By Tom Roebuck

January 22, 2010 5 min read

They once were considered a radical idea, but hybrid cars now have entered the mainstream. The Toyota Prius is among the 15 best-selling cars in the United States, with more than 100,000 sales a year, and by 2020, Toyota plans to offer a hybrid version of every model. Last year, Mercedes introduced its first hybrid, the S400, and will follow up with a hybrid SUV. Mercedes also has announced its intentions to convert all S-Class sedans to hybrids eventually, which would mark the first time an entire model line featured an alternative drivetrain.

A hybrid has the same -- but usually smaller -- internal-combustion engine found in a traditional car, but it also has an electric motor that runs on batteries. Most hybrids can go up to 35 miles per hour on the electric motor alone, ideal for city driving. No gasoline is used, and no exhaust fouls the air. As the batteries run low, the gasoline engine turns on and generates electricity that replenishes the batteries. When the car is stopped in traffic or at a stoplight, both engines are dormant, using no power at all when it isn't needed.

The gasoline engine takes over when the car speeds up, but the electric motor still plays a role. It not only can act as a turbo booster for the engine but also can generate electricity through regenerative braking. Traditional brakes slow the car through friction between the brake pads and rotors, generating heat. In a hybrid, pressing the brakes engages the electric motor to run backward. That not only slows the car but also allows the motor to generate electricity that can be stored in the batteries.

New hybrids can reach 50 miles per gallon in the city, so it's likely that their popularity will continue to grow. The market share for hybrids is about 3 percent, and that is expected to grow by 1 percent or more each year, according to Bradley Berman, editor of HybridCars.com.

"It's going to double in popularity in new-car sales about every four or five years," Berman says. "Hybrids are going to start going into the mainstream. Meanwhile, plug-in hybrids and electric cars are going to get introduced, mostly in certain markets like California and mostly to early adaptors willing to try new things."

A new generation of hybrids, called plug-ins, will hit the market later this year with the scheduled launch of the Chevy Volt in October. Plug-ins will have the ability to recharge the batteries through a standard home outlet, reducing the use of the gasoline engine even more.

"The nice thing about that is you get even more of the benefits of an electric vehicle. You use even less gasoline, and if you're plugged into a clean electric grid, you produce even less global warming emissions, depending on where you are around the country," says David Friedman, who is the research director of the Union of Concerned Scientists' Clean Vehicles Program.

The Volt will be able to travel 40 miles using the electric motor alone with full performance, including neck-snapping acceleration and the ability to reach highway speeds. After the 40 miles are up, the gasoline engine will fire up, and the car will work like a traditional hybrid.

"The idea there is most people don't travel much more than 20 or 40 miles a day," Friedman says. "So for most people, most of their daily commutes, in theory, could be on electricity alone."

The biggest challenge hybrids face is the cost of the batteries. Chevrolet plans on selling the Volt for $40,000, a princely price for a compact car. Government tax credits of $7,500 will help, but the high price tag is still a concern.

"At the end of the day, they have to drive the cost down of those batteries for plug-ins to ever make sense financially," Friedman says.

A report issued by the National Research Council in December predicted an uncertain future for plug-ins. It cited the high sticker prices -- as much as $18,000 more than equivalent conventional cars -- which are unlikely to come down much without a revolutionary breakthrough in battery technology. The council also expressed concerns about the nation's power grid if millions of cars are plugged in when people get home from work.

Despite the challenges, Friedman remains optimistic about hybrid technology.

"A lot of progress can happen over the next 10 years, which will give us a little time to see whether plug-in hybrid cost comes down or whether pure-battery electric vehicles make more sense or whether fuel-cell vehicles running on hydrogen make more sense," he says.

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