For 30 years, engineers have used the GM Aerodynamics Laboratory to improve fuel economy by helping cars and trucks slip through the air more easily. Put into operation in August 1980, the wind tunnel is the largest of its type dedicated to automotive work. Over three decades, engineers have cut the coefficient of drag (C D) of GM vehicles by approximately 25 percent.
To illustrate how that benefits owners of GM cars, that reduction in drag, without any other changes, would improve combined fuel economy by two to three miles per gallon. That’s the equivalent of saving the drivers between $100 and $300 per year on fuel, at $3.00 per gallon, and saving tens of millions of gallons of fuel per year for the entire U.S. fleet of 2010 model-year GM cars.
“There are three ways to improve fuel economy – reducing vehicle weight, improving powertrain efficiency, and improving aerodynamics,” said Charlie Klein, GM director of Mass, Energy and Aerodynamics. “Of the three, aerodynamics is often the most cost effective way to improve efficiency.”
Aerodynamics is the efficient management of air flow – measured as drag force – acting against a vehicle. The air flow around a vehicle affects vehicle acceleration, cornering, cooling, comfort, visibility, and especially fuel efficiency. For cars, cutting through the air accounts for 13 percent of all fuel consumed, according to the EPA-defined city and highway driving schedule. For full-size SUVs, the effect is even more pronounced, accounting for 22 percent of fuel used.
Designing for efficient aerodynamics is both science and art. Air must move smoothly around the vehicle’s shape and separate cleanly at the trailing edge. This is easy to say but difficult to do. This approach is best illustrated by the Sunraycer, a solar-powered research vehicle. At 0.143 CD, the Sunraycer remains the lowest drag vehicle ever tested in the GM wind tunnel.
Applying aerodynamics to production cars is much more complex, according to Max Schenkel, aerodynamics technical fellow for GM.
“The Sunraycer demonstrates what is aerodynamically possible, but it clearly is not intended for daily driving. For production cars, aerodynamics is compromised by necessity to accommodate aesthetic design, passenger accommodations, and even safety requirements.”
Aerodynamic work typically begins in 1/3-scale clay models to test the overall shape of a vehicle. At this stage, some of the things the team works on include:
- Smooth the transition from the front bumper to the vehicle sides, so air hugs the body
- Angle the windshield backward, and increase the curvature, to reduce air pressure on the glass
- Tailor the rear end of the vehicle to minimize pockets of low pressure behind the vehicle
Once the shape is coarsely defined, the team transitions to full-scale development, often testing aerodynamic changes as small as one millimeter. One of the best examples of that attention to detail is the 2011 Chevrolet Cruze Eco model that features a number of aerodynamic features, including:
- Segment-first active air shutters in the lower grille opening. When open, the air shutter provides additional cooling for the engine. When closed, the air shutter reduces wind drag.
- A front air dam, lower ride height, and underbody pans minimize underbody drag beneath the Cruze
- Tire blockers reduce the pressure build up in front of the tires and thus reduces drag
- A rear deck-lid spoiler increases the air pressure behind the Cruze Eco and reduces its CD
The changes reduce the coefficient of drag for the Cruze Eco by more than 10 percent – and contribute to the Cruze Eco’s expected 40 miles per gallon highway.
“The Cruze Eco demonstrates how aerodynamics will be increasingly important, as consumers and automakers look to increase their fuel efficiency,” Schenkel said. “Aerodynamics also play a critical role in developing electric vehicles, like the Chevrolet Volt, as better aerodynamics delivers more miles per battery charge. We expect the next 10 years likely will be the most innovative era in road vehicle aerodynamics.”