Riding Brakes Downhill

Qu 2 before Qu 1

2 - The hybrid control system doesn't like to allow the battery to go below 20% or above 80% (it'll happen but rarely). Around 80%, the regenerative braking will be used less and the car goes to mechanical braking. It can lead to less braking effect than you'd expect when coming off a motorway/freeway because you're just going off the mechanicals. The 80% hold is to avoid stressing the battery. The balance between regenerative and mechanical braking is another of those things that the car will adjust for you and is actually a bit more consistent than the hydraulic system in a normal car.

1 - B mode comes in when you've hit the limit of what will fit in the battery but have lots more downhill to go. The rest of the slowing down is done on the mechanical brakes but these will eventually overheat and start fading out. What B mode will do is hook up the engine but use very low gearing ... It'll spin the engine at 4000ish rpm to make engine braking happen. I don't think it's burning any fuel when it's doing that, it's just taking advantage of the pumping losses that come with pushing air into the engine and compressing it as the engine turns.

So - B mode comes in handy when the regenerative braking isn't allowed to squeeze more into the battery and the mechanical brakes are likely to overheat. It gives a third option for making the car not gain too much speed on the downhill. Instead of the energy going into the battery or into heating up the brakes, it's being thrown away* in pumping losses. (*better to be thrown away than be turned into kinetic energy speeding the car up)

PS There's a big Wall Of Text on these forums that explains the mechanicals of the Prius drivetrain we share ... not sure where it is now though.

bottom line= better to use "B" mode on mountainous downhills

This has been discussed ad infinitum in the Prius forums, here is one long thread see the third entry from

(B) braking. - PriusChat Forums

jdcollins5 answers your question pretty well (B) braking. - PriusChat Forums

bottom line= better to use "B" mode on mountainous downhills.

I went on a Colorado trip and the "B" mode worked very well.

There's 3 ways the hybrid can slow itself down :

Regenerative braking with the motors - the motors provide resistance and the energy goes into the battery;
Friction braking with the disc pads - the energy goes into heat (and is wasted)
Compression pumping losses in the engine - wasting energy again

The trouble with the Regen braking is that if the battery is full, there's nowhere for the energy to go. Friction braking is also good until there's too much heat (boils the brake fluid - which is bad news). So the third type of braking with B mode is intended for when the driver thinks they'll run out of Regen or Friction braking.

When the engine spins, it needs to put energy in to compressing the air & fuel in the cylinders. When that's all done, the air & fuel explodes and pushes the piston back which is where the engine power comes from. What (I assume!) happens in B mode is that the engine is allowed to spin up, energy goes from the car moving into compressing those pistons but because no fuel is going in there will be no power delivered by the engine.

If you watch programmes like Ice Road Truckers - where they refer to "Jake Brake" - that's what we have in B mode. It's treating the engine as an anchor, making it spin up (with all that compressing resistance) but deliver no power.