1. INTRODUCTION A wide variety of hybrid vehicles (HVs) have been proposed and commercialized by automakers around the world. Generally, these HVs shut down the engine while the vehicle is stopped to improve fuel economy and lower emissions, resulting in an extremely quiet interior environment. However, HVs will engage the engine while stopped to charge the main battery or to warm-up the engine or catalytic converter. In these cases, idling vibration is generated by 1st order of engine combustion force. However, other engine forces occur at frequencies lower than the 1st order of combustion (called low frequency idling vibration in this paper). Low frequency idling vibration in vehicles installed with the Toyota Hybrid System II (THS II) [1] has a unique generation mechanism based on the characteristics of the drive-line and the engine speed during idling. This paper describes the mechanism of this low frequency idling vibration in a V6 rear-wheel drive (RWD) HV . 2. DEFINITION OF STEADY-STATE LOW- FREQUENCY IDLING VIBRATION Idling vibration is generally described as a phenomenon caused by 1st order of engine combustion force. Low frequency idling vibration is the vibration that occurs at frequencies lower than the 1st order of combustion. There are two types of low frequency idling vibration: steady-state vibration felt as a swaying motion and intermittent unsteady-state vibration felt as a rattling motion. The former is mainly caused by differences in combustion pressure between each cylinder in the engine [2] or imbalances in engine revolution. In contrast, the latter is caused by unstable combustion pressure in each combustion cycle within the same cylinder [ 2][3][4]. This paper focuses on steady-state low frequency idling vibration ( Table 1). Table 1. Explanation of phenomenon. Figure 1. Floor vibration level of RWD HV under idling conditions. Figure 1 shows the frequency analysis results for the floor vibration level at an engine speed of 1,000 rpm in a V6 RWD HV . The normal peak of idling vibration peak is at 50 Hz, which is the 1st order of Mechanism of Low Frequency Idling Vibration in Rear-Wheel Drive Hybrid Vehicle Equipped with THS II Jun Kokaji, Masashi Komada, Masayuki Takei, and Masaya Takeda Toyota Motor Corp ABSTRACT 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. CITATION: 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):2015, doi:10.4271/2015-01-2255.2015-01-2255 Published 06/15/2015 Copyright © 2015 SAE International doi:10.4271/2015-01-2255 saepcmech.saejournals.org 910Downloaded from SAE International by Birmingham City Univ, Tuesday, August 21, 2018engine combustion (i.e., the 3rd order of engine revolution). However, there are multiple peaks at lower frequencies that are felt as a swaying motion. These peaks are examples of steady-state low frequency idling vibration. Vibration at the 0.5th and the 1.5th order of engine revolution are mainly caused by difference in combustion pressure between each cylinder. The 1st order of engine revolution is mainly caused by engine revolution imbalance. The level of vibration at 8.3 Hz, which corresponds to the 0.5th order of engine revolutions, is particularly large. The following sections focus