B Mode Mistake?

[Homebrewer]

I was driving in some mountains for the first time in the new Prius. I wasn't sure what B mode was and really needed to "downshift." When I got to the bottom of the hill I called a Toyota dealer and asked about B mode. The guy said I could shift into it on the fly and I would be braking with the help of the engine. He didn't mention anything about speed.

So I'm coming down the next hill, a long, steep slope. I'm in B mode at 45 mph, tapping the brakes. Then I hit 50mph, 55mph,..and when I hit 60mph the electric engine (i'm assuming) starts whining/spinning loudly out of control. It took me 20 seconds to realize what was happening and shift back to drive. I hope I didn't damage anything. All seems normal now.

Bottom line: There must be a speed cap on B mode and the computer isn't there to protect the engine. Is this right?

[efusco]

Originally Posted by Homebrewer

I was driving in some mountains for the first time in the new Prius. I wasn't sure what B mode was and really needed to "downshift." When I got to the bottom of the hill I called a Toyota dealer and asked about B mode. The guy said I could shift into it on the fly and I would be braking with the help of the engine. He didn't mention anything about speed.

So I'm coming down the next hill, a long, steep slope. I'm in B mode at 45 mph, tapping the brakes. Then I hit 50mph, 55mph,..and when I hit 60mph the electric engine (i'm assuming) starts whining/spinning loudly out of control. It took me 20 seconds to realize what was happening and shift back to drive. I hope I didn't damage anything. All seems normal now.

Bottom line: There must be a speed cap on B mode and the computer isn't there to protect the engine. Is this right?

That was the internal combustion engine spinning. You could leave it in B...it was doing what you asked it to do. Or go into D and use the brakes. Either way is fine and neither would do anyharm.

[JimboK]

No worries; neither the internal combustion engine (the ICE, as we call it around these parts) nor the electric motors (we call them motors, not engines) will over-rev in "B." The car protects itself from "red-lining" very well. It was doing its job and, from the sound of it, doing it well. What you were hearing was the ICE running at a higher than normal speed.

And welcome to PriusChat!

[Buckland]

I use the B mode when driving in the winter on icy roads. As soon as I let off the accelerator the car starts to slow on it's own and I have to use the brakes much less to stop, thereby limiting the risk of skidding through an intersection.

Buck

[qbee42]

As Evan and Jim implied, but did not say directly, in B mode the car will spin the engine without fuel to waste energy. Sometimes it will spin quite fast, which can be scary if you don't expect it. It's all completely normal.

You will also sometimes get this sort of spinning at the bottom of a hill. If the HV battery gets charged to a higher level than some trigger setting, the control system will spin the engine without fuel to bring the charge back down to the target level. So here is what happens: you come down a big hill and you get all green bars on the battery display. You stop at a light at the bottom of the hill with your Prius nice and quiet, when all of a sudden the engine switches on and races for a few seconds, then it turns off, then switches on again and races. The whole pattern repeats until the battery is back down to the target level. The control system is using the engine as a large frictional load for wasting energy.

Tom

[efusco]

Originally Posted by Buckland

I use the B mode when driving in the winter on icy roads. As soon as I let off the accelerator the car starts to slow on it's own and I have to use the brakes much less to stop, thereby limiting the risk of skidding through an intersection.

Buck

Just understand that if your car has VSC its FAR safer to leave the car in D and use the brakes and let the system that is specifically designed to prevent skids do it's job.
I firmly do not belive B-mode is intended for nor that it's particularly safe for this kind of use. I think we're going to, one day, hear reports of someone going into an immediate skid when entering B mode on an icy road due to the uncontrolled slowing it induces.

[9G-man]

Originally Posted by efusco

Just understand that if your car has VSC its FAR safer to leave the car in D and use the brakes and let the system that is specifically designed to prevent skids do it's job.
I firmly do not belive B-mode is intended for nor that it's particularly safe for this kind of use. I think we're going to, one day, hear reports of someone going into an immediate skid when entering B mode on an icy road due to the uncontrolled slowing it induces.

You are correct, there is a reason on manual transmission cars that "high" gear is recommended on slippery surfaces for starting and stopping. It's so the mechanical friction of the drivetrain will not lock the wheels on slippery surfaces, as "low gear" would....and cause skidding.
B mode, of course, is synonimous with "low" gear..........a no-no a slippery surfaces.

[Bill Merchant]

All the above is good information, particularly about not using B in icy conditions. But one more thing needs to be said:

Welcome to PriusChat, Homebrewer!

[Buckland]

The Prius in winter is much safer than the average automatic though, especially when the engine is still warming up. When it comes to winter driving I much prefer a standard transmission.

Buck

[scrambledeg]

just want to say that i am so grateful for this information. I was at a "prius class" at the dealership last night and listened as the service tech misinformed about 15 people as to the benefit of b-mode. I politely corrected him. I also corrected him when he stated that a driver can expect 40 to 80 mpg on their last PIP. It was so frustrating and scary because this really cool, helpful (seemingly) guy with 30 years experience working on cars (and 4 or so working with Prii) could have easily misled all of those people and caused some serious problems!

I'm not sure how receptive the group was to my information, but i tried!

i came back on here to find threads on b- mode and I only found 3. I would love to read Hobbit's "lecture" on it - how can i find it?


Thank God (karma, whatever) for PC!

heather