NACA Report No. 761

NACA Report No. 761 - Identification of Knock in NACA High-speed Photographs of Combustion was published by the UInited States National Advisory Committee for Aeronautics in 1943. It contained the results of high-speed photographic investigation into knocking in a gasoline-fueled internal combustion engine.
Summary
The study of combustion in a spark-ignition engine given in NACA Reports 704 and 727 has been continued. The present investigation was made with the NACA high-speed motion-picture camera, operating at 40,000 photographs a second, and with a cathode-ray oscillograph operating on a piezoelectric pickup in the combustion chamber.
Identity in time is established between the start of violent pressure fluctuations in the combustion chamber and the appearance of blur in the high-speed schlieren photographs. The blur was tentatively presumed to represent the occurrence of knock in the previous reports.
Photographs are presented showing that the origin of knock is not necessarily in the end gas.
The data obtained indicate that knock takes place only in a part of the cylinder charge which has been previously ignited either by autoignition or by the passage of the flame fronts but which has not burned to completion.
Mottled regions in the high-speed schlieren photographs are demonstrated to represent combustion regions.
Conclusions
The test conditions for the experiments of the present report included only one value of compression ratio and only one type of fuel. Usual engine operating conditions were not reproduced, inasmuch as the fuel charge was injected into the cylinder on the intake stroke and residual combustion products were not present in the chamber. The possibility exists that knock may take on different aspects under different conditions and the following conclusions should therefore be considered definite only for conditions approximating those of the tests:
1. The characteristic blurring in the NACA high-speed photographs of knocking combustion coincides in time with the start of knocking vibrations. The blurring may therefore be considered a result of the knocking reaction.
2. The preliminary pressure fluctuations of small amplitude that precede the violent knocking vibrations occur during the same period of time as the periodic variations in the configurations of the schlieren mottling, which have been observed on the projection screen.
3. The first evidence of knocking reaction is sometimes in a different position from the last part of the charge to be ignited, although always in the general end zone.
4. The knocking reaction can apparently originate in any region in which combustion is continuing, whether this is the last portion to be ignited or not. Knock apparently originates only in a part of the fuel charge that has been previously ignited, either by autoignition or by passage of the flame fronts, but which has not burned to cmpletion.
5. The mottling in the high-speed photographs undoubtedly coincides with the regions in which combustion is taking place. Serious doubt is therefore cast upon the theory that combustion is completed within an extremely thin flame front inasmuch as the mottling extends far back of the flame front.
6. Inadequacy or the simple autoignition theory of knock is indicated.

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