Iso-octane Combustion Research Articles

An experimental and kinetic modeling study of the impact of nitric oxide and hydrogen on iso-Octane combustion

The knowledge of the combined effects of exhaust recirculation and hydrogen enrichment on hydrocarbon fuels combustion is limited to the complexity of the ternary interaction. In this work, experimental and kinetic modeling techniques were combined to investigate the chemical effects of high-level NO, H2, or NO/H2 additions on hydrocarbon fuels. A series of jet-stirred reactor experiments were conducted on the oxidation of iso-Octane with NO/H2 additions at atmospheric pressure and over a wide range of equivalence ratios and temperatures. Experimental results reveal that high concentrations of NO improves iso-Octane oxidation in the temperature region of 800−1000 K and promotes its consumption more efficiently under the ultra-lean condition than stoichiometric and rich conditions. The addition of H2 slightly enhances iso-Octane oxidation by facilitating hydrogen abstraction reactions under the ultra-lean condition, but this effect is insignificant in the stoichiometric condition. The addition of combined H2 and NO demonstrates a more significant promoting effect on iso-Octane oxidation under both ultra-lean and stoichiometric conditions, indicating additive effect in the global reactivity. Existing kinetic models from the literature were incapable to capture these effects. We developed a detailed model which better accounts for the competing chain promotion/termination effects involved in NO sensitization on fuel oxidation, by modifying important reaction rate constants and introducing key nitrogen- containing C3 species. NO and combined NO/H2 additions both enhance the fuel initiation process, primarily through the NO+HO2=NO2+OH reaction, which promotes hydrogen abstraction pathways by converting HO2 to more active OH radical. The NO/H2 combination leads to intricate effects on intermediate species, driven by the competition between NO+HO2 and H+NO2 reactions for OH evolution, as well as the competition between H+O2 and H+NO2 for H consumption. This work contributes to the understanding of ternary hydrocarbon/NO/H2 interactions, providing insights into the design of improved fuel systems to optimize engine performance and mitigate emissions. Novelty and significance statementThis study investigates the effects of high-level NO, H2, or NO/H2 additions on iso-Octane oxidation through experiments and kinetic modeling. New JSR experiments were performed on the oxidation of iso-Octane with and without NO, H2, or NO/H2 additions at various equivalence ratios, 1 atm, and temperatures ranging from 600−1100 K. Experimental results reveal that high-level NO enhances iso-Octane oxidation under the ultra-lean condition in 800−1000 K. H2 has a moderate impact under the ultra-lean condition, but its effect is insignificant in the stoichiometric condition. Combined H2 and NO significantly enhance iso-Octane oxidation in ultra-lean and stoichiometric conditions. The developed detailed model outperforms existing ones in predicting these effects on fuel oxidation. This research contributes to understanding hydrocarbon/NO/H2 interactions and optimizing fuel systems.

Auto-ignition and knocking combustion characteristics of iso-octane-ammonia fuel blends in a rapid compression machine

Ammonia has attracted much attention because of its carbon-free nature. Ammonia also has a high research octane number (RON), making it a promising anti-knock additive for conventional engine fuels. In this study, to investigate the effects of ammonia addition on the knocking combustion of iso-octane, a series of spark ignition experiments were conducted in an optical rapid compression machine. And compression ignition experiments were also carried out to measure the ignition delay times. The molar fractions of ammonia in iso-octane-ammonia blends were set as 80% (A80), 40% (A40), and 0% (A0), and the initial thermodynamic conditions were 630–840 K and 8–25 bar. Chemical kinetics analysis of the end-gas was also performed with a blended reaction mechanism obtained by directly merging the LLNL iso-octane mechanism and the Glarborg ammonia mechanism. The experimental results showed that the ignition delay time increased with increasing ammonia fraction, but this trend could only be qualitatively predicted by the blended mechanism. At the same initial pressure and temperature conditions, the knock intensity (KI) decreased with increasing ammonia fraction, but the inhibitory effect was less pronounced at high initial temperature conditions. At the same initial energy density and high-temperature conditions, the KI of A40 and A80 was higher than A0, which was ascribed to the higher pressure requirement for the blended fuels with higher ammonia fractions to achieve the same energy density as those with lower ammonia fractions. At low temperature and pressure conditions, the auto-ignition showed a two-stage characteristic. With the increasing fraction of ammonia, the first-stage auto-ignition shifted from supersonic to subsonic, and the second-stage detonative auto-ignition gradually disappeared. The weakened auto-ignition with increasing ammonia fraction was attributed to the less reactive end-gas, as evidenced by the lower mole fraction of OH radical and lower heat release rate in the end-gas at the auto-ignition timing. When using the ε- ξ diagram to examine the auto-ignition mode, the experimental detonation cases were well distributed in the detonation peninsula, but the non-detonation and critical detonation cases were scattered irregularly.

Influence of n‐butanol and isomers on the combustion mechanisms of isooctane and coke formation based on ReaxFF simulation

AbstractFuel additives play a significant role in enhancing the thermal stability of fuel combustion. The effect of additives on the combustion of hydrocarbon fuels and the combustion performances of isooctane with three different isomer additives, (n‐butanol (1‐BuOH), diethyl ether (DEE), and 2‐butanol(2‐BuOH)), and additive‐free isooctane were explored by ReaxFF simulation in this work. The simulation system was heated to 3000 K at a heating rate of 10 K/ps and kept stable at 3000 K. A variety of combustion products (e.g., small gas molecules and C1‐C8 hydrocarbon compounds) in each system were analyzed, and the reaction paths were speculated based on the computed trajectory. The simulation results showed that the CC bond scission reaction dominated the combustion process of the three additives. All three additives promote the formation of toxic carbonyl compounds such as formaldehyde, while the pure DEE additive has the best inhibition effect on the formation of the coke precursor, C2H2, C3H4, and C3H6. The pure 1‐BuOH additive can shorten the initial reaction time of the reactants. The effects of DEE/1‐BuOH additive on the combustion of isooctane were investigated to obtain a desirable additive mixture with good performance. The expansion of the DEE proportion (80%/20% DEF/1‐BuOH) shows a slightly better coke (C3H4 and C3H6) reduction effect, while the inhibition effect is not as obvious as that of a pure DEE additive.

On Iso-octane Combustion with Ozone Addition under HCCI Engine-Like Conditions

Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time (IDT) of iso-octane/air mixtures under typical operating conditions of HCCI engines. 0-D and 2-D CFD simulations were carried out to compute IDT and to characterize compression, combustion, and expansion in an HCCI engine, respectively. A kinetic model was developed by merging a mechanism for iso-octane, a sub-mechanism for nitrogen oxides, and a sub-mechanism for ozone. The model was used to investigate iso-octane/air/ozone mixtures under typical operating conditions of HCCI engines running with very lean mixtures (equivalence ratio equal to 0.3). Parametric analyses were carried out considering different values of temperature (500 to 1200 K), pressure (15 to 40 bar) and ozone concentration (0 to 50 ppm). The results show that as ozone concentration increases IDT decreases, with a greater impact at low temperatures, and the NTC (Negative Temperature Coefficient) effect decreases. However, the reduction of IDT with ozone addition is less as ozone concentration increases, especially at low temperatures. An increased pressure, on the other hand, generally promotes faster ozone decomposition and enhances ozone effect on IDT reduction for all temperatures except in the range 760-840 K, where the opposite effect occurs due to NTC phenomenon. Finally, when temperature is very high, i.e. 1200 K, both ozone and pressure have little influence on the percentage reduction of IDT.

Quantitative evaluation of several chemical mechanisms for gasoline surrogate-ethanol blends and study of dominant chemistry under super-knock relevant conditions

A comprehensive database that covers wide ranges of temperature, pressure and equivalence ratio conditions was established for gasoline surrogate fuels composed of n-heptane, iso-octane, toluene with and without ethanol addition. The database includes ignition delay time measurements in shock tubes and laminar flame velocity measurements which are relevant combustion properties under super-knock relevant conditions. Due to the lack of reaction pathways for low-temperature iso-octane combustion and ethanol oxidation in Caltech mechanism, an updated mechanism has been constructed. The update has been carried out as follows: (i) changing the iso-octane sub-mechanism, (ii) adding an ethanol sub-mechanism, and (iii) modifying thermodynamic and kinetic parameters for few species and reactions for removing collision limit violation. The predictive capabilities of the modified Caltech mechanism and of three reaction models developed for n-heptane/iso-octane/toluene/ethanol based mixtures have been quantified using the model evaluation method of Olm. Significant improvement of the combustion properties predictions by the modified Caltech model has been demonstrated by the lower error value. For the prediction of ignition delay time, the updated Caltech mechanism shows better performance than the other models, especially in the high-temperature regime. Error maps were drawn over wide ranges of thermodynamic conditions to show the overall performance of all the evaluated mechanisms. For the prediction of laminar burning velocity, the modified Caltech mechanism performs better for most cases. Laminar burning velocity is always over-estimated by Livermore’s mechanism. The new mechanism was employed to determine the most important chemical pathways under super-knock relevant conditions corresponding to the following thermodynamic states: (630 K, 2.3 MPa), (750 K, 6.0 MPa), (720 K, 2.8 MPa), and (1000 K, 12.0 MPa). The modified Caltech model, or reduced versions of it, could be employed in future work to investigate super-knock for realistic gasoline surrogate.

Study of water and hydrogen addition on the pyrolysis and combustion characteristics of isooctane

Water injection and hydrogen combustion technologies have gained widespread attention in recent years. Combining H2O and H2 injection could bring more desirable combustion based on the needs of different engine conditions. To fully explore what effects the simultaneous injection of H2O and H2 would have on the combustion process of isooctane, three different subtests were performed in this study: investigation on chemical reaction mechanism, the laminar flame speed measurements and real engine tests. Firstly, H2O and H2 were added to the JSR (Jet Stirred Reactor) reactor to analyze the synergistic effect of H2O and H2 on the pyrolysis process of isooctane. The experimental results showed that the addition of H2O and H2 leads to an increase of OH and H radicals, which increases the concentrations of C1-C3 species and decreases the concentrations of C4 species. To further investigate the effect of H2O and H2 on the radical reaction of isooctane combustion process, this study then measured the laminar flame speed of IC8H18/H2/O2/H2O gas mixture by using constant volume combustion bomb, and found that water injection would reduce the laminar flame speed, and H2 would increase the laminar flame speed accordingly. Sensitivity analysis showed that it was more sensitive to reactions R1, R97 and R12, and was more sensitive to H2O than to H2. Finally, real engine tests were performed on a modified 1.5L gasoline equipped with port H2O and H2 injection systems at 1300 rpm with 30% throttle opening. The results showed that the use of H2O and H2 permitted the manipulation of engine combustion speed. Specifically, H2O injection led to a decrease in the peak cylinder pressure; and the addition of H2 increased the cylinder pressure again·H2O injection resulted in increased ignition delay and slower flame spread, slowing the engine's burn rate when needed. Using H2 addition led to a short ignition delay and accelerated combustion velocity, which counteracted the negative impacts of H2O injection and elevated the engine combustion speed to optimize efficiency. In summary, this paper revealed the complementary coordination between H2O and H2 through experimental and simulation studies. It will provide theoretical guidance for the development of H2O injection with H2 technology for gasoline engines.

Study of the heat release and exergy loss of iso-octane self-ignition under engine-like conditions

With the severe low-carbon policies, energy utilization from the maximum useful-work exergy becomes one of the breakthrough points towards engine high efficiency. Hence, for constraining the combustion which is the main exergy loss source, the energy analysis from second law is imperative for the further efficiency improving. In this paper, the influence of boundary conditions on the heat release and exergy loss of chemical reaction during the constant-volume combustion of iso-octane at engine-like conditions was studied. It was found that the boundary conditions: initial temperature of 700 K, 900 K and 1100 K, equivalence ratio of 0.5, 1.0 and 1.5, have the same trend effect on the heat release and exergy loss rates of the same chemical reaction. It is impossible to increase the heat release and reduce the exergy loss at the same time, but different equivalent ratios have different degree effects on the reaction. Hence, a parameter α, the ratio of exergy loss on heat release, which stands for the exergy loss of unit heat release was promoted to consider the cost feature of chemical reactions. The results show that enhancing the medium and high temperature reaction during the combustion process is beneficial to reduce the parameter α, and the lean mixture is beneficial to obtain a small α. From the perspective of reaction type, it showed that the α of Fuel Fragment series reaction is the largest and its α is controlled by both temperature and equivalent ratio, the rich mixture should be conducive to reduce its α. The α of H2O2 Loop, H-O and CO-CO2 series reaction are small, and they are mainly controlled by the equivalence ratio, and lean mixture is favourable for reducing their α. Therefore, under the constant equivalence ratio, the best way to optimize the heat release and exergy loss is to control the equivalence ratio as small as 1.0. Under the strategy of the variable equivalence ratio, it is the best to change the mixture from rich to lean in the combustion process, and the reactions would sequentially go through the best α region of Fuel Fragment, H-O, CO-CO2, H2O2 Loop reactions, which would keep the α of whole combustion process maintain at the lowest level.

Development of Chemical Kinetics Models from Atomistic Reactive Molecular Dynamics Simulations: Application to Iso-octane Combustion and Rubber Ablative Degradation.

Modeling the complex chemical phenomena resulting from multiple active species and long-chain polymers is limited by uncertainties in the reaction rate parameters, which increase rapidly with the number of active species and/or reaction pathways. Reactive molecular dynamics simulations have the potential to help obtain in-depth information on the chemical reactions that occur between the polymer (e.g., ablative material) and the multiple active species in an aggressive environment. In this work, we demonstrate that molecular dynamics (MD) simulations using the ReaxFF interatomic potential can be used to obtain the reaction kinetics of complex reaction pathways at high temperatures. We report two recently developed tools, namely, MolfrACT and KinACT, designed to extract chemical kinetic pathways by postprocessing reactive MD simulation data. The pathway extraction is based on a new algorithm, Consistent Reaction Stoichiometry via an Iterative Scheme (CReSIS), for the automated extraction of reactions and kinetics from MD trajectories. As a validation of the methodology, we first report the kinetic analysis and mechanisms for the high-temperature combustion of iso-octane. The observed reaction pathways are consistent with experimental models. In addition, we compare the activation energies of select iso-octane combustion pathways with experimental data and show that nanosecond timescale molecular dynamics simulations are sufficient to obtain realistic estimates of activation energies for different fuel consumption reaction pathways at high temperatures. The framework developed here can potentially be combined with time-series forecasting and machine learning methods to further reduce the computational complexity of transient molecular dynamics simulations, yet yielding realistic chemical kinetics information. Subsequently, the CReSIS scheme applied to ethylene-propylene-diene-monomer (EPDM) rubber ablative reveals that H2O, C2H4, and HCHO are the major products during the initial stages of the polymer degradation in high-temperature oxidative environments. While prior work involving ReaxFF simulations is restricted to overall rates of formation of any species, we extract kinetic information for individual reaction pathways. In this paper, we present several reaction pathways observed during the EPDM rubber degradation into the dominant products and report the pathway-specific reaction rates. Arrhenius analysis reveals that the dominant reaction pathway activation energies for the formation of water, ethylene, and formaldehyde are 34.42, 27.26, and 6.37 kcal/mol, respectively. In contrast, the activation energies for the overall formation (across all reaction pathways) of these products are in the 40-50 kcal/mol range.

Investigation on the dual-fuel active-thermal atmosphere combustion strategy based on optical diagnostics and numerical simulations

The dual-fuel active-thermal atmosphere combustion strategy has the potential to make the homogeneous combustion compression ignition more controllable, achieving low fuel consumption and engine emissions. However, the detailed combustion process and chemical interaction between the high- and low- reactivity fuels have not been fully understood. This work investigated the premixed iso-octane combustion assisted by the two-stage reaction of n-heptane on an optical engine. High-speed imaging technique was applied to visualize the natural flame luminosity and a data-processing method based on Cantera was proposed to evaluate the detailed chemical kinetics process. Results indicate that the high-temperature heat release stage changes from partially premixed combustion with distinct blue flames into the mixing-controlled combustion, showing evident signs of soot radiation when the two-stage ignition transits into three-stage ignition. Heat release from the premixed n-heptane is initially dominated by reaction HCO + O2 = CO + HO2 and turns into hydrogen-oxygen reactions with increasing temperature. In the primary reacting regions for the cases with latter injection timings, reactions C3H5 + H(+M) = C3H6(+M) and C3H3 + H(+M) = C3H4(+M) play the major role, which promotes the formation of soot precursors. Formaldehyde can represent the mixture reactivity. Reaction pathway of premixed n-heptane generated reactive species that promote the consumption of iso-octane. Reactions of iso-octane also enhance the formation of reactive species that further accelerate the combustion process. Retardation of the iso-octane injection timing reduces the local mixture reactivity and weakens the chemical interaction between iso-octane and n-heptane. This lowers the heat release rate, and, objectively, results in the transition of two-stage ignition to three-stage ignition.

Shock-Tube Laser Absorption Measurements of CO and H2O during Iso-Octane Combustion

The oxidation of iso-octane was studied in two different shock tubes by recording mole fraction time histories of CO and H2O at various equivalence ratios (0.5, 1.0, and 2.0) at around 1.5 atm, bet...

Skeletal Kinetic Mechanism Generation and Uncertainty Analysis for Combustion of Iso-octane at High Temperatures

A detailed mechanism for combustion of iso-octane with 116 species and 754 reactions has been reduced using a directed relation graph with error propagation (DRGEP) and DRGEP with sensitivity analysis (DRGEPSA) methods under high-temperature conditions. Two skeletal mechanisms, i.e., a 63-species mechanism with a maximum error of 7.2% and a 51-species mechanism with a maximum error of 28.5% on autoignition delay times have been generated. These two skeletal mechanisms are shown to reproduce ignition delays, laminar flame speeds, species and temperature profiles in good agreement with those of the detailed mechanism. Uncertainty in the ignition predictions by detailed and two skeletal mechanisms induced by the uncertainties in reaction rate coefficients has been studied. Probability distribution of autoignition predictions demonstrated that the 63-species mechanism can still keep the uncertainty characteristics, while the 51-species mechanism has significant discrepancy compared with the detailed one. Furt...

Various effects of reformer gas enrichment on natural-gas, iso-octane and normal-heptane HCCI combustion using artificial inert species method

Reformer gas (syngas) addition to main fuel is a practical solution for combustion timing control in HCCI engines. This study emphasizes the understanding of various effects of reformer gas (RG) addition, with composition of 75%vol H2 and 25%vol CO, in HCCI combustion by developing an artificial inert species method and using a detailed chemical kinetics multi-zone model. Three fuels (iso-octane, n-heptane, and natural gas) with different autoignition characteristics were used in this study. The developed multi-zone model was validated for mentioned fuels at various percentages of RG using six experimental cases of a single-cylinder CFR engine. The results showed that increasing reformer gas fraction in the fuel mixture advanced methane fuel combustion timing, retarded the combustion of n-heptane and had insignificant effect on iso-octane combustion. Thermal effect of RG in all fuels resulted in earlier start of combustion (SOC) because of the mixture specific heat ratio enhancement. The SOC for methane and iso-octane fuels was advanced by RG addition due to the chemical effect of RG. However, the chemical effect of RG for methane fuel is more significant. By adding 30% RG to methane fuel, H2 advanced the SOC by −5.6 °CA and CO retarded it by 1.4 °CA due to chemical effect and also the interaction effect of CO and H2 advanced the SOC by −1.8 °CA. For iso-octane fuel, adding 30% RG, both CO and H2 advanced the SOC by 0.4 CA due to their chemical effect. On the other hand, in n-heptane fuel, H2 is almost responsible for all of the chemical effects of RG for retardation of SOC.

Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/argon premixed flames

A modified one-dimensional model (Premix) in conjunction with Chemkin II and a detailed kinetic scheme combining the chemistries of hydrogen, dimethyl ether and iso-octane combustion were used to investigate the effect of hydrogen addition on the chemical composition of laminar premixed dimethyl ether/iso-octane/oxygen/argon flames. In the current modeling study, the key reaction mechanisms responsible for the observed variation in mole fractions of some major species, including radicals H, O and OH, species CO, CO2 and CH4 as well as oxygenated products CH2O and CH3CHO, were defined. The numerical modeling was focused on both chemical and dilution and thermal effects of hydrogen addition. It was found that chemical effect of hydrogen addition induced a boosting in the oxidation process whereas dilution effect led to its inhibition. The higher the hydrogen level the higher was the magnitude of the phenomenon. Besides, the chemical effect of hydrogen addition induced an enhancement in the OH, O, CO, CO2 and CH4 mole fractions, whereas the dilution and thermal effect led to a lowering in the amount of these species. This latter phenomenon was more important than the first effect yielding a net decrease in the concentrations of the four species. In addition, it was found that CH2O and CH3CHO levels showed a dramatic decrease in peak height with hydrogen doping and that the contribution of the thermal effect of hydrogen addition to this lowering was more important than that of its chemical effect. Thus it can be concluded that the synergism between chemical and dilution effects played a paramount role in the dimethyl ether/iso-octane/oxygen/argon premixed flames oxidation process as well as in the rise or the decrease in the pool radicals and the major species concentrations.

A comprehensive iso-octane combustion model with improved thermochemistry and chemical kinetics

Iso-Octane (2,2,4-trimethylpentane) is a primary reference fuel and an important component of gasoline fuels. Moreover, it is a key component used in surrogates to study the ignition and burning characteristics of gasoline fuels. This paper presents an updated chemical kinetic model for iso-octane combustion. Specifically, the thermodynamic data and reaction kinetics of iso-octane have been re-assessed based on new thermodynamic group values and recently evaluated rate coefficients from the literature. The adopted rate coefficients were either experimentally measured or determined by analogy to theoretically calculated values. Furthermore, new alternative isomerization pathways for peroxy-alkyl hydroperoxide (ȮOQOOH) radicals were added to the reaction mechanism. The updated kinetic model was compared against new ignition delay data measured in rapid compression machines (RCM) and a high-pressure shock tube. These experiments were conducted at pressures of 20 and 40 atm, at equivalence ratios of 0.4 and 1.0, and at temperatures in the range of 632–1060K. The updated model was further compared against shock tube ignition delay times, jet-stirred reactor oxidation speciation data, premixed laminar flame speeds, counterflow diffusion flame ignition, and shock tube pyrolysis speciation data available in the literature. Finally, the updated model was used to investigate the importance of alternative isomerization pathways in the low temperature oxidation of highly branched alkanes. When compared to available models in the literature, the present model represents the current state-of-the-art in fundamental thermochemistry and reaction kinetics of iso-octane; and thus provides the best prediction of wide ranging experimental data and fundamental insights into iso-octane combustion chemistry.

Analysis of entropy generation and exergy losses of iso-octane and n-butanol adiabatic constant-volume combustion process

Butanol is a promising bio-fuel. Using iso-octane as gasoline surrogate, the entropy production and exergy balance of n-butanol and iso-octane constant volume combustion process were analysed with the CHEMKIN software by considering detailed n-butanol and iso-octane chemical reaction mechanisms. The results of exergy balance analysis showed that, during the adiabatic constant-volume combustion process, there is a trade-off between exergy destruction and exergy loss due to incomplete combustion. The exergy destruction of n-butanol is obviously higher than that of iso-octane. The results of entropy generation analysis showed that, with high initial temperature, two apparent peaks of heat release rate and exergy loss rate are observed both for iso-octane and n-butanol. During combustion process, the internal energy exchange between high temperature product and the unburned reactant via a series of chemical reactions, especially the reactions of small molecule intermediate products, is the major reason for the exergy destruction.

A detailed kinetic mechanism of multistage oxidation and combustion of isooctane

A detailed kinetic mechanism of multistage oxidation and combustion of isooctane

Flame front analysis of ethanol, butanol, iso-octane and gasoline in a spark-ignition engine using laser tomography and integral length scale measurements

Direct-injection spark-ignition engines have become popular due to their flexibility in injection strategies and higher efficiency; however, the high-pressure in-cylinder injection process can alter the airflow field by momentum exchange, with different effects for fuels of diverse properties. The current paper presents results from optical studies of stoichiometric combustion of ethanol, butanol, iso-octane and gasoline in a direct-injection spark-ignition engine run at 1500 RPM with 0.5 bar intake plenum pressure and early intake stroke fuel injection for homogeneous mixture preparation. The analysis initially involved particle image velocimetry measurements of the flow field at ignition timing with and without fuelling for comparison. Flame chemiluminescence imaging was used to characterise the global flame behaviour and double-pulsed Laser-sheet flame tomography by Mie scattering to quantify the local topology of the flame front. The flow measurements with fuel injection showed integral length scales of the same order to those of air only on the tumble plane, but larger regions with scales up to 9 mm on the horizontal plane. Averaged length scales over both measurement planes were between 4 and 6 mm, with ethanol exhibiting the largest and butanol the smallest. In non-dimensional form, the integral length scales were up to 20% of the clearance height and 5–12% of the cylinder bore. Flame tomography showed that at radii between 8 and 12 mm, ethanol was burning the fastest, followed by butanol, iso-octane and gasoline. The associated turbulent burning velocities were 4.6–6.5 times greater than the laminar burning velocities and about 13–20% lower than those obtained by flame chemiluminescence imaging. Flame roundness was 10–15% on the tomography plane, with largest values for ethanol, followed by butanol, gasoline and iso-octane; chemiluminescence imaging showed larger roundness (18–25%), albeit with the same order amongst fuels. The standard deviation of the displacement of the instantaneous flame contour from one filtered by its equivalent radius was obtained as a measure of flame brush thickness and correlated strongly with the equivalent flame radius; when normalised by the radius, it was 4–6% for all fuels. The number of crossing points between instantaneous and filtered flame contour showed a strong negative correlation with flame radius, independent of fuel type. The crossing point frequency was 0.5–1.6 mm−1. The flame brush thickness was about 1/10th of the integral length scale. A positive correlation was found between integral length scale and flame brush thickness and a negative correlation with crossing frequency.

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.

Detailed kinetic mechanism of the multistage oxidation and combustion of isobutane

This study has been focused on the construction of a detailed kinetic mechanism of oxidation and combustion of isooctane (2,2,4-trimethylpentane) to describe both high-temperature reactions and the low-temperature multistage process with separated stages of “cool” and “blue” flames and hot explosion. In accordance with the proposed mechanism, isobaric autoignition, compression-induced autoignition, and flame propagation characteristics have been calculated; the calculation results have been compared with the experimental data. Satisfactory qualitative and quantitative agreement of the calculation and experimental results has been obtained.

CFD simulations using the TDAC method to model iso-octane combustion for a large range of ozone seeding and temperature conditions in a single cylinder HCCI engine

While HCCI engines have many advantages such as better efficiency and lower emissions, the control of these engines is still complex. In this context, cycle-to-cycle control using oxidizing species such as ozone (O3) presents a good potential. Only 1ppm of O3 has already a significant impact on the ignition. Predicting this effect as well as the low temperature combustion and ignition generally requires a detailed mechanism. Using CFD simulations with the Tabulation of Dynamic Adaptive Chemistry (TDAC) method and a mechanism including more than 1000 chemical species, we have predicted the effect of ozone seeding on the combustion of iso-octane in a HCCI engine. This paper presents the validation of the CFD simulations with a large range of ozone concentration and initial temperature. It shows that the TDAC method is able to reproduce accurately such complex chemical phenomena accounting for heat transfer and chemical reactions in a real geometry.