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Toyota Prius - Power Split Device (PSD)

The Toyota Prius is packed with some pretty high-tech stuff, but at the heart of the Hybrid Synergy Drive (HSD) is a simple little device called the Power Split Device, or PSD. The PSD is a planetary gear set that removes the need for a traditional stepped gearbox and transmission components, and also the familiar rev-lurch-rev-lurch of acceleration in an ordinary gas powered car. It acts as a continuously variable transmission (CVT) but with a fixed gear ratio.

If you have no mathematical or mechanical understanding of hybrid technology or even regular engine components, this will help you get a feel for how the PSD allows the car to use power from an internal combustion engine (ICE) , as well as 2 electric Motor/Generators (MG1 and MG2), all spinning at different and variable speeds. The PSD even allows the smaller of the two Motor/Generators, MG1, to act as a starter for the ICE, thereby eliminating another component of a traditional gasoline engine.

The model below allows you vary the speed of both the ICE, and MG2. MG2 is the larger of the 2 electric motor/generators and is also referred to as the traction motor because its speed (RPM, or revolutions per minute) has a fixed relationship to the speed of the wheels. Dragging the MG2 slider, you'll see that the speedometer changes, while changing the speed of the ICE does not directly affect speed. Once you master that concept you'll start to see how the CVT function works. The ICE can spin faster or slower, depending on how much power is needed, and with either resistance or assistance from the electric motors the car can reach the desired speed while always keeping the ICE running at the most efficient rate possible.

Rotation speeds of MG1, MG2, and ICE are inter-dependent, and the speed of MG1 will always change when you vary the speed of either of the other 2. MG1 has a maximum rate of 10,000rpm in either direction (positive or negative) with a software limit of 6500 RPM if ICE is off. Using the model below, you can see for yourself why this software limit means the ICE will always spin if you're travelling above 42mph. And in case you were curious, yes MG1 can and often does change spin directions under normal driving conditions.

ICE rotation is limited to speeds between 1000rpm and 4500rpm. It can also stop completely, but anything between 0 and 1000 will force the slider up or down. That's because the internal combustion engine can't operate effectively below that speed. The hybrid computer knows, and will stop the ICE when it decides you don't need to use any gas, and start it again when you need more power, or higher speed from MG1.

For a more technically in-depth discussion of the inner workings of the hybrid engine, as well as my inspiration and sources for this demo, look at Wayne Brown's Prius Simulator, and explore Graham Davies' Toyota Prius site. You can also get a pretty user-friendly description of the PSD in Word document format at john1701a.com

For a wealth of information and user experience with the Prius, visit PriusChat. A must for any Prius owner!

J. Kusnetz has taken an interest in my little toy, and combined it with his big toy, the Mac Prius. A variant of this demo now runs with realtime information being fed into it by his car.

You can see the Flash source files including graphics and ActionScript classes for this demo here