Braking by wire?

yes, the system is electronic, with a failsafe hydraulic backup.

the smoother transition is toyotas refining of the system over 5 generations

 

no, i'm just assuming it is a refinement. i haven't seen anyone who have owned previous gen's mention anything drastically different in feel or performance.
i know it started at least in gen 2 if not 1. it has been completely documented here, and of course, toyotas 16 billion dollar 'runaway acceleration' debacle5738a3c4-af69-11e3-9627-c65021d6d572_story.html
in which stepping on the brakes didn't slow the car.
and also discussions of how the brake pads work in conjunction with regen, and what happens when 'brake by wire' fails.

 

As far back as Gen 1, there has been a pedal that pushes brake fluid into a spring-loaded "stroke simulator" that feels like applying brakes. The pressure developed in that fluid is measured as an input to the brake ECU to indicate the driver's desired amount of braking.

The brake ECU negotiates that number with the hybrid vehicle control ECU (called the power management control ECU in some generations) to find out how much regenerative capacity is available, and whatever amount remains is achieved by controlling the valves in the brake actuator to apply pressurized fluid to the brakes.

If the ECU loses power, or loses confidence in itself, its electrical outputs to certain valves in the actuator drop, and those valves return to a fail-safe position where the fluid pressure produced by the pedal is routed to (some of) the brakes.

There have been changes over the generations: Gen 1, Gen 3, and Gen 4 have a 'booster' at the rear of the master cylinder where the system's pressurized fluid is used to assist the driver's pedal pressure (the way a vacuum booster would in a more familiar car). In those generations, the fail-safe braking is still assisted. Gen 2, by contrast, does not have such a booster (though the pump in Gen 2 is still called the "booster pump"); if a Gen 2 fully loses electrical power, its fail-safe braking is unassisted. To compensate for that, Gen 2 had a backup electrical power supply for the brakes.

You'll notice I haven't said anything directly about Gen 5 here. What usually happens around here after a new generation comes out is that someone will eventually be interested enough to download the diagrams and details from the new generation's New Car Features manual, and then we can pore over them and see what was changed from the generations before. For example, this thread shows the way that went when Gen 4 came out:

https://priuschat.com/threads/prius-4th-gen-brake-diagram.208330/

(Regrettably, the image attachments in that thread didn't all survive the recent PriusChat upgrade, which kind of hurts in a thread that's all about comparing details in diagrams.)

As a good thread like that generally starts with someone being interested enough to go get the new diagrams and details from the New Car Features manual, if there isn't such a thread here already, it may just mean that no one has been that interested yet.

 

There are 21 sub-sections of the braking section of the NCF for the gen5. If you can narrow things down a bit, I'd be more than happy to post some.

If I'm reading it correctly(no guarantee), the brake pedal is still connected physically to the master cylinder.

 

It kind of looks a lot like Gen 4, but if my eyes don't deceive me, they've gone completely without an accumulator for the first time in Prius history. Which must mean they're counting on the electric pump to instantly serve up your boost pressure just-in-time every time.

I suppose they were tired of accumulator leaks. Strikes me as a rather gutsy move though....

 

Accumulators, pumps, and combinations of each have been around longer than hybrids for diesels and forced air intake engines. May not be as gutsy as it appears.

 

I gotta admit, I kinda like having my accumulator, knowing it holds around 20 or 30 strokes worth of brake assist pressure, with the pump running occasionally to charge it up. If the pump flakes out, I'll get lights and a beep well in advance before it's hard to stop.

It feels like I'd have to be more gutsy to want to count on the pump for the actual braking in every instance.

 

This is a very interesting document thank you for sharing.

On pages 27/28 "Control during front circuit malfunction" The description on page 27 and the associated diagram depict only the rear brakes being used. I wonder why this is? If solenoid #15 fails in the open position the pressure from the booster in the master cylinder should actuate the brakes. If it fails in the closed position then pump #25 would be able to pressurize and operate the front brakes.

Also interesting (to me) is that the main pump and motor are directly connected to the rear brake actuator assembly but go through a valve (SLM1) before reaching the front brake actuators. During a brake actuator failure event I guess the rear brakes will have to lock up as only front brake pressure can be controlled via SLM1.

 

Out of curiosity what is the purpose of the connection between part #8 the stroke simulator cut valve and the master cylinder?

Also, what is the ball/check valve looking item within the master cylinder?

 

Have fun digging through those sections to find the answer to the question below .

It's not going to be easy to find the section that explains how everything in the system works together.
Some of the assumptions about the back wheel brakes sound premature (to me anyways).
Try an realize there are many more functions in the brake system than most of us can fully comprehend even with ALL the docs toyota publishes about them. Like the 3 levels of regen braking new in the gen 5. In full electric mode regen braking has a large set of thresholds for switching to engine on, engine braking plus the normal regen braking the goes with that mode (engine braking + regen). Couple that with three different levels of driver selectable regen and each one of those levels can have it's own preprogrammed thresholds that work in tandem with other systems thresholds for engine on system calls.

 

Hmm.. I wonder if the car automatically switches to engine braking once the battery is full outside of "B" mode. (/me ducks and hides )

 

If it's like gen 3 non-PHEV, it doesn't 'switch', it's a gradual transition starting around 77.2% battery charge, ending up all engine braking at 80% charge. (Charge % as displayed on an OBD-II scan tool, not as shown on the dash.)

A Prime, of course, has a lot more battery capacity to fill, which can complicate demonstrating this; have to make sure it's close enough to full to begin with.

 

why wonder, in the Primes full EV mode, it's obvious when the engine switches on and the ECU's change the combination a engine brake and regen. That combination is never under driver control anyway, even if the driver thinks they fooled the cars ECUs. when in reality the driver has only stayed within the parameters of the selected mode, per the other conditions the cars senor array detects at that specific time.
The stuff isn't static even if the driver thinks so because the obvious observable fact that the engine fired up at the same time during two or more different driving sessions. It's a fine line and I challenge anyone to nail down exactly what the ECU's are doiing for All the different combinations of sensor thresholds the Prii can get exposed to, locally or in the wild. (there are hints about this in the owners manual when the writers of the manual list conditions that will change a selected mode, most seem to have not understood the intent or the writers not listing every set of conditions and \specific thresholds the Prii "be heir to"). "read Hamlets soliloquy"

There are workarounds and driver hacks that can Usually keep the Prii from changing out of a driver selected mode (many times), but many owners don't have the skills or ;patience to figure out the more frustrating situations just to be faced with another set of thresholds that they weren't expecting within seconds or minutes from experiencing the first set of the ECUs override of the drivers selected mode.
That's when you'll read in a post a member saying "just drive the car or complaining about one or a set of estimate(s) the car gauge(s) display.

 

Sometimes, too, the way Toyota conceived of a given 'mode' might not be the way a particular driver assumes or wants it to work, and so the car may stay in one Toyota-designed 'mode' just fine, but then do something a particular driver didn't want or assume would happen in that 'mode', so the driver then concludes "i overrode my selected mode".

 

* it