vans, and box-like vehicles such as the Scion and Flex) can reduce Cd, although vehicle functionality is diminished. If the functionality is compromised, then the vehicle’s appeal to the consumer would be reduced.

As noted above, the aerodynamic drag is the product of the drag coefficient Cd, the vehicle frontal area, and speed. Reduction in the frontal area, reducing vehicle size, and lower speed limits would also improve fuel consumption; however, exploring these options is outside the committee’s statement of task.

Car Body Design and Interiors

Optimized car body design focuses on a balance between structural stiffness, noise/vibration/harshness (NVH), safety (crashworthiness), comfort (space), and mass. Today’s priority of reducing fuel consumption places an emphasis on mass reduction, with the assumption that other performance criteria will not be unduly compromised. Vehicle mass can be reduced without compromising size, crashworthiness, and NVH, although countermeasures are often required to restore NVH performance when mass is reduced.

The majority of vehicle mass can be attributed to the body structure, closure panels (doors, hood, and deck lid), interior seating and trim components, glass, power train components (engine, transmission, etc.), and the chassis (axles, wheels, brakes, suspension, etc.). Steel, cast iron, fiber/reinforced composites, glass, and aluminum have been the dominant materials for these components, with steel accounting for the majority of mass. Estimates for the amount of these materials in today’s average, high-volume vehicles are listed in Table 7.1 (Carpenter, 2008). The typical baseline vehicle used for comparison is described as a 3,600-lb model-year 2009 comparable to a Toyota Camry or Chevrolet Malibu.

High-volume vehicle manufacturing is generally associated with the production of more than about 100,000 vehicles per year (although some might say 50,000). Low volume might be under 25,000 vehicles per year. This is important because different materials become cost competitive at different volumes. Higher-cost materials (composites, aluminum, and magnesium) become more cost competitive at lower volumes because the forming tools in most cases have a lower investment cost offsetting the higher material cost. Steel requires high-cost forming tools but has a lower materials cost, making steel competitive at higher volumes. For example, for some non-structural applications, steel becomes cost competitive vis-à-vis plastic at around 50,000 units.

Two key strategies for achieving mass reduction are changing the design to require less material, or substituting lighter-weight materials for heavier materials. Assuming that the car size is essentially fixed, there are design techniques that can reduce mass. Several different body architectures are described below. Material substitution relies on replacing a heavier material with a lighter one while maintaining performance (safety and stiffness). For example, high-strength steel can be substituted for mild steel (and therefore a thinner gauge can be used), aluminum can be substituted for steel, plastic can be substituted for aluminum, and magnesium can be substituted for aluminum. It is often a misnomer to refer to this as material substitution. The part (or subsystem) often has to be redesigned, and the fabrication process may change and the assembly process may be different. In fact, the material cost differential may be insignificant relative to the costs associated with the changes in fabrication and assembly.

Body Design and Material Selection

The great majority of vehicles produced today are unibody design. The unibody design is a construction technique that uses the internal parts as the principal load-bearing structure. While the closure panels (doors, hood, and deck lid) provide important structural integrity to the body of the vehicle, the outer skin panels, defined as the metal outer panels on the entire automobile that are painted and visible to the consumer, do not. This design has replaced the traditional body-on-frame design primarily because it is a lighter. Body-on-frame designs, where an independent body structure (with its own structural integrity) sits on top of a separate frame (with its own structural integrity), still prevail on some heavier vehicles such as pickup trucks and larger SUVs because of its overall superior strength and stiffness. Another design, the space frame, was recently developed to accommodate aluminum. The forming and joining of aluminum cannot easily or cheaply be replicated in a steel unibody design. A typical space frame is composed of extruded metal connected at the ends, which are referred to as nodes. Both the unibody and the space frame have “hang-on” panels where the skin panels have little to no structural load. A final design architecture, the monocoque, relies on the outer skin surface as a principal load-bearing surface. The

TABLE 7.1 Distribution of Materials in Typical Vehicle (e.g., Toyota Camry and Chevrolet Malibu)

Material

Comments

Approximate Content in Cars Today, by Weight (percent)

Iron and mild steel

Under 480 Mpa

55

High-strength steel

≥ 480 Mpa (in body structure)

15

Aluminum

No aluminum closure panels; aluminum engine block and head and wheels

10

Plastic

Miscellaneous parts, mostly interior trim, light lenses, facia, instrument panel

10

Other (magnesium, titanium, rubber, etc.)

Miscellaneous parts

10



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