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Torque Converter

Transfer torque between two driveline axes as function of their relative angular velocity


Dynamic Elements


A torque converter couples two driveline axes, transferring torque and angular motion by the hydrodynamic action of a viscous fluid. Unlike a friction clutch, it cannot lock the axes together. The Torque Converter block models a torque converter acting between the two connector ports I and T as a function of the relative angular velocity of the two connected driveline axes. The Torque Convert block follows these conventions:

  • The I port represents the impeller or pump. The T port represents the turbine.

  • The input is the connector port into which power flows into the block. The output is the connector port from which power flows out of the block.

  • Forward power flow means power flowing from I to T. Reverse power flow means power flowing from T to I.

  • Forward motion means the relative angular velocity ω = ωT – ωI > 0.

Because the coupling of the axes occurs by viscous action, the torque transfer depends on this difference ω. In normal operation, the two axes have different speeds, and the output T axis speed never exactly reaches the input I axis speed (ω < 0). The torque transfer is largest when |ω| is large and shrinks as |ω| shrinks. Because |ω| can never reach zero exactly, a torque converter always transfers some torque.

Speed Ratio, Torque Ratio, and Capacity Factor

You specify the torque ratio and the capacity factor of the torque converter as discrete functions of the speed ratio with tabular vector entries. The three vectors of the independent and two dependent variable values must have the same length.

  • The speed ratio Rω is the output angular speed divided by the input angular speed. You specify a range of speed ratio values from 0 up to, but not including, 1.

    Rω = min[ωIT, ωTI]

  • The torque ratio Rτ is the output torque divided by the input torque.

    Rτ = τoutput / τinput

  • The capacity factor K is the input speed divided by the square root of the input torque.

    K = max[ωI, ωT] / √τinput

τinput is the torque flowing into the shaft with the larger speed, and τoutput is the torque flowing into the shaft with the smaller speed.

    Tip:   The Torque Converter block only accepts positive speed ratio values strictly less than 1. It assumes that output speed is strictly less than input speed. However, the identification of input and output switches from the I and T ports, respectively, to the T and I ports if the power flow is reversed.

    If you have torque converter data as functions of speed ratio values greater than 1, map the function values to the corresponding reciprocal speed ratio values, which are then less than 1. Specify the function values along with these reciprocal speed ratio values in the block dialog, instead of the original speed ratio values.

Using Dynamic Element Blocks

Use the blocks of the Dynamic Elements library as a starting point for vehicle modeling. To see how a Dynamic Element block models a driveline component, look under the block mask. The blocks of this library serve as suggestions for developing variant or entirely new models to simulate the same components. Break the block's library link before modifying it and creating your own version.

Dialog Box and Parameters

Speed ratio

Vector of values of the block function's independent variable, the dimensionless speed ratio. These values must be greater than or equal to 0 and strictly less than 1.

Torque ratio

Vector of values of the block function's first dependent variable, the dimensionless torque ratio. Each torque ratio value corresponds to a speed ratio value.

Capacity factor

Vector of values of the block function's second dependent variable, the torque conversion capacity factor. Each capacity factor value corresponds to a speed ratio value. The units are radians/second/√(newton-meters).

Torque Converter Model

Two functions specify the characteristics of the torque converter: the torque ratio Rτ and the capacity factor K, both as functions of the speed ratio Rω. You specify these as discrete tabulated functions in the dialog.

Rτ = Rτ(Rω)

K = K(Rω)

In normal operation (forward power flow), the input impeller (I) and output turbine (T) torques are

τI = sgn(1 - ωT / ωI) · [ωI / K]2

τT = τI · Rτ


The example model drive_torque_convert simulates a torque converter.

These Simscape™ Driveline™ example models include torque converters as part of complete drivetrains:


Society of Automotive Engineers, Hydrodynamic Drive Test Code (Surface Vehicle Recommended Practice), SAE J643, May 2000.

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