Modeling of Automatic Transmission Wet Clutches

Automotive automatic transmissions include several wet clutches. In the critical, transient intervals of shifts and engagements, the slip friction-induced clutch torque is transferred through the transmission. Thus, an accurate power train model, which is intended for the use in different automotive simulation studies, needs to include an accurate wet clutch model.

Different control-oriented power train models usually utilize a static clutch model, where the clutch torque is proportional to the applied hydraulic pressure and the speed/pressure/temperature-dependent friction coefficient. In order to provide more accurate description of wet clutch engagement, several distributed and lumped clutch models were proposed in the last decade. The models include the squeeze fluid-film dynamics which is described by the system of Reynolds equation and force balance equation.

Dynamic clutch models presented in the available literature have been experimentally validated only for the case of very fast, step-like change of the applied clutch pressure. In order to carry out a more comprehensive (more realistic) model validation, a thorough set of experimental data for both a grooved and an ungrooved clutch has been provided. Based on these experimental data, limitations of the static clutch model have been investigated. A simple empirical extension of the lumped dynamic clutch model from (Berger et al., 1997) has been proposed in order to improve the model accuracy. A clutch analysis has been carried out in order to gain the insight into the clutch dynamic behavior during the clutch engagement, and to facilitate tuning of some clutch parameters which are difficult to determine.

The below figure shows some of the validation results for the ungrooved clutch. The grooved clutch model has not been found to be as accurate as the ungrooved clutch model. Therefore, a special attention has been paid to improvements of the grooved clutch model. Also, modeling of clutch actuator has been considered.

Experimental validation results for ungrooved clutch.