Using "B" for dynamic braking

I am not sure what you mean by "dynamic braking." Here's what "B" does and when to use it:

Cars with transmissions always have a way to use the ENGINE to slow the car, for when you do not want to use the friction brakes. "Jake brakes" on an 18-wheeler, low gear on a manual or an automatic. This is useful for steep hills where the friction brakes would overheat.

The Prius is different:

Wen you step on the brake pedal the Prius uses MG2 as a generator to slow the car (unless you are going slower than 8 mph or step hard enough on the brake pedal to indicate an emergency stop; in both cases the friction brakes are used).

Thus in the Prius, stepping on the brakes normally re-charges the battery. This is good. It re-captures energy.

The exception to the above rule is if the battery is as full as the car's computer wants it to be. Then the friction brakes are used. This typically happens on a long downhill slope.

When you are going downhill AND you see all 8 battery bars green, THEN AND ONLY THEN should you use "B" in order to reduce the amount of usage of the friction brakes, and avoid overheating them.

"B" on the Prius stands for engine Braking: It uses the engine to help slow the car, just as first gear in a manual transmission car does.

NEVER use "B" except as above.

It is amazing and depressing how many wrong and downright stupid explanations some salesmen will give for "B." Many salesmen are hired off the street on a commission basis, and (unlike service people) the dealership may have no incentive to teach them anything about the cars they are selling, especially the unusual Prius.

 

My quick answer: I don't use "B" until my battery goes green on my MFD, and then only if I'm going down a steep hill.

Harry

 

I've never used "B".

 

<div class='quotetop'>QUOTE(Spike Jones @ Nov 4 2007, 09:16 AM) [snapback]534599[/snapback]</div>

Yes. For one thing, it's easier than hitting the brake pedal. We regularly drive mountain highways in California, and the "B" gear not only recharges the battery, it's kind to our brake pads. We got in the habit of doing this from our old Ford van, that had an Overdrive button on the gear shift. We punched "OD off" going down steep hills, and it did the same thing as the Prius, without the bonus of capturing that power in the battery.

 

I never use B. I don't have long downhill stretches on a steep grade.

You might want to read the manual about use of the B mode.

 

<div class='quotetop'>QUOTE(Spike Jones @ Nov 4 2007, 11:16 AM) [snapback]534599[/snapback]</div>

Hi Spike,

There is no "Dynamic" braking on the Prius, as there is no giant resistor with liquid flowing through the radiator to get rid of the heat. Might be a neat retrofit project though. Could extend mountain driving friction brake life. With the resistor plumbed into the engine coolant system, it would keep the engine warm for instant hybrid capability at the bottom of the hill.

The system would need to watch the CAN bus to get the battery SOC. When a switch on the instrument panel is switched to "Mountain Braking" mode, the following action would take place. When the battery is charged to the appropriat SOC (first green bar?) it switches in the resistor. Something more sophisticated might use the engine temp and gas pedal/brake pedal positions.

What B mode does is uses closed throttle engine drag to aid down-hill braking.

 

<div class='quotetop'>QUOTE(MO51 @ Nov 4 2007, 09:54 PM) [snapback]534667[/snapback]</div>

I may be misinterpreting your post, but going downhill in B mode will not generate more electricity than just coasting down the hill in D mode. The B mode runs the engine to slow down the car, not increase the regeneration. So you are wasting the energy unless the battery is full.

As long as the battery has some spare capacity, braking will capture the energy while B mode will throw it away. When the battery is full, energy must be wasted anyway, and B mode will not wear the brake pads, while braking will.

-Stein

 

Using B except when decending a hill which will overcharge the battery is like spilling petrol on the road. If you lightly touch the brake pedal you will increase regenerative braking and capture energy to reuse when climbing the next hill. Using B mode dumps energy as heat to the atmosphere by spinning the internal combustion engine generating heat which is dumped through the exhaust and cooling system.

 

But it also dumps power. What is the regen under light braking?

 

So braking lightly will give you more regen than B will?

 

Thankyou, that was my point.

 

<div class='quotetop'>QUOTE(Spike Jones @ Nov 5 2007, 07:35 AM) [snapback]534926[/snapback]</div>

Spike, your friction brakes are only used below 9MPH or in a panic stop. Using B in the manner you describe is a waste of time and effort.

 

After getting a $300 ticket, and then on another ocation a warning for going too fast in a school zone. I have deciede to use the "B" whenever I see a school zone so I will slow down faster after taking my foot off the gas. I get distracted watching for the kids and other cars that I don't watch the speedometer.

Joe