As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40 atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270 K. Results at temperatures above 900 K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900 K demonstrate a stronger negative temperature coefficient behavior for the FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex KAUST multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two-and three-component mixtures composed of n-heptane, iso-octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the KAUST multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition.

No abstract

Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer-Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been modeled using a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The importance of propene chemistry is highlighted as critical for correct prediction of high temperature ignition delay. The oxidation behavior of 2,5-dimethylhexane is also compared with oxidation behavior of other linear and branched octane isomers, in order to determine the effects of the number of methyl branches on combustion properties. Both experimental data and model predictions indicate that increasing the level of branching decreases fuel reactivity at low and intermediate temperatures. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane. FUEL or CNF, Sarathy et al. submitted July 2013Introduction Detailed chemical kinetic models for transportation fuels have reached a level of fidelity where accurate predictions can be made of combustion phenomenon relevant to the operation of practical devices. Schofield [1] states that these large scale models are adequate as engineering tools for studying the combustion of new fuel molecules. A recent review paper by Pitz and Mueller [2] describing the development of diesel surrogate fuel models concluded that major research gaps remain in modeling high molecular weight (i.e., C 8 and greater) aromatics, alkyl aromatics, cyclo-alkanes, and lightly branched iso-alkanes. The present study is concerned with the combustion of branched alkanes, specifically 2,5-dimethylhexane, which has been reported as a component of petroleum combustion exhaust, smog, and tobacco smoke [3]. Branched alkanes are important components of conventional diesel and jet fuels derived from petroleum [2,4]; synthetic Fischer-Tropsch diesel and jet fuels derived from coal, natural gas, and/or biomass [5,6]; and renewable diesel and jet fuels derived from thermochemical treatment of bio-derived fats and oils (e.g., hydrotreated renewable jet (HRJ) fuels) [7,8]. Detailed com...

Low-octane gasolines (RON  50-70 range) are prospective fuels for gasoline compression ignition (GCI) internal combustion engines. GCI technology utilizing low-octane fuels has the potential to significantly improve well-to-wheel efficiency and reduce the transportation sector's environmental footprint by offsetting diesel fuel usage in compression ignition engines. In this study, ignition delay times of two low-octane FACE (Fuels for Advanced Combustion Engines) gasolines, FACE I and FACE J, were measured in a shock tube and a rapid compression machine over a broad range of engine-relevant conditions (650-1200 K, 20 and 40 bar and  = 0.5 and 1). The two gasolines are of similar octane ratings with anti-knock index, AKI = (RON + MON)/2, of  70 and sensitivity, S = RON-MON, of  3. However, the molecular compositions of the two gasolines are notably different. Experimental ignition delay time results showed that the two gasolines exhibited similar reactivity over a wide range of test conditions. Furthermore, ignition delay times of a primary reference fuel (PRF) surrogate (n-heptane/iso-octane blend), having the same AKI as the FACE gasolines, captured the ignition behavior of these gasolines with some minor discrepancies at low temperatures (T < 700 K). Multi-component surrogates, formulated by matching the octane ratings and compositions of the two gasolines, emulated the autoignition behavior of gasolines from high to low temperatures. Homogeneous charge compression ignition (HCCI) engine simulations were used to show that the PRF and multi-component surrogates exhibited similar combustion phasing over a wide range of engine operating conditions.

No abstract