Regenerative Braking - D or B ?

Hill start in my 2010 definitely uses the hydraulic brakes.

 

I never looked into it in our car, not sure what does the holds other than not the emergency brake, because that's a classic manual rip-stick on the center console.

 

It's often possible to hear the clicky/squishy noises from the brake actuator.

You can also distinctly hear mine grab the brakes every time it's about to start or stop the engine while sitting parked, to minimize the body lurching. Clever trick. Surprises me how many people say they haven't noticed it; when you've paid a little attention to what, say, the 90-seconds-after-poweroff brake self-test sounds like, nothing else really sounds like it.

 

I haven't give that any critical listening attention, maybe I'll get a chance someday. (I'm on the road in a rented 2022 Camry this week)

Can a scangauge read anything useful on the topic? I've only briefly hooked ours up to the car, never bothered to program any prius-specific pids on the thing.

 

In this post @MTN describes how to use dynamic cruise control to Hold the Prime.
it's just a workaround for the Hold Button, put it might be a workaround someone likes to use.
Brake auto-hold function | PriusChat

 

Do you get pre-start "clicky/squishy noises" if you press the accelerator to wake up the engine promptly after pressing the POWER button (as opposed to waiting through the ~12-second delay before the engine otherwise decides to start)?

Maybe it makes a difference whether the brake pedal is applied when the engine starts.

The post-shutdown "clicky/squishy noises" are a lot more obvious.

 

I was describing my Gen 2, which actually, checking the manual, doesn't claim to have "hill start" at all. Possibly because it doesn't use the brakes.

But it does at least use the motor and resists moving backwards - sufficient to be a "hill start" at least on the sort of hills I meet, but maybe not enough to be declared a "hill start" feature...

Odd that they didn't do brake hold in the Gen 3 and 4 then, if they were already using the brakes. Maybe lack of electric parking brake back-up was the issue.

 

I think they were reluctant to make the car do things the driver won't expect.

Remember the first version of hill start in Gen 3 requires you to do something special on the brake pedal, get a beep, and see a blinky light. You wouldn't just find your brakes stuck on without thinking about it.

That got sort of accepted and people got used to it and then the later generations do it without you having to do the special dance, but still only on hills.

Then when they figured more people were used to that, they went ahead and made it hold on the flats.

 

I think you can see some brake fluid pressures and some solenoid valve status if you've got the right PIDs.

 

Well, that's never been when I've noticed it. If I press the accelerator right away, the car starts to go right away (on electric power), so of course it won't apply brakes to start the engine.

And if I'm stopped with my foot on the brake, the brakes are already applied, the car doesn't have to do anything special.

The times I notice the "apply brakes to restrain lurch" trick are the times you'd expect, when I'm just sitting in Park, not holding the brake down, and the car decides to cycle the engine on or off.

I can pretend to be psychic by announcing the engine start half a second in advance, when I hear the brake squish. (But I can't fool anybody who also heard the brake squish.)

 

I just used Hill Start this morning to refresh my memory of how it works in the Prime like explained by @fuzzy1 on pg 2 of this thread here Regenerative Braking - D or B ? | Page 2 | PriusChat
After stopping on a hill I took my foot off the brake and the Prime did not move for a few seconds. After a few (5 max) seconds it started to roll slowly backwards. Very slowing in my estimation. But my foot was already on the Go Pedal by than and I was able to stop the backwards roll by giving the Go Pedal some love.

 

Works well only if there is a vehicle in front of you.

 

So does this mean to hold the brake on a hill you need to press the "hold" button in the right of the picture? And then you get a full hold until you press the accelerator? And without pressing the "hold" button you still get the 2 second hold? Is that the way it works?
Also, I thought the easy way to think about the B drive mode for braking was analogous to downshifting in a traditional car, so you would only use it in a long downhill where you want the engine to brake. Is that the way to think about it, or is it more complicated in terms of the execution, when to use B mode?

 

You got it. Primary reason is to prevent runaway descent. The second layer is that once you tell the computer you need more drag by selecting B, it can make smarter choices in the real-time balance between engine braking and battery charging.

 

On B or D mode, like most people I took driver's ed and was told to put the car in a lower gear if you were going down a steep hill. There were what I considered steep hills all around and nobody ever did that. I didn't do it and the drivers ed instructor didn't tell me to.

Then in college, I went to Skinner State Park, and I drove up Mount Holyoke, which has inclines far higher than anything I'd ever seen and even in that very minor peak of less than a thousand feet, once we started down it became immediately apparently what they were talking about. That was the time to downshift and use the engine to slow you down and that's the time to use B mode.

Other than that, you should be using D exclusively and when you coast or use the brake pedal gently will initially use regenerative rather than physical braking--there's a meter on the speedo in the 2023 model. I've stomped a few times on the brakes when idiots pulled out in front of me, but in day to day driving, if the speedo is accurate, I almost never use physical braking.

 

I grew up in the San Fernando Valley in Los Angeles. The roads crossing the Santa Monica Mountains to the coastal basin tend to have gradual inclines on the south side and steep ones on the north side. In those times (late 1960s), automatic transmissions had 3 positions for forward driving, normal, middle, and low. I learned to use the middle for sections where I didn't want to go excessively slow but would need to constantly brake to stay in control. That middle drive is equivalent to the 'B' setting in the Prius, and I still follow that rule: if I need to control the car by constant braking, switch to 'B' and brake only occasionally.

In non-hybrids, the engine braking function provided by the mid-Drive setting was useful to keep the brakes from overheating by using engine resistance. In a hybrid, the effects of the 'B' setting and constantly using the brakes are the same (at least until the battery is full). However, using it does make driving easier since you don't need to constantly modulate the brakes. I now barely use 'B', usually only when driving down longer steep grades (more than 6%) on low speed roads in the nearby hills. This usually means I use it only a few times a year.

 

Side topic...

I tried using my B this morning and it felt like it wasn't doing anything. I double checked my Regen breaking settings and it was only on one instead of three so I changed it, but it still felt like it wasn't doing anything. I used it yesterday and worked fine. Any ideas what could cause that to happen? Should I take it in to have it checked out, or is there another setting somewhere I need to set that night have gotten reset somehow. (BTW I have no idea how it got changed to 1 from 3... It's been on 3 forever and I certainly didn't change it.)

 

I used B yesterday on a long (several mile) downhill. I found that in B unless it was REALLY steep it did too much braking, slowed the car too much, so I alternated between B and D rather than braking. I doubt I will use B much, and when I do it will just be on steep long downhills.