What really happens when one shifts into B?

that is essentially what happens, the computer connects the engine to the psd and runs it as an engine brake, 13 to 1 compression and injects enough fuel into the cylinders as determined by the O2 sensor on the cat's to keep them up to operating temp. The extra drag spins mg1 in the opposite direction and the engine provides the extra friction to help slow the vehicle down.

 

jamarimutt asked

"what really happens when one shifts into B?"

The gasoline engine (ICE) is allowed to spin, but without fuel or spark. With the throttle butterfly fully closed, there are "pumping losses", so is it very similar to engine compression braking in a conventional car. Perhaps a bit better, without ANY fuel being combusted to help the pump along.

I have also read that more regenerative braking happens at speeds below 20 mph, compared to descending a hill in "D", but I am not qualified to respond on this aspect. I can say that downhill w/cruise in "B" does an excellent job of speed control.

DAS, 58k miles in a 2001

 

Woo hoo! My chance to make my first post on the board (Hoping to get my Prius soon)

With a typical automatic transmission (or even a manual) what you'd do going downhill is put the selector into a lower gear to increase the engine braking effect. With a CVT type transmission, there isn't a "gear" so to speak so perhaps what the B mode does is tell the transmission computer that you'd prefer a lower "ratio" for enhanced engine braking rather than letting it select the best ratio to optimize engine economy, which is probably what it would normally do.

I've been driving one of Audi's Tiptronic automatic transmission cars for several years now and the ability to readily select gear ratios going up and down hills has been a big plus in my mind. There's one hill in particular near my house that almost everyone rides their brakes all the way down to keep their speed in check. I can use my Tip to go between 3rd and 4th gear and keep my speed right in line with the other cars (it's a single lane road), never using the brake OR the gas!