene/air ignition delay times at ϕ = 1. The large pressure range of the current measurements permits determination of the pressure dependence of ignition delay time at the temperatures and pressures of direct interest in HCCI engine simulations. Detailed examination of the pressure–time profiles shows evidence of significant pre-ignition energy release in both the iso-octane/air and toluene/air systems. Using current detailed mechanisms, the rate of this energy release is not correctly predicted either in iso-octane/air or toluene/air at temperatures above 850 K.

Kinetic Mechanisms of Ignition of Isooctane–Air Mixtures

A kinetic scheme was developed to describe the autoignition of isooctane in air. The scheme includes 976 reactions and 126 species and adequately describes the process at both low and high initial temperatures of the mixture. The results of numerical modeling agree with experimental data on isooctane pyrolysis and autoignition delay of isooctane–air mixtures at initial pressures of 0.1–4.5 MPa, initial temperatures of 700–1300 K, and a fuel‐to‐air ratio of 0.5–2.0 with an accuracy not worse than 30%.