Detailed kinetic models for the low-temperature auto ignition of gasoline surrogates (original) (raw)
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Ignition studies of n-heptane/iso-octane/toluene blends
Combustion and Flame, 2016
Ignition delay times of four ternary blends of n-heptane/iso-octane/toluene, referred to as Toluene Primary Reference Fuels (TPRFs), have been measured in a high-pressure shock tube and in a rapid compression machine. The TPRFs were formulated to match the research octane number (RON) and motor octane number (MON) of two high-octane gasolines and two prospective low-octane naphtha fuels. The experiments were carried out over a wide range of temperatures (650-1250 K), at pressures of 10, 20 and 40 bar, and at equivalence ratios of 0.5 and 1.0. It was observed that the ignition delay times of these TPRFs exhibit negligible octane dependence at high temperatures (T > 1000 K), weak octane dependence at low temperatures (T < 700 K), and strong octane dependence in the negative temperature coefficient (NTC) regime. A detailed chemical kinetic model was used to simulate and interpret the measured data. It was shown that the kinetic model requires general improvements to better predict low-temperature conditions and particularly requires improvements for high sensitivity (high toluene concentration) TPRF blends. These datasets will serve as important benchmark for future gasoline surrogate mechanism development and validation.
SAE Technical Paper Series, 2018
Toluene primary reference fuel (TPRF) (mixture of toluene, iso-octane and heptane) is a suitable surrogate to represent a wide spectrum of real fuels with varying octane sensitivity. Investigating different surrogates in engine simulations is a prerequisite to identify the best matching mixture. However, running 3D engine simulations using detailed models is currently impossible and reduction of detailed models is essential. This work presents an AramcoMech reduced kinetic model developed at King Abdullah University of Science and Technology (KAUST) for simulating complex TPRF surrogate blends. A semi-decoupling approach was used together with species and reaction lumping to obtain a reduced kinetic model. The model was widely validated against experimental data including shock tube ignition delay times and premixed laminar flame speeds. Finally, the model was utilized to simulate the combustion of a low reactivity gasoline fuel under partially premixed combustion conditions.
Combustion and Flame, 2019
1,3-cyclohexadiene (1,3-CHD) can be transformed into cis-1,3,5-hexatriene (1,3,5-HT) upon light irradiation, which makes it a potential additive able to change the reactivity of a conventional fuel. This paper presents the development of a detailed chemical kinetic model for the low-temperature (50 0-120 0 K) combustion of 1,3-cyclohexadiene and 1,3,5-hexatriene. Theoretical calculations were performed to compute the thermochemistry of a large number of intermediates involved in the reaction mechanism, and for several kinetic parameters. In particular, the pericyclic reactions of 1,3-cyclohexadiene, linking it to cis-1,3,5-hexatriene, were studied theoretically. It was shown that 1,3,5-HT is inherently a secondary molecule of the 1,3-CHD mechanism and a comprehensive set of its oxidation reactions were included. Simulation of literature data (ignition delays and products speciation) measured in rapid compression machines for 1,3-CHD were performed using the newly developed kinetic model. A good agreement with experiments was found, and kinetic analyses highlighted the decomposition mechanism of 1,3-CHD and the most sensitive reactions affecting the auto-ignition delay times. Simulations of cis-1,3,5-HT auto-ignition in an RCM were also performed and compared to the ignition behaviour of 1,3-CHD. The simulation results showed that 1,3,5-HT combustion involves an induction period characterized by the predominant formation of 1,3-CHD, whose decomposition starts the radical chain mechanism.
Autoignition of gasoline surrogates mixtures at intermediate temperatures and high pressures
Combustion and Flame, 2008
Ignition times were determined in high-pressure shock-tube experiments for various stoichiometric mixtures of two multicomponent model fuels in air for the validation of ignition delay simulations based on chemical kinetic models. The fuel blends were n-heptane (18%)/isooctane (62%)/ethanol (20%) by liquid volume (14.5%/44.5%/41% by mole fraction) and n-heptane (20%)/toluene (45%)/isooctane (25%)/diisobutylene (10%) by liquid volume (17.5%/55%/19.5%/8.0% by mole fraction). These fuels have octane numbers comparable to a standard European gasoline of 95 RON and 85 MON. The experimental conditions cover temperatures from 690 to 1200 K and pressures at 10, 30, and 50 bar. The obtained ignition time data are scaled with respect to pressure and compared to previous results reported in the literature.
Global reaction mechanism for the auto-ignition of full boiling range gasoline and kerosene fuels
Combustion Theory and Modelling, 2013
Compact reaction schemes capable of predicting auto-ignition are a prerequisite for the development of strategies to control and optimise homogeneous charge compression ignition (HCCI) engines. In particular for full boiling range fuels exhibiting two stage ignition a tremendous demand exists in the engine development community. The present paper therefore meticulously assesses a previous 7-step reaction scheme developed to predict auto-ignition for four hydrocarbon blends and proposes an important extension of the model constant optimisation procedure, allowing for the model to capture not only ignition delays, but also the evolutions of representative intermediates and heat release rates for a variety of full boiling range fuels. Additionally, an extensive validation of the later evolutions by means of various detailed n-heptane reaction mechanisms from literature has been presented; both for perfectly homogeneous, as well as nonpremixed/stratified HCCI conditions. Finally, the models potential to simulate the autoignition of various full boiling range fuels is demonstrated by means of experimental shock tube data for six strongly differing fuels, containing e.g. up to 46.7% cycloalkanes, 20% napthalenes or complex branched aromatics such as methyl-or ethylnapthalene. The good predictive capability observed for each of the validation cases as well as the successful parameterisation for each of the six fuels, indicate that the model could, in principle, be applied to any hydrocarbon fuel, providing suitable adjustments to the model parameters are carried out. Combined with the optimisation strategy presented, the model therefore constitutes a major step towards the inclusion of real fuel kinetics into full scale HCCI engine simulations.
Co-oxidation in the auto-ignition of primary reference fuels and n-heptane/toluene blends
Combustion and Flame, 2005
Auto-ignition of fuel mixtures was investigated both theoretically and experimentally to gain further understanding of the fuel chemistry. A homogeneous charge compression ignition (HCCI) engine was run under different operating conditions with fuels of different RON and MON and different chemistries. Fuels considered were primary reference fuels and toluene/n-heptane blends. The experiments were modeled with a single-zone adiabatic model together with detailed chemical kinetic models. In the model validation, co-oxidation reactions between the individual fuel components were found to be important in order to predict HCCI experiments, shocktube ignition delay time data, and ignition delay times in rapid compression machines. The kinetic models with added co-oxidation reactions further predicted that an n-heptane/toluene fuel with the same RON as the corresponding primary reference fuel had higher resistance to auto-ignition in HCCI combustion for lower intake temperatures and higher intake pressures. However, for higher intake temperatures and lower intake pressures the n-heptane/toluene fuel and the PRF fuel had similar combustion phasing.
The impact of adding an olefin to ternary mixtures of toluene and primary reference fuels to mimic the oxidation of a fully-blended gasoline was examined with kinetic modeling. Reactions for the oxidation of 2,4,4-trimethyl-1-pentene (DIB-1), which is the major constituent in diisobutylene (DIB), were added to a previously developed semidetailed mechanism for ternary mixtures. The merged kinetic mechanism was revised and successfully checked for validity against data for neat fuel components as well as fuel mixtures at conditions relevant to engine combustion. The validated kinetic model was then used to model a fully-blended research gasoline. By using a nonlinear-by-volume blending model for octane numbers, a four-component surrogate fuel was formulated which consisted of 51% isooctane, 18% n-heptane, 26.4% toluene, and 4.6% DIB-1 by liquid volume. The surrogate fuel reflected molecular-structure class composition, research octane number, motor octane number, density, and H/C ratio of the target gasoline. Ignition delay times for gasoline measured in a shock tube, rapid compression machine, and an HCCI engine were then compared to simulated results using the quaternary mixture and ternary mixtures with similar octane numbers and H/C ratio as the target gasoline. Adding DIB-1 to a ternary mixture had a small but significant effect on the autoignition of gasoline surrogate fuels. The quaternary mixture showed better agreement when compared to measurements, especially at higher temperatures. The simulated ignition delays at shock tube and rapid compression machine conditions were also well-correlated with the combustion phasing in an HCCI engine defined as the temperature required at bottom dead center to achieve 50% heat release (CA50) at top dead center. Similar results were achieved when comparing with other published mechanisms. Simulations with neat and binary mixtures combined with a rate-of-production and sensitivity analysis with multicomponent mixtures show that the reason for the increased reactivity and shorter ignition delay when adding DIB-1 to the ternary mixture is that DIB-1 promotes toluene ignition more than isooctane at these conditions. 65 chemistry has significant effects on engine performance under 66 HCCI conditions. 21 Having n-heptane and the olefin 1-hexene in 67 the gasoline surrogate fuel was beneficial to achieve satisfactory 68 combustion phasing (CA50) but detrimental to thermal
The journal of physical chemistry. A, 2016
Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxy-alkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate th...
Proceedings of the Combustion Institute, 2009
Ignition-delay times were measured in shock-heated gases for a surrogate gasoline fuel comprised of ethanol/iso-octane/n-heptane/toluene at a composition of 40%/37.8%/10.2%/12% by liquid volume with a calculated octane number of 98.8. The experiments were carried out in stoichiometric mixtures in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: Temperatures of 690-1200 K, and pressures of 10, 30 and 50 bar, respectively. Ignition delay times were determined from CH * chemiluminescence at 431.5 nm measured at a sidewall location. The experimental results are compared to simulated ignition delay times based on detailed chemical kinetic mechanisms. The main mechanism is based on the primary reference fuels (PRF) model, and sub-mechanisms were incorporated to account for the effect of ethanol and/or toluene. The simulations are also compared to experimental ignition-delay data from the literature for ethanol/iso-octane/n-heptane (20%/62%/18% by liquid volume) and iso-octane/n-heptane/toluene (69%/17%/14% by liquid volume) surrogate fuels. The relative behavior of the ignition delay times of the different surrogates was well predicted, but the simulations overestimate the ignition delay, mostly at low temperatures.
Detailed chemical kinetic modeling of surrogate fuels for gasoline and application to an HCCI engine
Gasoline consists of many different classes of hydrocarbons, such as paraffins, olefins, aromatics, and cycloalkanes. In this study, a surrogate gasoline reaction mechanism is developed, and it has one representative fuel constituent from each of these classes. These selected constituents are iso-octane, n-heptane, 1-pentene, toluene, and methyl-cyclohexane. The mechanism was developed in a step-wise fashion, adding submechanisms to treat each fuel component. Reactions important for low temperature oxidation (<1000K) and cross-reactions among different fuels are incorporated into the mechanism. The mechanism consists of 1214 species and 5401 reactions. A single-zone engine model is used to evaluate how well the mechanism captures autoignition behavior for conditions corresponding to homogeneous charge compression ignition (HCCI) engine operation. Experimental data are available for both how the combustion phasing changes with fueling at a constant intake temperature, and also how the intake temperature has to be changed with pressure in order to maintain combustion phasing for a fixed equivalence ratio. Three different surrogate fuel mixtures are used for the modeling. Predictions are in reasonably good agreement with the engine data. In addition, the heat release rate is calculated and compared to the data from experiments. The model predicts less low-temperature heat release than that measured. It is found that the low temperature heat-release rate depends strongly on engine speed, reactions of RO 2 +HO 2 , fuel composition, and pressure boost.