"brake by wire"?

Isn't all Power Split Device-equipped hybrids "brake by wire?"

A computer onboard the Prius decides when to use regen braking and when to use disc braking, after all.

 

You still have unboosted hydraulic brakes in the event of a complete power failure

 

Addition of:
1) Brake Assist
2) Electronic Brake Distribution with Cornering Brake Control
3) Vehicle Stability Control

 

The Gen 1 brake system is fundamentally a hydraulic system. The brake force to the front wheels is modulated to allow for regeneration, but basically the force at the wheel cylinder is directly proportional to the brake force from your foot.

Gen 2 and later completely separate sensing brake pedal pressure from applying braking force. This allows the car to control wheel cylinder pressure completely independently and to only use friction brakes where regeneration won't deliver enough brake force, or won't deliver it quickly enough. As a failsafe, there are valves allowing hydraulic fluid to flow between the master cylinder and wheel cylinders, but in normal, electronically-controlled operation, these valves are closed off and the master cylinder is only used to sense the driver's braking pressure.

Splitting sensing from application allows the car to do ABS with no judder, allows it to brake a slipping wheel for traction control (Gen 3 and later), supports Vehicle Stability Control (braking an inside or outside wheel to keep the car turning), Hill Start Assist (holding the brakes for a period after releasing the brake pedal). It also maximizes regeneration - if regen braking is possible, the car only uses enough friction braking to keep the car balanced. 'Brake Assist' basically means applying the brakes harder than the driver asked for, if they were applied suddenly - in emergencies, drivers often brake quickly enough, but not hard enough.

The Gen 3 system is much the same as the Gen 2 version, but the hydraulic pressure booster is fitted before the master cylinder, so in failsafe mode, you have boosted hydraulic brakes controlling all four wheels, not just unboosted controlling just the front wheels.

I would hope that the Prius c has the Gen 3 brake control system, and I'd be very surprised if it didn't. However, I haven't seen the 'New Car Features' documents for the c, which would confirm it. This document is available on TIS, which costs $15 for two days' access.

 

The Gen 3 system is much the same as the Gen 2 version, but the hydraulic pressure booster is fitted before the master cylinder, so in failsafe mode, you have boosted hydraulic brakes controlling all four wheels, not just unboosted controlling just the front wheels.

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Where does the power come from for the "boosted hydraulic brakes"? Is it electrical or some sort of vacume preasure source?


John

 

No vacuum system. 12V drives a pressure pump.

 

good info, thanks.