B or Jake brake Question

[Matt Herring]

Originally Posted by richard schumacher

Picky, picky... Pat means that the mechanical components are always engaged with each other, but of course the effect of "N" is exactly as in a conventional car: the drive system gives no resistance to motion. In a Prius coasting at high speed "N" has the added excitement of creating potentially harmfully high RPM and voltage in one motor/generator.

Correct...neutral in the Prius at high speeds is "potentially" harmful to the engine. Which is why I only do it at 60 mph or slower. The actual mathematically speed at which engine overspinning "could" be present is 63 mph but many in the mpg crowd suggest this number is a low estimate.

Back to the subject of the thread, B Braking is most useful in only heavy downhill situations which gets back to my original point in the first few comments on this thread. I said for MA drivers of which the OP and I are (except for the very few if any MA Prius drivers that happen upon the extreme north west part of the state over the highest point in the Berkshires at a less than impressive 3500 feet) B Brake is a useless feature. Except for that very small portion of the state, you will not find any heavy descents in MA that remotely require the use of B Brake.

[richard schumacher]

Originally Posted by codybigdog123

Why? When you take your foot off the gas, no fuel is consumed...whether in B or D.

"B" increases regeneration, which is inherently less efficient than gliding or coasting using light brake regeneration, and it will use some amount of engine braking, meaning that energy will be lost entirely. "B" really should only be used as an alternative to riding the brake on long steep downgrades. There's no benefit in it otherwise.

[codybigdog123]

Originally Posted by richard schumacher

"B" will use some amount of engine braking, meaning that energy will be lost instead of recovered and stored in the battery for re-use. "B" really should only be used as an alternative to riding the brake. There's no benefit in it otherwise.

This has nothing to do with what I said. I said that taking your foot off the gas pedal, and putting it in B will not use gas...ergo, your gas mileage during the decline (as long as your foot is off the gas) will not be any worse/better than had the car been in D. That's all I said in my prior post.

By the way, your statement regarding "energy will be lost instead of recovered and stored in the battery for re-use" is misleading...ie, energy will be recovered and stored in the battery, if the battery is not fully charged. But as long as the battery is not fully charged, some re-charging of the battery does occur. I've seen it on my car. I've read it. So it does happen.

[qbee42]

Originally Posted by codybigdog123

<snip>

By the way, your statement regarding "energy will be lost instead of recovered and stored in the battery for re-use" is misleading...ie, energy will be recovered and stored in the battery, if the battery is not fully charged. But as long as the battery is not fully charged, some re-charging of the battery does occur. I've seen it on my car. I've read it. So it does happen.

It is correct that some energy will be recovered by regenerative braking in B mode, but that would happen with non-B mode braking as well. B mode also deliberately wastes energy by spinning the ICE as a big air pump, just like a jake brake on a truck. This energy wasting begins well before the high limit on the battery, which is why it is often pointed out that B mode is a mileage loser. Non-B mode will wait until the high limit before switching to engine braking.

Tom

[codybigdog123]

Originally Posted by qbee42

It is correct that some energy will be recovered by regenerative braking in B mode, but that would happen with non-B mode braking as well. B mode also deliberately wastes energy by spinning the ICE as a big air pump, just like a jake brake on a truck. This energy wasting begins well before the high limit on the battery, which is why it is often pointed out that B mode is a mileage loser. Non-B mode will wait until the high limit before switching to engine braking.

Tom

Tom,

Thanks. I got all that, and yes, I understand that some regenerative energy is wasted in B. However, my point is still valid - when your foot is taken off the gas pedal, you do NOT consume fuel in the B mode, and unless the battery is at capacity, some of the regenerative energy goes back to the battery. Prior posts gave the impression that while in B, fuel is consumed, and no charging of the battery takes place.. That is wrong. I am not debating which method is superior. For some, with steep declines, the B mode is best. Under other circumstances, it isn't. That was my only point.

[fuzzy1]

Originally Posted by hill

The search tool can be your friend. Use 'B' for downhill / high speed descents and you'll get worse mpg than if you'd simply taken your foot off the pedal. .

In the places I use B-mode, mpg is irrelevant. It is a matter of throwing away energy with engine compression, or burning the brakes, or rear-ending the car ahead at high speed, or flying off a corner due to excessive speed.

On steep hills, regeneration fills the traction battery after just a few hundred vertical feet. Beyond that, the gravitational energy must be burned off somehow.

[qbee42]

Originally Posted by codybigdog123

Tom,

Thanks. I got all that, and yes, I understand that some regenerative energy is wasted in B. However, my point is still valid - when your foot is taken off the gas pedal, you do NOT consume fuel in the B mode, and unless the battery is at capacity, some of the regenerative energy goes back to the battery. Prior posts gave the impression that while in B, fuel is consumed, and no charging of the battery takes place.. That is wrong. I am not debating which method is superior. For some, with steep declines, the B mode is best. Under other circumstances, it isn't. That was my only point.

Fair enough. This topic has been beat to death in numerous other threads. B mode, like all of the modes, does not consume fuel when there is no call for energy, other than the special case of producing heat for the ICE, cat system, and cabin. B mode does waste energy, by design, which is exactly why it is used on long, steep downhills.

Tom

[hobbit]

Folks, please keep in mind that in the absence of other drag
factors and starting from a "virtual dead empty" of 40% SOC,
regen is only good for a MAXIMUM of 600 vertical feet. Then
you're all-green and done. Same with the 2010, as it's got the
same battery capacity and roughly the same weight. At that
point you'd better be B-ing or burning.
.
_H*

[patsparks]

I have found that in B mode, a full green battery, descending a hill and my foot lightly on the brake the engine revs exceed 4000 RPM! That is some serious energy dumping.

[qbee42]

Originally Posted by patsparks

I have found that in B mode, a full green battery, descending a hill and my foot lightly on the brake the engine revs exceed 4000 RPM! That is some serious energy dumping.

It makes the car sound like a hover craft. When it does that, I get a kick out of pressing the accelerator and listening to the engine slow down. It's very counter-intuitive to one raised on "normal" cars.

Tom