Lightweighting - Saving Fuel by Saving Weight

Six weeks ago I reported on the activities of a trade organization for the steel industry which has been promoting manufacturing techniques for the automotive industry which promise to reduce the mass of auto bodies by hundreds of pounds. So called "lightweighting" by whatever means happens to be a very important issue for the auto industry in an age of rising energy costs, and I promised to revisit topic. And so I have.

What's Different Now

The Oxford History of Technology, an admirable scholarly undertaking which is unfortunately decades out of print and thus firmly in the rare book category today, has a beautifully rendered line illustration—one of hundreds, incidentally—of an Egyptian racing chariot from the fourteenth century BC. It is an exquisitely fine piece of furniture made entirely of wood and leather with a frame fashioned of many pieces of wood glued together in intricate curved shapes. The vehicle, which weighs only a few pounds, survived a lifetime of use, and was still capable of supporting a man when it was unearthed from a tomb in the early twentieth century.

Some humans, like the master artisan who made that chariot, have always understood how to build strong and light. But strong and light has tended to be labor intensive and to make use of costly materials. And, most of all, it has tended to require incisive thought, that rarest and most precious of human possessions, and thus throughout history up until our own time, most vehicles have been built with far more structural material than what is required for physical integrity, and they have therefore been profligate in their energy requirements.

Even airplanes, which cannot exceed certain weight limitations and still remain aloft, are often unnecessarily heavy simply because they are light enough for their purpose and their customers are presumed to be not overly disaffected with respect to the fuel expenditures they are making.

But obviously, with the fuel price escalations of the present and still to come, mass carries a penalty that it did not before.

What follows is a mere consideration, a stimulus to further thought and to further study.

No one can summarize the art, science, and engineering of strong light construction in an article or even in a series of articles. But what one can do is to identify the major factors in changing manufacturing practices to achieve strength and safety without excessive mass. And those are primarily the cost of materials and the cost of assembling them.

Steel, the Legacy Structural Material

Steel is and always has been the primary structural material for automobiles although a fair number of wood bodies were manufactured in the earliest days of the industry.

Compared to certain other structural metals, namely aluminum, magnesium, and titanium, steel is relatively heavy per unit of volume. But steel is also strong, and one can use a lower volume of material to achieve the same end. An example from the past will illustrate this.

Most really premium bicycle frames made today no longer use steel, but I recall a Swedish manufacture of steel tubing for bicycle framesets who competed with purveyors of aluminum and carbon fiber tubes back in the late eighties when carbon fiber was just establishing itself in this market. His steel tubes were extremely large in diameter while at the same time extremely thin walled, admirably exploiting the structural properties of high temper steel. And the framesets were light, just as light as the state of the art carbon fiber frames of the time, and just as stiff. And just as expensive, unfortunately.

So why didn't steel retain its dominance in this application? Although these unusual steel tubes were strong, light, stiff, and durable, they dented very easily because they were so thin walled. One cycling journalist of the time likened them to beer cans. On something like the Tour of France or the Race Across America where a bike was likely to be used once and then discarded, a small dent was insignificant, but few racing amateurs wanted to campaign on bikes full of unsightly depressions. The market appeal of steel was limited in this rarified market.

Would one have the same problem in automobiles? Absolutely. Make the steel body panels sufficient thin, and you'll save a lot of weight but the appearance of the car will be easily degraded. A minor collision with a shopping cart could cause grave cosmetic impairment. So this isn't answer in the current marketplace.

It should be noted that steel can form a part of a composite structure including other materials, and such structures may in fact be utilized in the cars of the future. I will consider this possibility in a later section.

Other Metals

What about aluminum? Aluminum body panels are more widely used in automobiles today than in the past, but still carry a price premium. Aluminum is significantly lighter than steel per unit of volume, and a panel of equivalent strength will be less prone to surface deformation, i.e. denting. So it's a somewhat better solution, and one we'll see more of. Incidentally, aluminum is already widely used in automobile engines and appears in at least half of all motorcycle frames made today, so it's quite well proven across a range of automotive applications.

But is it the ultimate solution? Magnesium is lighter still—thirty-six percent lighter by volume—and is extremely well damped, that is, it does not transmit vibrations efficiently. Moreover, its stiffness to mass ratio is superior to that of all other metals. It is also highly resistant to permanent deformation. Unfortunately, extant processing techniques for both sheet and cast magnesium are less cost effective than those for steel or aluminum, and the production volumes are not such as to achieve optimal economies of scale. Particularly in Europe, auto manufacturers are seriously considering more extensive use of magnesium, but I see any major transition as years off, if it occurs at all.

Advanced Composites

Much more attention has been given of late to advanced composites and fiber reinforced plastics (these terms are almost interchangeable). These refer to structural materials made up of synthetic fibers such as fiberglass, Kevlar, and carbon fiber impregnated with synthetic resins, and often combined with foamed plastic cores in constructions where the resin impregnated fiber forms a skin over the relatively soft but light core. A fiberglass surfboard with a foam core is good example of this construction technique. Today most small pleasure boats and a growing number of personal aircraft make use of such materials, and they are often advanced as the best solution for reducing automotive curb weights.

In fact, advanced composites are not new in the automotive industry. The Chevrolet Corvette, introduced in 1953, has always had a fiberglass body, and various other manufacturers have attempted to follow suit. Weight savings have proved to be fairly modest however, and the body panels have been difficult to repair in the case of collisions. Behavior of the panels in crash situations is also disconcerting. Whereas sheet metal panels buckle, absorbing considerable energy in the process, resin impregnated fiberglass stores energy when deformed, and ultimately shatters abruptly. Which is one reason why no one is seriously advocating its widespread adoption today.

Resin impregnated carbon fiber, which is used today in certain exotic sportscars and also in aircraft, has similar bad characteristics, but it is much stronger per unit of mass than fiberglass, and its use permits really impressive weight savings over steel. Unfortunately, the cost of the material remains high, and it does not lend itself to the same high degree of automation we see in steel part forming.

The problems with carbon fiber are compounded when it is used in sandwich constructions having foam cores. Such constructions exhibit much greater structural strength per unit of mass than do sheets of resin impregnated carbon fiber, known as "layup" in the trade. The strength of a structural member is a function of its cross section, and thus a light, expanded foam core that is not particularly strong in its own right provides the basis of an ultra-light, ultra-stiff composite construction when it is combined with carbon fiber skins. Such constructions, which are common in the boating industry, are difficult and expensive to fabricate, however, and require extensive hand labor.

Moreover, most such composite sandwich structures only function optimally in flat panel constructions. In curved constructions the fibers are not properly tensioned and many of the advantages of the approach are lost.

There are two further significant disadvantages of this approach, especially in applications where the panels are repeatedly stressed as is the case with boat hulls. Composite constructions lose strength rapidly under repeated stress cycles. They also tend to delaminate, that this, the skins lose adhesion and are separated from the structural cores. This is particularly likely to happen with the lightest core materials, i.e. open cell honeycombs where the bonding surface is very limited.

The majority of transoceanic open class racing sailboats are made of carbon fiber skins laid over Nomex honeycomb cores which makes for a very light strong construction—when the hull is new. Some such boats are seriously degraded after a few months at sea, however, or even after weathering a single severe storm. They're generally the province of billionaire racing enthusiasts, and the fact that they are so expendable is not deemed much of a problem.

Advanced composite construction using light core materials has been very little used in automobile construction, and the only instance of which I am aware is the hood of the new Corvette which utilizes carbon fiber skins glued to a balsa core. A balsa core, interestingly, is the least likely to delaminate or weaken of any of the common core materials. Balsa, however, is relatively expensive, difficult to work with, and highly variable in quality, with the very best grades fetching premium prices. And although it is a fast growing wood, it is difficult to imagine that balsa could be raised in quantities sufficient to meet the demands of the automotive industry were it to switch to advanced composite construction.

How would other combinations of skins and cores perform in the automotive environment? Answering that question would require a lot of life cycle analysis which I'm sure the major auto manufacturers have performed. But I doubt that any of them will go this route. If they use composites, they'll go for layup, because it's more amenable to machine molding techniques, even though the weight savings are less impressive.

So what other options exist? Molded or die cast plastics could of course be used, and have been tried, but strength and stiffness are lacking in non-fiber reinforced plastics. They just don't perform as well as structural materials. Auto makers are looking at using Lexan, a tough, durable DuPont polycarbonate plastic, as a substitute for paint—just apply it as a film to the metal surface and then bake it on, for a highly scratch-resistant coating, but this is strictly a surface application.

A real possibility, though a fairly remote one at present is group of materials known as porous metals, which I'll consider at length in the second part of this series. These are actually metallic foams full of tiny air pockets, and they eliminate as much as 80% of the mass in a metal sheet for dramatic weight savings but fairly minimal losses in structural strength. Such materials will not be adopted quickly, and may not be widely adopted at all, but they're particularly attractive because, in many cases, they can be formed and shaped with ordinary metal bending equipment.