How the brakes on a Prius work

by | Feb 8, 2010 10:00 AM ET

If you have been watching the news, you know that the Toyota Prius is having problems with its brakes as well as with the accelerator. Here is a discussion of the brake problem - on bumpy roads the brakes don't work right:

They can also fail on icy roads:

Toyota set to recall Prius hybrid over brake failure

In a deepening of the crisis at the world's largest car manufacturer, Toyota will this week warn 300,000 Prius owners — 3,500 of them in the UK — that the brakes on their car may fail in icy conditions or on bumpy surfaces.

So what's going on? Hydraulic brakes have been in cars for a century, and provide a very reliable braking system. Anti-lock brakes are a fairly new development, but when they fail the brakes revert back to normal hydraulic brakes. How did something so solid get messed up on the Prius?

The Prius is unique because it is one of the few cars to have "brake by wire". When you push on the brake pedal, there is no direct connection to the actual brake pads the wheels. Instead, the pedal sends a signal to a computer, which decides whether the to activate the regenerative braking system, the actual disk brakes, or both. Because there is a computer in the middle, software problems can cause the brakes to work in bizarre ways. This article explains the situation:

Not your grandfather's braking system

Several users reported odd sensations while braking, and attributed it to a slight delay that can be produced when the Brake By Wire computers switch back and forth from the regenerative motor system to the disc brakes while deciding how much braking is required.

This article gives a sense of how involved the system is:

Hybrid Vehicle Brake System

One interesting feature is a backup power supply that uses capacitors to keep the braking computer and actuator operable even if the car loses power.

So if the software has bugs in certain situations, the brakes are not going to get activated properly. That creates a huge problem if you are trying to stop.

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