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How to set the Torque Converter Capacity Factor?


I am trying to specify some data for the Torque Convertor of a vehicle. The following parameters are needed to compute the Impeller Torque of the Torque Convertor:

Speed Ratio

csr = Nt/Ni 

Torque Ratio

ctr = Tt/Ti 

Where, Nt and Tt are the angular speeds and torques of the turbine and Ni/Ti of the impeller.

Capactity Factor

(Shows the ability of the convertor to absorb or transmit the torque)

Ktc = f(csr) -> Look-up table

The capacity factor is usually computed via linear interpolation in a look-up table, depending on the speed ratio. For example:

  csr         Ktc          ctr


   0        137.46      2.23

0.1         137.06     2.07

0.2         135.86     1.97

0.3         135.66     1.84

  .               .              .

  .               .              .

  .               .              .

  1           254.51     1.004


I found how to insert the Torque Ratio and Speed Ratio, but I need to know how to insert the capacity factor. Is it possible to do it?




The torque converter model defined in Vehicle Systems uses two tables, which you can set in the Dynamics Component -> Parameters -> Torque Converter properties:

Ctr Table: the torque ratio (as a function of speed ratio)

Lambda Table: the lambda function (as a function of speed ratio)


The torque ratio is basically what you expect. The table contains a column of speed ratios (the input variable) and torque ratios. You can check the exact format of the csv file by looking at the default table provided in:

C:\CM Labs\Vortex Studio 2019c\resources\VehicleSystems\TorqueConverter_CTR.csv


There is also a default lambda table in the same directory: TorqueConverter_Lambda.csv


The lambda factor contains the same information as the Capacity Factor that you are referencing, but scaled differently. I assume the model you are using is equivalent to what is provided in Matlab:


In that model:


Tt = (1/K^2) * Ni^2


We use a slightly different form, where the Capacity Factor is normalized by adding a coefficient, C, and a scaling factor, S:


Tt = S * C * lambda * Ni^2


S is the [Coupling Torque Scale Factor] input in the torque converter. It's normally 1, but you can change it to control the scaling of the torque.

C is a constant that is specific for each model of torque converter, which is calculated automatically in Vortex.

Lambda is a normalized table (as you can see from the default table, it's between 1 and 0).


In Vortex, it is not possible to directly specify C. It comes from a simple calculation to determine what value of C corresponds to the correct stall speed of the torque converter. For most vehicles, that is determined automatically from the max torque of the engine torque curve, which you can see in the Outputs -> Engine -> [Engine Max Torque] (T) and [RPM at Max Torque] (N):


C = T / N^2


This assumes that lambda is normalized to have a maximum value of 1, and S is also set to 1. Also note that the units of rotation are always rad/s (not RPM).


So you can convert your Capacity Factor table into a lambda table by using lambda = 1/K^2 / C, then normalizing it to have a max value of 1. Then you can calculate C using the expression above, and enter a value of S to that the values are equivalent:


1/K^2 = S * C * lambda

Unfortunately, this is a bit more work than just specifying the capacity factor table, but it was designed to help users get reasonable behaviour, even if they don't have exact tables or specifications available.

1 person likes this

Thanks, Martin.

I understood the explanation and it really helped me.

You're welcome. Glad it helped!

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