Abstract
This paper presents approach to widen torque converter clutch lock-up operation area to improve fuel efficiency of a vehicle with automatic transmission. Former studies on this theme have mainly focused on how to tune stiffness of damper clutch spring of the first stage, K1, to reduce the level of booming noise generated by engine firing excitation. But, if damper clutch spring rate is not tuned along with engine torque characteristics and transmission control as well as drive line vibration characteristics, sub-harmonic vibration can occur in specific lock-up driving conditions.
In this paper, we investigated how the sub-harmonic vibration occurs, and how to predict it and avoid it, so that vehicle can have wider lock-up operation area which contributes toward better fuel efficiency.
These are conclusions.
1. Lock-up damper clutches which consist of two stages of spring rate can introduce subharmonic vibration (half frequency of engine excitation), if engine torque meets the first stage maximum torque of the damper clutch during specific lock-up driving condition.
2. The sub-harmonic vibration (c1 component for I4 engine) can be developed enough to be a serious problem by resonance, if sub-harmonic vibration frequency is close to second mode frequency of driveline. And for almost all vehicles with I4 engines, the resonance is found in the vicinity of 1500~2000rpm
3. TCU lock-up control area and torque capacity of first stage of damper clutch should be set according to engine type and torque characteristics, such as number of cylinders, turbo charger and fuel.
4. We have carried out simulation with lumped mass drive line model, and predicted where sub-harmonic vibration can occur.
5. We designed torque converter damper clutch according to the simulation results, and tested in the vehicle. The test result showed that there was no sub-harmonic vibration for all lock-up diving conditions as predicted. And boom noise level of the vehicle reduced by more than 3dBC by optimizing first stage torque capacity of damper clutch as low as sub-harmonic vibration can be just prevented.
Keywords: Automatic Transmission, Torque Converter, Damper Clutch, NVH, Driveline