THIS paper describes studies of rumble with particular emphasis on the mechanism involved, the present and potential future extent of the problem, and means whereby the noise can be controlled or eliminated.
From the mechanism studies it is apparent that pounding is distinct from knock and is a form of combustion shock arising from too rapid burning of the charge as a result of secondary flame fronts introduced by deposit ignition. Objectionable levels of pounding have been observed in many of the 1958 model cars and, based on tests conducted in experimental high-compression engines, the problem can be expected to become worse as compression ratios are increased.
To date the most practical means which has been found for controlling pounding is the incorporation of tricresyl phosphate in the deposit fuel. The substantial increase in pounding-limited power afforded from the use of 0.3 theory of this additive is sufficient, in most cases, to eliminate pounding in 1958 and earlier model engines.
In more severe 11/1 and 12/1 compression ratio engines, however, pounding and other surface-ignition effects are only partially overcome. Thus, it would appear that unless further help can be provided from the standpoint of engine design, even more effective means for controlling surface ignition may have to be found for such engines.