Kokaji, J., Komada, M., Takei, M., and Takeda, M., "Mechanism of Low Frequency Idling Vibration in Rear-Wheel Drive Hybrid Vehicle Equipped with THS II," SAE Int. J. Passeng. Cars - Mech. Syst. 8(3):910-915, 2015, doi:10.4271/2015-01-2255.
Although idling vibration is usually caused by 1st order of engine combustion force, other engine forces also occur at frequencies lower than the 1st order of combustion (called low frequency idling vibration in this paper). The drive-line of the Toyota Hybrid System II (THS II) has different torsional vibration characteristics compared to a conventional gasoline engine vehicle with an automatic transmission. Nonlinear characteristics caused by the state of backlash of pinions and splines influence changes in the torsional resonance frequency. The torsional resonance frequency of the drive-line can be controlled utilizing the hybrid system controls of the THS II.