Ultracapacitor Use On Prius

<div class='quotetop'>QUOTE(efusco @ Jun 6 2007, 07:07 AM) [snapback]456496[/snapback]</div>

I'm pretty certain that Hobbit had the photo in one of his posts.

Yep the link is in post #31 of

http://priuschat.com/index.php?showtopic=3...citor&st=20

Dave M.

 

<div class='quotetop'>QUOTE(efusco @ Jun 6 2007, 03:07 PM) [snapback]456496[/snapback]</div>

Is that so? As I understood it, they were just back-ups, to provide power-assisted braking in the event of a 12V system failure. Mind you, that always seemed like overkill to me - why would the Prius need this more than any other vehicle?

 

<div class='quotetop'>QUOTE(fan-atic @ Jun 6 2007, 10:01 AM) [snapback]456493[/snapback]</div>

That sounds right. I don't know if the caps are used when regen braking; it makes sense to do so, but I haven't looked at that part of the system to see if handles that sort of power. It is true that the caps provide backup power for the brake system.

Evan, capacitors of that value are usually called ultra capacitors, just because they are so freaking huge. There may be some niggling technical definition, but none that I know.

Tom

 

Hi All,

An "Ultracap" as commonly understood by electrical engineers, is a device using a carbon foam (or aerogel) and electrolytic action to form a thin dielectric over down with the nano porosity of the carbon foam. This expands the capacitor surface area greatly. Check out: http://en.wikipedia.org/wiki/Ultra-capacitor .

 

<div class='quotetop'>QUOTE(KMO @ Jun 6 2007, 08:14 PM) [snapback]456960[/snapback]</div>

Traditional cars get the power assist for their braking systems from engine vacuum. But even if the engine dies while you're driving, there is usually enough residual vacuum in such a system for two or three stops before the power assist is gone. If you still have a traditional car around, you can prove this to yourself without killing the engine at speed. Just step on the brake pedal repeatedly while the engine is off. Press number one, normal travel and softness. Press number two, a little firmer. Press number three is usually rock hard. And now you know why sometimes you hear a "whoosh" from the area near the brake pedal when using the brakes in a regular car- that's air rushing into the power brake booster.

Since the power assist in the 2004+ Prius brakes is exclusively electric, they needed to put some provisions in there for you to at least get the car stopped if there were a sudden loss of power.

Ironically, I had BOTH of these conditions happen to me in a single instant in a traditional car in the 1990s. The battery terminal randomly snapped off the battery, so the car lost electrical power and stalled. While going 75 mph in the fast lane. Amazingly enough I made it to the right shoulder and got stopped without signals, brake lights, power steering, or trading paint with anyone.

When cars were designed without power steering and brakes, other mechanical advantage was provided so the driver didn't need superhuman strength. Steering wheels were huge so you had more leverage; you had to crank and crank and crank the wheel (the advantage of gearing)... brake pedals had long travel so you could get more advantage from the hydraulics. In a modern vehicle that's designed to have power steering and brakes, all that is gone, and when you lose your assist you better be feeling STRONG. I've had to stop a car that lost its power brakes and it took all my strength- using BOTH FEET- to get a moderate stop.

So yes I like very much that there is a huge capacitor bank to guarantee that the Prius brakes are not going to lose power. And I like that the final fallback to these electronic systems, the parking brake, is still entirely mechanical.