Investigation of Transient Torque Converter Behavior, 03-9264

Principal Investigators
Matt S. Castiglione
Brad P. Pohl
Robert A. Smithson

Inclusive Dates: 07/01/01 - 10/01/02

Background - The physical phenomenon that governs how the three-element torque converter operates is dependent on very complicated fluid and blade interactions and fluid angular momentum considerations. CFD has accurately predicted the steady-state performance curves one would obtain if the candidate converter were tested in the laboratory according to SAE Standard J643. Transient simulation of a torque converter over significant time intervals or varying conditions seen in automotive applications is still not fully realized with CFD. Transient CFD simulation of this nature requires modeling the entire fluid domain, which becomes computationally expensive and thus is not practical to use in conjunction with other models to simulate complete powertrain dynamics. An approximate method suitable for use in powertrain dynamic simulation strategies has found acceptance throughout industry. This method assumes that the transient response of fluid momentum and flow field inside the converter matches the steady-state response measured in the laboratory according to Standard J643.

Approach - The current strategy is to acquire transient test data to be used along with basic physical relations to develop an empirical model that predicts the behavior of the torque converter in a transient condition. The transient model will be created such that it can be implemented into existing dynamic system simulation codes, which have found use for total vehicle and powertrain simulation. The core of this model is based upon solving a set of nonlinear equations that have been previously derived and published in relevant literature. Using nondimensional variables, transient test and simulation results will be compared to steady-state test data as well as an existing approximation method to illustrate the differences.

Accomplishments - Six different torque converters currently used in automotive applications were successfully tested. Test results indicate that all torque converters exhibited similar transient performance characteristics over a range of transient conditions similar to that seen in application. A transient model of the torque converter has been implemented in a dynamic system simulation software. This implementation was made possible by developing a reverse engineering process to extract important geometric features from the tested torque converters. This process involved scanning the torque converter elements using a Dynamic Structured Light (DSL) three-dimensional imaging system developed by the Automation and Data Systems Division at SwRI. Additionally, an approximate quasi-static modeling method popular in industry was included in tandem with the transient model. The two models were able to reproduce transient test data very well. Minimal differences were seen between the transient and quasi-static models. This small difference indicates that for low-frequency conditions of less than one hertz, the transient fluid momentum effects inside the torque converter are insignificant. A linearized frequency response study over the range of speed ratios for three torque converters was conducted. The results of this frequency response study indicate that differences between the transient and quasi-static model occur primarily at high-speed ratios and frequencies between 1 and 10 hertz. Some amount of frequency response variability between torque converters was found at speed ratios equal to 1.0. The sensitivity of transient fluid momentum effects was studied, and it has been concluded that an order of magnitude increase in these parameters has a significant effect only at speed ratios near 1.0. Both models are unable to transmit torsional vibrations above 40 hertz as shown by the frequency response study and reinforced by a nonlinear torsional simulation on one of the converters. In general, transient fluid moment behavior inside the torque converter is only of consequence when the speed ratio is near or above the coupling point and the torsional frequency is between 1 to 10 hertz.

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