B Mode Questions

There is no maximum speed for "B" mode.

Mechanically (electronically) what "B" mode does is use the engine with no fuel injected to help slow the vehicle. It becomes an air compressor, exhausting the compressed air out the tailpipe. It also moves the regeneration up a notch or two - makes it more powerful.

When the battery gets "full" (it isn't actually full, just as full as the HSD wants it to be), the car stops regenerating and will do several things.
First, if you are still in "B" mode and going downhill the engine braking will still be used.
Second, the HSD system will try to use up the battery power. If you level out and apply the accelerator pedal the HSD will use electric power whenever possible, trying to drop the charge level back down to about the 60% range ("full" or indication of full is actually about 80% charge). It is limited by battery current and battery temperature, but you get the idea.

There are a few things you should keep in mind when using "B" mode for braking, or even just regenerative braking.
First, for a "what the heck?" moment, when going downhill in "B" if you press the accelerator pedal the engine will -drop- RPM. Very different than what you would expect.
Second, and very important in slippery conditions, is "B" and regenerative braking is only on the -front- wheels. It's -possible- this could destabilize the car (it might want to swap ends). So be careful when it's slippery. I haven't heard any reports of this and I suspect VSC will activate to prevent this (apply rear brakes for you), but something to keep in mind.

 

That is exactly what B mode is for. You do lose regeneration when the battery is full (actually not full, but over 80% SOC). B mode helps by using engine braking to supplement friction braking. Engine braking reduces the need to use friction braking and lessens the danger of overheating the friction brakes.

By the way, there is no reason you can't use B mode before the battery gets "full"; this will give you more control over your speed on the downhill sections. The only trade off is that you will be throwing some energy away and so your mileage will theoretically be lower. Safety vs. mpg ; safety should come first IMO.

There is no maximum speed limit for B mode.

 

... only if the grade is short enough to not fill the battery in B mode too. If it also fills in B mode, then MPG should be a wash.

 

No need to worry. The car will protect both the battery and the engine. It will not over-rev. The GII will not go over 5000 RPM and the GIII is higher.

The HSD will not allow you to discharge the battery too much as well as not charge it too much. The reason is a bit complex, but suffice to say it's to ensure long life.

 

Actually you don't need to select B mode to use it. If your battery gets full (aprox 80%) the car will go into B mode instead of continuing to regenerate when you push the brake pedal.

If you forget and leave it in B mode you will usually notice because the cruise control won't work.

 

No it doesn't. While D mode's synthetic drag will also spin up the ICE faster when the battery gets full, the resulting ICE RPM and braking drag force are well below what B mode produces in the same situation.

 

This is a matter of degrees. Pressing on the brake will produce more braking in either mode, although B mode is more aggressive with regeneration.

The main purpose of B mode is to forestall friction braking. If you know that regeneration will max out on a descent, switch to B mode early to dissipate excess energy. Early dissipation will save on friction braking.

Tom

 

I agree Tom, I have also noticed that using the brake pedal for regeneration in D can cause more current to flow to the battery than B mode without brakes applied. Using the brake pedal in D from a fairly high speed, with the battery not at full SOC, can generate over 100 Amps. My ScanGauge observations indicate the B mode will not generate that much, more in the neighborhood of 80 Amps as I recall, of course it could be higher with a very low SOC.

RPM in the B mode without applying the brakes seems dependent on speed. I have seen over 5000 RPM at higher speeds on a long steep hill using the B mode without the brakes. When using the brakes in D, with a full SOC, RPM seems dependent on both speed and pedal position.

 

What the "B" mode is, is a Jake brake ... like what semi trucks use. When a semi uses its engine brake, it shuts off the fuel and turns the diesel into a big air compressor, thus slowing the truck.

The Prius does the same thing. B (engine brake) cuts off the fuel and ignition ... turning the engine into an air compressor. IF you do not touch the GAS pedal, the computer will let the gas engine spin up against compression, making for a very effective way to slow the car.

The engine WILL wind up to what sounds like high RPM ... and hold it there ... to slow the car. If you slow down too much and need to go faster and push the gas pedal, the computer will bring the gas engine back to running speed, turn on the fuel and spark, and you will be driving ALMOST like being in Drive.

If you go down a loooong or steep grade in Drive with your foot OFF of the gas pedal, the computer will let the regenerative brake effect work until the battery is full ... and then you will HAVE TO use the brake pedal, possibly frying your brakes.

However, If you go down a loooong or steep grade in B (engine brake) with your foot OFF of the gas pedal, the computer will use the regenerative brake effect until the battery is full, BUT the computer will also use the engine as a brake until the hill ends or you have to speed up.

On one trip, I went across Interstate 8, over a pass west of Yuma, Arizona. On the downhill side was a sign saying 6% grade for 13 miles. I shifted into B (engine brake) and never had to touch the brakes for those 13 miles downhill. I DID have to use the throttle several times to keep from slowing down TOO much. Engine braking on a Prius is very strong.

By the way, the Prius DOES NOT use B automatically ... you have to shift into B. Just like if you want to back up, you have to shift into reverse.

There isn't a speed limit to when you use B, but the faster you are going when you shift, the more noticeable will be the sudden slowing down of your car.

 

A couple of clarifications for the above post:

1) While B mode is similar to a jake brake, B mode relies on the closed throttle plate to induce pumping losses. Since diesel engines lack throttle plates, jake brakes use a compression release valve system to dump pressure at the top of each compression stroke. This is an effective method for inducing pumping losses, but it is very noisy. Because of this, jake brakes are outlawed in some locations.

2) While the Prius will never automatically switch to B mode, it will switch to engine braking when the HV battery reaches a high SoC limit. This is effectively the same as B mode, although the details differ a bit.

Tom

 

Yes you can certainly use it at any speed you wish, I have used B at over 80MPH coming down long steep grades, works great.

But remember you are wasting energy if you use B mode instead of normal regeneration with the brake pedal to slow down unless you have a full SOC. To put maximum energy into the battery for future use press the brake pedal in D and keep an eye on the HSI, keep it in the regeneration band. There is no point in using the engine as an air compressor and throwing that energy away if the battery has room for it. As you probably already know the small area on the left side or the HSI indicates the amount of regeneration, if you completely cover that small area with the bar you have started using the friction brakes, also wasting energy.

 

Not sure on the specifics, but it's normal for mileage to be bad for the first 10 min or so after you start a cold car. It then improves dramatically. This is even more noticeable with a Prius.

Highway mileage is usually worse than city mileage, and the best mileage will be when you are on a road with a limit around 40 to 50 MPH with few stops (provided you obey the limit ) . Surprisingly this is true even when the road is a wild one with huge grades, such as the "Duffey Lake" road from Lilloet to Pemberton, BC.

 

Question; does B mode turn on the tail lights? I never really use B mode, but the times i have, it decelerates the car enough to make the tailights needed (IMO)

 

No.

The brake lights come on when you use the brakes.

 

Mine does not decelerate as fast as a two-gear downshift on any previous car, none of which activated the brake lights.