Abstract
To improve fuel efficiency of automatic transmissions, their torque converters are typically locked up at about 40 mph after which 100% of the torque is transmitted from the engine to the transmission. One cause of fuel inefficiency is the viscous losses in the torque converter at lower vehicle speeds due to slippage between the pump and turbine of the transmission’s torque converter. Consequently, the automotive industry has been experimenting with imposing an early lock-up of the torque converter to enhance fuel efficiency. However, damping in the driveline would drastically be reduced. A sudden excitation could then lead to oscillation of the drive shafts which is transmitted to the occupants of the vehicle. This could cause an unpleasant driving experience.
One proposed solution concerns an inline friction damper to be integrated with the drive shafts of the vehicle in such a way that friction damping only occurs when oscillations of the shafts occur. The study reported here deals with a comprehensive nonlinear transient analysis of the proposed Coulomb friction damper which was performed to assess the effectiveness of the damper.
Two cases involving a sudden loading of the drive shafts at two levels of disturbance (at second and third gear) are studied. One case considers the response of the driveline to a sudden application of torque when the torque converter is already locked. The second case examines the consequences of a sudden lock-up of the converter clutch.
The study shows that the second case will create more severe oscillatory response of the drive shaft. The results of the parametric study for the effects of the friction damper on the oscillations of the drive shafts reveal varied levels of generated damping and overshoot. For some parameters, the amount of damping and/or reduction of overshoot is not significantly improved. This has been found to result from a non-slip condition which occurs at the friction damper surfaces. To remedy this situation, a passive slip control mechanism is used to vary the normal force on the surfaces of the friction pads with the angle of shaft twist. Another approach was to use a preloading arrangement with the friction damper. A discussion is included to provide physical insight into the performance of the friction damper in controlling the transient response of the drive shafts.
