When Braking, how much is "Brake" how much is Regen?

Watch the regenerative braking bargraph on the display. When it goes to full, then further force on the brake pedal will be brake pad application/engagement. As long as the regenerative braking bargraph isn't up to the max on the display, then it is all regenerative braking until you come to a stop, then it will be the brake pads that keep your car stopped.

 

That's logical, but are you sure?

The reason I think this isn't quite the whole story is that I felt the brakes "grab" from being wet when the blue bar graph wasn't close to the bottom end-stop, but when the car got below about 8 mph. My 2004 engaged the friction brakes at 7 mph so this might be that reason (slow speed), but I'd like verification from someone with a vehicle monitor.

 

Well, the reason you will sometimes feel the pads engage when you are still at less than full regen. bar is because the pads are brought into touch with the disks, to "get ready for braking", or put another way, "to eliminate or reduce as much as possible" the perceived delay when the system switches from regen. to friction braking. With the pads that close, a slightly warped disk, water on the disk, or other will cause slight friction braking. And yes, regen. braking is combined with friction braking below about 7 MPH, according to Toyota. I do see regen. occuring right to 0 speed, just not as effectively as when moving faster, and in combination with friction braking.

 

A good way to tell is to switch it into neutral and feel how much easier it rolls. When it's in [N]eutral it's all physical brakes, from what I've read. Then when you switch it back to [D]rive feel how much it catches. Also what they said about when the battery is completely full, the regen turns off and it goes straight to physical braking(which can feel really uncomfortable if you're braking down a hill and suddenly the brakes feel like they're giving and you have to brake more)

 

All brakes have the pads in contact with the brake disk at all times, just without pressure. They float. To bring them into better contact requires pressure which results in brake force.

 

Actually no, the seals are designed to pull the pads back. Not much, but so they don't touch. Otherwise the disks would be running warm all the time. This is one of the "final tests" mechanics are supposed to perform after a brake job. Spin the wheel and see/hear no drag. At least that's what I was taught.

 

The pads are "floating" as I said, which usually keeps them to within a few thousanths of an inch (down to zero) of the disk. They can touch without significant contact force.

Regardless, this "grabbing" I felt was perhaps three times the force of the regeneration, which is typical of wet brakes (they slip like crazy, then grab once the water is boiled away). That wouldn't happen without substantial brake pressure.

 

At some small MPH, Motor/Generator 2 is not turning fast enough to regen. I was taught 7 MPH, but 8 MPH is just as likely.

 

Why pump-and-release at all in a modern passenger car? The total brake heating is the same either way. When the brakes are properly balanced, all four should share the heat evenly (at least side-to-side, maybe some adjustment front-to-back).

The only good reason I understand for pump-and-release is so that on vehicles with mismatched brakes, the lighter touch units don't take a disproportionate amount of the heat, causing premature failure. This is an inherent problem on detachable combination vehicles, such as commercial tractor-trailers and any other towed rig, and also somewhat common on older generation smaller vehicles.

But modern passenger cars, with correctly functioning brakes, should not be facing this issue. Then add in EBD (electronic brakeforce distribution) as an additional layer of protection. Then add regeneration braking on top of this, to almost completely take the heat off the friction brakes in the light braking cases where this uneven heating can became an issue.

My spouse habitually uses pump-and-release, ingrained from a long ago boyfriend's van where it may really have been needed. But she applies it in a way the partially defeats regeneration, as her pump phase significantly exceeds the regen limits and forces the friction pads. While this is a wash on the household's non-hybrids, lighter continuous braking could harvest several times more energy in my Prius.

But husbandly nagging was completely tuned out decades ago. The teaching attempts will never sink in until someone else presents them.

 

Regen and the rear brakes?

Braking needs to be balanced with four wheels. When the brakes are lightly to moderately applied, the fronts can switch the motor-generator to regeneration...OK. What about the rears? I haven't looked under the car yet. Are the rears pure friction? If that's the case, then switching to B mode for a downhill is pure regeneration. Braking is, at best, part regen and part friction. Or, (no surprise) am I missing something?

 

Maybe the rears brakes see more use than the front? I wouldn't doubt they're applied at least lightly with each brake application.

 

On past Prius generations, regenerative braking was with just the front wheels. If any friction braking was present, it was minimal and used only for smoother control and better user feel, and to help make the switchover to harder braking (stronger than available with regen alone) less disruptive.

 

The problem with your statement is that your premise (above) is wrong. Braking doesn't need to be balanced unless you are near the limits of adhesion.

 

About 7 MPH for Gen-2 and 3. Gen-4 will stay in regen down to about 2 MPH.

 

Where can I find the regenerative braking bargraph? I looked through the manual and all I could find was Regenerated amount of energy under the battery display on page 258.

Thanks!

 

It's called the "Charge area" in the manual.

 

The limits of adhesion vary with the condition of the road surface. A bit of spilled oil, or oil lifted off the road surface at the first rain, or frost, or mud put down by trucks coming out of a construction site greatly reduce traction. We need braking balanced on all four wheels.