CONTROLLING STEPS IN THE LOW-TEMPERATURE OXIDATION OF N-HEPTANE AND ISOOCTANE

The low-temperature oxidation of n-heptane and iso-octane in mixtures with air in a jet-stirred-how reactor have been compared under suitabl e high-pressure conditions, such that the two mixtures of hydrocarbon and air showed comparable fuel conversions and phenomenologies. The la rge presence of aldehydes in the products of the low-temperature oxida tion of n-heptane was attributed to a degenerate chain-branching path involving the addition of molecular oxygen to heptylhydroperoxy radica ls and isomerization by internal H-atom abstraction. The latter step i s particularly favored in linear alkanes where easy-to-abstract H-atom s are available. On the other hand, cyclic ethers and fuel-conjugate o lefins were the dominant products of the low-temperature oxidation of iso-octane. This is due to a lack of H-atoms for internal abstraction; this limits the degenerate chain-branching route and favors the propa gation path toward species having the same skeleton of the fuel, such as cyclic ethers and fuel-conjugate olefins. The prevalence of a degen erate chain-branching path for n-heptane compared with the propagation reactions in iso-octane oxidation is responsible for the different au toignition tendencies of n-heptane and iso-octane. (C) 1998 by The Com bustion Institute.