It takes many parts to make a car go, and right now, several key parts for natural-gas cars are sputtering.

Gasoline prices have eased, at least temporarily taking the public's mind off the issue. Automakers still haven't shown the desire to build the cars at large scale. And in March, the Senate voted down a bill amendment that would have sweetened incentives for natural-gas vehicles and infrastructure.

For the moment, new technology might be the car's best hope for advancement. And no technology looms as large as the one behind the back seat: the natural-gas tank.

Last week, the Department of Energy announced $30 million in new research awards for natural-gas vehicles; two-thirds of the cash was for projects focused on their tanks.

Scientists at Texas A&M University are working on a material that would let a natural-gas vehicle carry more fuel without becoming too expensive to buy. Photo courtesy of Texas A&M University.

The money was disbursed by the Advanced Research Projects Agency-Energy, the office that searches for breakthrough energy ideas. The focus on tanks is understandable: They are the biggest reason natural-gas cars cost thousands more than their gasoline cousins. As ARPA-E reasons, if there isn't a massive build-out of natural-gas infrastructure, then the cars will need to fuel up at home -- and hold every whiff of fuel they can.

Today's tanks make that difficult, according to David Friedman, deputy director for clean vehicles at the Union of Concerned Scientists. A regular Honda Civic gets about 400 miles on a tank of gasoline, he said, while the natural-gas version gets around 200 and has a tank crowding the trunk.

"The tank is clearly eating away a lot of cargo space -- which, along with the cost of the natural gas tanks, is why people are focused on new storage approaches," Friedman said in an email.

The tank's bulk comes from its cylindrical shape -- the same sausage form that is often used for oxygen, carbon dioxide and other gases, because it's a safe way to store high-pressure gas.

As a fuel tank for a car, however, it leaves much to be desired. Want enough fuel for a long drive? The gas will crowd into the steel tank, but the pressure will rise.

To fight that pressure, engineers typically wrap the tank with carbon composite. The material is as tough as they come -- Friedman said these tanks survive even when a car is dropped four floors -- but it's also expensive.

If the perfect tank is out there, it will carry plenty of gas, without an awkward shape, for low cost.

Shapes matter

The problem isn't as serious for natural-gas trucks and buses. For these commercial vehicles, the economics often pencil out nicely, and it's easier to plan their refueling.

Yet for passenger cars -- the sedans, pickup trucks and sport utility vehicles people drive every day -- ARPA-E and others believe a game-changer is needed. By ARPA-E's reckoning, natural-gas cars need to get within $2,000 of gasoline cars' price tag, or they break even in five years because natural gas is cheaper than gasoline.

To get there, ARPA-E funded projects that will try different tank designs and shapes. REL Inc., a materials company based in Calumet, Mich., will try filling a tank with a low-cost foam that takes some of the pressure the gas dishes out.

As the gas pushes outward, the foam will flex, softening the blow. That means the outside of the tank can use less carbon fiber, and the whole tank gets cheaper. DOE estimated it would cost one-third of current tanks, if it works.

Foam can also fit into a rectangular shape, which is more space-efficient than a cylinder, said Adam Loukus, a vice president at REL. By his back-of-the-envelope estimate, that would result in 10 to 20 percent greater gas storage.

Another strategy is to make more room in the tank. Researchers at Texas A&M University have found that natural gas clings to the inside surface of today's gas tanks, and they want to give it more to grab onto.

They are developing two materials with vast surface areas. "If you take 1 gram of what we're working with, you have basically the surface area of a football field," said Trevor Makal, a doctoral student of chemistry who is assisting on the project.

Fill the tank with these porous materials, Makal said, and gas suddenly has a massive structure to grab. The two materials -- metal-organic compounds and porous polymers -- hold the world record for hooking onto methane.

Though the research is just getting started, Makal said this style of tank already holds about 30 percent of the energy that a similar-sized tank of gasoline can hold. That's roughly where natural-gas cars are today, he said.

The other hitch is cost. Makal said some of the material in his lab can cost more than $4,000 per kilogram. The lab has made some modifications and thinks it can get to $800 per kg. That still stands a long way from ARPA-E's target of $10.