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Torque Converter

Viscous fluid coupling between rotating driveline shafts


Couplings & Drives


A torque converter couples two driveline axes, transferring torque and angular motion by the hydrodynamic action of a viscous fluid. Unlike a friction clutch, a torque converter cannot lock the axes together. The Torque Converter block acts between the two ports I and T. The block acts as a lookup function of the relative angular velocity of the two connected driveline axes. This function is defined at discrete angular velocities. For model details, see Torque Converter Model.

Ports and Conventions

The impeller or pump port I and turbine port T are rotational conserving ports.

The Torque Converter block follows these conventions:

  • The impeller port I is the port that connects to the engine, and the turbine port T is the port that connects to the load. In normal operation, power thus flows from the impeller to the turbine.

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

  • The power input is through the shaft with the larger speed. The power output is through the shaft with the smaller speed.


Torque Characteristics

Speed ratio vector

Vector of values of the independent variable, the dimensionless speed ratio Rω. You must order these values in ascending order.

Torque ratio vector

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

Capacity factor parameterization

Definition of the capacity factor, either K (ratio of impeller speed ωI to square root of impeller torque τI) or K* (ratio of τI to ω2I). The default is K.

Capacity factor reference speed

Choice of speed in the capacity factor definition, depending on speed ratio Rω. Select either:

  • Impeller speed ωI for all values of Rω.

  • Impeller speed ωI for Rω < 1, and turbine speed ωT for Rω > 1.

Capacity factor vector

Vector of values of the block function's second dependent variable, the torque conversion capacity factor K. Each capacity factor value corresponds to a speed ratio value.

From the drop-down list, choose units.

  • If you choose the default capacity factor definition K, the default units are radians/second/√(newton-meters) (rad/s/(N*m)^0.5).

  • If you choose the alternative capacity factor definition K*, the default units are newton-meters/(radians/second)2 (N*m/(rad/s)^2).

Interpolation method

Interpolates torque ratio and capacity factor functions between discrete relative velocity values within the range of definition. The default is Linear.

Extrapolation method

Extrapolates torque ratio and capacity factor functions outside the range of definition. The default is Linear.


Model transmission lag

Select how to model transmission lag from input to output driveshaft. The default is Specify time constant and initial torque.

  • No lag — Suitable for HIL simulation — Torque transfer is instantaneous.

  • Specify time constant and initial torque ratio — Torque is transferred with a time lag. If you select this option, the panel changes from its default.

     Time Constant and Initial Torque Ratio

Torque Converter Model

The torque converter is a mechanism for transferring torque between impeller and turbine shafts. Because the coupling of I and T occurs by viscous action, the torque transfer depends on the difference ω = ωTωI ≠ 0, or the speed ratio Rω ≠ 1. In normal operation, the two axes have different speeds, and the output axis speed never exactly reaches the input axis speed. The torque transfer is largest when Rω → 0 or ∞, and shrinks as Rω → 1. Because Rω can never reach exactly one, 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 variable values must have the same length.

  • The speed ratio Rω is the turbine angular speed divided by the impeller angular speed:

    Rω = ωT/ωI .

  • The torque ratio Rτ is the output (turbine) torque divided by the input (impeller) torque:

    Rτ = τT/τI .

  • The capacity factor K or K* is defined in two ways for Rω < 1, with either the default or the alternative definition:

    • Default, the input speed divided by the square root of the input torque:

      K = ωI / √τI .

    • Alternative, the input torque divided by the square of the input speed:

      K* = τI / ω2I .

  • The capacity factor reference speed for Rω > 1 is ωI by default. That is, the input speed ωI is used in the ratio that defines either K or K*.

    For Rω > 1, the alternative choice for reference speed is to replace ωI by the output speed ωT in this defining ratio.

The two dependent variables, Rτ and K, are functions of the independent variable Rω. They specify the characteristics of the torque converter:

Rτ = Rτ(Rω) , K = K(Rω) .

Continuation Near Speed Ratio of Unity

If you do not specify capacity factor data for a speed ratio of 1, the block uses a capacity factor value of 10*KMax, where KMax is the maximum value in the specified capacity factor vector. The corresponding torque ratio is assumed to be 0. For all other speed ratio values not explicitly specified in the lookup table data, the block uses the interpolation or extrapolation method selected in the block dialog box.

Instantaneous Torque Transfer

When there is no time lag, the input impeller (I) and output turbine (T) torques are:

τI = sgn(1 – ωT/ωI)·[ωI / K]2 , τT = τI·Rτ ,

in normal operation (forward power flow).

Lagged Torque Transfer

You can optionally include the effect of torque transmission time lag, caused by internal fluid flow and compressibility. Instead of τT and τI being instantaneously constrained to one another, a first-order time lag introduces a delayed response in the impeller torque:

tc·(dτI/dt) + τI = τI(steady state) .

The preceding instantaneous function of the capacity factor K determines the steady-state value of τI.


The impeller shaft must always rotate in a positive direction. Simulation is not valid for ωI < 0.

If you drive the Torque Converter from a torque source, such as the Generic Engine, you must include an inertia in the source, to represent the engine, shaft inertia, or other source components. To ensure that the impeller starts by rotating in a positive direction, set the initial speed for this inertia to a positive value.

Torque Transmission Time Lag

Torque converters lag in their response to changing input torque. By default, Torque Converter includes no time lag in its response. You can include a response lag by specifying a time constant. Time lag simulation increases model fidelity but reduces simulation performance. See Adjust Model Fidelity.


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

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