Temperature distribution induced by pre-ignition reactions in a rapid compression machine

The temperature after compression of reactive and unreactive gas mixtures are measured at different locations and times in a rapid compression machine by a thin wire thermocouple. Validation is done by laser Rayleigh scattering. Inspection of the temperature fields of unreactive gas and during the delays in three different patterns of spontaneous ignition of isooctane-''air'' mixtures shows that the gas temperature is homogeneous for some times after top dead centre. Then heat losses to the walls and gas recirculation induce temperature inhomogeneities. They are partially levelled as hot ignition is approached. The gas temperature before hot ignition is nearly the same and constant in time in both two-stage or one-stage ignitions. The levelling effect is attributed to the complex chemical kinetics of low temperature alkane oxidation in the range of the negative temperature coefficient of the global reaction rate.