economy increase owing to mass reduction would be only about 33 percent. To achieve 300
percent improvements or more, as envisioned in PNGV, weight reduction must be combined with
improvements in power plant efficiency, reduced rolling resistance, and more aerodynamic design.AERODYNAMIC DRAG REDUCTIONThe 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 CD 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.^33 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
(^32) Actually, with the relative speed of the air and the vehicle. If the vehicle is moving into a headwind, therefore, the relative speed-and thus the
“c drag-will be greater.
justment, vehicle performancewill increase, and the net fueleconomy benefit of the improvement in drag coefficient will be
somewhat less. 34
"Going With The Wind,” Car and Driver, August 1984.