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

BOX l-l: 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.^3

. 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.

(^1) To be precise, to the relative speed of the vehicle and the air. Thus, if a vehicle is moving into a headwind, the drag force is a function of the
speed of the vehicle plus the windspeed. 2
Vehic1e designs that seekto minimize aerodynamic drag must have sharply sloped rear ends that reduce the height and width of the rear passenger
space and the width of the trunk. 3
The U.S. automakers believe OTA’s values for fuel savingsassociated with rolling resistance reduction are too high, and propose a 3 to 4 percent
reduction for a 30 percent RRC reduction and a 5 percent reduction by reducing RRC to 0.005.