Transportation - Energy and the Environment - page 206

vehicle mass, while the latter depends upon vehicle size and shape, but not mass. The three major
parameters determining these components are vehicle mass m , rolling resistance coefficient C R ,
and the vehicle drag area C D A . All of these are important.
Vehicle Mass. In current conventional vehicles, mass is the parameter that best correlates fuel econ-
omy (see Table 8.2). Large, heavy vehicles require big engines to perform well; they consequently
consume more fuel. For a given vehicle size, reducing vehicle mass will permit reductions in en-
gine and transmission mass, tire and wheel mass, braking system mass, fuel storage mass, steering
system mass, engine radiator mass, and so on, compounding the gains in direct mass reduction of
the vehicle frame. The principal means for reducing mass is the substitution of lighter materials
of equal strength and stiffness, such as aluminum alloys or fiber-reinforced plastic for steel and
plastic for window glass, as well as the redesign of the vehicle structure to minimize structural
mass. Reductions of up to 40% of vehicle mass seem possible. 16
Reducing vehicle mass by material substitution may have implications for vehicle safety.
Vehicle frames are designed to absorb the vehicle's kinetic energy in a crash while protecting
the occupants from harm. Substitution of a lighter material of equal strength and energy absorbing
capacity in the body structure can maintain the same level of kinetic energy absorbtion and passenger
protection, while reducing overall vehicle mass. But in a two-vehicle collision of vehicles of
unequal mass, the lighter vehicle absorbs more than its own kinetic energy and thereby suffers
a safety disadvantage. In the current U.S. fleet of light-duty passenger vehicles and trucks, the
mass ratio of the heaviest to lightest vehicle is about 3. As long as vehicles of different size
persist in future fleets, this mass disparity will continue even if all vehicles are made lighter than
current ones.
Vehicles with propulsion systems that utilize heavy energy storage systems (electric storage
batteries, pressurized gaseous fuel such as natural gas or hydrogen stored in steel cylinders) incur
vehicle mass penalties. For lead-acid storage batteries, the battery mass per unit of energy storage
is 5000 times that of gasoline, so that vehicle mass for electric battery vehicles is dominated by
the battery mass needed to give the vehicle an adequate range. For hydrogen-fueled or natural-gas-
fueled vehicles, the storage of the energy equivalent of 60 L of gasoline would add about 300 kg
and 130 kg, respectively, to vehicle mass (see Section 4.4.5).
Aerodynamic Resistance. Reduction of aerodynamic resistance is limited to lowering the drag
coefficient C D by careful streamlining of the entire body, because the frontal area A is essentially
fixed by the requirements of providing an enclosure for the passengers (see Table 8.2). The bulky
form of the automobile limits what can be done to reduce aerodynamic drag, but attention to details
can bring the drag coefficient into the range of 0.20-0.25 for passenger vehicles. There does not
appear to be a weight penalty attached to low drag coefficients.
16 The cost of direct weight reduction by material substitution is in the range of $1 to $3 per kilogram (Mark,
Jason, 1999. Greener SUVs. A Blueprint for Cleaner, More Efficient Light Trucks. Cambridge: Union of
Concerned Scientists.). But each kilogram of direct weight reduction by material substitution provides an
opportunity for additional weight reduction if the engine and drive train are reduced in size in proportion to
the vehicle mass reduction, thereby providing a cost saving. This would lower the net cost of the material
substitution weight reduction.