The hydrodynamic modelling of torque converters

Online pdf Allison Transmision The hydrodynamic modelling of torque converters

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.

Between speed ratios of 0.8 and 0.9, the converter torque curve crosses that of the coupling, indicating that the turbine (or output) torque is less than the impeller (or input torque). Simultaneously the converter efficiency drops below that of the coupling. As this is not desirable, it is overcome either by the fitting of a one way clutch, or by the use of a mechanical lock-up device that automatically connects the impeller and turbine above the speed ratio that corresponds to a torque ratio of unity.
This paper describes a comprehensive one dimensional model to simulate accurately the flow processes in a torque converter and predict the performance characteristics.

Torque Converter Hydrodynamic Models
In the torque converter and fluid coupling field one dimensional models are fairly common. Lucas and Rayner
[2], Qualman and Egbert [3], Walker [4], Whitfield, Wal-lace and Patel [5], Wallace, Whitfield and Sivalingam [6], Lamprecht [7] and Adrian [8], have all produced one dimensional models which show more or less agreement with measured results, but all of these require the external specification of total pressure loss coefficients for the members (pump impeller, turbines and stators) and of blade incidence loss coefficients. Lamprecht [7] did however derive equations for the calculation of certain of the loss coefficients, but they were not implemented in the computer program listing of his thesis [7].
With the local availability of the Lamprecht [7] model, it was decided to utilise it as the basis for further development. The Lamprecht [7] model was obtained and completely re-written in the BASIC language to run on a standard IBM compatible personal computer. In the analysis, N&O JOERNAAL APRIL 1992 the input speed, the speed ratio and the geometry of the torque converter are given parameters. There are three main members, the impeller, the stator and the turbine, although each can have more than one stage. The stator in turn can have two phases of operation, namely fixed or freewheel. Further assumptions are:

  1.  steady state conditions throughout 
  2. incompressible fluid 
  3. work done by the fluid is positive and work received by the fluid is negative
  4. conservation of mass flow, momentum and energy
  5. the torque applied to the shell is not considered 

The process estimates a mass flow inside the torus and this is used to determine firstly, the torque and power transmitted (input power) and absorbed (output power) by the members and, secondly, the power losses due to friction, diffusion, shock and slip.
For conservation of energy, the sum of the input power, output power and power losses, bearing in mind the sign convention, must equate to zero, and the mass flow is then iteratively adjusted until this condition, or close to it, is achieved. The value of mass flow thus derived is then used to determine the input and output torques, torque ratio, and efficiency for different values of speed ratio. In calculating the power losses, values of the stagnation pressure loss coefficients for the impeller, stator and turbine, the shock (or incidence) loss factors and the impeller slip factor are required. At this stage values based on information in the literature were assumed, typically 0,35 for the loss coefficients, and 1,0 for the shock loss and slip factors. Wall friction losses were ignored.

Evaluation of the Initial Model with Externally Specified Loss Coefficients
An extensive series of computer tests was run on the standard Jandasek t9] torque converter geometry. Fig. 3 shows good agreement between calculated and measured torque ratio and efficiency using the loss coefficients and factors above, with maximum differences, as can be seen from Table 1, of about 3%. However, from fig. 4 it can be seen that only qualitative agreement was obtained be-

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