Three-stage autoignition of gasoline in an HCCI engine: An experimental and chemical kinetic modeling investigation

The alternative HCCI combustion mode presents a possible means for decreasing the pollution with respect to conventional gasoline or diesel engines, while maintaining the efficiency of a diesel engine or even increasing it. This paper investigates the possibility of using gasoline in an HCCI engine and analyzes the autoignition of gasoline in such an engine. The compression ratio that has been used is 13.5, keeping the inlet temperature at 70 °C, varying the equivalence ratio from 0.3 to 0.54, and the EGR (represented by N₂) ratio from 0 to 37 vol%. For comparison, a PRF95 and a surrogate containing 11 vol% n-heptane, 59 vol% iso-octane, and 30 vol% toluene are used. A previously validated kinetic surrogate mechanism is used to analyze the experiments and to yield possible explanations to kinetic phenomena. From this work, it seems quite possible to use the high octane-rated gasoline for autoignition purposes, even under lean inlet conditions. Furthermore, it appeared that gasoline and its surrogate, unlike PRF95, show a three-stage autoignition. Since the PRF95 does not contain toluene, it is suggested by the kinetic mechanism that the benzyl radical, issued from toluene, causes this so-defined “obstructed preignition” and delaying thereby the final ignition for gasoline and its surrogate. The results of the kinetic mechanism supporting this explanation are shown in this paper.     

Nitric oxide interactions with hydrocarbon oxidation in a jet-stirred reactor at 10 atm

The purpose of the present work is to better define the influence of trace amounts of NO on the oxidation of model fuels such as n-heptane, iso-octane, toluene, and methanol. This information is of interest for understanding and modeling autoignition whether for engine knock or for engines operating under compression ignition modes such as HCCI (homogeneous charge compression ignition) or CAI^TM (controlled autoignition). The experiments were performed in a jet-stirred reactor at 10 atm over a temperature range of 550 to 1180 K with a residence time of 1 s for stoichiometric mixtures highly diluted in nitrogen. The carbon content was about 1 molar percent and the added NO ranged from 25 to 500 ppmv. The effects of NO vary with the temperature regime. At the lowest temperatures NO inhibits the reaction. As temperature rises beyond 675 K, NO can considerably accelerate the reactivity of all fuels to an extent that can supercede the NTC behavior in the case of n-heptane. Modeling work indicates that in this temperature region at 10 atm the promoting effect of NO is largely due to the catalyzed production of OH, involving the dissociation of HONO, with the latter formed from reactions between NO₂ and HO₂, CH₃O, or CH₂O. In the intermediate temperature regime the intensity of the accelerating effects is observed to rise with the octane number of the fuel, with the exception of methanol. For toluene, the onset of oxidation drops down from 900 to 800 K with as little as 50 ppmv NO.     

Effects of buffer gas composition on autoignition

This work quantifies the chemical kinetic and thermal effects of buffer gas composition on autoignition of three fuels at conditions relevant to engines, combustors, and experimental facilities used to study ignition kinetics. Computational simulations of autoignition of iso-octane, n-heptane, and of n-butanol were used to characterize the effects of buffer gas composition on ignition delay time and heat release rate. Stoichiometric mixtures, ? = 1.0, and a temperature range of 600–1100 K were considered. Iso-octane and n-heptane were studied at initial pressures of 9.0 atm and 60.0 atm, and n-butanol was studied at initial pressures of 3.2 atm and 60.0 atm. Two dilution levels of buffer gas to O₂ of 3.76:1 (mole basis) and 5.64:1 were considered (∼21% and ∼15% O₂ respectively, mole basis). The fuels and simulation conditions were selected based on the relevance to engine operating conditions and previously published ignition studies. The buffer gases considered were argon, nitrogen, water, and carbon dioxide. Simulation results predicted changes of greater than a factor of 2 in ignition delay time and heat release rate as a function of buffer gas composition in the negative temperature coefficient (NTC) region for n-heptane and iso-octane. Outside the NTC region, the predicted effects of changes in buffer gas composition were small (<20%); however, experimental data for n-heptane indicate larger effects of buffer gas composition on ignition delay time at higher temperatures (>a factor of 2). The heat release rates were also sensitive to buffer gas composition, with carbon dioxide exhibiting relatively high levels of early and late heat release relative to the other buffer gases. Sensitivity analysis of the third-body collision efficiencies for the buffer gases showed the effects of uncertainties in the third body collision efficiencies on ignition delay time and heat release rate. The results highlight the significance of buffer gas composition on low-temperature combustion chemistry, particularly via H₂O₂ and HO₂ decomposition and recombination reactions.     

Low-temperature speciation and chemical kinetic studies of n-heptane

Although there have been many ignition studies of n-heptane—a primary reference fuel—few studies have provided detailed insights into the low-temperature chemistry of n-heptane through direct measurements of intermediate species formed during ignition. Such measurements provide understanding of reaction pathways that form toxic air pollutants and greenhouse gas emissions while also providing key metrics essential to the development of chemical kinetic mechanisms. This paper presents new ignition and speciation data taken at high pressure (9 atm), low temperatures (660–710 K), and a dilution of inert gases-to-molecular oxygen of 5.64 (mole basis). The detailed time-histories of 17 species, including large alkenes, aldehydes, carbon monoxide, and n-heptane were quantified using gas chromatography. A detailed chemical kinetic mechanism developed previously for oxidation of n-heptane reproduced experimentally observed ignition delay times reasonably well, but predicted levels of some important intermediate chemical species that were significantly different from measured values. Results from recent theoretical studies of low temperature hydrocarbon oxidation reaction rates were used to upgrade the chemical kinetic mechanism for n-heptane, leading to much better agreement between experimental and computed intermediate species concentrations. The implications of these results to many other hydrocarbon fuel oxidation mechanisms in the literature are discussed.     

Modeling of autoignition and NO sensitization for the oxidation of IC engine surrogate fuels

This paper presents an approach for modeling with one single kinetic mechanism the chemistry of the autoignition and combustion processes inside an internal combustion engine, as well as the chemical kinetics governing the postoxidation of unburned hydrocarbons in engine exhaust gases. Therefore a new kinetic model was developed, valid over a wide range of temperatures including the negative temperature coefficient regime. The model simulates the autoignition and the oxidation of engine surrogate fuels composed of n-heptane, iso-octane, and toluene, which are sensitized by the presence of nitric oxides. The new model was obtained from previously published mechanisms for the oxidation of alkanes and toluene where the coupling reactions describing interactions between hydrocarbons and NO_x were added. The mechanism was validated against a wide range of experimental data obtained in jet-stirred reactors, rapid compression machines, shock tubes, and homogeneous charge compression ignition engines. Flow rate and sensitivity analysis were performed in order to explain the low temperature chemical kinetics, especially the impact of NO_x on hydrocarbon oxidation.     

Modeling ignition and chemical structure of partially premixed turbulent flames using tabulated chemistry

Correctly reproducing the autoignition and the chemical composition of partially premixed turbulent flames is a challenge for numerical simulations of industrial applications such as diesel engines. A new model DF-PCM (diffusion flame presumed conditional moment) is proposed based on a coupling between the FPI (flame prolongation of ILDM) tabulation method and the PCM (presumed conditional moment) approach. Because the flamelets used to build the table are laminar diffusion flames, DF-PCM cannot be used for industrial applications like Diesel engines due to excessive CPU requirements. Therefore two new models called AI-PCM (autoignition presumed conditional moment) and ADF-PCM (approximated diffusion flames presumed conditional moment) are developed to approximate it. These models differ from DF-PCM because the flamelet libraries used for the table rely on PSR calculations. Comparisons between DF-PCM, AI-PCM, and ADF-PCM are performed for two fuels, n-heptane, representative of diesel fuels, and methane, which does not exhibit a “cool flame” ignition regime. These comparisons show that laminar diffusion flames can be approximated by flamelets based on PSR calculations in terms of autoignition delays and steady state profiles of the progress variable. Moreover, the evolution of the mean progress variable of DF-PCM can be correctly estimated by the approximated models. However, as discussed in this paper, errors are larger for CO and CO₂ mass fractions evolutions. Finally, an improvement to ADF-PCM, taking into account ignition delays, is proposed to better reproduce the ignition of very rich mixtures.     

Kinetic modelling of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows with comparison to experiment

A detailed chemical–kinetic mechanism is used to predict critical conditions of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows. The mechanism includes reactions that describe “low temperature chemistry” as well as “high temperature chemistry” for many high molecular weight fuels. The fuels considered here are n-heptane, n-decane, n-dodecane, n-hexadecane, and iso-octane. The kinetic model is validated by comparing its predictions of critical conditions of extinction and autoignition for these fuels with experimental data obtained in a counterflow configuration where a steady laminar flow of an oxidizer is directed over the vaporizing surface of a condensed fuel. The residence time in this configuration is given by the strain rate. The kinetic model predicts that n-heptane is most difficult to extinguish followed by n-decane, n-dodecane, and n-hexadecane. This is in agreement with experimental data. Computations show that the influence of low temperature chemistry on critical conditions of extinction is small. The kinetic model predicts that at low strain rates n-hexadecane is most easy to ignite followed by n-dodecane, n-decane, and n-heptane. At high values of strain rate, n-heptane is more easy to ignite in comparison to n-decane. This is again in agreement with experimental data, including the “cross-over” in relative reactivities of n-heptane and n-decane. Sensitivity analysis shows that at low strain rates autoignition is promoted by low temperature chemistry for all fuels. At high strain rates, autoignition for n-heptane is promoted by high temperature chemistry, whilst low temperature chemistry continues to play a significant role in promoting autoignition for the other straight-chain hydrocarbon fuels. The “cross-over” in relative reactivities of n-heptane and n-decane is attributed to competition between the rates of low temperature chemistry, rates of high temperature chemistry, and rates of molecular transport for these fuels.     

Experimental study and chemical analysis of n-heptane homogeneous charge compression ignition combustion with port injection of reaction inhibitors

The control of ignition timing in the homogeneous charge compression ignition (HCCI) of n-heptane by port injection of reaction inhibitors was studied in a single-cylinder engine. Four suppression additives, methanol, ethanol, isopropanol, and methyl tert-butyl ether (MTBE), were used in the experiments. The effectiveness of inhibition of HCCI combustion with various additives was compared under the same equivalence ratio of total fuel and partial equivalence ratio of n-heptane. The experimental results show that the suppression effectiveness increases in the order MTBE < isopropanol ? ethanol < methanol. But ethanol is the best additive when the operating ranges, indicated thermal efficiency, and emissions are considered. For ethanol/n-heptane HCCI combustion, partial combustion may be observed when the mole ratio of ethanol to that of total fuel is larger than 0.20; misfires occur when the mole ratio of ethanol to that of total fuel larger than 0.25. Moreover, CO emissions strongly depend on the maximum combustion temperature, while HC emissions are mainly dominated by the mole ratio of ethanol to that of total fuel. To obtain chemical mechanistic informations relevant to the ignition behavior, detailed chemical kinetic analysis was conducted. The simulated results also confirmed the retarding of the ignition timing by ethanol addition. In addition, it can be found from the simulation that HCHO, CO, and C₂H₅OH could not be oxidized completely and are maintained at high levels if the partial combustion or misfire occurs (for example, for leaner fuel/air mixture).     

Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures

Autoignition characteristics of n-heptane/air, gasoline/air, and ternary surrogate/air mixtures were studied behind reflected shock waves in a high-pressure, low-temperature regime similar to that found in homogeneous charge compression ignition (HCCI) engine cycles. The range of experiments covered combustion of fuel in air for lean, stoichiometric, and rich mixtures (Φ=0.5, 1.0, 2.0), two pressure ranges (15-25 and 45-60 atm), temperatures from 850 to 1280 K, and exhaust gas recirculation (EGR) loadings of (0, 20, and 30%). The ignition delay time measurements in n-heptane are in good agreement with the shock tube study of Fieweger et al. (Proc. Combust. Inst. 25 (1994) 1579-1585) and support the observation of a pronounced, low-temperature, NTC region. Strong agreement was seen between ignition delay time measurements for RD387 gasoline and surrogate (63% iso-octane/20% toluene/17% n-heptane by liquid volume) over the full range of experimental conditions studied. Ignition delay time measurements under fuel-lean (Φ=0.5) mixture conditions were longer than with Φ=1.0 mixtures at both the low- (15-25 atm) and high- (45-60 atm) pressure conditions. Ignition delay times in fuel-rich (Φ=2.0) mixtures for both gasoline and surrogate were indistinguishable in the low-pressure (15-25 atm) range, but were clearly shorter at high-pressures (45-60 atm). EGR loading affected the ignition delay times similarly for both gasoline and surrogate, with clear trends indicating an increase in ignition delay time with increased EGR loading. This data set should provide benchmark targets for detailed mechanism validation and refinement under HCCI conditions.     

HCCI experiments with toluene reference fuels modeled by a semidetailed chemical kinetic model

A semidetailed mechanism (137 species and 633 reactions) and new experiments in a homogeneous charge compression ignition (HCCI) engine on the autoignition of toluene reference fuels are presented. Skeletal mechanisms for isooctane and n-heptane were added to a detailed toluene submechanism. The model shows generally good agreement with ignition delay times measured in a shock tube and a rapid compression machine and is sensitive to changes in temperature, pressure, and mixture strength. The addition of reactions involving the formation and destruction of benzylperoxide radical was crucial to modeling toluene shock tube data. Laminar burning velocities for benzene and toluene were well predicted by the model after some revision of the high-temperature chemistry. Moreover, laminar burning velocities of a real gasoline at 353 and 500 K could be predicted by the model using a toluene reference fuel as a surrogate. The model also captures the experimentally observed differences in combustion phasing of toluene/n-heptane mixtures, compared to a primary reference fuel of the same research octane number, in HCCI engines as the intake pressure and temperature are changed. For high intake pressures and low intake temperatures, a sensitivity analysis at the moment of maximum heat release rate shows that the consumption of phenoxy radicals is rate-limiting when a toluene/n-heptane fuel is used, which makes this fuel more resistant to autoignition than the primary reference fuel. Typical CPU times encountered in zero-dimensional calculations were on the order of seconds and minutes in laminar flame speed calculations. Cross reactions between benzylperoxy radicals and n-heptane improved the model predictions of shock tube experiments for ?=1.0 and temperatures lower than 800 K for an n-heptane/toluene fuel mixture, but cross reactions had no influence on HCCI simulations.     

HCCI experiments with gasoline surrogate fuels modeled by a semidetailed chemical kinetic model

Experiments in a homogeneous charge compression ignition (HCCI) engine have been conducted with four gasoline surrogate fuel blends. The pure components in the surrogate fuels consisted of n-heptane, isooctane, toluene, ethanol and diisobutylene and fuel sensitivities (RON-MON) in the fuel blends ranged from two to nine. The operating conditions for the engine were pin=0.1 and 0.2 MPa, Tin=80 and 250 °C, ?=0.25 in air and engine speed 1200 rpm. A semidetailed chemical kinetic model (142 species and 672 reactions) for gasoline surrogate fuels, validated against ignition data from experiments conducted in shock tubes for gasoline surrogate fuel blends at 1.0?p?5.0 MPa, 700?T?1200 K and ?=1.0, was successfully used to qualitatively predict the HCCI experiments using a single zone modeling approach. The fuel blends that had higher fuel sensitivity were more resistant to autoignition for low intake temperature and high intake pressure and less resistant to autoignition for high intake temperature and low intake pressure. A sensitivity analysis shows that at high intake temperature the chemistry of the fuels ethanol, toluene and diisobutylene helps to advance ignition. This is consistent with the trend that fuels with the least Negative Temperature Coefficient (NTC) behavior show the highest octane sensitivity, and become less resistant to autoignition at high intake temperatures. For high intake pressure the sensitivity analysis shows that fuels in the fuel blend with no NTC behavior consume OH radicals and acts as a radical scavenger for the fuels with NTC behavior. This is consistent with the observed trend of an increase in RON and fuel sensitivity. With data from shock tube experiments in the literature and HCCI modeling in this work, a correlation between the reciprocal pressure exponent on the ignition delay to the fuel sensitivity and volume percentage of single-stage ignition fuel in the fuel blend was found. Higher fuel sensitivity and single-stage fuel content generally gives a lower value of the pressure exponent. This helps to explain the results obtained while boosting the intake pressure in the HCCI engine.     

Experimental and surrogate modeling study of gasoline ignition in a rapid compression machine

The use of gasoline in Homogeneous Charge Compression Ignition engines has propelled the need to better understand compression ignition processes for gasoline under engine-like conditions. In order to quantify low-temperature heat release and to provide fundamental validation data for chemical kinetic models, it is imperative to study autoignition phenomena under well-controlled conditions. However, there is a significant lack of autoignition delay data in the low temperature regime. Recognizing the need for kinetic information at high pressures and low-to-intermediate temperatures, this work aims to fill this void by conducting an experimental study of gasoline autoignition in a Rapid Compression Machine (RCM) to characterize the ignition response of gasoline + air mixtures over a wide range of compression temperatures at compression pressures of 20 and 40 bar with equivalence ratios ranging from 0.3 to 1.0. Results from the RCM experiments are also simulated using a four-component gasoline surrogate model which includes n-heptane, iso-octane, toluene, and 2-pentene. For the conditions investigated, good agreement between the experiments and the four-component surrogate model, in terms of first-stage and total ignition delay times as well as the comparison of measured and simulated pressure traces, is demonstrated. Kinetic analysis is further conducted to understand the role of the different hydrocarbon classes present in gasoline in controlling autoignition.     

Spectroscopic and chemical-kinetic analysis of the phases of HCCI autoignition and combustion for single- and two-stage ignition fuels

The temporal phases of autoignition and combustion in an HCCI engine have been investigated in both an all-metal engine and a matching optical engine. Gasoline, a primary reference fuel mixture (PRF80), and several representative real-fuel constituents were examined. Only PRF80, which is a two-stage ignition fuel, exhibited a “cool-flame” low-temperature heat-release (LTHR) phase. For all fuels, slow exothermic reactions occurring at intermediate temperatures raised the charge temperature to the hot-ignition point. In addition to the amount of LTHR, differences in this intermediate-temperature heat-release (ITHR) phase affect the fuel ignition quality. Chemiluminescence images of iso-octane show a weak and uniform light emission during this phase. This is followed by the main high-temperature heat-release (HTHR) phase. Finally, a “burnout” phase was observed, with very weak uniform emission and near-zero heat-release rate (HRR). To better understand these combustion phases, chemiluminescence spectroscopy and chemical-kinetic analysis were applied for the single-stage ignition fuel, iso-octane, and the two-stage fuel, PRF80. For both fuels, the spectrum obtained during the ITHR phase was dominated by formaldehyde chemiluminescence. This was similar to the LTHR spectrum of PRF80, but the emission intensity and the temperature were much higher, indicating differences between the ITHR and LTHR phases. Chemical-kinetic modeling clarified the differences and similarities between the LTHR and ITHR phases and the cause of the enhanced ITHR with PRF80. The HTHR spectra for both fuels were dominated by a broad CO continuum with some contribution from bands of HCO, CH, and OH. The modeling showed that the CO+O→CO₂+hν reaction responsible for the CO continuum emission tracks the HTHR well, explaining the strong correlation observed experimentally between the total chemiluminescence and HRR during the HTHR phase. It also showed that the CO continuum does not contribute to the ITHR and LTHR chemiluminescence. Bands of H₂O and O₂ in the red and IR regions were also detected during the HTHR, which the data indicated were most likely due to thermal excitation. The very weak light emission in the “burnout” phase also appeared to be thermal emission from H₂O and O₂.     

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 HO₂ and OH as temperature increases during the delay time. Termination reactions between stable radicals seem to have a minor impact at low temperature.     

A reduced chemical kinetic model for IC engine combustion simulations with primary reference fuels

A reduced chemical kinetic mechanism for the oxidation of primary reference fuel (PRF) has been developed and applied to model internal combustion engines. Starting from an existing reduced reaction mechanism for n-heptane oxidation, a new reduced n-heptane mechanism was generated by including an additional five species and their relevant reactions, by updating the reaction rate constants of several reactions pertaining to oxidation of carbon monoxide and hydrogen, and by optimizing reaction rate constants of selected reactions. Using a similar approach, a reduced mechanism for iso-octane oxidation was built and combined with the n-heptane mechanism to form a PRF mechanism. The final version of the PRF mechanism consists of 41 species and 130 reactions. Validation of the present PRF mechanism was performed with measurements from shock tube tests, and HCCI and direct injection engine experiments available in the literature. The results show that the present PRF mechanism gives reliable performance for combustion predictions, as well as computational efficiency improvements for multidimensional CFD simulations.     

Sooting tendencies of primary reference fuels in atmospheric laminar diffusion flames burning into vitiated air

In this study, the sooting tendencies of primary reference fuels (PRFs) are measured in term of yield sooting indices (YSIs) in methane diffusion flames doped with the vapors of PRFs. The present paper represents an incremental advance complementing the original methodology prescribed by McEnally and Pfefferle. The influence of both PRF formulation and CO₂ dilution of the coflowing air on the YSIs is also assessed. The diffusion flames burning in a coflowing oxidizer stream are established over the Santoro’s burner and vapor of the liquid fuel to be investigated is injected into the fuel stream. Laser extinction measurements are performed to map the two-dimensional field of soot volume fraction in the flame. For the pure liquid hydrocarbons investigated, i.e., n-hexane, n-heptane, isooctane, and benzene, the YSI reported in the original paper by McEnally and Pfefferle quantitatively predict the sooting propensities, measured here at much higher dopant concentrations. The present study therefore extends the consistency of the YSI methodology on the Santoro’s burner. For blends of n-heptane and isooctane, the sooting tendency of doped flames exhibits regular and monotonic trends and decreases with increasing n-heptane mole fraction or CO₂ dilution. Interestingly, the evolution of YSI with the isooctane mole fraction exhibits a strong similarity for varying CO₂ mole fraction. A quadratic least-squares fit is then derived, providing a phenomenological model of YSI as a function of both isooctane mole fraction in the fuel stream and CO₂ mole fraction in the oxidizer. A non-negligible cross effect of PRF formulation and CO₂ dilution on YSI is revealed. The method elaborated within the framework of the present paper could be extended to surrogate fuels. This would help develop a comprehensive database and empirical correlations that could predict the sooting propensities of different surrogate fuels, therefore their potentially mitigationed soot production through control of fuel composition and/or exhaust gas recirculation. This database would also be useful for the validation of CFD simulations incorporating sophisticated model of soot production.     

Ignition characteristics of heptane-hydrogen and heptane-methane fuel blends at elevated pressures

There is significant interest in using hydrogen and natural gas for enhancing the performance of diesel engines. We report herein a numerical investigation on the ignition of n-C₇H₁₆/H₂ and n-C₇H₁₆/CH₄ fuel blends. The CHEMKIN 4.1 software is used to model ignition in a closed homogenous reactor under conditions relevant to diesel/HCCI engines. Three reaction mechanisms used are (i) NIST mechanism involving 203 species and 1463 reactions, (ii) Dryer mechanism with 116 species and 754 reactions, and (iii) a reduced mechanism (Chalmers) with 42 species and 168 reactions. The parameters include pressures of 30 atm and 55 atm, equivalence ratios of ? = 0.5, 1.0 and 2.0, temperature range of 800-1400 K, and mole fractions of H₂ or CH₄ in the blend between 0 and 100%. For n-C₇H₁₆/air mixtures, the Chalmers mechanism not only provides closer agreement with measurements compared to the other two mechanisms, but also reproduces the negative temperature coefficient regime. Consequently, this mechanism is used to characterize the effects of H₂ or CH₄ on the ignition of n-C₇H₁₆. Results indicate that H₂ or CH₄ addition has a relatively small effect on the ignition of n-C₇H₁₆/air mixtures, while the n-C₇H₁₆ addition even in small amount modifies the ignition of H₂/air and CH₄/air mixtures significantly. The n-C₇H₁₆ addition decreases and increases the ignition delays of H₂/air mixtures at low and high temperatures, respectively, while its addition to CH₄/air mixtures decreases ignition delays at all temperatures. The sensitivity analysis indicates that ignition characteristics of these fuel blends are dominated by the pyrolysis/oxidation chemistry of n-heptane, with heptyl (C₇H₁₆-2) and hydoxyl (OH) radicals being the two most important species.     

Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions

An experimental investigation of the autoignition for various n-decane/oxidizer mixtures is conducted using a rapid compression machine, in the range of equivalence ratios of ?=0.5-2.2, dilution molar ratios of N₂/(O₂ + N₂) = 0.79-0.95, compressed gas pressures of PC=7-30 bar, and compressed gas temperatures of TC=635-770 K. The current experiments span a temperature range not fully investigated in previous autoignition studies on n-decane. Two-stage ignition, characteristic of large hydrocarbons, is observed over the entire range of conditions investigated, as demonstrated in the plots of raw experimental pressure traces. In addition, experimental results reveal the sensitivity of the first-stage and total ignition delays to variations in fuel and oxygen mole fractions, pressure, and temperature. Predictability of two kinetic mechanisms is compared against the present data. Discrepancies are noted and discussed, which are of direct relevance for further improvement of kinetic models of n-decane at conditions of elevated pressures and low-to-intermediate temperatures.     

Direct numerical simulations of NOx effect on multistage autoignition of DME/air mixture in the negative temperature coefficient regime for stratified HCCI engine conditions

Direct numerical simulations (DNSs), for a stratified flow in HCCI engine-like conditions, are performed to investigate the effects of exhaust gas recirculation (EGR) by NO_x and temperature/mixture stratification on autoignition of dimethyl ether (DME) in the negative temperature coefficient (NTC) region. Detailed chemistry for a DME/air mixture with NO_x addition is employed and solved by a hybrid multi-time scale (HMTS) algorithm. Three ignition stages are observed. The results show that adding (1000 ppm) NO enhances both low and intermediate temperature ignition delay times by the rapid OH radical pool formation (one to two orders of magnitude higher OH radicals concentrations are observed). In addition, NO from EGR was found to change the heat release rates differently at each ignition stage, where it mainly increases the low temperature ignition heat release rate with minimal effect on the ignition heat release rates at the second and third ignition stages. Sensitivity analysis is performed and the important reactions pathways for low temperature chemistry and ignition enhancement by NO addition are specified. The DNSs for stratified turbulent ignition show that the scales introduced by the mixture and thermal stratifications have a stronger effect on the second and third stage ignitions. Compared to homogenous ignition, stratified ignition shows a similar first autoignition delay time, but about 19% reduction in the second and third ignition delay times. Stratification, however, results in a lower averaged LTC ignition heat release rate and a higher averaged hot ignition heat release rate compared to homogenous ignition. The results also show that molecular transport plays an important role in stratified low temperature ignition, and that the scalar mixing time scale is strongly affected by local ignition. Two ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode (the D-mode) and a spontaneous, high-speed, kinetically driven ignition mode (the S-mode). Three criteria are introduced to distinguish the two modes by different characteristic time scales and Damkhöler (Da) number using a progress variable conditioned by a proper ignition kernel indicator (IKI). The results show that the spontaneous ignition S-mode is characterized by low scalar dissipation rate, high displacement speed flame front, and high mixing Damkhöler number, while the D-mode is characterized by high scalar dissipation rate, low displacement speeds in the order of the laminar flame speed and a lower than unity Da number. The proposed criteria are applied at the different ignition stages.     

Detailed numerical simulations of the autoignition of single n-heptane droplets in air

The autoignition process of single n-heptane droplets in air is simulated for spherical symmetry and at constant pressure. Using a detailed transport model and detailed chemical kinetics, the governing equations of the two phases are solved in a fully coupled way. The ambient gas temperature is varied from 600 to 2000 K. Simulations are performed for isobaric conditions. The initial droplet radius ranges from 10 to 200 μm. The influence of different physical parameters, such as ambient pressure, droplet radius, or initial conditions, on the ignition delay time and the location of the ignition is investigated. The gas temperature turns out to be the parameter dominating the ignition process. The droplet temperature shows a minor influence on the ignition delay time. The influence of the droplet radius on the ignition delay shows a high sensitivity to other ambient conditions, such as ambient temperature and pressure.