Confused about braking

I'm new to prius also - my v2 is on order ... but it is my understanding - that you may never really "know" ...
It's what my dad used to say -- it's PFM (Pure freakin' magic) ... it's all computer controlled.... and a delicate balance.
initially - when you let off the gas pedal - there is a little regen effect - that simulate engine drag on a normal car ...
as you lightly press the pedal, more and more regen force is added - if you press more, as in a panic stop - then the fritction breaks are applied. regen breaking only has so much brake force - and it's dependent on vehicle speed, battery charge, and amount of pedal travel as input from you on how hard you want to stop.

with that said - i read in a post somewhere on here - that below 7mph i think, it's friction brakes 100% as there is not enough "motion" if you will for the regen 'effect' to work.

hopefully I didn't confuse you more - but, just remember, if you use the brakes lightly - you will likely rarely use your friction brakes... below low speeds, i.e. in parking lots - you will likely only use your friction brakes...
the rest is dependent on too many variables - wheel speed, how hard you are pushing on pedal, battery charge, etc.

 

I think the post you are referring to said (meant) that friction brakes will only get used after 3 bars of regen are shown and you still need more braking force than they can provide. I found the wording a little confusing too.

I found code to make my ScanGuage II show the actual value of friction brakes applied. I was amazed how little the friction brakes are used. I can feel my wife's Gen II switch over to frictions brakes around 7mph, but the Prius v (and Gen III?) does not use friction brakes under normal braking until you are crawling around 2-3mph. I have to double check when the friction brakes come on during normal gradual braking.

I get to see the friction brake value jump up on my ScanGuage II when I hit a moderate bump while braking. My understanding is that the system detects the resulting change in regen power and shuts regen off to protect the systems from undulating (potential harmful) power conditions. You will feel a little surge because you were feeling deceleration, then an abrupt change to constant speed as you lose regen brakes quickly followed by friction braking deceleration. It is not really a surge in speed or acceleration. Your Prius is not actually accelerating, it just feels that way when you go from decelerating to coasting for a fraction of a second.

 

Simple rules (probably all you need for now):

* Friction brakes are used at low speeds (somewhere around 7 mph).
* Friction brakes are used after the regen section of the HSI is full.
* Friction brakes are used when regen brakes are slipping.

What this means, is that in order to maximize regeneration one needs to brake slowly enough so that you never fill the regen section, which means slowing early, and anticipating upcoming traffic patterns.

 

To the OP - welcome to Prius ownership! Hope it's treating you well!
Your question was a good one. Between the manual, this forum, and other tidbits I've picked up, I think I basically understand the friction vs regen. braking question as it's already been answered in this thread.
However, I am confused about another braking issue: What's the difference between letting off the gas pedal (i.e. coasting) to regenerate, vs. putting the car in the "B" gear? I understand it means "engine Braking", and it somehow increases the speed of the gas engine to regenerate power. But what does this mean for the overall charge going to the battery?
So let's say I'm coasting down a hill. If my foot is off the gas, the car is regenerating some of my forward motion to charge the battery. If I step lightly on the brake, I believe it's doing the same thing, the same way. But then when I put the gear selector in "B", the ICE revs up and the car slows down, but what's going on charge-wise? Does this charge the battery differently? Why is this system even available, if stepping on the brake also charges the battery? Is it just to let your brake foot rest? If so, why doesn't the "B" mode engage the same regenerative braking system as the pedal? Which is better or more efficient, if you have the choice?
Thanks,
Andy

 

Ah, the "B" gear or engine braking. In "B" gear all regeneration is turned off and no power is going to the battery. It is automatically engaged when the traction battery show full charge and further braking is detected. The vehicle can not send any more energy to the battery so the wheels are forced to turn the engine (but no gas or spark) and it slows the vehicle down by pushing air through the system. When the driver engages engine braking regen is turned off and the engine turns into an air pump/brake, same as when the vehicle turns it on. UPDATE: All regen is not turned off in "B" gear. I guess it is truely a confusing option, but does the trick to slow you down in a smart way.

So I don't understand why Toyota made this confusing option available when it automatically engages when it is needed.

 

The manual isn't much help on engine braking:

"Use engine braking (shift position B) to maintain a safe speed when driving down a steep hill. Using the brakes continuously may cause the brakes to overheat and lose effectiveness."

 

No, my GenIII liftbacks switch to friction at 7 mph just like the GenII.

As a veteran of many many mountain descents in B mode, I must also say 'No' on this claim. B mode sends some of the energy to the battery, if it isn't yet full, and sends the rest to spin the engine to pump air. When the battery fills up, all the energy goes to the engine, which then begins spinning even faster.

In the lift back, at least, this isn't the same as B mode. The ICE doesn't spin as fast this way as it does when B mode is selected.

 

I went out to test with my SGII and got some video of friction brakes coming on at 4 MPH twice, and one where the friction brakes came on slightly at 9 MPH and then more at 3 MPH. This might just be a Prius v tweek.

I stand corrected fuzzy1. In "B" regen is not turned off. I was seeing regen power going back into the batteries with "B" selected. I must get turned off when the battery shows full (and you are still braking), since there is no place to put the energy.

I thought (incorrectly) that the car would select "B" for you when you were going down a big hill and your battery was full on the display. Here is what I was trying to remember: B Mode Questions | PriusChat
I see you were in this thread fuzzy1, doing you best to educate us PriusChatters. Thank you again. I trust your answer from the thread;

 

...and
* Friction brakes are used in a panic stop.

Tom

 

Great thread, the helpful answers continue. Thanks for the clarification!
Andy

 

Try not to brake. It always wastes energy, even with regeneration. If you must brake, try to stay with regeneration. Your 3 bar strategy sounds good, but I don't have a Gen III to verify. Finally, safety first: if you really need the brakes, just stomp on them and forget about how any of it works.

Tom

 

No. This is wrong. It uses the gas engine (without any gas) as an air pump, to waste energy. The harder you hit the brakes, the faster it spins and the more energy you waste.

Why would you ever want to waste energy? Long downhills.

You can fill the HV Battery in a 600 foot drop, so all the vertical descent after that would be by friction of pads on the brake discs. Pads are cheap, so we would not ever use B, except that if the pads get hot enough, the brake fluid boils and you have no brakes. This is bad. Using B mode, the driver can add engine braking on hills he knows are long enough to cause trouble.

I drive this hill about twice a year, it is a 7 mile, 6% grade that loses 2000 feet. The speed limit on the freeway for trucks going downhill is 17 MPH. There are two runaway truck lanes, it is spectacular to see them use one.
http://www.oregon.gov/ODOT/MCT/docs/EmigrantHill.pdf?ga=t
While this pamphlet is aimed at truckers, this hill is an example of where it would be prudent to use B.

(As an aside, the Plug-in Prius can recharge it's HV Battery for 3500 vertical feet, so B mode would not be needed for a PiP)

 

I rented a liftback in California last year and took it over the mountains from Sunnyvale to Santa Cruz on Route 17. This drops 1800 feet from the summit to Santa Cruz in about six miles, and I found myself using B fairly often.

 

This may be a silly question, but I've not worried about seriously damaging my brake pads or "boiling" my brake fluid in any other car going downhill from a mountain. Is that a dangerous mistake?

 

Rew You don't list a location so it is hard to answer. Lets say there are more long steep descents in Colorado than in Mississippi so the answer is maybe.

Heavier loads mean a hill you could do empty may cause fade when fully loaded. Trucks are the most at risk, they have large loads and odd brakes. (air brakes and jake brakes)

downhill braking

The first two paragraphs of that would be true of cars as well, but after that it gets into techniques for air brakes with a trailer.

Here is a SUV example (in Colorado) Brakes get soft then smoke on long downhill. Should I be worried? - YotaTech Forums

 

On the Prius V, place gear selector on "B" -- the engine braking option

 

Thanks JimboPalmer and Air_Boss. I grew up in FL (not many hills) and live in NY now. It was more of a general questions. I've driven many mountain passes throughout NH and ME in other cars and hadn't concerned myself with boiling brake fluid or destroying the brake pads. These aren't huge decents compared to CO, but I wasn't sure how much I was damaging my car. I understand about using the "B" gear in the Prius, but this was a general question. Thanks!