2008 Chevrolet Tahoe Hybrid (© General Motors)

The 2-Mode Hybrid is the latest system to come from General Motors.

Most people know by now that hybrids use electricity to achieve good gas mileage, but how their advanced powertrains work and whether they help your pocketbook is a bit more complicated. Here we'll take a look at the basic technology and its variations. In a future article we'll discuss the cost, and potential savings, of owning a hybrid.

THE THEORY BEHIND HYBRID CARS

What sets hybrids apart from regular cars is that they essentially use two powertrains, an electric motor and a gasoline engine. The electric motor draws its power from a large battery pack that is recharged by the gas engine and by energy recouped from the brakes.

The electrically powered car has been a vision for the future for decades, but its development reached a virtual dead-end in the mid-1990s. That's when hybrids emerged, promising a compromise between the benefits and limitations of both electric and gasoline powertrains.

Electric motors are very efficient at accelerating, and unlike their internal-combustion counterparts, produce their maximum power from a dead stop. But batteries with enough energy to drive long distances cost too much, are too heavy, and take up too much space in a car. Theoretically, a hybrid solution is the best of both worlds. By relying on electric power, at least some of the time, there is a clear benefit in reducing emissions.

By using electric motors for acceleration, automakers could use smaller, more-efficient gasoline engines to supplement the motors under heavy load (i.e., acceleration, hills) and to cover long distances. By combining the two systems, the battery packs can be relatively small. The downside of a hybrid is that the dual drivetrains can be significantly more expensive than a traditional gas engine alone, the battery pack takes up space, and the pack adds weight.

FULL AND MILD HYBRID CARS

Hybrids today fall into one of two categories: full and mild. The main difference is whether or not the car can propel itself solely on electric power.

Full hybrids like the Chevrolet Tahoe, Ford Escape, Nissan Altima, and the Toyota Prius and Camry can start off silently, creep in stop-and-go traffic and putt around town at low speeds on just electricity. The gasoline engine kicks in as soon as the driver demands quicker acceleration to supplement the electric power. The engine can also turn off when the car is slowing down or coasting. Full hybrids are more expensive to produce, but they typically save more fuel and run cleaner, especially in city driving.

Mild hybrids like the Chevrolet Malibu and the Saturn Vue Green Line can shut the engine off when the car is stopped and restart it as soon as you take your foot off the brake pedal. They use electric power only to assist the gas engine on hills and when accelerating. Mild hybrids produce only a modest gain in fuel economy yet command a purchase-price premium.

Both types save gas by shutting down the gas engine at stops. The Prius uses a smaller gas engine than the similar-sized Toyota Corolla. Other vehicles typically use the smaller of the available gas engines in the equivalent nonhybrid models. All of them also use the electric motor to recoup some of the energy it takes to stop and use it to charge the batteries.

Driving these hybrids is really no different than driving other cars, except that the engine sometimes stops and starts by itself. At first it might seem a little strange but it's easy to get used to, especially if the transition to gasoline-engine operation is smooth. In fact, some hybrids feel smoother than comparable four-cylinder economy models because the electric motor is quiet and the gas engine is less strained under low-speed acceleration.

None of the hybrids on the market today need to be plugged in, and none can be. The batteries are recharged only by the gas engine and by braking. Just fill the tank with gas and go.

The best hybrids offer significant gas savings. The Toyota Prius, for example, returns the highest gas mileage of any five-passenger car we have ever tested - 44 mpg overall. The midsized Toyota Camry hybrid returns 34 mpg, better than many small cars. And the Honda Civic Hybrid returns an excellent 37 mpg. The only nonhybrid car that gets close to such excellent fuel economy is the 2009 Toyota Corolla, at 32 mpg overall.

Other hybrids, however, offer moderate gas savings in larger vehicles while improving acceleration and maintaining utility. The Toyota Highlander Hybrid and Lexus RX400h, for example, get 24 and 23 mpg, respectively. Not stellar, but about 25 percent better than their gas siblings. Both use relatively large 3.3-liter V6 engines.

In terms of emissions, hybrids typically produce 29 percent less smog-forming nitrogen oxide than conventional cars, according to a study by the American Council for an Energy-Efficient Economy. And every gallon of gas saved reduces carbon dioxide emissions by 19 pounds, which reduces global warming.