Roll-Down Considerations in Idle Quality 2001-01-1501

Idle quality is a complex phenomenon, involving both the average vibration amplitude perceived by the occupants and also its unsteady, erratic shake amplitude (here referred to as lumpiness). Besides the periodic combustion torque produced by an ideal engine, cycle-to-cycle combustion variation causes a random torque component. Together, these torque components cause periodic and random vibration response components. The random component, by its nature, has modulating amplitude. Further, depending on their dominant frequencies and amplitudes, the random and periodic components can modulate with each other. The resulting sudden change in amplitude is often interpreted as an engine “miss” or partial burn, when in fact it may not be. This and other idle phenomenon are also highly affected by the sensitivity of the vehicle, as determined by the powertrain mounting and body structure.