Contributing USMA Research Unit(s)
It is widely known that the rapid autoignition of end-gas will cause an engine cylinder to resonate, creating a knocking sound. These effects were quantified for a simple engine geometry in 1934 in a study where critical resonance frequencies were identified. That analysis, performed by Charles Draper, still forms the basis of most knock studies. However, the resonance frequencies are highly dependent on the engine geometry and the conditions inside the cylinder at autoignition. Since, engines and fuels operate at substantially different conditions than they did in 1934, it is expected that there should be a shift in knock frequencies. Experimental tests were run to collect knock data in an engine, representative of modern geometries, over a range of operating conditions for a number of different fuels. The operating conditions—intake air temperature, intake air pressure, and engine speed—were varied to identify shifts in the critical frequencies. Additionally, fuels were varied in octane number from 80 to 100. The resulting analysis found that the first circumferential mode, at approximately 6 kHz still played a substantial role in knock in modern engines. However, the analysis also found a decreased contribution from radial modes and an increased contribution from the axial modes. The distributions of frequencies did not shift significantly for changes in the intake air temperature or pressure; however, the axial modes became more significant at higher engine speeds. Additionally, the axial modes increase in frequency for higher octane fuels, which have an earlier knock-limited spark advance. These results show the increased importance of the axial modes in knock for modern engines; these modes are typically not audible, though they can still result in engine damage.
Proceedings of the Society of Automotive Engineers
Mittal, Vikram, "Distribution of Knock Frewquencies in Modern Engines Compared to Historical Data" (2018). West Point Research Papers. 108.