Advanced Automotive Technology: Visions of a Super-Efficient Family Car

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.^35

The tradeoffs 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 aluminum 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-efficients to high values. Manufacturers argue that

tapering the body and lowering their ground clearance would make them more like passenger

35 The effect of weight on fuel economy is obvious, but increased air intake complexity can lead to lower engine efficiency, while increased

cooling loads increase accessorypower requirements.