Automobile drag coefficients
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The drag coefficient is a common metric in automotive design, where designers strive to achieve a low coefficient. Minimizing drag is done to improve fuel efficiency at highway speeds, where aerodynamic effects represent a substantial fraction of the energy needed to keep the car moving. Indeed, aerodynamic drag increases as the square of speed. Aerodynamics are also of increasing concern to truck designers, where a lower drag coefficient translates directly into lower fuel costs.
About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient. Additionally, because drag does increase with the square of speed, a somewhat lower speed can significantly improve fuel economy. This was the major reason for the United States adopting a nationwide 55 mile per hour speed limit (90 km/h) during the early 1973 oil crisis as slower traffic would save scarce petroleum.
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[edit] CdA
While designers pay attention to the overall shape of the automobile, they also bear in mind that reducing the frontal area of the shape helps reduce the drag. The combination of drag coefficient and area is CdA (or CxA), a multiplication of the Cd value by the area.
In aerodynamics, the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient is called drag area. In 2003, Car and Driver adapted this metric and adopted it as a more intuitive way to compare the aerodynamic efficiency of various automobiles. Average full-size passenger cars have a drag area of roughly 8.5 ft² (.79 m²). Reported drag area ranges from the 1999 Honda Insight at 5.1 ft² (.47 m²) to the 2003 Hummer H2 at 26.3 ft² (2.44 m²). The drag area of a bicycle is also in the range of 6.5-7.5 ft².[1]
Automobile examples of CdA ft² are shown below: [2]
[edit] Drag in sports and racing cars
Reducing drag is also a factor in sports car design, where fuel efficiency is less of a factor, but where low drag helps a car achieve a high top speed. However, there are other important aspects of aerodynamics that affect cars designed for high speed, including racing cars. Notably, it is important to minimize lift, hence increasing downforce, to avoid the car ever becoming airborne and instead force the car onto the track -- allowing higher cornering speed for the vehicle. Also it is important to maximize aerodynamic stability: some racing cars have tested well at particular "attack angles", yet performed catastrophically, i.e. flipping over, when hitting a bump or experiencing turbulence from other vehicles (most notably the Mercedes-Benz CLR). For best cornering and racing performance, as required in Formula 1 cars, downforce and stability are crucial and these cars must attempt to maximize downforce and maintain stability while attempting to minimize the overall Cd value.
[edit] Typical values and examples
The average modern automobile achieves a drag coefficient of between 0.30 and 0.35. SUVs, with their typically boxy shapes and larger frontal area, typically achieve a Cd of 0.35–0.45. A very gently inclined windshield gives a lower drag coefficient but has safety disadvantages, including reduced driver visibility. Certain cars can achieve figures of 0.25–0.30, although sometimes designers deliberately increase drag in order to reduce lift.
Some examples of Cd follow. Figures given are generally for the basic model. Some "high performance" models may actually have higher drag, due to wider tires and extra spoilers.
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[edit] Selected Photographs
 2.1 - a smooth brick |
 0.7 to 1.1 - typical values for a Formula 1 car |
 0.9 -a typical bicycle plus cyclist |
 0.7 - Caterham Seven |
 at least 0.6 - a typical truck |
 0.57 - Hummer H2, 2003 |
 0.51 - Citroën 2CV due to the mudguard, despite the round roof |
 0.48 - Volkswagen Beetle due to the mudguard, despite the round roof |
 0.425 - Duple 425 coach |
 0.42 - Lamborghini Countach, 1974 |
 0.42 - Triumph Spitfire Mk IV, 1971-1980 |
 0.38 - Mazda Miata, 1989 |
 0.38 - Rolls-Royce Silver Seraph, 1998 [1] |
 0.374 - Ford Capri Mk III, 1978-1986 |
 0.372 - Ferrari F50, 1996 high drag due to aerodynamic aids and cooling ducts. |
 0.36 - Citroën DS, 1955, relative high drag despite the aerodynamic headlights, due to the rough windshield-roof transition. |
 0.36 - Ferrari Testarossa, 1986 |
 0.36 - Honda Civic, 2001 |
 0.36 - Citroën CX, 1974 (the car was named after the term for drag coefficient) |
 0.355 - NSU Ro 80, 1967 despite the edgy front |
 0.35 - Dodge Viper |
 0.34 - Ford Sierra, 1982, at least one combi in this gallery! The passat 2003, has 0.32 |
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 0.34 - Ferrari F40, 1987 |
 0.338 - Chevrolet Camaro, 1995 |
 0.33 - Citroën SM, 1970 low drag despite edgy front |
 0.32 - Buick Riviera, 1995 |
 0.31 - Citroën AX, 1986 low drag due to down pulled hood |
 0.31 - Citroën GS, 1970 |
 0.31 - Renault 25, 1984 |
 0.31 - Saab Sonett III, 1970 |
 0.30 - Audi 100, 1983 smooth nose to flow transition |
 0.30 - BMW E90, 2006 |
 0.30 - Porsche 996, 1997 |
 0.30 - Saab 92, 1947 - developed using wind tunnel testing |
 0.29 - Honda CRX HF 1988 |
 0.29 - Subaru XT, 1985 |
 0.29 - Lancia Dedra, 1990-1998 |
 0.29 - Lotus Elite, 1958 |
 0.28 - Rumpler Tropfenwagen, 1921 thin tires have low aerodynamic drag |
 0.28 - Toyota Camry and sister model Lexus ES, 2005 |
 0.28 - Porsche 997, 2004, and that with the wide tires |
 0.28 - Renault 25 TS, 1984, low drag despite its overall edgy shape |
 0.28 - Saab 9-3, 2003 |
 0.28 - Chevrolet Corvette Z06, 2006 |
 0.27 - Infiniti G35, 2002 (0.26 with "aero package") |
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 0.27 - Toyota Camry Hybrid, 2007 |
 0.26 - Mercedes-Benz W221 S-Class, 2006, low drag despite wheelarches an a large grill and a Mercedes sign in the airflow |
 0.26 - Lexus LS 430, 2001 (0.25 with air suspension) |
 0.26 - Toyota Prius, 2004 |
 0.26 - Vauxhall Calibra, 1989 even the transition from the bumpers to the head light is smooth |
 0.25 - Honda Insight, 1999, low drag due to down pulled hood and fastback |
 0.212 - Tatra T77 a, 1935 low drag despite an edgy windscreen-roof transition. |
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 0.195 - General Motors EV1 (electric), 1996 |
 0.11 - Aptera Motors Typ-1 (2008 planned) |
[edit] References
- ^ http://www.lafn.org/~dave/trans/energy/bicycle-energy.html#aero_drag (a bicycle's lower frontal area is offset by a higher drag coefficient)
- ^ The Mayfield Company Homepage - Coefficient of Drag Tables and Curves
- ^ Program: hpvplot (updated 11/12/07)
- ^ http://www.evworld.com/article.cfm?archive=1&storyid=911&first=2412&end=2411
- ^ http://autospeed.com/cms/A_108675/article.html
- ^ 50 000 km avec une Citroën Ax diesel, mon avis d'utilisateur ainsi que son coût kilométrique réel
- ^ Mercedes Bionic Concept Car
- ^ Daihatsu UFE-III | Concept Cars
- ^ D & H Enterprises, composites and fibreglass technology
