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NO FUEL IS SUPPLIED TO THE ENGINE IN B MODE. ICE is simply spun by MG into whatever RPMs suitable at the moment and engine braking is done by compressing air inside cylinders.
"B" mode -- it takes energy to turn a car engine that is otherwise not running. Friction in bearings and pistons and cams can be significant, as anyone who tries to turn an engine by hand is aware. But more significant is that it takes energy to pump air through the engine -- without fuel and spark, an engine is really nothing more than a big air compressor. The amount of energy needed to keep all that turning and pumping is what causes the "engine drag" felt in conventional cars when the accelerator is released. And if the transmission is geared down, the drag effect is more pronounced because the engine must then spin [i.e. be pulled around against air and mechanical resistance] that much faster.
Enter "B" mode. As in, "trucks use lower gear". By forcing the wheels to spin the engine and pump air, a good deal of that energy can be turned to heating the air going through the engine instead of heating the brake parts. Since fresh air is always coming into the engine, having it leave as much warmer air provides a convenient place to dump excess energy.
n a conventional car the wheels push the engine around through the transmission, but the Prius needs to help that process out a little bit by actually having its combination of electric motors spin the engine. In this case, the valve-timing in the Prius engine is advanced to increase the amount of air taken in and the suction against the throttle flap -- which uses much more energy than the coasting-in-"D" scenario above. Either way, stopping power now comes from a combination of things and the burden on the friction brakes is greatly reduced, allowing the hill to be descended safely.
"B" mode also increases regeneration current to 30 - 40 amps with no feet on the pedals, so the part about "more regeneration" is somewhat true. That is one of several mechanisms used to increase the "drag" feeling. That level also varies with the car's speed.
In general, the amount of extra resistance given by "B" mode is sort of staged upward depending on the car's speed and how charged the battery is. Some of these conditions can be utilized in entertaining ways. Under 20 mph, if the engine is not already running and your foot comes off the accelerator, B mode simply regenerates reasonably heavily [30A or so] into the battery. This drops off around 12 mph to a lower current, and is then similar to being in D until regen capability kicks out entirely around 7 mph. So between 19 and maybe 10 miles per hour, you can use "B" to slow down in an energy-productive way, and essentially drive around in electric-only mode with one pedal -- but be careful to not do something the person behind you doesn't expect without showing brake lights!
Driving around in "B" during warmup also tends to charge the battery a little faster, since electric-only mode is avoided, but again at the expense of burning more fuel to do it. Engine start/stop transitions are avoided. Sometimes this state feels more surefooted and responsive in snow and other tricky conditions.
Fuel usage in "B" is somewhat mitigated by the fact that when decelerating above some nominal speed, somewhere around 17 mph, no fuel is sent to the engine and it just spins "dry". It's still wasting energy and slowing the car, but there's no reason to throw away gas along with that. This is sometimes called "fuel-starve" mode, and is also used in some conventional cars during high-speed coasting conditions.
Many strange things happen when the battery pack gets up to "eight green bars" full level. The hybrid system begins doing several things to pull a little energy back out of the pack -- the engine will tend to spin in "D" mode even at low-speed, low-demand conditions, in fact just about in the same way as "B" mode does when the charge state is more normal. If "B" mode is selected in the full state during coasting, then the engine *really* screams and even more energy is pumped away. So while all the energy of a long descent cannot be captured, speed can be controlled in some interesting ways by creative shifting between "D" and "B" even after the pack is topped out.