Volt aero and styling: Touring the E-Flex design studio and GM wind tunnel
When the Volt concept was revealed almost a year ago, one of the big questions from readers here and elsewhere was about the aerodynamics. The exterior design team led by Bob Boniface had created a very striking design that looked absolutely nothing like the legendary EV1 teardrop. Judging from the first teaser shot we saw of the production design (above, and read more here), some things were going to have to change.
Read on past the jump to find out more.
[Source: General Motors]
In the wake of the enormous popular success of the original Volt concept, it quickly became apparent to GM that they needed to move ahead with production engineering and development of the car. One of the early moves made by the company in the spring of 2007 was to establish a new design studio for E-Flex vehicles. Bob Boniface, who led the exterior design team for the concept, was appointed to head the new studio.
Typically each brand such as Pontiac or Buick has exterior and interior design studios for their vehicles. Those studios are responsible for multiple projects. Because of the urgency of moving the Volt program forward and all the concurrent engineering going on, the decision was made to bring the exterior and interior designers together along with modelers, engineers and aerodynamicists in one location. Because the the decisions made on the outside can affect the inside and vice-versa, this would allow the teams to discover and resolve issues more quickly.
A nondescript building in the corner of the GM Tech Center campus in Warren, MI was made over for this purpose. An auditorium that from from the fifties through the seventies was used for dealer shows and other events was gutted; clay milling machines and computer workstations were installed. Currently forty-five people staff the E-Flex studio. The mathematical modelers who take the digital sketches generated by the designers and transform them into surface models work with Alias. That's the same software used by the animation gurus at Pixar to create movies like Finding Nemo and Ratatouille.
Those models are then transferred to the clay milling machines to be transformed into physical models of the car's exterior and interior. Where it used to take sculptors two to three weeks to create a clay model by hand, the machines can produce them accurately and repeatably in two to three days. This is critical for wind tunnel work where subtle changes in shape can have big effects.
The clay (and later fiberglass and metal) models and even actual vehicles eventually end up in GM's wind tunnel. The world's largest automotive wind tunnel sits on the Tech Center campus. It features a 43 foot diameter fan driven by 4,500 DC motor. To reduce electrical energy consumption, the tunnel is a closed loop system that takes advantage of the momentum of the huge mass of air that's being moved around. The maximum wind speed in the tunnel is 138mph. Construction of the tunnel began in 1977 and the tunnel went into operation in 1980. The test section of the tunnel measures 18 feet high by 34 feet wide so that a full size truck blocks only six percent of the cross section.
As Bob Lutz mentioned during the Los Angeles Auto Show, the Volt concept may have been pleasing to the eye, but not so much to the wind. The concept car was built with an extremely short front overhang and large wheels. The result was a blunt nose that did a poor job of moving air out of the way.
Before taking AutoblogGreen and others to the design studio on Monday, Frank Weber, the Vehicle Chief Engineer for the Volt, talked about the effect of aerodynamics and other factors on vehicle efficiency. Driver behavior is clearly a factor for both traditional and electric cars. Beyond that, on traditional cars, factors like mass, aero drag and rolling resistance come into play in that order. In testing and simulation GM has found that for electrically-driven vehicles mass actually drops to third on the list behind aerodynamics and electrical loads with rolling resistance coming in fourth.
Working from the Volt's forty mile target range, it has been determined that a 400 lb. swing in mass only has a two-mile effect on range on the city driving cycle and only a one mile effect on the highway. While every mile and pound counts, this turned out to be much smaller than the effect of aerodynamic drag. Intuitively, most people expect that aerodynamic drag has an impact on the range and efficiency of a vehicle at highway speeds.
This of course is true with a forty count reduction in drag coefficient (for example going from .300 to .260) having a six mile impact on range at highway speeds. The impact at city speeds is the big surprise. Here the same aerodynamic drag reduction causes the range to increase by four miles, more than double the effect of a 400 lb. weight reduction.
It turns out that the additional mass is partly offset by the extra electrical energy storage capacity helping the range. The extra capacity also means that more recaptured kinetic energy can be stored. With aerodynamics, any additional drag means more energy has to be used to keep the vehicle moving. When the accelerator is released, the extra drag also slows down the vehicle faster, absorbing some of the kinetic energy, leaving less for the regenerative braking to recapture. All these factors mean that aerodynamics are critical to the performance of an electric vehicle, even at urban speeds.
What does this all mean for the look of the production Volt? It's hard to say with certainty since car-makers like to keep some surprises up their sleeves. On Monday, we were able to see the same front corner of the full sized clay model that's in the teaser shot at the top of this post. The rest of the car was covered up. We were also shown a one-third scale model in the wind tunnel that was covered in duct tape to obscure details. Nonetheless some general assessments can be made about the styling direction from various hints around the design studio.
Clearly the front of the Volt had to change. Looking at the car in plan view (the view looking down from above), the concept was almost flat across. Not good. A more parabolic shape was needed to allow air to flow smoothly around the sides of the car without separating from the body (which causes turbulence). The headlights are now flush with the body rather than being set back as they are on the concept (as you can see in the gallery below). The main twin port grille is blocked off and engine compartment airflow comes from below the bumper. The car will have a full belly pan to manage airflow under the car.
From the start, Boniface and his team generated a list of design themes that characterized the Volt and needed to be maintained in order for the production model to be recognizable as a Volt. Chief among these was greenhouse and side glass look as well as the tail treatment. The clay models in the studio were kept covered aside from that one corner that's visible in the teaser. In spite of that, the profile of the car was clearly visible and some things could be gleaned from that.
The line from the roof to the rear deck is much like the concept. The base of the windshield has moved forward providing more rake to the glass. It appears the center line of the front axle has moved back a bit allowing for more front overhang while the front corners are swept back to provide better airflow as well as helping to meet European pedestrian safety standards. The other main visual difference that was apparent from hints around the studio was a reduction of the Coke bottle effect along the sides. While the concept had prominently flared fenders, this will be toned down significantly to help keep the airflow attached to the sides. One other design element that will be prominently maintained on the sides is the plug ports since this is such an important part of what this car is all about.
Detroit's channel 4 was allowed to bring a video camera into the E-Flex Studio
While side glass look is expected to be maintained, the design team isn't revealing exactly how they will manage it. Boniface also declined to say what kinds of materials would be used to construct the body and structure of the Volt. We won't be seeing anything of the production Volt at the Detroit Auto Show. But if what we're hearing from the team proves accurate, GM is on track to have those first test mules running early in the year and GM has indicated we'll have a chance to try them out in the spring.
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