Chemical Kinetics and Combustion Modeling

ion of H atoms from fuel or other species by R02 produces alkyl hydroperoxides ROOH that then decompose to produce RO and OH radicals. However, for hydrocarbon fuels more complicated than n-butane, a more rapid process for R02 is isomerization via internal abstraction of H atoms ( 1 94). The general features of this R02 isomerization theory provide the kinetic basis for global reaction schemes for engine knock in internal combustion engines ( 1 96-1 98). The major steps consist schematically of R02 +=t QOOH (internal H atom abstraction) QOOH � QO + OH (0-0 homolysis). At sufficiently low temperatures, molecular oxygen can add further to the QOOH radicals, leading eventually to an overall reaction QOOH + 02 = products + OH + OH. Both alternatives are important since they produce OH radicals through reaction sequences with relatively low energy barriers. ROz isomerization rates are determined primarily by the size of the ringlike intermediate transition state, by the bond energy of the H atom being abstracted internally, and by the equilibrium constant of the R02 addition reaction. For fuels of larger hydrocarbons many isomerizations are possible, and 0-0 homolysis of the QOOH product of the iso­ merization reaction yields a different stable oxygenated species for each isomerization reaction. Thus for n-alkanes, a 1 ,4-H -atom abstraction, followed by 0-0 bond fission, leads to a 3-membered oxygenated ring, an oxiran. Similarly, 1 ,5-processes lead to oxetans, 1 ,6-abstractions produce tetrahydrofurans, and l ,7-abstractions produce tetrahydropyrans. Cur­ rent models generally use activation energies tabulated by Baldwin et al ( 199), but with A factors slightly lower than the 1 0 1 2. 1 S 1 recommended by Baldwin et aI, closer to the value of 1 0 1 1 . 5 S 1 recommended by Benson ( 1 89) for unimolecular reactions involving a cyclic transition state. The isomerization reactions are reversible, and activation energies for the reverse isomerizations are easily computed from the activation energy of the forward (endothermic) reaction and the AH of the reactions ( 1 94). ROz isomerization through internal abstraction of an H atom from a site adjacent to the C-O bond, followed by breakage of the C-O bond, will lead to a conjugate olefin and H02_ Direct abstraction paths leading to the same products have been discussed by Gutman and co-workers (200, 20 1 ), favoring a path proceeding through R02 isomerization. The current work of Wagner et al (20 I) provides some insight into the diffi382 MILLER, KEE & WESTBROOK culties of this reaction, but this is one of the simplest of the R02 iso­ merizations, and there are many more such reactions for which complex analyses are needed to understand fully the detailed reaction rates and mechanisms. Reactions of the product epoxide and other oxygenated species must be included in kinetic models, but very few quantitative studies of H atom abstraction or other reactions for these species have been reported. Current models must estimate both the rates and products for reactions of the epoxides, primarily attributed to H atom abstraction by OH or H02•