A kinetic modeling study of self-ignition of low alkylbenzenes at engine-relevant conditions

The self-ignition of low alkylbenzenes at engine-relevant conditions has been studied with kinetic modeling. A previously developed chemical kinetic model for gasoline surrogate fuels [J.C.G. Andrae, R.A. Head, Combust Flame 156 (2009) 842-51] was extended with chemistry for ethylbenzene and m-xylene resulting in an overall model consisting of 150 species and 759 reactions. In model validation, comparisons were made between model predictions and experimental data of ignition delay times measured behind reflected shock waves, laminar burning velocities collected at elevated temperature and pressure and species profiles in a high-pressure single pulse shock tube. Generally good agreement was found and the model is sensitive to changes in mixture strength, pressure and temperature. Shock tube ignition delay modeling results for ethylbenzene and m-xylene also compare well to the ones for toluene. The rate controlling step for the ignition of ethylbenzene in the current mechanism is the reaction with ethylphenyl radical and oxygen. Ignition delay time for m-xylene was found to be very sensitive to reactions involving hydrogen atom abstraction from fuel by hydroxyl and oxygen and to branching reactions where methylbenzyl reacts with oxygen and hydroperoxide. The validated mechanism was used to study fuel chemistry effects when blending ethylbenzene with the paraffinic fuels iso-octane and n-heptane. A sensitivity- and flow path analysis showed that a higher consumption of hydroperoxide by ethylphenyl than expected from the contribution of neat ethylbenzene in a fuel mixture with iso-octane inhibits both iso-octane and ethylbenzene ignition. This can explain the observed increase in ignition delay time and octane number for fuel mixtures compared to neat fuels.     

A comparative experimental study of the autoignition characteristics of alternative and conventional jet fuel/oxidizer mixtures

Autoignition characteristics of an alternative (non-petroleum) and two conventional jet fuels are investigated and compared using a heated rapid compression machine. The alternative jet fuel studied is known as “S-8”, which is a hydrocarbon mixture rich in C₇-C₁₈ linear and branched alkanes and is produced by Syntroleum via the Fischer-Tropsch process using synthesis gas derived from natural gas. Specifically, ignition delay times for S-8/oxidizer mixtures are measured at compressed charge pressures corresponding to 7, 15, and 30 bar, in the low-to-intermediate temperature region ranging from 615 to 933 K, and for equivalence ratios varying from 0.43 to 2.29. For the conditions investigated for S-8, two-stage ignition response is observed. The negative temperature coefficient (NTC) behavior of the ignition delay time, typical of higher order hydrocarbons, is also noted. Further, the dependences of both the first-stage and the overall ignition delays on parameters such as pressure, temperature, and mixture composition are reported. A comparison between the autoignition responses obtained using S-8 and two petroleum-derived jet fuels, Jet-A and JP-8, is also conducted to establish an understanding of the relative reactivity of the three jet fuels. It is found that under the same operating conditions, while the three jet fuels share the common features of two-stage ignition characteristics and a NTC trend for ignition delays over a similar temperature range, S-8 has the shortest overall ignition delay times, followed by Jet-A and JP-8. The difference in ignition propensity signifies the effect of fuel composition and structure on autoignition characteristics.     

Insights relating to the autoignition characteristics of alcohol fuels

A detailed chemical kinetic modeling study was undertaken to characterize the auto-ignition behaviour of a full range of blends of both methanol and ethanol with a PRF 80 base fuel. The overall results were encapsulated in a computationally efficient semi-empirical formulation. The auto-ignition model was combined with an ASTM (CFR) engine simulation to explore the underlying causes for the octane number value exhibited by alcohol fuels. The model predictions were validated against experimental pressure traces obtained with a range of alcohol-PRF blends. These results confirmed the model’s general validity and provide valuable insights relating to the controlling role of the cool flame in the octane number determination and the use of alcohol fuels as octane blending components for use in modern and future gasoline engine technologies.     

Ignition delay times of ethanol-containing multi-component gasoline surrogates: Shock-tube experiments and detailed modeling

Ignition delay times for binary (ethanol/iso-octane, 25%/75% by liquid volume) and quinary (iso-octane/toluene/n-heptane/diisobutylene/ethanol, 30%/25%/22%/13%/10%) gasoline surrogate fuels in air were measured under stoichiometric conditions behind reflected shock waves. The investigated post-shock temperature ranges from 720 to 1220 K at pressures of 10 bar for the binary mixture and 10 bar and 30 bar for the quinary mixture. Ignition delay times were evaluated using side-wall detection of CH* chemiluminescence (λ = 431.5 nm). Multiple regression analysis of the data indicates global activation energy of ∼124 kJ/mol for the binary mixture and ∼101 kJ/mol for the quinary mixture and a pressure dependence exponent of −1.0 was obtained for the quinary mixture. The measurements were compared to predictions using a proposed detailed kinetics model for multicomponent mixtures that is based on the reference fuels (PRF) model as a kernel and incorporates sub-mechanisms to account for the chemistry of ethanol, toluene and diisobutylene. The model was tested using the measured ignition delay times for the surrogate fuels. Additional comparisons are based on literature data for other fuel combinations of the single constituents forming the quinary surrogate to insure that the modified mechanism still correctly predicts the behavior of simple fuels. The proposed model reproduces the trend of the experimental data for all pure fuels and blends investigated in this work, including the pressure dependence.     

Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR

This work investigates partial HCCI (homogeneous charge compression ignition) combustion as a control mechanism for HCCI combustion. The premixed fuel is supplied via a port fuel injection system located in the intake port of DI diesel engine. Cooled EGR is introduced for the suppression of advanced autoignition of the premixed fuel. The premixed fuels used in this experiment are gasoline, diesel, and n-heptane. The results show that with diesel premixed fuel, a simultaneous decrease of NO_x and soot can be obtained by increasing the premixed ratio. However, when the inlet charge is heated for the improved vaporization of diesel fuel, higher inlet temperature limits the operational range of HCCI combustion due to severe knocking and high NO_x emission at high premixed ratios. Gasoline premixing shows the most significant effects in the reductions of NO_x and soot emissions, compared to other kinds of premixed fuels.     

Partial oxidation (POX) reforming of gasoline for fuel-cell powered vehicles applications

As a part of the development of a gasoline processor for integration with a proton-exchanged membrane (PEM) fuel cell, we carried out the POX reforming reaction ofiso-octane, toluene and gasoline over a commercial methane reforming catalyst, and investigated the reaction conditions required to prevent the formation of carbon and the effect of fuel constituents and sulfur impurities in gasoline. The H₂ and CO compositions increased with increasing reaction temperature, while those of CO₂ and CH₄ decreased. It is desirable to maintain an O/C molar ratio of more than 0.6 and an H₂O/C molar ratio of 1.5 to 2.0 for vehicle applications. It has been found that carbon formation in the POX reforming ofiso-octane occurs below 620 °C, whereas in the case of toluene it occurs below 640 °C. POX reforming of gasoline constituents led to the conclusion that hydrogen production is directly related to the constituents of fuels and the operating conditions. It was also found that the coke formation on the surface of catalysts is promoted by sulfur impurities in fuels. For the integration of a fuel processor with PEM fuel cell, studies are needed on the development of new high-performance transition metal-based catalysts with sulfur and coke-resistance and the desulfurization of fuels before applying the POX reformer based on gasoline feed.     

New methods for the preparation of high-octane components from catalytic cracking olefins

A new method has been suggested for the preparation of high-octane components from the butane–butylene fraction (BBF) in two stages. At the first stage, the BBF olefins are oxidized with N₂O into carbonyl compounds with high selectivity without forming the products of deep oxidation and water. The process occurs in the gas phase in a flow reactor without using a catalyst at a temperature of 400°C and a pressure of 2 MPa with high conversion of both olefins and nitrous oxide. The blending octane number of the oxidation product is 118–133 (RON) and 99–104 (MON). At the second stage, the mixture of carbonyl compounds is hydrogenated with hydrogen in the presence of the Ni/Al₂O₃ catalyst. The hydrogenation occurs at 150–160°C in a flow reactor in the gas phase. The aldehydes are completely transformed into alcohols, while the ketones can remain in the product under certain conditions. The blending octane number of the hydrogenation product is 111–112 (RON) and 95–96 (MON), which is smaller than for the BBF oxidation product, but larger than for the alkylate obtained in the course of conventional butene alkylation with isobutane (RON is 95–97 and MON is 93–95). Synthesis of high-octane components by this procedure can be useful in practice, especially in productions with huge release of nitrous oxide.     

华北C4液化气芳构化的研究

以华北C₄液化气和华北C₄液化气与中国石油兰州石化公司炼油厂FCC汽油为原料，采用小型固定流化床为芳构化反应装置，考察了反应温度和空速对华北C₄液化气芳构化产物收率、转化率、马达法辛烷值、研究法辛烷值、液体产品组成的影响规律和华北C₄液化气与中国石油兰州石化公司炼油厂FCC汽油的不同进料形式对芳构化反应的影响。实验结果表明，随反应温度的升高，干气、液化气和焦炭收率呈上升趋势，而汽油和柴油收率呈下降趋势，液体产物的马达法辛烷值和研究法辛烷值随反应温度的升高先增大后减小，当温度为430 ℃时，马达法辛烷值和研究法辛烷值存在最大值；随着空速的增加，干气和液化气的收率逐渐增加，而汽油、柴油和焦炭的收率呈缓慢下降的趋势；华北C4液化气的转化率均在97%以上，且随空速的升高而逐渐增加；液体产物的马达法辛烷值和研究法辛烷值随空速的升高先增加后减少，在空速（1～9） h^－1内存在最大值。华北C₄液化气芳构化实验室内的最佳操作条件：反应温度（430～450） ℃，空速（3～5） h^－1。对于华北C₄液化气与中国石油兰州石化公司炼油厂FCC汽油混炼而言，先通入汽油后通入液化气的汽油和柴油收率和液体中的芳烃含量明显高于液化气和汽油同时进料和先通入液化气后通入汽油。     

Combustion of n-butanol in a spark-ignition IC engine

Alcohols, because of their potential to be produced from renewable sources and because of their high quality characteristics for spark-ignition (SI) engines, are considered quality fuels which can be blended with fossil-based gasoline for use in internal combustion engines. They enable the transformation of our energy basis in transportation to reduce dependence on fossil fuels as an energy source for vehicles. The research presented in this work is focused on applying n-butanol as a blending agent additive to gasoline to reduce the fossil part in the fuel mixture and in this way to reduce life cycle CO₂ emissions. The impact on combustion processes in a spark-ignited internal combustion engine is also detailed. Blends of n-butanol to gasoline with ratios of 0%, 20%, and 60% in addition to near n-butanol have been studied in a single cylinder cooperative fuels research engine (CFR) SI engine with variable compression ratio manufactured by Waukesha Engine Company. The engine is modified to provide air control and port fuel injection. Engine control and monitoring was performed using a target-based rapid-prototyping system with electronic sensors and actuators installed on the engine [1]. A real-time combustion analysis system was applied for data acquisition and online analysis of combustion quantities. Tests were performed under stoichiometric air-to-fuel ratios, fixed engine torque, and compression ratios of 8:1 and 10:1 with spark timing sweeps from 18° to 4° before top dead center (BTDC). On the basis of the experimental data, combustion characteristics for these fuels have been determined as follows: mass fraction burned (MFB) profile, rate of MFB, combustion duration and location of 50% MFB. Analysis of these data gives conclusions about combustion phasing for optimal spark timing for maximum break torque (MBT) and normalized rate for heat release. Additionally, susceptibility of 20% and 60% butanol-gasoline blends on combustion knock was investigated. Simultaneously, comparison between these fuels and pure gasoline in the above areas was investigated. Finally, on the basis of these conclusions, characteristic of these fuel blends as substitutes of gasoline for a series production engine were discussed.     

Determination of octane numbers for clean gasoline using dielectric spectroscopy

In the present work, dielectric spectroscopy (DES) in association with partial least squares (PLS) multivariate calibration method was employed to determine octane numbers (research octane number or RON and motor octane number or MON) of clean gasoline samples. The factor number included in PLS model was obtained according to the lowest sum of squares of predicted residual error (PRESS) in calibration set. The performance of the final model was evaluated according to PRESS and correlation coefficient (R). The optimal factor numbers are 9 in both RON and MON PLS calibration models, which were achieved with PRESS = 2.74 and R = 0.9598 in RON calibration set and the lowest PRESS = 2.72 and R = 0.8983 in MON calibration set. In validation set, PRESS = 1.00 and R = 0.9552 for RON and PRESS = 0.47 and R = 0.9105 for MON were obtained. Results indicated that PLS multivariate calibration models based on DES data were proven suitable as a practical analytical method for predicting octane numbers of clean gasoline.     

Pervaporation separation of n‐heptane/organosulfur mixtures with PDMS membrane: Experimental and modelling

In this paper, polydimethylsiloxane (PDMS) pervaporation membrane was employed to simulate the desulfurization process of gasoline where n‐heptane was selected to stand for gasoline. A modified solution–diffusion model is proposed to account for the mass transport of penetrants in the polymer membrane. In the model, the group contribution method (the UNIFAC‐ZM model) is applied to calculate the activity of penetrants in the polymer membrane, and the free volume theory with some modifications is employed to describe the diffusion behaviour of the penetrants. The effects of operating temperature and feed concentration on pervaporation properties were investigated to validate the model. It has been shown that the results predicted by the solution–diffusion model are in good agreement with the experimental values.     

Investigation of purified sulfate turpentine on engine performance and exhaust emission

This study deals with an experimental work that aims to examine effects of purified sulfate turpentine obtained from a kraft pulp mill in Turkey on an engine performance and exhaust emissions of a spark ignition engine. Three fuel samples are used to test the performance and emission of a 1300 cc engine manufactured by TOFA?. They are pure gasoline with 98-octane number and two other gasoline-turpentine fuel samples obtained by blending gasoline with turpentine in ratios of 5% and 10% on basis of total mass of the fuel. The thermophysical properties of the fuels are acquired by density, viscosity, flash and fire points, sulphur content, heating value and distillation tests. The experimental results showed that the turpentine has a positive effect on the engine performance parameters, such as brake power, thermal efficiency, mean effective pressure and specific fuel consumption. The turpentine also increases pollutant NO_x, unburned hydrocarbon contents and exhaust temperature, but it decreases CO concentration in the exhaust. It is observed that utilization of the sulfate turpentine alone is not viable and it needs to be used as an additive into gasoline to some extent, thereby providing a viable alternative to pure gasoline.     

Numerical investigation of the autoignition of turbulent gaseous jets in a high-pressure environment using the multiple-RIF model

The autoignition and subsequent flame propagation of initially nonpremixed turbulent system have been numerically investigated. The unsteady flamelet modeling based on the RIF (Representative Interactive Flamelet) concept has been applied to account for the influences of turbulence on these essentially transient combustion processes. In this RIF approach, the partially premixed burning, diffusive combustion and formation of pollutants (NO_x, soot) can be consistently modeled by utilizing the comprehensive chemical mechanism. To treat the spatially distributed inhomogeneity of scalar dissipation rate, the multiple RIFs are employed in the framework of Eulerian Particle Flamelet Model approach. Computations are made for the various initial conditions of pressure, temperature and fuel composition. The present turbulent combustion model reasonably well predicts the essential features of autoignition process in the transient gaseous fuel jets injected into high-pressure and high-temperature environments.     

灵活运用各项工艺措施实现清洁汽油的生产

对车用汽油清洁化生产的主要控制难点进行了分析,结合镇海炼化公司的情况,对工业生产中所采取的诸如催化原料脱硫预处理、MIP—CGP改造、应用降烯烃催化剂、调整控制汽油干点、提高重整汽油辛烷值与产量、扩大MTBE产能、重整汽油苯组分抽提、汽油组分资源的调合优化利用等多项清洁汽油生产工艺措施的应用及效果进行了总结与探讨。     

Effects of octane number on autoignition and knocking phenomena in a stratified mixture

The effects of the fuel concentration gradient and the octane number on the autoignition and knocking phenomena in a stratified mixture were studied experimentally on a using a rapid compression machine using stratified mixtures of air and fuels n-heptane, iso-octane, n-hexane, and n-pentane with different octane numbers (0, 100, 25, and 62, respectively). In the chamber, the lower the vertical location, the richer the fuel concentration of the mixture. The mixture contains no gradient in the horizontal direction. The experimental results show that rapid spread of the flame is caused not by flame propagation but by sequential autoignition. Although ignition delays of a stratified mixture are not dependent on the fuel concentration gradient in the mixture, they are constant as long as mean equivalence ratio is the same, and they decrease with the decreasing mean equivalence ratio. In excess of certain gradient value, the knock intensity is smaller as the gradient becomes larger for all fuels tested regardless of their octane number. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 93–100, July–August, 2009.     

Modeling Negative Temperature Coefficient region in methane oxidation

Standard kinetic models are essential tools for predicting and interpreting the evolution of oxidation processes and obtain useful information for designing and dimensioning practical combustion facilities. Quite often a large part of the development work consists in the determination of the most suited chemical kinetics scheme to use in numerical simulations. This step is even more critical in the case of innovative technologies. In fact, in this case, models are required to work in extrapolative conditions, i.e. in range of parameters outside the ones for which they have been optimized. This is the case of prediction methane autoignition at atmospheric pressure, in diluted conditions, corresponding to MILD combustion conditions, where no experimental data are available. The aim of the present work is to compare the efficacies in predicting the existence of Negative Temperature Coefficient (NTC) behavior of ignition time of methane at atmospheric pressure of several kinetic models available in the literature. Such phenomenology is extensively described in the literature for high molecular weight paraffin but few experimental evidences are reported about its occurrence in methane oxidation. Methane autoignition time in dependence of temperature, reaction pathways with rate of production, sensitivity and flow diagram analysis have been exploited in order to highlight the kinetic controlling steps of methane autoignition at different temperature ranges. It has been shown that the prevalence of either the oxidation or the recombination results in a speeding or a slowing down of the reactive process. In this reactive network, a key role is covered by the active oxidation pathway. At the same time, in dependence of working temperature, the branching routes of H₂/O₂ reaction mechanism supply a great part of radicals needed for ignition. Thus, numerical results presented in the paper clearly show that the Negative Temperature Coefficient region in the Arrhenius plot of methane ignition delay marks the shift from one principal reaction route to the others.     

Ignition characteristics of liquid hydrocarbon fuels as single droplets

An investigation into the effects of fuel boiling point and chemical type on the autoignition of single droplets of a number of pure hydrocarbons is reported. Ignition delay times were measured using the suspended droplet technique, and a numerical model was developed to extract reaction rate constants for the fuels. The results show that both fuel boiling point and chemical kinetics are important contributors to the ignition delay.     

Dimethyl ether autoignition in a rapid compression machine: Experiments and chemical kinetic modeling

Dimethyl ether (DME) autoignition at elevated pressures and relatively low temperatures is experimentally investigated using a rapid compression machine (RCM). DME/O₂/N₂ homogeneous mixtures are studied over an equivalence ratio range of 0.43–1.5 and at compressed pressures ranging from 10 to 20 bar and compressed temperatures from 615 to 735 K. At these conditions RCM results show the well-known two-stage ignition characteristics of DME and the negative temperature coefficient (NTC) region is noted to become more prominent at lower pressures and for oxygen lean mixtures. Furthermore, the first-stage ignition delay is found to be insensitive to changes in pressure and equivalence ratio. To help interpret the experimental results, chemical kinetic simulations of the ignition process are carried out using available detailed kinetic models and, in general, good agreement is obtained when using the model of Zhao et al. [Int. J. Chem. Kinet. 40, 2008, 1–18]. Sensitivity analyses are carried out to help identify important reactions. Lastly, while it is implicitly assumed in many rapid compression studies that chemical changes from the initial charge conditions that might occur during compression are negligible, it is herein shown with the help of Computational Singular Perturbation (CSP) analyses that chemical species formed during compression with little evolved exothermicity can considerably affect autoignition observations. Therefore, it is essential to simulate both compression and post-compression processes occurring in the RCM experiment, in order to properly interpret RCM ignition delay results.     

Fuel injection characteristics and spray behavior of DME blended with methyl ester derived from soybean oil

The objective of this work is to analyze the fuel spray injection characteristics and macroscopic behaviors of the dimethyl ether (DME) blended methyl ester derived from soybean oil at different blending ratios. The injection characteristics of the blended fuels such as injection delay, injection rate, and effective velocity in the nozzle flow passage were investigated under the various DME and its blended fuels. In comparison with the injection delay of blended fuels, the lower blending ratio of DME blended fuel with biodiesel showed a shorter injection delay than the higher blending ratio of the blended fuel. At the same energizing period and injection pressure, the DME fuel with a higher blending ratio showed a longer injection duration than that of the lower blending ratio. The higher DME blended with biodiesel also showed a low peak value of injection rate compared to the lower DME blended fuel at the same injection time. As the blending ratio of DME fuel was increased, the effective initial velocity of neat biodiesel and lower DME blended with biodiesel increased compared to the higher DME blended fuel. In comparison of spray penetration of blended fuel, biodiesel and blended fuel have a similar spray length at the same condition except the neat DME fuel.     

Assessment of Physico‐Thermal Properties,Combustion Performance,and Ignition Delay Time of Dimethyl Amino Ethanol as a Novel Liquid Fuel

Dimethyl amino ethanol (DMAE) contains both hydroxyl and amino functional groups, which may be introduced as a new liquid fuel with high safety and less toxicity with respect to common high performance liquid fuels. Physico‐thermal properties, combustion performance and ignition delay time of DMAE are compared with the usual high performance liquid fuels as well as ethanol and dimethylamine. Combustion performances of DMAE (specific impulse at sea level) with common liquid oxidizers including white fuming nitric acid (WFNA), inhibited red fuming nitric acid (IRFNA), nitrogen tetroxide (N₂O₄), hydrogen peroxide (H₂O₂), liquid oxygen (LOX), and the mixed oxides of nitrogen (MON) are also evaluated. Maximum and minimum specific impulses of DMAE are obtained with LOX (299.6 s) and WFNA (262.4 s), respectively. Maximum density‐specific impulse is obtained with DMAE‐N₂O₄ bipropellant. The ignition delay time of DMAE with several liquid oxidizers are measured with open cup test method. DMAE‐WFNA and DMAE‐IRFNA bipropellants are hypergolic where their ignition delay times are 26 and 42 milliseconds, respectively.