Prius Power Split Device (PSD)

(There is no torque changing transmission in this car!)

A simple set of three types of gears, about the size of your hand, shown in the photo above, connects the main electric motor and wheels (outer gear-called the ring gear, colored green), the internal combustion gasoline engine (planetary carrier with four pinion gears, colored blue), and the starter/generator (center gear-called the Sun gear, colored yellow). A great explanation of the PSD (and the entire Prius car) is available from Graham Davies' website. This website inspired me to create detailed animations of the PSD gears. While, Graham has great animations on his site, I have made ones of depictions of the gears themselves. The number of teeth on my images are very close to those found on the PSD.  In all cases, the animation frames were made by actually rotating the gear images and then saving them. They were then imported to make the animated gif files. The gears remain meshed. To see a larger version of the gears, you can click on any of the moving gears.

To help show the different modes together, I have placed the images along side of each other with the text below for each of the conditions. Remember: 1) The outer green gear is attached to the main electric motor and directly to the wheels. When the green gear turns, so do the wheels. 2) When the blue "carrier" turns, so does the gasoline engine. (There are no external shafts connected to the four blue idler grears.) 3) When the inner yellow gear turns, so does the small motor/generator shaft turns. Everything is engaged always.

Warm-Up (car not moving)

Electric Only (moving forward)

Normal (moving forward)

Overdrive (moving forward)


Warm-up: As the car starts moving, it usually starts on the electric motor alone. However, before the car starts to move, it usually warms up for a few seconds. for the car wheels to turn, the outer ring must rotate (actually at about a fixed 4 to 1 ratio, always). Notice that the ring gear (green) is not turning since the car is not moving. Electric Only: When you press the accelerator pedal, the car begins to move. Assuming it is warmed up, this is performed on the electric motor alone. Notice that the planetary gear carrier, connected to the gasoline engine is not moving. The generator is actually turning backward. Normal: As the car accelerates, usually the computer will apply power to the generator (center Sun gear), starting the engine. Overdrive: If the car is cruising at a relatively high speed, the computer can arrange for the sun gear to actually run backward, as shown here. Reverse: And to go backward, this must be done on the electric motor (there are no gears, remember), the following happens (note that the engine is not running)


So, there you have it. You can stop, start, and go backward, without changing any gears. Of course, the computer does have to control carefully the rotational speed of the motor/generator (central Sun gear, see above). But it does that automatically!

 All animations Copyright © 2005 Robert Foster.