Design and Optimization of an Automotive Torque Converter

ADT engineers performed the design of a torque converter to improve efficiency at multiple operating conditions against a baseline design

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Introduction

Torque converters are an atypical turbomachinery that recirculates flow from the pump into the turbine, through the stator and back into the pump. This means that there is a very high interaction between the stages and changes to one component, such as the pump impeller, could create an entirely different flow field back into the pump impeller. As a result it has become common practice to mainly rely on stator optimization, although major efficiency improvements can be achieved by optimizing all three components simultaneously.

A torque converter

ADT engineers were tasked to redesign an existing torque converter for a leading automotive manufacturer. The current design performance was used as a reference baseline but specifications regarding the number of blades and input speed were left to ADT’s choice during the optimization.

It is essential during the torque converter design phase to maintain both the pump and turbine head, at the specific volume flow rate and also the inlet and outlet conditions of each component at every design iteration. The risk is that mismatch between the components may cause incidence losses and deteriorate the torque converter performance.  

Using TURBOdesign1 the inflow and outflow distribution at each blade row, effectively the spanwise work distribution at the leading edge and trailing edge of each blade, are input in for each component and maintained throughout each and every design iteration. This is a major benefit during the design and optimization process and it allows designers to directly control the hydrodynamic performance of the components.

Using 3D Inverse Design as the blade parameterization it is possible to achieve high accuracy surrogate models using only a few design parameters and design cases.

A first design using TURBOdesign1 was generated as a starting point for the optimization. The design input parameters followed the generally established design guidelines illustrated below:  

 

Streamwise loading used for the baseline stator, pump and turbine respectively. The stator features a mid-loaded distribution while the pump is clearly aft-loaded to reduce profile losses. The turbine features a fore-loaded hub and aft-loaded shroud to control secondary flows

 

This initial design was run in CFD to evaluate the mechanical performance of the torque converter, some of the results can be seen below:

 

Comparison of rVt from 3D CFD (left) for the full stage and the input rVt* from TURBOdesign1 (right) showing a very good match between the specification and results

Velocity flow-field at midspan and 0.8 speed ratio

 

For the optimization of the entire torque converter the adopted design strategy involved blade parameterization of the stator, pump impeller and turbine runner using TURBOdesign1 and CFD analysis using Ansys CFX. In order to link the stage components together in an automatic optimization loop, TURBOdesign Link-WB was used to automatically set up the blade design, CFD and optimization environment within Ansys Workbench.

Using 3D Inverse Design as the blade parameterization it is possible to achieve high accuracy surrogate models using only a few design parameters and design cases. This allows for rapid multi-point and multi-objective design optimization with relatively inexpensive computational efforts. 

The final design performed significantly better than the original design.

The automatic optimization strategy adopted in this project involved generation of 30 design cases using TURBOdesign Optima’s Kriging sampling technique, automatic meshing and CFD analysis of the 30 stages at 3 operating conditions for a total of only 90 CFD points. The design optimization using Multi-Objective Genetic Algorithm (MOGA) based on the Response Surface Modelling (RSM) then allows multi-point optimization, at all three speed ratios, with no additional computational efforts. 

 

Automated torque converter stage optimization setup within Ansys Workbench with TURBOdesign Link-WB including all three components and three speed ratios of 0.8, 0.65 and 0.5

 

The 30 cases are run automatically with the CFD results captured by TURBOdesign Optima while the MOGA objectives were set up to improve the torque converter mechanical efficiency at all speed ratios.

The final design performed significantly better than the original design:

  • The pump efficiency was improved by 5%
  • The stator exhibited lower incidence losses thanks to a more aligned blade
  • Torque ratio towards stall was also improved significantly

Comparison of velocity flow-fields at 0.8 speed ratio and 0.9 span for the stage showing clear reduction of low momentum fluids in the pump and elimination of seperation in the turbine blade

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