Evaluation of idle stability through in-situ torque measurement in automatic transmission vehicles

Idle stability directly affects a vehicle’s NVH (Noise, Vibration and Harshness) and is closely related to driver satisfaction. The present study proposes a method of measuring an engine’s idle roughness, which is useful in quantifying the idle stability. Engine brake torque was measured directly using a torque sensor, which can be installed without modification of the engine’s mounting structure. In addition, angular acceleration was measured at the same position as the torque measurement, to compare dynamic characteristics of the angular acceleration with the torque variation. Both torque and angular acceleration oscillate between positive and negative values. In this study, torque data were divided into several regions, and each region starts from the point where the torque data changes its sign from negative to positive. The root mean square values of both torque and angular acceleration were calculated for each region. This calculation showed a very good correlation between the torques and the angular accelerations. The idle stability was evaluated with the standard deviation of the measured torque, and the cycle-to-cycle variation is a more dominant factor in the idle stability than is the cylinder-to-cylinder variation. Because it is easier to measure the angular acceleration than to measure the torque, the variations of angular accelerations are usually compared between engines. However, the present study showed that the moment of inertia of an engine and the angular acceleration should be considered together when comparing the idle stability between engines.