[B] - "Engine Brake" -- when to use

When descending in mountains where your regen braking will fully charge the battery pack, it is convenient to tell it to spin the ICE as resistance, too. This reduces regen rate. Also, you don't have to stay on the brake pedal that way. With full pack, you have the engine resistance and the friction brakes. Prolonged use of friction brakes is bad in any car due to heat/glazing/wear.

 

Also in snow/ice it's far more controllable to slow down using engine braking as the rears won't lock even for a split second as can happen with abs.

 

Yes it will but some people like the feeling of downshifting. You could downshift in your last car if you chose to but I suspect normal braking sufficed.

 

Why not just point to the original, instead of a warmed-over PDF
that doesn't have any of the clickable links or even a vague gesture
toward the courtesy of attribution?
.
_H*

 

Instead of all that verbosity, just say this:

The "B" mode helps to slow the vehicle by using the gas engine for additional drag.
It has no other value and does NOT increase the amount of energy regenerated.

The purpose is to save your brakes on long, steep hills where regen braking isn't enough.

 

I don't care for this technique. B mode braking uses only the front wheels, as does regenerative braking. While regenerative braking is limited in its power, with slippery conditions you can't brake hard anyway without invoking ABS.

It makes more sense to brake lightly with regeneration. If it's too slippery for that you are going to transition to ABS either way, so why not recover some energy.

Tom

 

I use the B mode to slow the car initially if I'm going 30 mph +. I'm used to down shifting in my last Outlander w/ 'shift-o-matic', and my current 5 spd Fit, makes driving more involved and rewarding, and maybe uses a little more gas, but the mileage is stll very high. I don't wand to have the highest mpg in the world, I want to have high mpg, and an enjoyable ride.

 

Thank you for the nice document.
Since this place is for Gen3 forum, would you please add Gen3 speed threshold on your document?

Ken@Japan

 

The B setting is to a Prius what a Jake Brake is to a Semi. In B, the car's computer changes the valve timing on the gas engine and shuts off the fuel ... which turns the engine into a large air compressor, slowing the car.

It is intended for use in mountain driving where you can encounter long and/or steep downgrades. If you have ever driven in the mountains and gone down a LONG grade, especially in the days before disc brakes, you will know what cooked brakes can do to your safety. If you have ever seen the Truck Runaway lanes in the mountains, you might get the idea about a Semi going downhill with NO brakes because they got cooked.

From actual experience driving in the Rockies of Utah and Wyoming, I can tell you that B is fabulous. I have driven down some long grades of 6% to 7% and greater. Using B, I actually have to use some throttle to keep from slowing down TOO much. PLUS, this slows you down WITHOUT burning your brakes.

Going downgrade in D, after your battery is as full as the computer will allow, you will be forced to use your brakes or else you will get going too fast as the regenerative braking shuts off.

 

I live in the mountains here and have 10-20 mile downhills. The "B" is great - in most cars around here you start to get brake shimmy after a couple of years of going down the hills, which results in having to have rotors turned, etc. (expensive brake work every couple of years). I'm lucky that one of my other cars will hold a speed of about 45 when downshifted, but that was not the case with the car that the Prius replaced. The brakes on that one needed constant work.

Plus a nice thing about the "B" mode is it seems that you can step on the gas while it is engaged and the car acts normally - the "B" only seems to have an effect when your foot is off the gas pedal and in the "regeneration" area of the display. If you hit a bunch of rollers with local law enforcement behind you - this is quite a nice feature - you can accelerate and slow down without having to mess with things or continually tapping your brakes. (Of course the cruise control also does a good job of this).

 

Just a minor clarification to make sure no one misinterprets this comment. When you get to a full SOC during regenerative braking, the Prius automatically shifts into what is essentially B mode. Manually selecting B mode gets you there sooner, and causes it to be a bit more aggressive. In either case friction brakes are only used as a last resort.

Tom

 

That is precisely the point.

 

A note for those not accustomed to these downgrades ...
Lightly pressing the gas pedal in this situation doesn't burn any fuel, it simply reduces the amount of engine & regen braking.

I.e., in the 2010, the amount of B-mode braking is easily controlled and modulated by the throttle.

 

I'm constantly driving up and down steep hills. If I'm using B-mode constantly will this cause any harm to my engine/transmission?

 

No. B Mode is harmless to everything but gas mileage. If it is steep enough to want B mode, gas mileage is not your major concern, safety is.

 

It won't cause harm, no. I recommend switching back to D when you don't need the extra drag from B, however. Staying in B will prevent you from using cruise control, will cause the engine to run more, and will reduce your mileage.

 

When coming down the Grapevine Hill in "B", after the traction battery got to full charge the regen braking was greatly reduced to the point I had to lightly use the brakes.

Also when in town in "B", the ICE doesn't shut down at stop lights, like it does in "D".

 

It is very useful to use B permanently while on dirt roads. it is like driving in a lower gear with your automatic or manual. My daughter has just undergone an intensive driver training course and the instructor insisted the students "downshift" while on gravel.