Reducing Tractive Forces

The tractive forces that a vehicle must overcome to stay in motion include:

Aerodynamic drag, the force of air friction on the body surfaces of the vehicle. Aerodynamic drag averages about 30 percent of total tractive forces, and is highest during fast highway driving (drag is directly proportional to the square of speed,1 so if speed doubles, the drag force quadruples). Drag forces may be reduced by reducing the frontal area of the vehicle, smoothing out body surfaces and adjusting the body’s basic shape, covering the vehicle’s underbody, and taking other measures that help air move freely past the vehicle.

The efficiency of a vehicle’s aerodynamic design is measured by the product of the drag coefficient CD and the frontal area, which designers seek to minimize. The CD of current U.S. automobiles averages about 0.33, with the best mass-produced vehicles achieving about 0.28. Experimental vehicles have achieved extraordinarily low CDS of 0.15 or better, but these low values have substantial costs in reduced passenger and cargo space,2  added complexity and weight in cooling systems, low ground clearance, and so forth. Most automakers view a CD of 0.25 as a feasible target for the next 10 to 20 years for an intermediate-sized sedan; this would yield about a 6 percent improvement in fuel economy from current average vehicles. Judging by some of the less-radical experimental vehicle designs, however, a more ambitious CD of 0.22, yielding about a 7 percent improvement in fuel economy, appears to be possible. Most automakers are, however, skeptical of the feasibility of a CD this low.

 

l Rolling resistance, the resistive forces between the tires and the road. These forces also average about 30

percent of total tractive force, and are of approximately equal importance in city and highway driving. Rolling

resistance may be reduced by: 1 ) redesigning tires and tire materials to minimize the energy lost as the tire

flexes, 2) lowering vehicle weight (see below), and 3) redesigning wheel bearings and seals. A major concern in tire redesign is to avoid compromising tire durability and handling capabilities.

The rolling resistance coefficient (RRC), like the aerodynamic drag coefficient, is a measure of the resistance to a vehicle’s movement—in this case, of the tires. Current mass-market (not performance-oriented) tires have RRCs of 0.008-0.010. By 2005, a 30 percent reduction in RRC, yielding about a 5 percent fuel economy improvement, should be possible with significant investments in research on tire design and materials and chassis technology. By 2015, an RRC of 0.005 may be possible, yielding a total 8 percent improvement in fuel economy over current levels.

Inertial force, the resistance of vehicle mass to acceleration or grade-climbing. This force is about 40 percent of total tractive forces, on average, and is largest in city driving and hill-climbing. Inertial force is reduced by

making the vehicle lighter—a 10 percent weight reduction yields as much as a 6 percent reduction in fuel

consumption, if performance is held constant and the vehicle design carefully handled. 

Although major reductions in vehicle weight have occurred since the 1970s, there remains substantial further

potential, by substituting lightweight materials—primarily improved high-strength steel, aluminum and, possibly,

composites—and by structural redesign using supercomputers. The complexity of vehicle structural design to

assure safety and the lack of industry experience with the new materials demand a careful program of testing and analysis, so that even aluminum will be introduced cautiously; an optimized design in a mass-market vehicle making full use of aluminum’s unique properties—and, therefore, achieving maximum weight savings—must probably wait until after 2005. By 2005, the Office of Technology Assessment projects that a highly optimized steel body with aluminum engine could achieve a 15 percent weight reduction over 1995 norms; an aluminum intensive body (but not an optimized, “clean sheet” design) could achieve a 20 percent weight reduction, at a price increment of about $1,500 for a mid-size car. By 2015, an optimized aluminum design could achieve a 30 percent weight reduction, at a similar $1,500 price. /f the severe manufacturing challenges of mass producing carbon fiber composites are overcome, a 40 percent weight savings could be achieved, though probably at high costs (an estimated $2,000 to $8,000 for an intermediate auto). Such a 40 percent weight reduction might increase fuel economy by one-third.

Advanced Automotive Technology: Visions

of a Super-Efficient Family Car

OTA-ETI-638

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