The development of surrogate mixtures that represent gasoline combustion behavior is reviewed.1 Combustion chemistry behavioral targets that a surrogate should accurately reproduce, particularly for emulating homogeneous charge compression ignition (HCCI) operation, are carefully identified. Both short and long term research needs to support development of more robust surrogate fuel compositions are described. Candidate component species are identified and the status of present chemical kinetic models for these components and their interactions are discussed. Recommendations are made for the initial components to be included in gasoline surrogates for near term development. Components that can be added to refine predictions and to include additional behavioral targets are identified as well. Thermodynamic, thermochemical and transport properties that require further investigation are discussed.

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The production of HO2 in the reaction of ethyl radicals with molecular oxygen has been investigated using laser photolysis/cw infrared frequency modulation spectroscopy. The ethyl radicals are formed by reaction of photolytically produced Cl atoms with ethane, initiated via pulsed laser photolysis of Cl2, and the progress of the reaction is monitored by frequency-modulation spectroscopy of the HO2 product. The yield of HO2 in the reaction is measured by comparison with the Cl2/CH3OH/O2 system, which quantitatively converts Cl atoms to HO2. At low temperatures stabilization to C2H5O2 dominates, but at elevated temperatures (> 575 K) dissociation of the ethylperoxy radical begins to contribute. Biexponential time behavior of the HO2 production allows separation of prompt, “direct” HO2 formation from HO2 produced after thermal redissociation of an initial ethylperoxy adduct. The prompt HO2 yield exhibits a smooth increase with increasing temperature, but the total HO2 yield, which includes contributions from the redissociation of ethylperoxy radicals, rises sharply from ∼10% to 100% between 575 and 675 K. Because of the separation of time scales in the HO2 production, this rapid rise can unambiguously be assigned to ethylperoxy dissociation. No OH was observed in the reaction, and an upper limit of 6% can be placed on direct OH formation from the C2H5 + O2 reaction at 700 K. The time behavior of the HO2 production is at variance with the predictions of Wagner et al.'s RRKM-based parameterization of this reaction (J. Phys. Chem. 1990, 94, 1853). However, a simple ad hoc correction to that model, which takes into account a recent reinterpretation of the equilibrium constant for C2H5 + O2 ↔ C2H5O2, predicts yields and time constants consistent with the present measurements. The reaction mechanism is further discussed in terms of recent quantum chemical and master equation studies of this system, which show that the present results are well described by a coupled mechanism with HO2 + C2H4 formed by direct elimination from the C2H5O2 adduct.

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Bioethanol is currently a significant gasoline additive and the major blend component of flex-fuel formulations. Ethanol is a high-octane fuel component, and vehicles designed to take advantage of higher octane fuel blends could operate at higher compression ratios than traditional gasoline engines, leading to improved performance and tank-to-wheel efficiency. There are significant uncertainties, however, regarding the mechanism for ethanol autoignition, especially at lower temperatures such as in the negative temperature coefficient (NTC) regime. We have studied an important chemical process in the autoignition and oxidation of ethanol, reaction of the alpha-hydroxyethyl radical with O2(3P), using first principles computational chemistry, variational transition state theory, and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation simulations. The alpha-hydroxyethyl + O2 association reaction is found to produce an activated alpha-hydroxy-ethylperoxy adduct with ca. 37 kcal mol(-1) of excess vibrational energy. This activated adduct predominantly proceeds to acetaldehyde + HO(2), with smaller quantities of the enol vinyl alcohol (ethenol), particularly at higher temperatures. The reaction to acetaldehyde + HO2 proceeds with such a low barrier that collision stabilization of C2O3H5 isomers is unimportant, even for high-pressure/low-temperature conditions. The short lifetimes of these radicals precludes the chain-branching addition of a second O2 molecule, responsible for NTC behavior in alkane autoignition. This result helps to explain why ignition delays for ethanol are longer than those for ethane, despite ethanol having a weaker C-C bond energy. Given its relative instability, it is also unlikely that the alpha-hydroxy-ethylperoxy radical acts as a major acetaldehyde sink in the atmosphere, as has been suggested.

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Kinetics for the chemical activation reaction of the OH radical with benzene and unimolecular dissociation of the adduct are analyzed using quantum Rice-Ramsperger-Kassel (QRRK) theory for k(E) and master equation analysis for pressure falloff. Thermochemical properties and reaction path parameters are determined by ab initio and density functional calculations. Molecular structures and vibration frequencies are determined at the B3LYP/6-31G(d,p) and MP2(full)/6-31G(d) levels, with single point calculations for the energy at the B3LYP/6-311++G(2df,p)//B3LYP/6-31G(d,p), composite methods of CBS-Q, CBS-QB3 and G3(MP2) and the G3 methods. The OH addition to benzene forms a chemically activated prereactive complex with a shallow well (ca. 3 kcal mol -1 ), which predominantly dissociates back to reactants. Additional reactions of the energized precomplex include stabilization, or forward reaction to form hydroxycyclohexadienyl radical, C • HDOH, which has a well depth of 16 kcal mol -1 . This C • HDOH adduct can either eliminate H atom to form phenol, undergo intramolecular addition of the radical to an unsaturated carbon site to form bicyclo[3.1.0]hexan-6-ol radical (I in Figure 2), or react back through the prereactive complex. The radical (I) can cleave a strained exocyclic, cyclopropane bond forming cyclopenta-2,4-dienylmethan-1-ol radical (II in Figure 2). Rate coefficients for reactions of the energized complex are obtained from canonical transition state theory. The high-pressure addition rate constant for OH + benzene f prereactive complex is calculated from variational transition state theory with a center-of-mass reaction coordinate approximation. A detailed mechanism with mass conservation and microscopic reversibility is assembled and used to identify the intermediates and products of the benzene + OH reaction for comparison with experiment. The prereactive complex has a small effect on the overall kinetics and can be considered negligible over the temperature and pressure range investigated. The most important product formation channel in the OH + benzene addition reaction system is formation of phenol plus H atom. Comparisons of our calculated rate constants with experimental data that exhibit complex temperature and pressure dependence of [OH] vs time shows very good agreement and illustrate that microscopic reversibility needs to be included in analysis of experimental data on this reaction system. The important products for benzene + OH addition are predicted to be C • HDOH and phenol + H. At 800 K, product formation from cyclopentadiene intermediates is at least 3 orders of magnitude lower than phenol + H.