The Power Split Device

The power split device is the heart of the Toyota Prius. This is a clever gearbox that hooks the gasoline engine, generator and electric motor together. It allows the car to operate like a parallel hybrid -- the electric motor can power the car by itself, the gas engine can power the car by itself or they can power the car together. The power split device also allows the car to operate like a series hybrid -- the gasoline engine can operate independently of the vehicle speed, charging the batteries or providing power to the wheels as needed. It also acts as a continuously variable transmission (CVT), eliminating the need for a manual or automatic transmission. Finally, because the power split device allows the generator to start the engine, the car does not need a starter.

The power split device is a planetary gear set (below). The electric motor is connected to the ring gear of the gear set. It is also directly connected to the differential, which drives the wheels. So, whatever speed the electric motor and ring gear spin at determines the speed of the car.


The Prius planetary gear set

The generator is connected to the sun gear of the gear set, and the engine is connected to the planet carrier. The speed of the ring gear depends on all three components, so they all have to work together at all times to control the output speed.

When you accelerate, initially the electric motor and batteries provide all of the power. The ring gear of the power split device is connected to the electric motor, so it starts to spin with the motor. The planet carrier, which is connected to the engine, is stationary because the engine is not running. Since the ring gear is spinning, the planets have to spin, which causes the sun gear and generator to spin. As the car accelerates, the generator spins at whatever speed it needs to in order for the engine to remain off. You can see all of this below:


Watch the Prius' power split device
as the car accelerates from 0 to 30 mph.

Once you reach about 40 mph (64 kph), the gasoline engine will turn on. The generator suddenly changes speed, causing the planet carrier to turn and start the engine. Once the engine is running, it settles into a constant speed while the generator varies its speed to match the output speed with the electric motor. If you are really accelerating hard, the motor will draw extra power from the batteries. Once you are up to freeway speed, the car will move under a combination of gas and electric power, with all of the electricity coming from the generator.

Like the Insight, the Prius never needs to be recharged; the onboard generator automatically maintains the proper level of charge in the batteries.

Both the Honda and the Toyota have long warranties on their hybrid components. The Insight has an eight-year/80,000-mile warranty on most of the powertrain, including batteries, and the Prius has an eight-year/100,000-mile warranty on the battery and hybrid systems. The motors and batteries in these cars typically don't require any maintenance over the life of the vehicle (however, if you do have to replace the batteries after the warranty expires, it will likely cost you several thousand dollars). The engine doesn't require any more maintenance than the one in any other car, and because both hybrids have regenerative braking, the brake pads may even last a little longer than those in most cars.

Achieving hybrid power is certainly more complex than using straight gasoline power or straight electric power. In the next section, we'll examine why hybrid technology is so desirable, both for consumers and for car makers.