FEA for MPGs

“Engine downsizing has long been known as one of the most effective available technologies for increasing efficiency,” according to Mark Stephenson, chief engineer, predictive analysis, MAHLE Powertrain. While a number of vehicle manufacturers are offering small, turbocharged engines—from the VW 1.4-liter TSI to the BMW 3.0-liter Twin-Turbo—MAHLE decided that they wanted to go one better.

“Engine downsizing has long been known as one of the most effective available technologies for increasing efficiency,” according to Mark Stephenson, chief engineer, predictive analysis, MAHLE Powertrain, Northampton, UK (mahle-powertrain.com).

While a number of vehicle manufacturers are offering small, turbocharged engines—from the VW 1.4-liter TSI to the BMW 3.0-liter Twin-Turbo—MAHLE decided that they wanted to go one better. That is, they started with a 2.4-liter port-fuel-injected V6, then sought to develop a 1.2-liter, in-line three that would be capable of producing 211 lb-ft of torque at 2,500 to 3,000 rpm and 193 hp @ 6,500 rpm.

This would require a twin turbocharger system, direct injection, and variable valve timing. It would be produced out of precision-cast aluminum, using the company’s proprietary Coscast process, which had been originally developed for Formula One car engines.

Stephenson noted, “As specific output increases, so do the technical challenges.” Among those challenges: developing a robust combustion system that allows a high compression ratio to maintain part-load efficiency; good low-speed torque and transient performance; real-world fuel consumption benefits through a reduction in full-load fuel enrichment; and engine robustness in durability.

They just didn’t start casting, machining, and assembling. “With a long list of technical challenges, we rely on finite element analysis to guide, validate, and optimize the design,” Stephenson said. For finite element analysis (FEA) work they use Abaqus FEA from SIMULIA (simulia.com), part of Dassault Systèmes. About the software, he said, “We originally chose Abaqus because we considered it the best tool for solving the day-to-day nonlinear problems we encounter, such as those involving plasticity and contact, as well as for its ability to perform thermal and NVH simulations.”

Here are key steps in developing the engine:

  • Crankshaft analysis. Given the fact that downsized engines require very high specific output, structural capability is key. To analyze the crankshaft, associated components affecting torsional oscillations—con rods, pistons, pulleys, and the flywheel—had to be taken into account. They developed with Abaqus a substructure of the crankshaft containing 340 K elements, 435 K nodes, and 1.66 M degrees of freedom, then imported the model into AVL Excite (avl.com), ran a dynamic simulation, then used the deformation results at the retained degrees of freedom to drive the full Abaqus model so to recover the stresses. With the stresses, they ran a fatigue analysis for a full 270° cycle to determine the fatigue safety factors for the crankshaft. Also, a submodel of the crankshaft journal fillets underwent an additional fatigue analysis that ran for 24 hours.
  • Head and block analyses. They looked at the entire assembly—block, bedplate, nut plate, head bolts, cylinder head, head gasket, valve guides, valve seats, valves, etc.—via a model that contained 1.01 M elements, 1.72 M nodes, and 8.8 M degrees of freedom. They performed thermal analyses as well as complex structural analyses (which in itself required 10 separate steps), durability, and more.
  • Exhaust manifold analysis. They constructed an exhaust model with 147 K elements, 410 K nodes, and 1.21 M degrees of freedom. Because the high-pressure turbo housing is integrated into the exhaust manifold, it was important to conduct thermo-mechanical analysis to determine the potential of manifold cracking due to thermal fluctuations.

After modeling, the group has built some demonstration engines, which are currently undergoing tests—in real cars, not on Dell blades running SuSE Linux.