The Toyota Prius
One of the main goals of the Toyota Prius is to improve emissions in urban driving. To accomplish this, Toyota has designed a parallel hybrid powertrain, called the Toyota Hybrid System (THS), that adds some of the benefits of a series hybrid.

Unlike Honda, Toyota has focused primarily on the powertrain to achieve its emissions and mileage goals. The Prius weighs 2,765 pounds (1,255 kg) and has as much interior space and trunk space as a Toyota Corolla. Figure 8 provides a layout of all the pieces.


Figure 8. Prius layout
Move your mouse over the parts for a 3-D view.

How Does the Prius Get Good Mileage and Lower Emissions?
The Prius mainly relies on two features to optimize efficiency and reduce emissions:

  • Its engine only runs at an efficient speed and load - In order to reduce emissions, the Prius can accelerate to a speed of about 15 mph (24 kph) before switching on the gasoline engine. The engine only starts once the vehicle has passed a certain speed. And once the engine starts, it operates in a narrow speed band.
  • It uses a unique power split device - Gasoline engines can be tuned to run most efficiently in certain speed and load ranges. The power split device on the Prius, which we'll talk about in a minute, allows the engine to stay in its most efficient load and speed range most of the time.

    Toyota designed the 1.5-liter engine in the Prius to run at a maximum speed of only 4,500 rpm, where it makes 70 horsepower. Keeping the maximum speed of the engine low allows for the use of lighter components that improve efficiency.

    The electric motor on the Prius is rated at 44 horsepower from 1,040-5,600 rpm. It produces 258 pound-feet of torque from 0 to 400 rpm, which is more than enough to get the car going without the aid of the gasoline engine.

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.


Figure 9. The Prius planetary gear set

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 (Figure 9). 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 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.

Driving the Prius
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 in Figure 10.


Figure 10. Watch the Prius' power split device
as the car accelerates from 0 to 30 mph.

Once you reach about 15 mph (24 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.