Why can't the world's car companies make a vehicle that gets 100 mpg? Automotive technology keeps improving and people keep asking the question, but—as with fusion reactors and comprehensible phone bills—the reality always seems to be just a few years away. Sure, student engineers have achieved 2000 mpg in design contests, but those vehicles have been exercises in automotive minimalism, not practical everyday cars.
Steve Lapp, a professor from Ontario, says the moment has nearly arrived. "I've actually gotten over 100 mpg on some trips in my 2001 Toyota Prius," he says. The secret? He mounted solar panels on the car's roof to keep the batteries charged when the sun is shining. If Lapp, a backyard big thinker, can get triple-digit mileage occasionally, why can't the world's carmakers hit the mark on every drive?
Recently, they've come close—in Europe. The Volkswagen Lupo 3L turbodiesel and the Audi A2, which use the same engine, have both edged close to 80 mpg. That's better than a hybrid. On the downside, that amazing three-cylinder diesel doesn't meet U.S. emissions standards and the vehicles are Ringling Bros. size by American standards.
So the question remains: Could manufacturers deliver a practical car that a typical American family could use as daily transportation, getting 100 mpg or better on every single trip? We asked some of the most inventive engineering minds in the country. We looked at designs, materials and drivetrains. The answer? Yes, it can be done.
Taking the Weight Out
Fuel economy is all about efficiency. The lighter the load, the smaller and more efficient a car's powertrain can be. That saves fuel, and using a lighter engine saves yet more weight. Marc Ross, a University of Michigan physicist, suggests that—other things being equal—reducing the mass of a typical light-duty vehicle by 10 percent would increase the fuel economy by 7 percent. That's good, but dramatic mileage gains would demand hefty weight savings. Where to find them? While most cars today are made essentially of steel, some rely on lighter structural aluminum. That metal, however, presents manufacturing and repair challenges. The best choice for building a 100-mpg car would likely be a carbon-fiber composite, which can weigh less than half as much as steel. Carbon fiber is both expensive and hard to work with, but it offers an outstanding combination of light weight and strength. Carbon fiber is used extensively in motorsports and on exotic cars.
"If you look at exotics, they're all engineered for high performance. But you can apply the same type of technologies to meet fuel-economy goals," says Tadge Juechter, assistant chief engineer for Chevrolet Corvette.
Low-volume production of such large automobile structures is expensive—up to $100,000 per copy—according to Jon Fox-Rubin, president and CEO of Fiberforge, a leading manufacturer of advanced composite structures. Part of the price comes from manufacturing difficulties and a high rate of rejects, says Paul Williamsen, Toyota's product education manager: "You still have to pay for what gets thrown away." However, as the technology advances and volumes rise above 50,000 per year, the unit cost for carbon-fiber body structures could eventually drop to around $4000, low enough to make mass-market cars feasible.
GM's Ultralite showcar, built back in 1992 by Scaled Composites, the aerospace firm that later built SpaceShipOne, was a testament to the potential of advanced lightweight materials. Ultralite had a carbon-fiber-skin/PVC-core sandwich panel structure for the chassis and body panels. The structural weight, with doors, front and rear bumpers, and interior components, was only 420 pounds.
Weight savings can be found throughout a vehicle. Glass, for instance, is one of the heaviest components of an automotive body—heavier per square foot than the most commonly used steel. "Polycarbonate glazing will be coming into production within the next 10 years," Juechter says. The material is already used to cover headlights. For windows, a plasma process will be used to deposit a thin layer of glass on the polycarbonate, he says. "To an ice scraper or carwash it will look like glass, but you have a 50 percent weight reduction."
Engineers also can save weight by paring down the hardware needed to support the electronics so prevalent in modern vehicles. "There is so much cabling and wiring in vehicles today that it's not trivial, weightwise," says Deborah Hopkins, staff scientist at Lawrence Berkeley National Labs. Given the current state of electronics, it's possible to imagine a car without bulky hardware for audio, video and navigation. Instead, you'd have a thin screen with a port. "The cellphones of the near future would already carry these functions in them. There'd be a wireless connection to the display screen," says Richard Plavetich, technical design manager of Nissan Design America.
In contrast, "airbags and belts don't add as much weight as you might think," says Greg Thomas, a senior engineer at Honda R&D Americas. All the components of a modern restraint system weigh less than 25 pounds.
To get around the high costs of making components of magnesium, carbon fiber, light steel and other alloys, David E. Cole, chairman of the Center for Automotive Research in Ann Arbor, Mich., suggests a model in which, "instead of buying these raw materials, the owner leases them all and gets credit for returning the car for recycling."
Rolling Easily Along
Any weight-reduction plan needs to account for wheels and tires. As these elements spin, their weight, in effect, increases. As a rule of thumb, the rotating weight of a spinning tire is 1.5 times its actual mass—so this is an area where trimming pounds really pays off. Dymag, a British wheel manufacturer, has produced a very light wheel with a magnesium center and carbon-fiber outer rim that's now being used by American exotic-car maker Mosler Automotive. "They cost about 40 percent more than our standard wheels," says Mosler engineering director Todd Wagner. "But we save about 11 pounds on each wheel."
For optimum fuel efficiency, tires must be designed for minimal rolling resistance: skinny, with a shallow tread and the ability to hold their shape at speed. "The lowest rolling resistance would be something like a locomotive wheel, which doesn't deform," Thomas says. The price paid is a harsher ride and somewhat compromised handling—necessary tradeoffs in the search for 100-mpg performance. A study by the Transportation Research Board, commissioned by the National Highway Traffic Safety Administration, found that cutting resistance by 10 percent yielded a 1 to 2 percent improvement in gas mileage.
On the market here since 1999, the small, two-seat Honda Insight was a harbinger of the hybrid mania and advanced technology to follow. Devotees continue to modify their Insights; some owners claim well over 100 mpg.
Tire manufacturers already use silica in the tread compound to help lower rolling resistance. David Van Emburg, Michelin North America's product marketing director, says we could soon see exotic tires with 20 percent lower rolling resistance than today's models. And, imagine a tire that could sense its environment and adjust air pressure to minimize rolling resistance—essentially a tire with active pressure management. "We have our group in Milan looking into technology embedded in tires. It's certainly thinkable," says Steve Carpino, director of R&D for Pirelli North America.
Slicing Through the Air
Aerodynamics are critical for good fuel economy—especially on the highway. "The aerodynamics become a factor exponentially the faster you go," says Nissan's Plavetich. A low-slung, top-speed machine isn't exactly the best design for shuttling the kids to school. But the Mercedes-Benz Bionic Car concept could do the job. Its fishlike profile is tall and skinny, but it is one of slipperiest designs ever conceived for a passenger car. The shape helps the 2888-pound concept deliver a claimed 84 mpg on the highway.
This concept car from Mercedes-Benz has a tall, narrow shape with a coefficient of drag of only 0.19. That's about 50 percent slipperier than most modern family sedans.
"You can get to a fairly small frontal area by going narrow and tall—a cross section that feels a little more upright and vertical but slender," says Stewart Reed, who chairs the Transportation Design Department at the Art Center College of Design in Pasadena, Calif. "We're noticing in the wind tunnel that what you do on the bottom of the car can be more profound than the roof shape," he says. In addition, the rear of the car needs to be either long and attenuated or abruptly cut off, Reed says. A car's wake can have a detrimental effect on the mileage by creating a partial vacuum behind the car, tugging it backward.
An interior view of the fuel-sipping concept car from Mercedes-Benz.
A teardrop shape with all four wheels enclosed would be ideal. But front wheels need to turn to steer the car. Skirts that move with the wheels would work. So would active aerodynamics. Juechter likes the idea of a car that changes shape. "As speed increases, the car would morph into a streamliner." Neither technology yet exists.
Powering the People
Small, fuel-sipping diesels are common in Europe. The three-cylinder 799cc unit in Mercedes-Benz's funky Smart FourTwo CDI is the world's smallest turbodiesel; the car gets around 60 mpg. Emissions regulations make passenger-car diesels rare in the United States. But Mercedes-Benz will soon introduce an E320 Bluetec diesel that meets new, even more stringent 2007 federal standards.
"Diesels are going to be more and more prominent in the years ahead," Plavetich says. "If you combine a clean-burning diesel with a hybrid electric drive system in a lightweight car, I think 100 mpg is doable."
The diesel portion of this drivetrain is well-established. The electric part is still evolving. Today's hybrids use nickel-metal-hydride (NiMH) battery packs. These deliver the needed high voltage, but are somewhat limited in their storage ability. Lithium-ion batteries—like those in your laptop, cellphone and, increasingly, power tools—are generating the biggest buzz.
"The idea for better batteries on hybrid-electric vehicles is on everyone's mind," says Dan Doughty, manager of the Advanced Power Sources Department at Sandia National Labs. Li-ion batteries have some distinct advantages over NiMH: They contain 50 percent more charge per unit mass and require only one-third as many cells to achieve a given voltage.
Volkswagen 1-Liter Car
VW developed this experimental two-seater to use less than 1 liter of fuel per 100 kilometers. That works out to about 240 mpg. The 0.3-liter 8.5-hp one-cylinder engine, however, just doesn't have what it takes to handle U.S. interstates.
However, today's lithium-ion batteries are two to four times more expensive than NiMH batteries. Also, currently, lithium-ion's life span isn't long enough for automakers' demands. Thermal management is an issue, too. "If a cell is damaged and you have an internal short circuit, the battery will release that energy and it could explode," Doughty says.
Doughty estimates that li-ion technology will be ready for hybrid use in two to four years. A nice thing about batteries is that they can be recharged from more than one source. Tapping into the electric grid is one option; there are also more self-sufficient technologies—which brings us back to Steve Lapp. Solar cells like his could offer the extra energy needed to push a diesel-hybrid car into the high-mileage end zone.
Large, roof-mounted solar panels like Lapp has on his Prius have limited appeal, but there are other approaches. Energy Conversion Devices is among a handful of companies that have developed amorphous photovoltaic material—a thin film that can be contoured to any surface. Some day, all of a car's horizontal surfaces could harvest energy to feed the battery pack.
Volkswagen Lupo 3L TDI
This small front-drive hatchback has a 60-hp three-cylinder engine, airbags, ABS and real seats for four. It is also 330 pounds lighter than a standard Lupo and gets about 80 mpg. However, it sold poorly and was dropped by VW.
Battery recharging can be enhanced further by reclaiming the regenerative braking power of all four wheels, as Toyota's Highlander Hybrid does, rather than just two, as in the Prius. Toyota's Williamsen speculates that using the four-wheel system can yield up to a 2 percent fuel-economy gain over a two-wheel system.
As an additional measure, using better insulation and electrochromic (self-darkening) windows could minimize the need for energy-gobbling climate control systems. Berkeley's Hopkins thoroughly insulated the passenger compartment of an experimental car when she worked on the government/industry collaborative Partnership for a New Generation of Vehicles project. Her team used ultralightweight, ultrahigh-performance, xenon-gas-filled insulating panels in the doors, roof, dash and floor of the project Taurus. Next, they coated the windows with solar-control film to block the hot rays of the sun. "We were able to reduce heating and cooling loads by 70 to 80 percent," she says.
Building What People Buy
Clearly, a car that gets 100 mpg every time you drive can be designed and built. But, absent government regulation or far higher fuel prices driving consumer demand, will any car company actually do it?
"There's no business case for it," says GM's Juechter. "How many people would spend $200,000 on a car that would ultimately save them a few thousand dollars on fuel over the life of the car?" That's the worst-case scenario in terms of price estimates, but there's little doubt that a 100-mpg car would cost thousands more than today's bigger, more powerful vehicles.
"Small, clean diesel engines run about $3000 and you can add another $4000 for the hybrid," Cole says. "So you're looking at a $6000, $8000 or even $9000 premium for just the car's powertrain. And we haven't even talked about the cost of the materials."
Few drivers would ever make up the cost premium at the pump. However, working out the requirements of a 100-mpg car makes it clear just how feasible it would be to build, say, a 75-mpg car—for far less money. Besides, saving money isn't the only reason people choose a car. It became fashionable to drive SUVs because they projected an image of power and an active lifestyle. It's possible to imagine drivers being drawn to the environmental and national security benefits of efficiency—and to the cutting-edge engineering, as well.
So perhaps people have been asking the wrong question all along. It's not, "Why can't they build a 100-mpg car?" but rather, "Do we really want one?"