Computer simulation helps Caterpillar improve torque converter performance - design tools

Diesel Progress North American Edition, Feb, 2004 by Youssef Dakhoul

Caterpillar is a leader in manufacturing a wide range of heavy equipment. The torque converter is an important powertrain component found on many Caterpillar vehicles such as track-type tractors, wheel loaders and off-highway trucks. Caterpillar's Transmission Business Unit (TBU) ks responsible for designing and manufacturing torque converters for these vehicles.

The three most crucial components of a torque converter are the impeller, turbine and stator wheels. These are bladed hydrodynamic devices arranged in a closed-loop as shown in the accompanying simplified diagram. Blade shape designs for the three wheels determine the performance of the converter. Finding the correct blade shapes is a major iterative effort involving: 3D blade geometry specification, 1D converter performance and thrust prediction and 3D CFD analysis of the oil flow in the converter.

Both CFX-TurboGrid and CFX-TASCflow from ANSYS Inc. are extensively used at TBU to conduct the 3D CFD analysis part of our converter blade design effort. Caterpillar engineers generate high quality, 3D grids for the three bladed passages with minimum effort using TurboGrid's powerful 'templates' which are designed to greatly reduce the grid generation effort for many standard blade arrangements.

They also find it easy to set up the initial and boundary conditions for this complicated flow problem. This is largely due to the strong compatibility between TurboGrid and TASCflow which automates most of the boundary conditions work. Also, the TASCflow pre-processor makes short work of setting up the rest of the boundary and initial conditions. It makes it easier to control all the analysis parameters, such as turbulence models, fluid properties, and convergence criteria and converges to a reasonably accurate converged solution in a short run time.

The powerful TASCflow postprocessing tools make it easy to examine the results in the form of contour plots, velocity vectors, or particle traces in the 3D space or in the 2D transformed cascade planes. As a result, a complete picture of the extremely complex flow emerges to help the blade designers.

TASCflow command macros can be quickly set up to estimate torque on the three wheels from the calculated pressure and velocity fields. This makes it simple to predict the performance of the converter at any desired turbine-to-impeller speed ratio.

The accompanying figures show some examples of calculated results. They provide excellent insight into the nature of the complex converter flow--insight that cannot be gained any other way due to the difficulty of direct measurement within the torque converter. The bottom line is that Caterpillar engineers find it relatively easy to understand the impact of the critical blade design variables on the performance of the torque converter and are able to optimize the design to a higher level than was possible in the past.

Youssef Dakhoul is senior engineering specialist, Caterpillar Transmission Business Unit, East Peoria, Ill.