EV "B"rake Mode

On the normal hybrid when driving manually or using DRCC the engine braking will automatically be engaged and the ICE will start spinning when the battery gets full. This is without changing to B mode. B mode does seem to change how aggressive engine braking will be used but I've seen the engine at over 4000RPM while going downhill in DRCC.

kevin

 

In HV mode, for best MPG, definitely don't use B for flat roads. It will disable engine auto-stop, wasting gasoline.

Not having a plug-in, I cannot really speak for EV mode, which differs dramatically from my Liftback. But I'm suspecting that for most typical drivers, B would end up regenerating some energy that would be better left to natural coasting or gliding. Remember that electric regen and propulsion are not perfectly efficient. Every time some energy is laundered back and forth between mechanical car motion and battery storage, a portion is skimmed off and lost as heat. B mode will encourage more of this back-and-forth conversion than will D mode, so more will get wasted.

 

Maybe this was discussed in subsequent posts, and if so, I apologize for a redundant post, but when I said that DRCC was equivalent to throttle/brake pedal usage, I meant that slowing down with DRCC would be done in the same way as pressing the brake pedal does it, which is some combination of regeneration, friction brakes, and engine braking, depending on many conditions. For example, when I have DRCC engaged and come upon a slower vehicle, I can see the hybrid display swing toward the regeneration portion as the car is slowed down. If did not have DRCC engaged, and instead just manually applied the brakes, i would see the same thing.

 

Concerning the safety aspects of just using regeneration to slow down when the brake pedal is pressed, you can be sure that if you slam on the brakes in an emergency, that the friction brakes will be engaged, subject to the anti-lock functionality. And due to some claims that the throttle got stuck and caused an accident, pressing the brake pedal deactivates the throttle. So if you slam on both the brake and throttle at the same time, you will slow down. Like in many modern cars, the throttle pedal in a Prius is just a sensor that provides input to the engine control computer, and it will be ignored if pressure on the brake pedal is detected.

 

You have about the right number of '9's - there are internationally agreed upon probability levels for such failures - they are referred to as ASIL (Automotive Safety Integrity Level) ratings.

For the brakes to fail and not be operative is probably an ASIL D which has a probability of failure of 10**-7 per hour.

kevin

 

Don't forget, there are backups for stopping the car:

Regeneration
Engine braking
Hydraulic friction brakes
Mechanical friction brakes (emergency/parking brake)

 

You left off the car in front or the tree at the side of the road.

 

@stevepea
To add my 5 cents after over 1000 miles in EV mode on my Prime I might offer the following feedback:

By priciple EV range maximization requires eliminating (if possible) of friction braking and also minimize re-gen (as @ fuzzy1 highlighted the re-gen - propulsion process is not lossless) therefore staying in D mode and applying "coasting" technique (keeping accelerator pedal slightly depressed to minimize/eliminate re-gen) gave me the best range for any given average e/v speed.

Nevertheless, since eliminating the use of friction breaks and "coasting" is not easily achievable in real day-to-day traffic (at least here in Houston) I have found valid reason to use B mode for my day-to-day driving as follows:

For Stop and Go traffic especially the unpredictable traffic when friction breaking is likely to be applied on regular basis (read Houston traffic) use of "B" in E/V mode maximizes re-gen (and changes the driving habit) and eventually minimize the use of friction breaks =at the end of the day better E/V range in such conditions.

I can't quantify objective gains, but I noticed E/V range improvements in really heavy traffic ( the 'kinda' one pedal like driving in heavy traffic might have also contributed to the range gains).

Other than that I found "D" and DRCC on highways sufficiently efficient and convenient for me to consistently achieve ranges over 30 e/v miles on flat Houston roads.

cheers

Kris