FUNDAMENTAL FEATURES OF HYDROCARBON AUTOIGNITION IN A RAPID COMPRESSION MACHINE

Results are reported for the autoignition characteristics of n-butane, n-pentane, n-hexane, and n-heptane and also of i-butane, i-octane, and toluene in stoichiometric mixtures with air following mechanical compression to gas temperatures in the range 600-950 K and pressures up to 0.9 MPa. Emphasis is placed on the dependence of ignition delay on compressed gas temperature, on the evolution of reaction as portrayed in the pressure-time records and on features of light output associated with single and two-stage ignition. Two-stage ignition is a clear feature of the n-alkane combustion at low compressed gas temperatures. Single-stage ignition is apparent at somewhat higher compressed gas temperatures, but there is evidence of the first stage reaction having occurred during the final stages of compression in some circumstances. Engine-knock related pressure waves are associated with the autoignition of the n-alkanes, to a lesser extent with the branched chain structures, but not at all with toluene under the present experimental conditions. These general features and also the relationship of the measured pressures to average gas temperatures attained during the ignition delay period are discussed. These data are relevant to the validation of numerical models for the autoignition of hydrocarbons.