The Toyota Prius

The Toyota Prius, which came out in Japan at the end of 1997, is designed to reduce emissions in urban areas. 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. The Prius meets California's super ultra low emissions vehicle (SULEV) standard. It is a four-door sedan that seats five, and the powertrain is capable of accelerating the vehicle to speeds up to 15 mph (24 kph) on electric power alone. This contributes to the better city mileage than highway mileage. The Prius was the 2004 North American Car of the Year.

Unlike Honda, Toyota has focused primarily on the powertrain to achieve its emissions and mileage goals. The Prius weighs 2,900 pounds (1,315 kg) and has as much interior space and trunk space as a Toyota Corolla. Here's a layout of all the pieces:


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

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 5,000 rpm, where it makes 76 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 67 horsepower from 1,200 to 1,540 rpm. It produces 295 pound-feet of torque from 0 to 1,200 rpm, which is more than enough to get the car going without the aid of the gasoline engine.

In the next section we'll learn more about the power split device.