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Aerodynamic Development of the 2011 Chevrolet Volt_图文

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Aerodynamic Development of the 2011 Chevrolet Volt
Nina Tortosa and Kenneth Karbon
General Motors Company

2011-01-0168
Published 04/12/2011

Copyright ? 2011 SAE International doi:10.4271/2011-01-0168

ABSTRACT
This paper presents some of the challenges and successful outcomes in developing the aerodynamic characteristics of the Chevrolet Volt, an electric vehicle with an extendedrange capability. While the Volt's propulsion system doesn't directly affect its shape efficiency, it does make aerodynamics much more important than in traditional vehicles. Aerodynamic performance is the second largest contributor to electric range, behind vehicle mass. Therefore, it was critical to reduce aerodynamic drag as much as possible while maintaining the key styling cues from the original concept car. This presented a number of challenges during the development, such as evaluating drag due to underbody features, balancing aerodynamics with wind noise and cooling flow, and interfacing with other engineering requirements. These issues were resolved by spending hundreds of hours in the wind tunnel and running numerous Computational Fluid Dynamics (CFD) analyses. The end result is a unique electric vehicle that, at start of production, has the lowest coefficient of drag, C D , of any Chevrolet sedan.

fuel efficient as possible for the total extended mileage were key goals during its development. Aerodynamics played a crucial role in that quest. Engineers spent over 500 hours in the wind tunnel and thousands of cpu hours on the computer to refine the air flow around the vehicle.

Figure 1. The 2011 Chevrolet Volt.

INTRODUCTION
The Chevrolet Volt (Fig. 1) is an electric vehicle with extended-range capability powered by the Voltec electric propulsion system (Fig. 2). It consists of a 16 kWh lithiumion battery, an electric drive unit, a 1.4-liter internal combustion engine generator, and a vehicle charge port. Able to re-charge the battery from a standard wall outlet, many drivers will consume no gasoline and utilize only battery power that provides the first 25-50 miles of range. Afterwards, the engine generator can deliver electric power for 300 more miles if needed. Extending the life of the battery to achieve the initial electric range and making the Volt as

Figure 2. The Voltec electric propulsion system.

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VOLT AERODYNAMICS Drag Requirements for Fuel Economy
Aerodynamic requirements on any given vehicle are determined by top speed, handling, and fuel economy needs. For efficiency cars, aerodynamic drag is to be minimized for vehicle energy conservation. Once the electric and extended range objectives were determined on the Volt, the required drag coefficient was backed out. On conventional vehicles, aerodynamic drag is usually the third largest energy loss, behind powertrain friction and vehicle mass. On the Volt system, losses from the electric drive unit are less significant, leaving mass and aero drag as the main fuel economy enablers. However, in an electric vehicle where much of the weight is fixed due to battery capacity, efficiency is much more sensitive to percentage changes in CD, rather than mass. This fact put aero development to the forefront of the Volt engineering process. Figure 3 shows aerodynamic drag performance of a number of production vehicles in the North American small car market since the 2001 model year. GM and competitor data were consistently gathered in the General Motors Aerodynamic Laboratory (GMAL). Clearly, minimizing drag area (CDA) is key to all hybrid and electric cars in this segment. A saleable Volt was tested per the new SAE Recommended Best Practice J2881, “Measurement of Aerodynamic Performance for Mass-Produced Cars and Light-Duty Trucks” [1], and achieved a CD of 0.28 in GMAL.

GM's small car platform. This provided the first aerodynamic evaluation as shown in the scale model of Figure 5.

Figure 4. Volt concept vehicle

Figure 3. Aerodynamic Drag of North American Small Car Market, 2001-2011

Figure 5. First reduced scale wind tunnel test. GM's aero methodology relies on several complementary tools. Drag reduction is developed mainly in the wind tunnel starting with reduced scale clay models and then full scale clay models, where design studies can be quickly sculpted and tested to a high degree of accuracy. Computational Fluid Dynamics (CFD) is the primary method to analyze and optimize front end airflow, with its faithful representation of
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Development Process
The development process began shortly after the Volt concept car debuted at the North American International Auto Show in January 2007 (Figure 4). Engineering efforts started in earnest as the concept design was proportioned to fit on to

SAE Int. J. Passeng. Cars - Mech. Syst. | Volume 4 | Issue 1

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underhood and underbody details. CFD evaluation of drag is also used when physical parts cannot be obtained or fabricated in the clay models. Lastly, functional prototypes, called IVER vehicles, allow for final fine tuning and confirmation of the aerodynamic performance. Each tool's strength is utilized to the fullest, and each play an important role in the process. Using all the available tools was necessary to achieve the Volt's aggressive CD requirement. The time line in Figure 6 shows the history of drag coefficient and some of the significant milestones in the development process. Also shown are Vehicle Technical Specifications (VTS) and the Glide Path. From beginning to end, vehicle drag was reduced by 150 counts.

enhanced with rapid prototype components. Testing of the Volt also showed that both underbody panels and an airdam would be needed to achieve aggressive CD targets, with follow up work planned for the full size model.

Figure 7. Reduced scale (1:3) model installed on the reduced scale balance of the GM Aerodynamics Lab (GMAL). Stereo lithography grille, mirror, and wheels enhanced the clay model fidelity.

Full Scale Testing
Once the aerodynamic drag of the 1:3 scale model was significantly reduced, development moved into the full scale testing phase. To check for consistency, the last surface used on the 1:3 model was milled on to the full scale clay and tested in the wind tunnel as the first data point. The CD results between reduced and full scale models were in agreement, paving the way for further tuning of the exterior surface and component details, including A-pillars, mirrors, underbody panels, and the airdam. Figures 8 and 9 illustrate some of the exterior aerodynamic drag features of the Volt. Nose curvature allows for smooth airflow around the front corners, hood, and wheel openings. A set-back airdam is also optimized for the front end to reduce underbody drag and to maintain a low stagnation point. At the back of the vehicle, sharp vertical edges and a refined decklid spoiler achieve clean separation and minimize drag within a short rear overhang design. One of the biggest challenges in developing the Volt was the outside rear view mirror. The styled mirror had to meet global market vision requirements, aero performance, and wind noise performance, all within the design theme of the car. Aerodynamic coefficients and sound pressure level are both measured and developed in GMAL. The swept, doormounted design initially had a low CD increment, but its aero-acoustics was not acceptable. Significant wind tunnel

Figure 6. Timeline of the Volt's Aerodynamic Development

Reduced Scale Testing
The majority of the exterior surface development was done on a 1:3 clay model in the wind tunnel, and over 500 hours of testing were spent improving the basic shape of the vehicle. Particular attention was paid to shaping the front and rear fascias while maintaining styling cues. Reduced scale testing was ideal for early aerodynamic development of the Volt because less clay needed to be sculpted for a given geometry change. It was also this early 1:3 scale testing that showed the pedestal door mounted mirror design had lower drag than the more traditional patch mounted design. GM's approach to vehicle aerodynamics development mandates simulation of the airflow through the engine compartment and underbody to account for any interaction with the exterior surface flow. This methodology is applied even in reduced scale, where grilles, radiators, and chassis components are represented while evaluating the styling surface drag. Figure 7 shows the model installed on the GMAL reduced-scale balance. The clay construction is

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optimization resulted in a quiet mirror that maintains a low drag contribution.

major drag regions on the car and were useful in side by side comparisons of different vehicle shapes. The surface restricted particle traces in Figure 11 shows how the body sides and wheel arches were streamlined during the course of development.

Figure 8. Front view of the full scale clay model installed on the full scale balance of the GM Aerodynamics Lab (GMAL)

Figure 10. Isosurfaces of Ptotal = 0, colored by turbulent kinetic energy

Figure 9. Rear view of the full scale clay model

Computational Fluid Dynamics (CFD)
Throughout the Volt aerodynamic process, non-intrusive visualization and in-depth analysis from CFD explained complex airflow behavior and complemented the hardware development in GMAL. The review of simulation results during wind tunnel tests was a great communication tool to help designers understand why modifications to the clay surfaces led to changes in drag coefficient. Over 1200 unique designs and operating points were computed in CFD to develop the aerodynamic drag and front end airflow. Engineers applied flow simulation to rank early studio themes and to assess the aero performance of styling features. Figure 10 shows isosurfaces of Ptotal =0, colored by turbulent kinetic energy. These regions of low energy airflow highlight the
SAE Int. J. Passeng. Cars - Mech. Syst. | Volume 4 | Issue 1

Figure 11. Surface restricted particle traces highlight body side streamlining In particular, the flexible nature of the virtual computer model was critical to designing the underhood layout and front end airflow (FEAF) performance. The grille openings, condenser/radiator/fan module (CRFM) orientation, and sealing were developed in detail with simulation. Figure 12 shows the heat exchangers for the five unique cooling loops in the Volt. CFD testing optimized the amount and direction of airflow to these heat exchangers as well as the engine air induction in order to meet high temperature driving conditions. The efficient air delivery system allowed

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designers to close much of the grille opening, thereby reducing the parasitic cooling drag through the engine bay.

Integration Vehicles
The first sheet metal prototypes are known as the integration vehicles or IVERs (Figure 14). It is here that all the modeling and simulation is proven out on functioning hardware. These vehicles had the final exterior surface and complete underhood components needed to validate the CD status established with the clay models. Excellent correlation was achieved between the final full scale clay and integration vehicles, indicating that all key aerodynamic features were captured throughout the entire development process. The integration vehicles allowed for final adjustments to the duckbills and confirmed the drag contribution of the styled wheels. Most importantly, interior sound pressure levels were measured in the fully-dressed cabin. Wind noise performance of the mirrors, wipers, and A-pillar were validated.

Figure 12. (Top) The majority of front end airflow enters through an optimized lower grille opening. (Bottom) An exploded view shows the velocity profile on the five heat exchanger cooling package. The high-definition CFD model helped design the underbody splash panels (“duckbills”) and wheel liners. Pressure and velocity results from the computations showed the airflow interaction under the hood, how the panels contribute to aerodynamic drag, and the areas of improvement (Figure 13). Further simulation focused on a venting strategy to reduce the drag while still allowing the splash panels to provide water protection to the Volt's electrical components.

Figure 14. Sheet metal “Integration Vehicle”

Summary of Key Aerodynamic Features
Many aerodynamic enablers were identified during the course of wind tunnel testing and CFD analysis. Some of them affected styling and are clearly visible to the customer, while others were more engineering oriented and not easily seen. ? Closed Front Grille - Due to lower cooling flow requirements and efficient baffling to the electric propulsion system, it was not necessary to maintain a traditional wide open front end. Much of the upper grilles were blocked, resulting in 0.010 CD improvement. Figure 13. High definition virtual models helped guide low-drag, vented splash panels (duckbills). Pressure contours are shown. ? 5mm Kick-up on Rear Spoiler - To help trip the separation on the rear deck-lid, a kick-up was added resulting in about 0.005 CD reduction.

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? Rear Sharp Trailing Edges - As with the spoiler, to help control the flow separation at the rear sides of the vehicle, sharp trailing edges helped reduce CD by about 0.010. ? Mirrors - Door mounted mirrors, as opposed to the more traditional patch mounted mirrors, helped cut another 0.008 off the CD value. ? Airdam - An enabler for both aerodynamics and cooling flow, the airdam shields underbody components from the airstream and improves flow through the several heat exchangers. This part alone reduced the CD by 0.025. ? Underbody Panels - Working with the airdam, the underbody panels also shield components from airflow. In total, the 4 underbody panels reduced the CD by 0.013. ? Lowered Ride Height - The ride heights were lowered to the minimum allowable by good engineering judgment and corporate standards, resulting in 0.010 CD improvement. ? Duckbills - Properly vented ‘duckbills’, a splash panel forward of the front wheel house liner and aft of the airdam, helped vent the engine compartment, reducing CD by 0.004. ? Body Sides and Wheel Openings - Minimizing the body side features within the styling theme also helped reduce drag significantly.

REFERENCES
1. SAE International Surface Vehicle Recommended Practice, “Measurement of Aerodynamic Performance for Mass-Produced Cars and Light-Duty Trucks,” SAE Standard J2881, Issued June 2010.

CONTACT INFORMATION
Lead Aero Development Engineer nina.tortosa@gm.com Lead Aero CFD Engineer kenneth.j.karbon@gm.com

ACKNOWLEDGMENTS
We would like to acknowledge the following individuals for their support during development in achieving very aggressive CD requirements. Greg Fadler, Aero Group Manager Max Schenkel, GM Technical Fellow for Aerodynamics Young Kim, Volt Lead Exterior Designer Robert Boniface, EREV Design Director

Conclusions: Lowest Drag Chevrolet Sedan
As the Volt hits the showroom, it is one of the most aerodynamic vehicles on the road and has the lowest drag of any Chevrolet sedan. The Volt's drag coefficient of 0.28 was measured per the SAE Recommended Practice J2881 at the GM Aerodynamics Laboratory. Designers and engineers worked hand-in-hand to deliver the necessary styling and aero performance to set the Volt apart from conventional cars. Hundreds of wind tunnel hours and thousands of computer cpu-hours were used in the development process. From the front grille openings to the rear trailing edge, all components exposed to airflow were studied. From start to finish, the Volt CD was reduced by 33 %, providing an additional seven miles of battery electric range and 50 miles of extended range.

DEFINITIONS/ABBREVIATIONS
CD Coefficient of Drag CFD Computational Fluid Dynamics GMAL General Motors Aerodynamics Laboratory

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