B Mode Mistake?

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!

 

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

 

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

 

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.

 

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! :welcome:

 

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

 

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

 

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

Congratulations for at least trying to set the tech straight. You might offer this paper to him.

 

What "B" mode does:

1. Wastes energy. Don't use if you don't want to be wasting energy. Use if you have too much (top of a mountain pass, for example). Note that the Prius is a slippery car aerodynamically with low mechanical drag as well. It will "take off" down hills. Use "B" to control this on longer hills.
2. Spins the engine to waste the energy. The car will -not- allow the engine to spin too fast. It "releases" the engine gradually to limit the top RPM to 5000. You would then have to use the brakes to keep the cars' speed under control.

What "B" mode doesn't do:
1. Provide controlled braking. It slows the -front- wheels only. If the road is icy this is a recipe for a spin! While the braking is gentle, it is still on the front wheels only. Use care!
2. Give you a "low gear". There are no gears in the HSD that "shift", just magnetic fields that shift the power flow in the HSD, providing the same effect as slipping a clutch or shifting a gear. So climbing a hill in "D" is the same as in "B".


As you descend a long hill such as a mountain pass, you can begin to use the brakes (gently), which recovers your kinetic energy using regeneration. If you watch the state of charge (SOC) bar graph you will see the bars increasing, eventually going all green. At some point shortly after this, you will seem to loose some braking effect (unless you press harder on the brake pedal). This is regeneration shutting down because the battery has reached about 80% SOC. If there is still a lot of hill to descend it's time to shift or switch to "B". Of course you could have started your descent in "B" as well, but just for a learning experience don't, rather do it this way. As soon as you shift to "B" you will hear the engine start to roar. Don't worry, it will not over-rev. and no damage will occur due to the roaring. It's just dumping energy for you, rather than that energy heating up the brakes. If you are descending a steep enough hill the engine will reach 5000 RPM, the maximum continuous RPM the HSD will allow. At this point, you will have to use the brakes to keep your speed under control if the car tries to continue to increase speed.

A curious effect of this design is, when the engine is being used to dump energy, if you press the accelerator pedal, the engine speed will -decrease-! Very funny!

 

A minor addition:

Even without going into 'B' mode, engine braking will be activated if the SOC becomes all green and you have your foot gently on the brake. It does not seem to be as aggressive engine braking as in B mode and I usually only see engine speeds up to about 3000 RPM.

kevin

 

Maybe its just too obvious, but here is another little detail I don't usually see mentioned. Since the size of the battery often limits the amount of regen energy you can store on a long downhill, the more battery you can use to help move the car the more regen you'll be able to capture. Obviously if you have 10 minutes of non-stop 10% grade, thats not possible, but more often than not even these long mountain down grades have flat spots. Any time I start slowing on a down hill run, I touch the accelerator very gently to get into warp stealth mode (arrows from battery to wheels only at highway speeds). That way I can maintain (or nearly maintain) speed through the flat without using any gas but more importantly using up battery power to make room for more regen on the next slope. The higher the SOC of the battery, the easier you'll find it to get into WS and the more you'll be able to press the accelerator w/o starting the ICE.

Also maybe too obvious, I often try to get to the top of a long climb with the battery as low as possible so I have as much room to store regen as possible. You don't want to overdo it, if you are still going uphill and the battery runs out you will take a FE hit while the car puts some charge back in the battery. By diddling the accelerator on the uphill you can usually find modes where you are still getting battery charging, where you are "dead banding" with no arrows to or from the battery, or where you are getting electric assistance from the battery. I usually try to drive uphill in the dead band, and then within a mile or so of the top start using electric assistance to bring the battery down.

I almost never use B mode at all. Since most of the downhill braking is being done by the electric motor, you don't generally have the regular concerns about overheating the brakes. I've really only ever used it coming down really steep hills, like the local 14% down into Tonto Natural Bridge State Park. Prius felt like it was on a ski jump ramp

Rob

 

My first experience with Pearl on mountain roads and the one I posted about above was on highway 99 in British Columbia, the "Duffey Lake" section, heading west. The downhill portion has grades up to 18%, the steepest I have EVER seen on a main road. My motorcycle bottomed out the front forks slowing down on that hill!

Lots of fun!

 

I can attest to the risk of shifting into B mode on ice. Last winter, on a particularly cold and icy night, driving home from work, I realized I had hit an area that was a complete sheet of ice. It was on the top of a hill and I could see that a couple of cars had already skidded off the road. I was going too fast and I started to feather the brakes. Then I remembered about B mode and I put the car into B mode. I immediately started skidding - due to the sudden change in speed associated with the shift. Fortunately I was able to steer out of the skid and move on - before going off the road. I learned a valuable lesson that night. If I'm going to use B in icy conditions I need to be in B from the start and not downshift into B when I hit the ice.

 

Don't use B mode in that sort of situation. There is no advantage. You discovered the potential disadvantage.

Tom

 

MY experience in "B" mode was in coming DOWN a mountain drive (steeper than Mt. Washington in NH). I began smelling VERY hot motor/electric only about a third of the way into the descent and had all the lights on, B-Mode grippin', the AC max-cool, and everything else and it STILL smelled like it was going to melt. We ended up stopping 3 times for an average of 5 minutes or more just to let stuff cool down. THAT is not the time to begin to understand compression, electric braking systems, and so on.

thanks for the read, Hobbit. It doesn't explain why my '08 Prius smelled like a fried sewing machine, but good to know.

 

Recently i visited hill station, here one of the track is very steep and rough, on uphill it was ok but on downhill i engaged b mode but still speed was way fast due to steep, my question is how to control speed on such steep hills, i miss first gear of manual in these hills, please help

 

b mode and brakes are your only options