Regenerative brakes versus friction (regular) brakes

[impreza]

It seems to me that one of the best ways to save energy is to NEVER use of the friction brakes, if safely able to.

I figure that if you can use the brakes in such a way that ALL of your forward enertia gets converted into electricity to recharge the batteries, and NONE of that inertia is wasted in heating the brake discs, then I've braked successfully. I've succeeded in minimizing waste since the energy that heats the brake discs/drums is just wasted energy.

And I've found that if I anticipate the stop soon enough, I rarely have to press all the way down except at the last second to get the car to totally stop. However, I really am not sure when I'm maximizing use of the regeneration and NOT using the metal to metal friction brakes.

I've found that pressing lightly on the brake does cause some drag and I'm sure that that is the regen brakes. I can increase that pressure and I then feel a bit more drag and more brake pressure/more drag. However, I'm not sure when I reach that point when that drag is no longer from the regen brakes and is instead from the friction brakes.

Is there a way to know for sure? Can I teach my foot to FEEL the difference? Maybe there is a sound that I could try to seek out? Maybe there is a point where the drag gets more abruptly different. Too bad there isn't some kind of light on the dashboard.

Can anyone tell me (reliably) how to tell when I'm using the regen only and not yet down to the friction brakes?

Thanks.

[efusco]

At anything less than 8mph you're using friction brakes.

Other than that, no, I can't tell you how to reliably tell. I will tell you that the graphs I've seen suggest that it requires very firm/hard braking to engage the friction brakes.

Coastal Dave has announced that he will be releasing an engine/brake indicator for the '04 Prius. This will allow you to see when the ICE is running and when you are using your friction brakes. Should be a very useful tool for all of us and I'm eagerly anticipating it's release.

[Jonathan]

It dawned on me today that anytime we're in regen braking only, we are only being slowed down by the front wheels, not all 4 as when traditional brakes are applied.

[m4prius]

Too bad Toyota does not show Regen vs Friction brake iindication on the graphic display. Would be a nice feature.

[rflagg]

This is one feature I'm very excited about Dave offering at CoastalETech.com. It's one of only a handful of things I think that should be included with the car that weren't. Hopefully Dave's mod is affordable and easy to install!

-m.

[impreza]

Originally posted by Jonathan
It dawned on me today that anytime we're in regen braking only, we are only being slowed down by the front wheels, not all 4 as when traditional brakes are applied.

This is not really a big deal. In any braking, the overwhelming majority of the braking power is in the front wheels 24/7 anyway. That's why whenever a car is sold with "disc brakes", but not 4 wheel disc brakes, the discs are always on the front. The back would remain drum brakes because they simply do less work. It's just physics.

The only problem might be if you are on ice and you use the front wheels only and they start slipping due to lack of traction.

But the Prius has antilock brakes. So if the computer senses the front wheels slipping, it will use the back (friction) brakes.



>>> I've made an assumption here that the anti-lock properties of the brakes are also in play for the regen brakes. If they are not, then this is indeed a flaw drivers need to be aware of. If someone is stopping slowly and only using the regen brakes and they start to slide, they might be inclined to let up on the brakes. But what they SHOULD do is press down harder so that the antilock system gets engaged. That would stop the sliding. I want to thank Jonathan for making me think of this.

[xlarimer]

Dumb question when reading this thread:

Does this mean that we won't have to replace our brakes as much if we use regen more than friction?

First, my service tech says that I don't have to replace the oil as much (since it runs less), next I learn that there is no steering fluid to deal with.

This is working out to be one of the BEST car ever!

[mboileau]

I pulled out my hardcopy of the Toyota THS II Technical Spec. This document is worth its weight in gold. I got a copy from my boss who I believe purchased and downloaded it from the Toyota technical site. I'd really like to see this document in the Files section...

Anyways, the spec shows two torque curves side by side, comparing the THS I system (in the 01-03 Prii) to the new THS II system. In THS I, there was always some hydraulic braking coming into play to supplement the regen braking. A slight tap on the brake pedal would first engage the hydraulic brakes, and then use regen braking upon further pedal depression. In THS II, the regen braking power range has been greatly expanded so that now it is almost exclusively the only means for braking. If I interpret the torque curve literally, in THS II the hydraulic brakes ONLY engage upon fully depressing the brake pedal. Anything short of that will exclusively regen brake. Note that the braking system, as controlled by the ECB module, is activated by a brake pedal stroke sensor, which trips any time you touch the pedal.

Given all that, unless you're a complete digital driver (full accel, full brake only), your brakes and pads should last an incredibly long time!

[rflagg]

Originally posted by xlarimer
Dumb question when reading this thread:

Does this mean that we won't have to replace our brakes as much if we use regen more than friction?

First, my service tech says that I don't have to replace the oil as much (since it runs less), next I learn that there is no steering fluid to deal with.  

This is working out to be one of the BEST car ever!

Also, According to the 2004 New Features Manual:

The engine coolant in the '04 is most likely the Toyota Genuine SLLC (Super Long Life Coolant), which is pink in color. This fluid does not need maintenance on it until the car reaches 100,000 miles for the first time, and then subsequently every 50,000 miles afterwards. In the 03, it was every 30,000 miles or 24 months, whichever comes first.

-However, it does state that it's possible that the car does have a mixture of the two, which means maintenance after 30k or 24 months. Does anyone know how to check and see what coolant you have in your Prius?

-m.

[toyoprius]

If you car has been sitting a while after a rainstorm, you can usually hear the brake pads scraping away oxidation on the rotor. It's a mild grating sound that disappears quickly. However, you can use these opportunities to "test" the point at which the friction brakes engage. With your Prius in stealth mode (the EV switch is great for this), you can clearly hear the point at which the "humming" of re-generation gives way to the "grating" sound.

Surprisingly, it takes more pressure than I would have guessed to engage the friction brakes.

Originally posted by impreza
Can anyone tell me (reliably) how to tell when I'm using the regen only and not yet down to the friction brakes?