2009 Honda FCX Clarity (© American Honda Motor Co., Inc.)Click to enlarge picture

Inside the Honda FCX Clarity is an amazing amount of room, unlike most cars featuring alternative drivetrains. The surprisingly small fuel stack fits between driver and passenger in a low center console.

It's not a call to arms, or a doomsday prophecy. It's a simple truth: Someday the planet will run out of oil. Whether the endgame comes within decades or a century, and despite whatever attempts are made to restrict global consumption and dramatically increase fuel efficiency, even the most optimistic experts admit it's only a matter of time.

For now, major automakers continue to roll out hybrid cars while teasing the public with more revolutionary models, such as GM's plug-in hybrid, the Chevy Volt, and the hydrogen-powered FCX Clarity from Honda. These might be glimpses of a brighter, more energy-independent automotive future. Or they might be marketing disasters in the making, and yet more reasons to defer the coming crisis for another few years. These are short-term concerns. In the long run, what will it take to achieve an entirely petroleum-free fleet of automobiles? What will the road look like when the gas runs out?

A complicated problem

"This question is likely to be a real concern in a 20- or 30-year time frame," says John Heywood, a professor of mechanical engineering at the Massachusetts Institute of Technology. "We have to start looking for alternatives immediately, but that's when it will really become critical." The problem, however, could hardly be more complicated.

Despite decades of research in battery technology and fuel cells, there is no clear-cut successor to petroleum-based fuels. Gas and diesel offer an irresistible combination of energy density and stability. The only drawbacks are the inevitable release of all those converted hydrocarbons as carbon dioxide and other emissions, as well as the fact they are made from oil. Despite all of the political and environmental collateral damage, petroleum is a tough act to follow.

Over the past decade, there has been no shortage of contenders in the race to replace gas and diesel, including cars powered by compressed air or recycled vegetable oil. For now, though, the world of alternative fuel research is essentially a two-party system, with the most high-profile projects falling under the umbrella of vehicles powered either by batteries or hydrogen fuel cells.

Both camps have their champions, decidedly flawed heroes like the Tesla Roadster electric sport car, which could reinvent the public image of electric vehicles (EVs), provided the fledgling automaker can survive long enough to deliver the first batch. Honda's hydrogen-powered FCX Clarity, which can be refilled at only a limited number of stations throughout the country, has a waiting list that's so restrictive the car seems more like a public relations stunt for eco-conscious celebrities than a step toward oil independence.

Awkward as they might seem, these are baby steps, some experts say, and only the opening round in a war between EVs and hydrogen for post-oil market supremacy. "It's unlikely we'll do both," says Heywood, who remains tentatively optimistic about battery-powered vehicles. "Electric vehicles can evolve in a grass-roots fashion. It grows bit by bit. There aren't real barriers to getting started."

EVs: available now, infrastructure issues

EV devotees can buy small, limited-range models right now, such as the $14,000 Xebra from California-based ZAP,a three-wheeler that can travel up to 40 miles on a charge. According to ZAP, sales are up — the company sold roughly three times as many Xebras in the third quarter as it did during the same period in 2007. When it comes to mainstream adoption, though, battery-powered EVs face significant barriers, such as the price and capacity of present-day batteries.

Norwegian EV maker Th!nk, which hopes to start selling its battery-powered two-seater City in the United States sometime next year, has unveiled its design for a more mainstream-friendly sedan. The Ox would have five seats, and as much as 50 percent more range than the City — up to 155 miles per charge — but according to Th!nk Chief Executive Richard Canny, "The Ox is a vision of where industry might be in 2012 or 2014. With today's batteries, it would be prohibitively expensive."