Engine runs while in B

First, why are you using "B"? It is only for simulating engine braking on long downhill runs.

"B" often runs the ICE to use up excess energy, but running the ICE while stopped has no purpose unless your HV battery is is discharged (as you mentioned) or the ICE (or Catalytic) needs heat.

JeffD

 

Don't use B. I can't imagine where in Iowa you need to use B mode.

As for shutting down, B mode uses the ICE for engine braking. Presumably it is kept spinning to be able to immediately shift to braking, although it could be just an interesting artifact. Likewise shifting to N will keep the ICE from shutting down.

Tom

 

I've found the engine will run when in B if I've decended a long steep hill, though it seems to do this only when the HV battery is very very full.

 

This is correct, although "run" may be a misleading term. In this case, the Prius control system spins the ICE as a big air pump to throw away excess charge in the battery. The ICE isn't running as in using gas, it is being spun by MG1.

Tom

 

There are some Iowas river towns with interesting street grades, like Snake Alley in Burlington. I've walked it but not driven it. They aren't long enough to "need" B but following the "use engine braking to maintain speed and the brakes to slow down" criteria, they are a reasonable use of B

The locals claim it's the crookedest street in the world, but we all know that Wall Street actually holds that record.

 

I echo Jeff and Tom.

A clarification on B mode: The ICE will not run if you shift into B at speeds below 20 MPH and stay there. It still offers added drag down to about 7 MPH, with that energy going exclusively to added regeneration (well, minus conversion losses).

See this for a complete description of B mode:

http://www.techno-fandom.org/~hobbit/cars/b-mode.html

Hobbit describes in this paper the running-while-stopped-in-B condition but states that no one really knows the reason for the design.

 

Friction brake wear & tear should be a very minor worry for Prius owners since regen braking takes care of most of the braking load until the car is down to a single-digit speed. To the extent that brake maintenance is an issue, the problem more likely is that the brake discs need to be replaced due to lack of use and rust forming on the disc surfaces.

By shifting into B, you are wasting energy spinning the gasoline engine that otherwise would be directed into the traction battery.

I have >110K miles on my 2004 and the original brake pads still have plenty of thickness left. I will probably change them at around 140K if I keep the car that long.

When on a long descent, you don't have to worry about excessive friction brake wear until you see eight green bars on the SOC gauge. When you get to that point, you'll notice that the transaxle automatically shifts into a quasi-B mode even if the shift indicator remains in D, as the engine will speed up to consume energy that no longer is needed to charge the friction battery.

 

At the downhill speeds I use B mode, above 20 mph, ScanGauge reveals that the ICE always spins. But it doesn't start impersonating a jet engine until the battery is full.

 

I tested this, and the engine does indeed not shut off if you shift to "B" and come to a stop. Regardless of the state of battery charge.

I wouldn't use "B" to try to enhance braking though. If you brake early enough you can use more gentle pressure on the pedal and keep it in regen rather than friction braking until you get quite slow.

I only use "B" when descending long mountain pass'. I'm talking of more than a mile to the bottom. That's what it's for.

 

(D'ohh!! somebody mentioned "N".) IABoy, note that in "N" the engine cannot recharge the battery. Don't use it except when being pulled through an automatic carwash.