  Ignition delay times of cyclohexane-oxygen-argon and
cyclopentane-oxygen-argon mixtures have been measured in a shock tube, the
onset of ignition being detected by OH radical emission. Mixtures contained 0.5
or 1 % of hydrocarbon for equivalence ratios ranging from 0.5 to 2. Reflected
shock waves allowed temperatures from 1230 to 1800 K and pressures from 7.3 to
9.5 atm to be obtained. These measurements have shown that cyclopentane is much
less reactive than cyclohexane, as for a given temperature the observed
autoignition delay times were about ten times higher for the C5 compound
compared to the C6. Detailed mechanisms for the combustion of cyclohexane and
cyclopentane have been proposed to reproduce these results. The elementary
steps included in the kinetic models of the oxidation of cyclanes are close to
those proposed to describe the oxidation of acyclic alkanes and alkenes.
Consequently, it has been possible to obtain these models by using an improved
version of software EXGAS, a computer package developed to perform the
automatic generation of detailed kinetic models for the gas-phase oxidation and
combustion of linear and branched alkanes and alkenes. Nevertheless, the
modelling of the oxidation of cyclanes requires to consider new types of
generic reactions, and especially to define new correlations for the estimation
of the rate constants. Ab initio calculations have been used to better know
some of the rate constants used in the case of cyclopentane. The main reaction
pathways have been derived from flow rate and sensitivity analyses.
