Hybrids make the best of two power sources

Electric cars, mandated in California and possibly Massachusetts by 2003, still have some practical drawbacks.

They can travel only 60 to 70 miles in everyday driving before the batteries die. And a full recharge takes as long as eight hours.

Their batteries are also expensive and heavy -they often double the weight and cost of the car. And they need replacement every two or three years.

To tame these troubles and provide much of the benefit of electric cars, automakers devised hybrids: Cars that use both a gasoline engine and an electric motor to save fuel and reduce emissions.

Hybrids run on gasoline, diesel, compressed natural gas, ethanol, or some other combustible fuel.

They also use a small bank of batteries that cost and weigh less than those in pure electric cars.

Most hybrids never need to be plugged in; the engine charges the batteries automatically.

Hybrids also use a much smaller gasoline engine than conventional cars use for better economy.

Early hybrids worked like electric cars, using an electric motor to power the wheels. The gas engine served only as a generator to keep the batteries charged.

But most modern hybrids, including the Toyota Prius, use a parallel configuration that alternately uses an electric motor and batteries, a gasoline engine, or both to drive the wheels.

This allows both the electric motor and the gasoline engine to do what they do best. Gasoline engines excel at traveling long distances at highway speeds. But they're dirtiest and get the worst fuel mileage accelerating from a stop.

Electric motors, by contrast, generate full torque from a stop and never waste energy or pollute while idling. But their batteries run down quickly during long drives.

So parallel hybrids start rolling silently under electric power. At about 10 miles an hour, the gasoline engine starts and drives the car.

Once the engine has started, most parallel hybrids recharge the batteries with excess engine power.

For instance, if a small hybrid engine develops 50 horsepower and only needs 20 to run on the freeway, it may run at full throttle anyway until the batteries have recharged.