Giant leaps of vehicle technology are the stuff of dreams: flying cars, sunmobiles that run solely on solar power or two-wheeled helicars held in balance by gyroscopes. But the path toward cleaner cars will be walked in tiny steps. There’s a place for all-electric and even semi-autonomous vehicles, but tweaks to designs that burn gasoline will deliver much of the fuel-economy gains expected in the coming decades.
Guzzlers are on their way out. This spring, the average fuel economy of all newly purchased cars climbed as high as it’s ever been, to 24.6 miles per gallon, according to an analysis from the University of Michigan Transportation Research Institute (UMTRI). Fuel economy will surely climb even higher: By 2025, national standards demand that automakers achieve a fleet average of at least 54.5 miles per gallon for cars and light trucks.
Better fuel economy can help reign in oil consumption and the more than 1.5 billion tons of greenhouse gas emissions coughed out annually by U.S. highway vehicles. And although cars and trucks with the best fuel economy often sell at a premium, improved gas mileage can help motorists save money at the pump, where a typical American household now spends about 4 percent of its annual income.
When gasoline prices exceed $4 per gallon, fuel economy tends to rise to be one of the top things people consider when purchasing a vehicle, says Bruce Belzowski, a research scientist at UMTRI. Prices have hovered around that mark nationally—though the national average has not crossed it since 2008--and shoppers are showing an appetite for better fuel economy. “Consumers may be saying, ‘We gotta get more outta this tank,’” Belzowski says.
A recent report from the National Research Council finds that it’s technically feasible to reduce petroleum use and greenhouse gas emissions from automobiles by 80 percent by 2050, compared to 2005 levels. Improving the efficiency of conventionally powered vehicles, however, will not be enough on its own to deliver such dramatic reductions. Cars would have to average upwards of an astonishing--and extremely unlikely--180 miles per gallon to reach that target based on efficiency gains alone. That’s where alternative fuels and all-electric vehicles will come into play.
All vehicles, no matter their power source, must become much more efficient if those goals are to be realized, but improving the efficiency of those that run on petroleum could have the biggest impact in the near term. These cars make up the vast majority of vehicles on the road today, consuming roughly one-third of all oil used in the United States. And there’s plenty of room for improvement, with as little as one-quarter of the energy in fuel for today’s cars actually being used to move them down the road. Most of the rest is lost as heat in the engine. Minimizing the amount of work that a gas engine must perform is one of the easiest and least costly ways to save fuel. Scientists, researchers and automobile manufacturers believe this can be accomplished through multiple strategies, many of which are catalogued below:
New Tire Technology
Tweaking tire designs can also deliver gains by cutting rolling resistance, or the force caused by the flattening of a tire as it rolls along the road. Cyclists know that a flat tire demands noticeably more legwork to roll along at a respectable clip. Similarly, minimizing the amount of flattening or deformation of a car tire through advanced materials and design can reduce the amount of energy required just to keep it rolling.
The most dramatic improvements, though, will probably come from changes to the engine transmission, says Alan Crane, a senior scientist for the National Research Council’s Board on Energy and Environmental Systems and the study director for the NRC report. Transmissions with a higher number of speeds, dual-clutch transmissions and friction-reducing coatings could help engines run at higher efficiency and cut energy loss.
A technology known as cylinder deactivation is one option for carmakers that desire a less thirsty product. This essentially kills half the engine when it’s not needed—during highway cruising, for example—but keeps the extra power on tap for acceleration, big climbs, boat hauling or other situations requiring a more powerful engine. “So you go from a six cylinder engine to a three,” says Brandon Schoettle, a researcher at UMTRI. Running on fewer cylinders lets drivers have it both ways, prioritizing power when you need it, and economy when you don’t.
Downsizing the engine is another way to gain efficiency, and it no longer has to come at the cost of performance. In conventional gas cars, the internal combustion engine takes a mixture of gasoline and air into a cylinder. A piston moves up to compress this mixture, and then a spark ignites it, producing an explosion that drives the piston downward. A valve opens for exhaust to leave the cylinder, and the cycle begins again: intake, compression, combustion, exhaust. Turbocharging, which forces extra air into an engine’s cylinders, can make it possible for smaller engines to generate more power from each of these tiny explosions.
Smaller usually means lighter, and a 10 percent reduction in a car’s weight yields about a seven-percent reduction in fuel economy, notes Crane. By 2050, the NRC report concludes cars could weigh 40 percent less. “That’s even without involving a great deal of [lightweight] carbon fiber,” Crane says. “Right now, almost everything in the car is just plain steel.”
Replacing Heavy Steel
Iron and steel alloys make up about 45 percent of most cars’ weight. But increasingly, advanced materials can be applied in a jigsaw fashion, with lightweight pieces inserted into various places in the steel structure. “You can reinforce the parts that are critical,” says Bill Reinert, national manager of advanced technology vehicles for Toyota. High-strength steels are being swapped in as thinner, stronger alternatives to ordinary steel, and aluminum content is on the rise. Carbon fiber and magnesium composites are relatively expensive and difficult materials to work with today, but further down the road they could help cut the weight of some components by as much as 75 percent.
Shedding weight can also have domino effects as few parts in a car run in isolation. “If you can save 100 pounds, you may be able to switch to a lighter, smaller engine, or reduce the size of the brakes,” says Crane. In turn, a smaller engine can mean simply less stuff under the hood, which allows more flexibility for aerodynamic design, leading to even better efficiency.
Optimized Part Production
Advances in computer-assisted design are making it easier to optimize individual parts and systems for a desired outcome. “The tools are improving,” says Crane. “When [automakers] come up with a revision for a car, they can feed a lot more information into the computer, and figure out what the best compromises are for fuel economy, as well as other factors.”
Tweaks to the curves and angles of a car, and the addition of active grill shutters that block air flow when it’s not needed for engine cooling, can minimize as much as 5 percent of a car’s drag at high speeds, enough to reduce a vehicle’s greenhouse gas emissions by about 1 gram per mile and yield extra fuel economy. But external changes needn’t be dramatic for cars to achieve 50 or more miles to the gallon. A fuel-sipper of the future, Crane says, “should look pretty much like current vehicles.”
Close inspection or a spin behind the wheel may reveal some differences, however. “Because it’s significantly lighter weight, [a more-efficient car of the future] may feel somewhat different. It’ll handle better, it’ll whip around a corner better,” Crane says. In analyzing the possible pathways to those 2050 goals, the NRC team assumed vehicles would continue more or less in their current form. Those cars will “be a little more windswept-looking,” Crane says, but nothing radical. Vehicles “don’t get smaller or so swept back that you can’t fit anyone in the back seat.”
A Helping Hand From Computers
More than a decade after the U.S. introduction of the Prius, hybrids still make up only a tiny sliver of the overall auto market—about three percent of vehicles sold in the United States. But some of the technology in today’s hybrids could help a broad swath of tomorrow’s cars get better gas mileage. One of the most important pieces is start-stop technology, which shuts off the engine when the vehicle is at rest, and then restarts when the driver steps on the accelerator.
In hybrids, this is often combined with regenerative braking, which harnesses kinetic energy during slowing and braking to charge a battery. The stored electricity can then be used to restart the engine. “Regenerative braking and start-stop are going to be basically very common design elements in the next few years,” Crane says.
Of course, when it comes to fuel economy, driver behavior matters, too. The difference in fuel use between an aggressive, lead-footed driver and an even-keeled, conservative one can be as much as 20 percent. To some extent, technology could nudge drivers away from their more wasteful tendencies. While autonomous driving is unlikely to result in driverless cars, at least not any time soon, the chief executive of Renault-Nissan, Carlos Ghosn, said at a recent event at Stanford University, “you’re going to see a lot of cars with less input from the driver.” Those cars can be optimized for fuel economy and efficient routing.
In the more distant future, intersections could be places where cars are programmed to slow down and weave their way through, rather than slamming the brakes or navigating roundabouts, UMTRI’s Schoettle suggests. “If no one is stopping, you’ve improved fuel economy,” he notes.
“It would be great if there was some magic bullet,” says Toyota’s Reinert—some technology that could turn a dirty car clean without us ever noticing a difference in performance, choice, convenience or pricing. The reality is multiple technologies in the right combinations can go a long way toward cleaning up our vehicles. “All these things are little,” Reinert says, “but it all adds up.”