Is Aluminum Affordable?
An aluminum vehicle based on either design would bring us closer to the goal of building a lightweight car at a relatively moderate increase in cost. A typical steel unibody weighs just under 600 pounds, while an all-aluminum unibody weighs about 325 pounds and various aluminum space-frame designs would weigh between 285 and 385 pounds. Thus either design could cut the weight of the body nearly in half; a lighter engine, suspension, transmission, and so forth could double the number of pounds saved. (Of course, weight may be added in other areas to compensate for the deficiencies of the new design-for instance, a lightweight car cannot rely on its structural components to protect passengers in the event of a crash and so will need to employ additional systems, like airbags, which add some weight.)Just how much fuel savings are generated by lightweighting the body alone? Reducing the weight of the vehicle by 300 pounds can increase fuel economy by as much as 15 percent. This would increase the gas mileage of a typical mid-sized car, such as the Ford Taurus, from about 22 to about 25 miles per gallon, and reduce carbon dioxide (CO2) emissions from about 410 grams of CO2 per mile driven to about 355 grams per mile. Secondary weight savings would double the improvement in fuel economy and the reduction in emissions. More dramatic improvements in fuel economy would result in proportional decreases in CO2 emissions, but these would require much more drastic measures than mere lightweighting: more efficient engine technologies, for instance, and probably less room and fewer conveniences than the American consumer typically expects.
A lightweight aluminum car based on either of these designs is likely to be somewhat more expensive than today’s steel car when produced in large volumes, according to cost analyses by members of the Materials Systems Laboratory at MIT. At very low production volumes (less than 20,000 vehicles per year), aluminum space frames are actually cheaper than a steel unibody: the least expensive space-frame design would cost about $4,500, compared with $5,800 for a steel unibody and $7,200 for an aluminum unibody.
However, these production volumes are much too low for mass-market vehicles. Popular models such as the Ford Taurus are produced in volumes of 300,000 to 500,000. Even niche vehicles-luxury cars like the Lincoln Continental-have production runs between 40,000 and 80,000. To be considered affordable, a lightweight vehicle must be able to be manufactured inexpensively in large quantities.
At production volumes of about 100,000, the steel unibody is the cheapest design, at an estimated unit cost of $2,500. Aluminum space frames are a bit more expensive-the cheapest design costs about $2,800-while the aluminum unibody costs about $3,600. For more typical production runs of 300,000, the cost of the steel unibody declines to an estimated $1,400, and the aluminum unibody becomes cheaper than the aluminum space frame ($2,000 compared with $2,400).
The changing cost profiles for the three designs result from differences in their manufacturing processes. Metal stamping-the process by which both steel and aluminum unibodies are made-is better able to capture economies of scale than extrusion. As a result, the unit costs of both kinds of unibodies decline as they are produced in greater quantity; the cost differential between them is largely explained by the difference in the cost of the raw material.
The space frame follows a different pattern. Because the capital costs of extrusion are far lower than those of steel stamping, space frames are less expensive than unibodies at low production volumes. But extruded parts require finishing and heat treating, which are time consuming. Furthermore, the rate at which extruded parts can be formed is far slower than the rate at which stamped parts can be made. As a result, unit costs do not decline as dramatically when production volumes increase. Higher production volumes ultimately shift the economics in favor of the unibody.
Given that a vehicle with an aluminum body is going to cost $300 to $1,100 more than a vehicle with a steel body, will increases in fuel economy make up for the increased cost over the lifetime of the vehicle? The answer depends on a variety of factors: the total weight (and cost) of the vehicle, the efficiency of its engine, and the price of fuel. However, the increase in fuel economy attributable to the aluminum body alone would pay for itself only if the price of gasoline were to rise. If the price of gasoline remains between $1.20 and $1.50 per gallon, the money saved on gas would not be enough to make up for the higher cost: the life cycle cost of an aluminum unibody produced in volumes of 300,000 would remain about $300 more than that of a steel unibody. But if the price of gasoline rose to $2.30 per gallon, the owner of the aluminum-based car would break even over the vehicle’s lifetime. It is reasonable to think that under these circumstances, consumers might be willing to pay the higher up-front cost of an aluminum-based car.