The Top 8 Clean Car Technologies Most Likely to Take Over for Gas

When the automobile was new in 1900, there was no clear consensus which technology would triumph. Would it be gasoline, steam or electricity? The smart money was on electricity, which shows that the smart money can be wrong.

We're in a similar period now, trying to find what comes after the straightforward, gas-burning internal-combustion engine. There's still a lot of fog, and it's unlikely to clear soon. But from where I sit today, here are eight leading technologies, listed in priority order from most-likely to could-be-a-contender:

1. Plug-In Hybrids. There's no question that plug-in hybrids, with 40-mile all-electric range and the ability to recharge from standard house current, will be on the market in the next two or three years. The leading (and only) mainstream players are General Motors (which plans on introducing a Saturn Vue plug-in hybrid) and Toyota (with an adapted Prius). Ambitious startups (Fisker, BYD) are also planning to field plug-in hybrids. The big challenge for all of them is developing a lithium-ion battery pack that can stand up to repeated discharge and recharge cycles and still demonstrate the longevity that today's nickel-metal-hydride hybrid battery packs have had. GM and Toyota talk about 2010 introductions, but battery development headaches could delay that.

2. Battery Electrics. Again, it's all about the batteries. Lithium-ion is the current leader, but is it ready to carry four passengers in a fully featured, crashworthy sedan more than 200 miles? It's time to be cautiously optimistic. Nissan has plans to bring an electric car to the U.S. by 2010. Chrysler, which has been lagging in green technology, surprised the world by suddenly announcing a concept car known as the Dodge EV, a sports car with a lithium-ion battery pack. It claimed 150-mile range and blistering acceleration of zero to 60 in less than five seconds. Some Chrysler electric is to be on the market by 2010. The sports car was clearly aimed at the Tesla Roadster, a California-built $100,000 exotic which (like the Chrysler) sports a Lotus-designed body.

3. Range Extenders. General Motors is making a big, bold step forward by building the Chevrolet Volt, with production slated for the end of 2010 (as a 2011 model). The Volt is something new: an electric car with a gas motor whose only function (it's not connected to the wheels) is to keep the electric motor spinning after the batteries are depleted. GM had this field (also known as "series hybrids") to itself, but Chrysler has jumped into the fray with range-extender versions of the Town and Country minivan and Jeep Wrangler. As with plug-in hybrids, 40 miles can be enjoyed in battery-only mode, but the gas engine extends that to 400 miles or more.

4. Very Small Cars. It doesn't have to be a hybrid; in fact, some of our current hybrids, based on SUVs, are actually gas guzzlers. High fuel prices have created a strong American market for very small cars, and carmakers such as Ford have been emboldened to start selling in the U.S. tiny, fuel- and space-efficient cars once relegated only to Europe or Asia. Consider the Toyota iQ. The minuscule car is just 118 inches long, but can carry three adults (plus a child)! It reportedly achieves 60/51 mpg fuel economy. The Toyota of 10 years ago would never have contemplated selling iQs in the U.S., but now it is definitely being considered.

5. Fuel Cells. The joke is that the hydrogen revolution is always at least 10 years away. And, well, it's still at least that far in the future. But the possibilities are endless, since hydrogen is the most abundant element in the universe. Big drawbacks now are the cost of the fuel and, of course, the cars themselves. Hand assembled by Ph.Ds, fuel-cell cars are way too expensive to be ready for showrooms before 2020. The leading players are General Motors (which has the cutting-edge Sequel and a fleet of Equinox SUVs under test) and Honda (the ready-for-the-road FCX Clarity).

6. Salad Oil. Americans love the idea of running their cars on used fryer drippings. And the technology totally works. Companies such as Massachusetts-based Greasecar have found a niche converting diesels to run on 100 percent biofuel. The problem is standardizing and ramping up the technology so we could seriously make a dent in gasoline power. We simply don't have enough available farmland to run our transportation fleet on soybean-derived oil, and there aren't enough fryers to greatly expand what is now a small cottage industry.

7. Liquid Hydrogen. BMW's Hydrogen 7 is a very impressive car, quiet and powerful. Its 12-cylinder engine will burn gasoline all day, but push a button and it switches without a hitch to a tankful of energy-dense liquid hydrogen. You can drive it anywhere. But things get complicated after that. Hydrogen is expensive to begin with, and it liquefies at -423 degrees Fahrenheit, meaning a super-cooled cryogenic tank is obligatory. Leave the car alone for too long (parked for a week at an airport, say) and much of the hydrogen will return to a gaseous state and vent out. The big challenges: affordable hydrogen production and liquification; easy refueling; and answering safety questions.

8. Ethanol. GM and Ford are world leaders in producing "flex-fuel" vehicles that can run on ethanol or gasoline. Millions have been produced, but most run on gasoline most of the time because of a still-embryonic ethanol station network. Cornell professor David Pimentel claims that ethanol has a net negative energy balance, which the industry heatedly denies. And there's the ramp-up question, too. According to Pimentel, if we converted 100 percent of the U.S. corn crop to ethanol, it would only replace six percent of current fossil fuel use. Because corn is in high demand, prices for it are escalating, creating a "food vs. fuel" controversy. Cellulosic ethanol, produced from plant fiber, has a much higher energy potential, but research is still embryonic.