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A newly developed measurement technique was employed to investigate flame propagation before and during knocking combustion. Two intensified CCD cameras were used in sequence to record the natural flame light during knocking combustion via a fused silica window, which was fitted to the cylinder head of a one-cylinder four-stroke SI-engine. This arrangement enabled acquisition of two images per cycle. While the first camera was triggered by the rapid rise of the cylinder pressure at the beginning of engine knock, the second camera was activated at a prescribed time of delay. Due to the high sensitivity of the intensified CCD cameras, extremly short exposure times of 100 to 250 ns proved to be sufficient. Consequently, acquisition rates of 200 kHz and more - which are substantially higher than those of conventional natural light photography - could be realized.

Numerical investigations are reported on the location of sites at which autoignition first develops in the end-gas ahead of a spark-ignited flame in a combustion chamber following rapid compression of an alkane + air mixture to high pressures and temperatures. Attention is drawn to the part played by the reactions that give rise to a negative temperature coefficient of reaction rate in an inhomogeneous temperature field. A ‘reduced’ kinetic mechanism was employed to model the spontaneous oxidation of n-alkanes. Flame propagation was described in terms of the ‘eddy dissipation concept’ and coupled to the more detailed mechanism by means of a switching algorithm. The CFD calculations were performed by use of KIVA II.