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How hybrids work

Hybrid cars use different systems to capture wasted energy, from full hybrids such as the Toyota Prius to mild hybrids which may only use the electric motor to add power during acceleration. In a full hybrid, both power plants shut off during traffic stops for an eerily quiet experience. Hit the accelerator and the electric motor pushes the vehicle to about 15mph or 20mph for fuel-free driving that doesn't burn an ounce of gas or cost a penny. After the car reaches a certain speed, the gas engine takes over to propel the vehicle to cruising speed, and if you need a little extra power to pass a truck, both power plants kick in.

Forget about having to plug a hybrid in to charge its batteries. The engine and brakes provide electricity. When the car cruises or slows down, regenerative brakes automatically charge the batteries by harnessing the car's speed and converting it to electricity. If the batteries get low, for example, during extensive uphill driving, the engine revs up to generate more electricity, though in this mode the hybrid is more inefficient than a normal gasoline--or diesel-only car.

Continuously variable transmissions

Many hybrids also run on advanced continuously variable transmissions with an infinite variety of gear ratios to choose from rather than the traditional gearbox's four or five. For hybrids, CVT has the great advantage of being able to allow both the electric motor and gasoline engine to run at its optimal speed regardless of vehicle speed. Toyota's Hybrid Synergy Drive, a CVT system used in all Toyota Hybrid vehicles, actually combines the power from both the internal combustion engine and the electric motor to a single turning force to the drivetrain of the vehicle. One very important piece of the puzzle is the power control module, a computer which makes decisions about where to send power in the car. This module monitors throttle, battery charge and other running parameters, then channels energy to the wheels or battery, from the engine or the motor, as needed. It also shuts down and restarts the engine during traffic stops and starts.

From mild to full hybrids

Mild hybrids are less robust. The mildest replace the car's generator with a strong motor/generator, so the engine can be turned off during traffic stops and the motor can quickly and seamlessly start it up again when it's time to accelerate. But, in this case, the motor doesn't actually move the car. There are other points on the scale from mild to full hybrid as well. For example, the full hybrid system of the Toyota Prius uses a motor connected to the power train through a special gear set, allowing for more flexibility in power use. On the other hand, the Honda Civic Hybrid has a motor integrated between the engine and the transmission, taking the place of the flywheel. This less robust system uses the motor to assist the gas engine and only runs the car under electric power for very short bursts. The full hybrid system made by Toyota, and used in the Toyota Prius, can drive the car under electric power during low-speed cruising and heavy traffic.

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