Dear Tom and Ray:
Recently my husband and I rented a Toyota Prius in California -- which we enjoyed driving, by the way. While driving in our many national parks, I was concerned that there seemed to be no way to downshift the car. Navigating the steep, winding roads caused me to worry that the brakes would overheat. Thankfully, this did not happen.
So, what's up with this vehicle? How does a driver prevent the brakes from overheating when cruising down these hairpin curves when there seems to be no way to drop the car into a lower gear? I really don't want to use the Fred Flintstone braking method! It would ruin my shoes! Thanks for answering this.
TOM: While you can't downshift the continuously variable transmission (CVT) in the Prius, they have created a simulated downshift mode just for this purpose, Susan.
RAY: If you look at the transmission selector, in addition to "P," "N" and "D," you'll notice there's a selection called "B." That's for "braking" mode.
TOM: Like most functions on the Prius, what it does is controlled by the car's main computer, and the details are pretty complex. But from the driver's point of view, what the "B" setting does is add a "braking" effect, just as if you were downshifting. You'd use it the same way, to prevent the brakes from overheating on a long, steep downhill grade.
RAY: If the car's battery is at less than optimal charge, the "B" mode will first engage regenerative braking, which uses the forward motion of the car to charge the battery. That creates a drag on the wheels, which slows you down a bit.
TOM: If the battery is already sufficiently charged, or the speed of the car is higher, and the computer determines that it needs more engine braking, the computer then engages the engine through its planetary gear set and makes use of the cylinder compression to slow the car, just like every other car does when you shift to a lower gear.
RAY: These two braking effects can be combined by the computer, depending on the speed of the car, the condition of the battery and the amount of braking that's needed.
TOM: So that's the complex answer, Susan. The simple answer is that you shift into "B" and drive.