Braking while car on jack stands

Not sure what you mean by "conventional." If you mean by hydraulic pressure, that's the only kind of braking there is on the rear wheels unless you use the parking brake.

The accumulator pump is just how the Prius develops the hydraulic pressure rather than using a 100-450 hp engine like most cars use for hydraulic pressure.

 

Well, and in most cars nothing moves the hydraulic fluid but pushing the pedal, and the vacuum created when the engine is running only serves to let atmospheric pressure boost your foot's effort on the pedal.

In the Gen 2 Prius, there is no such pedal-effort boosting. It's a plain unboosted master cylinder but you are normally only pushing brake fluid into a stroke simulator. The skid ECU measures the pressure of that, and electrically opens valves to send pre-pressurized brake fluid to the wheels. That pre-pressurized fluid is available because of the accumulator and pump.

Trivia bit: Gen 2 is the only Prius generation that works quite like that. In Gen 1 and again Gen 3 and later, there is a boosted master cylinder. Instead of atmospheric pressure, it taps the brake fluid under accumulator pressure to give the boost. So in those generations, the electrical power can fail and you still have not only braking but assisted braking, until you've used up the accumulator pressure. In Gen 2, losing electric power to the skid ECU would mean you lose braking assistance, and so Gen 2 (and only Gen 2) has a big box of capacitors in back next to the 12 volt battery, just to provide reserve electric power for the skid ECU.

 

Edit: at the time this post was composed, post #5 ended with a question about what the capacitor box is for, which is why the start of this post is phrased as a reply to a question about the capacitor box. Post #5 was later edited to remove the question this post was responding to.

No, the capacitor box only provides reserve power to the ECU. The 12 volt power system has to be available for the pressure pump to run and replenish accumulator pressure. With the pump out of the picture, you have power assist only as long as the stored accumulator pressure lasts, perhaps 20 or 30 uses of the brake.

(That's considerably better than conventional cars using engine vacuum for assist, where you typically run out of assist with two or three uses of the brake if the engine has stopped.)

Once you have used up the accumulator pressure in a Gen 2, you have only your pedal effort, unassisted, and working only the front brakes.

 

To be a fail-safe system, something needs to go to the 'safe' condition when it fails; it can't depend on power from the capacitors to actively put the solenoids in the "fail-safe" position.

An actuator hydraulic diagram indicates, for each solenoid valve, which side of the plunger the spring is on, thus showing whether each valve is normally open or normally closed. You can see that when electrical power fails, the SCSS valve springs closed (so no pedal effort is wasted working the stroke simulator), while SMC 1 and SMC 2 spring open, so that unboosted fluid pressure from the two master cylinder chambers will be directly applied to the two front brakes. (That's also why SLRFL and SLRFR are sprung normally closed, even though their rear-brake counterparts go normally open.)



That's the final fail-safe operation for Gen 2: when electrical power fails, those solenoids spring to their de-energized positions, and you have braking (a) to the front wheels only and (b) with zero power assist.

The capacitors are there to keep you out of that fail-safe mode as long as possible. As long as there is capacitor power, the ECU remains in control, those solenoids remain in their normal-operation positions, and for as long as there is accumulator pressure, the ECU and actuator can use it to operate all four brakes in the normal way.

The one thing it can't do on capacitor power is repressurize the accumulator. The capacitors don't power the pump. So once you have used up the 20 or 30 strokes worth of fluid the accumulator can hold, the system has no more choice but the unpowered fail-safe mode.

 

IF the rear brakes operate at all, then there is accumulator pressure available and power to the skid ECU- in that case the front brakes would operate at the same level of "normal" functionality.

The "fail safe" mode of manual braking (unboosted master cylinder pressure direct to the front brakes only) results when there is complete loss of power to the skid ECU or loss of accumulator pressure. This is the default mode when the car is OFF and the brake pedal is released. You can hook up a pressure tank to the master cylinder and bleed that part on the system- M/C to front calipers.

When you press the brake pedal with the car OFF, the skid ecu responds to a brake switch input and powers on enough to provide normal function at all four wheels. IDK if ABS will function in this case.

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I don't know.

I seriously set myself up for some jokes there, didn't I.

 

THIRD BASE.

 

Bit of a classic routine, for which many videos exist.

 

Ya know?

maybe you did

Posted via the PriusChat mobile app.

 

I understand even less because I'm American.

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As for the guy on first, here's a picture of him:


These guys talk fast, @MathWeijzen, but if you can follow it, they might just be the funniest people in history.