Regenerative braking

<div class='quotetop'>QUOTE(silver-machine @ Nov 18 2007, 11:05 AM) [snapback]541234[/snapback]</div>

I think efficiency is probably the most important issue. Braking is not efficient in any sense so I would be more concerned with how far you can glide and come to a slow stop using as little braking as possible. Braking firmly and quickly means you have been traveling too fast in the first place and thus using too much fuel getting to that "stop location". You could have let of the gas long before the stop, went into stealth mode. then coasted to a stop using regen to slow you down then slight braking.

I'm sure the hypermilers here will correct me if I am wrong but stopping quickly is inefficient in terms of regen and in overall efficiency.

 

Wow, fascinating stuff! I thought all along that the brakes were just typical passenger car hydraulic brakes, and that the regen braking was just passive - the generating effect picking up what it could as the brakes slowed the vehicle.

Boy was I wrong, and this makes so much more sense.

But... are we then driving a "brake-by-wire" car? If the HSD system uses intelligence to control regen braking when we apply the brake pedal, how / what engages the friction brakes when needed? If the friction brake is directly connected to the pedal through a master cylinder, wouldn't the frictions always engage on brake pedal pressure?

Thanks guys, the knowledge base here is incredible!

 

Yes, in normal operation it is brake by wire. When you press the brake pedal, several things happen: The brake pedal moves down, which actuates an electrical sensor (potentiometer), the brake light switches, and also applies pressure to the master cylinder. The potentiometer is used to measure pedal application speed, which is used to detect a panic stop. The master cylinder applies hydraulic pressure to the brake line, which in normal operation is connected to a stroke simulator. The stroke simulator is a spring loaded piston that lets the brake pedal move in a normal fashion with a normal feel, even when it is not connected to the brakes.

Two pressure sensors on the brake lines measure pedal pressure. These feed into the brake ECU along with the potentiometer. The brake ECU takes these inputs and has a little conversation with the inverter ECU to see who should do what. Normally the inverter ECU says "okay, I can take care of that", and applies sufficient regeneration. If it can't, then the brake ECU applies some hydraulic pressure to the brakes. This pressure comes from the accumulator tank which is pressurized by an electrical pump. The electrical pressure pump has a bank of super capacitors that provide electrical backup in the event of a total electrical failure.

In a panic stop, the brake ECU says "to heck with regen" and goes full on with the hydraulics. Likewise, once the car's speed gets down to around seven mph, the hydraulics kick in.

Here is where it gets interesting. You might be asking yourself "What happens if the brake by wire fails?" That's a reasonable question. The brake system contains five solenoid valves that fail to a safe position in the event of a failure. One of the valves removes the stroke simulator from the brake line, two of the valves disconnect the power braking system, and the remaining two connect the dual master cylinder to the two front hydraulic brakes. The short answer is in the event of a complete power braking failure, the system reverts to manual hydraulic brakes for the front wheels. The back wheel brakes are cable operated by the parking brake pedal.

It's an incredible system.

Tom

 

Wow, thanks so much for such a concise, well-written explanation! And yeah, as an ex-theme park designer, I'm always looking at FTA / FMEA and you nailed it for me.

Kudos to the engineers at Toyota for designing a regen system with such "feel", I would have sworn that the pedal was directly connected to a master cylinder and the regen was just along for the ride.

Thanks!