On Sat, 2 Feb 2008 15:05:45 -0800 (PST), The Tramp
Except for the rpm thing, any car would go downhill a bit faster if the
accelerator had some pressure on it, even when a lower gear.
I had a 1971 Chevrolet once which automatically opened the throttle when
the gas pedal wasn't being pressed at speed, so the car wouldn't suck
gas through the carburetor. The result was that the car would run
around at 30 mph without slowing down. If I applied a bit of brake or
lower gear, it would slow down some and the open throttle would go
finally close, permitting a slowdown to 2 or 3 mph, just like any gas
car with auto trans.
The reason the Prius doesn't suddenly accelerate when it reduces
regenerative braking is that as the regen resistance is reduced, the
mechanical brakes increase their effect by an equal, offsetting amount.
A similar thing happens every time you slow below 7 mph: at that speed
regen braking cuts-out, but you don't feel the transition.
The same is true for the motors. If you start moving slowing, on
electric only, and then the ICE kicks-in, you don't suddenly accelerate
because of the extra power; the EM reduces its output to equal the added
power from the ICE, so the transition is barely perceptible.
Shagnasty (firstname.lastname@example.org) says...
There is still a certain amount of regen (depending upon SOC), though it
is greatly reduced. This can be witnessed by the green arrows on the
MFD. The regen will stop when the SOC gets too high, but B-mode by
itself doesn't eliminate it.
B-mode is something I'm very familiar with, because I use it almost
every day. On my ride home from work, I have a long, steep downgrade
following by a significant upgrade, then a second, steeper downgrade.
By the time I get to the bottom of the first downgrade my battery is
usually all-green. After the upgrade I may have lost 1 green bar, so at
the start of the second downgrade I shift into B-mode. I'm usually back
to all-green about halfway down the second hill. At that point there is
an audible increase in RPM, as more of the braking load is shifted to
It's true that if you allow the SOC to get too high the car will start
spinning the ICE to slow itself. The Prius does a good job of
protecting the battery from over- or under-charge. But the purpose of
B-mode is for when the driver can anticipate an overcharge state, and
start dissipating some of the extra energy before hitting the upper
No, it does not regenerate electricity.
B mode is to get the added drag from the engine pumping air.