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CFD Yields EPA Okay

Article-CFD Yields EPA Okay

CFD Yields EPA Okay

Most people enjoy clean air, but it can give heavy-truck engineers a real headache now that truck builders have to comply with Environmental Protection Agency regulations that require them to reduce nitrogen oxide emissions by more than 40%. Coming up with trucks that meet current clean air regulations, and the even more stringent ones that go into effect in 2007, has already triggered changes in heavy-truck design. One of the first has been the adoption of new diesel engines fitted with exhaust gas recirculation (EGR) systems. These engines do run cleaner, but EGR's negative effect on engine efficiency would normally sacrifice some fuel economy. Engineers at Volvo Trucks North America Inc. managed to steer clear of this fuel economy trade-off when they redesigned the company's VN heavy trucks that appeared late last year.

They did so by applying more computer-aided-engineering muscle power than they ever had before and by using that CAE data to optimize aspects of the truck that affect its fuel use-including its cooling system, aerodynamics, and weight. "The new engines by themselves aren't as fuel efficient, but that doesn't mean that the truck can't be," says Chuck Pannell, the Volvo project manager who supervised the VN design team.

Powered by Volvo's own VED12 engine or Cummins ISX, the Volvo VN trucks were one of the first heavy trucks to gain EPA certification for meeting the 2002 emissions requirements. At the same time, the company estimates that they carry a fuel economy penalty ranging from just 0 to 2%, depending on the truck's configuration and how it's driven.

"Many customers won't notice any fuel economy penalty at all," says Pannell. And that's important for the truck to succeed commercially. "Fuel is the second highest expense for fleet owners after driver wages," notes Susan Alt, vice president of marketing for Volvo Trucks. She says large fleet owners have told her they would "replace their entire fleets for even a 2% savings in fuel costs."

Volvo Trucks has long used a variety of CAD and CAE tools to design its products. For this project, the design team worked in solids, using both CATIA and Pro/ENGINEER. As with earlier products, the design team also employed finite element analysis on the truck's structural components. And it used computational fluid dynamics (CFD) to optimize the aerodynamics of the exterior-though not to the point of completely eliminating wind tunnel testing of full-scale models. But this project also took CFD a step further. "Normally we use CFD just for the skin of the truck," says Pannell. "This time we used it inside the engine compartment to evaluate air flow under the hood and around the engine block."

Stay Cool

This extra CFO work under the hood came in handy as Volvo engineers tried to offset the most pronounced effects of adding engines outfitted with EGR systems. As Pannell points out, EGR systems raise the temperature in the engine compartment in two ways: They throw off some direct heat of their own and, because they must cool and pressurize the exhaust gas, they heap additional demands on the engine, reducing its efficiency. This reduced engine efficiency turns up as an increase in heat rejection, which Volvo measures as the difference between the temperature at the air intake manifold and of the coolant in its tank. Going into the new VN project, Pannell says that the company's computer models predicted that the reduced emissions engines would increase heat rejection by as much as 40%. "We didn't end up seeing that much," he says. "But we did see as much as 25 to 32%, depending on the engine model." If left unchecked, such heat rejection could have translated to as much as a 5 to 7% loss in fuel economy, Pannell estimates. What's more, unabated heat rejection could result in some "de-rating" of the engine, or limitations imposed on its horsepower and torque to prevent overheating.

Better Bumper: The VN's new bumper design, assembled from pieces of injection molded PC/ABS, helps manage the airflow under the truck. Pannell calls the latest VN the company's "most aerodynamic truck."

According to Pannell, the under-the-hood CFD prediction work helped the design team quickly come up with an front grill configuration that increases airflow into the engine compartment and helps dissipate heat. Volvo engineers, working in North Carolina and in Sweden, used the CFD to evaluate airflow under the hood and around the engine block. That information drove a variety of design decisions-in hood geometry, air vent location and sizing, air cleaner design, and the configuration of the intake manifold design. "Seventy percent of the CFD results confirmed our previous experience with cooling system design," Pannell says. "But 30% were things we hadn't considered in the past."

The design team also used computer modeling to finalize other aspects of the engine cooling system. Thanks to the use of computer modeling, the design team quickly culled dozens of cooling system design ideas down to just three strong candidates. Only these three had to be tested traditionally using calibrated engines and dynamometers. "I think we ended up with one of the best, if not the best, cooling packages in the business," Pannell says. He won't reveal too much about the company's proprietary cooling system design, except to say that it involves a larger radiator than usual as well as optimized air flow and component locations.

Systems Approach

Offsetting the effects of the reduced-emissions engine did not only come down to cooling alone. "We took a systems approach," says Pannell. For example, Volvo redesigned accessory drives in order to reduce parasitic losses. They also engaged in a weight reduction program that shaved weight from hundreds of components for an overall savings of about 1,500 lbs compared to the original VN models (see sidebar).

Thick Enough? Volvo engineers made extensive use of FEA analysis in their efforts to remove weight from the vehicle's hood and other exterior components. Engineering software also helped the company fine-tune the vehicle's exterior, which features aerodynamic improvements, such as repositioned cab that reduced the distance between cab and trailer, the elimination of drag-inducing gaps between body components, new aerodynamic headlights, flared fenders, sleeker mirrors, splashguards, and injection-molded bumpers that manage the flow of air under the truck.

Perhaps the most important efforts outside of the cooling system design related to aerodynamics. Pannell calls the latest VN the company's "most aerodynamic truck" to date with a 7% improvement in drag coefficient compared to the standard VN configuration. Though Volvo did do its share of wind tunnel testing using physical models, engineering software also helped the company fine-tune aspects of the vehicle's exterior, which features aerodynamic improvements such as a repositioned cab that reduced the distance between cab and trailer, the elimination of drag-inducing gaps between body components, new aerodynamic headlights, flared fenders, sleeker mirrors, splashguards, and injection-molded bumpers that manage the flow of air under the truck.

Pannell notes that some of these exterior components illustrate the delicate balance that the design team had to strike between technical considerations and styling. The new headlamps, to take one example, serve both styling and aerodynamic goals. So does the bumper (see DN 06.16.03, p. 87 for a more detailed look at this award-winning bumper design). "Styling was very important with this truck," he says, noting that Volvo wanted to create an "edgy, more masculine" look than the previous VN trucks. And sometimes these two realms-styling and engineering-did come into conflict with one another. Pannell points out, for instance, that hood configurations that would have been great for engine bay airflow could have interfered with styling goals or the driver's line of sight.

All the CAE tools-including not just CFD but also FEA analysis of exterior components-played a role in weeding out designs that in the past would have been rendered in physical models. And that weeding suited Pannell just fine. He says that the design team learned about the accelerated emissions reduction schedule more than halfway through an on-going redesign of the original VN trucks, which came out in 1996. "We decided to go ahead and accommodate the new engines," he continues. But that accommodation, with all the changes to the cooling system and exterior, compressed an already tight development cycle, driving Volvo to rely more heavily on CAE than in the past. "We didn't have time for a lot of trial and error," he says.

Contact Senior Editor Joseph Ogando can be at [email protected].

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