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.

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.

**Speed ratio vector**Vector of values of the independent variable, the dimensionless speed ratio

*R*_{ω}. 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

*R*_{τ}. Each torque ratio value corresponds to a speed ratio value.**Capacity factor parameterization**Definition of the capacity factor, either

*K*(ratio of impeller speed*ω*_{I}to square root of impeller torque*τ*_{I}) or*K*^{*}(ratio of*τ*_{I}to*ω*^{2}_{I}). The default is*K*.**Capacity factor reference speed**Choice of speed in the capacity factor definition, depending on speed ratio

*R*_{ω}. Select either:Impeller speed

*ω*_{I}for all values of*R*_{ω}.Impeller speed

*ω*_{I}for*R*_{ω}< 1, and turbine speed*ω*_{T}for*R*_{ω}> 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.

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* = *ω*_{T} – *ω*_{I} ≠
0, or the speed ratio *R*_{ω} ≠
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 *R*_{ω} →
0 or ∞, and shrinks as *R*_{ω} →
1. Because *R*_{ω} can
never reach exactly one, a torque converter always transfers some
torque.

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

*R*_{ω}is the turbine angular speed divided by the impeller angular speed:*R*_{ω}=*ω*_{T}/*ω*_{I}.The torque ratio

*R*_{τ}is the output (turbine) torque divided by the input (impeller) torque:*R*_{τ}=*τ*_{T}/*τ*_{I}.The capacity factor

*K*or*K*^{*}is defined in two ways for*R*_{ω}< 1, with either the default or the alternative definition:Default, the input speed divided by the square root of the input torque:

*K*=*ω*_{I}/ √*τ*_{I}.Alternative, the input torque divided by the square of the input speed:

*K*^{*}=*τ*_{I}/*ω*^{2}_{I}.

The capacity factor reference speed for

*R*_{ω}> 1 is*ω*_{I}by default. That is, the input speed*ω*_{I}is used in the ratio that defines either*K*or*K*^{*}.For

*R*_{ω}> 1, the alternative choice for reference speed is to replace*ω*_{I}by the output speed*ω*_{T}in this defining ratio.

The two dependent variables, *R*_{τ} and *K*,
are functions of the independent variable *R*_{ω}.
They specify the characteristics of the torque converter:

*R*_{τ} = *R*_{τ}(*R*_{ω})
, *K* = *K*(*R*_{ω})
.

If you do not specify capacity factor data for a speed ratio
of 1, the block uses a capacity factor value of 10**K*_{Max},
where *K*_{Max} 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.

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

*τ*_{I} =
sgn(1 – *ω*_{T}/*ω*_{I})·[*ω*_{I} / *K*]^{2} , *τ*_{T} = *τ*_{I}·*R*_{τ} ,

in normal operation (forward power flow).

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

*t*_{c}·(*d**τ*_{I}/*dt*)
+ *τ*_{I} = *τ*_{I}(steady
state) .

The preceding instantaneous function of the capacity factor *K* determines
the steady-state value of *τ*_{I}.

The impeller shaft must always rotate in a positive direction.
Simulation is not valid for *ω*_{I} <
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. You must set the initial speed for this inertia to a positive value to ensure that the impeller starts by rotating in a positive direction.

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.

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.

Was this topic helpful?