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.