SIMULTANEOUS RECORDINGS of cylinder pressure, audible sound, and crankshaft motion have shown that rumble is a noise associated with bending vibrations of the crankshaft. The vibrations are caused by abnormally high rates of pressure rise near the top dead center piston position. In this study the high rates of pressure rise were obtained by inducting deposits into the the engine. Thud is a torsional vibration of the crankshaft, similar in sound to rumble but resulting from much earlier occurrence of the maximum rates of pressure rise.
Rumble vibrations consisted of a fundamental frequency of 600 cps and higher harmonics in the 11/1 compression ratio V-8 laboratory engine used in the investigation. The audible noise of rumble was predominantly composed of the second harmonic or about 1200 cps.
Limited investigation of the influence of fuel and engine operating conditions on the mechanical aspects of rumble showed no modification of the basic character or frequency of vibration but did show major effects on the amplitude and damping. Lubricating oil bulk temperature changes of as little as 52 F (162–214 F) increased the initial rate of crankshaft deflection, amplified the higher harmonics, and greatly lengthened the persistence of the vibration.
Rumble induced by dropping combustion-chamber deposits into the carburetor was accompanied by power losses as high as 19%, cooling load increases of up to 50%, and a 10% reduction in exhaust gas temperature.
After having been subjected to an inordinate amount of rumble, approximately 3 hr, the engine was disassembled and examined. No signs of mechancial damage could be detected, implying that rumble while being undesirable is apparently not destructive.
This paper received the 1959 Horning Award.*