It’s been more than a year since General Motors unveiled its visionary Chevy Volt plug-in hybrid concept vehicle. GM promises that the Volt will allow most Americans to drive 40 miles each day without burning a single drop of petroleum. Instead, the energy will come from next-generation batteries recharged from a common household electric outlet.

Since the Chevy Volt’s introduction at last year’s 2007 Detroit Auto Show, the General’s PR machinery has buzzed with ever-increasing intensity, churning out news releases, keynote speeches, and television commercials extolling the virtues of the Volt. The intensity of the buzz is matched by an equal fervor from GM executives and engineers—a level of enthusiasm and dedication bordering on zeal. I’ve met with Volt team leaders on multiple occasions over the past year, and I’ve seen the fire in their eyes. But enthusiasm, and even a big budget, doesn’t ensure GM’s victory against those who would kill the Volt. Below is a futuristic whodunit; print it out and look at it five years from now to see which suspect proved to be guilty as charged.

Suspect #1: OPEC

Of course, our friends in Dhahran and Caracas are not literally trying to undermine GM’s plug-in hybrid project. But innovations like the Chevy Volt and other advanced renewable energy technologies are born of necessity. As long as the price of oil hovers around $100 per barrel, car companies will apply resources to develop fuel-efficient automobiles.

Some predict the price of oil to climb to even more staggering heights—but that’s not guaranteed. The oil price shocks of the 1970s, which brought the first wave of small gas-sipping Japanese cars to American shores, didn’t last forever. Subsequent oil spikes came and went, and subsided to pave the way for the SUV era. In the 1990s, the U.S. government invested $1 billion in hybrid research, but by the late 1990s, oil had dropped to $11 per barrel. With prices at the pump skirting $1 per gallon late in the decade, Detroit decided to leave those advanced technologies on the shelf, even though Toyota soldiered on and launched the Prius.

Another drop in oil prices by $20 or $30 a barrel, even if it lasts only for a few months, could deal a crippling blow to the Volt. Then again, oil-producing countries and companies say they don’t manipulate prices so…

Suspect #2: GM’s Stockholders

The Chevy Volt is an expensive endeavor. GM will invest tens, if not hundreds, of millions of dollars in the project before it sees a penny of profit. How long will GM be willing to subsidize a big and splashy project that doesn’t directly help the bottom line—especially if the company continues to suffer billion-dollar losses and dwindling sales in the shrinking U.S. car market?

I put that to Bob Lutz, GM’s product guru, on the morning before the Volt’s unveiling in Detroit. He said, “About as long as Toyota subsidized the Toyota Prius.” How long was that? About 10 years! Last year, Toyota sold its millionth hybrid vehicle—and only now is the Japanese company claiming to make a profit from its gas-electric cars. Recouping the investment on a plug-in hybrid with 40-mile all-electric range may take even longer. If the Volt comes to market by 2010, then GM won’t make greenbacks from its ultimate green car until 2020. That shouldn’t matter if the company has the long-term picture in mind. But stockholders want results this quarter, not in a decade.

GM’s broader financial challenges could pull the plug on the Volt.

Suspect #3: Sir Isaac Newton

The laws of economics are a formidable challenge to the Volt—but they’re nothing when compared with the laws of physics. GM is basing all its plans for the Volt on the development of lithium ion battery technology, which has never been used for powering a production vehicle. That technology is unproven, yet critical to allowing the Volt to carry enough energy for 40 miles of all-electric driving. Storing enough energy using the current hybrid battery chemistry, nickel metal hydride, would take a couple of trunk loads of battery packs. The company could squeeze in enough nickel metal hydride batteries for five or 10 miles of all-electric range, but GM is going for broke: not a drop of gasoline for everyday driving.

Testing of lithium batteries in the lab, and even on test vehicles, looks promising, according to GM executives. But before any car company can commit to more than a few prototype units, it needs to know that in the real world, those batteries will last the lifetime of the vehicle and will be safe. Any defect in manufacturing could be fatal to the project. Remember those Dell laptops that caught fire? Those were lithium batteries. When batteries are pushed to and beyond physical limitations—and there are limitations—then the longevity and safety of those batteries are compromised. Will GM be willing to take those risks? Or will the company face a perpetual series of delays because Newton’s essential law of physics doesn’t bend to corporate will?

Any one of those threats alone—lower gas prices, shaky company finances, or unproven battery durability—might not be enough to stop GM’s momentum with its plug-in hybrid project. But two out of three could mean the end of the Chevrolet Volt.