261 Miles Per Gallon, Anyone?

By Jonathan Marshall

In intellectual circles, “lightweight” is an epithet. But when it comes to car design, slashing weight is becoming the new gold standard for fuel-efficiency.

Case in point: Volkswagen claims that its experimental XL1 Super Efficient Vehicle achieves a mind-boggling 261 miles per gallon, more than double the fuel economy of today’s best electric vehicles. Think about it: You could commute 50 miles a day for a mere $4 a week in fuel.

A car like VW's XL1 Super Efficient Vehicle is one example of how automakers could achiieve much higher fuel economy numbers.

Although the XL1’s diesel-electric power train is highly efficient, and its aerodynamic shape cuts drag at high speeds, the real secret is the car’s weight—a mere 1,750 pounds. That’s well under half the curb weight of a Chevy Volt (3,781 pounds).

Isaac Newton probably never envisioned such cars, but he did discover the law of motion they must all respect. In a nutshell, the lower the mass, the less force is needed to accelerate it. In the real world, cutting vehicle weight 10 percent can improve fuel economy by 6 to 8 percent.

To lower the mass, Volkswagen is making much of the XL1 car body out of carbon fiber reinforced polymer, similar to what racing cars have used for years. By using a high-pressure mold, the manufacturing process can apparently be automated at a much more reasonable cost than production of one-off hot rods.

Safe as a race car

Although the body shell is a mere 1.2 mm thick, the manufacturer claims it’s just as stiff and strong as a skin made of steel. “The safest cars on the planet are race cars, and they’re made from carbon fiber,” Mark Gillies, a spokesman for Volkswagen, told a reporter for Energy and Environment Daily. “If you went flying into a wall at 230 miles an hour, you’re going to want to be doing it in carbon fiber rather than steel, because you’ll live.”

The XL1 implements the “hypercar” philosophy long expounded by Amory Lovins’ Rocky Mountain Institute. The idea is to benefit from a virtuous cycle: Lower overall weight in turn means less need for heavy batteries and other heavy-duty components.

“Instead of starting with the engine and making the engine more efficient and thinking about the fuel and how you’re going to deliver the energy, think about how to minimize the energy needed for propulsion in the first place,” one Institute consultant told Energy and Environment Daily.

You can’t buy the XL1 concept car in any showroom — yet — but Volkswagen might implement some of the same technology in its Up city car, which is still under development. That diesel-electric hybrid should weigh less than 2,200 lbs., according to early reports, and achieve over 90 mpg (by European ratings, which run a bit higher than US EPA figures).

Other automakers exploring lightweight options

VW isn’t the only manufacturer to experiment with how much weight they can strip away while still making a safe and economical car. Ford is also looking at applications for lightweight carbon fiber, including a prototype hood for the Focus wagon. General Motors says it is testing car bodies made of magnesium, which weighs a third less than aluminum and 75 percent less than steel. The car maker is hedging its bets by investing in exotic, lightweight steel technology as well.

“Every gram of weight reduction matters when it comes to improving fuel economy,” said GM’s executive director for global vehicle body engineering. “Being able to replace heavier metals with one of the lightest will help us deliver better fuel economy to customers around the world while also still providing the safety and durability they expect.”

Last August, Energy Secretary Steven Chu announced seven new project to accelerate the use of stronger and lighter materials in the next generation of American-made cars and trucks, citing the opportunity to help the United States “maintain its competitive edge in automotive design and manufacturing.”

The success of those projects will also help the country achieve the Obama administration’s historic fuel economy mandate of 54.5 miles per gallon in the 2025 model year. In particular, car makers have to figure out how to cut the weight without fattening the price, so consumers won’t face sticker shock when they consider more efficient vehicles. As the Alliance of Automobile Manufacturers pointed out when the new rules took effect last year, “Compliance with higher fuel-economy standards is based on sales, not what we put on the showroom floor.”

Email Jonathan Marshall at jonathan.marshall@pge.com.





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