CFD upshifts transmission valve design

Whitehall, MI -Acutex, a wholly owned subsidiary of Hilite Industries, designs and manufactures transmission-control solenoid valves for all major automotive companies-making between 30,000 and 40,000 valves every day.

"Customers specify the flow and pressure regulation requirements for the transmission fluid, and I start designing the valve based on those requirements," says Paul Christensen, advanced product development engineer for Acutex.

In the past, Christensen says, "We would use simple turbulent flow equations to choose the initial orifice sizes. We then ran the electro-hydraulic simulation without knowing the exact flow coefficient and laminar transition information, but found this did not represent accurately what's happening in a specific valve." Next, in the test lab, "We spent a very long time using the 'cut and try' method," he notes.

Now with the computational fluid dynamics package CFDesign from Blue Ridge Numerics (Charlottesville, VA), Acutex is not only sizing the valve orifices but more efficiently shaping (and integrating) them, as well as better determining flow coefficients. This information is then fed into Saber electro-hydraulic simulation software by Analogy (Beaverton, OR). Christensen says, "CFDesign makes it easy to change temperatures in the analysis so we can run several analyses to see which combination of size and shape provides sufficient and as constant a flow as possible over the desired temperature range."

For instance he notes, "We want to design valves that are as insensitive to viscosity changes as possible." Customers for a variable bleed solenoid (VBS) valve, for example, usually specify control pressure at 0.50A current at a temperature of 80C, as well as at the operating extremes of -30 and 135C. The viscosity of the transmission fluid changes with the temperature, and the valve needs to minimize pressure variation over the temperature range. "Because our valve's output pressure depends on the flow through both the fixed upstream orifice and the variable downstream orifice, the less sensitive they are to viscosity changes, the more constant the output pressure," Christensen says.

He can analyze the entire 3D valve design with CFDesign, or break it down into smaller subsystems for fast 2D analysis. Acutex engineers plot turbulent flow coefficients against Reynolds numbers over the temperature range. "We look for the 'knee' in the curve, where the flow transitions from laminar to turbulent," notes Christensen. "This transition Reynolds number should be as low as possible, so we can keep the flow turbulent to a very low temperature, which translates into reduced VBS output pressure variation over the temperature range." If the analyses show flow sensitivity to viscosity changes, Christensen looks for ways to sharpen orifice edges to minimize any long, thin flow paths. "We also look to add features that might keep the fluid in a turbulent state over a wider temperature range," he says. "For example, an angled hole."

CFDesign helps Christensen reduce design cycle time by making it possible to predict what will change before simulating the valve behavior. "The software has really saved a great deal of time and money over the past year especially in the amount of cold flow testing we have been able to avoid-on the order of $17,000 in testing and outside analysis that now can be done within CFDesign," he notes. "We use it to develop new products in less time, with fewer lab prototypes. I would estimate that the software has allowed us to shorten our development cycle by approximately 15%," Christensen adds. "But more importantly, it helps give our customers greater confidence in our products."