Computer power drives GM powertrain

Virtual validation testing

GM Powertrain is harnessing another kind of power these days - the power of the computer. They're utilizing computeraided engineering tools to simulate the validation testing processes of powertrain components and systems - a process that would normally have been done with hardware.

CAE testing not only improves function and performance, but by interfacing with the design process up front, there is a reduction in development cycle time and a dramatic savings in cost.

"The goal of synthesis and analysis," says John L. Givens Jr., director of GM Powertrain Synthesis and Analysis,"is to produce first-time capable designs. We want to come out with a design that we know will meet requirements and then take it into a confirmation phase right after the design phase."

Givens' 120 member team is divided into three functional sub-groups, Systems (engine, vehicle and transmission), Sub-systems (air flow, thermal management and transmission) and components (head and block, rotating and reciprocating, sealing and fastening and transmission), that do structural analysis for stress strain, fluid dynamics, noise and vibration, thermal analysis, stress and deflection analysis, fuel economy, drive quality and powertrain performance.

"We're gauging our supplier base to do more of this type of analysis," Givens adds. "As part of our sourcing process and statements of requirements, we're asking them to supply mathematical models in order for us to integrate their components into our system. That's becoming their responsibility."

While a large percentage of the math-based tools GM uses are commercially available, a special method team has developed fourteen specific tools.

"If we can't find anything commercially" Givens says, "we'll develop the software in-house. Some of the codes we have are pretty old, but they're still not available commercially to the level that we need.

"One of our internally developed tools, CRDA (connecting rod design analysis) is linked right to UG (GM's CAD design tool)," Givens says, "So we can feed our analysis right back to the designer."

Givens adds that even when commercial software is used it's often modified to meet special needs.

Senior Project Engineer Joe Bishop's group does internal structure analysis for cylinder heads, blocks and exhaust manifolds, measuring temperatures everywhere within the head and block and predicting stresses and strains off of those thermal loads. Bishop worked extensively on the Vortec 4200, inline-6, and is now working on the four- and five-cylinder variants.

Bishop says that once CAD designers build the models (two to six weeks, depending on the type of engine), they're boiled down to the fundamentals, called finite models (head, head bolts, valve seats, guides and head gasket) that are needed to do an analysis. He then collects data from all the other groups and runs a typical validation test.

"Once all of the thermal and stress analysis has been done," says Bishop, we zero in on every area of the head and block with sub-models and take a finely-detailed, high-fidelity look at stresses and strains." Results from these tests are fed back to the design group with design change recommendations.

From start to finish, a full thermal structural analysis can take up to a couple of months, but Bishop says that once the data is in the computer, it may only take a week to run a full simulation on design iterations. Bishop says that one of GM's goals is to model competitor's engines allowing engineers to do virtual comparative testing.

Isaac Du, senior project engineer for Noise and Dynamics, does multifunctional analysis. He designed the Powertrain Dynamics Simulator, a series of software tools developed for cranktrain, valvetrain, timing chains and accessory-drive line analysis that were used to test for crankshaft balance and noise an vibration during development of Displacement on Demand available on the next generation Vortec V-8 in 2004.

"Going to VA mode," Du says, compression is boosted in the remaining four cylinders, which changes every aspect of the engine's subsystems, causing unwanted vibration." Analysis of torsional rigidity over several sub-systems linked together called for the design of a special engine mount that created counter torque, reducing the noise and vibration

"The majority of our work is in components," says Rob McAlpine, group leader for air flow and combustion. "We monitor air flow and balance the engine to minimize losses so we can maximize fuel economy"

McAlphine has used synthesis and analysis to prove that what works on one engine may not work for all of them. For example, testing showed an intake port that needed an extra machining operation to improve air flow resulting in a significant gain in power and fuel economy. When his team applied the same solution to other engines, they found that the cost of the extra machining operation outweighed any benefits.

"Instead of cutting cylinder heads," McAlpine says, "we did the testing in the computer model. Once you gain confidence in the math," he adds, "then you can start to develop designs without hardware."

- John Peter

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"We're gauging our supplier base to do more of this type of analysis. As part of our sourcing process and statements of requirements, we're asking them to supply mathematical models in order for us to integrate their components into our system."

- John Given