Drive "By Wire" ???

The first portion of braking on the 2004-2009 Prius is "brake-by-wire" so to speak, but if you push the pedal down harder it engages the hydraulic braking which is still mechanical. The first portion being of course primarily regenerative braking which is handled by the electric motors.

Slamming on the brakes hard will engage the mechanical hydraulic brakes that have a redundant power system to keep them running momentarily even if your 12v battery dies. There's also the parking brake too if all else fails.

As for accelerating, the throttle pedal is completely drive-by-wire. This isn't a bad thing. Since it is electronically controlled, there are several sensors that all have to line up perfectly before the main computer will accept the throttle position. This allows for greater throttle accuracy and a higher degree of redundancy than a typical older car that was directly connected to the pedal.

No, there is no inherent problem with the drive-by-wire concept, the issue is when there is a mechanical problem that interferes and legitimately holds down the pedal. Not much you can do about that no matter how the throttle is connected.

The Prius is a well designed (and safe) car--it's just a shame that something as silly as the 2010 braking 'problem' somehow got through testing procedures...It should be a relatively simple thing to correct though.

Andrew

 

There is a parking brake, so, if all else fails, there is a physical link to braking.

 

To clarify, the accumulator for the brakes is for the power brakes booster. Even on a total unpowered Prius, the hydraulic brakes will still work, it will just take more force without the power boost.

Most cars these days are throttle by wire, and even when there was a cable to the throttle body, the cars relied on a throttle position sensor to control the rest of the engine.

 

The Prius is one of the few cars that I can think of that the term FADEC applies to.

[ame=http://en.wikipedia.org/wiki/FADEC]FADEC - Wikipedia, the free encyclopedia[/ame] (Full Authority Digital Engine Control)

While this is an aviation term, the Prius ICE is under the full control of a computer. While this seems normal in today's cars, most cars the engine is started and stopped by human control. In the Prius the engine is started and stopped entirely by computer control. No inputs to the engine are controlled directly.

 

and this is not a bad thing!!

 

No, it is a very good thing. The engine is very well taken care of!

 

I think some are asking Toyota to add one thing to the programming. When the brake is depressed, the throttle should be released. I'm not sure if there is any situation (other than force charging) that this would interfere with. Perhaps they could also add speed monitoring, to only release the throttle if the vehicle is moving thus allowing force charging.

 

This is not correct. In normal operation, Prius braking is entirely by wire, whether regenerative or hydraulic. Pressing harder does not mechanically operate the brakes. In normal operation, the brake pedal operates the master cylinder to push hydraulic fluid, but all the hydraulic fluid does is operate a "stroke simulator". This makes the brakes feel normal, but does not mechanically operate the brakes. A pressure sensor on the hydraulic line and a potentiometer on the brake pedal send control inputs to the brake ECU, which is the "brake by wire" controller.

In a failure situation, the stroke simulator is removed from the hydraulic system, and then the master cylinder mechanically operates the brakes.

Tom

 

It's not bad. The thing he thinks is a sensor when he's taking the stroke simulator apart isn't a sensor, it's the stroke simulator cut solenoid. When the system goes fail-safe, in addition to opening those two valves in the actuator so your pedal pressure can get to the front calipers, it also closes that valve in the stroke simulator, so your effort isn't being wasted there. (That solenoid was added in Gen 2. In Gen 1, in the fail-safe mode, you would be applying the front calipers, but also still squeezing the stroke simulator, so the pedal would go way down.)

He also mistakenly says there's an ECU in the actuator assembly, but he catches that later, when it's all sawn open and there's no ECU there. He would be right about Gen 3, where the ECU is in the actuator.

 

As Chapman suggests, you are brake by wire all the time unless the fail safe solenoids open properly allowing limited pedal driven hydraulic pressure to reach the calipers. My understanding is the solenoids are normally open but held closed for normal operation.

Of my three Toyota hybrids, only the gen3 brake by wire system failed. No collision because of significantly reduced braking but some are not so lucky. I would replace the system when the brake pump starts running every 30 seconds at a stop light.

 

The Gen 2 is the most purely brake-by-wire of all the generations. The other generations all have a booster—a small hydraulic chamber at the rear of the master cylinder piston, helping you push—and in fail-safe mode you can still have a boosted braking action, for as long as the pressure holds up.

In Gen 2, there is no booster (though they still call the pump the "booster pump"), and in ops normal your pedal pressure into the stroke simulator is completely isolated from anything else, and the measured pressure there is just an input to the ECU, which does all of the braking by applying system pressure through the actuator. That's why Gen 2 is also the only generation with a special capacitor back-up power supply to keep the brake electronics powered.

The valves involved in fail-safe have their normal (unpowered) positions chosen so that they really are fail-safe when the power is cut. That means the two "master cylinder isolation valves" you see in his diagram are normally open, and the system closes them in normal operation, and the one valve in the stroke simulator (that he thought was a sensor) is normally closed, and the system opens it for normal operation.

I'm not sure where he got that diagram; it's not quite the one from the New Car Features manual. In the diagram in the manual, you can tell which valves are normally open or normally closed, if you look closely at how the pintles and springs are drawn. In his diagram, not so much.