Q&A of Prius Brakes

Well when I looked at it in New Car Features there is a direct connection and that is stated as well. I need to read the chapter again I thought the Stroke Simulator had something to do with the ABS. I will reread it.

 

Getting in deep here,aren't we :lol: I will have to do some digging/research on that. I am not sure how to answer it right now. Let me get to work and do some checking

 

Dan in the stroke simulator you'll see a pitot valve it's not on the seat under nomal braking it allows the fluid to compress the piston at the end of it and what this does is allow the brake pedal stroke sensor to send the signal to allow regen. If you jam on the brakes the pitot valve closes and the fluid pressure builds faster to the wheel cylinder pressure switches which signals the brake control ecu to apply pressure from the accumulator to the system. This is modulated by the control valves which are many, these are in turn controled by the speed sensors on the brake rotors and in turn opened and closed as needed to keep from getting wheel lockup. The vsc system is also tied into the control valves to diagonallly apply pressure to counter yaw in a skid. The fastest way to find out if there are any brakes when the pump or accumulator fail is to find a large empty parking lot pull the fuse for the pump and drive the car and see if you have brakes, if not jump on the emergency brake. It'll have brakes, NTSA wouldn't allow the car to be on a public hiway with out backup braking.

 

the stroke simulator is just there to allow the pedal to depress up to the inch and a half that's required for the pedal sensor to provide the signal to the regen circuit that it needs to slow the car with regen instead of friction brake. When you jump on the pedal the pressure switch closes the pitot circuit to allow full master cylinder volume to activate the accumulator to pressure up the system.

 

Let me understand this. If I place little pressure on the break, as is the usual course the stroke simulator is not active. The regeneration is in full sway. If however the light goes yellow at the last minute, as it usually does in Lacy, and I put more pressure on the peddle the stroke stimulator cuts in the hydraulics to allow me to stop, all be it at a cost? Momentum converted to Evil heat. I hate it when that happens. Besides it means I may need to replace break pads at 80K. Daxx Lacy stop lights.

 

Henry it's just the opposite with slow depression of the brake pedal the stroke simulator works,brake fluid flows thru the valve depressing the piston and spring. This allows the brake pedal to go down and it operates the pedal sensor (connected to the brake pedal) to allow regen. With rapid depression the valve is closed and the piston and spring in the simulator don't compress.

 

I would like to thank all of you for the posts. I think Frank is right on and agree with what you said. It's not something that we have had any problems with so haven't had to do much with it since we went to Prius school when we covered it out of the NCF manual.
Thanks

 

just looking at the Master cylinder, I wonder if the volume displaced by the rear piston till the inlet check valve goes on seat is the same as the stroke simulator volume? The front inlet check valve looks to almost be on seat. It would have to be open slightly to prevent residual pressure in the front system. Also the stroke simulator cut valve would have to be closed when the car isn't Ready or there is a failure in the pump or accumulator.

 

Dan,

I got a 2004 Prius Technical Orientation DVD off of eBay.. I was watching it and you might find this interesting...

They say there is a "Power Source Control Backup Unit" in the 2004+ Prius. To quote
"that provides auxiliary power to the brake system. It contains 28 capacitor cells that store electricity.provided by the vehicles 12 volt system. If the 12 volt power supply voltage drops, this unit provides stabile voltage to the brake system"

On the video it looks like its located just aft of the 12volt battery stored in the rear left of the car. They do warn that this unit may retain a residual charge even if the car is off.

 

It is covered in New Car Features as well. This is a 3 battery car! There was some discussion in the past about the use of capacitor batteries (for the main HV battery) in the past on the Forum their usefulness and relative pluses and minuses, and it turns out we already have one. The more you learn about this car the more interesting it is. How much did you pay for the DVD? Sound interesting something I might want to look at. I am a very visual/aural learner. How did you search for it on E-Bay?

 

It was $35.10 after all the bidding wars. I found it just by doing a generic search of "Prius" . It has the tech stuff for the 2004's and and "archive" for the classic Prius.

 

What I know so far:

The hydraulic system is operating as a front/front/rear split system. Under normal conditions, the brake ECU proportions hydraulic fluid to the 4 brakes using accumulator pressure as a function of pedal travel, vehicle speed, rate of pedal travel, and vehicle deceleration.

If the accumulator were to fail, there is minimal affect to the system except that the pump will run each and every time the system demands hydraulic fluid displacement, such as when the brakes are applied.

If the pump were to fail, which is more likely to happen if the accumulator fails since the pump runs much more often, after the accumulator pressure (if there is any) is depleted, the brake ECU will notice this via sensors and open the solenoids feeding the front brakes and close the solenoid feeding the brake stroke simulator. This allows the master cylinder to directly apply brake fluid via its pistons to the two front brakes. This front brake apply will be without power assist, but meets the FMVSS-135 stopping distance requirements for no power stops. There are both distance and pedal effort requirements to this test. The rear brakes are non-functional as only accumulator pressure feeds the rears.

If you want to cycle through the various single point failures the system might experience, be they solenoid valves, a caliper, brake line or hose, etc., you will quickly see that you will be left with at least 2 hydraulic circuits for brake apply under any of these cases. The 2 rear brakes are considered one hydraulic circuit since accumulator pressure is their common source.

As for the brake stroke simulator, this device works in the same manner as the EV-1, which also had a stroke simulator. This device is nothing more than a passive displacement cylinder. The brake system on the Prius is technically a "wet" brake by wire system. In normal operation, the volume of fluid displaced by the master cylinder is absorbed by the stroke simulator, allowing for a normal feeling brake apply while the brake ECU has the solenoids from these circuits closed. This also allows the travel sensor pair on the brake pedal to provide a "driver intended deceleration" input based on travel and rate of travel. Based on the previously mentioned travel, master cylinder pressure, speed, etc., the brake ECU determines the percentage of regenerative braking versus hydraulic braking to meet the driver desired deceleration and meters hydraulic fluid to the brakes by opening and closing the solenoids fed by accumulator pressure (green in the mech.). It is the brake ECU which controls the flow of hydraulic fluid, thus if the brake stroke simulator were not there, the pedal would essentially not travel until some hydraulic braking were to occur under normal conditions as the fluid is not compressible. This would result in only a pressure input, based on pedal force, for determination of deceleration desired by the driver. It is very difficult to determine if the driver is requesting high braking force using only pressure, as impulse pressure can me misread. Further, the apply rate is used to determine panic brake situations and if no travel were available it would be nearly possible to implement panic brake assist. If a failure occurs, such as the brake ECU, the solenoids blocking master cylinder pressure open, allowing flow to the front brakes, and the stroke simulator is bypassed, allowing all fluid displaced by the master cylinder to go to the front brakes. Unassisted front braking remains, but ABS, EBD, VSC and panic assist are non-functional. Some Trac remains via powertrain control. Thus under "normal" conditions the master cylinder is effectively bypassed, and brake apply is metered by the brake ECU via the HCU based on sensor inputs, hence brake by wire. In a failed condition, the master cylinder becomes active, and is the source of front brake apply pressure, hence the "wet" brake by wire.

Back to Dan's frustration source, the Allante system is not much different than this, other than the fact that there are no blocking solenoids or a stroke simulator. The failure modes thereafter remain the same, and suggestions that no brakes are available are incorrect. The vehicle had to meet FMVSS-135 requirements for no power assist, thus manual (unassisted) braking is available if an accumulator or motor failure (or both) occur. I will concur that the pedal force allowed by FMVSS-135 is high and the stopping distance requirement is long, however these are the current federal requirements for no power stops and there is little variation in the pass margins of the various vehicle manufacturers. In other words, under no power assist conditions, most vehicles are more difficult to stop, but they will stop in a similar distance to each other.

 

<div class='quotetop'>QUOTE(mdacmeis\";p=\"67106)</div>

Great Post, Thank You! I get it. I like that.

 

<div class='quotetop'>QUOTE(hdrygas\";p=\"66626)</div>

Nah, when the light goes yellow at the last minute, that's what the OTHER pedal is for!

 

they also put a steering wheel in the car to use to avoid hitting the other vehicles that are starting into the intersection.