There are generally two types of hybrid cars available on the market. There are parallel hybrids, where there are several power options -- the electric motor can operate by itself to power the car, the gasoline engine can operate by itself or both the electric motor and the gasoline engine can work together. There's also the option of the series hybrid, where the gasoline engine, instead of directly powering the wheels, serves only to charge the vehicle's battery to provide power to the electric motor.
The Toyota Prius is somewhat unique among hybrids. It's a combination of both of the previous types and is known as a series-parallel hybrid. This is possible because of the Prius' power split device, a special gearbox that connects the gasoline engine, electric motor and generator together into one unit. The device, which was introduced in the second generation Prius, contains a planetary gear set, and it allows the car's various power technologies to work together to move the vehicle. The electric motor is attached to a ring gear, which transfers the power of the motor to a reduction gear unit that's connected to the final drive. The planetary gear set also contains a smaller sun gear and a series of planet gears. All of these gears moving together not only provide power for the car, but they also provide power for the vehicle's generator. When the sun gear simply can't spin any faster, this is the limit of the car's fully-electric operation and the vehicle's internal combustion engine is switched on. This setup allows for increased efficiency since the electric motor and generator run as long as possible before the gasoline engine starts up and begins to burn fuel.
The engine in the third generation Prius grew from 1.5 to 1.8 liters, giving the hybrid a boost in horsepower from 110 to 160, an important factor for those wary of the supposed weakness of hybrid cars. Also, the Prius doesn't have to be recharged, since its generator constantly supplies power to the battery. With all of this, the third generation of the Toyota Prius aims to get 50 miles per gallon (21.3 kilometers per liter) in combined city and highway driving, compared to 46 miles per gallon (19.6 kilometers per liter) for the 2009 Prius and 41 miles per gallon (17.4 kilometers per liter) for previous models.
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