NACA Report No. 727

Summary

The study of combustion in a spark-ignition engine given in NACA Report No. 704 has been continued. The investigation was made with the NACA high-speed motion-picture camera and the NACA optical engine indicator. The camera operates at the rate of 40,000 photographs a second and makes possible the study of phenomena occurring in time intervals as short as 0.000025 second. Photographs are presented of combustion without knock and with both light and heavy knocks, the end zone of combustion being within the field of view. Time-pressure records covering the same conditions as the photographs are presented and their relations to the photographs are studied. Photographs with ignition at various advance angles are compared with a view to observing any possible relationship between pressure and flame depth. A tentative explanation of knock is suggested, which is designed to agree with the indications of the high-speed photographs and the time-pressure records.

Conclusions

1. Through the use of the NACA high-speed motion-picture camera and the NACA optical engine indicator, knowledge was obtained concerning the phenomenon of fuel knock. The results strongly indicate the inadequacy of the commonly accepted autoignition theory of knock.
2. The photographs indicate that knock usually involves a sudden completion of combustion, although there is some indication that very light knocks may not always involve a sudden completion of combustion but that combustion may complete itself in the normal manner after knock has occurred. Time-pressure records, taken simultaneously with and having a determinable time relationship with the high-speed pictures, should be useful in clearing up this point.
3. There is indication of the gradual build-up of reflected pressure waves just before occurrence of knock. The simultaneous time-pressure records and high-speed pictures should provide more information about this.