(712a) Thermochemistry, Reaction Paths and Kinetics On the Isooctane Radical Reactions with O2: Kinetic Study At High Pressures

The increasing regulations to control the emissions of pollutants from gasoline-powered vehicles has lead to the need of implementing improvements in the vehicles with the aim of  optimizing the combustion, improving fuel economy and reducing emission of several pollutants. One of the innovations is the use of gasoline direct injection (DI) engines, where the fuel is compressed to higher pressure for coupled improvements in efficiency pollutant reduction. Moreover, if homogeneous charge DI engines are implemented, where high-pressure fuel injector sprays fuel directly into the combustion chamber early enough in the cycle to promote homogeneous fuel-air mixing, apart from higher fuel efficiencies, a reduction in pollutant emissions is achieved. Iso-octane, 2,2,4-trimethyl pentane holds an octane rating of 100 and is a major spark ignition engine fuel component; it is widely used as a model compound for detailed reaction mechanisms tha are applied to  modeling ignition and combustion in spark and HCCI ignition internal engines.   Isooctane is a highly branched molecule, where its thermochemistry and reaction kinetics are affected by its structure.  In this study the thermochemical properties of isooctane and its radicals corresponding to loss of hydrogen atoms, along with the peroxy and hydroperoxide-alkyl radicals formed by ³O₂ reactions with isooctane radicals have been evaluated. Calculations on transition state structures for the peroxy radical reactions have been performed using B3LYP and CBS-QB3 methods to identify differences from smaller chemical species model systems which are used for estimation of the higher molecular weight hydrocarbon fuels. The kinetics for reaction paths of the elementary reactions of isooctane radicals with O₂ have been constructed with use of chemical activation analysis for this association using quantum RRK analysis for k(E) and Master Equation analysis for fall-off. The isooctane radical systems have been evaluated over a pressure range up to 125 atm. in order to determine effects of pressure.