Flame propagation as well as the special role of detonation waves during knocking combustion are still unsolved questions. In order to examine these phenomena during cyclic resolved knocking combustion, high-speed schlieren photography and multi optical fiber technique were applied simultaneously. The pictures were taken at a rate of 200 000 frames per second, whereas the flame radiation signals of the knocking combustion, detected with the multi optical fiber technique at 49 measuring points, were recorded with a sampling frequency of 500 kHz.
The exact correlation between schlieren photography and optical fiber technique shows that knocking combustion is initiated by self ignitions in the unburned regions, clearly separated from the spark ignited flame. The complete autoignition (i.e. knocking combustion) proceeds in two stages thus showing distinct prereactions.
Nearly simultaneous self ignition processes in the entire unburned region as well as single self ignition kernels expanding into the unburned gas were observed.
In case of strong knock, the analog light intensity traces indicate that shock waves, caused by supersonic flame propagation, are responsible for initiating the characteristic high frequency gas oscillations. Detonative processes (suggested from supersonic flame propagation and correlating peaks in pressure traces simultaneously recorded) occur only for a very short time.