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.
The numerical study of engine combustion requires the coupling of advanced computational fluid dynamics and accurate chemical kinetic models. This task becomes extremely challenging for real fuels. Gasoline is a mixture of hundreds of different hydrocarbons. Detailed modeling of its chemistry requires huge numbers of species and reactions and exceeds present numerical capabilities. Consequently, simpler surrogate mixtures are adopted to approximate the behavior of the real fuels. Large kinetic models for surrogates are developed to characterize their chemistry, but these models still contain thousands of species and reactions and can usually only be used for simulating simple homogeneous systems. For multidimensional engine applications, they must be reduced. In this work, we propose a methodology for the formulation of a gasoline surrogates based on the intrinsic qualities of the fuel chemical behavior. Using the proposed procedure, a candidate surrogate containing four components has been identified to match a real nonoxygenated gasoline. Starting from this formulation, the LLNL (Lawrence Livermore National Laboratory) detailed kinetic mechanism has been reduced while maintaining its ability to reproduce targets of ignition delay times and flame speeds over a wide range of operating conditions. The reduction was carried by the construction of a preliminary version of a skeletal mechanism using the Computer Assisted Reduction Mechanism (CARM) code under a set of targeted conditions. Further reduction is made with a search algorithm that sequentially tests the importance of each species, leading to a much smaller mechanism. Finally, the resulting reduced mechanism has been validated against the detailed mechanism and available experimental data.
A detailed chemical kinetic reaction mechanism is developed for the five major components of soy biodiesel and rapeseed biodiesel fuels. These components, methyl stearate, methyl oleate, methyl linoleate, methyl linolenate, and methyl palmitate, are large methyl ester molecules, some with carbon-carbon double bonds, and kinetic mechanisms for them as a family of fuels have not previously been available. Of particular importance in these mechanisms are models for alkylperoxy radical isomerization reactions in which a C=C double bond is embedded in the transition state ring. The resulting kinetic model is validated through comparisons between predicted results and a relatively small experimental literature. The model is also used in simulations of biodiesel oxidation in jet-stirred reactor and intermediate shock tube ignition and oxidation conditions to demonstrate the capabilities and limitations of these mechanisms.Differences in combustion properties between the two biodiesel fuels, derived from soy and rapeseed oils, are traced to the differences in the relative amounts of the same five methyl ester components. † Lawrence
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 isooctane have been reassessed based on new thermodynamic group values and recently evaluated
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