New Prius owner here (2005, black, Package 3, LOVE it!)

I live in the SF Bay Area, in the Mt. Diablo foothills. My driveway is fairly steep, so I use the "B" setting going down. I do it, but I'm not exactly sure if it has value on other downhill slopes. I know that it does an engine-braking sort of thing, but don't really understand it and how to use it most efficiently.

BTW, while others in the general Bay Area are waiting for ordered Prius', I went to the dealership in Antioch and drove off with one of several they had for sale on the lot

 

B mode is in place to act like a low gear from a typical vehicle. And the way you describe using it, sounds like you get it.

Anytime you press the brake pedal the vehicle goes into a regenerative braking mode, unless you are braking at very low speeds, such as parking lot speeds. I believe the lower limit of regenerative braking effect is about 7mph. Below 7pmh the system goes to friction braking.

B mode has the vehicle spin the ICE without inducing ignition. Thus causing the engine to turn into an air compressor. This is akin to a large trucks compression brake called a Jacobs brake. Anyway, the air resistance created by the pumping pistons gives you a significant amount of additional control while coasting or braking down a long descent. Do not worry about friction or engine use during B mode operations, the engine is fully lubricated and functions in the same physical manner as if the engine was running, except that it isn't burning any gas and it isn't running ignition.

I do not use B mode in regular driving, however it is very useful if you don't want to ride the brake down a hill or want additional braking assistance down a hill or long incline. B mode works at all speeds that I typically drive, I'm not a speed demon so I can't speak to 80+ mph. However if you are doing 80 on a long downhill you have other issues that must be dealt with before B mode will be of much use.

 

Thank you.... very helpful reply!

 

B mode is very handy when going down mountains and long hills in the SW South Dakota, NE Wyoming, and W Montana areas. Saves alot of wear and whining on the engine.

 

Does anyone know how to assess whether pressing the brakes hard from high speed uses the friction brakes as well - I suppose the question is what is the braking potential of the regenerative system and is there a way to know when you are wasting heat energy in friction or regerating power (this assumes no heat loss due to braking from Tyre/road friction which of course is false)

 

You use it just like you would use the lower gear in any other automatic vehicle. The engine works harder but your speed is slower. More gearing allows you to go slower down hills without braking as much, as the engine slows you down.

 

hey there
I have a strange question, that I am sure you not used to getting. I am a Casting Director in San Francisco looking for Prius drivers , or soon to be Prius drivers that love their cars and are concerned about our global environment.We are looking for all ages/ all people/all walks of life.

We are shooting a short film about the Prius and environmental concerns in the bay area and was wondering if you are this person, or if you know of someone that is.
We have a great crew that will be sure to make this film outstanding, something that you would be very proud of, I promise.

There would be an audition next tues. 10/31, and if chosen would shoot one day the first week of December, it would pay $500 for the shoot day.

Please email if you know someone or have any ideas of where I should go.


Thank you so much for your time
Kristen
kristencast@gmail.com

 

Hi,

I'd like to add some details to this description:

I have an OBD scanner for recording data from my 03 Prius, the model before the 04-current Prius. With this I can report the actual engine power generated or 'absorbed' using "B":


The ICE power scale is ~10 hp per division, 1 hp ~= 745 watts. Fuel consumption is proportional to the injector timing times the rpm. You'll notice two slanty lines on the left, this is ordinary engine braking that happens even without using "B" but it is only good for about 3 hp. I have some unpublished charts going down Brindley Mountain at 65 mph using "B" and they show a 14 hp negitive (aka., braking) force.

Brindley Mountain is about a 6% grade and it looked like "B", 14 hp, was just about holding the vehicle at a slowly, decreasing speed. IMHO, grades steeper than 6% or at speeds above 65 mph may exceed the engine braking power capability.

Although we talk about 'shutting off the fuel', the data suggests there is still some fuel flow. To me, this makes sense to keep the catalytic converters at operating temperature. However, the timing values are reported by the engine ECU and I haven't used probes on the injectors to confirm they are still tracking the reported injector timing.

Bob Wilson

ps. If you are interested:

http://hiwaay.net/~bzwilson/prius/pri_power.html

 

What's really important to gather here is something I missed the first 3 or 4 times I read about the "B" setting:

"B" wastes energy as heat for you.

The second post by JeromeP, 3rd paragraph, explains how - but my initial read was "Oh it just puts it in max regenerative braking, that's useful." But that would be useful for short downhills like a driveway. Rather, it's for very long downhills where you're concerned about riding the brakes, and further, concerned about overfilling the battery to the point that regenerative braking ceases to function. The 10000ft drop from Flagstaff, AZ to Phoenix comes to mind. Oh - and "B" does work over 80mph.

So, to get the most out of regenerative braking, just use the brakes. To save your brakepads from unnecessary wear on very long downhills, use B. For something like a driveway:Well... that's up to you. You're probably not gaining anything with it, but if your driveway is that steep, you just might be saving your brakepads a bit.

One other danger of B - I'm fairly certain B doesn't engage your brakelights - but it sure does slow you down fast on a flat. Unless you're looking to file a big insurance claim, look behind you before using it on the highway.

That's addressed elsewhere on the forum - I'm getting used to saying this: Bob has a graph for that. Ken does as well. Basically, the answer is yes - the harder you press the more the friction brakes are used.