Why does the engine wind out when in B mode?

When you put it in B mode for hill descent, the computer assumes that there is more downhill road than space in the battery. It does so to prevent you from getting into a dangerous loss-of-brakes scenario once the battery fills up. So with B mode the engine is spun up and used as a torque sink. This is roughly the same as traditional engine braking in other cars.

It can be a bit noisy but it is safe and effective.

 

When the battery is full, then B mode is effectively the same as downshifting in a non-hybrid, winding up the engine's RPM. MG1 is used to spin it up, but the engine does the actual compression braking and makes the load jet engine / vacuuming / banshee noise.

 

Both MG1 and MG2 are mechanically connected to the engine, through the PSD which is effectively an open differential.

Both MG1 and MG2 can be either motor or generator, depending on the need at the moment. The ICE does the compression braking.

 

The engine is connected to the transaxle too.

MG1 starts the engine & charges the battery through the engine excess power. On the prime it can also drive wheels through a one way clutch.

MG2 drives the wheels & charges the battery through regenerative braking.

 

I never really understood why there was a "B" mode.

 

An ancient yet perfect Prius simulator:

Prius driving simulator

(It doesn’t like Chrome, so use Explorer.)

 

Prevent overcharging the traction battery when coming down Pikes Peak.

 

It also helps you from being put in the penalty box by a Ranger for overheated brakes too. Waiting for brakes to cool can really add some time to your descent.

Driving Down Pikes Peak Mountain - Pikes Peak Colorado Springs

 

As Jimbo frequently points out, federal regulations require some form of downshift available for hill descents. On all cars, not just hybrids. "B" mode serves that function.

Flatlanders don't need it, but those of us driving in the mountains do need it.

My spouse wanted to stop at the gift shop there. A minute later, an F-250 with Florida plates was ordered into the penalty box for a 30 minute cooling off period, stinking up the whole area with the aroma of burnt brakes.

 

You can find a German description of the different modes here:
PSD – Prius Wiki
(With Google translator or something similar you should understand it.)
Look for 7.2 for braking in B-mode in general and there for the link to the B-mode particularly, too.
There's a nice explanation, I think.

Edit: maybe this link with the translation will do: https://translate.googleusercontent.com/translate_c?depth=1&hl=de&nv=1&rurl=translate.google.com&sl=auto&sp=nmt4&tl=en&u=https://www.priuswiki.de/index.php%3Ftitle%3DPSD&xid=17259,15700021,15700186,15700190,15700253,15700256,15700259&usg=ALkJrhjDCWAW7Ij7qgSjnDtt6d-mtwwKgg

SM-G950F ?

 

Engine braking (whether in a Prius or any other car) is done by the wheels spinning the transaxle and the transaxle spinning the engine. Everything is spinning in its normal direction, and the only thing different is what's spinning what.

The engine is burning minimal fuel, or no fuel at all in modern cars, so it sure isn't spinning itself, and it takes plenty of work to externally spin an engine that's not under its own power, and that's what slows the car down.

One thing that's different in the Prius is it's all under computer control. In a different car, you can downshift to get some engine braking, and you can overrev the engine if your speed is too high or your gear is too low or you're not paying attention.

The Prius HV computer is coordinating all of this, and it knows the completely safe RPM range for the engine, and it will use as much of that range as it needs to, but never beyond it. Even if you think it sounds like it "winds out terribly", there's nothing terrible about it; it's well inside the engine's operating range and fully controlled at all times.

 

We didn't have to go look for them. The heavy stench announced itself to everyone in the area.