Despite a brief respite, gas prices are climbing again and consumers still have their eyes on fuel-efficient automobiles. The Chevy Volt, GM’s extended-range electric vehicle (E-REV), promises up to 40 miles of gasoline-free driving. The combination of lithium-ion battery technology and a cutting-edge combustion engine only account for part of that efficiency, though. The Volt’s body is the result of precise aerodynamic design. We recently spoke with Nina Tortosa, an aerodynamicist with GM, to find out more.
Electronic Design: Last fall, GM announced that the Volt would be hitting showrooms in late 2010. Is that still on target?
Nina Tortosa: Yes, this is still on target.
ED: Do you anticipate any changes in design between what we saw last fall and what will be hitting the roads next year?
NT: The design that was shown last fall for the GM centennial celebration is basically the production design. Due to manufacturability there may be a few adjustments, but the overall look and feel of the design will not change.
ED: Can you discuss the relationship between the aerodynamics and how they improve the vehicle’s electric range?
NT: The lower a vehicle’s drag, the further that vehicle can go on a single charge or tank of gas. It’s similar to using a cordless drill on concrete versus wood. If you’re using the drill to drill into concrete, the battery won’t last nearly as long as if you’re drilling into wood. The same is true for vehicles. The easier it is to cut through the air, the longer it can go on that single charge.
ED: The Volt’s grille has a distinct look. How did that grille evolve from what can be found on a typical gasoline car to what you have on the Volt now?
NT: The grille on the Volt was closed up to help reduce aerodynamic drag. By closing up the upper grille, the air gets diverted around the vehicle instead of through the vehicle, thereby reducing drag. However, not all of the grille can be closed off, since some air is still required to cool the fluid running through the radiators. We balance the radiator flow requirements with the aerodynamic drag to optimize the vehicle’s performance.
ED: Were there any other areas where computational fluid dynamics (CFD) analysis revealed where you could improve body design?
NT: CFD was instrumental in identifying some separation points behind the rear wheels. The CFD can show the pressure distribution on the surface, which is something not visible in the windtunnel. This allowed us to make specific changes that reduced drag.
ED: What kinds of software and hardware are involved in CFD testing?
NT: We use ANSA and a preprocessing tool, Fluent, as the CFD solver and post-processor and also Ensight as a post-processor. The preprocessor is what models the CFD model and provides the geometry definition. The solver is the number cruncher that actually calculates the drag coefficient. The post-processor generates all the colorful pictures that give us insight into what’s going on with the flow or pressure.
ED: How much of that testing system was developed by GM, and how much was contracted or bought “off the shelf?”
NT: I think by “testing system,” you refer to the CFD software, which was all off-the-shelf. But the software settings are customized to simulate the windtunnel that GM owns.
ED: Do you use this same testing system for the other cars in the GM fleet?
NT: All GM projects use the same development process and software.
ED: Going back to the shape of the Volt’s body, the grille design has an effect on the aerodynamics surrounding the front of the car. What effect does the spoiler have on the aerodynamics surrounding the rear?
NT: The spoiler had big impact on the performance of the rear of the vehicle. The spoiler location was optimized in the wind tunnel. In fact, the 5-mm kick-up on the rear spoiler was worth five counts of drag reduction, even though it was a small change.
ED: In addition to the grille, the spoiler, and the flared rear wheels, what other elements of the body were key to improving the Volt’s electric range?
NT: Aerodynamic drag is a dynamic system so it all matters. We spent a lot of time developing the exterior surface and the underbody to make sure that the vehicle will provide the customer with the 40 miles of electric range. The biggest “aero enabler” is the back of the vehicle and how it manages the flow as it separates off the surface.
ED: How important are the body’s materials to the aerodynamics of the design?
NT: The air doesn’t know what materials are used in a vehicle, so as long as the material doesn’t deform under aerodynamic loading, it has little importance. For parts like the airdam, material selection is important for part longevity. For example, the airdam, being as low as it is, needs to be flexible in case it hits something. But it also needs to retain its designed shape while the vehicle is moving.
ED: How closely do you work with the battery team or the other engineering teams in the overall design?
NT: I work with the other engineering teams as needed. For example, I work with the teams that have parts that might affect drag. The battery is covered up and isn’t “seen” by the wind so I didn’t need to interact with the battery team much. However, the battery covers are seen by the wind, so I tested those extensively and provided aero feedback for their design.
ED: How do you respond to some of the public misconceptions about electric and hybrid vehicle performance?
NT: I think of buying hybrids and electric cars like buying organic versus non-organic or fresh veggies versus canned veggies. We all know that non-organic and canned veggies are cheaper, and yet we all buy organic and fresh veggies because they are better for us. and as we consume more organic produce the price comes down. I think hybrid and electric vehicles follow a similar equation.
ED: Is GM taking any of the lessons it has learned in the Volt’s wind tunnel and applying them to the other cars in its fleet?
NT: We at GM share all our aero knowledge from one project to the next. We’re a small group and meet monthly to discuss all things aero-related. All projects go trough regular peer reviews to ensure that lessons learned on one project are passed on to the next.