THE ANALYSIS AND MECHANISM OF ENGINE "CRANK RUMBLE"

An investigation of crank rumble in a gasoline engine is described in which noise, vibration, and cylinder pressure development were measured. Crank rumble is not associated with the highest noise levels in the spectrum (800 to 2000 Hz.), but rather with the complex waveform present at the lower amplitudes and frequencies near 400 Hz. Rumble occurs when a minimum of three adjacent harmonics of similar amplitude, selected by a resonant part of the engine or vehicle structure, lie within the minimum bandwidth of the human ear and combine to form a highly modulated envelope waveform. The level at which rumble ceases to become audible is not only dependent on the amplitude around the rumble frequency, but also on the masking ability of the remainder of the noise spectrum. The mechanism of crank rumble indicates that this noise is not necessarily associated with impending mechanical failure. An electronic method of reproducing the exact form of crank rumble is described. Approaches to reducing rumble are discussed: changing the combustion process, which modifies the amplitude and phase of the harmonics of the engine cycle, and modifying the engine structure response.