The hydrodynamic modelling of torque converters

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Summary
One method of predicting the performance characteristics of torque converters is by means of a hydrodynamic model in which the geometry of the torque converter and the properties of the fluid are given, and the angular momentum flux over the members is calculated at specific operating points. A number of such models has been developed in the literature, all of which rely on empirical input data for determination of the losses and slip factor. This paper describes a hydrodynamic model in which the empiricism has been removed from the input data and built into the program in the form of empirical equations and loss models. The program can be applied to torque converters having profile and thin blades, and in addition a new shock (incidence) loss model is introduced which is employed in the hydrodynamic model to calculate the shock losses of the different members of a torque converter. Prediction of torque converter performance by means of this model agreed well with published experimental data for a wide range of torque converter geometries.

Introduction
Torque converters are widely used in automotive applications, from passenger cars to heavy commercial and military vehicles. The primary function of the torque converter is to provide torque multiplication. This is a maximum at stall and decreases without step to a value of unity at the coupling point. The configuration of a typical torque converter is shown in fig. I []. The torque converter is similar to a fluid coupling, but with the addition of a stator or reactor. In a fluid coupling, power is transmitted from the pump or impeller to the turbine without change in torque, but with the insertion of the stator in the circuit, the angular momentum of the fluid is changed between the turbine exit and impeller entrance, resulting in torque multiplication between impeller and turbine. As its name implies, the stator is norrnally stationary but in most modern torque converters it is usually fitted with a one-way clutch.

The characteristic performance curves of torque converters and fluid couplings are shown in fig. 2. The abscissa is speed ratio, the ratio of output (or turbine) speed to input (or impeller) speed. The ordinates are torque ratio i.e. output torque to input torque on the one side and efficiency on the other.