High speed photographs of the combustion process in an engine equipped with a quartz window, in the cylinder head are described. The optical path was so arranged that direct and shadowgraph images of flames could be separately photographed on the same film at the same time in conjunction with flywheel timing marks. Simultaneous pressure-time records were also obtained.
The photographs confirm the well-known fact that knock occurs immediately after the portion of fuel still unreached by the flame spreading from the spark plug spontaneously ignites and begins to burn very rapidly. Cylinder pressure then rises so fast that acoustic resonance develops in the combustion gases and is heard as knock.
Four types of combustion phenomena in the engine were investigated: normal flames, “cool” flames, “hot” flames, and knock.
With no spark ignition, cool flames start at the valve end of the L-head combustion chamber and end at the far side over the piston. The cool flames are followed by hot flames, which are easily distinguishable on the pressure-time records by the much greater pressure rise and the pressure oscillations which accompany them. Both the cool and the hot flames travel about four times as fast as normal spark-ignited flames.
The speed of the fast spontaneous reaction which is commonly associated with knock in engines is definitely less than that of a true detonation wave. The camera, which operates at speeds up to 7,200 frames per second, is possibly too slow to record a supersonic disturbance; however, the pressure records show no abrupt break in pressure rise such as would indicate the presence of a detonation wave.


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