Aerodynamic Drag Reduction

The aerodynamic drag force is the resistive force of the air as the vehicle tries to push its way through it. The power required to overcome the aerodynamic drag force increases with the cube of vehicle speed,32 and the energy/mile required varies with the square of speed. Thus, aerodynamic drag principally affects highway fuel economy. Aside from speed, aerodynamic drag depends primarily on the vehicle’s frontal area, its shape, and the smoothness of its body surfaces. The effect of the vehicle’s shape and smoothness on drag is characterized by the vehicle drag coefficient CD, which is the nondimensional ratio of the drag force to the dynamic pressure of the wind on an equivalent area. Typically, a 10 percent C D reduction will result in a 2 to 2.5 percent improvement in fuel economy, if the top gear ratio is adjusted for constant highway performance. The same ratio holds for a reduction in frontal area, although the potential for such reductions is limited by interior space requirements.

The CD of most cars sold in the United States in 1994 and 1995 is between 0.30 and 0.35, and the best models are at 0.29. In contrast, CD for most cars in 1979 to 1980 was between 0.45 and 0.50. The pace of drag reduction has slowed considerably during the mid-1990s, and automakers claim that the slowdown reflects the difficulty of reducing CD values much below 0.30 for a typical mid-size sedan. Meanwhile, however, highly aerodynamic prototypes have been displayed at motor shows around the world. Interesting historical examples include the Chevrolet Citation IV with a CD of 0.18, and the Ford Probe IV with a CD of 0.15, which is the lowest obtained by a functional automobile.34 (See figure 3-l).

In interviews, manufacturers pointed out that these prototypes are design exercises that have features that may make them unsuitable for mass production or unacceptable to consumers. Such features include very low, sloping hoods that restrict engine space and suspension strut heights. Windshields typically slope at 65 degrees or more from vertical, resulting in a large glass area that increases weight and cooling loads and causes potential vision distortion. Ground clearance typically is lower than would be required for vehicles to traverse sudden changes in slope (e.g., driveway entrances) without bottoming. The rear of these cars is always tapered, restricting rear seat space and cargo volume. Wheel skirts and underbody covers add weight and restrict access to parts needed for wheel change or maintenance, and make engine and catalyst heat rejection more difficult. Frontal wheel skirts may also restrict the vehicle’s turning circle. In addition, radiator airflow and engine cooling airflow systems in highly aerodynamic vehicles must be sophisticated and probably complex. For example, the Ford Probe IV uses rear mounted radiators and air intake ducts in the rear quarter panels to keep the airflow “attached” to the body for minimum drag. Liquids are piped to and from the front of the car via special finned aluminum tubes that run the length of the car. An attitude control system raises and lowers the chassis to minimize ground clearance at high speeds when aerodynamic forces are high and avoid clearance problems at lower speeds. While such designs may have minimum drag, the weight and complexity penalty will overcome some of the fuel economy benefits associated with low drag.

The trade-offs made in these vehicles may not be permanent, of course. Engineering solutions to many of the perceived problems will be devised: advanced design of the suspension to overcome the reduced space; thermal barriers in the glass and lighter weight formulations to overcome the added cooling loads and weight gain associated with steeply raked windshields; and so forth. Presumably, the more conservative estimates of drag reduction potential do not account for such solutions. Of course, there is no guarantee that they will occur.

Drag Reduction Potential

Manufacturers were conservative in their forecast of future potential drag coefficient. The consensus was remarkably uniform that for average family sedans, a CD of 0.25 was the best that would be possible without major sacrifices in ride, interior space, and cargo space. Some manufacturers, however, suggested that niche market models (sport cars, luxury coupes) could have CD values of 0.22. Other manufacturers stated that even 0.25 was optimistic, as maximizing interior volume for a given vehicle length, to minimize weight, would require drag compromises.

In contrast to these moderate expectations of drag reduction potential, some prototype cars not as extreme as the Probe, with shapes that do not appear to have radical compromises, have demonstrated drag coefficients of 0.19 to 0.20. For example, the Toyota AXV5, with a CD of 0.20, appears to offer reasonable backseat space< and cargo room. The car does, however, have wheel skirts and an underbody cover; it is also a relatively long car as shown in figure 3-2. Removing the wheel skirts typically increases CD by 0.015 to 0.02, and the AXV5 could have a CD of 0.22 and be relatively accessible for maintenance by the customer. This suggests that attaining a CD of 0.22 could be a goal for 2015 for most cars except subcompacts (owing to their short body), and sports cars might aim for CD levels of 0.19. For these cars, underbody and wheel covers could add about 40 to 45 lbs to vehicle weight, assuming they were manufactured from lightweight plastic or aluminium materials. This increased weight will decrease fuel economy by about 1 percent, although the reduced drag will offset this increase.

Light trucks have much different potential for CD reduction. Pickup trucks, with their open rectangular bed and higher ride height, have relatively poor CDS; the best of today’s pickups are at 0.44. Four-wheel-drive pickups are even worse, with large tires, exposed axles and driveshafts, and higher ground clearance. Compact vans and utilities can be more aerodynamic, but their short nose and box-type design restrict drag co-efficient to high values. Manufacturers argue that tapering the body and lowering their ground clearance would make them more like passenger cars, hence unacceptable to consumers as trucks. GM’s highly aerodynamic Lumina Van has not been popular with customers, partly because the sharp nose made it difficult to park; the Lumina Van was recently redesigned and its CD was increased from the previous value of 0.32.

Manufacturer’s projections of potential improvements in future truck CD are given in table 3-4.

Effect of Advanced Aerodynamics on Vehicle Prices

The costs of aerodynamic improvements are associated primarily with the expense of developing a low drag body shape that is attractive and then developing the trim and aerodynamic detailing to lower CD. The essential inseparability of drag reduction and styling costs makes it difficult to allocate the fixed costs to aerodynamics alone. Manufacturers confirmed that current body assembly procedures and existing tolerances were adequate to manufacture vehicles with CD levels of 0.25 or less.

Previously, aerodynamic styling to CD levels of 0.30 required investments in the range of $15 million in development costs. 36 Requiring levels of CD to be less than 0.25 would likely double development costs owing to the need to stabilize underbody airflow and control engine and internal air flow. Unit variable costs to an automobile manufacturer (from supplier data) are:

· Flush glass windows: $8 to $10 (for four),

· Underbody cover (plastic): $25 to $30,

· Wheel skirts: $5 to $6 each.

 Hence the retail price effect (RPE) is calculated as follows:

· Unit investment cost: ~$30,

· Variable costs: ~$48 to $64,

· RPE: ~$125 to $150.

These RPE’s would be associated with CD levels of 0.20 to 0.22, while RPE for achieving a CD levels of 0.24 to 0.25 would not require wheel skirts, reducing theRPEto$90to$115.

Price effects for trucks are expected to be similar to autos, for a similar percentage reduction in drag. Of course, the absolute values of CD will be higher.