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I am close to placing an order for an '04 Prius. I own a '99 Camry XLE and live at the bottom of a steep hill. I downshift to low coming down the hill to slow the car and save the brakes; still have the original brakes at 70,000 miles.

Question: How well does the '04 Prius transmission slow down the car while descending steep hills without acceleration and no brakes?

Thank you.

Joeb13
 

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You shift to "B" - for engine braking.
The engine spins (compressing air), which helps to slow you down.

You'll probably also see some nicely charged batteries from all the regenerative braking while going down that hill (which you'd get the same in either D or B driving).

Just remember to switch back to D once you're on the flat again, or your MPG will go down if you drive around in B all the time...
 

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Downhill braking with "B"

I have a 2002, but I suspect that the 2004's are similar. "B" keeps the gas engine engaged to the drivetrain and provides "engine" braking on long downhill runs. While this works reasonably well for normal highway driving in the mountains at up to 6 to 7% grades, the "B" setting does not provide enough braking at steeper inclines. I drove up Pike's Peak last fall and had to ride the brakes most of the way down, because "B" did not hold the vehicle speed down.

If you have a Camry, the "B" setting in the Prius will not provide the same braking force as "LOW" in the Camry. It would probably be more like "2nd".
 

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There is some very important difference between a Prius and a "regular" car concerning long downhill runs. As Phoenix says it is true that "B" does not provide enough braking for the steepest inclines, but "B" is not the only way to prevent excessive heating of the brakes : the brake pedal itself, if pressed moderately, does not engage brakes either. It is actually only a "fake" pedal that responds like that of regular brakes, but acts as an electric command sending a request to a computer, which will decide whether to use only electric motor braking (in most cases) or to add conventional brakes if you require a stronger braking force.

No car can save its brakes better than the Prius, because it simply doesn't use them most of the time, even if you think so... :wink:
 

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This is true. "Braking" in the prius does not necessarily mean that the wheel brakes are engaged. It's the electric motor that does the braking, and captures the energy to charge the battery. The brakes themselves engage in more urgent braking.
 

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Engine Braking

That's true about the brake pedal not actuating the brakes until pressed firmly. I "rode" the brakes (lightly) down Pikes Peak with the tranmission in "B", but got stopped by the Park Ranger in the run-out area about mid-way down the mountain. (They measure the temperature of your brakes at a level spot in the road, before allowing you to continue on down another steep incline. If your brakes are hot, they make you sit on the side of the road for half an hour to allow them to cool before proceeding.)

The brakes were cool, but after I tried to describe how the Prius "B" setting and electric motor braking works, the Ranger leaned inside the car to see what I was talking about, then looked confused and motioned us to the "waiting area". I guess I should have shut up and let the infrared temperature gage speak for itself. That was one time I shouldn't have boasted about the Prius' advanced technology. Obviously, he didn't understand and didn't want to risk letting us proceed.

I guess I'll have to learn when to shut-up...
 

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Pikes Peak

If you noticed, the area where your brakes are checked on the Pike's Peak Road for over-heating has a souvenir shop... Go figure.
 

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So, if engine compression alone is not enough to slow the car on steep inclines, what happens to all that regenerated electricity after the 201-volt battery is fully charged?

Energy needs somewhere to go. Conventional brakes dissipate it as heat. Engine braking also produces heat (by compressing the air) and then releases that heat. Regerative braking normally feeds charge from the generator to the battery.

So when the battery is full, and if engine compression is not enough (due to the steep incline) where is the energy going?
 
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